Vertical seismic shear wave profiling (VSSP) for engineering assessment of soils

Abstract

S-wave velocities in soils to depths of 60 m typically range from 50 to 1200 m/s. These velocities may be correlated with the results of other engineering tests, such as the standard penetration test (SPT), in order to provide an independent assessment of the dynamic elastic properties of shallow earth materials. Reliable S-wave velocities in soils may be obtained in cased boreholes by vertical seismic shear-wave profiling (VSSP), which employs a surface linear traction SH-wave seismic source, downhole multi-component geophones pneumatically packed against the hole wall, and special field procedures involving reversal of the source polarization, which are designed to enhance SH waves and assist their identification. Liquefaction assessment of alluvial materials under earthquake loading may be made independently using both SPT and VSSP. The problems of interpretation when both direct S waves travelling through the alluvial material and refracted S waves arising from the steeply dipping bedrock interface arrive at similar times can be resolved using refraction interpretation techniques. The S-wave velocities obtained from a dam site in the Solomon Islands suggested that the sandy gravels at the site were mainly in a medium dense state, not in a dense state as was expected from the SPT. The VSSP tests indicated that these materials may be subject to liquefaction under typical earthquake loadings likely to occur in the region.