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Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Paper No. K-099)
EFFECT OF BLAST LOAD ON SEISMIC SLOPE STABILITY USING FLAC
Ritika, PG student, Dept. of Civil Engg., IIT Bombay, Mumbai, India. email: [email protected]
Kaustav Chatterjee, PG student, Dept. of Civil Engg., IIT Bombay, Mumbai, India. email: [email protected]
Deepankar Choudhury, Associate Professor, Dept. of Civil Engg., IIT Bombay, Mumbai, India. email: [email protected]
ABSTRACT: In this paper, the necessity of counting blast load in seismic slope stability analysis is revealed. Typical soil
slope with embedded pipeline subjected to seismic and blast loads are modelled using finite difference based geotechnical
software FLAC2D. Results are shown in the form of vertical displacements along the face of slope and parametric variations for
types of soil, inclination of slope and location of pipeline are obtained. It is found that in clays, blast load must be considered
in the seismic slope stability analysis, whereas for sand, it is not essential. In clays gentle slope with pipeline close to the slope
face can be used, but in sands, pipeline must be away from slope face.
INTRODUCTION
A quantitative assessment of the stability of soil slope is
important when a judgment is needed about whether the slope
is vulnerable to failure due to various dynamic loads or not.
This assessment is made in terms of either determining the
displacement along the face of slope and/or critical
acceleration under seismic conditions.
Slopes can be subjected to various types of dynamic loads
such as earthquake load, blast load, wind load and others. A
complete slope stability analysis must consider the effects of
each dynamic load. Vibrations induced by bombing activities
of terrorists or blasting activities for tunnel constructions etc.
have detrimental effects on the existing important slopes in
seismically active zones. So the inclusion of the effect of
blast loads in addition to the seismic loads has become
today’s necessity for geotechnical researchers for safe design
of such slopes.
The analysis of slope subjected to seismic load can be done
by using either pseudo-static or pseudo-dynamic approach. In
pseudo-static approach, the effects of an earthquake are
represented by constant vertical or horizontal seismic
accelerations. The first explicit application of pseudo-static
approach is the analysis of seismic slope stability by limit
equilibrium method assuming planar failure surface [1].
However this force-based approach cannot provide any
information on deformations associated with slope failure. A
method for prediction of permanent displacement of slope
subjected to any ground motion known as ‘Sliding Block
Analysis’ was developed in 1965 [2]. It was further modified
by considering the movement of a rigid block on a slope in
1975 [3]. In 2003, limit analysis method [4] and in 2007,
vertical slice method [5] were used for the analysis of slope
by the conventional pseudo-static approach.
Human activities like mining, construction, and defense
works produce dynamic excitation and lead to the instability
of nearby geotechnical structures. Several incidents of slope
failures have been occurred by the explosion of oil pipeline
passing through the slope, terrorist attack, blast in mining etc.
The instability of rock slopes and underground structures
subjected to ground vibration produced by rock blasting has
been studied [6,7]. Moreover, as coal seams are being
excavated from deeper benches, possibility of slope
instability problems may increase. In order to understand the
causes of slope failure phenomena, in situ monitoring of
ground vibration and analysis of slope behaviour under
dynamic loading are absolute necessity. Generally, peak
particle velocity (PPV) is considered to be a reliable vibration
monitoring parameter for the assessment of attenuation
characteristics of blast wave and structural damage [8].
Dynamic responses of continuous rock masses under blast
loading have been studied to investigate blast induced wave
propagation and tensile damage to rock masses [9,10,11].
Numerical modeling of blast wave propagation through rock
mass and effects of water and joints were also studied [12].
Many researchers worked on the rock slope subjected to
vibrations due to blasting but the study on the effect of blast
loading on the seismic soil slope stability is scarce.
In present study, numerical models of soil slope have been
analyzed under seismic and blast loadings by using the finite
difference based geotechnical software FLAC2D (Fast
Lagrangian Analysis of Continua, version 6.0, Itasca Inc.
2008) [13].
PRESENT METHODOLOGY
The variation in displacement along face of the slope due to
the change in position of pipeline, soil type and inclination of
slope are studied. Soil slope of 6m height resting on the 3m
deep foundation along with the pipeline of 1m diameter is
modelled as shown in Fig 1. Pipeline was simulated as a
circular cavity to make the modelling simpler. The position
of pipeline (d) was varied as 2m, 3m and 4m from the face of
slope at a depth of 3m from the top. Analysis was carried out
on various types of soil having the physical properties as
given in Table 1.
