Diti Hengchaovanich, M.Eng., P.E.
CEO/Director, Erocon Sdn Bhd,
Kuala Lumpur, Malaysia
Use of vegetation for erosion control and slope stabilization has been made since time immemorial usually based on empirical methods or precedent cases. The popularity of vegetation over the last decade has been due to heightened awareness of environmental problems and availability of knowledge and information for use in design. Vetiver is, until recently, a relatively unknown plant with several unique features found to be ideal for erosion control and stabilization measures. Parameters obtained from recent experiments are presented that should assist engineers to assess with more confidence the effectiveness of his designs or construction incorporating vetiver. Successful applications of vetiver for erosion and slope stabilization on highway projects in Malaysia are described.
Over the millennia, Nature has 'designed' vegetation as a means to blanket and stabilize the good earth. In the humid tropics in which Malaysia is located, this has evolved into rainforests comprising complex multi-strata canopy, from big trees, shrubs and leaf litters, covering the organic humus-rich topsoils, that offer excellent overall protection.
With development, be it agricultural, infrastructure or townships, vegetation has to make way for this inevitable encroachment. As a consequence, there arise, and on an increasing rate, problems relating to erosion and stability in view of scarcity of land on the plains as people move further hinterland into the hilly and mountainous terrain.
In the light of the current awareness and conscientiousness of environmental issues, the preferred option to address the above problems would be to go back and seek solutions that Nature has already provided in the first instance to human prior to his ravaging; that is, by way of revegetation. This may be, as it were, a way of 'atoning' the wrongdoings to the ecology !
From ancient times, it has been on record that people have used vegetation as a means of erosion control and slope stabilization. For instance, it has been documented that the Great Wall of China, the ziggurat in Baghdad, used reed for soil reinforcement purpose. During the Ming Dynasty in China some 400 years ago history recorded that willow was used for embankment stabilization. In India, where the vetiver grass originated, the local farmers have for centuries used it as a soil binder; vetiver strengthens bunds and creates boundaries for paddyfields as well as fortifies river banks, canals, ponds etc. to keep the land from collapsing into water. The farmers know that it would work but cannot really explain how or why. When Indians moved overseas, presumably the grass was brought along by them and the usage continued in the new localities. Thus one sees Indian cultivars in far flungs corner of the globe.
Near Kuala Lumpur, Malaysia, it was mentioned that vetiver was in 1908 planted for the purpose of holding up steep banks (Ref. 1).
After the World Bank Agricultural Advisers stumbled upon and 'discovered' the vetiver grass, promotion of its use has gained impressive momentum since the 1980's but mostly in the field of 'soil and water conservation' in the agricultural sector with success stories to boot. However, it has also been observed that sloping lands planted with vetiver grass are less susceptible to landslips apart from having less or controllable erosion problems.
Use of vetiver grass in the engineering circle has yet to catch up with its agricultural counterpart. To date there are pockets of its use here and there with some beneficial results based on precedent cases, rule of thumb or trial and error basis. Part of the reasons for its limited use could be due to lack or
dearth of 'quantitative parameters' to plug into elegant mathematical formulae to come up with computed figures of which the engineer is so fond before he finds it acceptable or comfortable with and eventually convinced, like other engineering materials. With current trends now favouring 'green approach' to environmental problems and more databases now become available, this attitude is gradually changing for the better.
It is the aim of this paper to disseminate the information that there is now some quantitative parameters available especially for vetiver grass vis-à-vis other vegetation (albeit in the preliminary stage) and to present those findings and cases of successful applications of vetiver grass, in the Malaysian context.
2. PENINSULAR MALAYSIA - TOPOGRAPHY AND CLIMATE
Malaysia lies near the Equator of the South-East Asian region. Peninsular Malaysia, where development and population are mostly concentrated, is formed from mountain ranges with low-lying coastal riverine plains. The Main Range with a series of roughly parallel, north-south ridges rising to over 2 000 m, traverses the peninsular spine-like. Deep weathering characterizes most of the country ground profile (Fig. 1).
