Changes in Shear Strain and Subsurface Flow Prior to Rainfall-Induced Landslide in Flume Experiments

Two shallow landslides were induced in a large-scale model slope by artificial heavy rainfall, in which the heights of sand layers were set at 0.7 and 0.5 m. A total of 2,931 s of rainfall produced a relatively large landslide in a whole steep slope section in the 0.5 m high sand layer. On the contrary, only a small landslide was produced by a total of 4,000 s of rainfall in the lower half of a steep slope section in a sand layer that was 0.7 m high. Changes in shear deformation and subsurface flow were monitored in the experiments. By tracing the movements of markers imbedded inside the sand layers, the changes in shear strain were analysed and expressed in a form of Mohr’s circle. In addition, by approximating the equi-potential lines from the observed data of pore-water pressure, the changes in subsurface flow directions were calculated. It has been shown that, although the sand layers were packed to be uniform, the observed shear deformation and subsurface flow conditions were not homogeneous; just before the landslide initiation, water tables were formed in almost all slope sections, but a sliding surface was not necessarily formed below the water table. Directions of maximum shear strain and subsurface flow were more in general agreement with the direction of slope base in the parts inside the landslide, indicating their possible influence on the landslide initiation.