Cover photograph:
Marsaxlokk fishing port, Malta; courtesy of Joseph Alan Sciortino.

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ISBN 978-92-5-106696-6

E-ISBN: 978-92-5-107819-8 (MOBI)

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© FAO 2010

Preparation of this document

This manual was prepared by Mr Joseph Alan Sciortino, MICE, M. ASCE, C. Eng., the Food and Agriculture Organization of the United Nations (FAO) Consultant on Harbour Design and Management. The manual reflects the provisions of the FAO Code of Conduct for Responsible Fisheries in relation to fishing harbours and is an extension of Annex VI, No. 1, in the series of FAO Technical Guidelines for Responsible Fisheries. It contains proposals and guidelines to be followed with regard to the design, development, management and maintenance of ports and landing places for fishing vessels. It also provides guidance on the conduct of environmental auditing in relation to new construction and the upgrading of existing facilities. It was based on a manual prepared in electronic format in 1999, and updated and revised to integrate new guidelines, ideas, experiences and lessons from projects in different parts of the world.

The manual was technically edited by Mr John Fitzpatrick, C. Eng., C. Mar. Eng., MIMAREST, former Director of the FAO Fish Products and Industry Division. The draft of the manual was reviewed by Mr B.N. Krishnamurthy, FAO Consultant on Harbour Management and Institution Building and Mr V. Venkatesan, FAO Consultant on Community Participation in Fisheries, both under the Technical Cooperation Project on Capacity Building in Support of Cleaner Fishing Harbours in India; and Mr Simon Diffey, Chief Technical Advisor of the project on the Restoration and Improvement of Fish Landing Centres with Stakeholder Participation in Management funded by the Canadian International Development Agency in Sri Lanka.

The manual was edited by Maria Giannini. Susana V. Siar, Fishery Industry Officer, Fishing Operations and Technology Service (FIRO) of the FAO Fisheries and Aquaculture Department, provided overall supervision in the preparation and publication of this manual. The publication contributes to the achievement of the following organizational result: the operation of fisheries, including the use of vessels and fishing gear, is made safer, more technically and socio-economically efficient, environmentally-friendly and compliant with rules at all levels.

Abstract

The role of the fishing port may be considered as the interface between the netting of fish and its consumption. In many cases, the fishing harbour is also the focal point of pollution, both of the surrounding environment and the fishery products it produces. Many fishing harbours are also the source of major impacts on the physical and biological coastal environment.

Although the bulk of fish landed in fishing harbours in developing countries is destined for the local markets, it is every country’s wish to improve the health hazard-free quality of its landed catch in order to increase exports of seafood products to more lucrative overseas markets. In the not-too-distant future, the growth in local consumer rights advocacy will also increase demand for health hazard-free fish.

In today’s world of increased environmental awareness, a fishing port must be planned, designed and managed in harmony with both the physical and biological coastal environments. At each stage of the process, whether it is planning, design or management, both technical and non-technical persons become involved in the process. Within government departments, whether they be technical (fisheries or public works) or non-technical (budget or finance), it is not uncommon for non-technical persons to affect the outcome of technical decisions. Fisheries Departments worldwide generally have to manage and maintain harbours and landing places using non-engineering civil servants. The following manual was produced in order to tackle fishing harbours in a holistic approach.

This manual is useful to both technical and non-technical planners, both at national government level and at departmental level. It provides non-engineering staff within such departments with enough technical knowledge to better understand certain basic design requirements, which could otherwise be interpreted as superfluous and not cost effective.

The manual is of particular use to local independent consulting engineers and architects with no ports or fisheries experience involved in the design of locally tendered projects for the various international funding agencies. To technical staff of such firms, it provides a handy reference and the means for integrating Hazard Analysis and Critical Control Points (HACCP) and European Union Directive recommendations on hazard-free seafood directly into the fishing port’s design.

The fishing industry as a whole can ill-afford the economic losses from lower prices received for contaminated fish. Recent European Union rulings have even gone one step further by banning outright all fish imports from certain countries.

Sciortino, J.A.

Fishing harbour planning, construction and management.

FAO Fisheries and Aquaculture Technical Paper. No. 539. Rome, FAO. 2010. 337p.

Contents

Preparation of this document

Abstract

Abbreviations and acronyms

Executive summary

1.

Code of conduct, conventions and importing country directives

2.

Infrastructure needs assessment

3.

Port management

4.

Environmental auditing

5.

Hydrographic surveys

6.

Dredging

7.

Breakwaters

8.

Quays and slipways

9.

Construction materials

10.

Fittings and navigational aids

11.

Shore-based infrastructure and renewable energy

12.

Public health, hygiene and waste disposal

Annex 1

–

FAO Technical Guidelines for Responsible Fisheries – Fishing Operations – 1 (Annex VI)

Annex 2

–

Port hygiene checklist

Annex 3

–

Port hygiene deficiencies checklist

Annex 4

–

Training manual on seafood handling

Annex 5

–

Prevention of pollution

Abbreviations and acronyms

AC

alternating current

AISI

American Iron and Steel Institute

BOBP

Bay of Bengal Programme

CFC

community fishery centre

CFCs

chlorofluorocarbons

COFI

Committee on Fisheries

DC

direct current

DGPS

Differential Global Positioning System

EC

European Community

EEZ

exclusive economic zone

EIA

environmental impact assessment

EIS

environmental impact studies

EPS

Electronic Positioning System

EU

European Union

FAO

Food and Agriculture Organization of the United Nations

FMO

fisheries management organization

GHG

greenhouse gases

GPS

Global Positioning System

GT

gross tonnage

HACCP

Hazard Analysis and Critical Control Points

HDPE

high-density polyethylene

HHWL

Higher High Water level

HHW

highest high waters

IALA

International Assocation of Marine Aids to Navigation and Lighthouse Authorities

IATA

International Air Transport Association

IEE

Initial Environmental Examination

IMO

International Maritime Organization

IPCC

International Panel on Climate Change

ISO

International Organization for Standarization

ISPS

International Ship and Port Facility Security Code

IUU

illegal, unreported and unregulated

LAT

lowest astronomical tide

LDPE

low-density polyethylene

LED

light-emitting diode

LLW

lowest low waters

LLWS

Lower Low Water Spring

LUX

1 lumen per square metre

MARPOL

International Convention for the Prevention of Pollution from Ships

MCS

monitoring, control and surveillance

MLLW

Mean Lower Low Water

m/s

metres per second

MSL

mean sea level

MSY

maximum sustainable yield

N/mm^2

Newtons per square millimetre

NGO

non-governmental organization

PAH

polynuclear aromatic hydrocarbons

ppb

parts per billion

ppm

parts per million

PVC

polyvinyl chloride

PVN

Private Virtual Network

PV

photovoltaic

RO

reverse osmosis

SAR

search and rescue

SOLAS

International Convention for the Safety of Life at Sea (IMO Convention)

SWL

safe working load

UHMW

ultra-high molecular weight

UKC

under keel clearance

UNCED

United Nations Conference on Environment and Development

UNCLOS

United Nations Convention on the Law of the Sea, 1982

uPVC

unplasticized PVC

UXO

unexplored ordinance

W/C

water-cement ratio

WGS84

World Geodetic System 1984

WHO

World Health Organization

Executive summary

The unrelenting pressure on the world’s coastal resources and its coastal ecosystems is threatening the viability of fisheries as an industry. By its very nature, an ill-conceived port or fish landing structure has the potential to place a disproportionately large demand on both the local physical and biological resources and, in today’s world of increased environmental awareness, a fishing port or fish landing site must be planned, designed and managed in harmony with these environments.

Fishing ports and fish landing sites are complex dynamic interface zones involving the mixing of environmental, ecological, economic and social activities and problems. This heterogeneous mix of activities demands strong cross-sector interaction at the planning stage to ensure that the resulting infrastructure may be managed in a sustainable manner. This manual is not the first effort by FAO to set forth a set of cross-sector guidelines for the planning and/or management of a fish landing site or fishing port. In various forms, information related to fishing ports design has been available since 1976, the most recent being FAO Training Series 25 on the construction and maintenance of artisanal fishing harbours and village landings (1995).

In recent years, world fisheries have become a market-driven sector of the food industry and many coastal states have striven to take advantage of this new opportunity by investing heavily, and sometimes haphazardly, in fishing port infrastructure. When, in the early to mid-1990s various international conventions and importing country directives on food safety came into force, much of this same infrastructure was found to be non-compliant, requiring further investment. Also, in the intervening period, the requirement for proper Environmental Impact Studies became mandatory in most States, further complicating the added investments required to make such infrastructure compliant.

Why this manual? The manual builds on the earlier work by FAO and introduces for the first time the international conventions and importing country directives that have a direct influence on both the planning and design of fishing port infrastructure. It is intended as a reference not only for engineers and architects involved in the design and rehabilitation of fishing ports but also for non-technical staff at departmental level in government institutions that may influence technical decisions in the field. These institutions may comprise, directly or indirectly, policy and planning, budget and finance, export and public health authorities. It may also be of use to donor agencies and non-governmental organizations when planning fisheries-related investments.

The use of this manual should enable consultants, architects and contractors involved in any stage of design or construction of a fish landing or port to avoid the mistakes of old. This manual covers all the aspects of fishing port infrastructure, from inception to final design as well as the construction and management of the fishing port or landing once constructed. This manual does not replace the textbooks required to design the single elements that constitute the fishing port, such as the breakwaters, the quay walls, etc. Any costs quoted in the manual refer to United States dollars and generally apply to any country with good connections. Operational costs in some developing countries may differ significantly due to logistics. To technical staff in general, it provides a handy reference and the means for integrating HACCP and importing country directive recommendations on hazard-free seafood directly into the fishing port’s design.

The previous version of this manual was the basis of a distant-learning course. This edition is a good handbook based on the author’s extensive experience in the design, construction and operation of fishing ports in many developing countries.

There are twelve chapters and five reference annexes in this manual. Each chapter deals with a particular topic and is a stand-alone document.

The first chapter presents the technical guidelines to the Code of Conduct for Responsible Fisheries. It also lists the relevant international conventions to which a State may be a party, together with third country directives that jointly will have a direct bearing on the overall design of the port and its components. These include compliance with basic engineering principles regarding the morphological degradation of the coastal zone in respect of erosion and siltation (UNCED 92), compliance with the relevant conventions concerning pollution of the aquatic environment (MARPOL 73/78) and the provision of adequate monitoring of the effects of operations on the environment (UNCED 92). The Code also addresses post-harvest practices setting out minimum standards for food safety and quality assurance.

The second chapter describes how to plan shore-based facilities and fishing ports that are commensurate with the targeted resources, within the exclusive economic zone (EEZ) of a coastal State and distant water fisheries, as the case may be, that would be environmentally sustainable and financially justifiable. It addresses the major issues that influence the size of a new port and describes in detail the classification of fishing ports, from artisanal beach landings to coastal fishing ports, offshore fishing ports and distant water fishing ports. Guidelines on the preferred siting of a fishing port vis-à-vis land use are also discussed. The chapter also dwells on post-conflict reconstruction procedures and on overcapacity (redundant vessels) and potential income diversification for fishermen faced with an ever-dwindling catch.

The third chapter tackles port management, from the simple fish landing on a beach to a proper deep water fishing port. It stresses the need for a stakeholder approach to port management whereby all the port users are represented on the management body. Typical port management bodies are described in detail covering all scenarios, from the simple village landing to a sophisticated fishing port. It also highlights the importance of a fish quality assurance regime inside the port and lists best management practices for port and boatyard operators and the prevention of pollution.

Chapter four reviews the environmental auditing procedures to be followed for all new designs. Degradation of the marine environment can result from a wide range of activities on land. Coastal erosion and siltation are of particular concern. Hence, rational use and development of coastal areas as well as conservation of the marine environment require the ability to determine the present state of these ecological systems and to predict future conditions. Systematic collection of data on marine environmental parameters is needed for an integrated management approach to sustainable development and to predict the effects of the construction of a port on the marine environment. This chapter should be of use to government institutions involved in fisheries-related development seeking international investment funds. The guidelines are also useful for the development or expansion of commercial ports which may impact fishing operations.

Hydrography is the foundation for all coastal work and chapter five covers the topic in great detail. In the past, many artisanal shelters and fishing ports were built at convenient locations, with no particular attention paid to the underwater environment. Many such ports are now plagued by erosion and siltation problems that could have been identified earlier had proper hydrographic surveys been conducted. This chapter should provide planners and engineers with cost-effective options when faced with tight budgetary restrictions and difficult logistics. It describes the amount of hydrographic detail required for a project from inception stage to construction phase.

Dredging and underwater excavation, chapter six, are an important aspect in the design and construction of key elements in a fishing port. Dredging may be required to develop a port basin or to maintain a navigation channel open, but may also be undertaken for other purposes such as land reclamation, beach nourishment and environmental remediation of contaminated sediments. Dredging may be of a permanent nature, also known as capital dredging, or of a transient nature, also known as maintenance dredging. The nature of the material to be dredged determines the type of dredging equipment required or method to be adopted but, on the whole, dredging practice is site specific. Also, in recent years, the screening for the presence of potentially harmful chemical agents in the material to be dredged has become mandatory in many countries influencing not only the method of dredging but also the method of disposal. This chapter may be used as a guide to assist in the decision-making process for the selection of appropriate dredging and disposal techniques.

Chapter seven is dedicated exclusively to breakwaters. The sea is unforgiving and wave action continually degrades human-made structures erected in the sea and breakwaters form the bulk of this construction. Therefore, engineers are normally entrusted with the design and maintenance of large deep water breakwaters. However, cases also arise when artisanal structures are required at the local level and when engineering assistance is either not available or not with the right experience. This chapter is intended primarily for non-technical staff coming to terms for the first time with marine construction and purposely does not include advanced design aids for structures in deep water, the realm of professional engineers.

Chapter eight covers quays and slipways in detail and should provide the reader with a wide range of options for both quays and maintenance structures, such as slipways and boat gantries.

Chapter nine condenses the subject of materials knowledge and suitability into a single chapter while eliminating superfluous coverage. The chapter is confined to the properties of those materials and treatments or variations thereof that are applicable to port structures. Emphasis is placed on full coverage of the basic materials that have proved most durable in the highly aggressive marine environment inside port structures.

The tenth chapter is a comprehensive overview of all the mechanical fittings and quay furniture required to run a port efficiently, from bollards and fenders all the way to marker beacons, winches and fish boxes. It provides an ideal reference guide to both technical and non-technical readers.

Chapter eleven reviews the shore-based infrastructure typical in a port operation, from water supply, power, refuelling, ice, port buildings and paved areas. Any reader will find this chapter useful, either as a reference or as a guide on how to integrate HACCP and third country directives into a port’s infrastructure design.

Chapter twelve provides a holistic approach to public health issues in fisheries ranging from water and water-borne contaminants to sewage and sewage treatment options. The comprehensive detail of this chapter will enable the reader to better understand the ease with which fish, meant for human consumption, is contaminated. The chapter will also assist port planners in dealing with the prevention of pollution in fishing ports.

The manual also has a list of annexes for use as reference material. Annex 1 reproduces the text from the FAO Technical Guidelines, No. 1, Fishing Operations, on the procedures for the development and management of harbours and landing places for fishing vessels. Annexes 2 and 3 are checklists for port hygiene and port hygiene deficiencies, respectively. The fourth annex is a training manual on seafood handling prepared by Francisco Blaha, formerly FAO Fishery Industry Officer for the Technical Cooperation Project on Capacity Building in Support of Cleaner Fishing Harbours in India. The fifth annex is a useful tool already used with success in public awareness campaigns on the prevention of pollution in fishing harbours.

Part II

Construction

The publication deals with the planning, the construction and ultimately with the management of fishing ports. This publication should enable consultants, architects and contractors involved in any stage of design or construction of a fish landing or port to avoid the mistakes of old as it covers all the aspects of fishing port infrastructure, from inception to construction and management of the fishing port or landing once constructed. This manual does not replace the textbooks required to design the single elements that constitute the fishing port This e-version of the publication is divided into three parts.

Part II: Construction deals with the physical construction of the port itself and discusses issues relating to hydrographic surveys, dredging, breakwater construction, quays, slipways and construction materials. This section is intended primarily for non-technical fisheries staff coming to terms for the first time with marine construction and purposely does not include advanced design aids for structures in deep water, the realm of professional engineers.

Part I and Part III are published as separate e-books.

5.  Hydrographic surveys

SUMMARY

In the past, many artisanal shelters and fishing ports were built at convenient locations, with no particular attention paid to such environmental factors as wave heights, sudden changes in water depths, uncharted reefs, currents, tidal streams, seaweed and mobile beaches (sand drift). Many of the structures were subsequently expanded and, in countless cases around the world, many of the problems that used to be considered minor have now developed into major ones, with some shelters, for example, fouling up with seaweed or silting up (shelter mouth facing the wrong direction) or just being inaccessible in rough weather (reefs too close to entrance channel).

A hydrographic survey, also known as a bathymetric survey, is therefore essential if the correct design decisions are to be made right from the project inception stage to ensure that the landing is easy to use and free of major maintenance problems under all conditions.

This chapter (related to Chapter 7) reviews the various types of hydrographic surveys and equipment that exist in relation to the amount of detail required during the design stage and illustrates clearly how this can be applied at the artisanal level. Consequently, the reader will be able to draw up appropriate terms of reference for hydrographic surveys.

CONTENTS

5.1

Hydrographic standards and classifications

5.1.1

Introduction

5.1.2

Methodologies

5.1.3

Vertical depth measurements

5.1.4

Horizontal position fixing

5.2

Effects of vessel roll pitch and heave

5.3

The WGS84 datum

5.4

Specifying hydrographic surveys

5.4.1

Outline design and project formulation

5.4.2

Preliminary design

5.4.3

Final design

5.4.4

Construction phase of a port

5.4.5

Maintenance dredging and reclamation

5.5

Sample surveys

5.6

Tide surveys

5.6.1

Tides

5.6.2

Tide datum

5.6.3

Primary tide stations

5.6.4

Tidal streams

5.7

Bibliography and further reading

5.1 HYDROGRAPHIC STANDARDS AND CLASSIFICATIONS

5.1.1 Introduction

The results from a hydrographic survey are normally plotted to produce a bathymetric contour map, which is a plan of the depth of the sea bed arranged in such a manner as to show lines of equal depth from the coastline. In a hydrographic survey, the actual measurement of the water depth is the easy part. The main problem is not knowing how far the survey boat is from the coastline when the depth is recorded.

Boat A in Figure 1, for example, has no point of reference in relation to the coast. Boat B, on the other hand, is using a calibrated float line to obtain a “FIX” or position with respect to the coastline; in this case, 20 metres away on the straight line between the peg and the buoy. Hence, for each vertical depth recording, a horizontal position “FIX” is also required. Both vertical depth measurements and horizontal position measurements may be carried out either manually (low tech, low cost) or using sophisticated depth and position fixing equipment (high tech, high cost), depending on the end use of the survey.

Hydrographic surveys are required for a wide variety of purposes, ranging from simple reconnaissance (at project formulation, for instance) to payment for work carried out underwater, such as dredging or reclamation. The size of the area to be surveyed also influences the methodology to be used and hence the equipment required. In ascending order of accuracy, hydrographic surveys may be broadly classified as:

• Reconnaissance

or

Class 3

• Project condition or design

or

Class 2

• Contract payment

or

Class 1

Levels of accuracy for hydrographic surveys are intended to correspond directly to these three classes (Table 1). Class 3 is the lowest accuracy standard, Class 2 is a medium accuracy standard, and Class 1 is the highest accuracy standard.

Since expensive horizontal positioning and depth measurement equipment may be installed aboard a particular survey vessel, this same equipment may be typically used for all three classes. Any accuracy distinction between the classes is primarily a function of the field procedures used, along with recognized equipment limitations.

TABLE 1

Maximum allowable errors in hydrographic surveys

Type of survey

Class 3

Class 2

Class 1

Vertical accuracy

500 mm

300 mm

150 mm

Horizontal positioning

100 metres

12 metres

6 metres

5.1.2 Methodologies

There are various methodologies in use nowadays to carry out a hydrographic survey, depending on the end use of the survey and the size of the area to be surveyed. Vertical depth measurements may be carried out using:

- hand-held calibrated lead sounding line;

- simple engineering echosounder recording on paper; and

- advanced engineering echosounder recording on a data logger and linked to position fixer via integrated software (fully automated).

Horizontal position fixing measurements may be carried out using:

- hand-held optical square in conjunction with a float line;

- single theodolite in conjunction with a float line or twin theodolites;

- constant range tracking electronic positioning system (EPS); and

- differential Global Positioning System (GPS).

5.1.3 Vertical depth measurements

Figure 2 illustrates the hand-held calibrated lead sounding line, right, and on the left, the simple engineering echosounder (transducer not shown).

Figure 3 illustrates an advanced echosounder of the type used in modern Class 1 surveys. The echosounder is linked to a software package and yields electronic contour maps directly. This type of echosounder is also used for real-time monitoring of dredging works.

5.1.4 Horizontal position fixing

5.1.4.1 Method 1

The hand-held optical square in conjunction with a float line method, also known as the parallel line method (Figure 4), is the most basic and consists in setting out a straight baseline along the beach, say 100 metres long or more, depending on the extent of the hydrographic survey, with a ranging rod placed at either end. At every 5 or 10 metres from either end (5 metres for irregular terrain and 10 metres for flat beach), a steel peg is driven into the ground and, by means of an optical square (or theodolite), a buoy is dropped offshore at right angles to each peg.

One end of the float line is anchored to the steel peg and the other to its corresponding buoy offshore. By tying in the baseline to the topographic survey, the depth readings may be plotted on paper in the right place. It is always a good practice to extend the survey about 100 metres on either side of the proposed shelter or landing.

The actual depth of the water may be read by simply lowering the calibrated sounding chain every 5 or 10 metres and the person using the chain calls out the readings to another person in the boat who records the figures on paper in the correct order. This type of recording yields a grid with spot levels only. If an engineering echosounder is available with an experienced operator, the actual soundings may be recorded on the special thermal paper roll by the instrument itself. In this case, only the operator need accompany the boat pilot on the survey vessel up and down the graduated float lines. A continuous bottom profile is thus obtained on a continuous strip chart and sounding levels scaled off the chart.

When carrying out the hydrographic survey:

- The sounding chain must reach the bottom in a vertical line; when using a sounding chain the vessel must be still when the actual reading takes place. If the area is subject to strong tidal streams, the weight of the sinker should be increased by attaching further weights to the chain.

- The depth reading must be recorded with the time at which the recording was made to allow for changes in the level of the tide.

- If an echosounder is being used, inbound runs are preferred to outbound runs. By starting an inbound run, say 50 metres away from the float, the vessel's skipper will be in a better position to place the boat parallel to the floating line.

- In both cases, stormy or windy days should be avoided as should the flood and ebb periods in strong tidal areas. The sea should be perfectly calm.

- In rocky areas, before removing the float lines, a swimmer wearing a pair of goggles (or diver) should then swim up and down the surveyed line looking for submerged rock outcrops or wrecks. The diver should point these out by placing small floats near them and measure the depth of water over the obstacle. These floats should then be plotted on the survey map by taking a series of fixes from the baseline with the theodolite, Method 2, triangulation.

Figure 5 illustrates the type of plotted soundings derived by this method of survey. This method is considered acceptable for all three classes of surveys with a maximum offshore distance of 200 metres and is suitable for fish landing areas, small fishing ports, breakwater construction, minor reclamation and minor excavation and/or dredging.

The calibrated float line, illustrated in Figure 6, should be made up from a length of 12 mm diameter polypropylene rope with coaxial coloured floats at 5 and 10 metre intervals. One large spherical red float should be installed every 50 metres to ease chain readings from the survey vessel. The completed float line may be either wound on a purpose-made plastic drum or stowed inside a large fishermen’s basket and paid out over the bow or stern of the vessel.

The float line is rather difficult to keep stretched in a perfect line in the presence of wind, waves or current. It is therefore recommended that survey work involving float lines be carried out when the sea is perfectly calm (Figure 7).

5.1.4.2 Method 2

The single theodolite, in conjunction with a float line (ray method) or twin theodolite intersection (triangulation method), is the second most basic method of fixing positions offshore. In the past, positioning by these methods from baseline points onshore was often used to position vessels on near-shore projects.

Although it is no longer commonly employed, this method is considered acceptable for all three classes of surveys with a maximum offshore distance of 300 metres for the ray method, Figure 8, and around 1 000 metres for the triangulation method. Beyond this distance it may be difficult to collimate the theodolite on small objects, especially during hot summer days. More often, this method is used to perform EPS calibration when fixed points are inaccessible to the vessel. Triangulation positioning techniques are labour intensive. Two shore-based transit or theodolite observers are required, along with communication equipment with which to transmit the observed angles (or direction azimuths) to the surveyor. Due to the higher precision and stability of the instruments, the resultant positional accuracy can be quite good, provided observing procedures are properly executed.

Simultaneously observed positional “fixes” are usually radioed over to the survey boat. Fixes may be at equal time intervals or as called for on a random or as-needed basis, such as during EPS calibration. Advance warning is made of upcoming fix events so that observers can initiate precise tracking of the boat. A defined point aboard the vessel (mast or aerial) is normally tracked. This well-marked point should be centred over the echosounder transducer.

5.1.4.3 Method 3

The constant range tracking EPS used to be the most commonly employed positioning method. This method replaced the triangulation theodolites with electronic microwave ranging EPS which utilize range-range positioning techniques. This method has now been superseded by GPS techniques.

5.1.4.4 Method 4

Differential GPS is the primary survey reference for all types of present-day engineering and construction activities. GPS is a continuous, all-weather, worldwide, satellite-based electronic positioning system. It is available to the general public and is known as a standard positioning service. Over the past several years, a technique has been developed to process signals from two GPS receivers operating simultaneously to determine the 3-D line vector between the two receivers. This technique is known as “differential positioning” (DGPS) and can produce real-time positions of a moving vessel, Figure 9.

