Geng159-233 Dynamic Response of Laterally Loaded Piles in Clay

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Proceedings of the Institution ofCivil EngineersGeotechnical Engineering 159July 2006 Issue GE3Pages 233–241

Paper 14420

Received 06/09/2005Accepted 20/03/2006

Keywords:dynamics/field testing &monitoring/piles & piling

A. BoominathanProfessor, Department of CivilEngineering, Indian Institute ofTechnology, Madras, India

R. AyothiramanAssistant Professor, Department ofCivil Engineering, Indian Institute ofTechnology, Guwahati, India

Dynamic response of laterally loaded piles in clay

A. Boominathan PhD and R. Ayothiraman PhD

The behaviour of single piles under lateral dynamic

loading is critical, and has been an important field of

research since the 1950s. Many analytical or semi-

analytical linear and non-linear models are available to

estimate the dynamic lateral stiffness, but it is essential

to determine the dynamic characteristics of the soil–pile

system through full-scale lateral dynamic pile load tests

for important structures and for validation of existing

models. This paper presents the results of field lateral

dynamic load tests conducted on 33 piles of varying

types–driven precast concrete, driven cast-in-situ

concrete and bored cast-in-situ concrete–at different

petrochemical complexes in India. The results indicate

that driven precast concrete piles have stiffnesses that

are four to five times higher than those of driven cast in

situ piles. The lateral stiffness was also estimated using

the computer program PILAY for all piles and compared

with the stiffness determined from the field tests. The

estimated stiffness shows good agreement with the field

values for stiff clay sites, but greatly overestimates the

values for soft clay sites.

NOTATION

ax measured horizontal acceleration

Ax dynamic displacement amplitude

Am amplitude of vibration at mth cycle

Amþ1 amplitude of vibration at (m + 1)th cycle

e eccentricity of rotating mass oscillator

f forcing frequency

fn natural frequency of soil–pile system

Fd magnitude of dynamic force

Gmax maximum dynamic shear modulus

khp dynamic lateral stiffness of soil–pile system

me mass of eccentrically rotating body

N uncorrected SPT-N value

Navg average uncorrected SPT-N value

Vs shear wave velocity

�st static displacement amplitude

� magnification factor

r mass density of soil

�x damping factor

ø frequency of rotating mass oscillator

1. INTRODUCTION

Pile foundations are commonly employed in industrial

situations, such as power plants, petrochemical complexes, oil

refineries and compressor stations, to support a range of

structures. These piles are subjected to dynamic lateral loads

from operating machinery, wind and earthquakes in addition to

static loads, and hence the dynamic response of piles and pile

groups to lateral shaking has received considerable attention

from designers and researchers. The lateral capacity and

stiffness of piles depend mainly on the characteristics of the

top layers of soil (within a few metres of the surface), which

may be very soft to stiff in nature. At some industrial sites the

top layer is found to be soft clay or loose sand with a thickness

that may vary from 5.0 m to 30.0 m or more. Therefore the

lateral load criterion rather than the vertical load often dictates

the pile design. Hence evaluation of the lateral stiffness of a

single pile under dynamic loading becomes a crucial step in the

satisfactory design and performance of pile foundations.

Although many sophisticated linear and non-linear models—

theoretical, semi-analytical and numerical—have been

proposed,1–18 there are scant experimental data available to

confirm the reliability of these models.

The limited field testing carried out on piles embedded in clay

and sandy clay sites by various authors19–27 (details are

presented in Table 1) does not provide satisfactory calibration

for the existing models. Furthermore, designers have insisted

upon the need to evaluate dynamic pile parameters by means

of in situ tests for important and sensitive structures such as

nuclear power plants and industrial structures. This paper

discusses the results of lateral dynamic vibration tests carried

out on 33 piles located at various petrochemical complex and

oil refinery sites in India: the motor spirit quality (MSQ) unit

and cogeneration (COGEN) sites at Mathura (Uttar Pradesh), the

independent power producer (IPP) plant site at Panipat

(Haryana), the MSQ unit and hydro cracker unit (HCU) sites at

Haldia (West Bengal), and the pure teriphthalic acid unit 3

(PTA-3) site for Reliance at Hazira near the city of Surat

(Gujarat). The dynamic lateral stiffnesses determined from the

field tests are also compared with stiffness estimated from the

computer program PILAY.28

2. SOIL PROFILE

Table 2 summarises the site conditions explored through a

subsoil investigation, together with the measured average

standard penetration test (SPT) values (Navg) for the various

sites. It can be seen from the table that the MSQ unit and

COGEN sites at Mathura consist predominantly of stiff silty

Geotechnical Engineering 159 Issue GE3 Dynamic response of laterally loaded piles in clay Boominathan • Ayothiraman 233


