Load Transfer Characteristics of Bored Piles in Different Soil Conditions - Florian Bussert 04.2001

8/10/2019 Load Transfer Characteristics of Bored Piles in Different Soil Conditions - Florian Bussert 04.2001

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LOAD TRANSFER CHARACTERISTICS OF BORED PILES IN DIFFERENT

SOIL CONDITIONS

by

Florian Bussert

A thesis submitted in partial fulfilment of the requirements for the degree of

Master of Engineering

Examination Committee Prof. Dr. A.S. Balasubramaniam (Chairman)Dr. Noppadol Phien-wej

Dr. Kinya Miura

Dr. Tian Ho Seah

Nationality German

Previous Degree Diplom Ingenieur (FH)Fachhochschule Frankfurt am Main

Frankfurt, Germany

Scholarship Donor EEC (European PTS)

Asian Institute of Technology

School of Civil Engineering

Bangkok, Thailand

April 2001


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ACKNOWLEDGEMENT

The author wishes on this way to express his profound gratitude and appreciationto Associate Professor Chang Ming Fang who made it possible to fulfil the required in-

ternship at Nanyang Technological University (NTU), provided the topic and numerous

resources for the research. Also appreciated is the friendly welcome from all of the staffmembers at NTU and given help whenever questions raised.

Profound appreciation is expressed to Professor A. S. Balasubramaniam, for hiskindness and friendly support during the thesis, with valuable and unexpected help during

the internship in Singapore.

Thankful appreciation is also expressed to Dr. Tian Ho Seah for his constructive

ideas and encouragement throughout the duration of this master program.

Last but not least to Dr. Kinya Miura and Dr. Noppadol Phien-wej for being partof the thesis committee.


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ABSTRACT

Data from static pile load tests, conducted on 13 fully instrumented large diameterbored piles constructed in Bangkok, Singapore and Indonesia were evaluated. They pro-

vide useful information on the load transfer characteristics at the pile soil interface. A

calculation procedure to obtain reliable shaft friction values is recommended. It wasfound that the calculation method used has a significant influence on the results obtained.

The influence of the actual pile diameter and the concrete Modulus as a function of strain

are used to calculate the shaft friction is shown. This leads to the assumption that morereliable pile calculations can be done in future practice.

The authors results were compared with previous publications. Significant differ-

ences are noted. It is shown that these differences exist due to improper analysis, based

on insufficient information on the construction procedure.

Analysis of the results from piles in Bangkok show higher pile capacity possiblydue to improved installation procedure. A reduction in the construction time after the

boring and before the concreting is started could have a further influence on the capacity.


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TABLE OF CONTENTS

Chapter Title Page

Title Page iAcknowledgement ii

Abstract iii

Table of Contents ivList of Tables vi

List of Figures vii

List of Symbols viii

1.

Introduction 11.1 Scope 2

2. Literature Review 3

2.1 Load transfer principle 3

2.1.1 General 42.1.2 Shaft friction 5

2.1.3 End bearing 6

2.2 Factors influencing pile capacity 72.2.1 Drilling technique 7

2.2.2 Supporting agent 82.3 Actual design practice 9

2.3.1

Disadvantages of current approaches 11

2.4 Requirements for pile tests 12

2.4.1 Measurement devices for instrumented piles 13

3. Investigation of possible Improvement 14

3.1 Data evaluation 143.2 Reliable calculation procedure 16

3.3 Calculation method used for prediction 19

3.3.1 Advantages of load transfer method 203.3.2 Calculation of end bearing 20

3.3.3 Calculation procedure 21

4. Presentation and discussion of results 22

4.1 General 22

4.2 Soil conditions 23

4.3 Results of investigation 244.4 Results of prediction 27

4.5 Comparison with published data 28


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4.6 Increase in pile capacity 29

4.7 Constructive recommendations 30

5. Conclusions 32

References 27Tables 31

Figures 35

Appendix 54


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LIST OF TABLES

Table No. Title Page

3.1: Additional Data on Investigated Piles 133.2: Reference and Title for Additional Data 14

3.3: Shaft Friction Values Used for Calculation of Bangkok Soil

Condition 323.4: Shaft Friction Values Used for Calculation of Singapore Soil

Condition 32

3.5: Shaft Friction Values Used for Calculation of Indonesia Soil

Condition 32

3.6: Presumed Bearing Capacity Values under Vertical Static Loading(BS 2004) 33

3.7: Characteristic Tip Resistance Depending on the SettlementIndex s/D in Cohesionless Soils (DIN 4014) 34

3.8: Characteristic Ultimate Shaft Friction in Cohesionless Soils

(DIN 4014) 343.9: Characteristic Tip Resistance Depending on the Settlement

Index s/D in Cohesive Soils (DIN 4014) 34


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LIST OF FIGURES

Figure No. Title Page

2.1: Load Settlement curves for Piles Embedded in 1stSand (Bangkok) 36

2.2: Load Settlement curves for Piles Embedded in 2nd

Sand (Bangkok) 36

2.3: Load Settlement curves for Piles Embedded in Stiff Clay (Bangkok) 36

3.1: Flow Chart for Analysing Pile Capacity 373.2: Inconsistent Strain Gage Data Reading 38

3.3: Adjustment of Initial Strain Gage Data Reading 38

3.4: Concrete Youngs Modulus as a Function of Strain (Pile P1 P5) 39

3.5: Effect of Diameter Used in Calculation, Soft Clay (Pile P1) 39

3.6: Pile Section Properties as Function of Strain (P1 Bangkok) 403.7: Result of Improper Analysis vs. Right Procedure 40

3.8: Strain vs. Load (Pile P1 Bangkok) 413.9: Strain vs. Axial Load Initial Loading Cycle (Pile P1 Bangkok) 42

3.10: Strain Distribution with Depth (Pile P1 Bangkok) 43

3.11: Load Distribution with Depth (Pile P1 Bangkok) 433.12: Development of Shaft Friction by Top Settlement (Pile P1) 44

3.13: Development of Shaft Friction (Pile P1 Bangkok) 44

3.14: vs. cuCurve Proposed by Different Authors and Codes 453.15: Calculated & Expected Shaft Friction vs. Displacement (Soft) 46

3.16: Calculated & Expected Shaft Friction vs. Displacement (Medium) 47

3.17: Calculated & Expected Shaft Friction vs. Displacement (Stiff) 483.18: Calculated & Expected Shaft Friction vs. Displacement (1stSand) 49

3.19: Calculated & Expected Shaft Friction vs. Displacement (2nd

Sand) 50

3.20: Effect of Construction Time on Shaft Capacity of bored piles 51

3.21: Cost Development Curves of Foundation Expenses 52

4.1: Shaft Friction vs. SPT-N for Residual Soils (CHANG et al, 1989) 534.2: Critical Displacement (zsc) vs. SPT-N for Residual Soils 53

4.3: fs/zscvs. SPT in Residual Soils 53

5.1: Impact of Diameter used in Calculation 25


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LIST OF SYMBOLS

m: Shaft Friction: Adhesion Factorcu: Undrained Shear Strength

Nq: Bearing Capacity Factor

v: Effective Vertical Stress

Q: Pile Capacity

Qs: Shaft Friction CapacityQe: End bearing Capacity

qs: End bearing pressure

As: Area of Pile Tipdl: Length of Pile in Each Layer

Pt: Applied Load on TopAs: Cross Sectional Area of Reinforcement

Es: Youngs Modulus of ReinforcementAc: Cross Sectional Area of Concrete

Ec: Youngs Modulus of Concrete

e: Elastic Pile Shortening


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CHAPTER I

INTRODUCTION

Large diameter cast in-situ bored piles are extensively used in the foundations

design in Southeast- Asia. They provide several advantages compared to driven orprecast piles. In the past, lack of understanding on load transfer characteristics has lead to

extensive research made on this topic. It was observed that existing soil mechanics

theories cannot explain the complex mechanisms that take place at the pile soil interface.Investigation have been done to understand these mechanisms. More reliable

measurement devices that increase accuracy are being introduced. Nevertheless, better

understanding of the pile soil interaction has also not lead to reliable design procedure.

