15 YEARS OF BIO ENGINEERING IN THE WET TROPICS

from A (Acacia auriculiformis) to V (Vetiveria zizanioides)

Diti Hengchaovanich
APT Consult Company Limited, Bangkok, Thailand

(Paper prepared for the First Asia-Pacific Conference on Ground and Water Bio-engineering, Manila April 1999)

Key words: hydroseeding, acacia, vetiver, erosion, stabilisation

 Diti Hengchaovanich graduated in civil engineering from Chulalongkorn University, Bangkok, in 1967 and received his Master’s degree in geotechnical engineering from the Asian Institute of Technology (AIT) in 1969. Over the last 15 years he was responsible for remedial works for some 200 failed slopes in tropical mountainous terrain with extremely erodible soils, in regions prone to high and intense rainfall. Civil engineering solutions in combination with major bioengineering measures were the approaches rigorously undertaken. These had been proven effective both functionally as well as cost-wise. Diti Hengchaovanich has written technical papers and shared his experiences at several international conferences. He also serves as a specialist consultant both locally and overseas for slope stabilisation and erosion control projects. 

ABSTRACT

The Wet Tropics (i.e. the Southeast Asian monsoon belt) is subjected to one of the most intense and frequent rainfall incidences in the world. Infrastructure projects in this region are known to be fraught with rainfall-induced erosion, shallow mass movement or deep-seated stability problems caused by the absence of protective natural vegetation. Conventional ‘hard’ approach for solving these problems is gradually giving way to ‘soft’ or ‘bioengineering’ approach due to concern over the degradation of the environment coupled with the fact that more knowledge on vegetation, especially from the engineering aspects, has now come to light. The use of common grasses or legumes as surficial protection has long been practised but it has been found to be wanting in the Wet Tropics. As such, the use of trees with their root network to bind soil mass together is advocated. However, the problems in the interim before the establishment of even fast-growing species like Acacia auriculiformis are still extant. Vetiver (Vetiveria zizanioides), a unique grass with its penetrating root system that extends vertically down underground up to 3~4 m depths thus offers an equivalent if not faster and better alternative. Vetiver roots are very strong with an average tensile strength of 75 MPa(one-sixth the strength of mild steel). The massive root system therefore increases the shear strength of soil, thereby enhances slope stability significantly in particular the shallow-slip zone. Use of dead vegetation (e.g. bamboo) as a soil-reinforcing element (similar to geogrid) in steep embankments to counter deep-seated stability problems is described in this Paper.

  

1. INTRODUCTION

The Wet Tropics, a term of Australian usage connoting its north-east tropical area that receives huge amount of rainfall, is used in the context of this Paper to mean the Southeast Asian equatorial monsoon belt. This region sees one of the most intense and heavy rainfall incidences in the world with precipitation in excess of 2 000 mm per annum, rising to 4 500 mm in some exceptional wet years. Highway and infrastructure projects, constructed or founded on residual soils derived from the weathering of parent rock minerals, are known to be fraught with rainfall-induced erosion, shallow mass movement and/or deep seated stability problems caused in the main by the removal of natural vegetation in the first instance. Compounding the problems, some of these projects are located in the mountainous or steep topography, making runoff more rapid and hence accentuating the erosion and the destabilising processes.

Vegetation (dead or alive) for erosion control and slope stabilisation has been used since ancient times usually based on past experiences or empirical methods. The resurgence of this practice in a more scientific and methodical manner began in the 1930’s in Europe and became more popular over the last decade due to heightened awareness of environmental issues and availability of knowledge and parameters to aid as well as lending credence to the designs. In the Wet Tropics, grassing (inclusive of leguminous cover crops) by sodding/turfing or hydroseeding techniques as well as the planting of shrubs and trees have been used for quite a few decades for slope protection with varying degree of results. Grass or leguminous cover crops, although relatively good for slope protection, may be ineffective for intense rainfall area with highly erodible soils; hence the use of trees with its deep and stronger root networks to address the shallow mass movement problems. However, the problems in the interim (2~3 years) prior to the establishment of trees are still extant (even with fast-growing species like Acacia auriculiformis or Acacia mangium. Vetiver (Vetiveria zizanioides), a plant classified as a grass but behaves with tree-like features, is therefore suggested as a better alternative in the last 5 years. The concept of using plant for erosion control is well accepted for effectiveness and being environmentally friendly in addition to being aesthetically pleasing. However the use of plants, especially the utilisation of roots for the purpose of reinforcing soils in the shallow movement zones (slope stabilisation) in the manner of soil nailing/soil doweling starts to gain some grounds due to more parameters having come to light. The use of dead vegetation such as bamboo as a reinforcing element (akin to geogrids) for soil to address deep-seated stability problems have recently been tried with initial success, and described in this Paper. 

