Plant Soil (2009) 324:43–56 DOI 10.1007/s11104-009-0130-y REGULAR ARTICLE Root morphology and effects on soil reinforcement and slope stability of young vetiver (Vetiveria zizanioides) plants grown in semi-arid climate S. B. Mickovski & L. P. H. van Beek Received: 25 March 2009 / Accepted: 28 July 2009 / Published online: 21 August 2009 # Springer Science + Business Media B.V. 2009 Abstract Currently used in many countries in the world, vetiver grass (Vetiveria zizanioides) applications include soil and water conservation systems in agricultural environment, slope stabilization, mine rehabilitation, contaminated soil and saline land remediation, as well as wastewater treatment. The root system morphology of vetiver was investigated in a small plantation growing on abandoned marl terraces in southern Spain. Root distribution with depth, laterally from the plant, as well as root parameters such as root diameter and tensile strength were also investigated. The profile wall method combined with the block excavation showed that the vetiver grass grows numerous positively gravitropic roots of more or less uniform diameter. These were generally distributed in the uppermost soil horizon closer to the culm base. In situ shear test on blocks of soil permeated with vetiver roots were carried out and showed a greater shear strength resistance than the samples of non vegetated soil. The root reinforcement measured in situ was comparable to the one predicted Responsible Editor: Alexia Stokes. S. B. Mickovski (*) Jacobs Engineering UK Ltd, Glasgow G2 7HX, UK e-mail: S.B.Mickovski@jacobs.com L. P. H. van Beek Utrecht University, Utrecht, The Netherlands by the perpendicular root reinforcement model. The stability of a modelled terraced slope planted with vetiver was marginally greater than the one of a nonvegetated slope. A local instability on one terrace can have a detrimental effect on the overall stability of the terraced slope. Keywords Vetiver . Roots . Shear . Slope stability . Terraces . Eco-engineering Introduction Currently used in more than 120 countries, vetiver grass (Vetiveria zizanioides (L.) Nash, also known as Chrysopogon zizanioides (L.) Roberty) applications include soil and water conservation systems in agricultural environment, slope stabilization, mine rehabilitation, contaminated soil and saline land remediation, as well as wastewater treatment (Truong and Loch 2004). Vetiver grass is a graminaceous densely tufted perennial grass native to tropical and subtropical countries (India, China, Philippines, Indonesia). This species grows 0.5–1.5 m high stiff culms in large clumps from a highly branched root stock (Erskine 1992; Truong 1999) which is the most impressive characteristic of the plant. The root system is composed of fibrous roots which can reach depths of up to 3 m (Erskine 1992; Truong 1999; Hellin and Haigh 2002; Ke et al. 2003) and is 44 usually deep enough in the soil to provide the grip and anchorage needed to prevent surficial soil slippage in the event of heavy prolonged rainstorm events (Hengchaovanich 1999). Therefore, the use of vetiver is promoted by the World Bank (1990) and The Vetiver Network (Truong, pers. comm.). Vetiver grass is reported to be able to withstand climate extremes: drought, long periods of inundation, temperatures −10°C to 48°C) (Erskine 1992; Dalton et al. 1996; Truong and Loch 2004), soil acidity and alkalinity (pH from 3.3 to 10.5) (Dalton et al. 1996). It is cultivated in many areas in the Far East for the essential oil produced in the roots (Truong 1999). Vetiver oil is widely used in the perfume industry as a basic element in perfume blends and as a fixing agent for the odours of more volatile materials (Sethi and Gupta 1960 in Mucciarelli et al. 1993). Despite the extensive use of vetiver grass to control erosion (Truong 1999), the previous studies on vetiver grass usually describe hydraulic behaviour of vetiver hedges (Meyer et al. 1995; Dalton et al. 1996; Rodriguez 1997), or report root system yields that are of commercial use in perfume industry (Salam et al. 1993; Maheshwari et al. 1999). Although the ability of vetiver to control mass wasting and soil slippage is said to originate in the ability of its deep roots to penetrate and hold the soil together (Hengchaovanich and Nilaweera 1998), no quantitative study has been carried out on the rooting pattern or resistance of its roots, apart from the study of vetiver root system growth in an Oxysol (Salam et al. 1993; Cazzuffi et al. 2006; Cheng et al. 2003). Recognising the possibility that the morphology of the root system of vetiver grass in European conditions can be very different from the root morphology recorded in other parts of the world (Ke et al. 2003; Mickovski et al. 2005), we chose to carry out a morphological study on the vetiver root system in a dry Mediterranean climate where vetiver is planted to reinforce the soil as well as to prevent soil erosion and mass wasting. A study of the root system morphology of vetiver would also provide data necessary for the calculation of additional cohesion and hence the soil reinforcing effect of its root system. The increase in additional cohesion value termed ‘root cohesion’, or added cohesion, due to presence of roots in the soil is reported to be proportional to the amount and biomass of roots present, or the root area ratio RAR (Endo and Tsuruta 1969; Ziemer 1981; Gray and Ohashi 1983; Nilaweera 1994), which vary Plant Soil (2009) 324:43–56 greatly among species and in space, especially within the soil profile. The large variation in the value of the additional cohesion is due to the vegetation cover, soil properties, but mostly on the root distribution and strength properties. Efforts have been made to quantify the additional cohesion with