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clay mixed with kankars, having a minimum Navg of 9.0 even

at the shallow depth, except for the third (III) stratum of the

COGEN site, which has a low Navg of 3. The IPP plant site at

Panipat also consists of stiff brown clayey silt 4 m thick at

shallow depth, followed by loose to medium dense sandy silt or

sand deposits. The MSQ unit site at Haldia (Haldia I) consists

predominantly of moderate stiff silty clay, with minimum Navg

¼ 9.0 and thickness of 3.6 m even at shallow depth, followed

by soft silty clay (Navg ¼ 3) 3.9 m thick. But at the Haldia II

(HCU) site there is a very soft to soft clay layer 5.7 m thick

(underlain by a thin stiff clay layer 1.5 m thick from pile cut-

off level followed by loose to medium dense sandy silty clay

(4.7 m thick) and soft clay (6.3 m thick). At the Hazira site

medium dense silty sand and soft to medium stiff clayey silt

layers are found up to a depth of 10 m below pile cut-off level.

Seismic cross-hole or down-hole tests were not performed at

these sites, and so the shear wave velocity Vs of each layer,

required for using in PILAY to estimate the pile stiffness, was

evaluated by using the following correlation between shear

wave velocity Vs (m/s) and the uncorrected SPT N-value.29

Vs ¼ 91N0:3371

The low-strain shear modulus Gmax was then estimated using

the shear wave velocity Vs and mass density of the soil r by

the following equation.30

Gmax ¼ rV 2s2

As the routine soil parameters such as void ratio and plasticity

index were available, based on undisturbed samples collected

at particular depths for a few sites, the low-strain shear

modulus was evaluated using the equation proposed by Hardin

and Drnevich.31 For these soil layers, the shear wave velocity

has been back-calculated from the low-strain dynamic shear

modulus determined from the routine soil parameters. The

shear wave velocities and low-strain shear modulus of soil

layers determined for the various sites considered are also

provided in Table 2.

3. PILE DETAILS

The details of the various piles tested at the different

petrochemical complex and oil refinery sites are given in Table

3. The pile cap had dimensions of 750 mm 3 750 mm 3

750 mm and was cast monolithically with the pile head for

mounting the oscillator assembly for dynamic testing. A curing

period of 1 month was allowed for the pile before testing. The

pile cap was not in direct contact with the soil and a minimum

clearance of 150 mm was provided from the pile cut-off level.

4. DYNAMIC TESTING OF PILES

Free vibration or plucking tests and steady-state forced lateral

vibration tests were carried out on the 33 piles located at the

various sites listed in Table 3, in accordance with the procedure

recommended in India Standard 9716.32 The test set-up for

both forced and free lateral vibration tests and the test

procedure are discussed below.

4.1. Forced vibration tests (FVT)

A typical set-up for the steady-state forced lateral vibration

tests is shown in Fig. 1. A steady-state sinusoidal force was

generated by an eccentrically rotating mass oscillator of 5 t

capacity. The speed of the oscillator was controlled by a d.c.

motor and a speed control unit. The forced vibration response

of the piles was measured using two accelerometers, one fixed

at the mid-height of the pile cap and the other close to the pile

cut-off level, as shown in Fig. 1.

A data acquisition system consisting of a multi-channel carrier

frequency amplifier system and a digital storage oscilloscope

was used to monitor and record the time history of response of

the pile measured by the accelerometers. Each accelerometer

was calibrated before and after conducting the test. At the

conclusion of the first steady-state lateral vibration test, the

eccentricity of the oscillator was increased to raise the dynamic

force, and the test was repeated. The magnitude of the dynamic

force is related to the eccentricity of the oscillator as follows.

Fd ¼ meeø2 sinøt3

where Fd is the dynamic force, me is the mass of the

eccentrically rotating body in the mechanical oscillator, e is the

eccentricity of the rotating mass, and ø is the frequency. To

cover the wide range of lateral displacements expected as a

result of dynamic loading, the tests were repeated for three to

five levels of eccentricity.

