Lateral Dynamic Response and Effect of Weakzone

ORIGINAL PAPER

Lateral Dynamic Response and Effect of Weakzoneon the Stiffness of Full Scale Single Piles

A. Boominathan S. Krishna Kumar

RM. Subramanian

Received: 4 July 2012 / Accepted: 29 January 2014 / Published online: 13 February 2014

Indian Geotechnical Society 2014

Abstract The determination of dynamic characteristics

of soil-pile system by full-scale lateral dynamic pile load

testing is an important aspect in the design of pile foun-

dations subjected to dynamic/seismic loads. Field test

results are very useful for validating existing linear/non-

linear models, which are used to predict the dynamic

stiffness and damping of the soil-pile system. This paper

presents the results of field lateral dynamic load tests

conducted at two different petrochemical complex sites in

India and the measured dynamic constants of the soil-pile

system. A 3D finite element analysis is performed using

ABAQUS to predict the non-linear response of soil-pile

system under dynamic lateral loads. The lateral stiffness

estimated from the FE analysis shows good agreement with

stiffness measured in the field tests. Dynamic analyses of

single piles using improved Novaks method were per-

formed to study the effect of weak zone around the pile

shaft on the lateral stiffness of the piles.

Keywords Dynamic load Stiffness Weak zone Shear modulus

Introduction

There are many sources of ground-borne vibration such as

earthquakes, construction, machine-foundation design,

offshore structures, nuclear energy, and road and rail

development. These sources produce ground-borne vibra-

tions that can be transmitted into buildings via structural

foundations, resulting in disturbances and structural

damage. Pile foundations act as the major vibration trans-

mission paths, and as such understanding their dynamic

behavior is required. Analysis and design of piles subjected

to dynamic lateral loads is complex due to the non-linear

behaviour of soil and the soil-pile separation that happens

near the ground surface. Although, certain theoretical

models consider soil non-linearity, the effects of pile

installation procedure and the formation of soil-pile gaps

are not rigorously modelled. The stiffness of the pile in the

lateral direction is very low in comparison to its vertical

stiffness; hence, the lateral capacity/stiffness of the pile

governs the design in most cases, where the lateral loads

are dominant. The lateral stiffness and the bending

behaviour of piles depend mainly on the characteristics of

the top few meters of soil below the ground. The topsoil

layers mainly consist of weak deposits such as soft clay or

loose sand, resulting in high degree of nonlinearity on piles

subjected to lateral loading. Hence, the strong nonlinearity

and soil gapping becomes a crucial step in the satisfactory

design and performance of pile-supported foundations

subjected to dynamic loads. The lateral stiffness constant of

a pile is the most important parameter in the sub-structure

approach, which is used to analyse pile-supported struc-

tures subjected to seismic loading [27].

In the recent past, many sophisticated linear and non-

linear models were proposed to study the lateral response

of piles under dynamic loads [2, 4, 8, 10, 11, 14, 19, 2123,

25], but there are only a few full scale experimental data

available to confirm the reliability of these models. The

major full-scale field testing carried out on piles embedded

in clay and sandy clay sites by various authors [1, 57, 9,

12, 24, 28, 31] clearly demonstrate that the performance of

existing linear and nonlinear models are highly dependent

on in situ soil nonlinearity and dynamic loading charac-

teristics. Hence, it is important to perform in situ full-scale

A. Boominathan (&) S. Krishna Kumar RM. SubramanianDepartment of Civil Engineering, IIT Madras, Chennai 36, India

e-mail: [email protected]

123

Indian Geotech J (JanuaryMarch 2015) 45(1):4350

DOI 10.1007/s40098-014-0106-6

dynamic tests on piles in order to accurately assess the non-

linear dynamic characteristics of the soil-pile system. This

paper presents the results of two full-scale field dynamic

lateral pile load tests carried out at two different sites in

India (Chennai and Hazira) and the results of a nonlinear

three-dimensional finite element analysis of piles under

dynamic lateral loads using ABAQUS. Although the finite

element analysis used in this study includes important

features such as soil nonlinearity and gapping at the pile

soil interface, it does not account for the buildup of pore

pressure due to cyclic loading. Thus, neither the potential

for liquefaction nor the dilatational effect of clays and the

compaction of loose sands in the vicinity of piles is

accounted for, in the current analysis.