5.2 EFFECTS OF VESSEL ROLL PITCH AND HEAVE

Correcting observed depths for the superimposed effects of vessel roll, pitch and heave is perhaps the most difficult aspect of hydrographic surveying since all three conditions can occur simultaneously and at different periods. Roll and pitch introduce bias error in depth, resulting in a deeper reading over a level bottom (Figures 10 and 11). On side-mounted transducers, this bias error is compounded by the random up and down motion (heave) of the vessel.

Unless reliable motion compensation devices are used, the only practical method of minimizing vessel motion effects is to limit the maximum allowable sea states under which a particular class of survey may be performed.

Such limitations are highly subjective and can have significant economic impacts, due either to delayed survey work or to inaccurate payment when a Class 1 survey is performed under adverse conditions. Maximum sea state limitations must also factor in the size and relative stability of the survey vessel (Figure 12), along with the effects of the prevailing wave direction relative to the survey lines or cross-sections. Thus, a simple maximum allowable wave height criterion is difficult to specify. Hence, hydrographic surveys are best performed during calm weather spells.

5.3 THE WGS84 DATUM

In Figure 9, GPS readings of the roving antenna are given above a datum called the WGS84 (World Geodetic System 1984). A cursory look at the topographic and oceanographic details of the globe indicates that the Earth is a very irregular and complex shape. To map positions of those details, a simpler model of the basic shape of the Earth, sometimes called the “figure of the Earth”, is required.

The science of geodesy, on which all mapping and navigation is based, aims firstly to determine the shape and size of the simplified “figure of the Earth” and goes on to determine the location of the features of the Earth’s land surface – from tectonic plates, coastlines and mountain ranges down to the control marks used for surveying and making maps. Hence, geodesists provide the fundamental points of known coordinates which cartographers and navigators take as their starting point. The first question of geodesy, then, is “What is the best basic, simplified shape of the Earth”?

The Earth is very nearly spherical. However, it has a tiny equatorial bulge that makes the radius at the equator about 0.33 percent bigger than the radius at the poles. Therefore, the simple geometric shape which most closely approximates the shape of the Earth is a biaxial ellipsoid, which is the three-dimensional figure generated by rotating an ellipse about its shorter axis. The shorter axis of the ellipsoid approximately coincides with the rotation axis of the Earth. Because the ellipsoid shape does not fit the Earth perfectly, there are many different ellipsoids in use, some of which are designed to best-fit the whole Earth and some to best-fit just one region (Figure 13). The datum used for GPS positioning is called WGS84. It consists of a three-dimensional Cartesian coordinate system and an associated ellipsoid, so that WGS84 positions can be described as either XYZ Cartesian coordinates or latitude, longitude and ellipsoid height coordinates. The origin of the datum is the geocentre (the centre of mass of the Earth) and it is designed for positioning anywhere on Earth. The WGS84 datum is a set of conventions, adopted constants and formulae and includes the following items:

- The WGS84 Cartesian axes and ellipsoid are geocentric; that is, their origin is the centre of mass of the whole Earth including oceans and atmosphere.

- The scale of the axes is that of the local Earth frame.

- The orientation of the ellipsoid equator and prime meridian of zero longitude coincide with the equator and prime meridian of the Bureau Internationale de l'Heure at the moment in time 1984.0 (that is, midnight on New Year’s Eve 1983).

- Since 1984, the orientation of the axes and ellipsoid has changed such that the average motion of the crustal plates relative to the ellipsoid is zero. This ensures that the Z axis of the WGS84 datum coincides with the International Reference Pole, and that the prime meridian of the ellipsoid (that is, the plane containing the Z and X Cartesian axes) coincides with the International Reference Meridian.

5.4 SPECIFYING HYDROGRAPHIC SURVEYS

Although hydrographic surveys have become fully automated through the use of custom-designed software, they are still considered expensive surveys to carry out, especially when the survey equipment has to be moved in from far afield.

The size of the area to be surveyed depends greatly on the project being attempted and some basic rules-of-thumb are appropriate (Table 2).

TABLE 2

Class of hydrographic surveys

Project design stage

Outline

Preliminary

Final

Phase of studies

Formulation

IEE

EIS/EIA

Standard required

Navigation chart

CLASS 3

CLASS 2

type of construction

Port

Dredging

Reclamation

Standard required

Class 1

Class 1

Class 1

5.4.1 Outline design and project formulation

During project formulation no physical surveys are required. A sea chart at the appropriate scale is more than enough for selecting potential sites to insert in a shortlist for further detailed studies.

5.4.2 Preliminary design

At preliminary design or project preparation, a Class 3 survey is adequate. This can be an all manual survey carried out from a local fishing vessel using locally available equipment. If the project is located on or near a sandy beach, the area covered by the survey should extend at least 250 metres on either side of the project and down to the 7 metre contour.

5.4.3 Final design

During the final design of a port project, a physical or mathematical model may need to be built and run to determine the potential impacts of the port on the adjacent coastline. A Class 2 survey is required. If the port project is located on a rocky coastline, the area covered by the survey should extend at least 500 metres on either side of the project. If the port project lies on a sandy coastline, the survey should extend 1 000 metres on either side of the project. In both cases, the offshore extent of the survey must reach the 20 metre contour.

5.4.4 Construction phase of a port

During the construction phase of a port, a new and more detailed survey is required for the purposes of payment. Monitoring surveys may also be required in areas prone to erosion or accretion. Class 1 surveys are normally required at this stage and the extent of the survey does not need to exceed the submarine footprint of the port structures.

5.4.5 Maintenance dredging and reclamation

Only Class 1 surveys are required and the extent does not need to exceed the submarine footprint of the dredging works.

5.5 SAMPLE SURVEYS

Figures 14 and 15 are examples of the type of precision required at different stages in the design and construction process of a port. Figure 14 is a Class 2 survey that was commissioned during the environmental impact studies (EIS) for a new port to determine the behaviour of waves impacting the site via the construction and operation of a physical model. The vertical resolution required at this stage was 1.0 metre (with a Class 2 accuracy) with a horizontal grid of 100 metres (also with a Class 2 accuracy). Figure 15 illustrates a Class 1 survey utilized for the final payment of the contractor for the excavation work carried out for the construction of the breakwater. The vertical resolution is now 10 centimetres with a horizontal grid of 2.50 metres, both well below the maximum Class 1 accuracy.

5.6 TIDE SURVEYS

5.6.1 Tides

“Tides” should not be confused with “tidal streams”, although loose terminology has undoubtedly come to use the word “tide” for both. A tide is a periodic vertical movement in the level of the sea, whereas a tidal stream, even though resulting from a tide, is a periodic horizontal movement. Tides affect the depth of water at a place; tidal streams affect navigation courses.

A tide is a periodic vertical movement in the level of the sea. In consequence of the solar cycle, at times of new and full moon, at a place the highest high waters (HHW) and the lowest low waters (LLW) of a tide cycle – SPRING TIDES – will be experienced and 7-¼ days after these, with the first and last quarters of the moon, the lowest high waters and the highest low waters of a tide cycle – NEAP TIDES – will occur.

There are thus two separate tide cycles: height fluctuations from SPRINGS to NEAPS twice each in a lunar month (29 days), Figure 16, and height oscillations of each tide from high water to low water twice each in each lunar day.

5.6.2 Tide datum

The hydrographic surveys in tidal areas should be referenced to a base elevation or chart datum, Figure 16, normally taken as the lowest astronomical tide level, or LAT, also referred to as LLWS (Lower Low Water Spring) in some countries. This level is the result of the effect of celestial bodies only and does not include the additional effects of:

- atmospheric pressure (low pressure increases tide level);

- storm surge (increases tide level);

- wind (landward wind increases tide level); and

- heavy rainfall (increased flow in estuaries increases tide level).

In areas where this is not possible, hydrographic surveys in tidal areas should be referenced to a base elevation which has been determined by measuring the tide heights over the Tidal Datum Epoch. The Tidal Datum Epoch is the specific 19-year period or Metonic cycle (where the moon returns to exactly the same place at the same longitude and against the same constellation in the sky with the same phase) over which tide observations are taken and reduced to obtain mean values (e.g. Mean Lower Low Water or MLLW) for tidal datums.

Due to the long-term rise in global sea level and land subsidence, tidal datum readings are constantly changing and require continuous monitoring and updating.

To facilitate the process of establishing tidal datum readings, tide stations are operated at various locations (at all major and secondary port cities) for long- (primary), medium- (secondary), and short-term (tertiary) durations.

5.6.3 Primary tide stations

Long-term tide stations are referred to as primary control tide stations. These are tide stations at which continuous observations have been made over a minimum 19-year Metonic cycle. Their purpose is to provide data for computing accepted datums needed for a project and the predicted tides are normally published as a tide almanac. Nowadays, with the advent of the Internet, tidal predictions are available for most parts of the world, Figure 17. Tidal datum readings may also be determined by recovery of nearby tidal benchmarks from a former datum determination or by new tidal observations conducted in accordance with the correct procedures, Figure 18.

A length of plastic tape stapled to a length of steel tube makes an ideal tide board. This may either be buried in a concrete weight on a sandy bed or cemented to a rocky coast as illustrated in Figure 18. In both cases, the tide board should be installed in a sheltered area and observations carried out for at least one month. The longer the observation period, the more accurate the establishment of the tidal datum.

5.6.4 Tidal streams

The tidal phenomenon described above gives rise to tidal streams – periodic horizontal movements of the water. In open ocean this horizontal movement is either non-existent or negligible, but in inshore and coastal waters where there is any appreciable vertical movement a horizontal movement also can be expected.

The cause of tidal streams is primarily a change in water levels.

The average velocity of tidal streams depends upon the average height of the advancing tide wave: in deep ocean where this height is small the stream rate is either very feeble or negligible; where the height is large the velocity will be correspondingly great. As a tidal stream encounters any obstruction to its to-and-fro movement, its direction and velocity are affected.

So when a tidal stream meets a headland it is deflected around it, and usually immediately off the headland its velocity is locally increased. In Figure 19 above, its direction is deflected round the headland into the bay beyond, causing an indraught of indefinite direction and velocity. In the bay itself the spreading out of the effect of the stream, combined with the fact that the real strength runs from headland to headland leaving comparatively unmoved water in between, causes a diminished velocity.

In the absence of tides, feeble sea currents may be experienced during strong sea storms. These currents, though not very great when compared to tidal streams, should be observed closely because they usually carry weeds uprooted from offshore areas.

Currents in general make navigation more difficult but not impossible. In the presence of seaweed or flotsam (including timber and vegetation carried down by rivers), however, navigation may be hindered by weeds fouling up propellers. Flotsam, or floating debris, may also prove troublesome if it piles up inside a harbour by a prevailing tidal stream or sea current. Using a simple can-float, Figure 19 right, with a counterweight hanging about a metre below water level, the strength of a current may be measured by timing it to travel a known distance along the coast, or across a bay. When measuring currents at sea, the following points should be observed:

- the general direction and duration of the storm or incoming waves, if the currents are storm generated;

- if seaweed appears, after how many hours of storm does it make its first appearance; and

- if flotsam or driftwood appears, their landfall, in many instances one particular spot or bay may accumulate more debris than adjacent ones, in which case it should be discarded as a potential harbour site.

5.7 BIBLIOGRAPHY AND FURTHER READING

International Hydrographic Organization (IHO), 4 quai Antoine 1er, BP 445 MC 98011 Monaco Cedex.

International Hydrographic Organization (other publications) (www.iho.shom.fr)

- Promulgation of Radio Navigation Warnings

- General Bathymetric Chart of the Oceans (GEBCO)

- FIG/IHO International Advisory Board on Standards for Hydrographic Surveyors

- Technical aspects of the Law of the Sea

- Technical assistance to developing hydrographic offices

US Army Corps of Engineers. 1994. Hydrographic Surveying: Engineering and Design. Washington DC, US Army Corps of Engineers.

6.  Dredging

SUMMARY

Dredging and underwater excavation are an important aspect in the design and construction of certain key elements of a fishing harbour’s infrastructure. Capital dredging may also be an important aspect in the design of a new port if access channels or water deepening of a basin are required for the first time. Maintenance dredging is essential to keep certain access channels, fishing port basins and canals subject to high sedimentation open to navigation.

This chapter reviews the various types of dredging techniques that exist in relation to the characteristics of the material to be dredged. Thus the reader will be able to assess the dredging requirements for small ports and be able to suggest the most suitable method of dredging in whatever type of material.

CONTENTS

6.1

Requirements for dredging

6.1.1

Volume estimates

6.1.2

Characterization of materials

6.2

Dredging techniques

6.2.1

Pontoon-mounted grabbing

6.2.2

Dragline

6.2.3

Pontoon-mounted pumping

6.2.4

Trailing suction

6.2.5

Blasting

6.2.6

Bucket dredging

6.2.7

Cutter-suction

6.2.8

Backhoe dredgers

6.2.9

Chiselling

6.2.10

Other methods

6.2.11

Dredging adjacent to quay walls

6.3

Disposal of dredged sediments

6.3.1

Introduction

6.3.2

Classification of dredged sediments

6.3.3

Environmental impacts of dredging

6.3.4

Open water disposal

6.3.5

Confined shore disposal

6.4

Dredging of fairways

6.4.1

Minimum width

6.4.2

Minimum depth

6.5

Bibliography and further reading

6.1 REQUIREMENTS FOR DREDGING

The basic requirements to assess dredging works are:

- an accurate estimate of the volumes to be excavated or dredged; and

- an accurate evaluation of the nature of the material to be excavated.

6.1.1 Volume estimates

Accurate volume estimates are important for the choice of dredging plant, production estimates, times of execution and ultimately project costs. Expressed in quantities, a dredging operation can extend from a few hundred cubic metres to many millions of cubic metres. In order to arrive at an accurate dredging volume, the following is required:

- a detailed design layout showing areas to be dredged and the design depths required together with the relevant cross-sections;

- a Class 2 hydrographic survey of the area with bathymetric contours at 1 metre intervals for extensive projects (see Figure 14 in Chapter 5) or a Class 1 hydrographic grid with bathymetric contours at 0.25 metre intervals for isolated structures (see Figure 15 in Chapter 5); and

- a geotechnical survey, including borehole logs, in situ and laboratory test results and samples.

6.1.2 Characterization of materials

An evaluation of the physical, chemical and biological characteristics of the sediment to be dredged is necessary to determine potential dredging methods, beneficial use, disposal options and potential environmental impact.

6.1.2.1 Physical characteristics

The nature of the material to be dredged decides the type of equipment or method to be adopted to carry out the work thereby influencing the cost of the project, especially when mobilizing the plant from a distant site.

In practice, no sea bed material will fall precisely into a certain predetermined category, so combinations of types must be described accurately. When a specialist soil-investigation contractor is given the task of classifying the materials to be dredged, soil classification problems do not arise. Problems do arise, however, with artisanal-scale projects, where the volumes to be dredged are usually very small and the construction budgets are very limited.

In the absence of detailed geotechnical studies (i.e. on very small projects), clear descriptions should be noted down in addition to the collection of the samples in airtight glass jars. Typical examples of such descriptions include:

(i) In sand and silt deposits:

- compacted, coarse, angular sand mixed with scattered broken sea shells;

- loose, rounded, fine to medium sand with coarse gravel;

- brown, rounded, slightly compact fine sand;

- hard, brown clay containing sand and gravel;

- soft, grey/blue, sand silt; and

- stiff, fissured, grey clay.

(ii) In clay or clayey deposits:

- very soft, may be squeezed easily between fingers;

- soft, easily moulded by fingers;

- firm, requires strong pressure to mould by fingers;

- stiff, cannot be moulded by fingers, indented by thumb; and

- hard, tough, indented with difficulty by thumb nail.

In granular deposits, it is useful to remember that if all the sand grains are visible to the naked eye, then the material is entirely sand. If individual particles are invisible, the sample is classified as silt or silty sand. Gravel may be described as rounded, irregular, angular, flaky or elongated. Its texture may be classified as polished, smooth or rough. The general classification for granular material is as follows (Table 1):

TABLE 1

Classification of granular materials (diameter in mm)

6.1.2.2 Chemical characteristics

In 1972, the general interest in the importance of the environment resulted in the holding of the United Nations Conference on the Human Environment in Stockholm, Sweden. As part of the preparatory process for the conference an Inter-Governmental Working Group on Marine Pollution held its first meeting in London in 1971 and the Stockholm conference recommended that governments ensure that “ocean dumping by their nationals anywhere, or by any person in areas under their jurisdiction, is controlled and the governments continue to work towards the completion of and bringing into force as soon as possible of an over-all instrument for the control of ocean dumping…”. In response to this recommendation, the United Kingdom of Great Britain and Northern Ireland convened a conference which met in London from 30 October to 13 November 1972 and adopted the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (the “London Convention” for short).

The Convention entered into force on 30 August 1975 and the first meeting of Contracting Parties in December that year agreed to designate the International Maritime Organization (IMO) to be responsible for the Secretariat duties in relation to the Convention. The Convention’s purpose is to control all sources of marine pollution and prevent pollution of the sea through regulation of dumping into the sea of waste materials. It covers materials transported to sea for the purpose of dumping. The Convention is a living document that responds to new information, pollution issues and environmental concerns through a process of consultative meeting, scientific and legal debates, consensus building and the addition of new member nations.

Past amendments and proposed future actions to the Convention reflect our growing knowledge of or different approaches to waste management or of the potential harm from certain substances or processes. The London Convention, in October 1996, adopted revisions resulting in the 1996 Protocol to the Convention. This Protocol prohibits ocean disposal, except for a few types of wastes. Dredged material is one of these.

The screening for the presence of chemical agents in the material to be dredged is required to determine where and how the material will be disposed. The screening for chemical agents is based on a step-by-step investigation of any number of the following parameters:

- verifiable data from previous chemical tests on material in the vicinity of the proposed dredging;

- known major geochemical characteristics of the sediment;

- potential routes by which contaminants could reasonably have been introduced to the sediment;

- probability of contamination from agricultural and urban runoff;

- spills of contaminants in the area to be dredged;

- industrial and municipal point discharges, past and present; and

- prior use and source of the sediment.

The sampling of the sediments from the proposed dredging site should be representative of the vertical and horizontal distribution and variability of the properties of the materials to be dredged. The sampling procedures should also anticipate the information required for the evaluation and selection of the dredging and disposal techniques.

6.1.2.3 Biological characteristics

If the potential impacts of the dredged material cannot be assessed on the basis of the physical and chemical characteristics and available benthic information, biological or effects-based testing should be considered. It is important to consider information about species known to occur in the area which may be affected and the potential effects of the sediments to be disposed on the said organisms. Biological tests should incorporate species that are considered appropriately sensitive and representative and should determine, where appropriate:

- acute toxicity;

- chronic toxicity covering an entire life cycle;

- potential for bio-accumulation; and

- potential for tainting.

See Section 6.3.2 on the classification of the sediments to be dredged. Within the London Dumping Convention regulations, dredged material may be exempted from further testing if it meets one of the criteria listed below:

- dredged material is excavated from a site away from existing and historical sources of appreciable pollution, so as to provide reasonable assurance that the dredged material has not been contaminated; or

- dredged material is composed predominantly of sand, gravel and or rock; or

- dredged material is composed of previously undisturbed geological materials.

If any one of the above applies and if the geological records do not indicate the presence of potential leachates from heavy metal mineral deposits, it may be assumed that the dredged material has not been contaminated from human sources to an extent that necessitates further investigation. The evaluation and assessment procedure in these cases may normally be restricted to proper site selection and evaluation of physical impacts. Dredged material which does not meet any of the above criteria will require further investigation.

6.2 DREDGING TECHNIQUES

Dredging is most likely to be in one of the following types of sediment:

- cohesionless deposits, such as sand or weakly cemented granular deposits;

- cohesive clayey deposits;

- silty mud from inside existing port areas; and

- rock, ranging from soft coral limestone to the much harder granite;

Depending on the consistency of the deposits, various dredging techniques may be employed. The most common techniques or equipment are:

- pontoon-mounted grabbing;

- pontoon-mounted (or shoreline) dragline;

- pontoon-mounted pumping;

- self-propelled trailing suction dredging;

- blasting, using high explosives or low-power explosives;

- bucket dredging;

- cutter-suction dredging;

- backhoe dredging;

- chiselling; and

- airlift.

6.2.1 Pontoon-mounted grabbing

Figure 1 illustrates an ocean-going grab dredger, consisting of a self-propelled vessel with various on-board hoppers with a crane or cranes mounted on the deck. A pontoon dredger, on the other hand, consists of a barge-mounted crane, propelled by a tug and assisted by two hopper barges (see Figure 3).

A grab dredger lowers, closes and raises a single grab by means of wire rope. In operation, the clamshell grab, Figure 2 (left), is dropped to the sea bed in the open position and bites into the sediment due to its weight and the action of the closing mechanism. The materials thus dredged are either dumped into the on-board hopper wells (in ocean-going dredgers) or into adjacent hopper barges, Figure 2 (right), for transport to the authorized dump site.

The advantages of the grab dredger are:

- flexibility of use in maintenance dredging in all kinds of marine sediments, for which a variety of grabs are available;

- efficient to depths of 35 metres, beyond which the free-fall effect of the grab limits efficiency; and

- very suitable for working along quays and in the corners of harbour basins.

The disadvantages are:

- when stationary in a navigation channel it causes an obstruction to sea traffic; and

- it leaves irregular bottom topography.

6.2.2 Dragline

Figure 4 (right) illustrates the technique of dredging by dragline. This method consists in dragging an open steel bucket along the sea bed until it fills up. When full, the bucket is lifted and its load dumped on the shore or into a barge.

The dragline is not used much outside North America. It tends to spill much of the dredged sediment during operation and gives rise to considerable turbidity. Most of the functions of a dragline nowadays are performed more efficiently by backhoes, both from the shoreline as well as mounted on a barge, Figure 4 (left).

6.2.3 Pontoon-mounted pumping

The pontoon-mounted pump dredger consists of a large submersible pump suspended from a pontoon and dunked into the sea bed (see Figures 5 and 6). The slurry (11 percent sand by weight) is pumped away via floating pipes. For slurries containing extremely fine-grained sand particles, the velocity in the discharge pipe should exceed 1.50 m/s and for coarser sands 3 to 4 m/s. The advantage of the pontoon pump is that it is capable of a larger output than any other dredger of comparable size. Some pumps are also equipped with cutter blades to handle sea grass and weeds. This method, together with the airlift, is also very suitable for dredging in areas known to contain archaeological remains.

The disadvantages are:

- rough seas, especially swell, hamper this kind of dredging due to the fact that the pump does not always contact the sea bed;

- the range of sediments which can be dredged efficiently is very limited;

- it leaves an irregular bottom topography; and

- when stationary in a navigation channel it causes an obstruction to sea traffic.

6.2.4 Trailing suction

When dredging large volumes of cohesionless sediment, a trailing suction dredger is normally used. A trailing suction dredger is a hopper vessel with a trailing arm suspended over the side and dragged over the sea bed, Figure 7 and Figure 8.

The vessel usually steams forward at around 6 knots, automatically compensating for swell and tidal variations while maintaining the drag head in contact with the sea bed by means of a computerized hydraulic system.

The water/sand mixture is drawn on board by powerful pumps, passed through a series of decanters and the solids deposited inside the internal hoppers whose capacity ranges from 2 000 to more than 25 000 cubic metres. The relatively clear, decanted seawater is dumped overboard. The maximum dredging depth without intermediate pumps is around 35 metres. With one or even two intermediate pumps the dredging depth may be extended to 80 and 120 metres. These dredgers are fully automated and dredging generally takes place over a 24-hour period, non-stop. The dredged sand in the hoppers is either dumped offshore if clean or pumped onshore for reclamation via pipes laid out ashore.

The advantages of trailing suction dredgers are:

- minimum interference to sea traffic;

- versatility in handling both cohesionless and cohesive sediments;

- the dredged load may be pumped ashore as reclamation; and

- constructed in various sizes to suit most project sizes.

The disadvantages are:

- the final dredged depth is less precise, necessitating some overdredging; and

- mobilization costs can be considerable.

6.2.5 Blasting

When thick layers of rock or isolated rocky outcrops need dredging, blasting is an efficient method of excavation. In this method, a series of closely spaced holes is drilled into the sea bed, charged with explosives and fired. The holes in the sea bed may be drilled either from above water level, Figure 9, or directly from the sea bed, Figure 10. The hydraulic power to operate the driller is supplied from a shore-based power pack via floating or submerged lines. Both systems are generally carried out by specialist subcontractors.

Blasting is a relatively quick method for dealing with small amounts of very hard rock formations like outcrops. Large quantities of hard rock are nowadays excavated by rock-cutter suction dredgers or bucket dredgers.

The disadvantages are:

- the blasting must be accompanied by grabbing for the removal of the spoil;

- unless carried out from the sea bed, both drilling and grabbing cause an obstruction to sea traffic;

- the indiscriminate loss of sea life during blasting;

- the percentage of fines generated by the fragmentation of coralline rock are very hard to predict; and

- mobilization costs can be considerable.

6.2.6 Bucket dredging

The bucket dredger, also known as the ladder dredger, consists of an endless bucket chain scraping the sea bed while the dredger is moved across the area to be dredged by means of a series of anchor ropes (Figure 11).

Although most bucket dredgers nowadays have been replaced with cutter-suction dredgers, quite a few of these dredgers are still around. The maximum dredging depth is normally around 25 metres, although 34 metres is possible with the largest dredger of this type. The buckets range in size from 50 litres to 1 000 litres.

The advantages of a bucket dredger are:

- capability of dredging a level bottom topography;

- ability to work in narrow or restricted areas;

- versatility in handling a wide range of sediments; and

- side loaded barges are generally filled with a high solids-to-water ratio.

The disadvantages are:

- when stationary in a navigation channel it causes an obstruction to sea traffic;

- rough seas, especially swell, hamper this kind of dredging; and

- bucket dredgers are noisy and in urban areas they may be prohibited from working at night.

6.2.7 Cutter-suction

Cutter-suction dredgers are among the most popular type of dredger, available in a wide range of sizes ranging from the small portable units that fit on a large road trailer to ocean-going vessels over 100 metres long.

