DOI: 10.5433/1679-0359.2015v36n4p2453
Contribution of the root system of vetiver grass towards slope
stabilization of the São Francisco River
Contribuição do sistema radicular do capim-vetiver para
estabilização do talude do Rio São Francisco
Lorena Machado1*; Francisco Sandro Rodrigues Holanda2; Vanessa Sousa da
Silva3; Antonio Iury Alves Maranduba4; Janisson Bispo Lino4
Abstract
The control of soil erosion along the banks of the São Francisco River requires the use of efficient
and economically viable strategies. Soil bioengineering techniques may be an alternative to the
conventional methods as they provide good soil stabilization by mechanical reinforcement promoted
by the roots. The objective of this study was to evaluate the contribution of the root cohesion of vetiver
grass (Chrysopogon zizanioides (L.) Roberty) on slope stabilization in erosion control along the right
margin of the São Francisco river. Seedlings of vetiver grass were planted in the riverbank of the Lower
São Francisco located in Sergipe State, northeast Brazil, and plants were sampled after two years of
growth to evaluate the effect of grass on the shear strength of the soil. The monolith and cylinder
method was used to collect roots for the evaluation of Root Density (RL), Root Length Density (RLD),
Root Area Ratio (RAR), Root Tensile Strength (TR), and Root Cohesion (CR). Data were submitted to
analysis of variance (p < 0.05), with polynomial regression analysis. The results show that for RL, RLD,
and RAR, the layers of soil at depths of 0-0.10 m had the highest values of 4.84 kg m-3, 12.45 km m-3,
1.66%, respectively. The mean TR was 83 MPa and CR was 528 kPa. Vetiver increases shear strength
of the soil and slope stabilization.
Key words: Chrysopogon zizanioides, soil reinforcement, bank erosion
Resumo
O controle da erosão marginal do Rio São Francisco requer estratégias eficientes e economicamente
viáveis. A técnica de bioengenharia de solos pode ser uma alternativa, visto que proporciona maior
estabilização do solo pelo efeito do reforço mecânico promovido pelas raízes das plantas. Esse estudo
teve como objetivo avaliar a contribuição à coesão do solo da raiz do capim-vetiver (Chrysopogon
zizanioides (L.) Roberty), na estabilidade do talude para controle da erosão da margem direita do Rio
São Francisco. Mudas do capim-vetiver foram plantadas no talude da margem direita do Baixo São
Francisco no estado de Sergipe, com amostragem das plantas realizada após dois anos do plantio, a fim
de avaliar o efeito da gramínea no aumento da resistência ao cisalhamento do solo. Foram utilizadas a
metodologia do monólito e o método do cilindro para coleta de raiz, visando à avaliação da Densidade
Radicular (DR), Densidade do Comprimento Radicular (DCR), Razão de Raiz por Área (RAR),
1
2
3
4
*
Engª Agrª, M.e em Agroecossistemas, Deptº de Engenharia Agronômica, Universidade Federal de Sergipe, UFS, São Cristóvão,
SE, Brasil. E-mail: loremachado@globo.com
Engº Agrº, Prof. Dr., Deptº de Engenharia Agronômica, UFS, São Cristóvão, SE, Brasil. E-mail: fholanda@infonet.com.br
Discente do Curso de Graduação em Engenharia Florestal, Deptº de Engenharia Florestal, UFS, São Cristóvão, SE, Brasil.
E-mail: vanessa.sousas@hotmail.com
Discentes do Curso de Graduação em Engenharia Agronômica, Deptº de Engenharia Agronômica, UFS, São Cristóvão, SE,
Brasil. E-mail: iubam@hotmail.com; janissonlino@gmail.com
Author for correspondence
Recebido para publicação 09/04/14 Aprovado em 12/11/14
Semina: Ciências Agrárias, Londrina, v. 36, n. 4, p. 2453-2464, jul/ago. 2015
2453
Machado, L. et al.
Resistência de Raízes à Ruptura (TR) e Coesão da Raiz (CR). Os dados foram submetidos à análise de
variância (p < 0,05), com análise de regressão polinomial. Os resultados mostram que para DR, DCR
e RAR, na camada de 0-0,10 m apresentaram valores mais altos de 4,84 kg m-3, 12,45 km m-3, 1,66%,
respectivamente. O TR médio foi de 83 MPa e o CR de 528 kPa. O vetiver favorece o aumento da
resistência ao cisalhamento do solo, auxiliando na estabilização de taludes.
