experilmental lnvestxgation of tapered

EXPERIlMENTAL lNVESTXGATION OF TAPERED PILES

Jin Qi Wei

Faculty of Engineering Science Department of Civil Engineering

Submitted in partial ful fillment of the requirement for the degree of

Master of Engineering Science

Faculty of Graduate Studies The University of Western Ontario

London, Ontario August, 1998

O Jin Qi Wei 1998

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ABSTRACT

Tapered piles, which have greater top cross-sections than bottom ones, have not

ofien been considered as a design option because of the lack of laiowledge about their

behaviour. In this study, the characteristics of tapered piles performance were established

from experimental investigation. A relatively large laboxatory facility for testing model

instrumented steel piles was developed. In this facility, the sample soi1 was contained in

a steel chamber and pressurized to mode1 the laterai stress dong the different "segments"

of the pile. The effects of the pile taper on its response to axial compressive, tensile and

cyclic, and lateral loads were examined. The results of the study confirnied their

efiiciency over piles of uniform section with the same materid input in al1 loading modes

considered in this study. It was concluded that tapered piles represent a more equitable

distribution of pile material in several respects. A procedure was developed to calculate

the shaft resistance of tapered and straight-sided wall piles based on the experimental

observations. The shafi resistance for straight-sided wall piles estabiished 60rn the

experimental results compared well with the theoretical prediaions using the standard

design procedure, hence connmiing the validity of the experimental resdts.

KEYWORDS: Tapered piles, Experimental model testing, Axial response, Uplift

loading, Lateral response, Cyclic loadhg, Load transfer, Modulus of subgrade reaction.

I would like to express my sincere gratitude and appreciation to my Supervisor,

Dr. M. H. El Naggar, for his guidance, encouragement and support during the course of

study to this thesis.

Thanks are due to Mr. Wilbert Logan for his help in the setup of the data

acquisition system for this research and to Mr. Gary Lusk for his assistance as well. Ms.

Trudy Laidlaw designed the soi1 chamber, her help is greatly appreciated.

Sincere thanks are extended to the facdty and staff of the Department of Civil

Engineering and my fellow graduate students for their assistance and companionship.

1 am most gratefùl to my husband and my children for their love, understanding

and patience throughout the work.

This thesis is dedicated to my parents, Wang Lianying and Wei Guangcai.

TABLE OF CONTENTS

Page

CERTIFICATE OF EXAMZNATION

ABSTRACT

ACKNO WLEDGMENTS

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

C W T E R I INTRODUCTION

1 . 1 OVERALL REVIEW

1.2 OBJECTIVES AND SCOPE

2. 1 PILE LOAD CAPACITY 2. 1. 1 Axial Bearing Capacity

2. 1 . 1 . 1 Static analysis method 2. 1. 1.2 Pile load testing method 2. 1 . 1.3 Dynamic analysis method

2.1.2 Upiift Load Capacity 2. 1. 3 Lateral Load Capacity 2. 1.4 Effect of Cyclic Loadiog

2 . 2 RESPONSE ANALYSIS METHODS FOR SINGLE PILES 2.2. 1 Elastic Analysis Method 2. 2. 2 Subgrade Reaction Method 2. 2. 3 Finite Element Analysis Method

2 .3 RELEVANT STUDES ON TAPERED PILES 2.3. 1 Field Testing Resuits 2 .3 .2 Laboratory Testing Observations 2. 3. 3 Theoretical Analysis Results

4 MOTNATIONS

C . T E R 3 TESTING FACILITES AND PREPARATION

INTRODUCTION

TESTING FACILITES 3.2. 1 Testing Piles 3 .2 .2 Strain Gauge Installation 3 .2 . 3 Soil Sample 3 . 2 . 4 Soil Chamber (VLPSC ) 3 . 2 . 5 Soil Pressure Transducers 3 .2 .6 ûther Test Equipment

TESTING PREPARATION

EFFECT OF PILE INSTALLATION METHOD

CHAPTER 4 AXIAL COMPRESSIVE RESPONSE OF TAPERED PILES

INTRODUCTION

TESTING PROCEDURE

TESTING RESULTS 4 .3 . 1 Load-Displacement and Bearing Capacity

4 . 3 . 1. 1 First group of tests 4 .3 . 1.2 Second group of tests

4 . 3 . 2 Load Distribution 4.3.3 Unit Load Transfer 4. 3 . 4 Pile Tip Resistance

DISCUSSION: ARCHING PHENOMENON

SUMMARY

CHAPTER 5 UPLIFT BEHAVIOUR OF TAPERED PILES

5 . 1 INTRODUCTION

5 .2 TESTING PROCEDURE

3 TESTING RESULTS 5.3. 1 Uplift Load - Displacement S. 3.2 Ultimate Uplifi Load

5.3.2. 1 hosesandstatus 5 .3 .2 .2 Relatively medium dense sand status 5.3.2.3 EEect of placement method

5 . 3 . 3 Pile Head Stiffiiess 5. 3.4 Load Distribution 5 . 3 . 5 Shafi Friction 5.3.6 Downward and Uplifi Shaft Friction

5- 4 DISCUSSION: RESIDUAL STRESSES

C W T E R 6 RESPONSE OF TAPERED PILES S W C T E D TO LATERAL LOAD

6.1 INTRODUCTION

6 . 2 TESTING PROCEDURE

6 . 3 TESTING RESULTS 6.3 . 1 Load - Deflection 6 . 3 . 2 Ultimate Lateral Load 6 . 3 . 3 Bending Moment Distribution 6 . 3 . 4 Maximum Bending Moment 6. 3. 5 Soi1 Resistance 6. 3 .6 Pile Deflection 6.3.7 py Curve

6 . 4 PREDICTED ULTIMATE LATERAL LOAD

6 .5 PREDICTED py CURVES 6.5. 1 Analfical Background 6 .5 .2 Observations

vii

Page CHAPIER 7 CYCLIC RESPONSE OF AXIALLY LOADED

TAPERED PILES

7. 1 INTRODUCTION

7.2 TESTING PROCEDLJFE

7 . 3 TESTING RESULTS 7.3. 1 Cyclic Load-Displacement

7.3. 1. 1 Zero confinuig pressure 7.3. 1.2 20 kPa connning pressure 7. 3. 1.3 40 kPaconfiningpressure 7 .3 . 1.4 60 kPa confining pressure

7 . 3 . 2 Pile Head Stiffhess 7. 3. 3 Effect of Cyclic Load Amplitude 7 . 3 . 4 A c c d a t e d Pile Head Movement 7 . 3 . 5 Rate of Pile Movernent

CItQPTER 8 VALIDITY AND APPLICATION OF THE STUDY

8 . 1 INTERPRETATION OF TESTING RESULTS

8 . 2 VALDITY OF MODEL TESTING RESULTS

8 . 3 SHAFT RESISTANCE OF TAPERED PILES 8.3. 1 Relatively Medium Dense Sand Case 8 . 3 . 2 Loose Sand Case

8 . 4 DISCUSSION 8.4. 1 The State of Stress Inside of the Soi1 Charnber 8 . 4 . 2 Boundary Effect

C W T E R 9 CONCLUSIONS AND RECOMMENDATIONS

viii

Table

LIST OF TABLES

Description

Geometries of three piles

Sand properties

Test arrangement

Axial compression resdts (first group of tests)

Axial compression resdts (second group of tests)

Pile tip resistance at Qu (second group of tests)

Uplifi results (fbt group of tests)

Uplifi resuits (second group of tests)

Lateral loading resdts

Variation of K with deflection and relative density (Geosofi)

Amplitude of cyclic load applied at pile head

Pile head stifkess

Page

Figure

LIST OF FIGURES

Description

Geometries of three piles

Strain gauge circuits a: Axial loading test b: Lateral loading test

Strain gauge installation

Grain size distribution

Variable lateral pressure sand column (VLPSC)

Current - pressure relationships for soi1 transducers

Applied confining pressure vs meanwd stress around pile S

Axial loading equipment

Uplift loading equipment

Laterd loading equipment

Oblique view of the testing facility

Load-settlement curves of pile Tl with different confîning pressures ( k t group of tests)

Load-setdement curves of pile T ï with different confining pressures (fust group of tests)

Construction of the offset limit load (after Canadian Foundation Engineering Manual, 1992 )

Load-settlement curves of pile T 1 with different confuiing pressures (second group of tests)

Load-settlement curves of pile S with different confining pressures (second group of tests)

Load distribution dong the pile with different values of load applied at pile heads of Tl and T2 (first group of tests)

Load distribution d m g the pile with different values of load applied at pile heads of Tl and S (second group of tests)

Page

Figure Description

Unit load transfer to the soil when ultimate load was reached at piles Tl and T2 (fint group of tests)

Unit load t d e r to the soil when uitirnate load was reached at piles Tl and S (second group of tests)

Variation of unit load transfer to the soi1 for piles Tl and T2 at different confining pressures applied (fim group of tests)

Variation of unit load transfer to the soil for piles Tl and S at different confinhg pressures applied (second group of tests)

The distribution of the ultimate load between the pile point and the pile shaft for various applied confining pressures

Load- upward movement curves of piles at different confinùig pressure values in fîrst group of tests a: Tl b: T2

Load- upward movement curves of piles at different connnllig pressure values in second group of tests a: Tl b: S

The effect of confining pressure on the uplift pile head stiflhess a: First group of tests b: Second group of tests

5-4 (a) Load distribution along the pile at different load increments applied at pile head of T 1 in first group of tests

5-4 (b) Load distribution along the pile at different load increments applied at pile head of T2 in first group of tests

5-5 (a) Load distribution dong the pile at different load increments applied at pile head of Tl in second group of tests

5-5 (b) Load distribution along the pile at different load increments applied at pile head of S in second group of tests

5-6 Shaft friction for piles Tl and T2 at dBerent connning pressure values in first group of tests

5-7 Shaft friction for piles Tl and S at different confining pressure values in second group of tests

Page

45

48

50

Figure Description

5-8 Variation of shaft fiction at different confining pressure values in f is t group of tests a: Tl b: T2

5-9 Variation of shafi fiction at Merent confining pressure values in second group of tests a: Tl b: S

5- 10 (a) The comparison of shaft fiction at dtimate uplift and compressive capacity in first group of tests, pile Tl

5- 10 (b) The cornparison of shaft fiction at uîtimate uplift and compressive capacity in first group of tests, pile T2

5-1 1 (a) The comparison of shafi fi-iction at dtimate uplift and compressive capacity in second group of tests, pile Tl

5-1 1 (b) The comparison of shaft friction at ultimate uplift and compressive capacity in second group of tests, pile S

Load-displacement curves at the loading point in the push fonvard phase for different piles a: Pile S b: Pile M c: Pile Tl

Lateral load capacity versus confining pressure for three piles

Moment distribution dong pile S

Moment distribution dong pile T2

Moment distribution dong pile Tl

Normalized moment distribution dong pile S

Nomalized moment distribution dong pile T2

Normalized moment distribution almg pile Tl

Variation of maximum bending moment with applied load for three piles

Page

Figure Description

6- 10 Bending moment dong the pile sh& for the three piles subjected to the same load at different connning pressure

6-1 1 Soil resistance dong pile shaft for three piles under a typical load

6-12 Soil resistance along the pile shaft for three piles under ultimate load

6- 13 (a) Pile deflection along the pile shafi under a typical load

6- 13 (b) Pile deflection dong the pile shaft under ultimate load

6-1 4 The effect of pile taper angle on p y c w e s at 0.4 m depth

6- 15 The effect of confining pressure on p-y curves for different piles

6- 1 6 Degradation of modulus of horizontal subgrade reaction with deflection

6- 17 Unrestrained laterdly-loaded pile (after Poulos and Davis, 1980)

6- 1 8 Predicted p-y curves for piles Tl and S in relative dense sand (Geosoft)

7-1 (a) Characteristics of cyclic load at different values of confining pressure ( T 1 , R a n d S )

7-1 (b) Characteristics of cyclic load applied to pile Tl at 20 kPa confiring pressure

7-2(a) Load-movement curves for piles S, Tl and T2 at zero confining pressure

7-2(b) Load-movement c w e s for piles S, Tl and T2 at 20 kPa confuiing pressure

7-2(c) Load-movement curves for piles S, T1 and R at 40 kPa confining pressure

7-2(d) Load-movement curves for piles S, Tl and T2 at 60 kPa confIIiù1g pressure

7-3 Pile head load-movement curves in the first and the tenth cvcles

Page

1 O0

101

102

1 O3

1 O4

105

1 O6

107

108

108

119

119

120

121

122

123

124

Figure Description Page

7-4 Pile head stiffness at different confining pressure for piles Tl and S 126

7-5 (a) Load-pile movement curve of Tl at Merent loading value under 20 kPa confining pressure 127

7-5(b) Cornparison of pile Tl head Stifbess at 20 kPa confining pressure subjected to dflerent load amplitude 127

Accumulation of displacement for piles Tl and S under different confining pressures 128

Pile Tl head movement under cyclic load with different amplitude 129 a: Accumuiation b: Rate of displacement

Average unit load tramfer a: in compression b: in tension 138

(a) Unit tip resistance (b) Measured and prototype vertical stress 138

Cornparison of pile shaft resistance established fiom experiment and theory a: in compression b: in tension 139

Variation of Kt for pile T l with lateral pressure in relative dense sand 139

The shaft resistance of prototype pile established from experimental results with relative medium dense sand (a) in compression (b) in tension

Variation of l& for pile Tl and T2 with lateral pressure in loose sand 141

The compression shaft resistance of prototype piles in loose sand established fiom experimental results

(a) in compression (b) in tension

1.1 OVERALL REVIEW

Pile foundations are used extensively to support both inland and offshore

structures, including important structures such as nuclear power plants and oil-drilling

pladorms. Piles are usually loaded axially in compression to tramfer stnichual loads to

deeper competent soil layers. In some structures, like transmission towers and jetty

structures, pile foundations resist uplifi loads. Piles are also fiequentiy used to support

structures subjected to lateral forces and moments such as offshore structures, harbour

structures high rise bbuildings and bridge abutments.

Piles are generally classified according to the pile material (timber, steel or

concrete), the method of installation (driven, cast-in-place, bored, etc.), or are categorized

in terms of the load transfer mechanism. (a) Friction piles: the load capacity depends

mostly on the amount of fiictionai resistance developed at the interface between pile and

soil. (b) End-bearing piles: the loading capacity relies primarily on the concentrated soil

resiçtance at the pile tip for developing the resistance to axial load.

D i f f m t iypes of piles with different shapes such as circle, square or rectangle

cross sections are used in practice. Piles are mody used with straight-sided wdls. Most

of the design procedures and guidelines have been developed for straight-sided wall piles

with littie or no reference to tapered piles, aIthough tapered piles have the potential for

substantial cost advantages over straight-sided wall piles (Robinsky et al, 1964 and

Rybnikov 1990). Tapered piles are not widely considered as a design option due to the

lack of knowledge about their static and dynamic behaviour and the lack of appropriate

design tools similar to those available for straight-sided wall piles.

1.2 OBJECTIVES AND SCOPE

The objectives of the shidy are to explore the static response of tapered piles

subjected to axial, lateral and cyclic loads, and to provide a procedure for the design of

tapered piles. A research program for studying pile performance in cohesionless soils was

developed with emphasis on the pile shape effect on its capacity and displacement. Both

tapered and straight-sided wdl piles were examined in order to obtain a complete

comparative picture of pile actions.

1.3 ORGANIZATION OF THESIS

This thesis consists of nine chapters:

Chapter 1 presents a general introduction and the objectives of this thesis;

Pile's ultùnate load behaviour, displacement analysis methods and relevant studies of

tapered piles are reviewed in Chapter 2;

Chapter 3 contains a description of the experimental setup, test piles, sand properties

and test preparations;

In Chapter 4, the experimentd procedure and results for the axial compressive load

are presented three piles were tested with six different confining pressures;

Chapter 5 presents the experimental work and results for three piles subjected to axial

temile Ioad;

In Chapter 6, the experimental data and andysis for lateraily loaded piles are

descri bed;

The response of tapered piles and straight-sided wall pile subjected to cyclic load is

descnbed in Chapter 7;

Chapter 8 presents the validity and application of mode1 test results; and

Chapter 9 gives the conclusions and recommendations.

This literature review covers some s~rdies on straight-sided wall piles and tapered

piles. Pile load capacity, the analysis methods of single piles and relevant research works

on tapered pile are reviewed.