A small cutter-suction dredger is able to handle small volumes of weakly cemented sediments as well as weeds (Figure 12). The maximum discharge pipe diameter rarely exceeds 150 mm for the portable or demountable type of dredger.

Figure 13 shows the most popular range of cutter-suction dredgers, consisting of two demountable pontoons and a centrally hinged dredging ladder capable of dredging from -3.0 metres to -7.0 metres. The power on the cutting head (Figure 14, left) may range from 40 hp on the smaller versions to 1 200 hp on the larger models, making them suitable for dredging in soft limestone. Discharge pipe diameters range from 250 to 800 mm.

Figure 14, right, shows the full-size dredger commonly used for large-scale dredging operations. Typically, the sand/water mixture is pumped a distance away and longer-than-normal distances are achieved by the installation of booster pumps along the discharge line.

The advantage of a cutter-suction dredger is:

- suitability for dredging in a wide range of sediments.

The disadvantages are:

- in the presence of coralline limestone, the cutter head invariably generates considerable fines in the 10 micron range that are difficult to settle;

- the water-to-sand ratio of the dredged material is so high that it must be pumped directly to the disposal site;

- rough seas, especially swell, hamper the smaller dredgers; and

- when stationary in a navigation channel it causes an obstruction to sea traffic.

6.2.8 Backhoe dredgers

Backhoe dredgers are land excavators mounted on a pontoon equipped with spuds like a cutter-suction dredger (Figure 15). As with land-based excavators, various fittings are available for tackling different kinds of sediments.

The maximum operating depth is limited to around 18 metres.

The advantages of a backhoe dredger are:

- ability to work in narrow or restricted areas;

- versatility in handling a wide range of sediments; and

- side-loaded barges are generally filled with a high solids-to-water ratio.

The disadvantages are:

- when stationary in a navigation channel it causes an obstruction to sea traffic; and

- rough seas, especially swell, hamper this kind of dredging.

6.2.9 Chiselling

Rock breaking may be carried out by a steel chisel dropped through a height by a crane. Normally it is only used in low volume cases where trimming of rock is required. Production rates are very low.

6.2.10 Other methods

Various other methods may be encountered in practice but they range from specific experimental adaptations to exotic plant.

Airlifting, for example, is frequently used when clearing buried wrecks, but in construction, air lifting is only used to clear underwater foundation trenches from silt drift during construction. An airlift may be used to dredge very small volumes of fine cohesionless deposits only. In the presence of soft clays or clayey silts, airlifts merely form a vertical-sided hole and the material does not tend to slump towards the airlift unless assisted by independent water jets.

Figure 16 shows two types of assisted airlifts typically used in the excavation of foundations in clayey conditions. The airlift in Figure 16 (left) has been used with success on the excavation of bridge foundations in the United States of America. It consists of a 254 mm ejector pipe supplied with air through a 63.50 mm pipe at 11 cubic metres per minute, giving an up-flow velocity of 3 metres per second which is capable of lifting 60 mm gravel. The soil is loosened by a 150 mm jetting pipe operated independently of the airlift. The jetting pipe fed 4 swivelling nozzles of 38 mm diameter fed by two 7 cubic metre per minute pumps delivering water at 34 bar pressure.

6.2.11 Dredging adjacent to quay walls

Maintenance dredging adjacent to existing quay walls in fishing ports requires careful assessment of both the material to be dredged as well as the human-made rubbish discarded on the sea bed like tyre fenders, anchors, lengths of chain or wire rope, polyester rope and nets (Figure 17). In most cases, a visual inspection by divers is necessary to verify conditions on the sea bed to be dredged prior to deciding on the type of equipment to be used (cutter-suction versus grab).

In the presence of the aforementioned material, the dredging should be carried out by a grab dredger as illustrated in Figure 3.

6.3 DISPOSAL OF DREDGED SEDIMENTS

6.3.1 Introduction

The type of material to be dredged will greatly influence the method of disposal. However, costs can be turned into profits if the dredged material can be used to reclaim land (a process known as reclamation) for industry, roads, housing or leisure. Materials vary in their suitability for reclamation. Sand is the most suitable material for reclamation but all types of granular materials (crushed limestone, gravel, etc.) are suitable. Silt can be used for reclamation if sufficient time is available for drying out and settlement. Due to its fine texture, silt does not dry out as quickly as sand. Instead, before drying out to a fine powdery consistency, it goes through the “muddy” stage, which can take anywhere up to a few months to dry. However, mixed with sand, silty deposits tend to dry out much faster. Sand also improves the consistency of the reclaimed land. Soft clay, once churned up by a cutter-suction dredger, is not suitable as a fill unless it is dried out. This material is only suitable as agricultural fill and cannot be used for foundations or roads. Stiff clay forms into clay balls during the dredging operation and again it is only suitable as agricultural fill.

6.3.2 Classification of dredged sediments

Before deciding on the method of disposal, the dredged sediments must be first classified according to their potential to contaminate the environment where they are due to be deposited. Sediments may be classified under one of three classes:

- Class 1 – clean material, allowable for placing in any type of open water disposal site (open placement on sea bed);

- Class 2 – slightly contaminated, allowable for placing in certain open water disposal sites but requiring careful placing (inside a pit or depression of the sea bed); and

- Class 3 – contaminated material, in principle, not suitable for open water disposal but to be confined in either very strict or well-controlled disposal sites (capped atolls or reclamation).

Various tests on the standard of pollution are required by international legislation and range from the simple water leaching test to multiorganism benthic bioassays or laboratory-controlled tests on selected marine organisms.

The elutriate test is a water leaching test using one part sediment to be dredged to four parts water taken from the proposed dump site. This test, in use since 1973, has been evaluated under an extremely wide range of conditions and is useful for evaluating the short-term release of contaminants from dredged sediments into open water.

The multiorganism benthic bioassays comprise a range of laboratory tests on such organisms as algae, zooplankton, filter feeders (bivalves), deposit feeders (crustaceans) and burrowing species (polychaetes). The tests range from simple physical disruption to direct toxicity and bioaccumulation and consider the liquid, suspended and solid phases of the dumping. Even though bioaccumulation may occur from all three phases, the liquid (or leaching stage) and the suspended (dust plume) phases are of secondary concern due to their short-term occurrence. The solid phase, however, must be assessed for its bioaccumulation potential due to the long-term contact between the dumped sediment on the sea bed and marine organisms.

6.3.3 Environmental impacts of dredging

Strictly speaking, even capital and maintenance dredging in Class 1 sediment (clean material) are an environmentally disruptive exercise and an environmental impact assessment should always be carried out prior to any dredging. Dredging is characterized by three distinct operations:

- the removal of material from an existing environment;

- the method of removal of the material; and

- the dumping of the unwanted material in another environment.

The removal of the material in itself has many potential impacts on the environment and great care should be exercised in distinguishing each and every one of these impacts. For example:

- the sea bed may be used by fish for spawning (some fish require soft sand to dig a nest);

- most shellfish live in sand; and

- if seaweed is also anchored to the sea bottom, the area may serve as a nursery ground for juvenile fish.

The method of removal also has many potentially negative impacts and careful planning is required prior to mobilizing equipment. For example:

- A cutter-suction dredger is very fast but under certain conditions (especially in the presence of coral, coralline limestone and clay) it creates a dust plume visible for miles around the dredger; this plume blocks out sunlight (not particularly good for live coral) and chokes fish by depositing fine dust on their gills.

- High explosive blasting is a quick method of removing obstacles but it also kills marine life indiscriminately in the vicinity of the project.

- A leaking clam-shell grab is not suitable for dredging in a sensitive environment (clear water coral environment) due to the dispersion of fines in the water column.

Finally, the physical and chemical impacts of the actual dumping of the unwanted sediment must also be investigated. Ideally, if the material is clean material, then it should be used for reclaiming land or for filling. However, when the material consists of clay (not suitable for reclamation) mud or silt, it is usual to dump it offshore with a hopper barge, Figure 2. Dumping of Class 1 sediment, for example, may still:

- stifle growth in certain bivalve species through smothering with fine silt; and

- indefinitely render the water murky with the slightest wave agitation if the silt portion is too high.

Dumping of Class 3 sediment, for example, may:

- transpose contaminants from one environment to another and contaminate potentially fertile environments used in fisheries.

Class 3 sediments must be capped whether dumped at sea or on land.

6.3.4 Open water disposal

Since dumping of dredged materials at sea is still a major means of disposal of Class 1 and Class 2 sediments, the selection of dump sites should be carried out by matching the characteristics of the sediment and the site.

Figure 18 illustrates the four basic types of aquatic disposal of sediments up to Class 2 level of contamination. Type 1 consists of a human-made depression in the sea bed (dredged or excavated channel), Type 2 consists of a natural depression in the sea bed, Type 3 consists of a level sea bed and Type 4 is lateral confinement. In all cases, the material deposited may need to be capped with a layer of clean material to isolate it from the surrounding environment.

Permission for the aquatic disposal of dredged sediments at a specific site must be sought from the delegated competent authority.

The permit conditions for a site should be designed such that:

- only those materials which have been characterized and found to be acceptable by the environmental impact studies (EIS) are placed at the site;

- material is placed within the site boundaries;

- that the dredging and disposal management techniques have been identified by the EIS; and

- that monitoring requirements are fulfilled and the results reported to the competent authority.

The transboundary drift of the dredged sediments plume, Figure 19, during or after dumping has taken place is a matter of concern when the selected dumping ground is close to territorial boundaries (see Section 1.4.10 on transboundary impacts). In such cases, the permit conditions must include the installation of real-time GPS equipment on the dumping barge and monitoring requirements must ensure that seasonal currents do not transport the plume across national boundaries.

6.3.5 Confined shore disposal

If dredging (granular material) is being carried out by a grab dredger, hydraulic excavator or drag line, then the material may be loaded onto trucks, carted away and tipped behind a retaining structure (a quay wall) as backfill. Typically, such material will come to a rest at a slope ranging from 1 on 2 to 1 on 6 depending on the size and roughness of the individual particles; the smaller and smoother the particles, the flatter the slope, Figure 20.

If the dredging is being carried out by a suction or a cutter-suction dredger, then it may be pumped onshore via floating pipes or via a shore connection, Figures 8, 21 and 22.

Land reclaimed by forward tipping should be protected by a geotextile filter mat (to prevent sand loss and eventual settlement of the reclaimed area) and a layer of at least 2 metres thick of graded stone, as shown in Figure 23. A mass concrete kerb may also be placed at the summit of the rubble bank to prevent vehicles from driving into the water and falls built into the paving to drain surface water away to sea.

Small-to medium-sized earth retaining structures, such as bunds or quay walls, may be constructed in a variety of ways. The simplest bund is a rubble or stone bund, forward tipped by truck and sealed on the landside by an impermeable layer, Figure 24.

Small quay walls may also be constructed in mass concrete by pumping concrete inside a sheet pile cofferdam, Figure 25.

Quay walls may also be constructed with conventional rectangular blocks, Figure 26; “I” section blocks in-filled with stone (lighter to handle than a rectangular block), Figures 27 and 28; “L” shaped units, Figure 29; and annular rings, Figure 30.

Deeper water solutions may be in sheet piles or caissons.

6.4 DREDGING OF FAIRWAYS

6.4.1 Minimum width

Access channels or fairways are defined in Figure 31. In many dedicated channels the navigation markers are placed close to the edge of the channel to indicate the limits for safe navigation.

In determining the channel width, some or all of the following should be considered:

- basic maneuverability of the vessels intending to use the channel;

- cross-winds, which may cause vessels with high windage to drift sideways;

- cross-currents, which may also cause deep-draft vessels to drift sideways; and

- wave action, which may be heading or following (affecting pitch and heave) or beam (affecting roll and heave).

If a two-way channel is proposed, then allowance must be made for vessels to pass each other safely. Such a distance, Figure 32, must ensure that vessel-to-vessel interaction is reduced to an acceptable limit and it is usual to allow for a central safety strip, known as the “passing distance”.

Bank clearance should be large enough to reduce bank effects to a minimum, especially in the presence of mobile or unstable deposits such as sand or mud. As a rule of thumb, the minimum fairway width should not be less than eight times the width of the largest vessel to enter the fishing port.

In channels with a wide range of traffic, the fairway markers should be positioned in such a way as to allow the passage of smaller vessels on either side of the dredged channel. In some cases, both the deep-water dredged channel and the shallower outer lanes for smaller vessels may be marked.

There may also be a maximum permissible speed imposed to avoid bank erosion due to backwash from the passage of vessels.

6.4.2 Minimum depth

For a vessel to safely transit the access channel or fairway, it must have adequate clearance under the keel. The under-keel clearance, also known as UKC, depends on a variety of different parameters. These parameters are related to the vessel, the type of sea bed and the water level, Figure 33.

The net under-keel clearance is the minimum margin remaining between the channel bed level and the keel of the vessel in fully loaded condition and in the most unfavourable weather condition. The net under-keel clearance should not be less than 0.50 metre for a soft sea bed (silt or sand) and not less than 1.0 metre for a hard sea bed (rock).

6.5 BIBLIOGRAPHY AND FURTHER READING

International Maritime Organization. Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter 1972 and 1996 Protocol Thereto (available at www.londonconvention.org).

PIANC. 1977. Final Report of the International Commission for the Study of the Environmental Effects of Dredging and Disposal of Dredged Materials. Annex to Bulletin No. 27. Brussels, Belgium.

PIANC. 1989. Working Group No. 14. Economic Methods of Channel Maintenance, Permanent International Association of Navigation Congresses, Supplement to Bulletin No 67. Brussels, Belgium.

Vellinga T. 1984. Dredging and disposal of contaminated dredged material. Dredging and Port Construction, Vol. XI, No 12.

7.  Breakwaters

SUMMARY

A breakwater is a structure constructed for the purpose of forming an artificial harbour with a basin so protected from the effect of waves as to provide safe berthing for fishing vessels. There are many different types of breakwaters; natural rock and concrete, or a combination of the two, are the materials which form 95 percent or more of all the breakwaters constructed.

This chapter reviews the various cross-sections of the most common types of breakwaters and their method of construction. It does not go into great detail with regard to the design of breakwaters as this is best left to the professional engineers for specific applications. Nevertheless, a brief description of the design requirements is given for the sake of clarity.

The reader, however, will be well able to understand the different typologies of breakwaters in use nowadays and appreciate the complexity of choice and underwater construction.

CONTENTS

7.1

Parameters for the construction of a breakwater

7.1.1

Hydrographic survey

7.1.2

Geotechnical investigation

7.1.3

Wave hindcasting

7.1.4

Material needs assessment

7.1.5

Cross-sectional design

7.2

Construction methods

7.2.1

Land-based equipment

7.2.2

Floating equipment

7.2.3

Methodologies

7.3

Floating breakwaters

7.4

Bibliography and further reading

7.1 PARAMETERS FOR THE CONSTRUCTION OF A BREAKWATER

When a breakwater is to be built at a certain location, and the environmental impact of such a structure has already been evaluated and deemed environmentally feasible, the following parameters are required before construction can commence:

- a detailed hydrographic survey of the site;

- a geotechnical investigation of the sea bed;

- a wave height investigation or hindcasting;

- a material needs assessment; and

- the cross-sectional design of the structure.

7.1.1 Hydrographic survey

The hydrographic survey that is described in Chapter 5 is required for the calculation of the volumes of material required for the breakwater.

7.1.2 Geotechnical investigation

A geotechnical investigation of the sea bed is required to determine the type of founding material and its extent. The results of this investigation will have a direct bearing on the type of cross-section of the breakwater. In addition, it is essential to determine what the coastline consists of, for example:

- soft or hard rock (like coral reefs or granite);

- sand (as found on beaches);

- clay (as in some mangrove areas); and

- soft to very soft clay, silt or mud (as found along some river banks, mangroves and other tidal areas).

In the event that the harbour basin is to be formed by the breakwater itself, a proper advanced site investigation by a specialist contractor is recommended, particularly when project cost is expected to be considerable. On the other hand, if the proposed breakwater structure has no direct bearing on the outcome of a project (for example, if the breakwater is an added protection to a natural inlet) and if it is to be executed on an artisanal scale, then simple basic investigations may suffice.

7.1.2.1 Basic geotechnical investigations

Basic geotechnical investigations normally suffice for small or artisanal projects, especially when the project site is remote and access poor. A basic geotechnical investigation should be carried out or supervised by an experienced engineer or geologist familiar with the local soil conditions. The following activities may be carried out in a basic investigation using only portable equipment:

- retrieval of bottom sediments for laboratory analysis;

- measurement of bottom layer (loose sediment) thickness; and

- approximate estimation of bearing capacity of the sea bed.

The equipment required to carry out the above-mentioned activities consists of a stable floating platform (a single canoe is not stable enough, but two canoes tied together to form a catamaran are excellent), diving equipment, a Van Veen bottom sampler (may be rented from a national or university laboratory), a 20 mm diameter steel pricking rod and a water lance (a 20 mm diameter steel pipe connected to a gasoline-powered water pump).

Before the start of any work, the area to be investigated should be marked via a set of marker buoys or a scaffold pipe frame placed on the sea bed and the exact coordinates noted for future reference. To retrieve samples from the sea bed, a Van Veen hand-operated bottom sampler is required, Figure 1. Simply picking up samples from the sea bed with a scoop or bucket disturbs the sediment layers with the eventual loss of the finer material and is not a recommended method. The sediments thus collected should then be carefully placed in wide-necked glass jars and taken to a national or university laboratory for analysis. At least 10 kilograms of sediment are normally required by the laboratory for a proper analysis.

Sometimes, a good hard bottom is overlain by a layer of loose or silty sand or mud. In most cases this layer has to be removed by dredging to expose the harder material underneath. To determine the thickness of this harder layer, a water lance is required. This consists of a length of steel tubing (the poker), sealed at the bottom end with a conical fitting and connected to a length of water hose at the top end. The water hose is connected to a small gasoline-powered water pump drawing seawater from over the side of the platform. The conical end has four 3 mm diameter holes drilled into it.

The diver simply pokes the steel tube into the sediment while water is pumped into it from above until the poker stops penetrating, Figure 2. The diver then measures the penetration. This method, also known as pricking, works very well in silty and muddy deposits up to 2 to 3 metres thick. It is not very effective in very coarse sand with large pebbles.

Once the layer of soft sediment has been identified (sampled) and measured (pricked), it is then necessary to determine the hardness of the underlying layer. The underlying layer may be rock, clay or compacted sand. If the layer is rocky, the diver should collect a piece of the material for laboratory analysis using a hammer and chisel. For softer types of material, the diver (with a submerged weight of around 10 kilograms) should use a steel probe (1 metre long, 12 millimetres in diameter) or pocket penetrometer, Figure 3. An area of around 300 mm square should be cleaned of loose sediment and the probe or penetrometer placed vertically over it. The 10-kilogram exertion on the probe will cause the probe to penetrate into the material. The diver then notes the penetration for the engineer to estimate the bearing capacity. If a pocket penetrometer is used, the bearing capacity may be read off the penetrometer scale directly.

7.1.2.2 Advanced geotechnical investigations

An advanced geotechnical investigation normally requires the retrieval of undisturbed core samples, Figure 4, taken from the level of the sea bed down to a depth ranging from 10 to 30 metres, depending on the type of structure envisaged and the ground conditions obtaining at the site.

Advanced geotechnical investigations are normally carried out by specialist contractors or soil laboratories and require a mobile drilling rig. The drilling rig can travel to most destinations but must be installed on a stable platform before it can be used to drill for cores over water, Figure 5.

7.1.3 Wave hindcasting

The height of wave incident on a breakwater generally determines the size and behaviour of the breakwater. It is hence of the utmost importance to obtain realistic values of the waves expected in a particular area. Behaviour of water waves is one of the most intriguing of nature’s phenomena. Waves manifest themselves by curved undulations of the surface of the water occurring at periodic intervals. They are generated by the action of wind moving over a waterbody; the stronger the wind blows, the higher the waves generated. They may vary in size from ripples on a pond to large ocean waves as high as 10 metres.

Wind generated waves cause the most damage to coastal structures and if winds of a local storm blow towards the shore, the storm waves will reach the shore or beach in nearly the form in which they were generated. However, if waves are generated by a distant storm, they travel hundreds of miles of calm sea before reaching the shore as swell. As waves travel across the sea they decay (they loose energy and get smaller and smaller) and only the relatively larger waves reach the shore in the form of swell.

Wave disturbance is also felt to a considerable depth and, therefore, the depth of water has an effect on the character of the wave. As the sea bed rises towards the shore, waves eventually break. The precise nature of the types of wave incident on a particular stretch of shoreline, also known as wave hindcasting, may be investigated by three different methods:

- Method 1 – On-the-spot measurement by special electronic equipment, such as a wave rider buoy, which may be hired for a set time from private companies or government laboratories;

- Method 2 – Prediction by statistical methods on a computer – statistical hindcast models may be performed on the computer if wind data or satellite wave data are available for the area; and

- Method 3 – On-the-spot observation by simple optical instruments – the theodolite.

Methods 1 and 2 give very accurate results but are expensive, especially the hire of the wave rider buoys; they are usually reserved for big projects where precise wave data gathered over a period of time is of the utmost importance.

In Method 1, the observer is an electronic instrument capable of recording continuously on a 24-hour basis far out at sea where the waves are not yet influenced by the coastline (depth of water). Hiring a wave rider buoy and installing it may take anywhere up to six months, depending on the method of procurement and water depth and weather conditions at the site. A minimum of one year’s observations is required but generally three to five years provide more accurate data.

Method 2 is currently the standard worldwide method of establishing the wave climate along most coastlines. The huge amount of wind and wave data gathered by specialist agencies worldwide now enables most computer models to zero-in on most sites. Offshore wave climate data is nowadays compiled from hindcasting methods using detailed wind records available for most areas from weather information agencies. Inshore wave climates are then derived on a case-by-case basis from knowledge of the local bathymetry. At today’s prices, the cost of a detailed inshore wave climate is in the range of US$50 000, excluding the cost of the detailed hydrographic survey required for the area under study. Depending on how much raw data is already processed by the specialist agencies and if detailed bathymetry already exists, a good wave hindcast report takes about one month to produce.

Method 3 is not accurate but is cheaper and lies more within the scope of artisanal projects. It differs from Method 1 in one respect only, in that the observer is a normal surveyor with a theodolite placed at a secure vantage point observing waves close to the shoreline, Figure 6. This method, however, suffers from the following drawbacks:

- The wave heights thus recorded will already be distorted by the water depths close to the shoreline.

- A human observer can only see waves during daylight hours, effectively reducing observation time by a half.

- In very bad weather, strong winds and rain drastically reduce visibility making it difficult to keep the buoy under observation continuously.

- The presence of swell is very difficult to detect, especially during a local storm, due to the very long time (period) between peaks, typically 15 seconds or more.

Hence, this method of calculating wave heights is only suitable for minor artisanal projects with a very small capital outlay. To set up a wave monitoring station is easy and the equipment needed consists of two large buoys (one fluorescent and one white), say 750 millimetres in diameter, a large stone and concrete sinker weighing at least 1 tonne in water, a length of 12 mm nylon rope, a theodolite, a compass and a watch with a second hand or digital readout. At a vantage point, which should be just high enough above sea level to be safe and dry during a storm, a stone pillar should be erected with an anchor screw concreted in at the top so that every time the theodolite is set up it faces the same way in exactly the same position, Figure 6. Apart from the time it takes to set up the theodolite station, observations of major waves may only be undertaken during major storms. Hence this method may take at least one year to produce enough data to be useful for a study.

The two plastic buoys should then be moored a known distance offshore where the water depth is exactly 20 metres, the white buoy to the sinker and the red fluorescent buoy to the white buoy, as shown in the figure. The white buoy keeps the mooring line taut and vertical while the red fluorescent buoy floats freely on the incoming waves.

To calibrate the station, the theodolite should be pointed at the buoy on a very calm day. A witness mark should then be placed on something robust (a wall, for example, is preferable to a tree) in such a manner that the observer can re-point the eyepiece at the buoy in its rest position (even if the buoy is actually bouncing up and down with the incoming waves during a storm) at a later date. In this way the theodolite is not tied up completely with wave height observations but can be used for other work as well in between storms. During a storm, the buoy will float up and down with the passage of the waves. By following the base of the buoy with the same centreline hairlines, the theodolite is made to traverse a small angle, Z, as shown in the figure. Using basic surveying principles, the distance A and angle Z may be used to calculate the height H of a wave which, as a rule of thumb, is twice the height of the displacement above calm water level.

During wave height observations, the following additional information should also be recorded:

- direction of both the incoming waves and wind using the hand-held compass;

- the time difference between each successive wave peak, also known as wave period using the second hand on a watch;

- the exact position of the buoy with respect to the coastline; and

- time of the year when each storm was recorded.

It must be re-emphasized at this stage that this calculation and the method used are only very approximate and suitable for minor projects only.

7.1.4 Material needs assessment

Given that most breakwaters consist of either rock or concrete or a mixture of both, it is evident that if these primary construction materials are not available in the required volume in the vicinity of the project site, then either the materials have to be shipped in from another source (by sea or by road) or the harbour design has to be changed to allow for the removal of the breakwater (the site may have to be moved elsewhere).

To calculate the volume of material required to build a rock breakwater, for example, equidistant cross-sections are required. Each cross-section consists of the proposed structure outline superimposed on a cross-section of the sea bed. Figure 7 shows a grid map with five cross-sections. Figure 7 (middle) also shows cross-section number 2 of the sea bed, with the breakwater cross-section superimposed on it. Each cross-section may then be divided into known geometric subdivisions, like triangles (A and F) and trapezia (B, C, D and E), whose areas are given by standard formulae. In this way, area 2 is given by the sum of areas A + B + C + D + E + F. Similarly, areas 1, 3, 4, 5, etc. may be calculated from the hydrographic chart. The volume of material required is then the sum of volume 1 + volume 2 + volume 3 + volume 4, etc., as shown in Figure 7. Each segment of breakwater, say volume 1, is given by the average of the sum of (area 1 + area 2) multiplied by the distance between sections 1 and 2, in this case, 5 or 10 metres. Mathematically, this can be expressed as 1/2 [area 1 + area 2] x 5 metres. Once the volume of rock has been determined, the most likely source has to be investigated for:

- supply (must be large enough to supply all the rock);

- quality (not all rock is suitable for a breakwater);

- environmental impact (removing rock from the source must not cause negative impact there);

- mining methods (depending on the type of rock, it may have to be blasted, ripped or broken); and

- means of transport (if roads do not exist between source and project site, then other means of transport are required).