Reference(s) Details of experiments

19 Single steel pile 61 mm in diameter and 2.1 m long. Pile group consists of 102 piles 26.70 mm in diameter and1.06 m long

20, 21 Single steel tube pile 273 mm in diameter and 13.4 m long22 Group of six model piles 102 mm in diameter and 3.05 m long, and group of six larger piles 320 mm in diameter

and 7 m long23 450 mm diameter and driven to a depth of 17 m24 200 mm diameter and pile length equal to 15 m embedded in soft saturated peat25 Thin-walled steel pipe pile with inside diameter 98.3 mm and wall thickness 1.65 mm26 Driven precast concrete square pile of 400 mm size and driven cast in situ concrete piles 400 and 500 mm in

diameter with length about 30 m27 High-strength aluminium; consisted of solid rod with lengths of 136–142 mm, radii 4.5–4.8 mm

Table 1. Summary of reported experimental investigation on lateral dynamic response of pile foundations

234 Geotechnical Engineering 159 Issue GE3 Dynamic response of laterally loaded piles in clay Boominathan • Ayothiraman


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4.2. Plucking tests (PT)

Plucking or free vibration tests were generally carried out on one

set of piles. When one pile was tested, another one was used as a

dummy pile to create lateral load. The set-up for plucking tests,

which is shown in Fig. 2, consists essentially of a suitably

calibrated rotating pulling screw with a clutch-type

arrangement. The horizontal load was applied by suddenly

releasing the clutch. A load cell of 2 t capacity connected

between the rotating screw and the releasing system was used to

measure the horizontal load applied to the pile. The free vibration

response of the piles was measured using two accelerometers,

fixed as in the previous case. The applied load and the free

vibration response of the pile were recorded using the data

acquisition system, as for the forced lateral vibration tests.

5. ANALYSIS OF TEST RESULTS

5.1. Free vibration response

A typical free vibration record measured at the pile cut-off

level at the Panipat site is shown in Fig. 3. The natural

frequency of the soil–pile system was determined from the free

vibration records by the fundamentals of vibration. The natural

frequencies obtained for the various sites are presented in Table

4. The damping factor �x was computed from the time history

Site Stratum Thickness oflayer: m

Description Navg Vs: m/s Gmax: MN/m2

Mathura I I 3.0 Grey silty clay mixed with kankars 9 190.82 64.08(MSQ unit site) II 6.50 Yellowish silty clay mixed with kankars 12 210.24 84.86

III 11.50 Silty sand in yellowish colour mixed with kankars 32 292.60 170.37Mathura II I 0.70 Filled up soil – – –(COGEN site) II 6.80 Stiff brownish clayey silt 10.5 201.0 74.74

III 3.00 Loose brownish grey sandy silt 3 131.77 32.12IV 3.50 Brownish grey sandy clayey silt 15.5 220.60 92.95*V 1.0 Grey clayey silt with kankars – 240.15 110.15*VI 7.50 Dense to very dense grey brownish silty sand/

sandy silt with kankars43 234.68 110.15*

VII 4.50 Very stiff brownish clayey silt with kankars andfine sand

28 250.44 124.19*

Panipat I 4.0 Stiff brown clayey silt, medium plastic (ML) 8 183.39 60.54(IPP plant site) II 0.5 Loose to medium dense light brown sandy silt

with traces of gravel, low plasticity (CL)8 183.39 60.54

III 5.5 Loose to medium dense light brown silty finesand (SM)

9 190.82 66.63

IV 0.5 Loose light brown sandy silt with traces of gravel,low plasticity (CL)

6 166.45 50.70

V 1.0 Loose to medium dense light brown silty finesand (SM)

9 190.82 66.63

VI 4.5 Medium dense light brown sandy silt, lowplasticity (CL)

10.7 202.08 80.45

VII 1.5 Dense light brown silty fine sand (SM) 33 295.65 174.82VIII 3.0 Very dense light brown silty sand (SM) 47.5 334.26 223.46

Haldia I I 1.50 Filled up soil – – –(MSQ unit site) II 0.80 Moderate stiff brownish grey silty clay 9 190.82 75.00