Dynamic Testing of Piles

Steady state forced lateral vibration tests were conducted

on driven cast-in-situ piles installed, as per the procedure

recommended by the Indian Standard code of practice IS:

9716, at two different sites in India (Site-I: Chennai, Tamil

Nadu and Site-II: Hazira, Gujarat).

Soil and Pile Properties

The soil characteristics, pile dimensions, and properties for

both sites are described below.

Site-I: Chennai, Tamil Nadu

The test pile is 450 mm in diameter and it extends to a depth

of 20.15 m below the ground level. The length of the pile

below the cut off level is 18.25 m. The pile is a M30 grade

concrete pile, which corresponds to a dynamic Youngs

modulus (Ep) of 37,000 MPa. The soil profile and charac-

teristics of Site-I obtained from the borehole data are pre-

sented in Table 1. It can be observed from the table that four

distinct layers characterise Site-I. A 5 m thick very soft silty-

clay layer characterises the top 1/3rd length of the pile.

Multichannel Analysis of Surface Waves (MASW) is a

non-destructive technique that evaluates the elastic condi-

tion (i.e. stiffness) of the subsurface. The shear wave

velocity structure is typically derived from the fundamental

mode Rayleigh wavefield generated by an active and/or a

passive source. The frequency-dependent properties of the

Rayleigh-type surface waves can be utilised for imaging

and characterising the shallow subsurface [29]. MASW

surveys are typically used for geotechnical engineering

purposessuch as Vs30 profiling, bedrock mapping and

finite element modeling.

The MASW test was used to determine the shear wave

velocities of different layers up to a depth of 12.5 m below

the ground level. The shear wave velocity of soil layers

below 12.5 m depth was estimated using Eq. 1 based on

the average SPT-N value [17]. The maximum dynamic

shear modulus required for the finite element analysis is

determined based on the shear wave velocity using Eq. 2

[30] and the values are reported in Table 1.

Vs 91 N0:337 1Gmax V2s q: 2Site-II: Hazira, Gujarat

The pile is 500 mm in diameter and is 15.41 m in length

below the cut off level. Pile is made of M30 grade concrete

and the corresponding dynamic Youngs modulus (Ep) is

37,000 MPa. The typical soil profile of Site-II obtained

from the borehole record is presented in Table 2. It can be

observed from the table that Site-II is characterised by six

layers, wherein the top three layers are characterised by a

medium stiff silty-clay/stiff marine clay to a depth of 8.5 m

and is followed by a 1.5 m thick loose sandy layer. Since

direct measurement of shear wave velocity were not carried

out at this site, shear wave velocity of layers was estimated

using Eq. 1 based on the average SPT-N value. The max-

imum dynamic shear modulus was determined based on the

shear wave velocity. The values of shear wave velocity and

the shear modulus of the site are summarized in Table 2.

Table 1 Properties of soil at Site-I

Depth (m) Thickness (m) Soil type Avg. SPT (Navg) Vs (m/s) Density (kg/m3) Gmax (MPa) Cu

a (kPa)/friction angle

2.07.0 5 Soft silty clay 0 75b (91c) 1,800 10.1 2.0

7.012.5 5.5 Fine sand 40 170b (315c) 1,900 54.9 3812.518.0 5.5 Silty clay 60 362c 2,000 262.1 180

18.020.2 2.2 Yellow silty clay [100 430c 2,100 388.3 250

a based on SPT [20]b based on MASW testc based on SPT N value

44 Indian Geotech J (JanuaryMarch 2015) 45(1):4350

123

Test Setup and Procedure

Typical layout of a forced vibration test is shown in Fig. 1.