Design is still based on calculating a failure load. A high factor of safety (2.5 3.0) provides certainty against excessive settlement. In the design phase no information is

available on the estimated settlement that will take place. To check whether theassumptions made have been correct, load tests are conducted in every project. These

tests are mostly stopped when a specific settlement criterion is reached. Load tests are

time consuming and costly. More reliable estimation of pile capacity would lead to areduction in the number of load tests required. To gather such reliable information

knowledge on shaft friction values for different geological conditions are helpful.

When comparison is made between capacities of piles constructed under similar

geological conditions, a large scatter in the capacities of the shaft friction is noted. This ismostly due to the influence of the installation procedure, free stand-up time and varying

soil properties. These factors do not have such a great influence to alter the shaft friction

values by nearly 400% as noted in the scatter of the available data. With more accurate

measurement devices valuable information can be obtained, during the calculationprocess of converting the measured strains in the field into shaft friction values. To check

the reliability of the calculated data, SPT shaft friction correlations can also be used.

When ultimate shaft friction values and related critical displacement are within

acceptable range, improvement in the design procedure can be expected. Also the

differential settlement (which is most critical for high- rise buildings) can be estimated aswell.

This research tries to evaluate the reasons for the large scatter that exists inprevious interpretations. Thirteen instrumented large diameter bored piles were used to

investigate load transfer characteristics under different geological conditions. The piles

were instrumented with strain gages at several depths, providing information about

strains taking place at these different levels. With this knowledge, the load transfer can becalculated in the soil layers. Static load compression tests were conducted, which provide


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results with a high degree of reliability. Other load test methods are not fully reliable and

should be calibrated with the static tests of the type studied here.

1.1 Scope

This study deals with single axially loaded large diameter bored piles. The load

transfer characteristics for different soils in varying geological conditions are studied.

The main emphasis is placed on recommending a reasonable data evaluation process thatproduces reliable load transfer parameters. The consequence of improper analysis on the

pile design is also discussed. Pile improvement techniques like grouting, which is

conducted quite frequently to increase pile capacity after installation, are not evaluateddue to insufficient data.


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CHAPTER II

LITERATURE REVIEW

In this chapter an overview about achieved knowledge on the load transfer

characteristics from pile to soil is given. Factors influencing pile capacity are reviewedand their influence rated. To illustrate how the present uncertainties are taken in account,

the recommended procedures in various codes of practice are shortly introduced and the

limitations summarised. Necessary requirements that should be fulfilled to obtain reliable

data from fully instrumented load tests are given.

Research on behaviour of bored piles has been done in the Asian Institute ofTechnology (AIT) in the last years. The behaviour & performance of grouted bored piles(Analas S., 1995) and non- grouted piles (Apichai, W., 1993) was studied. The related

construction problems (NG, 1983) and the performance of piling practice in Bangkok

(OONCHITIKUL, 1990) provide the background for the work presented. Theseresearches give a view on achieved knowledge on pile behaviour and performance in

Bangkok. The results are discussed and suggestions for further research is made.

2.1 Load transfer principle

The geotechnical capacity of piles has been investigated in the last decades.Numerous publications available give a clear picture on the achieved knowledge. In the

last years mostly new design approaches have been published, representing regional

experiences and related findings. It has been seen that they cannot be used as overallvalid design procedures. This is due to the fact that a lot of influencing factors (that have

been constant during the presented investigations) were not taken in account.

2.1.1 General

In the beginning piles have only been tested measuring applied load and relatedsettlement. From this, conclusions were made, how the load is transferred to the soil.

With the development of new and reliable measurement devices a better understanding of

load transfer characteristics was achieved. Nevertheless, presented attempts to calculatepile capacity do not lead to reasonable results. It is still practice to conduct pile tests

before every project. With this it is checked if the assumptions made in the design phase

have been right. When necessary, improvements in design are made after the requiredparameters are obtained from the load test data.


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When the pile is loaded, a displacement field is introduced in the ground, causingsliding along the pile shaft until full shaft friction is mobilised. Compression and sliding

mechanisms occur in the soil volume in which the load transfer takes place

(SMOLTCZYK, 1993). Research has been done trying to explain pile behaviour by ideal

elastic theories. Even the use of non linear failure criteria in conjunction with finiteelement method does not provide reasonable results (FRANKE, 1996). All used methods

for design have the disadvantage that they require empirical adjustment based on the soil

conditions found and the drilling technique adopted.

Reasons for this are:

1. Due to pile installation in the ground, the initial ratio of vertical to horizontal

stress (k = x/z) is changed from the earth pressure at rest k0= x/(. z) tounpredictable values from k > k0or k < k0.

2. Due to the pile displacement occurring in the ground, end bearing cannot be

calculated anymore by the methods originally invented for shallowfoundations. The neglecting of compressibility in the formulas leads to muchlarger calculation errors than in shallow foundations. Higher overburden

pressure exists. These overburden pressures allow extremely high end bearing

values. When these values are reached, soil compressibility is increased bysoil grain destruction. This causes a decrease in effective friction angle below

pile tip. The failure mechanism taking place cannot be described by a linear

failure line.

3. The interaction between shaft friction and end bearing leads to a interlocking

effects in the ground, causing tensions forces near and above pile tip. They

disappear when shaft friction is around two times the end bearing (FRANKE,1996).

As these interaction mechanisms cannot be analysed theoretically, pile capacity is

calculated as a superposition of shaft friction and end bearing:

Q = Qs+ Qe (2.1)

2.1.2 Shaft friction

Shaft friction is determined by:

m= . cu (2.2)for cohesive soils and

m= . . z (2.3)

for cohesionless soils, with = k0. tan d


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It could be estimated from this formula, that shaft friction increases endless with

increasing depth. This is not the case. After a critical depth is reached, shaft friction doesnot increase anymore. Therefore the used formula is only applicable with empirical

adjustments depending on pile length, material, roughness of pile shaft, construction

method and others. All influencing factors have been summarised by POULOS &

DAVIS (1980) and BROMS & HANSBO (1981). Factors having main influence on pilecapacity are discussed in more detail later.

Only small displacement is required to fully mobilise shaft friction. The requireddisplacement are stated as 5 6 mm (HORVATH et al., 1979), 4 8 mm (CHANG et al.,

1991) or 5 10mm (ONEILL et al., 1972). Contrary results are given by STAMM

(1990) who reported possible displacements of 2 cm and more to fully mobilise shaftfriction.

2.1.3 End bearing

End bearing is calculated by:

Qb= v . Nq (2.3)

As for shaft friction it can also be estimated from this formula that end bearingincreases continuously with depth and inner friction angle of the soil (Nq increases

exponentially with increasing friction angle). This is not the case as well (compare shaft

friction). When end bearing reaches a critical depth no further increase is noted. This issimply due to the reason that overburden pressure is too large and the additional load

from the pile becomes small and negligible (FRANKE, 1996).

End- bearing requires a larger settlement to be fully mobilised, mostly assumed tobe circa 10% of the pile diameter (BS 2004, 1972, DIN, 1990). End bearing entirelydepends on the workmanship at the site, especially in cleaning process which can hardly

be calculated. To overcome the uncertainties extremely conservative end bearing values

are assumed in design.

WILLIAMS & PELLS (1981) and HORVATH et al. (1983) demonstrated on field

tests, that the majority of the load carried by a socketed pile is carried in side shear until

slip occurs at the interface. Once slip occurs, the majority of additional load is thentransferred to the base of the socket. In many instances, the shear stress distribution along

the socket is quite non uniform. When consideration is given to the potential variability in

side shear resistance, it can be expected that under normal conditions (extremely lowloads applied on the base of the pile) some slip might occur along the socket irrespective

of whether it is considered in design or not.


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2.2 Factors influencing pile capacity

With theoretical formulas it might never be possible to calculate all factors

influencing pile capacity, as even the initial values cannot be determined in a for an

analysis required range (FRANKE, 1996).

The existing difficulties are, that even definitions of different capacities are not

overall valid. Failure load is defined as a constant load under which the pile settlement

increases continuously. WEISS et al. (1983) reported from field tests, that failure loaddepends on in-situ soil properties. While piles in loose sand showed an extreme failure

mechanism, no ultimate load could be applied to piles in dense sand. Settlement was the

limiting factor. Load settlement curves from load tests in Bangkok (Fig. 2.1 2.3)indicate similar findings: no uniform and predictable failure mechanism is observed.

