Pile Testing_xyz

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University of Sydney

Analysis and Design of Pile Foundations, 2007

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PILE TESTING

1. INTRODUCTION

In relatively recent times, pile testing has been revolutionised largely as a consequence of high

powered computers. Fifteen to twenty years ago, testing options were restricted to static loading

tests, with some costly and slow forms of integrity testing available. Now, a variety of tests are

available to estimate or measure pile resistance, together with numerous methods available for

quickly and economically testing piles for structural integrity.

This paper presents details on the pile testing regime used in Australia today. Attention will be

focussed on:

static loading tests

dynamic testing Statnamic testing

integrity testing

2. STATIC LOAD TESTING

This test simply involves application of a static load to a pile. Tests are performed for

compression, tension and lateral loadings. The load is most commonly applied via a jack acting

against a reaction beam that is restrained by an anchorage system (comprising cable anchors or

reaction piles), or by jacking up against a mass (kentledge). The load is usually measured by a

calibrated hydraulic jack (now illegal under AS2159-1995) or a load cell. Pile movements arenormally measured by dial gauges acting off simply supported reference beams. Typical static

pile testing setups for axial loading are shown in Fig 1.

Compression Tension

Fig 1

Typical static pile test setup


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Static load testing remains the most reliable form of testing of a pile or pile group. However, a

number of sources of error may occur if the test is not carried out in accordance with proper

procedures. The sources of error are well documented (Pile Foundation Analysis and Design,

Poulos and Davis, Wiley, 1980) and will not be elaborated on here. Suffice to say that the major

sources of error are associated with interaction effects between the pile and/or the anchoragesystem; and also the interaction between the measuring system and the kentledge. It should be

noted that these errors may be accounted for by reasonably rigorous analyses. It is most

important to note however, that the above errors are usually not of significance when common

sense and normal sound testing procedures, as outlined in AS2159-1995, are adhered to.

Static loading tests in Australia are usually limited to about 4000kN (compression loading) for

small diameter piles. Tests to significantly higher loads have been performed, but those are

unusual because of the high costs involved.

It is important to note that the overwhelming majority of tests performed in Australia are not

performed to determine the ultimate capacity of a pile. Most tests are done to prove that a pilewill satisfactorily support the design serviceability load plus some measure of overload to ensure

the pile has a satisfactory reserve capacity (load factor) above the serviceability load.

Acceptance criteria for pile performance is written into the Specification, or simply referred to as

a requirement to comply with the criteria imposed of AS2159-1995.

The major problems with Static loading tests are the time required to setup and do the test, and

the high costs involved. For these reasons, static load testing has reduced dramatically in recent

times, in favour of less expensive methods.

3. DYNAMIC PILE TESTING

Dynamic pile testing was introduced into Australia in 1982, to test large diameter bored piles

socketed into rock. The results of those tests, when compared with static loading tests performed

on the same piles, were in good agreement (see Fig 2).

Fig 2

Dynamic & Static Loading Tests on

Bored Piles

WEST GATE FREEWAY: CORRELATIO

CAPWAP PREDICTIONS WITH STATI

LOADING TEST RESULTS

0

0.5

1

1.5

2

2.5

3

0 1 2 3

Static Loading Test Result (MN)

CAPWAPPrediction(MN)

Correlation lin

-30%

+30%


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By the early 1990's, dynamic pile testing had become the predominant means of pile testing in

this country.

3.1 The Test

The purpose of dynamic pile load testing is to ESTIMATE the performance of a pile. In the

process, an estimate of the suitability of the pile to perform its design task, is made. It is most

important to appreciate that direct measurements of the pile load and associated movements are

notmade with the dynamic test. The resistance mobilised during the test is a prediction, as is the

subsequent load-movement behaviour, of the pile performance.

As with static load testing, dynamic tests are seldom done to determine the ultimate loads.

They are normally done to prove pile performance.

The equipment used for carrying out dynamic pile loading tests comprises the following:

At least two sets of strain gauges and accelerometers (bolted at diametrically

opposite faces of the pile).

