The Foundation Engineering Handbook Chapter 9

The Foundation Engineering Handbook Chapter 9 Geotechnical earthquake engineering can be defined as that subspecialty within the field of geotechnical engineering that deals with the design and construction of projects in order to resist the effects of earthquakes. Geotechnical earthquake engineering requires an understanding of basic geotechnical principles as well as an understanding of geology, seismology, and earthquake engineering. In a broad sense, seismology can be defined as the study of earthquakes. This would include the internal behavior of the earth and the nature of seismic waves generated by the earthquake.

9.2 Construction Techniques Used in Pile Installation 365

9.4.2 Interpretation of Pile-Driving Analyzer Records 3759.4.3 Analytical Determination of the Pile Capacity 378

9.5 Comparison of Pile-Driving Formulae and Wave-Equation Analysis Using thePDA Method

384

9.8 Rapid Load Test (Statnamic Pile Load Test) 400

9.8.3.5 Segmental Statnamic and Derived Static Forces 407

9.10 Finite Element Modeling of Pile Load Tests 411

Page 364

9.13 Use of Piles in Foundation Stabilization 422

Page 365

FIGURE 9.2

Cast-in situ piling (Fromwww.gdonalcom With permission.)

ences, the degree of uncertainty regarding the actual working capacity of a pile foundation isgenerally much higher than that of a shallow footing Hence, geotechnical engineers

constantly seek more and more effective techniques of monitoring pile construction to

estimate as accurately as possible the ultimate field capacity of piles

In addition, pile construction engineers and contractors are also interested in innovativemonitoring methods that would reveal information leading to (1) on-site determination of pilecapacity as driving proceeds, (2) distribution of pile load between the shaft and the tip, (3)detection of possible pile or driving equipment damage, and (4) selection of effective drivingtechniques and equipment

9.2 Construction Techniques Used in Pile Installation

FIGURE 9.3

(a) Jetted piles.(From www.state.dot.nc.us.Withpermission ) (b) Preaugared concrete pile (From

www.iceusa.com With permission.)

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9.2.2 In Situ Casting

When the subsurface soil layers are relatively strong, it is common to install significantlylarge-diameter piles and using boring techniques For caissons, this is the only viable

installation method (Chapter 7) Depending on the collapsibility of the soils and availability of

casings, in situ casting can be performed with or without casings In cases where casing is

desired, drilling mud (such as bentonite) is an economic alternative More construction details

of cast-in situ piles are found in Bowles

9.2.3 Jetting and Preaugering

Although driven piles are installed in the ground mostly by impact driving, jetting or

preaugering can be used as aids when hard soil strata are encountered above the estimated tipelevation required to obtain adequate bearing However, the final set is usually achieved byimpact driving the last few meters, an exercise that somewhat restores the possible loss ofaxial load bearing capacity due to jetting or preaugering Nonetheless, it has been reported(Tsinker, 1988) that impact-driven piles have better load bearing characteristics than jetted-driven piles under comparable soil conditions This is possible due to the soil in the

immediate neighborhood first liquefying as a result of the excessive jet water velocity and

subsequently remolding with the dissipation of excess pore pressure The original in situ soil

structure and the skin-friction characteristics are significantly altered During the jettingprocess, some water also infiltrates onto the neighborhood maintaining a high pore pressurethere Thus, the creation of liquefaction and filtration zones, known as the zone of combinedinfluence of jetting, is expected to result in a reduction of the lateral load capacity

Consequently, although pile jetting may be effective as a penetration aid to impact driving insaving time and energy, the accompanying reduction in the lateral load capacity will be asignificant limitation of the technique Similar inferences can be made regarding preaugering

as well

9.3 Verification of Pile Capacity

There are several methods available to determine the static capacity of piles The commonlyused methods are (1) use of pile-driving formulae, (2) analysis using the wave equation, and(3) full-scale load tests A brief description of the first two methods will be provided in thenext two subsections

9.3.1 Use of Pile-Driving Equations

In the case of driven piles, one of the very early methods available to determine the loadcapacity was the use of pile-driving equations Hiley, Dutch, Danish, Janbu, Gates, and

modified Gates are some of pile-driving formulae available for use For more information onthese, the reader is referred to Bowles (1995) and Das (2002) Of these equations, one of theformulae most popular ones is the engineering news record (ENR) equation, that expressesthe pile capacity as follows:

(9.1)

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where n is the coefficient of restitution between the hammer and the pile (<0.5 and >0.25), Wh

is the weight of the hammer, WPis the weight of the pile, s is the pile set per blow (in inches),

C is a constant (0.1 in.), Eh=Wh(h), h is the hammer fall, and ehis the hammer efficiency(usually estimated by monitoring the free fall)

It is seen how one can use Equation (9.1) to compute the instant capacity developed at any

given stage of driving by knowing the pile set (s), which is usually computed by the reciprocal

of the number of blows per inch of driving It must be noted that when driving has reached a

stage where more than ten blows are needed for penetration of 1 in (s=0.1 or at “refusal”),

further driving is not recommended to avoid damage to the pile and the equipment

Example 9.1

(This example is solved in British units Hence, please refer toTable 7.9 for appropriateconversion to SI units.) Develop a pile capacity versus set criterion for driving a 30 ft concretepile of 10 in diameter using a hammer with a stroke of 1 ft and a ram weighing 30 kips

(kilopounds)

The weight of the concrete pile=¼ π(10/12)2(30)(150)(0.001) kips=2.45 kips

Assume the following parameters:

n=0.3

Hammer efficiency=50%

Substituting in Equation (9.1),

9.3.2 Use of the Wave Equation

With the advent of modern computers, the use of the wave-equation method for pile analysis,introduced by Smith (1960), became popular Smith’s idealization of a driven pile is

elaborated in Figure 9.4

The governing equation for wave propagation can be written as follows:

(9.2)

where ρis the mass density of the pile, E is the elastic modulus APis the area of cross section

of the pile, u is the particle displacement, t is the time, z is the coordinate axis along the pile and R(z) is the resistance offered by any pile slice, dz.

The above equation can be transformed into the finite-difference form to express the

displacement (D), the force (F), and the velocity (υ ), respectively, of a pile element i at time t

as follows:

D(i, t)=D(i, t−Δt)+V(i, t−Δt)

(9.3)

Δz=selected pile segment size at which computation is performed along the pile length.

Idealization of soil resistance In Smith’s (1960) model, the point resistance and the skin

friction of the pile are assumed to be viscoelastic and perfectly plastic in nature Therefore,the separate resistance components can be expressed by the following equations:

where PpDand PsDare static resistances at a displacement of D, V Pis the velocity of the pile,

and J and J' are damping factors corresponding to the pile tip and the shaft.

The assumed elastic, perfectly plastic characteristics of P pD and P sDare illustrated inFigure9.5

FIGURE 9.5

Assumed viscoelastic perfectly plastic behavior of soil resistance.

In implementing this method, the user must assume a magnitude for the total resistance (Pu), a

suitable distribution (or ratio) of the resistance between the skin friction and point resistance

(PpDand PsD), the quake (Q inFigure 9.5), and damping factors J and J' Then, by using

Equations (9.3)–(9.5), the pile set (s) can be determined By repeating this procedure for other

trial values of Pu, a useful curve between Puand s (such as inExample 9.1), which can be

eventually used to determine the resistance at any given set s, can be obtained.

The above system of equations ((9.3)–(9.5)) can be easily solved using a simple worksheetprogram, and the total static resistance to the pile movement during driving can be obtained.There are many commercially available wave-equation programs, such as GRLWEAP (Gobleand Raushe, 1986), TTI and TNOWAVE, that are available for this purpose However, thereliability of the above method depends on the estimation of soil damping constant along the

pile shaft (J'), soil damping constant at the pile toe (J), soil quake along the pile shaft (Qs),

soil quake at the pile toe (Qp), and the proportion of the force taken by pile toe (ξ) Smith

(1960) suggested that 2.5mm (or 0.1 in.) is a reasonable assumption for the skin quake (Qs)

and later it was suggested to take that the end quake at the pile bottom (Qp) as B/120 where B

is the pile diameter.Table 9.1shows the range of the skin damping constants used for

different soil types

Example 9.2

(This example is solved in the British system of units Hence, please refer to Table 7.9forappropriate conversion to SI units.) For simplicity, assume that a model pile is driven into theground using a 1000 Ib hammer dropping 1 ft, as shown inFigure 9.6 Assuming the

following data, predict the velocity and the displacement of the pile tip after three time steps:

Some Typical Damping Constants

FIGURE 9.6

Illustration for Example 9.2

As shown inFigure 9.6, assume the pile consists of two segments (i=2 and 3) and the time

step to be 1/4000 sec Then the following initial and boundary conditions can be written:

After the first time step From Equation (9.3),

In fact, wave-equation analysis of pile capacity can be supplemented by fabricating a piledriven by an impact or vibratory hammer as shown in Figure 9.7(a)to obtain records

FIGURE 9.7

(a) Strain gages and accelerometers attached to pile during pile driving (Courtesy of Applied

Foundation Testing, Inc.) (b) Field data showing pile-driving performance (1 kip=4.45 kN, 1psi=6.9 kPa, 1 in.= 25.4 mm, 1ft-kip=1.36 kJ, 1ft=0.305 m) (Courtesy of Applied

Foundation Testing, Inc.)

of the particle velocity and the longitudinal force at the pile top (Figure 9.7b) This techniqueknown as pile-driving analysis has now gained worldwide popularity and application Whenthe above instrument records are used in conjunction with wave-equation analysis, one would

be able to evaluate:

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1 The tip or end bearing resistance of the pile at a given stage of driving

2 The skin or shaft friction of the pile at a given stage of driving

3 The stresses induced in the pile

4 The pile integrity

The above evaluations are illustrated in the following sections in terms of numerical examplesformulated based on the concepts of pile-driving analyzer developed and published by Goble,Rausche and Likins Inc (Rausche et al., 1985; Goble and Rausche, 1986; Goble et al., 1970).The longitudinal wave propagation equation (Equation (9.2)) can be rewritten as

or

(9.8)

in which c, the velocity of compression waves in the pile medium, is expressed as

(9.9)

and R'(z) is the shaft resistance per unit mass of the pile.

The complimentary solution (without the shaft resistance term) to the above differentialequation can be expressed as

u=G(ct+z)+H(ct−z)

(9.10)

where G and H are the displacement pulses that sum up to form the resultant wave given by

Equation (9.8) If one assumes the propagation of a compression wave between the locations

P(z=z) and Q(z=z+Δz) within a time Δt, then for a given particle displacement pulse to move from P to Q or for the displacement pulse H to move from P to Q in time Δt, then

H(V c t−z)=H[V c (t+Δt)−(z+Δz)]

(9.11)

From Equation (9.11), it is seen that cΔt must be equal to Δz In other words, the disturbance

H travels between P and Q (i.e., Δz) within a time Δt at a velocity of c The above result

shows that H is the incident (or downward) velocity pulse that propagates in the positive z direction Similarly, it can be shown that G is the reflected (or upward) velocity pulse.

9.4.1 Basic Concepts of Wave Mechanics

The following facts on wave mechanics are useful in interpreting pile-driving records:

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2 In a tension stress pulse or wave, wave propagation occurs in a direction opposite to that ofparticle velocity

Based on the above facts, the following determinations can be made regarding wave

propagation in a driven pile due to a hammer blow which induces a compression wave:

Case 1 If the pile tip enters a stiffer medium relative to the medium surrounding its shaft

(Figure 9.8a), then the pile can be regarded as a fixed ended one with the following velocityboundary condition at the tip:

V=0

(9.12)

In order to ensure zero particle velocity resultant at the tip, the wave reflected at the tip, whichtravels in the direction opposite to the incident compression wave, must induce a velocitycomponent that is in the direction of the reflected wave (Figure 9.8a) Hence, one can

determine that the reflected wave has to be a compression wave thereby doubling the

compressive stress at the tip

Case 2 If the pile tip enters a softer medium relative to the medium surrounding its shaft

(Figure 9.8b), then the pile can be regarded as a free-ended one with the following forceboundary condition at the tip:

9.4.2 Interpretation of Pile-Driving Analyzer Records

Raushe et al (1979) present the following theoretical considerations that enable one to

comprehend the pile-driving analyzer As shown inFigure 9.7(a), the top of the

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monitored pile is instrumented with an accelerometer and a longitudinal strain gage duringpile-driving analyzer monitoring The accelerometer record is converted to obtain the particlevelocity of the pile top, in the longitudinal direction, as

(9.14)

On the other hand, the axial force on the pile top at a given instant in time can be obtained by

the strain gage reading (ε ) as

F=EAε

(9.15)

Since both the force and the velocity records are typically plotted on the same scale in PDA,

the particle velocity must be converted to an equivalent force (F*) by the following

conversion:

(9.16)

The EA/c term is denoted as the pile impedance or Z Hence, it is necessary to know the

elastic modulus of the pile material, the compression wave velocity in the pile material, andthe cross-sectional area of the pile in order to plot the equivalent force record Either these

parameters can be included in the input data or the velocity record can be calibrated a priori

against the force record to obtain the pile impedance

If the pile is unrestrained or completely free of shaft friction and end bearing, using basicmechanics it can be shown that

(9.17)

Then it is understood that both the force (F) and the equivalent force (F*) records due to a

hammer blow would coincide It is the above fact (Equation (9.17)) that is useful in

calibrating the V record due to a hammer blow to coincide with the corresponding F record (and indicate F*), before the pile is driven in.

When the pile is constrained particularly at the tip, the impact wave (downward) and thereflected wave (upward) together produce a resultant wave at a given location on the pile.Hence, what are recorded by the instrumentation are in fact the resultant force and the

velocity at the top of the pile The resultant longitudinal force on any pile section can be

desynthesized as follows to reveal the respective force components due to the downward (H) and upward (G) waves:

(9.18a)

(9.19a)

negligible tip velocity and a relatively high compressive force on the tip in response to a given

hammer blow However, if the pile length is L, since it takes a time of L/c for the stress pulse induced by the hammer to reach the tip and an additional L/c time interval for the tip response

to return to the top and get recorded by the instruments, the above response will be reflected

on the PDA monitoring after a time period of 2L/c from the instant of hammer impact This is

Case 3 Condition of high shaft resistance—Figure 9.9and Figure 9.10also clearly

illustrate that if the pile shaft is relatively free, i.e., with a minimum shaft resistance, R(z) (in

Equation 9.2), then both the force and equivalent force (velocity) records gradually attenuateshowing the expected decay of the hammer pulse at the pile top until the reflection of the tip

condition reaches the top at a time of 2L/c In fact, this can be seen numerically in Example9.2as well

On the other hand, if the shaft resistance is significantly high, one would expect the force

pulse to be constantly replenished by the reflected force pulses from the shaft resistance R(z).

Under these conditions, using basis mechanics, Equation (9.17) can be modified to:

FIGURE 9.9

Illustration of large tip resistance condition.

FIGURE 9.10

Illustration of minimal tip resistance condition.

This is illustrated inFigure 9.11where the difference between F and F* records until a time

of 2L/c indicate the cumulative shaft resistance R(z) A typical PDA record indicating

significantly high shaft resistance is shown inFigure 9.12

9.4.3 Analytical Determination of the Pile Capacity

Goble et al (1988,1996) presented a simple and approximate method of determining the pile

capacity based on PDA records This method is based on evaluating the parameters RTL, RS1,

RTL', and RS1', which are defined as follows:

Total resistance (both static and dynamic components) The total resistance (static and

dynamic) can be obtained from the following expression:

(9.20a)

Static resistance The static resistance can be obtained by subtracting the dynamic resistance

component from the total resistance as

RS1=(1+J)RTL−J[F1+ZV1]

(9.20b)

where (F1, ZV1) and (F2, ZV2) are PDA records at t=0 and t=2L/c, respectively (Figure 9.13),

and J is an empirical coefficient designated as the Case damping constant that accounts for

damping action of soil both at the tip and the shaft

The total resistance and its static component can be also evaluated by extending the 2L/c

time window considered in Equation (9.20) to other times in the PDA record as well

FIGURE 9.11

Wave effects of shaft friction and toe resistance.

where (F1', ZV1') and (F2', ZV2') are PDA records at t=t' and t= t'+2L/c, respectively, and t' is

a desired time selected on the record (Figure 9.13)

Typically, J is back-calculated based on correlation of PDA results with those of static load

tests Therefore, it must be noted that the Case damping constant cannot be considered as asoil property or a constant for a given soil As seen inTable 9.2, it is seen to vary within asignificant range of values even for the same type of soil depending on testing conditions.Finally, the maximum static resistance based on the entire record can be obtained as

RMX=Max(RS1')

(9.22)

FIGURE 9.13

Illustration of the desynthesizing of PDA record.

FIGURE 9.14

Idealization of a damaged pile.