2. VEGETATION FOR EROSION CONTROL AND SLOPE STABILISATION

2.1 Erosion and stability problems identified

Although erosion and stability problems are fairly distinct, common usage tends to overlap, as problems are intertwined. Erosion is the natural process whereby external agents such as wind or water remove soil particles. In the Wet Tropics this involves rainfall which is responsible for the removal of surficial layers, resulting in rills or gullies of about 10~60 cm depth. Over time, rills and gullies deepen and these cause slopes to oversteepen, thus precipitating instability. Instability or deep-seated problems can arise on their own depending on slope geometry, inherent soil strength, ground or porewater characteristics. These are basically geotechnical/geological problems that have to be addressed by proper studies and analyses. With a variety of computer programs available, the evaluation of the stability of slopes to determine their ‘factors of safety’ against sliding or failures has now become less tedious or laborious. On the other hand, shallow-seated problems, which lie in the 60~250 cm depths, do not lend themselves for such accurate computation with available software. They present a chronic problem in the Wet Tropics with the attendant heavy rainfall and inherent highly erodible slope materials. However, it is believed this problem can be dealt with very effectively by bioengineering measures which will be described in subsequent sections. 

2.2 Vegetation for erosion control and slope stabilisation

Over the millennia, Nature has ‘designed’ vegetation as a means to blanket and stabilise the good earth. In the Wet Tropics 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. In the light of 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. That is, to reinstate those areas ravaged by human beings by way of re-growing vegetation. The methods of sustainable revegetation have to be studied and applied where appropriate for the problems at hand as described below.

2.2.1 Control of surface movement by grassing and/or leguminous cover crops

The use of grasses and/or leguminous covers for surficial protection is common in civil engineering or infrastructure works. In fact, it is stipulated in most standard specifications. The methods popularly employed are by seeding (manual or hydroseeding) and turfing/sodding. On occasions, geotextile mats (made from jute, coconut coir or paddy straw, etc) may be incorporated to render temporary protective cover, and to serve as mulch prior to and after biodegradation.

Turfing is carried out using broad-leafed carpet grass (Axonopus compressus), as it is the most easily available. It is a relatively shade-tolerant grass which thrives well in residual soils with high rainfall. In the highlands, the use of Guatemala grass (Tripsocum andersoni) has been noted. Elephant grass (Pennisetum purpureum) is employed on a few projects and found fairly successful. Turfing is traditionally the best method as it gives instant coverage. However, due to various extraneous factors, such as heavy demand, lack of good nurseries, labour shortage, it is being overtaken by hydroseeding on projects requiring mass production.

Hydroseeding is a relatively novel method for planting grass and legume seeds in Southeast Asia although it had been used in the United States prior to World War II. Basically, it is a method of embedding seeds onto slope face by means of slurry jetted from pump-equipped mobile mixing tank, which is usually truck mounted. The slurry normally comprises water, grass and/or legume seeds, fertiliser, fibrous mulch (paper) and tackifier. Hydroseeding technique is ideally suited for slopes with little or no access, steep slopes (especially cut slopes) over 35 degrees where normal turfing would be either very difficult or impossible; rocky or gravelly slopes with cracks, crevices and totally devoid of topsoils. It is a very fast and efficient operation, with 0.5 hectare coverage in an hour of spray. One innovative method which has been introduced and would find applications elsewhere is the method of corrugating or grooving of cut slope face by means of a modified motor grader blade. Such practice helps avoid the slurry mix from drifting down the smooth, steep slopes. It helps retain moisture on the grooves which will induce early establishment of seedlings in the horizontal ‘niche’ so formed (Fig. 1). The prevalent practice of grooving slope face vertically is incorrect and should absolutely be avoided (Fig. 2). It will just hasten the drifting down of seeds and expedite the erosion process. In the Wet Tropics, residual soils are rather acidic or sodic. Hence, a soil ameliorant such as lime or gypsum may be needed to correct the pH and facilitate growth.