Even piles founded in the same soil layers show brittle as well as ductile failure

mechanism.

2.2.1 Drilling technique

Shaft friction depends on the shear resistance of the soil: cohesion and effective

stress perpendicular to the pile shaft. Before drilling, the soil in which the pile is placed is

in a medium, dense or even overconsolidated state, dependent on the geological history.Under these conditions, a bored pile could introduce extremely high shear and

compression loads to the ground. Every disturbance of the ground causes unknown

dilatancy and densification effects from the original parameters. This leads to a reductionin pile capacity. Differences in compactness in the order from 10 to 40 % were reported

by HARTUNG (1995). To reduce these disturbance effects drilling tools were optimisedin the last years (indicated by achieved increase in pile capacities). Aim is to conduct

drilling as fast as possible and with minimised soil disturbance. When specified

requirements are fulfilled during construction process (sufficient ahead installed, deeper

casing; sufficient high liquid column in the casing, limitation of suction during drillingprocess and small cutter head) smallest soil disturbance possible is achieved

(GOEDECKE, SCHULER, 1986).

2.2.2 Supporting agent

Every excavation in the ground causes a response: dilation and destressing of the

surrounding soil and therefore a change in the mechanical properties. To minimise these

changes as well as to provide a stable borehole support must be provided: casing, waterand slurry (bentonite and recently polymer) are used.

The effect of bentonite was discussed extremely controversy in the beginning.

Assumption that the shaft friction is virtually unaffected by the slurry to that itcompletely eliminates shaft friction have been stated. WEISS (1965) conducted

laboratory shear tests on vertically and horizontally loaded slabs cast on to a horizontal


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bed of dry sand and similar beds immersed in bentonite suspension. He concluded that a

significant greater load transfer is obtained at the latter interface. Several attempts havebeen made in field and laboratory tests to state the influence more clearly. It was found

that the results obtained are affected by the acting radial stress. Higher frictional

resistance is achieved with bentonite at lower displacements while at larger displacement

the resistance is lower (difference smaller than 10 %).The effect on pile capacity in theground depends on the interaction between bentonite and soil. Therefore on following

parameters: grain size of soil and slurry; the nature of the soil particles inevitably held in

the suspension and the rheological properties of the suspension, which dependsthemselves on the bentonite used and the chemical nature of the mixing water (FARMER

et al., 1970).

Due to several advantages, it becomes more popular to use polymer slurry as

supporting agent. It is more economical, requires less preparation at the side, can be

mixed and hydrated more rapidly and is environmental friendly. It has been shown in

several projects, that polymer slurry increases possible shaft friction. In field tests, ATA

et al. (1998) found, that polymer slurry, when introduced with the right dosage providesstable boreholes for more than eighteen hours. Developed unit side shear resistance of

polymer piles exceeded those constructed by bentonite at all depths. The average factoras recommended in the U.S. design practice (Fig. 3.14) for bentonite was exceeded of

more than 40 %, leading to an increase from 0.55 to 0.74. This increase in shaft frictiontook place under all investigated soil conditions.

2.3 Actual design practice

After it has been seen that no theoretical approach can be found, attempts were

made to state the influence of installation procedure more clearly for most geologicalconditions.

In the actual design of bored piles in Southeast- Asia it is widely adopted to

calculate pile capacity based on the effective stress principle. Test piles are used to check

the assumptions made. Preliminary piles should be constructed using the same methods,materials and dimensions as to be used in contract piles. Such tests are intended to verify

that the methods, materials and dimensions used in construction process provide an

adequate foundation. As the construction of the contract piles proceeds, further load testsare usually carried out on contract piles in order to ensure that the workmanship and

materials are of satisfactory standard. This produces high expenses that could be saved,

when a more reliable approach on pile capacity calculation could be given in the future.Actual pile design still consists to a high degree of uncertainties. These uncertainties are

taken in account in different ways in given codes of practice.

The American code, represented by the work of the U.S. Department of

Transportation, is based on partial safety factors. Depending on the reliability of

necessary information (e.g. ground conditions) the designer has to choose adequate

factors that are applied to the measured or estimated value. In most instances it can be


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chosen between a mean, lower and upper confidence line. The range of these lines is very

large, indicating the existing design uncertainties. Values from laboratory tests ondifferent soils provide the input parameters for the design charts, when pile tip or shaft

movement as a function of applied load is calculated. This approach requires profound

knowledge on pile behaviour. Every influencing factor has to be rated and a

representative partial safety factor chosen. When this is not done in an proper way,extremely large differences in calculated pile capacities are obtained. As in all other

presented codes it is therefore advised to conduct a load test in the beginning to check the

made design assumptions.

The German code follows a completely different design philosophy. To take all

existing uncertainties present in pile behaviour in account, design is completely based onanalysed data from field tests. They were conducted in the last decades and represent

therefore achieved piling practice in Germany. Shaft friction values are given for

different soil types based on the measured density. These values represent lower bound

values. With these a load settlement curve is calculated representing field behaviour that

can be garantied. Disadvantageous in this approach is, that pile strain is not taken inaccount even for long piles. Due to the lower bound values also large differences are

noted between designed and measured ultimate capacities. Load tests are required beforeevery piling project is started.

The English approach is the approach that is widely used in Southeast- Asia. Asthe interaction between shaft friction and end- bearing is not yet fully understood, pile

capacity is calculated as a superposition of both:

Q = Qs+ Qe

= qs. As+ . D . (m. dl) (2.4)

Shaft friction for cohesive soils is calculated by (2.1), while for cohesionless soils

(2.2) is used. is obtained from Figure 3.14, where only the recommended lines are

shown. They are rough average values, valid for highly scattered data points. These values have been slightly changed in the past to represent

The value stays nearly constant at a value of 0.25, as the lines for k0and tan dshow contrary results (BURLAND, 1973). This indicates that shaft friction for

cohesionless soils depends only on the overburden pressure, which would lead to a

continuous increase. MEYERHOF (1986) stated that values should decrease from acritical depth on in uniform soils, as capacity is not increasing linearly with effective

stress.

The and values have to take all uncertainties that arise during pile installationin account. Therefore they are stated in the literature in a large range, indicating regionalexperiences. It is clearly visionable that sound engineering judgement is required to

estimate these values in a realistic way. Knowledge about the construction process,

possible failure modes and the interaction between pile and soil conditions is required.PIMPASUGDI (1989) showed that even piles installed under comparable conditions


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show the same scatter. The and values are correlated to in-situ soil property data. Forcohesive soils the undrained shear strength provides the necessary information. SPT test

in cohesionless soils are correlated to the inner friction angle, which are again correlated

to the value.

With this shaft friction values an ultimate pile capacity is calculated. By applyingan overall safety factor, it is assumed that settlement and hazard against failure is in anacceptable range. That this is only a rough estimation can be seen from Figures 2.1 2.3

where it can be seen that every pile follows an own, non- predictable load- settlement

curve, which is independent from soil conditions below pile tip. A slightly differentapproach applies different factors of safety on shaft friction and end- bearing, taking in

account hazards of different failure probabilities. Disadvantages of this approach is

mainly, that no information on the load settlement characteristics is given. It is simplyassumed, that by the taken factor of safety settlement is in an acceptable range.

To overcome the existing difficulties in pile capacity design, semi-empirical

approaches are used. A lot of these approaches have been published, representingregional properties. They fit under special conditions, are independent from several

factors that were constant in this situation.

2.3.1 Disadvantages of current approaches

In Southeast- Asia the effective stress approach is mainly used. It has been seen in

the past, that pile capacity differs much from the predicted ones using the describedprocedure. Differences in order from 30% are noted. These changes are thought to take

place due to installation and workmanship and cannot be taken in account by thisapproach. To obtain more reliability, it has to be looked, why these differences betweencalculated and measured values takes place. An improved pile design procedure in the

future to overcome the existing uncertainties has to take all factors in account. Thereforeonly values derived from field tests can provide the necessary information

No information can be given to the structural engineer about settlement underworking load from soil mechanics theoretical point of view. Therefore no information is

available about possible maximum differential settlement as this is the most important

information for serviceability of the building.