III 2.80 Soft to medium stiff brownish grey silty clay 4 87.03 14.09*IV 3.90 Soft bluish grey silty clay 1 102.89 19.69*V 5.50 Medium dense bluish grey silty fine sand 17.7 241.03 106.31VI 5.50 Moderately stiff light grey silty clay 5.5 186.85 56.21*VII 3.30 Moderately stiff to stiff greyish brown silty clay 22 147.82 45.67*VIII 6.15 Medium dense to dense/very dense brownish

yellow fine sand31.3 289.49 175.15

Haldia II I 1.5 Firm to stiff brownish clay 8 183.39 62.50(HCU site) II 5.7 Very soft to soft silty clay 3 131.77 29.64

III 4.7 Loose to medium dense sandy silty clay 12 210.25 77.23IV 6.3 Soft to moderate stiff clay 3 131.77 30.20V 3.7 Stiff to very stiff sandy silty clay 13 215.99 91.52VI 12.1 Medium dense to very dense silty sand 20 249.74 112.35VII 1.9 Very stiff to hard silty clay 34 298.64 168.42

Hazira I 0.60 Filled up soil – – –(PTA-3 site) II 2.00 Medium dense black silty sand 16 231.65 101.42

III 6.90 Soft to medium stiff clayey silt 7 175.32 51.64IV 3.00 Medium dense black silty sand 16.5 234.06 104.09V 4.00 Dense dark brown silty sand 52 344.62 237.53VI 1.50 Hard black clayey sand with silt and kankar 76.5 392.49 323.50VII 2.00 Very dense black silty sand 89 413.03 358.25

*Dynamic shear modulus determined using method of Hardin and Drnevich31

ML, silt with low plasticity; CL, clay with low plasticity; SM, silty sand.

Table 2. Soil profile at different sites



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of acceleration using the logarithmic decrement method, as

given by the following equation.

�x ¼1

2�ln

Am

Amþ1

� �4

where Am and Amþ1 are the maximum amplitude of vibration

in two successive cycles. The damping factor estimated using

the above equation for piles located at different sites is also

provided in Table 4.

5.2. Dynamic response curves

The displacement amplitude of the vibrations, Ax , was

computed from the measured acceleration using the following

relationship,

Ax ¼ax

4�2 f 25

where ax is the measured horizontal acceleration of vibration

(mm/s2) at a particular frequency f (Hz). The computed values

of displacement amplitude corresponding to pile cut-off level

at each frequency for different eccentricities of the oscillator

were plotted as frequency response curves. A typical frequency

response curve obtained for the COGEN site at Mathura is

given in Fig. 4. It can be easily seen from the figure that the

resonant frequency is practically the same at low eccentricities

(e ¼ 16.48 and 24.68), which indicates that the soil–pile system

behaves linearly at low magnitudes of dynamic force. However,

the resonant frequency decreases as the magnitude of the

dynamic force increases, indicating a non-linear response of

the soil–pile system due to degradation of the stiffness of soil.

Site Pile size: mm Pile type Pile grade Length*: m No. of piles Cut-off levelfrom GL: m

Mathura I (MSQ unit site) 500 (circular) DrivenCast in situ

M25 11 to 15.5 4 1

Mathura II (COGEN site) 450 (circular) BoredCast in situ

M25 21.5 4 3.5

Panipat (IPP plant site) 500 (circular) DrivenCast in situ

M25 19.50 4 1

450 (circular) DrivenCast in situ

M25 20.05 4 1

Haldia I (MSQ unit site) 500 (circular) DrivenCast in situ

M25 28.58 4 1.4

Haldia II (HCU site) 400 (circular) DrivenCast in situ

M30 30.0 3 1.5

400 (square) DrivenPrecast

M30 30.0 3 1.5

500 (circular) DrivenCast in situ

M30 30.0 3 1.5

Hazira (PTA-3 site) 400 (square) DrivenPrecast

M35 17.0 4 0.6

* Pile length from cut-off level of pile.