Mostly the seeds used are signal grass (Brachiaria decumbens), narrow-leafed carpet grass (Axonopus affinis), Bermuda grass (Cynodon dactylon), Guinea/Hamil grass (Panicum maximum) etc imported from Australia, as well as Ruzi grass (Brachiaria ruziziensis) produced in Thailand. Legume seeds commonly used are Centro (Centrosema pubescens=CP), Calopo (Calopogonium mucunioides=CM) and Puero (Pueraria javanica=PJ) widely used in the plantation sector. One of the major problems faced by the hydroseeding specialist is the quality of seeds, which also has bearing on the overall cost. It is important that the seeds are genetically pure (i.e. true to label), physically pure (i.e. free from contaminating materials) and of fresh stock, with due consideration for dormancy period, to guarantee high germination rates (Yates, 1975). Seed germination tests are strongly recommended to be carried out to determine beforehand the likely germination rates that will result in field practice. Moreover, it can help preempt the pitfall of having large stock of bad seeds.

Hydroseeding and turfing is successful in areas where the soil is not highly erodible (i.e. it has some cohesion) and it is not carried out in the midst of a heavy monsoon period. Nonetheless, because grasses or legumes require certain time duration for growth they cannot be instantly of any benefit as intended in the design; therefore in the interim, it is sometimes proposed that some kind of synthetic or natural fabrics be used to blanket the bare slopes first prior to plant establishment. These products are marketed under the trade names of geojute (soil saver), fibromat, geocoir, etc to reflect their main constituents. The functions of these geofabrics are: to absorb raindrop impact, to help conserve moisture, to hold seed and soil firmly in place, to serve as mini-check dams trapping seeds and soil particles and reduces water runoff. The biodegradable types will also add humus for the establishment of grass stands in 2~3 years when they disintegrate.

2.2.2 Mitigation of shallow-seated instabilities (shallow mass movement) by shrub and tree planting

Although grass can provide effective slope protection (when soil/climatic conditions are not extreme), its roots do not extend deep enough into the soil (generally in the order of 20~40 cm) to provide the grip and anchorage needed to prevent surficial slip in the event of heavy, prolonged rainstorm. The residual soils in this region with little or no cohesion, once subject to fines (soil particles whose sizes are smaller than 74 microns) being washed out by rainwater will see the collapse of the soil structure and thus liquefaction or soil flow. On many highways one can often see random patches of stone pitching, gunite (shotcrete/cement spray) which are conventional rectification work carried out for the shallow-seated or shallow mass movement problems. It is not unusual to see a repeat of these repair works if the real root cause of the problems is not identified and solved accordingly.

In some cases, reconstruction was carried out by earthwork method. It was observed that if some timber stakes were stuck into the ground at the collapsed area it would help hold up the spot pretty well, a soil nailing/ doweling concept in the work but not well known at that point in time in 1985. Thus, analogically, the author conceived of the idea of ‘wickerwork’’ terracing which could be considered a modified form of contour-wattle staking on slope (Gray and Leiser, 1982). It was tried out to rehabilitate a spoil tip adjacent to a new embankment. The rehabilitated slope would help ensure that the new embankment would not be dragged down due to an adjacent collapse (Fig. 3).

Basically, the technique involves the staking of 30 mm x 75 mm x 1.5 m long timber posts into slopes along contour interval of 5 m. The stakes, driven at 600 mm centre-to-centre spacing, were woven in between by means of bamboo cuttings obtained from adjacent hills. Fast-growing shrubs and trees were grown behind the stakes. The aim was to create ‘hedgerows’ with the aid of stakes plus bamboo to minimise or block eroded sediments and to hold the slope up, pending the establishment of shrubs and trees. Some 12 years later, on a visit to the site, it was found that the site had turned into a mini-jungle as trees had already colonised the place. There was no trace of any slope failure or collapse to be seen.

