Proceedings of Indian Geotechnical Conference
December 15-17,2011, Kochi (Paper No. F- 379)
SEISMIC DESIGN CONSIDERATIONS FOR PILE FOUNDATIONS
A. Murali Krishna, Assist Prof, Dept of C E, IIT Guwahati, Guwahati-781039, Assam, India. Email: [email protected]
S. Bhattacharya, Sr.Lecturer in Dynamics, Dept.CE, UniversityofBristolBS81TR,UKEmail: [email protected]
ABSTRACT: Pile foundations are adopted commonly for various types of multi-storeyed structures when the founding
soil is weak and soft; and also in industrial structures, bridges, offshore structures. With increasing infrastructural growth
and seismic activities, designing the pile foundations for seismic conditions is of considerable importance for the efficient
function of the structures especially, the lifeline structures like bridges etc. Several studies were conducted by various
researchers on the seismic analysis and design of the pile foundations and evolved different theories on the same. Codes of
practice available in different countries suggested some procedures for seismic design of pile foundations. This paper
presents a short discussion on the various theories evolved on seismic pile performance concepts followed by outlines of
suggested procedures by selected international and Indian codes on the subject. A soil profile is selected from Assam,
Dibrugarh area as an exemplary case to demonstrating the seismic design of pile foundations. From this paper it can be
summarised the points that need to be amended to Indian codes of practice to meet the state of the art developments in the
subject.
INTRODUCTION
Following 1995 Kobe earthquake many pile supported
structures collapsed which led to extensive research on
seismic behaviour and analysis of pile foundations and the
supported structures. 2001 Bhuj earthquake is another
exemplary for many pile failures and associated damage.
Fig. 1 present a typical damage of the building due the
failure of pile foundation during 1995 Kobe earthquake [1].
Fig. 2 shows a revealed picture of pile foundation after 20
years of 1964 Niigata Earthquake highlighting the
formation of plastic hinges during earthquake loading [2,
3]. Many researchers explored different mechanisms that
pile foundations undergo during seismic event especially,
liquefaction. Some of the international codes adopted these
research contributions and are in the process of continuous
updating. In India, billions of money is being spent for new
infrastructure constructions involving huge numbers of pile
foundations of different types in different locations and for
different loading conditions. But the codal provisions are
not included the recent state of the art findings. This is the
high time to review the codes of practice and incorporate
the lessons learnt from the Japan and elsewhere.
This paper presents a short discussion on the various
theories evolved on seismic pile performance concepts
followed by outlines of suggested procedures by selected
international and Indian codes on the subject. Critical
comments are made on the need of the revisions of the
Indian codes of practice (IS 1893, IRC 78 etc.). A simple
design case study is also presented in explaining various
points to be considered to avoid the dynamic failure.
SEISMIC PILE PERFORMANCE: EVOLVED
THEORIES
Studies on seismic pile behaviour can be broadly divided
into two categories: Piles in liquefiable soil; and Piles in
non-liquefiable soil. In general, saturated loose to medium
dense cohesionless soils subjected to dynamic excitation
under undrained condition may liquefy depending on the
excitation level and depth of the soil layer with respect to
ground level.
Fig. 1 Tilting of building due to pile foundation damage
during 1995 Kobe earthquake and schematic of its failure
[1]
Fig. 2 Exemplary earthquake damage of piles [2]
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A. Murali Krishna & S. Bhattacharya
Different researchers focussed on these two different
categories: For example, [3-7] on piles in liquefiable soil
and [8-10] on piles in non-liquefiable soil. Bhattacharya
and Madabhushi [4] presented a prim review of the research
work on mechanism of pile failures. Major failure
mechanisms/modes can be outlined as: Bending mechanism
due to permanent lateral deformations or lateral spreading,
buckling instability, settlement failure mechanism and
dynamic failure.
Fundamental failure Mechanisms
The fundamental failure mechanism for pile foundations
are: 1. Bending mechanism–I (Inertial interaction due to
superstructure]; 2. Bending mechanism – II (Kinematic
interaction due to wave propagation]; 3. Bending
mechanism – III [Kinematic interaction due to soil flow]; 4.
Buckling failure mechanism in liquefiable soils; 5.
Settlement failure mechanisms [Serviceability Limit State];
6. End bearing failure mechanism [Failure in end-bearing of
the pile and/or not adequate fixity]; 7. Dynamic failure due
to change in soil properties and the change in first natural
frequency of the structure; and 8. Appropriate combination
of the above. Figure 3 depicts the schematics of typical
failure mechanisms of pile foundations under seismic
loading conditions [7].
CODAL PROVISIONS
Major international and Indian codes of practice which
discuss about the seismic design considerations can be
listed as: EN 1998 [11-12], JRA [13], NEHRP [14-15], IS
1893 [16] and IRC 78 [17]. Most of the international design
codes consider the lateral spreading that induces bending in
piles and suggest checking for bending moments.
Significant forces to be considered in the seismic design are
the additional forces due to kinematics of the superstructure
under seismic excitation. The international codes [11, 13]
also consider the liquefaction susceptibility for the site
under consideration and necessary aspects for the design.
Critical Comments on Indian Codes
The following are the critical shortcomings of the Indian
codes that warn the immediate revision.
1. IS 1893 [16] considers only three types of soils for
determining the design accelerations from response
spectrum, while the International codes five types.
2. In Indian Codes of practice, while considering the
seismic forces, the allowable stress is increased.
However, the soil capacity should be at best, equal to
the static case and should not be increased. It is
important to mention that International codes reduce
the soil capacity under seismic conditions.
3. Methodology for calculation of liquefaction potential
of the site should be explicitly specified.
