Numerical Simulation of Full Waveform Tomography to Evaluate the Geological Conditions beneath the Base of Drilled Shaft Excavations

Inspection of the geologic materials under the base of drilled shaft excavations is usually conducted to increase confidence in the anticipated base resistance, particularly, when the drilled shaft will rely significantly on rock sockets for its capacity. Current efforts rely on the use of probes inside corings at the bottom of the excavation. These assessments are qualitative, indirect, and spatially limited. A non-destructive testing (NDT) tool that provides information on the geometry and mechanical properties of the geologic materials under the drilled shaft excavation would potentially address these limitations. In this study, an NDT system that relies on full waveform tomography of stress wave data is examined for this purpose. FWI extracts as much information as possible from the recorded stress wave data by simulating wave propagation within the domain and matching the observed and simulated sets of waveforms in an iterative optimization scheme. In a series of numerical simulations, anomalous features were placed within a domain that represents competent base materials for a drilled shaft excavation. A source–receiver array at the bottom of the excavation was used to acquire waveforms for processing. Recommendations are provided on the selection of an appropriate FWI workflow given the constraints of this application. Finally, the results highlighted that an FWI NDT approach can accurately evaluate the spatial extent of any anomalous conditions and the contrasts in their mechanical properties relative to the surrounding bedrock.