An underground coal gasification (UCG) process is strongly exothermic, which will cause thermal damage on rock cap. We proposed a new thermal damage numerical model based on a two dimension particle flow code (PFC2D) to analyze the inception and extension of cracks on pre-cracked red sandstone, which were thermally treated at a temperature of 25~1000 °C. The results indicated that: (1) a thermal damage value D_T obtained by extracting the thermal crack area of scanning electron microscope (SEM), which can be used as an indicator of the degree of thermal damage of the sandstone; (2) a thermal damage numerical model established by replacing the flat-joint model with the smooth-joint model based on the thermal damage value D_T, this approach can properly simulate the mechanical behavior and failure patterns of sandstone; (3) the critical temperature for strength reduction was 750 °C. The peak strength increased as pre-treatment temperature increased from 25 to 750 °C and then decreased. The elastic modulus E₁ decreased with the increasing thermal treatment temperature; (4) micro-scale cracks initiate from the tip of the prefabricated fissure, and expand along the direction of prefabricated fissure, finally developing into macroscopic fracture. This approach has the potential to enhance the predictive capability of modeling and presents a reliable model to simulate the mechanical behavior of thermally damaged sandstones, thereby offering a sound scientific basis for the utilization of space after UCG.

The rheological properties and seepage laws of fractured rock masses have a highly significant impact on the long-term stability of engineering rock masses. To study the rheological properties and seepage laws, rheological experiments under different loading paths were designed and conducted on sandstone samples with pre-fabricated single fractures. The results indicated that (1) as the axial stress or pore pressure increases, or the confining pressure decreases, the creep strain (both axial and radial directions) during the creep tests increases, and their stain rate decreases; (2) compared to the pore pressure, the axial stress and confining pressure have a more significant influence on the flow rate during the creep process. As the axial stress increases or the confining pressure decreases, the flow rate shows an increasing trend; (3) the higher connectivity results in higher final flow in an exponential function, indicating that the connectivity ratios can well reflect the permeability of the fractures; (4) the changes in the stress field may lead to the transition of the surrounding rock from a stable state to an unstable state, i.e., the occurrence of creep failure, or water inrush. Therefore, when the state of the surrounding rock changes, strengthened monitoring and proper preventive measures should be implemented.