UNIVERSITY OF SHEFFIELD

DEPARTMENT OF CIVIL AND STRUCTURAL ENGINEERING

FURTHER STUDIES ON THE REPEATED LOADING OF PILES IN SAND

by

Fouad Ahmed A1-JumailY,B.Sc., M.Sc.

Thesis, submitted to the University of Sheffield for the Degree of Doctor of Philosophy.

July, 1981 v.

i

T 0

THE MEMORY OF MY FATHER

ACKNOWLEDGEMENTS

The work embodied in this thesis was carried out in the

Department of Civil and Structural Engineering of the University

of Sheffield, where the equipment, materials and facilities used

were gratefully provided.

The author wishes to express his sincere appreciation to

Professor T. H. Hanna for the opportunity of working with him and

for his invaluable advice, guidance and continual encouragement

throughout the work. The help of his Soil Mechanics group is gratefully

acknowledged.

Without the financial support of the Iraqi Government,

(University of Baghdad) and the encouragement and assistance of his

wife, this thesis would not have been possible, to whom the author

is obliged.

Much assistance was given to the,author during the research

programme by many technicians in the department. For this, he

would like to thank Messrs. D. J. Webster and P. Osborne. Thanks

are also due to Mrs J veasey for her skill and speed in typing the

thesis, to Mr J Biggin and Miss W Atkinson for their help in the

preparation of the drawings, to Mrs D Hutson for the photographic

prints, to the library and computing staff for their assistance

and to the author's colleagues for their constructive comments.

11

SUMMARY

The work presented in this thesis concerns the behaviour of

isolated piles subjected to repeated loading and placed at

various depths in a medium dense sand upon which either static or

cyclic surcharge acted. The piles, which were of laboratory scale,

were instrumented by strain gauged load cells located along the

inner surfaces of the pile shafts. The behaviour of tension as

well as compression piles was examined. It was found that the

behaviour of the pile was governed to a large extent by the repeated

load level, the number of load cycles and the initial boundary

stress conditions existing along the pile shaft. In compression,

the pile life-span decreased when the embedment depth increased

while the reverse trend was observed,for tension piles. The

movement of both tension and compression piles decreased when the

surcharge pressure was increased or was cycled, and it was of a

minimum value when the upper repeated load acted in-phase with the

higher surcharge pressure. For tests performed with static

surcharge pressure, repeated loading was found to decrease the

bearing capacity and the pulling resistance of the pile. ~le higher

percentage of reduction was recorded for the tension pile. In

contrast, after cyclic surcharge tests the pile capacity always

increased.

At any depth of embedment or surcharge pressure,' as the nun-her of

load cycles was increased the shaft load of a compression pile

increased up to a peak value then decreased gradually until it

reached a limiting value. This limiting value increased when the

load level, the pile depth or the surcharge pressure was increased

and it was independent of the pile loading history.

iii

For a tension pile the shaft load decreased progressively as the

number of cycles increased until failure occurred.

,

iv

CONTENTS

TITLE

ACKNOWLEDGEMENTS

SUMMARY

CONTENTS til '. ,

LIST OF FIGURES

LIST OF TABLES

CHAPTER 1

INTRODUCTION

1.1 1.2

CHAPTER 2

The Need for the Investigation. The Scope of the Investigation.

REVIEW AND DISCUSSION ",.OP SIGNIFICANT RELA TED WORKS . . 2.1 2.2 2.3

Introduction Repeated Loading on Metals. The Behaviour of Soil.

2.3.1. 2.3.2.

Under Static Loading. Under. Repeated ~ading.

;/

2.4 2.5

Repeated Loadings on Foundations Other Than Piled. The Behaviour of Isolated piles.

CHAPTER 3

2.5.1. Under Static Loading. " 2.5.1.1. Residual Stresses.

2.5.2. Under Repeated Loadings.

LABORATORY INVESTIGATION OF THE BEHAVIOUR OF A PILE ELEMENT EMBEDDED IN A TRIAXIAL SPECIMEN.

3.1 3.2 3.3 3.4 3.5

CHAPTER 4

Introduction. The Test Apparatus and Procedures. Test Progranune. Presentation and Discussion of Results. Conclusion.

THE TEST APPARATUS AND MATERIALS.

4.1 Arrangement of the Apparatus.

v

Page

i

ii

iii

v

viii

xiv

1 2

7 7 8

8 10

12 15

15 18 19

23 23 26 26 32

34

4.2 The Sand Container. 4.3 The Sand Hopper 4.4 Arrangement of Repeated Loading. 4.5 The Surcharge Pressure.

General The Pressure Controller.

4.5.1. 4.5.2. 4.5.3. The Magnitude of the Surcharge

Pressure. .. -

4.6 The Test Piles.

4.6.1. 4.6.2. 4.6.3. 4.6.4. 4.6.5.

General Measurement of the Pile Movement. Measurement of Pile Forces. Gauging Techniques. Calibration.

4.7 The Data Logger.

CHAPTER 5

EXPERIMENTAL PROCEDURES AND TESTING PROGRAMME.

5.1 5.2

5.3 5.4 5.5 5.6 5.7 5.8 5.9

CHAPTER 6

Properties and Sand Placement Assembly of Apparatus and Application of Surcharge Pressure. Depth of Embedment. Pile Driving. Static Loading Tests. Repeated Loading. Re-leve1ling of the Lever Arm Collecting of Data and Analysis. Test Programme.

PRESENTATION AND DISCUSSION OF STATIC-SURCHARGE TEST RESULTS.

6.1 6.2

6.3

General. Series 1: Preliminary Tests.

6.2.1. 6.2.2.

Series 11:

6.3.1. 6.3.2.

6.3.3.

Driving Resistance. Behaviour of Piles Under Static Loadings.

Compressi~e Repeated Loads with a Constant surcha2ge Pressure Equal to 100 kN/m •

Effect of Embedment Depth. Effect of S~atic Failure Loading on the Subsequent Beahviour of Piles Subjected to Repeated Loadings. Various Load Levels and Amplitudes.

vi

...

Page

34 35 35 36

36 37

38

39

39 39 39 40 42

42

44

45 46 46 47 47 48 48 49

54 5S 5S

58

62 63

65 6S

6.4 6.5 6.6 6~:7 6.8 6.9

CHAPTER 7

Series 111: Series lV: Series V: Series Vl: Series Vll: Conclusion.

Tensile Repeated Loadings. Effect of Surcharge Pressure.

Varying Load-Amplitudes. Compressive-to-Tensile Repeated Loads

Miscellaneous.

Page

67 70 71 75 76 78

PRESENTATION AND DISCUSSION OF CYCLIC SURCHARGE TEST RESULTS.

7.1 General 83 7.2 Piles Subjected to Static Loading. 84 7.3 Repeated Loads In-Phase with the Cyclic Surcharge. 88 7.4 Influence of Surcharge and Pile Loading Frequencies

on Pile Behaviour. 90 7.5 Various Combination of Loadings and Different States

of Surcharge Pressures. 91 7.6 Conclusion. 96

CHAPTER 8

MAIN CONCLUSIONS AND SUGGESTIONS, FOR FUTURE ''lORK.

6.1 6.2

REFERENCES

Main Conclusions. Suggestions for Future Work.

APPENDIX A: Measurement of Axial Loads in Vertical Piles

vii

99 103

105

116

Figure

2.1

2.2

2.3

2.4

2.5

2.6

2.7

3.1

3.2

3.3

3.4

--3.5

3.6

3.7

4.1

4.2

LIST OF FIGURES

'Title

Influence of stres~ path on strains to develop peak stress ratio of n01~ally consolidated weald clay.

Voids ratio versus average shear strain (~)

measured at the central third of dense sand tested in simple shear apparatus.

Schematic illustration of contribution of sliding friction, dilatancy and crushing to the measured Mohr envelope for drained tests on sand.

Stress distribution around a deep foundation in sand.

Lond-settlement curves for compression loading.

Changes in the distribution of shear-stresses on the shaft of pile (A) caused by soil movement set-up during the installation of pile (B).

Mechanistic interpretation of cyclic degradation process in axial pile-soil interaction.

Laboratory miniature pile test apparatus.

The sand sample after applying back pressure and removing the mould.

Procedure of loading.

Gene,ra1 view of the apparatus. .

Load versus displacement (compression).

Stress-strain relationship of compression tests.

Load versus displacement (tension).

The test apparatus ••

OVerall view of tll~a'n t:llJ'for repeated loading tests.

viii

Figure

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

5.1

5.2

5.3

5.4

6.1

6.2

6.3

6.4

6.5

6.6

6.7

'Title,

Section through test rig.

Arrangement for applying repeated loads.

Rollers for preventing accidental lever movement.

Schematic diagram of pressure controller.

Model piles.

Typical strain gauge circuits.

Model pile before and after assembly.

Measurement of pile movements.

S,train gauge mounting device.

General,arrangement of calibration of load-cell. '

Typical calibration results for lOud cells.

Particle'size distribution of sand.

Details of the swinging jack and electric motor.

Two positions of the jack allowing for pile insertion •.

Diagrammatic detail of the loading lever·

Variation in dtiving resistance of pile base with depth.

Influence of surcharge pressure on the base penetration resistance.

Variation of skin friction with depth of penetration.

Load-displacement behaviour for compression load tests.

Variation of ultimate resistance with depth.

Load-displacement behaviour for tension load tests.

Variation of ultimate shaft friction with depth.

ix

Figures

6.8

. 6.9

6.10

6.11

6.12

6.13

6.14

6.15

6.16

6.17

6.18

6.19

6.20

6.21a

6.2lb

6.22

6.23

6.24

Title

Influence of the surcharge pressure on the load-displacement behaviour of the pile.

Variation of ultimate capacity with surcharge pressure.

Variation of residual base load with surcharge pressure and with depth.

Influence of embedment depth on the pile life-span.

Influence of embedment depth on the pile life-span.

Variation of pile loads and rates of movement with log number of cycles.

Distribution of axial load and skin friction during selected cycles of Test 11.

Pile loads, movements and rates of movement again log. cycles (lId = 30).

, Effect of static failure loading on the subsequent behaviour under repeated loading (lId = 30).

Effect of static failure loading on the load­transfer of the pile during subsequent repeated loading (lId = 30).

Movement against log. number of load cycles (lId = 30).

Movement against log. number of load cycles (lId = 20).

Movement and rate of movement against log. number of load cycles (lId = 20).

Load-displacement behaviour (lId = 20).

Load.transfer during static load tests (lId = 20).

Variation of loads, movements and rates of movement against number of cycles.

Distribution of axial load and skin friction during selected cycles of Test 21.

Variation of loads, movements and rates of movement against number of cycles.

x

Figures

6.25

6.26

6.27

6.28

6.29

6.30

6.31

·6.32

6.33

6.34

6.35

6.36

6.37

6.39

6.40

6.41 -

. Title

Influence of embedment depth on the life­span of tension piles.

Variation of loads, movements and rates of movement against number of cycles.

Variation of loads, movements and rates of movement against number of cycles.

Variation of the rate of movement with the number of load cycles.

Load-displacement behaviour of tension pile (lid = 30).

Influence of surcharge on the behaviour of pile subjected to repeated compo loads.

Influence of surCharge pressure on the life­span on tension piles.

L~ad-displacement.relationship (lid = 20).

Variation of movements and rates of movement with log. number of cycles.

Variation of movements and rates of movement with log· number of cycles (lid = 30).

Variation of movements and rates of movement with log. nurr~er of cycles.

Variation of base load with log. number of cycles.

Variation of the pile loads with log number of cycles.

Variation of the pile loads with log number of cycles (lid = 30).

Variation of skin friction during repeated loadings.

Variation of movements and rates of movement with number of cycles.

Variation of movements and rates of movement with number of cycles.

xi

Figures

6.42

6.43

6.44

6.45

6.46

6.47

7.1

7.2

7.3

7.4

7.5

7.6

7.7

7.8

7.9

7.10

7.11

7.12

7.13

, 'Title

Load-transfer during repeated load of varying load-amplitudes.

Variation of movements and rates of movement with number of cycles.

Distribution of skin friction during O.lSQc/ O.lSQt repeated loading.

Variation of loads, movements and rates of movement against number of cycles.

Variation of loads and movements with number of cycles (l/d = 20).

Results of creep test.

Variation of loads movements and rates of movement with number of cycles.

Distribution of axial loading during cyclic surcharge.

/ '

Distribution of skin friction during cyclic surcharge.

Load-displacement.

variation of loads, movements and rates of JIX>vement with number of cycles.

Distribution of axial loading during cyclic surcharge.

Distribution of skin friction during cyclic surcharge.

Influence of, static failure loading on the subsequent behaviour of the pile.

Variation of loads, movements and rates of movement with number of cycles.

Variation of loads, movements and rates of movement with number of cycles.

Distribution of skin friction during cyclic surcharge.

Load-displacement b~aviour of tension piles.

Variation of loads, movements and rates of movement with nunber of cycles.

xii

Figures

7.14

7.15

7.16

7.17

7.18

7.19

7.20

7.21

7.22

7.23

7.24

7.25

7.26

._7·27

7.28

7.29

7.30

'Title

Variation of loads, movements and rates of movement with number of cycles.

Variation of loads, movements and rates of movement with number of cycles.

Influence of cyclic surcharge on the life­span of a pile subjected to repeated loading.

Variation of loads and rates of movement with number of cycles.

Influence of cyclic surcharge on the load­displacement behaviour of piles.

Influence of frequency on movement and rates of movement.

Influence of frequency on the load-transfer behaviour.

variation of loads, movements and rates of movement with number of cycles.

Influence of previous loading on the behaviour of piles.

Influence of previous loading on the behaviour of piles.

Variation of movement with log number of cycles.

Variation of loads, movements and rates of movement :wi th number of cycles.

Influence of cyclic surcharge on the life-span of piles.

Variation of movement and rate of movement with number of cycles.

Distribution of skin fr.iction (lId = 30, Test No 50).

Variation of loads, movements and rates of movement with number of cycles.

Variation of loads, movements and rates of movement with number of cycles.

xiii

Table 3.1

Table 5.1

Table 5.2

LIST ,'OF TABLES

Test pr,ogramme.

Static Surcharge Test Programme.

Cyclic Surcharge Test Programme.

, /

xiv

CHAPTER 1

INTRODUCTION

1.1 The Need for the Investigation

The loads on piles, in general, have two main forms, dead

and live. The former comprises the self weight of the structure,

foundation, and all other loadings that are independent of time,

whilst the latter includes loadings which are time dependent such

as wind' and wave pressures and other moving loadings.

The repetition and ratio of live to dead load may in certain

circumstances be so small that the pile experiences undetectable

changes in behaviour. In contrast, the drastic variations in the

system of forces that act upon off-shore structures and the high

live to dead load ratio plus the large number of loading cycles may

cause a sudden change in the performance of foundation piles :

(McClelland 1974). Not only off-shore structures encounter high .'. .

live to dead ratios but also narrow tall buildings, transmission

t~~ers, long span suspension bridges, and silos. During the life-

time, each of these structures is liable to a loading randomly

repeated in one or two directions with different~litudes. There-

fore the stability of these structures might also be severely

affected. Failure of piles under repeated loading was the subject

of several authors whose works are described in the following

paragraphs.

Chan (1976) described the state of failure in compression

model piles subjected to repeated loading as follows. Initially

the pile was stable, but after a certain number of cycles, which

increased with decrease of the load amplitude, the rate of pile

1

movement began to increase rapidly until another stable stage was

reached. In the case of tension piles~ be found that the rate

of movement initially decreased then after a stable stage it began

to increase until the pile was pulled out.

Matlock (1979) indicated that the resistance along the upper

portion of a pile subjected to tensile repeated loading was

progressively lost. The field observations by Kraft et al (1981)

showed that the load transfer Characteristics of piles were affected

when the loading was repeated. Even the theoretical study by

Poulos (1981) revealed that under the influence of repeated loading

both the ultimate capacity and the load transfer of the pile are

affected.

A search through the literature indicates that only a very

limited area of this wide subject of. repeated loading has been

investigated.

1.2 The Scope of the Investigation

In view of the limited published information the following

topics have'been considered in the present investigation:-

~) The influence of pile depth on the life span

of compression and tension piles when:~

(i) The pile is subjected to repeated

loading while the sand surface is

acted upon by a constant surcharge.

'(ii) The pile is subjected to sustained

loading while the sand surface is

acted upon by a surCharge cycled

between two limits.

2 '

(iii) The pile is subjected to repeated

loading while the sand surface is

acted upon by a surcharge cycled between

two 'limi ts. This type of loading

consists of the following three modes

of testing.'

a) The application of the cyclic

surcharge on the sand surface is

independent of the repeated load

level of the pile;

b) the application of the high cyclic

surcharge pressure is in phase with

the high repeated load level of the

pile, and

c) the application of the high cyclic

surcharge pressure is in phase with

the low repeated load level of the

pile.

(iv) Different combinations of the previous states of

loading.

ill) The influence of the previous states of loading on the

load displacement response of the pile.

~) The influence of the previous states of loading on the

load transfer characteristics of the pile.

To carry out such a research programme there are three main

approaches:-

(A) Theoretical approach.

(B) Full-scale field tests.

3

(c) Laboratory-scale model tests.

Unfortunately, soil is a very intricate medium. It is neither

homogeneous nor isotropic, "it is inherently a particulate system"

as Lambe and Whitman (1969) said when they differentiated between

solid, fluid and soil mechanics. Therefore, theoretical approaches

which treat the pile-soil system as a rigid plastic, perfectly

elastic or elastic up to failure and then yielded as a plastic

material are unlikely to lead to a proper solution. Although the

second approach is the more reliable for quantitative results it

has the following 1imitations:-

(i) Time of conducting the previous research programme.

(ii) Difficulties in conducting such a research programme.

(iii)The cost.

Therefore the only approach which can be adopted is the model

one. Hodel tests, in general, involve certain limitations. These

limitations are mostly due to the similarity between the state of

stress and strain in the model and that in the prototype, Rocha (1957),

Roscoe and Poorooschasb (1963) De Beer (1963, 1965). Hence the

results of model pile tests will be of quantitative value only if

the corresponding elements of soil in the prototype and the model

are subject~d to identi~ strains. However, there exists three

types of model testing:-

(i)

(ii)

(iii)

Free-stressed sand surface.

Pre-stressed sand surface.

Sruld subjected to centrifuge forces.

Concerning the first type of testing, this may be assumed as

the worst case of modelling. Piles tested by these methods are

usually subjected to a stress-level much lower than that of the

4

prototype and hence neither the load nor the deformation of the

model can be related to the prototype.

In the second type, the sand surface is acted upon by a

surcharge pressure that keeps the sand in a state of stress similar

to the average stresses of the prototype. Lack in Similarity is

very limited as compared with the first type.

Although the centrifuge is assumed to be the best testing

technique it also suffers fro.", certain defects such as:-

(i) Noises and vibration during the operation may

affect both the sand density and the

instrumentations.

(ii) .The ratio between the length of the sample

(iii)

'.

(the pile depth) to the average radius of the

centrifuge may affect the results.

The sample actually is subjected to an

ccceleration equal to the resultant of two

accelerations, the gravitational and the

centrifuge. Therefore the model is not

perfectly tested in a stress-path identical

to that of the prototype.

(iv) The influence of the particle size of tile soil.

That is, when a model is subjected to an

acceleration of N times the gravitational

acceleration, the size of each particle in

the model becomes in effect N times as large

. as the original size, which is equivalent to

anticipating the behaviour of piles in sand

from that in boulders.

5

(v) Any slight changes in the density of the sand

would be magnified N times.

(vi) Due to unexpected changes or a sudden stop of

the centrifuge motor, long-term tests such as

those of the present investigation cannot be

conducted.

(vii) The cost of the centrifuge per hour is relatively

high as compared with the other types of test.

Therefore, it was decided to carry out the present investigation

tests in a model of the pre-stressed sand surface. The topics have

been studied experimentally on isolated model piles driven into a

dry clean medium dense sand. The piles were instrumented by load­

cells located along the inner surface of the shaft. The load

transfer during any test was monitored and recorded by a data logger.

6

CHAPTER 2

REVIEW AND DISCUSSIONS OF SIGNIFICANT RELATED WORK

2.1 Introduction

In this chapter, the available literature is divided into four

main parts. In the first part the influence of repeated loading on

metals is considered. The second part deals with the behaviour of

sand under static and repeated loading. The third part, the response

of foundations other than piled to repeated loading is reviewed.

The last part is concerned with the general behaviour of a single pile

under both static and repeated loading.

2.2 Repeated Loading On Metals.

A repeated load is a force that is applied many times to a

member, causing stress in the material that continually varies,

usually through some definite range. The fatigue strength of a

material is often used to indicate its strength in resisting repeated

stress. Early in the study of strength of materials, it was found

from experience and tests that members usually failed under repeated

loads that were considerably smaller than similar static loads that

were required to cause failure.

The mechanism of fatigue failure of a ductile member caused by

repeated loads, and as described by Osgood (1970), is a gradual or

progressive fracture. The fracture seems to start at some point in

the member at which the stress is highest, usually at a point where

the stress is concentrated or highly localised by the presence of a

fillet, groove or hole, or some other abrupt discontinuity. As the

load is repeated, a small crack may start and gradually spread until

the member ruptures without measurable yielding of the member as a

whole.

7

2.3 The Behaviour of Band

The available information about the behaviour of sand will

be reviewed under the following two main sections.

2.3.l.Under static Loading

Roscoe (1970) demonstrated the following fundamental principles

of soil:-

(i) Results of triaxial compression tests on soils

tested under active and passive conditions

indicated that the soil behaviour was strongly

dependent on the loading path. (Fig. 2.1).

(ii) Tests on instrumented model retaining walls

indicated that the behaviour of the soil was

highly dependent on strain path.

(iii) Different granular materials of initially

different voids-ratio tests in a simple

shear apparatus results in the following

observations:-'

(a) Once the rupture surface had been

formed all the subsequent shear strains

occurred within that surface.

(b) The failure plane consisted of a thin

band of not more than 10 grains

thickness. Through that band the void

ratio attained the critical state and

beyond that it decreased in magnitude

as shown in Fig. 2.2.

'(c) Even the densist sand packing was found

to be compacted first before being

dilated.

8

0'3 =Const.= 120 p.s.i.

a I = Canst. = 120 p. s. i.

FIG. 2·1 INFLUENCE OF STRESS PATH ON STRAINS TO

DEVELOPE PEAK STRESS RATIO OF NORMALLY

CONSOLIDATED WEALD CLAY (AFTER ROSCOE 1970)

e True C. V.R. at (13 = 19 p.s.i. ( from ~-rays) o-a - - - ,tII'tIII'~ - - - - - -

,'''''''-- e local (from g-rays) I

I

0'7 / I

"

I I I I I I

I I

FIG.2·2 VOIDS RATIO VERSUS AVERAGE SHEAR STRAIN (~ )

MEASURED AT THE CENTRAL THIRD OF DENSE

SAND TE STED IN SIMPLE SHEAR APPARATUS

(AFTER ROSCOE 1970)

In an attempt to expla~n the phenomenon of the critical void

ratio of sand, Youd (1970) suggested that in randomly packed systems

both loose and dense arrays of particles are to be expected. Smaller

shear strains are required to collapse loose arrays than to dilate

dense arrays. After larger strains, the loose arrays of particles

have for the most part collapsed and the denser arrays dilate.

Dilation, in turn generates additional loose arrays which may eventually

collapse. Thus, at some stage of strain, the effect of expansion

and collapse of particle arrays counter balances each other and a

constant volume state is attained.

Based on triaxial compression drained tests, Lee and Seed (1967)

concluded that:-

(i) The state of critical void-ratio was a function

of the confining pressure. Very dense sand may

compress during. shearing if the confining pressure

is high enough.

(ii) The phenomenon of volumetric change ceased when

(iii)

the cell pressure was equal to a critical confining

pressure.

There was an unique relationship between the

critical void ratio and the critical confining

pressure. Critical void ratio of loose sand was

found to vary considerably with small changes

in confining pressure. In contrast, small changes

in void ratio of dense sand corresponded to

large changes in critical confining pressure.

(iv) The phenomenon of dilation" was dependent on the

strength of the sand grains.

9

(v) The failure strength of the sand was not

only dependent on friction and volumetric

change, it was also dependent on the

crushing and re-arrangement of the

grains" Eig. 2.3.

An investigation on the angles of friction between sand and

plane surfaces has been reported by Butterfield and Andrawes (1972).

From that study the following conclusions can be drawn:-

(i) The static coefficient of friction is dependent

on the relative movement between the solid and

the sand, microscopic toughness and hardness of

the so!jJmaterial. The gradation, density and

Qngularity of the grains and their packing are

also effected i~' the coefficient. I

(ii) The static coefficient of~fr/c.tion was always

greater than the kinetic coefficient by a

magnitude dependent on the material, the speed

of sliding and the stress level.

(iii) A phenomenon of "stick-slip" associated with a

non-steady deformation was noticed during the

failure stage of the tests. This phenomenon was

attributed to the difference between the static and

kinetic friction, which was found to depend on the

stiffness and damping of the loading system and

the speed of testing.

2.3.2.Under Repeated Loading

The behaviour of sand under the influence of repeated loading

is very complex as compared with that under static loading.

10

tf) tf) ClJ L--tf) L­

a ClJ

..c. \J)

Measured strength = Sliding friction

:!: dilatancy and re-arranging

o / ~e e":J/

..:> s.. c>'" e.,~/

~e ~e~ ~~~

'0<' ,0/ 6-'" ~ /

& '" C>« ~/ .f' ~CY -.: .'.' ~,e ... _ .. ~

'-' / i·· e., / .... ~~ ~/ ",.. ~SLiding friction

Normal stress

. re-arranglng

. FIG. 2·3 SCHEMATIC ILLUSTRATION OF CONTRIBUTION OF SLIDING FRICTION, DILATANCY

AND CRUSHING TO THE MEASURED MOHR ENVELOPE FOR DRAINED TESTS

ON SAND (AFTER LE E AND SEED 1967)

Parameters such as the amplitude of stress, number of stress

cycles, frequency and duration of the repeated stress, stress

history, stress level and density of the sand are important.

From the results of cyclic loading tests on the surface

of sand samples tested in a1arge oedometer, A1-Mosawe (1979) concluded

that the compaction of the samples increased with the number of

load cycles at a rapidly decreasing rate. Most of the compaction

took place within the first few load cycles. Ko and Scott (1967)

reached the same conclusion from a few loading cycles on samples

tested in the triaxial apparatus. They concluded that most of

the potentially unstable arrays of particles got a chance to be

re-arranged into a more stable configuration within the early

loading cycles. The investigation of Morgan (1966) on sand

samples subjected to 2,000,000 cycles of deviator stress confirmed

the conclusion of A1 Mosawe.

The observations of Silver and See (1971) of the behaviour of

sand under repeated loading in simple shear test indicated that the

sand sample was compacted by an amount dependent on the number of strain

cycles and the repeated shear strain level.

To explain the phenomenon of shear strength reduction

associated with vibrat~on of sand, Youd (1970) postulated that

--when a sand mass is subjected to stress fluctuated between two

limites, the inter-particle contact stresses would also fluctuate.

Repeating of the normal component of the contact stresses results

in fluctuations in the contact area. Separation between particles

due to the shearing stress component along the contact surfaces

may therefore be expected. Re-arrangement of the sand grains will

continue until a stable state is reached. The void ratio of this

11

stable state is equivalent to the critical void ratio. If the sand

is acted upon by increasing static loading during this stable state,

no volume change will be detected. The interlocking be~~een particles

will no longer contribute to the overall shearing strength of the

sand. The influence of confining pressure on the previous reduction

of shearing strength was also discussed by Youd. He argued that

increasing the confining pressure induces an increase in the initial

interparticle normal stresses which in turn increase the frictional

resistance along the interparticle surface.

The reduction in shearing strength of saturated sand was

extensively studied by many researchers such as Silver and Seed (197la,

1971b), Seed et al (1977), Peck (1979) Nemat-Nasser and Shokooh

(1979), Martin and Seed (1979), Blazquez et al (1980). The main

conclusion was that during the application of repeated shear-strain

the sand volume tends to decrease. Pore water pressure would

consequently increase by an amount depending on the relative density

the drainage condition of the sand, as well as the number of cycles,

,

frequency and amplitude of the repeated shear-strain. The effective

stresses, therefore, decreases which leads to a reduction in the sand

shearing strength.

2.4 Repeated Loadings On Foundations Other Than Piled

During its lifetime, a structural foundation may be subjected

to loading which fluctuates between different levels. This fluctuation

is mainly caused by the variation of the live-loading with time.

Repeated loading might severely affect the stability of the structure

depending on theamplitude, the level of the repeated loading, the

stress-history and desnity of the soil as well as the number of

loading cycles.

12

From a laboratory investigation Carr (1970) reported that the

ultimate pulling resistance of a plate anchor increased after

testing under repeated loading. In addition the movement of

the anchor increased with increase in the number of loading

cycles. The behaviour of plate anchors under repeated loading

was studied extensively by Hanna et a1 (1978). The tests were

conducted in dry ~and of different over consolidation ratios. The

main findings of that study were that:-

(i) The irrecoverable displacement was much greater

than that developed during static loading tests.

(ii) The over-consolidation ratio of tile sand was

found to have no effect on the accumulation of

the irrecoverable movement.

(iii) Even for the high level of repeated load which

had been used and the large number of loading

cycles applied no failure was observed.

(iv) The life-sp~~ of the anchor when subjected to

alternating loading was shorter than that for "

repeated loading.

Based on the results of .·.~ln tests performed on anchors,

Al-Mosawe (1979) supported the finding of Hanna et al. (1978) and

reported that.:-

(i) The irrecoverable movement of the alternating

loading, in contrast to repeated loading,

increased at an increasing rate, leading to

failure.

(ii) The ultimate load capacity of the anchor after a

repeated loading test increased but it always decreased

13

after an alternating loading test.

(iii) The performance of the anChor depended

mainly on the type and magnitude of the

previous loading.

(iv) When surcharge pressure was cycled, the

anchor system was always stiffer than that

of an anchor under static surcharge.

(v)· Pre-stressing the anchor was always

associated with a longer life-span.

The results of repeated loading tests on instrumented

reinforcing steel strips embedded in dry medium dense sand

have been reported by Al-Ashou (1981). He found that:-

(i) After an initial stable state, the strip .'

within a relatively small number of load

cycles, was pulled out.

(il) The life-span of the rei~forcing strip was

found to be a function of the amplitude~and

level of the repeated loading. "',

(ill) Increase In surcharge pressure always increased

the life-span of the reinforcing strip.

(iv) The ultimate pulling resistance decreased by as

(v)

muCh as 35% after the strip was subjected to

repeated loading.

5 Even after 2X10 cycles of surcharge, failure

had not been reached when the strip had been

subjected to a sustained loading as high as O.78Qt.

Qt being the static pull-out load.

14

(vi) Testing the strip under different combinations

of repeated loading and cyclic surcharge indicated

that the most severe loading condition occurred

when the upper load level.coincided with the lower

surcharge pressure.

Prevost et al (1981) conducted tests on a gravity platform

foundation bearing on normally consolidated silt. The tests were

carried out in a centrifuge. The foundation was subjected to

repeated, inclined and eccentric loading conditions (not more than

15 cycles). The results of those tests showed that the foundation

accumulated permanent vertical, horizontal and tilting movements

and that the accumulation of movement decreased as the number of

cycles of loading increased.