7.1.5 Cross-sectional design

Last but not least, a suitable cross-sectional design for the breakwater has to be produced taking into consideration all the previous data, for example:

- water depths (in deep water, solid vertical sides are preferred to save on material);

- type of foundation (if ground is soft and likely to settle, then a rubble breakwater is recommended);

- height of waves (rubble breakwaters are more suitable than solid ones in the presence of larger waves); and

- availability of materials (if no rock quarries are available in the vicinity of the project, then rubble breakwaters cannot be economically justified).

In general, expert advice should always be sought before embarking on the design of a breakwater cross-section. As was mentioned earlier, waves are one of nature’s least understood phenomena and considerable experience is required when designing breakwaters. If expert advice is not available, the following rules of thumb may be applied to very small projects with water depths not exceeding 3.0 metres:

For rubble mound or rock breakwaters:

- Unaided breakwater design should not be attempted in waters deeper than 3 metres.

- If the foundation material is very soft and thick, then a geotextile filter mat should be placed under the rock to prevent it from sinking and disappearing into the mud (Figure 8).

- If a thin layer of loose or soft material exists above a hard layer, then this should be removed to expose the hard interface and the breakwater built on this surface.

- The material grading should be in the range of 1 to 500 kilograms for the fine core, 500 to 1 000 kilograms for the underlayer and 1 000 to 3 000 kilograms for the main armour layer, Figure 9.

- Dust and fine particles should not be placed in the core as these will wash away and cause the breakwater top to settle unevenly.

- The outer slope should not be steeper than 1 on 2 and the inner or harbour side slope not steeper than 1 on 1.5 (Figure 8).

- In general, rock breakwaters absorb most of the wave energy that falls on them and reflect very little disturbance back from the sloping surface.

For solid or vertical breakwaters:

- Unaided vertical solid breakwater design should not be attempted in waters deeper than 2 metres and exposed to strong wave action, Figure 10.

- Vertical solid breakwaters are only suitable when the foundation is a firm surface (rock, stiff clay, coral reef); thick sand deposits may also be suitable under certain conditions.

- In the presence of thick sand deposits, a rubble foundation with adequate scour protection as shown in Figure 10 is recommended lest strong tidal streams, water currents or wave turbulence scour away the sand underneath the foundation.

- The core of a solid breakwater should be cast in concrete; not more than 50 percent of this concrete may be replaced by pieces of rock or “plums”.

Great care should be exercised when deciding the position of a solid breakwater. Solid vertical breakwaters do not absorb wave energy incident on them and reflect everything back, usually causing other parts of a harbour to experience “choppy-sea” conditions.

7.2 CONSTRUCTION METHODS

There are several types of equipment available for marine construction, both land-based and floating. The high cost of purchase, however, puts most of this equipment beyond the reach of village cooperatives, artisanal contractors and small general-building contractors.

Hence, it is assumed that most of the heavy plant will be made available through the government or public works department, or local contractors, and this chapter should be used as a guide to the general type of equipment required for marine work. Large specialist marine contractors often use floating equipment (all cranes mounted on barges, for example, and material like the core is often dumped using barges). When planning the construction of a marine-related project, it would be useful to know beforehand what type and size of construction plant is available in the vicinity of the village or landing.

7.2.1 Land-based equipment

Crawler crane

Figure 11 shows a typical crawler crane. As its name implies, a crawler crane moves forward on its steel tracks. This is the most ideal type of crane for building breakwaters because it is very stable, requires no outriggers (stabilizers which extend from the crane chassis of all rubber–tyred cranes) and is less likely to bounce off an uneven rubble surface into the water.

The most important characteristic is the nominal lifting capacity as this will dictate the maximum outreach that the crane can handle with a given jib size. The nominal capacity of a crane refers to the maximum safe working load that the crane can lift with the jib in the near-vertical position as shown in Figure 11. This load is dependent on the overturning moment of the load and is expressed as:

[Load at the hook] x [the lever arm La] / [Factor of safety] = A Constant

This value is factory-set and is usually displayed in tables inside the crane driver’s cabin. As the jib is lowered thus increasing the outreach (or lever arm La), the working load at the hook must be decreased to compensate for the greater overturning moment of the load. Failure to observe the proportional reduction in the suspended load as indicated in the crane’s tables will result in the crane overturning. Figure 11 also shows two typical attachments required for lifting and placing rock: slings and grapples. Most slings are made from steel wire rope and these should terminate in quick-release shackles to enable the crane driver to release the rock himself once it has been placed. Rock grapples are the industry standard for handling rock. If a rock grapple is used, the weight of the grapple (anything from 500 to 3 000 kilograms) must be subtracted from the safe working load specified for a particular crane.

SAFETY PRECAUTIONS

The crane driver should always wear a hard hat and soundproof earmuffs. Consequently, it is essential to put in place a signalling system between the load handlers and the driver. Load handlers should always wear hard hats and gloves. If the loads are handled with steel-wire ropes, then the appropriate gloves should be worn to prevent injuries.

Hydraulic excavators

Figure 12 shows a hydraulic excavator, which now forms the backbone of most marine work.

Most models offer interchangeable forearm lengths; for normal marine work, a long forearm is required to reach as far away as possible, Figure 12, right.

Excavators can be equipped with:

- hydraulic-powered chisels (for breaking hard material);

- hydraulic-powered rotating cutter-head (for digging in soft material); and

- a range of buckets to suit any condition that may be encountered on site (wide buckets, narrow buckets, small buckets, high-capacity buckets, etc.).

SAFETY PRECAUTIONS

Hard hats should always be worn around an operating excavator. The operator should wear soundproof earmuffs and a signalling system set up between the other workers and the driver.

Bulldozer

Figure 13 shows a typical tracked bulldozer.

Rubber-tyred equipment should not be used on breakwaters; this kind of equipment is prone to bounce around on dumped rock and likely to lead to fatal accidents (by falling into the water). The bulldozer, on the other hand, is slower moving and more stable. This kind of machine is essential when building breakwaters as it is required to level the fine core material as it is forward dumped into the sea. Bulldozers may be equipped with blades (for levelling the core of a breakwater) or buckets. The operator’s cabin may be sealed or open to the elements as shown in the figure.

SAFETY PRECAUTIONS

The operator should wear both a hard hat and earmuffs and a signalling system set up between other workers and the driver.

Tipper trucks

Figure 14 shows the recommended type of truck required for transporting and dumping of rubble.

If proper tipper trucks are not available for a project, then a farm tractor and trailer combination may be adapted to carry rock, aggregates and sand from a quarry to a project site. Considerably more use of direct labour is involved, but at local village level this should not present any problems. The trailers should preferably be made of steel and should be protected on the inside with timber planking. The timber prolongs the useful life of the trailer by absorbing the impacts of individual stones thrown onto the trailer. Care should be exercised when traversing the uneven surface of a rubble core with all rubber-tyred vehicles.

SAFETY PRECAUTIONS

All personnel should wear hard hats.

7.2.2 Floating equipment

Floating crane

Figure 15 shows a typical barge-mounted crane. The crane is either bolted or welded directly to the hull or driven on to the barge and lashed down with cable stays.

The crane can revolve through 360 degrees and the deck of the barge is usually lined with timber so that rock may be placed on the deck without damaging it. The stability of the crane in this instance is dictated by the stability of the barge and field conversions should always be checked for stability by an experienced naval architect. Normally, such cranes need a tugboat or fishing vessel to help them move from one place to another. Exact positioning is usually achieved by anchors.

SAFETY PRECAUTIONS

All personnel should wear hard hats. The barge should be equipped with the safety requirements stipulated for shipboard operations (life jackets, flares, raft, etc.), including MARPOL recommendations for the prevention of pollution at sea.

Tugboat

Figure 15, bottom right, shows a tugboat of the type generally used by marine contractors. The horsepower of these vessels may be anywhere from 200 to 2 000 hp, depending on the type of plant to be handled. Common sizes are in the range of 250 to 500 hp.

SAFETY PRECAUTIONS

The tugboat should be equipped with the safety requirements stipulated for shipboard operations (life jackets, flares, raft, etc.), including MARPOL recommendations for the prevention of pollution at sea.

Hopper barge

Figure 16 shows a general purpose hopper barge used for the transport and dumping of material at sea. Commonly, available barges have a capacity in the range of 500 to 1 000 cubic metres and are generally self-propelled. Hopper barges can be used for dumping the core of a breakwater in deep water (5 metres and deeper) and for dumping excavated or dredged material offshore.

SAFETY PRECAUTIONS

The hopper barge should be equipped with the safety requirements stipulated for shipboard operations (life jackets, flares, raft, etc.), including MARPOL recommendations for the prevention of pollution at sea.

7.2.3 Methodologies

The typical breakwater illustrated in Figure 8 (shallow water only) consists of a mound of coarse stone, also known as a core, covered or protected by blankets or layers of heavier stones.

7.2.3.1 The core

The core typically consists of stone weighing between 1 kilogram and 500 kilograms, without the fine particles (dust and sand) dumped in a heap out into the sea by a dump truck. To facilitate dumping by truck, the core should be ideally four to five metres wide at the top and approximately half a metre above mean sea level or, in the presence of a large tidal range, above high water spring level, Figure 17a. The top of the core should be kept level and uniform by a bulldozer to enable the dump trucks to travel the entire length of the breakwater. When tipped into the water, the core rubble comes to rest at a slope of approximately 1 on 1, i.e. it drops down 1 metre in level for every 1 metre forward. The rubble in the core is very light, so breakwaters should be built during calm weather only.

Environmentally speaking, the core dumping may have a large negative impact on the surrounding sea due to the fine dust that gets washed off the rubble. In environmentally sensitive areas, such as coral reefs, protected fish breeding areas and nursery grounds rich in certain species of protected vegetation such as Posidonia sea grass, the core must be sluiced or washed before placing to limit the dust plume that would otherwise be generated by the fine dust particles. This dust plume usually persists for many days and can cause a lot of damage by either blocking out sunlight or depositing fine dust on the gills of fish and suffocating them.

7.2.3.2 The underlayer

The underlayer of stone that protects the core rubble from being washed away, Figure 17b, usually consists of single pieces of stone whose weight varies between a minimum of half a tonne (500 kilograms) to a maximum of one tonne (1 000 kilograms). These are usually laid in a minimum of two layers at a slope which is generally shallower than that of the core; 2/1 on the outer slope and 1.5/1 on the inner slope. A slope of 2/1 means that the level drops 1 metre for every 2 metres forward. The first layer of stone may be placed by a hydraulic excavator as shown in Figure 17c. The excavator should place the heavier stone as quickly as possible without leaving too much core rubble exposed to wave action. If a storm strikes the site with too much core exposed, there is a grave danger of the core being washed away and spread all over the intended port area.

The figure shows the set up for a given stone profile, in this case a slope of 2.5/1: the distance H is the height of the top of the new sloping layer above the sea bed. A wooden pole should be conveniently placed at the tip of the underlying core and cemented into place with mortar. At a distance equal to 2.5 x H, a heavy stone sinker with a marker buoy should be placed on the sea bed. A brightly coloured nylon string should then be strung from the sinker to the required height on the pole. This procedure should be repeated every 5.0 metres to help the crane or excavator operator with the placing of the top-most layer. A swimmer wearing goggles (and in cold waters a wet suit) should ensure that each separate rock is placed within the profile outlined by the nylon string.

7.2.3.3 The armour layer

The main armour layer, as its name implies, is the primary defence of the breakwater against wave attack. The stone sizes for the cross-section in the shallow water example should be in the range of 1 tonne (1 000 kilograms) to 3 tonnes (3 000 kilograms). Any defects in the quality of the rock, grading (size too small) or placing (slope uneven or too steep) will seriously put the whole breakwater at risk. Hence, great care must be taken when choosing and placing the stone for the main armour layer.

Figure 17c shows main armour stone being placed by a crawler crane or tracked crane, which is by far the best equipment for placing large stones. The large stones should be lifted singly using a sling or stone grapple and placed in the water with the aid of a diver swimming over the placing area. The armour layer should be placed stone by stone in a sequence which ensures interlocking; in the figure, for example, stone 2 is held in place by stones 1 and 3 whereas stone 4 is jammed between stones 3 and 5. This ensures that waves cannot pull one stone out and cause the upper stones to topple down the slope, breach the armour layer and expose the smaller rubble underneath. To ensure proper placing, the swimmer or boat crew should direct the crane operator each time a stone is placed until the stone layer breaks the surface. As with the first underlayer, two layers of armour stones are required to complete the main armour layer. Slope profiles should be set up at regular 5 metre intervals using the same procedure as described previously. Figure 17d, bottom, shows how the nearly complete breakwater is closed off layer by layer. It shows the excavator backtracking to the root of the breakwater closing the top layers simultaneously. The end or head of the breakwater is the most delicate part of the breakwater and requires extra care. The outer slope of 2.5/1 should be increased to 3/1 to improve its stability.

7.2.3.4 Solid breakwater

Figure 19 illustrates how a vertical, solid breakwater may be built. A stone rubble foundation should first be laid on a hard sea bed (rock, coral deposits or stiff clay) using the appropriate equipment illustrated in Figure 18. If the foundation is a thick deposit of good sand (no silt or soft clay or mud), then a geotextile filter mat should be placed under the rubble foundation. The rubble should consist of a well-graded mix of 1- to 5-kilogram stones. A temporary profile of the proposed section should then be erected every 2 or 3 metres as shown in Figure 19.

Concrete filled jute bags, or locally available dressed stone, should then be laid on the rubble foundation, in line with the temporary profiles. Mass concrete should then be poured into the central cavity to form a solid structure. The deck and wave wall may be built to suit local conditions or as shown in the figure. Finally, after the removal of the temporary profiles, the sea side face of the breakwater foundation should be protected against wave scour by the application of concrete-filled jute bags as shown. In the case of a sandy bottom, these bags should come to rest on the geotextile filter. Bollards may then be cast into the deck as desired.

7.3 FLOATING BREAKWATERS

To be effective as a breakwater, the motions of a floating structure must be of small amplitude so that the structure does not generate waves into the protected harbour side. Although at resonance the oncoming waves can be out of phase with the transmitted waves (resulting in lower coefficients of transmission), the structure must respond to a spectrum of incident wave conditions. Hence, the design of a floating structure for resonance characteristics only is not possible given the wide spectrum of ocean waves.

The simplest forms of floating breakwaters are pontoon structures, although various modifications to their shape have been investigated in an effort to optimize the mass (and ultimately the cost).

The efficiency of a floating breakwater depends primarily on the ratio of the width of the pontoon to the wavelength of the oncoming waves (Figure 20) and, given that ocean swell has a very long wavelength, floating breakwaters are not suitable for creating protected areas along an exposed coastline and should never be installed. However, on lake shores, where the waves tend to be very short (choppy) and do not generally exceed 0.50 metre, floating breakwaters tend to work efficiently.

7.4 BIBLIOGRAPHY AND FURTHER READING

US Army Corp of Engineers. 1985. Shore Protection Manual. Washington, D.C., US Army Corp of Engineers.

US Army Corp of Engineers. 1995. Design of Coastal Revetments, Seawalls and Bulkheads – Engineering and Design. Washington, D.C., US Army Corp of Engineers.

8.  Quays and slipways

SUMMARY

For the purpose of this chapter, a quay is a general term used to describe a marine structure for the mooring or tying-up of vessels, and for loading and unloading of goods and passengers. It is generally contiguous with the shore. A pier or finger jetty is a quay that projects into the water. In contrast to a normal quay, a finger jetty may be used on both sides. In areas with a high tidal range, both quays and jetties are often of the floating type.

The traditional slipway in many small beach-side communities is still the natural beach where boats are hauled ashore for scrubbing, cleaning and repair. However, a beach is not always suitable or available for servicing a vessel, especially vessels larger than small canoes. On the other hand, inside fishing harbours, the recent technological advances in boat hoists has further widened the range of options available to a would-be designer of slipways.

This chapter reviews the cross-sections of a wide variety of quays and jetties to suit an even wider range of applications. The objective being that the reader will be able to appreciate the different applications of past and more recent technologies and be better prepared to assess the potential requirements for berthing and boat maintenance structures.

CONTENTS

8.1

Quays

8.1.1

Solid quays – minimum draft 1.5 metres

8.1.2

Solid quays – minimum draft 3 metres

8.1.3

Solid quays – minimum draft 6 metres and beyond

8.1.4

Open quays – minimum draft 1.5 metres

8.1.5

Open quays – minimum draft 3 metres and beyond

8.2

Jetties

8.2.1

Jetties – any draft

8.2.2

Floating jetties

8.3

Slipways

8.3.1

Traditional slipways

8.3.2

Slipways on beaches

8.3.3

Mechanical slipways – all sizes

8.3.4

Vessel repair facilities

8.4

Bibliography and further reading

8.1 QUAYS

The construction of quays falls broadly into two classifications: quays with a closed or solid construction, and quays with an open construction, where the deck is supported on piles. A key element inside a typical fishing harbour, however, is the draft, ranging from 1.5 metres to 6 metres may be required, depending on the type, size and number of resident fishing vessels (Table 1). An artisanal fishing port hosting small fishing vessels having a loaded draft of no more than 1 metre would not normally require a draft of more than 1.5 metres at low tide unless large vessels visit the port during the peak fishing season.

TABLE 1

Draft requirements

Draft at low tide

Vessels[1]

1.50 metres

Large canoes to small purse seiners
Maximum loaded draft 1 metre

3.0 metres

Trawlers and other coastal vessels
Maximum loaded draft 2.5 metres

6.0 metres

Ocean-going distant water trawlers
Maximum loaded draft

8.1.1 Solid quays – minimum draft 1.5 metres

Figure 1 illustrates one of the best ways to construct an artisanal quay with a draft of 1 to 1.5 metres. The earth-retaining structure, as the quay wall is known, consists of a number of layers of concrete-filled jute bags placed on a rubble foundation in a brick-wall fashion. This structure does not require any major cranage and may be built with the sole assistance of one or two divers. The major advantage of this type of construction is that an uneven sea bed or large boulders can be included in the foundation as shown in the front elevation in Figure 1. The jute bags should be filled with just enough concrete to form a pillow of uniform thickness, item A in Figure 1. Overstuffed bags, item B, should not be incorporated into the wall. Prior to commencing such work, a temporary guide frame should be built as shown in the construction method for solid breakwaters. The frame can be in scaffold pipes, bamboo or other timber sections.

Granular material only (no silt, mud or clay) should be used as backfill and the top surface should be blinded or sealed with graded aggregate. The blinding should be compacted properly using a vibrating plate compactor. The front or toe of the quay should also be protected against scour by both propellers and tidal streams. This protection can consist of concrete-filled jute bags laid side by side over the screeded rubble as shown in the cross-section in the figure. The concrete capping block should be cast in situ after the granular backfill has been placed. Each capping block should not be more than 5 metres long and should contain some reinforcement.

8.1.2 Solid quays – minimum draft 3 metres

Figure 2 shows a typical concrete blockwork quay built from concrete blocks placed by a crane on a screeded bed of stone (see Figure 18 in Chapter 7 on how to prepare a screeded bed) and rubble. This kind of earth-retaining structure is very common but requires the use of a suitable crane. The crane can either be the floating type or terrestrial.

The concrete blocks are first cast in a yard and after 28 days have elapsed, they are lifted and placed on the sea bed as illustrated in Figure 2. The blocks are placed to form pillars on the screeded rubble; Chapter 7 describes how the foundation bed is screeded. The block pillars should be kept about 50 mm apart in such a way that each pillar may settle without rubbing against adjacent pillars. To achieve this, it is common to nail wooden spacers, 50 mm thick, to one side of the blocks prior to placing. Figure 3 shows various lifting arrangements utilized in the block yard. Slings may either pass underneath the block or lift the block via hooks. The slings may be in wire rope or chain and the factor of safety in the lifting apparatus for safe working loads is 8. Some countries require a higher value to take the wear-and-tear of the slings into consideration.

Whereas rectangular blocks are the simplest to design and to cast, their weight is also a major disadvantage in remote areas with difficult access for heavy lifting plant (Figure 4, left). An alternative way to build the quay wall with lighter blocks (the design has to be wider to compensate for the loss of weight) is to use “I” section blocks and then to fill the voids with granular material (Figure 4, right).

Backfill should consist of clean granular rubble. If sand is available, then the rear of the block wall should be lined with a geotextile filter to prevent sand from leaching out from in between the block pillars. Silt, clay lumps and mud should never be used as backfill and all degradable material (timber pieces, logs and other vegetation) should be removed. The backfill should not be used as landfill for household rubbish because decaying rubbish causes the backfill to settle unevenly.

The concrete capping block should be cast in situ after the granular backfill has been placed up to the water level. Graded aggregate should be used to seal or blind the backfill if large stones were used. The capping block should be cast in lengths to cap two or three adjacent vertical pillars and should contain some reinforcement to bind it to the pillars.

8.1.3 Solid quays – minimum draft 6 metres and beyond

The cross-section shown in Figure 4 may be adapted for a quay with a draft of 6 metres by increasing the size and width of the concrete blocks; however, the required size of the blocks would be so large as to require very large and heavy lifting equipment. A more economical solution in terms of the equipment required is shown in Figure 5.

The earth-retaining structure in this case is a special corrugated sheet of steel, known as a sheet pile, which interlocks with adjacent units to form a continuous wall, Figure 6.

This wall is driven into the sea bed, sheet pile by sheet pile as shown in Figure 7, and the top tied back to an anchor wall, which may consist of a slab of reinforced concrete or a length of the same bulkhead. A temporary timber or steel guide frame is generally erected to help drive the sheet piles vertical and in a straight line. The crane used to drive sheet piles must have a long jib to enable it to pick entire lengths of sheet pile for driving. The crane may either be mounted on a barge, in which case the sheet piles are driven from the sea side of the bulkhead, or driven over a temporary reclamation and driven from the rear of the bulkhead, Figure 8. The temporary reclamation may then be used as backfill.

Sheet piles are suitable for driving into clay, sand and silt deposits, as well as some types of coral. Sheet piles cannot be driven in most types of rock and in the presence of large boulders. Hammers for driving sheet piles may be of two types: impact hammers or high-frequency hammers. Impact hammers, as their name suggests, are hammers which impart an impact to the sheet pile. In the presence of soft deposits or clay, impact hammers do not pose any problem. In the presence of difficult ground, however, such as when sand contains large boulders, the impact from the hammer may damage or bend the sheet pile.

Nowadays, high-frequency hammers are generally used to drive sheet piles in granular deposits. High-frequency hammers are not effective in the presence of clay.

The depth of penetration of a sheet pile depends on the type of material on the sea bed. The top of the sheet pile is usually anchored via thick steel rods, themselves anchored to anchor walls placed a safe distance away from the main bulkhead. The crown of the sheet pile should be finished off in an in situ concrete cope, typically up to 1 metre wide and 2 metres high, to accommodate quay services and allow the bollards and fenders to be bolted down in place, Figure 9.

Solid quays may also be constructed using reinforced concrete box caissons, cast on land or inside a dry dock and sunk in place to form a wall (Figures 10 and 11). By their very nature (wide), caissons exert a lower pressure on the foundation.

8.1.4 Open quays – minimum draft 1.5 metres

The deck of an open quay is supported on piles and the whole structure is open to full view. In view of this, an open structure is considered to be more delicate than a solid one and special fendering measures have to be incorporated in the design to prevent damage to the structure. Open quays may be constructed entirely in timber, concrete or steel, or a mixture of the three. Timber, however, may be attacked by insects; see Chapter 9 for more details.

Figure 12 illustrates how an artisanal open quay may be built using mainly locally available materials, such as timber or steel pipes. Given the small dimensions of the structure, a crane may not be needed if a light lattice tower or tripod and a piling winch are available to drive the piles, Figure 13. The figure also demonstrates the manner in which the pile heads should be prepared to receive the cross-beams.

The timber used in such a structure should be the right kind of timber and treated against decay and attack by insects as described in Chapter 9.

All fastenings should be either screws or bolts in brass or stainless steel. The heads of ordinary steel nails corrode very fast and come undone very suddenly as soon as a load is applied. Nails should not be used anywhere on a jetty. In order to render the structure shown more stable and readily accessible from the shore, graded rubble should be placed to a slope as shown in the figure. In the presence of very soft mud, silt or clay, a geotextile filter should be placed underneath the rubble to hold it in place.

8.1.5 Open quays – minimum draft 3 metres and beyond

Figure 14 shows conventional, deeper water open quays of the type traditionally found in larger fishing ports. The structures are typically subdivided into two categories: with and without tidal variation. Figure 14 shows a cross-section without tidal variation, where the impact load from a vessel is transmitted directly to the deck of the quay via a simple rubber fender. Figure 15 shows a cross-section with tidal variation. The open quay, in this case, is fronted by another structure, the rubbing fender pile, which has to absorb the impact from a vessel mooring at low tide without damaging the main quay piles immediately behind it. If the quay wall is solid (sheet piles), then timber or rubber strips are applied to the sheet pile for the vessels to rub against.

Piled quays are particularly effective at absorbing wave energy due to the presence of the rubble slope underneath the deck. The rubble is normally similar to a breakwater grading, requiring core material (1–100 kilograms), armouring (200–1 000 kilograms) and toe berm (1 000–2 000 kilograms) to prevent scour damage. If the reclamation behind the piled structure is not sealed properly with a geotextile membrane, fines tend to leach out of the rubble, leading to uneven settlement of the apron.