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2.4 Requirements for test piles

Testing of piles is in first line for the owners benefit. Exact information about load

transfer characteristics leads to a more sufficient design and a more economical

foundation. In the beginning piles were too small to implement reliable measurementdevices. Since large diameter bored piles have been introduced and equipped with

meaningful measurement devices more information about the load transfer is obtained.

In most load tests only top displacement dependent on the applied load is

measured. This does not provide any information about the load transfer principle to the

soil (NOWACK, GARTUNG, 1983). To be able to study behaviour at each pile soilinterface other requirements are necessary.

Careful planning, reliable equipment and instrumentation as well as proper set- upand a correct recording and repeating of measured data is necessary to obtain reliable

information (FULLER, 1983). Careful planning involves a geological examination. Whenthe geology is highly variable, it has to be decided if the location is chosen on average,worst or best conditions (CROWTHER, 1988). Boring has to be done near pile

location, to be able to install the strain gages dependent on the soil layers found. To

determine load transfer from the pile to the soil along the shaft strain gages or telltales are

placed at bottom and top, mid- or quarter points in uniform soil or the defined strata. Onetelltale or a strain gage level placed near the pile tip (around 2m depth) is necessary, to

obtain information on concrete Youngs modulus during loading. Telltales are rods

placed in shielded conduits that allow detection of movement at remote locations alongthe pile shaft. They require careful installation and securing. The protective conduit must

permit unrestricted movement of the internal rod to occur (CROWTHER, 1988).

When contract piles do not meet their required performance, the load ratio-

maximum settlement curve derived from the preliminary pile is invaluable. Such pilescan be downgraded to a design load at which their load- settlement performance will

meet the specific requirements. Decisive action can be taken immediately to provide an

adequate foundation (BUTTER et al., 1970).


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2.4.1 Measurement devices for instrumented piles

Reliable statement about load transfer principle can be obtained from following

devices:

Strain gages,

Load cell at pile tip,

Inductive displacement transducer,

Telltale,

Independent conventional geodetic measurement for control,

Inclinometer (for horizontally loaded piles).

Only a proper selection and combination of independent devices provide a high

degree of accuracy and reliability.

Optimum is to install our strain gages, put in 90 degree to each other, to obtaininformation about the bending moment. For reliable data, they should be installed,protected for damage during concreting process, at the reinforcement. The length of the

strain gages depends on the largest concrete grain size, as grains and concrete matrix do

not follow the same load- strain characteristics.

End- bearing is most reliable measured by load cell. Care has to be taken that no

load bridges occur. This would lead to wrong measured values and does not provide

reliable information about the acting load.


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CHAPTER III

INVESTIGATIONS FOR POSSIBLE IMPROVEMENT

To investigate a more reliable design procedure, several instrumented pile load

tests in Southeast- Asian countries (Thailand, Singapore, Indonesia) have been analysed.

Main emphasis is placed on the calculation process. Calculation is enhanced by taking a

different pile stiffness at every strain gage level into account. The observed diameter inevery layer and change in concretes youngs modulus (dependent on strain) are the most

important factors that influence pile stiffness. Consideration of these two factors show an

increase in accuracy of shaft friction values. The calculated results are checked forreliability and used for prediction by load transfer method (COYLE et al., 1966).

The piles have been tested by static compression test, which is mostly conducted,due to its simplicity in data interpretation and analysis of shaft friction and end- bearing.

3.1 Data evaluation

Thirteen instrumented tests on large diameter bored piles in Southeast- Asia,

constructed under different geological conditions have been evaluated.

Strain gage, load cell and telltale readings are available for the investigated tests.

They provide useful and reliable information in pile testing. It is necessary to check theconsistency of the telltale output versus strain gage data for the lowest measurement

level. The telltale is very sensitive to damage during the installation process and thereforemight not provide reliable information. When the difference between load calculated by

telltale and strain gage is too large, the strain gage reading is used for further calculation.

The results of this investigation are compared with published data in Chapter IV.

Details on all tests are given in Table 3.1:


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Table 3.1: Additional Data on Investigated Piles

No./Source

Country/Date

DesignDia.

(m)

Length(m)

Drillingprocess

Soil at PileTip

Testload

(ton)

Remarks/Pile Locations

BP02-A

NTU

Indonesia

03 - 1994

1.00 41.00 Wet (bucket,

temp. casing)

Cemented

Sand

1800 No concreting

report

BP03-A

NTU

Indonesia

04 - 1994

1.00 41.00 Wet (bucket,

temp. casing)

Cemented

Sand

1800 No concreting

report

UTP1

NTU

Singapore

06 - 1995

0.80 13.85 Wet (Flight

auger)

Weathered

Granite

350 No concreting

report

Soft toe

UTP2

NTU

Singapore

07 - 1995


auger)

Soft Toe 350 No concreting

report,

End bearing pilePTP3

NTU

Singapore

04 - 1997


auger)

Weathered

Granite

1160 No concreting

report

Track 10

NTU

Singapore

10 - 1996


auger)

Granite 350 No concreting

report

End bearing pile

P1

SEAFCO

Thailand

09 - 1997

1.20 46.25 Wet (bucket,

temp. casing)

Very dense

Sand

2000 Bang Na Trad

Road

P2

SEAFCO

Thailand

10 - 1997

1.00 43.00 Wet (bucket,

temp. casing)

Dense

Sand

1300 Asoke Srinakrin

Section

P3SEAFCO

Thailand05 - 1997

1.00 49.47 Wet (bucket,temp. casing)

Hard Clay 1200 Bang Pa In Pak Kret

ExpresswayP4

SEAFCO

Thailand

07 - 1994

1.00 41.00 Wet (bucket,

temp. casing)

Very dense

Sand

840 Bangkhlo Ram

III

P5

SEAFCO

Thailand

08 - 1997

1.20 57.10 Wet (bucket,

temp. casing)

Very dense

Sand

2000 Bang Na Trad

Road

PPLT 22

AIT

Thailand

11 - 1990

1.00 42.70 Wet (bucket,

temp. casing)

Very dense

Sand

1150 2ndstage

expressway

PPLT 23

AIT

Thailand

11 - 1990

1.20 46.30 Wet (bucket,

temp. casing)

Hard Clay 1500 2ndstage

expressway

Additional data from different investigators are used to increase the reliability of

pile capacity determination. Results from published material that differ from those

evaluated here are discussed to identify possible causes of inaccuracy. Those causinginaccuracy are not used in further calculation. Used additional data is given below:


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- 14 -

Table 3.2: Reference and Title for Additional Data

Name Title

RADHAKRISHNAN (1987) Foundations for Capital Square, Phase I, Kuala Lumpur

CHANG & BROMS (1991) Design of bored piles in residual soils based on field-

performance data

CHANG & GOH (1989) Design of bored piles considering load transferWACHIRAPRAKARNPONG

(1993)

Performance of grouted and non- grouted bored piles in

Bangkok subsoils

CHUN (1992) Performance of driven and bored piles in expressway

projects

SOONTORNSIRI (1995) Behaviour & performance of grouted bored piles in

Bangkok subsoils

BALAKRISHNAN (1994) Performance of bored piles in Kenny Hill formation inKuala Lumpur, Malaysia

THASNANIPAN, et al.(1998)

Large Diameter bored piles in multi- layered soils ofBangkok

3.2 Reliable calculation procedure

Reasonable results from load tests can only be achieved when the given procedure(Fig. 3.1) is followed. First, strain gage data has to be plotted versus applied load.

Inconsistent data has to be removed as it affects the outcome (Fig. 3.2). These

inconsistencies can be due to short- circuit or damage in the cable and transducer during

concreting and installation process. Wrong initial readings (Fig. 3.3) have to be adjustedas they do not represent actual pile behaviour. Their presence is due to incorrect readings

that can take place in the initial part of the test. To increase reliability, two strain gages

are installed in each layer. This provides useful information about bending moments.Engineering judgement is required to choose appropriate strain values for later analysis.

For the further process, strain gage readings are limited to the initial loading cycle.

Unloading and reloading procedures cause different stress fields in the ground that canhardly be calculated.

