Failure characteristics of rainfall-induced shallow landslides in granitic terrains of Shikoku Island of Japan

Soil slips, debris slides, and flow occurrences on hill slopes of particular areas are closely related to the mode of rainfall. Because of geological, geomorphological, and climatic settings, Japan is highly prone to rainfall-induced landslides. In 2004, Shikoku, the smallest island of Japan, faced extreme events of typhoon rainfalls and suffered huge losses of life and property because of floods and landslides. This paper deals with the synoptic descriptions of failures that occurred in the granitic terrain of northeast Shikoku Island, Japan, along with rainfall and failure relationships during the typhoon 0423 (Tokage) of 2004. Examples of typical failures that occurred in Moriyuki and Monnyu of northeast Shikoku are taken into consideration in this study. Data from laboratory and field were used to perform sensitivity and stability analyses with respect to varying slope angles, strength parameters, and thicknesses of saturated residual soil. This study attempts to employ a standard method of stability analysis of translational slides, which are very common in masa soil (weathered granite) during extreme rainfall. Average rainfall intensities and duration during typhoon Tokage were also used to interpret landslide-triggering thresholds for the study area.