Palavras-chave: Chrysopogon zizanioides, reforço do solo, erosão marginal
Introduction
The São Francisco River has undergone
significant changes to its hydrological regime due
to the construction of hydroelectric dams. These
changes have caused advanced soil erosion of the
Lower São Francisco riverbank, starting with the
caving of the slope base, which was triggered by
low water levels and the associated clearing of
riparian vegetation (HOLANDA et al., 2008).
The siltation of the river channel resulting from
erosion, whether increased or not by the removal of
riparian vegetation, has caused economic, social,
and environmental loss to the region and the riverine
population. Hence, the revegetation of the riverbank
with species that can withstand the ebb and flow
of the waves is required in order to reduce soil
movement toward the river channel (HOLANDA et
al., 2005; OLIVEIRA et al., 2009).
There are several possible methods to control
erosion on agricultural soils (SPERANDIO et al.,
2012) or to stabilize riverbanks; however, according
to Holanda et al. (2007), some techniques have
high costs, making their application to the whole
area unfeasible. Suleiman et al. (2013) claim that
the use of poaceae such as Vetiveria zizanioides
to stabilize riverbanks has become increasingly
important, particularly as a cost-effective technique
that is technically effective and environmentally
sustainable, compared with conventional methods.
In this context, soil bioengineering can be a viable
alternative for controlling bank erosion using live
or inert vegetable materials, such as geotextiles,
associated or not with rocks, concrete, or metal,
thus being environmentally sustainable in the
containment of riverbanks with different slopes.
Jones and Hanna (2004) state that soil
bioengineering can stabilize the soil and allow
living materials to develop a vegetation cover
starting the process of ecological succession.
Plants are considered one of the most important
components in the implementation of these
biotechniques. According to Gray and Sotir (1996),
roots and stems act as the main structure and are
mechanical elements in slope protection systems.
The vetiver grass (Chrysopogon zizanioides (L.)
Roberty) has been used as a structural component
of soil bioengineering techniques as root-based
reinforcement in the stabilization of slopes on the
right bank of the São Francisco River (HOLANDA
et al., 2012).
The vetiver grass is a perennial caespitose
grass that grows up to 1.5-2 m in height, with an
extremely fasciculated and deep root system, and
is originally from South India. It has dense, hard,
and deep roots that are capable of forming a natural
clamping system, thus stabilizing embankments
(CHONG; CHU, 2007). Due to the aggregating
potential of its root system, vetiver grass has been
widely used to contain erosion, providing a physicalmechanical consolidation of soil and increasing the
shear resistance due to soil-root interactions, thus
preventing shallow landslides (GOLDSMITH,
2006).
The effect of roots on slope stabilization
improvement has already been recognized in several
studies (CAZZUFFI et al., 2006). However, there
are still few published reports on the properties
of the root system of plants, especially those of
vetiver grass. Further studies are needed to improve
the understanding of root binding to the soil in
order to facilitate their suitability for stabilization
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Contribution of the root system of vetiver grass towards slope stabilization of th São Francisco River
of river embankments. Gyssels et al. (2005) state
that some soil properties such as infiltration rate,
moisture content, organic matter content, aggregate
stability, and shear strength may be influenced by
roots, allowing soil erosion to be controlled. The
reinforcing effect promoted by roots associated
with slope stability can be evaluated in terms of soil
shear strength. To estimate the increase in soil shear
strength, provided by roots, models using data from
root tensile strength and root distribution are used
(WU et al., 1979; DE BAETS et al., 2008).
This study is part of a broad investigation into
the behavior of species of the Poaceae botanical
family in stabilizing riverbanks. It aimed to evaluate
the contribution of root cohesion of the vetiver grass
(Chrysopogon zizanioides (L.) Roberty) on slope
stability, and thus controlling the erosion of the right
bank of the São Francisco River.
Material and Methods
Study area
The experimental area is located on the slope
of the right bank of the São Francisco River (UTM
coordinates N= 8.868.789,506 and E = 736.583,864)
in the city of Amparo de São Francisco, in the state
of Sergipe, in soil classified as Fluvisol (Table 1)
(HOLANDA, 2008). Seedlings of vetiver grass,
from the nursery of the Federal University of Sergipe
(UFS), were planted in pits fertilized with 8 g simple
superphosphate plant-1, in strands perpendicular to
the slope of the bank, spaced 0.9 m between rows
and 0.3 m between plants in each row.
