Volume of earth to be removed

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The labor and expense involved in moving soil can be substantial. Careful planning and volume estimation can keep the amount moved to a minimum.

When the land is essentially level, the volume to be removed from an excavation can be estimated by multiplying the cross section area of the excavation by the length.

Figure 2.11 Sight rails and traveller for boning.

Figure 2.12 Section between 2 sight rails on a gradient.

Figure 2.13 Section showing level excavation.

Often, however, the land has a considerable slope and must be leveled before construction can begin. In some cases the soil must be removed from the site, but frequently what is removed from the building site can be use for fill in an adjacent area. Estimating how much to "cut" so the soil removed just equals the "fill" required to give a level site is somewhat more difficult. Several approaches are explained in surveying books, but a graphical method using the information from the site contour map should be satisfactory for rural building construction.

A scale drawing of the building foundation is made and the contours superimposed on it (Figure 2.14a). A line is drawn through the center of the building plan and a section constructed using the values obtained from the intersections of the contour lines and the section line (Figure 2.14b).

Figure 2.14a Contours for establishing a cut and fill line.

A horizontal line is then drawn that is estimated to produce equal areas for cut and fill. The elevation of the line indicates an optimum elevation for the building. The approximate volume to be moved is given by the equation:

V = 1/2 hbw
= 1/2 x 0.06 x 6 x 6
= 10.8 mē


h = height above line
b = base of cut area
w = width of cut area

Figure 2.14b, c Estimating cut and fill

If the slope is not as uniform as illustrated in Figure 2.14b, the slope line must be averaged as shown in Figure 2.14c. In this example the volume to be moved is estimated to be 45.6mē.

V = 1/2 hbw
= 1/2 x 1.9 x 4.8 x 10

When excavated, the volume of firm soil will increase by approximately 20%. If this soil is used for fill, it must either be allowed to settle for some time or be compacted to reduce the volume back to the original before any construction work can begin. In addition, African soils are generally prone to settlement and erosion. Problems may be experienced in wet areas at the edge of the fill if it is not adequately stabilized with vegetation or a retaining wall. Therefore the 'cut and fill' technique should be avoided and if used, a reinforced concrete footing may be required.

Further reading

Clancy J., Site Surveying and Levelling, London, Edward Arnold Ltd., 1981.

Collett J., Boyd J., Eight Simple Surveying Levels, Agricultural equipment and tools for farmers designed for local construction, No. 42, London, Intermediate Technology Publications Ltd.

Olliver J.G., Clendinning J., Principles of Surveying, Volume 1: Plane Surveying, 4th edition, New York, Van Nostrand Reinhold Co., 1979.

Scott G.A., Construction Surveying, London, Longman Group Ltd., 1973.

Chapter 3 Building materials

Building materials

A wide range of building materials is available for rural building construction. The proper selection of materials to be used in a particular building can influence the original cost, maintenance, ease of cleaning, durability and of course, appearance.

Several factors need to be considered in choosing the materials for a construction job, including:


Wood is a commonly used building material in many parts of the world because of its reasonable cost, ease of working, attractive appearance and adequate life if protected from moisture and insects. However, forests are a valuable natural resource that must be protected, particularly in areas with marginal rainfall. Thus, as good a material as wood is, there are regions where other materials should be considered first simply on a conservation basis.

Wood for building is available from many different species with widely varying characteristics. Some species are used in the form of small poles for light construction while other species are allowed to mature so that timber (lumber in many countries) may be sawn from the large logs. The species that produce small inexpensive poles in rather short growing periods often grow in the fringes of agricultural land and can be used without danger to the ecology of the region.

The various species of wood have a number of physical characteristics that will be discussed in relation to the use of the wood in building construction.

Hardwoods vs Softwoods

Wood cut from deciduous trees (those which drop their leaves sometime during the year) is spoken of as hardwood, while that cut from coniferous (needle bearing) trees is spoken of as softwood. Unfortunately, there is no relation ship as to whether the wood is actually soft or hard in this classification. In this book, hardwood respectively, softwood, will be used to classify wood with hard characteristics.