2.1 PILE LOAD CAPACITY

2. 1. 1 Axial Bearing Capacity

The traditional study of single piles is directed towards the static load canying

capacity, assinning that the displacement or deformation is acceptable if an appropriate

safety factor is used in determining the allowable loads. There are three methods to

estimate or check the ultimate load capacity of a single pile: static analysis method, pile

loading test method and dynamic analysis method.

2. 1. 1. 1 Static anabsis method

The axial load canying capacity of single pile Qu is determined in practice as the

sum of the shaft resistance, Qs, fiom the pile-soi1 interface and the tip resistance, Qb, less

the weight of the pile, Wp, i. e. Qu is calculated as follows:

Q ~ = Q ~ + ~ ~ - w ~ = J f ~ d z + f b ~ b - w ~ (2 - 1)

where f, = ultimate unit shaft fiction; C = pile perimeter; fb = dtirnate unit base

resistance; Ab = area of pile base; Wp = pile weight. The h c t i o n f, a n d 5 are caicdated

fiom ernpirical correlation with standard geotechnical soi1 properties, such as the

undrained shear strength for cohesive soils and the fiction angle for cohensionless soils,

and the overburden pressure ( Meyerhof, 1 976).

For piles in sand or gravel, effective stress analysis of ultimate load capacity is

appropnate. When the cohesive component of drained strength is ignored, the ultimate

unit shaft fiictionf, and d h a t e unit base resistancefb can be expressed as foilows:

I;= Ks &tan 6 ( 2 - 2 )

fb = Nq ~ b ' ( 2 - 3 )

where &= vertical effective stress adjacent to the pile and a,b'= vertical effective stress

at the pile base. This approach will be m e r described in Chapter 8.

2. 1. 1.2 PiIe load testing method

Pile load testing is usually camied out to quanti@ pile load-settiement behaviour

and to detemine the uitimate bearing capacity as a check on the value obtained fiom

theoretical calculations. A number of empirical d e s have been used to serve as criteria

for detemiinhg ultimate load f?om the load test. Examples are the Davission criterion, the

Brinch-Hansen criterion and the Chin criterion (Canadian Foundation Engineering

Manual, 1992). The Davission cntenon defines the pile offset limit load as the load that

produces a movement, rn, of the pile head equal to:

where: m = the movement of the pile head at the offset limit load, mm, A = elastic

shortenhg of the pile, mm, b = diameter of the pile at the tip, m. The other two criteria

were described in Canadian Foundation Engineering Manual (1 992).

Many research achievements have also been made towards the understanding of

characteristics of compressive piles as repoxted by Nordlund (1963), Coyle and Reese

(1 966), Bane j e e and Davies (1 978), and O'Neill et. al. (1 982).

2. 1. 1 .3 Dynamic analysis method

The capacity of a single pile can be estimated by means of dynamic methods. The

objective with the dynamic methods of pile test is to relate the dynamic pile behaviour

(acceleration or driving resistance) to the ultimate static pile resistance. Pile axial capacity

can be obtauied based on dynamic monitoring, wave-equation analysis or dynamic

formula. However, dynamic anaiysis rnethods are better used to provide general guidance

due to its high dependence on competent person, local expenence and relevant

assumptions. For more detailed information about the dynamic analytic rnethods, see

Poulos and Davis (1980).

2. 1 .2 Uplift Load Capacity

There were considerable debates over the relative magnitude of pile shaft capacity

in tensile (uplift) loading as compared with compressive loading case. Generally, it is

assumed that the pile shaft capacity is identical under both tensile and compressive

loading case. However, there is widespread experimental and numerical evidence that in

sand, the straight-sided shaft capacity is significantly lower for tensile loading than for

compressive loading (e.g. Chattopadhyay and Pise, 1986 , Nicola and Randolph, 1993).

Pile uplift load testing is u s d y carried out to obtain pile uplifi resistance. In

practice, the interpretation methods used for estimating ultimate pullout load fiom pile

puilout load- pile upward movement ciwe are: (1) failure load may be taken as the Ioad

value that produces a net upward pile top movement of 6.25 mm; (2) upward failure load

occun at the intersection point of tangents on the load-movement curve; or (3) upward

failure load is the value at which upward movement suddenly increases.

2.1.3 Lateral Load Capacity

Broms (1964% b) provided solutions for the ultimate lateral resistance of a pile

assuming a distribution of the lateral pile-soi1 pressure and considering the statics of the

problem. He considered two modes of failure: yielding of the soi1 dong the length of the

pile (short-pile failure) and yielding of the pile itself at the point of maximum moment

(long-pile failure). Meyerhof (1995) accounted for the ef6ects of load eccentricity and

inclination on the ultimate lateral capacity.

Pile lateral load testing is performed to assess the load-deflection behaviour of a

pile. Methods for detemwiing failure load fiom lateral load testing Vary depending on the

tolerable movement of the structure supported by the pile. The general critena are: (1)

ultimate load can be taken al 6.25 mm Meral movement or deflection; or (2) ultirnate

load can be considered at the point of intersection of tangents on the load-movement

curve.

2.1.4 Effect of Cyclic Loading

The performance of the foundation piles under cyclic loading is an important

factor in the design of piles for offshore structures, transmission towen, and some ta11

buildings. Two major types of cyclic loading are considered in the design of piles. The

first type is dynamic or random loading whereby the dynarnic component is significant

compared to the rea of the forces, such as earthquake load. The second type is non-

dynamic or approximately systematic loading whereby a steady but slow variation of the

load is applied, as shown in wind and wave loads or tidai effects on piles resisting uplifi

forces. The cyclic loading has three possible effects on pile performance:

1. Accumulation of permanent displacement;

2. " Shake down" phenornenon reported by Poulos (1982), the pile defiection stabilizes

and react elastically to any further load cycle;

3. A possible reduction or " degradation" of pile resistance, especially shaft resistance;

It is acknowledged that the pile response to cyclic loaduig depends on the

characteristics of the pile-soi1 system. Chan and Hama (1980) investigated the effects of

the load amplitude, the type and the number of load cycles on a pile's response and

concluded that the response of the pile to cyclic load is cornplex. Podos (1 988,1989)

used soil degradation factors, which are defined as the ratios of soil properties after cyclic

loading to properties for static loading, to calculate the reduction of pile load capacity due

to cyclic axial loading, and showed their significance.

2.2 RESPONSE ANALYSIS METHODS FOR SINGLE PILES

Several approaches have been developed for the response analysis of axidly and

laterally loaded piles. These approaches assume either the theory of elasticity or the

theory of subgrade reaction. The former includes Poulos and Davis (19681, Mattes and

Podos (1969), Poulos (1971), Randolph (1981), Pise (1984) and Budhu and Davies

(1988), and the latter includes Coyle and Reese (1966), Kraft Jr. et al (1 98 1) and O'Neil

et al (1982). The load-settlement or loaddeflection behaviour of axially or laterally

loaded piles is highly nonlinear and hence requires a nonlinear analysis. Poulos and Davis

(1980) and Budhu and Davies (1987) modified the elastic solutions to account for

nonlinearity using yield factors. The modulus of subgrade reaction approach was

extended to account for the soil noniinearity. This was done by introducing p y curves

(Matlock, 1970; Reese et al., 1974; Reese and Welsh, 1975; Prakash, et al 1996).

2.2.1 Elastic Analysis Method

The elastic d y s i s of pile load-displacement behaviour under static axial or

lateral loading was based on the elastic theory for both the pile and surrounding soil. The

soil is considered as an elastic continuum, the pile is assumed to be a thin strip and

divided into nurnber of elements. Factors such as soi1 yield, soil layer depth, soil

inhomogeneity, stifk bearing stratum, and enlarged pile base were considered, the

dificulty of the application of the elastic method to practical problems is detemiining the

appropriate soil modulus.

2.2.2 Subgrade Reaction Method

The subgrade reaction method or load-tramfer method, correspondhg to lateral or

axial loaded pile, is based on the curves of soil resistance us. pile displacement. These

curves are commonly texmed as t- z, q-z curves, denoting load transfer vs setdement

curves dong the pile or at pile tip, respectively; and p-y curves, denoting lateral resistance

vs deflection. An apparent advantage of this method is that it can incorporate inelastic soil

behavior by using nonlinear curves while not complicating the analysis. The weakness of

this model is that the continuous nature of the soil medium is ignored and the pile

reaction at a point is simplly related to the deflection at that point.

The corresponding experimental curves have been develo ped for clay (Co y le and

Reese 1966, O'Neil et al 1982, Brown et al 1987) and sand (Brown et ai 1988). In

addition , an attempt was made to derive theoretical t- z curves (Kraft Jr. et ai 198 l), in

which the theoretical analysis provided a bais for t-z criterion that couid be applicable to

a variety of pile and soil conditions. Rakash et al. (1996) developed a method to predict

the load-deflection relationship @-y curve) for single piles embedded in sand, cons ide~g

soil nonlinearity using subgrade reaction, based on the analysis of 14 Ml-scale lateral pile

Ioad tests results.

2.2.3 Finite Element Analysis Method

The finite element method could be used for the response analysis of single piles.

It gives a better insight in the pile foundation behaviour, provided that adequate modeling

of the soil and the pile- soil interface takes place. The finite element method was used by

Ellison et al (1 97 l), Desai (1 974), Kuhlemeyer (1 979) and Randolph (1 98 1 ). Ellison et al

(1971) developed a general procedure to collocate the behavior of elements with the

cornplex foundation-soi1 system., where the h i t e elemect model was used to predict the

load capacity and load-deformation of a bored pile in stiff clay. Desai (1974) applied the

finite element method to predict load-deformations and stress distributions for

compressive steel pipe piles in sandy soils. Kuhlemeyer (1979) analyzed laterally loaded

piles and highhghted the dynamics solution. Randolph (1 98 1) examined pile static lateral

load behaviour and eeated the soil as an elastic continuum with a linearly varying soil

modulus.

2.3 RELEVANT STUDIES ON TAPERED PILES

A number of studies have been directed toward the response of individual

straight- sided wall piles with littie attention being paid to tapered piles. The vast rnajority

of the research on tapered piles focuses on the load-carrying capacity.

The previous studies of tapered piie subjected to axial compressive load include

full-scale field testing, laboratory testing and analytical procedures. Field testing results

are reported in Norlund (1 963), Appolonia and Haribar (1963) and Rybnikov (1990).

Laboratory testing observations include those conducted by Robinsky et al. (1964) and

Bakholdin (1971). Ladanyi and Guichaoua (1985) and Kodikara and Moore (1993)

suggested analytical solutions for tapered pile response; Poulos and Sim (1990)

conducted a theoretical anaiysis with five different pile types to assess their cyclic load

capacity.

2.3. 1 Field Testing Results

Norlund (1963) described a pile test program and a method for computing the

ultimate axial resistance of a pile in cohesioniess soils was developed. The test data

demonstrated the signifiant effect of pile taper, the roughness and the shape of the pile

sdace , and the volume of soil displaced by the piie on the pile bearing capacity.

Rybnikov (1 990) shidied the bearing capacity of bored-cast-in-place tapered piles through

a field experimental investigation. He suggested that the tapered piles had a specific

bearing capacity that exceeded the specific bearing capacity of straight-sided wall pile

having the same length by 20.30%.

2.3.2 Laboratory Testing Observations

Robinsky et al. (1964) investigated the effect of the shape and volume of piles

installed in sand on their capacity. In this study, instnunented model straight-sided wall

and tapered piles were driven into sand at different embedment depth to diameter ratios.

These tests revealed that the intensity of unit load transfer through the pile walls changed

continuously as the piles were advanced. Furthemore, tapered piles were found to be

appreciably more efficient than straight-sided wail piles. Robinsky and Momion (1964)

studied the effect of pile taper on the displacement and compaction of cohesiodess soi1

adjacent to fiction tapered piles. It was found that in relatively homogeneous

cohesionless soils, a tapered pile with most of the load being carried by skin fiiction can

support considerably greater loads than a sûaight-sided wall pile with a larger point.

2.3.3 Theoretical Analysis Results

Ladanyi and Guichaoua (1985) compared the response of tapered piles, straight-

sided wall piles and comgated piles in permafkost soils. They showed that tapered piles

were the safea because they display strain hardening characteristics as opposed to the

brittle failure that occurred in other types of piles. They also developed a model for the

analysis of tapered piles wherein the soil resistance was modeled by two components; the

first component was the fiction and adhesion dong the shaA (shearing resistance) and the

second component was due to the lateral soil reaction mobilized by the hole expansion

resdting fiom the pile penetrating the ground. A simila. mode1 was presented by

Kodikara and Moore (1993) for the analysis of the axial response of tapered piles, it

accounted for the nonlinearity conditions dong the pile-soi1 interface. Podos and S im

(1 990) conducted a theoreticai analysis with five different pile types to assess their cyclic

Ioad capacity. They concluded that the pile taper could have a favorable effect on its

cyclic performance as it reduced stress concentration.

2.4 MOTIVATIONS

In tapered piles, which have the great top cross section than the bonom one, an

increase in the side resistance can be expected when there is a slip of the pile relative to

the ground. This occurs because the ground adjacent to the pile is then forced to expand

radially, so that additional lateral pressures are developed and lead to an increase in the

shear stresses across the pile-soi1 interface. The flexural effects of deflection and bending

moment of a pile subjected to a lateml load at the top are also highest at the top and

decrease rapidly with depth. Hence, it is also expected that tapered piles represent a more

efficient distribution of the pile matenal in this loading mode as well.

In order to obtain a complete comparative picture of pile action, it is essential to

midy both tapered and straight-sided wall pile. A program was initiated to study fiiction

piles in cohensioniess soils, with particular emphasis on the effect of shape on pile

capacity. Four phases are included in this program, the fia phase examined the pushing

down bearing capacity of the tapered piles (Wei and EL Naggar, 1998); the second phase

dealt with the effect of the pile taper on tensile loading; the thKd phase examined the

response of test piles to the lateral loading and the forth phase explored pile behavior

under uniforni axial cyclic loading.

CHAPTER 3

TESTING FACILITIES AND PREPARATION

3.1 INTRODUCTION

The loading tests were performed on mode1 piles installed in dry sand in a

laboratory setup. The sand was enclosed in a steel chamber that ailowed the application of

variable confining pressure to the sand. The description of the experimentd setup,

hcluding the test piles, instrumentation, soil sample, soil chamber, loading equipment,

and testing preparation are given below.

3.2 TESTING FACILITIES

3.2.1 Testing Mes

Three inmumented structural steel piles of equal length and average embedded

diarneter but different taper angles were used in this mdy. Two piles were tapered wirh

different taper angles and the third was a straight-sided wall pile. Tapered pile number 1 ,

Tl, had a taper angle = 0 . 9 5 ~ ~ while tapered pile number 2, Tî, had a taper angle = 0.6'.

The piles were 1.52 m in length with diameters varying between 160 and 200 mm and a

wall thickness of approximately 6.4 mm. The length to diameter ratio for these piles was

approximately 9, typical of rigid piles. The geometrical properties of piles are given in

Table 3-1, and are shown schematically in Figure 3-1. The piles were fitted with slip - on

flanges at their heads to facilitate loading.

3.2.2 Strain Gauge Installation

The piles were instrwnented with electrical resistance saain gauges (CEA-06-

XOUW- 120). Full and half bridge electrical resistance strain gauge circuits were used in

axial and lateral loading tests, respectively, as shown in Figure 3-2.

Six pairs of sirah gauges were attached to the exterior walls of the piles using M -

Bond 200 adhesive. They were distributed over the length of the pile such that the first

strain gauge was 0.3m from the pile head (approximately at the surface of the sand) and

the last main gauge was close to the pile tip , 0.05 m fiom the pile tip. The rest of the

main gauges were spaced as shown in Figure 3-3.

3.2.3 Soi1 Sample

The soi1 w d in the tests consisted of coarse, anguiar particies of air dned sand.

The p i n size distribution for the sand is shown in Figure 3-4.

A standard test of the sand showed it had a maximum unit weight of 18.35 kN/m3

and minimum unit weight of 15.83 w / m 3 at a moisture content of 0.25 per cent. Two sets

of tests were perfomed on piles in this study for axial static load test. In the fint set, the

sand was loose with relative density, Dr = 18.4%. In the second set, the sand was

compacted by applying a 100 kPa confining pressure and then releasing the pressure to

zero before starting the testing procedure. The initial relative density in this case was

calculated as Dr = 32.7%. Grain size analysis and other data relating to the sand are given

in Table 3-2. The relationship between angle of fiiction and relative density of sand are

adopted fiom Das (1995).

