## 3 Results  
### Variations in Groundwater Level and Sinkhole Occurrence  
A series of small-scale laboratory tank model experiments was performed to examine the influence of clay content on sinkhole characteristics of sand-rich materials. It is well known that cavity development and surface collapse are dependent on soil density and variations in groundwater levels [20]; the variation in groundwater level is one of the main triggering factors in sinkhole formation. Even for the same density of sandy soil, the variation in groundwater level could impact the sinkhole risks in terms of the upward migration of the underground cavity, sinkhole size, influencing zones, and so on (Table 1). In this study, the relative density of the soil sample was fixed (i.e., \(\mathrm{D_{r}}\) = 90%), and the tested groundwater levels in the ground model were selected to be 5, 10 and 15 cm high (i.e., low, medium, and high groundwater level) for the two types of soil samples. After the test was complete, the sinkhole characteristics were examined. There was no surface collapse for the soil with a groundwater level of 5 cm. The cavity existed only under the groundwater level; for the lowest groundwater level case (i.e., 5 cm), the local surface appeared to subside (i.e., cover-subsidence sinkhole), regardless of the grain size effect. In the tank model, a cavity grew and reached the surface when the drainage valve was opened (Figure 1b). Figure 2 presents the test results of the variations in pore water pressure, volumetric water content, electrical conductivity and temperature in Jumunjin sands during sinkhole tests (also see Figure 1d,e. Figure 2a shows the pore water pressure measurements with time, for the groundwater levels (GWLs) of 5, 10 and 15 cm. The downward arrows in Figure 2 indicate the times when the valves were opened for each groundwater level. The open circles indicate the time of the surface collapse. The time required for complete water inflow at a desired groundwater level ranged from 1000 to 1500 s. After the water inflow, the saturation time for the soils in the ground model was approximately 300-800 s. In fact, for the commercial Jumunjin sand, it is expected that the time to reach full saturation is less than 10 min. In the same period, the volumetric water content, electrical conductivity and temperature were measured and reached a steady-state condition; in other words, constant values of these parameters were observed during this period. The detection range was approximately 5 cm around the sensor. The drainage valve was opened for the compulsory upward migration of the cavity at 1900 s (5 cm), 2000 s (10 cm) and 2100 s (15 cm). As shown in Figure 2a, the pore water pressures reacted almost immediately to the reduction in the water volume in the ground model. In fact, there was a delay between the valve opening and the observation of a sinkhole, e.g., 60 s for 10 cm and 30 s for 15 cm. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Measurements** & & **Sand 100\%** & & & **Sand 95\% + Clay 5\%** & \\ \hline GWL (cm) & 5 & 10 & 15 & 5 & 10 & 15 \\ \(\mathrm{u_{f}}\) (kPa) & 0.5 & 1.3 & 2 & 0.6 & 1.6 & 2 \\ \(\mathrm{\Delta u}\) (kPa) & 0.1 & 0.6 & 1.1 & 0.2 & 0.6 & 1.33 \\ \(\mathrm{t_{f}}\) (sec) & 50* & 60 & 30 & 150* & 110 & 100 \\ Surface collapse & X & O & O & X & O & O \\ Cavity (cm \(\times\) cm) & \(2.5\times 10\) & \(4\times 11\) & \(4\times 14\) & - & \(3\times 10\) & \(3\times 13\) \\ Failure type & - & Punching & Punching & - & Punching & Punching \\ Soil loss (g) & 150 & 430 & 560 & 305 & 450 & 565 \\ \hline \hline \end{tabular} Note: GWL = groundwater level; \(\mathrm{u_{f}}\) = pore water pressure at failure; \(\mathrm{\Delta u}\) = difference in pore water pressure between the drainage opening and collapse; \(\mathrm{t_{f}}\) = time difference; O = collapsed; X = not collapsed; and * = surface subsidence at the end of the test.  
\end{table} 
Table 1: Summary of the test results for the sand and sand-clay mixtures.