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TRANSPORT RESEAR TITLE CH LABORATORY Low cost engineering and vegetative measures for stabilising roadside slopes in Nepal by C J Lawrance / Overseas Centre Transport Research Laboratory Crowthorne Berkshire United Kingdom LAWIRANCE, C J (1994). Low cost engineering and vegetive measures for stabilising roadside slopes in Nepal. Paper presented to ICE Conference on Vegetation and Slopes: Stabilisation. Protection and Ecology. Oxford. 29-30 September 1994. Lawrance 1 LOW COST ENGINEERING AND VEGETATIVE MEASURES FOR STABILISING ROADSIDE SLOPES IN NEPAL C J LAWRANCE Overseas Centre, Transport Research Laboratory, Crowthorne, Berkshire, UK 1. INTRODUCTION Roads are regarded as being necessary to promote development in third world countries, as they always have been. Recently however, donor agencies have begun to take much greater account of the physical impact that a new road can have upon the terrain, and hence upon the lives of those who live within the road corridor. In 1984, the Overseas Development Administration set up a major maintenance project on UK-funded roads constructed in Eastern Nepal, under the management of the UK consultants Roughton International. ODA was concerned at the large number of shallow failures affecting slopes above and below its own (and others') roads in Nepal, and commissioned TRI, to examine the possibilities for 'low cost' engineering measures for slope protection that could be incorporated into the Nepalese Department of Roads' maintenance routines. TRL's brief turned out to be very wide. From the outset it was evident that vegetation would need to be included in the study, mainly because plants are best suited to the retardation of very shallow failures extending over large areas, of which there are many in Nepal. However, very little information was available about suitable plant species, about weathering processes or slope failure mechanisms, or about the effectiveness in Nepal of low cost engineering measures used in Europe and North America. Moreover, it was found that experimental trials -could-not be set up in the normal way, because the slopes are so variable in hydrology and in the depth and consistency of the regolith, that it was impossible to set up experimental plots with appropriate duplication and controls that would give statistically valid data. The approach adopted, therefore, was to make'regular observations of a large number of sites on which a wide range of techniques and species were implemented, on the assumption that the relative success of these would, in time, indicate those that tended to be most positive. The best circumstances in which to make observations is in heavy rain, when a site is put under stress. Visits to Nepal were therefore made annually in the late monsoon, in the years 1987- 1992. On several occasions, sites were watched in the process of destruction by a storm. Much was learned about slope processes and the response of stabilisation measures to heavy runoff at these times. Our tasks were basically twofold: -identify 'low cost' engineering techniques suitable for reducing shallow failures on Lawrance 2 slopes. - define a role for vegetation in slope protection in Nepal. Note that the brief for vegetation concerned its engineering function alone; it did not include any element of aesthetics or amenity. Later, the needs of local people in their utilisation of forest products was given greater recognition and incorporated into new planting schemes. 2. FIRST-ATTEMPT FAILURES Slope surface protection systems developed in the West fall into two broad categories: - Surface coverings (eg. mulch, brush matting, jute netting and similar mats). - Horizontal linear systems (eg. wattle fences, brush layering, fascines). As a starting point for experimentation in the absence of any guidelines for subtropical conditions', TRL, constructed wattle fences (Figure 1) and slope mulches made from local hessian sacking laid upon a trimmed, seeded, slope (Figure 2). The wattle fences were expected to reduce the rate of flow of water over the surface, hold back particles of soil washed down the slope, and form a haven for seedlings. The loosely woven hessian sacking was expected to formn a sun shade for seedlings, reduce raindrop energy, prevent the movement of material over the slope surface and ultimately rot down and provide a little organic goodness to the soil. The results were a complete failure, for both methods. In the case of the wattle fences, the soil behind the fence became so wet that the fence was either pushed over or the soil liquefied and flowed out from underneath (Figure 3). Fences that survived were Figure 1. Woven wattle bamboo fences found to last only one or-two-seasons before rotting or being eaten by termites; they simply did not last long enough to allow vegetation to become established. The hessian lay upon the ground like wet blotting paper, suppressing germination and preventing healthy drying out of the soil surface. The soil beneath would sometimes liquefy, causing the cover to bag and tear (Figure 4). These forms of failure were found to be common in horizontal systems and slope covering