3.2.4 Soil Chamber (VLPSC)

To mode1 the effixt of varying confining pressure on the response of piles

installed in cohesioniess soil a device termed the Variable Laterai Pressure Sand Column

or VLPSC was used and is depicted in Figure 3-5. This device was designed and used by

Moore et al (1995).

The sand was contained in a steel chamber, VLPSC, 1 Sm in intenor diameter,

with 10 mm thick walls, and 1.445m in depth. The top and bottom of the steel chamber

were covered by ngid steel plates. The top plate has a 397mm access hole for the test pile

and svain gauges leads to the data acquisition system. The edge of the access hole is

stiffened by a steel r i . fitted with an arrangement to facilitate the attachent of the

loading b e . The loading fiame (reaction fiame) was made of steel channel sections

with a fitting to fasten the hydraulic jack used for both axial and laterai loading.

The steel charnber was Iined with an air bladder so that sand inside the steel

chamber could be pressurized. The air bladder was used to vary the confining pressure to

simulate the typical embedded depth of piles. A manifold was applied to control the air

pressure through lines comected to the air bladder. The pressure variation was fiom O to

100 kPa.

3.2.5 Soil Pressure Transducer

Three soi1 pressure transducers were used in this study. Typical Current-Pressure

relationships of soil pressure tmnsducers are showed in Figure 3-6. The variation of the

stresses around the test piles were obtained before loading tests. Two soil transducers, Pl

and P2, were placed 150 mm fiom the pile tip; one to measure the vertical stress at the

pile tip elevation and the other to masure the lateml stress at the level of 200 mm above

the pile tip. A third soi1 transducer, P3, was installed in the sand 200 mm under the sand

surface, 150 mm fkom the pile to measure the lateral stress ~f the sand. The relationship

between applied confining pressures and measured stresses a r o d the tested piles are

illustrated in Figure 3-7.

3.2.6 Other Testing Equipment

Other testing equipment used in this study are listed as follow:

Blackhawk Holoram Hydraulic Jack, 178 kN (20 TON) capacity, 50.8 mm (2 in.) stroke

Simplex R106 Hydraulic jack ,89 kN (10 TON ) capacity, 127 mm (Sin.) stroke

Simplex Hydraulic Hand pump, RP6A, 0-68.95 MPa (10,000 Psi ) capacity

S train,ce~e

FL25U- 2SGKT 1 1 1.25 kN (25,0001b) loadcell

Donc Digital Readout Mode1 420

Data Acquisition System includes

Compter @oric 245A, &ta logger)

Strain gauge conditioner (UC- 19)

Beckmann "logger" software

3.3 TESTING PREPARATION

More than seventy testing configurations were considered on pile testing to

establish the behaviour of tapered piles in cohesionless soil, as indicated in Table 3-3.

Two sets of tests were performed in this study for investigating the behaviour of tested

piles under axial loading. In the first set, the sand was w d in a loose state, while in the

second set, the sand was relatively medium dense due to the application of a 100 kPa

c o n f i g pressure before testing started. Lateral and axial cyclic loading tests were

conducted with loose sand. The same preparation procedure described below was

followed for al1 performed tests.

The sand was spread in patches in the lab and air dried, it was placed into the

VLPSC to a depth of about 400 mm using a min technique. The pile was then placed at

the center of the chamber guided by a m e to assure centric vertical alignrnent. The pile

was slightly embedded in the sand such that the total embedrnent depth of the pile would

be 1.2m afier filling the soil chamber, and the first strain gauge was approximately at the

suface of the soil. As mentioned forgoing two pressure transducers were placed 150 mm

fiom the pile tip. Afier securing the pile and pressure transducers in place, more sand was

added around the pile until the chamber was filled to capacity. A third pressure

transducer was installed in the sand 200 mm under the sand surface, 150 mm fkom the

pile. The surface of the sand was leveled and the top cover plate was placed and screwed

to the chamber.

The reaction frame was placed across the access hole at the top plate and tightly

screwed to the rim of the access hole. A reference beam was then attached to the edge of

the chamber. Dia1 gauges were installed on the reference beam (separated fiom the

loading system) to meamre the pile head settlement or deflections. A hydraulic jack and

a load ce11 were placed between the pile head and the reaction fiame and adjusted to

ensure centric loading. A data acquisition systern was connected to the sûain gauges and

the load ce11 to read and record the strains and load appiied simultaneously durulg testing

every ten seconds. The loading equipment for axial downward, axial upward and laterai

loading are shown in Figures 3-8 to 3- 10, respectively, the axial downward and upward

loading equipment were used to conduct the axial cyclic loading tests. A typical oblique

view of the testing facility is shown in Figure 3-1 1.

The testing procedure descnbed and the pile load testing were repeated three

times for each testing configuration. The difference between the pile ultimate load in the

three sets was Iess than 10%. The results reported herei. represent one set of results that

was the closest to the average of the three sets.

3.4 EFFECT OF PILE INSTALLATION METHOD

The pile placement method may have important influence on pile performance.

When a pile is driven into sana the mil is usuaily cornpacted by displacement and

vibration. In loose sand, the load capacity of a pile is increased as a result of the increase

in relative density caused by the driving. Detailed investigations of extent of compaction

of sand and the increase in relative density around the pile have been carried out by

Robinsky and Monison (1 964).

Aiizadeh and Davisson (1 970) described a pile load testing program conducted to

determine the lateral Ioad-deflection behavior for individual vertical and batter piles, the

flexurd behavior of piles and the effect of sand density on the pile response. The results

showed the significant effect of the relative density of sand on the pile's behaviour.

To ensure that the placement method has no influence on the relative performance

of the piles, the same instailation procedure was w d for aii piles. In this procedure, as

was described, the pile was placed in the center of soi1 chamber and then the sand was

poured into i t Therefore, no densification was due to the placement method because no

soi1 displacement occurred. This installation method is more like a "bored pile" case and

hence, the results obtained do not represent the case of driven piles. However, it is

expected that the effect of the taper will be more pronounced in dnven piles. This is

because the wedging effect of tapered piles would result in more densification of the sand

narounding the pile that in tum would lead to a better performance under axial and

lateral loads as shown Iater.

Table 3-1 Geometries of Three Piles

Table 3-2 Sand Properties

'pi le Diameter (outside. ml IThickness(mrn) 'Length(rn) S~night sided wall pile. S Tapemi pile. T2 Tapend pile, Tl

Ettécrive diameter

Utirl'om~r~ coetticicnt

ASThn* Sieve Desipotion No. IO No. 20 No. 40 No. 60 No. 100

Mas~murn densi' Initial relative densin

Percentage f i bv weight 84.17% 45.96% 13.53% 4.66% 1.85%

D,o=0.35mrn

C,=2.85 26"-30"

1.615kg/rn3 28"-35"

l.873k~@? 18.4% (first group of tests) & 32.7% (second group of tests)

1.524 1.524 1.524

0.1683 0. & 968~Q0.165 1

_0.2032&0.1524

13 2"( tint poup or tests) &3S0(second p u p of tests) ASTM* 1993

7.1 12 6.35 6.35

Table 3-3 Test Arrangement

Apyilicd contininp pressiirr

Coinpression Lcwsr ssind Relritivrly rnaiium dense sanù

~rnsion Lwse sand Rrlritivelv medium dense sond

h e m 1 Lmse sand

C'!.clic Loc~sc smd T1.TZ.S TI.T2.S TI.T2,SI T1.T2.S I

O kPa

TI. ft

20kPn

T 1 . n

4OWa

T1.n

60kPa I 8 0 U a

TI.T2 S. Tl

T 1 . n S. TI

T 1 . n . S

100L;Pa

S. TI

TI,T2 S. TI

T1.R.S

S. Ti

1

S. Tl

T1.Tz.S

S. Tl

Tl. T2 S. Tl

TI.T2,S

S. Tl

T 1 . n S. TI

T l .

S. TI

S. Tl

T1.Tz.S

Dummy / \ Active / Signai 1

A c t C ive T

Act ive

Figure 3-1

T Act ive

Strain griuge circuits a: mial loading test b: Lateral loading test

Latera

L

I iood-

Strain lndicator

F e 3 - Strain gauge installation

Grain size in miIlimeters

Grain size distribution

O -! 1

0 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5

Pressure. psi

Figure 3-6 Typicai current-pressure relationship for soi1 transducer (1 psi = 6.895 kPa)

Applied conlining pressure (kPa)

Figure 3-7 Applied confining pressure vs. measured stress around pile S

To hydraulic

Figure 3-8

Figure 3-9

I /Reaction frame J

Load cell

Hydroulic jack

Bearing plate

Axial loading equipment

Uplifl loading equipmtnt

I ' VLPSC . .

I I 1 . .

I I I I I I 1 I I I

holdeî

beam

Figure 3-10 Lateral loading equipment

Figure 3-1 1 Oblique view of the testing facility

CHAPTER 4

AXIAL COMPRESSIVE RESPONSE OF TAPEIRED PILES

4.1 INTRODUCTION

Piles have been used to bansfer structural loads to deeper competent soil layers,

allowing construction in areas where the soil conditions near the ground surface are

unfavorable.

To investigate the effect of the pile taper on the pile axial response, two sets of

compressive loading tests, as defhed in Chapter 3, were performed in this study. The test

procedure and observations are discussed in this chapter, including failure load

detemination, load distribution dong the pile, unit load tramfer and the distribution of

pile shafl and tip resiçtance, etc.

4.2 TESTING PROCEDURE

At the beginning of pile load testing, al1 the instruments were reset to zero. The

fist axial loading was performed with zero confining presstire fiom the air bladder. The

pile was loaded in 20 increments. each about 10% of the expected pile capacity. Each

load increment was maintained for 2.5 minutes. The measurements of load and strains

were recorded every 10 seconds through the data acquisition system. The dial gauge

readings were taken at the middle and the end of time interval for each loading increment.

After the axial loading (downward) was completed, the uplift test was conducted.

Uplifi capacity was less than the compression bearing capacity for the same pile under the

same confining pressure and was attained at a smaller displacement. Hence, the pile had

to & pdled up to the initial embedded depth (1.2m) &er the uplift test. This was done to

ensure that the embedment depth was the same for ail loading tests. Also, any residual

stresses due to previous loading may dissipate due to that process. The pressure was then

increased by an additional 20 kPa, and the process repeated until the pile had been tested

at dl desired values of confining pressure.

in each group of tests, the confinùig pressure was varied fkom O kPa to 100 kPa.

However, before ninning the second group of tests, a connning pressure of 100 kPa was

applied to the sand for two hours and then released to O kPa. The top cover plate was

then removed and the drop of the sand sudace was measured and was found to be 30 mm.

More sand was added to f iU the chamber up again and the top cover plate was placed

back on the steel chamber. Therefore, the sand in the second set of tests could be

wnsidered medium dense. The same test procedure used in the first set was followed in

the second set. The axial load tests in the fkst group of tests were successfully completed

with 0-60 kPa confinhg pressure only. The testing at higher confining pressure was

intemipted due to a problem with the air bladder.

4.3 TESTLNG RESULTS

4.3.1 Load-Displacement and Bearing Capacity

4.3.1.1 First group of tests

The load applied at the pile head and the displacement of the pile head were

measured during the first set of tests and plotted in Figures 4-1 and 4-2. Figure 4-1 shows

the load-displacement curves at different values of confining pressure for tapered pile,

Tl, while Figure 4-2 shows the load-displacement curves for tapered pile, R. It can be

noted fiom Figure 4-1 and 4-2 th, as expeaed, the effect of the confining pressure was

to increase the pile capacity for both piles. Also, it may be noted that the axial stiffiess

(load divided by displacement) of pile Tl, with greater taper, at any loading increment

was greater than the axial sfiffness of pile T2 at the same level of loading for d l values of

confining pressure.

The ultimate load for each pile was detemiined fiom the load displacement c w e s

using Davisson cntenon (Canadian Fomdation Engineering Manual, 1992), illustrated in

Figure 4-3. The results are compared in Table 4-1 based on the pile bearing capacity ratio,

KQ, and the coefficient of effective utilization of the pile material, KV. The ratio KQ is

defined as the ratio of the bearing capacity of the two investigated piles, while the ratio

KV is defmed as the ratio of the specific bearing capacity (pile capacity per unit volume)

of the two piles. It cm be observed fiom Table 4-1 that pile Tl displayed higher axial

capacity for all values of confinhg pressure. This increase was manifested in terms of KQ

and KV values higher than 1. These values implied that the axial capacity of pile Tl is 17

to 27% higher than the axial capacity of pile T2 for the given values of confining

pressure, with the highea increase at a confïning pressure equal to 20 kPa. The same

trend was observed in the coefficient KV, which implied that the taper of the pile

increased the efficiency of the utilization of the pile material. These results are consistent

with the results obtained by Rybnikov (1990), who found that the specific bearing

capacity of a tapered pile was higher by a factor of approximately 1.3 compared with a

straight-sided wall pile.

4.3. 1.2 Second group of tests

Figures 4-4 and 4-5 present the load and displacement measurements at the pile

heads at different values of confining pressure during the second set of tests. Figure 4-4

presents the load-displacement curves for tapered pile, Tl, while Figure 4-5 shows the

load-displacement curves for dght -s ided wall pile, S. It can be noted fiom the figures

that the confining pressure was to increase the pile head sti&ess, for both piles.

However, this effect was less significant in this set of tests (dense sand) than it was in the

first set of tests (loose sand), especially at higher values of confining pressure (80 and 100

kPa). This observation could be used as an argument to support the arching effect for

piles installed in cohesionless soil. Further, It may be noticed that it was even less

significant for the pile Tl, which draws part of its capacity fiom the lateral resistance of

the soil. The comprison of the response of the tapered pile, Tl, and the straight-sided

wall pile, S, shows that Tl displayed a stiffer response at dl loading increments for al1

values of confinhg pressure.

In Table 4-2, the axial capacity of Tl and S were compared based on KQ and KV.

It c m be seen from the cornparison that the axial capacity of the tapered pile was higher

than the axial capacity of the straight-sided wall pile for al1 values of confining pressure.

The ratio KQ varied between 1.05 to 1.37, with the maximum value occurring at a

confining pressure of 40 kPa. The ratio KV varied between 1 .O9 to 1.42, with the

maximum value occurring at a confining pressure of 40 kPa. Both KQ and KV increased

as the confining pressure increased until a confining pressure of 40 kPa was reached. For

higher values of confining pressure, both KQ and KV decreased as the confining pressure

increased.

4 3 . 2 Load Distribution

The forces transmitted at different locations were caiculated fiom strain gauge

readings, i.e.

4, =E, E A , (4- 1)

where q, is the pile axial load at the location of strain gauge i, E is the strain measurement

of snain gauge i, E is the elastic modulus of the pile matenal and A, is the pile cross-

sectional area at the location of sûah gauge i.

Figures 4-6 and 4-7 show the load distribution dong the piles under various load

increments for the first and second sets of tests, respectively. It may be observed fiom the

two figures that the general trend of the load distribution was the same for al1 three piles

at al1 loading increments. It can also be noticed that most of the load was transferred to

the soii through the pile shaft with a small contribution fiom the pile tip. A closer look at

these figures showed that tapered pile Tl transferred more load to the soi1 along its upper

portion than did both tapered pile T2 and straight-sided wall pile S.

4.3.3 Unit Load Transfer

The unit load transfer of the pile shaft was calculated from the main

measurements during the pile test. When the ultirnate load of the pile was reached, the

readings of the strain gauges were recorded and used to calculate the load distribution

along the pile. The difference between the force calculated fiom any two sets of strain

gauges dong the pile wall represented the total load transferred to the surrounding soi1

between the two points. Dividing this ciifference by the correspondhg surface area, the

average of unit load tnuisfer was obtained

wheref;, is the average unit load transfer between stations i and j and Sv is the surface area

of the pile between stations i and j. The unit load transfer c w e was obtained fkom the

values of unit load transfer dong the pile.

