Jose M. Roesset

DYNAMIC RESPONSE OF INTACT PILES TO IMPULSE LOADS

The objective of this work was to evaluate the theoretical capabilities of the non-destructive impact–response method in estimating the length and cross-sectional area of intact piles. Three-dimensional (3-D) axisymmetric finite element models were developed to simulate the testing. The results obtained were compared to one-dimensional solutions to evaluate the importance of 3-D effects. Extensive parametric studies were then performed on piles without defects. In each parametric study, the results from the direct use of time histories of displacements or velocities, the mobility function and the Fourier transform of the recorded displacements (impact-echo method) were compared in order to assess their relative advantages and disadvantages. The effects of the relative stiffness of the surrounding soil to that of the pile and of the embedment depth were also investigated for all three methods. In a companion paper the use of these procedures to detect defects such as bulbs (increases in the cross-sectional area of the pile) or necks (decreases in area) is studied.

IDENTIFICATION OF DEFECTS IN PILES THROUGH DYNAMIC TESTING

The objective of this work was to evaluate the theoretical capabilities of the non-destructive impact-response method in detecting the existence of a single defect in a pile, its location and its length. The cross-section of the pile is assumed to be circular and the defects are assumed to be axisymmetric in geometry. As mentioned in the companion paper, special codes utilizing one-dimensional (1-D) and three-dimensional (3-D) axisymmetric finite element models were developed to simulate the responses of defective piles to an impact load. Extensive parametric studies were then performed. In each study, the results from the direct use of time histories of displacements or velocities and the mechanical admittance (or mobility) function were compared in order to assess their capabilities. The effects of the length and the width of a defect were also investigated using these methods.

Simulation of the crosshole method in isotropic and anisotropic media

The crosshole seismic method was simulated using a finite element model with a diagonal mass matrix and a direct integration of the equations of motion in the time domain. The results were compared to those of a more efficient but also more restricted formulation using discrete Greens functions. The effects of the type of excitation, the shape of the applied pulse and the position of the receiver with respect to the source on the shape of the recorded motions were investigated for isotropic and cross anisotropic soil deposits. The computed times of arrival of the different waves were compared to those predicted using curved ray path theory to assess the accuracy of this much simpler procedure as a means to interpret the experimental data and determine the soil properties.