Chapter 3 Forest road engineering

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What it is
Guiding principles
Potential consequences of improper road design, construction and maintenance
Recommended practices

What it is

Road engineering involves the specification of design standards and the actual engineering design, field layout, construction and maintenance of forest roads and subsidiary structures such as bridges and culverts.

Guiding principles

Forest roads are complex engineering structures upon which transport efficiency and reliable access to the forest both depend. They are unquestionably the most problematic features of timber harvesting operations since a major part of the total soil erosion resulting from timber harvesting operations can be attributed directly to roads, often because of design or construction flaws or poor maintenance practices. Nevertheless, except in cases where major waterways can be used, roads are essential, not only for industrial timber extraction, but also because they provide access to forests for management and monitoring purposes. In some cases forest roads may even form part of the planned network of public roads and thus can be an essential component of a country's development infrastructure.

Forest roads should be designed and laid out in the field by competent engineers who understand the need to minimize soil disturbance, maintain proper drainage and avoid stream crossings where possible. Construction and maintenance of forest roads is specialized work that should be supervised by engineers and carried out by specially trained work crews.


Forest roads that have been properly designed, constructed according to environmentally sound engineering practices and correctly maintained should:

- provide convenient, low-cost access to the forest for product transport and to serve the needs of forest management and protection while benefiting local communities at the same time;
- minimize soil erosion associated with roads, thereby reducing sedimentation in streams;
- be consistent with good design and maintenance practices, minimizing road and landing areas while providing adequate roadside clearings to permit the roads to dry out quickly after heavy rains;
- utilize natural drainage patterns;
-avoid passing through areas of cultural or religious significance or natural beauty or areas where wildlife or local communities or indigenous peoples may be disturbed by traffic;
- provide for the safety of workers and the general public who may use the roads or be affected by the traffic.

Potential consequences of improper road design, construction and maintenance

Among the potential consequences of improper road design, construction and maintenance are:
-high construction, maintenance and transport costs;
- short road life, with the consequence that new roads will have to be built for subsequent harvesting operations in the same area;
- excessive stream sedimentation, with potentially serious effects on water supplies, aquatic life and wildlife populations;
-excessive soil erosion, with a consequent loss of productivity in forest areas near the roads;
- increased potential for triggering landslides on steep mountain slopes with consequent damage to infrastructure, streams and land;
-disruption of breeding areas or migratory routes of animal species.

Using a tracked excavator to build a forest road in mountainous terrain. Such machines, properly used, can substantially reduce environmental impacts from road construction as compared with the use of bulldozers.

Recommended practices

Building forest roads involves clearing vegetation and moving soil and rock in order to create structures capable of supporting heavy vehicles that may have to operate during periods of adverse environmental conditions. Such actions are almost always accompanied by increased rates of erosion. To minimize the increase in erosion and reduce its destructive effects, the following practices are suggested:

- Utilization of competent engineers to conduct roadline surveys and supervise construction.

- Minimization of the total length of roads. This not only reduces the erosion potential of roads, but it also minimizes deforestation and enhances the profitability of forests by reducing the cost of road construction and maintenance.

- The total length of roads required for ground-skidding operations in a particular forest area is typically two or three times as much as that required for harvesting systems using either forwarders or skyline cable-cranes operating under similar conditions. Aerial systems, such as helicopters, can support even lower road densities. Both cable and aerial logging systems are generally more expensive than ground-skidding systems, however, except in unusually steep or rugged terrain. A trade-off therefore exists between the increased cost of road construction where road density is higher, and the increased cost of extraction where road density is lower.

- Reduction of the total area of disturbance associated with roads. Guidelines developed for tropical rain forests in northern Australia specify that, in gentle terrain where little excavation is needed, the maximum clearing width should be less than 7.5 m for major haul roads and less than 5 m for minor haul roads. Additional clearing is needed in steeper terrain to open cut and fill slopes and in areas of especially high rainfall to allow sunlight to penetrate so that it can dry out the roads after rainstorms. Although practices will vary in different regions, the general principle is that the average width of forest roads should be restricted to the minimum that will permit a properly constructed and maintained road on which hauling can be done efficiently and safely. This will reduce both soil erosion and the area of forest that must be dedicated to infrastructure.