2.5 The Behaviour Of Isolated Piles

2.5.l.Under Static Loading

Based on the results of compression tests carried out on H­

bearing piles driven through sand and gravel, D'Appo10nia and Romualdi

(1963) suggested that, during loading, pile deformation will be

resisted by shear-stresses mobilised along the pile/soil interface

surface. 'rhese stresses increase with increase in the relative

displacement between the pile and the soil and are a maximum near

the ground surface. Initially the soil sticks to the pile surface

and the relationship between pile top loading and load transfer, as

represented by the average shear stress on the pile, is approximately

linear. Once the loading reaches a certain level, the pile

begins to slip along the soil and a shear plane starts from the

ground surface and progressively extends towards the pile base.

Any subsequent increment of loading will be transferred to the pile

15

base in as. much as the pile is slipping along its entire le?gth.

The failure load will then be reached when the base starts to

punch into the soil and settle continuously.

The mechanics of load mobilisation have been comprehensively

studied by Hanna (1969). He confirmed most of the previous study

and postulated that the failure surface does not necessarily coincide

with the pile/soil interface surface. Concerning the pile load

transfer, Hanna stated the follOWing . fundamental concepts:-

(i) '!be shaft friction and base resistance

do not act independently, they are

interrelated and strain dependent.

(ii) Unloading friction is generated from the

ground surface and propagated down to the

pile base such as that which takes place

during the pile loading process.

(iii) After unload, the pile will be subjected to a

system of stresses (residual) distributed in

such a manner that keep the pile in a state of

equilibrium.

(iv) The behaviour of the pile in the subsequent

stages of loading is, to a large extent,

dependent on these residual stresses.

Field and model tests carried out by Vesic (1967) on instrumented

piles embedded in sand of different relative densities indicated that

both the unit ultimate point' " resistance and the aver,age skin

friction attain a limiting value beyond a certain depth of penetrati.on.

To explain this phenomenon Vesic s,uggested that when a pile

16

is loaded, the sand beneath the base is compressed whilst that around

the shaft tends to move downward. The latter action causes the

original horizontal stress which acts at any given point along the

'pile shaft to become inclined by an angle dependent on the

magni tude of the settlement and the depth of the investigated point.

If the depth of the pile is great enough, then a condition of constant

inclination after a certain critical depth will be attained., This

critical depth \'Tas found to vary from 10 times the pile diameter for

loose sand to 20 times the pile diameter for dense sand. Vesic

then reached the fo11CMing conclusion.. There is arching created

along the pile shaft such as that observed above a horizontal trap

door in a silo. This arching affects the state of effective

vertical and horizontal stresses around the shaft as shown in

Fig. 2.4. ,

A later published paper by Vesic (1969) stated that "very

little is known about the actual stress condItion around pLles'and

this problem will remain open".

The phenomena of arching and the limiting values of point and

shaft stresses have been supported by m~researchers such as Tan

(1971),Ooi (1980), Touma and Reese (1974).

Cooke (1974) described the initial strain field around loaded

piles as follows. The soil deforms in such a way that it consists

of a series of concentric cylinders centred on the pile. Shear

strains decrease with increasing distance from the pile surface.

Holmquist et a1 (1976), Parry and SWain (1977) attributed the

postulation of Hanna (1969) that the failure surface and the pileI

soil interface surface not being coincident to the reduction in the

shear strengtil of the soil in the direct vicinity of the pile.

17

n ....

~ Zo

Z

~ "-" " \ " ,

\ ~

\"

t::q:j t-- '5-:

~qz ./

7 7

/

I

Zo I

Z

,. '-'-

(a) - (b)

FIG.2-4 STRESS DISTRIBUTION ,AROUND A DEEP FOUNDATION IN SAND

(AFTER VESIC' 1967)

Gallagher and St. John (1980) suggested that the coinciding

of the two surfaces depend on the surface roughness of the pile.

Thorburn and Buchanan (1979) restricted the limiting value of

the skin friction to driven piles only~ This conclusion was

based on the following failure mechanism. During the process of

pile driving, sand in the immediate vicinity to the pile dilates

due to tile large applied shear strains. In contrast, small

shear-strains cause a compaction to the soils located a short

distance from the pile shaft, thus creating a thin, looser annulus of

sand along the shaft which affects the~sistance and distribution of

stresses along the pile shaft.

From field tension and compression tests carried out on

instrumented steel piles of 0.4S7m diameter driven into boulder

clay to 9.2m depth, Gallagher and St." John (1980) concluded that

the pile shaft capacity in tension was less than that in

compression. A similar conclusion was reported by Hunter and .

Davisson (1969).

2.5.1.1.Residual stresses

These stressed are generated along the piles during and after

the process of installation and have a magnitude and direction

dependent on many factors such as the depth, method of installation

"and flexibility of the pile. The important influence of these

residual stresses on the load-settlement behaviour of a pile has been

quantified by Hanna and Tan (1971). They carried out a series of

large scale laboratory pile tests. Two methods of test preparation

were followed. In one, the pile was permitted to float whilst in

the other method the top end of the pile was fixed and thus axial

movement at the pile was prevented. In this manner two extreme

initial residual stresses were created. The result of compressive

18

o

1

2

E 3 E ..

..... ~ l. E

Settlement of pile top after removal of clamp when sand stratum had been placed

Floating pile

~ top during :: 5

C1.I sand placement \f)

Pile length: 1 ·78 m Pile dia. : 2·55 em Wall thickness: 0·122 cm ~ = 15l.5 kG / m 3

FIG. 2-5 LOAD - SETTLEMENT CURVES FOR COMPRESSION

LOADING (AFTER HANNA AND TAN 1971 )

Shear stress J kNI m2

-50 -40 -30 -20 -10 0 ... 10 ... 20 ... 30 \ \ \. \

\ \ 1. Residual shear stress

\ , 1 \ J 2. Pile B at 2·0 m

3. Pile B at 3·0 m 4. Pile B at 4·0 m

. I \, L ,/}

/j " I " 1/ 2/ ,J" ,

, ' . I . \-",,,,'"

3. , ..... ..,. ... -', . I " , '" "4.1

'.... " ........ .... ' .... , \ \ ,

1.·6 m

Positive shear stress when soil moves upward relat ive to the pile

FIG. 2-6 CHANGES IN THE DISTRIBUTION OF SHEAR STRESSES

ON THE SHAFT OF PILE A CAUSED BY SOIL

MOVEMENT SET UP DURING THE INSTALLATION

OF PILE B (AFTER COOKE ET AL 1979)

tests, Fig. 2.5. indicated that the residual stresses greatly

affected the load carrying capacity and the settlement of the tested

piles.

Hunter and Davisson (1969) concluded that the evaluation of

load transfer based on zero residual loads may seriously be in error.

Measurements of load distribution (Cooke et al 1979) of instrumented

tubular steel piles embedded in London clay revealed that the state

of residual stresses of a given pile was also affected by the

installation of a nearby pile, Fig. 2.6.

2. 5.2. Under R epe a ted Loadings

Repeated tension loading tests on H-piles in sand have been

reported by Begemann (1973). Those tests revealed that the pile

tended to pullout of the ground when .a high alternating load was

applied. For repeated loading less than 35 per cent of the static

pull-out resistance, no Significant increase in movement was

observcd~ Begemann also highlighted the effect of load history

on the subsequent pile performance.

An extensive test programme to study the behaviour of a single

pile under repeated loading was conducted by Chan(1976). The pile

and the apparatus of testing is similar to that employed in the

present investigation except that Chan's tests were only performed

on sand subjected to a constant surcharge pressure. The main findings

of that work were:-

(i) The behaviour of the pile was found to be dependent

mainly. on the amplitude of the repeated loading and

the number of loading cycles •.

(ii) For compression repeated loading, initially the pile

was stable, but after a certain number of cycles

19

which increased with decrease of the load

amplitude, the rate of movement began to

increase rapidly until another stable

stage was reached. Repeated loding was

found to bring about a re-distribution of

pile load between the shaft and the base.

(iii) For tension repeated loading, the,pile

initially moved wi.th a decreasing rate then

after a stable stage, the rate increased

until eventually the pile was pulled out.

The intial compressive residual axial

loads changed to tensile during the early

stages of the repeated loading. These , '

tensile, residual loads were very small at

the final stage.

(iv) Failure due to repeated loading was

attributed to a reduction of normal

stresses that act upon the pile shaft.

(v) Repeated loading was found to decrease the

ultimate load capacity of the pile. This

, observation has been confirmed by

Bogard and Matlock (1979), Paulos

(1981) •

(vi) Increasing the surcharge pressure resulted

in a pile of longer life-span when subjected

to repeated loading.

Holmquist and Matlock (1976) conducted repeated loading (not,

more than 100 cycles) on 25nun instrumented aluminium tube piles

tested in a drum 750mm diameter containing remoulded clay.

20

The results of those tests indicated that the alternating loading

caused a severe reduction in skin friction as compared with that of

unidirectional repeated loading. Increasing the confining pressure

/ 2 2 .

from 68kN m to 340 kN/m corresponded to an increase in the life-

span of the pile . from 25 cycles to 75 cycles of repeated loading. When

comparing the life-span of driven and bored piles they concluded

that the. latter had the_ lot:lger life.

Although different techniques have been emp1ajed in testing~

the studies of t-1adhloom (1978) and Ooi (1980) confirmed the

finding of Chan (1976). Ooi indicated that the life-span of a pile

decreased when it was subjected to failure loading before being

tested under-repeated loading.

Matlock (1979) concluded that the resistance along the upper \ .

portion of a pile subjected to tensile repeated loading \'las--los t

progressively

Bogard and Matlock (1979), after. a cyclic loading test carried

out on an axially loaded pile, suggested a ,mechanistic model of

what may happen in a typical annulus of soil around a pile. ,Prior

to repeated loading, the distribution of shear strength, shear stress

and soil deformation are as shown in Fig. 2.7A. After repeated

loading the gradient of sheat- strength of the soil in the vicinity

of the pile may give rise to failure at a short distance from the

pile wall. The band in which failure is ini.tiated is shown hatched

in Fig. 2. 7B.

To solve the problem of a foundation pile subjected to

repeated loading many theoretical approaches have been proposed.

(For review see Smith 1979). Each of them attempted to produce a

model which combines some of the phenomenon observed in element

21

: \

.r:. ..... Ol C Q} L. ..... V)

L. a Q} .r:. If)

V) V) Q} L. ..... V)

L . a Q}

.c. V)

.t:. ...... en c Q} L. ..... If)

" .V) V)

2! -V)

c .2· -a E .... o

'to­Q}

"0

o V)

Radial distance Radial distance

~

A. Initial loading B. After cyclic degradation

FIG. 2-7 MECHANISTIC INTERPRETATION OF CYCLIC DEGRADATION PROCESS IN AXIAL PILE- SOIL

INTERACTION (AFTER BOGARD AND MATLOCK 1979)

behaviour such as a reduction in shearing strength. Most of these

solutions assumed the pile and the soil as an elastic continuum

and the problem W()S the" solved either by the solid or by the

transfer function approach. Gallagher and St." John (1980) did not

give credibility to any of the previous design approaches unless

it was used with an unrealistically high factor of safety.

Theoretical investigations by Poulos (1981) indicated that

.. the cyclic failure begins at the top of the pile and progresses

downwards as the number of load cycles and the level of the

repeated load are increased. This failure caused the load transfer

to increase gradually in the lower parts of the pile. Moreover,

Poulos found that the increase in the soil shear strength results

in a pile of longer life.

Recently, field investigation of Kraft et al (1981) on

instrumented (356mm) open-end steel piles, embedded in clay and

subjected to unidirectional repeated loading revealed tilat the load

history and cyclic loading affected the load-deformation response

of the pile. Concerning the load transfer of the pile the~e authors

found that initially the load was transferred to the deeper sections

but after several cycles of loading, the trend was reversed and

the upper portion of the pile began to carry more of the load again.

22

CHAPTER·3

LABORATORY INVESTIGATION OF THE BEHAVIOUR OFA PILE ELEMENT EMBEDDED IN A TRIAXIAL SPECIMEN

3.1 Introduction

One of the main parameters studied in the test programme presented

in this thesis is the pile shaft friction. The available information

has indicated that this friction depends on the load level and

history, surface roughness, pile material and rigidity as well as

the soil properties and many other factors, Hanna (1963, 1969), Coyle

and Sulaiman (1967), Vesic (1967), Touma and Reese (1974). Therefore

to establish a better understanding of this friction, a pile shaft

was tested in a modified triaxial cell. Thls was done in such a

manner that the base of the pile was freely extended out of the

sand sample and thus all the resistance to loading was taken by

shaft friction. Two levels of confining pressure were used in

these tests, 50 and 100 kN/m2• These·pressures were chosen to

simulate the conditions of the main research programme. Both

tension and compression piles were investigated. The properties of

both the pile and the sand that were used in this investigation are

the same as those used in the main research programme.

3.2 The Test Apparatus And procedures

A large triaxial cell of 300mm diameter. and 600mm high was

employed. The apparatus, shown in Fig. 3.1., consist of a

cylindrical sand sample of initially 200mm diameter and 497mm high

supported by a perspex pedestal of 200mm diameter and 50mm high. The

pedestal, which is rigidly connected to the base of the triaxial

cell, contains a hole of 25mm diameter located along its own axis

and through which the 19nnn diameter pile passed. The diameter of

23

Instrumented

r washer,

· " .. ' . · .. .. .' . . • •• ' 't ..

. ' .... , t •••• . .' · . . , . , . · . . .

'. . · . · , · . . ' . . ' . . ... . . " · . . . . " · . .. . . .. . .. o. , . . · . .

o • 0

o • o 0,

Pressu r.:;:iz:..::e:.::d:.......lc~~~rll ~~~~~ wa ter.

. ., , . . ' , · . . . . , . ,. ' .' . . . '. .­'. ' . . · .: " , , . . . ' . , " • •

00 · . . . . • • • t,

o ,

Packing of cotton with sit icon .

Stainl

Perspex edestal

FIG. (3'1). LABORATORY MINIATURE PILE TEST APPARATUS.

this hole was larger than that of the pile and ensured that in no

case would the pile surface touch the pedes tal.

In order to fix the bottom of the pile in its proper position

during installation and testing and to prevent the sand grains

from falling through the gap between the pile and the pedestal, a

rubber washer of lOmm thickness was stuck on the pedestal so that

the centre of the washer hole coincided with the centre of the

pedestal hole. A stainless-steel bushing 25mm inner diameter wa~

rigidly connected to the top cover plate. The outer diameter of

the bushing, 38mm was tapered at the top segment so that during

erection of the apparatus the bushing could easily and smoothly

pass through the cell cover plate.

The "model" pile was a tube of high strength aluminium alloy /

19mm outer diamter, 750mm in length, 'and a wall thickness of 1.6mm.

The soil was uniform, air-dried, mediwn dense sand. The

properties of this sand are stated in section 5.1. During the

sample preparat.ion, the sand was placed by a raining technique to

3 give an average density of 1.557 Mg/m which is equivalent to

51.2% relative density.

Before the first test was started many preliminary tests were

carried out to check the repeatability 'of the test and the vertical

displacement of the sand mass after applying the back pressure as

well as that during testing of the pile. It was found that the

tests do repeat to within approximately 5%. Regarding the vertical

displacement, this was found to vary from approximately 8.0mm to 20.Dmm.

Therefore an average of l4.Omm was considered when preparing the

samples ,to give a net depth of pile embedment equal to 475mm which

corresponded to a depth to diameter ratio of 25. The vertical

24

displacement was detected by a dial gauge mounted on the top end of

the bushing.

Before starting the test each individual component of the

apparatus was cleaned. The groove of the cell base, the hole of

the rubber washer and the inner surface of the threaded collar were

coated with silicon grease.

During the process of sand placement the pile was held at the

lCMer erid by the rubber washer. The upper end which was held by

four sets of thin plastic bands was adjusted to set the pile vertical

and hence coincide with the centre of the sanJ sample. The plastic

bands bridge the pile to four free-standing rods which were firmly

connected to the cell base and forming a cross-shape.

After the mould was filled with sand to a depth of 497mm and

the surface of the sand was levelled off, the top cover plate with

extreme care was placed.

In order to apply a back pressure (not more than 40 kN/m2) the

clearance between the pile surface and the inner surface of the

bushing was .gently packed by layers of cotton and silicon grease,

Fig. 3.3. The upper part of the triaxial cell, which was

suspended vertically from a crane, was lowered very slowly and

carefully guided so that the bushing could smoothly pass through

the collar outside the cell.

When the two parts of the triaxial cell were lumped tightly

together the cell as a whole was carefully lifted up and positioned

on the base of the triaxial machine. Water under low pressure (not

2 more than 5 kN/m ) was all.::Med to enter and fill the chamber. The

back pressure was then cut off and the cell pressure was raised

steadily to the required testing pressure. A load test was

conducted using. the arrangement shown in Fig. 3.4.

25

' . .

FIG . 3 . 2 . THE SAND SAMPLE AFTER APPLYING BACK PRESSURE AND REMOVING 1~E MOULD .

FIG. 3.3. PROCEDURE OF LOADING . FIG . 3.4 GENERAL VIEW OF THE APPARATUS .

. .­. . '~ ~ .

Load readings were taken each 30 seconds from the instrumented

proving ring which was monitored by a Solartron data logger. The

displacements were checked by dial gauge. The rate of displacement

2 was 1.36mm per minute. Confining pressures of 50 and 100 kN/m were

used in this investigation. During the compression phase of the

study the load was transmitted through a 25mm steel ball mounted on

the top of the pile. For the tension phase, a threaded metal bar

was used for this function, Fig. 3.3. Tension loadings were

generated by reversing the direction of movement of ~~e triaxial

machine, after the triaxial base was rigidly connected to the

base of the machine.

3.3. Test Programme

The test programme comprised nineteen tests performed on two

sand samples. The first five tcsts which were carried out on the

first sample, studied the shaft friction in compression loading. On

the second sample fourtcen tests have been conducted through which

the shaft friction in tension loading was examined. The detailed

test programme and sequence of tests are shown in table 3.1.

3.4 Presentation And Discussion Of Results

Fig. 3.5 shows a plot of load versus pile movement (average

skin friction versus pile movement) of the tests that were carried

out on the first sample. For all tests, at relatively small

displacements the skin friction increased rapidly until a peak

value was attained. The peak skin friction of the test Tl, which

2 2 was conducted under 100 kN/m confining pressure, reached 40 kN/m •

Beyond that the friction continuously decreased until it readled a value of

34.4 kN/m2• These values of skin friction correspond to an angle

of friction bebqeen the pile material and the sand equal to 22 o

o and 19 respectively. Moreover, the peak skin friction was found

26

Table 3.1

Test Programme

Sample Test Type of loading Cell Pressure Remarks No. No. kN/m2 .

Tl. Compression 100 T2 II 50

1 T3 II 100 T4 .. " TS .. 50

T6 Tension 50 I

T7 " T8 " T9 " T10 "

2 Tll " T12 , 100 T13 " T14 .. ..

TIS " T16 50 T17 100 T18 Compression ' .. '1'19 Tension ..

27

42·3

35·25

N E --z 28·2

.Y. .. c

.Q -u L. -c ~ 2",5

. <IJ tn o L. <IJ > «

14·1

7·05

I I I I I I

I /

T-3 T-4

T-18

T-I

Sample (1)

l:475mm

d: 19 mm

lid: 25

03 : 100

kN/m2

Ra1e of s1rain : 1-36 mm mn

__ T-S}. \ . - - T-2 °3 = 50kN 1m2

1200

1000

800

600

400

200

z "0 o o

..J

00 1 2 30

Displacement. mm

FIG.3-5 LOAD Vs. DISPLACEMENT (COMPRESSION)

occur at a displacement of 0.858mm which is equivalent to 0.05 of

the pile diameter or 0.18% shear strain.

The angle of friction between a reinforcing strip and the sand

'as measured by Al-Ashou (198l) using a load control method and for

o a wide range of normal stresses, was 20.5. This value may be

assumed equal to the average of the two aforementioned angles (19°

and 220).

The test T2 was performed on the same sand sample but after the

confining pressure was decreased from 100 kN/m2 to 50 kN/m2• The

results of this test showed the peak skin friction was 18.34 kN/m2

after which the skin friction dropped, then slightly increased as

the pile displacement was increased. The peak value was reached

when the pile moved 0.388mm which is equivalent to 0.02 of the

pile diameter. The reason for this smaller displacement as compared

with that of test Tl is believed to be due to the stiffening the

sand had gained due to the previous test Tl'where it was tested

2 under 100 kN/m confinement.

2 When the cell pressure was raised again to 100 kN/m and test

T3 conducted L~e peak skin friction reached a value equal to 44.3

kN/m2 which is much higher than that of test Tl. This value

",as attained when the pile head settled 0.655mm which corresponds

'to 0.034 of the pile diameter. The higher value of the peak skin

friction as compared with that of test T1 may be rel~ted to the

compaction of the s~ sample which took place after each ch~)ge

in the confining pressures Ko and Scott (l967).

The peak skin friction decreased and the load displacement

relationship became brittle when the test T3 was repeated as shown

by test T4, Fig. 3.5. A decrease in the peak skin friction due

28

to repeated loading has been reported by many authors, Chan (1976),

Ooi (1980), AI-Ashou (1980) and Bogard and Matlock (1979). The

first three authors indicated that the normal stresses which act on

the shaft surface decreased due to repeated loading while. the latter

:r."e1ated it to the reduction in shear strength of the soil layer

located very close to the pile shaft surface.

2 Again when the cell pressure was decreased to 50 kN/m and the

test TS carried out, the peak skin friction was greater than that

of test T2. This increases in the peak value may also be related to

the compaction effect that took place after changing the cell

pressure.

It may be argued that if an arch is generated in a horizontal

plane it wili affect the value of the horizontal effective stress

which acts on the pile shaft surface and therefore, the value of

the shaft friction will also be affected, Thorburn and Buchanan (1979)

Vesic (1967). Thus, the value of the shaft friction due to an increase

2 or decrease in cell pressure from and to 50 kN/m respectively should

not significantly be affected. From Fig. 3.5 it can be seen that

the shaft friction of tests Tl, T3 and T4 was always greater·than

2 twice that of tests T2 and T5 which were conducted under 50 kN/m

confinement. These results may indicate that the sand grains

did not form an arch around the pile shaft. A similar conclusion

may be drawn from the tension tests series as shown in Fig. 3.7.

At the start of each test, the load-displacement relationship

was non-linear. This behaviour may be attributed to the state of

residual stressE:!s which are generated after each loading test as well

as during the process of sand placement, Tan and Hanna (1974).

The intia1 displacement may therefore cause a relative movement

between the pile shaft and the soil, so that all stresses along the

29

shaft become in one direction such as that described by Hanna (1969).

Coyle and Su1aiman (1967) carried out a similar investigation

on 25.~4mm steel stube, instrumented model pile placed in a poorly

graded silty sand sample 150mm diameter and tested in a modified

triaxial cell under a wide range of cell pressures. The results of

two different initial densities tested under 70 kN/m2 1I0 psi) by

the authors together with the result of test T1 of the present

investigation are presented in Fig. 3.6. This plot shows the

relationship between the average skin friction and the displacement

ratio (displacement/diameter of the pile) of the pile. It will be

seen that there is close agreement between the displacement ratio

of the two investigations. With respect to the skin friction, the

peak value of the denser sample was much closer to the respcctiYe

value of the present investigation.

Fig. 3.7 shows the load displacement relationship of the other

test series which was carried out on the second sand sample and

designed to examine the shaft friction under tensile loading.

The general trend of this family of curves is similar to that

of the first test series (Fig. 3.5). The test, T6 which was the

first test in this series, was conducted under 50 kN/m2 cell pressure.

The peak skin friction of this test was about 14.1 kN/m2 and attained

at a displacement 0.021 of the pile diameter (0.13% shear strain).

Beyond that the skin friction increased slightly as the pile displacement . . 2

increased and reached 17.6 kN/m at the end of the test.

If the value of the post failure skin friction of the compression

test T1 (which was carried out on the first sru,a sample) and that

of the tension test T6 (which was carried out on the second sample)

are taken as a ratio of the applied confining pressure, the following

values of this ratio are obtained, 0.35 and 0.352 respectively. This

30

140

120

N E ~100 ~ . c o '--.~ 80

·c 3! V)

C1J 60 en c '-C1J > <t 40

0'04

Dr = Relative density

03 = Confining pressure

/l = Displacement

o = Diameter

Coyle an l · 0 '-100 0 /0 03=70kN/m2

d Su alman I r - f(, / I

Present T-1. Dr= 51,2%, 03 =100kN/m2

Coyle and SUlal'man, I 2 Dr=65%, 03=70kN m

0·08 0·12 0'16 0,20 D-2L. 0,28

Displacement ratio /lID

. FIG. 3-6 STRESS - STRAIN RELATIONSHIP OF COMPRESSION TESTS

revealed that, at post failure and after a large pile displacement,

the conventional coefficient of shaft friction in compression loading

and that in tension was the same and was independent of the cell

. pressure.

When the test T6 was repeated six times, the peak skin

friction was affected and the load displacement behaviour of any

test became stiffer than the preceding test.

During the first loading cycle, test T7, the peak skin friction

2 increased to 16.7 kN/m and then continuously decreased after each

2 loading cycle until it reached a value of 15.45 kN/m during test

Tll. The peak skin friction of test T7 was higher than that of

test T6 and is believed to be caused by the change in the direction

and magnitude of the residual stress which resulted after the test

T6 such as that demonstrated by Hanna' (1969). A similar behaviour

has been reported by Tan (1971) and Ooi (1980).

w~en the cell pressure was raised to 100 kN/m2 and the test

T12 performed the peak skin friction reached a value of 46.27 kN/rn2.

This value also decreased when the test T12 repeated three times

2 and reached a value of 39.32 kN/m during test Tls.

2 After the cell pressure was decreased to 50 kN/m and test T16

conducted the peak skin friction was higher than all the values of 2 .

the previous tests which had been carried out under 50 kN/m confining

pressure, as shown in Fig. 3.7.

The compression loading test T1S was carried out directly after

the tension test T17. The load displacement characteristic of this

test was more ductile as compared with the other compression tests,

as shown in Fig. 3.5. Moreover, in contrast to the other tests

which were conducted at 100 kN/m2 the post failure skin friction of

31

I

~ /--. T-12 ---'"", I _ -T-17 ---1/ ---T-13

42·3 A _--T-14 03 :100 1200

r" kN/m2 W - - T-IS /1/,,' -

I.v, II/ 111

35·2 III 1000 1[1 Sample (2) , I ' l = 475mm

I I' d = 19 mm N

II I ~ l/d= 25 II I z

/ Rate of strain =1·36mm/mn ~ 28·2 II 800 .. , c II .Q I -

"

z u ,II

.. '- "'0 - 0 c

I" 0 .Y.

II ' ...J

VI

0.1 21·1 " I 600 0'1 ,,: '0 '-0.1 >

'I' <{

I 03=50kN/m2

14·1 400 --"

200

Displacement, m m

FIG.3-7 LOAD Vs DISPLACEMENT (TENSION)

test T18 increased.

A similar trend was observed when the tension test T19 was

performed directly after the compression test TIS - see Fig. 3.7.

~1e change in behaviour of the tes~~ T18 arid T19 may also be attributed

to the state of the residual stresses as reported by Tan (1971) and

Hanna (1969). On examination of the mechanics of load mobilisation

in friction pile, the latter author found that the pulling resistance

of a pile which has not been subjected to a previous compression

loading is larger than an identical pile but loaded in compression.

3.5 Conclusion

From these few tests and within the testing limits, the following

conclusions may be dra\-ln: - .

(1) For small pile displacements, the skin friction

increased linearly with increases of pile

displacement.

(2) During the post failure stage and after a large

pile displacement the conventional coefficient

of shaft friction in compression loading and

that in tension loading was the same and was

independent of the cell pressure.

(3) The shear-strain required to attain the peak

skin friction increased as the confining

pressure increased.

(4) At a given confining pressure, the peak skin

friction was increased when the confining

pressure changed and, then, returned back to

its original value.

32

(5) The peak skin friction decreased and the load

displacement became more brittle when the

loading test was repeated.

(6) Within these few tests and under such conditions,

the influence of arching in a horizontal plane

and around the pile shaft was not detected.

(7) Alternating loading greatly affected the behaviour

of the pile shaft. The skin friction decreased and

the displacement required to mobilise a given skin

friction increased due to this type of loading.

33

, 1

CHAPTER 4

THE TEST APPARATUS AND MATERIALS

4.1 Arrangement Of The Apparatus

Three identicnl rigs have been employed in this investigation.

The arrangement and detail section through one of these rigs are shown in

Figs. 4.1, 4.2 and 4.3 respettively.-.The pile was placed in a

cylindrical steel container filled ... Ii th dry sand. A double hanger

and lever arm system acting together with a reciprocating machine

were used for repeated pile loading. In each minute during testing,

the machine steadily lifted and then released the lower hanger which

carried t~e amplitude of the repeated loading.

The mechanical advantage of the lever arm was 11.0 and 10.0

for compressive and tensile loads respectively.

(,:The stress condition around the pile was controlled by applying

a surcharge pressure on the sand surface.

The method of testing and the apparatus' adopted in this study

was similar to that of Madhloom (1978 ) except some modification

regarding the cyclic surcharge system which was found necessary

to fulfill the requirement of the test programme.

4.2 The Sand Cbntainer

The sand container was made of steel tube 380mm diameter,

965mm high IDld 17.5m thick. The diameter of the container was

20 times the diameter of the pile whilst the minimum depth of sand

beneath the pile base after installation was 18.S times the pile

diameter. In order to prepare a suitable space for the pressure

plate the sand was filled up to 920mm of the container height. The

influence of any external vibrations through the ground was minimised

by introducing a l2mm thick rubber sheet betwen the floor and the

34

FIG . /.-1 THE TEST APPARATUS

FIG . 4.2 OVERALL VIEW OF APPARATUS FOR REPEATED LOADING TESTS .

(After Chan 1976).

II

Hinges

Electric motor and beam

assembly·

rotate by 900

Surcharge of

To data logger

Supporting frame

To pressure controller

Klinger valve

for surcharge

-.--., p~, .~.!E ... W .. ,.lli .... ~ ... ~ .. ~. ~U---- 0 Ring · . -. ' ... , .

Perspex plate

Dial gauge

or transducer

o

. ," .;- ., " ';' '. '. ·"1 .: •. ' .. : ............ :.: Rubber :."~"'-' ..... ~. ',' " : '. ..... . ... :.: ~ membrane . ,. . . . ." ... • II '". '.

· '.' , .

-· .

. ", . . .'

.' ~<'.. Sand .... ' " · . " . . ' ......... ..'. '"'' ~ . . . . . . • • '1 " - •••• :.' ,. a

-, ..' '.-t • • • . ' •••••

,- :. " ., ••• 'e • _,' • ,', •••• ' ...

o

Sand container

Lateral strain

detector

FIG. 4-3 SECTION THROUGH TEST RIG

container structure.

Preliminary tests indicated that the ultimate load capacity

and the corresponding settlement of the pile were greatly

affected and became non-repeatable when the container wall

surface was not coated with silicone grease.

The influence of the container size on the state of stresses

in the soil; mass during the process of penetration was calculated

by Chan (1976). He found that the major principal stresses

decreased rapidly with increase of the radial distance from the

shaft and the vertical distance from the base. These stresses

became of negligible value near the boundary of the container.

4.3 The Sand Hopper

The sand hopper was made of 12mm thickness plywood with 40 x 40cm

cross-section and 60cm depth. The outlet of the hopper was a segment

of steel tube of 44mm outer diameter connected to a sOmm flexible

hose. The other end of the hose was fitted with a stainless steel

wire mesh of 3.3 x 3.3mm size through \,lhicl1 the sand was allowed

to flow. Hoses of two different lengths were used during the process

of sand placement, one of them was 70cm length and the other was 40cm.

