Liquefaction hazard assessment and building foundation safety for Chennai city, India

Disaster Advances
Vol. 7 (11) November 2014
Liquefaction hazard assessment and building foundation
safety for Chennai city, India
Rajarathnam S.*, Renu M. S., Santhakumar A. R. and Premalatha K.
Centre for Disaster Mitigation and Management, Anna University, Chennai-600025, INDIA
*[email protected]
water pressure causes the loose saturated soils to liquefy,
lateral spreading, sand boils, ground oscillations,
settlements etc. to appear at the surface and is major cause
of concern particularly with respect to destruction of
constructed facilities. It is one of the most destructive
phenomena caused by earthquake and has widely occurred
in loose saturated sands soil deposit. It tends to reoccur at
the same sites during successive earthquakes where
geological conditions and hydro-geological conditions
remain fairly stable16. The death toll in earthquake is
estimated to be more than half than due to all the disasters
combined. This is mainly due to the poorly designed and
badly built buildings which kill people. Such studies need
to identify liquefiable areas and map for urban cities which
are prone for moderate to severe earthquake hazard.
Abstract
Chennai is India’s 4th largest metropolitan city
having a multi-dimensional growth in development of
its infrastructures and population. The city is prone
for moderate seismic activity and the anticipated
earthquake is with magnitude of 6.5 based on past
earthquake history. The Chennai city with variety of
geological
deposits
and
the
geotechnical
characteristics of sediment deposits has its own
importance on ground movements. Liquefaction is one
of the most destructive phenomena caused by
earthquake and especially in loose saturated sand
deposit. Hence there is a need to prepare liquefaction
hazard map which will enable urban planners to
design earthquake resistant structures and strengthen
existing unstable structures. In this study, SPT data
from 666 boreholes was used to evaluate the
liquefaction potential.
Chennai city is the state capital of Tamil Nadu, India. The
city has a multi-dimensional growth in development of its
infrastructures and population. Chennai city became
Extended Chennai city to an area increased from 177 sq.km
to 425 sq.km in 2012. The study area is restricted to
Chennai city of 177sq.km. It is the 34th largest metropolitan
city in the world. Its population density is 24418 per sq. km
and this makes Chennai as one of the densest cities in the
World.
A Liquefaction Hazard Map is prepared and the result
has been correlated with various geological deposits.
It shows that the marine deposits has higher
liquefaction hazard compared to that of fluvial
deposits. Among marine deposits, the paleo tidal litho
unit is more prone for liquefaction compared to strand
flat and tidal flat litho units. The result shows that the
liquefaction layer thickness varies from 1m to 10m.
The Severity of Liquefaction (SL) is calculated for
Very Severe and Severe categories of Factor of Safety
(FS) and utilized to arrive the pile diameter for
preventing of buckling of building foundation piles.
In this study, over 50,000 buildings from 3 to 14
stories have been studied to evaluate the foundation
stability against the sand layer thicknesses of
liquefiable zones. Generic recommendations for
shallow foundation and deep foundation of multistoried buildings have been suggested to mitigate
against the effect of liquefaction hazard for building
foundation safety.
The seismic status of Chennai city was elevated to
moderate active zone (zone III) from low active zone (zone
II) in 2002 i.e. the constructed buildings in the city prior to
the year 2002 are not designed for moderate earthquake
hazard (IS1893 (part I):2002). Even a moderate earthquake
in Chennai city can be the source of high level socioeconomic disasters. Hence liquefaction hazard map based
on SPT N60 values from 666 boreholes was prepared. The
severity of liquefaction (SL) calculated considering the
layer thickness of liquefiable soil of Very Severe and
Severe categories of hazard. The SL was used for safety
design of shallow and deep foundation of buildings.
Geology of Chennai City
Chennai forms part of coastal plains of Tamil Nadu state of
India. Major part of the city is flat topography with very
gentle slope towards east. The elevations of land surface
vary from 14m above MSL in the southwest to sea level in
the east. The Chennai has river systems namely Adayar and
Coovum. Both drain to Bay of Bengal and remain flooded
during monsoon. Chennai is underlain by various
geological formations. It can be grouped into three units viz
Archaen crystalline bedrocks in southwestern part,
Gondwana (Lower Cretaceous to Lower Jurassic) in
western and north western and Tertiary sediments (Eocene
Keyword: Liquefaction Hazard, Factor of Safety, Severity
of Liquefaction, N- value, Marine and fluvial deposits,
Multi storied buildings, Building Foundation stability.
Introduction
The seismic shaking by the earthquake and increase in pore
* Author for Correspondence
1
Disaster Advances
Vol. 7 (11) November 2014
to Pliocene) and Recent alluvium (Pleistocene and SubRecent) in north, western, central and southern parts of the
city.
Various authors worked on deterministic seismic hazard for
Chennai city19 and arrived at different values for PGA at
basement rock level as 0.134g 34, 0.126g29 and 0.176g5.
