## 3 Results
### Role of Clay Content in Sinkhole Risk  
From the test results, a prompt response against cavity expansion and surface collapse is possible if the pore water pressure is monitored. Thus, it is very clear that the variation in pore water pressure in monitoring urban sinkholes is more applicable than other indicators. The water content, electrical conductivity and temperature changed when the ground materials were deformed. However, although it may be concluded that the sinkhole can influence these properties, the change in these properties was not directly linked to the sinkhole occurrence in the small tank model tests. Therefore, we focused on the coupling between the pore water pressure and indications of sinkhole risk. Figure 3 presents a comparison of the test results between the Jumunjin sand (100% sand) and the sand-clay mixture (95% sand and 5% clay). Both soil samples are very sensitive to variations in water pressure. There are several differences between the two types of soil samples, e.g., the variations in pore water pressure, time until surface collapse failure, cavity size, and amount of soil loss. Unlike the sand, it takes longer for the sand-clay mixture to become fully saturated (e.g., 1000-3000 s). Figure 3a,c,e show the full timescale of the test results; Figure 3b,d,f show selected timescales of the variations in water pressure with time, based on the starting point of soil outflow. Zero along the x-axis (0 s) is the time when the drainage valve was opened for the same test, and \(\Delta\)u is the variation in the pore water pressure (kPa). Like the sand, the sand-clay mixture undergoes a surface collapse in the 10 and 15 cm groundwater level tests. The variation in the pore water pressure of the sand-clay mixture is approximately the same or slightly greater than that of the sand. The soil samples have the same response to the variation in groundwater level: the higher the groundwater level, the higher the risk of a sinkhole. It is interesting to note that there is a large difference in the lag between the opening of the drainage valve and the observation of surface collapse. This lag may be used to detect sinkholes during field monitoring. In the tank model tests, the time needed to detect surface collapse in the sand-clay mixture is two to three times longer than in the sand (30-60 s) and sand-clay mixture (100-110 s). The sand-clay mixture can resist large deformation due to an apparent cohesive network in the ground model; thus, the cavity size in the ground model is slighter smaller in the sand than in the sand-clay mixture. Compared to the sinkholes formed in the sand, the sinkholes formed in the sand-clay mixture are smaller (Table 1). Therefore, in a way, the ground, with a larger percent of small particles, is more dangerous because it is difficult to detect cavity formation from the surface. However, the sinkhole risk is greater in sand than in clay; in other words, a small amount of clay in the ground can help to reduce cavity expansion and delay surface collapse. All cases exhibited the punching type of sinkhole failure, regardless of the groundwater level. The cavity sizes and soil losses were also very sensitive to variations in the groundwater level (Table 1). Figure 4 presents a summary of the test results of the sinkhole characteristics in sand and sand-clay mixtures, as a function of the variations in the groundwater level in the small tank model tests. Figure 4a shows the relation between the time until failure of the topsoil in the ground model and the groundwater level; the time until failure in both materials gradually decreased with an increase in groundwater level. However, the time until failure is delayed for the soil with clay, and is of the order of two to three times longer. Figure 4b presents the reduction in pore water pressure with the groundwater level. Figure 4c,d are the observed cavity height and soil loss as a function of the groundwater level. The observed height of the cavity was measured from the bottom of the plate in the ground model.