# Model Test Analysis of Subsurface Cavity and Ground Collapse Due to Broken Pipe Leakage
## 3 Results and Discussion
### Degree of Ground Collapse  
The ground collapse phenomenon caused by pipe breakage often proceeds through the evolutionary process of \"pipe breakage \(\longrightarrow\) soil erosion \(\longrightarrow\) cavity evolution \(\longrightarrow\) ground collapse\". However, the varying development of the soil cavity at different crack sizes inevitably leads to variations in the degree of ground collapse. By measuring and analyzing the ground settlement and ground damage area, Table 2 shows that as the crack size increases in the three sets of experiments, the ground settlement and the length of the ground settlement area increase as well, resulting in more serious ground collapse damage. In comparing the three types of cracks, it is clear that pipe breakage with vertical cracks causes more serious ground collapse than in the other two cases. The impact of surge and spray breakage on ground collapse was simulated in the circumferential damage group. In this case, a quarter-circumferential circular crack means a surge leak and a half-circumferential circular crack means a spray leak. The ground settlement above cracks of these two types was 12.7 mm and 20 mm, respectively. The length of the ground settlement area of these types was 339 mm and 490 mm, respectively. The difference in the degree of damage to ground surfaces can clearly be seen. Surge leaks make the soil around the pipe leak form a slender cavity. As the cavity exists at the interface between the sand layer and the experimental soil layer, the scouring effect on the overlying experimental soil is small, and the ground appears to exhibit moderate settlement. On the other hand, there is rapid flow out of the sand layer in the case of a spray leak case; therefore, the existence of a large cavity around the pipe further contributes to settlement and loss of the overlying experimental soil, resulting in significant ground settlement and a significant increase in the settlement area. In the pipe wall perforation group, the ground collapse length extended with increasing crack size, from 335 mm with a 20 mm-diameter circular crack to 435 mm in the case \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Cavity** & \
\begin{tabular}{c} **Settlement above** \\ **Cracks (mm)** \\ \end{tabular} &
\begin{tabular}{c} **Length of the Ground** \\ **Settlement Area (mm)** \\ \end{tabular} \\ \hline \multirow{3}{*}{Pipe wall perforation group} & 20 mm-diameter circular crack & 13 & 335 \\  & 30 mm-diameter circular crack & 18 & 402 \\  & 40 mm-diameter circular crack & 17 & 435 \\ \hline \multirow{3}{*}{Circumferential damage group} & 1/4 circumferential crack & 12.7 & 339 \\  & 1/2 circumferential crack & 20 & 490 \\ \hline \multirow{3}{*}{Vertical damage group} & 1 cm vertical crack & 31.6 & 434 \\  & 2 cm vertical crack & 32.7 & 452 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Settlement and length of ground collapse. of a 40 mm-diameter circular crack; the differences between the two adjacent groupswere 67 and 33 mm, respectively. Although the difference is not in this case as high as the 151 mm in the case of the circumferential crack group, it can be seen that the length of the ground collapse area increased significantly. Regarding the values for ground settlement above the cracks, there was not much change between circular cracks of 30 mm and 40 mm in diameter. In the vertical damage group, the ground settlement values above the cracks caused by vertical cracks of 1 cm and 2 cm were 31.6 and 32.7 mm, respectively, and the respective lengths of the ground settlement area were 434 and 452 mm. Compared to the previous two sets of experiments, there was a significant reduction in the difference observed in the data. The difference in the vertical damage group in terms of ground settlement was only 1.1 mm, and the difference in the vertical damage group in terms of the length of the ground collapse area decreased to 18 mm.