Portable field computer to condition and collect the data, and to store the signals.

The impact for the test is usually provided by a piling hammer. This causes a stress wave to be

propagated down the pile, to reflect off the toe. The downwards travelling wave may be partially

or completely reflected by irregularities or discontinuities in the pile shaft, and by interaction

with the surrounding soil to produce upward travelling waves. The field computer receives the

measured signals of strains and accelerations, and these are integrated to produce force and

velocity results. A number of relationships are used to model the passage of upward and

downward travelling waves and it is from these relationships that a prediction of pile

performance is made. The predictions are made initially by PDA (Pile Driving

Analysis/Analyser)methods and should be confirmed by signal matching methods.

3.2 PDA

The PDA produces an instantaneous prediction of the resistance mobilised during the blow,

using methods such as the Case or TNO or Impedance methods, the differences between

the methods being shown in Fig 3. The most commonly used PDA method is the Case method.

The Case method basically assumes a model incorporating a spring and dashpot at the toe of the

pile. The shaft performance is not modelled with this method, so it is best suited to piles

deriving essentially all resistance from end bearing.

The magnitude of the mobilised load can be heavily dependent upon the soil damping factor (the

so-called J value) adopted by the operator. Mathematically, the expression for mobilised pile

resistance is given as follows:


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Rtotal = Rstatic+ Rdynamic

&

Rdynamic= J. vtoe where J = damping constant

vtoe = velocity of pile

So it can be seen that the J value may have an important influence on the magnitude of the pile

resistance predicted by the Case method.

The appropriate J value is often little better than an educated guess and should always be

correlated to static loading tests to produce the most reliable results. Under no circumstances

should pile testing for prediction of load resistance comprise PDA testing only. A minimum

requirement should be to perform more detailed analyses using signal matching techniques

(discussed later) and preferably, correlated with well executed static loading tests for greatest

reliability.

It should be noted that a prediction of the load distribution between the shaft and the pile toe cannot be made with PDA methods only.

Fig 3

PDA methods

A typical PDA output, as most frequently viewed by the operator, is shown in Fig 4.


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Fig 4. Typical PDA Screen

3.3 Signal Matching

Signal matching provides the most reliable means of predicting the performance of a pile tested

by dynamic methods. The pile and the soil data are modelled according to the best estimates

made by the operator performing the analysis, and a calculation is made using wave equation

methods. The calculated signals are displayed on the computer screen along with the measured

signals. The operator then performs a number of iterations, varying the input data until a

satisfactory match between the measured and calculated signals is obtained. Once a satisfactory

match is obtained, a plausible model of the pile-soil system is deemed to be established, and

from this, the mobilised static loading can be predicted. An example of a satisfactory signal

match is shown in Fig 5.

Fig 5: Results of signal matching analysis


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A further advantage of signal matching methods is that the distribution of the resistance of the

pile down the pile shaft and the pile toe is predicted.

A further sub-routine of the signal matching process permits a prediction of the static load-

movement performance to be made, and example of which is depicted in Fig 6.

Fig 6

Prediction of Load-Movement Performance

3.4 Monitored Results

The information that is collected and may be displayed during a dynamic test is most impressive,

and includes the following:

(i) Force: The impact forces imparted to the pile plotted against real time for

each gauge. Average forces are also shown.

(ii) Velocity: The velocity of the pile at the measuring level against timedetermined from each gauge. Average velocity is also shown.

(iii) Force and Velocity times Impedance:

A graph of the force imparted to the pile, plotted against the

product of the velocity times impedance (impedance being defined

as the product of the pile modulus and area, divided by the wave

speed), plotted against time. An example of a Force - Velocity

curve is shown in Fig 4 above. The characteristics of the curves

may provide valuable information to an experienced operator,

including an idea of the relative distribution of the shaft and toeresistance of the pile. The shape of the curves also provides


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information on the structural integrity of the pile, as shown in Fig

7.

(iv) Downward wave: The computed downward travelling stress wave plotted against

time.