Table 3. Details of piles tested

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Fig. 1. Set-up for forced lateral vibration test (dimensions in mm)



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Fig. 3. Typical free vibration record observed at Panipat site

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Fig. 2. Set-up for free lateral vibration test (not to scale)

Site Pile size:mm

Pilelength:

Natural frequency: Hz Damping factor

m FVT PT FVT PT

Mathura I (MSQ unit Site) 500 (circular) 11–15.5 19.7–23.5 23.8–25.77 0.05–0.09 0.07–0.09Mathura II (COGEN site) 450 (circular) 21.5 10.5–20.0 24.04–26.6 0.03–0.07 0.02–0.06Panipat (IPP plant site) 500 (circular) 19.50 22.0–34.2 32.5–33.3 0.07–0.18 0.06–0.20

450 (circular) 20.05 20.0–28.5 23.8–30.12 0.08–0.14 0.07–0.16Haldia I (MSQ unit site) 500 (circular) 28.58 9.7–19.33 14.1–15.87 0.13–0.16 0.10–0.11Haldia II (HCU site) 400 (circular) 30.0 9.5–20.0 10.0–13.3 0.11–0.18 0.06–0.09

400 (square) 30.0 20.0–27.5 20.0–25.0 0.07–0.30 0.09–0.16500 (circular) 30.0 18.0–29.0 25.0 0.15–0.23 0.07–0.12

Hazira (PTA-3 site) 400 (square) 17.0 9.0–12.2 10.6–16.7 0.11–0.17 0.08–0.16

Table 4. Natural frequency and damping factor



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This observation is consistent for almost all piles tested at the

various sites. Similar observations were reported by

Boominathan and Ayothiraman33 based on experiments

conducted on model piles embedded in soft clay.

5.3. Natural frequency and damping factor

The range of natural frequencies of the soil–pile system

obtained from the frequency response curves for the various

sites is presented in Table 4. The damping factor of the soil–

pile system determined from the frequency response curve

using the bandwidth method is also given in Table 4.

It could be inferred from the table that the natural frequencies

obtained from forced vibration tests and plucking tests fall

almost in the same range, except for a few cases: the COGEN

site at Mathura, and one pile in the HCU site at Haldia II.

However, the lower value of natural frequency—that is, the

natural frequency at high magnitudes of dynamic force

obtained from the forced lateral vibration test—is always less

than the natural frequency obtained from the free vibration

tests owing to degradation of stiffness resulting from the strong

non-linear behaviour of the soil at high magnitudes of

dynamic force.

It can also be seen from Table 4 that the MSQ unit site at

Mathura I has a higher natural frequency than the COGEN site,

even though the pile length in the MSQ site is less than that at

the COGEN site. This may be attributed to the fact that the pile

diameter is slightly larger, and the soil contains kankars with

high stiffness. For the IPP plant site at Panipat, the natural

frequency of the 500 mm diameter pile is 10–20% higher than

that of the 450 mm diameter pile from the forced vibration test

data, and about 10–35% higher from the free vibration test

data, even though both piles are embedded at the same site and

have nearly the same pile length. This clearly demonstrates the

effect of pile diameter on the natural frequency of the soil–pile

system.

It can also be inferred from the table that the natural frequency

of the soil–pile systems with nearly the same length of about

30 m at both the MSQ unit site and the HCU site at Haldia is

less than that of the soil–pile system with length 20 m at the

Panipat site. This may be because the site conditions at both

Haldia locations consist of a thick, very soft clay layer below

the cut-off level of the pile, which thus leads to soil stiffness

degradation due to the strongly non-linear behaviour of the

soft clay layer.

It can also be seen from Table 4 that the 400 mm square driven

precast pile has a relatively high natural frequency compared

with the 400 mm diameter driven cast in situ pile, even though

both were driven at the same location in Haldia II. This may be

due to irregularities or defects in the pile, which is often

possible in cast-in-situ piles but not in precast piles, and thus

substantiates the effect of pile installation method on the

natural frequency of the soil–pile system. Table 4 also

indicates that the natural frequency of the 400 mm square pile

(30 m length) at the Haldia II site is nearly twice that of the

400 mm square pile (17 m length) at the Hazira site, even

though the Haldia II site consists of comparatively soft soil.

This clearly indicates the effect of pile length on the natural

frequency of the soil–pile system.

It can be seen from Table 4 that the damping factors

determined from forced and free vibration test data are fairly

well matched for almost all the sites. However, for the Haldia I

and Haldia II sites and the Hazira site, the damping factors

determined from forced vibration tests are always higher than

those determined from free vibration tests. This may be caused

by additional hysteretic damping due to the non-linear

behaviour of the soft soil layers present at shallow depths of

these sites, at the high magnitude of dynamic forces applied to

the pile.

