Performance of slope under seismic conditions

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 03 Special Issue: 06 | May-2014 | RRDCE - 2014, Available @ http://www.ijret.org 92
PERFORMANCE OF SLOPE UNDER SEISMIC CONDITIONS
Rajani Shurpali1
, L Govindaraju2
1
Assistant Professor, Dept. of Civil Engineering Sri Venkateshwar College of Engineering, Bangalore, Karnataka, India
2
Associate Professor, Dept. of Civil Engineering, UVCE, Bangalore University, Bangalore, Karnataka, India
Abstract
The seismic stability of earth slopes has been a topic of considerable interest in geotechnical engineering practice for the past few
decades. The failure of slopes during earthquakes can lead to the significant losses. The seismic stability of slopes is influenced by
many factors and complete slope stability evaluation must consider the effects of each like geological, hydrological, topographical,
geometrical, strong ground motion parameters during an earthquake and soil characteristics all influence the stability of a slope.
Hence, the seismic performance of these structures requires appropriate evaluation during their design. Such performance is best
evaluated through an assessment of the potential for seismically induce permanent displacements. This paper presents numerical
studies on response of unreinforced slopes due to dynamic loading based on finite element analysis. Model embankments of silty sand
and clayey sand have been considered. The parameters selected for the dynamic analysis includes embankment of height 5m and 12m
with gentle slope 300 and steep slope 500. The sub base depth (D) is varied with height of the slope (H) with D/H ratio from 0.5 to
2.0.The various past earthquake records are considered with different amplitudes ranging from 0.1g to 0.4g.The results are presented
in the form of factor of safety against slope failure under static and seismic conditions and settlement of crest of slopes. It is observed
that as the height of the slope increases the maximum crest displacement increases for both type of soil and sub soil conditions.
Keywords: Amplitude, Earthquake ground motion, Slope, Settlement, etc
-----------------------------------------------------------------------***-----------------------------------------------------------------------
1. INTRODUCTION
Earthquakes are the one of the most devastating natural hazards
in the world they causes a great loss of life and property every
year. The countries in earthquake prone area suffer a lot due to
frequent medium to large-scale earthquakes. Earthquakes can
play a triggering role for the slope stability problems as well
and can reactive the old landslides bringing about the
sequence of disaster. Analysis of the static and seismic
stability of natural and manmade slopes is a challenging
geotechnical problem. Often, different professionals analyzing
the same problem will estimate a wide variation in expected
performance. That variation results from variable levels of
care in site exploration, laboratory testing, and the
performance of stability analyses.Previously, many studies on
the response of slopes have been carried out based on
experimental and numerical analysis by various researchers
(Ambraseys N & Srbulov M, 1995; Ashford,S.A & Sitar N,
2002; Bijan Khazai & Nicholas Sitar,2000; Griffiths D.V, &
Lane P.A. 1999; Ka-Ching San & Lovlesh Chinsky, 1994;
Sawada T et al. 1993; Tan D & Sarma S.K 2008; Radoslaw L.
Michalowski and Liangzhi You,1999; Debabrata Giri and
Aniruddha Sengupta.
In the present study, the numerical analyses of model slopes
have been carried out based on finite element procedure using
the Geo Studio software. The effect of ground motion
parameters such as amplitude and frequency of vibrations for
the past history earthquake data like Bhuj earthquake and
Northridge earthquake.
1.1 Parametric Studies:
Sand and clay model embankments of height (D) 5m and the
ratio of depth of sub base (H) to the height of the slope (D),
H/D ratio are also considered for various slope 0.5, 1.0, 1.5
and 2.0 as shown in Fig. 1 are considered. Both the models
were studied for two slope angle (θ) gentle slope 300
and steep
slope 500
. The properties of soil have been given in Table 1.
Fig 1: Typical embankment with slope angle (θ)

