Semi-physical model tests of collapses induced by leakage from fractured underground pipelines in red clay areas under different flow rate conditions

Ground collapse triggered by leakage from fractured underground pipelines frequently occurs in red clay areas worldwide, posing severe threats to urban public safety and causing human casualties and property losses. Physical model tests investigating pipeline-leakage-induced ground collapse have mainly focused on sandy or silty soils in existing studies. Nevertheless, the collapse evolution mechanisms at different pipe flow rates under red clay geological conditions remain insufficiently understood. To fill this research gap, this study carries out targeted physical model experimental research.

Six groups of semi-structured physical model tests under different pipe flow rate conditions were performed using Guizhou red clay as the test material, with full-section pipeline fracture mode and constant overburden thickness kept unchanged throughout all groups. Multiple monitoring instruments, including high-speed cameras, soil pressure sensors, pore water pressure sensors, and laser displacement sensors, were deployed to continuously record multi-field information. The monitored items covered macroscopic soil deformation and failure patterns, wetting front migration behaviors, coupled responses of soil pressure and pore water pressure, and evolutionary characteristics of ground surface displacement.

Under identical pipeline fracture boundary conditions, the increase or decrease of the pipe flow rate exerted prominent control over the erosion and transport capacity of seepage water. As pipe flow rate decreased, soil erosion rate dropped, the migration mechanism of the wetting front transformed, and the dominant development direction of subsurface soil cavities shifted from horizontal toward vertical. Meanwhile, the critical overburden thickness required for collapse occurrence decreased accordingly. When the pipe flow rate fell below a critical threshold, soil cavities still developed inside the stratum, whereas surface collapse did not take place. Distinct soil arching behaviors were observed among different pipe flow rate groups. No significant soil arching effect occurred under high-flow-rate conditions, while well-developed soil arches formed during cavity expansion in low-flow-rate tests. Surface displacement remained extremely weak before collapse occurred, demonstrating the high concealment and suddenness of this geohazard. For those test groups where collapse finally occurred, the failure mode was characterized by overburden breaching driven by continuous seepage water. In addition, the first response location of pore water pressure sensors varied with the changing direction of wetting front migration.

This study reveals the mechanisms by which pipe flow rate controls cavity evolution, hydro-mechanical coupling responses, and final collapse modes in red clay strata. The findings can provide theoretical support for full-life-cycle safety monitoring of urban underground pipelines, ground-collapse risk early warning, and the formulation of corresponding engineering prevention and control measures.