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Khan et al. /Int.J.Econ.Environ.Geol.Vol. 8(2) 63-68, 2017
Assessment of Soil Liquefaction Potential in Defence Housing Authority,
Karachi, Pakistan
Sumaira Asif Khan1*
, Zubaid Saeed1, Adnan Khan
1, Gulraiz Hamid
1, Syed Waseem Haider
2
1Department of Geology, University of Karachi, Pakistan 2National Institute of Oceanography, Karachi, Pakistan
*Email: [email protected]
Received: 30 December, 2016 Accepted: 22 February, 2017
Abstract: The occurrence of liquefaction phenomenon may be induced in the event of a large magnitude earthquake
but sometimes loose, saturated and poorly graded sand may be subjected to liquefaction due to the vibration produced
by other sources. Liquefaction could cause damage to building and infrastructure due to sudden increase of pore
pressure in the loose layers of saturated sand causing the loss of bearing capacity and shear strength. Defence Housing
Authority (DHA) is the well planned residential scheme established by Pakistan Army along the coastal belt of
Karachi. The soil occurring in DHA is fine grained, poorly graded and mainly comprises of sandy silt and silty sand of
Recent age, where water table is encountered at very shallow depth. Hence, it is important to assess the geotechnical
behavior of the soil in DHA area, where most of the high rise buildings and mega civil structures are being constructed.
In present study, seismic soil liquefaction was evaluated at 15 sites (30 bore holes) in DHA by using simplified
empirical method in terms of Factor of Safety (FS). The Relative Density (RD) was determined with the help of
Standard Penetration Test (SPT) data. Grain size analysis was also carried out on each borehole samples. The results
revealed that the DHA area is vulnerable to liquefaction during severe seismic event of magnitude between 6.5 and 7.5
in Karachi.
Keywords: Silty-sand, liquefaction, factor of safety, DHA, Karachi.
Introduction
Soil liquefaction mainly occurs in sand, silty-sand,
sandy-silt and sensitive clays. Liquefaction is generally
ground shaking caused by strong earthquake, where
saturated, cohesionless granular soil is transformed
from a solid to nearly liquid state. The liquefaction
potential depends on the earthquake intensity,
magnitude, duration of ground motion, distance from
the source of earthquake, site specific conditions,
ground acceleration, type and thickness of soil
deposits, relative density, grain size distribution, fine
contents, plasticity of fines, degree of saturation,
confining pressure, permeability of soil layer, position
and fluctuation of ground water table, reduction of
effective stress and shear modulus degradation (Youd
and Perkins, 1978; Tuttle et al., 1999; Youd et al.,
2001). This study is based on consideration of factor
of safety (FS), relative density (RD) and grain size
analysis. The soil occurring in the study area is non-
plastic in nature.
Karachi is the largest city of Pakistan with population
of about 18 millions (CDGK, 2007). It is the economic
hub of country having a sea-port. As a result,
construction activities are exponentially increasing in
Karachi. Along the coast, Defense Housing Authority
(DHA) is the most fascinating and luxurious residential
scheme in Karachi. All the residential and commercial
settlements of DHA are founded on sediments
comprises of fine grained, saturated and poorly graded
sand of Holocene age (Hamid et al., 2015).The Factor
of Safety (FS), which is the resistance of soil against liquefaction can be determined by taking the ratio of
seismic demand and capacity of soil to resist
liquefaction (Seed and Idriss, 1971). Ability of soil to
resist against is calculated as Cyclic Resistance Ratio
(CRR), while seismic demand is determined by
computing Cyclic Stress Ratio (CSR). There are
numerous tests including Standard Penetration Test
(SPT), Cone Penetration Test (CPT), Beaker
penetration test and shear wave velocity (Vs) test could
be used to find out the FS of given soil layer (Youd et
al., 2001). Most commonly applied procedure for
assessing the liquefaction resistance of soil is SPT. Other FS is generally assessed by peak ground
acceleration (PGA), earthquake magnitude, SPT blow
counts, overburden pressure (σ), fine content (FC),
Atterberg limit and grain size distribution along the
depth of soil profile (Seed and Idriss,1971; Seed et al.,
1985; Youd et al., 2001). Liquefiable layer possesses
FS<1 and the soil layer with FS>1 falls in the category
of nonliquefiable soil (Seed and Idriss, 1971).
