Large-scale model test for detecting pile defects using the parallel seismic method

Although the parallel seismic (PS) method has been successfully applied to determine the unknown foundation depth of buildings and bridges, the ability to detect defects in pile foundations has not been verified. In this paper, three large-scale model piles with artificial defects are designed and constructed in multilayered ground. The capability of cross-hole sonic logging, low-strain pile integrity testing and PS method to detect the defects of pile foundation is compared. The effects of the pile-to-borehole distance and the excitation position on the stress wave transmission are analyzed in detail. Finally, the abnormal characteristics of stress waves corresponding to different types of defects are summarized. The results show that pile defects affect the difference in the stress wave impedance by the PS test, and the features of the first arrival waves can reflect the type of defect. When the distance between the borehole and pile is within 0.5 m, the wave form and amplitude of the first arrival waves can be used to distinguish the defects of necking, breaking and sedimentation. Increasing the pile-to-borehole distance by more than 1 m is not conducive to identifying defects. When the pile defect position is close to the side of the shortest path of stress wave transmission, the abnormal characteristics of the first arrival waves are much clearer than when the defect position is far from the side; otherwise, the feature of the defects will be greatly weakened.