systems, and demonstrated the need for a much more careful appraisal of slope degradation processes and of the way in which slope stabilisation measures function in this extreme environmnent. Lawrance 3 Figure 2. Hessian fabric slope cover and mulch Figure 3. Bamboo fences destroyed by shallow mudflows 3. CLIMATE AND GEOMORPHOLOGY Eastern Nepal receives 1000-2500mm of rain annually (depending upon orographic factors), most of which falls during the monsoon months of June through to September. Although it can rain continuously for many days, rainy periods usually last for a few hours only, often followed by a spell of intensely hot sunshine. With summer temperatures around 30-35 0C, the rocks are subjected to intense chemical weathering which often causes road cuttings to degrade within a few years of being opened up. The rocks of eastern Nepal are metamorphic rocks of Lawrance 4 Figure 4. Hessian slope cover damaged by mudflows beneath varying grade - phyllites, schists and gneisses, with subsidiary unmetamorphosed soft sandstones, siltstones and mudstones. All are highly susceptible to weathering. The downcutting of Himalayan streams causes continual undercutting of the valley sides, with attendant landsliding. This has created a landscape of slopes of 35-40o that are more or less straight from valley to crest, whose factor of safety is not far above 1. They are very sensitive to removal of support by, for instance, a road cutting. The slopes are mantled by 1-3m of more or less in situ weathered rock or old landslide debris (it is often difficult to tell them apart in the field). The materials are typically open-textured, of low cohesiveness and very stony. 4. LANDSLIDE APPRAISAL It is obvious that in a situation where permeable materials are subjected to sa'turation and short periods of very heavy rain, the hydrological regime of the uppermost few centimetres of the soil greatly affects the stability condition near the surface. Although it has not been possible to measure or predict hydrological conditions in this zone, their importance for slope stabilisation has been taken into account in the drawing up of a simple classification of failure modes and failure depths,-that provides a-first step towards identify'ing appropriate mitigation measures (Tables 1 and 2). The point to note is that, with some exceptions, vegetation is applicable only to erosion and failures in debris, as opposed to rock, and where the depth to the failure plane is less than 250mm. If the failure plane is deeper than this, even mature trees become at risk to sliding forces. Lawrance5 Failure mechanism Definition Consequence for engineering Erosion Removal of debris particles from Surface treatments the surface by water flow Shear failure ('Slide`) Mass movement of soil or debris A variety of slope support and down slope. Includes rotational slope drainage methods, plus slumps, translational slides, surface treatments flows of saturated material and soil falls Plane failure Mass movement in rock, whose Rock mechanics solutions - failure plane is controlled propping, pinning or grouting. principally by fracture planes Propping is most appropriate in within the rock mass Nepal Collapse (`Disintegration Type of rockfall in which rock Very difficult to stabilise other rockfall`) fractures play little part in than by reducing slope angle to controlling development of the one which will stand when rock shear plane. In sparsely-jointed, is weathered and saturated permeable and weatherable rocks (eg sandstone). Rock weakens and eventually fails by shear. Debris consists of mineral particles (eg, sand) containing soft, weathered cobbles Undermining Type of rock failure that occurs Prop walls between hard layers, in bedded sequences of soft and or surface rendering of the soft hard rocks (eg, sandstone and layers mudstone). Soft rocks weather back, leaving overhangs of hard rock which break off along a vertical joint plane. Thus face retreats Table 1. Slope failure mechanisms Depth to failure plane (mm) Principal mechanism of failure Scale of remedial measures Up to 25 Erosion Can be stabilised with vegetation alone, assisted by ____ ____ ____ ____ ____ ____ __ `light" engineering m easures 25 -100 Liquefaction Can be stabilised by normal engineering methods, plus _____ ____ _____ ____ _____ ___ ,Vegetation 100 -250 Sliding or plane failure Ditto 250 -1000 Sliding or plane failure Heavy engineering needed; expensive. Possible risk of failure. More than 1000 Sliding or plane failure Sophisticated engineering required. Major capital investment Table 2. Depth to failure plane, and scale of remedial measures 5 Lawrance 6 The use of timber and jute raised the question of the longevity of low cost engineering measures in a tropical weathering environment (Table 3). When a site is to be planted, it is obviously important to ensure that the material used in mechanical stabilisation will last long enough for the plants to become established and take over. The concept of handover from a mechanical to a vegetative system of stabilisation is itself an essential consideration for design; the engineering function must be the same for both systems. A list of engineering functions was