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ON THE SUBJECT OF STATIC PILE LOAD TESTSM. G. England and W.G.K. Fleming

Kvaerner Cementation Foundations Limited, United Kingdom

1. ABSTRACT

The popularity of static load tests in the UK would appear to be at an all time high, as deficiencies of all pile testing

methods have been exposed and verification of design and installation methods are continually sought and optimised

for both technical and commercial reasons. In Britain alone, more than 100 static load tests are performed per month.

Cementation have developed computer based equipment which both maintains loads constant to a high degree of

precision and electronically logs all settlements at prescribed short time intervals. This means that the deformation-

time can be accurately recorded and for any given load, the behaviour may be modelled mathematically and the final

settlement can be projected with high accuracy.

The deformation-time characteristic consistently observed at the pile head does not appear to governed by pore water

pressure dissipation alone. It would appear to be a function of a creep mechanism and is exhibited in most materials.

Further, by employing projected final settlements, interpretation of the load settlement behaviour can be done without

being affected by the test specification. Having projected the true long term settlements for a series of loads, it is found

that the characteristic of the load/deformation relationship can be interpreted according to the properties of the soil in

which the pile is embedded. This is of course subject to the settlement of the pile having been pursued to a point where

a significant proportion of the base load has been mobilised.

The methods have far reaching value both in the analysis and prediction of pile performance and may equally be

applied to the behaviour of spread footings, barrettes and other foundation types. The analysis methods also provide auseful diagnostic tool for those cases where piles do not perform according to expectation, for example arising from a

particular construction technique. The system demonstrates clearly the limitations of many testing methods and the

problems associated with rapid tests. It offers a method of determining appropriate soil parameters more reliably than

most small scale site investigation methods.

2. INTRODUCTION

Examination of old Civil Engineering text books suggests that the idea of load testing to prove the capacity of bearing

piles was not very common until the latter part of the last century and the early years of the present century when new

types of pile and pile driving equipment required some proof of performance. Indeed it may well have originated frompiling contractors who sought to give demonstrative proof of the ability of their particular piles to sustain impressive

looking heaps of railway lines or iron billets. There are a number of photographs from the 1920s and 1930s period

which show proud demonstrations of the ability of a specific company's piles to carry 40 or 50 tonnes of load.

The measurement of deflection under load in early tests was frequently carried out by wires which passed around

pulleys and actuated some sort of indicator or pointer arm which in turn moved against a crudely marked scale.

The First International Conference on Soil Mechanics and Foundation Engineering was held at Harvard University in

1936. It is of interest to note that driven piles were very popular at that time, with a large range of available

proprietary types. A near universal source of disquiet was the multiplicity of "Dynamic Formulae" which were then in

use and the fact that there was no consistency in the answers which they gave and no satisfactory correlation with

static loading tests. A great hope of the time was that soil investigation and load testing would eventually come

together to render pile capacity and performance more predictable.

In the British Isles clients and civil engineers sought comfort in the availability of the static demonstrations of

performance that were available, and about the middle of the present century the Institution of Civil Engineers in

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London saw fit to offer guidance on how to present the results of maintained Load Tests in some sort of formal

standard manner.

The main reasons for the popularity of full scale loading tests in the United Kingdom appear to have been (a) the

natural variability of British geology and soils, (b) the inborn scepticism of British civil engineers who always wanted

convincing proofs rather than the assurances of any contractor and (c) the very widespread criticism of driving

formulae.

Likewise for bored piles, Soil Mechanics was still not developed enough to predict pile performance with reliability.

Terzaghi in 1943 ("Theoretical Soil Mechanics") makes the statement that "Since the bearing capacity of piles cannot

yet be computed on the basis of results of soil tests performed in the laboratory, we are still obliged either to estimate

this value on the basis of local experience or else to determine it directly in the field by loading a test pile to the point

of failure". He says that persistent efforts have been made for more than a century to obtain the desired information

from the results of simple field tests involving the depth of penetration produced by a hammer of known weight falling

through a known height. He drew the conclusion that such formulae were utterly misleading but nevertheless enjoyed

great popularity among practising engineers.

The world has not changed very much since the days of Terzaghi. The search is still very much alive for a relationship

between static and dynamic pile resistances, although this quest bears similarity to that in ancient times for the

philosopher's stone.

Even in the field of pure static load capacity calculation and testing there is still only limited help from commercial

laboratory procedures and it does seem clear that the best design aids are really field instruments which seek to

emulate the mechanisms of pile bearing capacity (e.g. the Dutch Cone penetration equipment).

Two important issues need to be borne in mind in relation to Soil Mechanics and foundation performance.

Behavioural mechanism similitude is a vital issue to which much more attention should be paid in Soil Mechanics

than has been done to date. It has special significance for all non linear analysis systems. Secondly, Soil Mechanics is

just a branch of Materials Science and we should look more carefully at other materials and processes in order to helpsolve our problems. Some quite well accepted theories in our limited field of endeavour appear questionable in the

light of other fields of investigation.

Thus it comes about that after a century of mechanical equipment and theoretical development, there remain many

questions about pile performance prediction which are not well understood nor satisfactorily answered and these are

mainly at a fundamental level.

3. PURPOSES OF TESTING

There are several reasons for pile testing. It may be carried out for example:-

(a) as a research exercise to further knowledge of pile behaviour.

For this purpose large amounts of correlation information are generally sought and it would be usual to install

instrumentation to measure perhaps loads or strain distribution along the length of a pile. Good quality soil data from

investigation before pile installation would be regarded as an essential. However, it is not common for the soils to be

re-investigated after pile installation, and this is often a serious deficiency in such tests. The ground can be affected

significantly by the process of construction.

The use of instrumentation to determine pile load distribution within special test piles has become common but it is

not so reliable as might generally be supposed. Particularly if time functions are ignored in the resulting readings.

