Prediction method of surface subsidence due to underground coal gasification under thermal coupling

Underground coal gasification (UCG) is an essential part of the low-carbon green coal mining technology system. The implementation of the “double carbon” goal of the coal industry has brought excellent development opportunities for UCG. However, UCG will also cause rock movement and surface deformation, resulting in serious threat to safety of ground buildings (structures) when use UCG to recover the “three under” coal that is difficult to mine by underground mining methods. How to accurately predict the subsidence considering characteristics of UCG has become one of the critical bottlenecks limiting the industrial application of UCG. Based on this, combined with the characteristics of ‘strip mining-surface mining’ backward UCG process, this paper explores the causes of surface subsidence caused by UCG under the thermal coupling, and concludes that the root of surface subsidence caused by UCG is the deflection of rock strata and the compression deformation of coking barrier coal pillar. Further, the calculation method of deflection deformation of UCG roof under thermal-mechanical coupling is established, and the yield model and compression calculation method of gasification coal pillar based on D-P criterion are proposed. Then, according to the principle of equivalent subsidence space, an accurate prediction model of surface subsidence of UCG under thermal coupling is constructed, and the effectiveness and accuracy of the new method are verified by the measured data of UCG in Ulanqab. The research results have important practical significance for promoting the recovery of difficult-to-mine “three under” coal resources and the industrialization for UCG.