Malaysia is subjected to both South-West as well as North-East monsoons. The former lasts from April to September and brings with it rains to fall on the West Coast and some part of the interior, while the latter, developing from high pressure trough originating in Siberia, always brings the moisture-laden clouds from the South China Sea to rain on the East Coast states and their vicinity from October to March. Mean annual precipitation ranges from 1 750 mm to 2 500 mm. In the highlands in the interior, mean annual rainfall rises to 3 000 mm, as it is vulnerable to both monsoons and can go up to 3 500 mm in some wet years. Daily totals with a 10-year return period vary from 125 to 150 mm over most of the country (Fig. 2).
With such heavy and intensive rainfall pattern and the grounds being composed of residual soils derived from the weathering of granite or other rocks which are mostly sandy/silty in texture, erosion and slope stability problems are acute and chronic problems in Malaysia (Fig. 3, Fig. 4 & Ref. 2).
In fact, during the monsoon, the news items often appearing in Malaysian newspapers are stories of either earth slips or landslides.
3. VEGETATION FOR SLOPES STABILITY AND EROSION CONTROL
As stated in the Introduction, use of vegetation for slope stabilization started many centuries earlier, revival of the ancient practice and current development started in the Germanic-speaking countries (Austria, Switzerland, Germany) in the 1930's and spread to the U.S and Canada in 1970/80's with more relevant research being undertaken.
That a blanket of dense grass or herbaceous plant is able to protect against rainfall-induced erosion is generally well accepted by commonsense. However, it may not always be true for Malaysia, because, as mentioned in the Introduction, of the high and intense rainfall prevalent in the country compounded by presence of erodible soils. Even a well-designed slope (plus good grass cover) having satisfactory factor of safety (say F.O.S. >1.3) for mass stability still suffers from shallow mass moment with 0.3 -1.5 m depth of slip surfaces from time to time.
Confronted with this problem, this Author has since 1983 conceived of the idea of further stabilizing the slopes using trees, in particular those fast-growing species, e.g. Acacia spp and Eucalyptus spp, and others mostly originated from Australia. This is to arrest the shallow mass movement. It worked well on all the slopes planted (total no of tree planted > 100,000) and the results are published in Ref.
2. No theoretical or quantitative figures, however, were provided to back up the favourable qualitative outcome of erosion control and stability enhancement of slopes.
In recent years, a number of researchers (Ref. 3 & Fig. 5) have investigated the factors contributing to the stability of slopes by vegetation and they concluded that these comprise hydrological and mechanical mechanisms. The hydrological factors are rainfall interception and evapotranspiration, hence pore pressure reduction (positive effect) together with increase in infiltration, permeability (negative effect). The mechanical factors are surcharge of vegetation weight on slope plus resistance to wind (negative effect) and root reinforcement (positive effect). Although data are not yet so extensive but sufficient enough for conclusions to be drawn that the net effects are
Ref. 4, and Fig 6. shows the increase in factors of safety by the presence of roots vis-à-vis no-root scenario.
4. VETIVER AS A UNIQUE VEGETATION FOR SLOPE STABILIZATION AND EROSION CONTROL
Although vetiver (Vetiveria zizanioides) belongs to the grass family (Graminae), it is unlike other grasses, just as bamboo which is considered a special grass. Architecturally it looks like lemongrass and keeps its leaves up off the ground and is bottom heavy (no falling over). Its leaves are somewhat like sugarcane but smaller. The stems which act like the backbone of the erosion control barrier are strong, hard and lignified (as in bomboo), they act like a wooden palisade when planted on contour across the hill slopes (Ref. 5).
Of these characteristics, two properties stand out that make it ideal for erosion and slope stabilization work, namely,
1) It grows upright and with its stiff stems is able to form a dense hedge in 3 - 4 months and thus capable of slowing down rainfall run - off, distribute it uniformly, filter it and trap transported or eroded sediments at the hedge face. The hedge height is self adjusting in tandem with trapped silts.
2) It has a vigorous, strong, deep and massive root system that can penetrate up to 5 m underground (see Fig. 7), depending on soil condition.
