Geological background of shallow landslides induced by rainstorms

For many years, shallow landslides have been considered not to be related to bedrock geology. However, our experience clearly suggests that shallow landslides occur more frequently on specific bedrock types. This is because subsurface water migration behavior is strongly dependent on soil structures, most of which are derived from rock weathering. There are at least four types of surface structures that are prone to shallow landslides:

1) Some types of rocks are weathered with well-defined weathering front, which provides a common landslide model. In this case, porosity and permeability have high contrast between the soil layer and the bedrock, and therefore infiltrating rainwater commonly forms groundwater table within the soil and the resultant pore pressure build up causes shear failure. Piping erosion along the boundary might proceed and lead to the initiation of landslide. Vapor-phase crystallized ignimbrite, mudstone and gruss of granitoid form such structure.

2) Unwelded ignimbrite is weathered to become finer than the underlying fresh materials. Volcanic glass grains of unwelded ignimbrite interact with filtrating water and become finer than the original fresh one, forming a capillary barrier at the weathering front. Halloysite, which forms within the soil, are washed away by the groundwater and clogged in narrower spaces of pores to form clay bands, which prohibit downward water filtration. Weight increase of surface soil due to perched water on the clay bands and on the capillary barrier initiate landslide along with the suction decrease.

3) Surface materials that consist of dense rock blocks and soil have been prone to shallow landslides. Hornfels and spheroidally weathered granitoid form such surface materials. Subsurface flow washes away finer fraction to leave rock framework with open spaces, which might be collapsed and subsequent pressure buildup may cause landslide.

4) Horizontal impermeable beds overlain by permeable beds prohibits downward filtration of rainwater that comes through the permeable beds and the water moves laterally along the boundary and flows out of the slope. Such water flow out often induces landslide of surface soil. Tempestite that deposited on lower shoreface forms such structure.