Surface deformation and rupture characteristics of overburden during underground coal gasification under high temperature

During underground coal gasification (UCG), the in-situ coal seam is continuously consumed and converted into coal gas through high-temperature pyrolysis and gasification, forming a combustion cavity. The roof strata of the combustion cavity are subjected to stress concentration caused by high temperatures and chemical mining. The thermal-mechanical coupling effect significantly exacerbates the risk of rock mass instability, affecting the stability of the underground coal gasification process. Taking the sandstone from the coal seam roof of the Shanghai Temple Mining Area in Ordos City as the research object, uniaxial compression tests were conducted on standard sandstone samples after high-temperature heat treatment. By combining acoustic emission (AE) technology and digital image correlation (DIC) technology, the AE signals and surface deformation characteristics of the samples were investigated. The fracture mechanism of sandstone under thermomechanical coupling was revealed. The following main conclusions were obtained. ① The mechanical properties of sandstone exhibit significant temperature dependence, with the elastic modulus decreasing after the temperature reaches 200 ℃. The uniaxial compressive strength reaches a maximum value of 54.51 MPa at a temperature of 600 ℃, which is 57.8% higher than that at room temperature. The two exhibit a non-synchronous response, with the peak elastic modulus preceding the peak compressive strength. ② Observation using DIC technology revealed that the localisation of the strain field intensified at high temperatures. At a temperature of 600 ℃, the main crack migrated from the interior of the sample to the boundary, expanding the area of damage. ③ When sandstone samples are heated to temperatures ranging from 200 to 1000 ℃, tensile cracks predominate during the consolidation stage to the yield stage, while shear cracks predominate during the post-peak failure stage. At a temperature of 600 ℃, shear cracks are most active. ④ In the post-peak stage, the b-value of acoustic emission fluctuates rapidly and decreases, with medium- and small-scale fractures predominating. The rapid decrease in the b-value occurs earlier than the final failure stage of the sandstone sample. By combining AE and DIC technologies, complementary advantages are achieved at both the macro and micro levels. When the b-value continuously decreases to the critical threshold for this type of rock and surface strain becomes concentrated, an early warning signal is triggered. Measures such as adjusting gasification parameters are then implemented to prevent collapse. The experimental results provide scientific basis for controlling the stability of surrounding rock during coal underground gasification, predicting potential failures, and regulating temperature fields.