Wood Characteristics

Strength in wood is its ability to resist breaking when it is used in beams and columns. Not only is strength related to the species, but also to moisture content and defects. Strength is also quite closely related to density.

Hardness is the resistance to denting and wear. Hard woods are more difficult to work but are required for tools, tool handles, flooring and other applications subject to wear, or where a high polish is desired.

Woods that are stiff resist deflection or bending when loaded. Stiff woods are not necessarily very strong. They may resist bending to a point and then break suddenly.

Tough woods will deflect considerably before breaking. Even after fracturing, the fibres tend to hang together and resist separation. Tough woods are resistant to shock loading.

Warping is the twisting, bending, bowing distortions shown by some woods. The method of sawing and curing affects the amount of warping, but some species are much more prone to warping than others.

Nail holding resistance for hard woods is greater than for softer woods. However, woods that are so hard that they are subject to splitting when nailed, lose much of their holding ability. Pre-boring to 75% of nail size avoids splitting.

The workability such as sawing, shaping and nailing is better for soft, low density woods than hard woods but they usually cannot be given a high polish.

Natural decay resistance is particularly important in the warm humid regions of East and Southeast Africa. There is a wide range of resistance shown by different species. However, for all species, heartwood (darker center area of the tree) is more resistant than the sapwood (lighter outer area of the tree). In addition to selection for natural decay resistance, wood preservatives should be considered where contact with the ground is likely.

Paint holding ability differs between woods, and as a rule this should be considered when selecting materials.

Defects in Wood

Defects to watch for in selecting timber are:

Brittle heart, found near the centre of many tropical trees, makes the wood break with a brittle fracture.

Wide growth rings indicate rapid growth resulting in thin-walled fibres with consequent loss of density and strength.

Fissures include checks, splits, shakes and resin pockets. Knots are the part of a branch which has become enclosed in a growing tree. Dead knots are often loose thereby reducing the effective area which can take tensile stress. Knots also often deflect the fibres reducing strength in tension.

Decay, which results from moisture levels between 21 and 25% in the presence of air, reduces the strength of the wood and spoils its appearance.

Insect damage caused by borers or termites.

The fungi which feed on wood can be divided into three main categories: staining fungi, moulds and decay fungi. All these fungi thrive under moist conditions. The staining fungi live mainly on the sapwood but they may penetrate deeply into the wood and spoil the timber's attractive appearance. The moulds do not penetrate below the surface and they do not seem to affect the strength of the wood, but they look unsightly. The decay fungi eat the cell walls of the wood. This causes the tree to lose its strength and often reduces it to a crumbling, rotting mass. These decaying fungi never attack timber which is seasoned to a moisture content of less than 20% and which is kept well ventilated and dry.

The main species of borers which attach tropical woods are the pinhole borer and the Lyctus or powder post beetle. The pinhole borer attacks newly felled logs and sometimes standing trees. The attack can occur within hours of felling. The beetles do not normally continue to operate in seasoned timber. The powder post beetle attacks seasoned tropical hardwoods - particularly those which contain starch on which the larvae feed. Timber is sometimes sprayed in the yard to protect it until it is transported.

Termites are normally of two kinds, the drywood types which are able to fly, and the subterranean type. Termites usually operate under cover and it is only after the first signs of damage appear that the full extent is realised. Flying termites usually enter the end grain of untreated timber and build up a colony from inside, finally devouring all the interior wood and leaving only a thin skin behind. Some subterranean termites, white ants, operate from a central colony and travel in search of food. Their nests or hills sometimes achieve great size and house millions of ants. No timber is completely immune to attack from ants or other insects, but there are great variations among the speicies.. The density of the timber is no guide to its resistance to termite damage, some of the lighter timbers being more immune than heavier varieties.

Weathering is the disintegration of wood caused by alternate shrinkage and swelling due to rain, rapid change of temperature, humidity, and the action of sunlight. Painting, properly carried out, does much to prevent weathering. The paint must be of external quality, however, and applied according to the maker's instructions.

Figure 3.1 Pole connectors.

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