The Author would hasten to add the third characteristics (pending more investigation to verify his preliminary experiments) that because of its massive and deep root networks and abundant, long leaves, vetiver would tend to help increase the slope stability through soil moisture depletion (i.e. soil suction phenomena) via the process of evapotranspiration (Fig. 9). This theory might contradict the conviction of the agriculturist that it helps introduce and conserve moisture into the ground by filtering and infiltration at the hedge! (Perhaps the steepness of slope land in agriculture which is seldom more than 20% compared with > 100% for engineering works could account for such difference).
As to the erosion control properties, there have seen studies carried out by several Malaysian workers (Dr. K.F. Kon and Dr. F.W. Lim, Ref. 6) which showed that compared with bare soil, vetiver was able to control run-off to reduce by 73% and the total eroded soil (soil loss) by 98%. Recent study (Ref. 7) at the Universiti Kebangsaan Malaysia (UKM) revealed that vetiver could trap 600 gm/m< of surface soil loss against 18 gm/m< by cow grass (Axonopus compressus).
With regard to contribution of the vetiver root to the stabilization of slopes, it was reported by this Author and his co-worker (Fig. 7 & Fig. 10 and Ref. 8) that the tensile strength of vetiver was in the order of 75 MPa (or approx. one third of that of mild steel). Comparing with other types of the roots (Table 1), vetiver root is stronger.
Table 1 Tensile strength of roots
Botanical name Common name Tensile strength MPa
Salix Willow 9-36*
Populus Poplars 5-38*
Alnus Alders 4-74*
Pseudotsuga Douglas fir 19-61*
Acer sacharinum Silver maple 15-30*
Tsuga heterophylia Western hemlock 27*
Vaccinum Huckleberry 16*
Hordeum vulgare Barley 15-31*
Grass, forbs 2-20*
Moss 2-7 kPa*
Veitveria zizanioides Vetiver grass 40-120 (Average 75**)
* After WU (1995), Ref. 9
** After HENGCHAOVANICH AND NILAWEERA (1996), Ref. 8
Moreover, because of its dense and massive root system underground it offers better shear strength increase per unit fibre concentration (i.e. 6-10 kPa per kg of root per m> of soil) compared to 3.2 ~ 3.7 kPa per kg of root/m> of soil for tree roots (Ref. 4 and Ref. 8).
The fact that vetiver can grow vertically on steep slopes (more than 150%), faster growing and imparts more reinforcement to the soil makes it a better candidate to consider for slope stabilization than other plants. Another less well known characteristic and sets it apart from other tree roots is its power of penetration. Its 'innate' strength and vigour enables it to penetrate through difficult soils, hard pan or rocky layers with weak spots; it even manages to punch through asphaltic concrete pavement (vide Fig. 8).
Indeed, one can say that vetiver roots basically behave like 'living' soil nails or dowels of 2-3 m depth commonly used in 'hard approach' slope stabilization work.
5. APPLICATION OF VETIVER ON HIGHWAY PROJECTS IN MALAYSIA
A small trial on the cut slope of the North-South Expressway was carried out by others in 1991, followed by major planting by us (30 km of running hedgerow length so far) since 1993, on slope remedial works projects on the East-West Highway funded by Public Works Dept. (PWD), Government of Malaysia. Vetiver grass have been used on the above projects to complement the engineering designs to enhance the erosion control and slope stability aspects in view of the adverse soil and climatic conditions on the East-West Highway cited in Ref. 2 to provide added assurance or 'bonus'.
On these projects, vetiver grass has been used to trap silt at culvert inlets and outlets, alongside cascading drains, interceptor drains, canal banks and on the slopes as hedgerows on contours at 1 or 2 metre vertical height interval (vide Fig. 11 to Fig. 14). Planting of vetiver was governed by strict specifications as to the quality of planting material and the planting technique itself. From observations, the vetiver grew well, with hedgerow gaps closing in 3-4 months. Silts were trapped thus keeping drains and culverts clean and surface sloughing or shallow mass movement was non-existent. To dispel the doubt as to its long rooting depth, a wash-excavation was carried out for the client's benefit, which revealed an attainment of 3.6m penetration (Fig. 15).