Very substantial interpretive problems can remain unrecognised.

(b) to provide direct information to aid design of piles on a particular site.

Where the aim of a test is to provide information on which design can be based, it is essential to position the test pile

or piles where most information can be derived. This would certainly lead one to try and identify at least the worst

ground conditions likely to occur. Again, post pile installation investigation, especially in sandy soils and perhaps

using Dutch Cone apparatus can prove very helpful and enlightening.

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(c) to demonstrate that piles as designed and installed have adequate capacity or deformation performance for a

specific use.

The majority of piles which are tested fall into the "proof" or capacity demonstration area. This in the UK usually

requires that they are loaded to a design verification load which is 1.5 times the specified working load plus an

allowance for such items as downdrag and cut-off variations. The practice frequently does not allow a great deal of

basic design information to be back calculated but occasionally when the data can sensibly be analysed it may still

prove really useful.

4. TYPES OF TEST AVAILABLE:

Reference has been made above to the long quest for a method of assessing pile static capacity from observations made

during driving. The quest for simple methods had been fairly well exhausted by the middle of this century but in the

mid 1930's interest was aroused in an alternative approach to the interpretation of pile driving by means of the wave

equation. Work in this field was started by Granville, Grime, and Davies at the UK Building Research Station and

several papers were published at that time. Later work by Smith was taken up and developed by Goble, Rauche and

Likins, who were able to make use of electronic developments in signal handling to produce a "Pile DrivingAnalyser". This renewed hope that perhaps from the act of pile driving a means of assessing equivalent pile static

capacity might at last be found.

Rather as the piling industry originally seized upon the earlier type of dynamic formula because it seemed obvious and

practical, much the same history seems to have been repeated with this method. It is however becoming clear that such

methods have significant problems which are unlikely to be overcome in the near future. They are based on wave

equation theory as is appropriate, but they have not come to terms with the refusal of the ground to offer the same

resistance to fast loading as to slow loading.

An effort to avoid some of the problems of wave equation type testing has also been made in the "Statnamic" method

developed by Birminghammer Corporation of America in conjunction with TNO of Holland. In this system the

hammer blow is delivered by burning a propellant in a suitable chamber on the pile head. Whereas the conventional

hammer blow may take place within a period of say 20 ms, the Statnamic impulse force is several times longer induration. This in turn means that the whole pile can be in compression at the same time and therefore wave equation

methods of analysis become unnecessary. Other "pseudo-static" tests have been devised using spring systems which

appear to offer similar features.

However, the great problem of dynamic rate effects is still present in all such methods and it is still not possible to take

into account items which are of profound importance to static behaviour like consolidation and creep.

Contrary to common belief, creep is an important issue in all soil types and rapid testing always distorts ultimate load

values.

It is for these same reasons that the Quick Maintained Load Test of USA and the Continuous Rate of Penetration test,

commonly used in the UK, both give misleading results. The deformations they show are very short term and ultimateloads are either exaggerated or are interpreted downward by invented empirical rules.

When all the rapid test methods have been disposed of there still remains the common Maintained Load Test and this

is the only test which is likely to answer satisfactorily most of our questions.

However, as in every test, understanding and interpretation are fundamental to getting most benefit from results.

Strain equilibrium tests in which a load is applied by hydraulic jack and then subsequently allowed to decline until a

force/displacement equilibrium is reached, are similar in some respects to the maintained load test but are not quite so

pure because stress and deformation changes are occurring simultaneously and cannot easily be separated for analysis.

5. MODERN LOAD TESTING EQUIPMENT

Anyone who has had the experience of processing gauge readings from a long Maintained Loading test, and in

particular of trying to plot the time/settlement curves for each constant loading, will realise that it is tedious and leaves

much to be desired on the grounds of accuracy. It will also be appreciated that many specifications require different

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load holding periods at certain stages of a test and that this tends to concentrate time related settlement into the longer

periods. As a result erratic load performance becomes recorded as fact.

It was for these reasons that a programme of modernisation was begun by Kvaerner Cementation some 5 years ago.

The objectives were to read all movements and loading devices electronically, to maintain loads within fine tolerance

limits, and to transcribe all data direct from the test site to the client's report without manual intervention.

The method of collecting the data was seen also as an important advance in site safety because it removed observers

from the area where large loads were often being applied with consequent risks.

The results from the first set of equipment put into operation were so impressive in terms of both quality and cost that

additional advanced testing sets were commissioned and at present we have six system in the field in nearly

continuous operation. Not only has this changed the way the Company carries out tests but a growth industry has

sprung up with other specialist testing firms imitating the techniques. It is understood that there are some twenty five

sets operating in the UK at present and at least one similar system operated by A.G.I.S.Co (Italy). Many operators and

Consulting Engineers have adopted the CEMSET analysis methods described in the following sections, although often

perhaps using spreadsheet type software which is not so effective as the more fully developed and purpose built

programs.

The current stage of equipment development in testing is that the managing computer can be pre-programmed for a

complete loadig test without human intervention and the equipment has safety monitors which take evasive action in

the event of any accidental occurrence according to pre-set instructions. No safety protection will ever be perfect, but

this should go a long way towards avoiding potentially serious accidents.

6. THE MAINTAINED LOAD TEST:

Virtually every specification for this type of test calls for applied axial compressive load to be maintained constant, at

least until a certain slow rate of movement of the pile head has been reached. This stipulation has been included in test

specifications for a very long time but unfortunately has not commonly been applied with rigour.

Settlements in all soil conditions for any given load are strongly time related. Under any constant loading, the time

displacement is a smooth curve without erratic behaviour. If the time/settlement curve for say a pile base is

differentiated once in relation to time, we obtain a time/velocity relationship and if it is differentiated twice it becomes

a time/acceleration (or deceleration) function. When negative accelerations are calculated they are observed to be very

small and since Force = Mass x Acceleration it becomes apparent that we are also dealing either with enormous

masses or very small forces.