4. Suitable suggestions for the liquefiable soil case,
considering the recent research findings, should be
incorporated.
SEISMIC PILE DESIGN: GENERAL
CONSIDERATIONS
In general, for a seismic design of pile foundation, one
should have acquainted with the pile capacity requirement,
structural importance, and its seismic zonal information,
soil profile data etc.
Key Design Steps
1. Calculation of the structural loads that are going to be
transferred to the each pile (vertical, horizontal and
moment if any) considering the various load
combinations including the seismic loading. One
should predict the time period of the structure to
include seismic condition.
2. Soil profile analysis: Carrying out ground response
analysis i.e. liquefiable soil or layered soil with a
liquefiable layer or layered soil with no liquefiable
layer.
3. If the soil profile is non liquefiable:
a. Additional check for the lateral forces/moments
caused due to the passive pressure of soil around
the pile should be made.
b. If soil profile is layered with high stiffness
gradient additional checks the interface layers for
additional moment should be performed.
Fig. 3 Different failure mechanisms: a) Typical building with pile foundation; b) Shear failure mechanism; c) Bending
failure mechanism; d) Buckling mechanism; and e) Dynamic amplification mechanism [7]
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Seismic design considerations for pile design according to International and Indian codes of practice
4. If the soil profile is liquefiable:
a. Pile should be designed as a column
considering: the change in effective length of the pile
(column) taking into account of appropriate end
conditions prevailing at the site; the change in the time
period of the structure-foundation system due the
degradation of soil strength.
b. Soil resistance available in the liquefied
zone should completely be disregarded.
c. In case of the sloping ground lateral
spreading may occur, necessary checks and measures
should be adopted.
PILE DESIGN CASE STUDY
As a design case study a residential building is considered
in Northeast part of the country, Dibrugarh area of Assam.
Axial load on a typical pile is determined as 482 kN for a 5
storey building with a building height of 14.5 m above
ground level.
The soil profile at the site along with SPT N values
recorded is shown in Fig. 4. Ground water level at the
location is considered at 2.5 m from ground surface.
Liquefaction potential was evaluated according to Idriss
and Boulanger [18] for an earthquake of 6.5 magnitude
with a peak ground acceleration of 0.36g. Factor of safety
(FOS) against liquefaction is presented in the Fig. 4. It is
observed that the soil layers between 4 m to 9.5 m depth
from the ground surface are susceptible to liquefaction.
0 10 20 3 0 40 50 0 1 2 3 4 5 6 7 8
20
16
12
8
4
0
SPT N
Lique fiable zone
FO S
SP
SM
SC
CI
Soi l Type
Dep
th, m
Fig. 4 Soil profile data considered and its liquefaction
susceptibility
Pile design normal conditions
Under normal conditions considering a circular concrete
driven cast-in situ pile, the pile design suggests 0.6 m dia
pile with pile length of 12 m for a factor of safety of about
3.0 as per IS 2911[19]. The standard also suggests for
checking the maximum moment and lateral load capacity
which were satisfied for the selected pile section.
Pile design under seismic conditions
Under the seismic conditions the ground under
consideration may liquefy for the design excitation levels
considered. Under such situation the following conditions
should be checked:
i) Neglecting the frictional capacity for part of the pile
that passes through the liquefiable zone. This implies,
neglecting the frictional capacity of pile between 4m to
10 m depth which leads to an increase in the pile length
to maintain the same factor of safety.
ii) Change in the natural period: Natural period of the
structure will change due to liquefaction that can be
accounted to two reasons: Reduction in the strength
(stiffness) of the soil; Change in the fixity point.
iii) Change in the critical load capacity of the pile: Due to
change in the fixity point after liquefaction and loss of
lateral confinement to the pile in the liquefied layer the
pile is essentially to be designed as a column against
buckling. Critical load capacity of the pile is the main
parameter for the design under this consideration. As
the effective length of the pile (depth of fixity from
ground surface) increase significantly depending on the
thickness of the liquefiable stratum (in this case, 6 m),
critical load capacity of the pile (Pcr) reduces. To keep
the P/Pcr ratio (where P is the working axial load of the
pile), flexural rigidity (EI) of the pile need to be
enhanced. Bhattacharya and Bolton [20] suggested
minimum diameter that needs to be adopted based on
the thickness of the liquefiable layer (Fig. 5).
iv) Consideration of lateral movement of the soil layer
above the liquefiable layer: When the ground is
liquefied, soil layers above the liquefied zone moves
according to the liquefied zone movement, resulting
passive pressures on the pile. These additional passive
pressures rise the moments at the fixity point
necessitating a higher moment capacity. This can be
achieved whether increasing the reinforcement in the
originally adopted section or increasing the pile section
to meet the requirement.
Fig. 5 Minimum pile diameter for buckling consideration
[20]
Considering the above points, to avoid dynamic failure, the
pile section is to be modified as 0.75 m diameter with a pile
length of about 15 m. Typical calculation details can be
referred at Bhattacharya [21] and Adak et al. [22].
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A. Murali Krishna & S. Bhattacharya
CONCLUSIONS
Recent research findings on the seismic behaviour of pile
foundations are discussed along with the codal provisions
for seismic design of pile foundations. A design case study
is presented highlighting the key points to be considered for
seismic design of pile foundation in liquefiable soil. A few
major shortcomings of the Indian codes are listed which can
be considered for the revision of codes to include the state
of the art findings in the area and to meet the international
standards.
ACKNOWLEDGEMENTS
First author acknowledges the Department of Science and
Technology for providing the financial support to visit and
collaborate with the co-author.
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