2.1 ROAD-NET PLANNING
2.2 AERIAL PHOTOGRAPHS AND MAPS
2.3 ROAD TYPES
2.4 ROAD STANDARDS
2.5 ROAD-NET DENSITY AND SPACING
2.6 INTEGRATED PLANNING OF ROAD NETS AND CABLE SYSTEMS
2.7 ROAD LOCATIONS IN HILL AND MOUNTAIN SIDE AREAS
The integrated planning of the opening up of mountain forest resources is essential in order to ensure a steady industrial roundwood supply on the one hand and to pursue forestry work in such a manner as to guarantee the preservation of the forests and the environment on the other. This is especially important with the introduction of modern machinery for both road construction and forest harvesting, and the ever increasing demand for wood.
Very often, in addition to these basic objectives, land tenure and local people's rights and requirements must be taken into consideration in such matters as the procurement of fuelwood and fodder or interests of a village, district, county or the country in such matters as water supply, scenery, wildlife, etc. For all needs, be they of a productive or protective nature, a well planned forest road-net is the basic ingredient needed to carry out the required work and to maintain the forests as a renewable resource.
Generally, the layout of a road-net depends on the timber resources, terrain conditions, type of forest operations (afforestation, silvicultural treatment, fire protection, cutting system, logging and transport methods), technical equipment and machinery, labour techniques and costs, as well as other resource benefits to be considered. Careful attention has to be paid, when planning and locating roads in steep terrain, to avoiding and minimising the erosional impact of roads on the environment.
Forest roads should, as much as possible, be planned with the view of conducting wood harvesting operations (present and future). A road-net where manual extraction is to be used, has to be laid out differently from one where cable cranes and/or tractors are operated. Figure to determine the efficiency of a road net for a certain area are derived when relating the road length to total roundwood removals per year (m/m3/year) or length of road net per hectare of forest area (m/ha).
When planning and surveying a forest road, the use of aerial photographs facilitates the location of forest roads as one may get some ideas about possible routes in the office by viewing the pictures through a stereoscope and unacceptable variants can be disregarded. In addition Global Information Systems are now more widely available. Therefore, the amount of labour and time consuming road survey work on the ground can be reduced considerably. However, field reconnaissance and field checks are required in any case to ensure that something has not been missed, and to arrive at an optimum road location.
For forest road planning, good topographical maps are essential, preferably with scale not larger than 1:10 000. If good contour maps are not available, then extensive field work is required to measure all main control points barometrically and to record detailed terrain features.
Figure 1 shows four different types of terrain, from easy to difficult, which can be recognised from the spacing and curvature of contour lines.
FIGURE 1
Easy to medium terrain
Medium terrain
Difficult terrain
Very difficult terrain
Example of different types of topography in Bhutan (Southern and Western, Central Region)
Scale 1: 50 000
Contour Interval = 40 m
Within a forest road-net one can classify roads:
(a) According to their position:
- main valley roads
- secondary valley roads
- slope roads
- feeder roads
- mountain ridge roads
(b) According to the construction:
- earth roads
- gravelled roads (mechanically stabilized roads)
- chemically stabilized roads
- roads with bituminous or oiled surface
(c) According to the intended use:
- truck roads
- tractor roads
- purely opening-up forest roads
- access roads
- multiple use forest roads
(d) According to importance:
- main roads (permanent roads and all-weather roads)
- secondary roads, feeder roads (seasonal roads)
2.4.1 Access Roads
2.4.2 Main Forest Roads
2.4.3 Secondary Forest Roads (Feeder Roads)
2.4.4 Skid Roads (Skid Trails)
The standard of a road depends largely on its proposed end use, on the amount of harvestable and marketable wood per unit area as well as on terrain conditions. The following road classification is suggested for steep terrain:
- Access roads
- Main forest roads
- Secondary forest roads, feeder roads
- Skid roads
- Skid trails
The main purpose of access roads, as their name suggests, is to provide access to the forests for the transport of people from villages to the forests and for the transport of roundwood from the forests to the wood processing sites or terminals. Very often these roads are the links between the public roads and the main forest roads.
The basic road-net consists of main forest roads, usually where wood transport is possible during the whole year. The location of such roads must therefore be decided upon from the point of view of their use. Very often they are needed for long term intensive management, where they are the key to all forestry operations. Therefore, adequate engineering structures and drainage facilities are required. If the in situ road base material does not have enough bearing capacity to support heavy traffic throughout the year, it would need to be reinforced or entirely gravelled with adequately graded road material.