(v) Upward wave: The computed upward travelling stress wave developed by

reflections at discontinuities (eg pile toe, mechanical joints, cracks)

and the interaction with the surrounding soil.

(vi) Total static and total dynamic resistance:

These are computed from the force-velocity curves.

(vii) Displacement: The displacement of a pile is shown for the period prior to impact

through the testing period, to residual displacement.

(viii) Energy transfer: The energy transfer from the impact, as calculated at the measuring

level, displayed against time.

(ix) Driving stresses: Compression and tension stresses can be continuously monitored

for every blow during driving if desired, to provide a check that

driving stresses do not become excessive so as to cause possible

structural damage to a pile. This is especially important for

concrete piles where tensile stresses are easily established during

driving through soft soils in particular.

All of the above items are stored automatically during the test, and may be displayed during the

test, with selected items being presented for reporting purposes.

Sound pile

Damaged pile

Fig 7: Typical Force and Velocity times Impedance Curves showing damaged pile.

3.5 Comments

The following selected items may assist in clarifying a number of misconceptions relating to theperformance and subsequent analyses of dynamic pile loading tests.


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3.5.1 Accuracy

The accuracy of dynamic testing, when compared with static loading tests performed on the

same pile have been reported at numerous venues to be unerringly accurate; often within a few

percent, thus demonstrating that exceptional results can be achieved. However, a series of wellconducted comparative tests performed during various contests both locally (eg 4th ANZ

Conference) and internationally (Brussels, 5th Stress Wave Conference) provide not so glowing

results. The results of such a contest, held at the 4th Stress Wave Conference are shown in Fig 8

below.

Fig 8

Results of Prediction Exercise - 4th Stress Wave Conference

Fig 8 shows large discrepancies in the pile performance as predicted from dynamic tests and the

actual static test results, but it is emphasised that the majority of participants managed to be in

reasonable agreement with their predictions.

Results such as the above tend to be less convincing than the glowing reports that have been

issued by us practitioners in the past, but overall it can be concluded that dynamic pile loading

tests can usually predict the static test result within an order of accuracy of around 10% to 25%,

which should be regarded as being acceptable for geotechnical work. Also, there appears to be

no doubt that the order of accuracy increases for tests performed on preformed piles incomparison to cast insitu piles. Notwithstanding, good results have been reported for all pile

types.

When the above-mentioned accuracy is put into context with the comparison of the costs of

dynamic pile load testing to static pile load testing, the value of dynamic testing should become

immediately apparent.

3.5.2 Operator Error

Contrary to claims made by some practitioners with the introduction of dynamic pile loading

tests, the solution/prediction obtained upon completion of the signal matching process is by nomeans unique. Different operators achieve different results. Fellenius (1988), in a study that


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involved 18 participating CAPWAP practitioners evaluating the same data for preformed piles,

showed standard deviations of between 5% and 14% for the four piles studied.

Personal experience of the author suggests that a greater variability can be expected for bothpreformed and cast insitu piles.

It is clear then, that different operators will produce differing answers. There is no one unique

answer to any set of monitored data, but generally, experienced operators will producereasonably similar predictions given the same set of data.

3.5.3 PDA Results vs Signal Matching

There appears to be two schools of thought with respect to how many pile should be subjected to

analyses involving detailed signal matching on any specific project.

Probably the most common practice, nationally and internationally, is to test a proportion

(typically 5 to 10%) of piles on a project and a representative number of these selected for the

more rigorous CAPWAP/TNOWAVE analyses. It should be noted that the more piles tested and

subjected to signal matching, the greater the confidence in the piles and hence higher reduction

factors can be incorporated into the design or the required proving load. This is clearly

highlighted in AS2159-1995.

Upon completion of these analyses, the average soil characteristics via the J value, are then

applied to all other piles tested by PDA methods only, to come up with a more reliable estimate

of the mobilised pile capacity than that indicated from raw PDA methods only. The intention of

this practice is simply to test a large number of piles; establish an average soil characteristicappropriate for the site/area and to use this value to predict the performance of all piles. The

objective of restricting the number of CAPWAP/TNOWAVE analyses is quite simply to reduce

costs, ostensibly without compromising technical standards.

