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: amurali@iitg.ac.in

S. Bhattacharya, Sr.Lecturer in Dynamics, Dept.CE, UniversityofBristolBS81TR,UKEmail: S.Bhattacharya@bristol.ac.uk

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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