Abstract
To study fracture evolution and peak stress in burst risk coal samples (BRCSs) under true triaxial loading and unloading conditions, experimental and numerical research was applied to BRCSs under true triaxial stress paths entailing “x-direction displacement fixed, y-direction loading, z-direction unloading.” Both the experimental and the numerical results demonstrated that the peak stress borne by the BRCSs was not only affected by the initial stress but also had a negative exponential relationship with the ratio of the unloading rate and the loading rate (RURLR); therefore, peak stress equations of BRCSs under true triaxial loading and unloading conditions were established. The triaxial stress-time curves obtained by experiments and simulations exhibited an “elasticity-yield-destruction” phase, and the characteristics of the yield phase were determined by the RURLR. A typical BRCS was selected for velocity tomographic imaging to analyze the fracture evolution characteristics under true triaxial loading and unloading. The results showed that when the BRCS was subjected to a triaxial state of stress, the high- and low-velocity regions existed alternately due to the presence of the crack; during the elastic phase, the crack closed during loading in the previous phase was reopened upon unloading, so that the velocity of the sample decreased and a wide range of low-velocity regions could be formed; when entering the yield phase, the original crack continued to expand into a hole-through crack, leading to wider extreme values and ranges of these low- and high-velocity regions; at the breaking phase, multiple microcracks were generated around the hole-through cracks, decreasing the overall velocity, and showing point distributions characteristics of high- and low-velocity regions. Overall, many low-velocity regions with similar normal directions to the unloading direction were formed; these correlated well with macrofractures (postfailure).
Highlights
As the mining depth increases, rock burst becomes a serious mine disaster, which restricts the safety production of the coal mine [1,2,3]. e occurrence of rock burst is induced by many factors
Based on the stress path of the underground engineering excavation, Du et al [12] carried out true triaxial unloading compressive test and found the failure modes of the granite and red sandstone specimens changed from shear to slabbing with the increase of σ2
Two schemes were adopted. (1) e effect of RURLR: the samples are loaded to initial stress of 25 MPa, and the Y-axis is loaded at a certain rate; the ratio of the Z-direction unloading rate to the Y-direction loading rate is shown in Table 1; the X-direction displacement is kept constant until the sample is destroyed. (2) e effect of initial stress: the samples are loaded to different initial stresses; the ratio of the unloading rate in the Z-direction to the loading rate in the Y-direction is set to 1.5 for true triaxial loading and unloading tests. e displacement in the Xdirection is kept constant until the sample is destroyed
Summary
As the mining depth increases, rock burst becomes a serious mine disaster, which restricts the safety production of the coal mine [1,2,3]. e occurrence of rock burst is induced by many factors. In order to better understand the stress characteristics and fracture mechanism of rocks under true triaxial loading and unloading conditions, many experimental and numerical investigations on rocks have been carried out. Based on the stress path of the underground engineering excavation, Du et al [12] carried out true triaxial unloading compressive test and found the failure modes of the granite and red sandstone specimens changed from shear to slabbing with the increase of σ2. The experimental and numerical methods are used to study the mechanical and fracture evolution characteristics of BRCSs under the triaxial loading and unloading, so as to provide a reference for the risk prediction of roadway driving in burst risk coal seams
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