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). For slopes prone
to failure under fully saturated conditions, in situ shear
box tests combined with a full morphological study of
the root system are expected to provide input parameters for modelling and assessment of the reinforcement effect of the vetiver roots.
Acknowledgements The work on this paper was funded under
the framework of the ECOSLOPES project (EU, 9 QLK5—
55
2001-00289). Planting material was kindly provided by Mike
Pease, the European and Mediterranean Vetiver Network
Coordinator. The help and suggestions of Dr P Truong are also
kindly acknowledged.
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