thus drawn up, to help in the landslide appraisal process (Table 4). Material Life Untreated bamboo, jute net, hessian 1 rainy season Timber (branches up to 50mm diameter), local 2 rainy seasons roofing felt Bitumen-coated jute net At least 4 years Wire horizontal fences (with steel posts) At least 6 years Gabion, masonry At least 20 years (provided that ground wateri not corrosive and stone is not weatherable) Table 3. Life of low cost engineering materials in the tropics Engineering function Engineering treatments Vegetative treatments REDUCE STRESS Remove load; reduce slope Not applicable, but weight of angle vegetation on slope should be minimised CATCH material moving over Tightly-pegged wire netting; Stout grass; broad-based surface catch wall shrubs ARMOUR slope against Revetment; surface rendering; Grass mat; aerial canopy rainsplash and erosion jute netting SUPPORT slope from below Toe wall; prop wall; fence Trees; shrubs; bamboos ANCHOR slope by pinning Rock anchors; anchored earth Anchoring effect of individual through to layer below (both hardly used in Nepal); tree root systems cannot be cable lashing of boulders guaranteed, though this mode of operation is often assumed REINFORCE soil by increasing Reinforced earth; soil nailing; Strong, dense rooting system its shear strength soil-filled fabric cells. (Not used of grasses, shrubs and trees in N epal)__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ DRAIN slope by means of sub- Gravel-filled drains Root systems carry water down surface waterways into soil -can be a dis- advantage; live fascine drains LIMIT extent of slope failure Loosely-draped wire mesh (limits Change in rooting depth from should it occur, or of damage to hazard but not extent of failure) shallow to deeper layer can property _________________prevent lateral spread of failure IMPROVE environmental or Not applicable Site and micro-climate of soil ecological condition of site improve as vegetation develop Table 4. Engineering functions of treatments Lawrance 7 5. PLANT SPECIES FOR SLOPE STABILISATION Since there was no indigenous source of supply of seed available on a commercial scale, consideration was given to importing the quantities required. However, importation was rejected on the grounds that: - costs could be high and would utilise foreign exchange. - the introduction of exotic species could have an adverse effect on the ecology. - local people, with their agrarian background, had the potential to develop their own nursery industry. By including farmers in the slope protection programme, both they and the project could benefit. - importation is against the principle of sustainable development. As with the bioengineering techniques, expert advice on suitable species was completely lacking. Agriculturalists M and foresters in Nepal had no knowledge of the bioengineering characteristics of any of the wild plants found growing within the road corridors that were thought to be potentially useful. They could not even name them; specimens were sent to the Royal Botanic Garden in Scotland for proper identification. Initially,' about thirty species of grasses, shrubs and trees were selected, primarily on the basis of their rooting depth, ease of propagation, speed of growth and ability to colonise poor sites (Figure 5). As a range of plants for all bioengineering purposes, the list had its shortcomings: --one is a weed which spreads in an unwelcome way into adjacent farmland. - at least two are colonisers of opengrond hic arquiklyFigure 5. Plants for engineering. Butea minor, a supressed b inangdeep-rooting shrub, protects drain margins, and species. ~~Neyraudia arundinacea, a clumping grass, - no creeping grasses, having a peet rso dense, binding root mat, occur peet rso locally. (Only clumping grasses are available, although some are very substantial). - the plants had to be unpalatable to animals, because it was impossible to prevent goats and cows from eating any palatable species that was planted. This situation has since been rectified by employing local farmers as resident guards to protect new sites. This practice has given access to many more species as potential bioengineering candidates. Lawrance 8 Questions of ecological compatibility, maintenance and value to the community as a source of forest products, were all deferred in order to pursue engineering effectiveness. These wider issues are now being addressed by current projects in Nepal. 6. PLANTING CONFIGURATION Deciding upon an ideal planting configuration proved to be the most difficult area of research because, to the variables in the landscape and species is added the need to allow tune for the plants to become established; only then can they demonstrate their ability to hold a- slope that is coming under stress. The following observations were made regarding plant development: - the often spectacular rate of growth of the aerial part of a plant belies the much slower development of an extensive root system. At least three seasons must pass before a plant is capable of performing an engineering function. A site containing immature plants can be wiped out by a single heavy storm. Many experimients were lost in this way, hampering progress in research. - growth rate is appreciably slower than normal on dry, rocky, exposed sites, as most landslide scars and road Figure 6. Rills formed by a rush of surface water - cuttings are*. 