First, the rebar stress data and the section properties for the first measurement

level are used to calculate concrete Youngs modulus which is dependent on themeasured strain. When the trend is acceptable (decrease of modulus with strain, Fig. 3.4)

these values are used to calculate pile stiffness in each level as a function of measured

strain. Pile strain under self weight is not taken in account, it is assumed that it is part of

the initial readings. Once zero reading is done under the assumptions of completebonding between steal and concrete; equal strain in steal and concrete in the specific area;

known cross section in the measured section and a constant steal Modulus.

Load = (EsAs+ EcAc) .i (3.1)

Where i= measured strain in each strain gage level.


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- 15 -

From the load acting at each layer the axial shortening is calculated. Measurement

from telltales are converted to load by:

P = 2 (AsEs+ AcEc) . e/L - Pt (3.2)

The importance in knowing the section properties to calculate load in each level isshown in this formulas. From the measured amount of concrete used for every filling

step, the cross sectional area is calculated. Figure 3.5 verifies the importance of the

difference between the obtained in-situ diameter and design diameter. It shows that asmall difference (in a range of 5 - 8%) leads to erroneous output for shaft friction values.

With calculated Youngs modulus and reinforcement properties, reasonable information

about pile stiffness in each layer is obtained (Fig. 3.6). The differences in results ofcalculation procedures with an without consideration of actual strain is compared in

Figure 3.7.

From the difference in load between measured strain gage levels, shaft friction for

each layer is derived. Pile shortening as a function of existing load is analysed for eachlayer and compared with the telltale results. Proper telltale readings should give the same

axial shortening value as the calculated one from overall pile strain. Otherwise smallcorrections on pile stiffness for each layer are necessary. The telltale is very sensitive to

destruction during pile installation process. Lower data output can also occur due to

overfilling with concrete, restricting its movement. Nevertheless, telltale data can be usedto verify the quality of pile concrete, when abrupt changes in readings take place. When

no changes are noted, concrete quality is good and no failure occurred in the pile itself.

After uniformity of measured and calculated strain is obtained, pile movement at the mid-point of each layer is calculated. Shaft friction development curves for each layer are

obtained with this information. A complete set of Figures, illustrating the whole processto be done, is given in Figures 3.8 3.13. Results of all tests investigated are presented

according to country and summarised in Appendix A - C.

The results have to be checked for consistency after the calculation process. For

cohesive soils, the - versus cucurve is used (Fig. 3.14). For soft clay, results of the vane

shear test provide information about initial soil properties. For stiff clay, correlation ofshaft friction with SPT numbers is given by cu =

2/3 N. In Figures 3.15 3.17 the

estimated shaft friction for cohesive soils, based on SPT meanvalues are shown, together

with shaft friction values obtained from the pile tests. It is noted that the range is smallerthan expected. Details of the initial load development curves indicate the small

displacement required to mobilise main part of shaft friction. It can be seen that the shape

of the shaft friction activation curve is not similar, even for similar soil layers. This canbe caused by improper conducted pile test.

Figure 3.20 states the influence of construction time on shaft friction degradation.Shaft friction values decrease with free stand-up time due to swelling in cohesive soils

and caking effects in cohesionless soils. This rough trend shows the importance of a rapid

and efficient pile installation process. It can be used in practice when the degradation of

shaft resistance due to delayed pile installation needs to be compensated.


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- 16 -

In cohesionless soils, the established correlation fs= N/3 N/5 is used to checkthe calculated shaft friction. Figures 3.18 3.19 show the range of acceptable shaft

friction values. Those outside this boundary indicate an error in either the calculation

procedure or construction process.

3.3 Calculation method used for prediction

Based on obtained shaft friction values, the load transfer method by COYLE et al,

(1966) provide a more reliable pile capacity design than the effective stress procedure

currently in use. The calculated data from different countries indicate that the approach inpile design is to some extent regional and cannot be generalised. Several factors that are

relevant to local piling practice and experience influence pile behaviour. These results are

consistent with the literature. It is stated in DIN 4014 (1990) and BS 2004 (1972) that

regional experience is always more reliable than overall valid approaches and should be

used for design whenever available. Tables 3.3 3.5 present shaft friction valuesobtained for each analysed soil condition in the investigated locations and have been used

in design process. These values were taken carefully from the calculated loaddisplacement curves and represent lower bound values. The values for Bangkok are

comparable to the design values used in Bangkok 2nd

stage expressway- project. They

represent actual piling design practice under the given soil profile and show the highdegree of accuracy obtained from the described method.

Further investigation (for Singapore and Indonesia) is necessary to prove thereliability of these data as they might contain shaft friction values that are too low or

high. No data was available here to compare the calculated data with actual design valuesused in these countries.

3.3.1 Advantages of load transfer method

The values used in the calculation are based on lower bound values. This seems to

be fairly uneconomical in the beginning compared with the effective stress procedure.The load transfer method is made more economical by using a lower factor of safety to

compensate for the lower bound values used. Advantages of this calculation are:

Settlement is taken into account,

Artificial load settlement curves for different pile length are generated.

In this approach, a factor of safety equal to two is sufficient to prevent the

foundation from failure in practice. When this factor of safety is applied to the generated

load settlement curve, the difference between required load and design load is smaller

than expected. Comparison of predicted ultimate load divided by the given factor ofsafety (Fig. 3.21, 3.22) with design load indicate the reliability of the approach. Cost for

expensive load tests can be saved as design procedure contains less uncertainties. For


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- 17 -

large projects it is still preferable to use the normal procedure: conducting load tests in

the beginning to prove if the design load is reliable under in-situ installation procedures.Initial cost for load tests are high and as overall costs of small construction projects are

low this approach would prove fairly economical (Fig. 3.23).

3.3.2 Calculation of end bearing

End bearing capacity can be obtained from undrained shear strength or effectivestress calculation. The evaluated piles where mostly tested based on a settlement

criterion. Therefore the number of load tests available does not provide reliable

information about required settlement necessary to fully mobilise end bearing as well asmaximum possible end bearing. It was chosen here to use bearing capacity values from

BS 2004 (1972) and DIN 4014 (1990) (Tables 3.6 3.9) as they provide simple and

acceptable formulation of end bearing values. End bearing failure is in both codes

assumed to take place at:

s = 0.1 .D (3.3)

Values for end bearing resistance are chosen carefully in these codes as it entirely

depends on in-situ installation practice. Therefore it is not convenient to predict

accurately the end bearing values to be used. Reasonable cleaning time and installationschedule should be adopted from the beginning. Negative results from improper cleaning

is described in Chapter IV in detail for pile BP02-A (Indonesia) as workmanship was the

reason for early failure of this pile.

3.3.3 Calculation procedure

In this paragraph the procedure to calculate pile capacity based on proposed

design values is presented. After appropriate design values have been chosen, predictionof pile capacity under similar soil conditions can be made. The pile is divided into several

segments each having a thickness of the corresponding soil layer. Calculation is done

starting from pile tip. For this segment, a settlement is assumed which will cause acorresponding reaction from the ground in the form of end bearing and shaft friction. The

sum of both resisting forces is applied as the load acting on the top of the segment in the

first iteration step. This load in turn causes a strain of the pile segment, mobilisingadditional shaft friction. The addition of the load acting of the top of the segment and

additional shaft friction causes further pile strain. This again mobilises additional shaft

friction. An iteration process is applied in this segment until the additional shaft frictiondue to pile strain becomes negligible. Then the final total force acting on the top of the

first segment is applied as the reaction force on the base of the next segment. Similarly

this reaction mobilises shaft friction. The iterative process is applied in this segment as

well. This procedure is followed through the entire pile length to obtain the load acting onthe pile top and its corresponding settlement.


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CHAPTER IV

PRESENTATION AND DISCUSSION OF RESULTS

In this chapter the results of the investigated piles and the predictions are presented

and discussed. Actual piling practice is reviewed critically and possible causes for

inaccuracies in predicted shaft friction values are presented. Suggestions for

improvements are given then.

4.1

General

The investigated piles have been constructed under various geological conditions.