4.4 Arrangement 0.f Repeated Loading

The system by which the pile loading was repeated is shown

in Fig 4.4. It consists of a lever arm, two hangers connected

together through a ball bushing guide and a recipro~ating electric

machine. The frequency of the repeated loading was one cycle per

minute for all tests except one in which the frequency was 3

cycles per minute. An adjustable press button relay was fixed to

the skeleton of the reciprocating machine by which the machine

could be stopped when the lever arm became inclined due to the

35

FIG. 4.4 ARRANGEMENT FOR APPLYING REPEATED LOADS.

(After Chan 1976).

displacement of the pile by more than a certain amount. Scanning of the

top load cell of the pile during the 60 seconds duration of the

loading cycle indicated that the pile loading was held constant

for approximately 23 seconds while the transition period of

increasing or decreasing the loading was about 7 seconds. This

relative period of the pile loading was kept approximately constant

in each cycle by re-levelling the lever arm as discussed in

section 5. 7.

The influence of impact on the pile during application of

the repeated load was minimised by introducing rubber dampers at

the surface of contact between the pile and the lever arm and at the

top of the ram of the reciprocating Inachine. By use of these

dampers the stiffness of the lever-hanger system was artificially

.' decreased.

The problem of lateral movement of the lever arm due to

eccentric load application was overcome by providing the bottom of the

llangcr with a small semi-spherical steel element Fig. 4.4 and by two'

frictionless roller guidea located at the top of the container

as shown in Fig. 4.5. These' guides also prevented any undesirable

lateral movement of the load lever. Measurement of the frictlcn

in the guide between the lower and the upper hangers indicated that

this friction was negligible.

4.5 The Surcharge Pressure system

4.5.l.General

Three types of surcharge pressure have been used in this

investigation.

(i) Constant surcharge pressure.

(ii) Surcharge pressure cycled between two limits

36

· FIG. 4.5 ROLLERS FOR PREVENTING ACCIDENTAL LEVER MOVEMENT.

(After Chan 1976)

(iii)

and independent of the pile repeated

load level, and

the cycle of the surcharge pressure was

conditioned by the pile loading level •

. This type consisted of two modes of

testing:-

a) The high surcharge pressure was in

phase with the high repeated load

level of the pile.

b) The high surcharge pressure was in

phase with the low repeated load

level of the pile.

Since the availa ble system which had been built by Madhloom

(1978) could only fulfill the first two types of surcharge

pressure the system was subjected by the writer to a simple

modification through which the third type of. surcharge could be

applied. F.ig. 4.6, shows the technique' by \O/hich the surcharge

pressu~e, on the sand surface was controlled. The high pressure of

the air which \V'as supplied by the compressor was regulated through

the pressul'e controller, then applied on the water surface in the

"change-over pot". The pressurised water then, through: the pressure

plate, acted upon the sand surface as a surcharge.

4.5.2. 'J'he Pressure controller

Schematic diagrams of the pressure controller, by which the magnitude

and the time of application of the surcharge pressure were regulated

are shown in Fig. 4.6. The pressure diagram, Fig. 4.6b consist of:-

(i) Two Norgren regulators, type Rl3 -

400 RNBM

(ii) Two Solenoid valves, type 454TA

37

. __ . ____ ..... _. ___ --- ______ .. __ ~ ___ ~._~ ____ .~ -4-_~._ . ...-.-~ __ ~__. ~-.;;.,. . - .-. ----.---.-~.

FROM COMPRESSOR

PRESS CONTROLLER BOX

FROM COMPRESSOR Kl::= ~

~ ___ ---J

PRESS. CHANGE ·OVER POT

, , '.' .. · . . . . · " .' . '. · .... '. . . .. ' ... · .. : .. '. , , · .

. ' ...... . .' .. , " , .. "

THE PILE

PRESS (ONTROllNG DIAGRAM.

TO PRESS CHANGE • OVER POT.

1. NORGREN PRESS' REGULATOR. ,'. SAND 2. SOLENOID VALVE OPENED AT ON·ELEC· PHASE.

(a)

CONTAINER . , · . . .' ..... . · .. ' .. .... . .. . '. . ' ...... . . " . · '.' .' . . '.' .. '. . ..... '.'

PRESS - BUTTON RELAY

J RELAY COIL

1. SWITCI-t TO CHANGE DIRECTION.

3. DITTO. BUT AT OFF . PHASE. 4. SHUTTLE TEE . 5. PRESSURE GAUGE.

LEVER ARM

P.S. -=- 2. SOLENOID Vf>tVE OPENED AT ON- PHASE. 3. Dino. BUT AT OFF - PHASE.

(e) Elec. Circuits. /.

FIG. (4' 6). SCHEMATlC DIAGRAM OF PRESS ,CONTROLLER,

(d)

...

and 457TA. The first one is open when

the electrical current is "on" whilst

the other is open when the current is

"off".

(iii) Shuttle T which was provided with two

valves. When one is opened the other

is shut.

According to this diagram the frequency of the cyclic surcharge

is dependent on the frequency of the electrical current. From the

electrical circuits diagram, Fig. 4.6c it will be seen that the

current is controlled either by the timer through which the current

is repeated regularly at a given frequency or by the press-button

relay. This relay was attached to the head of the reciprocating

ram as shown in Fig. 4.6d. Therefore; the electxic current may be

"on" or "off" depending on the contact between the ram and the

bott.om of the lower hanger.

4.5.3 ~e ~xgnitude Of The Surcharge Pressure

In reality tilis research programme is a part of a continuing

programme to study the behaviour of piles under repeated loadings. In

1976 Chan published the results of the first part of that programme

in which the sand surface was subjected to a constant over-burden

2 pressure equal to 100 kN/m. Madhloom (1978) presented another set

2 of results in which the surcharge was cycled between 50 to 100 kN/m •

In the present investigation, therefore, the surcharge pressure

.22 was either constant at 100 kN/m or cycled between 50 and 100 kN/m •

2 Several additional tests were carried out under 0.0 kN/m and 200 2 .

kN/m . to:.:studyti1e inf1uence 'of the surcharge pressure on the behaviour

of the pile.

38

4.6 The Test Piles.

4.6.1. General

FOllt: model piles have been used in this investigation. They

were made of high strength aluminium alloy tube of 19mrn outer

diameter and lS.8mrn inner diameter. They were of three different

lengths, the longer of l, oOOmm, two of 700mrn and the shorter pi le

of 600mrn. Three depths of penetration were examined namely, 570,

380 and 285mm which corresponded to 30, 20 and 15 times the pile

diamet.er respectively. The instrumentation and details of the

piles are shown in Fig. 4.7 Glnd Fig. 4.8.

In order to facilitate the gauging work, the shaft of each pile

was divided into segments. These segments were connected together

by means of threaded internal collar joints as shown in Fig.4.9.

The pile base consisted of conical segment of 60 degrees apex angle,

and was threaded onto the lowest pile segment. The pile head,

whicll comprised a thick tubular section, was welded to the shaft.

The two threaded collars, located near the pile head, were used

to transfer the load from the lever arm to the pile head, Fig. 4.8a.

4.6.2. Measurement Of pile Movements

The pile top and base movements were directly measured by dial

gauges, Fig. 4.10. A "tell-tale" strain rod was used to detect the

movement of the pile base. This rod was threaded to the pile base

and extended out of the pile top. Through the pile body, the rod

was separated from the strain gauge wires by a tube of relatively

greater diameter.

4.6.3. Measurement Of Pile Forces

Measurements of the axial load dis~ribution along the pile depth

were made by strain gauges mounted on the inner surface of the pile

39

Cell No. .1.- -

2.- -

E E

·0 r--­LO

3.- -

4-: ----'-

No. 3D . Length 1000 mm

Diem. 19 mm

Cell No. 1.-

2.- -

3.- -

4.- -

No. 20

Length 700 mm

Diam. 19 mm

Cell No. t-

E E o co M

Cell No. 1.- -

2:-

3.-

1..- -

5:-

E E tn co N

2.-

No.22 Length 700 mm

Diam. 19 mm

No. 15

Length 600 mm

Diam. 19 mm

Each load -cell consist of 4 orthogonal rossette strain gauges of 2 mm gauge length and 120 ohms resistance

FIG. 4-7 MODEL PILES

(0)

~~SLOTTED WIRE OUTLET.

HREADEO COLLARS.

AXIAL

LOAD CELLS 2.3.4.5.

l TANGENTIAL

(e)

IV<IAL

I I I I I. I , I I I ~ )

.... ------"" TYPICAL LOAD CELL.

TOP LOAD CELL. (1 )

FIG.(4·8J. TYPICAL STRAIN GAUGE CIRCUITS.

(b)

:=:E

shaft • Therefore, each instrumented segment of the pile was actually

. acting as a load-cell. Each load-cell consisted of four tem prature

compensated strain. gauge rosettes. The individual rosette comprised

two orthogonal foil strain gauges type FCA-2 manufactured by the

Tokyo Sekki Kenkyujo Company. 'I'he length, the nominal res is tance

and tile gauge factor of each strain gauge were 2mm, 120 ohms and 2.01

respectively. Except the top load cell which was always located

above the sand surface, the axial and tangential strains at the position

of the load cell were evaluated by two independent full bridges.

These bridges were combined into a single bridge at the top load-

cell of each pile Fig. 4.8.

The analysis by which the pile loading at any given depth,

in terms of tile axial and tangential ~trains, was calculated is

presented in Appendix A. The influence of the lateral soil

pressure was taken into account in this analysis.

To increase the sensitivity of the load cells, the inner

shaft surface at the location of the strain gauges were machined

to a thickness which varied from 0.63Srum at the base cell to 0.89mm

at the top cell location. The length of the machined part was

designed so that the shaft was always safe from local buckling, and

the positions of the strain gauges were always far enough 'from the

zones at which the shaft wall thickness were changed. This length

was found to be equal to the pile diameter.

4.6.4. Gauging Technique.

Before mountin~ the loads of each strain gauge were extended by

soldering a sufficient length of 0.2Smm wires in order to be longer

than the length of the pile segment under consideration. The pile

segments were then assembled and the outside position of each strain

40

FIG. 4.10 MEASUREMENT OF PILE MOVEMENTS

(After Chan 1976)

FIG. 4.11 STRAIN GAUGE MOUNTING DEVICE

a) Pile top section .

b) Intermediate pile section.

c) Holding down clamp.

d) Strain gauge mounting rubber pad .

.. ...,.. (After OOi 1980).

gauge was marked before dismantling for cleaning and gauging. A

perspex plate on whtch the rosette axes have been marked was used

to prepare the rosettes so that they became suitable for handling.

Double side sticky tape was first placed on tile plate, and after

tracing the rosette axes a hole was then cut from the tape. The

back of the rosette was then carefully positioned on that hote

so that the axes on the tape coincided with those of the rosette.

The leads were lined up along the tape length and coated with a

layer of adhesive to insulate them. The rosette as well as the

leads were covered by a layer of sticky tape. The latter layer also acted

as an extra precaution to separate the wires from each other. The

rosette, after cutting and shaping the holding tapes, became

ready for mounting on the pile shaft surface. A strain gauge

mounting device which consisted of a.central rod on which a metal

plate was stuck as shown in Fig. 4.11. This plate was a segment

of a cylinder 25mm length and had an inner diameter equal to the

diameter of the rod and an outer diameter equal to the inner diameter

of the pile shaft. On the outer surface of thls segment a rUbber

pad which had been marked with the axes of tbe rosette, was stuck.

A part of this device was the sliding clamp which was used to apply

the pressure on the rosette during the process of mounting. The

method of mounting a strain gauge on the inner shaft surface was as

follOWS. After covering the rubber pad with silicone paper, the

rosette with the backing facing upwards was carefully aligned and

positioned on the pad. with the central rod inside the pile .

segment and after applying the CN adhesive to the back of the rosette,

the pile segment was slid into place and clamped down on to the rosette.

A suitable pressure was applied by the sliding clamp to keep the

41

rosette in contact with the pre-determined position of the inner

shaft surface. The assembly was left two hours to cure before

another rosette was mounted. To release the residual stresses which

were generated during and after the rosette placement, each load­

cell was subjected to many cycles of loading-unloading in a hydraulic

testing machine. Through these cycles, the load response of the

cell was checked. The stability of the load cell over a period of

two weeks was also checked before the calibration test was carried

out on that cell.

4.6.5. Calibration

The calibration of each load-cell was conducted by using dead

weights as shown in Fig. 4.12. A typical result of a calibration

is presented in Fig. 4.13. from which it can be seen that the response

of the load cell was linear. Unloadtng readings were within 0.5% of

those of the loading whlch was in full agreement with that quoted

by Chan (1976). A repeated calibration test after the pile had been

acted upon by a large number of loading cycles indicated that the

calibration factor was not affected appreciably.

4.7 The Data LOgger

The wires of the load cells were connected to the data logger

through terminal boxes. These boxes were provided with an excitation

voltage from a King Shill constant voltage power supply. Therefore

all the output of the load-cells of the four piles can be recorded ..

directly by the data logger. This Solartron Compact 33 data logger

comprises four elements:-

(i) The analogue scanner.

(H) The control unit. Through this unit

the various mode of operation can be

42

FIG. 4.12 GENERAL ARRANGEMEN'r OF CALIBRATION OF LOAD-CELL.

a) Pile s e ction.

b) Dead wei.ghts.

c) Guide .

d) Portable logger.

e) Power supply.

120

~ 100h y )'

.. \I)

-0 0 B eOr I I I

-0 I II OJ

~ 60L ~ J'J' «

I I

I 11/ L.O

y y ~

/ /' //

/ /' // / / ~~

, F --r .--r

//~.

/"' /"'

Pile No. 20

lid = 20

11·0

I • 0·5 I

r7 '0 00 200 L.OO 600 800 1000 1200 1L.00 1600 1800 2000

Response of load cell,)J volts

FIG. L.-13 . TYPICAL CALIBRATION RESULTS FOR LOAD CELLS

Z ~ , -0 0 0 -I

(iii)

selected.

The digital Voltmeter with the display

screen, and

(iv) An auto-mode switching out-put unit

which prints on paper and/or punches

on tape.

43

CHAPTER 5

EXPERIMENTAL PROCEDURES AND TESTING PROGRAMME

5.1 Properties And sand placement

The sand used in this investigation was the same as that

used by Chan (1976) and Moilllioom (1978). In order to clean the

sand it was washed with running water and dried in an oven. Prior

to testing, the sand was sieved between No. 10 and 100 sieves

to remove the coarser and finer materials respectively. According

to the particle size distribution curve shown in Fig. 5.1. the

sand is a medium, uniformly graded sand. The sand was in an air-

dry state with a moisture content of 0.2%. The specific gravity

of the sand grains was 2.64 and the maximum and minimum densities , obtained following the method of Kolbuszewski (1948) were 1.84 and

1.34 Mg/m3• These densiti~s corresponded to void ratios of 0.434

and 0.971 which were slightly different from those reported by

Chan.

preliminary tests indicated that the relative density of the

sand \'las a function of both the intensity of raining and thcheight

of fall, Walker and Whitaker (1967), Vesic (1967). For a 40cm

height of fall with 3.3 x 3. 3mm mesh and 50mm hose diameter, the

3 density was 1.557 Mg/m which corresponded to a void ratio of 0.696

and a relative density of 51.2%.

Before starting a test the sand container wall was coated with a

thin layer of silicone grease. The hopper, which was filled with

sand, was lifted by a crane to be at a suitable height and suspended

directly above the container. This helght was determined so that

the mesh was 40cm above the container bottom. To obtain a uniform

44

).J m mm t A ~ ~

O OlnO cO M N ONO ~ \0 ~NM~\o':- N

I I

~ b?"" 90 I V V I

I / I J

80 i if I

I I I

t7I 70 c

J I , I

III • I III 50 0 a. • ell 50

, : C1' 0 - 40 c ell 0 '- 30 ell

0..

I J

I I : I I J

I I

I I 1

20 I I 1 , I I I

10 I III L If £ I

.....-(/ I

0-1 1-0 Particle size (mm)

AG.5-1 PARTICLE SfZE DISTRIBUTION OF SAND

I

..

embedment the sand was distributed uniformly over the surface of

placement while the hose was continuously tapped gently by hand.

The height of fall was controlled by a small steel washer

suspended from the mesh, which was kept always a few millimetres

above the sand surface.

5.2 Assembly of Apparatus 'And 'Application Of Surcharge pressure.

When the container was filled, the sand surface was levelled'

with a hand rotating scoop which also provided a suitable space .

for the pressure plate. After the pressure plate had been positioned

and its bushing coated with a layer of silicone grease, the cover

plate which was lifted by a crane, was gently lowered, positioned

and bolted to the container. The lever was placed, levelled and

supported on the stopper. With great care the upper loading

frame was mounted and firmly bolted to the lower steel frame.

In order to set the pile vertical and to adjust the jack so

that its centre line became vertical and coincided with the centre

line of the pile, two plumbs were used. By means of these pltuubs the

vertical! ty in two perpendicular planes could be checked.

Initially the pile was set randomly and was supported laterally by

two special guides located at two different ,elevations. The first

guide was at the lever arm while the other was at the top of the

"bushing of the pressure plate. Both guides rested in a pistol'l-fit

fashion in their holes which allowed only vertical movement of the

pile during the driving process. When the lever arm moved in a

horizontal plane" the pile which ... ,as supported by the lever-guide,

rotates in a given vertical plane about its base. Therefore,

the verticality of the pile in this vertical plane was adjusted by

adjusting the horizontal alignment of the lever.

45

After the pile was set vertical in this plane the lever was clamped

to the roller supports. Since the container axis was originally

set vertical and the pile base was located on this axis, hence

, the vertical axis of the pile coincided with that of the container.

To align the axis of the loading jack with that of the sand container

the jack was driven 'down until its lower end became close to the

pressure plate bushing. After the verticality of the jack was

checked and adjusted the centre lines of the jack, the pile and the '

container were made coincident. After holding firmly, the jack was

redriven up and the pile then repositioned before the jack was

brought into contact with the ballbearing located at the top of the

2 water at a pressure not greater than 10 kN/m was introduced pile.

into the space between the pressure plate and the container cover.

To flush out any air bubbles which might be trapped underneath the

container cover a large amount of water was allowed to run through.

The bleed valve was then closed and the chaIDber pressure increased

to Ule required level. This level was monitored by a regulator

mounted on a pressure cylinder. The surcharge pr~ssure was maintained " 2

constant at 100 kN/m in most of the penetration tests except for

2 a few tests"in which the pressure was either 0 or 200 kN/m as

shown in the test programme, table 5.1.

'5.3 Depth Of Embedment

Three different depths of embedment have been examined in this

investigation. They were 570, 380 and 285mm which are equivalent to 30,

20 and 15 times the pile diameter.

5.4 pile Driving

The driving process was carried out by a jack of lOOOmm length

and 25mm diameter driven by an electrical motor of 2HP capacity

with a constant rate of penetration 'of 36mm per minute. Both the

46 '

motor and the jack were carried by a rigid steel frame, Fig. 5.2.

Because of the large resistance encountered during driving additional

lateral supports were provided specially in those tests where the

longer pile was tested and/or a high s~rcharge pressure was applied

to the sand surface.

Since the length of the longer pile was greater than the space

between the lever arm and the cross beam of the upper loading

frame, the cross beam was designed in such a manner that it could

o be rotated through 90 as shown in Fig. 5.3.

5.5 Static Loading Tests

These tests were carried out to study the load-displacement

response of the pile before and after the pile was tested under

repeated lOAding. ~~e method of testing was by incremental

loading, Whitaker and Cooke (1961), in which each load increment

was maintained until settlement had ceased before adding the next

load increment. The top and base displacement of the pile were

measured by dial gauges. After the pile movement had stIDstantia11y

ceased, scannn,ing of load-cell readings was carried out for each

increment of load.

5.6 Repeated Loading Tests

The arrangement of such loading, whieh was discussed in

section 4.4 is shown in Fig. 4.4. The repeated load was generally

taken as a percentage of ultimate capacity of the pile and applied

in the form of dead weights.carried by the load hanger. The lower

repeated loading was placed on the upper part of the hanger while

the lower part of the hanger carried the amplitude of the repeated

load. Thus, in a complete loading cycle the reciprocating machine

carried the lower hanger up, then down to the original position in

a 60 seconds period. The measurements made during each test were

47

Hi nge

ain Ch dr ive

E u

M M

-.n

1 ~

L ..

-

I .

Screw jack .'-.".P-~ @ ~ ~

~ -== ~ ~

900 Swinging ~ ~ ~

support beam ~

/ ~ ~ ~

I

....... J l ..Il'lllill I I I I I I liD I

V ~ ~ ~ E ~ u ~ 0 ~ a

,\ ~ ..... ~

~ ~ r- r§ ~

\. I ./ ~ ~

El~ctric ~ ~ .:;.

V motor ~ ~ / ~

Upper part of the ~

70 em loading frame .1

E u

M M

E u ..-C1'I

FIG. 5- 2 DETAILS OF THE SWINGING JACK AND ELECTRIC MOTOR

Position (2)

, .

~--=-=- =~.:=-=:: ==..=:=--

o

Dead weights to counterbalance the weight of the motor

., r FIG.5-3 TWO POSITIONS OF THE JACK ALLOWING FOR

PILE INSERTION

the reading of the load-cells and the pile movement which were taken

at the end of the following cycles, 1, 2, 5, 10, 20, 50, 100, 200.

In each of these cycles measurements were taken when the repeated

loading was on the high load level and when it was on low load

level. When the pile moved a large amount (generally more than

0.7 of the pile diameter) or when the rate of movement of the

compression pile changes·from unstable to another stable stage,

the repeated load test was terminated.

5.7 Re-levelling of The Lever Arm

With increase in the number of cycles the pile movement

increased and the lever arm was no longer horizontal. The re­

levelling of the lever was achieved by the aid of the thrust

bearing screw Fig. 5.4 and was done slowly and smoothly during

the lower loading phase. The horizontality of the lever was

checked frequently by a water level mounted on the top edge of

the lever.

5.8 collecting Of rata And I ts Analysis

From the data logger output and the dial gauge readings

togetl1er with a computer programme the following were calculated :-

(i) Variation of the rate of movement with

the number of load cycles.

(ii) Variation of the axial pile loading with

the number of load cycles.

(iii) Distribution of the average skin

friction along the pile depth and

the variation of this friction with

the number of load cycles during the

high and during the low repeated

loading.

48

RoUer bearing

Sand container

r;=== Hydr~static 1-----=nI ,"==--,"r:..~~ pre ssure

Pressure plate

Pile

Sand

O.Ring Rubber membrane

FIG. 5-4 DIAGRAMMATIC DETAIL OF THE LOADING LEVER

5.9 Test programme

The test programme was concerned with the behaviour of both

compression and tension piles when subjected to different types

of repeated loading. Variation in the state of the effective

stresses around the pile was also considered in this programme.

The test programme was divided into the following main parts :-

Part A,wh(,h deals with the general behavlour

of piles under the condition of static

surcharge pressure, table 5.1.

Part B, which deals with the general behaviour

of the pile when the sand surface 1s

subjected to cyclic surcharge, table 5.2.

49

TEST 5.1

STATIC SURCHARGE TEST PROGRAMME

Test Type Range of Past Surcharge of Repeated Load pres2ure R,

Loading Loading -Series No Name History kN/m d Remarks

1 - Compression - . - 100 15 2 - " - - 0 20 Preliminary 3 - " - - 100 " 4 - " - - 200 " Tests for

1 5 - " - - 100 30 6 - Tension - - 100 15 static-7 - " - - 0 20 8 - " - - 100 " capacity 9 - " - - 200 "

10 - " - - 100 30 determination

** V+ 11 3015 Compression 0.3Qc/0.e, 100 30 12 3013 " A++ " " 13 2005 " V 20 14 2202 " A "

11 15 1507 " V 15 16 1505 " A " 17 3023 0.5Qc/0,.0 V 30 18 3017 0.7Qc/0.0 V " 19 2003 0.5Qc/ A 20

0.2Qc 20 2015 " 0.5Qc/0.0 V " "

*** 21 3020 Tension 0.3Qt/o.0 V 100 30 22 3016 " A " 23 2210 " V 20

111 24 2205 " A " 25 1509 " A 15 26 2016 O.5Qt/0.O V 20 27 2209 0.5Qt/ V "

0.2Qt

28 2009 Compres.sion 0.3Qc/0.0 A 0 20 29 2008 " " A 200 I "

1V 30 2207 Tension 0.3Qt/0.0 A 0 " 31 2010 " " A 200 "

so

TABLE 5.1 (cont'd)

Test Type Range of Past Surcharge of Repeated Load pres~ure R,

Loading Loading History -Series No Name kN/m d .Remarks . . . , .. . . . .

, 0.7Qc/0.0 32 3017 Compression 0.5Qc/0.O V 100 30

0.3Qc/O.O

0.3Qc/0.0 33 3018 " 0.5Qc/0.0 V " 30

0.7Qc/0.0

V 0.3Qc/0.0 . 34 2007 " 0.5Qc/0.0 V " 20

0.99Qc/0.0 0.3Qc/0.0

0.3Qc/O.O -0.3SQc/0.0 . 0.3Qc/O.O

35 2008A " 0.4Qc/O.0 A 200 20 0.3Qc/0.0 O.SQc/O.O 0.3Qc/O.0

36 2011 Alter 0.lSQc/0.15Qt V 100 20 'Vl 37 2012 " .15Qc/.3Qt V " " .,

38 2013 " .5Qc/.15Qt V " "

39 2015 Compression 0.5Qc/O.0 V 100 20 V11 Alter .5Qc/.3 Qt

40 3025 Compression 0.7Qc A 100 30 Creep Test

*. R, denotes ratio of the enbedment depth to the diameter of the pile. -d

** Qc denotes static ultimate load in compression.

*** Qt denotes static ultimate load in tension.

+ V denotes test carried out after installation.

++ A denotes test carried out after ultimate loading test.

51

61)

'"

Series

V1ll

lX

X

Xl

Test

No

41 42 43 44

45 46

47

48 49 50 51

52

53

54

Name

3014 3019 2205 3021

2204 2208

2004

2000 2014 3022 3024

2208;.

20l4A

3022;.

Type of

Loading

Compression

" "

Tension

Co:::pression "

" -

Co:r:pression "

Ta.'1sion Ccr::pressio!l

Co:;:pressio!l

Co:::pression

Tension

TEST 5.2

CYCLIC SURCHARGE TEST PROGRAMr-1E

Past Surcharge kN/m2 R- '.

Ra.'1ge of Load Repeated Loading History Max - Min Frequency d Remarks

C,.D_.m

0.7Qc A 100/50 1 30 R.=depth of

" V " 1 " embedment

" V . " 1 , 20 d=diameter of pile 0.7Qt V n 1 30

i

O.3Qc/0.0 , " V 100/50 1 20 High surcharge

0.5Qc/0.0 V " 1 " pressure coincides - with high load level. O.3Qc/0.O V 100/50 1 " High surcharge

pressure coincides with lCM load level.

i

O.3Qc/0.O V 100/50 1.5 20 ~ Surcharge " V " 3 "

0.3Qt/0.0 A II 3 30 ) frequency

0.3Qc/0.0 V " 1 30 Loading frequency. (3 c.p::n.) .

O.5Qc/O.0 I V 100/50 1 20 Different O.3Qc/0.0 coIrlbinations 0.3Qc/0.0 0.5Qc/0.0 0.7Qc/O.0 V 100/50 3 " 20 0.7Qc . O.9Qc 0.3Qt/O.0 O.5Qt/0.0 A 100/50 3 30 O.5Qt . 0.5Qt/0.0

VI

'"'

Test

Series No

Xl 55

56

:

Type _of

Name Loading

3023A Compression

2016A Tension

-------- -

TABLE 5.2 (cont'd)

Past Sur0arge kN/m 2

Range of Load R. Repeated Loading History Max - }lin Freauency d Remarks c;p.m.

O.5Qc/0.O 100 - "

O.5Qc V 100/50 1 30 O.5Qc/0.O 100 I

O.5Qt/O.O ,

, 100 ".

O.5Qt V 100/50 1 20 ,

O.5Qt/0.O 100 , - I I i

I

I - _ .. - --------------- --- - --- ______ L --- ------- I

..

CHAPTER 6

PRESENTATION AND DISCUSSION OF STATIC-SURCHARGE TEST RESULTS

6.1 General

This chapter is mainly concerned with the results of tests

carried out on piles subjected to repeated loadings and embedded

in a sand on which a constant surcharge pressure acted. OUt of

forty tests in this part of the investigation ten tests, series 1,

were performed to study the static load-displacement characteristic

of both compression and tension piles.at different depths of

enmedment. Through the second and the third test series, the

influence of depth and load level on the behaviour of piles

subjected to 'repeated loads was examined. The effect of

surcharge pressure was examined in the fourth series. Different

combinations of repeated loadings were studied in the other three

test series. In the following discussion standard terms used are

defined as follows:-

(i) Qc and Qt are the ultimate bearing capacity

and the ultimate· pulling resistance of the

pile respectively.

(11) The load amplitude is the difference between

the upper and the l~~er repeated load levels.

(lii) The pile movement is the irreversible

accumulated. displacement of the pile which

is produced from repeated loading. It was

measured in that half cycle at which the

~agnitude of the repeated load was at the

lower level.

54

. 6.2

(iv) The pile life-span is the number of

~ycles beyond which the pile movement

exceed a certain allowable lim! t. For

comparison, this limit was taken lOmm

in this investigation.

(v) The rate of movement is the pile

movement per load cycle.

(vi.) The depth ratio is the ratio of the

embedment depth to the pile diameter.

(vii)

(viii)

The base load is the load that is

applied on the pile base when the

repeated load is at its upper level.

The residual base-load is the load that

is applied on the pile base when the

repeated load is at its lower level.

( ~x) Virgin pile is a pile that has not

been subjected to any loading after

installation.

series 1: Preliminary tests •

These tests were performed to determine the magnitude of

the ultimate capacity of the pile from which the repeated load

levels were established. They were also used to check the

performance of the test equipment and the repeatability of the

. results. The influence of depth of penetration as well as the

surcharge pressure on the driving resistance were also

examined in this test series.

6.2.1 Driving resistance

The variations in driving resistance of the pile base with

ss

depth are presented in F,ig. 6.1. 'rhese tests were conducted with

2 100 kN/m surcharge pressure. It is clear that the resistance was

independent of the lengthof pile employed in the test, that is at

lOOmm penetration, for example, the driving resistance of the

longer pile and that of the shorter pile were approximately the same.

Initially tl1e pile base resistance increased rapidly with depth

until it reached a maximum limiting value after a penetration which

varied between 5 to 10 times the pile diameter. Beyond that it

decreased slightly and reached an almost constant final value. 2 .

Tests carried out with 200 kN/m surcharge pressure, Fig. 6.2,

revealed the same trend but the maximum driving resistance was

attained within a penetration of 4 to 7 times the pile diameter.

2 At 0.0 kN/m surcharge pressure the resistance steadily increased

at a rate which decreased as the penetration increased. A comparison

between the three curves of Fig. 6.2, indicated that the depth at which

the pile base resistance reached a maximum value (critical depth)

decreased as the surcharge pressure increased. 'l'he present

2 observations at 100 kN/m surcharge pressure are supported by

Chan (1976) and Madhloom (1978). They attributed the high initial

driving resistance to the presence of the rigid pressure plate which

restricted the upward movement of the soil during the shearing

stages. The driving resistance, therefore, increased due to the

change in the pattern of the slip failure s~rface, IIanna (1963).