5 cnieal during a storm retarded during monsoons in which less rain falls compared to normal monsoons. As noted above, horizontal. systems -tend- to be. affected. by- liquefaction. and undermining. In heavy rain, water pours downslope in rills (Figure 6), carrying all before it. Two forms of improvement were, and still are, sought, in preparation for those occasions when a site is subjected to a downpour. One is to allow excess water to escape, the other is to build an element of 'survivability' into any new site, to minimise the destructive effect of a storm early in life. It was observed that the only plants to escape a deluge tend to be those growing on the ridges between waterways. A theory was conceived, that it might be better to place plants in lines down slope, in order to allow excess runoff to flow between the lines of the seedlings without impacting upon them. It was reasoned that, by allowing rills to develop or even creating them during slope preparation, excess water could be removed more quickly from the slope. This would increase the risk of deepening of the rills, but after the storm, Lawrance 9 provided the site remained intact, the rills could be stabilised by stone pitching or equivalent, to prevent them from deepening further. The theory was put to the test by planting about six sites vertically with grasses (Figure 7). It was found that, although water may flow between the lines, the new configuration carries its own disadvantages that prevent the plants from becoming established: - plants on ridges seem to suffer from drought. Growth is very slow -even after two years they remain small and can be pulled out with one hand alone. - water, sometimes travels preferentially down the line of plants, following the lightly- compacted planting holes and excavating the seedlings as it goes. - enlargement of the rills causes the ridges to rapidly become undermined, exposing the roots and killing the plants. The plants cannot grow fast enough to protect the ridge , from erosion or liquefaction 7- before the ridge is destroyed. Figure 7. Grasses planted down slope on a Experiments with down slope planting prepared ridge and rill surface were thus abandoned. However, a yariatin was tried that does seem to offer some advantages. When plants are placed in closely-planted diagonal lines (Figure 8): - overland flow volume is kept up, while the rate of flow is reduced because the water is intercepted at intervals by the grass lines. -water impacton--the grass, line is minimised. because. the. water- strikes the line at an angle, and the grass slips support each other. - water enters the ground along the line of plants, feeding the roots. The main disadvantage of diagonal lines as presently implemented is that the plants are overcrowded, causing many ultimately to die, which is wasteful of planting material. 7. CONCLUSIONS If vegetation is to serve a true engineering role, it is essential that the intended engineering function of the vegetation is firmly established and incorporated into the design of the remedial works. This implies that the failure mechanism that the plants are intended to resist is correctly predicted, which requires a careful assessment of soil and water conditions on the Lawrance 10 Figure 8. Grasses (Neyraudia arundinacea) planted diagonally on a trimmed slope. A line of Indigofera atroturpurea, a small leguminous tree, is planted along the toe slope, and of slope degradation processes. In the tropics, where storms can be very intense, it is important to design an element of 'survivability' into the planting scheme, so that if the site is subjected to heavy runoff in its early life, loss is not total. A design to improve a site' ability to withstand heavy runoff may reduce its capacity to stabilise the slope, but since maximium efficiency in this respect cannot be achieved in less than five years, survival to maturity is at first more important. The engineering function of the vegetation is also important when a planting scheme is combined with a 'hard' engineering measure, especially if the latter has a life of only a few years. The engineering function of the hard structure must be equivalent to that of the vegetation, and its life span should be sufficient to afford protection to both the site and the planting scheme, until the plants reach maturity and take over the engineering role from the decaying temporary measure. ACKNOWLEDGEMIENTS The research programme was -carried out-as part~ of the work of the -Overseas Centre, TRL, funded by ODA. ODA's liberal view of the research procedure made it possible to gain experience on the broad front described in this paper within the short space of a few years. The degree of co-operation between the research teams and Roughton International was exceptional, and contributed substantially to the rate of progress and high level of success. Crown Copyright 1994. Thne views expressed in this paper are not necessarily those of the Overseas Development Administration.