Table 1. Granulometric composition of Fluvisol from the São Francisco River bank (Amparo de São Francisco – SE).
Depth (m)
0-0,10
0,10-0,20
0,20-0,30
0,30-0,40
0,40-0,50
Sand (g kg-1)
874
838
755
659
716
Silt (g kg-1)
82,00
98,00
137,00
173,00
167,00
Sampling of vetiver grass was performed in
August 2013, two years after planting, to evaluate
the effect of the grass on soil shear resistance. It was
considered important to test adult plants in order to
analyze the potential of these specimens to stabilize
the slope, in the medium and long term, after being
established.
Monolith method
The monolith method was used to collect roots
according to Böhm (1979). The root system of five
specimens of vetiver grass was sampled at different
depths from 0.50 m wide, 0.50 m long, and 0.50
Clay (g kg-1)
44,00
64,00
108,00
168,00
117,00
Texture
Sandy
Sandy
Sandy Loam
Sandy Loam
Loamy Sand
m high monoliths, amounting to 0.025 dm3 in each
monolith at different depths. Before initiating the
removal of monoliths from the soil, it was necessary
to level the surface and the walls of the block. A
straight shovel and sharp cleaver were used to assist
in cutting and thus obtain the soil portions including
the roots at the bottom of each monolith. Samples
were taken from layers 0-0.10; 0.10-0.20; 0.200.30; 0.30-0.40, and from 0.40-0.50 m, from each
vetiver grass specimen. Each monolith was then
placed in plastic bags, along with the aerial part of
the five sampled plants.
Subsequently, the roots were separated from the
soil by washing with a water jet, using sieves with
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a mesh of 1.0 mm to minimize root loss (BÖHM,
1976). After washing, the roots were placed in paper
bags and dried in an oven with forced air circulation
at 65°C for 72 h. The aerial biomass was also dried
at 65°C for 72h.
projection of the plant’s aerial biomass (DE BAETS
et al., 2008).
Thereafter, the weighing of root and shoot
biomass took place with an analytical scale, thus
determining Root Dry Matter (RDM) and Aerial
Dry Matter (ADM). From the RDM, Root Density
(RD) could be calculated using Eq. (1), where MR
is the mass of roots per class of depth (kg) and V is
the volume of soil in each class (m3). RD aims to
quantify the biomass of roots for each parsed soil
depth.
A key step in evaluating the increased root-based
resistance is the Root Tensile Strength (TR) test.
For this, samples of whole vetiver grass roots were
collected using the cylinder method. The cylinders
were made of a PVC pipe that was 0.50 m in height
and 0.30 m in diameter. Undisturbed soil samples
containing roots and aerial parts were collected,
inserting the cylinder in the soil profile when 25
vetiver grass specimens were removed. Then, the
soil portions and the roots from the samples were
washed with water jets, using sieves with a mesh
of 1.0 mm in to minimize root loss at the time of
washing (BÖHM, 1976).
(1)
To determine the presence of roots in the soil,
root length density (RLD) was calculated, and for
this purpose, the total measured length of dried
roots of each individual layer was required. This
was determined with the help of a graduated ruler.
The RLD was calculated using Eq. (2), wherein Cr
is the total length of roots per class of depth (km)
and V is the volume of soil (m3).
(2)
Based on the RLD of each depth class, RAR
(Root Area Ratio) was calculated using Eq. (3) to
estimate the contribution of the roots to the increase
in soil resistance.
(3)
where RL is the total length of roots per class of
soil depth (m); ai is the mean cross-sectional area
of the root section of a representative plant (m2); P
is the class of soil depth used (0.10 m); and A is
the reference area (m2), calculated by the vertical
Cylinder method
Among the 25 plants sampled, 50 roots were
selected for the root Tensile Strength tests. The
following root selection criteria were used:
undamaged roots, constant diameter, less than 8
mm root diameter (maximum diameter at the root
that can be tested in the laboratory), and a minimum
length of 0.10 m. The roots were stored in alcohol
(15% ethanol) at 4°C to maintain viability (DE
BAETS et al., 2008).
