Slope Stability Analysis of East Ring Road Construction at Sadawarna Dam With Shear Strength Reduction Method

Abstract

Slope stability has been a problem that studied on geotechnical works for the uncertainties such as varieties of soil behaviours to unpredicted failure of measurements and samplings. The Limit Equilibrium Method (LEM) has been popular for decades for its convenience but cannot determine displacement thus the result could be uncertain. As computational calculations have been developed, the Finite Element Method (FEM) began to use as a tool to not only determine factor of safety, but also determine displacement and forces that affect slope’s stability. This research was done to analysed slope stability using Shear Strength Reduction (SSR) and compared it to general method such as Limit Equilibrium Method (LEM). The research was conducted on a section in Sadawarna DAM ring road area, Subang, West Java. Slope on research area was divided into two layers with the bottom layer relatively non-cohesive compared to the top of layer. Both soil however dominantly consist with fine grain soil such as silt and clay. The upper layer of slope can contain more water with liquid limit of 73,46% compared to the lower layer with 68,27% liquid limit. Simulation result showed slope that analysed using SSR method has Factor of Safety (FoS) 0,12 lower than LEM method. SSR method could be used to analysed pessimistic value at worst scenario and could predicted deformation of slope.

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JOURNAL OF GEOLOGICAL SCIENCES AND APPLIED GEOLOGY VOL. 7 NO. 3 DECEMBER 2023
1 E-ISSN : 2579 – 3136
SLOPE STABILITY ANALYSIS OF EAST RING ROAD CONSTRUCTION AT SADAWARNA DAM
WITH SHEAR STRENGTH REDUCTION METHOD
Rafino Kurnia Putra1*, Zufialdi Zakaria1, Muhammad Kurniawan Alfadli11, Raden Irvan Sophian1, Nur Khoirullah1,and Yusi
Firmansyah1
1Faculty of Geological Engineering Universitas Padjadjaran, Indonesia
*Corresponding author: rafino18kurniap@gmail.com
ABSTRACT
Slope stability has been a problem that studied on geotechnical works for the uncertainties such as
varieties of soil behaviours to unpredicted failure of measurements and samplings. The Limit
Equilibrium Method (LEM) has been popular for decades for its convenience but cannot determine
displacement thus the result could be uncertain. As computational calculations have been developed,
the Finite Element Method (FEM) began to use as a tool to not only determine factor of safety, but
also determine displacement and forces that affect slope’s stability. This research was done to
analysed slope stability using Shear Strength Reduction (SSR) and compared it to general method
such as Limit Equilibrium Method (LEM). The research was conducted on a section in Sadawarna
DAM ring road area, Subang, West Java. Slope on research area was divided into two layers with
the bottom layer relatively non-cohesive compared to the top of layer. Both soil however dominantly
consist with fine grain soil such as silt and clay. The upper layer of slope can contain more water
with liquid limit of 73,46% compared to the lower layer with 68,27% liquid limit. Simulation result
showed slope that analysed using SSR method has Factor of Safety (FoS) 0,12 lower than LEM
method. SSR method could be used to analysed pessimistic value at worst scenario and could
predicted deformation of slope.
Keyword: Limit Equilibrium, Strength Reduction Factor, Finite Element Method, Soil Mechanic,
Displacement
INTRODUCTION
Slope stability is an issue that commonly
studied at geotechnical engineering. Slope
stability studies are conducted to obtain
factor of safety, which commonly associated
with infrastructure development (Zakaria and
Jihadi, 2017). Slope stability becomes an
interesting study where analyses to determine
stability of the slope often encounter
uncertainties (Zhao et al., 2020).
Uncertainties in slope can be cause by several
factors, including variations in soil properties,
changes in environmental conditions,
geological anomalies, and unexpected
measurement and sampling failures (El-Ramly
et al., 2002).
Over the decades, slope stability has posed a
challenge for experts to accurately assess field
conditions. Therefore, slope stability analysis
methods continue to evolve, with earlier
analyses involving slopes with soil materials
(Zakaria et al., 2018). One of the first
methods is the Limit Equilibrium Method
(LEM), which is used to analyse dam failures.
Since then, this method has undergone
developments such as the use of vertical slices
in slope analysis (Fellenius, 2023) and force
equilibrium between slices (Bishop, 1955).
Another development is the determination of
safety factor with the assumption of non-
circular slipe surface (Morgenstern and Price,
1965).
The LEM principle on slopes can be explained
as the ratio of soil shear strength to slope
shear stress required to achieve equilibrium
(Duncan, 1996)(Dawson et al., 1999). This
principle is considered simple enough to
describe slope stability and remains popular
until now. However, there is a limitation which
the assumption of the slip surface is not
always accurate as well as the shear strength
of the material along the slip surface is
assumed to be uniform. (Duncan and Wright,
1980).
As computational calculations have advanced,
other methods have been developed, such as
the Finite Element Method (FEM) (Veubeke,
1964). This method can determine equilibrium
on slopes without applying conformity of the
formula (Veubeke, 1964). This method then
became known as a good method in slope
stability analysis because it can estimate
deformations and displacements on the slope
(Veubeke, 1964)(Salunkhe et al., 2017). With
the advancement of technology, slope stability
can be simulated with this principle, and this
method has been developed to reduce shear
strength on the slope until it reaches failure
(Dawson et al., 1999; Matsui and San, 1992).
Sadawarna Dam is one of the recent dams in
Indonesia than serves as a water source for
three districts in West Java. It serves as
reservoir for water that mostly arrived from
Cipunagara River. Despite being operated,