Response spectrum provided the IS 1893 (Part 1): 2002
which is based on the concept and past earthquake
attenuation consideration and the zonal factor of 0.16 for
Chennai city and estimated PGA as 0.2 g. For the present
study the PGA at rock level arrived is 0.16g based on the
study carried out by various authors.
Most of the geological formations are concealed since they
are overlain by the alluvial materials excepting for a few
exposures of crystalline rocks of charnockites in
southwestern part of Chennai. The thickness of sediments
varies from a few meters in south western to 20 to 30m in
the northern, eastern and southern parts to as much as
>100m depth of sediments of Gondwana age. The Chennai
city represents the flood plain of fluvial environment of
Gondwana periods and the other types of sediments namely
strand flat deposits, tidal flat deposits, paleo flat deposits
deposited under marine environment. All the deposits are
parallel to coast of Bay of Bengal. The Cooum River and
Adyar River flow west to east direction and it has fluvial
deposit of leeve, point bar and channel bar deposit in the
courses of rivers and estuary deposits close to the sea.
Based on Probabilistic approach, Vipin et al35 arrived PGA
0.25g for site class D at surface level for 10% probability of
exceedance in 50 years and Uma Maheswari et al34
estimated PGAmax at surface level based on site-response
studies as 0.3g.
Methodology
The present study involves two aspects in the methodology
adopted viz. liquefaction hazard assessment and building
foundation safety based on Severity of liquefaction hazard.
Sediments also contain at places peat bogs and fossiliferous
horizons, at varying depths, indicative of fluvial, estuarine
and marine conditions. The sediments consist of sand, clay,
sandy clay, silt and occasional gravels and are underlain by
crystalline rocks mostly of charnockite suites. Fig. 1 shows
the geology map of Chennai city18.
Liquefaction hazard assessment: Scientists have
conducted extensive research and have proposed many
methods to predict the occurrence of liquefaction.
Undrained cyclic loading laboratory tests had been used to
evaluate the liquefaction potential of a soil under a special
cyclic loading which simulates an earthquake excitation.
But due to difficulties in obtaining undisturbed samples of
loose sandy soils, since 1980s many researchers have
preferred to use in situ tests to evaluate liquefaction and
lateral spreading potential. Robertson and Companella22,
Shibata and Teparaska26 and Stark and Olson30 adopted
the Cone Penetration Test (CPT) for evaluating liquefaction
potential. The Standard Penetration Test because of its
simplicity was one of the first in situ tests to be widely
used27.
Seismicity in and around Chennai
Peninsular India is a shield region which has maintained its
continental structure since the Permian age. An earthquake
is not supposed to be well-known phenomena in a shield
area. However, the earthquakes of Coimbatore (1900),
Koyna (1967) and Lattur (1993) with intensity of 6 to 7 on
the Ritcher scale have exploded the myth that Peninsular
India is earthquake free.
The information on past earthquakes gives an idea of the
seismic status of a place or region. It requires a variety of
geological and seismological information such as details of
epicenters origin, time, focus depth and magnitude; the
various fault systems along with earthquakes had occurred
as well as those which are currently active 31.
In this study, to evaluate the liquefaction potential, the SPT
(field test) has been used. Nearly 666 borehole data drilled
for design foundation to residential/commercial multi
storied buildings and bridges were collected from various
sources. Liquefaction susceptibility was evaluated based on
the primary relevant soil properties such as grain size, fine
content and density, degree of saturation, SPT N60 values
and age of the soil deposit in each of the borelogs. These
susceptible areas have been identified by considering the
approach of Pearce and Baldwin17.
To know about the regional seismicity of around 300km of
Chennai, past earthquakes data were collected for a period
of over 200 years (1815 onwards). Microseismic event
data are from the seismicity array of Gauribidanur,
Karnataka state6, moderate seismic events from Indian
Meteorological Observatories and National Geophysical
Research Institute (NGRI) observatories. These are
earthquakes with magnitude of <3 to 6 on the Ritcher
Scale. The available earthquake data were plotted over such
a lineament and geotectonic elements map, the major
percentage of the data is aligned along the NE-SW and
NW-SE trend of fractures. A few of them fall on the E-W
and N-S trend of fractures. The NE-SW lineaments are
longer in length compared to those with other trends 21.
Soil is susceptible for liquefaction if (1) presence of sand
layers at depths less than 20m, (2) encounter water table
depth less than 10m, (3) SPT filed “N” blow counts less
than 20 and (4) Clayey sand with <10% of clay content and
Liquid Limit is <322 . By using ArcInfo GIS software,
interpolate susceptibility of map has been prepared. To
prepare the liquefaction susceptibility map the N value
observed in the field, using the SPT must be necessarily
corrected for various corrections1,12,14,15,20,23,25,28,32,33 such
as: (a) Fines Content (Cfines), (b) Overburden Pressures
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Disaster Advances
Vol. 7 (11) November 2014
(CN), (c) Stress Reduction Factor (CS), (d) Hammer energy
(CE) and (e) Bore hole diameter (CB). Corrected N value
i.e. (N60) is obtained using the following equation:
N60 = N * Cfines* CN * CS * CE * CB
(6)
Magnitude correction factors for cyclic stress approach are
shown in table 1.