-Avoidance, where possible, of areas of wet soils and high erosion risk. Routing roads through such areas is costly, both in terms of initial construction and the extra maintenance that will be required over the life of the road.

- Minimization of cutting and filling. On steep, erosive slopes, hydraulic excavators should be used to avoid the need to side-cast excavated material. Appropriate blasting techniques are to be used when needed and excavated material deposited in stable disposal areas that are well away from streams.

- Adequate compaction of the road base and allowing the road to dry thoroughly before use. When feasible, roads should be constructed only during the dry season and they should be allowed to go through an entire rainy season before permitting their use by heavy machinery. Suitable local materials should be used to prepare a firm foundation and, when possible, a sealing layer of gravel or similar material applied on the running surface of primary roads.

Road crew excavating a site for emplacement of a culvert on a forest road. Providing for adequate drainage is one of the most important considerations in the engineering of forest roads.


Effective density of forest roads

Regardless of the type of harvesting system, pre-planning of the road system as described in Chapter 2 will almost always result in a substantially lower road density than if harvesting is done without the benefit of a comprehensive harvest plan. The appropriate road density for a particular area will depend upon the type of forest, the cost of road construction and maintenance, the cost of extraction, and other factors. In general, because of the relatively low volume of harvestable timber per hectare, the optimum road density in tropical forests is much lower than it would be in temperate forests. Under European conditions, commonly reported forest: road densities for ground skidding are about 25 m of road per hectare of forest. Expressed in terms of the volume of timber extracted, this corresponds to an effective density of roughly 100 m of road per 1000 m of extracted roundwood. In a mixed broad-leaved tropical forest under conditions typical of West Africa, where harvest volumes average about 10 m per hectare, an effective density of 100 m of road per 1000 m of extracted roundwood would imply only 1 m of road per hectare of forest. This is many times lower than actual road densities commonly encountered in tropical forests, suggesting that effective road densities may be higher in some tropical forests than in temperate forests in industrialized countries.

- Use of roadside ditches and properly spaced cross-drains to channel water away from the road structure and into the surrounding vegetation. These are relatively simple and inexpensive practices that are often neglected on forest roads. They are absolutely essential if soil erosion associated with roads is to be minimized, especially in areas with intensive rainfall. Furthermore, such practices can pay for themselves many times over through reduced road reconstruction costs as well as reductions in the delays associated with muddy or washed-out roads.

- Ensuring that roadside slopes become revegetated as soon as possible after construction. Low, shrubby vegetation or grass is preferable to fast-growing trees for this purpose since the latter will shade the road surface and prevent it from drying out after rainy periods. In many tropical regions, the fast-growing pioneer tree species that rapidly grow up after clearing may have to be controlled along roadsides in order to permit the roads to dry out properly.

- Keeping roads and disturbed areas away from streams and outside streamside buffer strips. Where a stream crossing is necessary, the crossing structure should be designed on the basis of a detailed site survey and disturbance to the stream minimized during construction.

- When culverts or similar drainage structures are used for cross-drains or stream crossings, determination of their proper size and spacing based on local rainfall intensity and runoff rates.

- Keeping the road grade as low as possible, consistent with the need to provide adequate access to the forest. Maximum grades of 10 to 20 percent (6 to 11) are recommended in some countries, except for short pitches, where steeper grades are necessary to provide access to higher ground. In mountainous areas, relatively steep roads may be preferable to flatter grades, which require more total construction. However, maintenance costs and erosion problems tend to rise sharply with steeper grades.

- Utilization of stable ridge-top locations for roads, wherever feasible, except when long-distance cable-cranes will be used in combination with valley-bottom roads.

- Proper maintenance of road surfaces, roadside ditches, cross-drains and stream crossings. Secondary roads can be closed off if they will not be used again until the next felling cycle; in this instance, the road surface should be ripped up if necessary and revegetated with grass or shrubs. All roads that are considered essential for the management or protection of the forest should be maintained for continuous use.

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