# Experimental Study on Cavity Formation and Ground Subsidence Behavior Based on Ground Conditions
## 5 Analysis of Ground Subsidence Mechanism  
The ground subsidence mechanism was analyzed based on the results of the laboratory model test where the model ground was experimented under various conditions. As sufficient parameters were not considered, it is difficult to perform a quantitative mechanism analysis through a correlation analysis. Therefore, the ground subsidence mechanism with respect to ground conditions was studied by analyzing the overall tendency. The main cause of ground subsidence is damage to sewer pipes [10]. Therefore, in this study, an experiment was performed by simulating damage to the upper part of a sewer pipe. During the installation of a sewer pipe, backfilling is performed on top of the pipe using sand. It is difficult to produce an appropriate level of compaction due to concerns regarding pipe damage [32]. The relative density indicates the degree of compaction. The experimental results for ground subsidence with respect to the relative density revealed that the occurrence of ground subsidence tended to accelerate with decreasing relative density; however, the cavity volume increased with increasing relative density. This appears to be due to the difference in the rate of soil loss caused by the friction resistance between soil particles resulting from the relative density [33]. In the ground with a low relative density, a reduction in the friction resistance between particles causes rapid soil loss, thereby accelerating the occurrence of ground subsidence. In grounds with a high relative density, the soil loss is gradual due to the relatively strong bonding between particles, causing delays in cavity formation and ground subsidence [34]. In addition, structural stability prevents the collapse of the ground surface and inhibits cavity expansion; however, the use of the same material is expected to cause consistent soil loss and result in larger cavities and ground subsidence. The ground subsidence mechanism was analyzed with respect to the groundwater level. Figure 7 shows the time until cavity formation and ground subsidence (stabilization) with respect to the groundwater level. Figure 7a shows the results for relative densities of \(60\%\) and \(75\%\) (Figure 7b), respectively. Clearly, the time until cavity formation and \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Factor**} & **Time Until Cavity Formation** & **Time Until Ground Subsidence** & \multicolumn{2}{c}{**Cavity Volume**} \\  & **Corr** & \(p\)**-Value** & **Corr** & \(p\)**-Value** & **Corr** & \(p\)**-Value** \\ \hline Relative density & \(-0.023\) & \(0.946\) & \(-0.082\) & \(0.809\) & \(0.612\) & \(0.014\) \\ Groundwater level & \(-0.366\) & \(0.268\) & \(-0.472\) & \(0.143\) & \(-0.340\) & \(0.306\) \\ Fines content & \(0.320\) & \(0.337\) & \(0.327\) & \(0.326\) & \(-0.837\) & \(0.001\) \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Correlation analysis results. ground subsidence decreased with increasing groundwater level, and no ground subsidence occurred under a groundwater level of 10 cm. Soil discharge measurements revealed that the cavity volume decreased with increasing groundwater level. This appears to be because cavities occurred above the groundwater level. Figure 8 shows the images for the time until cavity formation with respect to the groundwater level. As shown in Figure 8a, for a groundwater level of 10 cm, the cavity occurred above the groundwater level and expanded upward. In Figure 8b, for a groundwater level of 20 cm, expansion of the cavity that occurred near 15 cm slightly below the groundwater level can be seen. As shown in Figure 8c, for a groundwater level of 30 cm, a cavity is formed at a height of approximately 25 cm. These results indicate that cavities tend to form in the vicinity of the groundwater level. Figure 9 shows a conceptual diagram that helps visualize the location of cavity formation with respect to the groundwater level based on the laboratory test results. Clearly, the location of cavity formation varies with increasing groundwater level.