The observation as mentioned above convinced the author that he had been on the right tract in the use of trees and shrubs to arrest the shallow-seated stability problems. The advantages of trees and shrubs are to bind and reinforce the shallow portion of the soil surface by their root networks, to induce the soil water depletion (pore pressure dissipation) through evapotranspiration as well as buttressing and soil arching action from embedded stems. Besides, trees also provide canopy by their leafy branches to absorb or break the impact energy of raindrops. Studies by Greenway (1978) as well as CIRIA (1990) provide detailed researches on the roles of vegetation on slope stability.

One major factor responsible for slope stability is the role of roots. Roots (or ‘inclusions’) impart apparent cohesion (cr) in similar fashion to ‘soil nailing’ or ‘soil doweling’ in the reinforced soil principle, thus increasing the factor of safety of slopes permeated with roots vis--vis no-root scenario (Gray, 1994).

Interestingly, Wright and Upadhayaya (1998), at the Agricultural Research Service of the U.S. Dept of Agriculture (USDA), have discovered a unique fungal protein that may be the primary glue (nicknamed ‘superglue’) that holds soils together. The protein is named ‘glomalin’, for Glomales, the scientific name for the group of common root-dwelling fungi that secrete the protein through hairlike filament called hypha.

Shrubs and trees which have been used by the author on a number of highway slopes are Mexican lilac (Glyricidia sepium), wild tapioca (Glochidon wallichianum), yellow wattle or acacia (Acacia auriculiformis), Acacia mangium, Eucalyptus camaldulensis, with some trial on Leucaena leucocephala (on low land), Anthocephalus chinesis and Duabanga grandiflora for rocky slopes, etc.

Mexican lilac is a leguminous plant normally used on cocoa plantations as shade-provider while wild tapioca can commonly be found in villages in Southeast Asia. These plants are easy to grow simply by inserting cuttings onto side slopes. They are able to thrive on infertile, compacted materials of roadway embankments. Due to its slenderness, Mexican lilac does not uproot in strong winds nor does it block sunlight to the grass undergrowth.

Wild tapioca, on the other hand, is fairly bushy to offer canopy to screen raindrops and is extremely resilient to poor soils and drought. The two types of plants are therefore used complementarily, along 3-m contour at approximately 3 m spacing. Excavation for rooting length determination has been conducted on Mexican lilac and it was found to extend 5 m along the slope inclines; while it was 7 m for wild tapioca (Narayanan and Hengchaovanich, 1986). It was also found that wild boars that abounded in the locality liked to feed on wild tapioca. As such, they have ravaged the ground in search of the tubers and thereby ‘pockmarked’ the slopes. The use of wild tapioca was unfortunately therefore abandoned after some time.

Yellow wattle was first used on roadside shoulders, toes, and platform areas. As it tended to shed too many leaves, this was later changed to Acacia mangium. A. mangium grew fast, shed few leaves, and seedlings could be produced easily in site nurseries. Subsequently, to effect a ‘reforestation’ on deep embankments, acacias were planted onto slopes (Fig. 4) on a major scale (to date some 200 000 trees have been planted). It can be said that reasonably good successes have been achieved with Acacia spp. One drawback that is often encountered with Acacia mangium is that its branches are rather brittle and prone to snap in the event of strong winds which occasionally accompanies rainstorms. From another perspective, Hill (1998) reckoned, however, that acacia has not solved erosion problems because ‘it initiates substantial stem-flow, possibly even hundreds of litres a day during rain, resulting in local scouring. Work by Zhao (1996) in China seemed to suggest that at high rainfall intensities, drip from leaves is even more erosive than direct exposure to rain.’

Even though acacia is fast growing, it still takes at least 2~3 years for the plant to mature, ‘establish’, and to become effective. Moreover, in steep-battered portions (transition zones between man-made and natural slopes), it is rather difficult to grow acacia there. Thus when vetiver (Vetiveria zizanioides) was introduced some 5 years ago, it was decided to try out its effectiveness and to see whether it can overcome some of the deficiencies of acacia.

