# Model Test Analysis of Subsurface Cavity and Ground Collapse Due to Broken Pipe Leakage
## 3 Results and Discussion
### Degree of Ground Collapse
As listed in Table 2, in the case of the 30 mm and 40 mm diameter circular cracks and the 1 cm and 2 cm vertical cracks, the difference in the settlement above the cracks and the difference in the length of the ground collapse settlement area were both significantly smaller than the differences in the other adjacent groups. When the crack was larger than a certain critical size, the hydraulic action in a short duration formed a rapid erosion cavity on the soil around the pipe, causing the ground to collapse. The ground settlement above the crack and the length of the ground collapse settlement area did not increase significantly, and a large amount of water flooded the ground. The 3D laser scanning of the circumferential and vertical crack groups on the ground before and after the collapse and the post-processed contour images (Figure 9) can help more intuitively to compare the influences of the type and size of cracks on ground collapse. The blue, green, yellow, and orange colors represent the severe settlement area, general settlement area, general uplift area, and severe uplift area, respectively. The contour image shows that the entire ground area of the model exhibited varying degrees of damage due to pipe breakage, with the settlement value at the center of the cracks, the settlement area, the height of the elevation at both ends of the ground, and the degree of ground damage all being significantly higher in the vertical damage group than in the circumferential damage group. In the circumferential damage group, the quarter-circumferential damage was more toward the right end of the ground in terms of both ground settlement and ground uplift caused by the surge leak. The underground soil cavity formed in the surge leak was an elongated cavity to the right of the crack, resulting in the presence of a dehiscence area in the overlaying soil at the right end of the ground and the formation of ground settlement and uplift. Therefore, the half-circumferential damage was significantly greater than the quarter-circumferential damage in terms of ground settlement at the center of the crack, the height of the rise at the ends of the ground, and the settlement area caused by the spray leak. In the vertical damage group, the ground collapse contours were extremely similar for both sizes, with no significant differences in the settlement values at the center of the crack, the settlement area, or the height of the rise at the ends of the ground surface. This further shows that when the crack is larger than a certain critical size, there is no significant change in the severity of the ground collapse caused by pipe breakage. The more concentrated area of settlement in the vertical damage group compared to the circumferential damage group indicates that larger cracks cause more abrupt ground collapse due to the cavity. During the evolution of the ground collapse, the soil around the pipe erodes more rapidly, forming an underground cavity that rapidly erodes and expands. When the surrounding soil is not affected, the soil above the crack center then begins to collapse.