12.1 SLOPE PROTECTION AND STABILIZATION
12.2 SLOPE DRAINAGE
12.3 REVEGETATION OF ROAD EMBANKMENTS AND SLOPES
12.4 STABILIZATION OF DRAINAGE-WAYS
12.5 SLOPE PROTECTION WITH ENGINEERING STRUCTURES
12.6 PROTECTION OF ROAD EMBANKMENTS AGAINST TORRENTIAL WATERFLOWS
The basic requirements for modern forest management, especially wood harvesting, are well planned and designed forest road networks. Careful attention must be paid when planning and locating roads, especially in steep terrain, to avoid or minimize the erosional impact of roads on the environment.
Areas particularly susceptible to erosion problems such as very steep slopes with easily erodible soils and rock strata dipping towards the slope should be avoided as much as possible.
Erosion caused by road construction and soil disturbance can be avoided by using biological means and/or engineering structures. Slope and gully erosion adjacent to the road is very often a result of over-grazing and denudation of hills which expose the soil to wind and rain and endanger the road structure. Erosion often occurs on the cuts and embankments, as well as on the outlets of cross drains, water flows and on the surface of the road itself.
First of all, it is most important to determine the source of factors influencing slope instability in order to be able to design appropriate control and rehabilitation measures. Very often a single measure may achieve the desired results but sometimes it may be necessary to combine measures to restore the stability of the slopes. For instance, on a seepage slope, it may only be necessary to drain off the water with open ditches or stone filled drains. On other occasions, it may also be necessary to revegetate the slope in order to fix the slope surface because vegetation would not come back at all or it would take too long a time, and a retaining wall would be required. In a mountain road project in the USA (Idaho) it was noted that 60 percent of the surface erosion occurred within one year of the disturbance of the slope; thus it is important to stabilize slopes at once, or shortly after the construction of a road.
The simplest method to safely drain off springs and surface water is by means of an open ditch or a system of open ditches. The main ditch is located in the direction of the slope gradient (downhill); secondary or lateral ditches are located in a fishbone pattern. Water should be collected as closely as possible from its origin and be channelled safely to the road ditch, culvert or any other nearby water course. In areas with steep gradients and a large amount of water run-off, pitched ditches 20 may be required. The excavation of the ditches should start at their lowest point in order that the accumulating water may drain off immediately. A very effective method to drain off the sub-surface water is by means of so-called "covered drains". On cut slopes the drains may at the same time act as a kind of retaining structure if made in a "Y" or arch shape, thus further increasing slope stability. The most common types are stone or gravel-filled drains with or without pipes.
20 A ditch in which stones have been placed in the bottom and sometimes sides, depending on the volume of water and gradient.
To ensure the efficiency of the drains and for maintenance purposes, it is advisable to have a standing pipe at the junction of the main drain and secondary drain. Pipes may be made of concrete, brick or PVC material. The excavation of the drains should start at the lowest point, the lining of pipes, however, should be started at the top. The pipes should be placed as tightly as possible, one to each other, and they should be located in water-tight soils in order that maximum water drainage can be achieved. Piped drains are the most efficient and their effectiveness is long-lasting; however, they are more expensive, or often not available. Normal stone drains may silt up after some time; therefore it is advisable to form a drainage channel of stones at the bottom of the drain, or to put on a bundle of brush wood at the bottom of the drain. The tope of the drain may be covered with a layer of grass in order to prevent the siltation of the drain more effectively.
Besides being effective in stabilizing fill and cut slopes, drains are very useful behind retaining walls.
FIGURE 57 Drainage Systems to Drain Off Surface and Sub-surface Water on Slopes
FIGURE 58 Cross-Sections of Different Types of Drains
Revegetation measures for the stabilization of cut and fill slopes of roads may be grouped as follows:
(i) Seeding, grass turfing and mulching to obtain a grass cover;
(ii) contour wattling, wicker work fencing, contour planting and fascines21 to allow the start of shrub vegetation;21 Long bundles of sticks bound together.
(iii) reforestation with pioneer species.
Very often before sewing grass seeds on barren slopes, soil and site preparation such as shaping the slope, spreading humus and application of fertilizer may be required. The seeds may be either sown on the entire area, in rows or in certain places only. About 3 kg of grass seeds are needed to seed an area of 100 m2 It is an advantage to have legume seeds mixed with grass seeds as they are nitrogen fixers. It will take 0.5 to 1.0 working hour to seed 100 m2. A mixture of deep rooted and flat rooted strong, quick growing pioneer grasses will give the best results in fixing the soil.