A steady state sinusoidal force was generated with a

5-tonne capacity mechanical oscillator. The speed of the

oscillator was controlled by a DC motor and a speed

control unit. The forced vibration response of the piles

were measured using two acceleration transducers fixed at

the mid height of the pile cap, and at the pile cut off level

as shown in Figure 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

accelerometers. Each acceleration transducer was cali-

brated before and after conducting the tests. After every

steady state lateral vibration test, the eccentricity of the

oscillator was increased to raise the dynamic force and the

test was repeated to cover a wide range of lateral dis-

placements expected during a typical dynamic loading of

the pile. The tests were repeated with five different

eccentricities in the machine load. More information about

interpretation of test data is presented in [6].

Test Results

The displacement amplitude of vibration (Ax) was com-

puted from the measured acceleration using Eq. 3.

Ax ax4p2f 2

3

where, ax = measured horizontal acceleration of vibration

(mm/s2) at a particular frequency, f (Hz).

The computed values of the amplitude of displacement

corresponding to the 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 Site-I and Site-II are presented in Figs. 2

and 3, respectively.

It can be noticed from Fig. 2 that the resonant frequency

of soil-pile system at Site-I ranges from 10.5 to 14 Hz. The

resonant frequency reduces from 14 Hz to 10.5 Hz as the

magnitude of the dynamic force increases, indicating a

non-linear response of the soil-pile system due to the

degradation of soil stiffness. This observation is consistent

for almost all the test piles at Site-I. The observed non-

linearity could be due to the presence of a 5 m thick very

soft silty-clay layer at the top with a SPT-N value of zero.

It can be observed from Fig. 3 that the resonant frequency

of soil-pile system at Site-II ranges from 13 Hz to 15 Hz,

Fig. 1 Typical forced vibration test layout (After [6])

0 10 20 30 40 50Frequency (Hz)

0.0

0.2

0.4

0.6

0.8

Disp

lace

men

t Am

plitu

de (m

m)

e=Eccentricity

e=24 deg. (Experiment)e=24 deg. (Numerical)e=32 deg. (Experiment)e=32 deg. (Numerical)e=48 deg. (Experiment)e=48 deg. (Numerical)e=64 deg. (Experiment)e=64 deg. (Numerical)

Fig. 2 Dynamic amplitude vs. frequency for Site-I

Table 2 Properties of soil at Site-II

Depth (m) Thickness (m) Description Avg. SPT (Navg) Vs (m/s) Density (kg/m3) Gmax (MPa) Cu (kPa)/Friction angle

0.05.5 5.5 High plastic silty clay 5 157 1,800 44.4 25.0

5.58.5 3.0 Marine clay 8 183 1,700 56.9 45.0

8.510.0 1.5 Loose silty sand 17 236 1,900 105.8 3310.015.5 5.5 Dense silty sand 25 269 2,000 144.7 3515.518.0 2.5 Stiff silty clay 21 254 2,100 135.5 150.0

18.020.5 2.5 Silty coarse sand 32 293 2,100 180.3 40

Indian Geotech J (JanuaryMarch 2015) 45(1):4350 45

123

i.e., the resonant frequency remains practically same irre-

spective of varying magnitudes of the dynamic force. This

indicates that the degree of nonlinearity for Site-II is less,

which is due to presence of relatively medium-stiff to stiff-

clay layers in the top 8.5 m depth below the ground

surface.

