Slope Stability Analysis by Shear Strength Reduction Method: J C E U
Slope Stability Analysis by Shear Strength Reduction Method: J C E U
Slope Stability Analysis by Shear Strength Reduction Method: J C E U
ABSTRACT: This research uses the shear strength reduction method to study soil slopes stability. In this
ORIGINAL ARTICLE
method shear strength is considered to be reduced as less as failure occurs. It uses Plaxis, which is capable of
PII: S225204301500008-5
Accepted 20 Dec. 2014
To cite this paper: Farshidfar N., Nayeri A. 2015. Slope Stability Analysis by Shear Strength Reduction Method J. Civil Eng. Urban., 5 (1): 35-37.
Journal homepage: http://www.ojceu.ir/main/
35
minimum shear stress required for creating preliminary of slopes. Table 4 studies the effects of groundwater level
failure. on slopes stability.
In shear strength reduction method, soil shear A slope with angle 1 to 3 and internal friction angle
strength is gradually decreased by applying finite element of 30° was considered in the calculations. Values of
and finite difference programs as long as the first groundwater level are of downstream.
indications of failure appear. Safety factor is defined as
the ratio of real shear strength of soil to reduced shear
strength.
The shear strength reduction method is superior to
the other methods investigating slopes stability. One of
the advantages is that there is no need to the primary
guess at determination of critical failure surface. Due to
the high-speed computer systems, this method is used
increasingly today than before. Figure 1. The scheme of constructed soil slope
To examine slopes stability through shear strength
reduction method, modeling is carried out using a set of Table 1: Characteristics of soils
safety factors as trial and error. In this method (RRS), Parameters γ γsat E ν C Characteristic
slopes stability is defined using soil strength 1-Sand 16 20 3000 0.3 1 Drained
characteristics as follows: 2-Clay 15 18 1000 0.33 2 Un Drained
3-Peat 8 11 350 0.35 5 Un Drained
1
C trial C (1)
F trial Table 2. Safety Factor vs. slope angle
1 β
trial
arctan( tan ) (2) 15 30 45 60 75 90
F trial F.S 1.557 1.262 1.020 0.702 0.500 0.259
Where Ctrial and φtrial are soil reduced strength Table 3. Safety Factor vs. slope friction angle
characteristics (adhesion and friction angle) in proportion
φ 10 20 30 40 45
to the real mode (C, φ).
As it is seen from Eq 1 and Eq 2, the increase in F.S 0.969 1.305 1.384 1.463 1.505
safety factor causes the reduction of soil strength
parameters and such strength reduction makes the slope Table 4. Safety Factor vs. slope level of water
closer to instable behavior. Evidently, higher safety factor Level of ground
0 1.5 3 4.5 6
water
denotes much more tendency of a slope to be stable
F.S 1.900 1.890 1.884 1.749 1.384
against driving and destructive forces.
As per the results of Table 4, with the groundwater
RESULTS AND DISCUSSION
level increasing, the slope’s safety factor lowers and the
slope exposes to further risk of failure. The results
A cohesion less soil slope with 4-meter height was
achieved so far have been obtained from analyzing slopes
built on a bed of cohesive soils with 6 meter height
under static mode and no seismic force was imposed to
(Figure 1). Table 1 shows characteristics of soils
the slopes. However, as far as we know, most slopes
strengths. The backfill was built in two steps. Execution
destructions are caused by earthquakes and under the
period was considered as 7 days for each step and there
influence of seismic forces of an earthquake.
was a 100-day interval between the executions of the two
One of the problems of studying the slopes under
steps. Construction of the soil slope was modeled by
seismic forces has always been how to model seismic
Plaxis software. Lattice work of the model is in course
forces. Different methods have been proposed for
mode.
modeling seismic forces. One of the frequently used
The analyzed backfill along horizontal direction
methods is pseudo-static method. Some assumptions are
was considered as fixed with zero displacement. The back
considered in pseudo-static method for simplifying
fill’s lower boundary along horizontal and vertical
calculations; some of them are: ignoring time, ignoring
directions was also considered as fixed. Soils’ behavior
phase difference, and ignoring oscillatory nature of
was modeled using elasto-plastic and Mohr–Coulomb
earthquake forces.
model.
Despite the unreal assumptions of pseudo-static
Table 2 shows the results obtained from
method in seismic analysis of slopes, it has had frequent
investigation slope stability with friction angle of 30 o and
applications, as it provides acceptable results during a
adhesion of 2 KN/m3 and 6-meter groundwater level from
short period. Here, seismic forces were modeled in a
downstream for different values of backfill slope (β). The
pseudo-static manner. Horizontal acceleration coefficient
results showed that slope’s safety factor lowers with the
(Kh) and vertical acceleration coefficient (kv) were
slope angle increasing and slope exposes to further risk of
considered equal to 0.2 and zero, respectively.
failure.