Geology of Study Area
Coastal part of Sindh lies in the seismically active zone
(Tatheer and Yasmeen, 2012). The coastal margin of
Karachi appears to depict regional conformity with the
folding pattern of the region in general and with the
Int. J. Econ. Environ. Geol. Vol. 8 (2) 63-68, 2017
Journal home page: www.econ-environ-geol.org
Open Access
ISSN: 2223-957X
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Khan et al. /Int.J.Econ.Environ.Geol.Vol. 8(2) 63-68, 2017
faulting pattern in particular. There is a major
geological fault, which runs from Ahmadabad and
Bhuj to Ormara along Makran coast. Besides, some
minor faults are also reported in Karachi. One of the
minor faults is Allah Bund fault, which passes through
coastal town of Shah Bundar and runs through eastern
parts of the city ending near Cape Monze. Another
fault lies in the Rann of Kutch near southeastern border
of Sindh with India. The third one is Pub fault, which
lies near the Makran coast (West of the city) while
fourth fault is located in Dadu district on the northern
boundary of Karachi (PMD, 2007). South eastern part
of Karachi city is mainly resting on the Quaternary
deposits such as beach sand, coastal sand dunes and
tidal mud flat underlain by Tertiary rocks including
Nari, Gaj and Manchar formations (Mohsin et al.,
1995). The coastal belt has significantly thick
Quaternary deposits, which need to be assessed in
order to understand geotechnical behavior. Drainage
conditions are poor and the problem of salinity and
water logging are also common in the coastal areas as
the ground water occurs at very shallow depth (< 3 m).
The soil occurring in DHA area is found to be poorly
graded, which is mainly comprised of sand, silty sand
and sandy silt of Recent age with negligible clay
fraction, most probably due to coastal geographic
control and dominance of aeolian deposits from the
beach (Mohsin et al., 1995).
Materials and Methods
Assessment of liquefaction potential was evaluated in
terms of Factor of Safety (FS) using SPT based
simplified empirical procedure originally proposed by
Seed and Idriss (1971).
Estimation of Factor of Safety
The Factor of Safety (FS) against liquefaction can be
determined by using following equation:
F.S=
Values of CRR (Cyclic Resistance Ratio) and CSR
(Cyclic Stress Ratio) vary with depth, so the FS was
calculated at different depths within the soil profile.
Estimation of Cyclic Stress Ratio
Seed and Idriss (1971) proposed a simplified equation
for computing the horizontal shear. This equation is
expressed in term of Cyclic Stress Ratio (Seed et al.,
1983, 1985).
Fig.1 Location map of the study area.
Fig. 2 Geological map of Karachi area, Sindh, Geological Survey of Pakistan (after Qureshi et al., 2001).
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CSR = 0.65*(amax/g) * (σt/σˊ)*rd
Where:
amax=Peak horizontal acceleration
g = Acceleration due to gravity
σt= Total vertical over-burden stress
σˊ = Effective vertical overburden stress at the depth
of interest.
rd=Stress reduction factor
Average value of Stress reduction factor (rd) is given
as:
rd=1.0- 0.00765z; for z ≤ 9.15 m
rd= 1.174 - 0.0267z; for 9.15 m < z ≤ 23 m
rd= 0.744 - 0.008 z; for 23 m < z ≤ 30 m
rd= 0.50; for z > 30 m
The mean value of rd calculated from above equation
is shown in Fig 3.
Estimation of Cyclic Resistance Ratio from SPT Blow
counts
Evaluation of cyclic resistance ratio (CRR) requires
value of fine content (FC, particles <0.007 diameter)
and corrected values of (N1)60 (from the measured
blow counts). This equation is suggested by Idriss and
Boulanger (2006) for estimation of CRR for cohesion
less soil with fine content.
CRR=exp
(N1)60cs is an equivalent clean sand standard
penetration resistance value.
(N1)60cs= (N1)60 + ∆ (N1)60
The measured SPT blow counts (NSPT) is normalized
for the overburden stress at the depth of the test and
corrected to a standardized value of (N1)60. Using the
recommended correction factors given by Robertson
and Fear (1996). The corrected SPT blow count is
calculated with:
(N1)60=NSPT.CN.CE.CB.CS.CR
The correction factors CN, CE, CB, CS and CR are
described below.
The first correction factor (CN) normalizes the
measured blow counts to an equivalent value under
one atmosphere of effective overburden stress.
CN=
Where:
σ =Vertical effective stress
Pa=1atm of pressure (0.000101325 KN/m²)
The factor CE is used to correct the measured SPT
blow counts for the level of energy delivered by the
SPT hammer. Using 60% of the theoretical maximum
energy as a standard.
CE=ER/60
By assuming ER=60, CE = 1
CB is another correction factor for borehole diameters
(Table 1, Robertson and Fear, 1996).
Another correction factor CS is for SPT samplers used
without a sample liner, CS=1 is used for a standard
sampler.
Table 3. Correlation among SPT-N value, angle of friction
and relative density (Meyerhoff, 1956).
SPT-N
(Blows/0.3m
or 1ft )
Soil
Packing
Relative
Density
(%)
Angle of
Friction
<4 Very loose <20 <30
4-10 Loose 20-40 30-35
10-30 Compact 40-60 35-40
30-50 Dense 60-80 40-45
>50 Very Dense >80 >45
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CR correction factor is used for rod length correction
with respect to depth (Table 2).
The correction factor ∆ (N1)60 is computed with the
linear function.