Non-Linear Finite Element Analysis

Finite-element models are extremely versatile, allowing for

the introduction of nonlinear behavior in the soilpile

system and contact surfaces. Full 3D geometric models

were used in ABAQUS to represent the pilesoil system

[16]. The non-linearity in the lateral response of piles is

because of the geometric and material non-linearity of the

pile and the soil surrounding the top 1/3rd length of the

pile. In the present study, the pile is idealised as an elastic

linear isotropic material without any damping. The solid

circular pile is modelled to the same scale as in the field

conditions. The pile is discretized using solid tetrahedral

elements. The pile is assumed to be pinned at its base and is

free at the top simulating a free head condition similar to

the large-scale field test. The soil profile in the FE analysis

is modelled identical to the field conditions. The non-linear

stressstrain behavior of soil is modelled using an elasto-

plastic Drucker-Prager criterion. The input parameters,

such as the Youngs modulus that is determined from the

shear modulus, are presented in Tables 1 and 2. Soil is

essentially a Tresca material, if the un-drained behaviour is

being considered. The un-drained strength of the clays is

used to model it as a Tresca material. Sand was modelled

using the friction angle calculated (See Tables 1, 2) based

on the correlations with SPT N value [20]. The soil

matrix of size 20D is adopted, where D is the diameter of

the pile. Kelvin-Voigt elements are used at the soil mesh

boundary to prevent reflection of stress waves on the

boundaries and to eliminate the box effect (i.e., the

reflection of waves back into the model at the boundaries)

during dynamic loading, and they are assumed to be fre-

quency independent given the low frequencies considered

(020 Hz). The boundaries are restrained in the vertical

direction. The base of the FE model is fixed in all direc-

tions and the top of the soil mass is not restrained. The

discretized soilpile model is presented in Fig. 4.

The modelling of the pilesoil interface is crucial

because of its significant effect on the response of piles to

lateral loading. The interface between the pile and the soil

was modelled using surface-to-surface contact elements.

The pile is considered as the contact surface and the soil as

the target surface, to allow for the interpenetration of pile

nodes into the soil surface. Penalty contact method with

small sliding was utilised to simulate the normal and tan-

gential contact behavior. The separation of contact under

reversal of loading (tension) was also simulated.

The verification process followed incremental steps to

ensure that pile, soil, and boundary conditions were sepa-

rately accounted for to minimise error accumulation. The

size of the mesh was mainly dependent on the loading

conditions and geometry of the piles. The mesh was refined

near the pile to account for the severe stress gradients and

plasticity encountered in the soil, with a gradual transition

to a coarser mesh away from the pile in the horizontal X

and Y directions. The vertical Z direction subdivisions

were kept constant to allow for an even distribution of

vertically propagating SH waves. The maximum element

size was less than one-fifth to one-eighth the shortest

wavelength to ensure accuracy.

A cyclic ramped load similar to that in the field test was

applied at the top of the pile at 0.75 m above the ground

0 10 20 30 40 50Frequency (Hz)

0.0

0.1

0.2

0.3D

ispla

cem

ent A

mpl

itude

(mm)

e=Eccentricity


Fig. 3 Dynamic amplitude vs. frequency for Site-II

Fig. 4 The discretized FE model


123

level. The acceleration, stresses, and displacements were

recorded throughout the soil-pile system. The typical dis-

placement contour obtained from finite element analysis is

shown in Fig. 5.

The dynamic amplitude versus frequency response

curves obtained from the finite element model for Site-I

and Site-II are presented in Figs. 2 and 3, respectively. It

can be observed from the figures that the lateral dynamic

response of piles predicted by the FE simulation matches

the field measurements. For the given soil conditions the

FE simulations over-predict the amplitude by about

510 %. The amplitude predicted by the FE simulations for

the given soil conditions increases at higher eccentricities

and near the resonant frequency. The inhomogeneity that

presents in the soil means that experimental results will

invariably differ from the idealized conditions. Another

peak, but smaller in amplitude, is observed at a frequency

of about 5 Hz, which is due to the resonant frequency of

the pile in its 4th mode (see [3]). It can be noticed that the

shear-wave velocity estimated from the SPT N values are

much higher than the direct measurements (see Table 1).