To regenerate successful vegetation through placement of grass sods, it should be borne in mind that grass sods need to be placed horizontally on the slope when the surface is wet and during the vegetative period- Depending on the availability of grass sods, slopes may be entirely covered by them or only in strips. the latter application would require additional seeding. On very steep slopes fixation of the grass sods may be necessary to get a firm hold on the slope surface. This could be done by means of sticks prepared from tree branches, twigs, or bamboo, when available.
This is a very quick method of regaining grass cover on sterile, bare soils. This method of revegetation requires a layer of straw, wood fibre or other organic material which is spread onto the soil. seeds and fertilizers are added and finally the layer of mulching is fixed by spraying cold asphalt suspension. The advantage of mulching is that the grass cover comes up after a relatively short time because through this method a favourable micro-climate and growing conditions are created; it reduces water losses, surface temperature and solid crust formation. It prevents seeds from rolling down the hill, and it also preserves the fertilizer. In the USA and Japan, machines (hydro-seeders) have been developed which can spray the mixture of mulching material mixed with water and an adhesive as well as seeds and fertilizer onto the slope in one operation.
In the Alpine Region of Middle Europe revegetation by mulching techniques has been successful using the following method of application: the slope is covered by a layer of straw (2-4 tons/ha), which is spread by hand, utilizing a ladder which is placed on the slope. Seeding and fertilizing is carried out by spreading seeds and fertilizer by hand, again utilizing ladders. Seeds and fertilizer fall through the straw layer onto the ground. For the fixation of the straw layer onto the slope surface, an asphalt suspension of 50 percent asphalt in water is watered at a 25 percent solution which is applied onto the straw by means of a portable rucksack type sprayer. About 0.5 litre of asphalt suspension per m2 is applied. Spraying cannot be carried out during heavy rain and wind. Normally it takes 2 to 3 hours after spraying for the suspension to fix the mulching material, in general, by the time the asphalt suspension covering the straw layer has disintegrated, the grass vegetation is well established.
FIGURE 59 Mulching
Contour wattling, also called "wattling and staking", is one method of achieving a brush vegetation on steep slopes to be applied where a grass cover would not be strong enough to stabilize the soil of the slope. The idea is to sub-divide the slope with dense brush rows and, if necessary, in between the rows, grass seeding could be applied for additional soil fixation. Before starting with the wattling and staking, slope preparatory work should be carried out such as levelling of small gullies or removing such obstacles as big loose boulders and branches. Then stakes should be driven in along the contours at certain distances from each other within the contour line as well as from row to row. It is desirable to have every fourth stake as a sproutable stake. Staking should be started from the lowest part of the slope, moving uphill. Trenches should be dug just above the stakes and wattling consisting of sproutable twigs and branches are then put in the trench, overlapping each other. Part of the twigs and branches should be above the surface to prevent soil from moving down the slope. The soil dug out is used to cover the lower contour wattling. Some technical data are given below, as well as production data of an example of contour wattling carried out in Jamaica under the supervision of Mr. Sheng, FAO Watershed Management Officer. Stakes sharpened at the bottom ends 1 -1.2 m long and with a diameter of about 5 cm with a row interval of 1.2 m and 0.50 m from each other were driven into the soil, leaving about 15 cm of the stake above the solid surface. Thus a hectare would require about 17 000 stakes. Contour trenches 20 cm wide and 25 cm deep were dug and bundles of wattling 13 cm in diameter and 3 m long were laid in the trench. A ten-man crew may be able to carry out contour wattling work on an area at a rate of up to 250 m2 per day. Within the working crew, six labourers staked, two labourers trenched and covered wattles, and two labourers transported and carried out other duties.
FIGURE 60 Contour Waffling
Another example of where several species to be used as cuttings have been tested (by Mr. Tautscher, FAO) was in Nepal. Those found to be most suitable for regeneration by cuttings were Salix tetrasperma. Salix vallichiana and Viburnum.
Wicker Work Fencing
The system is similar to the one mentioned above and is widely used in the Alpine Region of Middle Europe. The difference is that the sproutable material is not put in bundles into the soil, but is put around the stakes like a fence and the ends of the sproutable twigs are put into the soil.