An alternative practice, particularly promoted by the TNO organisation of The Netherlands, is to

subject all piles to the more rigorous signal matching procedure. The philosophy adopted in

promoting this practice is to obtain the highest possible degree of confidence in the results, at a

relatively low cost. In further support of this argument, it can be stated that once an accurate

signal match has been obtained for one pile, relatively little work has to be done to obtain

satisfactory matches for other piles tested.

Many practitioner argue as to the correct practice that should be adopted. It is probably best

left to the designer and the testing authority to arrive at an acceptable testing regime best suited

for the project, prior to testing and subsequent analyses.

Under no circumstances however, should piles be tested by PDA methods only, as significant

errors in mobilised pile resistances may result. It is not sufficient to adopt J values from the

literature as being sacrosanct, for considerable departures from the published values are

common. A minimum requirement should be to perform a representative number of signal

matches to obtain a representative J value. A preferred alternative is to correlate dynamic test

results with a well executed static loading test.


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3.5.4 Mobilised vs Ultimate Load

One of the major problems with dynamic pile load testing is that hammer blows of insufficient

energy might be used to predict/prove the load capacity of the pile. Most dynamic tests areused as a means of providing a satisfactory proof load, not to predict the ultimate load. As a

consequence, sufficient energy is seldom imparted to the pile to produce geotechnical pile

failure, nor is it required.

Many engineers appear to have difficulty in accepting the concept of mobilised load. Put

simply, a light tap from a hammer will move the pile toe nil to a negligibly small distance, and

will not then mobilise anywhere near the available geotechnical resistance. With a heavy blow,

the pile toe can be made to move a greater distance and hence realising a higher load as a

consequence of mobilising more end-bearing resistance. In short, if the pile toe can not be made

to penetrate the end-bearing layer, then only a proportion of the maximum available pile

resistance will be mobilised. It stands to reason then, that if a pile is founded on strong rock anda nil set is registered with an appropriate blow from an appropriate hammer, then the pile will

fail structurally before reaching the maximum available geotechnical support.

As a general guide, a 6 tonne and 8 tonne hydraulic hammer can be relied upon to mobilise

around 3000kN and 4000kN respectively for precast piles commonly used in the Australian

piling market. A 20 tonne drop hammer has been used to mobilise almost 30MN!

3.5.5 Time effects

The resistance of a driven pile usually exhibits some form of set up or increase in capacity,

with time. Usually these set up effects are the consequences of the dissipation of pore waterpressures and can be most dramatic. In clay soils, increases in capacity of 2 to 6 greater than that

registered upon completion of driving, have been obtained. Even in sands, increases of up to

70% have been reported.

These aspects are most important for dynamic pile load testing. If the magnitude of proof

loading is required to prove a maximum load factor, then clearly, dynamic testing should not be

performed until an appropriate time delay and the test is referred to as a restrike test. Often a

restrike test is performed 1 to 2 days after completion of driving, but obviously this time can

be varied to suit site circumstances. The practice of firstly mobilising a pile by subjecting it to a

number of blows prior to restrike testing should not be followed, as this directly conflicts with

the objective of performing the restrike test.

Testing performed at the completion of driving can usually provide better information relating to

the potential end-bearing resistance. Restrike testing, which incorporates set up effects, will

provide a better indication of maximum available shaft friction. It is acceptable to assume that

the maximum mobilised pile resistance is the sum of the shaft resistance determined from

restrike testing, and the end-bearing resistance as obtained from the end of drive conditions.

Some specifications require that dynamic tests performed at the completion of driving and with a

restrike test to be carried out at a nominated time (typically not less than hours) later. The value

of such testing may be insufficient for set up effects of any significance to be established. Often

an indication of the set up effects can be gauged simply by comparing the traditional set at the

end of drive and at restrike testing.