It is perhaps obvious that the force producing the

acceleration is in fact the differential forces

between the applied load and the soil reaction. If

the decelerations are plotted together with the

displacement and velocities for a particularsimple case (Figure 1) it will be observed that

deceleration (and hence force differential)

changes very rapidly close to the time origin.

Change in applied force must therefore be

expected to have strong effects on the

displacement function in the same region.

Experience indicates that to obtain smooth

regular time/displacement curves, applied load

needs strict control (within approximately 0.2%).

If this level of control is maintained then

time/displacement curves for normal piles are

invariably found to be of the type shown in

Figure 2 for each load stage. This is true of all

loads but if the deformation under any specific load is very small then accuracy of measurement becomes an evident

practical limitation.

Figure 1

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0 1 2 3 4 5 6 7 8 9 10 11 120

1

2

3

4

5

6

7

8

TIME [Hours]

Displacement [mm] RELATIVE DISPLACEMENT TIME DIAGRAM

DRIVEN

CASTIN-SITU

IN

CHALK

Constant load = 3052 kN

Time offset = 39.86 hours

Disp offset = 21.24 mm

Measured displacement

Figure 2 Typical relative displacement-time diagram.

0 1 2 3 4 5 6 7 8 9 10 11 120

1

2

3

4

5

6

7

8

TIME [Hours]

Displacement [mm] RELATIVE DISPLACEMENT TIME DIAGRAM

DRIVEN

CASTIN-SITU

IN

CHALK




Measured displacement

Function S + Function B

Function S

Function B

Asymptote of S: Ws = 2.94 mm

Asymptote of B:

Wb = 4.84 mm

Asymptote of S+B: 7.78 mm

Figure 3 Data from Figure 2 modelled mathematically

Provided one can formulate mathematical relationships which accurately track the recorded time-deformation

functions over a long period as shown by way of example in Figure 3, the final settlement for each given load can be

projected from the data collected over a relatively much shorter period which might be say 3 to 12 hours.

This consistency of behaviour became observable once computers could give accurate control of loads and recording,

although of course it will be recognised that direct dead loading might perhaps more easily maintain true constancy.

Direct dead loading has not been encouraged for many years for reasons associated with safety and the magnitude of

loads which might be required to balance on pile heads. Figure 4 shows a current arrangement for a normal computer

controlled pile loading test.

Hydraulicpump

SafetyGauge

Displacement

Transducers

Link to site computer or direct

telephone link to head office.

Jack

Load Cell

Oil Supplyto Jack

Controller& DataLogger

Electronic Safety Barrier

Reaction System

PILE

Site

Monitor

Pile cap

Figure 4 Functional diagram of load test arrangement

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6.1 ANALYSIS OF DISPLACEMENT-TIME BEHAVIOUR

6.2 Time/displacement functions

Early attempts to track time-displacement

functions used the Chin (1975) method of plotting

time against time/settlement. A typical example is

shown in Figure 5 (the data employed is that

displayed in Figure 2). According to Chin, plotting

in this manner should yield a straight line, yet

accurately recorded experience showed that in fact

the result of such a procedure is almost invariably

to produce a gentle smooth curve unless either

shaft friction or end bearing can be discounted.

With much improved accuracy of load control and

recording, it became obvious that a single linear-

fractional function was not a reasonable

assumption, but instead there was a high

probability that a double function (in which one

function was related to shaft friction and another

to end bearing) would prove much more successful

as typically shown in Figure 3.

Note: Linear-fractional (hyperbolic) functions

Vyalov (1986) comes to the conclusion that such models are probably the best available for representing creep.

The generalised time function K(t) suggested by Vyalov is of the form

( )K tT

T t

n

=+

2

1

It is of interest to the current findings that setting the exponent n to unity, gives a linear-fractional relationship for the

deformation.

He refers to the form of the relationship as "linear-fractional" which is probably a more appropriate term because it

more correctly describes a function in which the secant from the origin falls linearly with the proportion of the

distance the function has travelled from its start point towards the asymptote. The term "hyperbolic" could imply that

the relation is of the form sinh, cosh or tanh in mathematicians terminology and this is not what is intended. The

mathematical functions bear similarity but are not identical in form.

This observation led to the development of a double linear-fractional analysis method (M. England) which has been

tested in literally thousands of cases and which routinely can track pile behaviour for periods of 24 hours or more. The

method has been given the name TIMESET.

0 1 2 3 4 5 6 7 80

.1

.2

.3

.4

.5

.6

.7

.8

.9

1

1.1

1.2

1.3

TIME [Hours]

Time/Disp [hours/mm] TIME/DISPLACEMENT vs TIME DIAGRAM

DRIVEN

CASTIN-SITU

IN

CHA

LK




Figure 5 Typical Chin plot of relative displacement/time

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Current research on these functions is proving very interesting and enlightening and shows promise of having a

profound influence on the way we think about soil consolidation and creep behaviour.

Once accurate tracking of real pile behaviour becomes routinely possible, then the final settlement of a pile under any

given load can be fixed with sensible reliability. The asymptote of the time/deformation curve represents final long

term settlement for each given load and when all applied loads are similarly treated, the unique long term load-settlement characteristic of the pile is displayed, as shown in Figure 6 for a site where piles were founded in chalk.