Determination of the above parameters from a given PDA record will be illustrated in

Example 9.3 RS1, RS1', and RMX parameters offer the pile construction engineers with the

facility of estimating the approximate static resistance on-site without having to use the equation analysis

wave-9.4.4 Assessment of Pile Damage

Pile damage due to tension cracks can be idealized by two pile segments with different crosssections (Figure 9.14)

The cross-section reduction factor βcan be defined as follows:

(9.23)

Considering the vertical force equilibrium and continuity of velocity continuity at the

damaged section, the following expression can be derived to obtain β(Figure 9.14):

Silt 0.2–0.5

damage based on the βvalue.

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TABLE 9.3

Assessment of Pile Damage

Using Equation (9.18), the instant force records due to the upward and downward wavescan be obtained as shown inFigure 9.17

Based on Figure 9.17, it can be seen that the minimum compression pulse of 1.433 MN due

to the downward compression wave occurred on the top at a time of 1.5L/c This compression pulse would move toward the pile tip at a velocity of c.

Similarly, it can also be seen that a maximum tension pulse of 2.225 MN reached the pile

top at a time of 2L/c traveling upwards at a velocity of c Hence, the two pulses (the minimum compression and the maximum tension) must have encountered each other at a time of T

creating a net maximum tension of 2.225−1.433 or 0.792 MN

FIGURE 9.16

Illustration for Example 9.4

FIGURE 9.17

Desynthesized force components.

If the location where the two pulses encountered each other is at a depth of Z from the pile top, one can write the following expressions to compute Z:

(T−1.5L/c)=time taken for the minimum compression pulse to reach Z from the

top

(2L/c− T)=time taken for the maximum tension pulse to reach the top from Z

c(T−1.5L/c)=c(2L/c−T)=Z

By solving, T=1.75L/c and Z=0.25L.

Thus, it can be concluded that a maximum tension of 792kN occurred at a distance of 2.5 m

at a time of 5.3 msec after the input

Example 9.5

Based on the PDA records indicated inFigure 9.18, assess the extent of concrete pile

damage and the location of damage Assume that the pile is of length 80m

The wave velocity can be estimated by using Equation (9.9) by knowing the elastic

modulus of concrete as 27,600 MPa and the mass density as 2400 kg/m3 Therefore,

c=(27,600,000,000/2400)0.5=3391 m/sec

L=80 m

2L/c=47.2msec

Therefore, the expected time of arrival of the return pulse=47.2msec

The time of occurrence of the tension pulse (identified by the sudden increase of

velocity)=15.7msec<<47.2msec Hence, one can assume that the pile is damaged If the

effective length of the pile is L* (up to the damaged location), then

2L*/c=15.7msec=0.0157sec

Hence, L*=26.6m

Using Equation (9.24) to determine the cross-section reduction factor β ,

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FIGURE 9.18

Illustration for Example 9.5

Based on Table 9.3, it can be deduced that the tested pile is broken at a depth of 26.6 m Theallowable stresses for pile in common use are provided inTable 9.4

A more precise evaluation of the pile capacity can be performed in conjunction with thewave-equation analysis One of the popular methods currently used to perform this type ofanalysis is the Case Pile Wave Analysis program (CAPWAP) computational method (Gobleand Raushe, 1986) Basically, in this technique one determines the set of soil resistance

parameters (ultimate resistance, the quake and damping constants) that produces the bestmatch between the instrument recorded and the wave equation based force and the velocity ofthe pile top One of the two records (pile top velocity or force) is used as the top boundarycondition and the complimentary quantity is computed using an analytical procedure similar

to that presented in Section 9.3.2and compared with the corresponding record Further details

of this technique can be found in Goble and Raushe (1986)

9.5 Comparison of Pile-Driving Formulae and Wave-Equation Analysis

Using the PDA Method

Thilakasiri et al (2002) report a case study in which the pile capacity predicted at the time ofdynamic load testing of driven piles together with the measured sets were used to verify

Driving damage likely 0.25 Fy

Driving damage unlikely 0.33 Fy

Concrete-filled steel pipe

during the application of the hammer blow were obtained in the field using the piledrivinganalyzer Subsequently, the acquired data is processed using the CAPWAP to obtain the staticload-settlement curves from the measured force and velocity data Moreover, the resultingpenetration of the pile due to the hammer blow was independently measured as one parameterfor checking the accuracy of pile-driving formulae