Piles in Bangkok are friction piles while in Singapore piling practice is based on end

bearing piles. This difference is due to varying underlying soil conditions. First the most

reliable pile test data are discussed, before the less reliable ones are analysed. Due to the

of lack of reliable data available, no probabilistic method can be used to obtain reliable

mean values for design. Lower bound values are used for calculation. The ultimate load in

the recommended procedure appears to be much higher (Fig. 3.21, 3.22) than those

obtained from pile tests. Once a factor of safety is applied, the working load for both cases

does not seem to differ significantly. This is due to the factor that a lower factor of safety

is applied in the more reliable method recommended.

Installation of all analysed piles was done using wet drilling process. This method

is mostly adopted in Southeast- Asia. During installation a temporary casing prevents soil

with low undrained shear strength from collapsing. Excavation is done by bucket. This

process is flexible in situations where pile diameter and length have to be altered due to

in-situ soil conditions that differ from prediction. To reduce construction time and degree

of disturbance, different drilling techniques have been introduced resulting in an increase

in the achieved pile capacity. Bentonite slurry was used as stabilising agent in all tests.


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4.2

Soil conditions

Bangkok subsoil condition is formed by a geological depression filled with alluvial

and deltaic sediments as well as shallow sea sediments (LIMANHADI, 1992). Layers of

clay with varying strength (increasing with depth) and sand layers alternate. Thickness of

these layers vary, but show remarkable uniformity over the whole Bangkok area. The

engineering properties of each soil layer have been investigated and reviewed in several

publications (PIMPASUGDI, 1989; LIMANHADI, 1992). A cross section and a typical

soil profile are given in Appendix A together with calculated results.

The investigated piles in Singapore have been constructed in residual soils.

Varying strength and thickness of soil layers which depend on the weathering grade cause

main problems under these conditions. The soils are highly variable, spatially

heterogeneous, often unsaturated, overconsolidated with respect to the overburden and

have a high strength and low compressibility (CHANG & GOH, 1988). Piles are mostly

drilled down to bedrock. Therefore only end bearing is considered in design (CHANG &

WONG, 1987). Piles in this analysis were embedded in strong granite and weathered rock.

One pile was constructed with a soft toe for investigation of shaft friction development.

The piles placed in weathered rock show behaviour comparable to piles founded in very

dense sand. Soil profile from pile locations and obtained results are given in Appendix B.

Soil conditions for the constructed piles in Jakarta (Indonesia) are to some extent

comparable with the Bangkok subsoil profile. Jakarta lies on an alluvial flood plain with

sand and silt layers sedimented in alternating sequence. The uppermost layers show low

resistance to SPT test, but these values increase with depth. Analysed piles were founded

in a very dense cemented sand layer. Because of artesian water in the alternating sand

layers, care has to be taken during pile installation. Soil profile and obtained results under

these conditions are given in Appendix C.


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4.3

Results of investigation

As can be seen from load displacement curves of different soil layers (Fig. 3.15

3.19), the displacement required to fully mobilise develop shaft friction does not follow a

general trend. Shaft friction values are fully mobilised under very small displacements for

most layers (Detail Fig. 3.15 3.19). In a limited number of cases it has been observed

that larger displacement at soil pile interface can be required for full mobilisation. This is

consistent with the literature, where MEISSNER (1979) observed that ultimate shaft

friction can be achieved at displacements larger than 2 cm. STAMM (1990) reported that

only small changes in grain size and grain size distribution is required for significant

change of shaft friction. Strain softening or hardening that takes place depending on the

soil properties. Finally it can be said that even under small displacements (5 8 mm)

where pile friction is not entirely mobilised, its magnitude is sufficient to provide required

pile resistance.

The results from the investigations in Bangkok have a high degree of reliability.

All necessary information for proper calculation was provided. The recommended

procedure was adopted, producing results with a high degree of reliability. Comparison of

calculated shaft friction with values obtained from SPT test correlations show good

agreement in both procedures (cohesive and cohesionless soil). This indicates good piling

practice. It is observed that the SPT correlation for cohesionless soils (fs = N/3 N/5)

leads to good results in medium dense to dense sand (1stsand layer). In very dense sand

(2nd

sand layer) the calculated and correlated results differ slightly. Possible reasons is the

high SPT number required to penetrate the ground. Common practice in the high

blowcount range is that the SPT test is terminated before the required 30 cm penetration is

achieved. The SPT number is recorded then as a value higher than the actual blowcount by

correlating the achieved and the required penetration. It has to be investigated, whether

this is a common difference or only existent in these tests.

Shaft friction decreases with free stand-up time. Previous findings

(THASNANIPAN et al., 1998) and those found in this analysis are shown clearly in

Figures 3.20, 3.20a. It can be observed that there are differences in the initial estimation of

shaft friction values in both findings. This is due to the fact that shaft friction values are

derived here from SPT test correlations which do not represent real values. The results of


29/84

these correlation is usually presented in a range of values. Therefore the results strongly

depend on the exact values taken within these upper and lower limits. Observed residual

shaft friction values have to be used in the ratio (expected shaft friction/ observed shaft

friction), so that pile testing must be done until failure load is reached. Apart from these

differences in soil properties, swelling and caking effects are other possible factors

causing different results.

Further, it is observed that shaft friction values for each pile do not show the same

characteristics. A low shaft frictional resistance in one layer does not indicate that the

other layers shows similar behaviour. From Figures 3.15 3.17, it can be seen that low

shaft frictional resistance is mobilised in soft clay for pile P3 whereas shaft friction in

medium clay is high. Pile PPLT 23 shows high resistance in stiff clay and lower resistance

in medium clay and 1stsand.

End bearing resistance indicates a large scatter in results. The piles investigated in

this report show high as well as low end bearing values. The limited data available caused

large scatter in results. This scatter could not be analysed logically.

The Results from Singapore were analysed carefully, as no information was

provided on the amount of concrete used. Nevertheless, analysis on these piles show

results that differ quite significantly from those in Bangkok. This enables a useful

platform for comparison and deduction. As explained in Chapter III, improper analysis has

lead to some unacceptable values of shaft friction in piles investigated in Singapore. To

check the reliability of these values, expected shaft friction values were derived from the

mentioned SPT correlation. The correlation was found for sedimentary soil. A fair

agreement is observed in this comparison.

The vs. cu correlation by CHANG et al. (1988, Fig. 3.14) shows that stays

constant at 0.3 for all cuvalues. This value is derived from the relationship fs= 2 . N found

by plotting all data points like shown in Figure 4.1. No comparable relationship could be

found for critical displacement (zsc) vs. SPT. Also the often used ratio fs/zsc does not

provide a clear trend. This indicates the large scatter obtained in residual soil. The

formulation fs= 2 . N might therefore be a usable formulation of average shaft friction in

Singapore. This formula does not indicate overall valid design in residual soils.


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BALAKRISHNAN (1994) found slightly different values for Kuala Lumpur, Malaysia

soil condition. The especially when lower shaft friction is present. Failure mechanisms

indicate that the ratio of shaft friction/ undrained shear strength is coincides with

decreasing cu. The proposed curve does not show this behaviour.

Load carrying principle of the piles evaluated here are different from piles

constructed under Bangkok soil conditions. The developed end bearing provides high

support. Therefore shaft friction cannot be fully mobilised under working load in the

lower pile segments, as movement is restricted by end bearing. From the given pile tests

no conclusion can be made on interaction of end bearing and shaft friction. This

relationship can be found from findings by WILLIAMS et al. (1981) and HORVARTH et

al. (1983). It is found from calculation, that the settlement observed at ultimate testing

load is sufficient to mobilise substantial shaft friction throughout the entire pile length.

It can also be concluded that proper base cleaning is important. To investigate this

a pile constructed with soft toe (UTP 1) can be used to simulate improper bas cleaning.

The settlement observed is significant until the settlement reduced the soft toe effect. After

this point further load increase takes place when the end bearing is obtained from the

underlying rock layer.

The Test results from Indonesia are not reliable. Calculated shaft friction values

contain errors due to incomplete data. The calculation procedure could not be done in a

proper manner. The achieved results sometimes do not fit with the correlations used to

check shaft friction. Piles showing failure characteristics provide valuable information.