0 400 800 1200 1600 2000 2400 2800 3200 360032

28

24

20

16

12

8

4

0

LOAD [ kN ]

Displacement [mm] LOAD DISPLACEMENT DIAGRAMLOAD DISPLACEMENT DIAGRAM

DRIVEN

C

ASTIN-SITU

IN

CHALK

Figure 6

It has not been possible to track pile behaviour in practice for more than about 7 days under field conditions for two

reasons. Firstly, the movements become very small, and secondly, the disturbance caused by site traffic and

temperature changes assumes proportionately larger importance. Nevertheless, presented data on the long term

settlement of whole buildings is available and can be tracked by the same method, as shown in Figure 7 (Monadnock

Block, Chicago; Skempton et al). The similarity between the recorded data and the best fit linear-fractional curve

demonstrates that in this case even a single linear-fractional function can be applied reasonably well throughout the

recorded period of about 50 years.

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The parameters which form the basis of the time/deformation tracking system under constant load are shown with the

basic equation in the Appendix at the end of this paper.

6.2.1 IDEALISED DISPLACEMENT-TIME BEHAVIOUR OF PILES

A simplified model of the reactions of the pile/soil elements to an induced loading at the pile head is shown in Figure-

8. The particular aspect of interest is the response in time of each element to a change of load; the development of the

reaction of each element is described and discussed in the following paragraphs.

As a result of an application of a constant axial compressive load (P) at the pile head, it can be deduced that the pile

head displacement will instantaneously exhibit elastic shortening of the pile length corresponding to the portion of pile

shaft where little or no friction exists (Lo). The immediate change in pile head displacement will be directly

proportional to the elastic response of the friction free length and the change in load applied.

Application of a load increment to the top of a pile, will apply the same increment of force along the pile until someresistance is encountered. Initial resistance would normally be the top of the layer in which friction exists. The short

term response of the friction zone would generally be to exhibit a higher strength than its long term value, as a

consequence, practically none of the load increase is transferred immediately to the base. As the shaft resistance

decreases, any of the load applied which will not finally be carried in friction, would be transferred to the base.

IfPs . f(t) is assumed to describe the shaft resistance as mobilised in time, where Ps represents the long term shaft

friction, we can conclude that the time function f(t) must approach unity for large values of time (t), to give the long

term value ofPs.

0 12 24 36 48 60 72 840

2.4

4.8

7.2

9.6

12

14.4

16.8

19.2

21.6

24

TIME [Years]

Displacement [cm] RELATIVE DISPLACEMENT TIME DIAGRAM

MONA

DNOCKBLOCKSKEMPTON

etal(1955

)

Time offset = 5 years

Disp offset = 32.02 cm

Proj rel disp = 28.93 cm

Proj tot disp = 60.95 cm

Figure 7 Settlement-time recordings on Monadnock Block, Chicago; Skempton

et al (1955)

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In terms of load distribution, the load that may be

expected to be applied at the base of the pile can now

be expressed as P - Ps . f(t) or F.f(t), i.e. the load

transferred to the base must be whatever remains of

the total P applied which is not carried in friction and

in time will rise to an asymptotic value according tothe rate at which the excess load carried by the shaft

decreases.

IfPb .f(t) represents the base resistance as mobilised

in time under constant load, wheref(t) characterises

the individual base time component, then, if friction

exists along the length of the pile shaft, the load

transferred to the base is not constant and is a

function of the way in which the shaft resistance is

mobilised in time and therefore the response of the

base alone will be governed by two independent time

functions.

Since the load applied to the base, is governed by f(t),

it follows that the reaction from the base to the load

applied can be represented by F. f(t). Pb . f(t). The

combination of these two functions is manifest in the

displacement-time behaviour of the base. It may be

reasoned that the time constant associated with the mobilisation of the shaft, directly affects the rate at which the base

reaction is mobilised.

The above holds true whether the load applied is less than or exceeds the ultimate skin friction. It may be reasoned

that for loads less than the ultimate skin friction, a further time function may be needed to characterise the slower

development of friction along the pile length until it finds its long term equilibrium and distribution along the pile

length. It may be expected that the time function may be different according to the load applied. In contrast, for loads

applied which are above the ultimate skin friction, the time function characterising the transfer the load through to the

pile base may be expected to be constant. In practice, previous loading history distorts the time function exhibited.

Elastic shortening is dependent on the distribution of the applied load between the shaft and the base. Since the load

distribution is dependent only on the development of the shaft resistance, the influence of time on the elastic

shortening must follow the time component associated with the skin friction, with the final long-term elastic

deformation of the full pile length (Lo +Lf) occurring once the skin friction is fully mobilised.

Since the pile base deflection is linked to the pile head by just the elastic shortening, it may be reasoned that the

displacement-time response of any part of the pile to a new load must exhibit two time functions simultaneously. A

particular case is the displacement-time response of the pile head, often available for measurement.

Two important conclusions may be drawn:

1. The load applied to the base is a function of how the shaft resistance develops in time.

2. The displacement-time response of any part of a pile will exhibit the time constants associated with the

development of skin friction and end bearing.

Thus far, no particular shape or response characteristic has been associated with the time functions. If the development

of skin friction is considered, in the short term f(t) needs to give an enhanced value for frictional resistance, and

therefore the time function must start at a value greater than unity and tend towards an asymptote as time progresses.

This would be consistent with the Whitaker & Cooke (1966) data.

Figure-8 Components determining time effects

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A further point worthy of note is that the load applied need not be greater than the total shaft friction for the

elementary model described to be applicable. Each element of friction, associated with either each soil stratum or part

thereof, must also suffer the same phenomenon of gradually reducing its enhanced resistance and progressively

allowing a larger portion of the applied load to be transferred to greater depths. It can therefore be deduced that there

are practically always some elements contributing to the shaft friction which are fully mobilised, these are most

prevalent at the top of the pile. In practice, if the load applied is lower than the ultimate skin friction, the timeconstant for the development of the shaft becomes longer; and since the application of load to the base is affected by

the shaft time function, the base time function also takes longer to develop and reach a condition of equilibrium.