(1)
Factor of safety against liquefaction of soil has been
evaluated based on the revision of the simplified procedure
of Seed and Idriss 24,25, Youd et al36 and Cetin et al3
considered for evaluation.
Since the liquefaction resistance increases with increasing
confining stress, a correction factor, Kσ, is applied such that
the values of CSR correspond to an equivalent overburden
pressure (σv) of 1 atmosphere.
In this method, the earthquake induced loading is expressed
in terms of cyclic shear stress and this is compared with
liquefaction resistance of soil. The two variables were
defined for evaluation of factor of safety against
liquefaction. They are (1) the seismic demand of a soil
layer and is represented by cyclic stress ratio (CSR) and (2)
the capacity of soil to resist liquefaction represented by
cyclic resistance ratio (CRR).
(7)
where
Kσ = 1 - Cσ ln
Evaluation of Cyclic Stress Ratio: The CSR induced by
earthquake ground motions, at a given depth z below the
ground surface, is calculated using the following
equation24:
amax
CSR = 0.65
g
γz
σv’
rd
Cσ =
(2)
(3)
α (z) = -1.012 – 1.126 sin z
5.133
11.73
(4)
β(z) = -0.106 + 0.118 sin sin z
5.142
11.28
(5)
Pa
≤ 1.0
1
(9)
18.9 – 2.55√ (N1)60
(8)
≤ 0.3
(9)
Pa is the atmospheric pressure = 100 kpa and (N1)60 is the
corrected SPT-N value, limited to a maximum value of 37.
where amax is the peak horizontal acceleration on the
ground surface, γ is the bulk unit weight of soil, σv′ is the
effective overburden pressure, g is the acceleration due to
gravity and rd is the stress reduction factor that measures
the attenuation of peak shear stress with depth due to the
non-elastic behavior of soil. All the boreholes considered in
the present work being less than 34 m depth, the stress
reduction coefficient (rd) is calculated using the following
equations7,8:
Ln(rd) = α (z) + β(z) M
σ'v
Evaluation of Cyclic resistance ratio: The following
expression was used for determining the CRR for cohesion
less soil with fines content (FC):
(10)
(N1)60cs= (N1)60 + Δ(N1)60
(11)
where Δ(N1)60 is the correction for fines content present in
the soil in percent (FC)20 and is given as:
As practiced, the values of CSR that pertain to the
equivalent uniform shear stress induced by an earthquake
having a moment magnitude Mw , are corrected through the
Magnitude Scaling Factor so that the adjusted values of
CSR would pertain to the equivalent uniform shear stress
induced by an earthquake having a moment magnitude Mw
= 7.5, i.e. (CSR)Mw = 7.5. The magnitude of factor of
resistance indicates the degree of resistance to liquefaction.
(12)
Factor of Safety: The factor of safety against liquefaction
is calculated using the following formula:
The MSF is calculated using the equation proposed by
Idriss8 as:
Factor of Safety =
3
CRR
CSR M=7.5, σ = 1
(13)
Disaster Advances
Vol. 7 (11) November 2014
The subscript 7.5 for CSR denotes CSR values calculated
for the earthquake moment magnitude scaling 7.5. The
factor of safety against liquefaction has been grouped into 4
categories as shown in table 2.
foundation for buildings upto 3 stories is 2 to 2.5m. They
are classified as shallow foundation.
If SL is of the order of 10, the depth of liquefiable layer
will be>12m. In shallow foundation this thickness of
liquefiable layer will induce large settlements causing
structural damages to shallow buildings of 3 – 4 stories.
However, for tall multistoried buildings which are
generally founded on piles the larger SL values will induce
loss of lateral support for the pile which can cause buckling
of the pile. Therefore the effect of larger liquefaction depth
can manifest different types of foundation distress in
shallow and deep foundations.
Severity of liquefaction hazard and building
foundation safety
The Severity of Liquefaction in a particular area is directly
related to the layer thickness of liquefiable soil, both under
very severe and severe categories of hazard and inversely
to the depth of the layers from the surface.
SL = Φ1*tvs + Φ2 * ts + Φ3* d1
d2
(14)
When SL value is in the range of 5-10,
liquefiable layer
(
thickness can be in the order of 6 – 8m and the possibility
of severe distress in deep foundations can be estimated
based on actual depth of liquefaction and the lateral
pressures it develops. When SL<5, the maximum thickness
of the liquefiable layer will be 6m. The effect of this on
shallow foundation will be severe if this layer is occurring
close to the surface. On the other hand when this layer
occurs deep, its effect on pile foundation will depend on the
pile diameter used.