_______________________________________________________________________________________
Table 1: Properties of Soil
Soil Type Silty Sand Medium Clay
Cohesion,c kPa 1 24
Angle of Friction (θ) 20 5
Poisson’s Ratio(μ) 0.3 0.3
Static young's modulus (
MPa)
18 28.7
Dynamic shear modulus
( MPa)
24 30
1.2 Numerical Analysis
Numerical analyses of the embankments in static and dynamic
conditions were carried out using finite element procedure
employing computer program Geo Studio. The analyses of the
both soil models was carried out based on Mohr Coulomb
yield criteria. The embankments were discretised by iso
parametric elements. In the static condition, gravity analysis
was performed and the stresses are allowed to reach
equilibrium state under self weight only by applying
appropriate boundary conditions. The boundary at the bottom
of the embankment was fixed. The two vertical side
boundaries were assumed to be on roller allowing for vertical
deformation. Dynamic analyses of the embankments were
carried out using QUAKE/W. In dynamic analysis, the two
vertical side boundaries were made free to allow deformations
and fixed at the base in order to simulate the field condition.
The vertically propagating seismic waves from base of the
embankment that reaches the crest of the embankment induce
the displacements.
Initially, static analysis has been carried out to check the
stability of the slope in terms of factor of safety. The analyses
were carried out for both silty sand and medium clay
embankments. Table 2 shows the stability of slopes in terms of
factor of safety embankments of silty sand and medium clay.
It is observed that the slopes are stable under static condition
for slope angles 300
and 500
for both types of embankments.
2. DYNAMIC ANALYSIS
The dynamic analyses have been carried out on embankments
of both silty sand and medium clay materials using various
ground motions for past history earthquake Northridge and
Bhuj earthquakes 25s and 134s respectively as presented in fig
2a,b c, d and fig 3a,b,c,d respectively. These ground motions
were excited at the base of the embankment and the resulting
deformations at the crest of the embankments due to
propagation seismic waves were evaluated. It is observed that,
as the amplitude of acceleration increases and ratio of depth of
sub base (H) to the height of the slope (D), H/D ratio
increases, the magnitude of the crest settlement increases at a
given frequency.
0 5 10 15 20 25
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
ACCELERATION(g)
TIME(s)
A=0.1g,f=1.5Hz
(2a)
0 5 10 15 20 25
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
ACCELERATION(g)
TIME(s)
A=0.2g,f=1.5Hz
(2b)
0 5 10 15 20 25
-0.2
-0.1
0.0
0.1
0.2
0.3
ACCELERATION(g)
TIME(s)
A=0.3g,f=1.5Hz
(2c)

_______________________________________________________________________________________
0 5 10 15 20 25
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
ACCELERATION(g)
TIME(s)
A=0.4g,f=1.5Hz
(2d)
Fig 2: Northridge Earthquake motion for varying amplitude
0 20 40 60 80 100 120 140
-0.10
-0.05
0.00
0.05
0.10
ACCELEATION(g)
TIME(s)
A=0.1g,f=1.1Hz
(3a)
0 20 40 60 80 100 120 140
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
ACCELERATION(g)
TIME(s)
A=0.2g,f=1.1Hz
(3b)
0 20 40 60 80 100 120 140
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
ACCELERATION(g)
TIME(s)
A=0.3g,f=1.1Hz
(3c)
0 20 40 60 80 100 120 140
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
ACCELERATION(g)
TIME(s)
A=0.4g,f=1.1Hz
(3d)
Fig 3: Bhuj Earthquake motion for varying amplitude
Fig 4 shows the Variation of Max Crest Displacement Vs
Acceleration for Northridge Earthquake, 300
& 500
, silty sand.
It is observed that as the amplitude of acceleration increases
and H/D ratio, magnitude of displacement increases. Also as
the inclination of slope increases the displacement increases.
Table 3& 4gives the comparison of factor of safety of slope
under static and dynamic conditions

_______________________________________________________________________________________
0.10 0.15 0.20 0.25 0.30 0.35 0.40
0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
300
MAX.VERTICALDISPLACEMENT(mm)
ACCELERATION(g)
30
0
,H/D=0.5
30
0
,H/D=1.0
30
0
,H/D=1.5
30
0
,H/D=2.0
50
0
,H/D=0.5
50
0
,H/D=1.0
50
0
,H/D=1.5
50
0
,H/D=2.0
Fig 4: Variation of Max.Crest Displacement Vs Acceleration
for Northridge Earthquake, 300
& 500
, silty sand.
Acceleration for Bhuj Earthquake, 300
& 500
, silty sand. It is
observed that increase in amplitude of acceleration increase in
crest settlement. However, it is interesting to note that for H/D
ratio1.5 for 500
slope, maximum crest displacement is
observed. This may be attributed to the close matching of the
excitation frequency with the natural frequency of the
embankment.
0.10 0.15 0.20 0.25 0.30 0.35 0.40
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
ACCELERATION(g)
30
0
,H/D=0.5
30
0
,H/D=1.0
30
0
,H/D=1.5
30
0
,H/D=2.0
50
0
,H/D=0.5
50
0
,H/D=1.0
50
0
,H/D=1.5
50
0
,H/D=2.0
for Bhuj Earthquake, 300
& 500
, silty sand.
Acceleration for Northridge Earthquake, 300
& 500
, Medium
Clay. It is observed that as the amplitude of acceleration
increases and H/D ratio, magnitude of displacement increases.
Also as the inclination of slope increases the displacement
increases.
0.10 0.15 0.20 0.25 0.30 0.35 0.40
0
50
100
150
200
250
ACCELEARTION(g)
30
0
,H/D=0.5
30
0
,H/D=1.0
30
0
,H/D=1.5
30
0
,H/D=2.0
50
0
,H/D=0.5
50
0
,H/D=1.0
50
0
,H/D=1.5
50
0
,H/D=2.0
Fig 6: Variation of Max .Crest Displacement Vs Acceleration
for Northridge Earthquake, 300
& 500
, medium clay
0.10 0.15 0.20 0.25 0.30 0.35 0.40
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
ACCELERATION (g)
30
0
,H/D=0.5
30
0
,H/D=1.0
30
0
,H/D=1.5
30
0
,H/D=2.0
50
0
,H/D=0.5
50
0
,H/D=1.0
50
0
,H/D=1.5
50
0
,H/D=2.0
for Bhuj Earthquake, 300
& 500
, medium clay
Acceleration for Bhuj Earthquake, 300
& 500
, Medium Clay. It
is observed that increase in amplitude of acceleration increase
in crest settlement. However, it is interesting to note that for
H/D ratio 2.0 for 300
slope, maximum crest displacement is
observed. This may be attributed to the close matching of the
excitation frequency with the natural frequency of the
embankment