2.2.3 Erosion control and slope stabilisation by vetiver grass

Unheard of by most people up to the early 80’s until being actively promoted in the agricultural sector by Dick Grimshaw, then of the World Bank; vetiver grass had actually been used a few centuries earlier in India, apart from the extraction of its aromatic oils, to strengthen bunds, creates boundaries in paddy field, fortified pond banks, etc. When Indians moved overseas, vetiver was brought along with them to new localities. Thus one finds that vetiver has already been used in places like Mauritius, the Caribbean, Fiji, etc for roadside stabilisation for well over half a century, in addition to being used on sugarcane plantations for erosion control or as hedge barriers against weed or for boundaries purpose (NRC, 1993). In Kuala Lumpur, Malaysia, it was on record that vetiver was planted in 1908 for the purpose of holding up steep banks (IBRD, 1995).

With funding and support from the Public Works Department (PWD), Malaysia, four trial embankments were constructed in August 1993 for the purpose of observing the field performance of vetiver to evaluate its potential use for engineering purposes. Three months after planting, an exceptionally heavy monsoon hit Peninsular Malaysia, which caused numerous failures along major highways and hillside development projects. It was found that slopes planted with vetiver grass were not significantly affected although some slopes in the vicinity failed. In mid-April 1994, an excavation exercise was conducted to determine the rooting depth of vetiver. It was found that the massive root networks had reached a record depth of 3.6 metres after 8 months of growth (Fig. 5). Other conclusions drawn from these trial included: vetiver could grow rapidly to form a complete hedgerow which managed to trap wash-off soil material; from rooting depth monitoring exercise, it was evident that the roots managed to penetrate the harder stratum (with fragments of rocks). They not only grew vertically, but some seemed to incline themselves, following the side slope profile. From the limited trials carried out, results appeared encouraging and promising indicating the tremendous potential of vetiver for slope protection and stabilisation work. In a way, to draw a parallel to the rediscovery of acupuncture in medicine, the exercise above and the one which is going to be mentioned below is, to cite Dafforn (1998), in fact, an exercise in ‘scientifically validating an effective traditional practice’.

In response to a number of engineers’ call for more parameters for a wide range of vegetation categories, so that they can plug them into elegant mathematical formulae in their designs, the author and his colleague decided to carry out some experiments to include vetiver among the list of vegetation with available parameters. One of the experiments involved tests on gain in shear strength in soils by the presence of vetiver roots versus identical soils which are root-free. By conducting large-scale direct shear tests at an embankment at varying depth levels, the increase in shear strength can be determined. This is plotted in Fig. 6. It is also important to determine the root tensile strength properties in the process of evaluating a plant species as a component of slope stabilisation. This is because when a plant root penetrates across a potential shear surface in a soil profile, the distortion of the shear zone directly resists shear while the normal component increases the confining pressure on the shear plane.

For the determination of root tensile strength, mature root specimens were sampled from 2-year-old vetiver plants grown on an embankment slope. The specimens were tested in fresh condition, limiting the time elapsed between the sampling and testing to two hours maximum. The unbranched and straight root samples about 15~20 cm long were connected vertically to a hanging balance via wooden clamp at an end while the other end was fixed to a holder that was pulled down manually until the root failed. At failure, the maximum load was monitored. The tensile strength of root is defined as the ultimate root tensile force divided by the cross-section area of the unstressed root (without bark, as it has weaker strength properties). Fig. 7 illustrates the test set-up and the actual relationship between the root tensile strength and root diameter. As shown, the mean tensile strength of vetiver roots varies from 180 to 40 MPa for the range of root diameters 0.2~2.2 mm. The mean tensile strength is about 75 MPa at 0.7~0.8 mm root diameter which is the most common diameter class for vetiver roots. This is approximately equivalent to 1/6th(one-sixth) of the tensile strength of mild steel. Compared to many hardwood species, the average tensile strength of vetiver roots is very high. Even though greater tensile forces are required to break hardwood roots, their average root tensile strength values are lower than that of vetiver because their average root diameter is higher than the 0.7~0.8 mm average diameter of vetiver roots, vide Table 1 hereunder.