laboratory shear tests on rooted soils (Waldron 1977) or soils reinforced with fibres (Gray and Ohashi 1983; Wu et al. 1988; Shewbridge and Sitar 1989, 1996). In situ shear tests on soil blocks rooted with roots of woody species have also been carried out (Endo and Tsuruta 1969; Ziemer 1981; Abe and Iwamoto 1988; Nilaweera 1994; Wu and Watson 1998) and produced data that show increases in shear strength due to soil-root interaction. However, only rarely have grasses being investigated for their reinforcing effect on the soil (e.g. Tobias 1994; Hengchaovanich and Nilaweera 1998). In order to assess the reinforcing effect of vetiver roots on the slope stability in an arid Mediterranean region, eco-engineering study was carried out where the soil shear resistance was measured in situ for both fallow soil and soil rooted with vetiver roots. The test results were evaluated against an analytical model (Wu et al. 1988) and slope stability modelled using Limit Equilibrium (LE) method in an attempt to quantify the stabilising effect of roots on slopes where they are not anchored in a firm base. Materials and methods Site location and characteristics Vetiver grass (Vetiveria zizanioides (L.) Nash.) experimental plots were planted on a site in the proximity of Almudaina, Spain (X=729275 Y=4293850 and Z=480 m on UTM 30 s) during springtime. The climate characteristics of the site are described elsewhere (van Beek 2002; Mickovski et al. 2005) and fall well within the tolerances of vetiver (World Bank 1990). Vetiver plants were planted on a bench terrace (Fig. 1) which is potentially endangered by runoff and soil slippage after intense rainfall events. Cuttings of vetiver were planted in rows in the nursery with spacing of 0.3–0.4 m. The spacing between the rows of vetiver grass was approximately 0.4 m and their length 3 m to 4 m each. A nursery of vetiver plants was planted on the flat near the toe of the terrace in the same plant distribution as on the terraces to provide re-planting Plant Soil (2009) 324:43–56 ~ 0.4 45 ~ 0.4 ~ 0.4 ~ 0.35 ~ 0.35 crest toe slope ~ 0.35 Fig. 1 Plan and cross sections of the terraces planted with vetiver grass. All dimensions in metres material for the terraces and comparable plant material for investigation. To better estimate the potential effect of vetiver roots on the soil shear resistance at the time when the majority of the soil instability events occur (the beginning of the rain period), the present investigation was carried out in November. The 6months-old plants were well established on the site, and have proliferated multiple culms from the ones originally planted. It was considered important to test relatively young plants as the soil stability on the slope would be at higher risk in the earlier development stages of the plants where relatively little reinforcement is offered by the young roots. Soil The soil on the site where the vetiver nursery was established derives from Miocene marl (Mickovski et al. 2005). Albeit having a high clay content in which smectites dominate, most of the soil particles fall in the silt fraction due to a carbonate content of 60 per cent or more. The bulk weight of the soil was 18.0 kN m−3 (dry bulk weight 14.6 kN m−3). The shear strength of the soil was determined in the laboratory by means of a standard small shearbox tests (BS 1377:1990). Thirty undisturbed soil samples with dimensions 60×60×20 mm were taken from the site, packed in air-tight containers and transferred to the laboratory where they were put in a small shearbox testing machine and sheared at a strain rate of 0.2 mm hr−1. Because of the dominance of the silt fraction, the soil had a high angle of internal friction of 34.5°±2° and cohesion of 4.5± 0.8 kPa. Profile wall method To obtain a close estimate of the overall root distribution as well as detailed information on the spatial patterns of vetiver roots, profile walls with vertical observation planes (Böhm 1979; van Noordwijk et al. 2000) were prepared for two plants (AA1 And D1) growing on the terraces. An investigation trench 0.8 m long, 0.8 m wide, and 0.6 m deep was excavated near the plant (approximately 10 mm from the culm base) and the observation plane smoothed with a long knife blade providing that the disturbance to the surrounding plants was minimal. No further preparations were needed since the soil and the roots had very good contrast in colour. Maps of root occurrence were also produced and analysed. The position of each root in the observation plane was mapped on a polythene (transparent plastic) overlay with a permanent marker pen (Böhm 1979). Each root was represented as a ‘dot’ with different colours used for different diameter classes. The diameter of each root on the profile plane was measured with callipers and the cross sectional area (CSA) was calculated as an area of a circle with a given diameter. Block excavation With an aim of investigation of the distribution of vetiver roots in depth, four plants growing in the nursery were completely excavated using the block excavation method (van Noordwijk et al. 2000). These plants were randomly selected from the plot, the soil surface in a radius 30 cm around them was carefully cleared from the litter, and the soil block with dimensions 0.3 m×0.3 m×0.5 m containing their roots was manually excavated using a spade. To minimise root loss or damage, the plants together with their root systems were hand-washed gently 46 Plant Soil (2009) 324:43–56 from the remains of the soil several hours after they have been excavated. Root systems were sprinkled under a low water flow from a sprinkler. For separating the last remnants of soil on the roots, it was necessary to soak the root systems in water basins and remove the soil by gently agitating the sample after which the