# Surface Subsidence Modelling Induced by Formation of Cavities in Underground Coal Gasification
## 1 Introduction  
Underground coal gasification (UCG) is a process in which coal is converted into product gas without mining by artificially enhancing gas permeability in a section of a coal seam, igniting the coal, partially combusting and gasifying coal by means of injected oxidants, and producing the product gas for cleanup and processing for a variety of end uses. During the UCG process, the volume of the cavity increases progressively with coal consumption and thermo-mechanical spalling, if any, from the roof. The formation of an underground cavity will lead to the redistribution of stress field near the cavity and the disturbance will lead to surface subsidence. Hence, it is significant to investigate the effect of the formation of a cavity on surface subsidence. [PERSON] et al. [1] more accurately simulated surface subsidence during UCG by introducing new elastic parameters and empirical correction factors. [PERSON] et al. [2] studied the issues of surface settlement due to the mining of a horizontal coal seam by using the elastic thin plate theory. [PERSON] et al. [3] proposed a new thermal-mechanical-chemical displacement multifield coupled numerical simulation method, which was based on controllablepumping injection point (CRIP) technology to numerically simulate the coal seam gasification working face, and analyzed the evolution law of temperature and displacement fields after the gasification reaction. [PERSON] et al. [4] applied the complex function analysis method to solve the stress field of an elliptical formation cavity under a self-weight stress field and a vertically uniformly distributed load. By developing a thermomechanical coupling model, [PERSON] et al. [5] simulated heat transfer, stress distribution, and surface subsidence during underground coal seam gasification using ABAQUS. The material properties of rocks and coal were obtained from the existing literature and geomechanical tests were carried out on samples derived from the demonstration site in Bulgaria. Three days of gasification ere simulated by assigning a moving heat flux on a cell of size 2 m \(\times\) 2 m \(\times\) 2 m at a velocity of 2 m/day. Results of temperature and stress distribution showed that the developed numerical model was able to simulate the heat propagation and the stress distribution around cavities under a thermal-mechanical coupled loading during the UCG process. Also, the surface subsidence was found to be 0.08 mm after three days of gasification for the case studied. [PERSON] et al. [6] studied the influence of different coal seam constitutive models on the thermal-mechanical response during the UCG process by coupling thermodynamic and geomechanical modules and used stress rebalancing technology to simulate the elastic-plastic behavior of coal seams. [PERSON] et al. [7] proposed an elliptical cave expansion model for estimating the bearing capacity and settlement of circular shallow foundations on clay. [PERSON] et al. [8] developed a soil flow protector (SFP) through field experiments and numerical analysis to reduce soft soil settlement caused by cave formation. [PERSON] et al. [9] simulated the movement of a coal seam roof and surface subsidence during shallow underground coal gasification process using a finite element model coupled with a heat transfer module in COMSOL. [PERSON] et al. [10] developed a coupled thermomechanical numerical model to simulate the underground coal seam gasification (UCG) process and simulated and predicted the ground settlement of the test site under different working conditions. [PERSON] et al. [11] discussed the risk assessment and protective measures for ground subsidence caused by underground mining. [PERSON] et al. [12] evaluated the impact of cave types on ground stability through a finite element analysis. [PERSON] et al. [13] proposed an approximate solution for predicting the settlement of underground cylindrical caves based on empirical and analytical methods. [PERSON] et al.