 

 

Table 1: Tensile Strength of Roots of Some Plants

 

Botanical name

Common name

Tensile strength (MPa)

Salix spp

Willow

9-36*

Populus spp

Poplars

5-38*

Alnus spp

Alders

4-74*

Pseudotsuga spp

Douglas fir

19-61*

Acer sacharinum

Silver maple

15-30*

Tsuga heterophylia

Western hemlock

27*

Vaccinum spp

Huckleberry

16*

Hordeum vulgare

 

Barley

Grass, forbs

Moss

15-31*

2-20*

2-7kPa*

Vetiveria zizanioides

Vetiver grass

40-120 (Average 75**)

*After Wu (1995) ** After Hengchaovanich and Nilaweera (1998)

 

 

Moreover, because of its dense and massive root system underground, it offers better shear strength increase per unit fibre concentration (i.e. 6 ~19 kPa per kg of root per m3 of soil) compared to 3.2 ~3.7 kPa/kg per m3 of soil for tree roots. For a detailed account of these tests, the reader is recommended to refer to the paper by Hengchaovanich and Nilaweera (1998).

Concurrent with these tests, some preliminary experiments were also carried out to assess the evapotranspiration and soil moisture depletion characteristics by vetiver, as it was anticipated that vetiver would be able to deplete moisture in the soil thus lowering porewater pressure and thereby increasing suction under partially saturated conditions. This situation will have beneficial effects on slope stability from geotechnical point of view, apart from mechanical reinforcement by roots. With the above results, one can say that vetiver roots behave like ‘living’ soil nails or soil dowels that are now being widely used for steep slope stabilisation processes. The difference is that there should not be any concern over long-term corrosion prospect of metallic nails to worry about. On the contrary, ‘living nails’ even improve with the passage of time.

While the dense underground root webs offer shear strength increase to soils resulting in slope stability enhancement, the upper parts of vetiver or the stiff stems act like a living barrier to trap silt in the erosion control process. Because of its rapid and vigorous growth rate, it can form a dense hedgerow if planted in that pattern in only 3~4 months and is capable of slowing down rainfall runoff, distribute it uniformly, filter it and trap eroded sediments at the hedge face. (The author wished he had been introduced to vetiver much earlier! cf Sect. 2.2.2 on wickerwork hedgerow). It is noteworthy to highlight the fact that the hedgerow adjusts itself in tandem with trapped silt on the upslope, thus ensuring that it will never get itself buried and die off. A number of studies have been conducted on the erosion control properties of vetiver with figures of its silt-trapping capacities being 6~35 times greater than that of other grasses or plants. Fig. 8 portrays a completed slope with vetiver hedgerows functioning both for slope stabilisation and erosion control.

Another salient feature of the vetiver grass root is its power of penetration. In a research conducted by Chalothorn et al (1998), vetiver was found to break through hardpans as thick as 15 cm, with root extending down to 74 cm below ground level. In other words, ‘ breakthrough through breaking through’! (Vetiveria, 1997).

On slopes underlain with weathered rock, boulders, the penetrating vetiver roots will provide anchorage by root tendon action. Coupled with ‘soil superglue’ mentioned in Sect. 2.2.2 earlier, this should enhance slope stability even further.

Other outstanding characteristics of vetiver which make it an ideal plant for bioengineering as well as environmental purposes are described in NRC (1993) and Truong and Baker (1998). In a nutshell, it can be said that it is a grass which behaves more like a tree for erosion control and slope stabilisation purposes. It is also a versatile, hardy plant that is fast growing; it can survive in harsh environment and yet never poses as a weed, hence some like to call it Miracle Grass or Supergrass. The reader is recommended to visit the website on the Internet maintained by the Vetiver Network (https://www.vetiver.org) for more information. 

2.2.4 Use of bamboo strips as reinforcing element for deep-seated instability

Very much earlier before the reinforced earth concept as initiated by Henri Vidal (1966) becomes widely accepted, ancient people had already used vegetation (albeit dead ones) to enhance slope stability (Yamanouchi, 1986), e.g. the Great Wall in China (where twigs of tamarisks were used), the ziggurats in Mesopotamia (where reeds were used); and the use of willows to stabilise earth dykes by a certain Engineer Pan in China during the Ming Dynasty (Barker, 1994). Even as recently as a few decades ago, anecdotal account had it that tin miners in Malaysia always included grass stems in mine tailings for stabilisation, in the construction of spoil bunds.

