Integrity evaluation of drilled shaft by stress wave propagation method

Abstract

Numerical and experimental model studies were carried out to evaluate the integrity and the end condition for drilled shafts by applying elastic impact on the top of the shaft in the stress wave propagation method. One-dimensional finite element studies were carried out using mock-up shafts including four types of defects: (1) axisymmetric void, (2) non-axisymmetric void, (3) neck, and (4) bulb. Experimental studies were carried out to verify the finite element models in the air and in soil, and impact responses were analyzed in both time and frequency domains. It was shown that axisymmetric void, non-axisymmetric void, and neck could be detected in the frequency domain when the reduction in shaft cross-sectional area was more than 50%. Alternatively in the time domain, it was possible to identify the defect reliably when the reduction was 30%. Subsequently, typical impact responses corresponding to the various end conditions including free, fixed, rock-socketed, and soft-bottom with good and poor side contact conditions, were investigated. In order to simulate these conditions, mock-up shaft models made of cement mortar were used. Three-dimensional finite element studies using axisymmetric model were performed to evaluate each end condition. Small-scale laboratory experiments were also performed, and field tests were carried out for the shafts that were socketed into weathered rock. It is found that the rock-socketed condition and depth of penetration into rock can be identified from the reflection at the interface between the soil and rock in the waveform. The soft bottom condition can be identified, only when the side contact between shaft and surrounding rock is poor, whereas it cannot be identified when the side contact is good because the waveform is similar to that of fixed end condition.