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It should be obvious, but nonetheless needs to be stated, that long term effects such as creep can

not be assessed by dynamic testing methods.

3.5.6 Driving stresses

One of the major advantages offered by dynamic testing is that driving stresses may be

continually monitored, hence providing a control measure to ensure that piles are not damaged

through over driving. This is true in the overwhelming number of tests performed. However,personal communication between the author and a number of practitioners indicates that most

practitioners have reported that tension stresses as indicated by the PDA are not always correct.

It has been reported that when driving reinforced concrete piles onto rock, unrealistically high

(>10MPA) tensile stresses have been indicated by the PDA. That magnitude of tensile stress, if

real, would result in pile damage which would be detected by the shape of the Force-Velocity

curve. This phenomenon, whilst not being infrequent, is not what might be termed as being a

common occurrence.

3.5.7 Youngs Modulus

The PDA program requires the value of Youngs Modulus to be input, from which the computer

estimates the Forces in the pile using conventional stress - strain relationships. The value of

Youngs Modulus, if over-estimated, will result in higher forces and hence an over-prediction of

the pile resistance.

Correct values of Youngs Modulus can be deduced during signal matching, where the average

values can be determined by conventional stress wave mechanics.

3.5.8 Impact equipment

Dynamic pile load testing is a relatively complex task, but as with all technical data, the accuracy

of the predictions is very much dependent upon the quality of the data obtained during the tests.

Dynamic testing equipment, regardless of the brand name used, is of comparable high quality.

It is most important then, that the energy delivered to the pile is delivered by a hammer that has a

high degree of control. It is not satisfactory to believe that any mass, dropped from a nominated

height, will produce good signals. A piling frame, because it can be manoeuvred to reduce force

eccentricities as measured by the gauges, is preferred. This will result in higher quality signals,

which in turn will provide operators a better chance of making higher quality predictions and

help to reduce uncertainties that may otherwise result.

3.5.9 Concluding statements

The dynamic pile loading test procedure has been outlined in simplistic terms which hopefully

will provide those who have not had direct experience with the test, some insight of the value of

the test.

Dynamic testing has been a rapidly evolving field that has justifiably established itself as a cost

effective test that can produce results of sufficient accuracy to be regarded as a valuable means

of testing piles.

However, it is the authors opinion that the test enjoys a greater reputation for precision than can


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be reliably produced for the all cases in the field. The test enables estimates of sufficient

accuracy to be regarded as an invaluable asset to the construction industry, particularly when the

low costs of the test are considered.

4. PILE INTEGRITY TESTING

As mentioned above, dynamic testing provides an indication of the structural integrity of a pile.

As this requires a large hammer to provide the impact, relatively large pile movements result.

For this reason the test is referred to as a high strain test. The more common means of testingpiles for integrity only is termed a low strain test and this is elaborated on in this section.

The sonic pile integrity test is a non-destructive test that quickly and economically checks the

structural integrity of a pile shaft. The test can be used on cast insitu piles and preformed driven

piles (concrete, steel, timber). The test does not, and cannot, give any information on the

load capacity of the pile.

4.1 The equipment

Testing equipment comprises a field computer, hand-held transducer and a plastic mallet. The

equipment is robust and portable (fitting into one briefcase) and requires only one person to carry

out the tests.

4.2 Basic principles

The hammer blow induces a stress wave which travels down the pile shaft as a packet of energy,

reflects off the toe and is registered by the transducer at the surface as a toe reflex (Fig. 6).

If the pile material is homogeneous, the wave will travel at a generally constant velocity. The

time taken between the hammer blow and the wave to travel down, then up the pile shaft will be:

t = 2 L / c where t = time (ms)

L = pile length (m)

c = wave speed (m/s)

The wave speed c for say concrete is dependent upon concrete quality as follows:

c = ( E/) where c = wave speed (m/s)

E = Youngs Modulus (N/m2) = density (kg/m3)

It is well known that the density of poor concrete is about the same as that for high quality

concrete. However, Youngs Modulus for poor quality concrete is much lower than for high

quality concrete, and this is reflected by the wave speed. In many cases the pile length is known

with a reasonably high degree of certainty. The operator performing the test then adjusts the

wave speed until the pile length and the toe reflex correspond. In this way, the concrete

consistency is indirectly indicated.


