# Model Test Analysis of Subsurface Cavity and Ground Collapse Due to Broken Pipe Leakage
## 3 Results and Discussion
### Cavity Initiation and Evolution Processes
Although each group of erosion processes had a similar development pattern, under different crack sizes the seepage force of the flowing water from the pipeline was different. Due to the three stages of cavity development in terms of time and extent, the impact of eventual ground collapses will be different. From Table 1, it can be seen that:  
1. In the pipe wall perforation group with smaller cracks, the crack diameters were 20, 30, and 40 mm, though the cavity height hovered between 65 mm and 70 mm and the cavity width increased from 174 to 215 mm. As the crack size increased, the water flow from the pipe into the soil cavity increased, thereby increasing the seepage force of the flowing water on the soil; the underground cavity increased with the crack size, the area of the soil cavity continued to expand, and the underground cavity exhibited an oval shape (Figure 8a). Figure 7: Pressure curves corresponding to each measurement point in a 20 mm-diameter circular crack. 2. In the experimental group of circumferential damage, the width and height of the 1/4 circumferential crack cavity was 490 mm and 40 mm respectively, while the width of the 1/2 circumferential crack cavity was only 154 mm; the height was 68 mm. The width and height of the cavity varied considerably in both the groups, and the soil cavity formed by soil erosion damage exhibited two different shapes (Figure 8b). From the analysis of the pressure curve corresponding to Stage 3, the water flow in the pipeline from left to right caused the wall above the crack and the right side to be broken first. Therefore, when the supporting force of the left side wall was greater than the water flow pressure and when the right-side wall of the soil cavity was continuously damaged and extended, the cavity that was formed by the circular crack with a relatively small size of one quarter the circumference elongated and extended in the direction of the water flow. In the case of seepage due to the relatively large half-circumferential crack, the amount of water flowing out of the pipe was immediately excessive, the erosion damage to the soil was more serious, the upper covering soil fell off faster, and the soil volume was high. Consequently, the soil cavity formed a ground collapse without a lengthy and gradual development process, and the soil cavity was slightly oval. 1. In the vertical damage experimental group, the width and height of the cavity formed in the two groups of experiments were similar. The cavities formed by 1 cm and 2 cm vertical cracks were 280 mm wide, 59 mm high and 270 mm wide and 58 mm high, respectively. The cross-section of the pipe was broken, because of which there was water seepage from the pipe in the area surrounding the pipe, and the soil cavity resembled an inverted \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Cavity** & **Cavity Height (mm)** & **Cavity Width (mm)** \\ \hline \multirow{3}{*}{Pipe wall perforation group} & 20 mm-diameter circular crack & 66 & 174 \\  & 30 mm-diameter circular crack & 70 & 190 \\  & 40 mm-diameter circular crack & 65 & 215 \\ \hline \multirow{2}{*}{Circumferential damage group} & 1/4 circumferential crack & 40 & 490 \\  & 1/2 circumferential crack & 68 & 154 \\ \hline \multirow{2}{*}{Vertical damage group} & 1 cm vertical crack & 59 & 280 \\  & 2 cm vertical crack & 58 & 270 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions of cavity for different damage types. Figure 8: Soil cavity development: (**a**) wall perforation group; (**b**) circumferential damage group; (**c**) vertical damage group (yellow font for scale reading). triangular shape (Figure 8c). The boundary of the soil cavity was not well defined, the overlying soil was close to the pipe, the sand bedding beneath the pipe had been heavily eroded, and cracks in the pipe could be seen through the cavity.