Impact of Soil Layer Distribution on the Morphology Characteristics of Underground Cavities

Urban road collapse disasters pose a serious threat to public safety and national economic development. The underground cavities formed by drainage pipe exfiltration are the primary cause of road collapse disasters in Beijing, with soil layer distributions playing a crucial role in their formation and development. In this study, a physical model test system for road collapse induced by drainage pipe exfiltration was developed, which comprised a drainage control unit, pressure regulation unit, experiment platform, multiple information monitoring unit, and water circulation unit. Internal and external imaging, coupled with waterproof laser rangefinders, were employed to study the formation, development, and collapse morphologies of the underground cavity influenced by clay, sand–clay, and clay–sand distributions. The results indicated that the underground cavity formed in four phases: soil–water mixture adhesion, seepage-induced runoff generation, irregular void development, and arched cavity formation. Furthermore, the physical and mechanical properties of the soil in proximity to the drainage pipe exerted the most significant influence on cavity development. The vertical cross section of the underground cavity was mushroom cloud-shaped, characterized by an upper bearing structure zone, middle saturated rheology zone, and lower stacking repose zone. The findings provide theoretical support to reveal the mechanisms of road collapse.