Multi-phase Temporal Seismic Imaging of a Slope Stability Mitigation Project at Newby Island Sanitary Landfill, San Jose, California

Without slope stability mitigation, liquefaction-induced settlement in bay mud and Pleistocene alluvial deposits may lead to the collapse of levee walls surrounding sanitary landfills that are located adjacent to the San Francisco Bay. To analyze the effectiveness of a slope stability mitigation project involving deep soil mixing at Newby Island Sanitary Landfill in San Jose, California, we acquired P- and S-wave seismic surveys along a transect through the mitigated region during, and two years after, completion of the mitigation project. Deep soil mixing involves the injection of a cement slurry in augered holes, resulting in groups of soil-cement columns (elements) that are intended to increase the strength and rigidity of the subsurface materials.

For our seismic investigations, we used accelerated-weight-drop (AWD) and hammer impacts to generate P- and S-wave seismic sources, respectively, at 57 geophone locations, spaced 5 m apart. The resulting seismic data were recorded using 40-Hz, vertical-component (P-wave) and 4.5-Hz, horizontal-component (S-wave) sensors. Initially, we developed tomographic refraction (velocity) images along a progressive transition from a yet-to-be-mitigated area into a more recently mitigated area, located along the base of a steep slope composed of compacted landfill. The initial survey revealed an increase in seismic velocity in the treated area, seismic velocity increases with curing time for soil-cement elements, and a high-velocity zone beneath the active injection zone. The influence of the mitigation was most apparent from increases in V_p/V_s and Poisson’s ratios. To assess the long-term effects of the mitigation project, an identical, follow-up survey was acquired in July 2014, 23 months after the initial survey. We present a comparative analysis of the tomographic images from the two surveys, variations in V_p/V_s and Poisson’s ratios over time, and a comparison of in situ, time-varying seismic parameters with laboratory-based observations of curing times as a function of cement density. We also compare S-wave velocities with cone penetrometer testing (CPT) data, acquired prior to the mitigation project, to infer changes in the subsurface over time.