Use of estimated shear-modulus instead of direct mea-

surements of dynamic properties of the soil would have

resulted in an estimation of a much stiffer response by FE.

The dynamic stiffness of the soil-pile system is typically

obtained from a plot of dynamic force versus the equivalent

static amplitude using the procedure described in [6] and as

per IS 9716 [18]. The variation of the static amplitude with

the dynamic force measured from the field tests and those

predicted from the FE simulations for both the sites are

presented in Figs. 6 and 7, respectively. It can be observed

from Figs. 6 and 7 that the simulated response matches

with the response observed in Site-I. In Site-II, although

the simulated response matches with the measured

response at low to moderate force levels, the simulated

response at higher force level is about 30 % more than

what is measured in the field-test. This might be due to the

estimation of dynamic properties, especially for the soil in

the top 1/3rd length of the pile, from SPT N values,

instead of a direct measurement of the dynamic properties

of the soil. This signifies the importance of measurement of

dynamic soil properties.

The lateral stiffness of soil-pile system predicted by the

finite element model and those measured from field tests

are presented in Table 3. It can be observed from Table 3

that the FE simulation matches with the results obtained

from the field tests. Site-I characterised by soft clay

deposits at the top with a low shear wave velocity (Vs) of

75 m/s has lower stiffness in contrast to Site-II. Hence, it is

evident that the site conditions, especially the top 1/3rd

length of the pile, play an important role on the lateral

stiffness of the pile, although the resonant frequency

remains practically unchanged for piles of approximately

same dimensions located in different strata. Comparison of

FE simulations and large-scale field tests signifies the

importance of measurements of dynamic characteristics of

the sites.

Effect of Weak Zone on Lateral Stiffness

It is found that the site conditions, and in particular, the

properties of the top soil layers greatly govern the degree of

non-linearity and the dynamic lateral stiffness of the soil-

pile system. Dynamic analysis were performed using

Improved Novaks method, where a non-reflective

Fig. 5 A typical displacement contour obtained in FE simulation

0.0 0.5 1.0 1.5 2.0Static Displacement (mm)

0

2

4

6

8

10

12

Dyn

amic

For

ce (k

N)


Stiffness (Experiment) (BestFit)Stiffness (Numerical) (BestFit)

Fig. 6 Force vs. static amplitude plot for Site-I


123

boundary is formed between the near field and the far field

to account for the mass of soil in the boundary, to under-

stand the effect of presence of weak zone in the top layers

(Fig. 8) on the lateral stiffness of the piles. DynaN [13] is

used to study the effect of weak zone on the dynamic

response of piles. DynaN employs well-established ana-

lytical solutions based on the improved Novaks approach

to describe the soil-structure interaction under dynamic

loading conditions. A non-reflective boundary is used

between the near field and the far field to account for the

mass of soil in the boundary.

Both theoretical and experimental studies have shown

that the dynamic response of the piles is very sensitive to

the properties of the soil in the vicinity of the pile shaft

[15]. A rigorous approach to the nonlinearity of soil-pile

system is extremely difficult and therefore approximate

theories have to be used. Novak and Sheta [26] proposed

including a cylindrical annulus of softer soil (an inner weak

zone or so called boundary zone) around the pile in a plane

strain analysis. The ideal model of boundary zone should

have properties smoothly approaching those of the outer

zone to alleviates wave reflections from the interface. The

model of non-reflective interface assumed that the

0.0 0.1 0.2 0.3 0.4Static Displacement (mm)

0

2

4

6

8

10D

ynam

ic F

orce

(kN)

e=16 deg. (Experiment)e=16 deg. (Numerical)e=32 deg. (Experiment)e=32 deg. (Numerical)e=48 deg. (Experiment)e=48 deg. (Numerical)e=64 deg. (Experiment)e=64d eg. (Numerical)Stiffness (Experiment) (BestFit)Stiffness (Numerical) (BestFit)

Fig. 7 Force vs. static amplitude plot for Site-II

Fig. 8 Weak zone around pile

Table 3 Measured and simulated lateral stiffness

SITE Lateral Stiffness (MN/m)

Field test ABAQUS

Site-I 10.75 10.28

Site-II 56.98 52.85

Fig. 9 Dynamic amplitude vs. frequencyeffect of weak zone


123

boundary zone has a non-zero mass and a smooth variation

into the outer zone by introducing a parabolic variation

function, which may be best fit with use of experimental

data. In the present analysis, DynaN is used to determine

the stiffness of the soil-pile system considering the mass in

the boundary zone.

