Katsuhiro Kitagawa1 and Shoji Okawara2
1 Nagoya University, School of Agricultural Sciences, Chikusa, Nagoya 464-013-18-8, Japan.
2 Iwate University, School of Agriculture, Ueda, Morioka, Iwate Pref., 020 Japan.
The aim of this paper is to illustrate a forest road with a newly designed sub-base structure and to discuss its advantages. This method was originated by Dr Okawara in 1983, and nowadays an increasing number of actual forest roads are constructed by this method in the national forest every year.
In this method, a ballast road is constructed by the cut on the mountainside with no side ditch and with the embankment on the valley side. The surface of roadbed (subgrade) soil is formed by a gentle super-elevation of about 3 to 7 percent down towards the valley side from the mountainside, and the waterproof polyester mat for separating the roadbed soil and sub-base ballast covers the entire surface of roadbed soil. Then, the ballast is scattered on the mat and the road surface is levelled.
In this new type road, the sub-base itself has a function of drainage, and the rainwater, which falls onto the road surface and comes into the road from the mountainside slope, passes through the ballast, flows on the oblique mat and is drained dispersely onto the valley-side slope, so that unlike the concentrated drainage system with side ditch, no erosion occurs on the valley-side slope. Moreover, the forest land below the forest road is irrigated.
Long-term observations of actual forest roads constructed with this method revealed that the new type forest road was still in sufficiently good conditions for practical use many years after construction. The water permeability of the sub-base under tracks gradually declines year after year, but the rainwater permeates freely through another portion of the sub-base. In this way, the smoothness of the road surface is maintained without erosion and there is no need to repair the road.
The new type forest road has many advantages over the traditional one, for example, quick and abundant drain from the mountainside to the valley side, less earth volume to be removed, no rutting, less forest clearing damage in constructing a forest road, etc.
In order to avoid certain problems encountered in the construction method of the new type forest road, applied research should be carried out on, for example, the best composition of macadam sizes according to the location of a forest road, the desirable methods for protecting the cutting slope, etc. In addition, in order to lower the construction cost of the new type road, the cost of the mat should be reduced mainly as a result of an effort on the part of mat makers.
In the construction and maintenance of forest roads in a mountainous country like Japan under severe conditions (rainfall above 2 000 mm/a, brittle geographical features) drainage is one of the most important problems. For maintaining the drainage of a forest road in a good condition, we need to look at the drainage system itself from a different angle than the traditional one. We believe that the introduction of a new forest road with a newly designed sub-base structure would not only be very useful in making the sub-base of a road strong and stable but also environmentally sound, and that the effort for its introduction would adapt to the social needs for environmental conservation.
The new type forest road with the newly designed sub-base structure was originated by Dr Okawara, one of the authors of this paper, in 1983 (Okawara 1986), and has been tested in our laboratories and in the field (university forests). Nowadays, roads of this type have been constructed every year in the national forests in Japan and some have been used in harvesting operations. Based on the results of actual traffic, the construction method has been gradually improved and we believe now that we have achieved our goal as expected.
This paper deals with the outline of the new type forest road with the newly designed sub-base structure and discusses its advantages.
Fundamental ideas of a newly designed sub-base structure
A ballast forest road is generally damaged by the ballast sinking into the roadbed (subgrade) and the roadbed soil going up into the sub-base; both are caused by heavy vehicle traffic. This causes erosion along wheel tracks on the road surface. In the new type road, we introduced a special, durable and waterproof polyester mat spread between the roadbed soil and the ballast to prevent their movement. We changed the traditional drainage system, which collects water through drains from the upper slope and the road surface, with a new one, which disperses water along the road by using gaps in the ballast.
Structure and function of the new type forest road
The new type forest road is constructed on a structure whose roadbed soil surface inclines slightly (3-7%) towards the valley side (see Figure 1a). Compared with the traditional road type (see Figure 1b), the new one has the following characteristics:
1. As no side ditch is constructed, the whole earth cutting volume can be reduced. Making the roadbed soil surface incline towards the valley side brings less earth fill volume into the embankment. These two features reduce the cost of earthwork and slope destruction.