TR tests were conducted at the Laboratory of
Ecophysiology, Federal University of Sergipe (UFS),
using a Universal Press. This machine combines the
functions of tensile strength generation, load and
displacement measuring, and data acquisition. To
conduct the tests, the roots were fixed to the gears
of the machine with tapping clamps. To circumvent
problems in the tests, adjustments were made to the
clamps of the equipment to ensure the best grip,
avoiding rupturing of the roots in the clamping
position. Tests in which the roots broke whilst in the
clamping position were considered invalid.
The sample was then subjected to a constant
tensile speed of 10 mm×min-1. The load cell of the
equipment was capable of measuring up to 500 kN,
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Contribution of the root system of vetiver grass towards slope stabilization of th São Francisco River
with an initial force of 1 kN. Tensile strength was
measured and recorded on a computer connected to
the test machine. Equation (3) was used to calculate
root Tensile Strength (N×mm-1) (BISCHETTI et al.,
2003).
(3)
where Fmax is the maximum force required for root
breakage (N) and D is the mean diameter of the root
close to the point of breakage before the application
of tensile strength (mm). For this purpose, before
the test, the root diameter was measured at three
equidistant points using a digital caliper.
Effect of soil reinforcement by vetiver grass roots
The determination of soil reinforcement provided
by vetiver roots was based on the predictions of a
perpendicular root reinforcement model using the
means of the parameters of the root system from the
surface shear and root tensile strength (WALDRON,
1977; MICKOVSKI et al., 2008). In this model, it is
assumed that all the roots crossing the shear plane
break during the shear process. The magnitude of
the reinforcement due to the presence of roots in
the soil was determined according to Gray and Sotir
(1996), using the following equation:
CR = 1.2 × TR × RAR
(5)
where CR (N m-2) is Root Cohesion or the increase
in shear strength due to the presence of roots in the
soil, TR (N m-2) is the mean Tensile Strength of the
average number of roots with a mean diameter per
unit soil area, and RAR (m2 m-2) is the root area
ratio (the ratio between the total root cross sectional
area and the total shear area). The CR of specimens
collected by Böhm’s monolith method (1979) was
calculated, taking into account the mean TR for
the respective mean diameters of RAR by class of
depth.
Statistical Analysis
Data were subjected to analysis of variance
to determine significance at a level of 5% (F
test). Next, a polynomial regression analysis was
performed, adjusting mathematical models and
coefficients of determination (R²) for each variable.
All analyses were performed using the statistical
program SISVAR (FERREIRA, 2011).
Results and Discussion
The total dry matter of five specimens of
vetiver grass, sampled by the monolith method,
presented maximum values of 2.08 kg and 0.31 kg
for the shoots and roots, respectively. In contrast,
the minimum values were 0.41 kg for shoots and
0.21 kg for the root system (Figure 1). For these
specimens, the mean ratio between shoots and the
root system was 4:1. This indicates that vetiver
grass is capable of producing high levels of shoot
and root biomass, as suggested by Manoel et al.
(2013). The aerial biomass helps to minimize soil
loss by providing greater protection to the soil as a
result of the strong tillering of the species, thereby
reducing the direct impact of raindrops on the soil,
whereas root biomass promotes strong anchoring to
the ground, which helps to prevent landslides.
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Figure 1. Aerial Dry Matter (ADM) and Root Dry Matter (MSR) of five specimens of vetiver grass.
The distribution of root depth was evaluated
by the variables Root Density (RD) and Root
Length Density (RLD). The biomass of the roots of
individual plants analyzed was proportional to root
length (Figure II). The change in root density was
inversely proportional to depth, presenting a strong
negative correlation. RD showed the best fit to the
cubic model (p < 0.05) with R² = 0.9994, while for
the RLD, the model was quadratic (p < 0.01) with
R² = 0.9892 (Figure 2).
Figure 2. Root density (RD) and Root Length Density (RLD) of five specimens of vetiver grass with soil depth.