For each soil, analysis was done for different inclinations of
slope ( ) i.e. = 30˚, 35˚ and 40˚. The analysis of entire
assembly was done for two types of dynamic loads i.e.
639
Ritika, Kaustav Chatterjee and Deepankar Choudhury
seismic load and blast load. For seismic analysis, pseudo-
static approach was used and the model was subjected to
horizontal seismic acceleration coefficient (kh) of 0.1g and
vertical seismic acceleration coefficient (kv) of 0.0kh.
Table 1 Physical soil properties considered in present study
Soil
Type
Density
(kg/m3)
( )
Poisson’s
Ratio
( )
Elastic
Modulus
(Pa)
(E)
Cohesion
(Pa)
(c)
Soil
Friction
Angle
( )
Dense
Sand 1800 0.30 5.6x108 1000 32˚
Soft
Clay 1420 0.25 3.0x106 25000 5˚
Stiff
Clay 1690 0.20 10.0x106 75000 5˚
Foun-
dation
soil
1900 0.30 6.0x108 1000 32˚
Fig. 1 Schematic diagram of soil slope with embedded
pipeline as used in present analysis using FLAC
Blast load was applied at the inner boundary of the cavity in
the form of pressure wave. Pressure can be characterised by
the exponential decreasing time histories as shown in Fig. 2.
The arrival time for blast pressure at any point R, away from
point of detonation as per TM5-855-1 [14] is,
aRt
v (1)
R is equal to 0.5m because point of detonation was at the
centre of the pipeline and was applied at the boundary surface
of pipeline. And v is the wave velocity. The peak pressure is
given by,
n
W
RfcP )()(160
30 (2)
From the peak, the pulse decays monotonically with time as,
)(
0at
t
ePP (3)
where P0 is the peak pressure in N/m2, f is the ground
coupling factor ranging from 0 to1 depending on the depth of
explosion, c is the acoustic impedance , W is the charge
weight in kg, n is the attenuation coefficient. In the present
study 6 kg of TNT charge is applied. Blast pressure increases
linearly up to the peak value and then declines exponentially.
Dynamic time for the explosion is taken equal to 3 ms.
Fig. 2 Blast pressure vs. time history with ta as rise time and
td as damped time.
Table 2 gives the dynamic properties of the soil used to
estimate the peak blast pressure using formula given by TM5-
855-1.
Table 2 Blast input parameters for dry soils as per TM5- 855-
1 [14]
Soil
Type
v
m/s n c x 105 f
ta
ms
Dense
Sand 1000 2.50 18.40 0.75 0.50
Soft
Clay 1500 2.50 21.75 0.75 0.33
Stiff
Clay 2000 2.25 34.60 0.75 0.25
RESULTS AND DISCUSSIONS
Results of FLAC2D analysis are elaborated below in the terms
of vertical displacements of slope face as shown in Fig. 3(a)
and 3(b). Mohr-Coulomb failure model was selected for the
analysis. Firstly the model was solved for the static condition
to equilibrate it, and then the dynamic loadings were applied.
(a)
(b)
Fig. 3 Vertical displacement contours of soft clay slope with
embedded pipeline at 3m away from slope face with slope
inclination of 30˚ subjected to (a) Seismic Load (b) Blast
Load
Pipeline is modelled as a cavity
6m
3m Foundation soil
d
3m
Soil slope
640
Effect of blast load on seismic slope stability using FLAC
Table 3 shows the vertical displacement of slope in dense
sand for both seismic and blast loads. Fig. 4 depicts the
variation of vertical displacement due to seismic and blast
loadings with the distance of pipeline from the face of slope
for various inclinations in case of dense sand and it is
observed that with an increase in slope angle, displacements
due to seismic and blast loadings are increasing and are
vertically downwards. However as the location of pipeline is
moving inwards, displacement is gradually reducing for both
the cases. Displacements due to seismic loading is more than
due to blast, so no further analysis for blasting is required for
dense sands.