The piles may either be in normally reinforced concrete, prestressed concrete, or steel. Whereas most concrete piles are solid, steel piles are usually hollow pipes. In most cases, if only small lengths of pile can be handled by the crane, piles can be jointed in situ to form longer lengths as required. Concrete piles are generally glued with special epoxy glues, whereas steel piles are commonly welded together via simple butt-joints. If hollow pipes are used, these may either be filled up with concrete and reinforcing steel (to prevent corrosion on the inside and add strength to the pile) or, if the pipe thickness is enough, left as open-ended piles.

Piles in general may be divided into two broad categories: driven and in situ. Driven piles are piles driven into the ground by a piling or vibrating hammer. In situ piles are piles formed in the ground by first drilling a large diameter hole and then filling it with concrete and reinforcement. A combination of the two, also known as a cased or jacketed pile, is when a steel pipe pile is first driven into the ground and then emptied of the contents and refilled with reinforced concrete. The bearing capacity of piles may be increased by the inclusion of a bulb at the foot of the pile. Figure 16 illustrates the various techniques used to drive piles and their most common applications.

8.2 JETIES

A finger jetty is a structure that projects out into the water and is generally suitable for mooring on both sides. The draft alongside a finger jetty may range from 1.5 metres in an artisanal port to any desired depth in large industrial fishing ports. Since there is no earth to retain, a finger jetty derives its stability from the type of foundations utilized. A finger jetty may be constructed in timber, in concrete, in steel, or a mix of any two. The construction equipment required to build jetties is similar to that used for solid and piled quays. Solid jetties in concrete or stone rubble should never be constructed on mobile sediments, like beaches, as they interrupt the littoral flow and lead to problems with erosion.

8.2.1 Jetties – any draft

Unlike quay walls that depend on a gravity structure for their stability, jetties may be constructed with relatively small equipment. Figures 17, 18 and 19 illustrate typical small-scale jetties of the types used in many ports. Figures 20, 21 and 22 illustrate various examples of jetties built in relatively remote areas with small equipment.

8.2.2 Floating jetties

Floating jetties come in a whole range of sizes and shapes to cater for vessels ranging from a small canoe to an ocean-going trawler.

Figure 23 illustrates a jetty in timber which may be manufactured from locally available materials like plastic or oil drums, timber and fibre rope. Figure 24 illustrates the Jetfloat unit, a commercially available system, consisting of modular floats in plastic that join up to form a floating platform of any desired shape.

Ready-made pontoons in concrete or a combination of timber, metal, concrete or plastic may be purchased from a variety of sources (Figure 25). Sizes range from 6 metres to 12 metres and widths vary from 1.5 metres to 3.5 metres. All pontoons should be installed in sheltered waters with little or no current.

Floating jetties may also be of the heavy duty type, suitable for the mooring of heavier vessels and able to withstand greater impact loads (Figure 26). The floating element in this case is a reinforced concrete caisson, built on a river bank and launched at high tide.

Due to the size and weight of the caisson, proper piled anchoring systems are required, especially in the presence of high tidal streams or river currents. The anchor system may consist of a reinforced concrete beam held in place by anchor piles driven into the river bank. The caisson is held away from the river bank by two outrigger lattice girders perpendicular to the caisson. Two further inclined girders absorb the transverse loads imparted to the caisson by the water current. The girders are anchored to both beam and caisson by stainless steel hinges bolted to the concrete. If longer lengths of quay are required, extra, independently anchored and restrained caissons should be placed adjacent to each other. Loose caissons should not be hinged together to anchored caissons to form a single long jetty. The caissons should be internally divided to provide separate water-tight compartments. Each compartment should be equipped with a manhole and water-tight manhole cover for internal inspection. The gangway should be hinged to the anchor beam and allowed to roll over a 5 mm steel wearing plate bolted to the caisson. Depending on the tidal range, the gangway should have timber battens at half-metre intervals to improve foot grip at high slopes during low tide. It is important to equip the caisson with proper fenders. A floating tree trunk, if available, provides very good defence against impact; otherwise, large, truck-size tyres should be employed. Due to the rather permanent nature of such structures, they are usually equipped with water and electricity.

8.3 SLIPWAYS

8.3.1 Traditional slipways

Traditional slipways in use all over the world still make use of greased timber skids as a means of retrieving and launching vessels, Figure 27. This kind of slipway has no mechanical parts and no maintenance and the vessel occupies the slipway until it is relaunched after maintenance work. Typically, the slipway can handle boats up to 5 tonnes in weight. It usually cuts through the quay as shown in Figure 27. The width should not be less than 5 metres and the depth of water at the toe should not be less than 1.5 metres. The portion above mean sea level may be cast in situ and should not be less than 250 mm in thickness in Grade 35 concrete. The submerged portion should consist of precast units, also in Grade 35 concrete, such as the ones shown, laid to a slope on a bed of screeded stone chippings or aggregate. The toe should always be protected with heavier stone armour or concrete-filled jute bags. The timber skids, 100 mm x 100 mm in section, may then be bolted to the concrete as shown in the figure. In some countries, fishermen construct their own custom-made trailers and timber skids may not be required.

Inside harbour or river basins, it is recommended to install a flotsam barrier as shown in Figure 28 to prevent floating rubbish from fouling up the slipway.

There is no ideal slope for a slipway; slopes may be anything from 4 percent to 17 percent steep (a 4 percent slope falls 4 metres vertically in 100 metres horizontal). Compound slopes, with the submerged part at 17 percent and the dry part at 9 percent, are also common. However, one must remember that the steeper the slope the more powerful the winch has to be and that the submerged part may be steeper (the vessel weighs less when in the water) for the same type of winch. Slipway equipment is illustrated in Chapter 10.

8.3.2 Slipways on beaches

It is not uncommon for heavy boatyards, which handle vessels above 5 tonnes, to be located out of town in the vicinity of a sandy coastline. Ideally, no obstructions should be constructed on a sandy beach as these give rise to erosion. Such a structure will invariably need constant dredging to keep the axis free of accumulated sand. Decked boats, including small trawlers, may be safely beached on sand if this method had been taken into consideration at the vessel design stage. Where bilge keels are attached to the sides of the hull to keep it upright they should be so designed to support the weight exerted by the vessel and the keels of wooden hulled and fibreglass vessels should be protected by a skid bar. This might be a stainless steel rubbing plate bolted to the bottom of the keel on a wooden vessel or more probably hard wood in the case of a fibreglass vessel. The end of the skeg may be curved to avoid digging in on launching. A small tractor or winch may be used to haul the vessels ashore (Figure 29).

Launching is clearly a reverse process with the same winch or tractor hauling the boat against a secure anchoring point in the water. In both landing and launching, wooden skids or rollers below the keel are often used; see section 10.7 of Chapter 10.

8.3.3 Mechanical slipways – all sizes

Mechanically operated slipping facilities may be divided into three categories for all vessels up to 500 tonnes displacement. The three methods are:

- mechanized slipway;

- mobile gantry or travelift; and

- synchrolift or ship lift.

8.3.3.1 Mechanized slipway

A mechanized slipway is a large slope with two or three parallel beams running down 4 to 5 metres below sea level, each carrying a heavy-duty steel rail (Figure 30). A steel cradle, Figure 31, runs up and down the rails by means of a special winch.

The vessel to be slipped is floated over the cradle and secured. The cradle is then winched up the slope until it is clear of the water and then locked in the desired position. The vessel stays on the cradle throughout the maintenance period and all servicing is carried out at the water’s edge. Some mechanized slipways cover the work area with a permanent building, while others with sliding or even inflatable structures to protect the work area from the harshest weather conditions only (Figure 32).

The steel cradle runs on foundation beams laid to the appropriate slope. Figures 33, 34 and 35 illustrate a typical plan and cross-section for a slipway utilizing precast beams bedded on a rubble bed. The precast beams are laid on a screeded bed of ballast or coarse aggregate and the adjoining spaces packed with 5 to 50 kilogram rock. A wheel side-restraint in galvanized angle ensures that the cradle does not run off the concrete surface. The wheel or group of wheels or bogies are normally in solid steel coated with rubber. Steel rails may also be used over the beams.

8.3.3.2 Mobile gantry or travelift

A mobile gantry or travelift facility typically consists of a rubber-tyred gantry running over a twin pier construction (Figures 36, 37 and 38). The vessels to be slipped are floated in between the piers and then hoisted out of the water by the mobile gantry or travelift using polyester straps.

The piers may protrude from the quay out into open water or the vessel pen may be incorporated in the quay structure as illustrated in Figure 37.[2] Boat servicing is carried out some distance away from the water’s edge at an open work area or inside a covered workshop. The number of vessels handled at any one time is only a function of the size of the work area, whether covered or open. No cradles are utilized as the vessels are placed directly on timber frames or special steel props and often by laying their keel on the ground. There should always be an arrangement in the repair area with sumps in the ground as an aid to repairing/replacing the keel of wooden hulled boats.

8.3.3.3 Synchrolift or shiplift

Until the advent of high-capacity mobile gantries, synchrolifts were the standard equipment of established boatyards (Figures 39 and 40). Synchrolifts, however, have the capacity to lift up to 1 000 tonnes and this makes any yard equipped with a synchrolift able to offer its services to a wider range of vessels. In a synchrolift the vessels are floated over the transfer cradle that sits on the elevating platform. The platform with the cradle and vessel on top is then hoisted out of the water and brought level with the transfer rails. A tractor can then push the cradle down the line to a repair bay which may be an open work area or a covered workshop.

The number of vessels handled at the yard at any one time is a function of the number of cradles and/or work area available.

8.3.3.4 Fixed or mobile cranes

Fixed or mobile cranes at the quayside may also be used for launching or recovering vessels (Figure 41). In this case, the vessel workshops may be located a distance away from the port.

8.3.4 Vessel repair facilities

8.3.4.1 Best management practice

Vessel repair facilities pose special environmental concerns in a fishing port because of the processes and chemical materials that they use and their proximity to areas where fish meant for human consumption is handled. The area of major concern in a boatyard is hull stripping and painting.

When a vessel’s hull is prepared for painting the process typically starts with pressure or mechanical (hand brush) washing to remove the marine organisms and slime accumulated between maintenance intervals, Figure 42.

Next, depending on whether the hull is metal or timber or glass reinforced plastic (GRP), the antifouling paint is stripped by grit blasting or mechanical sander. Both grit blasting and sanding tend to raise particulate matter into the surrounding environment and in windy conditions the entire port may be enveloped in a cloud of potentially harmful particulate matter.

Similarly, if it rains, the dust settled under the vessel may be washed back into the sea. Figure 43 illustrates how a covered work area around the vessel may be set up using simple plastic sheeting clamped to the vessel’s coaming and held down at ground level with simple upright poles driven into the ground.

The following recommended work practices should be observed at the slipway:

Hull cleaning

- perform abrasive or sanding under covered tarpaulin enclosures or boat skirts;

- perform abrasive or sanding over a horizontal hard impermeable surface, such as concrete, to enable proper cleaning of surface and collection of wastes;

- whenever possible, use vacuum sanders to limit the amount of dust generated;

- sanding dust and paint chippings must be removed on a daily basis and appropriate covered waste containers should be provided within the facility;

- workers under the boat skirt should be provided with appropriate full-face masks; and

- a list of the above work practices should be posted at the work area for the benefit of the “do-it-yourself” vessel owners who may not be aware of the port’s environmental regulations.

Hull painting

- techniques such as brushing and rolling are preferred to spraying to reduce overspray and solvent emissions;

- all painting should be performed under covered tarpaulin enclosures or boat skirts;

- all painting should be performed over a horizontal hard impermeable surface;

- whenever possible, solvents and coatings with low volatility should be used;

- waste paints, solvents and rags should be stored in covered waste containers to prevent evaporation to the atmosphere; and

- workers under the boat skirt should be provided with appropriate full-face masks and solvent-resistant gloves.

8.4 BIBLIOGRAPHY AND FURTHER READING

British Steel. 1988. Piling Handbook. British Steel, PO Box 30, Motherwell ML1AA, UK.

Committee for Waterfront Structures. 1982. Recommendations of the Committee for Waterfront Structures. Berlin, Wilhelm Ernst & Sohn.

Naval Facilities Engineering Command. 1989. Marine Railways. Alexandria, Virginia, NavFacEngCom.

Quinn, A.D. 1972. Design and Construction of Ports and Marine Structures. McGraw-Hill, Inc., New York.

[1]It should be kept in mind that many fishing vessels have what is known as “rake of keel”; therefore, the draft should be measured from the deepest part of the keel. This is particularly important when such a vessel is trimmed by the stern.

[2]The sketches do not show in detail the arrangements to keep the wheels from running “off track” and for the wheel stops.

9.  Construction materials

SUMMARY

Construction materials for port and coastal structures may be classified into five general categories: steel, timber, concrete, stone and synthetics. However, subject of materials and their suitability for application within a port facility is very wide and, for this reason, the intention is to condense the subject into a single chapter covering the basic principles while eliminating superfluous coverage. The chapter is confined to the properties of those materials and treatments or variations thereof that are applicable to port structures. Nevertheless, emphasis is duly placed on full coverage of the basic materials that have proved most durable in the highly aggressive marine environment inside port structures. The overall objective being the provision of sound advice to the reader regarding the selection of materials for port structures and acceptable practices as well as other less suitable materials and practices that should be avoided.

CONTENTS

9.1

Metals

9.1.1

Ferrous metals

9.1.2

Non-ferrous materials

9.2

Corrosion

9.2.1

Introduction

9.2.2

Corrosion and steel

9.2.3

Influence of design on corrosion

9.3

Anti-corrosion metallic coatings

9.3.1

Introduction

9.3.2

Hot-dip galvanizing

9.3.3

Metal spray coatings

9.3.4

Electroplated coatings

9.3.5

Sherardizing

9.3.6

Stainless steel

9.4

Paint coatings

9.4.1

Introduction

9.4.2

Paint systems

9.4.3

Main generic types of primers

9.4.4

Main generic types of paint

9.4.5

Methods of applying paint

9.4.6

Preparing a steel surface for coating

9.4.7

Typical compatible paint cycles

9.4.8

Painting on galvanized surfaces

9.4.9

Painting of sheet piles

9.5

Cathodic protection

9.6

Timber

9.6.1

Introduction

9.6.2

Infestation

9.6.3

Durability

9.6.4

Timber preservatives

9.7

Concrete

9.7.1

Introduction

9.7.2

Cement

9.7.3

Aggregate

9.7.4

Mixing water

9.7.5

Batching

9.7.6

Reinforcement

9.7.7

Placing, vibrating and curing

9.7.8

Admixtures

9.7.9

Polymer-modified cement mortars

9.7.10

Non-shrink grouts

9.7.11

Epoxy flooring compounds

9.7.12

Epoxy concrete coatings

9.7.13

Bituminous coatings

9.7.14

Site practice and safety

9.8

Rock

9.8.1

Introduction

9.8.2

Quarries

9.8.3

Geological characteristics of rock

9.9

Plastics, rubber and bituminous compounds

9.9.1

Polyvinyl chloride

9.9.2

Polypropylene

9.9.3

Polycarbonates

9.9.4

Low-density polyethylene

9.9.5

High-density polyethylene

9.9.6

Polyurethane

9.9.7

Polyesters

9.9.8

Rubber

9.9.9

Bituminous compounds

9.10

Bibliography and further reading

9.1. METALS

9.1.1 Ferrous metals

Many ferrous (iron) alloys are used for engineering purposes. Despite this, iron alloys are used in larger quantities than the alloys of any other metal. This arises from the relative cheapness with which steels (an iron alloy) and cast iron can be produced with a variety of useful properties, by variations of composition, heat treatment and heat working. Steel is an alloy of iron and carbon, silicon, phosphorus, manganese and sulphur. Inclusion of nickel and chromium produces stainless steel.

Wrought steel components are produced by hot- or cold-working processes, although some machining may be involved. In civil engineering, the largest use of wrought steels goes into making reinforcement steel and structural sections such as beams and channels, sheet piles and pipes.

Forged steel involves hot shaping between dies and the deformation sequence is chosen so that adequate deformation is given to all parts of the component and the metal flow direction is controlled to give optimum fibre structure and properties. A limitation on the shape of forgings arises from the need to remove them from the dies. Forged steel is used in making chains, shackles, boat moorings, as well as other products.

Cast iron provides an alternative to forging for the production of complex shapes. Instead of working the metal in a die, casting is achieved by pouring molten metal into a recyclable sand mould. In general, forgings have better properties than castings because the properties of castings are determined by the solidification characteristics of casting alloys. Typical products in cast iron are manhole covers and bollards.

9.1.2 Non-ferrous materials

Non-ferrous metals and alloys are available for engineering and building purposes. Pure non-ferrous metals (like aluminium) have properties that make them suitable for engineering applications and as a building material.

However, care must be taken when, for example, aluminium is to be used in conjunction with steel in order to ensure that cathodic action will not take place.

9.2 CORROSION

9.2.1 Introduction

Corrosion is the destructive attack on a metal by chemical or electrochemical reaction with its environment (Figure 1). Rusting applies to the corrosion of iron or iron-base alloys with formation of corrosion products consisting mainly of hydrous ferric oxides. Non-ferrous metals, therefore, corrode as well but do not rust.

Most corrosion of steel can be considered as an electrochemical process which occurs in stages. Initial attack occurs at anodic areas on the surface, where ferrous ions go into solution. Electrons are released from the anode and move through the metallic structure to the adjacent cathodic sites on the surface where they combine with oxygen and water to form hydroxyl ions. These react with the ferrous ions from the anode to produce ferrous hydroxide which itself is further oxidized in air to produce hydrated ferric oxide: red rust.

The sum of these reactions is described by the following equation:

4Fe + 3O_2 + 2H_2O = 2Fe_2O_3H_2O

(Iron and steel) + (oxygen) + (water) = rust

Two important points emerge:

- For iron or steel to corrode it is necessary to have the simultaneous presence of water and oxygen; in the absence of either, corrosion does not occur.

- All corrosion occurs at the anode; no corrosion occurs at the cathode.

However, after a period of time, polarization effects such as the growth of corrosion products on the surface cause the corrosion process to be stifled. New, reactive anodic sites may then be formed thereby allowing further corrosion. Over long periods the loss of metal is reasonably uniform over the surface and so this case is usually described as “general corrosion”. Nevertheless, various types of localized corrosion can also occur such as:

Pitting corrosion occurs in some circumstances where the attack on the original anodic area is not stifled and continues deep into the metal, forming a corrosion pit. Pitting more often occurs with mild steels immersed in water or buried in soil rather than those exposed in air. Pitting can also occur on stainless steels in certain environments.

Crevice corrosion leads to crevices that can be formed by design-detailing, welding, surface debris, etc. Available oxygen in the crevice is quickly used by the corrosion process and, because of limited access, cannot be replaced. The entrance to the crevice becomes cathodic since it can satisfy the oxygen-demanding cathode reaction. The tip of the crevice becomes a localized anode and high corrosion rates occur at this point.

Bi-metallic corrosion occurs when two dissimilar metals are joined together in an electrolyte, like seawater, an electrical current passes between them and corrosion occurs on the anoxic metal. Some metals (e.g. nickel and copper) corrode preferentially themselves, thereby protecting the steel. The tendency of dissimilar metals to bimetallic corrosion is partly dependent upon their respective positions in the galvanic series (in descending order, the galvanic series for seawater lists magnesium – zinc – aluminium – cadmium – mild steel – wrought iron – cast iron – stainless steel – lead/tin solder). The further apart the two metals are in the series, the greater the tendency. In the presence of seawater, cadmium, aluminium, zinc and magnesium all corrode preferentially to mild steel, with magnesium corroding the fastest and cadmium the slowest. Other aspects which influence bi-metallic corrosion are the nature of the electrolyte and the respective surface areas of the anodic and cathodic metals. Bimetallic corrosion is most serious for immersed or buried structures but should also be considered for steel in the atmosphere.

Stress-corrosion cracking occurs under the simultaneous influence of a static tensile stress (the metallic element is under load), which may be well below the yield strength of the steel and a specific corrosive environment. This type of corrosion is not common with ferrous metals, though some stainless steels are susceptible in chloride environments and mild steels can exhibit stress-corrosion cracking in the presence of nitrates or in highly alkaline solutions.

Bacterial corrosion can occur in soils and water as a result of microbiological activity. The most commonly encountered is that arising from the presence of sulphate-reducing bacteria. These reduce sulphates in the soil to sulphides and cause corrosion under anaerobic conditions (i.e. in the absence of oxygen). They are characterized by black corrosion products having the distinctive “rotten-egg” smell of sulphide. Bacterial corrosion is most commonly encountered in pipelines, sheet piles and other buried structures.

9.2.2 Corrosion and steel

The principal factors that determine the rate of corrosion of steel in air are:

- type and amount of pollution;

- “time of wetness”, i.e. the proportion of total time during which the surface is wet; and

- temperature.

Within a given local environment corrosion rates can vary markedly. For example, steel may corrode more on a particular side of a building because it is in the shade and so remains wet for longer periods. Prevailing winds may carry airborne contaminants (e.g. exhaust from a power station or sea spray from the coast) predominantly on to one face of a structure. It is therefore the “microclimate” immediately surrounding the structure which determines corrosion rates for practical purposes. Steel is most commonly used in the following environments:

- rural atmospheric – essentially inland, unpolluted environments;

- industrial atmospheric – inland polluted environments;

- marine atmospheric – the 2 kilometre strip around the coast is broadly considered as being a marine environment;

- marine/industrial atmospheric – polluted coastal environments;

- seawater immersion:

(i)

the splash zone, immediately above high-tide level, is usually the most corrosive zone;

(ii)

the tidal zone, between high tide and low tide, often covered with marine growth; and

(iii)

the low-water zone, a narrow band just below the low-water level;

- the permanent immersion zone, from low-water level down to the sea bed;

- freshwater immersion – corrosion rates are lower in freshwater than in saltwater;

- soils – the corrosion process is complex and very variable; various methods are used to assess the corrosivity of soils, such as resistivity (generally high-resistance soils are least corrosive), redox potential (to assess the soil’s capability of anaerobic bacterial corrosion), pH (highly acidic soils, e.g. pH less than 3.0, can be corrosive), water content (corrosion depends upon the presence of moisture in the soil and the position of the water table has an important bearing).

Long-buried steel structures, e.g. pipelines, are most susceptible to corrosion. Steel piles driven into undisturbed soils are much less susceptible due to the low availability of oxygen.

9.2.3 Influence of design on corrosion

Design can have an important bearing on the corrosion of steel structures. The prevention of corrosion should therefore be taken into account during the design stage of a project. The main design points to be considered are:

Entrapment of moisture and dirt:

- avoid sharp edges, sharp corners, cavities, crevices;

- welded joints are preferable to bolted joints;

- avoid or seal lap joints;

- provide drainage holes for water, where necessary;

- seal the ends of hollow sections, such as pipe piles, box sections, etc.; and

- provide free circulation of air around the structure.

Contact with other materials:

- avoid bi-metallic connections or insulate the contact surfaces;

- provide adequate depth of cover and quality of concrete; and

- separate steel and timber by the use of coatings or sheet plastics.

Coatings application:

- design should ensure that the selected protective coatings can be applied efficiently;

- radius all edges and corners;

- provide vent-holes and drain-holes for items to be hot-dip galvanized;

- do not drill or cut through items that have already been galvanized; and

- provide adequate access for metal spraying, paint spraying, etc.

In general:

- large flat surfaces are easier to protect than more complicated shapes;

- ideally, locate load-bearing members in the least corrosive locations;

- provide access for subsequent maintenance; and

- provide lifting lugs in the design to reduce damage during handling and erection.

9.3 ANTI-CORROSION METALLIC COATINGS

9.3.1 Introduction

Anti-corrosion coatings for steel fall into two broad categories:

- metallic coatings, and

- paint coatings.

Due to their very thin nature, the coating thickness is measured in microns:

- 1 000 microns = 1.0 millimetre or 1 micron is equivalent to one thousandth of a millimetre.

The term galvanizing has its origin with the concept of galvanic or electrochemical corrosion, which is demonstrated by placing two dissimilar metals in contact with each other in an electrolyte, such as seawater.

The principle is the same as that in an electrochemical cell or battery of cells, corrosion being the chemical attack that occurs at the anode. Conversely, batteries make use of the electrical energy produced by such a reaction. Applying this principle to galvanic protection, metals have individual electrical characteristics known as electrode potentials. The difference between these potentials is the driving force for electrochemical action such as corrosion. When dissimilar metals are connected in the presence of an electrolyte, the driving force for the corrosion reaction is released and electronic conduction (with no mass transfer) occurs by the route of least resistance through the metals from the electropositive to the most electronegative. To satisfy the requirement for more electrons at the electronegative metal interface with the electrolyte mass transfer occurs by dissolution of the metal of most electropositive potential, creating ions in the solution.

The main advantages of using zinc are the low melting point (around 420 °C) and the fact that zinc is anodic to steel: that is, when in contact with iron or steel in the presence of an electrolyte, zinc will corrode in preference to the iron or steel. The zinc acts as a “sacrificial anode”, Figure 2, and corrodes by the mass transfer of zinc ions into the electrolytic solution, releasing electrons to travel the electrically conducting path through the metals to the steel/electrolyte interface where they are consumed in a reverse cathodic reaction, forming compounds with the positively charged ions from the solution. In some instances these deposit as a chemically stable film on the exposed steel surface, thereby protecting it from further corrosion and limiting reaction.

There is another factor of importance when considering the corrosion activity of zinc. The inherent corrosion resistance of zinc relies mainly on the formation on its surface of insoluble carbonates, which act as a protective film against further corrosion activity.

There are four commonly used methods of applying metal coatings to steel surfaces:

- hot-dip galvanizing;

- metal spraying;

- electroplating; and

- sherardizing.

Hot-dip galvanizing and metal spraying are best suited for large items, whereas the latter two processes are used for fittings, fasteners and other small items.

9.3.2 Hot-dip galvanizing

The most common method of applying a metal coating to structural steel is by galvanizing. The galvanizing process involves the following stages:

- The steel is cleaned properly; surface preparation of steels is described in detail further on in this text.

- The cleaned steel is dipped into a bath of molten zinc at a temperature of about 450 °C at which the steel reacts with the molten zinc to form a series of zinc/iron alloys on its surface, Figure 3.