The rows must not necessarily follow the contour lines; very good results have been achieved with rows placed at an angle of 45 forming rhomb22 shapes with sides 1.5 - 4 m long. The stakes are driven into the soil with a spacing of 40-50 cm, having a length of 1-2 m and diameters of 5-10 cm. The stakes should be driven into the soil three-quarters to two-thirds of their length. The spacing of the wattle rows very much depends on the gradient of the slope and soil. Normally they are 1-4 m apart, laid out in parallel rows. In Europe good results have been achieved using Salix spp. and Alnus spp. as sproutable fencing material.
22 An equilateral parallelogram usually having oblique angles.
FIGURE 61 Wicker Work Fencing
Contour Planting (Cordons)
Sproutable plant material of 0.9 to 1.5 m in length is placed in horizontal cross layers into the contour terraces. Terrace digging starts from the bottom of the slope proceeding to the top. The lower cross layers of sprouting material are covered with soil gained from the excavation of the upper terrace. The spacing of the terraces depends on the gradient and the soil; it may be up to 3 m. The width of the terrace should be 0.5 - 0.6 m. Cordon layers may either continuously follow the contour line or be of a certain length, say 5 m, and overlap each other. With the indicated spacing of contour planted rows, 3 500 to 5 000 of cordons per hectare would be required for the rehabilitation of the eroded slopes.
FIGURE 62 Contour Planting (Cordons)
The technique is similar to the one used in contour planting. It differs in that instead of putting cross layers in the contour terraces, brushwood is laid in. This can be mixed with cuttings to achieve a green brush row. In between the brush rows, shoots are put or seedlings are planted. Terraces should have a gradient of 20 to 25 percent towards the slope having a width of 0.6 -1.2 m. The brushwood and cuttings should be about 20 cm longer than the width of the terrace.
FIGURE 63 Fascines
Revegetation work should be carried out with pioneer23 species to stabilize slopes subject to landslides, or as a preventative erosion control measure on severely degraded slopes. In Alpine Regions pioneer plants such as alnus. Betula, Fraxinus and Prunus have proved to be most successful as far as their survival rate on eroded slopes is concerned. When considering plants for use as slope stabilizers it should be borne in mind that they have strong, deep roots to bind as much soil as possible. Wherever possible it would be desirable to select species for afforestation on bare slopes which could be used as fodder or fuelwood trees, since there is a desperate need for such trees in many developing countries.
23 In order to ensure the fastest possible start of growth.
Unprotected drainage-ways crossing the roads are very often the source of major erosion problems. Erosion mainly occurs on unprotected outlets of the drainage-way where runoff water frequently develops gullies through its erosive force, which in some cases even cases landslides and damage to the road structure. Protection of water drainage outlets and channels can best be done by fixing the soil surface with dry stones or cement-bonded stones. In channels with steep gradients, it is advisable to have some stones cemented along the channels which are above the bed of the cement stone channel, thus reducing water velocity and its destructive erosion forces.
A cheaper way of stabilizing channels and outlets of water crossings is to provide rook riprap which in most cases gives satisfactory results.
For the protection of bridges, culverts and fords, structures such as rock riprap, dry stone or cement stone retaining walls, or where applicable, wooden protection structures may suffice. Very often revegetation treatment on the slopes of cross-water drainage-ways already gives satisfactory protection.
Simple engineering works for forest road construction such as drystone structures, gabions, log crib revetments and timber retaining walls, have proved very useful in many countries. They are inexpensive and easy to construct at the required sites with local material. Cement is often difficult to get or is not available in remote areas of developing countries, transport costs are usually high and skilled masonry labourers are often scarce. Therefore, in this paper heavy emphasis has been placed on drystone structures and timber construction works.
Stones are placed in the form of arches into the soil of cut slopes. The width of such arches may be 0.60 to 1.20 m and they may be up to 1 m in depth. In between the arches and above them, cuttings of Salix spp. may be planted to achieve additional stabilization.
Drystone Retaining Walls
Stones from 20 to 30 cm in size are placed next to each other into the surface of the slope. For the setting of stones into the soil surface a productivity of 2.5 - 4 m2 per man per day may be achieved. However, provisions for obtaining and transporting the stones must also be made.
PHOTO NO. 27 Drystone Retaining Wall
Gabions are structures made of stones which are normally set up by hand labour and covered with wire mesh to keep them together.
