Imaging of voids beyond boring depth with 3D SPT-seismic full-waveform tomography

Abstract

Identification of voids in bedrock is important for the design of deep foundations. Driven piles are often socketed near the top of bedrock due to driving refusal, thus it is crucial to image voids below the pile tips for the reliability of foundations. This study investigates the capabilities of a new SPT-seismic testing method in imaging voids beyond the boring depth. The method utilizes a 3D full-waveform inversion (3D FWI) of wavefields induced by the standard penetration test (SPT) blows at depths (in-depth source) to characterize subsurface material properties. After being verified on synthetic data, the method is applied to field data recorded by 72 vertical geophones placed in a 36 × 18 m array on the ground surface during a SPT. Using the seismic data from a single SPT to 11-m depth, subsurface soil/rock properties were characterized in a large 3D domain of 24 × 36 × 18 m (depth × length × width), or up to 18 m away from the SPT location. A deep void at 13 to 17-m depth is successfully imaged, showing the capability of the method to detect a void beyond the depth of an SPT boring. The results suggest the SPT-seismic method is a powerful tool for site investigation particularly for imaging of voids in weathered and karst rock that are difficult to identify by conventional geophysical methods or invasive tests.