Soil fluidisation induced by fine particles migration: Insights from the Shenzhen 2015 landfill landslide

Naturally completely decomposed granite (CDG) soil typically exhibits strain-hardening behavior under undrained shear conditions. Nevertheless, flow-type landslides are not uncommon in CDG landfills. This paper endeavors to address the observed contradiction by conducting a case study of the 2015 Shenzhen landslides. Based on field investigations, we propose a hypothesis for the initiation and evolution of flow-type landslides in CDG landfill slopes, termed ‘clay particle argillization, mud-water migration, and static liquefaction’. This hypothesis was verified by element-scale internal erosion tests and triaxial tests, and further elucidated by microscale particle analysis. It was observed that the internal erosion-induced removal of plastic fine particles and retention of low-plasticity fine particles from CDG soil promotes the sliding and reorganization of coarse granules under shear stress, thereby increasing the soil's susceptibility to fluidization under undrained conditions. The proposed hypothesis and experimental findings provide new insights into the instability and subsequent extensive runout of CDG landfills and analogous broadly graded landslides.