Water movement in unsaturated multi-layered slope under heavy rainfall conditions in wetting and dry process

Abstract

In 2013, a number of shallow landslides triggered by heavy rainfall affected a mountainous area which located on Izu-Oshima island (Eastern Japan). These slopes are consist of fine soil layers and coarse soil layers which have different permeability coefficient and soil water characteristic curves. To clarify the characteristics of water infiltration in such unsaturated multi-layered slope and to assess the influence of the water content distribution on slope failure, 4 groups of slope model experiments were conducted. Silica No 1(D50=3.10 mm) and silica No 7(D50=0.16 mm) were used as the slope materials. The results indicated that wetting front apparently stopped at the interface between the silica No 7 layer and silica No 1 since the capillary barrier works and then lateral water flow occurred along with the interface and infiltrate into next layer when soil approaches saturation. It was found that lower water content in fine and coarse layer, unsaturated permeability coefficient K in coarse layer (5.54E-06 cm/s) is smaller than fine layer (1.08E-04 cm/s) since matric suction is 2.5 kPa, which result that capillary barrier works. In addition, higher water zone still exists at the bottom of the fine layer after drainage for a long period which results in the failure in this zone firstly when second rainfall was applied. Piping occurred at the bottom of the model when amount of water exists inside the slope.