Regarding the decrease in the penetration resistance after the peak,

Chan stated that this decrease was mainly related to the reduction

in the magnitude of the effective vertical pressure which was

caused by the container wall friction. The b~se penetration resistance

of the pile has been investigated by Biarez and Foray (1977). They

56

E E

c .2

100

200

C 300 '--GJ C GJ

a..

LOO

500

Base driving resistance "NewtonsI' 400 800 1200 1600 2000 2400 2800

o lid = 15

A lid = 20

c tid = 30

Surcharge = 100 kN/m2

Rate of penetration = 36 mm/min.

Dia. of pile, d = 19mm

5

10

15

20

25

30

6001~----------------------------------~

FIG.6-1 VARIATION IN DRIVING RESISTANCE OF

PILE BASE WITH DEPTH

~ -.. 0 -0 '-'-GJ -GJ E 0 'U 0 -.c -a. GJ

0

l

00

100

E E 150

c .2 15 200 "--<11 C <11

a... 250

300

, , 1.00

-.

\ \

\ \ \ \

Base driving resistance t

800 1200 1600 2000 21.00 ..--- .. -...........

- .... '" ...

•

'\ .. "

\ t

• \ \ 1 \

\ \

\ 0 kN/m2 \ \ \ \ \ \

, ,

,/ , i 100 kN/m2

, ,

/ • •

I lid = 20

3600 1.000 1.1.00 I I 1

200kN/m2

5 0 .--a "-

"-0) -0)

E a .-

10 "0 o -.c -c-O)

o

15

: Rate of penetration = 36 mm/min. \ \

350' ,

FIG.6-2 INFLUENCE OF SURCHARGE PRESSURE ON THE BASE PENETRATION RESISTANCE

--~'" ", .. ""'-'--.----"' .. .,.., .... _. ------.- .-..

found that,' with zero surcharge pressure, the resistance always

increased with penetration but, when a surcharge prssure was applied

on the sand surface, the resistance reached a maximum at a certain

critical depth and then decreased as the penetration was increased.

Moreover, the critical depth was found to decrease as the

surcharge pressure increased. To explain the phenomenon of limiting

value of penetration resistance, Biarez and Foray postulated that,

for a homogeneous sand medium the limiting value was related to the

critical confining pressure, (Lee and Seed, 1967). At shallow

depths of penetration, the meun applied stress under the pile point

was smaller than the critical confining pressure and therefore the

sand dilates. As the. penetration increases the mean stress

increase~ too and hence, after a certain depth, this stress becomes

equal to or higher t.han the critical confining pressure and thus

the sand compresses. Beyond the critical depth the point resistance,

therefore, becomes constant. The phenomenon of limiting resist.ance

attracted the attention of many researchers. Vesic (1967) attrjhuted

this phenomenon to some kind of arching assumed to take place along

the pile depth and hence affecting the state of stresses around the

.pi1e shaft. Meyerhof (1976) indicated that the soil compressibility

crushing and stress history may also contribute to such a limiting

value. Based on field observations Meyerhof (1977) reported that

the critical depth, which ranged from 5-15 times the pile diameter

depended on the sand density.

The values of the skin friction during identical driving tests

were much less repeatable than the base resistance as shown in Fig.

6.3. At the beginning of penetration, the average skin friction

was relatively high but after a short penetration, generally not

57

E E ..

Average skin friction, kN/m2

OO~ ___ 4rO __ ~8~0~~1~2~0~~16~0~~20rO~~2TL~0 __ ~2~8~0~~3~2~0~ ., .. ~:~': :.::':< \:.~"; : : ": ::.:;."> .. : ... ,' ~ ~~~...:......-___ .-.~~. _' __ --

LO

160

400

1.1.0

1.80

. :-~ ......... : ". ..' ... ;-..... " .. .. I ,. • .... .. .. ... , ,-: i~::; .. ; ~.:'. :.... .'.:

,_ I •• " .t .-t :'.::"{.:~:;j,/~ / ::'/ ;1,:',

" ~. ~~;.~;'~.' ::.. .. -to ~ ;;. -' .- .. " .. ' tt

.~ .... s :.. '! '." .. ,t: .... .... ., -'" :." ,v:, •• -- .:~ .. :t, 't .. . :' .' .......... Ie .. 't "1_' ,:,., '.~ ": :.,'" ......... .. :'I~" '. ~.~! :,;'::~~:::':,~ ',: ..... ,... "f ., ..... ' .. ' '-., ., ,,'. .' .. .. "'.:' "t

't ,t " ;': '!'.. .., .. ~ .. :l ;', =' .. " ,

.... ," 't.. ~ ••• '

... : ,-':.':,.. " Ie ' .. ,',_. " ••

":. .!..·t, : ... :. -._ .. ,'" ,.. .. , ..

.. . ', : ',.,' .. ... .. ...... .. . .. .. .. .. ' ... -.. ;..,' " .. :' '.~', . : ..

.. .. ... ': .. ' .. .... .. " .. 't, ,t " I' : .... " :. It.... :',.

.. .. I 't' , .. ' " ." I, • n. \ ..... .:. ..

...... " .. It : .'; ... .".. • It.. ..

, .. ,. ': .. " , ..

. 0· .". , •. , ..... • • • • f .. . , . . . .. ' . . . · . .

'.

.. . . . . . ,

" . , ,

Surcharge pressure

q =100 kN/m2

FIG, 6-3 VARIATION OF SKIN FRICTION WITH DEPTH OF PENETRATION

more than two pile diameters, it decreased to a value between 20 to

2 80 kN/m. It is believed that the early high skin friction was

due to the presence of the pressure plate as concluded by Chan (1976)

and Madhloom (1978). The scattering in skin friction during the

subsequent penetration stages is thought to be attributed to the

random change in properties and state of motion that took place in

the sand during the process of pile penetration. The local skin

friction at any given depth, therefore, depends on the void ratio

as well as the magnitude and direction of the relative movement

between the soil and the pile shaft at that depth. Results of similar

tests carried out by Madhloom (1978) revealed that the value of the

skin friction did not repeat at any given depth when the penetration

test was repeated. Furthermore, the scattering in many times

was greater than that quoted in this investigation. The estimation

of the pile shaft capacity by means of the Cone Penetration Test is

generally based on the cone resistance rather than the local sleeve

friction, Thorburn and Buchanan (1979). 'I'his is because the local

sleeve friction is not repeatable such as the cone resistance

(Beringen et al (1979).

6.2.2. Behaviour of piles under static loadings

Results of typical static compression load tests carried on

piles embedded at various depth ratios under the influence of

2 100 kN/m surcharge pressure are presented in Fig. 6.4. It can be

seen that the load-displacement relationship of the pile became stiffer

and the ultimate capacity was increased as the depth increased.

Piles tested at a depth corresponding to 30 times the pile diameter

failed at 3.13 kN at which the recorded ultimate base resistance and

shaft friction were 1.60 and 1.53 kN respectively. At 20 diameters

58

320r' --------::=====:::-:-------(30) -13

280~ / ____ (20)

21.0t- I / ( 15 )

.2 200' " , 0 ~

... I III '.

Z ~ .. "'0 a 0 -'

120~11 Surcharge = 100 kN/ m2 11 III

80

40

0' I I '0 o 1 2 3 4 5 6 7 8

Displacement. mm

FIG. 6-4 LOAD - DISPLACEMENT BEHAVIOUR FOR COMPRESSION LOAD TESTS

depth the ultimate total, base and shaft friction were 2.75,

1.75 and 1.00 kN respectively, while they were. 2.45, 1.72 and

0.73 kN respectively when the pile was tested at 15 diameters.

The variation of these capacities with depth is shDwn. in Fig.

6.5. The curve of the total ultimate capacity indicated that this

capacity increased with depth but at a decreasing rate, which is

in agreement with Vesic (1967). Although the state of equilibrium

involved in static tests is different than that of driving tests,

Yong (1970), the variation of the ultimate base resistance with

depth had a similar trend in both modes of testing. The maximum

limiting value was smaller and was attained at a greater depth in

the case of the static test. The results of driving and static tests

conducted by Chan (1976) revealed that the final penetration resistance

of the pile base was approximately 2350 newtons •. This value

dropped to 2100 newtons when the pile was subjected to a subsequent

static load test. The reduction in the ultimate base resistance

after the pile was placed in dense sand was also reported by Yong (1970).

He related this phenomenon to the relaxation of the pile. Since dense

sand tends to dilate during shearing and the static load test is

generally performed many days after the placement of the pile therefore,

due to stress relaxation, under subsequent static loading the pile

will experience a smaller base resistance. The Ultimate shaft

friction was found to increase approximately linearly with depth as

shown in Fig. 6.5. This variation implies that tile average unit

skin frict:i.on did not change with depth. In an attempt to explain

this phenomenon the following is suggested. After the dense sand

is sheared during the process of penetration by the pile base it

receives high monotonic shear strains (36mm/min) from the pile shaft.

59

L. OJ -OJ E 0 0

"-.c. -c. OJ

"0

"0 OJ "0 "0 OJ .0 E UJ

Load, kG

00 100 200 300 \ -....;;; \ ....... \ ....... , ....... , "-

\ , \ , , ,

\ , .\ 10

, \ \ , \ \

\ \ \ , ,

\ , I \ \ I 20 \

\ I \ \ . I . /

Shaft '\ I Base \ \ I \

\ , \

30

L.O Surcharge = 100 k N / m2

·0 1 2 3 Load,kN

FIG. 6-5 VARIATION OF ULTIMATE RESISTANCE WITH DEPTH

I· i I~

I I

'I l

I

These strains decrease in magnitude as the radial distance from

the pile increases, Cook (1974). The sand grains along this

distance will, therefore, be set-up in different packings. Close

to the pile shaf.t and due to the high s]1ear strains the grains are

arranged into a loose packing, Thorburn and Buchanan (1979) or

more precisely into a state of critical void ratio (Youd (1970). This

state covers a distance of approximately ten grains and beyond that

the void ratio decreases as the distance from the failure surface

increase, Roscoe (l970). Since the sand was initially homogeneous

and subjected to a relatively high surcharge pressure, the critical

void ratio will not change with depth. Because the behaviour of the

pile at failu~is governed by the strength and deformation properties

of the small volt~e of soil close to the pile shaft, Gallagher and

St John (1980)1 therefore the variation of the ultimate skin

friction with depth under these conditions of testing will be

constant.

Fig. 6.6 shows the load-displacement curves of pull-out tests

conducted on piles at different depth ratios and tested under the

. / 2 influence of 100 kN m surcharge pressure. At each depth the test was

repeated to assess the effect of the residual stresses on the behaviour

of the pile. This family of curves revealed that the ultimate pulling

resistance of the pile increased as the depth of embedment was

increased •. They were 1.20,0.55 and 0.40 kN when the pile was

first tested at 30, 20 and 15 depth ratios r~spectively. The residual

base load after penetration at the depth ratios mentioned above were

compression of 318, 182 and 93 newtons respectively, and in tension

of -42, -33 and ·-20 newtons respectively after the first tests had

been performed. In fact, the residual base load after testing the

60

i'

I

160,~-------------------------------------------------------------------------,

C) ..::t.

"'0

100

o 80 o ..J

60

20

. \'1,';;\ ~\(<;\

I I I , I

l20 l Repeated - -I

I I I

Surcharge = 100 kN/m2

. (15) Repeated

1-5

1·0

0-5

a' r I I '0 o 1 2 3 I. 5 6 7 8 9 10 11 12

Displacement, mm

FIG.6-6 LOAD-DISPLACEMENT BEHAVIOUR FOR TENSION LOAD TESTS

Z ..::t.

... "'0 o o

..J

pile in tension must be zero. However, because the base load cell was. actually

one pile diameter above the base, the recorded load represents the

friction of the last one pile diameter segment of the pile. When the

test at each depth ratio was repeated, the resulting load-displacement

relationship became stiffer and the ultimate pulling resistance

was increased. This increase in the pile capacity may be related

directly to the change in state of the residual stresses. At the

beginnlng of the first test the pile was entirely subjected to

compressive residual stresses. These stresses became entirely

tensile at the beginning of the repeated·test. To compare the

failure shaft friction during the compression loading with that

during tension loading, the results of the corresponding tests are

drawn together in Fig. 6.7. From these curves it may be concluded

that the shaft friction in compression was always greater than that

ill tension by a magnitude dependent on the depUl of embedment and the

state of ~~e residual stresses. The ~nfluence of surcharge pressure

on the load-displacement response of compression piles tested at

20 diameters depth is shown in Fig. 6.8. It can be seen that the

increase in surcharge pressure produced a stiffer load-displacement

relationship. The ultimate load capacity of the pile increased as

the surcharge pressure increased. When the results of these

tests were plotted together with those of tension tests conducted

on the same pile as shown in Fig. 6.9 it was noticed that the

ultimate shaft friction in compression tended to reach a limiting

value beyond which it became independent of the surcharge pressure.

In contrast, the tension shaft friction, which was initially of

smaller value as compared with that of the compression test, was

continued to increase when the surcharge pressure increased. The

61

Load. kG _. O~ 20 40 60 80 100 120 140 160 180

. 10

o -o '- 15

..c. -a. <V o

20

25

. 30

350

~o

\.~ \ 0

\\. \~" , \ ..

, ". , 0

, 0

, " \ 00

'" ",-.s-,S'/Q ' .. ~ ~ ~~ , "~I} .

, oO~1) "" ........ .If?E>1J

" ~%i . ~E>.o.tE>(j) .... !?/) II) ~ .. _ " ...... , ~ -...... --

0-5 Load. kN

1-0 1·5

FIG. 6-7 VARIATION OF ULTIMATE SHAFT FRICTION WITH DEPTH

700~i ----------------------------------------------~

600 6·

500 200 kN/m2 lId = 20

(!)

~ .. 1.00 -u. "'0

I / c z 0

~

..J ..

"'0

100kN/m2 a 0

..J

2

I o kN/m2

Compression

.°0 1 2 3 4 5 6 7 8 9 .100

Displacement, mm

FIG.6-8 I NFLUENCE OF THE SURCHARGE PRESSURE ON THE LOAD - DISPLACEMENT . BEHAVIOUR OF THE PILE

difference between the trend of the compression and that of the

tension friction ,may be attributed to the residual stresses as

indicated above. The effect of surCharge pressure seems to

increase the ultimate base resistance at an increasing rate as

shown in Fig. 6.9.

In Fig. 6.10 the residual base load as a percentage of the

ultimate base resistance is plotted against the surcharge pressure

and against the pile depth ratio. It is clear that the residual

base load increased at a decreasing rate when the surcharge pressure

was increased, and rapidly reaChed a limiting value. In contrast,

with increase of the depth ratio the residual base load increased

at an increasing rate up to a depth ratio of 20. Beyond that it

began to decrease gradually which indicated that the residual base

load was also tending to reaCh a limiting value.

6.3 Series 11: Compressive repeated loads ~ith a constant surcharge pressure equal to 100 kN/m2

As mentioned earlier, the present investigation is part of a

continuing research programme to study the behaviour of piles under

repeated loads. The first part which was conducted by Chan (1976)

was concerned with the behaviour of piles embedded at 30 diameters

depth in sand upon which a constant surcharge pressure of 100 kN/m2

was acting. The behaviour of compression as well as tension piles was

examin~d under a wide range of load amplitudes. Therefore, in

this section, as well as in the subsequent sections of this chapter

the investigation was concentrated on the influence of both the depth

of embedment and the surcharge pressure on the performance of piles

subjected to repeated loads. The repeated loads chosen for this

purpose were 0.3 Qc/O.O or 0.3Qt/0.0 which means that the pile load

was cycled between zero and 0.3 of the ultimate compression and

the ultimate pulling resistance of the pile respectively. The

62

800

700

600

~ 500 .. -.-o ~ 400

·0 ·0

Q}

"0 300 E

100'--",.....--'

/ /

/

/' ;/

/

""

Camp. total

/ Camp. base ./ /'

Tension repeated ",._---- -- Comp.shaft

.,..,.,..,."... ...... Tension first

8

6

4

2

~----~--~~--~----~----------~----~O 100 200 300

Surcharge pressure t kN/m 2

FIG. 6'9 VARIATION OF ULTIMATE CAPACITY

WITH SURCHARGE PRESSURE

z .x .. "'0 a 0

...J

Depth rat io , ( Yd ) Q)

1000 5 10 15 20

Ol 0 .....

OJ C Q) U

~ C 0

Q) ..... 80 c. V)

a V)

OJ en L-

a OJ "'C V)

60 0 0 0 .0 -Q)

Q) - , en a 0 E .J:l 40 -- --0 :J :J OJ

"'C .J:. en .....

20 Q) -0: 0 surcharge ( lfcj = 20 ) ----------"",----0

0 100 200 300 Sur~harge pressure, kN/ ~2

. FIG. 6-10 VARIATION OF RESIDUAL BASE LOAD' WI TH

SURCHARGE-PRESS. AND WITH DEPTH

." .......

reason behind the choice of such repeated loads is that the design

load of a foundation pile is, in general, limited within this range.

piles were also subjected to a range of loading conditions in this

part of the investigation.

6.3.1.· Effect of embedment depth

The movement of piles examined in Tests 11, l3and 15 at

30, 20 and 15 depth ratio respectively and subjected to 0.3Qc/0.0

repeated loads are shown in Fig. 6.11. Those of Tests 17 and 20

which were at 30 and 20 depth ratios respectively under a repeated

loading of 0.5Qc/0.0 are presented in Fig. 6.12. It is concluded

that the life-span of the pile decreased when the embedment depth

increased. the variations of the rate of movement and the pile base

load against the logarithm of number of load cycles of Tests 11,

13 and 15 are shown in Fig. 6.13. At any depth, the pile was stable

during the initial stage of repeated loading. After a certain number

of load cycles the rate beg~ to increase until it reached a maximum

value then decreased as the number of cycles was increased. The

value of the maximum rate appeared to increase as the depth of the

pile increas~d. It was 0.0115, 0.0062 and 0.OO43mm/cycle for 30,

20 and 15 diameters depth respectively at approximately the same

number of cycles (around the l300th cycle). During the intial few

CYcles an upwards movements were observed before the pile began to

penetrate into the sand. These movements may be attributed to the

change in state of the residual stresses from those generated after

penetration (failure loading) to those corresponding to a loading

of 0.3Qc. Because the upper repea.ted load level wa:3 always O.3Qc

the difference between this load and the base load was equal to the

embedded shaft friction. Similarly and because the lower repeated

load level was zero in each cycle the residual base load was equal

to the residual shaft friction but with negative sign. As shown in riff.

63

Number of cycles (Log)

100 '10' 102 103 104

2

4

E 6 E . -c Q)

E8 Q)

> o :E

10

12

R.L. = 0·30 Oc /0·0 Virgin Surcharge = 100 kN/m2

'.

-=:<: i

'4' ,

FlG. 6-11. INFLUENCE OF EMBEOMENT OEPTH ON THE PILE LI FE SPAN

0' 10' 2 No. of cycles (Log) I 10 103 . 4 == I 10 I

2

-C <lI E ~ 8 o ~

10

12

14

R.L = o·sac/o·o Virgin

Surcharge pressure = 100 kN/m2 ~\ .. w 0

, ~ . ---\~ ,0 ,

\ , , 16~'----------------------------------------------------------------~

FIG.6-12 INFLUENCE OF EMBEDMENT DEPTH ON THE PILE LIFE-SPAN

(])

u ~ u

A

No. of cycle s (Log)

10 1 . O' 10 .102

. _', 104

RES. 15. Test 15 '> .>" _-- - ___ ~:---- a 2 =. -..= :".--=--.-==:-.:-RES:io. --:-. -Te~t:·13 -- -:-: ==--::- ::.:.:::.5.;:- -- - «-(}~'!Y

RES.30. Test 11 '. 7 /;;() ,. ""j, \ ",. ... ... <l ()'\v

I. \ .. ' '" ~ .. \ ' '" , / " , I

Test 15 ' --,... Base Load 15 - ____ ---- __ - ------------ 1 , 6·~----------:.=-:='::---- -- -- -- ----,' r-- __ ._ -- I I

200

1.00

____ -.~ d 20 ----- --,.- I Base L~ Test 11 .'~ -,!-----------",

• I ease Load 30 ' 8·

" E ME 10 -

600 V)

c o -.~ (}J

000 z

-o

0'3Qc/O'0 VIRGIN. q. = 100 kN 1m2

-"'0 o o

1000 ~

1200

141 '1400

RG. 6-13 VARIATION OF PILE LOAD AND RATE OF MOVEMENT WITH LOG NO. OF CYCLES.

6.13 at any depth, when the number of load cycles increased the shaft ,

friction increased up to a peak value after which it decreased

progressively and reached a limiting value. This limiting value

increased as the embedment depth of the pile was increased. The

number of cycles through which the shaft friction dropped from the peak

to the limiting value appeared to increase when the pile was placed

at a greater embedment. If the rates of movement of each test are

related to the shaft frictions it will be seen that the rate began

to increase at the cycle in which the shaft friction started to

decrease from its peak value and continued to increase so long as

the value of the shaft friction was decreasing.

The distribution of axial loads and residual axial loads

with the corresponding skin friction during three selected cycles

of Test 11 are presented in Fig. 6.14. The selected cycles were the

. first cycle, the cycle at which the shaft friction reached its peak

value, and the last cycle in the test •. As can be seen in Fig. 6.l4B,

initially and up to the cycle number 50, the skin friction of all poin~9

along the pile shaft increased. Beyond that cycle it decreased

progressively as the number of cycles was increased until it reached

almost a constant value along the upper half of the pile. Along the lower

half the· skin:·friction increased as the depth of the pOint was

increased, and became equal to that of the first cycle for points

located within the last three diameters of the pile. 'l'he

distribution of the residual skin friction, unloading phase, revealed

a continuous decrease from the first cycle until the end of the

test. In contrast to the loading phase the repeated loading

reduced the residual skin friction of the lower half of the pile

significantly.

64

Load • Newtons

o 1.00. 800

Skin friction,kNj'm2

o 10 20, r -- T - T----.----y-----. r- -1-1

I : I

'\

h! • I , I

/ I , I

N= 1

N= 50 ,

!----N=1930 I

I : 1 , •

(A) ( B)

At 0·3 Qc

Loading phase

Load I Newtons Skin friction,kNjm 2

o 1.00 -20 -10 0 ,- ~ -~

I~\ I! I ~

i I ~ I ,

I \ \ , I I ~. , I

\\ ,

I r I ' ~ ,

\ \ \.\ (C) (D)

Residual

Unloading phase

FIG. (6-11.) DISTRIBUTIONS OF AXIAL LOAD AND SKIN FRICTION DURING SELECTED CYCLES OF TEST 11

6.3.2.Effect of static failure loading on the subsequent, behaviour , of piles subjected to repeated 'loads.

The results of Test 12 which was performed on a pile of 30

diameters depth and subjected to a static failure load test before

being tested under repeated load of 0.3Qc/0.0 are presented in

Fig. 6.15. When the movements and the rates of movement of this

test are compared with that of Test 11 as shown in Fig. 6.16, it

will be seen that the pile life-span was reduced when it was

previously subjected to a static failure loading •. The load transfer

characteristic was also affected by the previous failure loading.

Fig. 6.17 shows the load transfer of this pile together with that of

a virgin pile, Test 11. At the beginning of the test, the shaft

load of the previously loaded pile was a maximum while it was a

minimum in the case of the virgin pile. After about 600 cycles the

shaft load of both piles reached a limiting value which appeared

to be independent of the pile loading history. The residual shaft

load of the previously loaded pile was always smaller than that of

the virgin pile. The difference was a maximum at the first cycle

and a minimum at the end of the test. Similar conclusions were

drawn when the results of Test 14 and Test 16 in which the pile had

20 and 15 diameters depth respectively, are compared with those of

Test 13 and Test 15 respectively.

6.3.3. Various load levels and amplitudes

Fig. 6.18 shm.,rs the pile movements for Testsll, 17 and 18

against the logarithm of the number of load cycles for various

repeated load levels applied to a pile of 30 diameters depth. It is

clear that the life-span of the pile red~ced when the load level

was increased. The number of cycles required to develop the state

of maximum rate of movement decreased from 1300 to 400 when the

65

"

No. of cycles ( Log)

1 101 102 103 Or 0, 0 ft \::" 0: ~ _ I . 10'0

4r 2--0- - - -<>- __ -0-- - __ ....Q....a_ -0-0--

Base Residual. 200

81- 4'r- I \~ \~ --1400 Q)

0 ~

~ 12~E 5k- ~,--- .... Base Load. \ \ -1600

~ ~~ ~ I ~ooo tJ)

C M C 0 '0 Q) -~. 16 ~ 8

~ (1)

z C

~ 20~L10'F t ~ ~1000 ~ 0·3Qc = 971 N \ > 0 E

..... 0 e/d = 30 Q) p = 0·3 Qc 10·0 . -c After Qc a::

q. = 100 kN/m2 2 -11400 (

321... 15L '1600 FIG. 6-15 PILE LOADS, MOVEMENTS AND RATE OF MOVEMENT AGAINST LOG CYCLES. (tid = 30 )

E E -c C1I E C1I > ° :::E

No. of cycles (Log)

100 101 102 103

01 I I ::::: = ---~ I ,0 -----'" ~o

4·

"

\ '1) L..

\~ .. Y-\~ ~ \. ~ \~ \-ro \"'" \--~. \c \"'" \~ \0 \%. \S· f.l \l3 \<3 \w \­\ \ \ \ \ I \/

\

'1)\ ~\ tn' . \ -, v>, %, -\ ,

\ , \

l.

16·0' '32 FIG. 6-15 EFFECT OF STATIC FAILURE LOADING ON THE SUBSEQUENT BEHAVIOUR UNDER REPEATED

LOADING. (fld =30)

1100

10

c ..E 60 ~. <U

Z

"'0 o o

--.J

o

o·3Qc := 971 N

ease \oad after static failure. ---------------- ---- ------- ----------.-

Base '\}\fg\O'

Residual" . _ _ R VIrgin'

- - - - ~2i.fJuot - __ after - __ ~..t.9t;c tanure . --- ........ ------- --. ------_I

100 200 300 400 500 600 700 800 900 1000 No. of cycles.

FIG. 6-17. EFFECT OF STATIC FAILURE LOADING ON THE LOAD-TRANSFER OF THE PILE DURING SUBSEQUENT REPEATED LOADING. (l;d = 30)

load level increased from 0.3Qc to 0.5Qc. At O.7Qc the rate of movement

1ncreased continuously and the pile did not last more than 80 cycles.

A similar behaviour was observed when the load level on a pile of

20 diaM~ters depth was increased from OA3 Qc to 0.5Qc as shown in

Fig. 6.19.

The movements and the rates of movement of Test 19 together

with those of Test 14 are shown in Fig. 6.20. Although the load

amplitude was the same for both tests the pile tested under 0.5Qc/

O.2Qc repeated loads experienced a longer life-span than that subjected

to O.3Qc/O.0 repeated load. The reason behind this behaviour may

be related to the state of residual stresses. In test 14 the skin

friction alternating in Sign from positive (upward) during the

application of the upper repeated load to negative during the lower

repeated load, 0.0, whereas it was always positive in Test 19. Since

alternating stresses severely affected the life-span of the pile,

Chan (1976). then the movements which resulted from 0.5Qc/0.2Qc

loading should be smaller than those of o.3Qc/0.0 repeated loads.

A similar conclusion was reported by Al-Ashou (1981) when the

results of 0.7Qt/0.2Qt loading were compared with those of 0.5Qt/0.0

repeated load.

The influence of 0.5Qc/O.2Qc repeated loading on the static

load displacement response and on the load transfer characteristics

of the pile are clearly demonstrated in Fig. 6.21. ~1e results of

this test confirmed the conclusion stated in section 6.3.2. that both

the ultimate shaft friction and the ultimate base resistance

decreased after the pile was tested under repeated loads. The load­

displacement relationship became stiffer as compared with that of

a virgin pile.

66

LOG. NO. OF CYCLES. 100 101 102 103 104 II==-----------~ ________ __... I , ------------ ....

2·

4-

. 6-~ ~

I-Z UJ 8-L UJ > o ~ 10·

12

eld = 30·

------- '--- , '" , ..... ", \

, \ , , \\\ \

\ '0 \ ' . \ 0 ,l1' \ . <=> \-1 Q. \0 "0 \0 \. \- 0 o \ . -\0 'lJJ \ '0

\~ \~ ,<3 -\-1 '-\ ,- 'CO \ -\ \0\ \CO -,\fa .Z ,- '0 ,Z \-~ .? :-oJ ,..> • ,0:> •

\" \ , , , , • ,

\ 1~' ,

F1G.(6-1BL MOVEMENT AGAINST lOG. NO. OF LOAD CYCLES. (eld =30)

--\ co lI\ -5

..> ~

LOG. NO. OF CYCLES.

100 10' 102 103 10' I I ____ I . """"'" I I

2·

4·

~ 6· :;E

...... z w :;E 8· w > o ~

10·

12·

tId = 20 Surcharge Pressure = 100 kN I ~

11.. 1 "

FIG. (6·19). MOVEMENT AGAINST LOG. NO. OF LOAD CYCLES. (fld =20)

o ~

" ~ o o

LOG. NO. OF CYCLES ..

100 101 102 103 104

I 1\ -- -BATE 03 I I \ --.. -~Qg..Q'Ql2202) --2'r I 1 \ ~ Q.Cjad..Q ·I.Qc (2003)'~ _ '\. '"

. I -12'

1\ - •

I ~ ~--- \ ---~7 .

I ·1 \ ./ \ W // \ --l

4,"- I I ...... 4· ~ --I I \ u

\ \ -. , \

(Yl

\ 10 . I , \ ~

~ 6, 6, x ::E I \ , \ ~

\ ::E t- \ I z I \ .-: w \ I \ Z :E 8· \ 8' w w I \

, ,- ..I' :E > I w 0 \ > ::E I I 0

10- '-,/ ....-:. 10· ~ LL 0

tid =20 ~ \ w

~ .A ~12' ~ 12+- Surcharge Pressure = 100 kN/m2 . ~ .- ,n

1ftl ' \ .- '14,

FIG. (6,20). MOVEMENT AND RATE OF MOVEMENT AGAINST LOG. NO. OF LOAD CYCLES. (lId =20)

3.00 Before repeated loads

----- d __ -- -- t d \00 s ,.. __ --After repea e / .

Test No.19 q = 100 kN/m2

p = 0·50c/0·2 Qc

1 2 3 4 5 6 Top piLe displacement, mm

FIG. 6-210. LOAD - DISPLACEMENT BEHAVIOUR (lid = 20 )

1'0

Test No.19 -- q = 100 kN/m2

p. = 0·5 Qc/O'2Qc o 0.2 .c. Vl

. °O~--~0~·2~~0~·4--~0~·6~--O~·8~--1~·0----~"2~--~1·4

Base load/Ultimate base resistance

FIG. 6-21 b. LOAD TRANSFER DURING STATIC LOAD TESTS

( Yd = 20)

6.4 Series Ill: Tensile repeated loadings

This section deals with results of test carried out on piles

embedded at various depths and subjected to repeated tensile loads.

The influence of previous static. failure loadiug on the subsequent

behaviour of piles under repeated loads is also examined.

The results of Test 21 which were performed on a pile of 30

diameters depth and subjected to 0.3Qt/0.0 loading are presented in Fig.

