Estimating Spatial Variations in Bedrock Depth and Weathering Degree in Decomposed Granite from Surface Waves

In Singapore, sudden changes of bedrock depth and highly variable weathering degrees in residual soils are commonly observed. This study employed the dispersive characteristics of Rayleigh-type surface waves from combined active-source and passive-source surface-wave techniques to (1) generate S-wave velocity (Vs) profiles of residual soils and (2) estimate spatial variations in the weathering degree and bedrock depth of residual soils in Singapore, ranging from completely weathered soil to weathered rock. Finally, the feasibility of detecting bedrock depth in Bukit Timah granite was examined by several approaches based on (1) preselected Vs, (2) normalized Rayleigh wave phase velocity (VR), and (3) average Vs. The results of the field experiments showed that this combined noninvasive multichannel analysis of surface waves (MASW) method can be successfully used to determine the Vs profile of residual soils, which includes approximate bedrock detection. The newly proposed average Vs-based approach using the ratios of average Vs values of top soil layers (10–40 m) especially appears to be a simple, automatic, objective, and standardized method to identify depth to bedrock (within about 5-m resolution), which is confirmed by conventional boring investigations.