Vetiver planting (4.5 km) was extended to another PWD-funded contract on Jalan Gunung Raya, Langkawi, on similar slope remedial works project involving substantial number of soil nails. The results have also been successful (vide Fig. 16).
On the Kuala Lumpur-Karak Highway Privatisation Project, vetiver was used for cut slope stabilization at locations where soil is known to be relatively weak. However, a majority of plantings are on spoil disposal areas (approx. 42 km). This is to check the loosely-dumped spoil from collapsing and to trap silts from washing downslope into nearby rivers, a Dept. of Environment directive. This has proven effective to date (vide Fig. 18 to Fig. 21)
A small-scale trial, was carried out to stabilize and to render erosion control for a very highly erodible embankment built of silty sands on Kuala Kangsar-Gerik Road Upgrading Project. This is shown in Fig. 17. The results have been impressive to the Government-appointed consultant supervising the works.
The projects cited above were undertaken by us and proven effective and satisfactory to the clients. It has been brought to attention that there were instances where vetiver hedgerows were not successful elsewhere in Malaysia. On inquiry, it was found that mostly those failures could be attributed to poor agricultural practices e.g. poor planting material, incorrect planting techniques, stunted growth due to competition for sunlight or space by other grasses, legumes or weeds due to no maintenance etc. It is not within the ambit of this paper to discuss failures but suffice it to say that like other engineering undertaking, quality control and supervision play a vital role for the achievement of success.
6. SUMMARY AND CONCLUSIONS
Over the last decade, people are adopting 'green approach' for erosion control and stability problems due to greater accent being placed on environmental issues.Vetiver, for a long time unknown and ignored, started to gain acceptance prominence due to heavy promotion by the World Bank, mostly in the agricultural sector with reported success stories. Application in engineering is still somewhat limited due to lack of knowledge and availability of design parameters which now start to emerge from recent researches. Compared to many countries, Malaysia has made good stride in the use of vetiver grass for erosion control and slope stabilization in highway engineering prompted by necessity due to the nature of its highly erodible residual soils and adverse rainy weather conditions. However, the design is still somewhat conservative, treating vetiver as 'a bonus' or added assurance. Once more data and track records come to light, especially on the evapotranspiration and hydraulic aspects, bolder and more innovative designs maximizing the full potential of vetiver grass should be adopted.
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2. NARAYANAN, A and HENGCHAOVANICH, D (1986). Slope Stabilization and Protection for Roads in Mountainous Area with High Rainfall, Proc. 13th ARRB-5th REAAA Combined Conference, Road Engineering Association for Asia and Australasia / Australian Road Research Board, Adelaide.3. GREENWAY, D.R. (1987). Vegetation and Slope Stability, in Slope Stability, edited by Anderson and Richards, John Wiley and Sons, New York.
4. GRAY, D.H. (1994), Influence of Vegetation on the Stability of Slopes, in Vegetation and Slope, edited by D.H. Barker, Institution of Civil Engineers, London.
5. NATIONAL RESEARCH COUNCIL (1993) Vetiver Grass: A Thin Green Line Against Erosion. National Academy Press, Washington, D.C
6. KON, K.F. and LIM, F.W. (1991), Vetiver Research in Malaysia - Some Preliminary Results on Soil Loss, Runoff and Yield, in Vetiver Newsletter. No 5, March 1991, Vetiver Information Network, ASTG, World Bank, Washington D.C.
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Kebangsaan Malaysia, Appendix 4.
8. HENGCHAOVANICH, D. and NILAWEERA, N.S. (1996), An Assessment of Strength Properties of Vetiver Grass Roots in Relation to Slope Stabilization, Proceedings, 1st International Conference on Vetiver, Chiang Rai, Thailand (in press).
9. WU, T.H. (1995), Slope Stabilization, in Slope Stabilization and Erosion Control : A Bioengineering Approach, edited by R.P.C. Morgan and R.J. Rickson, E&FN Spon/Chapman and Hall, London.