As global temperature rises and weather pattern changes, sediment disaster risks have been increasing in Japan. When Typhoon Hagibis hit East Japan in October 2019, about 30% of landslides occurred outside the designated landslide-prone areas. Such sediment disasters outside the designated areas draw public attention for fear of their increases with advances in climate change. This research aims to clarify characteristics of slope failures that have occurred outside the landslide-prone areas in the Kanto region (Greater Tokyo region) in recent three years. Intensities of heavy rainfall as well as basic topographic and geological features that may have caused failures are examined. The analysis indicates that such slope failures are more likely to occur as a result of long-term rainfalls rather than short-term rainfalls. Moreover, long-term rainfalls with a higher Soil Water Index (SWI) exceeding 10-year return period may trigger larger-scale landslides and are accompanied with a longer runout distance. It may suggest that large-scale slope failures of “slide” and “flow” types have a relatively high probability to occur on gentle slopes with a gradient of less than 30 degrees as a result of abnormal long-term rainfall.

The strength of a slip surface is an important parameter when considering countermeasures against landslides. However, the shear strength of slip surfaces is difficult to determine directly owing to the small number of samples of the slip surfaces themselves. Therefore, the back-calculating method has set shear strength parameters without a clear basis. In this study, the shear strength obtained from natural and pseudo-slip surfaces was compared with shear and physical properties to clarify the validity of the shear strength of pseudo-slip surfaces (artificially cut and shaped specimens made from the same rock type as the slip surface, boring cores). The experimental methods used included mechanical tests such as cyclic direct shear test, and direct box shear, as well as surface observation of the sheared surfaces using a scanning electron microscope and measurement of surface roughness using three-dimensional surface analysis. In the sandstone, the shear strength (based on one-sided shear tests) for grit size #150 (abrasive grain size of sandpaper 125 μm) obtained a clear correspondence with the fully softened strength of the natural slip surface, and for grit size #600 (abrasive grain size of sandpaper 28 μm) with the residual strength of the natural slip surface. Therefore, in the future, the shear strength of pseudo-slip surfaces is expected to replace natural slip surfaces because of the simplicity of testing and the fact that it reflects the properties of the rock types.