Mechanisms of Shallow Rainfall-induced Landslides in Residual Soils in Humid Tropical Environments

Abstract

Landslide mechanisms and processes, particularly within the basal deformation region during failure, are poorly understood in the current knowledge. The aim of this research is to understand the initiation processes of shallow rainfall-triggered landslides under humid, tropical conditions. The major objectives are (1) to investigate the actual processes of rainfall-triggered landslides; and (2) to examine the relationship between the rates of pore water pressure increase and the rates of soil movement. Field sampling has been undertaken at two landslide sites on weathered volcanic slopes on Lantau Island, Hong Kong (22°N, 114°E). Reinflation tests have been conducted on the undisturbed soil samples using a triaxial stress path cell to replicate the field conditions during rainstorm events. Various rates of increasing pore water pressure have been designed with reference to the field monitoring data. Landslide initiation processes have been deduced by examining the relationship between the increasing pore water pressures and soil deformation. The results demonstrate that rainfall-induced landslide development on weathered slopes undergoes three stages with distinctive movement patterns towards failure. The key controlling mechanism appears to be plastic deformation. The varied velocities are dependent on the stress state (changes in the mean effective stress, under constant shear stress), the pore water pressure reinflation rate and soil permeability, stage 1 represents a perfectly plastic state below yield. Velocity remains low or at zero as the soil particles are strongly interlocked. The fluctuating to constant strain rates at stage 2 are primarily controlled by the reinflation rate and permeability after exceeding the yield state, with localised interparticle sliding. Rapid acceleration to failure occurs at stage 3, facilitated by the general remoulding process. The systematic movement patterns have also been observed in shallow rainfall- induced landslides in plastic soil materials. The steady-state behaviour could be a precursor for the acceleration to the final failure.