8.1 MECHANIZATION IN FOREST ROAD CONSTRUCTION
8.2 MAIN MACHINERY USED IN FOREST ROAD CONSTRUCTION
8.3 CALCULATION OF MACHINE COSTS
The major mechanization for earth moving in forest road construction was characterized by the introduction of the crawler tractor. Over the years, in many countries of the world, due to environmental reasons and rising labour costs, more refined working and construction methods have been developed and a wide range of diversified road construction equipment is now available, allowing mechanization of all major activities in road construction.
Depending on local circumstances, such as workers' salaries, availability of manpower and skilled machine operators, equipment, fuel costs, soil and terrain conditions as well as the size of operation, different levels of mechanization are used in forest road construction, ranging from labour-intensive methods with only basic machinery, up to fully mechanized techniques employing a whole fleet of specialized road construction machines.
PHOTO NO. 21 Manual road construction, removing slope cut material
With the application of mechanized road construction techniques the planning and surveying techniques also had to be adapted to give adequate response to the high construction capacity of these machines. Generally, for forest road construction purposes, surveying by means of pocket compass, clinometer and measuring tape gives quick results which are accurate enough to act as a guide for the machine operator. The two principal methods in road surveying are the centre line and the grade line methods (the latter is also called the "zero line" method).
8.2.1 Tracked Tractors
8.2.2 TRAXCAVATORS
8.2.3 EXCAVATORS
8.2.4 Rock Drills
8.2.5 Graders
8.2.6 Loaders
8.2.7 Dump Trucks
8.2.8 Rollers/Compactors
Generally, crawler tractors in the 140 to 270 fw hp (105 labour to 225 labour)9 range are used. These are equipped with either an angle or a straight blade, and usually a winch in the rear to move trees and logs and to pull itself out in case it gets stuck. The latter is especially important in wet, soft soils and difficult terrain. The angled blade allows for greater flexibility in the shaping of the road. Some blades also have the ability to tilt in a vertical plane which helps with camber and drainage cuts.
9 fw hp = fly-wheel horse power; labour = kilowatt
In mountainous terrain, because of its versatility and for environmental reasons, the smaller tractors such as 125 and 140 fw hp (95 to 105 labour) are preferable. One should also bear in mind that construction sites in forest areas in mountainous terrain are often difficult to reach and therefore for transport reasons and economy of operations, a smaller tractor is called for.
Depending on the road standard (and width), soil and terrain conditions, the productivity of a crawler tractor for the above-mentioned activities varies considerably.
Generally in tropical forests, roads are wider than in temperate forests, and in mountainous areas road width is limited to 4.5 - 5 m due to the amount of material to move and erosion hazards. The productivity of a 140 fw hp (105 labour) tractor used on secondary forest roads for clearing and formation work is given below.
TABLE 8 Production Output in Machine hours/km of Road. for Clearing and Formation Work based on a Cat D6C tractor in Austria 10
|
Type of terrain |
Road class |
|
|
Secondary forest road (4.5 m) |
|
|
Work output in machine hours per km of road |
|
Easy |
70 |
|
Average |
100 |
|
Difficult (steep) |
130 |
10 Based on an average elevation of 1 000 m. It should be noted that at higher elevations the work output will be lower, similarly, the experience of the operator has a bearing on work output.
The main difference in construction techniques between a public road and a forest road is determined by the earth movement technique employed. Generally, in public roads, lengthwise earth movement by tractor or transport of soil (cuts and fills) is applied; whereas in forest roads the material is moved sideways (sidecast) over short distances by tractor only, and dispersed on the downhill slope. This operation has to be done with care as steep slopes can cause widespread distribution of debris. In forest road construction it is not normally economic to move soil for more than 50 m and for an average distance of not more than 30 m.
PHOTO NO. 22 Crawler tractor building a road
Based on time observations of a 180 fw hp (135 labour) crawler tractor used in forest road construction in developing countries, the time distribution for the different work elements was as shown in Table 9. The basic data are given above the table.
TABLE 9
Average Time Consumption of a Crawler Tractor in Forest Road Construction
Basic Data: Assumptions: 180 fw hp (135 labour) crawler tractor with angle blade; 7 machine hours per working day; one tractor operator; one helper; sand and clay soils; flat and undulating terrain.
|
Work element |
Operator's time in percentage of the total time consumption |
|
Clearing |
19 |
|
Earth moving |
31 |
|
Formation |
13 |
|
Miscellaneous work |
21 |
|
Service and maintenance |
9 |
|
Site preparation |
7 |
|
Total |
100 |
Traxcavators11 are very useful in mountain forest road construction, especially in rocky areas where the main aim is to place the soil and/or blasted rock material with a minimum risk of erosion, and for loading rock and gravel.
11 Also called front-end loaders.
Traxcavators can also be used very effectively in shaping mountainside cut slopes of forest roads when using a multi-purpose bucket.
