Geophysical ground truthing with geotechnical investigations

Abstract

Geotechnical engineers traditionally rely on test boring investigations to obtain information needed for foundation design. Geophysical surveys are well suited for screening large areas of a site, targeting locations for test borings, filling in the gaps between borings, and getting a snap shot of the unknown subsurface; however, geophysical results cannot be directly correlated to geotechnical properties. This means there is a level of uncertainty when describing subsurface conditions based solely on geophysics. Ground truthing, through targeted intrusive investigations, alleviates some of this uncertainty and allows geotechnical engineers to better understand the subsurface, and to make appropriate foundation and design recommendations. This work presents several case studies where geophysical surveys were conducted prior to intrusive geotechnical investigations. In the case studies presented, test boring locations were chosen based, in part, on anomalies in the geophysical results that were likely related to karst features, variable depth to rock, and variable rock composition. One case study presents electromagnetic (EM) and electrical resistivity imaging (ERI) results for a mid-sized airport runway expansion in karst, where variable depth to rock and a large, weathered rock zone were identified and confirmed through test borings. Another case study includes a seismic refraction (SR) survey across a former coal strip mine with a highwall that was followed up with test borings to confirm the depth to rock and edge of the highwall. Challenges come from correlating geophysical results with actual lithology or geotechnical characteristics when using non-unique geophysical methods. Ground truthing with subsurface sampling can reduce the level of uncertainty. In these correlations, we must balance providing the geotechnical engineer with a sufficient description of the uncertainties, while still describing the likely materials the geophysical results are characterizing, so that the results can be used to guide design. It is important to pair geophysical investigations with ground-truthing geotechnical investigations in order to provide results that geotechnical engineers can use with a high level of confidence.