6.22. The curve of the movement shows that initially the movement

increased at a decreasing rate. After a stable stage, which was

between the cycle number 200 and 40,000, a drastic change in the

pile behaviour took place. The movement began to increase at a

rate which increased very rapidly as the number of load cycles was

increased. It was found that in none of the tensile repeated loading

tests the pile was entirely pulled-out. However, even at large

movements the rate of movement still had a finite value.

The changes in the pile base loads with' the number of load

cycles are also shown in Fig. 6.22. During the initial stage of the

repeated lQad, and because the pile base was subjected to a

compressive load (due to penetration) the shaft load was greater than

the applied repeated load. As the pile load was repeating the

compressive base load decreased gradually and became tensile after

1900 cycles. ~nis tensile load then increased and reached a peak

value of 80 n.::wtons around the 40,OOOth cycle. Beyond that it

decreased as the pile progressively pulled-out. As stated earlier

although the true base load cannot have negative values in a dry sand

the tensile load recorded was due to the fact that the base load

cell was one pile diameter above the base. The trend of the

residual base load was similar to that of the base load as shown

in the same figure.

67

-1

cv -0-12 ~

" E-10 E ~ o ~-8 )(

"' -c ~-6 Q)

~ E _[. -o

-14, . d

-12' \ 0-3Qt: -353N • -1-[.00

I \ Test No.21 . . lid = 30 -g I II -1-300

\. q : 100 kN m2 ;E; ..9 -101-

\ p ': a·3Qt/O oa Virgin LJ) :::: C --200 E 0 Mao (/)

E -8 I- Movement !..J:.·Vi § .. ~ 0 tJ) c_

C i \ Rate of movement·- ~ ~ - -100 ~ Q) '"0. B l d 0 _1--- ...... 1 Q)

E 6 a \ ase oa - - - - -- - " z - I- __

o . """ . ..-----. ~ Q) - ResIdual base load --------- ~ , ...... -- '"0 > ~ -- 0 o .-... a ~ - '"0 :: \. . __ .....- ... '-".... 0

-[. I- . C a a Q) "'0" 0 ___ -- ___ ... -- - ...J :J O.J:. til 0 . --- -- ...

'"0 -' tJ) a ." -- -- _-_ ... ---.- cv ,..,.. 0 __ -- __ ...... til til &..L.I -' • _ ... __ - ...

t'I, a ~ - --"" ---- ........ -2- OC.o _- ... -_ ... -_-I.-- . __ ------- .. ~~ ..--- -- -... ----- ~ ---- • I ------ ~ ' . . - --- --- . - ./ a ----- - . - . . . . . .

I i

1 101 102 ~

103 No. of cycles

I

10[. (Log)

• -t 100 c. E o u ~200

~300

105

FIG.6-22 VARIATION OF LOADS. MOVEMENTS AND RATES OF MOVEMENT AGAINST No. OF CYCLES

, The distribution of axial pile loads and the corresponding

skin friction durind the loadi.ng and unloading phases of selected

cycles of Test 21 are shown in Fig. 6.23. These cycles were the

first, a cycle during the stable stage, and the last cycle in the

test. Before testing, the pile was subjected to compressive

residual loads which developed after the process of penetration,

Fig. 6.23A. These loads increased as the depth of embedment was

increased. During the first cycle the compressive residual load

along the upper portion of the pile became tensi~whi1e that of

the~lower portion remained compressive but decreased from 450 to

194 newtons. The corresponding skin friction revealed an increase

at all points located along the pile shaft, Fig. 6.23B. Due to

repeated tensile loading and up to cycle number 835 the compressive

load of the lower portion was decreased progressively. The skin

friction of the upper half of the pile decreased whereas it

increased along the lower half during.this period. The corresponding

residual skin friction, Fig. 6.23D showed a continuous decrease.

This decrease was. greater along the upper half and it became even

of positive sign. During the loading phase of the last cycle,

number 50275, the skin friction along the upper part of the pile was

not altered while it decreased progressively along the lower part.

The residual skin friction during this cycle experienced the same

trend.

The results of Test 23, in which the pile was at 20 diameters

depth and subjected to O.3Qt/O.O repeated load, are illustrated in

Fig. 6.24. When these results are compared with those of Test 21,

Fig. 6.22, it will be seen that the general feature of the pile

behaviour did not alter when the depth of embedment decreased from

68

, ,

, , N 835~

. (A)

~----~-----,------~-----.

Res. after pent.

N=l

Loading phase (0·3 at)

• •

! ~ I

l

l

(B)

N=835 N=50275

load, Newtons Skin friction, kN/m2

-10 0 10 20 -20 -10 0 10 , , 'i •

(e)

N=l

, •

\ I I

I , ,

/

, 'I ~ " I N=835 - '1' I, I

, I I

l (0)

Unloading phase

(0'0)

FIG. (6-23) DISTRIBUTION OF AXIAL LOAD AND SKIN FRICTION DURING SELECTED CYCLES OF TEST 21

-,

18~ 18~ \ Test No.23 11

r \ . lid =20 J -\500

~16 16 q = 100 kN/m2 u . >.

\ p = 0-30t/O-O Virgin J il.OO u

""-11. 11. "- Movement E

\ Rate of movement -1300 E _.-

C M 12 E12 i- Base load ---- 0

'0 E VI ~

\ Residual base toad c x -------(1) - 200 VI .. - 0·3 Qt = 161-8 N I- c

C 10 a;10 r \ 0 ..... (1) E 0 - 100 ~ E - -0 (1)

-0

~\ :J-o 0 0

(1) > 0 :g 0 .c. 0 Z ~ 8 0 (/) -~8 - til 0

(1)-...--.. - ---- --- --- ..........

... E ..... a:: -- ~ ...... -0 - _. - cO ... ---- '-- ~(1)i \ --, -- '-I 0 0 0 .c. til ........ ...."..-------- 0 ell 6 6 (/) 0 --- --- ...J - ro ___ - - - --- - .... ..._--.----0 .-- - \;:. ------------. '. - -100 0:: -- -

I ------- Q.

I. I. E 0

"" I u +200

21- . '............ ) +300 ----._. a' '"---_. . .--"

I I I I I '-1.00

101 102 103 104 105

No. of cycles (Log)

FIG. 6-21. VARIATION OF LOADS, MOVEMENTS, AND RATES OF MOVEMENT AGAINST No. OF CYCLES

30 to 20 diameters. It may also be concluded that the pile life­

span increased when the depth of embedment increased. Moreoever,

the npmber of cycles at which the pile base experienced zero

load was increased from 500 to 1800 when the pile depth increased

from 20 to 30 diameters.

As concluded in section 6.3.2. for compressive piles, the

life-spans of tension piles were also reduced when the piles were

previously subjected to failure static loadings before being tested

under repeated loads. For piles of 30 diameters depth it was

reduced from about 50000 cycles, Test 21, Fig. 6.22, to about 8000

cycles in Test 22, Fig. 6.25. A similar behaviour was observed at

20 diameters depth and the pile life-span was reduced from about

30000 cycles, Test 23,' Fig. 6.24, to about Gooo cycles, Test 24,

"

Fig. 6.25.

'Fig. 6.26 shows the results of Test 26 in which the pile was

subjected to 0.5Qt/0.0 repeated load and embedded at 20 diameters

depth. When these results are compared with those of Test 23,

Fig. 6.24, in which the pile was subjected to 0.3Qt/O.0 loading,

it will be found that the pile life-span was reduced more than 16 fold

due to this increase in the repeated load level. The pile base

load, Test 26, reached a peak tensile value of 80 newtons before

aropping to 20 newtons at the end of the test. It was also

observed that the decrease in the base tensile load from its peak

value coincided with an increase in the rate of movement.

In contrast to that observed in compression tests, when the

repeated tensile load levels were increased from 0.3Qt/0.0 to 0.5Qt/0.2Qt

Test 27, the pile life-span decreased from about 30000 cycles, Fig.

6.23 to about 18000 cycles as shown in Fig. 6.27. This change in

69

81 II II 116

lid = 15 (1508)

n- lid = 20 ( 2205) \I II I ,14

tid = 30 (3016)

Tension = 0·3Qt /0·0 ~I Q)

Lf) -- u 0 LO >-Lf) . ~

Surcharge pressure = 100 k N/m2 ~I N 0 12~ N M

Movement E

....r N E

~I N N

Rate of movement o. cj CT> _.- z 10 $2

1/). il _ x

If ~I

I/)

~ +-' C QJ E

I 8 Q)

> 0 E -0

5 Q) -/I I I 0 a::

4

2 ~.

I ?~.---. .--.~.~-? .~<-A.;;;;:.c=~-:,/ I o 101 102 103 10£

No. of cycles (Log)

FIG.5-25 INFLUENCE OF EMBEDMENT DEPTH ON THE LIFE SPAN OF TENSION PILES

--~--------------~~----------------------------------~qOO Test No.26 lid = 20

- 1201- 241- q = 100 kN/m2 -1600

p = 0·5 Q t/O-O (Virgin)

Movement 100 20 Rate of movement

Q) 500

U Base load ----~

~ Residual base load -- - - ---- -E80 E . ~ E \ a E ..--

x,

~60 c12 E ClJ

ClJ E ~,\:-----------~----~~~~~~~~-----------------------------------I-----.----~---: > ClJ o >

~40 ~ 8 \

41- \

0·50 at = ?7n N

z a ~ N

'400 til C 0 -~

~300 ClJ z '"0 a 0

200.J I

C 0

II -\ o _'>.,----.,c..~~-----------­--.- ......

- ______ -- - ___ - __ ---------- - - - - _- - _-_- _-... -~ _- - -- - - ·0-o

a LO

6

I r t

101 102 103

No. -of cycles (Log)

FIG.6-26 VARIATION OF LOADS, MOVEMENTS, AND RATES OF MOVEMENT AGAINST No. OF CYCLES

V)

~1100 t-

~

E o u

o

,100 10'+

18

_16 QJ

u ~11.

~ E E

cr> 12 o .....

)(

-o 6 QJ -a

oc4

2

o

9,.

8

7

6

,

\

\ ,

.,

"

O-SQt = -270N

0·20t=-108N

E5 ' -' -1---------------El ~ -- ---------------------------------------'" L ---- - - - - - ---:::t-

E

--

~ I~

::E.

3

\ Test No. 27 (2209) I '\ lfd = 20

q = 100 kN/m 2 ,

\ /'''', 'p = O'20t/O'SQt Virgin. I _ \ Movement '

\ /' . Rate of movement _. - I . "Base load - --- .

',. Residual base load -- ----- /

I "--.- ,_ ./ O~ ,_.-. .-".

2

1

I --1 ~

1 101 102 103 104

No. of cycles (Log)

-400

-300 --I c 0

.~ +200 .~

V) c

-100.2 ~ QJ Z

0 "0 0 0

ci. ~100 ...J E 0 u

~200

300

400

500 105

FIG.6-27 VARIATION OF LOADS, MOVEMENTS,AND RATES OF MOVEMENT AGAINST No. OF CYCLES

behaviour may be related to the state of the residual stresses.

After penetration the direction of the'residual stresses was

downward and remained so during the repeated loading. Therefore

the pile life-span will be controlled by the load level, Chan

(1976). Measurements of the rates of movement during the failure

stage, that is from the cycle number 4000 to the end of Test 27,

are presented in Fig. 6.28. It is clear that the rate did not

increase regularly hut fluctuated in value as t.he number of cycles

was increased.

The influence of repeated tensile loads on the static load-

displacement behaviour of the pile is shown in Fig. 6.29. It can

be concluded that repeated loading decreased the ultimate pulling

resistance of the pile. The reduction is greater in the case of

tension piles than in the case of compression piles. It was

greater than 50% for the former piles whereas it seldom reached

30% for the latter piles.

6.5 Series lV: Effect of surcharge pressure

From the results of two identical tests performed on piles

subjected to repeated compression loads of 0.3Qc/0.o and placed in

a sand acted upon by a constant surcharge pressure of either 0.0 or

100 kN/m2 Chan (1976) concluded that the effect of increasing the

surcharge pressure was to delay the onset of the critical stage of

maximum rate of movements, but the main features of pile behaviour

were not significantly altered. Based on repeated tension load tests

conducted on reinforcement bars embedded in a sand upon which a surcharge , 2

pressure not more than 100 kN/m acted, A1-Ashou indicated that the

surcharge pressure caused an increase in the life-span of the

reinforcement but, otherwise it had little or no'influence on the

70

~. '

3'0,' ~~~~---------------------Test No.27 lId: 20

2·8

2·6

2·4

~ 2·2 -\) ~2'0 "-E 1·8 E

M '0 .1-6 ~

x = 1'4 c OJ E 1·2 OJ

~ 1'0 E

'0 ·8 OJ

C ·6 0::

'4

·2

q = 100 kN/m2 p = 0·5 Qt 10·2 at.

°4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 No. of cycles.

FIG.6-28 VARIATION OF THE RATE OF MOVEMENT WITH THE NUMBER OF LOAD CYCLES.

o .:::£

"0

·Tension.

140

120' .

100

I

11 I

~

Test No. 22. -11·5

~ T I I

'? 1·0 I

0 I

C 80 o I I

After R.L .. I ?

---l

I a I I I

I <} I I

9 I Vd=30 I

q = 100 kN/m2 I 0 I I

Qtl = 120 KG.

·1 d Qt2 = 1~ I

60

0·5

I I I I

or I I I I I El I I I Q I I I a 0·4 0·8 1·2 1·6 2·0 2·4 2·8 3·2 3'6 4·0 4· 4

t Displacement mm FIG. 6-29. LOAD DISPLACEMENT BEHAVIOUR OF TENSION PILE. ( Vd =30) I

Z .Yo .. 'U o o -l

behaviour of the reinforcement. In view of these studies it was

decided to examine the behaviour of both compression and tension

piles embedded at shallower depth, with a surcharge pres~ure which

ranged from 0.0 to 200 kN/m2

•.

Fig. 6.30 shaws the movements and the rates of movement of

Tests 14, 28 and 29 versus logarithm of the number of load cycles

for piles subjected to repeated loads of 0.3Qc/0.0 at 0.0, 100 and

2 200 kN/m surcharge pressure respectively. The number of cycles

at which the rate of movement reached its maximum value increased

from 600 to 850 cycles when the pressure was increased from 100 to

2 2 200 kN/m and it was only 200 cycles at 0.0 kN/rn pressure.

The number of cycles at which the pile moved 6.0mm also increased

when the surcharged pressure increased. It was 300, 900 and 1800

2 . . cycles at 0.0, 100 and 200 kN/m ~ressure respectively. A similar

conclu .sion can be drawn from Fig. 6.31 which sho'vlS the results of

Testr24, 30 and 31 where the pile was subjected to tension repeated . , . 2

loads of 0.3Qt/0.0 with 0.0, 100 and 200 kN/m surcharge pressure

respe d.ively. The pile life-span increased from 5800 to 36000

cycles when the surcharge pressure was increased from 100 to 200

kN/m2• At 0.0 kN/m2 pressure the pile did not last more than

50 cycles.

Fig. 6.32 shows that the ultimate bearing capacity of the

pile also decreased after having been tested under repeated loads

2 wi th both 0.0 and 200 kN/m surcharge pressure.

6.6 Series V: Varying load amplitudes

A foundation pile is not always subjected to a repeated load

cycle between two fixed .limi ts. In many cases these lim! ts vary

wi th time. In order to assess the influence of such kinds of

71

.,

1 2 No. of eycles (Log) 3 l. o 10 ·10 10 100 , I >cc:::: ==-- I :os < _____ I I

2

4

---. . ""--- . -- . .""" -.~

./ '"".. 0-/ \ ~

/ \ ~ i Yct =20 . ~ . 0·3 Qe/O.O After ultimate \

/ 2

4 OJ

U >­u 6" E E

c-r c8 OJ

I test . q =0·OkN/m2

1 =100kN/m2 \ 8 ~

E OJ > o ~10

. =200kN/m2 .

I Movement \~ Rate of movement - . - .;.

-,...> t \ % '\ ~

16 "3,...>

--c Q)

E 10 ~ o

E -o

12 2

14

16

o a:

FIG. 6-30 INFLUENCE OF SURCHARGE ON THE BEHAVIOUR OF PILE SUBJECTED TO REPEATED CaMp. LOADS

E E

7. II 111.

6

5

.

\ /? . / 0-\ . J

-lid = 20

O'3Qt/OO t""--IO ~ ci After ultimate test N Z a _ Movement . til

a Q) Rate of movement-·­II I-0'

- . If') /....: a N N ' N 0 .z

~II] 0'

~I~ a 0 N Z

alVi a Q) N l-II

0'

OJ 12 U ~ u

"-10 E

E era

..-x ..

cl.

I 8 C

Q)

E ell > o

Q)

E Q}

:!E 3 I j

> o 6 E -o

Q} -2 I. ~ .

/ .

I 1 2

" ,..--.J 0' rig I '-' -c===::r::: ----<-, 10 1 . 101 102 103 101. 105 106

No. of cycles (Log)

FIG. 6-31 INFLUENCE OF SURCHARGE PRESSURE ON THE LIFE-SPAN OF TENSION PILES

~ ~.- : ... --.- --'---~-- - :&-'''"-.~,..

700----~----~----~----~--~~--~-----

600 6

500 \'200<0' £>e~o{'e 0 '29. \es\ ~ .

400 C)

'2008' z ~ ~\\er \ .Yo .. "' ~ , -0 0 0 0 0

-.oJ -J

/

2

f e l2009 ) Test No.28. Be or

After l2009) I I

1-0 ' 2·0 3·0 4·0 5·0 6·0

Displacement mm.

FIG. 6-32 LOAD - DISPLACEMENT RELATIONSHIP. (Vd = 20)

loading on the behaviour of piles four main tests were conducted,

within this series of tests. The'influence of load amplitude,

subsequently decreased or increased on the behaviour of piles

placed at 30 pile diameters has been investigated in Test 32

(3017) and Test 33(3018) respectively. The sequence employed

in the first test was as follow3:-

(32 - a)

(32 - b)

(32 - c)

0.7Qc/0.0

O.SQc/O.O

0.3Qc/0.0

In the second test the reverse sequence was applied

to the pile, that is :-

(33 - a)

(33 - b)

(33 c)

O.3Qc/0.0

O.5Qc/0.O

0.7Qc/0.0

During each of these tests the surcharge pressure was maintained

2 at 100 kN/m. Each individual test was stopped l'/hen the rate of

movement began to decrease after having reached a maximum value

or when it decreased progressively as the number of load cycles

increased.

To appreciate the effect of previous repeated loads on the

subsequent behaviour of the pile subjected to repeated load, the

results of Test 33-a together with those of Test 32-c are plotted

in Fig. 6.33. It is clear that the pile which was being tested

under higher repeated load levels experienced a smaller rate of

movement end therefore had a longer life when compared with the

virgin pile. The' amount of the maximum rate of movement decreased

from O.OllSmm/cycle for the vi,rgin pile to 0.002lrnm/cycle for

the previously loaded pile. After 2000 cycles the displacement of

the virgin pile was four times that obtained from the other pile.

72

100 .

2,..-

4·1-

6· E E . -·c 8· OJ E

. OJ > 0

:E 10

12'r

1L..~

101

0·3 Qc/O'O tId = 30 . q = 100kN/m2

No. of cycles (Log]

102

: .

103

\ \ ~ ::)[.-'" \ \ ~ \ \ ~ 0t.. \ \ -; \S',..-.

:0\ \~ .~ o \. '\ -\ \~ (\" ro, -=;. I ~ 1 \~. I ~.I \? , :l I \-4 I - ro I rot I \~ I ~I \~ I ~I \b I I I \ I 0, \ I

I \ I V

\ 1\ '-~/ \

\ \ \

RG. 6-33. VARIATION OF MOVEMENTS AND RATES OF MOVEMENT WITH LOG. NO. OF CYCLES.

10 4

,2'

4· ~ u ~ 0 -

5· E E

M '0 or-. x .

8, C OJ E OJ. > 0

10· E .... 0

OJ -c 12· 0::

-114·

----~ .. --" .. " ... --.. ~"'-.---

This change in behaviour is probably due to the stiffening and

densification of the sand beneath the" pile base caused by the previous

repeated lodding. When the movements and the rates of movement of

Test 32-a and Test 33-c in which the repeated load level was O.7Qc

are compared as shown in Fig. 6.34 the reverse trend will be seen.7he

rreviov..sly" loaded pile had a shorter life-span. This behaviour

is nttributed to the initial movement and to the trend of the rate

of movement. Because repeated loads decrease the ultimate capacity

of the pile therefore the initial movement of the previously loaded

pile was larger than that of the virgin pile. On the other hand,

the rate of movement of the previously loaded pilo being a

maximum at the beginning of the test and decreased as the number of

load cycles increased while it was a minimum for the virgin pile

and then increased during the advancement of the test. Therefore

the virgin pile experienced the smaller movement. Based on the

results presented in Figs. ~.33 arid 6.34 it may be concluded that

the life-spa~ of a pile previously loaded dependS on the load level

in the succeeding test. If it was low, O.3Qc,the previous load

resulted in a pile of longer life, whereas a shorter life was

caused by a higher load level suCh as O.7Qc.

A similar behaviour can be seen in Fig. 6.35 in which the movements

and the rates of movement of tests 32-b and 33-b together with those

of Test 20 are plotted. It may also be concluded that, the higher

the previous load level the smaller will be the rate of movement of

the succeeding test. The influence of the previous loading on the

pile shaft load and its residual values is shown in Flg. 6.36. Both

of these loads decreased due to the previous loading. The higher

the previous load level the greater was the reduction.

73

E

No. of cycles (Log)

1 101 102 103 104 a. .0

-- ................ - ........

2 ' ...... ,

4

6

" , , , \

\ \

\

l{j = 30

q = 100 kN/m2

0'7Qc/0'0

100

200 ~li~I, >-OJ u "i 1 E'

300 E 1 M , 10 i ~ ,

x,

e: 8

\ ,

\ /~\

/ <,...-\ (\)

/ ·L\

.. , " 400 C -c

CLJ E

~ 10 o ::E

12

14

~CJ/ ;fry

-P1 tjq,

. (;]

t:l:

~.\

~.\ ro -::> Vl ..A \ -~\Z ,...- 0 -z,\ ·w 0\", .~\ 0

0\

\ \ .

CLJ: E; CLJ, >' 0'

500 E _., o CLJ -

GOO 'aE

700

FIG. 6-34 VARIATION OF MOVEMENTS AND RATES OF MOVEMENT WITH LOG NO. OF CYCLES. (Yd = 30)

--------... ~-- ...

No. of cycles (Log)

0\ 101 102 10' 104

-==- , -- --,.,.. I I 10 - Mov. Test N 0·32

21- ~ ~ ro\\ -120 ~'i. ~

~\\~- -,.- -,to' 1f~t N 32-b - QdO'O -I ' -_€ _0.;... - - 0-1 40 .SE

u

~ ~ u

'[.. ""-

6 .~

60 E . ~U' ME E '/ / E ~

'0 I -. - -U> I x

c ...... -Vl --<lI 8 - 80 . E (5 , ~ ..--- c OJ .., Q)

'> -' E 0 :> lId =30 OJ ::E - > 10 0 q = 100 kN/m2 100 ~

~

R I 0-50c/0-0 -/ 0

~ / OJ ..-

121- 0, 120 &.

I ~

I 14' l ~ } ~t.O

FlG.6-35 VARIATION OF MOVEMENTS AND RATES OF MOVEMENT WITH LOG NO. OF CYCLES.

No. of cycles (Log)

1 101 . 102 103 104 o ,_, I

Test NO.33-b Res. (3018) ------------------...... a .c.

200 TestlJo.32-b_Res. (3017) ------- --vi

en c o 600 -~ QI

Z

"0 800 a .9 OJ til

<l1 0::

R~~._V.!.r9.in 301~ _ Te~tJ'J2..'£O __ -

o 100l co _ Base Load. Test. No. 20

1200t- Base Load. \es~o. 33-b -1400

Base Load. Test No. 32-b

\:J a o

....J

<l1 til a en

---- ........ -" ---..--".,,'"

-"

~ If)

.;'

FIG. 6-36. VARIATION OF BASE LOAD WITH LOG. NO OF CYCLES.

.--.----

Vd =30 q = 100 kN/m2

O·SOc/O·O

The magnitude of the shaft load and its' residual load tended to

reach a limiting value which seemed to be independen t of the previous

loading history of the pile. The same conclusion may be drawn

from Fig. 6.37 and 6.38 in which the succeeding loads were O.3Qc/O.O

and O.7Qc/0.O respectively.

Fig. 6.39 shows the variation of skin friction along the pile

shaft during the two phases of Tests 20, 33-b and 32-b in which

the repeated loads were 0.5Qc/O.O and TestS32-b and 33-c where the

loading was O.7Qc/O.O. Three load cycles were chosen for comparison.

The first cycle, an intermediate cycle and the last cycle of each

test. The results shown supported the conclusion stated earlier that

repeated loading brought about a redistribution of loads along the

pile length wh:i.ch resulted from a deterioration of the frictional

resistance. .'

In Test 34 the depth ratio and the surcharge pressure were 20

2 and 100 kN/m respectively. The pile was subjected to the following

sequence of loadings:-

(34 - a) 0.3Qc/O.O

(34 - b) O.5Qc/O.0

(34 - c) Static compression load test

(34 d) O.99Qc/O.O

(34 - e) O.3Qc/O.O

The results of this test, Figs. 6.40 and 6.41 confirmed the

conclusions drawn from Test 33, that is the rate of movement

decreased as the load level of the previous loading increased

and that the pile life-span of a previously loaded pile depended

on the load level in the succeeding load~g.

74

100 101 102 103 104

- I I I • -o c

(j, .Q Res. Base Loe d Test No. 32-c . --- -:,..-.----I- ---- -. ~--- - ----- ---~- - - - - - - ~ .... ~-- ....

1./'1 U ------------..--- .,,~

- OJ .;: • -.,..

0::: u.. Res. Base Lo( ld_T,g~.ll.Q.13-Q..-- -------- -- ------- - ---- --- -- --

200

-0 0

400 r- e -' -OJ 1./'1 0

co -r- Bose Load, Test No. 33-0

t'-0"1 -II

. \J) 600 c o

-0 U

0 d

l-e (Y')

-' Bose Load. Test No. 32-0 0 -. -"-0

£ (j)

-~ OJ

z .. 800 "8 o -1

:... 0'3Qc=971N -m 1000 o

co

tid = 30 1200 "- q = 100kN/m2 -

O'30eI 0'0

1400

FIGJ6-37) VARIATION OF THE PILE LOADS WITH LOG. NO. OF CYCLES.

100 101 No_ of cycles (Log)

102

400

800

~ 1200 .E ~ Q)

z ~ 1600 o o Q) til ~ 2000

I I Res_ After o-sac/o·o _----------------------- 1--

(Vir9l~L~Q1]L--- - ---- --------------Test No_ 32- a

!-- "0 0 0 -OJ til 0

I- m

130se load virgin (32-0)

~Base load after 0·50c/O·0 i3018) ~ ::::---.J ,

33-c) - r--"0 0 0 -- --0

'J:. (f)

'103 I

l/d=30 q = 100 kN/m2

0-7Qc/0-0 - After (0-5ac/O'0) - ' Virgin (3017 )

, (,0 (,0 N N II

u d t:' 0

2400

I 0-7 Qc = 2266 N

,2800

FIG.(6-381. VARIATION OF THE PILE LOADS WITH LOG NO_OF CYCLES.(Yd=30)

(3018)

----

~I

I

104 Ii:

It'

Il

-

-

-

!

-

-

-

---

:1

II I' ,I ;! I' Ii I:

kN/m2 30 20 10 0 , , i •

o 10 20 30 kN/m2 , iii 60 50 L.O 30 20 10 0 o 10 20 kN/m2

i i. .

Test

t No. J 32-b

After / O'7Qd 0·0 I ~/I

High RL. Low RL (a) O·SOc/O·O

iii , i i • •

High RL Low R.L

(b) O·7Qc/O·O

FIG. 6-39. VARIATION OF SKIN FRICTION DURING - REPEATED LOADINGS.

E E . -C

No_ of cycles (Log)

1 101 102 103 104 01 '" I S ~ ,0

( It !I

2

4

6 Yd = 20

, ...... a::c::::::::: ~ .....

" , \

\ \

\ \

' ...... --

\-:g \:;A' \~ \~ \~ , .....

\% \-,u' \~ \(:J

--...... ----/' / /<~

/ / ~ / /~~

/ / "-/ /~e

///«:-0

.A

~ ,... %

2 :,1 'I I :, :\ ;1

4 ,..... 11 ~ :: U I, I,

'>. ': Uf

-"""",1 6 E i~

E I: If Ii ,..,... II 'i I II

01> ~ 'i )( Ii

QI 8 E

q = 100kNI m2 0-30c/0'0

\ \

- Virgin "

-0.> r , ....

...." j! 8 - II C " Q} H QI

> o :E

10 - After (0'5QdO-0 ,Q-99QdO'O) '\ - After Qc Test only 2202 ,

\ \

\

12

, , " " "' ,/ ,----"

o ~ '1

E ') OJ :; > " " o ;;

10 E ::

'0 Ii QI Ii -t 0,

12 0::: i: l' I I' I, !

14' , '11. ! !

FIG_ (6-40) VARIATION OF MOVEMENTS AND RATES OF MOVEMENT WITH No_ OF CYCLES

101 2 No. of cyc(es (Log/ 103 10' 1 10

0

. lid = 20 / 21- . p = 0·5 Qc/O·O -14

I -. .\ q = 100 kN/m2

( Q)

\ Movement -u 4i- Rate of movement -'- 8 ~ ,

I \ '-......

E · E

~ 61 I 12M 0 ~

· x

I §/ . ... -- c

~8 ~. 16 Q} · E E I ~/ Q} OJ ..A > > (Q 0 0 · ,~ \; E :£10 I 20 '0

~.

7 f .~ Q) · -I

a U' (5 a:: 12~ / ............. / f~ 24 ·

I . I "'--.--'" v.> ~

tJ) ../

· .0

I .

14r I ~ \ \~ 128 .

I I '-16' \ '32

FIG.(6-41) VARIATION OF MOVEMENTS AND RATES OF MOVEMENT WITH No. OF CYCLES

The results of Test 35' in which the surcharge pressure was

200 kN/m2 and the pile of 20 diameters depth, revealed that the

main features of the pile behaviour did not change when the

, 2 2 surcharge pressure increased from 100 kN/m to 200 kN/m. It is

of interest to note in this test that the residual base load

reached a limiting value which appeared to be independent of the

repeated load level as shown in Fig. 6.42.

6.7 Series VI: Compressive-to-tensile repeated loads

This section deals with the results of Tests 36, 37 and

38 in which the repeated loadings were 0.15Qc/0.15Qt, 0.15Qc/

0.3Qt and O.5Qc/0.15Qt respectively. The piles were tes b!? ... ~ at 20

2 diameters depth with a surcharge pressure of 100 kN/m •

The movements and the rates of movement versus the number of

load cycles of these tests together with those of Tests 23 and

39 are presented in Fig. 6.43. In Test 36 the initial stage of the

pile life was characterised by a very small or even zero downward

movement. After a certain load cycle, and as tile nurrber of cycles

was increased the rate of movement increased and reached a maximum

value of O.OI4Irun/cycle around the l700th cycle. The behaviour of

the pile during this period was similar to that of test Series II •.

However, after the pile had moved more than 12mm the direction of

the pile movement was reversed with a rate which increased very

rapidly as the repeated loading progressed. When the test was

terminated after 200 cycles of upward movements the pile head

had risen more than double the downward movement.