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Fig. 9. Stress Wave Propagation for Intact Pile

The stress wave paths for a pile with a reduced cross section (neck) are shown in Fig. 7 below.Here, the stress paths are more complex, with waves being reflected off the necked section

eventually meeting waves reflecting off the full cross section. The depth to the neck is calculated

by the computer knowing the time for the wave to travel down to, then up from, the neck and

also the wave speed.

Fig. 10. Stress Wave propagation for Necked Pile

Reflectograms are displayed as velocity versus pile depth. The shape of the reflectogram

provides a qualitative indication of major pile discontinuities.

Reflections of stress waves occur not only at the locations of pile discontinuities, but also at the

boundary of soil layers. A soft soil may, for example, produce a reflection similar to a pile neck.

It is essential therefore, that the operator be provided with all available information, which

includes the following items:

piling records showing nominal pile depths and geometry

construction details (eg lengths of temporary casing, concrete consumption, piledriving records.

soils details


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structural details (eg concrete strengths, reinforcement details)

any unusual occurrences during construction (eg delays during concreting)

4.2 Soil Effects

Shaft friction usually plays a major role in pile performance. It also plays an important role in

pile integrity testing. Together with internal damping characteristics of the pile material, shaft

friction dampens or decreases the magnitude of the signal. This reduction in amplitude depends

on the pile length, pile type, soil type and consistency and the length to diameter ratio of the pile.

A practical on length for sonic pile integrity testing is about 50 diameters, although in stronger

soils a limit of 30 diameters may be appropriate.

4.3 Phenomena Detected

reflections from the toe (in most instances)

reflections from significant inclusions ( 5 to 10% of pile diameter)

reflections from horizontal cracks

reflections from joints (as for precast concrete piles)

reflections from increases and decreases in cross sections

reflections from changes in soil layers

reflections from changes in material properties

4.4 Phenomena NOT Detectable

gradual increases or decreases in cross section

curved forms

small inclusions of foreign materials

local loss of cover

debris at the toe of the pile

cracks parallel to the pile axis

4.5 The Test

The test is performed by pressing the transducer on the pile head and hitting the pile head with a

sharp blow from the plastic mallet. The induced stress wave travel down, then up the pile shaft,

to be registered on the screen of the field computer. The signals (reflectograms) are reflected

off discontinuities in the pile, such as necks, cracks, enlargements, etc. Once satisfactory signals

have been obtained, they are stored in the internal memory of the computer, from which they can

be down loaded onto a PC for further enhancement and reporting at a later stage.

The signals are recorded either in the time domain (eg as used by TNO, Pile dynamics, IFCO).

Testing authorities using this method are Ground Engineering, Franki and Wagstaff Piling). The

alternative method records signals in the frequency domain (eg using equipment developed

initially by CEBTP, France), as used by Pile Test International, Vibropile and Integrity Testing.

Recording in the time or frequency domain produces similar results, with one method having no


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major advantage over the other, although it could be argued that measuring in the frequency

domain provides and indication of the pile head stiffness, which is basically the limit of elastic

stiffness. Pile head stiffness is not to be confused with static load capacity. The value of Pile

Head stiffness is to compare numerical values of different piles. Obviously, piles having

significantly lower values require further investigation. The following presentation is orientedtowards results produced in the time domain, for the sake of brevity.

An experienced operator can, in most cases, provide an immediate on-site interpretation of the

test result. The test usually takes between 1 and 5 minutes per pile, so production rates of 100 to300 piles tested in one day are theoretically achievable in ideal conditions.

It is a wise policy to test a number of piles on a project, so that reflectograms which differ from

the norm, may be targeted for further investigation.