The bulk of RD and RLD were concentrated in
the first 0.20 m above the ground with the 0-0.10 m
layer being the densest. These data corroborate the
findings made by De Baets et al. (2007) for all species
analyzed. In the 0-0.10 m layer, the mean values of
RD and RLD were, respectively, 4.84 kg m-3 and
12.45 km m-3; however, the 0.40-0.50 m layer had
a mean RD of 0.83 kg m-3 and a mean RLD of 3.32
km m-3. It was apparent that there was a reduction in
biomass and root length with increasing soil depth,
which may have been caused by the lower aeration
and nutrient availability in the deeper soil layers,
and by the emergence, in depth, of more compacted
layers, which would have hindered root penetration
and development. This behavior of the soil can be
attributed to vertical and horizontal variability of
soil layers of different textures and cohesion, which
are characteristic of a Fluvisol.
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Contribution of the root system of vetiver grass towards slope stabilization of th São Francisco River
The root system of vetiver grass is known to
reach depths of up to 4 m (CHONG; CHU, 2007).
However, in this study it was noted that the density of
roots was reduced, on average, by 82.76% between
the 0-0.1 m to the 0.4-0.5 m layer. This suggests
that the greatest reinforcement promoted by the root
system to the soil occurs in the surface layers where
root density is the greatest. Due to this, the shear
strength of the embankment’s soil, promoted by the
roots, decreases with greater depths.
However, even with this decline in root density,
the high values found for RD and RLD, especially
for the deeper layers, indicated a higher rooting
depth of vetiver grass compared to other grasses.
De Baets et al. (2007) showed that grass presented
a higher density of roots only up to the first 0.25 m
of soil. Because of this dense and deep root system
observed in vetiver grass, this grass has become
well known for its ability to conserve soil and
stabilize slopes in several studies (DONJADEE;
TINGSANCHALI, 2013).
To better assess the contribution of the roots to
soil strength in relation to depth, the RAR was used.
This index represents the soil area that is occupied
by the roots, i.e., the concentration of the roots in
the soil. Figure III represents the change in the RAR
for the five vetiver grass specimens sampled from
every 0.10 m of soil depth. The best-fit regression
model was quadratic (p < 0.01) showing a strong
negative correlation (R ² = 0.9717) of the RAR with
soil depth (Figure 3).
Figure 3. Root Area Ratio (RAR) and root Tensile Strength (TR) of vetiver grass with soil depth.
The high mean percentage of RAR (1.66%)
found for the 0-0.10 m layer when compared to other
grasses, can be explained by the extremely dense
and fibrous root system of vetiver grass. Abernethy
and Rutherfurd (2001) and Bischetti et al. (2005)
stated that RAR could reach values close to 1%.
This high percentage observed for RAR is another
feature that helps to increase the shear strength of
the soil, as according to Wu et al. (1979), shear
strength increases with increasing concentration of
the roots in the soil.
The mean values of RAR in the deeper layers of
the soil were significantly (p < 0.01) lower than those
closer to the surface. This can be explained by the
lower density of root length, and by the lower mean
cross-sectional area of representative roots found
in the deeper layers of the vetiver grass specimens
evaluated. Although the mean RAR of 0.04% for
the 0.40-0.50 m depth is much lower than that of the
0-0.10 m soil layer, it falls within the range of RAR
obtained for all grasses evaluated by De Baets et al.
(2008), at a depth of 0.40-0.50 m, and was in many
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cases, higher. This decrease in RAR in the deeper
layers confirms that the reinforced shear strength of
soil that is promoted by roots decreases with depth.
Figure 3 also shows the TR values, which are
essential to evaluate the increase in shear strength
through the reinforcement provided by the roots to
the soil. Regression analysis showed that 89.33%
of the variation in root tensile strength could be
attributed to changes in the diameter at root breakage
(p < 0.01).
As found in the mechanical testing of TR with
vetiver grass performed by Mickovski and Van
Beek (2009), the roots evaluated in this study with
smaller diameters had a higher tensile strength than
the thicker roots. To break roots with less than one
millimeter (mm) diameter, a mean tensile strength
greater than 100 MPa was used. On the other hand,
roots longer than 1 mm broke with less tensile
strength (< 100 MPa), reaching a minimum mean
tensile strength of 16.4 MPa for 2.7 mm roots
(Figure 3).
Other studies with different species also found
that root resistance is very high in thin roots, and
that this decreases significantly with increasing
diameter (BISCHETTI et al., 2005; TOSI, 2007).
