# Experimental Study of Ground Subsidence Mechanism Caused by Sewer Pipe Cracks
## 3 Test Results and Discussion
### Summarized Ground Subsidence Mechanism and Influence Factors  
As observed from the experiments, the main influencing factors on the mechanism of ground subsidence were the water supply mode and its periodicity. The effect of the water supply mode on the cavity expansion and contraction cycle was different for continuous water supply and cyclic water supply. When leakage occurred continuously from the pipeline, the cavity formed around the crack, gradually growing by the ground deformation. Water leakage occurred without cease in this case, and ground loosening due to drainage of leaked water did not occur with a particular trend, as shown in Figure 15a. Therefore, the speed of the ground subsidence process was slower than that of the cyclic leakage from the pipe. On the other hand, when the leakage and drainage cycle occurred from the pipeline, the cavity formed or expanded during the water leakage, contracting due to downward seepage during the drainage stage of each cycle, as shown in the Figure 15b. Because of the existence of the drainage stage in the cyclic water leakage, the ground loosening occurred faster in the cyclic leakage case than in the continuous leakage case. In short, the existence of downward seepage and buoyancy according to the water supply mode had an influence on the ground subsidence mechanism.  
Figure 14: Effect of buoyancy shown by the upward settlement: (**a**) Settlement of Case 4; (**b**) Settlement of Case 5; (**c**) Close-up of black circles in (**a**); (**d**) Close-up of black circle in (**b**).  
The risk level according to the water supply mode is summarized in Figure 16. The risk level was assumed according to the collapse time. Continuous leakage is less risky than the repeated leakage, as the collapse happens more slowly than with cyclic leakage. Additionally, provided that the water leakage and drainage cycle occurs without cease after each cycle as in the test cases, the faster the speed of the repetition of the cycle (or the water leakage and drainage time of each cycle), the riskier the leakage of the pipeline becomes, as shown in the collapse times in Table 3. Each cycle of supply/drainage corresponds to a cavity formation/contraction. However, the cycle of the water leakage and drainage required for the collapse increased as the water supply and drainage time decreased.  
The ground subsidence process in the case of the pipeline leakage is summarized in Figure 17. After leakage from the pipeline occurred, a cavity formed around the pipeline crack, and that cavity  
Figure 16: Risk according to the water supply mode.  
Figure 15: Cavity expansion and contraction cycle by water supply mode: (**a**) Typical cavity expansion and contraction process (continuous water supply); (**b**) Typical cavity expansion and contraction process (cyclic water supply).  
underwent a cavity expansion and contraction cycle. In the case of continuous water leakage, the cavity expansion and contraction occurred only by the deformation of the adjacent ground. Whereas in the cyclic water leakage mode, the cavity expansion occurred due to the water leakage, and the cavity contraction occurred due to the downward seepage force applied by the drainage of leaked water and the loosening of the soil, as shown in Figure 17b. As the cycle of cavity expansion and contraction continued, the cavity ceiling collapsed and the cavity grew bigger than the initially formed cavity, as shown in Figure 17c. This cycle led to the final ground sinking or settlement as shown in Figure 17d.