Numerical simulation of strip mining under thermal-mechanical coupling of underground coal gasification

Underground coal gasification (UCG) combined with the strip mining method has strong adaptability and is helpful to the stability control of surrounding rock，with a good application prospect. In order to study the evolution law of temperature field，stress field and displacement field of surrounding rock of the cavity during strip mining with thermo-mechanical coupling effects of UCG，COMSOL multiphysics was adopted to simulate the UCG strip mining of No.4 coal seam in Shanjiaoshu Mine，Panjiang，Guizhou Province. The stress and deformation characteristics of the surrounding rock after gasification and conventional mining were compared and analyzed. The results show that，along with the UCG working face being advanced，the temperature in the area about 2 m inside the surrounding rock first increases and then decreases，and the temperature conduction range has been expanding，with the maximum influence range of 10-12 m. The conduction range of temperature in the roof is the largest，followed by the coal on both sides，and the smallest at the floor. The high temperature of UCG will produce thermal stress in the surrounding rock of cavity，which makes the vertical stress of strip pillars of UCG obviously higher than that of conventional mining，and makes the overhanging surface of the cavity and the interface between the pillar and roof and floor bear horizontal compressive stress. The displacement trend of the roof and floor near the cavity is the same as that of the conventional method，but the displacement of the deep part of the roof and floor is opposite，that is，the roof moves up and the floor sinks. The horizontal displacement of the surrounding rock of the UCG cavity is obviously larger than that of the conventional strip mining，and the left surrounding rock in the mining area moves horizontally to the left and the right part moves horizontally to the right. The fluctuation amplitude of the immediate roof deformation during the UCG strip mining process is obviously larger than that of the conventional mining，and it moves up in the area with coal support，and the top of the model moves upward as a whole，and its deformation is greater than that of the conventional mining，which indicates that the thermal stress formed in the UCG process can prevent the subsidence and deformation of overburden rock to a certain extent. The results of numerical simulation can provide theoretical guidance for the engineering design of the UCG strip mining.