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Sadawarna Dam is still undergoing
construction, including construction of road
relocation and other essential infrastructure
(Sitepu and Pontan, 2021). Road construction
on steep slopes then has the potential for
landslides to occur (Allison et al., 2004). FEM
can be used as a method to estimate slope
failure and its deformations using the Shear
Strength Reduction (SSR) approach.
RESEARCH METHOD
The research area is located on the slope of
the East Ring Road development at the
Sadawarna Dam, West Java. The research
involves a section of the natural slope. The
slope consists of two layers with a high degree
of weathering (from the Completely
Weathered Zone to the Top Soil).
The soil mechanical properties are obtained
from direct shear test, while the soil unit
weight values are obtained from soil physical
property tests. Slope stability simulation is
conducted by using the shear strength
properties of soil, such as cohesion and angle
of internal friction, as well as the unit weight,
into the calculations. The slope is simulated
under four different conditions, representing
increasing moisture content in each condition:
dry, optimum, natural, and saturated. The
determination of the optimum moisture
content for each layer is done through a light
compaction test on disturbed soil that passes
through a No.4 sieve.
Shear Strength Reduction
Shear Strength Reduction (SSR) is a method
to reduce the shear strength of slope until
failure (Matsui and San, 1992) (Dawson et al.,
1999). This method becomes popular when
computer became more advanced. (Cala M. &
Flisiak J., 2003).
The slope factor of safety is based on the
safety factor values resulting from a slope that
fails as the shear strength of the slope is
continuously reduced (Dawson et al., 1999).
This method uses Strength Reduction Factor
(SRF) as parameter to show slope’s stability
(Sihotang et al., 2019). By applying the
Figure 1. Slope’s Observation Point
Figure 2.Slope’s Geometry