Table 1
Magnitude correction factors
where SL is Severity of Liquefaction, tvs is thickness of
layers coming under Very Severe category, ts is thickness of
layers coming under Severe category and d is depth of the
layer. This parameter SL can be used to assess the effect of
liquefaction severity on the building foundation. A high
value of SL will indicate severe condition with respect to
the depth of liquefied layer and directly to the total
thickness. The severity of liquefaction is influenced by
three parameters Φ1, Φ2 and Φ3.
The very severe condition has been identified as having
factor of safety <,1, such a condition will induce nearby
100% effect of liquefaction on the foundation. Hence Φ1
factor is taken as 1.0 for the thickness of layer having factor
of safety <1. The parameter Φ2 relates to severe condition
and the corresponding factor of safety is 1 to 1.5. Since
factor of safety is 1.5, its effect on foundation is less severe
than the thickness containing FS<1.0 (Φ1). Therefore the
parameter is chosen as 1/1.5 as it will induce same amount
of severity as a lesser thickness having FS <1.0. Hence Φ2
factor is chosen as Φ2=1/1.5.
MSF
5.25
1.50
6
1.32
6.75
1.13
7.5
1
8.5
0.89
Table 2
Factor of safety and criticality index of liquefaction
The severity of liquefaction is influenced by the depth at
which the liquefiable layer occurs and the total depth up to
the bottom of liquefiable layer. Its influence is much less
compared to factor of safety related to thicknesses.
Therefore the severity components coming from the depth
of occurrence of liquefiable layer are given 20% weightage
(i.e. Φ3 = 0.2) and the depths are chosen as (d1/d2) where d1
is depth at the bottom of the liquefiable layer and d2 is
depth at the top of the liquefiable layer. Thus the equation
for severity of liquefaction (SL) is derived as:
SL = tvs + ts + 0.2d1
1.5
d2
Magnitude(Mw)
Group
1
2
3
4
(15)
Group
According to the SL value, liquefiable areas are classified
in to three viz. Very High Severity, High Severity and
Moderate Severity of Liquefaction as shown in the table 3.
Over 50,000 buildings of 3 stories to 15 stories were
studied. The foundation systems adopted in buildings in
Chennai are summarized in table 4. The depth of
4
Factor of safety
range
<1
1 to 1.5
1.5 to 2
>2
Criticality index
Very Severe
Severe
Medium to Low
Not Liquefiable
Table 3
Severity of liquefaction
(15)
SL
Category
1
> 10
2
5 to 10
3
<5
Very High Severity of
liquefaction (VHSL)
High Severity of liquefaction
(HSL)
Moderate Severity of
liquefaction (MSL)
Disaster Advances
Vol. 7 (11) November 2014
areas and infrastructure of bridges. However, it is to be
noted that the liquefiable layers are not restricted to shallow
depth but also continue their distribution down to the
maximum depth of 20m. The shallow depth liquefiable
layers are in the flood plain by the influence of the rivers.
But the deeper sands are from the depositional environment
of the deposits of the various litho-units.
Results
The liquefaction study is attempted for deposit of
sediments, based upon 666 boreholes which are spread out
to all types of litho-units of Chennai city. For example, 53
number of boreholes in strand flat deposit, 73 number of
boreholes in tidal flat deposit, 173 number of boreholes in
paleo tidal deposits, 138 number of boreholes in fluvial
deposits and 37 number of boreholes in thin sediments
underlined by Charnokites (Fig. 1). The areal distributions
are 17, 24, 44, 61 and 16sq.km respectively. No borehole is
available in sand flat deposits and medium grey brown sand
with levees litho units of thin layers and is not considered
for the present study. Table 5 is the typical representative
borehole and shows the methodology adopted for the
liquefaction hazard assessment of Chennai city. The bore
hole encountered fine to medium sand, stiff clay, fine to
medium sandy silt down to the borehole depth of 22.5m.
The water table is at 2m depth. N value corrected and
calculated FS for corrected N60 value of <37.
Table 6 shows the various litho-units grouped in to three
categories viz. Charnokites, fluvial/terrestrial deposits of
Gondwana age group and deposits of marine origin.
Charnockite rock area is underlined by thin sediment
maximum to 5m and is free from any hazards of
liquefaction. Terrestrial deposits of Gondwana Group
cover 33.93% of the study area but the very severe and
severe categories occupied 3.53% and 13.07% respectively
whereas the marine deposits occupied 23.03% and 62.13%
of the said hazard categories. The study identified that the
marine deposits are more liquefaction prone than the
terrestrial origin of deposits (Fig.1 and Fig.2).
The calculated Critical Index is >2 to the depth of 4.5m and
the FS is <1 to >1.5 category between the depths 5m to
20.5m, following the hard rock at depth of 22.5m of the
borehole. Liquefaction zone is between 5 to 20.5m. The
different category of critically index arrived from 666
boreholes upto the depth of 20m and the least Factor of
Safety was considered to represent the respective borehole
and mapped. The prepared liquefaction hazard map of
Chennai city based on Inverse Distance Weighted tool of
ArcInfo uses a method of interpolation in GIS platform
(Fig.2). The study reveals the Chennai city prone for
liquefaction hazards 9, 27, 33, 86 sq.km of liquefaction
zone as Very Severe, (FS: <1), Severe (FS: 1-1.5),
Moderate to Low (FS 1.5 – 2) and Not Liquefiable (FS :>
2) as detailed in fig.2 and table 6.