5.4. Dynamic lateral stiffness

The soil–pile stiffness was evaluated in accordance with Indian

Standard 9716.32 It is based on the assumption that, for all

practical purposes, there is a unique variation in the static

displacement amplitude �st and dynamic force Fd, irrespective

of variation in the forcing frequency f and natural frequency

fn, as these are taken corresponding to larger amplitudes. In

accordance with this procedure, the equivalent static

displacement amplitude �st corresponding to different dynamic

force Fd and forcing frequency was calculated from the

frequency response curves by using the following formula.

�st ¼Ax

�6

where Ax is the dynamic displacement amplitude (mm) and �is the magnification ratio, given by

� ¼ 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 � f =ð fn½ Þ2�2 þ 2�x f =ð fn½ Þ�2

q7

in which �x is the damping factor, obtained from free vibration

1 �2��3!��

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Fig. 4. Typical frequency response curve (COGEN site,Mathura)



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records; f is the forcing frequency (Hz); and fn is the natural

frequency under lateral vibration (Hz).

The dynamic force was plotted against the static displacement

amplitude for piles tested at different sites, and a typical plot of

dynamic force against static displacement amplitude plot is

shown in Fig. 5 for a 450 mm diameter pile tested at the

COGEN site, Mathura. The tangent modulus of this plot gives

the soil–pile stiffness khp under dynamic loading conditions.

The dynamic lateral stiffness of the soil–pile system obtained

by this procedure for the various different sites is summarised

in Table 5. It can be seen from the table that the MSQ unit site

at Mathura has a lateral stiffness that is nearly 12 times higher

than that at the COGEN site, even though the pile length at the

MSQ site is very small. This is due to the presence of kankars

in the soil, and the very high shear modulus of the soil at

shallow depth. The low stiffness of the COGEN site at Mathura

may be due to the pile installation by boring, in which defects

and irregularities are often possible.

For the IPP plant site at Panipat the lateral stiffness is

practically the same for both the 500 mm and the 450 mm

diameter piles. This indicates that, although the pile diameter

affects the natural frequency of the soil–pile system, it does

not significantly influence the dynamic lateral stiffness of the

soil–pile system of the piles tested.

It can also be inferred from the table that the stiffness of the

500 mm circular pile embedded at the MSQ unit site is nearly

half that of the 500 mm pile at the HCU site in Haldia. This is

due to the low shear modulus (less than 20 MN/m2) of the soft

grey silty clay layer, about 6.7 m thick, at shallow depth at the

MSQ unit site, Haldia. Similarly, the lateral stiffnesses of long

piles embedded at both the MSQ and HCU sites at Haldia are

much lower than the stiffness of the short piles embedded at

the IPP plant site, Panipat. This may be attributed to the soil

conditions at both Haldia sites, which consist of a thick, very

soft clay layer below the pile cut-off level, and the low shear

modulus values within the shallow depth.

It can also be seen from Table 5 that the 400 mm square driven

precast pile has four to five times higher stiffness than the

400 mm diameter driven cast in situ pile, even though both

were driven at the same location at Haldia II. This is due to the

good quality of the precast concrete pile, which is always

controllable in the casting yard. Also, the stiffness of the

400 mm square pile (30 m length) at the Haldia II site is about

three to five times higher than that of the 400 mm square pile

(17 m length) at the Hazira site, even though the Haldia II site

consists of very soft soil in comparison.

6. COMPARISON OF FIELD DATA WITH PILAY

The dynamic lateral soil–pile stiffness was estimated using the

PILAY program,28 and was compared with the stiffness

obtained from the field vibration tests. PILAY is a computer

program developed by extending the Novak elastic continuum

approach34 to layered soil35 for evaluation of the dynamic

impedance of a single pile. The variation of shear wave

velocity and the corresponding dynamic shear modulus with

respect to depth listed in Table 2 were used in the PILAY

program. The various other input parameters representing the

pile characteristics are the pile length, the pile unit weight, the

Poisson’s ratio of the pile, the coefficient of rigidity in shear

for the pile material, Young’s modulus of the pile and the static

load on the pile (weight of the pile cap and oscillator). The

respective values for piles embedded at the various sites were

adopted in the PILAY program.

