# Model Test Analysis of Subsurface Cavity and Ground Collapse Due to Broken Pipe Leakage
## 3 Results and Discussion  
### Cavity Initiation and Evolution Processes  
A circular crack 20 mm in diameter was used as an example to analyze the development process of the underground cavity; other groups had similar erosion development patterns of the pipe and soil structures. Figure 6 clearly shows the development of the underground cavity erosion under the hydraulic action of pipe leakage, which eventually causes the ground to collapse. This process can be divided into three stages. Stage 1 is the soil erosion stage. When the experiment was started, the pipe water flowed out from the crack and scoured the sand bedding layer around the pipe, gradually saturating the surrounding sand and forming an arch-shaped infiltration surface followed by the loss of sand particles with the pipe water under hydraulic action, and tiny fissures began to appear in the sand layer. Stage 2 is the stable stage of cavity development. As the pipeline water continued to flow out and induce scouring, under constant hydraulic erosion, the sand layer was continuously hollowed out and lost. The cavity area gradually expanded to the sand layer and the pipeline overlying soil interface, and the weakly permeable experimental soil was equivalent to a waterproof layer. 1. During the loading process, the seepage force of the flowing water could not cause effective erosion on the experimental soil, the scouring effect on the cavity walls was reduced, and the cavity was in a stable state. 2. During the unloading process, the pressure of the flowing water on the cavity wall decreased, the experimental soil layer was lost, an increasing amount of soil around the cavity became loose, and the cavity shrunk. 3. Reloading caused the cavity to expand under sudden pressure. During the expansion and shrinkage process, the structure of the cavity wall gradually deteriorated, and the flowing water gradually eroded the soil. The scale of the cavity was in a stable development state. Stage 3 is the ground collapse stage. The scale of the cavity continues to develop during the continuous expansion and shrinkage process. When the supporting force of the cavity wall becomes lower than the scouring effect of the water flow, the pressure around the cavity changes suddenly (Figure 7). Figure 6: Evolution of an underground cavity formed by pipe seepage erosion from a circular crack 20 mm in diameter: (**a**) \(t=60\) s, first appearance of cavity; (**b**) \(t=140\) s, cavity becomes larger and stable; (**c**) \(t=160\) s, cavity shrinks after unloading; (**d**) \(t=490\) s, cavity expands suddenly; (**e**) \(t=530\) s, topping up the ground and water bursting onto the ground; (**f**) \(t=560\) s, ground collapse. 1. From 480 s onward, the pressure in the 502# and 202# pressure boxes directly above the crack exhibited the same waveform; however, there was a significant drop in the pressure, indicating that the cavity wall above had been damaged. 2. Due to the direction of the water flow from left to right, the pressure in the 503# pressure box on the right side also decreased due to the pressure release from the cavity wall damage. The pressure in the 501# pressure box on the left side decreased; because the wall of the cavity above and on the right side was destroyed first, an increasing amount of soil around the cavity was lost, and the cavity gradually expanded, because of which the water flow formed a swirl on the left side and the pressure dropped after a sudden increase. 3. The 201# and 203# pressure boxes were located on the left and right sides of the vertical plane of the upper crack, where the uplift of the ground surface caused the earth pressure to rise. Finally, an elliptical cavity was formed, and the ground surface formed an area of settlement similar to an inverted triangle, with the ground surface sinking on the vertical surface of the pipe crack and rising on both sides, eventually forming a ground collapse.