The advantages of gabions are:
(i) their construction is simple; with proper supervision, unskilled labours can set up these structures;
(ii) they are cheap;
(iii) stone material which is available in many places at the construction site can be used;
(iv) only wire mesh or wire needs to be purchased and transported to the construction site;
(v) their construction time is short;
(vi) they are very durable; in comparison to cement masonry walls, they are more resistant against mass movement without breaking, because they are flexible;
(vii) water drains off easily, thus increasing the shear strength of the soil and reducing the erosion hazard of the slope to be protected, thus preventing the mass movements mentioned in (vi) above;
(viii) sooner or later, grass grows between the stones, thus making the gabions even more stable and integrating them well into the environment.
Basic data used in the cost estimate are as follows:
Average construction output per m3 of gabion = 1.9 man-days which comprises preparation of wire mesh, collecting stones near the construction site, transport and setting up the stones, forming the construction, as well as rock fill.
PHOTO NO. 28 Road base and surface destroyed by landslip, road repaired by gabion
Log Crib Revetments
These structures may be of use where wood is easily available and where there is no adequate stone material or where the construction costs of stone structures are excessively high because of long transport distances for the stones. Log crib revetments are made of roundwood, consisting of logs laid parallel to the slope and cross-layers, which fix the structures with the sub-soil of the slope. The cross-layers should be placed at a spacing of 1 to 2 m. In between the log layers, which lie parallel to the road, stone filling and additional sproutable material may be placed protecting the road from stone and earth material. Log layers and the ends of the cross logs must be fixed either by nails or cut (notched) to fit each other. In severely sliding areas, it is advisable to construct log crib revetments consisting of front, back and cross-layers of logs, which would form a cage and would be thus more resistant to the gravity force of the slope material. The advantage of log crib revetments are that they can be set up in a short time, they are cheap, local tree species can be used and they are more resistant to slope movements than inflexible masonry constructions. Their disadvantage is that they have a limited lifetime, generally 10-15 years. However, by that time it is expected that the treated slopes will be stable.
PHOTO NO. 29 Log crib revetment under construction
Timber Retaining Walls
This simple type of structure may be built to protect slopes from erosion. It is composed of stakes driven into the sub-surface of the slope and of timber nailed onto them from the mountainside. They are placed near the road and, if necessary, higher up on the slope along the contour lines of the cut slope.
Pre-cast Concrete Crib Revetment
These structures have been developed for areas where neither stone nor timber is economically available. Concrete beams of 250 cm x 12.5 weighing about 90 kg each and cross-beams of 125 cm x 12.5 cm weighing about 45 kg are used for this type of structure. This example is only mentioned to give a more complete picture of the development in this sector of construction. At present their application may not be economically feasible in many countries, except for locations close to a source of cement.
The damage to roads caused by torrential waterflows may occur when the roads are located along or across the torrents. Erosion caused by the running force of water may endanger or destroy the bank or embankments of roads or the road itself by its scouring effect and erosion of the toe of torrent banks. When crossing torrents or gullies, the road may be blocked by sedimentary material or destroyed by downhill mass movements.
Necessary rehabilitation measures in controlling erosion caused by torrents are to reduce the velocity of water by engineering structures and rehabilitating slopes of the gully or torrent banks. Thus, a combination of biological and structural bank stabilization, as well as putting in check dams or sills and check dams, may be required to fully protect the road from erosion and sedimentation caused by torrential waterflows.
Embankments may be constructed with different materials. The most common type is made of rocks. Stones protecting the toe and bottom of the channel should have a diameter of at least 0.5 m and those protecting the banks should be 0.3 m and even greater. If only smaller stones are available, paving with stones covered by a wire mesh is very effective.
A very quick method of stabilizing embankments is by putting boulders on the banks of the torrents - these structures are called "rip rap". In torrential flows with big hazards of bed erosion and scouring, the paved toe may be protected additionally by placing boulders on it.
A combination of layers of boulders and layers of fascines with sproutable material may give very good results, as the water velocity is reduced by the facines on one side and the embankments are made more stable because of the vegetative cover. The cuttings should be put about two-thirds of their length into the sub-soil.
Boulders in combination with grass turfing, or grass turfing and planting of brushes and trees on the embankments, may provide good results in stabilizing embankments.
In areas where wood is available, log crib revetments with stone fillings in between the logs may be constructed. At the bottom of the timber crib revetment a layer of logs should be placed to prevent the filling material from being washed out from the structure.
Bank revetments and retaining walls made of concrete are very effective -however they are more costly than the structures mentioned above.
PHOTO NO. 30 A series of check dams to protect the forest road from erosion