Calculations were made for a slope with angle 1 to
Table 3 shows the results obtained from studying
3 and internal friction angle of 30° and groundwater level
slope stability with angle 1 to 3 and 6-meter groundwater
of 6 meters. Figure 2 compares the results obtained for
level from downstream for different friction angles (φ).
such a slope under the influence of seismic forces using
The results of Table 3 shows that the increase of friction
pseudo-static method and the ones obtained from static
angle leads to increasing safety factor and further stability
To cite this paper: Farshidfar N., Nayeri A. 2015. Slope Stability Analysis by Shear Strength Reduction Method J. Civil Eng. Urban., 5 (1): 35-37.
Journal homepage: http://www.ojceu.ir/main/
36
analysis for the same slope without considering seismic seismic aspects, in Geosynthetics and their
forces. applications, Chapter 14, Thomas Telford, pp: 327-
As per the results of Figure 2, the slopes under the 392.
influence of seismic forces have safety factor lower than Chen, J., Yin, J. H., and Lee, C. F., (2003), Upper bound
the static mode, which indicates lower stability of the limit analysis of slope stability using rigid finite
slopes under seismic mode. Here, we study stability of a elements and nonlinear programming, Canadian
slope with height of 10 meters and angle of 45° using the Geotechnical Journal, 40: 742–752.
FEM and Plaxis. Table 5 shows the results of this research Cheng, Y. M., Liang, L., Chi, S. C., and Wei, W. B.,
and the results of the analysis of Dawson et.al (1999) (2008), Determination of the Critical Slip Surface
performed by FDM and Flac. Using Artificial Fish Swarms Algorithm, ASCE
It showed that the results obtained at lower angles Journal of Geotechnical and Geoenvironmental
of internal friction are almost similar; however, with the Engineering, 134 (2): 244-251.
internal friction angle of soil increasing, FEM obtains Cheng, Y. M., Liang, L., Chi, S., and Wei, W. B., (2007),
bigger safety factor. Particle swarm optimization algorithm for the
location of the critical non-circular failure surface
in two-dimensional slope stability analysis,
Computers and Geotechnics, 34: 92–103.
Choudhury, D., Nimbalkar S. S., (2006), Pseudo-
Daynamic approach of seismic active earth
pressure behind retaining wall, Geotechnical and
Geological Engineering, 24: 1103-1113.
Dawson, E. M., Roth, W. H., and Drescher, A., (1999),
Slope stability analysis by strength reduction,
Geotechnique, 49(6): 835-84
Jahanandish, M., and Keshavarz, A., (2005), Seismic
bearing capacity of foundations on reinforced soil
slopes, Geotextiles and Geomembranes, 23 (1): 1–
25.
Keshavarz, A., (2006), The Analysis of seismic stability
reinforced soil structures, PhD Thesis, Shiraz
Figure 2. Comparison of Safety Factor obtained from University, Shiraz, Iran.
Static & Pesudo-Static method by SSR technique Li, X., (2007), Finite element analysis of slope stability
using a nonlinear failure criterion, Computers and
Table 5. Comparison of Safety Factor obtained from Geotechnics, 34: 127–136.
FEM&FDM method Lo, S. C. R. and Xu, D. W., (1992), A strain based design
φ 10 20 30 40 method for the collapse limit state of reinforced
F.S (This Study) 0.898 1.071 1.357 1.699 soil walls and slopes, Canadian Geotechnical
F.S (Reff. 11) 1.019 1.026 1.031 1.080 Journal, 29 (8): 832–842.
Zheng, H., Tham, L. G., and Liu, D., (2006), on two
definitions of the factor of safety commonly used
CONCLUSION in the finite element slope stability analysis,
Computers and Geotechnics, 33: 188–195.
The studies and calculations of the earlier sections
show some facts on the slopes behavior, a summary of
which is explained here:
The use of shear strength reduction method of
soils led to some simple analyses with acceptable results.
With the slopes angle increasing, safety factors of
slopes reduce and slopes expose to further destructions
threshold.
Increasing internal friction angle of a backfill
leads to slopes stability.
Rising groundwater level increases failure risk of
slopes to some extent.
The results of FEM and FDM at lower internal
friction angles are similar.
Soil shear strength reduction method predicts the
deformations occurred over time.
REFERENCES
To cite this paper: Farshidfar N., Nayeri A. 2015. Slope Stability Analysis by Shear Strength Reduction Method J. Civil Eng. Urban., 5 (1): 35-37.
Journal homepage: http://www.ojceu.ir/main/
37