Coupling Influence of Inclination Angle and Moisture Content on Mechanical Properties and Microcrack Fracture of Coal Specimens

Abstract

Abstract Waterproof coal pillar in the underground reservoir is affected significantly by inclination loads and water erosion. Comprehensive understanding of the influence of moisture content and inclination angle on the mechanics properties and microfracture behavior of coal specimens plays an important role for revealing the instability mechanism of waterproof coal pillar. In this paper, a new modified combined compression and shear test (MC-CAST) system was firstly developed for achieving the specimen inclined loading and then used to carry out the inclined uniaxial compression test of water-bearing coal specimens at the inclination angle of 0°, 5°, 10°, and 15° and the moisture content of 0%, 2.42%, 5.53%, 7.55%, and 10.08%. The mechanical and microfracture behaviors of water-bearing coal specimens were analyzed and discussed by AE techniques and SEM. Research results indicate that ① the peak shear stress, peak strength, and elastic modulus of coals showed obvious moisture content weakening effect, and the weakening effect was more significant at larger inclination angle; ② the peak strength and elastic modulus decreased nonlinearly as inclination angle increased, of which decreasing amplitude was greater at high moisture content than that at low moisture content; ③ the peak shear stress gradually increased with the increasing inclination angle for 0%-7.55% moisture content, while it increased first and then decreased, and reached the maximum value at 10° inclination angle for 10.08% moisture content; ④ the stress thresholds of crack closure, initiation, and damage of coal specimens decreased gradually with the increase of moisture content and inclination angle, but the ratio of above three thresholds to their corresponding peak strength was basically unchanged; and ⑤ the erosion-stripping effect of bound water and pore pressure effect of free water, respectively, caused initial damage and secondary superposition damage of coal specimens before and after inclination loading test. Combining with the promotion effect of additional shear stress on crack initiation, propagation, and coalescence, the coupling effect of the above three factors was the fundamental reason for the strength deterioration of water-bearing coals under inclined loading. The research results can provide an important basis for the strength and stability evaluation of waterproof coal pillars of underground reservoirs in ecologically fragile areas.