Among the marine deposits, paleo tidal and tidal flat are
higher liquefaction hazards than the strand flat deposits
(Table 6). In case of Tidal Flat deposits, the very severe
category areas restricted to two locations are in the estuary
deposits of Coovum River and Marshy area converted into
residential land areas. But the areas of very severe
category in paleo tidal deposit are distributed all over the
areas of the deposit. Based on liquefaction hazard
distribution, it is to be stated that paleo tidal deposits are
comparatively higher hazard prone than tidal flat deposits.
The study also reveals that the marine deposits of south of
Adyar River, south east of Chennai, are not prone of hazard
possibly due to their layer of sediments overlying the
Charnockite bedrock.
From fig. 2, the FS of <1 (Very Severe category) areas are
selected to represent in table 7 for discussion e. g.
Tondiarpet where the thickness for FS <1 and FS 1 – 1.5
are 9.5m and 3m respectively. The sands occupy in the
depth range 4 to 20.5m. For mapping of liquefaction hazard
assessment the least level of FS i.e. < 1 is assigned to the
borehole of Tondiarpet. The distribution of liquefiable soil
varies from 1m in Kolathur and down to 20 m. The soil
layers of thickness FS < 1 are in the range of 2 to 12.5. The
arrived pile diameter from SL correlates that higher is the
liquefiable thickness layer, larger will be the pile diameter.
The Severity of Liquefaction (SL) calculated for Very
Severe and Severe categories of Factor of Safety of the
respective boreholes and the result for selective areas are
presented in table 7. The result is used to design the
shallow and deep foundation for earthquake resistant
buildings.
Discussion
The result of the study is discussed on two aspects viz.
liquefaction hazard correlated with geological deposits and
vulnerability of buildings and liquefiable layers for shallow
and deep foundation.
The vulnerability of buildings and liquefiable layer for
shallow and deep foundations: Preliminary study has
been attempted to correlate the shallow and deep
foundations stability and their impact pertaining to the
liquefaction hazard of the site. Fig. 2 shows the liquefaction
hazard map in which the red areas have most vulnerability
with the factor of safety in the region of <1 and those in
brown colour have factor of safety against liquefaction 11.5. These areas are figuratively in potential hazard zone
among the borehole data areas which have been chosen for
Liquefaction hazard assessment: The liquefaction hazard
areas are distributed almost within the flood plain of rivers
viz. Adayar, Cooum and Otteri. The hazard area in north
eastern of Chennai is also located in the areas of converted
marshy land in to the residential areas (Fig.2).
The distributions of liquefiable sands are in the range of 1m
to maximum depth 20m, considered for the study as evident
from the drilled boreholes for development of residential
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Disaster Advances
Vol. 7 (11) November 2014
a detailed study. The areas chosen are listed in the table 7
along with the approximate liquefiable depth with factor of
safety less than 1 (FS < 1 is very severe) and factor of
safety less than 1.5 (FS 1 - 1.5 is severe).
as shown in fig.4. It is also to be noted that the pile bends
in double curvature mode. The maximum thickness of
liquefiable depth of soil would make the piles to fail by
buckling. The diameter of piles required to avoid instability
failure based on the liquefiable depth is proposed below:
Deep foundation adopted in Chennai is mostly bored cast
in-situ piles of depth 5 to 8m in some areas like Chepauk
and Adyar and 12 to 15 m in areas like Koyembedu and
Thondiarpet. An examination of table 7 shows that the
depth of liquefiable layer is less in Koyembedu and Anna
Nagar whereas it is significantly more in Raja
Annamalaipuram and Saligramam. When liquefiable depth
is less, it does not pose a serious problem to pile foundation
since the length of affected soil will not change the pile
behavior from short to long. Table 4 also shows that two
different types of piles are in use depending on type of soil.
In areas like Anna Nagar, the use of under reamed piles of
depth 5 to 8 m is common. Buildings up to 3 to 4 storeys in
Chennai city are supported by shallow foundation
(individual or strip footing) or raft slab whereas tall
buildings are supported by deep foundation. Normally piles
are used for deep foundation.
The behavior of the pile will depend on the thickness of
liquefiable layer. If the thickness of liquefiable layer is less
than 12 times the diameter of the pile, then the pile will
behave as a short column (IS:456, 2000). Hence failure will
not occur due to buckling. If the thickness of liquefiable
layer is larger than 12 times diameter, there is a chance for
the pile to buckle and fail prematurely. Hence if we choose
the diameter of the pile greater than liquefiable
thickness/12, then the pile will behave as a short column
and hence the foundation will be safe against buckling as
per design.
For example, the liquefiable depth at Egmore is 4.5m
(Fig.5). For this location the diameter of the pile required
based on the above principle is D = (4.5/12) = 0.375m.