soil particles settled on the bottom of the basin, and the broken roots, if any, floated on the surface. After the root systems were thoroughly cleaned, they were placed on a paper mat and left to dry in the open air for half an hour and the maximum lateral spread for the root system was measured for each plant. All the roots were then carefully cut off from the root bole with scissors; their diameter at the base (di) and near the tip (dj) was noted together with their length. The root cross sectional area (CSA) at a given depth was calculated as an area of a circle with an average radius d=0.5 (di + dj). Observing the strong positive gravitropic tendency in the rooting pattern of vetiver, it was assumed that all of the roots grow more or less vertically downwards and the length of each root was assumed to represent the maximum depth reached by the root itself. After the root data was recorded, a number of root samples from each excavated plant was taken and subjected to mechanical testing. In situ soil shear resistance To ascertain the effect of vetiver roots on the shear resistance of the soil, a stainless steel in situ direct shear box with dimensions 315 × 315 × 200 mm (plan area 0.1 m2, volume 0.02 m3) was used to measure soil shear resistance of four samples rooted with vetiver roots and of two fallow soil samples. These investigations were carried out in the same vetiver plantation where the block excavations took place, i.e. on a site where the roots grow in a thick layer of fairly uniform soil and shear failure could be forced through this layer. For the rooted samples, the plant that was to be investigated was first cleared from the debris around the culm taking care not to disturb the surrounding soil. The shear box was gradually slid in the soil using the knife-shaped lower edge until the top edge of the shear box was on the same level with the soil surface. Special care was taken to have the plant culm in the middle of the shear box, as well as not to inflict any damage to the surrounding root system. After the mounting of the shear box, the soil surrounding the box and towards the projected shear box movement was manually dug out to the depth of the shear box using a shovel. Before the start of the test the soil was saturated with water and left to drain for 2 h, simulating undrained conditions when most of the instabilities are likely to occur. A hand winch (Fig. 2) was connected to the shear box via an analogue spring scale (RS Components Ltd., Corby, UK) and strong non-elastic mountaineering cord and attached to the box at a specially designed link on the box. The other end of the hand winch was anchored on the trunk of a tree which was in line with the projected movement of the shear box. Each turn of the winch was equal to 1 cm of horizontal displacement of the cord, and the winch was turned for a whole turn every 30 s, thus gradually applying the shearing force while manually reading the scale every 10 s. The displacement of the shear box was measured using a measuring tape connected to the rear of the shear box. Turning the hand winch exerted shearing force on the soil block in the box, pulling it gradually towards the winch anchor until the soil finally yielded. For three shear tests (two on rooted and one on fallow soil samples) a normal load in form of concrete plates of 150 N was applied perpendicularly on the top of the box, while for the other three tests, the normal pressure applied was 300 N. During the shear process, Fig. 2 A schematic of the in situ measurement of the shear resistance of soil rooted with vetiver roots Normal load Vetiver plant Displacement Pulling direction Load cell Anchored winch Shearbox Plant Soil (2009) 324:43–56 the force and displacement were recorded, and any sounds of root failure were monitored. After the soil failed, the shear box was overturned and the shear plane was investigated for the overall root failure mode (slippage or breakage), and the number of roots intersecting the failure plane was recorded together with their corresponding diameters. Root mechanical testing Fifty root samples were taken from the excavated vetiver root systems and tested in tension in order to obtain their mechanical properties. Each of the samples that ranged between 50 mm and 80 mm in length and between 0.3 mm and 1.4 mm in diameter were clamped on both sides with the clamps of an universal testing machine (Adamel Lhomargy, Paris, France) equipped with a load cell capable of measuring forces of up to 1 kN and sensitive to 0.01 N. In order to minimise the damage to the roots and at the same time to provide better grip, the clamps were padded with a 2 mm thick layer of cork. The samples were then pulled apart at a rate of 1 mm min−1 while the resisting force was measured and plotted against the displacement on a computer connected to the testing machine. The tensile strength of the root was calculated as a ratio of the ultimate tensile force and the cross section of the root. A test was considered as valid if the root broke in the middle third of their length when subjected to tensile loads. Modelling of the root reinforcement effect of vetiver Measured root reinforcement was compared with predictions from a perpendicular rooting model using averaged root system parameters from the shear surface and the root tensile strength, (Waldron 1977; Wu et al. 1988; Mickovski et al. 2008). In this model, assuming all the roots crossing the shear plane break during the shear process, the magnitude of reinforcement due to presence of roots in soil is determined by (Gray and Sotir 1996): 47 Slope stability assessment of terraces planted with vetiver The stability of the slope where the plants were grown (Fig. 1) with and without the root reinforcement effect was calculated