All phases of the road construction process are described in detail for both construction techniques as found at the construction sites while carrying out the work and time studies and distinct construction activities are compared in Chapters 5.1–5.6. The road template terminology which is used throughout the report is given in Figure 4.
Figure 4. Road template (FAO, 1998)
Prior to clearing of the construction area, the design information had to be moved from the plan to the ground. In both road projects under review the grade line method, often referred to as “zero-line method”, was employed for fitting the road to the natural terrain as closely as possible. The grade line itself represents the intersection between subgrade of the road and the slope. This method can be described as a step-by-step procedure for fixing the location line with a given longitudinal gradient directly on the terrain by means of a hand-held clinometer.
Such simple methods must not be confused with inadequate planning as skill and experience are required to find the most suitable road alignment, both from the environmental and economic point of view (Sedlak, 1985b).
To avoid losing the road alignment in the field if stakes become lost, which often is the case if much time elapses between staking and construction, the grade line is projected onto suitable trees outside the downhill clearing limit. These trees have been marked at road centreline intervals of 30 to 40 m with red oil paint. This way the grade line can be re-located accurately with the help of a clinometer even if all stakes are lost (Roetzer, 1995).
Additionally, the marked “grade line-trees” form a valuable aid to the machine operator to follow during construction (Roetzer, 1995). These marked trees are useful for road construction in remote areas of Bhutan where close supervision of the ongoing construction process cannot be ensured.
Where operators are familiar with road construction in mountainous terrain and the forest stand condition is favourable, marked grade line-trees outside the clearing limit are sometimes used as the sole guideline for mechanized road construction. This manner of road location in the field is referred to as “zero-level method” and helps increase the productivity of fixing the location line in the terrain without assistance.
After the grade line had been marked for the total length of the road project, clearing of the construction area by felling the trees within the clearing limits (see Figure 4) was carried out for that part of the road assumed to be constructed before the start of the rainy season. In general, road construction activities are cancelled from June to September during the monsoon rainfalls. Winter conditions also force suspension of activities in most parts of Bhutan.
Although a modified full-length method is normally recommended when timber will be extracted by excavator (FAO, 1998), trees had been cut into length at the felling sites in both road projects under review. This was due to the heavy weight of broad-leaved trees in natural forests in the project areas. Clear grading instructions and the availability of measuring tapes for the felling crews would have increased the wood recovery rate considerably.
The major step towards environmentally friendly road construction practice was the introduction of hydraulic excavators in forest road construction. In Austria, for example, the use of excavators not only replaced bulldozers in forest road construction in hilly terrain but also improved the quality of roads while reducing environmental impacts of these complex engineering structures. In steep terrain their use is the only option to make road construction even feasible (FAO, 1998).
Road construction technique by hydraulic excavator comprises the following five distinct phases stated below:
Some operators will follow the order stated above and arrange single activities in sequence. Others who change and mix tasks are at risk of burying organic materials in the fill. They also risk failing to separate excavated local materials taking account of their anticipated use in building up the fill in layers. Even if a well-trained and experienced professional operator follows his personal optimum sequence of construction activities, the first phase in road construction will always be the log removal from the construction area.
Photo 10. Log removal from construction area by moving the excavator backwards and either by balancing logs put on the bucket or by attaching logs by means of chain to the excavator's bucket
Close supervision should ensure the compliance with the project plan and that best construction practices are followed. In particular, the road gradient should frequently be checked to ensure that the grade line is followed since stakes marking the grade line will be obliterated during topsoil removal. Lack of supervision may easily lead to reckless construction work destroying the beneficial effect of even the most carefully selected road location by the forest engineer as seen in the Korila road project. There, the bulldozer operator left the marked grade line and followed the most comfortable route with regard to construction conditions in order to increase his work performance.
The five distinct phases of road construction by excavator provide a step-by-step comparison of the two distinct construction techniques as applied in the Kharungla road and Korila extension. It is noteworthy that some disadvantages of road construction by bulldozer cannot be considered inherent features of this technique but of poor construction practice.