2. As the durable and waterproof polyester mat is covered on the roadbed soil to separate it from the sub-base ballast, the sub-base is kept separate from the roadbed. In this way, the rainwater, which falls onto the road surface and penetrates through the sub-base ballast, is prevented from infiltrating into the roadbed soil. This provides a good condition to stabilize the sub-base because it keeps the roadbed soil from muddying.
3. Rain passes through the ballast, flows on the oblique mat and is drained dispersely onto the valley-side slope, so that unlike the concentrated drainage system with side ditch no erosion occurs on the valley-side slope. Moreover, the forest land below the forest road is irrigated.
4. In the drainage of the traditional forest road, it is difficult to drain in a flat section where there is no profile gradient, but the new type can easily drain regardless of the profile gradient.
5. Although the roadbed soil surface of the new type forest road has an inclination of three to seven degrees, the road surface itself is flat. Therefore, vehicle drivers can pass on the new type road with no bumping. The sub-base at the valley side has a different thickness from that at the mountainside. That is to say, ballast filling on the embankment (valley side) is thicker than on the cut (mountainside) which has a stronger resistance of roadbed soil to the tyre pressure of passing vehicles than that of the fill. As a result, it produces a desirable ballast distribution according to soil resistance. This means that the sub-base of the new type forest road is better than that of the traditional one.
6. The ballast used for the sub-base is angular and hard macadam of mixed sizes in diameter, so that it can be fastened tightly by only the tyre pressure of the passing vehicles; however, it is desirable to scatter sandy materials of small-size diameter only under tracks on steep gradients.
7. The improved, coarse-meshed and hinged wire net used to hold and stabilize the ballast at the valley side of the sub-base is coated with aluminium and zinc to prevent rusting, so it has good durability and, as the type pressure brings about ballast stabilization, the sub-base is permanently stabilized and maintained. The traditional forest road has a weakness in the road shoulder at the valley side since it is subject to collapsing. The improved wire net, however, can overcome this problem and strengthen the road shoulder.
8. If we occasionally scatter manure or fertilizer on the surface of the new type forest road using vehicles, they would be spread by rainfall dispersely and widely on the filled slope along the road. This may stimulate the growth of shrubs on the filled slope. If we plant certain tree species along the new type forest road to protect the filled slope, the slope may be better preserved by this treatment than in traditional ways. It would be desirable also from an aesthetic viewpoint.
The progress of the research
1. Fundamental investigations in university forests
The first forest road of the new type was constructed in 1983 in the Gomyojin University Forest of Iwate University, Iwate Prefecture, which lies in the northern part of Honshu Island, Japan. A part of the existing forest road, Akazawagawa forest road, was converted into the new type forest road. It is 15 m long and has a gradient of 1 percent. The second forest road, which is 50 m long and has a gradient of 1 percent, was constructed in the same university forest in 1985 (Okawara 1986, 1987).
Investigations on the second road were carried out for six years concerning the following points: drainage ratio (the ratio of water drained from the roadside, concentrated by the special gutter built along the road shoulder at the valley side and measured with an automatic water metre, to precipitation; moisture content (electric resistance of the roadbed soil measured through an electrode sheet w buried into the roadbed soil; and changes in the durability of the mat. These investigations revealed that the new type forest road could work well for a long period under certain road conditions.
Unfortunately, this road was insufficiently managed and the water permeability of the sub-base declined significantly owing to a great quantity of mud brought to the road from the roughly cut slope at the mountainside and also by the vehicle tyres from parts of the traditional road adjacent to both ends of the road section. Moreover, the ballast did not stabilize sufficiently so that the sub-base was widened out sideways by the type pressure and the tracks finally collapsed. Investigations on this road were interrupted when the sub-base structure collapsed.
In 1988, another forest road of the new type was constructed in Sado University Forest belonging to Niigata University, Niigata Prefecture, near the central part of Japan. A part of the existing forest road, Okura Forest Road, was converted into the new type road. It is 50 m long and has a profile gradient of 12 percent. This forest road has a special gutter along the road shoulder at the valley side to collect the water drained dispersely from the roadside. The drained water is measured by a self-registering water metre with a turnover measure and recorded with the precipitation.
