See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/351499936
Effectiveness of Vetiver Grass on Stabilizing Hill Slopes: A Numerical Approach
Poster · February 2020
CITATIONS
READS
11
514
3 authors, including:
Md Azijul Islam
Mohammad Shariful Islam
University of Texas at Arlington
Bangladesh University of Engineering and Technology
60 PUBLICATIONS 355 CITATIONS
176 PUBLICATIONS 1,267 CITATIONS
SEE PROFILE
All content following this page was uploaded by Md Azijul Islam on 12 May 2021.
The user has requested enhancement of the downloaded file.
SEE PROFILE
Effectiveness of Vetiver Grass on Stabilizing Hill Slopes: A Numerical Approach
Md Azijul Islam*, Mohammad Shariful Islam, Ph.D. and Tausif E Elahi
*Corresponding Author (mdazijul.islam@mavs.uta.edu)
ABSTRACT
NUMERICAL MODELING
Vetiver grass (Vetiveria zizanioides) is easily available in Bangladesh and widely known for its long fibrous roots
•
which significantly increases shear strength of soil. This paper aims to numerically analyze the stability of hill
slopes and effectiveness of vetiver for protection of slopes using PLAXIS 2D software. Soil samples were collect-
•
ed from four different hill slopes in Chittagong Hill Tracts (CHT) which were prone to landslides. Three soil samples were classified as sandy soil (SM, SC and SM-SC) and another one was lean clay (CL). Results from numerical
analysis showed that the factor of safety (FS) decreases with the increase in slope angle and for soft clay soil, slope
fails at different slope angles. However, presence of vetiver grass on hill slopes increases FS by 2%-15% for sandy
soils but for clay the increase in FS is insignificant because of deep seated base failure. Although vetiver alone can-
•
not stabilize a hill slope but can protect the slopes from rain cut erosion which leads to failure. Additionally, analysis has been performed for different root zone depth considering the age of the plantation. Therefore, for sandy
•
INTRODUCTION
Slope failures are very common in CHT of Bangladesh, which ultimately lead to landslides.
•
Landslide that occurred in 2017 at CHT, killed 152 people and caused property damage worth $223 million.
The soil was modelled in PLAXIS according to Mohr-Coulomb model
which involves five input parameters,
E and µ for soil elasticity, φ' and c' for
soil strength parameters and angle of
dilatancy.
Standard fixities have been used for
modelling which sets boundary conditions.
•
•
•
•
•
•
There are several methods available for slope stabilization including structural and non-structural solutions such
as retaining wall, soil nailing etc.
However, bio-engineering solution like vegetation is now becoming popular all over the world because of its
simplicity, efficiency and cost effectiveness.
Effect of vegetation in preventing slope failure is well known and used effectively to prevent landslides especially shallow landslides.
Dense cover of vegetation on the slope plays an important role in preventing surficial rainfall and wind erosion.
•
Parameter
Soil Classification (USCS)
Moist unit weight
Saturated unit weight
Initial void ratio (eint)
Young’s modulus (E)
Poisson’s ratio
Cohesion (cꞌ)
Friction angle (ϕꞌ)
Horizontal Permeability (kx)
Vertical Permeability (ky)
The studies on vetiver grass for determining rooted shear strength of soil are conducted based on some laboratory tests of rooted specimen (Islam et al. 2016) or field model scale test.
However, numerical simulation of rooted hill slope with parametric study is very limited.
Furthermore, applicability and effectiveness of locally available vetiver grass has not been assessed much in the
context of landslide prevention.
This study has been carried out to determine soil characteristics of CHT hill slopes and analyse the stability of
hill slopes having different slope angle.
•
•
•
•
•
Time
(Day)
Analyzing these results soil characteristics of CHT
will be observed and these properties will be used for
numerical analysis in PLAXIS 2D.
30
60
90
120
180
Existing hill slopes of the selected location was
found to vary within 10-15m, and slope angles are
found within 70-80o.
In this study, the dimensions of the model have been
adopted from average existing slopes in CHT.
The rooted behavior of slope in this study is simulated by added cohesion provided by roots.
After determining root diameter of vetiver plant,
tensile strength, root area ratio (RAR) and added apparent cohesion were determined from Voottipruex
(2008).
View publication stats
Average Root length
(cm)
Islam et al.
Parshi
(2016)
(2015)
50
5
90
15
100
22
120
30
40
•
Average Root diameter (cm)
•
Islam et al. (2016)
0.11
0.12
0.13
0.135
-
Tensile
strength Root Ar- Added apparTime Diameter,
of root, ea Ratio, ent cohesion,
(Day) D (mm)
Tr
RAR
cr (kPa)
(Mpa)
30
1.1
16.0
0.010
1.92
60
1.2
15.2
0.022
4.01
90
1.3
14.5
0.035
6.1
120
1.4
13.9
0.049
8.2
1.5
•
Unit
kN/m3
kN/m3
kN/m2
kN/m2
°
m/day
m/day
Grain size distribution of Soil-A, B, C and D
Soil-A
SM
16.4
18.5
0.60
3.0×104
0.3
0.1
40
16.7×10-03
16.7×10-03
Soil-B
SC
16.7
19.0
0.58
2.0×104
0.3
9
37
4.6×10-03
4.6×10-03
Soil-C
CL
15.1
19.0
0.75
1.0×104
0.3
18
2.6×10-05
2.6×10-05
•
Soil-D
SM-SC
14.5
16.8
0.85
2.5×104
0.3
2
30
4.5×10-03
4.5×10-03
•
•
•
•
13.3
0.079
12.6
•
•
For Soil A, FS increases with the increase of root
zone depth for all slope angles.