according to Bishop’s method (Bishop and Morgenstern 1960) using the Slope/W software (GeoSlope Ltd, Canada). For the purpose of this simulation, the stability problem was drawn to scale and material properties of the soil (unit weight, cohesion, angle of internal friction) were assigned to match the ones measured in situ and outlined above. The soil permeated with vetiver roots (the uppermost 0.3 m of the soil on the slope) was modelled as an area with increased soil cohesion (‘root cohesion’ cr, added to the existing soil cohesion) due to presence of the roots. The value of the root cohesion was calculated using the perpendicular root model procedure described above. Additionally, the global stability of a terraced slope was also investigated. For modelling purposes and to illustrate the effect of terrace width and spacing, a slope with two 5.0 m wide terraces was assumed, as opposed to terrace width of approximately 20 m observed in situ. Local terrace and global slope stability in terms of slope factor of safety (FoS; defined as the ratio between the stabilizing and destabilizing forces acting on the slope) was assessed for short-term (undrained; equivalent to sudden buildup of pore water pressures in the slope during and immediately after a rainfall event) and long-term (drained) conditions for fallow and reinforced slope. To investigate the possible ‘domino effect’ where the failure of the uppermost terrace and the surcharge it poses to the other terraces triggers global slope failure was explored. The surcharge value was taken to mimic the load imposed by a locally failed terrace. Results cr ¼ 1:2  Tr  RAR Vetiver grass root system morphology where cr [N m−2] is the ‘root cohesion’ or the increase in shear strength due to presence of roots in soil, Tr [N m−2] is the mean tensile strength of the average number of roots with an average diameter per unit area of soil and RAR [m2 m−2] is the root area ratio (the ratio between the total root cross sectional area and the total shear area). Profile wall method The analysis of the profile wall maps showed that the investigated plants had fibrous root systems with numerous roots (290 and 368 for both investigated plants respectively) of diameters ranging from 0.3 mm to 1.4 mm. Table 1 shows that the root systems of the vetiver plants consisted of shorter roots with larger diameters, while the diame- 48 Plant Soil (2009) 324:43–56 Table 1 Root system distribution with depth and laterally for Vetiveria zizanioides plants investigated with profile walling: number of roots with lengths corresponding to the depth classes, their total cross sectional area (CSA) and its percentage of the total CSA at different depths Plant: AA1 D1 Distribution with depth number of roots CSA (cm2) % CSA number of roots CSA (cm2) % CSA 0–5 cm 66 364.99 46.73 109 392.45 42.2 127.3 5–10 cm 49 16.3 73 180.27 19.4 10–15 cm 47 82.83 10.6 72 192.75 20.7 15–20 cm 46 79.79 10.21 55 117.29 12.62 20–25 cm 43 75.25 9.2 30 22.42 2.41 25–30 cm 30 30.77 3.94 30 23.97 2 2.58 2 Lateral distribution number of roots CSA (cm ) % number of roots CSA (cm ) 0–5 cm 73 263.97 33.8 165 450.00 48.38 5–10 cm 70 236.04 30.1 109 249.57 26.85 10–15 cm 69 173.57 22.23 56 114.17 12.22 15–20 cm 61 107.32 13.67 38 115.43 12.42 ters of the roots penetrating deeper in the soil were smaller. The profile wall investigations showed that the most of the roots of vetiver plants are distributed in the uppermost soil horizons (Table 1). The uppermost 15 cm of soil contained more than 70% of the root CSA for both of the plants investigated. Regression analysis showed that the root CSA decreased exponentially with depth in both investigated plants (CSA=69.84 e−0.093 depth with R2 = 0.50, and CSA=118.75e−0.125 depth with R2 =0.74 for AA1 and D1, respectively), and only 2.5–4% of the root CSA was present between 25 cm and 30 cm depth. Lateral distribution of the root systems in these two plants showed that most of the larger roots were distributed closer to the culm. In AA1, there were 160 roots with length between 10 mm and 100 mm and these contributed to the almost 64% of the total root CSA, while in D1 274 roots with lengths less than 100 mm contributed to more than 75% of the total root CSA. Regression analysis showed that the number of roots (N) decreased linearly with the distance (D) from the culm (N=−3.7D+77.5, R2 =0.87 and N=−43.4D+200.5, R2 =0.96 for AA1 and D1 respectively), as does the total root CSA (CSA=−53.24D+328.33, R2 =0.97 and CSA=−113.91D+517.07, R2 =0.86 for AA1 and D1 respectively). % Block excavation The dimensions of the excavated blocks proved to be suitable for this investigation and no visible damage was done to the root systems since, as in the profile wall investigation and elsewhere (Mickovski et al. 2005), the root systems of vetiver did not grow wider than 0.3 m from the culm base or deeper than 0.5 m. Detailed root system investigation showed that the number of roots originating at the culm base ranged from 140 to 200 or 176.2±0.3 on average for the four investigated plants. The root diameter at culm base ranged from 0.2 mm to 2.4 mm or 1.17±0.05 mm on average for all roots investigated. Root diameter at the culm base did not significantly differ from the one at its tip for most of the analysed roots which showed that vetiver roots have a low rate of taper. Maximum root system depth for the excavated plants was on average 0.22±0.01 m with mean lateral spread of 0.26±0.02 m. On average for the four investigated plants 76.2 ± 0.1% of roots were present in the uppermost 0.1 m of the soil. Shear strength of the soil In situ soil shear resistance Under undrained conditions, the shear resistance of the soil block in the shearbox first increased with the force applied to the box and peaked when the