Figure 4-8 shows the comparison of the unit load transfer for piles T l and T2 in

the f is t set of tests, while Figure 4-9 shows the comparison between Tl and S in the

second set of tests. These two figures iliustrate clearly that, as expected, the unit load

tramfer increased as the confining pressure increased, for dl piles. However. for higher

confinhg pressure (Le. greater than 60 kPa) the unit load tramfer for tapered pile Tl

leveled off at a maximum of 40 kPa. These results were in good agreement with the

results obtained by Robinsky e t al. (1964). It may be noted fiom the wo figures that the

effect of the taper was to increase the unit load transfer especially for the topmon part of

the pile and the lower confining pressure range. Furthemore, comparing the unit Ioad

transfer for the tapered pile Tl in the two sets of tests (Figures 4-8 and 4-9), it could be

observed that the initial sand density has a significant effect on the unit load transfer in

the lower confining pressure. This effect was less significant, however, in the higher

confiking pressure range.

Figure 4-1 0 shows die unit load transfer curves for

different values of confining pressure, derived fkom the £ïrst

tapered piles T l and T2

set of tests, while Figure

11 shows the unit Ioad transfer for piles Tl and S, derived fiom the second set of tests.

These two figures confirrn that the intensity of the load transfer increased with an increase

in the confining pressure until a maximum value of unit load transfer was reached. The

increase in the confining pressure beyond this point has no effect on the load tramfer

through the pile shaft. This observation suggests there was a limiting fiction value that

depends on the of shearing resistance of sand and the pile Wction angle. Similar

conclusions were made by Yoshimi and Kishida (1 98 1) in their experimental study on the

characteristics of the fiiction between sand and metal surface.

4.3.4 Pile Tip Resistance

Table 4-3 compares the distribution of the ultimate load between the pile point

and the pile shaft for various applied confining pressures plotted in Figure 4-12. It can be

observed fiom the figure that the contribution of the pile tip was higher for the maight-

sided wall pile than for the tapered pile. For zero confiring pressure, the tapered pile T1

derived approximately 58% of its support from the pile tip while the straight-sided wall

pile denved about 66%. As the confining pressure value increased, the pile tip

contribution decreased and the shaft contribution increased. This is in a good agreement

with Abendroth and Greimann (1 WO), where the end-bearing capacity of the mode1

fiction piles was as large as 70% and 49% of the total vertical capacity for the piles

ernbedded in loose and compacted sand, respectively.

4.4 DISCUSSION: ARCHING PHENOMENON

The effect of pile taper can be explained by the arching phenomenon. The pile

compression test was accompanied by some sand loosening dong the pile walls, which

was believed to cause a complex systern of arching in the soil s w r o ~ d h g the piles

(Robinslq, 1964). The pile shape and the initial soil density detemüned the systern of

arching and the efficiency and capacity of the piles. Tests with the tapered pile permitted

the development of wide arches, thus transferring the pile load to a greater volume of

sand than in the tests with the straight-sided wall pile. With the kght -s ided wdl pile,

the load had to be carried by a smaller volume of sand. The failure stresses in the sand

surrounding a &&t-sided wall pile were tt.s reached at a lower total load than in the

case of the tapered pile.

At low confinhg pressure values, the axial capacity of both piles increased

significantly with the increase of the appiied confining pressure. At hi& confining

pressure values, this effect is much less significant. The experimentd investigation by

Sirnonini (1996), on the pile behaviour offered a possible explanation for this trend.

When a pile is ioaded, the effective stresses inside the soi1 mass range fiom low values,

co~responding to the initial overburden stress, to very high ones; at high confining

pressures, dilatancy vanishes and crushing becomes the only mechanism of deformation

in addition to simple slip. These combined effects lead to a decrease of the sheax-ing

strength of sand. This explariation was supported by the fact that the cnishing sound was

heard during the experimental work as the dtimate load was reached, at confining

pressure values greater than 60 kPa At one stage during the pile loading at a 100 kPa

confinhg pressure, two soil pressure transducers with a capacity of 700 kPa were failed.

That meant the stress in the soil exceeded the tramducers capacity, which resulted in the

sand crashing.

4.5 SUMhlARY

An experimental investigation of the axial response of tbree steel piles with

different taper angle installed in sand was presented and discussed in this chapter. The

analysis of the results showed consistent agreement with the results obtained by other

researchers. Based on the results presented and the cornparisons between the response of

tapered piles and straight-sided wall pile, the following conclusions were made:

1. The pile axial capacity increased with an increase in confinulg pressure for d l piles

examined in this study.

2. The resdts indicated a beneficial effect of the pile taper in ternis of an increase in the

axial capacity and niffness.

3. The analysis of ihe test results indicates that there was a limiting fiction value that

depends on the angle of shearing resistance of sand and the pile friction angle.

4. As the effect of the taper was much less significant for higher confixing pressure values

(Le. greater embedment depth), it may be recomrnended that the taper be confied to the

topmost part of longer piles.

5. The pile tip contribution 10 the total pile capacity was less for tapered piles than for

straight-sided wall pile.

Tcrbte 4-1 Axial Compression Results ( first group of tests)

Applied confining pressure O kPa 20 kPo 40 kPa , 60 kPa t3canng Cnpacity h QT2 1.70 7.10 1 1.50 14.50

2 .OQ 9 .O0 17.00

Table 4-2 Axial Compression Results ( second group of tests)

Table 4-3 Pile Tip Resistance at Qu ( second group of tests)

Applicd Confininp Pressure Rearing Cepacity

Qu (W QS QT 1

KQ=QT I /QS Capacit)lNolume 1 ( Q W s

Applied Confininp Pressure l'ip Kcsisionce (kN)

Shaîl Resistance (kN)

Perceniage of Shaft Resistancc Perccniaee of Tip Rctsisiancc.

TI S

T I S

T I S

Tl S

O kPa 4.54 5.00

O kPa 2.89 2.96 2.1 1 1.58

42.20 34.80 57. 80 65.20

40 kPa 15.36 2 1 . 0

20 kPa 1 1.50 14.50

1.10 1 .O3

60 kl'a 1.82 2.29 2 1.78 17.2 1 92.29 88.26 7.7 1 1 1.74

1.37 3.50

1,26 2.62

20 W u 5.86 6.08 8.64 5.42 59.59 47.13 40.4 1 52.U7

60 kPa 19.50 23.60

40 kPa 2.32 2.64 1 N.68 12.72 88.95 82.8 1 1 1 .O5 17.19

80 kPa 0.71 1.4

24.58 22.60 97.19 94.17 2.81 5.83

1.21 4.44

100 kPa I .58 1.8 1

26.42 24.19 94.36 93.04 5.64 6.96

80 kPn 24.00 25.29

100 P a 26.00 28.00

1 ,O5 5.47

1 .O8 5.93

Figure 4- 1

confining pressure

10 1 S 20 Settlement (mm)

Load-seulement curves of pile T 1 wirh different codining pressures (firsr group of tests)

1s 20

Setticment (mm)

Figure 4-2 Load-settlement curves of pile T2 with different confining pressures (first group of tests)

Figure 4-3 Constmction of the offset limit Ioad (after Canadian Foundation Engineering Manuai, 1992 )

15 20 Settlemmt (mm)

Figure 4 4 Load-senlement curves of pile Tl with dEerent confhing pressures (second group of tests)

Figure 4-5 Load-seulement curves of pile S with different confining pressures (second group of tests)

-1.4 - a: O kPa confining pressure

c: JO kPa confining pressure

b: 20 kPri confining pressure

Load (kN)

- - * - -5.OkN (Tl)

- - * - -1O.OW~l) -1.4 1

d: 60 kPa confining prrsswe

Figure 4 6 Load distribution along the pile with different values of load applied at pile heads of T 1 and T2 (first group of tests)

Load (kN) O 10 20 30

a: 0 kPa cotirinirig pressure

c: JO kPa confirring pressure

b: 20 kPa coiifiiiitig prcssurc

Load (kN) O 20 40 60

d: 60 kPa conlining pressure

Figure 4-7 Load distribution dong the pile with different values of load appiied at pile heads of T 1 and S ( second group of tests)

Load (kN) O 1 O 20 30 40

- - * - -2û.O kN(T1)

- - * - -30.0kN(T1) -1.4 -

e: 80 kPa coiifinirig prcssiirc

Figure 4-7 (continued)

Load (kN) O 20 40 60

Unit load transfer (kPa)

O 2 4 6 8 10 12 14

a: O kPa c o ~ i i i n g pressure

Unit luad transfer (kPa)

O 20 40 60 80

- 1

c: 40 kPa confining pressure

Unit load transler (kPa)

O I O 20 30 40 50 60

J

b: 20 k f a conliniiig pressure


-1 J

d: 60 kPa confining pressure

Figure 4-8 Unit load transfer to the soi1 when ultimate load was reached at piles T 1 and T2 (first group of tests)

Unit load transfer(kPa)

O 5 1 O 15 M


O M 40 60

c: 40 kfa confiiiing pressure


O 10 20 30 40


0 2 0 4 0 6 0 8 0 1 0 0

d: 60 kPa confining pressure

Figure 4-9 Unit load transfer to the soi1 when ultimate load was reached at piles Tl and S (second group of tests)


O 20 40 6 0 8 0 1 0 0 1 2 0 1 4 0

C

e: 80 );Pa confining prcssurc

Fisure 4-9 (continued)

Unit k d transfer (kPa)

O 20 40 60 80 1 0 0 1 2 0 1 4 0

Unît ioad transfu ( kPa)

10 20 30 40 50

Unit ioad transfer (kPa)

1 O 20 30

b: Tapcred pile. T2

Figure 4- 10 Variation of unit load transfer to the 5011 curves with piles Tl and T2 at different confining pressure applied (first group of tests)

Unit load tnnsfer (kPa) O 10 20 30 40 50 60

-1 J

a: Tapercd pilc. Tl

Unit load transfer (kPa) O 10 20 30 40 50 60 70

-1 J

b: Straiglitaided wall pile. S

Figure 4- 1 1 Variation of unit load transfer to the soi1 curves with piles TI and S at different confining pressure applied (second group of tests)

O 1 O 20 30 40 50 60 70 80 90 100 Confining pressure (kPa)

Figure 4- 12 The distribution of the ultimate load between the pile point and the pile shafl for various applied confining pressures (second group of tests)

CHAFTER 5

UPLIFT BEKAVIOUR OF TAPERED PILE

5.1 INTRODUCTION

In practice, a working pile is not aiways subjected to a compressive load. Piles

supporting transmission towers and jetty structures have to resist uplifi loads.

Tapered piles have a substantial advantage with regard to their load-carrying

capacity in the downward frictional mode. The uplift performance of tapered piles has not

been fully unùerstood. This chapter describes the results of the experimental investigation

into the characteristics of the uplift performance of tapered piles. The observations

include the load-displacement behaviour, ultimate uplifi load, ratio of uplift to

compressive !oad and load -fer patterns.


The pile axial compressive loading test started after the installation procedure was

completed as described in Chapters 3 and 4. The pile was fîrst loaded ciownward with

zero applied confinhg pressure. After the downward axial loading was completed, a

pullhg jack was set and al1 the instruments were reset to zero, the uplifi test was

conducted. The testing procedure and readings for the axial pullout tests were similar to

those for axial compression tests described in Chapter 4 except that the load was applied

in tension. Each loading increment was about 10% of the expected pile uplift capacity

until 15 mm upward pile movement was attahed or the failure (significant change in

displacement due to a small load increment) occurred first.

5.3 TESTING RESULTS

5.3.1 Upiift Load-Disphcement

The load applied at the pile head and the displacement of the pile head were

measured during the loading tests and plotted in Figures 5-1 and 5-2 for the first and

second group of tests, respectively. Figures 5-1 (a) and 5-2 (a) show the load-

displacement cuves at different values of confining pressure for tapered pile, Tl , in the

fkst and second sets of tests, respectively. Figure 5-1 @) shows the load-displacement

curves for tapered pile, T2, and Figure 5-2 (b) shows the load-displacement curves for

straight-sided wall pile, S. It can be noted fiom the figures that the pile's uplift capacity

increased due to the increase in the confining pressure. It may also be noted that the piles

with larger taper angle, Tl, displayed a softer response manifested by larger

displacements at the sarne load level, except for initial loading which was afTected by the

res idd stresses as discussed later.

5.3.2 UItimate Uplitt Load

The ultimate pullout Ioad for each pile was detemiined fiom the load

displacement Cumes. The fdure load of a pile was considered to be the load that resulted

in 6.25 mm upward movement. The results were compared in Table 5-1 based on the pile

uplift capacity ratio, KP, and the net uplift capacity ratio, KF$J. The ratio KP was defined

as the ratio of the uplifi capacity of the two investigated piles, while the ratio KPN was

defmed as the ratio of the net uplift capacity (pile upiift capacity subtracted by pile-self

weight) of the two piles. The ratio of the net uplift to push down shaft capacity for the

same pile under the same confining pressure was also obtained. The results of the fkst

group of tests, piles Tl and T2 in loose sand, and the resulu of the second group of tests,

piles T 1 and S in medium dense sand, are given in Tables 5- 1 and 5-2, respectively.

5.3.2.1 Loose sand status

It can be observed fiom Table 5-1 that pile Tl displayed lower uplift capacity

manifested in values of KP and KPN lower than 1, for dl values of confining pressure.

The uplift capacity of pile Tl is 7 to 12% lower than the uplifi capacity of pile T2 for the

given values of confining pressure, with the lowest capacity at confining pressure e q d to

20 and 40 kPa. The same trend was observed in KPN. The ratio of net uplift capacity to

push down shaft capacity for Tl varied between 41% at zero confining pressure to 33% at

40 kPa, while it varied fiom 66% at zero confhing pressure to 46% at 40 kPa for T2.

These values suggested that this ratio was less for piles with a larger taper angle and

higher confming pressure.

5.3.2.2 Relatively medium dense sand status

It can be seen fiom Table 5-2 that the axial uplift capacity of the tapered p lile was

lower than the axial uplift capacity of the straight-sided wall pile for al1 values of

confining pressure. However, the difference was insignificant, especially at higher

confining pressure. The ratio KP varied between 0.86 to 0.98, with the maximum value

occurring at a confining pressure of 20- 40 kPa The ratio KPN varied between 0.83 to

0.99, with the maximum value occuning at a confining pressure of 20 kPa. The

cornparison between the renùts of the two sets of tests suggested that the eEect of the

taper angle on the uplifi capacity of prototype piles installed in dense sand wodd be

small, especially for longer piles (as the confining pressure increases with depth). The

variation of the ratio of net uplift capacity to push down shaft capacity was small in the

case of the straight-sided wall pile, S, mtween 59% and 70%) while a larger variation

was calculated for the tapered pile, Tl, (between 37% and 58%). A cornparison with the

results of the first set of tests suggested that this ratio was higher for piles installed in

dense sand.

5.3.2.3 Effect of pile placement method

The ratios of net uplift capacity to push down sh& capacity in this study were

lower than the results referred by Nicola and Randolph (1 993). In their study, the ratio of

tende and compressive shaft capacities varied with an average of about 0.7 for piles

driven into sand. In the current study, the pile was placed in the centre of the soi1

chamber and the sand was then poured around it, resulting in no densification due to the

method of placement. In the case of pile driving, the soi1 is displaced and the sand

becomes denser in the close vicinity of the pile, and consequently, the ratio of uplifi

capaci~j to pushdown capacity becomes higher. Levacher and Sieffert (1984) investigated

the axial performance of piles installed in sand. They concluded that the placement

method had a significant effect on the axial performance of piles.

5.3.3 Pile Head Stiffness

The eflect of confinhg pressure on the pile head e e s s is illustrated in Figure

5-3. Severai observations can be made fiom this figure. FirstIy, as the connning pressure

increased both the initiai and secant stifniess of ail piles increased, as expected. However,

the inmease in the secant *ess (at ultimate load) was much more significant.

Secondly, piles with smaller taper angle, Tï, in the first set of tests and S in the second

set, had smaller values of initial stiffhess and higher values of secant &ess. The higher

initial stifhess values of piles with larger taper angle may be attributed to lower rrsidual

stresses developed during the pushdown loading tests. However, as the pullout loading

continued, the residual stresses were dissipated and piles with smaller taper angle

displayed higher secant stifniess values. This behaviour was more evident in the second

group of tests because more significant residual stresses were developed due to

application of higher loads to piles in dense sand. Thirdly, the secant stifhess was 15-

20% of the initial stiffhess for al1 piles, which represented a highly nonlinear behaviour in

this loading mode. This nonlinearîty was more pronounced in loose sand and piles with a

larger taper angle.