While enhanced friction is to be expected in the short term, it might not be obvious that this is manifest in the typical

gradual reduction of settlement rate normally experienced after application of a different load to a pile head.

To illustrate this point a function,

assumed to represent the displacement-

time characteristic is displayed in Figure

6-9. It is represented as a linear fractional

(hyperbolic) function of the form

rb

b

W t

T t=

+

........... (1)

The function tends towards a total

displacement, Wb, of 4 mm as shown; the

time scale is represented over

18 divisions, each representing a 1 hour

interval.

This displacement-time function

(equation 1) can be characterised by

quoting the asymptote and a point on the curve which may be arbitrarily chosen. If the time taken to get to 50% of theasymptotic value is selected Tb, in this case this is reached in 1 hour.

The differential of the function (1), represents the decreasing velocity in time, which in the illustration above is

indistinguishable from zero after approximately 10 hours. The function can be written as

( )

( ) ( )

r b b b

b

b b

bt

t T W W t

T t

W T

T t=

+ +

=+

2 2 .......................................(2)

A further differentiation of equation 2 can be performed. The result, shown in equation 3, is displayed graphically in

Figure 6-9, where the change of acceleration in time is represented by:

( )

2

2 32r b b

bt

W T

T t=

+......................................................(3)

This varying deceleration (deceleration is indicated by the minus sign) is directly proportional to a change in force,

since force = mass x acceleration. Since the mass of the pile (considered with or without a boundary soil layer)

remains constant, the change in acceleration may be attributable directly to a change in the forces involved in the

pile/soil system. Conveniently therefore, only the displacement-time function need be addressed when trying to model

the behaviour of a pile in time, and attempting to deal with the specific changes of forces in the pile-soil system with

time, can be circumvented.

The magnitude of the deceleration is a function of the difference between applied and reactive force. It is therefore

clear the importance of true constant application of load to reveal how reactive forces develop in time.

Figure 6-9 Displacement, velocity and acceleration variation

in time

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The following summarises the main issues which have been deduced by the authors from the displacement/time

analyses:

1. The time-displacement model appears to match measured field data from pile loading tests with high accuracy.

The model is a continuous function exhibiting no transitions from one particular mechanism to another. Thetime-displacement behaviour for the period typically recorded must be governed by a single or mechanism.

2. The characteristic behaviour of time-displacement under constant load for piles founded in clays and sands and

gravels appear to be so similar that any differences are imperceptible.

This similarity of behaviour of materials of vastly different permeabilities would lead to the conclusion thatpermeability cannot alone govern the time-displacement.

Since conventional consolidation theories are generally based on excess pore water pressure dissipation, it maybe concluded that this water expulsion effect only plays a secondary role and it is cncluded the mechanism is

creep.

3. The Timeset model employs two distinct time functions and the pile/soil interaction problem has two elements

which are likely to contribute in different ways to the displacement-time behaviour.

While it is difficult to obtain evidence of the single functions independently unless one of the contributingfunctions is effectively constant, each of the Timeset components can be consistently equated to mobilisation of

the shaft and base in time.

It is therefore apparent that the skin friction is not a passive component as might be assumed. As, for every loadincrement beyond the ultimate skin friction, it is apparent that the skin friction carries the additional load

temporarily and then, in time, returns to its ultimate capacity.

A mechanism which explains the short term, enhanced skin friction capacity, is sought.

A Viscous effect can be considered: however, the resistance typical of viscous drag is generally a function

of velocity. If the load applied is less than the ultimate capacity of the pile, end bearing is likely to bepartially mobilised and the displacement-time, and pile displacement continues and therefore so also

would the viscous effect.

Dilation has been explored and it appears to contain the range of necessary behaviour characteristicsbeing sought. It also provides an insight into a mechanism of rupture of skin friction, evident in some tests

on piles in a cohesive stratum with little end bearing.

6.3 ANALYSIS OF LOAD-DISPLACEMENT BEHAVIOUR

6.4 Load-settlement relationships

It had been suggested by several authors, including Professor Chin in the early 1970's, that both time/deformation and

load/deformation for piles followed hyperbolic (linear-fractional) laws. This observation had been made with regard

to the deformation of other structural members, going back into the 1930's and possibly earlier. Professor Southwell

in a lecture to the Royal Society in 1933 had drawn attention to the fact that steel struts behaved according to such

laws and several other workers in following years began to look for situations where the same rules might apply.

Many papers have appeared in geotechnics demonstrating that foundations have very nearly followed such laws but

usually not quite, and this led to doubts on the part of those who sought to use the simple single hyperbolic methods

for prediction or analysis.

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It has only recently been recognised that there is a very good reason for the non-compliance of many piles with such a

function. Piles in general exhibit shaft friction, end bearing, and elastic shortening. It would therefore be perverse of

the soil to behave in terms of a single linear-fractional function as pointed out by Fleming (1992) and a twin function

accounting for the two main aspects of behaviour, allied to a simple elastic shortening model, has therefore to be

applied. The results are remarkable and no pile has been found in over a thousand cases which fails to comply with

the discussed formulations in respect to time-deformation and load-deformation, unless the pile has suffered somephysical damage, disturbance or there has been some error in the testing process.

The parameters required to fully represent the load/settlement behaviour of a pile are given in the Appendix. A full

account of the development of the equations is given in the above reference and the method has been given the name

CEMSET. A version of the program used for back analysis is called CEMSOLVE.

6.5 The pile performance analysis process

An example analysis using the CEMSOLVE method is shown in Figure 10.

