# Experimental Study on Cavity Formation and Ground Subsidence Behavior Based on Ground Conditions
## 2 Research Materials and Methods
### Research Method  
#### 2.4.1 Experimental Method  
The experiment was performed using the relative density, groundwater level, and fines content as factors. The sequence of the laboratory model test with a soil box was as follows. First, the standard sand was compacted to relative densities of 60% and 75%. After calculating the sample amount required with respect to the soil box volume, samples were prepared with a water content of 5%. The water content of the sample was adjusted by first measuring the mass of the completely dried soil and then mixing the amount of water corresponding to a target water content of 5% with the soil. The valve in the soil box was used to prevent the loss of soil through the crack at the bottom of the box, and filters made of wire mesh were installed on both sides of the soil box to prevent soil discharge into the water tanks. The prepared samples were compacted by hand using a tamping rod. A uniform degree of compaction is required for simulating the model ground. Thus, the compaction was performed by dividing the ground into nine layers. One layer was set to 5 cm, and the sample amount for the corresponding volume was calculated and added into the soil box. For every 5 cm, colored sand was placed on the walls of the soil box to distinguish the layers and examine the ground behavior. The model ground with a total height of 45 cm was simulated. Upon the completion of compaction, the target groundwater level was formed using a tape measure attached to the water tanks. After the compacted ground was fully saturated, the cavity formation test was started. The groundwater level was maintained by supplying water at a constant flow rate through a hose, and a measuring tape attached to the water tank was used to ensure that the groundwater level remained unchanged. The groundwater level was sustained from the preparation of the model ground until the completion of the experiment by continuously supplying water. The saturation of the ground was confirmed by observing whether the water reached the target groundwater level through the soil box walls. The experiment was performed after confirming that there was no change in the saturated ground for 5 min following saturation.  
In the experiment, points at which cavities and ground subsidence occurred were determined to identify the ground subsidence mechanism. Since it was difficult to clearly observe cavity formation inside the soil box, the ground behavior on the walls of the soil box was observed. The ground subsidence process was divided into cavity formation and ground subsidence. Figure 5a shows the point at which cavity formation occurred in the model ground due to soil loss. It was defined as the moment when the movement of the colored sand occurred on the soil box observation surface (front). Figure 5b shows an image of the scene defined as ground subsidence. Ground subsidence was defined as the moment  
Figure 4: Outlet at the bottom of the soil box.  
when the ground surface completely collapsed. In addition, the time until cavity formation and ground subsidence after the start of the experiment was measured using a stopwatch and video to analyze ground subsidence tendencies under experimental conditions. The dry weight of the soil discharged through the bottom of the soil box until cavity formation was measured to estimate the cavity volume under these conditions. Figure 6 shows a flowchart of the experimental process.