Pile BP02-A failed earlier than expected due to the soft toe effect. It can be seen from

Appendix C that load capacity increases slowly with settlement. This indicates slow

densification with settlement at pile tip. The observed behaviour indicates the importance

of careful pile installation. End bearing is very sensitive to the installation process.

Possible failure reasons are: inflow of water when artesian water is present, and

supporting liquid in the column does not have sufficient height; loosening possibly due to

suction during fast bucket removal. The results of an optimised process can be seen from

pile BP03-A which reached the required load settlement criterion easily.


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4.4

Results of prediction

The lower bound shaft friction values calculated from the analysed pile tests in

Bangkok can be applied for all piles within the same soil conditions. From the prediction,

some tested piles in practice show agreement indicating good prediction. Others show fair

agreement. Capacity reached under good workmanship is higher than predicted values.

This indicates that for further design shaft friction above the lower bound can be used. The

results show an acceptable agreement with field behaviour at working load. In the early

stage of design, this can be used as a reliable approach to estimate working load as a

function of settlement.

Results of Singapore show different behaviour. Although measured and calculated

ultimate loads differ significantly, a good agreement is achieved under working load,

which to an acceptable degree coincides with the measured value. This is due to shaft

friction taken in account in the calculation. Prediction of pile UTP 1 (soft toe), show that

values chosen for shaft friction are reasonable. The difference between predicted and

measured ultimate load is due to conservative assumptions for end bearing. Low values

from DIN 4014 (1990) and BS 2004 (1972) were chosen for design because of lacking

regional data. Local experience on end bearing capacity should be used for further design.

Substantial cost can be implemented by good prediction and by reducing the number of

pile tests conducted.

Actual design neglects shaft friction to some extent in Singapore (CHANG et al.,

1987). The results presented here indicate this highly uneconomical approach. In Figure

3.21, 3.22 predicted load settlement curves are shown based on lower bound values as

well as measured field behaviour. The working load and the calculated ultimate capacity is

indicated as well. It is noted that the working load for the prediction still provides a factor

of safety of around 3. The corresponding settlement to this working load is in a reasonable

range, and indicates the reliability of the design procedure.

Shaft friction should be considered in further design.

No conclusions can be taken from Indonesian tests. Reliable data is necessary to

give a final statement and a reliable design. Calculated shaft friction values depend on

reliable soil parameters obtained from field tests. Due to this, data of both tests analysed


32/84

are considered in choosing lower bound design values. This leads to underestimation of

pile capacity. Careful research has to be done to overcome the lack of data. The

importance of proper piling practice and clear construction records is described later.

4.5 Comparison with published data

The analysed shaft friction values are compared with previous publications.

However, due to insufficient material, this could only be done for piles in Bangkok.

A large difference between calculated and published data is observed. Shaft

friction indicate erroneous shaft friction values for several layers. Shaft friction obtained

from calculation and published material are given in the table.

Soil layer Soft clay

[t/m2]

Stiff clay

[t/m2]

1stsand

[t/m2]

2nd

sand

[t/m2]

Recommended values 1.5 2.0 3.5 9.1 6.6 12.1 12.1 25.1

THASNANIPAN (1998) 6.0 13.9 7.5 18.8 11.2 19.9 15.6 25.1

2nd

stage expressway project 1.3 - 2.0 3.7 6.8 6.3 7.6 11.4

Analas S. (1995) 3.5 6.6 3.1 5.9 2.8 9.7 0.2 2.6

CHUN, 1992 - 9.1 24.2 15.1 27.4 15.2 18.1

NG, 1993 6.23 8.5 28.0 5.81 7.8 8.6

Table 4.1: Comparison of different proposed shaft friction values

Most shaft friction values given for different soil layers in previously published

material cannot be used in design practice. These published calculation procedures should

be checked as incorrect sublayer shaft friction values somehow produce overall pile

capacity that agrees well with pile tests. Load was calculated under constant diameter andconcrete Youngs modulus value obtained from test cube. Youngs modulus from test

cubes are taken at a fixed strain. This cannot lead to reliable field data evaluation. As

described in Chapter III, small changes in diameter and Youngs modulus (function of

strain) lead to different results.


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THASNANIPAN et al. (1998) presented shaft friction values obtained from pile

tests that ranged from 50% higher to 27% lower than values obtained from SPT test

correlation. Possible reasons are stated. They expect test load (not yet fully mobilised

shaft friction as failure load was not reached), to cause the lower difference. These

conclusion contain reasonable and non reasonable assumptions. As it was shown that only

small pile displacement is necessary to fully develop shaft friction, a top settlement from

3 4 cm already develops full shaft friction in most layers. The assumption made can only

be valid for the lowest soil layers of very long piles.

Possible reasons for too high shaft friction values are stated as increase in diameter

in the field due to casing removal or an increase in cu due to higher values in the

weathered crust. The assumptions made are correct. When the influence of the weathered

crust is neglected, an increase in diameter in order from 5-10% due to the casing would

lead to reasonable calculation result. When calculated shaft friction values are too high,

used diameter in calculation is too small. The reverse effect is observed when calculated

loads are too small.

Comparison of results with presented values for 2nd

stage expressway project show

good correlation with the observed values given in this research.

Based on the achieved knowledge, data used in predictions should be reviewed

carefully and checked for reliability. With the amount of data available for these soil

conditions, reliable pile capacity estimation must be possible.

4.6 Increase in pile capacity

It is shown that pile capacity and load- settlement characteristics depend to a high

degree on the construction process. Higher pile capacities would be preferable for more

economical foundations. In order to achieve this more emphasis has to be put on the

construction process. Higher shaft friction values can be achieved, by use of different

stabilising agent (polymer), shorter construction time and toe and shaft grouting. Research

is necessary on these improvements to state definite values of the higher achieved

capacity.


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4.7

Constructive recommendations

During data evaluation, several points were observed, that needed to be improved

in further test pile construction and presentation of data.

Mistakes in pile tests can occur in every stage. The importance of a proper

conducted load test is obvious from strain gage data output of pile PPLT 23. The strain vs.

load graphs contains discontinuities that indicate a too fast loading process (Fig. 4.1). The

load was not kept stable for a sufficient time to build up static equilibrium. When the next

load increment was applied strain (and therefore load) in these layers increased drastically.

Measurement can be optimised, by taking the given settlement criterion (next load

increment is applied after a specific settlement rate is reached) and a strain gage criterion

in account. Load should not be applied when strain gage readings for lower pile sections

still show an increase in strain. The effects of these strain gage data output are present in

the whole calculation process. In the load displacement curve shaft friction values that are

not representing real behaviour (Fig. 4.2) are observed. They are higher for upper layers

than the expected values, in lower layers they are smaller. These values have to be

removed for further calculation, leading to less reliable data.

It is further noted from the available reports, that less care is taken on construction

reports. Only this information leads to reasonable statements on possible failure modes.

The same conclusion is valid for soil profiles. Less care is taken to clearly describe soil

conditions at test pile location. Test piles are mostly constructed outside the foundation

area, a soil profile equal to the foundation report is attached. As strain gages are usually

installed on top of each layer, differences exist between given strain gage level and soil

profile. In the worst case it is impossible to gain reliable information which soil exists

between adjacent strain gage levels. Reasonable soil investigation must be adopted as

well. SPT test does not provide reliable information on strength of soft clay.

Emphasis should also be put on the concreting record. Measurement after each

filling step provides necessary information about the existing diameter. The record must

be written clearly, additional drawings must lead to the same result. Concreting record

should be extended to provide information about concrete level while the casing is


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removed. From this the diameter in the uppermost level can be calculated, and information

on the interaction of fresh concrete pressure to unstable soil is provided as well.

For later investigation, all these records should be attached to the load test report.

Useful drilling records provide information about the entire construction process. When

reports are written in the field they should refer to one fixed point, several measurements

to different locations does not provide reliable information (top casing, ground level and

mean sea level).