Secondary forest roads are the connective lines from the landings to the main roads. They are normally only used temporarily and therefore gravelling is generally not done. If the soil material is soft, a single truck can completely destroy the road surface by making ruts in the road which will become a source of heavy erosion, often leading to the total destruction of the road. Therefore, it is better to close down these parts of the road net during adverse weather conditions. Through careful planning, logging could be scheduled in bad weather for those roads which are surfaced and will not be damaged.
PHOTO NO. 1 Secondary Forest Road - truck loaded at log landing
These are used entirely by wheeled skidders and/or crawler tractors for wood extraction only. The width of the road (trail) generally does not exceed 3.5 m. On skid roads with steep gradients water-bars1 should be built into the road to prevent gully erosion caused by concentrated surface water flow before the rainy season or at least after logging operations have ceased.
1 Ridges of packed earth formed diagonally across the surface of the skidding road.
Table 1 gives suggestions for a possible road classification in mountainous terrain
TABLE 1
Road Classification
|
Road Type |
Road Use |
Road Formation Width |
Carriage Way Width |
Maximum Gradient in |
|
|
Transport Direction |
Adverse Direction |
||||
|
Main Forest Road |
Truck, Pickup permanent |
5.0 |
4.5 |
9 |
6 |
|
Secondary Forest Road |
Truck, Pickup temporary |
4.5 |
3.5 |
10(12)* |
8 |
|
Skidding Road |
Wheeled skidder, Wheeled tractor, Crawler Tractor |
3.5 |
- |
12(20)* |
10 |
() * maximum gradient for short distances only
In hilly and steep terrain the road spacing and location is predominantly determined by the terrain and the wood harvesting system feasible under the prevailing conditions. Road construction and maintenance costs are generally higher in difficult and steep terrain than in flat and hilly areas. Formulae are often used for the estimation of optimal road spacing.
The basic formula visualizes a road net with parallel, equally spaced and infinite roads which of course deviates very much from the actual road patterns, especially in mountains. Various authors have introduced modifications of the basic model in order to adapt it to reality.
The need for roads, the standard of roads and the quantity of roading will vary for each site. What will not vary is the need to ensure that only the minimum financial investment is made to fulfil all of the criteria the manager has identified in formulating his roading policy. Roads are not cheap and due consideration must be given to the requirements for the user as well as the geometric and structural design to minimize the overall cost.
Roads are installed to improve the efficiency of the running of the site and to give access to undeveloped areas. Everything can be done without roads if there is sufficient labour and horses or cross-country vehicles and machinery. But labour can be expensive and long uncomfortable travelling, either on foot or on a tractor is non-productive and a poor use of skilled workers' or managers' time. There are also cases where other factors, such as opening up new land or providing avalanche protection where transport to and from the areas affected is a requirement.
It is therefore necessary to make a judgement on the benefits which can be obtained from the expense of constructing roads, which can be high in comparison to other activities, against the expense of the alternative methods of fulfilling the objectives of the site. In some cases this will be fairly straightforward, in others some guesswork may be involved, but the exercise should always be carried out to help in reaching final conclusions.
Planning for Roads
The object of a planning system is to minimize total transport costs. These include:-
a. cost of road, both capital and maintenance;b. cost of carrying goods and materials from the point of growth or collection to the nearest roads;
c. cost of vehicle on the road;
d. cost of people's time.
The quantity of roading, generally, does not affect the number of times of handling the produce or transferring produce to the vehicle. Handling or transferring need not be taken into account in any appraisal, other than to provide the manager with an indication of the total cost of the operation.
The influence of terrain, landscape, material sources and social requirements may provide a number of alternative routes for new roads. It is necessary to examine these routes and to cost the variables listed above for each road network and the most economic will, in normal circumstances be the selected solution.
The Quantity of Roading with Forestry as the Prototype
Timber harvesting has been used for the example given of the need to give consideration to the total cost of the operation, but the formulae and type of exercise work equally well for other products.
Assuming parallel access roads across the site, the following can be derived:
h Cost of movement from stump to roadside ($/m3/100 m travelled).S Distance between roads (metres).
R Capitalised road cost, including allowance for maintenance in future years, ($/km).
V Total volume of product to be extracted during the life of the road, discounted to year of road construction (m3/ha).
Average straight line extraction distance = 
The area served by a road 1 000 m long is S x 1,000, when extraction takes place on both sides of the road. (If only one side of the road is usable the area is reduced to 
x 1,000.)
