Relatively shallow surface failures due to subsurface infiltration of rainwater can occur on road slopes, triggering large-scale collapses. It is important to investigate the topography, geology, rainfall conditions, etc. at the site where slope failure is likely to occur in advance to predict the deformation and strain properties due to external forces on the slope. More precise prediction of risk assessment will lead to measures to protect the lives and property of citizens from slope hazards. Here, we investigated the disaster site in northern Kyushu caused by the July 2008 torrential rainfall, conducted in-situ strength evaluation experiments with the topography, geology, and ground conditions at the site, and conducted numerical analysis by introducing the values obtained from the in-situ tests into finite element analysis. In addition, a road slope deformation analysis was conducted to investigate the slope failure phenomenon by setting up a model ground assuming the site and numerically analyzing the soil constants obtained from the in-situ tests. At a result, under conditions in which displacement is applied to the top of the embankment, a shear plane is generated at the boundary with the cliff cone deposit, and strain is concentrated at the toe of the slope.