5.3.4 Load Distribution

The forces transmitted at different locations were calculated from strain gauge

readings as

4, =E, E A , (5-1)

where q, is the pile axial load at the location of main gauge i, E, is the main mesurement

of strain gauge i, E is the elastic modulus of the pile material and Ai is the pile cross-

sectional area at the location of strain gauge i.

Figures 5 4 (a) (b) and 5-5 (a) (b) show the load distribution dong the piles under

various load incrernents (given as ratios of the ultimate uplift load, P.) for the first and

second groups of tests, respectively. It may be observed from both figures that the

general trend of the load distribution was the same for ail three piles at ail loading

increments. It can aIso be noted that the load was transferred to the soi1 gradually except

for a distinct change close to the pile tip. This change was the result of residual stresses

developed during the downward loading test that was performed before the uplift loading

test. The presence of the residual stresses was evident fiom the compressive stresses

shown near the pile tip. It may also be noted that the residual stresses were more

pronounced in piles installed in the dense sand.

5.3.5 Shaft Friction

The shaft friction of the pile was calculated from the strain measurements during

the pile test. The readings of the strain gauges recorded when the ultimate uplift load was

applied were used to calculate the load distribution dong the pile. The difference

between the force calculated fiom any two sets of strain gauges dong the pile wall minus

the corresponding weight of the pile represented the total load transferred to the

surrounding soi1 between the two points. Dividing this value by the conespondhg

surface area. the average of shaft fiction was obtained as

wheref, is the average shaft friction between stations i and j , S, is the surface area of the

pile between stations i and j, and Wg Ys the pile weight between stations i and j. The shaft

friction curve was obtained fiom the values of shaft fiction dong the pile.

Figure 5-6 shows the comparison of the shaft fiction for piles Tl and T2 in the

first group of tests, while Figure 5-7 shows the comparison between Tl and S in the

second group. Both figures illustrate that the shaft friction was slightly lower for piles

with larger taper angle. Comparing the shaft fiction for the tapered pile Tl in the two

sets of tests, the effect of initiai sand density on the shaft fiction could be observed. As

expected, the pile in the dense sand had a higher shaft fiction, however, this effect was

Iess significant at higher confining pressure.

The shafl friction curves at different values of confuiing pressure are shown in

Figures 5-8 and 5-9 for piles Tl and T2 in the first group of tests, and Tl and S in the

second group of tests, respectively. Both figures show that the intensity of the shaft

fnction increased with an increase in the confining pressure. However, the increase in the

shaft friction was less in the higher confuiing pressure range (greater than 60 kPa). This

suggested that there was a limiting fiiction value in this mode of loading.

5.3.6 Downward and Upiift Shaft Friction

The cornparison of shaft friction at ultimate tensile and compressive capacity are

s h o w in Figures 5-10 (a) (b) and 5-1 1 (a) (b) for the first and second groups of tests,

respectively. The compressive results were extracted fiom Chapter 4. The general trend

was that for most of the pile Iength the tensile shaft fiction was lower than the

compressive shaft fiction, but close to the pile tip the compressive shaft fiction

decreased and the tensile fnction increased. This trend was similar to that reported in

Nicola and Randolph (1993), where the theoretical basis for a consistent difference in

tensile and compressive sh& capacity of straight-sided wall piles in sand was explored.

Their work showed that there are sound reasons for expecting the tensile shaft capacity to

be significantly lower than the compressive shaft capacity for straight-sided wall piles in

fiee draining soils. The increase in the tende niction close to the pile tip could have been

fictitious and codd be ateibuted to the presence of the residual stresses which affected

the strain measurements used to calculate the shaft fiction along the pile.

5.4 DISCUSSION: RESIDUAL STRESSES

The existence of residual stress has k e n known and investigated by other

researchers such as Stewart and Kulhawy (1 98 11, Briaud and Tucker ( 1 984) and Poulos

(1987). In the field, these stresses develop during the driving of piles, where their value

could be significant, or as a result of the load testing of bored piles.

During driving or downward axial loading tests, a pile moves downward, and the

pile-soi1 friction along the shaft and the point soi1 resistance acts upward on the pile to

resist the pile's peneûation. Mer dnving and during the unloading that follows, the soi1

under the pile tip pushes the pile back and stresses dissipate. However, a significant

residuai point load c m exist in the pile toe, especially with lower confining pressure

applied since point capacity is larger and a large rnovement is needed to unload the pile

tip, wMe little movement is needed to unload the pile shaft.

Poulos (1987) emphasised the importance of considering the residual stresses in

the interpretation of btrumented pile loading tests. He noted that if zero r e s i d d

stresses were assumed, only the incremental stresses and loads were measured, and a false

picture of the shaft and toe resistance was obtained. Hence, substantial ciifferences

appeared to exist between the skin Wction values in compression and tension, whereas

the values were the sarne (in his opinion). He also pointed out that the effect of residual

stresses was more signifiûant on the initial uplifi stiffness of piles dnven in sand.

In the current study, the pile installation method did not result in any residual

stresses. However, the piles were tested in compression prior to the uplift test which

might have renilted in sorne residual stresses developing dong the lower part of the pile.

The load transfer c w e s obtained in this study varied considerably as s h o w in Figures 5-

6 and 5-7. This variation could be amibuted partially to the residual stresses. However,

it is the author' opinion that the soi1 reaction to the pile motion was inherently different in

the two loading modes, especially for tapered piles.

5.5 SUMMARY

An experimentai investigation of the axial uplifi response of three steel piles with

different taper angles installed in sand was presented and discussed in this chapter. The

q l i f i performance characteristics of the piles were investigated and the following

conclusions were drawn:

1. The pile axial uplift capacity increased with an increase in the confinuig pressure for al1

piles examined in this study;

2. The ratios of uphft to compressive load and load transfer patterns for straight-sided

wall piles were similar to those obtained by other researchers. These ratios were less for

tapered piles than saaight-sided wall piles since tapered piles possessed much higher

bearing capacity and slightly less uplifi capacity;

3. The uplifi capacity of tapered piles was comparable to that of straight-sided wall piles

at higher confining pressure values, suggesting that the performance of actual tapered

piles (with greater length) would be comparable to that of straight sided wall piles;

4. Residual stresses developed during the pushdown loading phase and their effect were

more significant on the initial uplift capacity of piles. This effect was more pronounced in

the case of straight-sided wail piles in dense sand.

Confining pressure

O S 10 15 Upward movement (nm)

O 5 10 15

Upward movement (mm)

Figure 5- 1 Load- upward movement curves of piles at different confining pressure values in first group of tests a: Tl b: T2

Confining pressure

O kPa - o . - M kPa -_-- 40 kPa ; - * . o . - 60 kPa ---

L 80 kPa -1WkPa j

O 5 10 1s O 5 1 O 15 Upward movement (nm) Upward movcmmt (mm)

Figure 5-2 Load- upward movement curves of piles at different confinhg pressure values in second group of tests a: Tl b: S

1 lntial stiffness I

O 20 40 60

Confining pressure (kPa)

1 Secant stiffness at Pu (


a: First group of Tests

lntial stiffness I

O M 40 60 80 100


- - --

Secant stiffness at Pu

O 20 40 60 80 100


a: Second group of Tests

Figure 5-3 The effect of cunfining pressure on the uplifi pile head st if iess a: First group of tests b: Second group of tests

Load (kN)

O 0.2 0.4 0.6 0.8 Load (kN)

O 1 2 3

Load (kN)

-2 O 2 4 6

Figure 5-4 (a) Load distribution dong the pile at different load incrernents applied at pile head of T 1 in first group of tests

Uplift Ioad applied at pile liead (w I

Load (kN)

O 0.2 0.4 0.6 0.8

Load (kN)

-2 O 2 4 6 8

Figure 5-4 @) Load distribution dong the pile at different load incrernents applied at pile head of Tl in first group of tests

Uplifi load appIied at pile lmd (kN)

Load (kN)

-5 (W

-1 O 1 2 O S

a. O i ù ? ~

Load (kN)

I . a ,

c: JO kPn

Load (kN)

Figure 5-5(a): Load distribution along the pile at different load increments applied at pile head of Tl in second group of tests

Uplifi load applied at pile liead (khi)

Load (kN) -3 -1 1

Figure 5-5(b):

L.

E Cr

al U

3 3 w c 2 Q 5 t w C =I

5 O O

Load (kN)

1 O

Load (kN)

1 O 20

Load distribution along the pile at difEerent load increments applied at pile head of S in second group of tests

Shaft friction (kPa)

O 0.5 1 1.5 2

Shan friction (kPa)

O S 10 1s m 25

Shan friction (kPa)

O 2 4 6 8


O 10 20 30

Figure 5-6 Shaft friction for piles Tl and T2 at different confining pressure values in first group of tests

Shan friction (kPa)

O 1 O 20


O M 40


O

Shaft friction CkPa) Shan friction (kPa)

O 50 100 O 50 100

Figure 5-7 Shaft friction for piles T1 and S at different confining pressure values in second group of tests

Shaft bicüon (kPa) O 5 10 15 20 25

Shafi friction (kPa)

O 10 20 30

Figure 5-8 Variation of shafl friction at different confining pressure values in first goup of tests a: Tl b: T2

Shaft friction (kPa) O 20 40 60 80

ShaR friction (kPa)

O 20 40 80

Figure 5-9 Variation of shaft fiction at different confining pressure values in second group of tests a: T 1 b: S

Loading direction

1 +TB~BM +Cornpmwon 1

Shan friction (kPa)

O 1 2 3 4

Shan friction (kPa)

O 1 O 20 30 Shan friction (kPa)

O 1 O M 30 40

Figure 5-1 0 (a) The cornparison of shafl friction at ultimate uplift and compressive capacity in first group of tests. pile T 1

Deplh under the sand surface (m)

üeplh unclsr the sand surface (m)

Deplh undet Ihe Sand surface (m) , b b b b b b b b b d ~ m ~ m ~ O W ~ - a

I

Dtpth under Iha sand surface (m)

Loading direction

+Tension 4 C o m p m s i o n

a: O kPa

Shan friction (kPa)

4

E 0 -0.2 .- u

-1 .-

b: 20 kPa

Shan friction (kPa)

O

4 .1 n

E --O2 O

Y 'tO.3 2 z4.4 8 f -0.5 - L

8-05 C 3 5-0.7 a

84.8

4.9

- 1

c: 40 kPa

S h d t friction (kPa)

Figure 5- 1 1 (a) The cornparison of shafi friction at ultimate uplifi and compressive capacity in second group of tats, pile Tl

Loading direction

-u Tension -Cornpnssion

ShaR friction (kPa)

O 1 O 20


O 50


O 20 40

Shaft hiaion (kPa)

Figure 5-1 1 (b) The cornparison of shaft friction ar ultimate uplifk and compressive capacity in second group of tests, pile S

CBAPTER 6

RESPONSE OF TAPERED PILES SUBJECTED TO LATERAL LOAD

6.1 LNX'RODUCTION

Piles are fkequently used to support structures subjected to lateral forces and

moments such as offshore structures, harbour structures, high lise buildings and bridge

abutments. Laterdly loaded piles have received very extensive attention due to their

common use in practice.

The governing criteria in designing pile foundations to resist lateral loads in most

cases is the maximum deflection of the foundation rather than its ultirnate capacity. The

maximum deflection at the pile head and the distribution of the bending moment dong

the pile are important idonnation for the successful design of pile foundations that

support lateral loads. Knowing the maximum deflection at the pile head is important to

satisfy the serviceability requirement of the superstructure while the bending moment is

required for the structural sizing of piles.

Laterd loading tests were conducted on mode1 steel piles to establish the lateral

behaviour of tapered piles in cohesionless soil. This chapter describes the loading test

procedure and observations. The results include the horizontal pile load and

conesponding movement at the loading point, the bending moment distribution dong the

pile, and the soil resistance and pile displacement relationship which was developed in

the fonn of p-y curves.


The lateral loading tests of the three piles were carried out starting with zero

confining pressure. The lateral load was applied incrementally to the pile until 10 mm

deflection occurred at the loading point which was 0.195m above the sand surface. The

load was applied at a two and a half minute time intervals. At each connning pressure, the

lateral push forward test was followed by a push backward test until the pile came back to

its initial vertical position (confirmed with level meanirement). This was done to ensure

that the initial condition was the same for al1 loading tests. The pressure was then

increased by an additional 20 kPa and the process repeated until the pile had been tested

at al1 desired values of confïning pressure (fkom O kPa to 100 kPa). The loading

uicrement values were 0.22,0.35, 0.53, 1.0, 1.0, 1.0 kN for confining pressures 0, 20,40,

60, 80 and 100 kPa, respectively.

6.3 TESTmG RESULTS

6.3. 1 Load-Deflection

The load and displacement at the Ioading point were measured and recorded

during the loading tests at different values of confining pressure. Figures 6-1 (a), (b) and

(c) show the load-displacement curves for straight-sided wall pile, S, tapered piles, T2

and TI, respectively. It can be noted from the figures that the confining pressure had a

significant effect on the load-displacement behaviour. As expected, the response of al1

piles was stiffer (i.e. smaller displacement at the same load level) for higher confining

pressure values. It may also be noted fiom the figures that pile Tl was stiffer than pile l2

which in nim was stiffer than pile S, for dl values of confining pressure.

6.3.2 Ultimate Lateral Load

Generally, detemüning the ultimate load from lateral pile load tests depends on

the tolerance of the structure supported by the piles. Where no such criterion is available,

the criterion usually accepted for estimating the ultixnate lateral load is the load

comesponding to 6.25 mm lateral movement or displacement normal to the pile a i s . The

ultimate lateral load capacities of the piles were established based on this criterion and are

ploned in Figure 6-2. The figure shows clearly that pile Tl had a larger ultimate load

than pile T2 which in tum had a larger ultimate load than pile S. To evaluate the

eficiency of tapered piles under lateral loading conditions, the results were compared in

terms of the pile lateral capacity mtio, KH, given in Table 6-1. The ratio KH was defmed

as the ratio of the laterd capacity of the tapered pile to that of the straight-sided wall pile.

It can be observeci h m Table 6- 1 that pile Tl displayed an increase in the ultirnate lateral

I d ranging between 77% at zero confining pressure to 21% at 100 kPa. A similar trend

was obsewed for pile T2 with a maximum increase of 25% at zero confuing pressure to

6% at 100 kPa. These results are significant because moa of the lateral resistance of piles

is derived from the topmost soi1 layers where the confining pressure is low. Therefore, it

is expected that prototype tapered piles wül have a lateral capacity increase closer to the

upper end of the range (e.g 77% for piles with 0.95' taper angle).

6.3.3 Bending Moment Distribution

The bending moment values at the location of the strain gauges were calculated

from strain rneasurements as

where M, is the pile bending moment at the location of strain gauge i, is the strain

measurement of sbab gauge i, E is the elastic moddus of the pile materid , Il is the

moment of inertia of the pile cross-section at the location of strain gauge i, and c, is the

exterior radius of the pile at the location of the strain gauge i.

The distribution of bending moment dong the pile at different load increments

had the sarne pattern as shown in Figures 6-3, 6-4 and 6-5 for T 1, T2 and S , respectively .

The maxirnum moment for al1 piles occurred at a depth about 0.41m below the sand

surface (about one third of the pile embedded depth) and the bending moment along the

pile increased as the loading increased. Aiso, these figures indicated that the bending

moment increased with the confining pressure, as higher loads were sustained. The

bending moment along the pile was normalized by the applied lateral load (Le. bending

moment / laterd loading, MM) and the results are shown in Figures 6-6 to 6-8. It can be

observed fiom the figures that the normalized moment slightly increased as the lateral

load increased for piles S and M. However, pile Tl showed Wnially no increase in the

normaiized moment with the load level. This can be explained as follow: pile Tl whiçii

has a larger cross section at the top transferred more load to the soi1 near the top reducing

the load resisted by the pile dong its shaft and consequently resulted in no increase in the

normalized moment dong the shaft.

6.3.4 Maximum Bending Moment

The maximum bending moments as a fiinction of lateral load applied at the pile

head are shown in Figure 6-9 for the three piles at al1 confuillig pressures. It c m be

concluded that for any speciiïc applied load, the three piles experienced approximately

the same maximum bending moment. The plots of bending moment with depth for the

three piles subjected to different values of laterai load are s h o w in Figure 6-10. The

maximum moments occurred at about the sarne location for the three piles, the top third

of the pile's embedded length, where the cross-section of tapered piles is larger than that

of the straight-sided wall pile. Hence, the stresses developed in the tapered piles were less

than those developed in the straight-sided wall pile supporting the fact that tapered piles

represent a more equitable distribution of the pile material.