0 400 800 1200 1600 2000 2400 2800 3200 360032

28

24

20

16

12

8

4

0

LOAD [ kN ]

Displacement [mm] LOAD DISPLACEMENT DIAGRAM

DRIVEN

CASTIN-SIT

UIN

CHALK

CEMSOLVE

ANALYSIS

X = Input Data

Ds = .435 m

Db = .435 mUs = 1172 kN

Ub = 3021 kN

Lo = 5 m

Lf = 12 m

Ms = .001

Eb = 381169

Ec = 3E+07

Ke = .5

Base Elastic shortening

Figure 10 CEMSOLVE analysis

These models mean firstly that a long term load/settlement curve can be drawn from relatively short observation of

pile deformation under each constant load. Secondly, provided all work is carried out to high standards and that the

observed settlement has been sufficient to mobilise a reasonable proportion of the pile base resistance, then a back

analysis can be carried out to separate the main parameters.

Clearly if one had a full load/deformation curve one could form a set of simultaneous equations and solve for all the

variables. However, this would not be very sensible because some parameters may be important while others are not

and could be subject to large proportional errors if determined in this way. The procedure for analysis which has been

developed is therefore based on screen curve fitting and numerical optimisation, recognising that many of the

parameters are either closely bounded or known.

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For example, if one considers the parameter list for the CEMSET program as shown in the Appendix, the items Ds,

Db,Lo,Lfare geometrical dimensions andMs,Ec, Ke are mostly closely known (The first four items are known from on

site measurements, the Site Investigation and the installation records). Ec, the pile material modulus is generally

known or can be measured easily. Ke, representing the position of the centroid of friction in relation to the friction

length, is usually an insensitive number provided it is roughly correct.

Ms is a dimensionless number and has been found to lie normally within the limits 0.001 and 0.0015. This applies

throughout the whole range of experience in soils of all stiffness and it is suspected that it really is a constant in

naturally occurring soils which could only be determined with high accuracy by a series of instrumented pile tests in

different soils. Its constancy probably explains why trials indicate that it is not necessary to use iterative methods to

discover the exact value of the parameter Ke in most analyses. This leaves three important items which have

potentially wider ranges, though not unlimited. It is usually best to insert known median figures for all the other

parameters and then solve for the last three on screen. Semi automatic optimisation to match the model with the data

is built into the computer program for analysis but allowance is made for engineer intervention when necessary.

Having arrived at a solution it is then possible to carry out a sensitivity study in order to gauge the reliability and

accuracy of the solution.

Occasionally one finds anomalous pile behaviour which does not comply with any reasonable choice of parameter.

This usually enables one to search for and identify the anomalous item. Some examples of anomalous behaviour are

briefly indicated in Section 7 of this paper.

A consequence of this kind of analysis is that it demonstrates that instrumentation of piles and the recording of say

shaft loads and base loads, cannot reliably be carried out without the aid of a satisfactory behavioural model. The

ultimate base load is not just the last reading which a load cell showed.

7. NON LINEAR ANALYSIS CONSEQUENCES

7.1 General understanding

It is not until one works with good non-linear behavioural models that one comes to appreciate their benefits. There

are a number of features of pile behaviour which rapidly come to notice.

(a) Ultimate load definition

It will be recognised that the asymptotic definition of ultimate load is the only practical one that works. It was first

proposed by Terzaghi and many attempts have been made to supplant it with strain related definitions over the last

fifty years. When dealing with piles in a wide range of diameters and soils of varying stiffness, it is clear that neither

fixed settlement numbers nor "percentage of diameter values" can give consistent results.

The theory of plasticity on which most bearing capacity factors are based has in any case no strain related base and

implies an asymptotic failure definition.

(b) Validity of plotted points.

It is common to unload and re-load piles in the course of a test. It is noticeable that on return to a load which has

previously been applied, the projected deformation is not the same as that obtained from the first visit to that load. If

all the other sequentially determined points are also plotted, it will be observed that the point which is inconsistent is

that referring to the re-load. This is due to the second approach to that load being by a different stress path which has

been conditioned largely by shaft stress reversal.

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(c) Recovery from applied test loads.

Recovery from any test load is determined by the reversal or part reversal of the shaft friction and by the pile base

reaction. There appears to be a close relationship between recovery and the familiar process of hysteresis in other

materials (paper in preparation - M. England). Using a hysteresis linear-fractional model reasonable predictions of

pile recovery from load appear to be possible.

(d) The uniqueness of solutions.

It quickly becomes evident when solutions are attempted that if a pile has not been made to settle enough to mobilise

say 20 to 30% of the available base load, then unique solutions for this fraction of the load will be difficult to find and

will be very dependent on the precise accuracy of the records.

As a generality, reasonably good solutions for conventional piles carrying load mainly by friction can be arrived at

where the settlements are perhaps 15 to 20 mm. Where the pile rests on sand, then the load will usually be large in

end bearing and perhaps 20 to 40 mm settlement will be required in order to distinguish between the effects which are

separately due to base load resistance and base stiffness.

It is always possible and desirable to test the sensitivity of any solution against the credible range of parameters which

might possibly fit.

(e) Parametric observations.

It is found that there is a consistency of behaviour which is related both to the ground conditions and to the

construction method.

It will be noted that because of the way in which the data are processed, all results represent long term behaviour and

the deduced parameters have the same characteristic. This means that one can classify certain geological conditions as

likely to produce particular values of strength and stiffness.

Observation implies that the ratio of stiffness to strength is probably one of the most demonstrative ratios in the whole

of soil mechanics.

It also becomes apparent from many analyses that for very low strength and low stiffness soils, deformations required

to reach ultimate loads can be very large. On the other hand deformations in high strength high stiffness rocks can be

tiny and ultimate loads very high. In the former case large or very large factors of safety or partial factors would be

necessary to control movement of a supported structure within strict limits, while in the latter case one could have a

near unity factor of safety allied with negligible movement. This is not in accord with common factoring systems but

nevertheless needs to be considered.

7.2 Construction improvement.

The method makes clear that the history of pile installation and of the construction of adjacent piles can have

significant effects.

