Numerical Analyses of Erosion in Sand-Gravel Mixtures Caused by Buried Defective Pipeline under Intense Rainfall

In recent years, it has been frequently reported that with the advent of extreme rainfall events, sudden ground subsidence or even cave-in of roadway occurred in densely populated urban areas of China, in particular in sand-gravel strata. This had caused massive damage to pre-existing structures or even fatalities. The relevant investigations disclosed that many of these incidents largely arose from seepage erosion of the cohesionless strata surrounding defective sewer pipeline under intense infiltration of rainwater. Ground cavity formed in the strata and ultimately collapsed with washout of fine particles into the defective pipeline over time. To explore the relevant erosion mechanism, a coupled two-dimensional discrete element model was adopted for analysis. The granular material was modeled in Particle Flow Code (PFC); the flow field was computed in Fast Lagrangian Analysis of Continua (FLAC) and was then imported into PFC for seepage force simulation. Because the morphology of gravel affects its mechanical behaviors in the erosion n process, a random generation algorithm of polygonal gravel shape was implemented into PFC. Parametric studies were conducted to investigate influences of major factors, including initial ground water level, size of pipe defect, gravel content, and gravel shape (angularity). Simulation results suggest that high-ground water level after intense rainfall infiltration accelerates erosion process significantly. Moreover, the gravel content and gravel shape both have important roles in the formation of internal cavity and the timing of sudden collapse. The average rate of cavity expansion and ground surface settlement decreased as gravel content or angularity increased. In the end, conclusions were drawn based on these results, which could serve as references for practitioners to cope with relevant geohazards in the future.