# Model Test Analysis of Subsurface Cavity and Ground Collapse Due to Broken Pipe Leakage
###### Abstract
[PERSON] et al. [13] conducted a statistical analysis of ground collapse cases in China, and pointed out that 55% of all collapse accidents were caused by pipeline breakage. Due to the concealment of underground pipeline breakage, the process of water seepage and erosion and the impact on the ground are difficult to observe after the occurrence of breakage. In terms of the disaster mechanism of ground collapse caused by pipeline breakage, scholars have focused on the disaster-causing process of \"pipeline breakage \(\longrightarrow\) soil erosion \(\longrightarrow\) cavity evolution \(\longrightarrow\) ground collapse\". The disaster-causing mechanism has been studied from four aspects: type of pipeline breakages, hidden forms and causes, the loss process of water and soil interpenetration, and cavity evolution before ground collapse. Based on computer fluid dynamics (CFD) and discrete element method (DEM), [PERSON] and [PERSON] [14] used the flow-solid coupling method to explore the loss process of soil under the action of water flow. Similarly, [PERSON] et al. [15] adopted the CFD-DEM method to investigate the patterns of stratum deformation and foundation loss as well as typical particle flow processes. [PERSON] et al. [16] performed CFD simulations on pipe leaks of different sizes using a 3D turbulence model. [PERSON] et al. [17] used the pipe flow simulator HYSTEM-EXTRAN coupled with the groundwater flow simulator OpenGeoSys to study the effects of pipe flow and duration on breakage. [PERSON] et al. [18] proposed a method for estimating the location of pipeline seepage and leaks in conjunction with a ground penetrating radar (GPR) algorithm. [PERSON] et al. [19] used numerical calculations to simulate the change in the cavity stability in the presence of pipe breakage. [PERSON] et al. [20] used DEM and PFC software to simulate cavity expansion by quasi-static stepwise material removal in the cavity expansion domains of different geometries at different depths. [PERSON] et al. [21] combined the finite element model (FEM) and field data, and concluded that considering the results of both R- and P-wave analysis can efficiently identify cavities. [PERSON] and [PERSON] [22] compared the propagation mechanisms of the longitudinal (P), Rayleigh (R) and shear (S) waves, and found that low frequencies or passive methods should be used to detect deep cavity. [PERSON] et al. [23] coupled a Darcy fluid model with a DEM approach to study the erosion process of the surrounding soil by a sewage pipe. Due to the suddenness and concealment of pipeline breakage and the complexity of the causative factors and mechanisms, most scholars have studied ground collapses induced by pipeline breakage through numerical simulations. The mechanism of ground collapse caused by pipeline breakage can often be more intuitively analyzed through physical model tests. In physical model experiments, dry sand is mostly used as the experimental overlaying soil for indoor tests in order to make the test effects more evident. [PERSON] et al. [24] used a combination of numerical calculations and model tests to study the damage process in sandy soils due to pipeline seepage. [PERSON] et al. [25] used dry sand as the experimental material to study the formation and evolution of cavities after pipeline breakage through indoor tests. [PERSON] et al. [26] used sand to conduct physical experiments on the hydraulic characteristics of flow. [PERSON] and [PERSON] [27] used dry sand as the soil overlaying the pipe to study the surface settlement and collapse pattern after pipeline breakage, considering the soil type, water flow type, and breakage location in the pipe. However, field investigations on urban ground collapse accidents have shown that in addition to being prone to ground collapse accidents in sandy soils, ground collapses can occur in clay as well.