Thursday, November 22, 2012

Pile Driving Analysis - Driven Piles.

Improvements in electronic instruments  permit the measurement of data  for evaluating hammer and driving system performance, pile driving stresses, structural integrity, and ultimate pile capacity.  The required data may be measured and pile performance evaluated in fractions of a second after each hammer blow using pile driving analyzer equipments.  PDA is also useful when restriking piles after some time following pile installation to determine the effects of freeze or relaxation on pile performance.  The Case method (Pile Buck, Inc. 1988) developed at Case Institute of Technology (now Case Western Reserve University) is the most widely used technique. The CAPWAPC analytical method is also applied with results of the PDA to calibrate the wave equation analysis and to lead to reliable estimates of the ultimate static pile capacity provided soil freeze, relaxation, or long-term changes in soil characteristics are considered.   The CAPWAPC method quakes and damping factors, and therefore, confirms input data required for the wave equation analysis.

(1)  PDA equipment.  PDA can be performed routinely in the field following a schematic arrangement shown in Figure 6-2. The  system includes two strain transducers and two accelerometers bolted to the pile near its top, which feed data to the pile driving analyzer equipment.  The oscilloscope monitors signals from the transducers and accelerometers to indicate data quality and to check for pile damage.  The tape recorder stores the  data, while an optional plotter can plot data.  Digital computations of the data are controlled with a Motorola 68000  microprocessor with output fed to a printer built into the pile driving analyzer.  The printer also documents input and output selections.

(a)   The strain transducers consist of four resistance foil gauges attached in a full bridge.

(b)  The piezoelectric accelerometers measure pile motion and  consist of a quartz crystal that produces a voltage proportional  to the pressure caused by the accelerating pile mass.

(c)  Data can be sent from the pile driving analyzer to other equipment such as a plotter, oscilloscope, strip chart recorder, modem for transmitting data to a distant office or analysis center, and a computer.  The computer can be used to analyze pile performance by the Case and CAPWAPC methods.

(2)   Case Method.  This method uses the force F (t) and acceleration ä (t) measured at the pile top as a function of time during  a hammer blow.  The velocity  v (t) is obtained by integrating the acceleration.  The PDA and its transducers were developed to obtain these data for the Case method.

integrating the acceleration. The PDA and its transducers were developed to obtain these data for the Case Method.

(a) The total soil resistance during pile driving R is initially calculated using wave propagation theory and assuming a uniform elastic pile and an ideal plastic soil as

tl  is often selected as the time at the first maximum velocity.  R is 1 the sum of the static soil (displacement dependent), Qu and the dynamic (velocity dependent) D components are of the capacity.

where Vtop  is the velocity of the wave measured at the pile top at top time tl   Approximate damping constants Jc  have already been  determined for soils as given in Table 6-2 by comparing Case method calculations of static capacity with results of load tests. Jc  can be fine tuned to actual soil conditions if load test results  are available.

(c)  Proper calculation of Q requires that the displacement  obtained by  integration of the velocity at time tl v(tl ), exceeds  the quake (soil compression) required for full mobilization of soil r esistance.  Selection of time tl corresponding to the first maximum velocity is usually sufficient.

(d) A correction for ealy skin friction unloading causing a negative velocity may be required for long piles with high skin friction. The upper shaft friction may unload if the pile top is moving upward before the full resistance is mobilized.  A proper correction can be made by adding the skin friction resistance that was unloaded to the mobilized soil resistance.

(e)  Proper calculation to static resistance requires that freeze or relaxation effects are not present.  Piles may be restruck after a waiting period such as 1 day or more to allow dissipation of pore water pressures.

(f)  The driving force must be sufficient to cause the soil to fail;  otherwise, ultimate capacity is only partially mobilized and the full soil resistance will not be measured.

(3)  CAPWAPC method.  This is an analytical method that combines field measured data with wave equation analysis to calculate the static ultimate bearing capacity and distribution of the soil resistance.  Distribution of soil resistance, Qu , and the  pile load-displacement behavior calculated by the CAPWAPC
method may be used to evaluate the damping constant  Jc   , quakes and soil resistances required in the Case method, and to confirm  the determination of Qu calculated using the Case method.  The CAPWAPC method is often used as a supplement to load tests and may replace some load tests.

(a)  The CAPWAPC method is begun using a complete set of  assumed input parameters to perform a wave equation analysis.  The hammer model, which is used to calculate the pile velocity at the top, is replaced by a velocity that is imposed at the top pile element.  The imposed velocity is made equal to the velocity  determined by integration of the acceleration.  The CAPWAPC method calculates the force required to give the imposed velocity.  This calculated force is compared with the force measured at the pile top.  The soil input parameters are subsequently adjusted until the calculated and measured forces and  calculated and measured velocities agree as closely as practical such as illustrated in Figure 6-3.  The CAPWAPC method may also be started by using a force imposed at the pile top rather than an imposed velocity.  The velocity is calculated and then compared with the velocity measured at the pile top.

The CAPWAPC method is applicable for simulating static and dynamic tests.

(b)  A simulated static load test may be performed using the pile and soil models determined from results of a CAPWAPC analysis.  The pile is incrementally loaded, and the force and displacements at the top of the pile are computed to determine the load-displacement behavior.  Actual static load test results can be simulated within 10 to 15 percent of computed results if the  available static resistance is fully mobilized and time dependent soil strength changes such as soil freeze or relaxation are negligible.

(c)  Dynamic tests with PDA and the CAPWAPC method provide detailed information that can be used with load factor design and statistical procedures to reduce factors of safety and reduce foundation cost.  The detailed information on hammer performance,  driving system, and the pile material can be provided  to the contractor to optimize selection of driving equipment and cushions, to optimize pile driving, to reduce pile stresses, to reduce construction cost, and to improve construction quality.  The foundation will be of higher quality, and structural integrity is more thoroughly confirmed with the PDA method because more piles can be tested by restriking the pile than can be tested by applying actual static loads.  PDA can also be used to simulate pile load test to failure, but the pile can still be used as part of the foundation, while actual piles loaded to failure may not be suitable foundation elements.

Table 6-2 Recommended Soil Parameters for Wave Equation.

 Figura 6-2  Schematic of field pile driving analyzer equipment

 Figure 6-3 Example results of CAPWAPC analysis.

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