soil reached its ultimate Plant Soil (2009) 324:43–56 49 shear resistance. After the soil yielded, the resistance levelled off to its residual shear resistance value, a behaviour which is typical of normally consolidated clays. During the winching process sounds of root breakage were recorded after the maximum resisting force had been reached. Typical force-displacement curves of the direct shear test on a rooted and an unrooted sample are shown on Fig. 3. These tests confirmed the soil mechanical parameters of the fallow soil obtained by the laboratory shear tests described in the “Materials and methods” section. Figure 4 shows that the maximum soil shear resistance for fallow samples was 6.5 kPa under 150 N normal pressure (normal stress 4.4 kPa), and 10.2 kPa under 300 N normal pressure (normal stress 5.9 kPa). For the rooted samples, the peak soil shear resistance was 9.9 kPa and 10.1 kPa under 150 N normal load and 11.4 kPa and 12.5 kPa under 300 N normal load. The four tests on soil rooted with vetiver yielded a significant increase in soil shear resistance due to the presence of vetiver roots. On average, the presence of roots in the soil increased soil shear resistance by 2.7 kPa (ranging between 2.1 kPa and 3.7 kPa), or by 36% (ranging from 12% to 55%) compared to the shear resistance of fallow soil determined through laboratory shear tests (van Beek et al. 2005). Soil failure of the rooted samples occurred at strains that ranged from 11% to 24%, which was not significantly different from the strains when the soil Fig. 3 Shear resistance of a rooted and an unrooted blocks of soil tested in situ under 300 N normal pressure failed in fallow samples (16–17%). However, it is worth noting that soil rooted with vetiver roots in two samples exhibited higher ductility and was able to withstand much higher displacements without yielding. The investigation of the shear plane after the direct shear tests on rooted samples showed that the majority of the roots intersected the shear plane at right angles and that the overall mode of failure was a combination of root slippage and breakage, with more roots breaking than slipping which may be the reason for sudden drops in the shear resistance of the soil as shown on Fig. 3. The number of roots intersecting the 0.1 m2 shear plane (both broken and pulled out) for the four rooted soil samples ranged from 60 to 93, with an average diameter that ranged between 0.73 mm and 0.81 mm. The number of roots and the average diameter did not differ significantly (p>0.05) between the four rooted samples. The root area ratio (RAR) at 0.20 m depth ranged from 0.034% to 0.071% which is comparable to the RAR at 0.20 m depth of the profile wall plants AA1 and D1 (0.0305% and 0.0325% respectively) and to the four excavated plants (0.048%, 0.110%, 0.0961% and 0.0851%). The correlation between the maximum shear force and the RAR was positive but not statistically significant, while there was no significant correlation between the number of roots intersecting the shear plane and the shearing resistance of the soil sample. 11 rooted 10 9 Shear strength [kPa] 8 7 unrooted 6 5 4 3 2 1 0 0 10 20 30 40 50 Displacement [mm] 60 70 80 90 50 Plant Soil (2009) 324:43–56 Fig. 4 Shear strength of rooted and fallow soil measured in situ 13 rooted shear strength [kPa] 12 unrooted 11 10 9 8 7 6 3 3.5 4 4.5 5 5.5 6 6.5 normal stress [kPa] Root reinforcement model From the root dimensions measured at the shear surface analysis of the in-situ tests, an average root diameter for each test was calculated. The tensile strength Tr of this ‘average’ root was calculated using the diameter-stress relationship (Fig. 5) for each in situ shear test. RAR recorded for each shear surface was also used for this calculation, and the values were multiplied with a factor of 1.2 from the perpendicular root model (Wu et al. 1988). According to this model when using averaged and derived parameters, the average increase in soil shear strength due to root presence was 2.4 kPa, which compared well with the root reinforcement values measured in the in situ tests (2.7 kPa). Fitting the measured root cohesion into the perpendicular model with the observed near vertical orientation of roots (80°–90°) and soil friction angle Root mechanical testing Roots with smaller diameters were breaking under higher stresses when submitted to tension, while thicker roots broke under lower tensile stresses. Overall, the tensile resistance of the tested vetiver roots decreased from around 17 MPa to around 2 MPa for roots with diameters ranging from 0.3 mm to 1.4 mm (Fig. 5). Regression analysis showed that 71% of the variability in root tensile strength can be ascribed to the changes in root diameter (τ=3.3d−1.31, R2 =0.71). It should be noted that even though every step was taken to minimise stress concentration in the zones near the clamps, a large proportion of the sampled roots that were tested (30 out of 50) actually broke in or near the clamps. The results presented here are the cases when the roots broke in the middle third of their length when subjected to tensile loads. 20 18 tensile strength (MPa) Fig. 5 Tensile strength of vetiver roots with different diameters. The tensile strength decreased exponentially with increasing root diameter (y=3.30× −1.31, R2 =0.71) 16 14 12 y = 3.3016x-1.3134 R2 = 0.7114 10 8 6 4 2 0 0 0.2 0.4 0.6 0.8 diameter (mm) 1 1.2 1.4 1.6 Plant Soil (2009) 324:43–56 measured in the laboratory tests (34.5± 2°), the coefficient of root cohesion in Wu’s model (Wu et al. 1988) should have ranged between 1.0 and 1.1 which was 10–20 % lower than the average value proposed. Slope stability model Considering the fact that the soil strength parameters and the value of root cohesion obtained from the in-situ tests varied considerably, it was considered prudent to adopt the values for cohesion and angle of internal friction of the soil from the laboratory tests, and to use a root added cohesion of 2.5 kPa to reflect the values of both in-situ measurements and the perpendicular root model prediction. Hence, the fallow soil modelled had a bulk unit weight of γ=18 kN/m3, angle of internal friction ′=34.5° and an undrained cohesion Fig. 6 Bishop’s Limit Equilibrium analysis in Slope/W was used to calculate the factor of safety (FoS) of slopes a) with no vegetation, b) planted with vetiver and c) progressively failed slope. 