By incorporating elements having tension into the soil (which lacks tension), a new composite construction material is created whose mechanism of reinforcement is derived from the generation of frictional forces between the soil and the reinforcement (Ingold, 1982).

Reinforced soil structures constructed because of the need to steepen side slopes to overcome space restraint are normally very costly, which can be attributed to the costs of reinforcing elements, either imported stainless steel strips or grids made from synthetic polymers. Costs can be substantially cut if local materials can be used in lieu thereof. Bamboo, an abundantly available, replenishable material native to Southeast Asia, which was used to substitute steel because of acute shortage during World War II, therefore comes to mind.

Bamboo as a material possesses a very high strength to weight ratio, with its tensile strength (265~388 MPa) nearing that of mild steel at 480 MPa (Abang Abdullah and Abang Abdulrahim, 1984). The chief drawbacks of bamboo as an engineering material are its long-term durability (i.e. prone to attacks by fungi, insects etc if not properly treated by chemicals) and its variability due to nonhomogeneity and anisotropy, being a naturally occurring and not manufactured material. However, using proven techniques applied to timber piles that have been long-lasting (> 50 years; Low, 1994) and when designing reinforced embankments using bamboo, due and generous allowance is provided when applying the tensile forces reckoned to be taken by bamboo strips (to account for variability), steep embankments using bamboo reinforcement la geogrid can be designed and constructed. Six steep, bamboo-reinforced embankments with side slopes varying from 1:1.2 to 1:0.85(v:h) have been successfully formed (Fig. 9), and open to service on an expressway in Malaysia for some 2 years already. To date there has been no faulting regarding their performance and yet they were constructed at considerably lower costs than the conventional reinforced soil walls. For slope protection, vetiver and geojute laid in between hedgerows and cover crops (such as Arachis pintoi and/or Arachis glabrata) are planted on top of the geojute. This ensures a good outer ‘armour’ for the reinforced soil block.

 

3. DISCUSSIONS AND CONCLUSIONS 

Once a slope is well designed geotechnically with appropriate factors of safety, it behoves that slope protection be properly implemented to ensure long-term stability especially for those high rainfall area and faced with highly-erodible soil materials such as the Wet Tropics as mentioned in earlier sections. There are 2 approaches whereby this can be implemented: the conventional ‘hard’ or ‘inert‘ approach favoured by the conservative engineers: using shortcrete/gunited surface or stone pitching (mortared riprap) or wire mattress, etc each of which is highly costly; or one can opt for the ‘soft’’ or ‘green’ approach which is much less expensive, aesthetically pleasing as well as environmentally-friendly.

Even for the ‘green’ or bioengineering solution, one has also to be judicious in selecting which measure one wants to employ to suit one’s needs based on climatic, soil types and budgetary constraint. For example in fairly dry weather condition, with the soil being relatively strong and cohesive, and also a nursery with good turf is located in the vicinity, it may suffice just to have turfing as slope protection, with decorative shrubs thrown in for landscaping or beautification purpose, budget permitting. Or in extreme conditions, on steep slopes where soils are highly erodible, rainfall copious and time is of the essence (otherwise the constructed slope can give way in no time), then vetiver planting should be seriously considered. On the other hand, if deep-seated stability is required of an embankment, an inexpensive reinforced soil embankment using treated bamboo as a reinforcing element should come in handy.

In implementing a bioengineering scheme, it must be borne in mind that it is a hybrid or cross-disciplinary approach and one is dealing with living things which can wilt or perish. It is therefore highly recommended that consultation with, or better still, the involvement of an agriculturist be sought. Otherwise the scheme will fail for incorrect or improper implementation and will give bioengineering a bad name. On the other hand, because the plant is being called upon to perform engineering functions that sometimes are quite ‘strenuous’, it too has to be subjected to stringent specifications, which can be enforced through good quality control procedures just like other engineering materials.

In addition, like all living things, plants need time to grow, mature and establish before they can truly ‘function’, plus the fact that some maintenance is required so that the ‘functioning’ is sustainable, a good maintenance programme in terms of fertilising, watering, weeding, etc is essential depending on the type of plants one is dealing with.