According to Genet et al (2005), the variation in
tensile strength with the increase in diameter is
directly linked to the structure of the root; thinner
roots have a high cellulose content. Hence, roots
with smaller diameters are stronger and are essential
to increase the shear strength of the soil.
The mean TR of 83 MPa obtained during testing
of the vetiver grass specimens was very close to the
mean resistance values found by Hengchaovanich
and Nilaweera (1996) and Cheng et al. (2003) who
measured 85 MPa for the same grass. However, the
variability in TR may occur due to the age of the
root, growth rate, growth orientation by changes in
soil moisture content, and soil texture (DE BAETS
et al., 2008).
Lower TR values were found by Mickovski and
Van Beek (2009). The authors presented values for
vetiver TR of 17 MPa to about 2 MPa, with root
diameters of 0.3 mm and 1.4 mm, respectively.
These low values of tensile strength differ from
those observed in the present study; the variable
tensile strength varied, on average, from 353 MPa to
16 MPa for roots of 0.4 mm to 2.7 mm in diameter,
respectively (Figure 3).
This discrepancy may be due to the period in
which vetiver grass specimens in those experiments
were assessed, which was six months after planting,
unlike the assessment of individual plants in the
present study, which occurred two years after
planting. Barbosa and Lima (2013) reported that
increasing time from vetiver grass planting increases
the shear resistance parameters of soils.
The presence of roots in the soil tends to
increase soil cohesion by increasing soil shear
resistance, thereby increasing the surface stability
of slopes (Van Beek et al., 2005). The increased
shear resistance due to the presence of roots in the
soil, also called CR, was calculated as a function of
depth classes (Figure 4). The relationship between
CR and depth of soil is represented by a seconddegree polynomial model (p < 0.01), with a strong
negativ.
In accordance with Figure 4, the relationship
between CR and soil depth is represented by a
second-degree polynomial model (p < 0.01) with a
strong negative correlation (R² = 0.9738). Similar
to other indices analyzed, CR also decreases as the
depth increases. This is explained by decreasing TR
and RAR in the deeper layers, since CR is directly
proportional to these variables. It is worth noting
that the result found for CR confirmed that the
higher the root density in the soil, the greater the
increase in soil reinforcement against shear force.
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Contribution of the root system of vetiver grass towards slope stabilization of th São Francisco River
Figure 4. Root cohesion (CR) by soil depth in five vetiver grass specimens.
The mean CR found for the first 0.50 m soil depth
of vetiver grass was 528 kPa (Figure 4). Using the
same method for calculating CR, De Baets et al.
(2008) also found the same tendency with increasing
soil depth, but the maximum root reinforcement of
the species analyzed by the authors was 304 kPa.
This variability can be explained by the different
characteristics of the species evaluated in the two
studies; the vetiver grass showed higher values of
TR and RAR.
The result shown in Figure 4 indicates that the
vetiver grass contributed to the increase in CR or
soil shear strength of the riverine bank under study
from 126 kPa (at a depth of 0.40-0.50 m) to 1280
kPa (0-0.1 m). Reinforcement of the soil by roots
is good evidence that vegetation improves the
stability of riverbanks, while the magnitude of the
reinforcement, as shown in this study, depends on
the morphology of the root system, such as the
distribution of roots at different depths, diameters,
and tensile strength.
Conclusions
Fasciculate roots such as the vetiver grass, with
proven concentration of small diameter fibers,
occupy a large contact surface area, which promotes
high tensile strength and hence provides increased
shear strength to the soil.
Root Distribution (RD and RLD) presented
higher values at the surface due to the higher density
of roots in these layers, which is characteristic of
species with fasciculate roots.
The high Root Area Ratio (RAR) found for
vetiver grass confirms the importance of this
gramineae in increasing shear resistance.
The high root Tensile Strength (TR) contributes
to the increase in soil resistance to erosion, with
emphasis on thinner roots, which have higher TR
values.
Root cohesion (CR) increases shear strength in
the upper soil layers due to higher root density.
Direct shear tests of this soil with and without
revegetation, coupled with deterministic slope
stability analyses, are essential to prove the efficient
action of vetiver grass suggested by this study.
The use of grasses with dense and fibrous roots is
an alternative to the makeup of soil bioengineering
techniques for slope stabilization and containment.
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Acknowledgements
The authors thanks to the Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq)
on financial support for the development of this
research.
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