Static mechanical properties and microstructural damage characteristics of CO<sub>2</sub> adsorbed coal mass

In order to explore the influence of different CO₂-load coupling effects on the mechanical properties and meso-damage and failure characteristics of coal mass, the uniaxial loading tests of raw coal under different CO₂ pressures were carried out by using industrial CT scanning system, MTS 816 rock mechanics loading device and self-developed gas-solid coupling test device. The influence of CO₂ pressure on the mechanical properties and failure mode of coal was analyzed. Based on image processing technology, the meso-damage evolution characteristics and crack propagation laws of coal bodies are explored from two-dimensional and three-dimensional scales respectively, and the mechanical response mechanism of coal bodies to CO₂-load coupling is revealed from the macro-meso-level. The results indicate that: (1) the stress-strain curve of coal mass under gas-solid coupling can be divided into four typical stages, and both peak strength and elastic modulus exhibited a degradation trend with increasing initial CO₂ pressure. (2) Coal fracture expansion can be divided into fracture adsorption and expansion stage, fracture compaction and closing stage, new fracture initiation and development stage, and fracture rapid expansion and penetration stage, the fracture volume, fracture rate and three-dimensional fractal dimension are positively correlated with CO₂ pressure, additionally, influenced by the CO₂ gas wedge effect, the spatial distribution of fractures inside the loaded coal mass becomes complex, and its failure form gradually changes from a single tension failure to a compound failure in which tension and shear coexist. (3) The damage variable was defined based on three-dimensional porosity, and it was found that its value gradually increased from 0 to 0.34, approximately exhibiting an exponential rise. By comparing the peak strength of the coal under gas-solid coupling with the theoretical strength obtained from the damage variable, the rationality of the damage variable was validated.