- As the steel item is removed from the bath a layer of relatively pure zinc is deposited on top of the alloy layers.

As the zinc solidifies it assumes a crystalline metallic lustre, usually referred to as “spangling”. The thickness of the galvanized coating is influenced by three major factors:

- size of the component;

- surface roughness of the steel; and

- chemical composition of the steel.

In general, thicker, heavier sections tend to produce heavier coatings.

TABLE 1

Zinc coatings

Base sheet thickness (mm)

Minimum average coating (grams/mm^2)

Coating thickness (microns)

Over 5 mm

610

85

Between 2 and 5 mm

460

65

Between 1 and 2 mm

335

47

Cast iron

610

85

Centrifuged components

305

43

The greater the surface roughness of the steel, the heavier the coatings (Table 1). The surface of a component may be purposely abraded by different means such as grit or shot blast to increase the surface area of the component by as much as 50 percent.

Silicon, a major chemical component of steel, can have a marked effect on the coating weight deposited. The thickness of the coating varies with an increase in the silicon content of the steel and, sometimes, these coatings may have a dull dark-grey appearance and can be brittle and less adherent.

Since this is a bath-dipping process there is obviously some limitation on the size of the components which can be galvanized. Generally, filling, venting and draining holes, with a diameter of at least 10 mm, must be provided in hollow items such as tubes to allow rapid access for the molten zinc, venting of hot gases and subsequent draining of the zinc.

For most applications, galvanized articles do not need to be painted. Where, for reasons of decorative effect, there is a need to paint, then special etch primers are normally required.

In seawater immersion, zinc coatings are effectively resistant for protecting steel against rusting, with each 0.03 mm of zinc roughly equivalent to about one year of useful life.

9.3.3 Metal spray coatings

An alternative method of applying a metallic coating to structural steelwork is by metal spraying of either zinc or aluminium. The metal, in powder or wire form, is fed through a special spray gun containing a heat source which can be either an oxy-gas flame or an electric-arc. Molten globules of the metal are blown by a compressed air jet on to the previously blast-cleaned steel surface. No alloying occurs and the coating which is produced consists of overlapping platelets of metal and is porous. The pores are subsequently sealed, either by applying a thin organic coating which soaks into the surface, or by allowing the metal coating to weather, when corrosion products block the pores. The adhesion of sprayed metal coatings to steel surfaces is considered to be essentially mechanical in nature.

It is therefore necessary to apply the coating to a clean roughened surface for which blast cleaning with a coarse grit abrasive is normally specified, usually chilled-iron grit. Coating thickness varies from 100–250 microns for aluminium to 75–400 microns for zinc. The metals perform similarly in most environments but aluminium is more durable in highly industrial environments.

Metal spray coatings can be applied in workshops or at site and there is no limitation on the size of the component, as there is with hot-dip galvanizing. Since the steel surface remains cool there are no distortion problems. However, metal spraying is considerably more expensive than hot-dip galvanizing. For many applications metal-spray coatings are further protected by the subsequent application of paint coatings. A sealer is first applied which fills the pores in the metal-spray coating and provides a smooth surface for application of the paint coating.

9.3.4 Electroplated coatings

Together with galvanizing, electroplating is the second most important method of applying a metal coating to a steel item (Figure 4). The method consists in suspending steel articles from a cathode in an aqueous electrolyte of the metal to be plated to the article. A current is then passed through the solution and the positive metal ions migrate to the cathode and hence the article in steel to be plated. Physically, a very thin layer a few microns thick is deposited; thicker layers are obtained by prolonging the electrolyte bath. Due to its simplicity, this method can also be utilized to electrodeposit a number of noble metals such as tin, nickel, chromium and gold. This system, however, is best suited to small engineering components where precision in film thickness is of the utmost importance.

9.3.5 Sherardizing

A sherardized coating is produced by a process called sublimation. In this process, heat is applied to steel components in contact with zinc powder, which then deposits itself on the steel. Sherardizing gives a hard and wear resistant surface and is generally reserved for small components like nuts and bolts.

9.3.6 Stainless steel

Stainless steels are alloys of iron, chromium and other elements that resist corrosion from many environments. In general, steel cannot qualify for a stainless prefix unless it has at least 10 percent chromium. Stainless steels can be very complicated from the metallurgical standpoint because they can contain significant concentrations of eight or more elements and the properties of the various alloys vary from alloy to alloy.

The most popular stainless steel used in marine applications is the 316 series, known as AISI 316 (American Iron and Steel Institute).

The basic AISI 316 has extra molybdenum added to the basic alloy to improve its corrosion resistance, whereas the AISI 316L has a lower carbon content to improve its welding characteristics (Table 2). AISI 316 LN has less carbon and more nitrogen to improve its strength.

Wrought stainless steel components for the fabrication of mooring rings, step ladders and railings should be specified in AISI 316L. Fittings may be in AISI 316.

Stainless steel reinforcement is also commercially available. Stainless steels in the series AISI 304 and AISI 308 are not suitable for a marine environment and should be avoided.

TABLE 2

Properties of some stainless steels

9.4 PAINT COATINGS

9.4.1 Introduction

Paints are made by mixing and blending three main components:

- Pigments: finely ground inorganic or organic powders which provide colour opacity, film cohesion and sometimes corrosion inhibition;

- Binders: usually resins or oils but can be inorganic compounds such as soluble silicates. The binder is the film-forming component in the paint; and

- Solvents: used to dissolve the binder and to facilitate application of the paint. Solvents are usually organic liquids or water.

Since, in the broadest terms, paint consists of a particular pigment, dispersed in a particular binder, dissolved in a particular solvent, the number of generic types of paint is limited. The most common methods of classifying paints are either by their pigmentation or by their binder type.

Primers for steel are usually classified according to the main corrosion-inhibitive pigments used in their formulation, e.g. zinc phosphate, zinc chromate, red lead or metallic zinc. Each of these inhibitive pigments can be incorporated into a range of binder resins, e.g. zinc phosphate alkyd primers, zinc phosphate epoxy primers, zinc phosphate chlorinated rubber primers.

Intermediate coats and finishing coats are usually classified according to their binders, e.g. vinyl finishes, urethane finishes. Paints are applied to steel surfaces by many methods but in all cases they produce a “wet film”. The thickness of the wet film can be measured, before the solvent evaporates, using a comb gauge, Figure 5.

As the solvent evaporates, film formation occurs, leaving the binder and pigments on the surface as a “dry film”. The thickness of the “dry film” can be measured usually with a magnetic induction gauge. The relationship between the applied “wet film” thickness and the final “dry film” thickness (d.f.t.) is determined by the percentage volume of solids of the paint, i.e.

d.f.t. = “wet film” thickness x % vol. solids

In general, the corrosion protection afforded by a paint film is directly proportional to its dry film thickness.

9.4.2 Paint systems

Paints are usually applied one coat on top of another, each coat having a specific function or purpose. The primer is applied directly on to the cleaned steel surface. Its purpose is to wet the surface and to provide good adhesion for subsequently applied coats. Primers for steel surfaces are also usually required to provide corrosion inhibition. The intermediate coats (or undercoats) are applied to build the total film thickness of the system. This may involve the application of several coats. The finishing coats provide the first-line defence against the environment and also determine the final appearance in terms of gloss, colour, etc., Figure 6. The various superimposed coats within a certain system have, of course, to be compatible with one another. They may be all of the same generic type or may be different, e.g. chlor-rubber-based intermediate coats may be applied on to an epoxy primer. However, as a first precaution, all paints within a system should normally be obtained from the same manufacturer.

9.4.3 Main generic types of primers

Shop primers (also referred to as blast primers, temporary primers, weldable primers, temporary primers or holding primers) are used on steel, immediately after cleaning, to hold the reactive cleaned surface in a rust-free condition until the final painting cycle can be undertaken. The main requirements for a primer are:

- The primer should be capable of airless-spray application (see text on cleaning further on) to produce a very thin even coating. Dry-film thickness is usually limited to 15–30 microns. Below 15 microns the peaks of the blast profile are not protected and “rust-rashing” occurs on weathering. Above 30 microns the primer affects the qua1ity of welds and produces excessive weld fume.

- The primer must dry very quickly. The interval between priming and handling is usually keep as is 1–10 minutes and hence the primer film must dry within this time.

- The primer coating should provide adequate protection. It should be noted that many manufacturers make misleading claims about the durability of their blastprimers and suggested exposure periods of 6–12 months are not uncommon. In practice, such claims are never met except in the least arduous conditions, e.g. indoor storage. In aggressive marine conditions, durability can be measured in weeks rather than months.

- The primed surface, after weathering, should require the minimum of preparation for subsequent painting and must be compatible with the intended painting system.

Many proprietary blast primers are available but they can be classified under the following main generic types:

- Etch primers are based on polyvinyl butyral resin reinforced with a phenolic resin to uprate water resistance. The curing agent contains phosphoric acid or alkyl phosphates and can be added as a separate component immediately before application. Alternatively, the primer can be supplied in a single-pack form. In general, two-pack etch primers provide better durability than the one-pack types which are usually preferred because of their handling convenience.

- Epoxy primers are two-pack materials utilizing epoxy resins and usually either polyamide or polyamine curing agents. They are pigmented with a variety of inhibitive and non-inhibitive pigments. Zinc phosphate epoxy primers are the most frequently encountered and give the best durability within the group.

- Zinc epoxy primers can be subdivided into zinc-rich and reduced-zinc types. Zinc-rich primers produce films which contain about 90 percent by weight of metallic zinc powder. These primers provide the highest order of protection of all primers. Reduced-zinc primers are formulated with metallic zinc content as low as 55 percent by weight on the dry film, the remainder of the pigmentation usually being made up with siliceous extenders. This reduces the cost of the primer, avoids possible difficulties with intercoat adhesion in marine environments, but slightly reduces the standard of protection that can be achieved. Zinc epoxy primers all produce zinc oxide fumes during welding and gas cutting which can cause a health hazard. When exposed in marine environments, zinc epoxy primers are prone to the formation of insoluble white zinc corrosion products which must be removed from the surface before subsequent overcoating.

- Zinc silicate primers can be based upon either ethyl silicate or inorganic silicates e.g. sodium or potassium. Only the ethyl silicate primers are suitable as primers. Ethyl zinc silicate primers produce a level of protection which is comparable with the zinc-rich epoxy types and they suffer from the same drawbacks, e.g. formation of zinc salts and production of zinc oxide fumes during welding. They are however more expensive and usually are less convenient to use.

9.4.4 Main generic types of paint

The main generic types of paint are:

- Air-drying paints, e.g. oil-based, alkyd, epoxy-ester, dry and form a film by an oxidative process which involves absorption of oxygen from the atmosphere. They are therefore limited to relatively thin films. Once the film has formed it has limited solvent resistance and usually poor chemical resistance.

- One-pack chemical-resistant paints, e.g. chlor-rubbers, vinyls, form a film by solvent evaporation and no oxidative process is involved. They can be applied as moderately thick films, although retention of solvent in the film can be a problem at the upper end of the range. The film formed remains relatively soft and has poor solvent resistance but good chemical resistance. Bituminous paints also dry by solvent evaporation. They are, essentially, solutions of either asphaltic bitumen or coal-tar pitch in organic solvents.

- Two-pack chemical-resistant paints, e.g. epoxy and urethane, are supplied as two separate components, usually referred to as the base and the curing agent. When the two components are mixed, immediately before use, a chemical reaction begins. These materials therefore have a limited “pot life” by which the mixed coating must be applied. The polymerization reaction continues after the paint has been applied and after the solvent has evaporated to produce a densely cross-linked film which can be very hard and has good solvent and chemical resistance. Liquid resins of low viscosity can be used in the formulation thereby avoiding the need for a solvent. Such coatings are referred to as “solvent-less” or “solvent-free” and can be applied as very thick films.

9.4.5 Methods of applying paint

The standard methods used for applying paint to steel are brush, roller, conventional air-spray and airless-spray.

- Brush. The simplest and also the slowest and therefore most expensive method. Nevertheless, it has certain advantages over the other methods, e.g. better wetting of the surface; can be used in restricted spaces; useful for small areas; less wastage; less contamination of surroundings; and can be used for application of certain toxic materials like lead-based primers which cannot be sprayed.

- Roller. Much quicker than brushing; useful for large flat areas; demands suitable fluidity of the paint.

- Air-spray. The paint is atomized at the gun-nozzle by jets of compressed air; application rates are quicker than for brushing or rolling; paint wastage by overspray is high.

- Airless-spray. The paint is atomized at the gun-nozzle by very high hydraulic pressures; application rates are higher than for air-spray and overspray wastage is greatly reduced.

9.4.6 Preparing a steel surface for coating

Structural steel is a hot-rolled product (that is, it is rolled into sections while it is still hot). Sections leave the last rolling pass at about 1 000 °C and as they cool the steel surface reacts with oxygen in the atmosphere to produce mill scale, a complex oxide which appears as a blue-grey tenacious scale completely covering the surface of the as-rolled steel section. Unfortunately, mill scale is unstable. On weathering, water penetrates fissures in the scale and rusting of the steel surface occurs. The mill scale loses adhesion and begins to shed.

Mill scale is therefore an unsatisfactory base and needs to be removed before protective coatings are applied. As mill scale sheds, further rusting occurs and this also needs to be removed before protective coatings are applied. Surface preparation of steel is therefore principally concerned with removal of mill scale and rust. Various methods of surface preparation are available:

- Manual preparation: The simplest form of surface preparation involving chipping, scraping and brushing with hand-held implements. This method is not very effective (although only about 30 percent removal of rust and scale can be achieved it is nevertheless often used, usually for economic reasons). The degree of cleaning achieved can be specified by reference to photographic standards included in ISO 8501 (International Organization for Standardization No. 8501).

- Mechanical preparation: Similar to manual preparation but utilizes power-driven tools, e.g. rotary wire brushing. A marginal improvement in efficiency can be achieved (up to 35 percent), and the same photographic standards can be used. Care must be taken to avoid confusing burnished scale with clean steel, both of which have a similar appearance. The above methods are used on site, usually after a weathering period to promote loosening of mill scale. A suitable primer must then be applied which is tolerant of poor surface preparation. Many modern primers are quite unsuitable for such surfaces and, indeed, the old tried and trusted red lead in oil primers cannot be bettered for manually cleaned surfaces.

- Flame cleaning: Not used extensively. An oxy-gas flame is applied to the surface. Differential thermal expansion and steam generated behind the mill scale serve to loosen the mill-scale layer, which can then be removed by mechanical scraping.

- Acid pickling: The steel is immersed in a bath of suitably inhibited acids which dissolve or remove mill scale and rust but do not appreciably attack the exposed steel surface. It can be 100 percent effective. Acid pickling is always used on structural steel intended for hot-dip galvanizing but is now rarely used as a pretreatment before painting.

- Blast cleaning: Abrasive particles are projected at high speed on to the steel surface. The abrasive can consist of either spherical particles, described as “shot”, or angular particles, described as “grit”. The abrasive is projected towards the surface either in a jet of compressed air or by a centrifugal impeller wheel. The particles impinge on the steel surface removing scale and rust, producing a rough, clean surface. The size and shape of the surface roughness produced is largely dependent upon the size and shape of the abrasive used; angular grits produce angular surface profiles, round shots produce a rounded profile. Grit-blast abrasives can be either metallic (e.g. chilled iron grit) or non-metallic (e.g. slag grit). The latter are used only once and are referred to as “expendable”. They are used exclusively for site work. Metallic grits are expensive and are used only where they can be recycled. Grit blasting is always used for metal-sprayed coatings, where adhesion is at least partly dependent upon mechanical keying. It is also used for some paint coatings, particularly on site and for primers where adhesion may be a problem (e.g. zinc silicates). Shot-blast abrasives are always metallic, usually cast steel shot, and are used particularly on shot-blast plants, utilizing impeller wheels and abrasive recycling. They are the preferred abrasive for paints, particularly for thin film coatings (e.g. prefabrication primers). Blast-cleaned surfaces are normally specified in terms of surface cleanliness and surface roughness. A number of standards have been used in the past but these are now superseded by ISO 8501-1: 1988, which utilizes photographic replicas of four grades of surface cleanliness after blast cleaning: Sa1, Sa2, Sa2 1/2 and Sa3. Surface roughness of blast-cleaned surfaces is defined in ISO 8503-1: 1988 and Parts 2, 3 and 4 of this standard describe methods of measuring surface roughness.

- Wet blasting: A further variation on the blast-cleaning process. In this process, a small amount of water is entrained in the abrasive/compressed air stream. This is particularly useful in washing from the surface soluble iron salts that are formed in the rust by atmospheric pollutants (e.g. chlorides and sulphates) during weathering. These are often located deep in corrosion pits on the steel surface and cannot be removed by conventional dry blast cleaning methods. Wet blasting has proved to be particularly useful in marine environments.

9.4.7 Typical compatible paint cycles

Paint cycles should be designed for compatibility and Table 3 illustrates some typical cycles used in a marine environment.

To avoid mistakes with incompatible paint cycles, paints should always be purchased from the same manufacturer.

TABLE 3

Typical compatible paint cycles

Surface preparation

Paint cycle

Total thickness

Method of application

Manual or mechanical

Primer:
Oil-based alkyd (red lead)

140–160 microns

Brush or roller

Undercoat:
Oil-based alkyd

Finish:
Oil-based alkyd

Blast cleaning

Primer:
Zinc epoxy

120–140 microns

Brush, roller or spray

Undercoat:
Chlor-rubber

Finish:
Chlor-rubber

Blast cleaning

Primer:
Epoxy polyamide

120–140 microns

Brush, roller or spray

Undercoat:
Epoxy vinyl

Finish:
Vinyl

Blast cleaning

Primer:
Zinc epoxy

300–350 microns

Brush, roller or spray

Undercoat:
Epoxy tar

Finish:
Epoxy vinyl

9.4.8 Painting on galvanized surfaces

Whether thermally sprayed or hot dipped, the zinc coating can provide several years of protection by itself. Components may need to be painted or it may be necessary to apply a paint system to extend corrosion protection after the zinc is consumed. Quality painting after erection is both difficult and expensive, so it is always best to apply organic coatings beforehand, preferably in a shop setting. Surface preparation and painting of components in a shop can be accomplished under controlled conditions to provide optimum protection of the metal. Shop cleaning of zinc-coated surfaces is normally limited to detergent washing to remove loose contaminants and/or solvent cleaning to remove grease or oil. Sometimes, a thin film of grease or oil is applied at the factory to protect galvanizing from corrosion during exterior storage. Also, new galvanizing is sometimes treated with chromate corrosion inhibitors for corrosion protection during storage. Such treatment should specifically be excluded in specifications for galvanized steel components to be coated.

Galvanized steel components are best protected with one coat each of epoxypolyamide and aliphatic polyurethane coatings. If a delay of over four days occurs before the top coat is applied, the finish coat of polyurethane may not adhere because of the solvent resistance of the nearly fully cured epoxy coat. A thin film of the epoxy primer applied and allowed to cure to a tacky finish (e.g. four hours) will provide a suitable surface for the polyurethane finish coat. Epoxy and urethane coatings must have at least a six-hour pot life for practical coating. Oil-based paints (including oil/alkyds) are not recommended because of the inherent incompatibility of oil-based paints with the alkaline surface of galvanizing. Premature failure by peeling is predictable.

A two-coat latex system can also be used on galvanizing, but the protection and gloss and color retention may not be quite as good as with the epoxy/polyurethane system. The corrosivity of the exposure environment should be considered when choosing between the two systems.

9.4.9 Painting of sheet piles

Sheet-pile surfaces are subjected to abrasion from direct and indirect sources which may damage a normal coating. For long-term performance immersed in seawater, the coating system must be of high quality and is often specified in combination with a cathodic protection system (see further on). The recommended paint cycle for sheet piles is shown in Table 4.

TABLE 4

Painting of sheet piles

Surface preparation

Paint cycle

Total thickness

Method of application

Blast cleaning

Primer:
Polyamide cured
Epoxy primer

500 microns

Airless spray

Undercoat:
Polyamide cured
Coaltar epoxy

Finish:
Polyamide cured
Coaltar epoxy

9.5 CATHODIC PROTECTION

The corrosive effect on steel and other metals when submerged in seawater is known as galvanic corrosion and is analogous to the conditions prevalent in a battery, i.e. the presence of two dissimilar metals in an electrolyte. In the case of immersed steel (like a jetty’s steel piles or a quay’s sheet piles), the dissimilar metal consists of the non-uniformities or non-ferrous impurities in the steel. These impurities establish a flow of current from the steel (positive pole or anode) through the electrolyte (seawater) to the impurity (negative pole or cathode).

Cathodic protection consists in reversing this current flow, i.e. to the steel and not from the steel, thus preventing the iron ions from flowing out of the steel and causing its corrosion (galvanic action).

Cathodic protection is accomplished by establishing a direct current (DC) voltage between the pile or sheet pile and an auxiliary anode suspended in the water in the vicinity of the structure, Figure 7. The density or strength of the current required depends on the rate of corrosion (specified in millimetres per year) and differs from place to place. Corrosion rates for a given type of steel generally depend on the salinity of the seawater, the ambient temperature and other environmental factors. If the steel has been coated, the efficiency of the coating, will also have a major effect on the rate of corrosion (the better the paint coating, the slower the corrosion rate). Various experiments worldwide indicate that typical current densities are of the order of 15 to 50 mA/m^2 (milliamperes per square metre of steel surface to be protected). This impressed current may be achieved in one of two ways:

- by using a galvanic anode or sacrificial anode, made of zinc, aluminium, magnesium or their alloys; and

- by forcing an external DC current through the auxiliary anode which may consist of a soluble metal, such as iron, or an insoluble material, such as graphite.

Galvanic anodes are generally supplied as ingots (weights from 2 to 100 kilograms) with cast-in suspension brackets which may be bolted or welded directly to the steel piles to be protected. Typical characteristics of galvanic anodes are listed in Table 5.

TABLE 5

Characteristics of sacrificial anodes

Metal

Zinc

Aluminium

Magnesium

Specific gravity

7.14

2.91

1.84

Potential in millivolts

-1 100.0

-1 150.0

-1 500.0

Current ampere/hour/kg

780

2 700

1 103

Current efficiency

95%

95%

50%

Kilogram of metal consumed to produce 1 ampere/year

11.23

3.24

7.94

On large structures, however, to gain better control of the current and regulate the magnitude thereof more accurately, an impressed external source of current is usually applied. Figure 7 illustrates a schematic diagram of an impressed-current cathodic protection installation, consisting of an alternating current (AC) supply, a rectifier (to convert the AC to DC) and the suspended graphite anode slung underneath the jetty in between the steel piles. Graphite anodes also become decomposed in time, primarily due to electrochemical oxidation, and must be replaced every 5 to 15 years, depending on the quantity, size and location of the anodes. Theoretically, when applying the recommended current density, the graphite anode life is of the order of 44 000 to 176 000 ampere hours per kilogram. Assuming the lower value of 44 000 ampere hours as a conservative value, a 30-kilogram graphite anode submerged in seawater would be completely consumed only after providing 7 amperes continuously for [(44 000 x 30) divided by (7 x 24 x 365 days)] or 22 years. In practice, however, replacement must be made before the anode is greatly reduced in size in order to avoid an excessive current density as the exposed surface of the anode diminishes. The realistic useful life of a 30-kilogram graphite anode described above would be between 5 to 15 years.

Although the initial cost of the external impressed-current system is greater than that for galvanic anodes, the long-range economics over a ten-year period indicate that overall costs for impressed-current systems are considerably less than for galvanic anode systems.

The step-down transformer and rectifier are usually mounted inside a weatherproof building not far from the structure to be protected.

9.6 TIMBER

9.6.1 Introduction

Timber is cut and machined from trees, themselves the product of nature and time. Humans have found timber to be a cheap and effective material and continue to use it in vast quantities.

However, criticisms levelled at timber as a material are a consequence of humankind’s misuse of one of nature’s most important products. Unlike so many other materials, especially those used in construction, timber cannot be manufactured to a particular specification; instead, the best use has to be made of the material already produced, though it is possible to select timbers with the most desirable range of properties.

A tree trunk consists of two distinct sections: the inner section or heartwood, and the outer section, or sapwood, Figure 8. The periphery of the trunk is formed of bark or, more correctly, an outer bark and an inner bark. The outer bark is rough in texture and dense enough to provide a protective “coat” covering the vital growth areas immediately inside it. The inner bark is soft, moist and spongy and transports the converted sap from the leaves to the growing parts of the tree. Between the inner bark and the actual growing timber is a thin layer of cells called the cambium.

It is here that growth takes place by the splitting of single cells into two cells, each of which grows and splits in a process which continues throughout the growing season, eventually forming a sheath of cells which in cross-section appears as a ring, referred to as an “annual ring”. These cells, which make up the wood tissue on the inner side of the cambium, are tubular in shape with diameters between about 0.02 mm and 0.50 mm, and vary in length from about 1 mm in hardwoods to 6 mm in softwoods.

The inner layer consists of cells with comparatively large cavities and thin walls. This cellular structure is due to a more rapid spring growth and, not illogically, is referred to as springwood or earlywood. Later in the year, cells grow more slowly and have thicker walls and smaller cavities, resulting in heavier, harder and stronger material called summerwood or latewood. The amount of summerwood may vary in different species of tree and as a result of different weather and soil conditions. This affects the overall density of the timber, which has a direct relationship with the strength of the timber. A group of cells known as the medullary rays run at right angles to the main cells from the outer layers inward. These carry food material by transporting the excess towards the centre of the tree, where it is stored in cells in the inner rings which cease to function as a live part of the tree. This older timber is known as heartwood and is usually dark in colour as well as being drier and harder than the living layer, known as sapwood.

Heartwood is composed of dead tissue, its cells being completely filled, and its function is the mechanical support of the tree. Sapwood, containing more moisture, is not as strong in the “green” state as heartwood but, after seasoning, when both heartwood and sapwood are reduced to the same moisture content, the difference in density and strength is very small.

Sapwood is inferior to heartwood in respect of durability, containing starches which may attract insects and fungi. Sapwood, however, is very permeable and more easily impregnated with preservative and, where the service conditions demand treatment, it may be beneficial to use sapwood as a deliberate choice.