1) Slope and reinforcement layer geometry; 2) Shortterm (undrained) analysis: Slope material γ = 18 kN/m3, 51 cu =4.5 kPa. The root reinforced soil was modelled with a bulk unit weight of γ=18 kN/m3, angle of internal friction ′=34.5°, undrained cohesion cu = 6.5 kPa with included root cohesion of cr =2.0 kPa, and drained cohesion of c′=2.5 kPa due to the presence of the roots. For a fallow slope with geometry based on the insitu measurements (Fig. 1) and modelled as shown on Fig. 6.1, the FoS against failure was around or below unity for undrained and drained conditions respectively (Table 2, Fig. 6a.2 and 6a.3), showing that even the slightest changes could lead to instability. This compares well with the observed behaviour of the terraces where failure occurred on the non-vegetated section during the winter period of the preceding year, and even affected the section where the young vetiver plants were planted (Fig. 7). cu =4.5 kPa, Reinforced material γ=18 kN/m3, cu =6.5 kPa. 3) Long-term (drained) analysis: Slope material γ=18 kN/m3, c′ = 0 kPa, ′ = 34.5°, Reinforced material γ = 18 kN/m3, c′=2.5 kPa, ′=34.5° 52 Plant Soil (2009) 324:43–56 Table 2 Slope stability analysis of aa fallow fallow and and rooted rooted terraced terraced slope using method in Slope/W. Slope materialwith modelled c′=0 kPa,Bishop’s ′=34.5°; Reinforced material modelled γ=18 drainedSlope c′=0 kPa, ′=34.5°; Reinforced material modelled kPa, drained with γ=18 c′=2.5 kn/m3, kPa, undrained ′=34.5° cu slope with γ=18 usingkn/m3, Bishop’s undrained method cu =4.5 in kPa, Slope/W. material kn/m3, undrained cu =6.5 =6.5 kPa, with drained c′=2.5 kPa, ′=34.5° modelled γ=18 kn/m3, undrained cu =4.5 kPa, drained Slope stability analysis Factor of safety Fallow slope Local, undrained 1.01 Local, drained <1.0 Global, undrained <1.0 Global, drained Failed uppermost terrace (surcharge 7.5 kN/m2), undrained Failed uppermost terrace (surcharge 7.5 kN/m2), drained Failed two uppermost terraces (surcharge 20 kN/m2), undrained Failed two uppermost terraces (surcharge 20 kN/m2), drained 2.39 <1.0 1.44 <1.0 1.39 The FoS against slope failure increased with the addition of a 0.3 m thick layer with added cohesion due to the presence of vetiver roots to 1.13 for shortterm and 1.06 for long-term stability (Fig. 6b.2 and b.3). The type of failure simulated with Slope/W was sought to correspond to the failure type observed in situ (circular slip below the root system, and slumping approximately at the middle of the slope length, Fig. 7). To achieve this, a range of slip surface radii were tried for a broad slip surface grid (Bishop and Morgenstern 1960; GEO-SLOPE/W International Ltd. 2008). Local instability of one of the terraces (in this case, the top terrace) leading to an additional surcharge on the next terrace was shown to render the whole slope unstable immediately after failure (FoS = 0.57, Fig. 6c.2), causing failure of the lower terraces in a ‘domino effect’ fashion, although the global (deep-seated) long-term slope stability was not compromised (Fig. 6c.3). Discussion The morphological study on the root systems of vetiver grass showed that the numerous, positively gravitropic roots rarely penetrated deeper and laterally further than 0.3 m on abandoned marl terraces in southern Spain, being heavily concentrated in the Vegetated slope, modelled with added root cohesion (cr) 1.13 1.06 <1.0 2.40 <1.0 1.50 <1.0 1.43 shallow soil horizons where nutrients are more readily available (Schenk and Jackson 2002). Arid Mediterranean conditions with shallow soils and scarce water supply in the growing seasons seem to affect the root development of vetiver in a way that it is more economical for the plant to grow its roots closer to the soil surface where they can exploit most of the available nutrients and water from natural rainfall. In the Mediterranean climate, soil dries out and becomes Fig. 7 Failure observed on a plot adjacent to the vetiverplanted terrace during the winter period of the preceding year. The type of failure simulated with Slope/W was sought to correspond to the failure type observed in situ: circular slip below the root system, with slumping approximately at the middle of the slope length. The height of the terrace was approximately 1.85 m with an angle of approximately 55°. The spacing between the terraces was approximately 20 m Plant Soil (2009) 324:43–56 harder during the growing period (Norris et al. 2008) and only limited amounts of water are available to the plant (Schenk and Jackson 2002). Therefore, such stresses may have caused modifications in the root systems of vetiver, as they were shorter and less branched than those found in wetter, tropical conditions (Salam et al. 1993; Mishra et al. 1997; Truong 1999). However, we only examined a small number of young plants, therefore, more focussed research should be carried out on this subject. In this study, the profile wall method combined with the block excavation proved to be well suited to describe the spatial variations of the root morphology. Both methods showed that the vetiver grass grows numerous positively gravitropic straight roots of more or less uniform diameter. The