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principle of shear strength reduction on the
slope, the mechanical properties of soil that
use the Mohr-Coulomb failure criteria can
have their reduction values determined using
the following equation.
󰆒
 󰆒  󰇛
󰇜 ………(1)
Where C and C’ are initial cohesion and
cohesion when failure occurs. ϕ and ϕ’ are
initial internal angle of friction and angle of
friction when failure occurs. With this
equation, the minimum shear strength of soil
before failure can be determined.
RESULT AND DISCUSSION
Based on the physical and mechanical
property tests on two samples representing
each layer of the slope, it was found that UDS
8.1, which represents the lower layer of the
slope, has a clay percentage of 60.78%, silt of
26.92%, and sand of 12.3%. The relatively
high sand percentage makes this soil
considered as non-cohesive soil. In the upper
layer of the slope (UDS 8.2), the percentage
of clay is 51.72%, silt is 47.12%, and sand is
1.16%. The lower sand content in comparison
to UDS 8.1 suggests that this soil has a higher
cohesion value compared to UDS 8.1.
Table 1. Sieve Analysis Result of Slope Materials
No
Sample
Weathering
Sieve Percentage
Gravel
(%)
Sand (%)
Silt (%)
Clay (%)
1
UDS 8.1
CWZ
0
12,3
26,92
60,78
2
UDS 8.2
Top Soil
0
1,16
47,12
51,72
The soil consistency based on the liquid limit,
plastic limit, and plasticity index, it was
determined that the soil type of both layers is
high plasticity silt (MH), as obtained from the
Cassagrande curve. Layer UDS 8.2 has a
greater soil consistency compared to UDS 8.1,
which means it can retain more moisture
content before reaching the saturation phase.
Table 2. Soil Consistency of Slope’s Materials
No
Sample
Weathering
Soil Consistency
Figure 3.Simulation Result Using Shear Strength Reduction

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(LL) (%)
(PL) (%)
(PI) (%)
(USCS)
1
UDS 8.1
CWZ
68,27
24,26
44,01
MH
2
UDS 8.2
Top Soil
73,46
47,76
25,7
MH
The unit weight and shear strength of soil are
used as parameters on slope stability. The test
results show that unit weight and cohesion
values of slope’s top layer (UDS 8.2) has
higher value than slope’s below layer (UDS
8.1), meanwhile internal angle of friction of
UDS 8.1 has higher value than UDS 8.2.
The slope stability analysis was then
conducted using Shear Strength Reduction
Method. The result shows that the critical
Strength Reduction Factor (SRF) has a value
of 0,8, which indicates failure of slope when
shear strength parameters reduced 0,8 times
from the initial shear strength value.
Another analysis was conducted on the same
slope using same shear strength values using
Limit Equilibrium Method (LEM) with the
Bishop method. The result was a more
optimistic with the values of 0,92 compared to
the SSR method, indication a higher factor of
safety.
Table 3. Unit Weight and Shear Strength Values of Slope’s Materials
Layer
Unit Weight (kN/m3)
Cohesion
(kPa)
Internal Angle (°)
UDS 8.1
13,366
8,300
15,32
UDS 8.2
13,827
27,939
10,50
CONCLUSION
Based on the analysis, several conclusions can
be drawn as follows:
1. The grain size of the slope materials is
dominated by clay. The lower layer of
the slope contains a larger proportion
of non-cohesive particles compared to
the other materials.
2. This non-cohesive behaviour is further
reflected in the shear strength values,
where UDS 8.1 material exhibits lower
cohesion but a larger internal friction
angle, which is reflects non-cohesive
soil behaviour.
3. Based on the slope stability
simulations, it was observed that the
safety factor values obtained using
Shear Strength Reduction (SSR)
method are more pessimistic that
those obtained using Limit Equilibrium
Method (LEM). This difference arises
from the fact that in the SSR method,
the slope body is subdivided into small
grids, resulting in different soil
displacements until failure occurs. As
a result, the SSR method provides a
more accurate and pessimistic safety
factor compared to LEM, which only
calculates shear strength within the
potential failure surface.
4. The SSR method predicts
deformations on the slope with a
maximum material displacement of

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0,110 meters when the slope is stable
and 1,9 meters when the slope fails.
ACKNOWLEDGEMENT
The Author would like to thank Fakultas Teknik
Geologi Universitas Padjadjaran and
Laboratorium Geologi Teknik Universitas
Padjadjaran for the aids to the research on
disaster identification in Sumedang Regency,
West Java.
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APPENDIX
Figure 4. SRF vs Maximum Total Displacement at SRF 0,8