Figure 5a. Road construction technique by excavator (balanced road construction)
Figure 5b. Road construction technique by bulldozer (full bench road construction)
Comparison of five distinct phases of road construction by excavator and by bulldozer
| Excavator | Bulldozer | |
| i) Log removal from construction area | ||
| Logs were pulled or carried out of the construction area to roadside storage by moving the excavator backwards either just by balancing logs put on the bucket or by attaching logs by means of chain to the excavator's bucket. | Logs were pushed aside either manually before road construction commenced or by means of the bulldozer's blade during operations. The removed logs were often buried by excavation material and therefore not available for utilization. | |
| ii) Topsoil removal from construction area | ||
| Wood residues and organic topsoil were removed from the construction area and spread out on the fill slope established during previous work cycles. Stumps, tree tops and other vegetative debris were placed by the operator along the base of the fill slope in order to form a filter windrow. The additional width between the construction limit and forest edge ensures that debris is deposited outside the construction area to prevent organic material from mixing into the base of the fill (Figure 5a). | Wood residues and organic topsoil were pushed aside by the bulldozer. In case of balanced road sections this means that organic material is often mixed into the fill resulting in uneven settlement and increasing the potential for fill failures as organic materials decompose. In full benched road sections the materials are side cast and wasted downhill. | |
| iii) Excavating base for fill foundation | ||
| At the toe of the anticipated fill slope a base of about 1 m width and up to 1.5 m height for fill foundation (FAO, 1998) will normally be excavated (Figure 5a). Due to soil conditions characterized by a lack of cohesive materials and boulders required to build up a solid foundation, a preliminary road was necessary to provide a solid base for the fill. | At the Kharungla site a preliminary road had been constructed. However, this preliminary road cannot be considered a proper fill foundation due to placement of unsuitable fill material and unsatisfactory compaction. The organic material often mixed into the road structure is likely to decompose and will result in uneven settlement and increase the potential for failure. | |
| iv) Fill slope construction | ||
| The main feature of road construction by excavator is balanced road sections where cut excavation is incorporated into the fill which needs to be built up in well compacted layers to develop strength since it must support traffic. The suitable materials were spread by the excavator's bucket on the base to form a 30 to 50 cm layer which is to be compacted by several excavator passes before being covered by the next layer of less coarse material. | The subgrade which can be achieved by the bulldozer seems to be quite favourable at first glance but in road sections where the road is not established by full bench construction, major settlements in the subgrade can be expected as a consequence of lack of compaction and decomposition of organic material mixed into the road structure during construction operations. | |
| v) Subgrade and cut shaping | ||
| One of the last excavator activities is to fill smaller holes in the uppermost fill layer while the layer gets continuously compacted through excavator passes. Finally smoothened and compacted by the excavator's bucket, a firm subgrade will be achieved. A sealing layer of gravel or similar material could not be applied to the coarse subgrade as suitable excavation was simply not available at the construction site. However, final shaping of the cut was the last activity carried out by the excavator. The hillside ditch, normally built by excavator, was constructed after completion of the excavator's work by two labourers equipped with shovel and hoe. | Final smoothening of the subgrade is carried out by means of the bulldozer's blade. In this way smaller holes are filled but, unlike excavator construction, not by placement of material suitable in size and free of organic debris. Cut shaping by means of bulldozer can be described as unsatisfactory as the resulting cut slopes are too steep and most unfavourable for revegetation without further measures. The potential for erosion is highly increased. The hillside ditch had to be constructed manually after surfacing in the traditional way (for details see Chapter 5.5). |
Some observations made when carrying out the time and work studies at the Korila extension site which are considered features of poor construction practice are stated below. They are neither inherent features of bulldozer construction technique nor covered by the above description of the construction process.
Photo 11. Organic materials are mixed into the fill if not clearly separated in topsoil removal by bulldozer
Photo 12. Due to soil conditions characterized by a lack of cohesive materials and boulders required to build up a solid foundation, a preliminary road had become necessary at the Kharungla construction site to provide a solid base for the fill - one can see the ongoing fill construction
Photo 13. Wood residues and organic topsoil are often mixed into the fill in bulldozer construction resulting in uneven settlement and increasing the potential for fill failures as organic material decomposes
Photo 14. One of the last exercises by excavator is to fill up smaller holes in the uppermost fill layer while the layer gets continuously compacted through excavator passes. Finally smoothened and compacted by the excavator's bucket, a firm subgrade will be achieved
Photo 15. Final smoothening of the subgrade is carried out by means of the bulldozer's blade. In this way holes are often filled by material unsuitable in size and mixed with organic debris
Photo 16. Trees were often pushed down by the bulldozer when circumventing obstacles which increased the clearing width considerably in certain road sections
| Road features | Observations | Reason for poor practice |
| • Road location | Marked grade line has not been followed | The most comfortable route favouring high work performance has been chosen by the bulldozer operator |
| • Road gradient | Gradients too shallow (0–2%) | Most comfortable way has been chosen in order to avoid the need for longitudinal mass transport |
| • Subgrade width | 3.9–4.8 m instead of 5.0 m as specified in the project plan | Removal of stumps and boulders was avoided by reducing road width |
| • Clearing width | Increased clearing width | Trees were pushed by the bulldozer when circumventing obstacles |
The above-stated observations seriously jeopardize a comparison of work performances found by the time studies for both construction techniques. Even for parts of the road sections under review where side slopes where quite similar, 60–65 percent for bulldozer and 65–70 percent for excavator, the bulldozer production rate which was about twice as high as in excavator construction (for details see Chapter 8.1) cannot be considered a general finding since the required road standards were not met in the Korila extension where the bulldozer was employed.
