# Experimental Study on Cavity Formation and Ground Subsidence Behavior Based on Ground Conditions
## 3 Experimental Results  
### Experimental Results with Respect to Relative Density and Groundwater Level  
The model ground was simulated with relative densities of 60% and 75% using the standard sand. The time until cavity formation and ground subsidence, as well as the amount of soil discharged until cavity formation, was measured with respect to the groundwater level. The time until cavity formation and ground subsidence was measured by visual observations of the soil box. This approach, however, cannot determine the accurate time at which cavity and ground subsidence occurred. Since the experiment was performed under the same conditions (sample, equipment, experimenter, and measurement method), it was deemed possible to identify the occurrence tendency under the experimental conditions. As for the amount of soil discharged, the soil discharged until cavity formation was dried, and its weight was measured. The amount of soil discharged was converted into volume (V) through Equation (2) [28].  
\[\mathrm{V}=\frac{m_{s}(1+e)}{G_{s}\times\rho_{w}} \tag{2}\]  
Here, \(\rho_{w}\), \(G_{s}\), \(m_{s}\), and \(e\) are the water density, specific gravity, dry mass, and void ratio, respectively.  
Table 4 presents the results of the time until cavity formation with respect to the relative density under varying groundwater level conditions. Here, the \"Cavity formation (min)\" and \"Ground subsidence (min)\" represent the time elapsed from the start of the experiment to the formation of the cavity and to the occurrence of ground subsidence, respectively. Clearly, the time was relatively delayed under all groundwater level conditions at a relative density of 75%. Similarly, the time until ground subsidence was also relatively delayed for the ground with a relative density of 75%.  
The comparison of the time until cavity formation and ground subsidence with respect to the groundwater level revealed that the time was inversely proportional to the groundwater level. In addition, no collapse of the ground surface was observed at a groundwater level of 10 cm. After soil discharge for a certain duration, only the water that saturated the ground leaked downward.  
Table 5 presents the cavity volume calculated using the amount of soil discharged. The cavity volume tended to increase with increasing relative density; however, this tendency was not clear at a groundwater level of 30 cm. This could be attributed to the early occurrence of cavity due to the high groundwater level, and measurements were distorted in the process due to human errors during the model ground simulation and experiment.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Relative** & **Groundwater Level** & **Cavity Formation** & **Ground Subsidence** \\
**Density (\%)** & **(cm)** & **(min)** & **(min)** \\ \hline \multirow{3}{*}{60} & 10 & 10.45 & 15.20 (none) \\  & 20 & 2.17 & 4.16 \\ \cline{1-1}  & 30 & 2.01 & 2.13 \\ \cline{1-1}  & 10 & 6.30 & 10.00 (none) \\ \cline{1-1}  & 20 & 3.30 & 6.30 \\ \cline{1-1}  & 30 & 3.20 & 4.20 \\ \hline \end{tabular}
\end{table}
Table 4: Experimental results with respect to the relative density and groundwater level.