## 1 Introduction  
Urban sinkholes pose considerable threats to people and property because these cavities might develop underground without notice [1; 2]. An aged and damaged sewer pipe may cause local subsidence and sudden surface collapse in urban areas, especially along roads [2]. These events may be related to the age of the infrastructure; insufficient proactive management may induce cave-ins in urban areas. Sudden surface collapse in urban areas can also occur around construction work due to unreliable groundwater management around soil foundations or civil infrastructures. Sinkhole classification and case histories in karst and cavern rocks in engineering practice have been examined by numerous studies [3; 4; 5; 6]. Sinkholes can also form in volcanic sandy deposits [7]. A proper monitoring system should be developed to minimize catastrophic urban sinkholes. To develop a proper monitoring system, sinkhole mechanisms and influencing factors are first examined in terms of disaster prevention and mitigation measures. Many sinkhole studies have been previously performed with respect to sinkhole prediction using synthetic aperture radar (SAR)-related imagery data [8; 9; 10],geophysical approaches [11; 12; 13] and geophysical modeling [1; 14]. In particular, it is well known that interferometric synthetic aperture radar (InSAR) monitoring and light detection and ranging (LiDAR) measurements are effective early warning techniques which enable the detection of ground subsidence before the catastrophic sinkholes [15; 16; 17; 18].  
In physical experimental approaches, numerous works have been performed to gain a better understanding of sinkhole mechanisms in sand-rich materials, because cave-ins are often observed in roads overlying areas backfilled with sand-rich materials. Therefore, soil compaction and water management are very important issues in the field of urban sinkholes. In addition, the grain-size distribution may be very important, because the clay content in sinkhole materials is also related to the loss of small particles, e.g., the loss of clay content in fine-grained sediments in permeable soil may create water pathways and reduce the soil strength [19]. However, the influence of grain size on sinkhole formation, especially the role of clay content, is poorly understood. In this paper, sinkhole formation is examined with respect to sand and sand-clay mixtures, using a small tank model apparatus. In addition, a proper technique to monitor the occurrence of sinkholes is suggested, based on test results.  
The objectives of this paper are to examine the cavity development and sinkhole monitoring factors in sand (100% Jumunjin sand) and sand-clay (95% sand and 5% clay) mixtures. To determine a proper monitoring system in urban areas, a simple technique is used to simulate the sinkhole. A series of small tank model tests, which can simulate cavity expansion and surface collapse by the repeated inflow and outflow of soil-water mixtures, was performed to understand the sinkhole triggering mechanisms and their consequences. Regarding the sinkhole indicators, variations in the pore water pressure, volumetric water content, electrical conductivity and temperature during cavity development are examined with respect to the cavity expansion and surface collapse in the tank model. However, the soil-water interactions between a buried structure and the ground are beyond the scope of this study.