This can be rounded off and a safe 0.4m diameter of the
pile can be adopted. This will avoid possibility of
instability of the pile due to liquefaction.
Shallow foundation
Maximum number of buildings having less than 4 floors is
supported on shallow foundation. The bulb of pressure for
buildings with shallow foundation is shown in figure 3. As
can be seen, the depth to which the bulb of pressure goes
will be about 2.5 + 2 = 4.5 m below ground level (Fig.3 and
table 8).
If we take another example at Raja Annamalaipuram
(Fig.6), the liquefiable depth as per table 6 is 10.7m. For
long piles effective length of buckling can be assumed to be
0.75 times the free length of pile losing lateral support.
Hence effective length of pile for the site at Raja
Annamalaipuram becomes 10.7*0.75=8.025. Thus, the
diameter of the pile required to prevent buckling works out
to 8.025/12=0.668m and this is rounded off as 0.7m for
adoption. Adoption of 0.7m diameter will avoid instability
of pile due to liquefaction at Raja Annamalaipuram.
On examination, table 7 shows that the liquefiable soil in
the boreholes examined starts from a depth of 1m. Except
Mandaveli, Raja Annamalaipuram and Virugambakkam all
other areas have liquefiable soil within 5m from the
surface. The shallow foundation will be affected by the
liquefiable layer present in these areas.
The example noted at Egmore and Raja Annamalaipuram
demonstrates how the pile diameter can be chosen
according to a particular location based on liquefaction
depth to prevent instability failure of the pile. In other
areas, it is necessary to use pile diameter of not less than
300 mm. If the suggested pile diameters are used, the
possibility of buckling failure of piles can be avoided. In
the above description, it was assumed that all piles are
bearing piles. It is also suggested to avoid friction piles for
tall buildings founded in the liquefiable zones identified in
fig.2.
Fig. 3 shows the bulb of pressure for a shallow foundation.
It can be seen that the pressure bulb is at a depth of about
2.5m to 5m. If the liquefiable layer occurs in the zone of
2.5m to 5m, the shallow foundation will be affected
severely. If the liquefiable layer occurs below 5m as in
case of Mandaveli, Raja Annamalaipuram and
Virugambakkam, the foundation will suffer only
subsidence and may not collapse (Table 7).
Deep foundation
Fig.7 shows a pile driven through two layers i.e. top
unliquefied and bottom liquefied layer. Due to movement
of pile laterally it will experience passive earth pressure at
the top and flow of sand at the bottom 13.
Of late, there are many multistory building in the form of
flats using deep pile foundation. The failure of the pile can
be caused by either the entire depth or a small layer losing
its resistance due to liquefaction. These foundations are
taken to the rock bottom and are supported on the rock.
However they pass through belts of liquefiable soil whose
thickness and depth from ground level vary depending
upon the area in which the building is located. When large
depth gets associated, the end conditions of the pile are
likely to reduce the effective length due to end continuity9
The lateral pressure p(z) on a pile which acts in the vertical
direction can be determined by using the expression,
p(z) = az2 + bz + c
6
(16)
Disaster Advances
Vol. 7 (11) November 2014
in which the parameters a, b and c are unknown. This
equation is integrated twice to obtain the bending moment
as:
M = - [(az4/12) + (bz3/6) + (cz2/2) + dz + e]
parameters a, b, c, d and e are arrived at 74.12, 94.9,
34*103, 58.2*104 and -45*105. The moment is calculated as
-39.2 *106, the negative and positive sign denote the
direction of pile bending. The bending moment will act in
presence of axial compression. For this moment, using
interaction curve the required steel for RCC pile should be
arrived at and adopted which will give enough bending
strength against failure.