FS increases up to 15% for rooted soil compared
to bare soil in case of Soil A.
•
On the contrary, for Soil C, change of FS is only
0.7% for rooted soil.
•
For clayey soil (like Soil-C), deep seated base failure occurs i.e. failure surface lies at greater depth
which is similar to the findings of this study.
•
Variation of FS with different root zone depth
•
Therefore, it is observed that increase in FS due
to vegetation is maximum for Soil A followed by
Soil-B and Soil-D which indicates that vetiver is
mostly effective for shallow slope failure.
Roots of vetiver penetrate the failure surface and
thereby increases the shear strength of soil resulting overall increase of FS.
Whereas for Soil A, B and D this decrease is
52.1%, 48.3% and 47.4% respectively.
Chok et al. (2015) studied the effect of root reinforcement on slope stability using FEM.
Variation of FS with slope angle for soil- A, B, C and D )
It is observed that FS obtained from Chok et al.
(2015) are slightly higher than FS obtained from
this study.
Chok et al. (2015) applied the added cohesion
from root matrix on slope surface, upper and
lower slope region.
However, in this study, it is assumed that the
vegetation covers only on the slope area for
which results were found lower FS for all type of
soil except soil C.
Numerical analysis of the influence of vegetation
on slope’s stability has been studied by Kokutse
et al. (2016) using classical shear reduction method with PLAXIS 2D.
Comparison of FS between this study and Chok et al. (2015)
Kokutse et al. (2016) investigated the combined
effects of different rectilinear slope geometries,
soil types and vegetation mechanical parameters.
It is seen that for both the studies, mean values
of FS decreases significantly with increasing
slope angle for all types of rooted soil.
For Soil-A, the FEM model of this study gives
higher FS whereas for other soil types the results
of this study resemble Kokutse et al. (2016).
Comparison between Kokutse et al. (2016) and this study
The observed results from the study shows that, for the clay soil with lower cohesion failure occurs for all slope
angles whereas for soil B, FS is relatively higher than remaining soils due to higher cohesion and angle of internal friction.
The application of vetiver grass on hill slopes increases FS up to 2-15% for sandy soils but for the clayey soil,
increase in FS is negligible.
For milder slope, increase of FS due to application of vegetation is found high compared to steeper slope. Although overall FS is increased by vegetation, vegetation can be applied only for mild sandy slope because of its
shallow failure pattern.
For steeper clayey soil, vegetation with other preventive measures should be considered.
REFERENCES
For Soil-B, FS increases about 2-4% while for Soil
-D FS increases 3-8% compared to bare slopes.
•
•
For Soil-C, change of FS with slope angle is very
small and decrease in FS from 1V:2H slope to
2V:1H slope is only 12.6%.
SUMMARY AND CONCLUSION
Effect of root zone depth for slope stability
Table: Added apparent cohesion for vetiver root
180
•
RESULT AND DISCUSSION
Table: Morphological characteristics of vetiver grass with time obtained from different literature
Laboratory tests were conducted to determine physical and engineering properties of these soils.
•
•
Modulus of elasticity, E and poisson’s ratio, µ
of soil types used for numerical analysis are
assumed from literature (Bowels 1996).
•
FEM models for proposed hill slope angles have
been analyzed to determine the FS and obtained
values of FS for bare slopes.
Validation of FEM Models
Schematic model used for FEM analysis
On the other hand, Soil C is classified as soft
lean clay (CL).
•
Vetiver grass is a promising type of grass that works very well in many different environments and makes hill
slopes resistant against erosion.
Disturbed and undisturbed samples have been collected from four different locations of CHT of
Bangladesh
•
From sieve analysis and Atterberg limit test
performed, Soil A, Soil B and Soil D are identified as silty sand (SM), clayey sand (SC) and
silty clayey sand (SC-SM) respectively as per
Unified Soil Classification System (USCS).
METHODOLOGY
•
•
FS is calculated using SRM method in PLAXIS where the strength parameters tan
ϕ and c of the soil are successively
•
•
•
SOIL PROPERTIES
•
•
Effect of slope angle on stability of slopes
PLAXIS 2D with plain strain model
was used to model vegetated slope.
•
mild hill slopes, vegetation can be an effective and green solution for slope protection.
•
RESULT AND DISCUSSION
•
•
•
Percentage increase in FS due to vegetation for different
slope angles
•
Chok, Y.H., Jaksa, M.B., Kaggwa, W.S., and Griffiths, D.V. (2015). “Assessing the influence of root reinforcement on slope
stability by finite elements,” Int. J. of Geo-Engineering, vol. 6, no. 12.
Islam, M.S., Arif, M.Z.U., Badhon, F.F., Mallik, S., and Islam, T. (2016). “Investigation of Vetiver Root Growth in Sandy
Soil,” Proc., BUET-ANWAR ISPAT 1st Bangladesh Civil Eng. SUMMIT, Dhaka, Bangladesh.
Kokutse, N.K., Temgoua, A. G.R., and Kavazovic, Z. (2016). “Slope stability and vegetation: Conceptual and numerical investigation of mechanical effects,” Ecological Eng., vol. 86, pp. 146-153.
Parshi, F.N. (2015). “Strength-deformation charecteristics of rooted soil.” M. Sc. Eng. Thesis, Dept. of Civil Eng., Bangladesh
University of Engineering and Technology (BUET).
Voottipruex, P., Bergado, D., Mairaeng, W., Chucheepsakul S., and Modmoltin, C. (2008). “Soil Reinforcement with combination roots system: a case study of vetiver grass and acacia mangium willd,” Lowland Technology Int., vol. 10, no. 2, pp. 56-67.