Requirements for testing are simple and oriented towards achievement of high quality signals

necessary for interpretation. Firstly, access to the heads of the piles tested should be such as toallow a hammer blow to be delivered, preferably without impediments such as spiral wire or pile

cap reinforcing cages. Cracks or voids in concrete under the transducer or hammer impact

locations can produce false signals, so the surface of the pile should be trimmed back to sound

material and be free of water or other debris. No surface grinding or other special treatment is

required.

Best results are achieved when tests are carried out as soon as possible after installation. For

driven piles this means that piles should preferably tested immediately after driving, when shaft

friction will be minimal. Cast insitu piles can not be tested until the concrete or grout is cured to

a sufficient degree (about 80% of its ultimate strength), which is normally reached 5 to 14 days

after installation.

At this stage, results of tests on piles which have been cast into a pile cap or floor slab have

usually not been successful. So where possible, access to the head of the pile is desirable.

4.6 Typical Results

The results of a reflectogram for an intact pile is shown in Fig. 11. This cast insitu pile was

constructed in soils comprising 6m of soft clays overlying very stiff clays and medium sands.

The reflectogram clearly shows the influence of the soil strata. The pile toe is visible with a wave

speed of 3900m/s, which is typical of that expected for high quality concrete. No shaft defects

are indicated.

Fig 11. Reflectogram of a sound pile


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By contrast, Fig. 12shows a reflectogram of a 600mm diameter bored pile that did not exhibit

the characteristics expected for the pile in the given soil conditions.

Fig 12. Reflectogram of an unsound pile

Soil details for the pile depicted in Fig 12 were understood to comprise 2 to 3m of loose or soft

soils overlying stiff clays, in turn overlying weathered rock at 7 to 8m depth. Temporary casing

was used during construction. During the concreting process, some piles were reported to be

making water and concrete was discharged directly into the excavation. The reflectogram

indicates a change in impedance at 2.7m, consistent with a change in soil conditions or reduced

pile diameter. A further change in impedance occurred below 4m, consistent with a reduced

diameter or loss of concrete consistency. The pile toe is visible at 7.5m depth with a wave speed

of 2200m/s only, which is not typical of that expected for good quality concrete. Subsequent

coring of the pile proved the concrete to be porous. Below 4m depth, clay inclusions of up to

100mm were found down the length of the pile shaft. The total pile length was found to be 7.4m.

The pile was rejected and replaced.

4.7 Advantages of Sonic Pile Integrity Testing

tests are performed quickly and economically.

an immediate indication of pile integrity may be provided, permitting

immediate rectification work to be carried whilst piling equipment is still

on site, thus eliminating costly re-mobilisation costs and delays to the

project.

no special treatment is required to prepare pile surfaces prior to testing.

Software is now available to assess the influence of defective piles on the

pile group, thus providing an indication of the extent of desirable remedial

works, if any.

4.8 Future Directions

Quantification of pile defects, using computer simulation techniques, are available. A new

development is that Coffey Geosciences have undertaken some Research and Development toassess the influence that defective piles may have on the performance of single piles or pile

groups.


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dramatic than conventional static load testing, the procedure is safe and quiet. Comparative tests

on piles subjected to Statnamic testing and conventional static loading tests have provided very

good agreement in load-settlement performance. Statnamic devices are now available for

routine testing of piles to loads in excess of 3000 tonnes and the technology exists to increase

loadings well beyond 3000 tonnes. It is understood that a device that would enable test loadingto 6000 tonnes is under consideration for manufacture at present. Tests have been conducted in

the UK, USA, Canada, Malaysia, China, Japan, Korea, Indonesia, Germany, Israel, and now,

Australia.

5.2 Typical results

Results from the Statnamic tests are immediately visible on computer screen during the test, and

stored for subsequent reporting purposes. Results from three tests are shown, as reproduced

from the field data, are shown in Fig 15. For the tests shown, the pile working loads were

around 5.2MN, thus all piles were effectively proof loaded to a load factor of about 3.

Fig 15

Statnamic Test results, Quay West Project, Melbourne.

