Abstract
Under coupled compression–shear loading, the failure and instability behavior of inclined pillars is different from that of horizontal pillars. To enhance the reliability and accuracy of pillar strength design, the influence of different inclination angles and loading rates on mechanical property and the failure behavior of inclined pillar should be studied. In this paper, the combined compression and shear test (C-CAST) system was developed, and mechanical properties and macro failure behavior of coal samples under different inclination angles and loading rates were studied, and acoustic emission (AE) technology was used to determine the internal cracking mode of the sample. The results show that with the increase of inclination angle, the peak shear stress of coal sample increases gradually, while the peak axial stress and elastic modulus slightly increase first and then decrease, and reach the maximum value at an inclination angle of 5°. Within the inclination angle range of 0°–15°, with the increase of loading rate, the peak axial stress and elastic modulus of coal samples first increase and then decrease, while the loading rate corresponding to peak axial stress and elastic modulus decreases. Within the inclination angle range of 20°–25°, the peak axial stress and elastic modulus of the sample gradually decrease with the increase of loading rate. The failure mode of coal samples changes from tension-splitting failure (0°–5°), tension–shear composite failure (10°) to single shear failure (15°–25°). Meanwhile, the loading rate has little effect on the failure mode of coal samples, but has a significant effect on the failure degree. When the loading rate is 1.0 and 10 mm/min and the inclination angle ranges from 0°–5°, the proportion of tensile crack is significantly greater than that of the shear crack, and tensile failure is the main failure mode; when the inclination angle ranges from 10°–25°, the proportion of shear crack is more than 50% and increases gradually with the increase of inclination angle, and shear failure is the main failure mode. This law is consistent with the macroscopic failure mode of the sample.
Highlights
IntroductionThe stability of coal pillars is the premise of safe and efficient mining
During the mining process, the stability of coal pillars is the premise of safe and efficient mining
The results showed that with the increase of strain rate, the peak strength of the sample increases gradually; when the strain rate is less than 1 × 10−5 s−1, the single shear failure mode is presented on the failure surface, and when the strain rate is greater than 1 × 10−5 s−1, the surface cracks are crisscross and interlinked and distributed in a complex form
Summary
The stability of coal pillars is the premise of safe and efficient mining. Uniaxial compression [7,8,9,10,11,12], biaxial compression [13,14], triaxial compression [15,16,17,18,19], Brazilian splitting [20,21] and shear tests [22,23,24] are widely employed to study the deformation and failure behavior of rock samples under different loading conditions These studies have certain limitations, namely the actual situation of the coupled compression–shear loading on inclined pillars cannot be effectively simulated. It is important to study the effect of loading rate on the mechanical property and failure behavior of coal samples under the coupled compression–shear loading to reveal the instability mechanism of the inclined pillar under the mining
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