The data collected in the field during the dynamic pile load testing program consisted ofmobilized soil resistance, weight of the drop hammer, height of drop of the hammer, and thepenetration of the pile per blow In addition, the actual energy transferred to the pile,

maximum compressive stress, and the maximum tensile stress developed during the hammerblow were also estimated from the PDA measurements The skin frictional resistance and theend bearing resistance mobilized during the hammer blow were separated using the CAPWAPanalysis

The test results showed that for driven concrete piles in many residual formations, a

significant part of load is carried by skin resistance Test results also show that the efficiency

of the hammer has varied between 15% and 60% for the piles tested When crawler craneswere used with four-rope arrangement, where the hammer falls when the brakes are released,the efficiency factor was in the range of 60% to 40% Similarly, when the hammer is raisedand dropped using a mobile crane with a sixrope arrangement, where the hammer falls whenthe brakes are released, the efficiency dropped to the range of 30 to 15% The measuredefficiency factors are much smaller than the values quoted in literature For example, Poulos

and Davis (1980) recommend an efficiency of 75% for the drop hammer actuated by rope andfriction winch.

In the estimation of the mobilized resistance using different driving formulae, the efficiencyfactors estimated from the PDA are used with the driving formulae containing such a factor.The mobilized resistance during the dynamic load testing was independently estimated usingcommonly used driving formulae and the measured set For comparison purposes

“Engineering news record” (ENR), Danish, Dutch, Hiley, Gates

deviation, maximum and minimum of μf or ten driven piles tested.

Based on the pile test program conducted in the Thilaksiri et al (2002) study, it appearsthat predictions from Dutch, Hiley, and Janbu methods have a high scatter indicating that theyare not very reliable for estimation of carrying capacity of driven piles in residual formationsand the driving formulae, which are good and comparable to the reliability of the wave

equation in residual formations, are ENR, Danish, Gates, and modified Gates

9.6 Static Pile Load Tests

The static pile load test is the most common method for testing the capacity of a pile and it isalso considered to be the best measure of foundation suitability to resist anticipated designloads Procedures for conducting axial compressive load tests on piles are pre-

FIGURE 9.19

Variation of μfor different dynamic formulae.

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TABLE 9.5

Mean and Standard Deviation (STD) of μfor Driven Piles in Residual Formations

These tests involve the application of a load capable of displacing the foundation and

determining its capacity from its response Various approaches have been devised to obtainthis information When comparing these approaches, they can be sorted from simplest to mostcomplex in the following order: static load test, rapid load test, and the dynamic load test.These categories can be delineated by comparing the duration of the loading event with

respect to the axial natural period of the foundation (2L/C), where L represents the foundation length and C represents the strain wave velocity Test durations longer than 1000L/C are considered static loadings and those shorter than 10L/C are considered dynamic (Janes et al., 2000; Kusakabe et al., 2000) Tests with duration between 10L/C and 1000L/C are denoted as

rapid load tests The static and rapid load tests will be discussed in Sections9.6and9.8,

respectively The dynamic load test was discussed in Section 9.4

Although there are a number of different setups for this test, the basic principle is the same;

a pile is loaded beyond the desired strength of the pile There must be an anchored reactionsystem of some sort that allows a hydraulic jack to apply a load to the pile to be tested Ideally,

a test pile should be loaded to failure, so that the actual in situ load is known The load is

added to the pile incrementally over a long period of time (a few hours) and the deflection ismeasured using a laser sighting system The pile can be instrumented with load cells at varieddepths along the pile to evaluate the pile performance at a specific location Instrumentation

of the pile load cells, strain gages, etc can provide a great deal of information All the dataincluding time are collected by a dataacquisition unit for processing with software (Figure9.20and Figure 9.21)

It is clear from the discussion inSection 6.5that if a load test is performed on a pile

immediately after installation, irrespective of the surrounding soil type, such a test wouldunderestimate the long-term ultimate carrying capacity of the pile Therefore, a sufficient timeperiod should be allowed before a load test is performed on a pile Moreover, the additionalcapacity due to the long-term strength gain allows the designer to use a factor of safety on thelower side of the normal range used Establishing a trend in the strength gain of driven pileswith time will boost the confidence of the designer to consider such an increase in the

capacity during design and specifying the wait period required from the time the pile is

Test load 200% of design load 300% of design load or up to

failure Load increment 25% of the design load 10–15% of the design load Load duration Up to a settlement rate of 0.001 ft/h or 2h, whichever

occurs first

2.5 min