6.3.5 Soii Resistance

The soil resistance along the pile shaft was determined from the bending moment

values using an approach simila. to that developed by Matlock and Ripperger (1956). In

this approach, the distribution of the bending moment along the pile shaft was c w e fitted

by a cubic polynomial funchon, i.e.

M(x) = ax3 + bx2 + cx + d (6-2)

where x is the depth below the sand surface and a, b, c and d are constants obtained fiom

the curve fitting process. The distribution of the soil resistance along the pile shaft was

then obtained by a double differentiation of the bending moment, Le.

The curve fitting procedure introduced a mild degree of smoothing to reduce the scatter of

experimental error.

Figure 6-1 1 shows the soil resistance for the three piles nibjected to the same

typical load value, while Figure 6-1 2 displays the soil resistance at the ultimate load level.

It can be noted nom Figure 6-1 1 that at O kPa confining pressure, the piles pivoted about

the pile tip (ngid body movement typical of short piles). As the confining pressure

increased, the pivoting point moved upward leading to more deflection, curvature and

stresses in the pile simulating the behaviour of longer piles (flexible response typical of

longer piles). This is due to the increased soil resistance that resulted from the increase in

the confining pressure. The tapered piles' response, however, was more rigid than the

straight-sided wall pile (the pivoting point was aiways closer to the pile tip for al1

confining pressures compared to pile S). This was because tapered piles transferred more

load to the soi1 through the top most part of the pile which had a larger cross-section.

Figure 6-12 showed that at the ultimate load, pile Tl expenenced the highest soil

resistance followed by T2 then S. AIso, the same observations noted in Figure 6-1 1 were

confinned in Figure 6-12, i.e., the pivoting point moved upward as the confinuig pressure

increased, but it was closer to the pile tip for piles with a larger taper angle.

6.3.6 Pile Deflection

The deflection of the pile dong its shaft was obtained by a double integration of

the bending moment hct ion , Le.

which yielded

In Equation (6-5), a, b, c and d are the curve fining constants, El is the flexural rigidity of

the pile which was constant for pile S and was taken as constant equal to the average

value over the pile length for tapered piles, and F and G are integration constants which

were obtained frorn the boundary conditions as

F = EI @(O) (6-6)

G = El y (O) 05-71

where 8(0) and y(0) are the slope and deflection of the pile at the sand surface.

Figure 6-13 (a) displayed the deflection of the piles at some typical load values

applied to the three piles. It can be noted fiom Figure 6-1 3 (a) that tapered piles Tl and

T2 expenenced srnaller deflections dong the shaft than pile S, for the same load and

cofining pressure. Figure 6-1 3 @) showed the same behaviûur observed fiom Figure 6-

12, i.e.. pile Tl had a more rigid behaviour with the pivoting point closer to the pile tip

for al1 confining pressures.

6.3. 7 p-y Curve

The analysis of lateral load-deflection behaviour of piles using the subgrade

reaction method is performed by considering the pile as a bearn on an elastic foundation

and replacing the soil with a series of elastic, closely spaced but independent springs. In

this analysis, the term subgrade reaction rnodulus which represents the stiffiess of the

spring used to mode1 the elastic soil medium is dehed as kt,= ply, where p is the soil

resistance and y is the pile deflection. The relationship between the pile deflection and the

soil resistance at the same level is defhed as the ÿ y c w e .

Figure 6-14 compares the p y curves at a depth of 0 . 4 ~ close to the position of

maximum bending moment, for the three piles. The cornparison showed that for lower

confinhg pressures (e.g. 0-20 P a ) , tapered piles had higher soil resistance than straight-

sided wall pile for the same pile deflection. This difference, however, was less significant

for higher confinuig pressures (e.g. 80-1 00 kPa). This observation is important as most of

the soi1 resistance to the lateral deflection stems fiom the top most soil layers where the

cod~ning pressure is low. Figure 6-15 presents the vanation of the p-y curves at 0.4m

with the value of the confining pressure for the three piles. Several observations can be

made fiom this figure: fxst, as expected, the rnodulus of the subgrade reaction (the dope

of the p-y curve) increased as the c o n f i g pressure increased for al1 piles and deflection

values; second, the modulus of the subgrade reaction decreased with deflection for al1

piles under the sarne confining pressures; third, the p-y c w e s of Tl showed larger soi1

resistance than T2 and S. These observations were illustrated more clearly in Figure 6- 16

where the modulus of subgrade reaction was presented. Figure 6-16 shows that the

modulus of subgrade reaction significantly increased as the confining pressure and taper

angle increased. It can also be noted fiom the figure that the modulus of subgrade

reaction was strongly nonlinear as it decreased with the increase of the pile deflection,

especidly at lower confïning pressures.

6.4 PREDI-D ULTlMATE LATERAL LOAD

For a straight-sided wall pile with a unrestrained head the ultirnate lateral

resistance may be estimated nom statics for the situation shown in Figure 6-17. The

u l h a t e horizontal load, Hu, and Moment, MK may be approximately written in the forms

where Ru is the ultimate soi1 pressure at a depth Z below the soi1 surface, D is the pile

diameter. Assume the ultimate soi1 pressure is uniform dong the entire length of the pile

(similar as our experimentai setup) then Eqs. (6-8) and (6-9) give:

Broms (1964 a, b) suggested that, for piles in sand ,

(6- 1 2)

where Kp= Rankin passive pressive coefficient = (1 +sin@)/(l -sin$), o, is effective vertical

stress and fi = coefficient of lateral eaah pressure.

The ultimate laterai resistance and the rotation point for the zero confining

pressure were calculated using Eqs. (6-10) , (6-1 1) and (6-12). It was found that the

calculated lateral resistance of the pile Hu = 6.27 kN and the rotation point is at a depth

0.8 1 m below the sand surface. The parameters used in the calculation were: L/D = 7.25, y

4 6 . 3 2 kWrn3, e= 0.195m, t$ = 3 2 O . The failure mode assumed in this analysis involved

the soil failure dong the entire length of the pile. In the current study, however, the

measured ultimate load was defined as the load correspondhg to 6.25 mm deflection at

the loading point. The straight-sided wall pile ultimate load was evaluated to be 1.34 Iùrl.

At this load Irvel, soil failure over the entire length of the pile was not expected to occur.

6.5 PREDICTED p-y CURVES

An analytical study was performed to calculate p-y curves based on the

mathematical expressions developed by Geosofi(l987).

6.5.1 Analytical Background

The p-y curve describes that the soil responses are nonlinear and depends on

several parameters: soi1 depth, shearing strength and stress-deformation charactenstics. A

subgrade reaction modulus, Kh, is defined as the ratio of soil reaction to pile laterai

deflection, i. e.

K h = PIY (6- 1 3)

For sand. Kh is generally assumed to vary linearly with depth such that

K h = K x (6- 1 4)

where K = constant in kN/m3. Combining equations (6-1 3) and (6-14), we have

p = K x y (6- 1 5)

where p = soi1 reaction per unit length in M m ; x = depth at which p-y c w e is defined in

m; y = Iateral deflection at depth x in m. The major factors affecthg K are relative

density of the sand and lateral deflection.

6 - 5 2 Observation

Based on the relationship shown in Table 6-2 for variation of K with y/D

(deflection, y, normaiized with respect to the pile diameter, D), theoretical p-y c w e s

were obtaùied at the sand relative density 32.7% and 0.4m depth below the çand surface

with different confining pressures. The computational procedure for soil reaction p is:

(1) use the linear interpolation method to obtain density factor F fiom Table 6-2 @) at the

relative density of 32.7%; (2) compute the equivalent depth x for different confining

pressure from the soil properties and the lateral stress surrounding the pile; (3) multiply y/

D, shown in Table 6-2 (a), by the pile diameter D to get the deflection values y; (4)

calculate soil reaction p from equation (6- 1 5).

Variations of the soil reaction with the pile deflection in Figure 6-18 were

predicted using Geosoft approach for stmight-sided wall pile S and tapered pile Tl. It can

be seen fiom the figure that Kh is approximately 1.5 - 2.0 times greater than that observed

from experiment (Figure 6- 15) which may be due to the initiai relative density effect. The

experirnentai p-y curves were actually obtained under loose sand condition, where the

relative density was 18.4%. Hence, this cornparison suggests tbat soil subgrade reaction

modulus is higher in dense sand than that in loose sand, indicating the reliability of the

experimental results.

6.6 SUMMARY

Lateral load tests were conducted on mode1 piles in a laboratory setup to

experimentaily investigate lateral pile behaviour for a variety of pile geometries and soil

confining pressures. Pile ultimate load, soil resistance and pile deflection relationships,

moduius of subgrade reaction and bending moment distribution dong the pile were

obtained from measured pile responses and strains. For each test, remonable correlation

existed between the ioad and displacement relationships and between the pile e s .

The following conclusions could be drawn from the analysis of the results of the lateral

load tests:

1. The pile ultimate lateral load increased and the pile head deflection decreased as the

soil confinhg pressure increased.

2. The pile taper had a significant effect on its lateral load capacity. A taper angle of

0.95' resdted in an increase of 77% in the pile lateral capacity at O kPa confining

pressure.

3. The p-y c w e s of tapered piles represented a much stiffer soil resistance at al1 load

levels, especially for lower confining pressures that exist in the topmost soil layers.

This is significant b e c a w the lateral response of piles is generally controlled by the

resistance of surface soil layers.

4. The moddus of subgrade reaction was highly nonlinear for shallow soil layers. It was

less noniinear for deeper soils. Also, the moddus of subgrade reaction of tapered piles

was much higher than thai of straight-sided wdl piles.

5. Measured maximum bending moments occurred within the upper third of the pile

embedded length where the tapered piles cross-section was larger than the cross-

section of saaight-sided wall piles with equivalent average diameter. This represents a

more efficient distribution of the pile material and resdted in lower bending stresses in

the piIe.

In general, it can be concluded tbat the tapered piles represented a more efficient

distribution of the pile materid and the lateral loading behavior of pile foundations could

be improved by using tapered piles instead of straight-sided wall piles.

Table 6-1 Lateral loading results

Applicd Confininfi Prcssiirc

Lateral Loading Capaciîy (kN)

Ralio of'capacily I

Table 6-2 Variation o f K with deflection and relative density (Geosoft)

y / D (inh., d m )

0.0000 0.00 1 O 0.0025 0.0050 0.0100 0.0 166 0.0250 0.0750

Dr. Pcrcent

HS HT2 HT I

HT2MS HTlMS

1.97 2.29 2.95

M a ~ i u m Bcnding Moment (kN.m)

lcnsiiy facior, F

1,250 1.000 0.750 0.400 0.125

20 kPa 2.93 3.54 3.82 1.2 1 1-30

O kPa 1.34 1.67 2.37 1.25 1.77

0.52 0.64 0.92

MS MT2 MT I

Notc:

1.29 1.34 1 .56

3.16 3.3 l 1 .O8

y = dcflcciion D = pile diainetcr

40 kPa 4.98 5.91 7.37 1 , l Y I .A8

4.22 1.34 5.43

60 kPa 7.83 8.55 10.23 1 .O9 1.3 1

5.36 5 .fi

6.56

80 kPa 10.33 10.84 13.65 1 .O5 1.32

100 kPa ' 13.55 14.37 16.33 l .O6 1.21

8 12

Dispkcanent at loading point (m)

8 12 Displacement at loading point (mm)

Figure 6- 1

8 12 Displacement at loading point (mn)

Load-displacement curves at the loading point in the push forward phase for different piles a: pile S b: pile T t c: pile Tl

O 20 40 60 80 100 120


Figure 6-2 Lateral Ioad capacity versus confining pressure for three piles

Bsnding moment (WmI

O 2 4

Bending moment Bending moment Bending moment (kNm) (kNm ( k N m

O 2 4 6 0 2 4 6 0 2 4 6

Figure 6-3 Moment distribution along pile S

Bending moment (kN.m)

O 2 4 6

Bending moment ( k N 4

O 2 4 6

Figure 6-4 Moment distribution along pile T2

Bmding moment ( W ~ n l

O 0.5 1

Bending moment (kN.rn)

Bending moment ( k N m

Bending moment (kNm)

O 2 4

Bending moment ( k N m l

O 5 10

Figure 6-5 Moment distribution along pile Tl

Mornenu Load (kN.mn<N)

Mamenti Load (kNMN)

O 0.2 0.4

Figure 6-6 Normalized moment distriLution along pile S

Figure 6-7 Normalized moment distribution along pile Tt

Figure 6-8 Normalized moment distribution along pile Tl

O 0.2 0.4 0.6 0.8 1 Maximum bending

mofnent(kN-rn)

O 1 2 3 4 Maximum knding momtnt(kNm)

O 0.5 1 1 .S 2 Maximum bending

rnoment(kNm)

O 1 2 3 4 5 Maximum ôending moment(kN.m)

O 2 4 6 8 Muhtrar knding

moment (kNm)

Figure 6-9 Variation of maximum bending moment with applied load for the three piles

Bending manent(kNm)

O 0.5 1

-1.4 I

a: applicd ioad 2.0 kN

Bending momerit(kN.m) O 0.5 1

c: applicd load 2.0 kN

Bending mament(kN~n)

O 1 2 3

e: applied load 6.0 kN

b: appIied load 2.0 W

Bending momtnt(kN.m)

apptied load 6.0 kN

Bandhg mamcnt(kNm)

f: applied load 6.0 kN

Figure 6-1 0 Bending moment along the pile shaft for the three piles subjected to the same load at different contining pressures

Soil resistance (kNh)

-1 0 -5 O 5

a: Pile S. applied load 2.0 kN

Soil resistance (kNlm)

c: Pilc T2, applicd load 2.0 kN

Soil Resistance (kNlm)

e: Pile T 1. applied load 2.0 kN

Soil resistance ( k W ) 4 -30 -20 -10 O 10 20

b: Pile S. applied load 6.0 kN

Soit residance (kNlm)

d: Pile T2. applicd load 6.0 kN

Soil residance (kN/m)

f: Pile Tl. applied load 6.0 k.N

Figure 6- 1 1 Soil resistance dong pile sh& for three piles under a typical load

Soil resisbnce ( k m )

-1 5 -1 O -5 O


I 7


Soi4 resisbnce (Wm) -1 5 -1 0 5 O 5

Soil resistance(kWm)

Soil resistance (kNhn)

Figure 6-1 2 Soil resistance along the pile sh& for three piles under ultimate load

+O kPa + M kPa

I I +4û kPa

a: Pile S. applied Ioad 2.0 kN

c: Pilc T2. applicd load 2.0 kN

DefIection(m)

- a * - ,

e: Pile Tl. appiied had 2.0 kN

.. - b: Pile S. applied load 6.0 kN

Defledion(m)

- . -

d: Pilc T2. applicd load 6.0 kN

hflection(m)

f: Pile Tl. applied load 6.0 kN

Figure 6-13(a) Pile deflection dong the pile shaft under a typical load

Deflection (m) Defidon (m) -0.005 O 0.005 0.01 4.005 O 0.005 0.01

Defiection (m)

-0.005 O 0.005 0.01

Defiection (m)

O 0.005 0.01

Figure 6- 13(b) Pile deflection along the pile shaft under ultimate load

Deff ection (m)

O 0.002 0.004 0.006

Deflection (m)

O 0.002 0.004 0.006

Ocflection (m)

Defiection (m)

O 0.002 0.004 0.00o Deflecton (m)

O 0.002 0.004 0.006

Defiection (m)

Figure 6- 14 Effect of pile taper on p-y curves at 0.4 rn depth

Pile S

Q

Pile T2

O 0.00 t 0.002 0.003 0.004 0.005 0.006

Oeflection (m)

L.

