Evaluation of strength distribution at cut slope of decomposed granite with the use of sounding method and geophysical exploration method

Natural slopes and cut slopes frequently collapse due to heavy rain and earthquakes, causing disasters. Countermeasures must be applied to mitigate such disasters. There is an especially high risk of a collapse at the surface layer of slopes, and thus, evaluating the strength distribution in the surface layer in detail is important to mitigating and preventing disasters. As simple investigation methods for these purposes, there are sounding methods and geophysical exploration methods. In the present study, dynamic cone penetration (DCP) is selected as the sounding method, and the surface wave method (SWM) is selected as the geophysical exploration method. The strength parameters are generally assumed based on standard penetration tests (SPTs), but DCP tests are simpler than SPTs and can be applied to narrow spaces. On the other hand, the SWM can be used to investigate wide spaces in a short time. We developed a synthesized approach to the geophysical exploration method and the sounding method. The two results obtained from the SWM and the DCP tests—namely the shear velocity and the DCP blow count, respectively—need to be converted to the standard penetration test blow count in order to be synthesized. An indicator simulation, one of the geostatistical methods, is employed to simulate the random field of N values by synthesizing the two results. The proposed procedure is applied to evaluate the strength of the weak surface layer of a cut slope composed of weathered granite, and its applicability for practical use is verified.