With good design during the planning stage, careful selection of quality planting materials that meet specifications, correct planting and maintenance techniques, it can confidently be said that good outcome will be achieved with the full potential of plants being realised. 

4. REFERENCES 

Abang Abdullah, A A and Abang Abdulrahim, A A (1984). Basic strength properties of a few selected Malaysian bamboos, J. Institution of Engineers, Malaysia, Kuala Lumpur.

Barker, D H (1994). Discussion to Session 4. D H Barker, Editor. In: Vegetation and slopes, Thomas Telford, London.

Chalothorn,C, Tapmat,P, Paen-oi,V and Nuraks,D (1998).Study on the penetration of vetiver roots in boring hardpans. Paper presented at the Internal Vetiver Seminar organised by the Dept of Land Development, Kanchanaburi, Thailand.

CIRIA (1990). Use of vegetation in civil engineering. N J Coppin & I G Richards, Editors. CIRIA/Butterworths, London.

Dafforn, M (1998). Know your hedge vetiver: environmental concern about Vetiveria zizanioides. Proc. First Int.Conf. on Vetiver, Chiang Rai, Thailand, pp 293-303.

Gray, D H (1994). Influence of vegetation on the stability of slopes, D H Barker,Editor. In: Vegetation and slopes

Thomas Telford, London.

Gray, D H and Leiser, A T (1982). Biotechnical slope protection and erosion control. Van Nostrand RheinholdNew York.

Greenway, D R (1978). Vegetation and slope stability , Anderson and Richards, Editors. In: Slope stability, John Wiley and Sons, New York.

Hengchaovanich, D and Nilaweera, N S (1998). An assessment of strength properties of vetiver grass roots in relation to slope stabilisation, Proc. First Int. Conf. on Vetiver, Chiang Rai, Thailand, pp 153-158.Hill, R D (1998). Personal communication.

IBRD (1995). Vetiver grass for soil and water conservation, land rehabilitation, and embankment stabilization,R G Grimshaw and L Helfer, Editors. IBRD/World Bank, Washington, D C.

Ingold, T S (1982). Reinforced earth. Redwood Burn Ltd, London.

Low, K S (1994). Personal communications.

Narayanan, A and Hengchaovanich, D (1986). Slope stabilisation and protection for roads in mountainous terrain with high rainfall. Proc. 5th REAAA/13th ARRB Combined Conf., Adelaide, Australia, 13(3): pp 152-161.

NRC (1993). Vetiver grass – a thin green line against erosion. N D Vietmeyer, Editor. National Research Council, National Academy Press, Washington, D C.

Truong, P and Baker, D(1998). Vetiver grass system for environmental protection,Technical Bulletin No 1998/1,

Chomchalow, N and Henle, H V, Editors. Pacific Rim Vetiver Network, Office of the Royal Development Projects Board, Bangkok.

Vetiveria (1997). Editorial in Newsletter No 2, October 1997, Pacific Rim Vetiver Network, Office of the Royal Development Projects Board, Bangkok.

Vidal, H (1966). Reinforced earth (in French), Ann. de l’ Inst. Tech. du Batiment et des Travaux Publics, France,No 228-229, pp 888-938.

Wright, S F and Upadhayaya, A (1998). A survey of soils for aggregate stability and glomalin, a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi, Plant and Soil, 198:97 - 107, Kluwer Academic Publishers, the Netherlands.

Wu, T H (1995). Slope stabilisation. Morgan, P C and Rickson, R J, Editors. In: Slope stabilisation and erosioncontrol: a bioengineering approach. F N Spon/ Chapman and Hall, London.

Yamanouchi, T (1986). The use of natural and synthetic geotextile in Japan. IEM-JSSMFE Joint Symposium ongeotechnical problems, 27-28 March, Kuala Lumpur, pp 82-89.

Yates (1975). Better pastures for the tropics. Arthur Yates and Co Pty, Queensland, Australia.

Zhao, G (1996). Function of forest with respect to water and soil conservation: theory of canopy influence on the distribution of rainfall kinetic energy. Acta Phytoecol. Sinica, 20(4), in Chinese with English abstract.