Estimation of shallow subsurface structures on granitic hillslopes based on electrical resistivity distribution

Abstract

Electrical resistivity (ER) survey is an efficient method for investigating subsurface structure without destruction. However, this has not been used for estimating shallow subsurface zones of hillslopes that is related to the occurrence of shallow landslides. If the ER survey can alter conventional methods such as cone penetration test and boring to investigate soil thickness, an effective method for detecting landslide-prone area would be developed. In this study, ER survey was conducted on two granite hillslopes in Japan with low soil-moisture conditions to verify the usefulness of the ER survey to estimate the shallow subsurface structures. To compare the subsurface structure and the ER distribution, sounding by using a boring stick and a cone penetrometer were conducted along each ER survey line. Two soil pits were also referred near the survey lines. The subsurface structures were composed of topsoil, saprolite, and bedrock in our study sites. Results of ER survey with the unit electrode spacing of 0.5 m demonstrated that the ER increases with depth at the depths of the bottom of the topsoil layers, but the ER values were unconstrained. Below the bedrock surface, the ER generally decreased with depth, showing low ER less than 12800 Ω·m excepting an exposed bedrock indicating more than 20000 Ω·m. These spatial changes in ER are consistent with changes in grain size distribution and electrical conductivity of soil of each subsurface layer measured in this study and literature on ER of granite. ER distribution can visualize shallow subsurface structures and potential sliding surfaces for shallow landslides because the bottom of topsoil layer that was successfully estimated exhibits a discontinuity of shear strength, which generally corresponds to sliding surfaces for shallow landslides. Therefore, ER survey is applicable as an alternative to conventional methods for estimating shallow subsurface structures on granite hillslopes. Furthermore, ER survey can be a useful tool to estimate where shallow landslides will occur and how we should assess the risks of landslide hazards if a targeted subsurface zone has a distinct contrast in physical properties between each layer.