Mechanisms of shallow landslides on soil-mantled hillslopes with permeable and impermeable bedrocks in the Boso Peninsula, Japan

Rainfall-induced shallow landslides play a vital role in hillslope denudation processes in humid temperate regions. This study demonstrates the contrasting mechanisms of landslides in adjoining hills with permeable (sandstone) and impermeable (mudstone) bedrock in the Boso Peninsula, Japan. The characteristics of slope hydrology were inferred from pressure-head monitoring and rainfall–runoff observations. An analysis of slope stability provided critical conditions for several previously occurring landslides. The results are as follows. (1) In slopes with the permeable sandstone, infiltrated rainwater percolates through the bedrock as an unsaturated gravitational flow. The wetting front migration results in a decrease of soil cohesion and causes landsliding at the steep lower part of the hillslopes. (2) In contrast, the impermeable mudstone beneath a thin soil layer causes a transient positive pressure head that generates a saturated subsurface storm flow. The reduction in effective normal stress triggers shallow soil slipping at the uppermost part of a hollow.