# Experimental Study of Ground Subsidence Mechanism Caused by Sewer Pipe Cracks
## 3 Test Results and Discussion  
### Continuous Water Supply vs. Cyclic Water Supply  
In Table 2, test cases are summarized. Cavity formation cycles for the continuous water supply cases and cyclic water supply cases both had similar and different behaviors. For both water supply methods, cavity formation and closing was observed. The cavities formed and closed directly above the water opening at the early stages of the experiments. This process of cavity formation and closing continued during the experiments, increasing in size. As the cavity size increased, it led to a collapse of its ceiling and the formation of a cavity above the previous one in the middle of the soil specimen (Figure 6). This mechanism was pursued until the collapse of the upper layer of the soil specimen. The difference between the water supplies triggered the cavity contraction. For the continuous water supply case, the cavity expansion and contraction cycle continued with a continuous water supply from the bottom, and did not follow any particular trend. Meanwhile, for the cyclic water supply case, cavity contraction occurred when the water was drained, while the cavity expanded when the water was supplied. Thus, each cycle corresponded to a cavity formation and contraction. For the continuous water supply cases, the increase in pore pressure was not uniform (Figure 7a). Pikes were observed and corresponded to cavity formation and collapse throughout the test until the experiment was stopped due to the failure of the soil. The same trend was observed with the suction measurements, as shown in Figure 7b. This indicates that the ground over the initial cavity ceiling was gradually saturated, leading to a decrease in effective shear stress, which was related to a decrease in the shear strength of the cavity ceiling. With this, the saturated area over the cavity ceiling collapsed after repeated initial cavity expansion and contraction. Although the water head applied to Case 2 was higher than the water head applied to Case 1, no collapse was observed for Case 2. The soil specimen in Case 1 collapsed, while only a small settlement was observed for Case 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Water Head (cm)** & **Continuous Water Supply** & **15 s Supply \& Drainage Cycles** & **60 s Supply \& Drainage Cycles** \\ \hline
177 & Case 1 & Case 3 & Case 5 \\
130 & Case 2 & Case 4 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Test cases. Figure 6: Cavity initiation and development around the opening. During the cyclic water supply cases, the cavity expansion and contraction process followed the water supply and drainage cycles. Whenever the water was supplied, the pore pressure on the opening increased and the cavity expanded, whereas cavity contraction and pore pressure decreased following drainage (Figure 8a). This cycle is well expressed in Figure 8b, where Part A in Figure 8a is enlarged. Part A corresponds to the early cycles and shows the increase and decrease of the pore pressure values following the supply and drainage cycles. In Figure 8c,d, the cavity contraction can be seen with the cessation of the water supply and the drainage of leaked water. After the water was supplied again in the 18 th cycle, the cavity expanded again, and when the supply of water ended and pore pressure decreased, the cavity contracted again. Part B of Figure 9a corresponding to the last cycles is enlarged in Figure 9b, and represents the cavity propagation upward, preceded by the sudden increase in the groundwater level, as represented by the sudden increase in pore pressure. The higher values of pore pressure correspond to larger cavity size. During this stage, the cavity expansion and contraction cycle was repeated on a larger scale and higher in the soil specimen. As shown in Figure 9c,d, when the water supply stopped and leaked water was drained, the ground over the cavity ceiling settled downward and additional cracks occurred over the cavity, which indicated that the cavity contracted with drainage and pore pressure dropdown.