An advantage of road construction by hydraulic excavator is that balanced road construction with excavated material incorporated into the road structure can be carried out on steep slopes whereas in road construction by bulldozers full bench construction techniques would have to be applied.
Operating distances in longitudinal mass transport by excavator with distances of up to 70 m were found at study sites in Austria (FAO, 1998). This distance was considered acceptable on road projects where little mass movement is needed and therefore the costly use of a dump truck can be avoided.
In contrast, although end hauling of excess material is unquestioned practice in environmentally friendly road construction, side casting excess material even in very steep terrain was found to be common practice in full bench construction by bulldozer on the Korila road project.
Photo 17. Side casting excess material even in very steep terrain was found to be a common practice in full bench construction by bulldozer on the Korila extension project
Mass transport by dump trucks did not occur during the studies and was not considered necessary in the road sections under review.
Forest roads are essential for providing convenient access to the forest for applying sustainable forest management practices and for monitoring purposes while often benefiting local communities at the same time.
Some measures of environmentally friendly forest engineering practice to provide satisfactory water drainage and consequently to prevent erosion which are considered both suitable as well as achievable for road projects in remote areas of Bhutan, are compared to what was found in the Kharungla road and Korila extension projects.
The following table lists some construction items by which satisfaction of environmental objectives are measured. Satisfactory results are achievable even in remote areas of Bhutan.
| Measures for satisfactory water drainage | Kharungla road | Korila extension | |
| • | Road fitted as closely as possible to the terrain | Achieved | Deviations from marked grade line |
| • | Road width is to be restricted to the absolute minimum for safety and anticipated use | Achieved | Road width less than specified in project plan |
| • | Soil disturbance and surfaces exposed to erosion are to be minimized by balance of cuts and fills | Achieved | High disturbance by extensive use of full bench construction |
| • | Road gradients should be varied to reduce concentrated flow on road surfaces and in drainage facilities | Achieved | Gradients were too shallow (0–2%) for proper road drainage |
| • | Gravel should be applied on the running surface to provide a more weather resistant sealing surface | Planned | Traditional way of surfacing will be applied (Chapter 5.5) |
| • | Ditch gradients should be adjusted to specific soil conditions at the construction site to keep collected waters moving to culverts and to prevent sediment deposition and ditch erosion | Manually constructed hillside ditches were found unsatisfactory in size, shape and gradient | Manually constructed hillside ditches were found unsatisfactory in size, shape and gradient |
| • | Road drainage features are to be designed and spaced so that peak drainage flow from surfaces will not exceed the capacity of the individual drainage facilities | Will be achieved if recommendations in project plan are followed | Care must be taken as road location in the field has been altered |
| • | Prefabricated steel culverts pipes are to be installed in preference to articulated concrete pipes as the latter are at risk of collapse under the load of heavy hauling during periods of weakened road strength and consequently to obstruct or to block water drainage | Prefabricated steel culverts are simply not available for the time being | Prefabricated steel culverts are simply not available for the time being |
| • | Culverts are to be protected from plugging by using sediment catch basins and debris racks where needed and water is to be prevented from eroding and undercutting the culvert by rock armoured inlets | Will be achieved if recommendations in project plan are followed (Photo 18) | Will be achieved if recommendations in project plan are followed |
| • | Outlets of culverts are to be armoured with rock boulders to prevent emerging water from eroding the fill slope where water will not be released onto a stable area | Will not be achieved as pipes used are too short and therefore discharge onto the fill | Will not be achieved as pipes used are too short and therefore discharge onto the fill |
Photo 18. Culverts are to be protected from plugging by using sediment catch basins and/or inlet protection - the latter manually constructed at the Kharungla construction site
Photo 19. Individual drainage facilities are to be inspected periodically and in particular after heavy thunderstorms, after logging and/or hauling activities for need of maintenance - in particular culverts in order to avoid failure due to plugging by debris
A significant advantage in road construction by excavator over bulldozer construction is the fact that drainage facilities and erosion control features will either be provided by excavator in each single work cycle of the ongoing road construction process or can easily be established by means of excavator at any time required without need for additional machinery. This advantage was partly offset since the hillside ditch was constructed manually in the Kharungla road project after completion of the excavator construction work.