The botanical name for those plants which grow outwardly, acquiring a new sheath of cellular tissue during each growing season, is exogens and this classification can be subdivided into:

- angiosperms, or dicotyledons, which have broad leaves shed in the autumn and which are normally classed as hardwoods; and

- gymnosperms, or conifers, which have needlelike leaves, broadly evergreen, and which are generally classed as softwoods.

It should be noted that the terms hardwood and softwood in relation to species of tree do not necessarily indicate relative hardness or density; balsa, for example, although soft in texture, is a hardwood by classification whereas yew, softwood by classification, has a density six times that of balsa.

In the living tree, water is to be found not only in the cell cavity but also within the cell walls. Consequently, the moisture content of green wood (newly felled) is high, varying from about 60 percent to nearly 200 percent. Green or newly felled timber will yield moisture to the atmosphere with consequent changes in its dimensions and, with moisture contents above 20 percent, the timber is also subject to attack by fungi. For these reasons it is necessary to dry or season timber following felling of the tree.

9.6.2 Infestation

Although all timbers are susceptible to attack by at least one species of insect or fungus, in practice only a small portion of the timber in service actually becomes infested. The major types of attack are:

- Insect attack can take one of two forms. In certain insects the timber is consumed by the adult form and the best known example of this mode of attack are the termites. Few timbers are immune to attack by these voracious eaters. They are found principally in the tropics but certain species are also present in the Mediterranean region. The other mode of attack is by the grub or larval stage of certain beetles. The adult beetle lays its eggs in superficial cracks on the surface of the timber; the eggs hatch and produce grubs which tunnel their way into the timber, remaining there for periods up to three years.

- Fungal growth may be of two types: destructive or non-destructive. Some fungi, such as moulds, are present only on the surface and do not affect the strength of timber. Sapstain fungi live in the sugars present in the ray cells and impart a distinctive coloration to that region of the timber and may cause a considerable reduction in the strength of the timber. By far the most dangerous to timber are the fungi that cause decay by chemical decomposition; this is achieved by the digesting action of enzymes secreted in the fungi. The best known examples are dry rot or Merulius lacrymans and wet rot or Coniophora cerebella. Dry rot is the most destructive; during attack the wood usually darkens and in the advanced stages tends to break up into cubes and crumbles under pressure. A moisture content of 30 to 40 percent with an ambient temperature of 23 °C and lack of ventilation are required for growth. In temperatures above 26 °C, the fungus remains inactive. Wet rot may occur in timber which is excessively wet, whether located inside or outside a building. The fungus attacks all the chemical constituents of the cell wall and, unlike attack from dry rot, timber attacked by wet rot does not crumble under pressure but separates into a fibrous mass.

- Marine borers such as the shipworm (Teredo sp.) and the gribble (Limnoria sp.), Figure 9, are marine-boring animals that attack timber used in sea or brackish (salty) water. Marine borers are widely distributed, but they are particularly destructive in tropical waters. Most timbers do not have sufficient resistance to marine borers to be used untreated. Figure 9 also illustrates how the gribble and the shipworm destroy timber structures.

9.6.3 Durability

Durability is a term which has different meanings for many people: it is defined here in the broadest sense to embrace the resistance of timber to attack from weathering, salt water, corrosion of fastenings, fungi and insect attack.

In weathering, the action not only of light (especially ultraviolet radiation) but also of rain and wind render the timber silvery-grey in appearance: part of the process involves the degradation of the cellular fibres by ultraviolet radiation. However, the same cell walls that are attacked act as a filter for the intact cells below and the rate of erosion from the combined effects of ultraviolet radiation, light and rain is very slow indeed.

As a general rule, timber is highly resistant to a large number of chemicals. When in contact with steel in the presence of seawater, timber gives rise to a condition often known as “nail sickness”. Porous timbers absorb seawater, an electrolyte, and assume the role of an electrical conductor. Alkalis are produced at the cathodic surfaces and soluble metallic salts at the anodic areas. The metallic salts so formed degrade timber.

The principal factor conferring resistance to biological attack (insect, fungi and marine borers) is undoubtedly the presence of extractives in the heartwood. The far higher durability of the heartwood of certain species compared with the sapwood is attributable to the presence in the former of toxins (many of which are phenolic in origin), decreased moisture content, increased density and deposition of gums and resins. The decay resistance of most timbers varies a great deal and even pieces cut from the same tree will often show wide differences. For this reason, timbers have been classified into five broad grades based on the performance of their heartwood in contact with the ground. Table 6 illustrates the five grades.

Untreated timber in direct contact with sea or brackish water (jetty piles for example) should be heartwood of one of the species listed as very durable. Durable and moderately durable timbers should only be used if treated.

Table 7 presents a list of timbers suitable for marine construction. The heartwood is naturally resistant to marine borers and does not need treatment. The sapwood, however, is perishable unless treated.

TABLE 6

Grade of durability

Grade of durability

Useful life in contact with ground (Years)

Very durable

More than 25

Durable

15 to 25

Moderately durable

10 to 15

Non-durable

5 to 10

Perishable

Less than 5

TABLE 7

Typical timber species suitable for marine construction

Note:These and other hardwood species suitable for marine construction are now strictly controlled to limit indiscriminate deforestation. In general, the design of a port should contain as little tropical timber as possible and that used should be sourced from plantations only. The species used should not be on the Convention on International Trade in Endangered Species of Wild fauna and Flora (CITES) list.

9.6.4 Timber preservatives

As stated above, not all timbers require treatment with a preservative to enable them to resist infestation and decay. Many timbers, like the ones mentioned in Table 7, owe their natural resistance to attack to the presence of toxic oils and resins. However, environmental considerations (most species come from virgin tropical rain forests) and cost mean that it is sometimes necessary to use timbers not so well endowed naturally and to subject them to some form of preservative treatment which will increase their durability. Timber preservatives must, by nature, be poisonous to the agents of decay: they must also satisfy conditions of permanency, economy, availability, penetrability and be non-corrosive and non-toxic to humans, animals and plants. Preservatives are generally classed into three groups, depending on the solvent used:

- tar-oil preservatives;

- water-soluble preservatives; and

- organic solvent preservatives.

9.6.4.1 Tar-oil preservatives

Tar-oil preservatives consist essentially of mixtures of distillate oils from coal tar (an aromatic hydrocarbon) and are known as coal tar creosotes or creosote and are the most widely used. Its efficacy as a preservative lies not in any natural toxicity, but rather in its supreme water repellence. It has a very distinctive odour, stains timber, and once treated timber cannot be painted unless primed with a metallic paint. The major drawback with this type of preservative is that it is a polycylic aromatic hydrocarbon (PAH) which is a highly toxic compound. This product is gradually being phased out due to the pollution of the water environment.

9.6.4.2 Water-soluble preservatives

Water-soluble preservatives are generally odourless and non-staining. Because of the waterborne nature of this type of preservative it may be necessary to re-dry the timber to an acceptable moisture content. These preservatives function as toxins and commonly contain formulations of copper, chromium and arsenic salts. These chemicals are “fixed” in the timber and cannot leach out.

9.6.4.3 Organic solvent preservatives

These preservatives tend to be more expensive than the previous two, and are generally formulations of pentachlorophenol, tributyltin oxide or chlorinated naphtalenes. These preservatives also function as toxins and have good penetration characteristics. As with tar-oil creosotes, however, these preservatives are toxic to humans and should not be used.

Timber which has been treated with both water soluble and solvent type preservatives can be painted; it is also possible to glue together treated components. Preservatives may be applied by brush, by steeping or by pressure. Brushing is the least effective method but is better than none and, provided that the preservative is flooded over the surfaces to encourage absorption, reasonable penetration is possible in very permeable timbers. Steeping or dipping involves placing the timber sections inside a bath of preservative for a number of days. In this method of application, the timber section to be treated should be prepared in its final form, with all chamfers, holes and recesses cut. Some preheating of the preservative may be necessary to assist penetration. Pressure application is by far the most efficient and controllable method of preservation. Pressure application is the timber industry standard. In the process, the timber section is placed in a large enclosed pressure vessel and subjected to a vacuum for about an hour. While the vacuum is maintained the preservative is introduced into the vessel until it is filled. Pressure is then gradually increased until the required amount of preservative has been introduced into the timber. A further vacuum is applied for a brief period only long enough to clean the surface of the timber from excess preservative. This method assumes that timber seasoning has taken place beforehand to ensure that during subsequent storage cracks do not expose untreated timber.

9.7 CONCRETE

9.7.1 Introduction

Concrete is a human-made composite, the major constituent of which is natural stone aggregate such as gravel, sand or crushed rock. The other principal constituent of concrete is the binding medium used to bind the aggregate particles together to form a hard composite material. The most commonly used binding medium is the product formed by a chemical reaction between cement and water. When this chemical reaction takes place (also known as hydration), heat is given off (also known as heat of hydration). In its hardened state concrete is a rock-like material with a high compressive strength but a low tensile strength. Compressive and tensile strengths in concrete are measured in N/mm^2 (Newtons per square millimetre) or MPa (Mega Pascals).

1 MPa = 1 N/mm^2

The tensile strength of normal concrete is low and this can be improved by incorporating steel bars to resist tension (reinforced concrete). The density of concrete is measured in Tonne/m^3 (Tonne per cubic metre) or kN/m^3 (kilonewtons per cubic metre).

1 Tonne/m^3 = 10 kN/m^3

It is not the intention here to teach concrete mix design but rather the site practice involved in producing good-quality concrete that is durable in time and compatible with the marine environment. Almost all the topics discussed are dealt with in greater detail in national or international standards for quality control, use of which requires access to a well-equipped concrete testing laboratory.

9.7.2 Cement

Portland cement is a finely ground powder developed in 1824 and derives its name from Portland limestone because of its close resemblance to this rock after hydration has taken place. The basic components used in the manufacture of Portland cement are calcium carbonate (found in limestone rock) and iron oxide, alumina and silica (found in clay). As a result of the chemical changes which take place inside a cement-making kiln, these constituents give rise to several compounds, only four of which are considered to be important:

- tricalcium silicate, also known by its chemical abbreviation as C_3S for short;

- dicalcium silicate or C_2S;

- tetracalcium aluminoferrite or C_4AF; and

- tricalcium aluminate or C_3A.

Over time, a variety of cements have been developed to ensure good durability of the concrete under different conditions and these cements each contain different amounts of the above constituent compounds. The main types of cement currently available in practice are:

- ordinary Portland cement;

- rapid hardening cement;

- low heat Portland cement;

- sulphate resisting cement; and

- portland blast furnace cement.

Marine works require a concrete which is durable in a marine environment and to achieve this only cement which performs well in this environment should be used. Both sulphate resisting cement and Portland blast furnace cement have these required characteristics.

During the process of hydration, the chemical reaction between the tricalcium aluminate and gypsum (CaSO_4.2H_2O) produces a compound called calcium sulphoaluminate. In hardened cement, calcium aluminate hydrate can react with a sulphate salt from outside the concrete in a similar manner: the product of addition is calcium sulphoaluminate, forming within the framework of the concrete’s cement paste. Since the increase in volume of the solid phase is over 200 percent, gradual disintegration of the concrete results.

A second type of reaction is that of base-exchange between calcium hydroxide and the sulphates, resulting in the formation of gypsum with an increase in volume of the solid phase of 124 percent.

These reactions are known as sulphate attack. The salts particularly active are magnesium and sodium sulphate. Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying, as in the case of marine structures. The remedy lies in the use of cement with a low content of C_3A and such cement is known as sulphate resisting cement. Portland blast furnace cement is also low in C_3A and suitable for marine concrete.

When large volumes of concrete are being poured, in excess of around 15 m^3, the heat of hydration in ordinary Portland cement and rapid hardening cement will give rise to thermal cracking in the cast, depending on the shape of the cast, the ambient temperature and the temperature of the batched materials. Both sulphate resisting cement and Portland blast furnace cement have low heat characteristics and should be the preferred choice in the mix design.

9.7.3 Aggregate

Aggregate is much cheaper than cement and maximum economy is obtained by using as much aggregate as possible in a concrete mix. Its use also considerably improves both volume stability and durability of the resulting concrete. The properties of the aggregate known to have a significant effect on concrete behaviour are:

- shape and surface texture;

- grading;

- strength;

- deformation (toughness and hardness);

- porosity;

- specific gravity or the ratio of its unit weight to that of water; and

- impurities.

9.7.3.1 Shape and surface texture

The shape and surface texture of an aggregate can affect the properties of concrete in both its plastic (fresh) and hardened states. These external characteristics may be assessed by observation of the particles and classification of their particle shape and texture as shown in Boxes 1 and 2.

The best aggregate for concrete is one that is irregular and angular. Rounded aggregate provides a smaller mechanical bond with the hydrated paste, whereas the flaky and elongated aggregate tends to have little strength across the thinner sections, influencing the strength of the concrete.

The most suitable mechanical bond is obviously achieved with rough surfaces, followed by granular, smooth, crystalline and glassy. Honeycombed aggregate is generally very porous and is not suitable for good quality concrete.

BOX 1

Classification

Description of shape

Rounded

Fully water-worn such as beach shingle or pebbles

Irregular

Naturally irregular such as breccia and having rounded edges

Angular

Possessing well-defined edges such as crushed from rock

Flaky

Having a thickness considerably smaller than its length or width

Elongated

Usually angular but very long

Flaky and Elongated

A combination of the above two conditions

BOX 2

Surface texture

Characteristics

Glassy

Conchoidal fracture such as in broken basaltic rock

Smooth

Water-worn such as found on beach pebbles

Granular

Fracture showing more or less uniform rounded grains, as in sand

Rough

No visible crystalline constituents, such as crushed limestone

Crystalline

Visible crystalline structure such as in granite aggregate

Honeycombed

Visible pores and cavities such as in some types of coral stone

9.7.3.2 Grading

Grading of an aggregate defines the proportions of particles of different size in the aggregate. The size of the aggregate particles normally used in concrete varies from 0.15 mm to 37.5 mm. Generally, fine aggregate or sand has particles the majority of which are smaller than 5 mm. The rest are known commonly as the coarse aggregate. The grading of an aggregate can have considerable effect on the workability and stability of wet concrete and is a most important factor in designing a concrete mix. A mix with too little fine particles is defined as harsh and does not place easily because the main aggregate tends to segregate away from the cement paste. A mix with too many fine particles on the other hand requires more water, resulting in a concrete with inferior properties.

9.7.3.3 Strength

Strength of an aggregate limits the attainable strength of concrete only when its compressive strength is less than or of the same order as the required strength of the concrete. In other words, the strongest and not the most convenient aggregates should be used in the concrete mix. Typically, coral, which only attains strengths of the order of 20 N/mm^2 cannot be used to produce concrete with strength of 30 N/mm^2, which is considered to be the minimum strength suitable for a marine environment. Typical values of strength for good parent rock (from which the aggregates are extracted) are:

Granite

181 N/mm^2

Basalt

200 N/mm^2

Limestone

159 N/mm^2

Sandstone

131 N/mm^2

Quartzite

252 N/mm^2

Porphyry

230 N/mm^2

9.7.3.4 Deformation

Deformation characteristics of an aggregate are seldom considered in assessing its suitability for concrete work although they can easily be determined from compression tests on specimens from the parent rock. A commonly used standard is toughness or brittleness, i.e. its resistance to failure by impact. Another common standard is hardness, i.e. its resistance to wear or abrasion. Soft rock such as coral, for instance, abrades very quickly and a concrete surface made with coral aggregate ruts very easily. A good guide in the field for hardness is the “penknife scratch”; a steel penknife should not easily scratch the surface of a piece of aggregate.

9.7.3.5 Porosity

Porosity is an important factor which influences the amount of water required for a given mix and makes it very difficult to obtain good quality concrete. Porous aggregate tends to absorb the mixing water quickly, depriving the cement paste, and in the process rendering the wet concrete too dry to handle and place in the process. More water has to be added to make up for this “loss” thereby reducing the quality of the finished concrete.

For this reason, porous aggregate stockpiles should not be exposed to rain. Direct measurement of porosity is difficult and in practice a related property, water absorption, is measured.

9.7.3.6 Specific gravity

Specific gravity of a material is the ratio of its unit weight to that of water. Generally speaking, the higher the specific gravity of the parent rock from which the aggregate has been extracted (e.g. limestone, granite, basalt, etc.), the denser the concrete and hence the more durable.

Figure 10 illustrates the various terms used in defining the “weights” of the various components. The basic unit for specific gravity is water and everything else is referred to unity. For example, a cube measuring 1 metre cubed (1 m^3) of solid granite weighs 2 630 kilograms; granite is a very dense material. Its specific gravity is 2.63. The following provides the average specific gravity for parent rock.

Parent rock

Average specific gravity

Basalt

2.793

Granite

2.636

Hard limestone

2.60

Soft limestone

2.20

Porphyry

2.73

When a 1 cubic metre cube of solid granite is crushed to make aggregate, the bulk or the overall volume of the resulting aggregate occupies more than 1 cubic metre. The reason for this is that inside the aggregate there are now also voids and the volume occupied by these voids is the amount of leftover aggregate that will not fit inside the 1 cubic metre. The 1 cubic metre of granite aggregate typically weighs only 1 800 kilograms and the “weight” or more appropriately the bulk density of granite aggregate is specified as being 18.0 kN per cubic metre (1.8 t/m^3). The density of other types of aggregate depends mainly on the specific gravity of the parent rock, the shape of the particles (some particles fit tightly together making the aggregate denser) and the relative size of the particles (if an aggregate has a lot of fine particles, i.e. dust, it weighs more). Dry cement powder typically weighs 18.0 kN/m^3 or 1.80 Tonnes/m^3.

9.7.3.7 Impurities

Natural aggregates may be sufficiently good in themselves and yet they may not be satisfactory for concrete making if they contain organic impurities which interfere with the chemical reactions of hydration. The organic matter found in aggregate consists usually of products of decay of vegetable matter and such materials are more likely to be present in sand than in coarse aggregate, which is easily washed. Clay may be present in aggregate in the form of surface coatings which interfere with the bond between aggregate and the cement paste. Another type of fine material present in some fine aggregates (i.e. in the sand) is silt and crusher dust. Both silt and fine dust (particle diameters in the range of 1 to 60 microns) may form coatings on the aggregate similar to those of clay. The presence of loose silt and dust in a mix necessitates the use of more water in the concrete, thereby decreasing the strength.

Sand from the seashore or from a river estuary also contains salt. This has to be washed away using freshwater. Due to their porosity, coral aggregates generally contain high levels of salt and should not be used, especially with reinforcing steel. A concrete surface containing coral also exhibits efflorescence and cannot be painted or rendered.

Other types of unsound materials, such as coal, mica and iron pyrites (iron sulphide), should also be removed or limited to trace levels. Some forms of aggregate are totally unsuitable for inclusion in concrete in that they react over a length of time with the hydrated cement paste causing the concrete to deteriorate.

9.7.4 Mixing water

The quality of the mixing water also plays a vital role in the strength of the concrete because impurities in the water may interfere with the hydration of the cement paste. In many specifications worldwide, the quality of water for mixing concrete is covered by a clause saying that water should be fit for drinking. Such water rarely contains dissolved solids in excess of 2 000 ppm or parts per million (i.e. 2 000 grams of dissolved salts per cubic metre of water), and as a rule less than 1 000 ppm. While the use of potable water is generally safe, water not fit for drinking may often be used to make concrete. As a rule, water with a pH of 6.0 to 8.0 which does not taste saline or brackish is suitable for use, but dark colour or bad smell does not necessarily mean that organic substances are present. A very good way of determining the suitability of dubious water is to compare the setting time of cement made with say, tap or distilled water (the control test) and the dubious water. Brackish water contains chlorides and sulphates. When chlorides do not exceed 500 ppm and sulphates (SO_3) do not exceed 1 000 ppm, the water is harmless.

Seawater has a total salinity of around 35 000 ppm (i.e. 35 000 grams or 35 kilograms of dissolved salts per cubic metre of seawater) and 78 percent of these salts is sodium chloride (NaCl) and 15 percent magnesium sulphate (MgSO_4) and magnesium chloride (MgCl_2). When used for mixing concrete, seawater:

- may lower the long-term strength of concrete by about 15 percent;

- will increase the risk of corrosion of steel inside the concrete; and

- will cause persistent dampness and surface efflorescence.

9.7.5 Batching

Concrete is made up from the various constituents mixed together according to a specific mix; the mix may either be a standard mix or a designed mix. Designed mixes are usually reserved for use on large projects and the science of designing a mix to suit local conditions is beyond the scope of this manual.

Concrete mixes are generally quoted by weight of the constituent materials but, knowing the densities of the constituents (remembering that cement always comes in 50-kilogram bags), mixes may also be converted to volumetric proportions; volumetric proportions are easier to follow in artisanal situations and will be dealt with in greater detail. A standard volumetric mix may be any one of those shown in Table 8.

TABLE 8

Batching volumes

Nominal proportions

Quantities of aggregate

Typical strength measured (per 50-kg bag of cement after 28 days in n/mm^2)

Fine (Sand)

Coarse

1 : 2 : 4

0.07m^3

0.14m^3

20.0 (general mass concrete)

1 : 1½ : 3

0.05

0.10

25.0 (not in much use)

1 : 1 : 2

0.035

0.07

30.0 (minimum marine concrete)

The above table gives the volumetric dosage of the constituent materials referred to a 50-kilogram bag of cement. The recommended minimum grade or strength for marine concrete is 30 N/mm^2 as this provides a good all-round mix with good durability. This mix is generally known as a “one one two” mix.

Batching is the term used for preparing the various dry constituents for mixing. To batch the aggregates by volume, a wooden measure box should be constructed with inside dimensions of 400 mm by 400 mm by 200 mm high as illustrated in Figure 11.

Each such level box contains 0.035 cubic metre of aggregate. Therefore, for each 50-kilogram bag of cement loaded into the concrete mixer, one box measure of sand (0.035 m^3) and two box measures of aggregate (0.070 m^3) are required. This hence completes the batching of the dry constituents.

The nominal mix table purposely leaves the water dosage out. In engineering practice, the strength of the concrete at a given age (28 days are generally prescribed as the minimum for testing) is assumed to depend primarily on two major factors in the field:

- the amount of water in the mix (known as the water-cement ratio or W/C ratio); and

- the degree of compaction.

Assuming that the concrete is fully compacted (by mechanical vibration, as described later on in the text), the strength is a function of the W/C ratio (i.e. the amount of water introduced into the mix divided by the weight of the cement in the mix), Figure 12.

This means that the less water is added to the mix (beyond the necessary minimum to turn the cement into a paste, albeit a stiff one), the stronger the concrete is. However, very stiff concrete is difficult to pour from a mixer and difficult to place inside a given mould and, generally speaking, more water has to be added beyond the minimum to make the mix workable.

In practice, if the aggregates have been chosen carefully, the W/C ratio can be set at around 0.35 to 0.40 as a starting point. A W/C ratio of 0.35 means that for every 50 kilograms of cement present in the mix, 17.50 litres (0.35 x 50 = 17.50) of water should be added. This will undoubtedly result in a very stiff mix and further amounts of water will be needed to make the mix workable. Generally, water in the concrete consists of that added to the mix and that held by the aggregate at the time it enters the mixer (for example, aggregate which has been exposed to rain will automatically introduce more water into the mix, increasing the given W/C ratio). Conversely, when the aggregate is very dry (i.e. left exposed to the mid-day sun), a part of the water added to the mix will be taken away from the cement paste and absorbed by the hot aggregate during the first few minutes after mixing, decreasing the W/C ratio and resulting in a very stiff mix.

The workability of concrete has never been precisely defined. For practical purposes it generally implies the ease with which a concrete mix can be handled from the mixer to its finally compacted shape. The three main characteristics of the property are:

- consistency (a measure of the wetness or fluidity);

- mobility (the ease with which a mix can flow into and completely fill the formwork); and

- compactability (the ease with which a given mix can be fully compacted).

In this context, the required workability of a mix depends not only on the characteristics and volumetric proportions of the constituent materials but also on:

- the methods employed for conveyance and compaction;

- the size, shape and surface roughness of the formwork; and

- the quantity and spacing of any reinforcement.

A simple test devised to measure the consistency and workability of fresh concrete is the “slump test” and the apparatus involved is inexpensive, portable and robust. Figure 13 illustrates the dimensions of the steel cone required for the test. Figure 14 shows how a slump test is carried out.

When just enough water has been added to the concrete mixer to make the concrete appear wet but stiff, the cone should be filled with three layers of concrete from the mixer, each compacted by hand using a 20 mm diameter steel poker. The top of the cone should then be towelled level and the cone lifted off. As soon as the steel cone is lifted off, the concrete will slump or settle down as shown in Figure 15. The ideal slump for most practical work is 50 mm. Should the concrete not slump by 50 mm, a bit more water should be added to the mixer with a measuring can, half a litre at a time (and not directly through a water pipe). The test should then be repeated until the desired slump is achieved. This test is suitable for the quality control of consistency. It is not suitable for stiff mixes and very wet mixes.

Figure 15 illustrates the three types of slump usually observed; true, shear and collapse slumps. A true slump is observed with cohesive and rich mixes for which the slump is generally sensitive to variations in workability (the ideal case). The shear slump tends to occur more often with leaner mixes and indicates a lack of cohesion generally associated with harsh mixes (low on fines). A collapse slump is an indication of a very wet mix and is generally taken to indicate poor workmanship or poor concrete.

The standard slump apparatus is only suitable for concretes in which the maximum aggregate size does not exceed 37.5 mm. It should be noted that the value of the slump changes with time after mixing owing to the normal hydration processes and evaporation of some of the free water and it is desirable, therefore, that tests are performed within a fixed period of time, Figure 16.

On a larger scale, concrete is batched by concrete mixers and concrete mixers come in many sizes and shapes but generally fall into three main categories:

- portable, tilting drum, mini-mixers, capacity not exceeding about 150 litres (0.15 m^3);

- transportable, fixed horizontal drum mixers, capacity not exceeding 1.0 m^3; and

- pan mixers, used mainly by ready-mixed operators, with much larger capacities.