vast majority of these roots were distributed closer to the culm base, similarly as in other plants and trees (Norris et al. 2008) which justified the use of the profile wall method very close to the plant base. However, using the profile wall method for the estimation of root distribution with depth must be treated with caution not only because of the risk of not mapping the nearly vertical roots that may not cross the profile plane. Compared to the block (monolith) excavation of a whole plant which is time consuming, tedious, not always possible and also sometimes flawed by the loss of fine roots when washing, the profile wall method for root distribution assessment is faster and, in cases where little to no disturbance to the site is required, more practical but tends to underestimate root distribution with depth—a crucial parameter in calculation of RAR of the root and, in turn, of root reinforcement effect. In this study, the block excavation method produced results for root distribution with depth similar to the results obtained via the direct shear test on rooted samples and, therefore, the block excavation was considered to be useful. However, for future studies and where more detailed root 3D distribution in the soil is needed as a part of root system characterization, a complete 3d soil monolith sampling (Kuchenbuch et al. 2009) may be more applicable. The root morphology and distribution of vetiver grass seem to be very well suited to resisting soil shear. Numerous long vetiver roots growing almost vertically downwards are able to penetrate and reinforce the soil that might be prone to surficial failure. This investigation showed that the vetiver 53 roots can grow with their particular pattern even in climates that are very different from its natural conditions. However, extreme climate condition can pose a large obstacle for the unimpeded root growth. The maximum rooting depth recorded for the investigated plants was much lower than the one reported for the same species in the tropics (Truong 1999; Hengchaovanich 1999) which may be due in part to the cold winters and long dry summer periods in this area of the Mediterranean. In spite of the low rooting depth, direct in situ shear tests showed that the presence of vetiver roots does significantly increase soil shear resistance. The reinforcement effect of vetiver roots in case of soil slippage (usually after a rainstorm event, when the pore water pressures are built up and increase the risk of instability) was evident in the in situ shearbox tests. The effect of the roots present in the soil, manifested as an increase in the apparent cohesion (‘root cohesion’) compared well with the results of the laboratory tests. In all of the tests, the soil reinforced with vetiver roots resisted greater shear forces than the unrooted soil samples (Fig. 4). The values for the ‘root cohesion’ obtained in this study were lower than the ones reported in the literature for vetiver (e.g. Cazzuffi et al. 2006), but still higher than the reported values for other grasses (Norris et al. 2008). This could be partially explained with the differences in the soil conditions which, in turn, affect the development and the morphology of the root system. It should be noted that it is common to have a degree of variability in the in-situ direct shear tests, especially for rooted soils. This variability could be attributed to the natural variation in the local soil and root system properties as well as the nature and application of the normal load. Although every care was taken to minimise the variability in the external factors (e.g. load application), the variability in the soil and root parameters had an effect on the results of the tests. While the average value of the ‘root cohesion’ obtained from in-situ tests matches the one predicted by the perpendicular model, there was a difference between the values obtained under different normal load. Similarly, the friction angle of the fallow soil recorded in the in-situ tests was higher than those for fallow soil tested in laboratory conditions. Such differences may have been reduced, or at least their source could have been localised, with the undertaking of more highly controlled in-situ tests. However, 54 due to the fact that this was the largest amount of testing permitted on the terraces, the above remains to be explored in the future. Realising the shortcomings of the sole application of highly variable in-situ results in the theoretical stability models, we used a value of the ‘root cohesion’ that lies within the error margins of both insitu test and theoretically predicted results. The root reinforcement model (Wu et al. 1988) assumes that roots are long enough to ensure sufficient anchorage depth and, during soil slippage, roots break rather than slip can be used to estimate the reinforcement effect. Knowing that the tensile strength of vetiver roots depends on their diameter, it is not hard to imagine that thinner roots that have high tensile strength would slip while the thicker roots with lower tensile strength would break during soil shear, a mechanism that was indeed observed on the shear planes of the reported in situ direct shear tests. Assuming that all of the roots intersecting the shear plane were fully mobilised in tension during the shear tests, knowing the relation between the tensile strength and the root diameter, as well as the RAR, Wu et al.’s (1988) fibre breakage model would yield an increase in shear strength of the root-soil composite comparable to the measured increase in shear strength. The advantage of this approach is that known root morphologies and readily measured strength properties from one or more test plants can be used for a stability assessment of the slope without major physical