(17)
Using the boundary conditions in the above equation, the
parameters a, b, c, d and e are determined. When the
liquefiable depth of Chepauk is analyzed, the value of
Fig. 1:
Fig. 1: Geology map of Chennai City
7
Disaster Advances
Vol. 7 (11) November 2014
Fig. 2: Factor of safety against liquefaction for Chennai city
Fig. 3: Bulb of Pressure for Buildings with Shallow
Foundation
8
Disaster Advances
Vol. 7 (11) November 2014
Table 4
Summary of foundations adopted for various types of Buildings in Chennai
No. of
Stories
1
2
3
4
5
6
Type of
construction
Shallow Foundation
Depth of Foundation (m)
Brick
Brick
RC
RC
RC
RC
1-1.5
1.5-2
1.5-2
2-2.5
3
-
Deep Foundation
Depth of Pile
Depth of under
rear piles (m)
5
Upto bed rock >10m
8
Upto bed rock >10m
8
Upto bed rock>10m
-
Table 5
Table showing calculation of FS after N corrections in a sample borehole
Depth
Material
WL
N
CS
CE
CB
CN
CR
N60
NCRR
CSR
CRR
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.0
5
1.7
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
0.7
5
29.88
49.8
0.062982
28.92
48.2
0.062967
81.93
NC
NC
NC
NC
NC
NC
NC
2.89
136.5
5
125.3
1
160.6
5
4.82
0.47759
4
0.42477
4
NC
0.117501
0.63
2.89
4.82
0.130004
1.93
3.21
0.151084
2.89
4.82
0.149931
3.75
6.25
NC
0.07456
7
0.07475
1
0.06971
5
0.10384
8
NC
1.93
3.21
0.181516
0.38
17.38
28.96
0.110338
14.71
24.52
0.117007
2.7
4.5
0.171487
3.59
5.98
0.158504
10.22
17.03
0.126631
13.96
23.27
0.117483
11.71
19.51
0.124549
96.39
160.6
5
NC
0.06971
6
0.17752
8
0.15372
1
0.07356
7
0.07825
9
0.11957
9
0.14760
8
0.13029
6
NC
0.75
Fine to medium sand
2
31
1.2
0.6
1.5
Fine to medium sand
2
30
1.2
0.6
2.25
Fine to medium sand
2
85
1.2
0.6
3
Fine to medium sand
2
78
1.2
0.6
4
Fine to medium sand
2
1.2
0.6
5
Fine to medium sand
2
10
0
3
1.2
0.6
6
Fine to medium sand
with silt
Fine to medium sand
with silt
Fine to medium sand
with silt
Firm clay
2
3
1.2
0.6
2
2
1.2
0.6
2
3
1.2
0.6
2
5
1.2
0.6
Fine to medium sand
with silt
Fine to medium sand
with silt
Fine to medium sand
with silt
Fine to medium sand
with silt
Fine to medium sand
with silt
Fine to medium sand
with silt
Fine to medium sand
with silt
Fine to medium sand
with silt
Fine to medium sand
with silt
2
2
1.2
0.6
2
34
1.2
0.6
2
29
1.2
0.6
2
4
1.2
0.6
2
6
1.2
0.6
2
22
1.2
0.6
2
34
1.2
0.6
2
28
1.2
0.6
2
10
0
1.2
0.6
7
8
9
10
11.5
13
14.5
16
17.5
19
20.5
22.5
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.3
2
1.7
0.9
0.8
9
1.1
9
1.0
6
0.8
2
0.7
2
0.7
4
1.7
NC: Not calculated due to N60 value >37 or clay layer
9
75.18
96.39
Factor
of
safety
7.58
6.75
NC
0.57
0.46
0.69
NC
1.61
1.31
0.43
0.49
0.94
1.26
1.05
NC
Disaster Advances
Vol. 7 (11) November 2014
Table 6
Liquefaction hazard areas in different geological soil types
Soil Type
Total Area
(sq.km)
%
Area
0-1
Sq.km
%
2.12
3.53
1-1.5
Sq.km
%
7.85
13.07
1.5-2
Sq.km
%
12.55 20.90
>2
Sq.km
%
36.82 61.31
Fluvial deposit of Gondwana
Group
Black clay and sandy clay
60.06
33.93
Paleo tidal deposit
Black clay under sandy cover
44.04
24.88
2.94
6.68
11.80
26.79
10.35
23.50
16.57
37.62
Tidal Flat deposit
Black clay
24.4
13.79
2.95
12.09
4.98
20.41
7.35
30.12
8.12
33.28
Strand flat deposit
Medium grey brown sand
16.41
9.27
0.70
4.27
2.45
14.93
2.52
15.36
8.86
53.99
Charnokites
15.93
9.00
0.00
0.00
0.00
0.00
0.05
0.31
15.88
99.69
Fig. 6: Influence of liquefaction depth on pile
diameter
Conclusion
Fig. 4: Pile susceptible for Buckling
The liquefaction hazard assessment of Chennai city is
based on geology, seismicity and geotechnical
characteristics of soil collected from 666 boreholes in
Chennai and various sources. The distribution of
liquefaction hazard categories is Very Severe, Severe,
Medium to Low and Not liquefiable and percents are as
5.62%, 17.48%, 21.19% and 56% respectively. It is
observed that 22% of Chennai city is prone for very severe
and severe liquefaction hazard showing significantly low
N60 values and high water table conditions. All type of
sediments has liquefiable sand layer and it was identified
that marine deposits are higher hazard prone than the
terrestrial deposits. Among the marine deposits the hazard
graded as paleo tidal deposits tidal flat deposit and strand
flat deposits. The severity of liquefaction (SL) calculated
for the very severe was severe areas and divided into three
levels viz. SL is >10, 5 to 10 and <5. The derived SL value
is utilized to study about the building foundation safety for
Chennai city.