5.3 Advantages of Statnamic testing

The advantages of Statnamic testing include the following:

the test is quick and easily mobilised.

pile performance is measured cost-effectively

high loading capacity is available

the system is flexible and adaptable eg single piles or pile groups can be tested for


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compression loading and also lateral loading characteristics.

the test is quasi-static, and does not produce harmful compression and tension stresses thathave the potential of damaging a pile.

$ Statnamic test results are in good agreement with static loading test results, particularlyfor piles founded in stiff soils or rock.

the test can be used not only for testing pile foundations, but also to confirm the bearingcapacity of soils or rock suited to pad footings, thus enabling optimisation of footingdesign.

RELATIVE MERITS OF VARIOUS METHODS OF LOAD TESTING

Static load testing remains the definitive testing method, with the development of all of the more

modern methods resulting in comprehensive comparative testing with static methods required

to gain acceptance in the engineering community.

However, static load testing is slow and expensive and generally being suitable for relatively lowloads. It is for this reason that other methods such as dynamic and Statnamic have been devised

and accepted. It may be of use to compare the three main methods of testing, which is discussed

below.

Fig. 16 summarises the three main methods schematically.

Fig. 16

Major load testing methods


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The resultant phenomena associated with the tests are shown in Fig. 17.

Fig 17.

Phenomena resulting from various test methods.The following observations become apparent:

$ the load for Statnamic testing is significantly longer than for dynamic testing, typically being

10 to 15 times longer. For this reason, it is closer to modelling static load performance.

$ the stresses resulting from the tests are similar for static and Statnamic testing, whereas stresswave phenomena result from dynamic testing.

$ velocity effects in dynamic testing vary down the length of the pile; are relatively constant

during Statnamic testing; and non-existent during static testing.

$ resultant displacements during dynamic testing vary down the length of the pile, but are

relatively constant during static and Statnamic testing

In short, the static test is closely simulated by Statnamic testing, especially for piles founded on

an end-bearing stratum compared with dynamic testing.

Poulos (Pile testing - from the designers viewpoint, 2nd Statnamic Seminar, Tokyo, 1998)

Comparison Load TestsLOAD

TIME

DEPTH

H

DLT STN

DLT

STN

SLT

v

v

v

u

u

u

z z z

z z z

z z z


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tabulated a summary of the capabilities of various forms of load testing, which are reproduced

below.

Table 1. Summary of capabilities of various pile load tests with respect to the results

obtained.Test

TypeUlt.

Axial

Geot.

Capacity

Ult.

Lateral

Geot.

Capacity

Load-

settlmtLateral

deflnGroup

EffectsStruct.

Capacity

&

Integrity

Special

Load-

ings

Ground

Movs.

Static

Uninstrumented

3 0 3 0 1 1 1 0

Static

Instrumented

3 0 3 0 2 2 2 2

Static Lateral 0 3 0 3 1 2 2 0

Dynamic (PDA) 3 0 2 0 0 3 1 0

Osterberg Cell 3 0 2 0 0 1 1 0

Statnamic

Uninstrumented

3 2 2 2 2 1-2 1-2 0

Statnamic

Instrumented

2 2 2 2 2 2-3 2 1

Legend:

3 = very suitable; 2 = may be suitable under some circumstances; 1= possible but unlikely to be suitable; 0 = not

suitable

Table 2. Summary of Various Pile Load Tests with Respect to the Accuracy and Relevance

of the Results

Test

TypePile Loaded in

Same Way?Additional

Stress Changes

(Side effects)

Accuracy of

Movement

Measurement

Accuracy of

Load

Measurement

Similar

Duration of

Loading to

Prototype?

Static

Uninstrumented

3 2 2 3 3

Static

Instrumented

3 2 2 3 3

Static Lateral 3 2 2 3 3

Dynamic (PDA) 3 2 1 1 1

Osterberg Cell 2 2 2 3 3

Statnamic 3 3 3 3 2

Legend:

3 = good; 2 = may be adequate; 1 = generally not good

This paper has not presented information on Osterberg Cell testing as this is not a test that has


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been used to any great extent in Australia.

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