Road costs per hectare is:
If the discounted volume per hectare to be carried by the road is V m3 per hectare:-
Road costs per m3 = 
The best solution occurs when the total cost per m3 is at a minimum ie when
is at a minimum
This occurs when extraction costs and road costs are approximately equal.
Thus:-
Example:-
Let the road cost be $30,000 per km and h be $1.60 per m3 per 100 m.A crop of Yield Class 12 Sitka spruce discounted to 3 years after optimum timing of first thinning yields a volume of 202 m3 per hectare.
Optimum road spacing =
Therefore optimum road spacing for parallel access roads would be just over 600 m.
An alternative method of calculating roading requirements is to consider the optimum density of roading. Where a block of land, because of geography and land form, does not lend itself to parallel roads, the use of a total economic length of road for the block, distributed in a manner which takes account of the land form, may be a better solution.
If the same figures are used as before.
The optimum density (m/ha)
= 16.41 m per ha
By multiplying the number of hectares in the block of land by this density, the quantity of economic roading can be decided for the block. It can then be distributed to suit the crop and land form in the block.
From experience gained in mountain logging operations some guideline figure for road-net densities (truck roads) have been developed which may serve for planning purposes. However, these would have to be checked and modified according to local conditions before being used for purposes other than planning.
TABLE 2
Examples of road-net densities versus terrain features
|
Description of Terrain, Forest and Infrastructure Conditions |
Road-net Density (Truck Roads) in Metres per Hectare |
|
Hilly Terrain | |
|
Slopes of up to 40% with 60-80 m skid roads per ha |
7-10 |
|
Steep Terrain | |
|
Use of cables cranes |
15-25 |
|
Steep Terrain | |
|
With intensive forest management |
25-35 |
In steep terrain2 the recommended road-net density is in the order of 20 m/ha, for economic reasons, productivity, erosion problems, forest protection areas, etc. In order to arrive at the most economical mix of road and off-road transport, the utilization of short-distance cable cranes, especially mobile and radio-controlled cable cranes, is a good supplement to forest roads for opening up of mountainous areas. The maximum possible skidding distance of a short-distance cable crane is about 500m; the lateral skidding distance on both sides of the cable is about 20m to 25m. By using tractor attached winches, wood can be skidded up to 150m uphill onto the road or landing.
2 Slopes over 50%
In general, slopes over 70 percent have a tendency to landslides, especially after heavy rains and where soil and rock strata dip towards the valley; therefore, it is advisable to open up those forest areas by means of long-distance cable crane systems. Long-distance cable cranes (skyline length, generally more than 1500m) and medium range cable cranes (skyline, 700m - 1500m) may substitute partly or entirely for roads so that they more or less completely take over the function of timber transport, especially in extreme situations.
PHOTO NO. 2 A forest area opened up by a combination of forest roads and cable cranes
Generally, nature and the harvesting system dictate the road-net pattern and there are endless variations of different road-nets as much as different terrain configurations. However, over the past years, some standard route patterns have been developed. Some of these are shown in Figures 2 to 7.
Valley Roads
These are usually main roads which are designed to open up the bottom of the valley. The road should be routed along one bank, as bridges are expensive.
FIGURE 2 Routing of Valley Roads
Valley road in steep terrain A
Valley road in steep terrain B
Valley Bottoms
These should be opened up by means of a circular road system, provided that the terrain is not too difficult. This system is used at the headwaters of a river. The road should be at least 50 metres from the water edge to prevent road debris affecting the water quality.
FIGURE 3 Circular Routing for Valley Bottoms
Slope Roads
These start from valley roads and subdivide slopes. A distinction can be made between serpentine and diagonal systems.
FIGURE 4
Routing of Slope Roads
Serpentine system (on steep, long slopes)
Diagonal system (on gentle slopes)
Ridge Roads
Ridge roads are often the best solution in hilly terrain insofar as logging with light cable systems is concerned; however, they often only open up the forest area to a very limited extent. In many developing countries they are frequently used in combination with skid trails and/or cable systems. Ridge roads should probably be constructed when hillside slopes are prone to erosion or valleys are inaccessible because of extremely wet and swampy terrain. Care should be taken with the drainage patterns to ensure that land slips are not encouraged.
FIGURE 5 Ridge Road
Logging Areas on Ridges
These should be opened up by crossing the ridge, then descending the other side.
FIGURE 6 Logging Areas on Ridges
Mountain and Hill Tops
Circular routing, starting from a saddle, should be used to open up mountain and hill tops.
FIGURE 7 Circular routing for Tops
(Figures 2 to 7 by O. Sedlak)