When the repeated load level of the tension phase was increased

to O.3Qt, Test 37 (2012) the downward movement ceased. The

behaviour of the pile in this test was similar to that of Series

rIr. If a comparison is made between the life-span of this pile

and that of Test 23, in which the load was repeated between zero and

75

01 101

2 No. of cycles CLog} 3 10 10 104

--, 1----- - ------ -- -- ----

O·3SQc, O·4Qc,O·SQc ----------------------------------------------------------

L.00t-

---- - -------------~---------------------------------------0·30 Qc

BOOr lJ)

c I 0 0·3 Qc -~ 12001-z -"0

0·35Qc a .3 1600, 0·4Qc

20001- 0·5 Qc

21.001-

Test No.35

lid = 20

q = 200 kN/m2

Base load Residual base load------

2BOoL. ----------------------------------------------------------~ ..

FIG. (6·1.2) LOAD TRANSFER DURING REPEATED LOAD OF VARYING LOAD-AMPLITUDES

1 2 No. of cycles (Log) 3 4 1 10 10 10 10 iii . i I Ii ,

-16 Yd =20 q = 100 kN/m2

-12 Movement

Rate of movement _.-

-8

-I.. E E

c 'CJ)

~\; 0·1 N z C; _ N

~·I ::-~ a

M '0

I~ 1~

~ u a t.n ., ..--6

-160

--120 ClI -u >.

~ E -80 E

era ..--x

-1..0 :: c <lJ E <lJ

.0-

C 0 ClI E

4" I Mov. Test 23

---- ~. JI R.o.M. Test 23 10 > o E

ClI > 0

~ 4 /'~

/ \ / \~ , "\5'

12f- I . ~.g, 16J '\

8

~ ~ -- ...-\ Q <'> ~ ~ ~ -t, ~ '? (3, '& ~ L-. ~ ~.

~

o~

~ 0'>-

<'> ~~ 0: -t, 't 0

/ .~. ~. ~ ~ ~. ~

FIG.(6-t.3) VARIATION OF MOVEMENTS AND RATES OF MOVEMENT WITH No. OF CYCLES

-o <lJ -40 !E.

80

120

160 •

O.3Qt it will be seen that the latter pile had longer life. The

small component of compressive repeated load, O.lSQc, caused

a reduction in the life-span of the pile from 30000 cycles to 800

cycles. The upward movement ceased and the pile penetrated

continuously into ti1e sand when the compression phase of the repeated

loads was increased from 0.15Qc to O.5Qc in Test 38. The behaviour

of the pile in this test was similar to that of Series II. The

life-span of this pile also decreased, as compared with that of

Test 39 (2015) due to the presence of the small component of tensile

repeated load, O.l5Qt.

Fig. 6.44 shows the variation of the skin friction during the

first,two intermediate and the last cycle of Test 36. The redistribution

caused by repeated loading is well demonstrated in this figure.

Along the upper third of the pile and after a few cycles, the

direction of skin friction during both phases was upward while along

the lower third it was alternating as . the load was repeated. Over

the middle third the skin friction was always downward and

fluctuated in value with the progress of the repeeated loading.

6.8 Series Vll: Miscellaneo~q

In this section the results of Tests 39,and 40 are discussed.

~uring Test 39 the pile, which was of 20 depth ratio was first

tested under repeated loads of O.5Qc/O.O and after the rate of

movement began to decrease from its maximum value and became

relatively small the pile was immediately subjected to a repeated

load fluctuated between O.5Qc/0.3Qt. This test was aimed at

studying the influence of the previous compression-to-zero repeated

loads on the succeeding compression/tension repeated loads.

76

Skin friction, kN/m2 35 30 25 20 15 10 5 0 -5 -10 -15

I _ I _~ i_ ~ _ 1 ___ ----'- __ 1 ___ 3 ,_, , r~l--.- - --I --- T- ------.---- I - ,

15 10 5 0 -5 -10 -15 -20

A

Compression phase

1

1

0·15 Qc ,/. ,I( 'I iii \\ \ , . ~ '"

~" N=1 , . ,~/" N =10 ~7y N=2530

)j\ V N=3230 ·V 'n

'fi;'

/., "/

" ~ .............. ~, " \ \ .\ )

) J / '/' /

" ./' -'

~ , . \

\ .

n /.

, I ". IJ

I ~

~

~

8

Tension phase

0·15 Qt

Test No.36 (2011)

Vd = 20

. q = 100 kN/m2

p = 0·15 Qc /0'15Qt

FIG.(6-44J. DISTRIBUTION OF SKIN FRICTION DURING 0·15 Qc/0'15Qt REPEATED LOADING

In Test 40 the creep characteristic of the pile under 0.7Qc was

examined. This test was conducted to evaluate the influence of

creep on a pile subjected to a sustained load of.the same

magnitude while the sand surface was acted upon by a surcharge

2 pressure cycled between 50 and 100 kN/m such as those of Series I

Table 5.2.

The variations of loads, movements and rates of movement wi th

the logarithm of the number of load cycles of Test 39 are

presented in F.ig.6.45. It can be seen that the behaviour of the

pile during the first part of the test, .in which the repeated

loads were 0.5Qc/0.0 was similar to those of Series II. Once the

pile was subjected to the second part of the test, in which the

repeated loads were OfSQc/0.3Qt, a change in its behaviour took

place. In spite of the compression repeated load being the same,

0.5Qc, the rate of movement i which was decreasing during the

previous part began to increase rapidly as the number of cycles of

the succeecUng repeated loads was increased. After a maxlmum value

was reached the rate of movement again decreased with .increase of

load cycles. The maximum rate of movement of the previous repeated

loads, which was 0.0366nm/cycle and attained after 281 cycles,

became 0.052mm/cyc1e and was attained after 100 cycles. At the

beginning of the second part of the test the base load, during tile

compression phase of the repeated loads, increased as the rate of

movement was increased. After a maximum value the base load

gradually decreased and returned back to the value that had been

reached at the end of the first part of the repeated loading, as shown.

in the figure. When the movement of the pile in the second part

77

I

\ Or 0

1

s '?' 102 No. of cycles (Log) 3 7 "-.. I '0 10' . ----... ,0 -----t. "- ---------'-----1 I ---- - - - - -,,- - -.------- -4

C1I

u8 ~

" ~ rr 12 D

)(

-~16 E C1I

E; E '020

C1I -o a::

4

.. -c C1I E C1I8 E;

:E

',,--. . ~ .

! . \ I . I "'. . \ I . . .\ ---:-----------. - - -- . 1------

\ o· SOc = 1348 N

Test No.39

Yd=20 p = D-SQc/O·O Virgin q = 100 kN/m2

Movement Rate of movement --.­Base load - -­Residual base load------

.~ osoc/0#OC/0.3Ot

I: . I I

I I I I I

- I I

;-_A(' __ :-_

, ' I : , ,

,

200

1.00

11)

c 6002

:= C1I z

BOO\:) o o

...J

1000

1200

1400

I \ -'\: I 32L '6· I . I - i'600

FIG.(6-t.S) VARIATION OF LOADS MOVEMENTS AND RATES OF MOVEMENT AGAINST No. OF CYCLES

is compared with that of Test 38, despite the tensile repeated loading

being higher in Test 39, it will be seen, Fig. 6.46. that a longer

life was obtained from Test 39'. The' reason for this behaviour

is probably due to the stiffening effect which resulted from the

previous repeated loads of Test 39.

Fig. 6.47 shows the movement versus logaritl1m of the time in

hours of a pile subjected to a sustained load of O.7Qc. Initially

the movement increased at a rate which decreased rapidly. After a

few minutes, the rate of movement with respect ot the logarithm

of the time in hours reached more or ,less constant value. At

the time of writing the thesis, and after about 4510 hours, the

pile had not settled more than l.45mm.

6.9 Conclusion

From this part of the investigation the following conclusions

, may be drawn:-

(1) Preliminary tests carried out on piles

at various depths of embedment with a

constant surcharge pressure of different

values indicated that:-

(a) The driving resistance of the pile

base increased rapidly with the depth

of penetration, reached a maximum

then decreased slightly to a limiting

value. The maximum value increased

and was attained earlier when the

s~rcharge pressure was increased.

(b) During each;. identical driving

test, the value of skin friction at

78

E E

~ No. of cycles (Log) 01 _ __ __ _ 10 102 103 101. F __ i _____ i i a __ I

""""­

4

8

" " " 1.00

,10 ~ \% ~~/

200

\~ (J)

c -"12

\~ ~ \0 z 600 .8 c

(l)

E (l)

> ~ 16

\'& (l

\-­o \~

Base load ·Comp. phase" l20'3L Test"N \g. 20t-- - - - - - - - - - - - _0':]8 \

Base load ·Comp. phase" (2015) Test No.39 - \ - - - -

21.

.- I

::::1"0 o 0 .I::. 0 l/)-

0·5 Qc = 131.8 N

\~ \~ ~

\9 ~

~ (l) co z ~

M - ... 800 -g II

U 0

..J 0 .,....\ In ~ 0

.,..,

1000 % .v> u:>

\ I 1200

28 1 , '1400

FIG.6-1.6 VARIATION OF LOADS AND MOVEMENTS WITH No. OF CYCLES (Yd = 20 )

~

-1 dr-a-I.

0-81-

~ 1'2t "'-----c

Q) 1-61-E Q)

> 0

:E 2'0 ...

2·1.1-

2·8t-

1

"

Time, hours (Log)

10' 102 103 104

-"" ~

....

Test No.1.0 Load = 0'7Qc

.... --

Surcharge pressure = 100 kN/ m2

3'2'~------------------------------------------------------------------~

FIG. (6-1.7). RESULTS OF CREEP TEST

r I ! t ! i

I

ally. given depth below the sand surface

was much less repeatable than the pile

base resistance.

(c) The ultimate static load capacity

of the pile increased and the load­

displacement became stiffer when the

depth of embedment and/or the surch~ge

pressure was increased.

(d). The variation of the ultimate

base resistance with depth had the same

trend as that during penetration. The

peak value was smaller and was attained

at a greater depth in the case of the

static loading.

(e) The ultimate capacity and the load

displacement behaviour of the pile were

affected by the state of the residual

stresses before testing.

(f) The shaft friction in compression

was generally greater than that in

tension loading by a magnitude dependent

on the depth of embedment, the surcharge

pressure and the magnitude alld direction

of the residual stresses before testing.

(2) The results of repeated comoression loading tests

performed on piles placed at various depths revealed

that:-

(a) Initially the pile was stable, but

79

---"------< ... - "' .... _-& ~- .- .----

after a certain load cycle,· it b.egan to

move at a rate which increased rapidly to

a maximum then decreased as the number

of cycles increased.

(b) For a given percentage of loading,

0.3Qc, the pile life-span decreased when

the embedment depth increased •

(c) The number of cycles at which the

rate of movement reached a maximum value

appeared to be independent of the pile

depth. It was around cycle number 1300.

(d) At any depth, ac:; the number of

cycles was increased, the shaft load

increased up to a peak value then

decreased gradually to a limiting value.

This limiting value increased when the

depth of embedment was increased.

(e) The reduction in shaft load from its

peak value was associated with an increase

in the rat e of pile movement. This

increase reached a maximum value at the

cycle during which the shaft load

reached its limiting value.

(f) 'I'he rate of movement increased and

the life-span was reduced when the pile

was subjected to a static failure loading

before being tested under repeated loadings.

80

~.-.-----.. -- --- -- ~-~--. ~ - - - ..

(3) ~ests performed on tension piles at different

depths of embedment showed that :-

(a) The pile moved initially at a decreasing

rate. After a stable stage the movement began

to increase at a rate which increased very

rapidly until failure occurred.

(b) For a given percentage of loading,

O.3Qt, the pile life-span increased when

the depth of embedment increased.

(c) , At a given pile depth and a given

loading the pile life-span reduced when

it was previously . Subjected to static

failure loading.

(4) For both compression and tension piles and at any

given depth of embedment the life span increased when

the load amplitude and/or the load level was

decreased.

(5) For both compression and tension piles tests; at any

depth the life span increased when the surcharge

pressure was increased.

(6) Previous repeated loading did affect the behaviour

of the pile \mder the succeeding loading. Smaller

rates of movement and longer life-spans generally

resulted. Both the pile shaft load and its residual

value decreased due to the previous repeated loading,

the higher the previous load'level the greater the

reduction. At·,~the last stage of the repeated

loading, both the shaft friction and its residual

value reached a limiting value which appeared to

be independent of the loadi,ng his tory of the

pile.

(7) Compressive-to-tensile repeated loading greatly

reduced the pile life-span as compared with that

of the unidirectional repeated loadjng.

(8) The repeated loading was found to decrease the

bearing capacity and the pulling resistance of

the pile for all depths of embedment and all

surcharge pressures examined in this part. of

the investigation. The highest reduction was

observed in the case of tension piles, being

more than 50% sometimes, whereas in compression

piles this reduction seldom exceeded 30%.

82

CHAPTER 7

Pru~SENTATION AND DISCUSSION OF CYCLIC SURClffiRGE TEST RESULTS

7.1 General

In the tests presented in chapter 6 the sand surface was

subjected to a constant pressure while the effect of repeated loading

on the pile performance was investigated. At some locations the

soil surface may be acted upon by a cyclic surcharge varying in

magnitude regularly or randomly. An example of such a condition

of loading may be encountered with offshore piles. The frequencies

of the cyclic surcharge pressure on the sand and tl1e repeated

loadings on the pile may not necessary be the same. Thus a phase

difference might be expected. The influence of such complex

conditions of stress on the behaviour'of a pile was examined and

the test results are presented in this chapter. Madhloom (1978)

carried out a series of tests on compression and tension piles' of

30 diametero dCptll embedded in a sand upon which a cyclic surcharge

2 varying between 50 and 100 kN/m acted. He found that:-

(i) The application of the cyclic surcharge

caused compaction of the sand over a

limited number of cycles beyond which

there was little or no further

compaction. The ultimate bearing

capacity and the pulling resistance

of the pile were, therefore, increased.

(ii) Under a static compressive loading test

two trends of th~pi1e movement were

observed. At 0.6Qc or less, the

83

pile moved at a decreasing rate as the

number of surcharge cycles was increased.

In contrast, at O.8Qc load the rate was

continuously increasing. In the case

of static tensile loading, even at 0.55Qt

loading, after a stable stage the pile

movement increased by a rate which increased

rapidly until failure occurred •. ,

(iii) The behaviour of the pile subjected to

repeated loading and with cyclic

surcharge was similar to an identical

pile but with a constant surcharge.

(iv) Schematic diagrams indicated that the

most severe situation occurs when the

load on the pile is at a maximum while

the cyclic surcharge is at its minimum.

'value.

7.2 Series Vlll: Piles subjected to static loading.

In order to continue the research programme on the behaviour

of piles under refeated loading the two limits of the cyclic

surcharge pressure adopted by Madhloom (1978) have been used in

this investigation.

The variation of the loads, the movements, and the rates of

movement ~gainst logarithm of the number of surcharge cycles for

Test 42 are shown in Fig. 7.1. '!'he pile examined in this test, had

a,depth ratio of 30 and was subjected to a static compression load

2 of 0.7Qc where Qc is the ultimate bearing capacity at 100 kN/m

surcharge.pressure. The pile loading was applied in four increments

2 while the surcharge pressure was kept constant at 100 kN/m •

84

o No. of cycles (Log) 3 10

4 1 102 10 10 10 1 __ • 1 ___ . 01 _. Movement

,1 11--

-;,2 2~ -u >..

" E E'3

r;-> o

31-E E 1=-= =-=-=- == ..-

)( -~.1. ~ I. ... Q) E E QJ Q) > > 0 o ::::E

E'5 5!--o OJ -o

a:: ·6... 61-

,71- 71-

z \.0 \D N N

II

~/' e~'

o~,

?i-7

./"

/'" /' -1400

~800

&1 ;;-0/' Base load at 50 kN/m 2 1200 ~ . «:-'/ /100 kN/m2 {/) 0, /~./ c ----- --t- =-=./: ----------- ------------- ___ 2 -- .----- ----- ==

/ / I

/ /

/ 1600 ~

0·7 Qc :: 2265 N

Test No. 1.2

lid:: 30 q :: 50/100 kN/m2

p :: 0·7 Qc Virgin

" "0 o

-12000 0 .-J

~21.00

~2800

-aL al I 13200

FIG, 7-1 VARIATION OF LOAOS,MOVEMENTS.ANO RATES OF MOVEMENT WITH No. OF CYCLES

(

To minimise the effect of creep on the observed movements,

the test did not start until at leaSt 10 hours after the load had

been fully applied to the pile. At the' initial stage of the cyclic

2 surcharge and during the lower phase, 50 kN/m ,the pile moved at a

rapidly decreasing rate. The reason behind this movement is

probably that at the lower pressure the pile loading,which was 0.7

of the ultimate capacity at 100 kN/m2 surcharge pressure, became

2 close to or even higher than that at 50 kN/m pressure. Regarding

the decrease in the rate of movement, this may be explained as ' .. , , . ~ . ~ .... foll~~s. Initially in each surcharge cycle and under the influence of

the higher pressure the sand mass was compressed. This compression

consisted of two components, reversible elastic and irreversible

plastiC movements. The first component resulted from the elastic

deformation of both the sand grains and the arrays of grain, while

the plastic movements were caused by rearrangement of the potentially

unstable grains which slip into more stable locations and/or

crushing, KO and Scott (1967), Lambe and Whitmnn (1969). When the

pressure was decreased to its lower value only the elastic movemen't

was recovered. With the repetition of surcharge cycles the number

of unstable grains decreased rapidly, Ko and Scott. This implies that

the sand grains were arranged in such a way that they became

approximately stable during the two phases of the cyclic surcharge.

The rate of movement, therefore, would consequently decrease as

the number of cycles increased.

The distribution of the axial pile loading during the two

phases of the first, the lOath and the last cycle (32643) in

this test are shown in Fig. 7.2. The axial load at any given point

85

~

o r

load • Newton s 800 1600 21.00

\ , I

N=1

yN=326L3

SO kN/m2

/ ,I

, / rl I

/

o

~ ,

load t Newtons 800 1600 21.00

--.--~- ,-- T-

100 k~/m2

Virgin Test No.t.2 Surcharge pressure = 50/100

kN/m2

Load = 0-7Qc

lid = 30

FlG.7-2 DISTRIBUTION OF AXIAL LOADING DURING CYCLIC SURCHARGE

(

Skin friction, kN/m 2

-10 0 10 20 30 1.0 50 60 -20 -10 0 10 20 30 1.0 50" 60 70 I I I I I I I I i I I I I I I I I I

1

1

1

50 kN/m2

Virgin

Test No_ 42 (3019) Surcharge pressure

=50/100 kN/m 2

Load = 0-7Qc

lid =30

",

(

/'" I

\

" " 1

1

1

N = 32643

~

" " " " \ ) "--- ~/ ----

100 kN/m 2

FIG.7-3 DISTRIBUTION OF SKIN FRICTION DURING CYCLIC SURCHARGE

(

l

700,~------------------------------------------------------------------~

C) ~

600

SOO

.; 1.00 c o -J

300

so kN/m2 -I

I

Typical Virain I

100 kN/m2

Test No. 1.2

YcI = 30

After cyclic . surcharge 50/100 kN/m2

p = 0·7 Q c

I 2 . 0 \<oN m - 1

I I

L~O\\ __ ~ ,t . - - I /~ I $0.:-- -- I

/' 1",....-'/ I I

I I /' I ~i~_ 1-100H r 1.".,- /.

I I 1/ /

6

4

2

I I / a

00 1 2 3 I. 5 6 7 8 9 . 10 11 12 Displacement, m m

FIG. (7-4) LOAD - DISPLACEMENT

z ~ ..

I I I

"0 C 0: ...J .

along the embedded shaft increased as the number of surcha,rge cycles

increased. This load was greater at 100 kN/m2

than that at 50 kN/m2

pressure. At the end of the test, the axial load at points located

within the upper third of the pile increased and became even

larger than the applied pile loading. Therefore a negative skin

friction developed along the pile at these points as shown in Fig. 7.3.

The skin friction along the upper half of the pile decreased and

along the lower half it increased when the number of cycles

increased. At the end of the test, a load test was carried out

2 on the pile at 100 kN/m surcharge pressure and then repeated at

2 50 and 0.0 kN/m pressure. Some interesting results were obtained

as shown in Fig. 7.4. The ultimate capacity of the pile at 100 kN/m2

pressure increased and it was approximately the same for both 100

2 and 50 kN/m surcharge pressure. Even at zero surcharge pressure

the pile load capacity increased and became close to that of the

2 virgin pile at 100 kN/m pressure. This phenomenon may be attributed

to the structure of the sand grains. As stated above, after each

pressure cycle the sand grains moved into more stable locations.

With increase in the number of cycles, the structure of the sand

grains, which supported the pile became more stable. Therefore at

the end of the test the static load capacity of the pile at 50 kN/m2

. 2 will be close to that at 100 kN/m surcharg9 pressure.

.. From the results of Test 41, in which the pile was load tested

before the beginning of the cyclic surcharge test, similar conclusions

may be drawn as shown in Figs. 7.5, 7.6 and 7.7. When the results

of this test are compared with those of Test 42 it will be seen,

Fig. 7.8,that the rate of movement decreased smoothly to zero during

the virgin pile test whereas it fluctuated and reached a state of

86

A ~ ~... • •

o 2 No.ofcydes (Log) 103 104

101

10 10 01 I! ! L'S r Movement

1~ \ z -g_1 \j

I.D 0 Y I.D ~ ~ ~ <II 0 $ III a 0 ~ " a ~ LJ) o· u co _ £27

_·1 Q.I

u >-. u

"'--2 E E

-u.OO

-1800

a 0 - ..... r-... a 0

- . co"" B ~ -=_-==_ _ __ _ ___;#1 ase loed at 50 kN/m

2

1';'4 "'E N - ==-.---i-:.::.::-- ____ ;(:100kNjm

2

1/1 E "0 ..... E I __ ---L./. 1200 c _ 0 ............ - ----- 0 o 0 Z Z -- ------ -<II - -'< -'< . -- ---- ~ 0.

5 5 ::: :5 0 I ~ Vl _ 1500 ...

o _ "0

C'j') a "; '3~E

E

n: 0 ~'" .I:-o 0

o ..J

2000

'6

·7

·8

0·7Qc = 2266 N

I 61- -12400

\ I I 71- i . h200 . f

81 \

-12800

FIG.7-5 VARi,l.TION OF LOADS,MOVEMENTS,AND RATES OF MOVEMENT WITH No. OF CYCLES

~

Load I Newtons

a 800 1600 21.00 I I I ,

50 kN/m2

N:1

Test No 1.1 -(3011. )

Surcharge pressure = 50/100

kN/m2

Load: 0-7Qc

Vd = 30

Load, Newtons

o 800 1600 21.00

100 kN/m2

1\ \\

FIG_ 7- 6 DISTRIBUTION OF AXIAL LOADING DURING CYCLIC SURCHARGE

Test No.41 (3014 )

Surcharge pressure = 50/100

kNjm2

Load =0·7Qc

lid = 30 1

1

Skin friction, k N / m 2

o 10 20 30 1.0 50 60 -30 -20 -10 0 10 20 30 40 50 60 iii Iii i " i , iii i I ( I

N = 1

N =10 N =100

(/--7~~ \ ,). ,

" ' " N = 21832

:::,.. ...........

'~~: .

. , \ ~~\ \ p. } . ,

1

1 ~I L - ..,.a>//

~=-====-.~

50 kN/m2 100 kN/ m2

FIG.7-7 DISTRfSUTION OF SKIN FRICTION DURING CYCLIC SURCHARGE

zero value rapidly in the case of the previously loaded pile. The

number of surcharge cycles at which the rate of movement of Test 42

attained zero value was approximately ten fold of that of Test 41.

The figure shows also that greater load was transferred to the pile

base during Test 42, virgin pile, as compared with that of Test 41.

Fig. 7.9 5.l1ustrated the variation of loads, movements and

rates of movement of Test 43,' in which the pile was also subjected

to 0.7 of the ultimate capacity at 100 kN/m2 surcharge pressure but

buried at 20 diameters depth. \'lhen the results in this figure are

compared with those of Test 42, Fig. 7.1, in which the pile was

embedded at 30 diameters depth, it may be concluded that both the

pile movement and the rate of movement decreased as the depth of

embe&nent was decreased. Concerning the pile base load it can be

seen that the difference in this load aD registered during the

higher and the lower surcharge pressure of any given cycles was

more pronounced as compared with thos,e of the longer pile. This is

because the relative movements between the sand and the pile, due to

the change in surcharge pressure was a maximum at the sand surface

and decreased as the depth of the sand was increased.

The influence of cyclic surcharge on the behaviour of a tension

pile subjected to a static load of 0.7Qt, where Qt is the ultimate

2 pulling resistance of the pile at 100 kN/m surcharge pressure, and

embedded at 30 diameters depth was investigated in Test 44. The

results of this test presented in Fig. 7.10 revealed that the

general feature of the pile behaviour was similar to that observed

in section 6.4 where the tension piles subjected to repeated loadings

,87

Or 01 101 2 No. of cycles (Log 1 I i 10 103 4 " / 10 .0

0-1 I

-u 0 0 Z

to OJ to tn N 0 N

'" CD II -C u

E3 0 r--.

OJ a - ~ ----c ~ OJ ~0-4 41

"0 0

> 0 0 E

5~ :(}5~ --a £. Vl -0

a::

06

0-7

~ _____ ~e

d -8/ OJ/ -~I

I

Movement 3019 -- -- ----Movement 3014

__ Base load at 100 kN/m2 After ~ 3014

Base load at 100 kN/m2 Virgin 3019

0·7 Qc = 2266 N

lid =30 p = 0-7Qc q = 50/100 kN/m2

400

800

tn c

12002 ~ OJ

Z

1600 -g o -J

o-aL. 8' , I '3200

FIG.7-B INFLUENCE OF STATIC FAILURE LOADING ON THE SUBSEQUENT BEHAVIOUR OF THE PILE

-u ~ u

Or-

~O-2 E E

--~ (}4 E <lJ > o

- E ()

No. of cycles (Log)

O' '01- 102 103 10'

I ,

\1 I~ Iv '-....

. 't-

.

2t-2 .

Bose at 50 kN/m 2 ,.

- L5100 kN/m '-4' _______________ -_________________ _ 3 . / / _________ __ _ -----------::.:7=-.=::.:. __ __ _ _ ________ _

~ - - - - - 0·7Qc =1888 N "~-------=~~~;-~------~~t-----~~-------1 Test No. 1.3 ., ClJ5

i> Yd =20 2 ~ q = 50/100 kN/m 61- p = 0.7 Qc Virgin

7'" Movement Rateof movement _.-

o-st- 8t-

1Q5 10

-I L.OO

-iroJ

IJ) -112m c

o -~ (l}

-i1fJ1Jz ...

"'0 a

-I2CXDo .....J

-I2tJJJ

-I26X)

-13200

O·gL g' 13faJ

FIG. 7-9 VARIATION OF LOADS, MOVEMENTS, AND RATES OF MOVEMENT WITH No. OF CYCLES .

No. of cycles (Log)

18r 3~ 101 102 103 104

I • I I

\ 0-7Qt = 824 N

105 900

-

-c (1)10 E (1) > o E 8 -o QI

06 a::

-I.

2

o

32 -t- /_ \ J. \. Test No 41.. 28

E E21. ~ -c

E20 QI

~ ~ 18

1

8

4

o

- . \ lid = 30 -

.\ P = 0·7 Qt . - . q = 50/100 kN/m 2 Virgin -

\ Movement .-............ Rate of movement -. - § -t-, Z __

"\ ...: ~ N (1) co ~-t- • II

\ ~ t- "\ 0 -

Base load q= SO kN/m2

r .", / q= 100 kNjm2 -

", LL ./ 1=-- ---------------=-=_-::::-- ____ ~_ -.il:l: ________ ~ -----

- ---------- ~,- --------------------. I I .~ .1. )

FIG. (7-10) VARIATION OF LOADS MOVEMENTS AND RATES OF MOVEMENT WITH No_ OF CYCLES

800

700

\J\

600 c o -~ (1) 500 Z ..

'U o

400 .3

300

200

100

a

with static surcha,rge pressure.,

The distributions of skin friction duri,ng each phase of

the cyclic surcharge of the first, the lOth, the l070th and the

'last cycle, cycle number 18300 are illustrated in Fig. 7.11. It

can be seen that the residual compression skin friction mobilised

after penetration decreased and became entirely tensile when the

loading O.7Qt was fully applied to the pile. During the first

cycle, therefore, the skin friction was entirely in tension with

a value which increased as the depth of the pile increased until

it reached a maximum value at a point located 3.5 diameters above

the base. Beyond this it decreased rapidly and became zero at the

lower end of the pile. Except for the initial few cycles of the lo ... rer

surcharge pressure, where the pile was progressively pulled out,

in each cycle of the surcharge pressure the negative skin friction

along the upper half of the pile increased whilst along the lower

half it decreased. The results of a static pulling load test '

carried out on the pile after the end of the cyclic surcharge test

'2 2 at 100 kN/m and then repeated at 50 kN/m surcharge pressure are

'shown in Fig. 7.12. For comparison typical load-displacement

2 curves at 100 kN/m pressure are also presented in this graph.

It may be concluded that the pulling resistance of the pile

increased after it had been tested under cyclic surcharge. Although

the resistance at 50 kN/m2 pressure was relatively high it was

2 smaller than that at 100 kN/m pressure.

7.3 Series lX:' Repeated loads in-phase with the cyclic surchClrge.

Through this test series the pile which embedded at 20

diametezsdepth, was subjected to repeated compressive loads. These

loads acted upon the pile in-phase with the cyclic surchargerr~ssure.

TWO modes of testing were therefore employed within this series.

88

Sk in friction kN/m 2

-1.0 -30 -20 -10 . 0 -50 -1.0 -30 -20 -10 0 10 20 iii iii i I Iii I I

--- -" ......-- " ~ ..,...,.

~ , ./' , I ,

.

, J ( .

, I I

.

\ I 'l I

\ \ \ b

,. 7

,~

rt

,. t " ~

,\ ~ \ ,

\ I l I \ I , \ \

\ \ , .-N = 1

N =10 N = 1070 N : 18300

50 kN/m 2 100 kN/m 2

FIG. 7-11 DISTRIBUTION OF SKIN FRICTION DURING CYCLIC SURCHARGE

Test No.!.!. (3021 )

Surcharge pressure :50/100 kN/ m2

Load = 0·7Qt

'!d = 30

1·6,---------------------.,

1-4 After cyclic surcharge (100 kN/m2 )

1-2 ~.--/

Virgin repeated (2)

~ 10/ ... . /'

0-6

0-4

0·2

I I

1

virgin(1)

Repeated

at 50 kN/m2

2 5 Displacement, mm

6

FIG.7-12 LOAD - DISPLACEMENT BEHAVIOUR OF

TENSION PILES -

7

·._ .-41'- ~---------' -_."-

In Tests 45 and 46 the upper repeated load was in-phase with the

higher surcharge pressure,' 0.3Q'c/100'whereas in, Test 47 the upper

repeated load was in-phase with the lower surcharge pressure O.3Qc/50.

The results of these three tests are presented in Figs. 7.13, 7.14

and 7.15 respectively. These results indicated that the pile

experienced the same behaviour as that under static surcharge

pressure. During the initial stage of the repeated loading the

pile moved slowly into the sand but after a certain number of

load cycles the rate of movement began to increase until it

reached a maximum value beyond which it decreased as the number

of load cycles was increased. The pile shaft load first

increased and reached a peak value before decreasing to a limiting

value.

