Mechanism and failure model of group-occurring loess falls induced by domestic sewage discharge: insight from field investigation and flume experiment

Group-occurring loess falls are common catastrophic geological hazards on the Loess Plateau, typically triggered by heavy rainfall, excessive irrigation, or earthquakes. However, group-occurring loess falls induced by sustained sewage discharge are exceedingly rare. To better understand the failure mechanism of group-occurring loess falls caused by persistent domestic sewage scouring in Liudian Village, Qingyang City, Gansu Province, multi-temporal remote-sensing images and UAV-derived DEMs, together with field investigations, were used to track slope evolution. ERT and drilling were used to characterize subsurface moisture and stratigraphy. A flume test was set up using analogous materials, and water infiltrated from the constant-level tank at the slope toe. Displacement and inclination were monitored. The results show that basal loess saturation increased progressively and caused collapsible settlement. Once the basal layer became fully saturated, the slope toe gradually softened, and capillary action promoted the upward migration of moisture, saturating the upper slope. This hydrological process induced plastic deformation and slow creep, eventually resulting in an overall slope fall and sliding. The entire failure process can be divided into three stages: steady-state deformation, accelerated deformation, and final failure, each displaying distinct characteristics. These observations indicate an erosion-controlled, time-dependent cumulative failure pattern with recurrent collapses under sustained toe water supply. In the future, mitigation can focus on sewage diversion and toe flow interception, together with localized toe protection, to reduce long-term scouring and infiltration.