# Experimental Study on Cavity Formation and Ground Subsidence Behavior Based on Ground Conditions
## 1 Introduction  
Ground subsidence is a geological hazard characterized by the downward movement or sinking of the earth's surface. Such incidents, particularly in densely populated urban areas, can cause human casualties and extensive property damage, and various social problems such as the paralysis of transportation infrastructure and anxiety among residents [1]. Ground subsidence is a phenomenon where the ground surface collapses, triggered by both natural processes and human activities. It has a similar accident pattern to sinkholes. Unlike sinkholes, which are caused by the dissolution of soluble bedrock (such as limestone strata), ground subsidence occurs due to the loss of soil particles under the ground [2]. Hence, ground subsidence can be prevented in advance through the management of underground soil particles and cavities. An effective prevention of ground subsidence would require a thorough understanding of its causes and mechanisms. Several countries experiencing rapid urbanization are currently implementing management strategies to counter the threat of ground subsidence. In Florida, U.S.A, information on how to use the ONE-CALL SYSTEM and the locations of underground facilities isprovided to the managers of underground facilities and excavation or demolition workers to prevent damage to underground facilities caused by excavation or demolition in underground spaces [3; 4]. In Japan, periodic cavity inspections are performed on roads, and research has been conducted to prioritize maintenance activities through risk assessment [5; 6]. In Singapore, roads are regularly inspected, along with the operation of 24 h reporting hotlines and the construction of a repair system within 24 h [7]. In South Korea, the \"Special Act on the Safety Management of Underground Facilities\" has been in effect since 2018 to manage disasters in underground spaces. As part of the underground safety information system construction related to the act, an integrated underground space map and an underground space safety management system have been constructed [8]. For ground subsidence management, ground penetrating radar has been used to detect underground cavities in advance [9; 10]. According to a survey conducted by the Seoul metropolitan government in Korea, the main cause of ground subsidence is damage to underground pipelines [11]. Damage to underground pipes causes a mechanism whereby a cavity occurs due to the loss of underground soil particles in the damaged area, and the ground surface collapses with its gradual expansion [12]. The construction of underground facilities is facilitated by backfilling with sand. In Japan, several experimental studies have been conducted to simulate underground cavities using standard sand. Through the results of laboratory model tests, the relevant relationships between ground conditions or cavity geometry and the bearing capacity of the ground have been established [13]. Laboratory model tests were conducted to simulate ground subsidence using silica sand, along with research on cavity formation, expanded geometry, and subsidence mechanisms [5; 14; 15; 16; 17]. The drainage time and ground behavior characteristics with respect to the permeability were identified using samples with different particle sizes. To identify the mechanism of cavity formation caused by water leakage from sewer pipes, which is known as the main cause of ground subsidence, cavity formation characteristics were identified by repeatedly injecting water under a constant pressure into the bottom of a soil box. In addition, the relationship between the ground stress and displacement in the presence or absence of cavities was established by simulating underground cavities in advance [18]. Laboratory model tests were conducted under various conditions to identify the ground subsidence mechanism. However, these tests were conducted on a small scale (a width of 10 cm or less). This makes it difficult to exclude the interference of the soil box while identifying the ground subsidence mechanism [19].