Excavators 12 are machines with a mounted boom and a bucket which can be moved vertically. They are ideal in extremely steep terrain, and for construction of hairpin bends or turns, and shaping high-sided cut slopes, for the formation of the sub-grade, especially where blasted rock must be removed and loaded on to a truck or for digging side ditches and putting in culverts efficiently. These machines are replacing crawler tractors in some areas for the main excavation on road lines. Because the excavator can place the excavated material either on to lories for transportation away from the site or neatly and carefully in preselected positions, excavators are more environmentally acceptable. In very steep terrain the debris from road excavations can fall down hill and be both dangerous and damaging. The use of Excavators is emphasised in later Chapters.
12 Also called backhoes.
Since excavators are more costly than comparable-sized dozers and traxcavators, construction work for excavators must be more carefully planned and, in addition, requires very skilled and experienced operators.
PHOTO NO. 23 Excavator clearing a road. loading blasted material onto a truck
Depending on the scale of rock drilling and blasting operations, various types of machines are used in forest road construction. The smallest unit for rock drilling is a compressor mounted on a two-wheeled trailer, with hand operated hammer drills (nowadays mainly used for drilling and removal of single rock outcrops or big stones which cannot be removed by earth moving equipment). Hand held drills are only used for depths of drilling less than 3.5 metres. For minor rock drilling a 15 to 20 labour compressor with a supply of 2 to 3 m3 of compressed air per minute is sufficient.
For larger scale operations, however mounted rock drills are used. They can be mounted on wheeled or crawler tractors and on trailers or trucks. Depending on the type of road material, different rock drill systems are used. For hard rock the external hammer drill gives satisfactory results with a drill speed of 10 to 50 cm/minute requiring, however, a 150 person labour, producing compressed air of about 20 m3/minute. In soft rock the internal hammer drill is used, requiring much less air. When the latter technique is used a 50 to 60 person labour is required, producing compressed air of about 7 m3/minute.
For blasting rock with small drill holes gelignite is widely used, but for those areas where mounted drills have been used prepackaged slurry explosives are available. These explosives are safer to handle and to use. Depending on the scale of operation, different detonators are employed. For small outcrops of rock or stumps, detonation is achieved by fuse; whereas for rock blasting over larger areas, electrical ignition with milli-second detonation is used.
Use of explosives is a dangerous operation and should be restricted to experienced labour. It is necessary to ensure that all of those involved in explosive operations are aware of the safety procedures developed by the Supervisor and that these procedures are implemented.
The quantity of explosives and the depth and pattern of the drilled holes are appropriate for the shotfirer
Depending on the slope of the terrain, hardness and type of material to be drilled and blasted, machines used, and drilling method applied, rock blasting costs may be judged from information given in Chapter 10.
PHOTO NO. 24 Pneumatic Hammer Drill in operation
For gravelling and surfacing as well as constructing V-section drains and maintaining forest roads, self-propelled graders are used. In modern forest road construction, graders with an engine power of about 100 labour are used.
On roads where blasted rock is spread, small crawler tractors are preferred to spread the base material over the sub-grade; then the grader makes the final shaping with surfacing material.
Main roads in tropical countries on clay and lateritic soils will have to be gravelled in order to be able to keep them permanently open for logging operations, otherwise they would be open only during the dry season or dry periods.
In forest road construction, road material is generally taken from natural gravel pits nearby to reinforce low bearing material of the original ground surface and sub-grade. In the event that no suitable paving material can be found in gravel pits nearby the construction site, rock has to be crushed into coarse (5 to 6 cm) and fine material (1 to 2 cm) must be added as surfacing material to seal off the road and to allow for maintenance.
The structural design of the pavement is a function of the quality of the subgrade, the materials used in the pavement, and of loading intensity.
The subgrade or formation, when completed and compacted, can be assessed on the Californian Bearing Ratio scale (CBR). The CBR results are not reliable in wet cohesive soils, but the table below shows a correlation between CBR value and soil type with a wide variety of subgrades in their equilibrium moisture condition for high and low levels of water table. For most purposes this information will be sufficient to give the appropriate CBR values.
|
Type of Soil |
Depth of Water Table Below Formation Level |
|
|
More than 600 mm |
Less than 600 mm |
|
|
Heavy Clay |
2 to 3 |
1 to 2 |
|
Silty Clay |
5 |
3 |
|
Sandy Clay |
6 to 7 |
4 to 5 |
|
Silt |
2 |
1 |
|
Sand (poorly graded) |
20 |
10 |
|
Sand (well graded) |
40 |
15 |
|
Well-graded sandy gravel |
60 |
20 |
The following table indicates total pavement thicknesses (including surfacing) which have been found to be suitable for the type of loading we are considering:-
|
CBR (Per Cent) |
Pavement Thickness (mm) |
|
under 2% |
750 mm |
|
2% |
600 mm |
|
3% |
500 mm |
|
4% |
450 mm |
|
5% |
400 mm |
|
6% |
350 mm |
|
7% |
325 mm |
|
10% |
250 mm |
|
20% and above |
150 mm |
The pavement may be formed of a base course with a wearing course of 50 mm thickness of fine material or be constructed to its full depth in a single course.
