Study on the macro-meso degradation mechanism of mechanical properties of siltstone under different temperatures

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 1 000 ℃, 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.