E % " m O

3 20 L II) - II)

!! = 10 O U)

O O 0.001 0.002 0.003 0 . w 0.005 0.006

Deflection (m)

Figure 6- 15 Effect of confining pressure on p-y curves for different piles

0 ' I

O 0.002 0.004 0.006 O 0.002 0.004 0.006 ûetlection (m)

Dtflection m)

O 0.002 0.004 0.006 O 0.002 0.00s 0.006 Deflection (m) Deflection (m)

0.002 0.004 0.006 Dcfltcüon (m)

O 0.002 0.004 0.006 Defieetion (m)

Figure 6- 16 Degradation of modulus of horizontal subgrade reaction with deflection

Pila diameter=d

Figure 6- 17 Unrestrained lateral-loaded pile ( d e r Poulos and Davis, 1980)

Figure 6-1 8 Predicted p-y curves for pile Tl and S in relative dense sand (Geosoft )

CHAPTER 7

CYCLIC RESPONSE OF AXIALLY LOADED TAPERED PILES

7.1 INTRODUCTION

From previous chapters, it is observed that tapered piles have a substantial

advantage over straight-sided wall piles with regard to their load-carrying capacity and

lateral load resistance, however, their behaviour under axiaily cyclic loading has not been

investigated. Structures such as offshore platfonns and towers for power transmission

lines are commonly supported by piles and are subjected to environmental loads (i.e.

wind, waves, tides and earthquakes). These environmental loads are cyclicd in nature.

The design of piles subjected to axial cyclic loading comprises consideration of both the

permanent deformation of the pile head and the effects of cyclic loading on the axial load

capacity.

This chapter presents the results of the fourth phase of this study that explored the

pile behaviour under axial cyclic load. The pile characteristics investigated include the

pile head load-movement, the accumulation of displacement and the pile head stifniess.


The instruments were reset to zero at the beginnuig of the pile load testing. Each

pile was subjected to 10 two-way cycles of axial loading test at each value of confining

pressure. The confinkg pressure was varied from O to 60 kPa. Figure 7-l(a) shows the

characteristics of cyclic loads that were applied at the pile head for three piles at different

values of confning pressure. Figure 7-1@) shows the characteristics of cyclic loads

applied in two different tests for Tl with 20 kPa confining pressures to examine the effect

of load amplitude on the cyclic response to be discussed later. For the purpose of

cornparison, the amplitude of the cyclic load is expressed in terms of Q,, the static

ultimate load of the pile in compression, and Pu, the static uplift capacity of the pile, as

s h o w in Table 7-1. The meanirements of load and strain were recorded every 10 seconds

using the data acquisition system. The dia1 gauge readings were taken at the end of each

loading increment.

7.3 TESTING RESULTS

The response of a pile to cyclic loading is complex, however, several aspects of

pile behaviour can be identified with regard to changes in the movement of the pile head,

accumulation of displacement, pile head stifniess and pile head movement within each

cycle.

7.3. 1 Cyclic Load - Displacement

The load applied at the pile head and the movement of the pile head were

measured during the tests. The same load amplitude was used in testing the three piles for

each value of the confining pressure. However, because the capacities of the three piles,

both in compression and tension were not equal, the load amplitude represented a

different ratio fiom the pile capacity as shown in Table 7-1. This could explain the

substantial difference in the behaviour of the three piles observed in Figure 7-2. The load-

movement curves of the three piles at confinhg pressures equal to 0, 20,40 and 60 kPa

are shown in Figures 7-2 (a), (b), (c) and (d), respectively.

7.3. 1. 1 Zero confming pressure

It can be noted fiom Figure 7-2 (a) that at O kPa confining pressure, Tl

experienced a large upward movement startùig nom the first cycle. This upward

movement continued to increase through dl the loading cycles. The pile behaved in this

manner because the amplitude of the applied load represented 0.354. / 0.88P, (i.e. 35%

of Qu and 88% of P.). This meant the load in compression was well below the pile

capacity, however, it was close to the pile capacity in tension. Consequently, the pile

movement in tension was much larger than that in compression. On the other hand, in the

case of pile S, the same load amplitude represented OSQ, and 0.72P.. Therefore, the

pile's movement in tension was comparable to its movement in compression and

consequently, the pile's final upward movement was much less than that of Tl. The

behaviour of T2 was similar to that of Tl, however, the final upward movement was even

larger as shown in Figure 7-2 (a). This could be attributed to the variation in the initial

state of the sand. The tests on pile Tl and S were conducted with loose sand but were

different for T2, where the cyclic loading tests were perfonned afier pile lateral loading

tests.

7.3. 1.2 20 kPa confrning pressure

Figure 7-2 (b) shows the response of the three piles to a cyclic load of the same

amplitude, at 20 kPa confining pressure. This amplitude represented 0.37QU/1.26P,,

0.65Qu/0.99P, and 0.48QJl .l lP, for Tl, S and T2, respectively. As seen in the figure,

the three piles showed excessive upward movement because the load amplitude reached

or exceeded the uplift capacity of the piles. Hence, the load amplitude, as a ratio of the

pile capacity, was reduced in the subsequent tests (i.e. for 40 kPa and 60 kPa confïning

pressures).

7.3. 1.3 40 kPa confining pressure

The response of the three piles to cyclic loading at 40 kPa confinuig pressure is

s h o w in Figure 7-2 (c). The cyclic load had a maximum amplitude equal to 0.32QnU,

0.45QJ0.78Pu and 0.38Q&88PU for piles Tlo S and R, respectively. Although the load

amplitude reached the uplifi capacity of Tl, the upward movement was small. This was

because the uplift behaviour of tapered piles at higher confining pressures is similar to

that of straight sided wall piles in that uplift displacements are not excessive.

7.3. 1.4 60 kPa confining pressure

The response of the three piles at 60 kPa is s h o w in Figure 7-2 (d). The load

amplitude in this case was 0.29Q,,/0.71PU, 0.40Qu/0.65Pu and 0.34QJO.66P. for Tl, S and

R, respectively. It c m be seen fiom the figure that the response is not govemed by the

uplift characteristics in this case, especially for Tl. It may also be observed that the pile

movement did not increase with the repeated loading. This could be attributed to the

"shake down" phenomenon as described in Chapter 2. One possible cause for this

phenomenon is the densification of the sand adjacent to the pile due to the repeated

loading.

7.3.2 Pile Head Stiffness

The pile head load-movement curves in the fmt and the tenth cycles for the three

piles are illutrated in Figure 7-3. As mentioned previously, the tests on piles Tl and S

had the same initial relative density for the sand but were different for T2. The efEect of

the initial state of sand on the pile response was significant, which was manifested by the

substantial difference between the general trend of the response of T2 on the one hand

and Tl and S on the other, as shown in Figure 7-3. However, the emphasis of this study

is to explore the effect of the pile taper, not the initial state of the soil, on the cyclic

response of piles and hence, the performance of T2 was not discussed M e r .

The slope of the hysteretic loop approximated the pile head stiffness in each cycle

(the hysteretic loops for the first and tenth cycles are s h o w in Figure 7-3). Hence, the

pile head stifkess was computed as

K, = ( pc - pt )/ ( dc - dt ) (7 - 1)

where Ki is the pile head stiffiess in the i" cycle, pc and pt are the maximum downward

and uplifi loads applied at the pile head in the i" cycle, respectively, and dc and dt are the

corresponding maximum downward and uplifi displacements at the pile head,

respectively. The stiffhesses of Tl and S were compared in Figure 7-4 and Table 7-2. It

is interesting to note that the cyclic loading had caused the stiffiiesses to increase for both

piles at 0, 40 and 60 kPa connning pressures except the stiffness decreased slightly for

pile S with zero confining pressure. The increase of stiffness was due to the densification

of the sand surrounding the pile that resulted fiom the process of cyclic loading.

However, the stifiess of both piles decreased for 20 kPa confining pressure, it may be

due to excessive cyclic load amplitude, which reached or exceeded the pile uplifi

capacity.

7.3.3 EfTect of Cyclic Load Ampiitude

To M e r investigate the effect of the load amplihide on the performance of

tapered piles, a cyclic loading test with reduced amplitude (O.25Q,,/û. 86P,) was conducted

on Tl at a 20 kPa confining pressure. The pile head load-movement is displayed in Figure

7-Sa. The pile head stiffiess is presented in Figure 7-5b, and for cornparison, the pile

stiffness fiom the fxst set of tests (at the same confining pressure, Le. 20 kPa) is also

presented. It can be observed fiom Figure 7-5a that the final upward movement was

much smaller compared to the movement with the higher load amplitude (Figure 7-2b).

Also, the pile showed some 'shake down" behaviour in this case. Furthemore, the

stifiess in the repeat test showed the same trend observed with 40 and 60 kPa c o n f i i g

pressure values, i. e. it was increased by the sand densification caused by cyclic loading

process. On the contrary, the pile head stiffhess obtained with higher applied load

amplitude detenorated with the increase in the number of cycles. This was attributed to

the excessive displacement of the pile in the uplifi loading phase. Clearly, the loading

magnitude had a signifiant efkct on the pile response. The results also implied that the

behaviour of the pile subjected to cyclic loading was dominated by the compression

characteristics of the pile, as long as the load amplitude did not approach the uplift

capacity of the pile.

The behaviour of piles subjected to compressive and tensile loads, described in

Chapters 4 and 5 showed that tapered piles have higher compression d f h e s s and slightly

lower tensile stiffhess than straight-sided wall piles. The entire cyclic loading procedure

consisted of compression and tension. Therefore, it was expected that higher pile head

stifiess would be obtained for tapered piles than for hgh t - s ided wall piles, especially

when the load amplitude does not approach the pile uplift capacity.

7.3.4 Accumulated Pile Head Movement

The accumulated pile head movernent is plotted vs. the number of cycles for piles

Tl and S in Figure 7-6. The upward accumulation of displacements was less for pile Tl

dian for pile S, except for zero confining pressure.

With zero confinhg pressure, the accumulation of displacements was downward

for pile S and upward for pile Tl. The axial static loading test data (in Chapters 4 and 5)

could explain this behaviour. The residual stresses at the pile tip were higher for S than

for Tl at lower confinulg pressures. These residual stresses resulted in a softer response

in the upward loading phase, and hence larger downward movement. Furthemore,

because of the beneficial effect of the pile taper, which was most significant at zero

confinllig pressure, the downward movement of the tapered pile was less than that of the

straight sided wall pile. Hence, it was reasonable to have an accumulation of downward

displacements for the S pile and upward displacements for TI, with zero confinhg

pressure.

7.3.5 Rate of Pile Movement

The rate of pile movement was defined as the net movement per cycle. To

evaluate the effect of the load amplitude on the cyclic behaviour of tapered piles, the

accumulation of displacement and the rate of pile movement for Tl under two different

load amplitudes with 20 kPa confining pressure were plotted in Figure 7-7. For a cyclic

load with an amplitude 0.37QJ1.26Pu (the load amplitude exceeded the uplift static

capacity of the pile), both the accumulation of displacement and the rate of pile

movement increased in d l loading cycles. when the load amplitude was reduced to

0.25Qd0.86P. the pile response was more stable. The rate of pile rnovement decreased

substantidly (practically diminished), and consequently the total displacement

approximately maintained a constant value starting fiom the third loading cycle. It was

clear that the pile behaviour was highly dependent on the amplitude of the cyclic load. A

similar conclusion was reached in Chan and Hanna (1980). It should be noted that the

load amplitude as a ratio of the pile capacity was the dominating factor. It can be

concluded that the load amplitude should not exceed 25% of the axial capacity and 75%

of the uplifi capacity to ensure satisfactory performance of tapered piles under cyclic

loading.

7.4 SUMMARY

Three Uistnimented mode1 piles W e d in sand enclosed in a soil chamber w w

tested to investigate the effect of the taper on the cyclic response of axially loaded piles.

The confming pressure applied to the sand in the soil chamber was varied fiom zero to 60

kPa in 20 kPa inc~ements. The t h e piles weze subjecîed to 10 twa-way cycles of axial

l&g at each value of cannnllig premue. The load arppIied at the pile head and the pile

head movement were measured simultaneously. The cyclic response of the piles was

investigated and the following conclusions were drawn:

1. The amplitude of the cyclic load has a significant effect on the cyclic response of piles.

It is recommended that the amplitude of the cycIic Ioad should not exceed 25% of the

static axial capacity and 75% of the static uplift capacity of the pile. This requirement is

readily satisfied in most pile foundations as the static component of the load carried by

the pile eliminates or at least padally offsets the cyclic component of the load. In this

case, the performance of tapered piles would be superior to that of straight-sided wall

piles.

2. The stifThess of tapered piles increases with the number of load cycles as long as the

load amplitude does not approach the uplift capacity of the pile. This behaviour rnay be

attributed to the densification of the sand surrounding the pile. However, this may not be

the case for tapered piles uistalled in dense sand.

3. Residual stresses developed during the downward loading phase and their efYect are

more significant on the cyclic response of straight-sided wall piles at higher confining

pressure. This may not be the case for piles insralled in dense sand.

Table 7-1 Amplitude o f Cyclic Load Applied at Pile Head

Pu = the siatic ultimatc load in tension **: The absolule maximum load applied at pile head = 2.3 l kN.

Table 7-2 Pile Head Stiffness ( MNlm)

60 kPa 0.40Qu/0.65Pu 0.3SQidO.66Pii 0.29Qi1lO.7 1 Pu

*: Qu = tlic stalic ul~imate load in compression

Confining Prcssurc Applicd Pile S Pilc T2 Pik TI Pilc TI

20 kPa 0.65QidO.99Pu O.48QidI. I I Pu 0.37QidI .26Pii

Oq25Qid0.86Pii**

0 kPa 0.50Qu/0.72Pu* 0.42QuN.8 1 Pu O. 3 5Qtd0.88 Pu

Applied C ,.--------, - - ------ -

l ~ i l c TI The f i n t cyclc 1 1 25.99 1 1 1

40 kPa 0.45Qd0.78Pu 0.38Qiil0.88Pu 0.32Qii/1 .OPu

Pilc S Tlic first qclc Tlic tcn~li cycle

Tiic icnili mclc

I Tlic tenili cycle 1 1 47.70 1 1 1

Pile TI - - - - - . . - . . - - . .

I -

1 I

16.18 13.82 7.6 1 21.9

6.9 1 I .7J

85.92 121.55

8.57 2.55

26.17 27.98 23.76 38.37

93.13 155.4

Figure 7- 1 (a) Characteristics of cyclic load at different values of confining pressure ( T I , T2 and S)

Figure 7-1 (b) Characteristics of cyclic load applied to pile Tl at 20 kPa confining pressure

Displacement (mm)

-0.5 -0.4 -0.3 -0.2 O. 1 O 0.1

Displaccmcrit (m)

4.6 -0.5 -0.4 -0.3 4-2 4 . 1 O 0.1

0.8 t

Figure 7-2 (a) Load-movernent curves for piles TI, T2and S a? zero confining pressure

Displacement (mm) -6 -5 4 -3 -2 -1 O 1

Pile T l

I IV :

Displacement (mm)

d -5 4 -3 -2 - 1 O 1 2 3

Figure 7-2 (b) Load-movement curves for piles Tl. T2and S at 20 kPa confining pressure

Displacemmt {mm)

-0.3 -0.2 4.1 O 0.1 0.2 0.3

Displacement (m)

-0.3 -0.2 -0.1 O 0.1 0.2 0.3

.- Pile T2

f 2 . -

F -

-4

I

Figure 7-2 (c) Load-rnovement curves for piles Tl, T2and S at 40 kPa confining pressure

4 - -

Pile S

Displacement (rini)

4

L

g 2

z - n O n iO 'b 8 -2 A

4 Pile T l

Displacement (mm)

-3 -2 - 1 O 1

1

Figure 7-2 (d) Load-movernent curves for piles Tl. TZand S at 60 kPa confining pressure

j -pite s - - - P ~ I ~ T Z ..--- - Pile Tl

Disphcement (m)

-0.6 4.4 -0.2 O 0.2 7

" 1st cycle

L c

1 I

1 1

/

/ /'

/

1 st cycle

i

Displacernent (nm)

-5 4 -3 -2 -1 O 1 2 3

Figure 7-3 Pile head load-movement curves in the first and the tenth cycles

1 P i l e s --- Pile T2 * - * - - -Pile Tl ,

Displacement (mm)

-1 -0.5 O 0.5 1

1st cycle

I I

1

I I . I /

I / 1' ;

I I

f

/ 0

/

,, Y

Displacement (mm)

-0.5 O 0.5 6 1

Figure 7-3 (continued)

1 2 3 4 5 6 7 8 9 1 0

Number of cycles

1 2 3 4 5 6 7 8 9 1 0 Nurnber of cycles

1 2 3 4 5 6 7 8 9 1 0

Number of cycles

1 2 3 4 5 6 7 8 9 1 0 Numbcr of cydes

Figure 7-4 Pile head stiffhess with different confining pressure for Tl and S

Displacement (mm)

0.2 O. 1 O

Load amplitude: 2.31 kN

Figure 7-5 (a) Load-pile head movement curve of pile Tl at decreased loading amplitude under 20 Wa confining pressure

i + Load amplttude: 3.38 kN

-4- Load amplitude: 2.31 kN ! A

1 2 3 4 5 6 7 8 9 10 Number of cycles

S

Figure 7-5 (b) Cornparison of pile Tl head stifiess at 20 kPa confining pressure subjected to different load amplitudes

Numkr of cycles

0.2 1

Number of cycles

O 2 4 6 8 1 0 1 2

N u n k r of cycles

O 2 4 6 8 10

Number of cycles

Figure 7-6 Accumulation of displacements for piles Tl and S under different confining pressures

r

i A Load amplitude: 3.38 LN X Load amplitude: 231 kN 1 Nunber of cycles

1 2 3 4 5 6 7 8 9 10

Number of cycles

1 2 3 4 5 6 7 8 9 1 O

Figure 7-7 Pile T 1 head movement under cyclic load with diKerent amplitude a: accumulation b: rate of displacernent

CHAPTER 8

VALIDITY AND APPLICATION OF TEE STUDY

The curent chapter has two objectives. First, the validity of the experimental

study was to be verified by comparing the bearing capacity of straight-sided wall piles

predicted fiom the experiments with the bearing capacity established using the standard

design procedures. Second, the experimental results were interpreted to develop a

procedure for the design of tapered piles with taper angles similar to those considered in

the study.