As recommended in Almas et al. (1993), ditch gradients should range from 2 to 8 percent, just steep enough to keep collected waters moving without carrying excessive sediments. Gradients steeper than 8 percent might easily result in too much momentum of collected water and the carrying of sediment and debris for long distances, whereas gradients which are too shallow lead to silting of ditches.
The individual drainage facilities are to be inspected periodically, in particular, after heavy thunderstorms, after logging, and after hauling activities in order to start immediately needed maintenance and repair work. Attention should be paid particularly to culverts to avoid failure due to plugging by debris and sediment.
Once the road subgrade has been constructed and culverts have been established, the road is normally allowed to go through an entire rainy season before surfacing takes place during the subsequent construction season.
In the Kharungla road project it is planned that settlements in the road subgrade will be filled with coarse gravel. Well-graded gravel will be applied to the running surface by dump trucks hauling from local quarries. Compacting, final shaping and smoothing will be performed by the excavator rather than by grader and roller as these machines are now not available for use in forest road construction in Bhutan.
Photo 20. Reasonable result - compacting, final shaping and smoothing will be performed by the excavator rather than by grader and roller as these machines are now not available for use in forest road construction in Bhutan
Photo 21. Use of stone crushers to provide suitable gravel material should not be restricted to public road construction
In contrast, for the Korila extension the traditional way of surfacing with rocks placed in upright position (see Photo 22) will be applied. Suitable rock outcrops along the road corridor were quarried with hand tools and suitably shaped and sized for their anticipated use in surfacing. Manually transported from the forest stand, the rocks were piled alongside the road corridor. After the road was constructed, the rocks were manually loaded onto dump trucks and delivered to road sections where needed for surfacing.
Photo 22. Rock outcrops along the road corridor were split by means of hand tools into rocks suitable in shape and size for their anticipated use in surfacing
In the Kharungla road project the only slope protection and stabilization measures observed were those performed in each work cycle as they are inherent features of proper road construction technique by excavator. These measures were the following at the construction site under review:
finally shaped, smoothed and compacted cut and fill slopes produced in each work cycle ensured that loose material was removed from slope surfaces that otherwise would have been exposed to erosion (Photo 23).
a fill slope cover, which provided immediate erosion control, was continuously provided during the phase of topsoil removal from the construction area in each work cycle of the road construction process (Photo 24);
Photo 23. Compacting the fill after final shaping and smoothening is an important measure to ensure that loose material is removed from slope surfaces that otherwise would have been exposed to erosion
Photo 24. A fill slope cover to control erosion is continuously created during the phase of topsoil removal from the construction area in each work cycle of road construction by excavator
In contrast, bulldozer construction as applied in the Korila extension does not even provide this minimum standard of slope protection and stabilization. Loose, unconsolidated side cast material and unprotected surfaces exposed to erosion were features of road construction by bulldozer at the construction site along with steep cuts most unfavourable for natural or man-made revegetation. The only measure undertaken in the Korila project was the manual removal of overhanging floor vegetation and tree stumps at the top edge of the cut as already mentioned in Chapter 5.2.
Out of the wide spectrum of bioengineering measures, which range from simple seeding procedures by hand, mulching, etc., to combined use of retaining structures and living plants, no measures have been undertaken to stabilize slopes and to prevent erosion from exposed surfaces. This is true at both the Kharungla road project and the Korila extension project.
Employing bioengineering methods would not only prevent erosion and ensure a better blending into the landscape but would also provide an additional benefit to the draining effect of structures incorporating living plants (Litzka and Haslehner, 1995).