An important item which it has elucidated is the practice of base (and possibly shaft) grouting of piles. It shows clearly

that the process is akin to prestressing in general. A consequence is that it is unprofitable to grout the base of a pile

where the pile is shallow and there is little friction. Likewise grouting the base of a pile where the available base load

is small and the shaft friction is large is also unhelpful. By the use of the method it becomes possible to predict the

effects of the base grouting technique and reasonably to plan the grout pressures that may best be used.

Techniques of construction can be examined in regard to the variance of pile performance and there is much to be

learned about machine performance, operator variability, and the general nature of the construction process.

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This method of Partial Factoring has been in use now for 5 years within Kvaerner Cementation Foundations and we

would not wish to take the backward step of using the methods of EC7. It is very simple to use, asks the right

questions and appeals to our young engineers who appreciate its simple logic. It also has a continuing refining effect

on the general process of design.

7.4 TEST OPTIMISATION

It will be appreciated that to carry out high quality tests it is necessary to have careful regard to both

time/displacement and load/displacement behaviour.

Final settlement under a specific load cannot be determined without obtaining sufficient time/displacement data to

project the answer. If load stages are restricted in duration to half and hour or 1 hour, it will often be found that only

say 1/3 to 1/2 of the final settlement will have occurred in such a period and final settlement will be difficult to define.

This generally means that in maintained load tests, loads need to be held for several hours.

Various guidelines have been suggested for deciding the length of load holding periods (e.g. specific pile head

settlement rates) but the answer really is to obtain sufficient data to give a good and consistent fix on the final

settlement. Using computer monitoring techniques this is not difficult, and a method for exploiting the benefit of using

normalised time constants allows further reduction of the load durations.

It should be realised also that where instruments are installed within piles (for research purposes or otherwise) the

same considerations apply and that short term readings have a distinct possibility of being misleading because load

distribution has to adjust with time.

8. IDENTIFICATION OF ANOMALIES

As one might expect anomalies arise from time to time in analysis. These are almost always traceable to the conditionsunder which the particular pile was installed. Some examples are shown below for illustration of typical cases.

(1) Poor base construction on a bored pile.

The example shown in Figure 11 is one of a relatively short pile in stiff clay. There is no doubt that the base load of

the pile should have been over 200 kN, but the pile moved downward abruptly and there was no indication of any

response from its base.

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This pile was bored by a rotary drill and it is

speculated that on the final journey to unload

excavated spoil, some fell back into the base of the

bore. The moral is that short piles are particularly

prone to this sort of accident and that either special

base cleaning tools should be employed or the pileshould have been made longer to allow for the risks

associated with ensuring a properly constructed base.

In this case the pile specified working load was

360 kN and the requirement was for an overall Factor

of Safety of 2.

It might be thought that it is nearly impossible to find

a deficient pile base on a driven preformed pile, yet

such cases are to be found. They are traceable to

heave of clay soils overlying a sandy or rocky

founding layer. The effect is to unseat the pile by a

small amount and hence the potential performance is,at least initially, without end bearing. Heave is

particularly likely to cause such problems for

displacement piles in stiff clay and it is sometimes

necessary in design to give a pile sufficient penetration into the underlying founding stratum so that it is effectively

tied down.

(2) Hammer damage to a driven pile.

It is often not recognised that the driving of precast concrete piles requires careful consideration, especially in clay

soils or where a hard and resistant soil layer overlies one which is much softer.

It will be appreciated that end resistance to hammer impact in sand layers can be very high but if the pile suddenly

passes from the sand into a soft clay layer underneath and if the hammer drop is not smartly curtailed, then large

tensile reflections travel upward in the pile and can break it into a series of relatively short cracked sections.

Figure 14 shows a case of a precast pile driven into clay. The driving resistance was high and the pile length was 20

metres. However, quite stringent sets were being expected and a five tonne hammer was being used at about 0.5 m

drop.

0 100 200 300 400 500 600 700 800 900 1000 1100 120022

20

18

16

14

12

10

8

6

4

2

0

LOAD [ kN ]


OVER-DRIVEN

PRECASTPILEIN

CLAY

CEMSOLVE

ANALYSIS

X = Input Data

Xs = .27 m

Xb = .27 m

Us = 1128 kNUb = 1 kN

Lo = 10 mLf = 10 m

Ms = .0015

Eb = 50000

Ec = 1.24E+07

Ke = .5

Figure 14

0 100 200 300 400 500 600

22

20

18

16

14

12

10

8

6

4

2

0

LOAD [ kN ]


B

Pile-

StiffClay-PoorBase

CEMSOLVE

ANALYSIS

X = Input Data

Ds = .5 m

Db = .5 m

Us = 550 kN

Ub = 0 kN

Lo = 1 mLf = 9 m

Ms = .001

Eb = 1

Ec = 3.1E+07

Ke = .5

Figure 13

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developed by the pressure necessary to push the clayey material from between the blocks. Thus it is possible for the

rock to mimic the pile behaviour. Figure 16 illustrates the mechanism.

Figure 16 Blocky rock behaviour mechanism

(4) Inadequate design method or data.

A feature of the large number of analyses to date is that whereas many of the methods which we use are reasonably

reliable others frequently do not conform with expectation. Particularly it has been found that many of the sandy or

stony clays which are often treated as clays, in practice behave much more like sands. This is true of many, though notall, of the "boulder clays" of the United Kingdom. Often these same soils show stiffness values (Eb) which are also

very like sands.

Further, in regard to differences between sands and clays, the time constants used in the TIMESET analyses, are

frequently found to be not nearly as large as classical theory would lead one to expect. The time taken to carry out a

good quality test in sand is in practice not much different to that required in say a stiff clay.

(5) Shaft friction loss due to drilling disturbance.