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- 25 -

CHAPTER V

CONCLUSIONS

The load transfer characteristics of bored piles investigated in this study show a

high degree of dependency on proper calculation procedure. Results calculated by theauthors recommended method show smaller scatter than in the previous literature. The

differences were discussed and it is concluded that they are due to improper calculation

procedure when the measured strains where converted to load. The significant differencein the calculated shaft friction values as a result of the authors method (e.g. for soft clay

layer) is shown below:

Analysis based on design diameter does not lead to reasonable results. The shaft

friction values are higher than actual because obtained in-situ diameter is usually higher

due to removal of casing and inconsistencies in installation procedure. Although largerdiameter is used by taking the average amount of concrete used throughout pile length in

order to compensate for this to some extent, the reduced shaft friction values obtained

still are higher than actual. This could be due to the inconsistent diameter and concrete

Youngs modulus throughout the pile length. In the method recommended by the author,the diameter and concrete Modulus are assigned according to pile segment and full

compensation for casing observed, producing highly representative shaft friction values.

Results similar to those obtained for soft clay layer were observed for all layers and soilconditions.

The given correlations for shaft friction based on SPT blowcounts provideacceptable results. Shaft friction values for cohesive soils in Bangkok, calculated by the

procedure recommended by the author show values slightly higher than those presentedin previous literature (Fig. 3.14). Another fact supporting this observation is the noted

increase in shaft friction in all layers for more recently constructed piles (P1P5). This

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

1

ShaftFriction[t/m2]

Design Diameter

Average Diameter Based on

Total Amount ofConcrete

Segmental Diameter based

on Amount of Concrete in

each Pile Segment

cufrom Site

Investigation


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could indicate that installation process has been improved in the last 6 8 years. At

present, pile installation process could still be improved by decreasing construction timeand increase efficiency. A further increasing in capacity is expected when achieved

theoretical understanding is applied to practice.


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REFERENCES

ATA, A. A., ONEILL, M. W. (1998): Side- wall stability and side- shear resistance inbored piles constructed with high- molecular- weight polymer slurry, Proc. of

Deep Foundations on bored and auger piles, Rotterdam

BALAKRISHNAN, E. G. (1994): Performance of bored piles in Kenny Hill formation

(weathered meta- sedimentary) in Kuala Lumpur, Malaysia, M. Eng. Thesis,

GT934, AIT, Bangkok

BAUER (2000): http://www.bauer.de/deutsch/gbm/prod_frame.htm

BRITISH STANDARD INSTITUTION (1972): CP 2004: Code of practice for

foundations, British Standard Institution, London,

BROMS, B., HANSBO, S. (1981): Foundations on soft clay, Soft clay engineering,Developments in Geotechnical Engineering 20, pp. 417 - 468

BURLAND, J. (1973): Shaft friction of piles in clay a simple fundamental approach,Ground Engineering, Vol. 6, No. 3, pp. 30 - 42

BUTTER, F. G., MORTON, K. (1970): Specification and performance of test piles inclay, Behaviour of piles, Proceedings of the conference by the Institution of

Civil Engineers, London

CHANG, M. F., BROMS, B. B. (1991): Design of bored piles in residual soils based on

field- performance data, Canadian Geot. Journal Vol. 28, pp. 200 - 209

CHANG, M. F., GOH, A. T. C. (1988): Behaviour of bored piles in residual soils and

weathered rocks of Singapore, Research Report CSE/1988/17, Nanyang

Technological Institute, Singapore

CHANG, M. F., GOH, A. T. C. (1989): Design of bored piles considering load transfer,

Geot. Engineering, Vol. 20, No. 1, pp. 1 - 18

CHANG, M. F., WONG, I. H. (1987): Shaft friction of drilled piers in weathered rock,

Proc. of the 6thInt. Congress on Rock Mechanics, Canada, I, pp. 313 - 318

CHUN, H. H. (1992): Performance of driven and bored piles in expressway projects, M.

Eng. Thesis, GT913, AIT, Bangkok

CROWTHER, C. C. (1988): Load testing of deep foundations, John Wiley & Sons, New

York


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DIN (1990): DIN 4014 German Industrial Code of Practice (in German)

COYLE, H. M., REESE, L. C. (1966): Load transfer for axially loaded piles in clay,

Journal of Soil Mechanics and Foundation Division, ASCE, Vol. 92, No.2, pp.

1 - 26

FARMER, I. W., BUCKLEY, P. J. C., SLIWINSKI, Z. (1970): The effect of Bentonite

on the skin friction of cast in place piles, Proc. of the conference behaviour ofpiles by the institution of civil engineers, London

FINDLAY, J. D., BROOKS, N. J., MURE, J. N., HERON, W. (1997): Design of axiallyloaded piles United Kingdom practice, Proc. of ERTC3 Seminar, Brussels,

1997

FRANKE (1996): Load carrying principle of piles under axial loading,

Grundbautaschenbuch, Teil 3, Ernst & Sohn, Berlin (in German)

FULLER, F. M. (1983): Engineering of pile installation, McGraw Hill, New York

GOEDECKE, F.-J., SCHULER, G. (1986): Foundations on bored piles Experiences,

examples and evaluation of test loading, Bautechnik 63, H. 5

HARTUNG, M. (1995): Quality ensurance during pile installation, Pile Symposium of

TU Braunschweig, Germany (in German)

HORVATH, R.G., KENNEY, T.C. (1979): Shaft resistance of the rock- socketed drilledpiers, Proceedings on the Symposium on Deep Foundations, ASCE National

Convention, Atlanta, pp. 183 - 214

LIMANHADI, B. (1992): Prediction of pile settlement with quantified uncertainty for thesecond- stage Bangkok expressway, M. Eng. Thesis No. GT 91 9, AIT,

Bangkok

MEISSNER, H. (1979): Load carrying principle of axially loaded bored, Geotechnik 2,

H. 2, (in German)

MEYERHOF, G. G. (1986): Theory and practice of pile foundations, Proc. of the int.

Conference on deep foundations, Beijing, 1986

MYINT, Z. W. (1989): Pile driving criteria for residual soil in Thailand, M. Eng. Thesis,

GE 98 32, AIT, Bangkok

NG, K. C. (1983): The construction problems and performance of large bored piles insecond sand layer, M. Eng. Thesis No. GT8226, AIT, Bangkok


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NOWACK, F., GARTUNG, E. (1983): Instrumentation of cast- in- situ concrete piles for

vertical and horizontal load testing, Geotechnik, 6, Heft 1 (in German)

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Thesis No. GT8910, AIT, Bangkok

ONEILL, M.W., REESE, L.C. (1972): Behaviour of bored piles in Beaumont clay,

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195 - 213

PIMPASUGDI, S. (1989):Performance evaluation of bored, driven and auger press piles

in Bangkok subsoils, M. Eng. Thesis No. GT 88 -12, AIT, Bangkok

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& Sons, New York

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WEISS, K., HANACK, S. (1983): Der Einfluss der Lagerungsdichte des Bodens und der

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TABLES


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Table 3.3: Shaft Friction Values and Related Crit. Displacement, lower bound, Bangkok

Soil Shaft friction Critical

Displacement

[-] [t/m2] [mm]

Soft Clay 1.5 10

Medium Clay 5 41

stSand 8 12

Stiff Clay 6 9

2nd

Sand 16 10

Table 3.3a: Shaft Friction Values and Related Crit. Displacement, upper bound, Bangkok


Displacement

[-] [t/m2] [mm]

Soft Clay 2.1 6

Medium Clay 6 7

1stSand 10 8Stiff Clay 8 7

2nd

Sand 25 9

Table 3.4: Shaft Friction Values and Related Critical Displacement, Singapore


Displacement

[-] [t/m2] [mm]

Fill Soil 12 15

Peaty Soil 12.5 16

Loose to Silty Fill Sand 6.5 7Dense Silty Sand 12.5 19

Hard Clayey silt 13 15

Very Dense Sand 18 9

Stiff Silty Clay 9 13

Fractured Siltstone 12 14

Very Hard Siltstone 20 15

Table 3.5: Shaft Friction Values and Related Critical Displacement, Indonesia

Soil Shaft Friction Critical

Displacement[-] [t/m

2] [mm]

Sandy Silt, Stiff 5 11

Clayey Silt, Very Stiff 6 13

Sandy Silt, Hard 7 9

Sand, Very Dense 15 12


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Load Transfer Characteristics of Bored Piles in Different Soil Conditions - Florian Bussert 04.2001

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