H=4.5m
Fig. 5: Liquefaction depth influencing pile
diameter
10
Disaster Advances
Vol. 7 (11) November 2014
Table 7
Severity of Liquefaction calculated for selected sites and Diameter required for preventing buckling of piles
Name of the
Depth
Layer
Layer
Depth at
SL Remark
Total
Length Diamete
Area
of
Thickness Thickness
which
s
liquefiabl
of
r
Bedro
(FS<1
(FS 1 - 1.5 liquefiabl
e
affecte required
ck
Very
Severe)
e soil
thickness d pile
(m)
severe)
exist
Thondiarpet
26.9
9.5
3
4.0 – 20.5 12.5
VHSL
16.5
12.37
1
(4+1+4.5)
(1+2)
Egmore
16.5
2.8
---5.4 –
3.27
MSL
7.2
7.2
0.6
(1.2 + 1.6)
12.6
0.9
1
3 -7.5
2.07
MSL
4.5
4.5
0.4
Chepauk
17.5
7
2
4.5– 17.5
9.1
HSL
13
9.75
0.8
(2+5)
Adyar
14.0
2.3
6.5
1.0-13.5
9.33
HSL
12.5
9.375
0.8
3.7
---5.8 – 9.5
4.02
MSL
3.7
3.7
0.3
Perambur
22.0
1
1
3-7
2.13
MSL
3
3
0.3
Vyasarpadi
2.5
---5.5 – 10
2.86
MSL
4.5
4.5
0.4
(1+1.5)
Veperi
2.3
1.4
2.99 – 9.5 3.86
MSL
6.51
6.51
0.54
Purusawakkm
21.0
1.3
2.1
4.5 – 11
3.18
MSL
6.5
6.5
0.54
4.4
---4.0 – 10
4.9
MSL
6
6
0.5
(3+1.4)
Triplicane
4.5
8
3.5 – 16
10.7
MSL
12.5
9.375
0.8
(1.5 +2+1) (1+2+2.5+2.
4
5)
Mandaveli
17.5
4.5
----18.5 – 23
4.75
MSL
4.5
4.5
0.4
Raja
8.9
---2.3 -13
10.0
VHSL
10.7
8.02
0.7
Annamalai
(3.3
3
Puram
+0.9+4.7)
Virugampakkam
Koyambedu
Anna Nagar
Kolathur
Kodambakkam
Chetpet
Teynampet
Alwarpet
3.8
(1.5 +2.3)
1.5
1.3
2
1.5
1
1.7
(0.9+0.8)
1.1
2.8
9.3
5.8
4.7
(2.2 +2.5 )
3
1.3 +1.7
---2.6
1.2
1
7.5 – 24.5
7.58
HSL
17
12.75
1 .06
6.8 – 13
3.88
MSL
6.2
4.65
0.4
3.7 – 5
6.0 – 10.6
2.8 – 8.8
1.0 – 3
1.57
4.08
2.92
2.27
MSL
MSL
MSL
MSL
1.3
4.6
6
2
----
6.4 – 11.2
2.05
MSL
4.8
1.3
0.3
4.6
0.4
6
0.5
Shallow
Foundation
4.8
0.4
2.2
1.1
3.7
----
6 – 9.3
4.8 – 14.3
2 – 15
8.6 – 14.4
2.87
4.12
13.2
6.13
MSL
MSL
VHSL
HSL
3.3
9.7
13
5.8
3.3
9.7
9.75
5.8
11
0.275
0.8
0.81
0.48
Disaster Advances
Vol. 7 (11) November 2014
Table 8
Depth of pressure bulb for shallow foundation
S. N.
1
2
3
Width of
foundation
1
1.5
2
Depth of
foundation
1
2
2.5
Size of
bulb
1.5
1.75
3
Chennai is prone to have foundation failures due to
liquefaction based on potential areas if high SL>10. The
failure can be due to either buckling or excessive moments
developed due to lateral pressure. The liquefaction affects
both shallow foundation and deep pile foundation. In many
areas liquefiable depth occurs at a shallow depth. The bulb
of pressure is at a depth of 2.5m to 5m. Hence buildings in
these potential zones with shallow foundations will be
affected by liquefaction. The areas where such a problem
can arise have been identified and listed.
Depth at which pressure
bulb extends
2.5
3.75
5.5
Acknowledgement
The authors are thankful to the
Department
of
Information Technology, Government of India for
funding the sponsored research project entitled “ Remote
Sensing & GIS Application for Chennai city on EGovernance Aspects”. Authors also acknowledge Anna
University and Govt. of Tamil Nadu for their support. The
authors extend their thanks to M/s Geotechnical Solutions,
Chennai, M/s Neo Geocons, Chennai, M/s GeoMarine
Consultants (P) Ltd., Chennai, M/s GeoEngineers, Chennai,
M/s Josmer, Chennai and M/s Nagadi Consultants Pvt.
Ltd., Chennai for providing the borehole data of the regions
reported in the paper.
Deep foundation like piles can fail buckling when the depth
of liquefaction becomes large. The diameters to be adopted
to prevent buckling failures in such areas have been
identified in the potential areas such as Egmore and
Kotturpuram. Deep foundations can also be affected due to
lateral pressure caused by liquefaction. An expression to
calculate the lateral has been included. The effect of
bending moment on the pile due to liquefaction can be
taken care by proper design of pile using required steel
requirements.
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(Received 14th August 2014, accepted 20th September
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13