It has been found that in sands, excessive rotation relative to penetration of continuous flight augers can cause

loosening of the soil and a noticeable loss of shaft friction. Additionally Dutch Cone Penetration tests have verified the

loosening effect.

These observations led to an attempt to model the digging mechanism used for this type of pile. The model has been

published (Fleming, 1995) and has been successfully applied in practice as a means of assessing the effects of the

various dimensional and other parameters.

In the United Kingdom and in many other countries common practice is to calculate shaft friction in sand on the basis

that unit shaft friction is reasonably represented by

f Ksu s v= tan

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where Ks an earth pressure coefficient, v is the mean effective vertical pressure in the sand layer and is the angle offriction between the pile and the soil which is usually taken as equal to the angle of internal friction of the sand inthe case of a continuous flight auger pile.

It has been found that the value ofKs can fall from a normal value of say 0.7 to about 0.3 depending on the control

exercised over the digging process, though it is of course also a function of the initial ground condition, the auger

diameter, its pitch and other matters.

9. CONCLUSION

The basis of much of our foundation design rests on the results of pile loading tests. In spite of many attempts to find

cheaper or simpler ways of interpreting pile behaviour, such as through dynamic and similar tests, there is no doubt

that the static Maintained Load Test remains the main source of direct and interpretable information. All tests which

do not account for rate effects in terms the overall framework of time - load - deformation give misleading results and

in the end only cause confusion.

The objective of this paper has been to show that there is a simple way of carrying out and interpreting tests and that it

depends on carrying out the work to high standards under computer control and not trying to hurry natural soil

behaviour. The linear-fractional function has been shown to be the most satisfactory for interpretive purposes and this

is proved now by a large amount of high grade evidence.

The effect of adopting a systematic computer based control and analysis system is far reaching and it opens a door to

understanding of real pile behaviour which has been hidden to date. Among the consequences one of the great

opportunities is to cast away crude factoring systems which have existed to date and which seem to be entering into the

Eurocode EC7.

It is believed that many of the findings which have resulted from the development of computer controlled testing and

analysis systems allow insights into the design and construction processes, as this paper seeks to illustrate, and willeventually lead to improvement of the whole basis of foundation work.

The cycle of Design - Installation Monitoring - Testing - Analysis - Design improvement is now a routine and we are

beginning to look like other industries where this cyclic improvement (Figure 15) is routine.

FIGURE 15

DESIGN INSTALLATION

DETAIL

ANALYSIS TEST

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APPENDIX:Extracts from published papers

Note: The definition of all ultimate states is asymptotic

TIMESET

Relative settlementat timetW t

T t

W t

T tr

s

s

b

b

: =+

++

Ws asymptotic value for shaft related deformation

Wb asymptotic value for base related deformation

Ts mobilization time for half shaft related deformation

Tb mobilization time for half base related deformation

t is time elapsed from application of load

CEMSET/CEMSOLVE

Ds Diameter pile shaftDb Diameter pile base

Us Ultimate pile shaft load

Ub Ultimate pile base load

Lo Length pile not frictional

Lf Length pile with frictionMs Shaft/soil flexibility factor

Eb Secant modulus base(25% u load)

Ec Pile material elastic modulus

Ke Equivalent column length/Lf

Applied shaft load at any load stage = PsApplied base load at any load stage = PbTotal applied load at any stage = PTTotal settlement corresponding to Load PT = t

a = Us

b =DbEbUbc =MsDsd = 0.6Ube =DbEbf = e PT - a e - b

g = d PT + e c PT - a d - b c

h = c d PT

( )tg g f h

f=

2 4

2

using positive root.

Total elastic shortening e is additive to settlement For applied loads (PT) up to Us

( )eT o e f

s c

P L K L

D E

=+42

For applied loads (PT) greater than Us

[ ( )es c

T o f f s e

D EP L L L U K = +

4 112

Total settlement for load PT is = t + e

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10. References

Goble, G. G, Rauche, F. and Likins, G.E., "The analysis of pile driving: A State of the Art", Int

Conf Stress Wave Theory on Piles", Stockholm, 1980.

Vyalov, S.S., "Rheological Fundamentals of Soil Mechanics", Elsevier Press, 1986

Chin, F.K., "Estimation of Ultimate load of piles not carried to failure", Proc. 2nd SE Asian

Conf on Soil Eng., 1970

Chin, F. K., The seepage theory of primary and secondary consolidation, 4th

Southeast Asian

Conference on Soil Engineering, April. 1975

Southwell, R, "On the analysis of experimental observations in problems of elastic stability",

Proc. Royal Soc., London, 1933

Kondner, R.L, "Hyperbolic stress-strain response; cohesive soils", ASCE Jnl SM & FE, Feb

1963

Skempton, A.W, Peck, R.B. and MacDonald, D.H, (1955) "Settlement analysis of six structures

in Chicago and London", Proc. I.C.E., Vol 4, pt1, p525

Fleming W.G.K.,"A new method for single pile settlement prediction and analysis",

Geotechnique, Vol XLII, No.3, Sept 1992

Fleming,W.G.K., "The improvement of pile performance by base grouting", Civil Engineering,

Thos. Telford, London, May 1993

Fleming,W.G.K., "Limit States and Partial Factors in Foundation Design" - Civil Engineering,

Thos. Telford, London, Nov 1992.

England, M.,"A method of analysis of stress induced displacement in soils with respect to time",

M. England, Deep Foundations on Auger Bored Piles Conference, Ghent, 1993

England, M. & Fleming, W.G.K, "Review of Foundation Testing Methods and Procedures" -

Geotechnical Engineering,Thos.Telford, July, 1994

Fleming,W.G.K., "The Understanding of Continuous Flight auger Piling, its monitoring and

control" , Geotechnical Engineering, July, 1995, Discussion Geotechnical Engineering,

October 1996

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