Table 3 Vertical displacement (m) along slope face of dense
sand at the pipeline position 2m, 3m, and 4m from the face of
slope. kh=0.1g, kv=0.0kh
Slope
Angle
Load
type
Vertical displacement (m) for
location of pipeline from face (d)
2m 3m 4m
30˚
Seismic -12.0 -9.0 -0.15
Blast -0.1 -0.1 -0.10
35˚ Seismic -40.0 -22.5 -15.0
Blast -20.0 -4.0 0.10
40˚ Seismic -40.0 -35.0 -30.0
Blast -30.0 -15.0 -12.5
Fig. 4 Vertical displacement (m) v/s distance of cavity from
the face of slope (m) in the case of dense sand.
Table 4 and Fig. 5 show the vertical displacement in the case
of soft clay. From Fig. 5, it can be observed that
displacements due to blast loading is vertically up in all the
case, but in pseudo-static approach for slope angle = 30˚, it
is vertically up and for other two case , it is downward. So in
seismic case as slope angle is increasing, displacement is
increasing in vertically downward direction. Nature of
displacement due to blast loading is completely different and
large as compared to pseudo-static approach. So analysis for
blast loading must be considered. However, as location of
pipeline is moving inwards, displacement due to blasting is
increasing for slope angle = 300. And for = 350 and 400
slope angles, it is showing almost same displacement. It
indicates as slope is becoming steeper, displacement is
increasing but location of pipeline is not the matter of
concern. So for all slopes in soft clay, location of pipeline
near to the slope face can be used.
Table 4 Vertical displacement (m) along slope face of soft
clay at the pipeline position 2m, 3m, and 4m from the face of
slope. kh=0.1g, kv=0.0kh
Slope
Angle
Load
type
Vertical displacement (m) for
location of pipeline from face (d)
2m 3m 4m
30˚
Seismic 0.03 0.03 0.03
Blast 0.03 0.20 0.20
35˚ Seismic -0.04 -0.05 -0.025
Blast 0.23 0.23 0.24
40˚ Seismic -0.05 -0.10 -0.40
Blast 0.25 0.25 0.25
Fig. 5 Vertical displacement (m) v/s distance of cavity from
the face of slope (m) in the case of soft clay
Table 5 Vertical displacement (m) along slope face of stiff
clay at the pipeline position 2m, 3m, and 4m from the face of
slope. kh=0.1g, kv=0.0kh
Slope
Angle
Load
type
Vertical displacement (m) for
location of pipeline from face (d)
2m 3m 4m
30˚
Seismic 0.013 0.035 0.013
Blast 0.100 0.100 0.100
35˚ Seismic -0.025 -.0.025 -0.025
Blast 0.150 0.170 0.200
40˚ Seismic -0.025 -0.050 -0.050
Blast 0.200 0.250 0.250
Table 5 and Fig. 6 show the vertical displacement in the case
of stiff clay and it is observed that nature of results in stiff
clay are similar to those in soft clay but displacement in stiff
clay is much lesser than that in soft clay, as expected.
641
Ritika, Kaustav Chatterjee and Deepankar Choudhury
Fig. 6 Vertical displacement (m) v/s distance of cavity from
the face of slope (m) in the case of stiff clay
CONCLUSIONS
The present study provides the necessity of inclusion of blast
load in the seismic slope stability analysis for different kinds
of soil slopes by comparing the vertical displacements. It is
found that for the case of clays, there is a need for
considering blast loading in the seismic analysis, because
vertical displacement for blast load is significant. However,
in sand it is optional to include blast load, because slope may
not be stable in the seismic case itself due to shear
fluidization [15]. And if also dense sandy slope becomes
gentler, the vertical displacement in seismic case is coming
more significant as compared to blast load. As per the
location of pipeline, in clays gentle slope with the pipeline
close to the face of slope is acceptable, but in sands more the
distance of pipeline away from the slope face, less is the
displacement of slope face and hence preferred in design.
LIST OF NOTATIONS
c Acoustic impedance ta Arrival time for blast pressure in ms
n Attenuation coefficient W Charge weight in kg
c Cohesion in Pa
Density in kg/m3
E Elastic modulus in Pa
f Ground coupling factor kh Horizontal seismic acceleration coefficient
kv Vertical seismic acceleration coefficient
P0 Peak pressure in N/m2 Poisson’s ratio
d Position of the pipeline from the face of slope in m
R Radius of the pipeline modelled as a cavity in m
Soil friction angle in degree
Slope inclination in degree
v Wave velocity in m/s
ACKNOWLEDGEMENT
Authors are thankful to SERC division of DST, Govt. of
India, for sponsoring the research project number
SR/FTP/ETA-41/2008, from which the above technical study
has been carried out.
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