_______________________________________________________________________________________
Table 2: Stability of soil slope of 5m high
Type of Soil Ф in degrees H/D ratio Critical FOS
0.5 0.9
1.0 0.882
1.5 0.863
2.0 0.87
0.5 2.544
1.0 2.779
1.5 2.592
2.0 2.696
0.5 0.561
1.0 0.551
1.5 0.527
2.0 0.513
0.5 17.01
1.0 2.101
1.5 20.53
2.0 3.179
Medium Clay 50
Silty Sand 30
Medium Clay 30
Silty Sand 50
without reinforcement Reinforced (Nos)
0.5 0.561 1.635 (4)
1 0.551 2.581 (2)
1.5 0.527 2.116 (3)
2 0.513 2.535 (3)
0.5 0.713 3.760 (4)
1 0.678 2.572 (2)
1.5 0.693 2.447 (2)
2 0.841 4.507 (1)
0.5 17.01
1 2.101
1.5 20.539
2 3.179
Silty sand 50 5m
Type of soil
Angle of inclination
in 0 Height of slope D/H ratio
Critical Factor of safety
Clayey sand 50 5m
Medium clay 50 5m
3. CONCLUSIONS
A numerical study on the seismic response of slopes of silty
sand and medium clay has been carried. The effect of ground
motion parameters such as amplitude and frequency of
vibrations on the response of these slopes were explored.
Based on the present study the following conclusions may be
drawn.
1. As the height of the slope increases the maximum
crest displacement increases for the various type of
soil. The sub soil condition effects the amplification
of the slope.
2. As the slope angle increases, the crest settlement of
the slope during dynamic loading increases and vice
versa.
3. Increased crest settlements occur at increasing
acceleration levels. The significant crest
displacements are observed at the resonant
frequencies.
REFERENCES
[1] Bray, J. D., and Rathje, E., [1998]. “Earthquake
Induced Displacements of Solid Waste Landfills,”
Journal of Geotechnical and Geoenvironmental
Engineering Vol. 124, GT 7, July 1998, pp. 242-253.
[2] Franklin, A. G. and Chang, F. K., [1977].
“Earthquake Resistance of Earth and Rock-Fill Dams.”
Report 5: Permanent Displacements of Earth
Embankments by Newmark Sliding Block Analyses,
Miscellaneous Paper S-71-17, Soils and Pavements
Laboratory, U.S. Army engineer Waterways
Experiment Station, Vicksburg, Mississippi
[3] Newmark, N. M., [1965]. “Effects of Earthquake on
Dams and Embankments.” Geotechnique, London,
England, Vol. 5, No. 2, June 1965, pp. 139-160.
[4] Makdisi, F.I. and Seed, H. B., [1978]. “A Simplified
Procedure for Estimating Dam and Embankment
Earthquake-Induced Deformations.” Journal of the
Geotechnical Engineering Division, ASCE, Vol. 104,
No. GT7, July 1978, pp. 849-867.
[5] Jibson, R.W., [1993]. “Predicting Earthquake Induced
Landslide Displacements Using Newmark’s Sliding
Block Analysis,” Transportation Research Board,
National research council, Washington, D.C.
[6] Martin, G.R., and Qui, P., [1994]. “Effects of
Liquefaction on Vulnerability Assessment,” NCEER
Highway Project on Seismic Vulnerability of New and
Existing Highway Construction, Year One Research
Tasks - Tech. Research Papers
[7] ANSAL, A “ Lecture Notes On Soil Dynamics’’
[8] KRAMER, S.L. “Earthquake Geotechnical
Engineering’’ (1996)
[9] Design of Foundation in Sesmic Area: Principle and
Application.
[10] Debabrata Giri, Aniruddha Sengupta (2009)
“Dynamic Behaviour of small scale nailedsoil slopes”,
Geotechnical & Geological Engineering 27: 687-698.
BIOGRAPHIES
Mrs Rajani Shurpali graduated from UVCE,
Bangalore Unversity and later did Post
graduation in UVCE ,Bangalore University.
Currently working as Assistant Professor in
Sri Venkateshwar College of Engineering,
Bangalore 562157.
Dr. L.Govindaraju is currently Associate
Professor, in the Department of Civil
Engineering, UVCE, Bangalore University,
and Bangalore.

Performance of slope under seismic conditions

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Performance of slope under seismic conditions