For a 4 m wide road, an amount of 1 m3 of ballast per metre of forest road would be necessary.
The productivity of a motor grader spreading gravel on the road may range from 150 to 250 m per machine hour. For grading 1 km of forest road, around half to one machine operating day would be sufficient.
PHOTO NO. 25 Grader employed in shaping the road surface
For loading gravel from grave! pits onto dump trucks in large-scale forest road construction operations, high-powered wheeled front-end loaders are used; whereas for smaller works, agricultural tractors with a bucket are sufficient. IN quarries, tracked loaders of about 110 labour are preferably employed.
Loading costs vary considerably, depending on machine and labour costs and the
Size of the operation. Obviously larger quantities will be considerably less costly than smaller volumes.
For transporting gravel, either side or rear dump trucks are used, preferably heavy duty with three axles (25 t).
The gravel can be spread by dumping the load while driving over the sub-base. The final shaping can thus be done by crawler tractor, although it is preferable to use a grader.
Very often in forest road construction rollers are not used. When applying gravel on top of the natural ground surface to reinforce the low bearing capacity of the local material, compaction of the gravel should be carried out, one way or another. The cheapest way is for loaded logging and/or gravel trucks to drive over the newly shaped road surface, making a different path each trip. When for weather reasons immediate compaction is required, self-propelled vibratory rollers (95 labour), 9 t, are used successfully in forest road construction. Productivity may be 80 to 100 m per machine hour.
The degree of compaction that can be achieved by a given compactive effort acting on a given volume of soil is governed by the moisture content of the soil. If the soil is too dry the friction between the soil particles tends to resist closer packing of the particles. If the soil is too wet the spaces between soil particles are completely filled with water which prevents the particles fitting together. For a given compactive effort per unit volume there is thus an "optimum moisture content" between these 2 extremes at which maximum density is achieved. This figure is of the order of 8% for sandy gravels with fines, 10% for coarse gravels with fines, 13% for clean fine sand, 16% for sandy clay and 26% for heavy clays, which can be easily determined in the field by a simple test, the so-called "calcium-carbide" test13.
13 For further details see FAO Forestry Paper No. 14, "Mountain Forest Roads and Harvesting".
8.3.1 Basic Data Used in the Calculations
8.3.2. Cost Calculation15 Cost per hour in US$
An example of the methodology of calculating costs for operating a crawler tractor, based on recent information, is given below. Some assumptions have to be made which are derived from similar operations in forest road construction, and these are included.
Acquisition cost of 105 labour (140 fw hp) crawler tractor with Angle-Blade, winch and protective ROPS 14 delivered to the operation site, without customs duties:?
14 ROPS - Roll-Over Protective StructuresAssumed life of machine: 8 000 machine hours
Operating hours per year: 1 500 machine hours
Interest, insurance and taxes: 15 percent
Average fuel consumption per machine hour: 23 litres
Cost of diesel fuel per litre: US$
Cost of lubricant per litre: US$
Assumed hourly consumption of lubricants: 0.45 litres
Assumed hourly consumption of grease: 0.02 litres
Average daily machine hours: 7 hours
Operator's salary: US$ per month
Helper's wages: US$ per day
Working time per day: 7 hour shift, plus 1 hour for lunch
15 In calculating hourly machine costs the method presented in the FAO publication "Logging and Log Transport in Tropical High Forest" has basically been used.
|
Deprecation acquisition cost x 0.90 |
| ||
|
|
110 x 000x0.90 |
- | |
|
estimated life in operating hours |
8000 | ||
|
Interest, acquisition cost x multiplier factor16 - |
| ||
|
insurance and taxes |
1 000 |
| |
|
- 110 000 x 0.06 = |
| ||
16 For 15% and 1 500 effective working hours/year the multiplier factor for calculating hourly costs of interest, insurance and taxes equals 0.06.
|
1 000 |
|
|
|
Fuel: 23 x Fuel cost = |
$ |
|
|
Oil and Grease: 0.45 x Lubricant Cost= |
$ |
|
|
Repairs and maintenance = |
$ |
|
|
Total hourly machine costs = |
$ |
sum of the above |
|
Operator's hourly costs 17 = |
$ |
|
|
Helper's hourly costs18 = |
$ |
|
|
Total costs: |
$ |
|
17,18 Operators and helpers hourly costs do not include fringe benefits such as insurance, housing, subsidies for basic food.
With an average machine operating time of 7 machine hours/day, machine operating costs would thus amount to the hourly costs multiplied by 7.
When assuming 100 hours are required for 1 km formation work of 4.5 m wide secondary forest road, formation costs would thus amount to hourly cost by 100.
x 0.10=