1. For FC ≤ 5% ; ∆(N1)60 =0
2. For 5<FC<35 % ; ∆(N1)60=7*(FC-5)/30
3. For FC ≥ 35 % ; ∆(N1)60=7
Where: FC= Fine Contents.
Grain Size Analysis
American Society for Testing and Material (ASTM, D-
422) is used for grain size analysis. This procedure
reveals the size, shape and sorting of sand. Soil having
uniformity coefficient less than five is more susceptible
to liquefaction (Ross, et al., 1969; Lee and Fitton, 1969).
Atterberg Limit
Atterberg limit test was performed to determine the
nature of soil (plastic or non plastic) and Plasticity
Index (PI). Soil with substantial plastic fine should be
appraised on the Atterberg limits for the liquefaction
potential (Seed and Idriss 1981, 1982, Seed et al.,
1983).
Relative Density
Liquefaction occurs principally in loose saturated clean
sands and silty sands having a relative density less than
50%. Relative density was determined by using the
SPT-N values because the number of blow counts has
a direct relation with relative density.
Results and Discussion
This study attempts to evaluate liquefaction potential
of the soil occurring in DHA area, for which different
parameters were used including Grain Size Analysis,
Factor of Safety (FS) and Relative Density. Factor of
Safety (FS) was calculated against the liquefaction
potential in DHA using SPT based semi empirical
procedure. Soil liquefaction potential was determined
at 15 sites (30 bore holes) across the DHA area for the
earthquakes of magnitude Mw=6.5, Mw=7 and
Mw=7.5 with peak ground acceleration of 0.2g. The
soil deposited at these sites comprises layers with filling material composed of silty sand, medium to stiff
clayey silt, sand and soft to medium stiff silty clay of
Holocene age (Hassan et al., 2015). It is widely
accepted that only recent sediments or fills of
saturated, cohesionless soils at shallow depths will
liquefy in a large magnitude earthquake. Soil gradation
curve depicts that the soil of all sites may be
categorized as sand (medium to fine) and there is
negligible amount of clay with the plasticity range of
30 to 35%. Loose sand with less amount of clay
(particles <0.005) is more prone to liquefy, if it
encounters with 0.9 times water content (Seed and
Idriss, 1982).
Fig. 4 Boundaries of liquefiable and non-liquefiable layers.
Loose uniformly graded materials are more susceptible
to liquefaction than well-graded materials (Ross, et al.,
1969; Lee and Fitton, 1969). While, Tsuchida (1970)
proposed ranges of grain size curves separating
liquefiable and nonliquefiable soils (Fig. 4). Factor of
safety (FS) against the earthquake of magnitude
Mw=6.5, Mw = 7 and Mw=7.5 reveals that all the 15
sites have FS < 1 which are liable to liquefy because
liquefiable layers possess FS < 1. A few layers depict
FS=1 which indicates the critical point and FS >1
indicates the safe zone. However, these layers are
relatively very thin. Calculations for FS (Mw=6.5, 7,
7.5) are given in Table 4.
Table 1. Correction factor for borehole diameter.
Diameter of Borehole CB
65 mm to115 mm (2.5 to 4.5 inch) 1
150 mm (6 inch) 1.05
200 mm (8 inch) 1.15
Table 2. Rod length correction with respect to depth.
Depth (m) Correction for Rod length (CR)
3 0.75
3-4 0.8
4-6 0.85
6-10 0.95
10-30 1.0
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Conclusion
Based on computation of FS from 15 sites of DHA, it
is concluded that study area is highly vulnerable to
liquefaction. Factor of Safety (FS) was computed
against earthquake magnitudes of 6.5, 7 and 7.5.
Moreover, the detailed textural analysis and relative
densities of borehole sediment samples also supported
the liquefaction potential favoring the earthquake
magnitude of 6.5, 7 and 7.5. Presence of groundwater
table along with the presence of non-cohesive soil (silt
and sand) further supported the likelihood of
liquefaction phenomena to occur in DHA during any
noticeable seismic event. Therefore, high rise buildings
should be designed under the provision 2B (Uniform
Building Code) for moderate seismic zone in Karachi.
Acknowledgement
We gratefully acknowledge Managing Director of Soil
Mat. Lab., Mr. Kazim Mansoor and Engr. Noor
Ahmed for their valuable suggestions and technical
support. We are also thankful to Mr. Khalid Alvi,
Museum Curator Department of Geology, University
of Karachi for his constant support.
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Fig. 5 Textural analysis of 15 sampling sites.
Table 4. Calculation for FS of a representative site.
Site Borehole Layer Thickness
(ft) CSR NSPT CRR
FS
Mw=6.5 Mw=7 Mw=7.5
1
1
6 0.19 9.00 0.10 0.81 0.63 0.53
10 0.19 13.00 0.10
0.78 0.60 0.51 14.00 0.06
GWT
(m)
2.286
5 0.21 14.00 0.10 0.73 0.56 0.47
17 0.20 22.00 0.06
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2
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