To assess the effect of such a condition of loading on

the perfor:mo.ce of the pile, the movements of Test 45 and 47

together with those of Tes~ 13 in which the surcharge pressure

2 • was static and equal to 100 kN/m are drawn in Fig. 7.16. It is

clear that the pile life-span increased when the surcharge was

cycled and it was longest when the upper repeated load acted in-

phase with the high cyclic surcharge pressure, O.3Qc/1OO. This

. latter conclusion is in line ... lith that reported by Al-Ashou (1981)

and also with the analysis suggested by MadhloolD (1978). The

magnitudes of both the maximum rate of movement and the maximum shaft

load were also affected by the state of the surcharge pressure as

shown in Fig. 7.17. The value of the maximum rate decreased when

the sand was subjected to cyclic surchC>.,rge and it was the smallest

in the case of lOO/0.3Qc. As demonstrated in this figure, the value

of the maximum shaft load was largest in the case of lOO/O.3Qc.

89

o

~2 u >.

~ E E4

M '0 .-

:Ie

(]J -o a:

10

12

No. of cycles (Log) 10t. 103

0 1 - 102

_>" '" __ ._., .--1 10 I _ ____ -....... /

1 . i - --- ~--7----0, ______ .-1---. . . . ____________________ -______ ----.\ _ ~200 -------------- / ------------

\.-

1--------2~

z 0')

o cO 41-

E E6~

.. ~ M 6

....... J.---- 1

-

C (]J

-- ---~ ---- \ ---- \ ---------- \

-1400

II)

c -16002

~ (]J

z ~8 -0:3Qc =-809 N 800-0 ~ Test No.l.S g ~ . /, V' - -l 0·3Qc/ 0·0 Irgin

101- Surcharge pressure = 50/100 kN 1m2

12~

~ :20 Movement Rate of movement Base load Residual base load -------

-11200

1l.L. 14' '11.00

FIG. 7-13 VARIATION OF LOADS, MOVEMENTS, AND RATES OF MOVEMENTS WITH No_ OF CYCLES

"

o

l.

<11 u8 ~ u

~ E

cp 12 o ~

x

-~ 16 E <11 > o

~20 o <11 -o a:: 24

28

1 2 No. of cycles (Log) 3 '

01 '0 'p 'P '95

.--......., / '",

/' "'-. I \

----------

Test No.46

Yd= 20 q = 50/100 kN/m2

p = O·SOc/O·O Virgin

Movement

Rate of movement --.-Base load

Residual base load - - -- - -

1\ .', --~---------~-----

I \ .

I O·SOc = 1348 N

\

200

400

II)

600 § -~ (])

z ,

800" o o -'

32L 16' , '1600

FIG. 7-14 VARIATION OF LOADS,MOVEMENTS,AND RATES OF MOVEMENT WITH No. OF CYCLES

Or 01r==============~10~1~========~~~2~N~o~.~O~f~ey~e;le~S;{L~O~9~)~~ ____________ ~ 10 104

_____________ 0 ..... _.... ...... ...... .."......---.---- ~ OJ U >.

2

~4 E E

cr>o ';6

-

E E

2

4

c6 OJ E ------------ - - - --

.~---------

til c 600B

~ c OJ E ~ 8 E

OJ Z

~ f- 0-3 Qc \ ~800-g L8 ~ ---o OJ -&. 101- 10

12t- 12

Test No.47 0·3Qe/0·0 Virgin

q = 50/100 kN/m2

Yd =20 Movement Rate of movement Base load

14L 14' Residual base load '1400

FIG.7-15 VARIATION OF LOADS I MOVEMENTS,AND RATES OF MOVEMENTS WITH No. OF CYCLES i I , d

E E

01

2

I.

6

.. 8 -c . OJ E OJ

~10 ~

12

1[,

101

Residual load = 0·3 Q c 0·0

lfcj = 20

No. of cycles (Log) 3 102 . 10

"

104

l

16~' ----------------------------------------------------------------------~

~

~~-~.--~~ ...... ----------------

FIG.7-16 INFLUENCE OF CYCLIC SURCHARGE ON THE LIFE-SPAN OF A PILE SUBJECTED

TO REPEATED LOADING

1'1"

-

No. of cycles (Log) 3 lOt.

102

10 :::::l 0 10

1

~100kN/m2/0'3Qcl o ~ r # (J)

~2 ~ E E

200 1:2; '), ~

~ ~~ o ~.

(J) ~ cr> C) I. 400 ~ o ..- ~ x .... ct __ -- \

Tes! No.13", = 100 kN/m 2 = cons!. _ \ 1.7~~ -L~ ________ -==--_ \

-'-~~ =--=-:--~ ~ = " ~ ~-;;:;/~3~---=--=---=-~-=---::..--.-::-:::::.. - - - l00kN/m2/0·3Qc - -o 81 s:

~ ~ a 0.30 Q 600 Z ~ c •

'U C o

Isoo ...J

.... c (J)

E 6 (J)

> o E

....

10

12

Residual load = 0·30 Qc/O·O

Vd = 20

Rate of movement --­

Base load ---

1000

1200

fl.' '1400

FIG.7·17 VARIATION OF LOADS AND RATES OF MOVEMENT WITH No. OF CYCLES

I -1

-,

5------------------------------------~

4

.. 3 "'0 100 \<~m2 o o

..J

/ I

2 / / I I

1 I I

1

. co\ Virgin ~~l;-- -

2 3

-

4

Te st 46

q = 50/100 kN/m2

p = 0'5 Qc/O'O

100/0'5 Qc

Yd =20

5 6 Displacement I mm

FIG. 7-18' I NFLUENCE OF CYCLIC SURCHARGE ON THE

LOAD -DISPLACEMENT BEHAVIOUR OF PILES

7

During the last stage of the repeated loadi?g, the shaft load

reached a value which appeared to be independent of the' state of

the surcharge pressure.

Fig. 7.18 confirmed the previously stated conclusions which

indicated that the ultimate capacity of the pile increased after

being tested with cyclic surcharge and that the capacity at

2 ' 50kN/m static surchargefr~ssure was very close to that at 100

kN/m2 pressure.

7.4 Series X: Influence of surcharge and pile loading frequencies on pile behaviour.

Based on the results of tests carried on sand samples tested

in a simple shear apparatus and subjected to cyclic shear-strain,

Youd (1972) reported that there was little or no effect on the amount

of compaction when the frequency increased up to 115 c.p.m. Simllar

tests conducted by Lee and Focht (1975) revealed that the cyclic

loading shear strength of sand was slightly affected when the .

frequency iilareased up to 10 lIz. Because the stress-paths and the

strain-path involved in the case of a pile subjected to repeated

loadings or tested with cyclic surcharge are different from that of

a simple shear condition it was decided to check whether the frequency

had any effect on tile performance of the pile. In this respect,

three tests were conducted, namely Tests 48, 49 and 50. In the

first two tests the influence of the cyclic surcharge frequency on

the behaviour of a pile at a depth ratio of 20 and subjected to

0.3Qc/0.0 repeated loads was examined. In the latter test the

2 surcharge pressure was maintained constant and equal to 100 kN/m

while the effect of the repeated loading frequency on the behaviour

of a pile had 30 diameters depth was investigated.

90

The variations of movement and rate of movement with l.ogarithm

of the number of surcharge cycles of Test 48 and 49 are shown in

Fig. 7.19. In spite of the frequency in Test 49 being two times

that in Test 48 but the difference between the movement and the

rate of movement of the two tests was relatively small. HONever,

these minor differences indicated that after any number of

surcharge cycles a smaller amount of movement resulted from the

1011 frequency test. 'l'he measurements of base loads of tl1e two

piles during the two phases of the cyclic surcharge, Fig. 7.20

also revealed minor differences. More improvement in the pile

shaft resistance was observed during the test of the low frequency.

The difference in pile behaviour of these two t.est.s may bE.

attributed to the fact· that with lCM frequency the sand grains had, ,

a'longer time to arrange into a more compacted and stable structure •

. In contrast,' when the frequency of the repeated loading was

increased from 1.0 c.p.m. in Test 11' to 3 c.p.m. in Test 50 the

pile movement was slightly larger during the test of lON frequency

as shown'in Fig. 7.21. The pile base load did not alter significantly

when the frequency increased three times. However, it may be

concluded that from the practical point of view and within the

tes ted liroi ts, the frequency of both the cyclic surcharge and the

repeated loading did not affect Significantly neither the movement

nor the load transfer. characteristics of the pile.

7.5 Series Xl: . Various combinations of loading and different states of surcharge pressure.

This section deals with the results of Test Series Xl which

consists of five main tests. In these tests the performance of

the pile was examined under various load histories and with cyclic

and/or static surcharge pressure. During the first test in this

91

... - j

r

o 101 2 No. of cycles (Log) I I 10 10

3 4 :::::: I 10 __ ::::::::-.... 0 I

21- \" \ <'1 OY "/ 1"\ CJ~- ,2 ClJ

u \ >.. ,

~ 41-

3· 0 C. P. M. (T est 49) V'\ 4 E E

--- 1· 5 C.P.M. (Test 48) M

E \~ '0 ...-

E 6 x

p = 0·3 Qc/OoO l.GP.M '!, \0t,. 6 ... 0", \?\j' -- q = 100/50 kN/ m2 c

c ClJ OJ

. ,,~.-<)

E Lid = 20 ol E

'(1)

~ 8 0 .~

8 > (1 ". 0

0

" E

::E: 0..-0 .~

"- -0

101- ... 10 2 I

a 0::

121- -n2

14' '14

FIG.7-19 INFLUENCE OF FREQUENCY ON MOVEMENT AND RATES OF MOVEMENT

~ 2 No. of cycles (Log) 3 4 i

10 10 10 10 i I

~~£f 5~kNL~ - ::::::,...:...-::;...-------::~~---- -- - === -- --------- --- /-2

t- - - - - - - - - - - _____ - - Lower 100 kN m 200~ - -I-~,-g a 0 0)-u

a M o 400 ....

U)

c .2 _ -=: -.:::-=. _ ___ -- - - - - - - - - Upper 50 kN 1m2 ~ --- --~-OJ ..... 'U a a Z 6i g Upper 100 kN/m2

" -g 800b j , 0·3 Qc = 809 N . o

-oJ

1000'"

1200'"

lid = 20

--~

3·0 C.P. M

1·5 C.P.M

(Test 49)

(Test 48)

1400~'-------------------------------------------------------------------------~

FIG. 7- 20 INFLUENCE OF FREQUENCY ON THE LOAD -TRANSFER BEHAVIOUR

pr.

Or 01 101 2 No. of cycles 1 Log 1 i 10 103 <::::~ i 10

4 10

Residual base load 1 c.p.m. \\\ .. .r;_---Q; 2l- 2 3 c.p. m.~ ... :;~

_----------------------------..:-,:-:..:-->;5:.. ... , \ \\ Movement u >. u

'E'l. E

rr> o ..-

~6 -C <lJ E <lJ >8 o E

-o

_------- \ /1 c.p.m. 3c.p.m.

Base load 1 _ c.p.m. 130151

~ ~ . 3 c.p.m. (3024)~· E ------ --<lJ - ---------- -- -- --- - - .=::::-> -- - -08 --:::E --

-----~ .. ~

0·30c = 971 N . <lJ 10r 10 Rate of movement 1 c.p.m.

3c.p.m~ -o c::

12 Vd=30 q=100 kN/m 2 = CONST.

,200

400

~ <lI

Z

800 -oA o o

...J

1000

16L 16' '1600

FIG.7-21 VARIATION OF LOADS,MOVEMENTS,AND RATES OF MOVEMENT WITH No. OF CYCLES

series, Test 52 the pile, which was embedded at 20 diameters

depth, was subjected to 0.5Qc/0.0 repeated loading and after the

rate of movement had reached a maximum value and began to

decrease, repeated loads of 0.3Qc/0.0 were then applied. During

testing the surcharge pressure was cycled between 50 and lOOkN/m2

so that the high pressure was applied in-phase with the upper

repeated load. When the results of the latter part of the test

are compared with those of Test 45 it will be seen, Fig. 7.22 that

the life-span of the previously loaded pile is much longer than that of

the virgin pile, Test 45. The value of the maximum shaft load was

larger in the test of the virgin pile while the limiting value

appeared to be the same for both piles. These two conclusions are

in line with those observed under constant surc~arge pressure in

section 6.6.

In Test 53 the pile was subjected to the following sequence of

loading:

53 - a 0.3Qc/0.0

53 b 0.5Qc/0.0

53 - c 0.7Qc/0.0

53 - d 0.7Qc

53 - e 0.9Qc

The surcharge pressure in all these parts of the test was

cycled every 20 seconds. The results of part 53-b together with

those of Test 46 are plotted in Fig. 7.23 as a function of the

logarithm of the number of load cycles. As ill the case of static

surcharge, Fig. 6.35, when the repeated load became as high as

O.5Qc the virgin pile exhibited a longer life-span. During the

initial stages of repeated loading the shaft load of the virgin

92

/

I I I i I 1 I .

I !

o

2

I.

6

E E

C 8 QJ

E QJ

> ~ 10

12

11.

16

101 2 - No. of cycles (Log) 3 10 10

-----~--.-~.,l....;;~ __ --------Mov-:-"" Test 52

- \~

\~ \~

~ \?. \0>'

1es0~--\ \'D.. Base toad _ - - -- \ \~

Test 52 \ ,!J' ~ ------------ ~----- \ r==-- ----

0·30c = 809 \ \

\ 0·30c 0-0

,.. Surcharge pressure:: 50/100 kN/m2

100/0·30c In - phase

~ ~=20_

~ •

FIG.7-22 INFLUENCE OF PREVIOUS LOADING ON THE BEHAVIOUR OF PILES

\

101. o

\ I

\i I

-

-

200

400

III C o

600 i QJ

Z

800 15

1000

1200

1400

1600

o -..J

0, 101 102 No. of cycles (Log) 3 10 101. .0

2

I.

E E6

-ffi E ~8 o ~

- --- -------., --- .........

leS! Iv. o SJ"'

6

Base load T, est No.53

-------------Base toad Test No. 1.6

0·5 Qc/O·O

" " "-\~ \~ ~ t ~ f 't-\~ ~

=- ----,--\4C.

~. \ \

\ \

lid =20

q = 50/100 kN/m 2

p = O'SOc / 0·0 o.SQC/l00 .

(/)

600 § -3: (l)

z

800 -

1000

1200

"'0 o o

.....J

11.00

16' '1600

FIG.7-23 INFLUENCE OF PREVIOUS LOADING ON THE BEHAVIOUR OF PILES

- -~- -. - - . - ... ~ .--' -.~ ,~-.- _.-- '- ~- _.--- - -- '--'-'~-----'-

pile, Test 46, was smaller than that of the previously loaded

pile, but during the last s~age the" shaft load of both piles

reached approximately the same value."

Fig. 7.24 compares the movements of part 53-d with that of Test

43 in which the virgin pile was first loaded with O.7Qc. It will

be seen that the virgin pile experienced smaller settlement at the

beginning of the test. During the cyclic surcharge the movements

of the virgin pile were always smaller than those of part 53-d.

The results of part 53-e shown in Fig. 7.25 indicate that even

under a high static load level, the pile moved at a decreasing rate.

Initially, the pile settled 1.5mm and after 7043 cycles the

settlement did not exceed 2.7mm.

Test 54 was directed to a study of the influence of cyclic su~charge

on the perfo~~ance of a pile subjected to repeated tensile loads.

During testing the following sequences of loading were applied:

(54 - a)

(54 - b)

(54 - c)

(54 - d)

0.3Qt/O.O

o. 5Qt/0~O

o.5Qt

O.5Qt/0.0

2 The surchargeprE'ssure was cycled between 50 and 100 kN/m

during all these parts. In part {54-a) the pile was only subjected

to 8700 load cycles. At the end of this part the pile was in

a stable state. For comparison the movement and its rB.te of this

part together with those of Test 22, in which the surcharge pressure

2 was constant and equal to 100 kN/nl are presented in Fig. 7.26.

It. can be seen that when the pile in Test 22 had failed, the pile in

part (54-a) was still in a stable condition. This result confirmed

the previously stated conclusion that cyclic surcharge increased the

pile life-span.

93

1

No. of cycles. (log)

100 101 102 103 104

Test 43 (Virgin) ~------------------------------------------

Test (53-d) 1·01-

2·t-

E 3'1-E

- 4.L c ..

01 ~

E 01. >

sl 0 q = 50/100 kN/m :;E

p = 0·70 Qc I/d= 20

I 6·

FIG. 7-24. VARIATION OF MOVEMENT WITH LOG NO. OF CYCLES.

~

Or 0'

<lI

M

I~ lE ~3 E

- ~--c C <lI <lI E E <lI <lI

~L. ~ E ~ -o <lI

0 5 c::

6

7

r--

I'---~r-- Z

r--. N -..t N II

If- u a 0')

~-==-=-~ .-

....

)~

f-

8L 8

"'0 0 0

~-QJ Vl 0

CD

F= -

"0 0 0 ---0

..c. U)

10' 2 No.of cycles (Log) 3 10 10

I I . .,---.--t._ .

..... / !i .§J.

t7 -0/' Movement

cf

--. ,/'. ~ ----- ---------- ----------7 -----------...........:-~ ~- - -:.=..:. - - - - -- -- - ----

I Base load at 50 kN/m 2

. 100 kN/m2

/ . .

I ,

.. -

I O·g Qc = 2427 N . I I i

FIG. 7-25 VARIATION OF lOADS,MOVEMENTS.AND RATES OF MOVEMENT WITH No. OF CYCLES

104 o

-

-

-

i -

-

-

-

400

800

II)

1200 § -~ <lI z

1600 "'0'"

o o

..J

2000

2400

2800

3200

r

101 No. of cycles (LogJ

1 . 102 103 101. 71 \

I • I I 17 lid =30 I

6~ p =0·3Qt 0·0 I -16

Test No.51. q = 50/100 kN/m2

I ~ sr \

Rat e 0 f movement I -i5

/ , -~l. f

-it. E OJ > , 0

::E3 -13. I

Test No.22~

21- , . I \ Test No.54 Movement -12

11- t \ I /""-.

Rate of mo~ -11 .

~ , ---- I

" O· ___ -"10 ~_~~ -~ - --=-- -- , - ----=- - - ~---- - - -- .... 'a

FIG.7-26 INFLUENCE OF CYCLIC SURCHARGE ON THE LIFE SPAN OF PILES

. .... '

When the pile was subjected to.O.5Qt/O.O repeated load,

part (54-b), the movement increased at a decreasing rate. After

cycle number 60 the rate of movement increased rapidly with

increase in the number of load cycles. At cycle number 430, the

last cycle in this part, the pile had pulled out a distance

of IO.74mm at a rate of 0.043mm per cycle, as represented by

points B1 and B in Fig. 7.27. The pile loading was then held

constant· and equal to O.5Qt whereas the cyclic surcharge was

continued, part (54-c). After 1900 surcharge cycles the pile

loading was then repeated, part (54-d). It was found that the

rate of movement reversed its trend and began to decrease as the

number of load cycles was increased. When the test was terminated

after 100 load cycles, the rate of movement had decreased from

0.043 to 0.017mm per cycle as shown by point C.

The distribution of skin friction along the pile shaft during

the first and the last cycle of each part of Test 54 is shown in

Fig. 7.28. In part (54-a) the skin friction along the upper

half increased whereas along the lower half it decreased which is

similar to that observed in section 7.2. During part (54-b) where

the loading was O.5Qt/0.0 the reverse trend was observed. The 1900

cycles of surcharge pressures, part (54-c),caused an improvement in

friction along the upper half of the pile. This improvement again

deteriorated when the pile was subjected to repeated loading of

o. 5Qt/0. 0, part "( 54-d) •

Flg. 7.29 shows the results of Test 55 in which the

.influence of cyclic surcharge on the behaviour of a pile

subjected to compressive repeated loads and embedded in a.sand

2 upon whlch a static surcharge pressure of 100 kN/m acted.

94

16' 10' 102 No. of cycle s (Log)

I I B 103 j

104 I L.O

I I

lL.l- I I I

35 -

12f-

I ,C1

~ u

I I

~

I

u

I , 30 ~ , E C'j'>

0 .-I I'

E 10 E \ -c

<lJ --c <lJ 8

\ .. E <lJ

E

>

\ <lJ

0

20 ~

~ \C

E -0

ROf / / 15 2

E;\ Of

L.l-"'0","

lId: 30 I ~

~ef)f

2 5

01 10

FIG.7-27 VARIATION OF MOVEMENT AND RATE OF MOVEMENT WITH No. OF CYCLES

. '< N I ml _0

o 20 40 ii'

kN I ml o 20 40 I I ,

N:1_...u

J / .L...- N =8700

/

100 kN/m2 50 kN/m2

(a) 0 0 3 a tlOoo During the upper repeated load level

kN / m'L o 20 40

100 kN/m2

(e) O·SOt

IOJ I m' o 20 40

SO kN/m2

kN 1m' o 20 40 i I I

100 kN/m2

(b) o·sat/ooo

I(N I m'l o 20 40 • I I

50 kN/m2

During the upper repeated load level

0' 0

kN Im1. o 0 . 20 40

100 kN/m2

(d) O'SOt/O'O

'kN 1m" o 20 40

50 kN/m2

During the upper repeated load level

FIG. 7 -28 DISTRIBUTION OF SKIN FRICTION (Vd = 30 I TEST N o.SO

-o OJ

No. of cycles (Log) 1-~ 101. 10

2 ~ a Or 0' § _._._. /

- ~ " 81

16

E E .. 21.

-C <LI E <LI ~32 ~

-------

z co ~

lD ~

II

U a I/')

6

g :f \ ~' - -b---'" \) - ,. I· -D2 .- - . ,; \ Vl 0 . \' .... OJ .L: ......... I " 0:: til .... _ .... _ " •

---,-R;sid~;\- -b~;~ -\~;d----- \ { Dr \ B2 . ~ I

I \~ \1 * ~ I m ~

. 0

200

400

VI

600 § -~ OJ z

800 .. "U o o

....J

oSOr 40 0::

\\ .-_~a~loOd' \ 1000 -----

t.8

56

"U o o

--o .L: til

~---........ . Ie A1

............ \ t\ 1 ". ". 't--. __ ..JB . ---, C2 . I e \ . . f \ .

.-..J

Test No. 55

0·5 Qc/O'O

1200

q = 100 kN/m2-11400

~=30

6l.~ t t O·SOc = 1618 N d1600

FIG.7-29 VARIATION OF LOADS MOVEMENTS AND RATES OF MOVEMENT. WITH No. OF CYCLES

The pile was first subjected to 0.5Qc/0.0 loadi?g and a£ter the

rate of movement had reached its maximum value and began to decrease

the load was held constant and equal to 0.5Qc. After 2300 cycles

2 with the surcharge cycled between 50 and 100 kN/m the pile

loading was then repeated while the surcharge pressure was held

2 constant and equal to 100 kN/m. As illustrated in Fig. 7.29,

when the surcharge was cycled the shaft load and its residual value

increased as represented by the path C-C,land D-Dlrespectively.

During the last part of the test, when the pile loading was repeated

these values decreased from CI-C2 and from DI-D2 respectively.

On the other hand, a sudden change in trend of the pile movement

resulted after the cyclic surcharge as represented by the patll

A-Al. The corresponding rate decreased from 0.065 to 0.007mm per

cycle, B-Bl then began to increase during the last part of the test , -until it reached 0.020, point B2;at the end of the test.

cyclic surcharge loading also improved the performance of

a pile subjected to a tensile repeated load of 0.5Qt/O.0 and with

2 a constant surcharge pressure of 100 kN/m , Test 56, as shown in

Fig. 7.30. During the first part, after the pile had pulled out

l8.7l1mll at a rate of 0.083nnn per cycle the Jpa d was then held

constant and equal to 0.5Qt. After 1300 cycles of cyclic

surcharge was appllied to the sand surface the pile loading was

then repeated. It was found that the rat.e of moyement decreased

from 0.083 to ~ro rom/cycle as sho .. m by the path B-Bl. The pile

behaviour, therefore, entered another stable stage which lasted

more than 2700 cycles before the rate b,egan to increase. When

this number of cycles compared with that of the first part, 1300

cycles, it will be found that the pile life-span had increased by

95

,/

1 140r 28

1201- 24

-1001- 20 ~

~ ~ E80 16 E E

C';'l E a -..-- -x C

~O ~2 (j) (j)

E > (j) 0 > ~ o El.O 8 -o (j) -a

0:201- l.

01- 0

101 2 No. of cycles (Log) 3

10 10 I I I

l. 10700

r- Test No. 56 A ~ 1 .

lid =20

600

P = 0·5 Qt/O·O

r- q= 100 kN/m2 A-

\ Movement ~ Rate of movement -' - B ._

I \ Base load - - - § '\ Residual base load ------ ~

I r-_ ~ , 0·5 Qt = 270 N

\ ,I -\ I f2 -

\ N I

" \ . ------- .\. ~-\ ----- --- I· .' Sa . ':=-a- -- ~b 2 ......... --~-........ . ------. --- I __ J I -_':>', -,,' ........... -'-' - ./"""- -.....-- ./'

..-- ----,' 81' --------- ~ -------- E ' ---- - ------- , 0 I

~----- U

500

l/)

400 § -~ (j)

z I 1300 '0

a 0

--l

200

100

o

FIG.7-30 VARIATION OF LOADS, MOVEMENTSJAND RATES OF MOVEMENT WITH No. OF CYCLES

more than ·two fold.

7.6 ·Conclusion

The followin9 conclusions may be drawn from the results of the

. tests performed with cyclic surcharge pressure:-

(1) When the pile is subjected to static loading

and a cyclic surcharge acted upon the sand

surface the follo\ving were observed:

(a) For compression piles, the movement

increased at a decreasing rate until it

reached an approximately constant value.

For a given percentage of loading, the

pile movement increased when the depth

of embedment increased. Due to cyclic

surcharge the axial load 'at any given

point along ti1e pile depth increased.

This increase was greater along the

upper part of tile pile. After a certain

number of surcharge cycles, fue axial

l~ads of points located along fue upper

part of the pile became larger than the

applied pile loading and thus a negative

skin friction developed along the pile

at these points.

(b) For tension piles, the pile movement

first increased at a decreasing rate

then, after a stable stage, the rate of

movement increased very rapidly until

failure occurred. Cyclic surcharge loading

96

caused an increase in the .n.egati ve skin friction

al~ng the upper elements of· the pile and a decrease

in that of the lower elements.

(2) From repeated loading tests,· the following

conclusions were reached :-

(a) The behaviour of piles subjected to

repeated loadings with cyclic surcharge

was similar to that of . identical piles

but tested with a static surcharge pressure.

Yor compression piles, the rate of movement~:

after having reached a maximum value decreased

wnen the number of load cycles was increased.

For tension piles, the rate of movement, after

a stable stage, increased rapidly until

failure occurred. ".

(b) The cyclic surcharge resulted in a pile

of longer life. This life was longest when

the upper repeated·load.acted,in-phase with

the higher surcharge pressure.

(c) For compression piles,the value of

the maximum rate of movement decreased when the

surcharge pressure was cycled. The smallest

value was observed when the upper repeated load

was in-phase with the higher surcharge pressure

(d) During repeated loeding, the pile shaft

load increased to a peak value then decreased

until it reached a limiti?g value. The value

of the peak was largest in the case of the

97

upper repeated load being. in-phase with the h,igher

surcharge pressure.' The limiting value appeared to

be independent of the state of the surcharge, i.e.

whether it was in or out of phase, static at 100

2 2 kN/m or cycled between 50 and 100 kN/m •

(e) From a practical point of view and within

the tested limits, the frequency of the cyclic

surcharge or the repeated loading did not alter

significantly the movement nor the load transfer

characte:dstics of the pile.

(f) The previous pile loading caused a reduction

i~ the rate of movement of the pile during the

succeeding loading.

,(g) The peak shaft load of the virgin pile was

generally greater than that of the previously

loaded pile but, at the latter stages of the

test, the shaft load of both piles attained

approximately the same limiting value.

(3) The cyclic surcharge was found to increase the

bearing capacity and the pulling resistance of

the pile. Moreover, the pile load capacity,

after a large number of surcharge cycles was

not changed significantly when the pile load

tested at 100 kN/m2 or at 50 kN/m2 surcharge

pressure (the two limits of the cyclic surcharge).

98 '

CHAPTER 8

MAIN CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK

0.1 Main Conclusions

The purpose of this investigation was to st.udy in detail the

influence of repeated loading on the behaviour of isolated piles.

The tests were performed on laboratory scale ~nstrumented piles

driven. in a dry medium dense sand. The influence of the depth of

embedment, the surcharge pressure and the loading history on

pile behaviour were examined in this investigation. The followin9

conclusions can be drawn from these tests:-

(1) The results of tests carried on a pile

element embedded in a triaxial specimen,

chapter 3, have shown that:-

(a) The skin friction increased linearly

with increase of pile displacement up to

failure.

(b) The value of the ultimate skin

friction decreaged and the load­

displacement became more brittle when II

the loading was repeated.

(c) Alternating loading greatly affected

the behaviour of the pile shaft. The

ultimate skin friction decreased and the

load-displacement became more ductile with

this type of loading.

(2) The results of preliminary tests carried out

with static surcharge pressure revealed that:­

(a) The driving resistance of the pile

base increased rapidly with the depth of'

99

penetration, reached a peak value and

then decreased sl.ightly to a limiti.ng

value. The peak value increased and

'was reached earlier when the surcharge

pressure was increased.

(b) The shaft friction in compression

was generally greater than that in

tens ion loading by a magni tude

dependent on the depth of embedment,

the surcharge pressure and the

magnitude and direction of the

residual stresses before testing.

(3) Repeated loading tests performed on

compression piles with static

surcharge pressure have shown that:-. . (a) lnltially, the pile was stable

but after a certain number of cycles

it bego.n to move at a rate which

increased rapidly to a maximum value

anel then decreased as the numl1er of

cycles increased.

(b) The pile life-span increased when

the embedment depth decreased or the

surcharge pressure increased.

(c) At any depth of embedment or

surcharge pressure, as the number of

load cycles was increased the shaft

load increased up to a peak value and

100

then decreased gradually, to a 1imi ti,ng

value.

(d) The rate of movement increased and

the life-span reduced when tne pile was

subjected to failure loading before

being tested under repeated loading.

(4) Repeated tension loading on piles with

static surcharge pressure has indicated

that:-

(a) Initially, the pile moved at a decreasing

rate. After a stable stage the movement began

to increase at a rate which increased very

rapidly until failure occurred.

(b) The pile life-span increased when the

depth of embedment and/or the surcharge

pressure was increased.

(5) For both compression and tension piles and at any

depth of embedment or surcharge pressure, the pile

life-span was reduced when the load amplitude and/

or the load level were increased.

(b) Previous loading affected the behaviour of

the pile during the succeeding loading and a ~ma11er

rate of movement generally resulted. Both the

shaft load and its residual value decreased when

the pile was subjected to previous loading, the

higher the previous repeated load level, the

greater the reduction.

101

,

(7) Compressive-to-tensile repeated loads greatly

reduced the pile life-span.

(8) Repeated loading was found to decrease the ultimate

beari.ng capacity and the pulling resistance of the

pile for all depths of embedment and at all

surcharge pressures examined. The la.rgest reduction

was observed in the case of tension piles.

