## 3 Results
### Role of Clay Content in Sinkhole Risk
For both materials, the differences in the cavity height and soil outflow were very small at the highest water level; therefore, the ground model was fully saturated (i.e., GWL goes to 15 cm). At the lowest water level (5 cm), however, the sinkhole characteristics are affected by the clay content. The clay content can improve the resistance of the soil to cavity expansion. In Figure 4b, the lowest water level may be a typical condition in urban sinkhole-prone areas in Korea (e.g., approximately 7 m below the ground surface in Daejeon Metropolitan City, Republic of Korea). In addition, the highest water level (15 cm) is considered for extremely heavy rainfall events. The cavity size and soil loss during the tests are significantly affected by the groundwater level. Figure 3: Comparison between the sand and sand-clay mixture results. The arrows indicate the start point of upward migration of the cavity in (**a**), (**c**), (**e**) which is the same as the time of 0 s in (**b**), (**d**), (**f**). \(\Delta\)u = variation in the pore water pressure (kPa). To prevent future sinkholes in urbanized areas, the mechanism and consequences of grain-size-dependent sinkhole risk should be established. Recently, for human-induced sinkhole occurrences in Ecuador, [PERSON] et al. [21] proposed a new risk-management approach, including (1) determining the hydrological and hydrogeological characteristics of sinkhole-prone areas, (2) performing a geotechnical characterization in terms of the material properties and (3) creating accurate monitoring and prevention techniques. To implement this approach, we need a sinkhole database and inventory map. One of the important issues is determining the causes and consequences of urban sinkholes, e.g., the rainfall-induced ground deformation and uncertainties in the migration of the fine content in the soil matrix due to natural causes or the breakage of outdated pipes and insufficient maintenance of underground facilities from artificial environments. Additionally, cavities can form underground due to damaged buried structures, aged sewer pipes, drainage problems surrounding underground tunnels, and deteriorating retaining walls with broken concrete; therefore, these conditions should be considered. In terms of assessing the soundness of civil infrastructures, the corrosion and damage of underground sewer pipes is difficult to determine. In this study, one of the simplest cases was considered. Thus, loosening around the cavity is not easy to determine in a small tank model test. The outflow of fine-grained sediments can be gradually observed during the inflow and outflow cycling in the tank model tests. According to [PERSON] and [PERSON] [2], soil and water loss is not as easy in dense conditions, but can easily expand the loose soil conditions. They stressed that the maximum fine loss that can be reached is 50% of the initial fines in the model ground. In addition, the rate of subsurface erosion in the process of piping is approximately one order of magnitude greater than the surface erosion [22]. In a future study, the infiltration effect on the damage to underground structures and sinkhole occurrence should be examined. Grain deformation and crack mechanisms in sinkholes are crucial. Numerical analysis for predicting future sinkhole events in urban areas may be recommended using grain-based models, such as Particle Flow Code (PFC) and related numerical techniques [23; 24; 25]. Figure 4: Sinkhole characteristics as a function of clay content: (**a**) failure time; (**b**) pore water pressure; (**c**) cavity height; (**d**) soil outflow.