Study on stress response and collapse mechanism induced by shallow-buried defective pipelines in river alluvial plains

In response to the frequent occurrences of urban surface collapse in fluvial alluvial plains, this study takes the collapse event on Changfeng Road in the Binjiang Plain area of Anqing City as a typical case. Through scaled physical model experiments and mechanical response analysis, the stress transfer patterns of the soil surrounding defective pipelines and the chain-evolution mechanism of collapse disasters are systematically investigated. The results indicate that the essence of pipeline-induced collapse lies in the progressive structural instability of soil caused by seepage-induced erosion. The dynamic evolution of erosion cavities-including their formation, expansion, and eventual breakthrough-is identified as the dominant mechanism leading to sudden surface collapse. Stress evolution displays spatial variability: vertically, layers nearer to the leak exhibit greater stress reduction and disturbance compared to overlying strata; horizontally, disturbance diminishes radially from the collapse center, accompanied by delayed stress release in peripheral zones. Pore water pressure dynamics are controlled by seepage forces and hydraulic gradients, with sharp variations near the leak point signaling imminent instability. Groundwater flow promotes soil skeleton weakening, particle transport, and collapse channel development. The morphology of the collapse evolves through three stages: truncated cone, truncated cone + conical body, and conical body, all of which display significant precursors and characteristics of instability prior to surface collapse.