8.1 INTERPREITATION OF TESTING RESULTS

The strain measurements taken during the Ioading tests were used to predict the

load distribution dong the piles. The load distribution was then w d to determine the

shah fiction (in terms of unit load transfer to the soil). The unit load tramfer for piles Tl

and T2 in loose sand and piles Tl and S in medium dense sand were shown in Figures 4-

10 and 4-1 1 (in Chapter 4) for compression, respectively; the results of the shaft fiction

were given in Figures 5-8 and 5-9 (in Chapter 5) in tension. The unit load transfer curves

were obtained at different confining pressures.

ï h e unit load transfer was averaged for each value of confining pressure. Also, the

readings fiom the soil pressure transducers (Figure 3-7) were used to calculate the

average lateral stress comsponding to each value of confining pressure applied to the soil

chamber. The average unit load transfet in compression and tension was plotted versus

the average lateral pressure in Figures 8-l(a) and 8-l(b), respectively. These plots were

cuve fitted to introduce a mild degree of smoothing to the scatter of experimental results

and to allow a continuous relationship between the shaft fiction and lateral pressure. T'bis

continuous relationship could be used to calculate the shaft fiction dong the shaft of

prototype piles.

The pile S tip resistance in second group of tests was determined from the load

distribution obtained from the strain measurements during the loading tests. The unit tip

resistance was then calculated by dividing the tip resistance by the area of the pile tip.

Figure 8-2(a) shows the variation of the unit tip resistance with the measured vertical

stress at the pile tip. As can be seen fiom the figure, the unit tip resistance increased with

the vertical stress up to 20 kPa and then decreased. Figure 8-2(b) shows the measured

vertical stresses and the vertical stresses for the prototype pile (consistent with the

measured lateral stresses assuming normally consolidated soil) vs. the applied confining

pressure. It can be seen from Figure 8-2(b) that the vertical stress was representative of

the prototype state of stress (for normally consolidated soil) up to 20 kPa. As the

confinhg pressure increased, the measured vertical stresses were much Iess than the

prototype messes. Hence, the erroneous trend of the unit tip resistance noted in Figure 8-

2(a) rnay be attributed to the inappropriate modeling of the vertical stress in this

experimental setup. However, the focus of this research was the effect of the pile taper on

the shafi resistance. Therefore, al1 the cornparisons and calculations will be confined to

the shaft resistance.

8.2 VALIDITY OF MODEL TESTING RESULTS

The shaft resistance of cylindncal piles in cohesionless soils stems mainly fiom

the friction between the soil and the pile skin. The sh& resistance, Qs, can be caiculated

assumuig Coulomb friction, i.e.

where K, = coefficient of lateral earîh pre

(8- 1

ssure, 4 = angle of interna1 fiction of the sand,

cv = the vertical stress, 6 = 41 -5' = the friction angle between the soil and the pile

material, p = pile perimeter and L = pile length. The magnitude of Ks varies with depth

(state of stress) and depends on the installation method of piles. For bored piles (close to

the installation method used in the experiments), K, can be calculated as

Ks = I -sin( (8-2)

Equation (8- 1) is used to calculate the shafl resistance in the design of straight-sided wall

piles installed in cohesionless soils. Alternatively, the shaft resistance can be calculated

fiom the unit shaft fiction, i.e.

where f = unit ultimate shaft fiction which varies with depth. Equation (8-3) is

equivalent of Eq. (8-1) as the unit shaft fiction equals

f = K s a , tan6 (8-4)

Equation (8-3) can be readily used to calculate the shaft resistance of a prototype pile

fiom the experimentally evaluated unit SM fiction.

In cohesionless soils, it has been customary to assume rather lower shaft

resistance capacity under tensile loading than compressive loading (Nicola and Randolph,

1993), the ratio of tensile and compressive shaft capacity varied with an average of about

0.7, e. g.

QIy = 0.7 Qs

where Q,' is tensile shaft resistance and Q, is compressive shaft resistance.

The validity of the mode1 test redts could be assessed by comparing the sh&

resistance of a prototype straight-sided wall pile estabiished fkom both the standard

procedure, i.e. Eqs. (8- 1) and (8-5), and the experimentally evduated unit shaft fiction,

i. e. Eq. (8-3). The parameters used in the analysis were 4 = 35*, y = 16.57 kN/m3 and f

obtauied fiom Figures 4-10, 4-1 1, 5-8 and 5-9. The variation of the shaft resistance of a

prototype pile predicted from the two approaches with the pile length is shown in Fig. 8-

3. It cm be seen fiom the figure that the shaft resistance denved fiom the experimental

results compared well with the theoretical predictions, indicating the reliabiiity of the

experimental results.

8.3 SHAFT RESISTANCE OF TAPERED PILES

The shaft resistance of tapered piles stems fkom two mechanisms: the Wctional

resistance (i.e. shear resistance dong the pile skin) and the wedging effect due to the pile

taper (i.e. direct bearing on the projected area of the pile shaft). However, it would be

useful to express the shaft resistance of tapered piles in a format similar to that used for

straight sided wall piles, i.e.

Qs (tapered) = [ K, Kp,, ta6 p &

where Kr is a coefficient that represents the eEect of the pile taper on the shaft resistance.

K, is defined as

Unit shafi &idon of tapered piles Kr = Unit SM fiction of straight - sided wall pile

It was found nom the experirnentai results that Kt depended on the taper angle, the

density of the sand and the lateral earth pressure. There were two more factors that might

affect Kr significantly but they were not investigated in this study: the method of pile

installation and the pile materiai.

8.3. 1 Relativeiy Medium Dense Sand Case

The coefficient Ki was calculated fiom the experimental results as the ratio

between the average unit shaf? fiction of the tapered pile, at a given value of the

confining pressure, and the average unit shaft fiction of the straight-sided wall pile, at the

sarne confining pressure. The variation of Kt for pile Tl with the lateral pressure is shown

in Fig. 8-4 for both compression and tension. It can be seen f?om the figure that in

compression, Kr varied b e ~ e e n 1.62-1.1 with a slow rate of reduction meaning that the

effect ?f the taper decreased with the depth but its effect was still significant. In tension,

Kr varied between 0.62-0.95, however, the rate of increase was large, indicating that the

negative effect of pile taper on the uplift resistance diminished quickly with lateral earth

pressure. Consequently, the uplift capacity of prototype tapered piles (with larger depth)

would be comparable to that of straight sided wail piles. The shaft resistance of a

prototype straight sided wall pile and a tapered pile (0.95' taper) in relative medium dense

sand was established nom the experimental results using Eq. (8-6) and is s h o w in Figs.

8-5(a) and (b) for both compression and tension.

8.3.2 Loose Sand Case

The same procedure was used to obtain coefficients Kt for piles Tl and T2 in the

axial compressive and tensile load in loose sand conditions. Since the experimental

results were not available for straight-sided wall pile, the unit shaft fiction of pile S was

estimated from theoretical calculation, the parameters used in the analysis were 4 = 32', y

= 16.32 kN/m3. Variations of Kt with the lateral pressure are shown in Fig. 8-6 for piles

TI and T2. It can be seen fiom the figure that the pile taper effect decreases with the

depth for the compression case; in the tension case, the negative effect of pile taper on the

uplift resistance diminished quickly with lateral earth pressure. Hence, in the case of

loose sand, the taper effect on the bearing capacity of prototype piles would be

tremendous and not significant in uplift capacity. The cornparison between two sets of

results (dense and loose sand case) suggested that the coefficient K, was higher for 0.95'

taper angle pile in dense sand under both compression and tension. Figs. 8-7 (a) and (b)

showed the shaft resistance of a prototype straight-sided wall pile and two tapered piles

(0.95' taper and 0.6' taper) in the loose sand case.

8. 4 DISCUSSION

8.4.1 The State of Stress Inside of the Soi1 Chamber

It is important to note that the purpose of this device is to mode1 the state of lateral

stress along different "segments" of the pile rather than the state of stress along the entire

length of the pile. Hence, the state of stress in the soi1 .surrounding the pile in the soil \

chamber when contining pressures were applied ciifferrd from the state of stress that

existed at larger soil depths in two aspects: the lateml stress was almost uniform in the

soil chamber while it varies parabolidy in a normally wnsolidated sand; and vertical

stresses in the soil chamber were not equal to the vertical stresses in deeper soi1 deposits

condition did not exist). However, as mentioned before the focus of this research was

the effect of the pile taper on the shafi resistance under axial loading, where lateral stress

is the key factor. In the lateral loading case, the response of piles is strongly controlled by

the response of the top most soi1 layers, up to a depth corresponding to 10-1 5D where D

is the pile diameter. The state of stress for this depth was correctly modeled within this

experimentai program (Le. 0-20 kPa confining pressure). For longer piles, the results

fiom higher confining pressure values can be used approximately in terms of p-y curves

(at the point of maximum moment) to represent the soil resistance at a depth

representative of the confining pressure at which the p-y cuve was obtained.

8.4.2 Boundary Effect

In the loading facilities the stiffening efTect of the wall of the soii chamber was

considered to be small for two reasons: f k t , the wall was at a distance greater than 4D

fiom the pile exterior wall; second, the region of high strain at the level of loading

sustained in this experimentai program was considered to be confined to the close vicinity

of the pile and hence the boundary effect was not significant.

8.5 SUMMARY

The resdts of axial and uplifi load tests on straight-sided and tapered piles were

analyzed to assess the validity of the experimental sehip and to develop a procedure to

calculate the shafi resistance of tapered piles based on the experimentd results. The

following conclusions were drawn:

1. The shaft resistance for straight-sided w d piles established fkom the experimental

results compared well with the theoretical predictions using the standard design

procedure;

2. The negative effect of the pile taper on the uplift capacity diminished quickly with

depth and hence the uplift capacity of prototype pile is expected to be comparable to that

of straight-sided wall piles; and

3. The VLPSC appeared to be a valid device for assessing pile shaft resistance and Ioad

transfer patterns in piles in granuiar soils.

Lateral pressure (kPa)

(b)

Figure 8- 1 Average unit load transfer (a) in compression (b) in tension

t O a) 30 40 50 60 Vertical stress it pile tip (Wa)

'-t O Measured Prototype ' i

0 2 0 4 0 6 0 8 0 1 0 0

Applied confining pressure (kPa)

Figure 8-2 (a) Unit tip resistance @) Measured and prototype vertical stress

O 2 4 6 10

Length (ml

Figure 8-3 Cornparison of pile shaft resistance established from experiment and theory a: in compression b: in tension

O-" t O 10 20 30 40 50 60 70

Lateral pressure (kPa)

Figure 8 4 Variation of Kt for pile TI with lateral pressure (relative medium dense sand)

1 ; t Tapered pile + S m sided buail pile 1

Figure 8-5 The shaft resistance of prototype piles established from experimentai results in relative medium dense sand a: in compression b: in tension

loose sand

- O 5 1 O 15 20 25 30 35

kteral pressure (kPa)

Figure 8-6 Variation of Kt for pile TI and T2 with lateral pressure (loose sand)

Compression Sand: Dr=1 8A0r6,

Pile Tl : 0.95' taper Pile T2: 0.6" taper

O 1 2 3 4 5 6

Cength (m)

Tension Sand: Dr=18.4 %,

Pile T l : 0.95' taper Pile T2: 0.6' taper

Figure 8-7 The shaft resistance of prototype piles established frorn expenmental results a: in compression b: in tension ( Loose sand)

CHAPTER 9

CONCLUSIONS AND RECOMMENDATIONS

A relatively large laboratory facility for testing mode1 piles was developed, three

imtrumented steel piles with difTerent degrees of taper were tested in this study. The

study suggests the following conclusions and recommendations:

Tapered piles have a substantial advantage with regard to their load-cmying

capacity in the downward fictionai mode. It was found that the shaft resistance of the

tapered pile was up to 40 % larger than that of the straight sided-wall pile and as the taper

angle increased the shaft resistance increased. The merence in the shaft resistance of the

two types decreased for higher values of confining pressure. It was also found that the

load distribution dong the pile shaft for both pile types had the same pattern. However,

this pattern varied as the confining pressure increased. Furthemore, the unit load transfer

was significantly afTected by the initiai sand density for both pile types at low confining

pressure, but as the confining pressure increased this effect diminished. It is concluded

that the tapered piles offer a larger resistance than the straight-sided wall piles.

The results of uplift investigation indicated that the pile axial uplift capacity

increased with an increase in the confïning pressure for all piles examined in this study.

The ratios of uplift to compressive load and load transfer patterns for tapered piles were

less than those for dght-s ided wall piles of the same length and average embedded

diameter. The uplift capacity of tapered piles was found to be comparable to that of

straïght-sided wall piles at higher confining pressure values, suggesting that the

performance of achial tapered piles (with greater length) would be comparable to îhat of

maight-sided wall piles. Also, the results indicated that residual stresses developed

during the pushing downward loading phase and their effect was more significant on the

initial uplift capacity of piles. Aiso, this effect was more pronounced for straight-sided

wall piles in dense sand.

Pile lateral loading tests were conducted to establish the lateral behaviour of

tapered piles in cohesionless soil. It was found that tapered piles carried up to 77% more

lateral load than straight-sided wall piles with the same average diameter. The maximum

bending moment occurred in d l piles at alrnost the same depth of one third of the

embedded length of the pile. Hence, the cross-section of tapered piles at the location of

maximum bending moment was larger than that of straight-sided wall piles, resulting in

lower stresses in the pile. It was concluded that the tapered piles represent a more

efficient distribution of the pile materiai, and display a better performance under laterai

loading conditions.

The characteristics of the cyclic response of tapered piles were established from

experimental investigation. The results of this study indicated that the pile stifiess

increased through cyclic loading due to the densification of the sand surrounding the pile

when the load amplitude is limited to a certain range. The amplitude of the cyclic load

had a sipifkant effect on the performance of the piles. As a resdt, it is recommended

that the amplitude of the cyclic load be iimited to 25% of the static axial capacity and

75% of the static uplift capacity to ensure satisfactory perfomiance of tapered piles. This

requirement is readily satisfied in the design of most piles. In this case, the performance

of tapered piles under cyclic axial load was found to be supenor to that of straight-sided

wall piles. Also, the results indicated that residual stresses developed during the

downward-loading phase and their effect were more significant on the cyclic behaviour of

straight-sided wall piles at higher confining pressure.

The Iast part of this study presented the validity of this study and developed the

procedure to calculate the shaft resistance for tapered piles.

Recornrnendations for fiiture research are:

1. Additional experimental investigation of tapered pile behaviour is very desirable,

particularly under higher confining pressure (higher than 100 kPa) to complete the

prediction of tapered pile shaft resistance by laboratory testing;

2. Future research is needed on the distribution of stress in sand column;

3. There were two more factors that might affect tapered pile behaviour significantly and

need to be investigated: the method of pile installation and the pile material;

4. Theoretical research is needed to cla* the complex features of tapered piles predicted

fiom the experimental investigation;

5. To investigate the effect of pile taper on pile dynarnic response.

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