The analysis of the yearly drained water showed that the drainage function of the new type road had still been sufficiently maintained for practical use seven years after construction (Kuze et al. 1995). The water permeability of the sub-base under tracks has gradually declined year by year, but the rainwater permeates freely through another portion of the sub-base. In this way, the smoothness of the road surface is maintained without erosion and there is no need to repair the road.
2. Actual working results in the national forest
The first forest road of the new type for actual harvesting use was constructed in 1987 in the Akita Regional Forestry Office, Akita Prefecture, which lies in the northern part of Honshu Island and adjacent to Iwate Prefecture. It was newly constructed in a horizontal section of 100 m in length as part of the Negisawa Forest Road. Since then the length of the new style forest road in the national forest has been increased by 100 m every year. Nowadays, there are 11 new type forest roads for practical use in the whole country.
In the case of the Negisawa Forest Road, the oldest among them, there has been no need to repair it since its construction. Its road surface has maintained smooth and stable, and is almost similar to that of a paved road although a total of 2 500 trucks with ten tons in weight have passed on it up to now. For the parts of the traditional forest road adjacent to both ends of the new type road, the traffic condition is the same as the new type one, but the road surface needs to be levelled by grader once every two months and macadam must be added to the concave parts two or three times a year. If the costs of those repair works are added up and converted into money each year, the total expenditure over a certain number of year would exceed the extra money spent for the construction of the new type road (mainly, the cost of the waterproof polyester mat).
When the new type forest road was at first constructed, the method for stabilizing the ballast could not be established and the ballast had to be stabilized by using thinned logs so that this method had shorter durability. But in the latest road building (1996), a newly devised, coarse-meshed and hinged wire net was used for stabilizing the ballast to overcome the above-mentioned weak point.
It seems that there is a clear relationship between the declining curve of the drained water after rainfall and the permeability of the sub-base. In order to prove the above-mentioned assumption, we are now considering establishing a drainage theory in the sub-base applicable to the new type forest road by applying the Darcy theory. If the drainage theory is established, we would be able to check easily the sub-base condition only by attaching a water sensor to the mat at the valley side and converting the measured value into the water declining degree. It will contribute much to the simplification of the road maintenance method.
After this, in order to establish the construction method of the new type forest road, we should develop applied research; for example, the best composition of macadam sizes according to the location of a forest road; the desirable method for protecting the cutting slope, etc. In addition, in order to reduce the construction cost of the new type road, the cost of the mat should be reduced. Therefore, our most urgent task may be to ask mat-makers to reduce mat prices by introducing mass production.
The authors thank Dr S. Onwona-Agyeman, United Nations Centre for Regional Development, Nagoya, Japan, for his kind assistance in checking the manuscript.
Okawara, S. 1986. A new structure of forest roads in mountains (I) - Developmental purpose and characteristics of the new structure. Trans. Jpn. For. Soc. 97: 631-632 (in Japanese).
Okawara, S. 1987. A new structure of forest roads in mountains (II) - Changes in structure and function during about past two years after its construction. Trans. Jpn. For. Soc. 98: 699-700 (in Japanese).
Kuze, T., Kitagawa, K., Yamamoto, M. & Okawara, S. 1995. A study on the permeability of the forest road with a new structure sub-base (II) - Relationship between precipitation and the drained water. Trans. Chubu Branch Jpn. For. Soc. 43: 179-180 (in Japanese). (The title is a tentative translation from the Japanese title by the authors of this paper.)
List of technical terms
1. drainage system
2. new sub-base structure
3. ballast stabilization
5. road construction method
6. wheel track
7. waterproof polyester mat
8. no side ditch
10. rainwater drainage
11. drainage ratio
12. moisture content
13. sub-base collapse
14. self-registering water metre
15. sub-base permeability
16. road surface smoothness
17. coarse-meshed wire net
18. road maintenance
Figure 1. Cross-section of forest roads of two types
(a) New Subbase Structure Type
(b) Traditional Subbase Structure Type