Portable mixers, Figure 17, are available with a wide range of power options, such as petrol, diesel and electricity. They are small enough to be loaded on a light truck or towed behind a suitably equipped vehicle. The capacity of the tilting drum is usually enough to hold the batch for a 50-kilogram cement bag.

Allowing for loading and a minimum mixing time of about five minutes, a crew of five people can produce an average of about 1 m^3 of fresh concrete per hour (suitable for small pours, jute-bag filling, etc.). Petrol-powered versions tend to be lighter than diesel-powered ones. Electric-powered mixers are the lightest and easiest to maintain.

The fixed horizontal drum mixer is quite large and heavy and when towed to a site it is generally set up on a prepared concrete platform to ease loading and discharging of concrete, Figure 18. Modern horizontal drum mixers are quite sophisticated, generally incorporating a load cell on the tipping hopper (so that the mix can be batched by weight instead of by volume) and an automatic water dosimeter for dispensing the exact volume of mixing water directly into the drum. The drum capacity can range from 0.50 m^3 to 1.0 m^3 and needs a fairly large crew to operate. A fleet of wheelbarrows is generally needed to haul enough aggregate to keep it working at full capacity and enough wheelbarrows or a motorized skip or tipper to haul away the concrete to the formwork. The wheelbarrows loading the aggregate may be replaced by a small pay-loader and the wheelbarrows loading the batched concrete may be replaced by a tipper.

Pan mixers are generally used by ready-mix companies for discharging concrete into truck mixers. Pan mixers are the most efficient way to mix concrete and need mechanized feeding of all the dry constituents.

9.7.6 Reinforcement

As mentioned earlier, the tensile strength (resistance to pulling) of concrete is very low and reinforcing steel in the form of bars or mesh is used to increase this strength. Reinforcing bars are manufactured as deformed or ribbed bars to better grip the concrete as illustrated in Figure 19. Commonly available sizes are illustrated in Table 9 and the common types of steel mesh are illustrated in Figure 20.

TABLE 9

Commonly available sizes of steel bars and steel mesh

SQUARE STEEL MESH FABRIC

STRUCTURAL STEEL MESH FABRIC

When exposed to a marine environment, the steel bars inside the concrete may corrode if:

- The concrete is of poor quality (porous) and not very dense.

- The concrete cover to the steel bars is too small, allowing salt to reach the steel.

- The concrete (aggregates or mixing water) contains too much salt.

Assuming that the concrete has been manufactured to the right standard (i.e. good quality, dense and salt-free), the minimum cover to the steel should not be less than 50 mm as shown in Figure 21.

9.7.7 Placing, vibrating and curing

Placing, vibrating and curing complete the cycle of concrete construction. The formwork should be sturdy and properly anchored to prevent it from moving during the placing of concrete. It should be free of dust or organic matter (vegetation, cigarette butts, timber) and it should be properly oiled on the inside with an appropriate, water-based formwork release agent. Diesel is not suitable for this purpose and should not be used. The steel should be kept clean and free of oil-based compounds, such as diesel and oil. The reinforcement should be tied firmly to the sides of the formwork and workers should not be allowed to walk over it. Suitable planks should be provided across the formwork for this purpose. Concrete should not be dropped from a height exceeding 1.50 metres as this will lead to segregation of the aggregate and, if reinforcement is present, it may push the steel bars out of position. When concrete has to be placed underwater, a tremie pipe should be used, Figure 22 (left). The concrete should be made richer (cement content increased by about 25 percent from the standard nominal mix) and slightly wetter as no vibration of the concrete is allowed underwater. The flexible pipe should be raised along with the pour until the concrete breaks the surface of the water. Concrete should never be tipped into water, Figure 22 (right), as this will cause the cement to wash out. If a concrete pump is available, concrete may be pumped directly underwater.

The process of compacting the concrete consists essentially of the elimination of entrapped air bubbles. The use of vibration as a means of compaction makes it possible to use drier mixes than can be compacted by hand. Of the several types of vibrators, the poker vibrator, illustrated in Figure 23, is perhaps the most common one in use.

It consists essentially of a poker, housing an eccentric shaft driven through a flexible drive by a small petrol-driven engine. The frequency of vibration varies up to 12 000 cycles per minute, whereas 3 500 to 5 000 is considered as desirable minimum. The poker is easily moved from place to place, and should be applied every half a metre. The poker should be withdrawn from the concrete very gradually and in fresh layers of concrete not exceeding 300 mm in thickness. To achieve the best results it should be worked into the concrete at regular intervals in an orderly fashion, as illustrated in Figure 24. The actual completion of compaction can be judged by the appearance of the surface of the concrete, which should neither be honeycombed nor contain an excess of mortar, which is generally due to overvibration. For heavily reinforced sections, a 50 mm to 60 mm diameter poker should be used. Mass concrete pours, such as quay wall blocks, should be vibrated with pokers at least 80 mm in diameter.

In order to obtain good concrete, the placing of an appropriate mix must be followed by curing during the early stages of hardening. Curing is the name given to procedures used for promoting the hydration of cement, and consists of a control of temperature and of the moisture movement from and into the concrete. Specifically, the object of curing is to keep concrete saturated with water until the original water-filled space in the fresh cement paste has been filled to the desired extent by the products of hydration of the cement. The necessity for curing arises from the fact that the hydration of cement can take place only in water-filled capillaries. This is why a loss of water by evaporation from the capillaries must be prevented at all costs. Furthermore, water lost internally by self-desiccation (hydration of cement releases a lot of heat which in turn increases the rate of desiccation of the fresh concrete) has to be replaced by water from outside. Loss of water from the surface of freshly-poured concrete depends on:

- the ambient temperature (the higher the temperature, the greater the evaporation);

- the relative humidity (the drier the air, the faster the water evaporation);

- the wind speed (fast moving wind dries the surface very rapidly); and

- the speed at which the formwork is removed (early removal increases the exposed surface area).

Depending on the shape of the cast, curing may be achieved in a number of ways, such as:

- by flooding (suitable for large areas such as paving slabs, which can be flooded by surrounding them with an impermeable bund);

- by spraying or misting (requires a fine-mist sprinkler system and a good supply of freshwater);

- by covering the cast concrete, with damp hessian or plastic sheeting; and

- by impermeable curing membrane, a rubber latex emulsion sprayed over the cast. The membrane, provided it is not punctured or damaged, will effectively prevent evaporation of water from the concrete but will not allow ingress of water to replenish that lost by self-desiccation.

The period of curing cannot be prescribed by theory but it is usual to specify a minimum of seven days. The formwork should not be removed until at least 24 hours have elapsed since the casting operation and only if measures are taken immediately to establish a curing regime.

9.7.8 Admixtures

Nowadays, instead of using special cement, it is possible to change some of the properties of the cement in a mix by the use of a suitable additive. A vast range of proprietary products is available and these fall into three major groups:

- accelerators (they accelerate the setting of the concrete);

- retarders (they slow down the setting of the concrete); and

- water-reducers (they decrease the amount of mixing water, thereby increasing the strength of the concrete by virtue of lowering the W/C ratio).

There are other, less commonly used admixtures, such as air-entraining and waterproofing agents. An important feature of the majority of admixtures for concrete is that they are used primarily on the basis of experience or ad hoc tests: this is largely due to the marketing of admixtures as proprietary products.

9.7.8.1 Accelerators

The addition of calcium chloride to a concrete mix increases the rate of development of strength, and this accelerator is, therefore, sometimes used when the concrete needs to be “hardened” quickly, such as when concrete has to be placed underwater. Calcium chloride increases the rate of heat liberation during the first hours of mixing, but the normal process of hydration of cement is not changed. A calcium chloride content in a mix of 1 percent of the cement by weight (i.e. 0.50 kilogram per 50-kilogram bag) is generally sufficient for most purposes. Using commercially available calcium chloride flakes, the admixture should be prepared in a concentrated aqueous solution and added to the concrete in the mixer. Calcium chloride must not be used in reinforced concrete as this will increase the risk of corrosion of the steel from the chloride ions. Chloride-free proprietary formulations also exist for use with reinforced concrete.

9.7.8.2 Retarders

A delay in the setting time of the cement paste can be achieved by the addition to the mix of a retarding admixture. These admixtures slow down also the hardening of the concrete. Retarders do not alter the composition of the products of hydration. Retarders are useful in concreting in hot weather, when the normal setting time is usually shortened by the high ambient temperatures. Retarding action is exhibited by sugar, carbohydrate derivatives, soluble zinc salts, soluble borates and others. Great care is necessary in using retarders as in incorrect quantities they can totally inhibit the setting and hardening of concrete. Sugar content in a mix of 0.05 percent of the cement by weight (i.e. 0.025 kilogram or 25 grams per 50-kilogram bag) is sufficient to delay setting by about four hours. In important concrete work, the performance of sugar as a retarder should be determined by trial experiments with the actual cement which is to be used in construction. Sugar may also be used to advantage in the event of a mixer breaking down; a 1 percent addition by weight of cement will “kill” the mix inside the drum, allowing repairs to be carried out without the risk of seizure.

9.7.8.3 Water-reducers

As their name implies, these admixtures allow concrete to be mixed with less water, thereby decreasing the W/C ratio. The reduction in quantity of the mixing water that can be achieved varies between 5 and 15 percent. A part of this is in many cases due to the entrained air introduced by the admixture. The actual decrease in the mixing water depends on the cement content and the type of aggregate used. Lignosulphonic acids and their salts are water-reducing admixtures only; hydroxylated carboxylic acids and their salts are water reducing and set retarders at the same time.

High-range water reducers are a modern type of water-reducing admixture and commonly know as a superplasticizer. Chemically, they are sulphonated naphtalene formaldehyde condensates, and at a given W/C ratio they typically increase the slump from 75 mm to 200 mm without compromising the cohesiveness of the mix. The plasticizing action of superplasticizers only lasts about 10 minutes: after about 30 to 90 minutes the workability returns to normal.

9.7.9 Polymer-modified cement mortars

Polymer-modified cement mortars are special proprietary formulations for the repair of spalled concrete. These cement mortars are generally supplied as a one-pack or a two-pack product. This type of mortar has increased mechanical strength, is very resistant to abrasion and chemical attack, and does not shrink during setting, making it ideal for repairing reinforced concrete. Its adhesive properties to old concrete are superior to those of a normal sand-cement mortar.

Spalled sections of a reinforced concrete member should be thoroughly cleaned of any residual cracked concrete, Figure 25 (middle), a tight-fitting formwork applied to the damaged sections and the cement mortar poured in from the top. Certain cement mortars are very stiff, in which case they need to be applied with a trowel.

9.7.10 Non-shrink grouts

Non-shrink grouts are generally used for grouting in of anchor bolts and bearing plates, such as bollards, as illustrated in Figure 26. These grouts actually expand on setting and provide a very good mechanical bond with the base concrete. They are free-flowing (pourable), harden very rapidly, and can reach strengths of the order of 60 N/mm^2. They are all sold as proprietary products and generally come prebatched in 25-kilogram waterproof bags, needing only mixing water.

9.7.11 Epoxy flooring compounds

An exposed concrete finish is not suitable as a floor inside “wet” fish markets and fish processing halls in general for the following reasons:

- Some fish oils attack concrete and lead to pitting in the surface texture of a concrete floor, Figure 27.

- Concrete is not waterproof and as blood soaks in it gives rise to bad odours.

- A concrete floor has to be laid in bays or sections to prevent it from cracking due to shrinkage; the resulting joints are difficult to clean and generally harbour bacteria.

Current food hygiene legislation in many countries stipulates that the floor finish in such cases should be:

- resistant to chemical attack, including the bleaching agents used to disinfect the premises;

- waterproof; and

- of seamless construction.

The only floor finish (the base is still concrete, in the form of a slab 200 mm to 250 mm thick) that satisfies the above requisites is that made from solvent-free epoxy resins. Typically, epoxy-resin flooring compounds are self-levelling and available in two-pack or three-pack form and come in grey, green, red and yellow pigment. Epoxy-resin flooring compounds are very strong and durable; after 14 days the compressive strength typically reaches values in the region of 80 N/mm^2 and their bond to the underlying concrete is superior to that of concrete over concrete. The components are usually mixed just prior to use (the pot life being in the region of 30 minutes depending on the ambient temperature) and applied by roller or trowel in a thickness not exceeding 3 mm to 4 mm. The finish may be rendered non-skid or non-slip by the application of a second roller just before the final setting takes place and has excellent resistance to abrasion. Ideally, epoxy-resin floor finishes should be applied to newly-laid concrete floor slabs. In the event that the epoxy floor needs to be retrofitted to a floor inside an existing market, and assuming that the concrete floor slab is still homogenous, the existing floor must first be “scabbed” (the upper 15 mm to 20 mm of the existing concrete removed by grit blasting to expose a fresh concrete surface) and a suitable solvent-free epoxy primer applied prior to the flooring compound. All epoxy flooring compounds are marketed as proprietary products and specifications vary from one manufacturer to the other.

9.7.12 Epoxy concrete coatings

Epoxy concrete coatings are specifically designed to prolong the life of reinforced concrete sections exposed to a harsh marine environment by rendering the surface waterproof. Most coatings are two-pack, water-based epoxy paints, which may be applied by brush, roller or airless spray. Both grey and colourless paints are available. These coatings may be applied to either dry or damp concrete surfaces in two to three separate coats. All coatings in this category are marketed as proprietary products.

9.7.13 Bituminous coatings

When the concrete surface to be waterproofed is not visible to the eye (such as the underside of a piled jetty, concrete piles, the inside of a concrete potable water tank, etc.) and not exposed to direct sunlight (i.e. ultraviolet radiation), bitumen provides the most cost-effective surface-sealing treatment. Prior to treatment, concrete surfaces must be thoroughly dry and free of dust particles. Bituminous coatings may be applied by brush, roller or airless spray. These coatings are suitable for waterproofing surfaces that come in contact with potable water, such as water reservoirs. Sunlight degrades bitumen.

9.7.14 Site practice and safety

Careful housekeeping on site ensures that all building materials remain suitable for construction purposes. In particular:

- Building materials should not be stored in areas subject to flooding.

- Some materials must not be exposed to direct sunlight.

- Cement should be purchased sealed in good quality bags; punctured bags should not be accepted.

- Cement should not be stored on site for more than six weeks.

- Cement should be stored indoors, 150 mm off the ground; if stored outside, it should be covered with plastic sheeting with plenty of space for air to circulate.

- Steel bars should not be stored in contact with the ground.

- Aggregates should be shaded from direct sunlight with netting or plastic sheets to keep them cool.

- If rain is forecast for the day, concreting should be postponed.

- Concreting should be avoided during hours of peak temperatures; early morning is ideal.

Although concrete mixing may be considered by some as a hazard-free activity, safety on site is in fact quite a serious matter. Cement, concrete, concreting and the various epoxy formulations in use nowadays do present a hazard to human health; for example:

- Airborne cement powder is toxic to human health.

- Skin bleeds when it comes into contact with cement and fresh concrete.

- Most epoxy formulations are toxic to human skin.

Figure 28 shows basic safety equipment to be used on site. In particular, persons handling cement (breaking bags and pouring out the contents into the mixer) should always wear goggles, a proper face mask, gloves and boots. People handling concrete should wear gloves and boots. Everybody on site should wear a safety helmet.

9.8 ROCK

9.8.1 Introduction

Stone for coastal structures should be sound, durable and hard. It should be free from laminations and weak cleavages, and should be of such character that it will not disintegrate from the action of air, seawater and undesirable weathering, or from handling and placing. In general, stone with a high specific gravity should be used to decrease the volume of material required in the structure and to increase the resistance to movement by the action of waves or currents. Characteristics that affect the durability of stone are texture, structure, mineral composition, hardness, toughness, and resistance to disintegration on exposure to wetting and drying and to freezing and thawing. Ordinarily, the most durable stone is one that is dense or fine textured, hard and tough, but exceptions to this general rule occur. The character of the stone for any project depends on what is available, and often the choice of material involves weighing the relative economy of using a local stone of lower quality against using a better quality stone from a distance.

Where the local stone is markedly inferior, the greater cost of transporting durable, high-quality stone from outside the immediate area may be justified and advisable.

9.8.2 Quarries

Rock is generally obtained from quarries but quarries do not normally supply rock in the sizes required for the construction of breakwaters, Figure 29.

This is generally due to the fact that blasting systems, drilling patterns and equipment are tailored to produce only small-size aggregates for concrete or road construction. Retooling a quarry to produce breakwater-type rock sizes is very expensive and should only be carried out if the potential yield of the quarry is deemed sufficient for the purpose; experienced geologists should be called in to carry out such investigations. The potential yield in terms of breakwater-sized rock of a quarry depends almost entirely on the geological formation and the natural bedding of the deposits.

Figure 30 illustrates some commonly encountered geological formations or rock outcrops. The left figure illustrates regular block formation, heavily fractured, producing regular sizes of rock. The middle figure illustrates an irregular outcrop and the right figure shows heavily bedded layers that produce tabular rocks. Therefore, prior to constructing a rubble breakwater it is necessary to:

- Collect or obtain information on the geological formation of the quarry site, including faults, natural bedding of layers, etc., Figure 30;

- Obtain details of the vertical extension of the overburden (the rock strata may be underneath a top soil, which may be fertile and cannot be discarded); and

- Carry out laboratory tests to determine the geotechnical characteristics of the rock.

In addition, the quarry must be accessible without too much investment in access roads; and have adequate storage space for stockpiling and sorting of the rock into the required sizes.

9.8.3 Geological characteristics of rock

The strength of the parent rock essentially decides the degradation mechanisms of the individual armour rock. These mechanisms may be grouped into:

- spalling, commonly associated with salt attack, alteration of minerals within the rock and expansion or decay of clay minerals;

- fracture, linked to incipient planes of weakness in the rock deposit; and

- abrasion, caused by adjacent stones rubbing together under wave action or by much smaller particles of sand and rock thrown against the stone during wave action.

Typical deposits suitable for the production of breakwater rock are granite, basalt, limestone, sandstone, quartzite and porphyry. In the laboratory, the geotechnical characteristics of the rock are usually defined by the parameters, shown in Table 10, details of which are found in various national standards.

It is a generally accepted fact that all marine structures will degrade with time and that only adequate monitoring during the lifetime cycle will prevent serious damage from occurring to the structure.

The evaluation of a potential stone source should consider the extent that a quarry development might detract from natural beauty and otherwise cause environmental concern during and after operation. The quarry area should be graded and landscaped as practicable to restore a natural appearance and to control erosion upon closure.

TABLE 10

Geological characteristics of rock

Property

Suggested limit

Specific gravity

2.60 minimum

Water absorption

2.50 maximum

Particle shape

Angular

Surface texture

Crystalline

Impact value %[1]

25 maximum

Abrasion value %[2]

25 maximum

Magnesium sulphate

Soundness test [3]

12 maximum

[1] Standard test for resistance to chipping from impact loads.

[2] Standard test for resistance to abrasion by other particles.

[3] Standard chemical test for resistance to chemical alteration of the minerals.

9.9 PLASTICS, RUBBER AND BITUMINOUS COMPOUNDS

Nowadays, both plastics and rubber compounds play an ever-increasing role in marine works, typically replacing steel articles such as pipes, pipe fittings, gratings and covers.

9.9.1 Polyvinyl chloride

This material is available in two forms – plasticized or unplasticized. Both types are characterized by good weathering resistance, excellent electrical insulation properties, good surface properties and they are self-extinguishing. Plasticized PVC is flexible and finds applications in wire covering. Unplasticized PVC (uPVC) is a hard, tough material which is widely used for pipes and gutters.

PVC has a specific gravity of 1.38 to 1.45 and a surface hardness (Shore D) of 70 to 90, equivalent to that of aluminium.

9.9.2 Polypropylene

Polypropylene is an extremely versatile plastic and is available in many grades and also as a copolymer (ethylene/propylene). It has the lowest density of all thermoplastics (plastics that deform with an increase in temperature) in the order of 900 kg/m^3 and this combined with strength, stiffness and excellent fatigue and chemical resistance make it attractive in many situations. Current uses of this plastic is in making fish boxes and fibre ropes.

9.9.3 Polycarbonates

The outstanding feature of these materials is their extreme toughness. They are transparent and have good temperature resistance but are attacked by alkaline solutions and hydrocarbon solvents. Typical applications include “vandal-proof” street lamp covers and lenses in marine lanterns.

9.9.4 Low-density polyethylene

Low-density polyethylene (LDPE) is one of the most widely used plastics. It is characterized by a density in the range 918 to 935 kg/m^3 and is very tough and flexible. Major applications include pipes and cold water tanks.

9.9.5 High-density polyethylene

High-density polyethylene (HDPE) has a density in the range 935 to 965 kg/m^3 and is more crystalline than LDPE. It has a surface hardness (Shore D) of 63. It is slightly more expensive than LDPE but it is much stronger and stiffer and applications include waste bins and pipes.

9.9.6 Polyurethane

This material is available in three forms: rigid foam, flexible foam and elastomer. They have high strength and good chemical and abrasion resistance. Foams find applications in insulation and elastomers in solid tyres.

9.9.7 Polyesters

The main application of this material is as a matrix for glass fibre reinforcement. The fibres (E-glass) are generally calcium aluminium borosilicate with a specific gravity of 2.05.

9.9.8 Rubber

Rubber is a unique engineering material possessing an ability to deform elastically by several hundred percent without failure. Raw rubber would in most cases be relatively unstable, so for engineering applications it is generally cross linked or vulcanized with a chemical such as sulphur. Rubber components are thus normally manufactured by means of a cure process in a metal mould. Most types of rubber used in engineering also incorporate at least one type of filler, such as carbon black or soot. Fillers generally reinforce rubber, improving its resistance to tear, abrasion.

The life and performance of a rubber component may be strongly influenced by its service environment (sunlight, weathering, ozone cracking), which can cause changes in stiffness and surface texture. Normally, effective protection against sunlight is given by the carbon black filler incorporated in the rubber; this acts by filtering out the ultraviolet radiation. If rubbers are not protected by the incorporation of appropriate antioxidants and antiozonants, then atmospheric ozone, sunlight and oxygen can damage rubber.

Nitrile rubber has the best resistance to petroleum products and is generally used for refuelling hoses. Neoprene rubber is used in the form of pads under precast concrete structural elements such as beams, lintels or roofing slabs to avoid concrete-to-concrete abrasion.

9.9.9 Bituminous compounds

All bituminous materials are for the most part used in mixtures with aggregates as binders. Binders all have certain valuable properties in common: they are water-resistant, have good adhesive properties and can withstand ordinary weathering. All binders, whether tars or bitumen, are exceedingly complex materials chemically. Different tars and bitumens have been characterized by separating them into fractions according to their solubility in a series of solvents of increasing dispersing power.

- Tars are obtained from the destructive distillation of coal or shale, where for engineering purposes all the lighter oils are distilled off leaving a hard, semi-solid residual material – pitch. Pitch is in itself too hard and viscous to use so lighter oils are fluxed back into the pitch to produce tar.

- Bitumens are obtained by the fractional distillation of petroleum (crude oil). In some cases this process occurs naturally, producing rock or lake asphalt, but the bulk of the world’s bitumen is produced by refining crude oil. Chemically, bitumens are similar to tar and are also highly resistant to weathering agents.

- Cutbacks are mixtures of binders with light volatile oils, the resultant mixture having a much lower viscosity than the original binder, allowing various handling operations to be carried out at a much lower temperature than would otherwise be the case.

- Emulsions are mixtures of bituminous binders and water in a dispersant. The dispersant may consist of sodium stearate, soap or trimethyl-ammonium bromide.

- Rubberized binders consist of unvulcanized rubber powder, 0.50 to 5.0 percent, dispersed in bitumen. Rubberized bitumens are less susceptible to temperature changes and are more elastic.

- Trinidad Lake Asphalt is a natural asphalt. This asphalt has superior drying properties to those of refinery bitumens but is considerably more expensive.

The very term binder used to describe bituminous materials suggests an ability to act as adhesive and “bind” other materials together. As with all adhesives when tars or bitumens are used it is important that the materials to be bound together by these binders should be clean, dry and free from dust. If, for example, in coating stones for road mixes the binder is too thick or viscous, then it will not “wet” or coat the stone efficiently. If the stone is dusty, then the binder may not reach the stone surface. If there is water on the stone, then the binder will not adhere to the stone. Bituminous materials are used in engineering in a wide variety of consistencies or viscosity. For practical purposes, the viscosity may be measured empirically and expressed in terms of a simple unit, usually either seconds or degrees Celsius, when the liquid has been tested in a standard apparatus under specific standard conditions.

The standard test for ordinary bitumens is the penetration test. In this test a loaded needle is allowed to penetrate a sample of bitumen; the viscosity is then a measure of the depth of penetration at that temperature and under those loading conditions.

For normal test conditions in the penetration test, a 1 mm diameter needle is used, ground to a sharp point and loaded with 100 grams. The needle just touches the surface of the bitumen sample at the start and is allowed to fall under gravity for five seconds, the ambient test temperature being 25 °C. The penetration is measured in tenths of a millimetre.

If a bitumen is referred to as 70 pen, it is understood that a penetration of 70 tenths of a millimetre was obtained under the above conditions.

At ordinary temperatures most binders are much too stiff and hard to handle. In order that they may be sprayed, pumped and mixed, or compacted in a stone-binder mixture, their viscosity must be greatly reduced.

9.10 BIBLIOGRAPHY AND FURTHER READING

Dinwoodie J.M. 1981. Timber: Its nature and behaviour. New York, Van Nostrand Reinhold.

Hornsby M.J. 1995. Hot-dip galvanizing: A guide to process selection and galvanizing practise. London, Intermediate Technology Publications.

Jackson N. 1981. Civil engineering materials. UK, Macmillan Press Ltd.

Moffat & Nichol Engineers. 1983. Construction Materials for Coastal Structures. Virginia, USA, US Army Corps of Engineers Coastal Engineering Research Center.

Neville A.M. 1981. Properties of concrete. UK, Longman Scientific & Technical.