The top soil layers were assumed to have developed

weak zones due to pile driving. The weak zone effect is

considered by reducing the shear modulus ratio by 0.5 %

and increasing the material damping of the soil layer by

0.5 % (or reduced by 10 % of the original damping) for

every subsequent increase in eccentricities. Weak zone

shear modulus ratio is the ratio of shear modulus in the

disturbed zone around the pile to the shear modulus of soil

present outside the disturbance zone. Weak zone thickness

ratio is the distance of the disturbed zone from the outer

diameter of the pile to the radius of the pile. A zone of 1.25

times the radius was assumed to be weak zone in the top

layers. The soil and pile properties were similar to the non-

linear Finite Element Analysis. The pile is assumed to have

a fixed connection with the pile-cap. The dynamic ampli-

tude versus the frequency plots for Site-I and Site-II are

presented in Fig. 9.

It can be observed from Fig. 9 that the presence of weak

zone shifts the predominant frequency to higher frequency

range by about 5 Hz, but cannot be precisely attributed to

the weak zone because of approximations involved in

Novaks approach. However, the lateral extent of the weak

zone around the pile shaft has negligible effect on the

frequency. The force versus the static amplitude plots is

presented in Fig. 10. It can be observed from Fig. 10 that

the stiffness of the soil-pile systems for both the sites

decreases due to presence of weak zone. The stiffness of

the soil-pile system obtained using improved Novaks

approach is 7 and 20 MN/m for Site-I and II, respectively.

The decrease in the stiffness, due to the presence of the

weak zone in the topsoil layers is significant for the very

stiff soil site (Site-II).

Summary and Conclusions

Based on the lateral dynamic pile loads tests carried out on

full scale single piles at two different sites in India, it is

found that the site conditions, and in particular, the prop-

erties of the top soil layers greatly governs degree of non-

linearity and the dynamic lateral stiffness of the soil-pile

system. It is found that the piles installed in medium-stiff to

stiff clay have higher lateral stiffness compared to the piles

in very soft clays, however the resonant frequency of the

pile-soil system is found to be unaffected by the stiffness of

the soil strata. The finite element analysis is able to predict

the dynamic lateral response of pile for soft as well as stiff

soil sites, thus signifying the efficiency of non-linear FE

models in simulating the different degrees of nonlinearity

of soil and pile separation. The inhomogeneity that presents

in the soil means that field-test results will invariably differ

from, say, the idealised conditions, making it more difficult

to identify shortcomings in the models. Despite this,

Fig. 10 Force vs. static amplitudeeffect of weak zone


123

experimental investigations remain a key aspect of model

validation; a comparison to experimental results is essential

for gauging prediction accuracy. The presence of weak

zone around the pile shaft significantly decreases the

stiffness of the soil-pile system, especially in the case of

stiff-soil sites. The variation in the lateral extent of the

weak zone around the pile shaft has negligible effect on the

stiffness degradation.

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Lateral Dynamic Response and Effect of Weakzone on the Stiffness of Full Scale Single PilesAbstractIntroductionDynamic Testing of PilesSoil and Pile PropertiesSite-I: Chennai, Tamil NaduSite-II: Hazira, Gujarat

Test Setup and ProcedureTest Results

Non-Linear Finite Element AnalysisEffect of Weak Zone on Lateral StiffnessSummary and ConclusionsReferences

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