Mechanism of a rainfall-induced slide-debris flow: constraints from microstructure of its slip zone

The microstructure of the slip zone of a rainfall-induced slide-debris flow landslide, developed within a volcanic saprolitic soil, was comprehensively investigated using optical and backscattered electron microscopy (BEM) techniques, and a digital image analysis system. The slip zone had generally looser structure with higher relative porosities and greater abundance of discrete kaolinite aggregates than its confining materials. Particles are generally random or weakly arranged. Distinct microstructural characteristics suggest that the slip zone has undergone dilation and microstructural destruction. The dilative deformation should have began at the tip of tensions crack(s), where high pore water pressure was generated during heavy rainfall, and gradually propagated down the slope under sustained water level in these crack(s). The soil in the dilated zone should have facilitated shear deformation during which it experienced contraction under undrained conditions. Rapid and significant increase of pore water pressure within the toe section of the slip zone and sudden release of strain inhibited by the shotcrete cover should have been responsible for the transformation of the fore mass of the slide into a debris flow. During sliding, particle movement within the slip zone should have taken place as a fluid-like particulate flow, resulting in a random or weak particle alignment in general.