disturbance on it. A careful examination of the root tensile strength would prove to be crucial for this calculation since the usage of, for example, ‘average’ value of vetiver root tensile strength (75 MPa, Hengchaovanich and Nilaweera 1998; Truong and Loch 2004) would significantly affect the calculations of root reinforcement and produce results which are an order of magnitude higher than the realistic ones. From the applied rooting model (Wu et al. 1988), it is clear that with an increasing depth of the failure surface the contribution of root reinforcement will diminish because of the decreasing RAR—the deeper the root system penetrates, the larger the destabilising forces that the slope can withstand without failure. Hence, the vetiver root system will be able to provide better ductility to the root/soil composite with higher resistance to breakage in tension at small strains as seen in other in situ tests reported in the literature (Wu and Watson 1998), and confirmed with the measured Plant Soil (2009) 324:43–56 shear force vs displacement (Fig. 3). In order to achieve better root growth which, in turn, will make the deeper soil horizons more resistant to shear, it might be advisable to give the vetiver plants in the plantation/hedge an optimal spacing. Judging from the results of the profile wall investigation and the block excavation, 0.3 m spacing between the plants in these climatic and soil conditions seems to be the optimum spacing distance for planting vetiver hedges, as root spread was limited to 0.3 m. The stability of a typical terraced slope was shown to increase when the effect of the vetiver roots was taken into account. The FoS calculated for the vegetated slope was higher than that for the fallow slope. This is especially significant in the cases where the FoS of a fallow slope is around unity, i.e. even the slightest change in the slope morphology or the hydrology could trigger instability. Successful application of stabilising eco-engineering techniques such as vetiver contour hedges could significantly contribute towards an increase of the FoS, rendering the slope acceptable in terms of stability. Slope stability models showed that vetiver grass roots with morphological and mechanical characteristics as recorded in this study have a positive effect on the shallow stability of the terraces. The young root systems of vetiver permeating the uppermost 0.3 m of the soil were shown to contribute to the minimal increase in FoS both short- and long-term which, for the model presented here, rendered the slope marginally stable (FoS>1.0). As expected, the root systems of the vetiver plants in this study did not have an effect on the FoS against a slope failure running below the reach of the root systems. The minimal effect vetiver roots had on the FoS in the slope model presented here was due to the specific root system morphology recorded in situ. Longer, and more numerous branched roots would have arguably increased the RAR, provided better soil bonding at greater depths and, in turn, increased the shear strength of the root-soil composite. Plant species with such roots should be investigated for eco-engineering application for stabilization of terraced slopes in the future. The investigation of a progressive failure of a series of 5 m wide terraces showed that the failure of one of the terraces and the subsequent surcharge the failed terrace will pose on to the slope can be a trigger for a global slope instability in short-term (the failure occurs rapidly, the surcharge is posed immediately Plant Soil (2009) 324:43–56 and the underlying soil can not drain). This investigation showed that terrace width, height, as well as type of failure are factors affecting the global slope stability. widely spaced relatively low terraces with shallower slope angles will be less likely to be at risk of catastrophic slope failure as the weight of displaced soil from a failed terraces will be less and distributed close to the terrace toe. The above suggests that an optimal terrace ratio defined as the ratio between terrace height and spacing for a specific soil and vegetation types may exist. Although the natural succession and the hydrogeological stability of mostly abandoned terraces has been investigated at catchment scale (Kienholz et al. 1984; Ruecker et al. 1998; Rowbotham and Dudycha 1998; Crosta et al. 2003; Cameraat et al. 2005), further practical research and modelling studies at slope scale are needed to explore the concepts in the stability of vegetated terraces. In order to model and assess the stability of a vegetated slope, an inter-disciplinary approach is needed where soil mechanical properties are well defined and the morphological and mechanical characteristics of the root systems of the plants growing on the slope are characterised. While soil sampling and characterization processes are fairly standardised (e.g. British Standard 2004), a similar approach to the investigation and classification of the root systems, especially of the plant species commonly used in ecoand ground bio-engineering techniques, is lacking and presents itself as a challenge for the future. In our study, it was not possible to use more plants since more disturbance in the experimental plots would have produced significant gaps in the vetiver plantations established for soil conservation and protection. It should be noted that the vetiver plants used in this study were sterile and thus non-invasive, and any damage to the formed hedgerows would have had to be repaired with planting of new stock. Future studies should concentrate on the investigation of the vetiver root systems in large plantations where a full vetiver system is installed (Truong and Loch 2004). 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