Field investigation and numerical study of a siltstone slope instability induced by excavation and rainfall

Weak rock slope instabilities are a common engineering problem during highway construction in South China. This study focused on a siltstone slope instability, which was induced during the construction of an expressway in Guangdong Province of China. Field monitoring and numerical simulation analyses were performed to examine the failure mechanism and formation processes of this landslide which is associated with construction activities and a period of prolonged rainfall. According to the characteristics of the slope deformation and the monitoring data, the slope deformation can be divided into two stages: a period of slow creep caused by excavation and an accelerated sliding period triggered by rainfall. Numerical simulation results show that during the excavation process, large horizontal displacement occurs at the front edge of the slope, and the initial plastic zone develops, resulting in a shallow landslide. During 20 days of continuous rainfall, the water content in the shallow layer of the slope increases continuously, and a transient saturated area forms at the surface of the slope. Within 7 days after the rain stops, the zero pore pressure surface of the slope gradually moves towards the interior of the slope, and the plastic zone begins to extend to the top of the slope. In addition, rainwater seeps down along the cracks to form a penetrating zone, thus accelerating the process of rock and soil mass softening, which further reduces the factor of safety of the slope. The combined effects of the excavation and rainfall ultimately lead to the failure of the siltstone slope; however, continuous rainfall is the key factor triggering deep sliding. The deformation and failure of the slope mainly undergo four stages: local collapse of the slope surface, formation of the plastic zone at the foot of the slope, bulging at the toe, and formation of tension cracks in the crown of the landslide. The failure mode of the siltstone slope belongs to be a retrogressive-type of the front edge bulging and trailing edge tension cracking. Based on the deformation characteristics and the failure mechanism of the landslide, comprehensive control measures including interim emergency mitigation measures and long-term mitigation measures are proposed.