The single pile stiffness for the pinned head condition was

computed considering a layered soil profile for the various

sites, and the results are summarised in Table 5. It can be seen

from the table that the lateral stiffness estimated by PILAY for

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Fig. 5. Typical plot of dynamic force against staticdisplacement (COGEN site, Mathura)

Site Pile size:mm

Pile length:m

Lateral stiffness: kN/m

Field test PILAY

Mathura I (MSQ unit site) 500 (circular) 11 to 15.5 12.8–13.7 3 104 14.5 3 104

Mathura II (COGEN site) 450 (circular) 21.5 0.60–1.67 3 104 2.3 3 104

Panipat (IPP plant site) 500 (circular) 19.50 26.5–29.4 3 104 31.5 3 104

450 (circular) 20.05 22.6–30.4 3 104 29.8 3 104

Haldia I (MSQ unit site) 500 (circular) 28.58 1.47–2.45 3 104 4.2 3 104

Haldia (HCU site) 400 (circular) 30.0 0.50–1.23 3 104 16.2 3 104

400 (square) 30.0 3.9–4.9 3 104 17.1 3 104

500 (circular) 30.0 2.9–5.0 3 104 18.4 3 104

Hazira (PTA-3 site) 400 (square) 17.0 0.69–1.77 3 104 8.9 3 104

Table 5. Dynamic lateral stiffness of soil–pile system



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the MSQ unit site at Mathura and the IPP plant site at Panipat

matches the field test values closely , even though PILAY has

considered only the linear behaviour of the soil. This is because

the site conditions at the MSQ site, Mathura, are predominantly

stiff silty clay with kankars, particularly at shallow depths, and

hence the soil is expected to behave in a linear fashion under

dynamic working load (which is normally 20–30% of the

maximum static load). This also holds true for the Panipat site.

However, for the COGEN site at Mathura and the MSQ unit site

at Haldia, PILAY overestimates the stiffness by about two

times. This may be due to the presence of a loose sandy silt

layer, 3 m thick, at the COGEN site, Mathura, and a soft silty

clay layer 6.7 m thickness at the MSQ unit site, Haldia, which

may behave non-linearly. Though PILAY considered the

layered soil profile nature, it has not accounted for the soil

non-linearity of each layer. Similarly, for piles embedded at the

HCU site at Haldia and the PTA-3 site at Hazira, PILAY greatly

overestimates the lateral stiffness: by about four to five times

for the 400 mm square and 500 mm circular pile embedded in

the Haldia II site, and by about seven times for the 400 mm

square pile embedded in the Hazira site. For the 400 mm

circular bored cast in situ pile PILAY overestimates the lateral

stiffness by about 18 times, because the program did not

account for the effect of the pile installation procedure.

It can be concluded here that, for very stiff clay and dense

sand sites, the PILAY program, which is based on the linear

elastic continuum approach, can be utilised for the estimation

of lateral stiffness. However, for soft clay and loose sand

deposits, in which soil non-linearity prevails, PILAY may be

used for a rough estimation of lateral stiffness, but field

dynamic lateral load tests must be conducted on test piles for a

realistic determination of lateral pile stiffness.

6. CONCLUSIONS

Based on lateral dynamic pile loads tests carried out on 33

piles located at different sites in India, it is found that the site

conditions and, in particular, the properties of the top soil

layers greatly influence determination of the dynamic lateral

stiffness of the soil–pile system. It can also be concluded that

the natural frequency of the soil–pile system is significantly

influenced by the size of the pile (diameter and length), the pile

installation procedure, and the stiffness of the top soil layers.

Driven precast piles have four to five times higher stiffness

than driven cast in situ piles. PILAY accurately estimates the

dynamic lateral stiffness constant of piles embedded in stiff

clay sites, but grossly overestimates the stiffness of piles

embedded in soft clay sites. It can be concluded that PILAY can

be used for estimation of the stiffness of quality-assured

(precast) piles embedded in stiff or dense soil deposits, but

immense care is required when using it for soft or loose soil

deposits and for cast in situ piles. Under these circumstances,

lateral dynamic load tests on full-scale piles must be carried

out along with the use of PILAY for practical recommendation

of dynamic parameters that can be used for design of piles

subjected to lateral dynamic loads.

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Geng159-233 Dynamic Response of Laterally Loaded Piles in Clay