This study investigates the degradation effect of high temperature during underground coal gasification on coal seam roof rock. Using a specific heat capacity measuring instrument, XTDIC three-dimensional full-field strain measurement system and scanning electron microscope(SEM), six groups of rock specimens were treated separately for uniaxial compression tests.Results show that 400 ℃ is the critical threshold for the transition of mechanical properties of specimens: At 25 ℃, 200 ℃ and 400 ℃, increasing temperature led to decreasing porosity, more tightly packed mineral particles, postponed occurrence of deformation localization, decreasing value of maximum displacement dislocation evolution and more intense ejection failure; At 600 ℃, 800 ℃ and 1000 ℃, specimens exhibited aggravated internal damage, significantly increasing porosity, degrading mechanical properties, earlier occurrence of deformation localization and intensified spalling failure. With rising temperature, their uniaxial compressive strength, elastic modulus and thermal conductivity showed initial increase and subsequent decrease. The transformation process of the failure mode is accompanied by varying degrees of ejection-spalling damage. This study reveals the degradation patterns and threshold of siltstone under different temperatures during underground coal gasification, and its macro-meso damage evolution mechanism, providing a theoretical basis for roof stability evaluation and prevetion of engineering disaster in high-temperature surrounding rock during gasification.

To investigate the influence of interfacial crack angle on the mechanical properties of coal-rock composite samples, uniaxial compression tests were conducted on samples with interface crack angles α (the angle between the interface crack and vertical direction) of 0°, 15°, 30°, 60°, 75°, and 90°. Combined with acoustic emission system and XTDIC three-dimensional full-field strain measurement system, the characteristics of characteristic stress, deformation and failure characteristics, and energy evolution of rock-coal composite samples with interfacial crack were studied. under axial stress, the fractured zones within the coal sample are more prone to developing stress concentrations, leading to crack initiation and propagation. As α increases, the crack initiation stress and damage stress of the composite specimen gradually decrease.Interfacial crack facilitate the initiation, development, and propagation of internal cracks within composite samples, thereby weakening their overall load-bearing capacity.With the increase of α, the pre-peak elastic energy density of the composite sample gradually decreases, and the uniaxial compressive strength and elastic modulus of the sample show a decreasing trend. The propagation and coalescence of internal cracks in coal and rock samples lead to the failure of composite samples.With the increase of α, the macroscopic failure mode transitions from a tensile-shear mixed mode accompanied by localized ejection failure to one predominantly characterized by tensile failure, with a gradual decrease in post-peak surplus energy density. Concurrently, the failure of coal sample leads to the rebound deformation of rock sample, and with the increase of α, both the magnitude and rate of rebound deformation in the rock sample progressively rise.