(9) When the pile was subjected to static loading

and a cyclic surcharge pressure acted upon the

sand surface the following trends were

observed:-

(a) For compression piles initially the

movement increased as the number of

surcharge cycles increased but at a

decreasing rate IDltil it reached an

approximately constant value.

(b) For tension piles the pile movement

first increased at a decreasing rate then, after

a stable stage, the rate of movement increased

rapidly until failure occurred. Cyclic

surcharge loading caused an increase in the

negative skin friction of the upper elements

of the pile and a decrease for the lower

elements.

(10) The results of tests carried out on piles

subjected to repeated loading with cyclic

surcharge have shown that:-

102

(a) The behaviour of the piles was similar

to that of loen tical piles but tested with

a static surcharge pressure.'

(b) The cyclic surcharge load1.ng results in

a pile of longer life. ~1e life was largest

when the upper repeated load acted in-phase

with the higher surcharge pressure.

(c) During repeated loading the shaft load

increased to a peak value then decreased until

it reached a limiting value. The peak value

was largest when the repeated load acted in­

phase with the higher surcharge pressure.

(d) From a practical point of view and within

the tested limits, the frequency of the cyclic

surcharge or tlle repeated loading did not alter

significantly the movement nor the load-transfer

characteristics of the pile.

(11) Cyclic surcharge loading was found to increase the

bearing capacity and the pulling resistance of the

pile under static loading.

(12) The test apparatus, the instrumentation and the

testing techniques developed and adopted during

this investigation were satisfactory and in

general gave repeatability to within 3% with

respect to both loads and pile movements.

8.2 sugQestions for future work

Among the most important s,uggestions for future investigations

are the followi?g :-

103

(1) The influence of sand density on the behaviour

of piles subjected to repeated loading.

(2) The behaviour of a pile group under repeated

10adi!lg.

(3)' Combinations of horizontal and vertical repeated

loading.

(4) The behaviour of piles in cohesive soil and the

influence of pore water pressure set up during

repeated loading are also suggested.

(5) The behaviour of belled piles subjected to

repeated loading.

(6) The influence of the method of installation on

the behaviour of piles s~bjected to r~peated

loading.

(7) The carrying out of some tests at full-scale or at

" least at a much larger scale, in order to

establish the scale effect, if any, which may be

present.

(8) Extending the study of the pile element in a triaxial

specimen to includ2 repeated loading. This will

help in understanding the behaviour of the pile shaft

under such a type of loading.

(9) The influence of the surface roughness and the

slenderness ratio of a pile 'on its behaviour when

subjected to repeated loading.

104'

REFERENCES

Al-Ashou, M.O. (1981). "The behaviour of reinforced earth under repeated lading" Ph.D. Thesis, University of Sheffield, England.

Adams, J. I. and Hanna, T.H. (1970). "Ground movements due to pile driving". Conf. on Behaviour of Piles, Inst. Civ.Engrs. London. pp 12.

Al-Mosawe, M.J. (1979). "The effect of repeated and alternatir.g loads on the behaviour of dead and prestressed anchors in sand". Ph.D. Thesis, University of Sheffield, England.

Balaam N.P., Poulos, H.G. and Booker, J.R. (].975). "Finite element analysis of the effects of installation on pile load-settlement behaviour". Geot.Engr., Vol. 6. No.1. pp 33-48.

Barkan, D.D. (1962). "Dynamics of bases and foundations". New York: McGraw-Hill.

Begemann, H.K.S. (1973). "Alternating loads and pulling tests ,-on steel I-beam piles. Proc. 8th Int.Conf.S.M. and F.E., Vol. 2: pp 13-17.

Beringen, F.L., Windle, D. and Van IIooydonk, W.R. (1979). "Results of loading tests on driven piles in sand". Conf. on recent developments in the design and construction of piles, Inst.Civ.Engrs. London.

Biarez, J. and Foray, P.(1977). Discussion to paper by Mcyerhof. Proc.ASCE, Vol. 103, No. GT4: pp 348-349.

Bishop, A.W. and Henkel, D.J. (1962). "The measurement of soil properties in the triaxial test." Arnold, London.

Bishop, A.W. (1971) • "The influence of progressive failure on the choice of the method of stability analysis.".' Geotecnnique, 21, 2: pp 168-172.

Bjerrum, L. (1973). "Geotechnical problems in foundations of structures in the North Sea." Geotedmique, 23, 3: pp 319-358.

Blazqueze, R.M., Krizek, R.J. and Bazant, Z.P. (1980). "Site factors controlling liquefaction. II Proc. ASCE, Vol. 106, No. GT7: pp 785-801.

Bonin, J.P., Del.euil, G. and Zaleski-Zamenhof, L.C. (1976). "Foundation analysis of marine gravity structures submitted to cyclic loading." Proc. 8th Off-shore Tech. Conf.Houston, Tex. pp 2475.

105

Bowles, J .E. (1977). "Foundation analysis and design." MCGraw-IIill, New York.

Broms, B.B., and Hellman, L. (1970). "Methods used in Sweden to evaluate the bearing capacity of end-bearing pre-cast concrete piles." Conf. on Behaviour of Piles, Inst.Civ. Engrs. London. pp 4. ,

Brown, S.F., Lashine, A.K.F. and Hyde, A.F.L. (1975). "Repeated load triaxial testing of a silty clay." Geotechnique 25, 1: 95-114.

Butler, F.G. and Morton, K. (1970). "Specification and performance of test piles in clay." Conf. on Behaviour of Piles, Inst.Civ.Engrs. London. PI' 3.

Butterfield, R. and Andrawes, K.Z. (1972). "On the angles of friction between sand and plane surfaces." J. of Terramechanies, Vol. 8, No 4: 15-23.

Campanella, R.G. and Vaid, Y.P. (1973). "Influence of stress path on the plane strain behaviour of a s~nsitive clay". Proc. 8th Int.Conf.S.M. and F.E., Vol. 1: pp 85-92.

Chan, S.F. (1976). "The behaviour of piles subjected to static and repeated loads." Ph.D. Thesis, University of Sheffield, England.

Chandrasekaran, V., Garg, K.G. and Prakash, C. (1978). "Behaviour of isolated bored enlarged base pile under sustained vertical load." Soils and Found. Vol. 18, No.2: pp 1-15.

Chan, S.F. and Hanna, T.H. (1980). "Repeated loading on single piles in sand." Proe.ASCE, Vol. 106 No. 01'2: pp 171-188.

Chaplin, T.K. (1977). Discussion to paper by Meyerhof. Proc. ASCE. Vol. 103, No. GT3: pp 253-254.

Chin, F.K. (1970). "Estimation of the ultimate load of piles from tests not carried to failure." Proc. 2nd South East Asiru~ Conf. on Soil.Eng. Singapore: pp 81-90.

Chin, F.K. (1978). "Diagnosis of pile condition." Geot.Eng. Vol. 9: pp 85-104.

Cooke, R.W. (1974). "The settlement of friction pile foundations." Proe. Conf. on Tall Buildings. Kuala Lumpur: pp 7-19.

Cooke, R.W., Price, G. and Tarr, K. (1979). "JackedpiH~s in London Clay: a study of load transfer and settlement under working cond! tions • " Geoteehnique, 29, 2: pp ·113-147.

Cooke, R.W., Price, G., and Tarr, K. (1980). "Jacked piles in London clay: interaction and group behaviour under working conditions." Geotechnique,.30, 2: pp 97-136.

106

cox, W.R. and Heese, L.C. "Pullout tests' of grouted piles in stiff clay". Proc. 8th Offshore Tech.Conf. Houston, Tex. pp. 2473.

coyle, H.M. and Reese, L.C. (1966) "Load transfer for axially loaded piles in clay." Proc. ASCE, ,!ol. 92, No. SM2: pp 1-26.

coyle, H.M. and Sulaiman, LH. (1967). "Skin friction for steel piles in sand." Proc. ASCE, Vol. 93, No. SM6: pp 261-277.

D'Appolonia, E. and Romualdi, J.P. (1963). "I.oad transfer in end-bearing steel II-piles." Proc. ASCE, Vol. 89, No SM2: 1-25.

D'Appo10nia, D.J., Whitman, R.V. and D'Appolonia, E. (1969). "Sand compaction with vibrating rollers." Proc. ASCE, Vol. 95, No. SM1: pp 263-284.

De Beer, E.E. (1963). "The scale effect in the transposition of the results of deep-sounding tests on the ultimate bearing capaci ty of piles and caisson foundations." Geotechnique 13, 1: pp 39-76.

De Beer, E.E; (1964). "Some considerations concerning the point bearing capacity of bored piles." Proc. Sym. on bearing capacity of piles, Roorkea, India: 178-204.

De-Beer, E.E. (1965). "The scale effect on the phenomenon of progrcs:Jive rupture in cohesionless soils." Proc. 6th Int.Conf. S.M. and F.E., Vol. 2, Div.3: pp 13-17.

De-Beer, E.E. (1965). "Influence of the mean normal stress on the shearing strength of sand." Proc. 6th Int.Conf.S.M.and F.E.Vol.~ Div.2: pp 165-169.

De Beer, E.E. (1979). "Analysis of the results of loading tests performed on displacement piles of different types and sizes penetrating at relatively small depth into a very dense sand layer." Proe. Conf. on Recent Developments in the Design and Construction of Piles.lns.Civ.Eng: pp 199-211. London.

Drnevich, V.P. and Richart, F.E. (1970). "Dynamic prestraining of dry sand." Proc. A.c)CE., Vol. 96, No. SM2: pp 453-469.

Eden, W.;;. and Law, K.T. (1980). "Comparison of undrained shear strength results obtained by different test methods in soft clay."Can.Geo. J. Vol. 17: pp 369-381.

Farmer, I.W., Buckley, P.J.C., and Sliwinski, 2. (1970). "The effect of Bentonite on the skin friction of cust in place piles." Proc. Conf. on Behaviour of Piles, Inst.Civ. Eng. London, pp. 10.

107

------

Gallagher, K.A. and St. John, H.D. (1980). "Field scale model studies of piles as anchorages for buoyant platforms." European Off-shore Petroleum Conf. and Exhibition, London: pp 1 - 14.

Gaskin, P.N., Raymond, G.P., Addo-Abedi, F.Y. and Lau, J.S. (1979). "Repeated compression loading of a sand". Can. Geot. J. Vol. 16: pp 798-802.

Gouvenot:, D. and Grimoult, o. (1976). "Offshore tests on jetted piles." Proc. 8th Off-shore Tech. Conf. Houston.Tex. pp 2476.

Green, P.A. and Ferguson, P.A.S.(1971). "On liquefaction phenomena, by Professor A Casagrande: Report of lecture~" Geotechnique, 21, 2: pp 197-202.

Grivas, D.A. and Harr, M.E. (1980). "Particle contact in discrete materials." Proc. ASCE, Vol. 106, No. GT5: 559-564.

Hanna, T.H. (1963). "Model studies of foundation groups in sand." Geotechnique, 13: pp 335-351.

lIanna, T.H. (1968). "The bending of long H-section piles". Cand.Geot.J. VOl.5, No.3: pp 150-172.

Hanna, T.H. (1969). "The mechanics of load mobilization in friction piles." J. of Materials, Vol. 4, No.4: pp 924-937.

Hanna, T. H. (1970). "Contribution on settlement and construction aspects. Session E, Conf. on Behaviour of Piles, Inst. Civ. Engrs, London.

lIanna, T.H. and Tan, R.H.S. (1971). "The load movement behaviour of long piles." J. of Materials, Vol. 6, No.3: pp';S32-554.

Hanna, T.H., Sivapalan, E. and Senturk, A. (1978). "The behaviour of dead anchors subjected to repeated and alternating loads." Ground. Eng. Vol. 11, No.3: pp 28-34 and 40.

Ho1rrquist, D.V. Thompson, R.W. and Matlock, H. (1976)."Resistance displacement relationships for axially-loaded piles In soft clay." Proc. 8th Off-shore Tech. Conf. Houston.Tex. pp. 2474.

Ih.Ulter, A.H. and Davisson, M.T. (1969). "Measurements of pile load transfer." Symp. on Performance of Deep Found. ASTM. STP. 444: pp 106-117.

Idriss, I.M., Dobry, R., Doyle, E.n. and Sing, R.D. (1976). "Behaviour of soft clays, u,nder earthquake loading condi tions." Proe. 8th Off-shore lJ'ech. Conf. Houston. Tex. pp 2671.

108

Idriss, I.M., Dobry, R. and Singh, R.D. (1978). "Non-linear behaviour of soft clays during cyclic loading." J.GT.E.D.Proc. ASCE, Vol.104, No. GT. 12: pp1427-1446.

Ismael., N.F. and Klym, T.W. (1979). "Uplift and bearing capacity of short piers in sunde !,'. Proc.ASCE. Vol. 105, No. GT5: 579-593.

If

Kallaby, J. Capanoglu, C., Earl and Wright (1976). Liquefaction considerations in the design of piled off-shore structures." Proc. 8th Off-shore Tech.Conf.Houston,Tex. pp 2670.

Kerisel, J.L. (1961). "Deep foundations in sands: variation of the ultimate bearing capacity with soil density, depth, diameter and speed." Proc. 5th Int.Conf. S.M.and F.E. Vol. 2: 73;83.

K~risel, J.L. (1964). "Deep foundations - basic experimental facts." Proc. North American Conf. on Deep Found. Hexico City •

.. Kerisel, J.L. (1965). "Vertical and horizontal bearing capacity of

deep foundations." Proe.Symp. on bearing capacity and settlement of foundations. Duke University, Durham, North Carolina.

Kezdi, l'l.. (1970). "Increasing the bearing capac! ty of floating piles: settlement observations on a large silo on piled foundations." Conf. on Behaviour of Piles, Inst. of Civil Engrs. London. pp. 8.

Kolbuszewski, J.J. (1948). "An experimental study of the maximum and minimam porosities of sand.~' Proc. 2nd Int.Conf.SMr""E. Vol. 1: pp 150-165.

Ko, H. Y. and Scott, R. F. (1967). "Deformation of sand in hydrostatic compression." Proc. A8CE, Vol. 93, No. 8M3:' pp 137-155.

Kraft, L.M., Cox., W.R. and Verner, E.A. (1981). Pile load tests, . cycli.c 10ad3 and varying load rates." Proc. ASCE. Vol. 107, No. GTl: pp 1-19.

Kulhawy, F.H. Kozera, D.W. and Withiarm, J.L. (1979). "Uplift testing of model drilled shafts in sand". Proc. ASCE, Vol. 105, No. G'rl: pp 31-47.

Lambe, T.W. and Whitman, R.V. (1969). "Soil mechanics." John Wiley & Sons, New York,

Larew, H.G, and Leonards, G.A. (1962). "A strength criterion for repeated loads." Proc. Highway Research Board, Vol. 41: pp. 529-556.

Lee, K.L. and Focht, J.A. (1975). "Liquefaction potential at Ekofisk Tank in North Sea". Proc. ASCE, Vol. lOll. GT1: pp 1-18.

109

"

Lee, K.L. and Seed, H.B. (1967). "Drained strength charactris.ti C s­of sands." Proc. ASCE, Vol. 93, S116: pp 117-141.

Madh100rn, A. (1978) "Repeated loading of piles in sand." Ph.D. Thesis, University of Sheffield, England.

Mandl, G. and Luque, F. (1970). "Fully developed plastic shear flow of granular materials." Geotechnique, 20, 3: pp 277-307.

Mansur, C.l. and Kaufman, R.I. (1956). "Pile tests, low-sill structure, Old River, Louisiana." Proc. ASCE. Vol. 82, SM4, pp 1079.

Martin, P.P. and Seed, H.B. (1979). "Simplified procedure for effective stress analysis of ground response." Proc. ASCE, Vol. 105, GT6: pp739-757.

Matlock, H. and Foo, S.H.C. (1979) "Axial analysis of piles using a hysteretic and degrading soil model." Conf. on Numerical Methods in Off-shore Piling. Inst.Civ.Engrs. London. pp 16.

Matsui, T., Ito, T., Mitchell, J.K. and Abe, N. (1980). "Micro­scopic study of shear mechanisms in soils." Proc. ]\SCE. Vol. 106, GT2: pp 137-151. ,

McClelland, B. (1974). "Design of deep penetration piles for ocean structures". Proc. ASCE, Vol. 100, GT7: pp 709-747.

Mcnc1, V. and Kazda, J. (1957). "Strength of sand duting vibration." Proc. 4th. Int. Conf. S.M. and F.E. Div. 3, pp 3a/25.

Meyerhof, G.G. (1956). "Penetration tests end bearing capaci t.y of cohesion1ess soils." Proc. ]\SCE. Vol. 82, SM1: pp 1-19.

Meyerhof, G.G. (1956). "Compaction of sand and bearing capacity of piles." Proc. ASCE, Vol. 85, 5M6: 1-29.

Meyerhof, G.G. (1976). "Bearing capacity and settlement of pile foundations." Proc. ASCE, Vol. 102, GT3: pp 196-227.

Mohan, D., Jain, G.S, and Kumar, V (1963). "Load bearing capacity of piles." Geotechnique, 13, 1: pp 76-86.

Morgan, J .R. (1966). "The response of granular materials to repeated loading." Proc. 3rd Conf. Australian Road Research Board. Vol. 3, Part 2: pp 1178-1193.

Morz, Z., Norris, V.A. and Zienkiewicz, O.C. (1979). "Application of an anisotropic hardening model in the analysis of e1asto­plastic acfonnation of soils." Geotechnique, 29, 1: ppl-34.

Moussa, A.A. (1975). "Equivalent drained - undrained shearing resistance of sand to cyclic simple shear loading." Geotechnique, 25, 3:

485-494.

110

,

Nair, K. (1967). "Load settlement and load-transfer characteristics of a friction pile subjected to a vertical load." l' 3 d roc. r Pan. Americ. Conf. S.M. and F.E. Vol.l: pp 565-590.

Nemat-Nasser, S. and Shokooh, A. (1979). "A unified approach to densification and liquefaction of cohesionless sand in cyclic shearing." Cando Geot. J. Vol. 16, No.4: pp 659-679.

Nogami, T. and Novak, M. (1980). "Coefficients of soil reaction to pile vibration." Proc. ASCE, Vol. 106, GT5: pp 565-569 ••

Norlund, R.L. (1963). "Bearing capacity of piles in cohesionless soils." Proc. ASCE. Vol. 89, SM3: pp 1-35.

Neville, A.M. (1970). ,"Creep of concrete, plain, reinforced and prestressed." North Holland Publishing Cornpnny, Amsterdam.

Ooi, T.A. (1900). "The loading behaviour of long piles." Ph. D. 'l'hesis University of Sheffield, England.

Parry, R.H.G. and Swain, c.w. (1977). "A study of skin friction on piles in stiff clay." Ground Eng. Vol. 10, Nov: pp 33-37.

Paunescu, M. and Mateescu, G. (1970). "Study on the behaviour of piles thrust into soil by means of vibrato:r:y equipment." Conf. on behaviour of piles, Inst. Civ. Engrs. pp 11.London.

" Peacock, W.B. and Seed, H.B. (1968). "Sand liquefaction under cyclic loading 5 imple shear condi tions." Proc. ASCE, Vol. 94, SM3: pp 689- 700. "

Peck", R.B. (1942). Discussion to pile driving formulas. Progress Heport of the Committee on the Bearing Value of Pile Foundations. Proc. ASCE. Vol. 68: pp 322-324.

Peck, R.B. (1965). "Bearing capacity and settlement: certai.nties and uncertain ties." Proc. Symp. on Bearing Capacity and Settlement of Foundations. Duke University, Durham, North Carolina.

peck, R.B. (1979). "Liquefaction potential: science versus practice." Proc. ASCE, Vol, 105, GT3: pp 393-397.

Potyondy, J.G. (1961). "Skin friction between various soils and construction materials." Ceotechnique, 11, 4: pp 339-359.

Poulos, H.G. (1981). "Cyclic axial response of single pile." Proc. ASCE, Vol. 107, GT1: pp 41-58.

Poulos, H.G. and Davis, E.H. (1980). "pile foundation analysis and design." John Wiley and Sons, New York.

111

Prater, E.G. (1980). "Cyclic shear resistance of non-cohesive soils." Proc. ASCE, Vol. 106, GT1: pp 111-116.

Prevost, J.H. Cuny, B. and Scott, R.F. (1981). "Off-shore gravity structures: centrifugal modeling." Proc. ASCE. Vol. 107. GT2: 125-141.

Prevost, J.H. Cuny, B, Hughes, T.J.R. and Scott, R.F. (1981). Off-shore gravity structures: analysis." Proc. ASCE, Vol, 107, GT2: pp 143-165.

Randolph, M.F. and Wroth, C.P. "Analysis of deformation of vertically loaded piles." Proc. ASCE, Vol. 104, GT12: pp 1465-1487.

Raymond; G.P. and E1.Komos, F. (1978). "Repeated load testing of a model plane strain footing." Cando Geot. J. Vol. 15: pp 190-201.

Reese,L.C. (1964). "Load versus settlement for an axially loaded pile." Proc. Symp. Bearing Capacity of Piles, Roorkee, India. Part 2: 10-38.

Reese,L.C. and Cox, W.R. (1976). "Pullout tests of piles in sand." Proc. Oth Off-shore Tech. Conf. pp2472.

Reese, L.C. Ht;dson, B.S. and Vijayvergija, B.S. (1969). "An investigation of the interaction between bored piles and soil." Proc. 7th Int. Conf. S.M. and F.E. Vol. 2: pp 211-215.

Robinsky, E.l. and Morrison, C.F. (1964). "Sand displacement and corr.paction around model friction piles." Cando Geot. J. Vol. 1, No.2: pp 81-93.

Rocha, M. (1957). "The possibility of solving soil mechanics problems by the use of models." Proc. 4th Int. Conf. S.M. and F.E. Div.l, pp lb/ll.

Rodger, A.A. and Littlejohn,' G.S. (1980). "A study of vibratory driving in granular soils." Geotechniques, 30, 3: 269-293.

Roscoe, K.I1. and Poorooshasb, H.B. (1963). "A fundamental principal of similarity in model tests for earth pressure problems." Proc. 2nd Asian Regional Conf. S.I-1. and F.E. Japan. Vol.l, pp 1/28.

Roscoe, K.ll., Schofield, A.N. and Wroth, C.P. (1958). "On the yielding of soils." Geot. Vol, No.1: 22-53.

Roscoe, K.H. (1970). "The influence of strains in soil mechanics." Geotechnique, 20, 2: pp 129-170.

RCM, P.W. (1963). "Stress-dilatency, earth pressures, and slopes. 1t

Proe. ASCE, Vol. 89, SM3: 37-61.

112

Schofield, A.N. (1980). "Cambridge Geotechnical Centrifuge Operations". Geotechnique, 30, 3: 227-268.

Seed, H.B. (1979). "Soil liquefaction and cyclic mobility evaluation for level ground during earthquakes." Proc. ASCE, Vol. 105, GT2: 201-251.

Seed, H.B. (1979). "Considerations in the earthquake-resistant design of earth and rock fill dams." Geotcchnique 29, 3: pp 215-263.

Seed, H.B. and Chan, C.K. (1961). "Effect of duration of stress application on soil deformation under repeated loading." Proc. 5th Int. Conf. S.M. and F.E. Vol. 1: 341-345.

Seed, H.B., Morris, K. and Chan, C.K. (1977). "Influence of seismic his tory on liquefaction of sands." Proc. ASC'E, Vol. 103 GT4: pp 257-270.

Seed, H.B. and Peacock, W.H. (1971). "Test procedures for measuring soil liquefaction characteristics". Proc. 1\SCE, Vol. 97 SM~: pp 1099-1119.

Silver, M.L. and Seed, H.B. (197l-a). "Deformation characteristics of sands W1der cyclic loading." I"roc. ASCE, Vol. 97, SM9: pp 1091-1097.

Silver, M.L. and. Seed, H. B. (1971-b). "Vo1urr1e changes in sands during cyclic loading." Proc. ASCE. Vol. 97, S~19: pp 1171-1182.

Smi th, I.M. (1979). "A survey of numerical methods in off-shore piling." Conf. on Numerical Methods in Off-shore Piling. Inst. Civ. Engrs. London, pp 1.

Tan, R.H.S. (1971). "piles in tension and compression.p Ph.D. Thesis, University of Sheffield, England.

Tan, R.H.S. and Hanna, T.Il. (1974). "Long piles under tensile loads in sand." Geot. Eng. Vol. 5: pp 109-124.

Tavenas, F.A. and Andy, R. (1972). "Limitations of the driving formulas for predicting the bearing capacity of piles in sand." Can. Geot. J. Vol. 9: 47-62.

Terzaghi, K. and Peck, R. B. (1948),. "Soil mechanics in engineering practice." John Wiley & Sons, New York.

Thorburn, S. (1976). "The static penetration test and the ultimate resistances of driven piles in fine-grained non-cohesive soils." The Structural Eng. Vol. 54: pp 205-211.

Thorburn, S. and MacVicar, R.S.L. (1970). "Pile load tests to failure in the Clyde alluvium." Conf. on Dehaviour of Piles, Inst. Civ. Eng. London: pp 1-8.

113

•

Thorburn, S. and Buchanan, N.W. (1979). "Pile embedment in fine­grained non-cohesive soils." Conf. on Recent Developments in the Design and Construction of Piles. lnst. Civ. Engrs.: pp 191-190.

Thurairajah, A. and Lelievre, B. (1971). "Undrained shear strength characteristics of sand." Geot.Eng. Vol. 2: pp 101-117.

Thurairajah, A and Sithamparapillai, V. (1972). "Drained deformation characteristics of sand." Geot. Eng. Vol.: 91-104.

Tomlinson, 1-1.J. (1970). "Some effects of pile driving on skin friction." Conf. on Behaviour of Piles, lnst. Civ. Engrs. London, pp 9.

Tomlinson, M.J. (1978). "Fowldation Design and Construction." pi tman, London.

Touma, F.T. and Reese, L.C. (1974). "Behaviour of bored piles in sand. 1I Proc. ASCE, Vol. 100, GT7: pp 749-761.

Vaid, Y.P. and Liam Finn, W.O. (1979). "Static shear and liquefaction potential." Proc. ASCE, Vol. 105 GT10: pp 1233-1246.

Van der Veen, C. and Boersma, L. (1957). "The bearing capacity of a pile pre-determined by a cone penetration tcst.". Proc. 4th Int. Conf. S.M. and F.E. Vol. 2, pp 3b/15.

Vesic, A.S. (1965). "Ultimate loads and settlments of deep foundations in sand.". Proc. Symp. on bearing capacity and settlement of foundations, Duke University, Durham, NorUl Carolina.

Vesic, A.S. (1967). "A study of bea.ring capacity of deep foundations. 1I

Final report, Project B-189, Georgia. lnst. of Tech. Atlanta, U.S.A.

Vesic, A.S. (1970). IITests on instrumented piles, Ogeechee River site." Proc. ASCE, Vol. 96, SM2: pp561-583.

Walker, B.P. and Whitaker, T. (1967). "An apparatus for forming uniform beds of sand for model foundation tests. II Geotechniquc, 17, 2: pp 161-167.

Whitaker, T. (1970). "The design of pilcd foundations." Pergamon Press, London.

Whitaker, T. and Cooke, R.W. (1961). "A new approach to pile testing. 1I Proc. 5th Int. Conf. S.M. and F.E. Vol. 2.

171-176. London.

Whi taker, T. and Cookp-, R.W. (1966). "An investigation of the shaft and base resistance of large bored piles in London clay." Symp. on Large Bored Piles, lnst. Civ. Engrs. pp 7-49.

114

Whitman, R. V. (1957). "The behaviour of soils under transient loadings." Proc. 4th Int. Conf. S.M. and F.E. Vol. 1. Div. 1. pp 16/15.

Wu, T.H. (1957). "Relative density and shear strength of sands." Proc. ASCE. June 1957. pp 1161.

Yamaguchi, H. Kimura, T. and Fujii, N. (1977). "On the scale effect of footings in dense sand." Proc. 9th Int. Conf. S.M. and F.E. Vol. 2.: pp 2190.

Yang, N.C. (1970). "Rel:!1xtion of piles in sand and inorganic silt." Proc. ASCE. Vol. 96, SM2: 395-409.

Youd, T.L. (1970). "Dcnsification and shear of sand during vibration." . Proc. ASCE, Vol. 96, SM3: 863-879.

Youd, T.L. (1972). "Compaction of sands by repeated shear straining." Proc. ASCE, Vol. 98, SM7: pp 709-725.

Youd, T.L. and Craven, T.N. (1975). "Lateral st.ress in sands during cyclic loading." Proc. ASCE. Vol. 101. GT 2: 217-221.

115

APPENDIX A

MEASUREMENT OF AXIAL LOADS IN VERTICAL PILES,

A~l Stress-Strain Relationship

Q Q

p p p

-I-

Q

For·a given load-cell embedded in sand and subjected to an

axial force, Q, and a sand pressure, P, Fig. A-l, the axial and

tangential strains are :-

e "" E - l.l E t a a ..... A-l

e "" t ••••• A-2

respectively

116

in wh1ch:-:.< ,

E = axial strain due to subjecting the load-cell to axial a

force only.

Et

= tangential strain due to subjecting the load-cell to

lateral sand pressure only.

~ = Poisson's ratio

From equation A-2

= E a

Therefore, equation A-l becomes

e = €: 11 (et + 11 €: )

a a a

or,

e ::: (1 - 112) €: 11 et a a

But the load, Q = stress x area

e:.E.1r (rl 2 2

Q = r2

)

E.1r (rl 2 2

• = r 2 )

Q 112 (e 1

A-3

••••• A-6

+ a II e' ) t

Where E is Young's modulus of the cell material.

A-4

A-S

• ••••

When the strains are monitored at 2 points·, equation

A-7 becomes

E.1f (rl 2 2

II (et1 r 2 ) e + ea2 Q = ( al 2 +

1 - 1.1 2

• ••••

or

E:Jf (rl 2 2

Q == r 2 ) (e

al + ea2

) + l.1(etl 2(1 _ 112)

• ••••

A-7

e t2 ) + -)

2

A-a

+ et2

)

A-9

Therefore the axial pile loading can be obtained by measuring the

axial strain and the tangential strain separately. Such a measurement

117

cml be done by using rosette orthogonal strain gauges.

Equation A-9 is related to voltage output as follows:-

I

/~ h· 4~ l>V

1 _-~2

~_)t __ ~ ______ ~ :3

Fig. 1\-2

The bddge shown in Fig. A-2 consists of four identictl!

strain gauges, two of them are active which are ~ocated on the

pile surface and the other two are dummy and are located outside

the pile and are supplied by an excitation voltago, Vo, under

the influence of straining the resistance of the active strain

gauge, R, will be changed by ~R which corresponds to a drop in

voltage ~V between points 2 and 4. If the electrical current, i,

which is equal to Vol (R + D), is assumed constant, then

~V = -i ..... A-10

but

6R/R = K.e ••••• A-ll

where

K is the gauge factor of the strain gauge which is constant

e is the measured strain

118

... e ..... A-l2

equation A-9 then becomes

Q = R.K.i. +

..... A-13

or

Q = C X (' 6Va + 116V ) A-l4

Where C is the calibration factor,

6Va is the drop in voltage of the axial strain bridge due

to straining of the load-cell, which is also evaluated

and recorded by the data logger.

In the case of the top load-cell, where the sand pressure is equal

to zero, equation A-14 becomes,

Q = C. twa

Vo

Fig. A-3

To increase the sensitivy of the top load-cell bridge and

to eliminate the need for dummy strain gauges the strain gauges

are connected as shown in Fig. A-2 in which the two bridges are

combined in to one.

119