Damage accumulation of coal caused by cyclic loading and creep impacts may ultimately lead to rock failure and dynamic disasters. Quantifying this behavior is crucial for evaluating the mechanical response under creep and cyclic loading processes. Here, creep tests were conducted after cyclic loading and unloading at 10 MPa confining pressure. The elastic modulus and Poisson's ratio indicated fatigue damage during the entire loading process. The elastic moduli in the unloading stage were slightly higher than those in the loading stage. The gap in Poisson's ratio decreased in the cyclic loading procedure. The axial and volumetric strains exhibited a slightly negative exponent trend. The stress–strain response indicated hardening and softening effects under constant axial stress. The hardening effect dominated the peak strength at lower axial stresses. The creep-hardening deformation was 0.17 and 0.21 under 88 and 100 MPa, respectively. However, the softening effect played a more significant role at higher applied stresses. Additionally, the peak strength decreased by approximately 2.7 MPa when the axial stress increased from 100 to 102 MPa. The coal near the mining-induced stress boundary experienced cyclic damage, and the creep stress gradually decreased. Thus, the mining-induced stress boundary evolution rate initially increased and was mitigated as the mining-induced stress reduced. Next, the boundary expanded outward and gradually stabilized until the applied loading did not lead to obvious rock failure. Finally, the mining-induced stress boundary in the entire gob may be flat during the long-term evolution. Considering the creep failure time and choosing a suitable time to determine the boundary are essential for preventing dynamic disasters.
Abstract The revelation of the distribution characteristics of stress and fractures in the floor strata is of great significance to improve the regional gas drainage efficiency and to eliminate or reduce the risk of coal and gas outburst in adjacent coal seams during multi‐seam mining. In this paper, the stress distribution in the floor under mining condition was derived, and the characteristics of stress zones in the floor were elaborated while mining the 22201 working face in Shaqu coal mine in Shanxi province, China. Besides, the propagation laws of fractures in different stress zones were theoretically analyzed, and the distribution law of fracture angles in the floor strata under different mining‐induced stresses was obtained. The research results demonstrate that the compression and transition zones show high vertical stress and less developed fractures and are dominated by fractures with a large angle (>60°), which is unfavorable for gas drainage. The expansion zone in the floor within 2‐40 m in the rear of the working face is characterized by high unloading degree of vertical stress and developed fractures. Moreover, fractures with a small angle (<60°) dominate the spatial distribution of the fractures, which is favorable for gas drainage. The research results can not only be a beneficial supplement to the theory of coal and gas co‐exploitation in multi‐seam mining but also provide theoretical support for optimizing the design parameters of cross‐measure boreholes under similar conditions.
Abstract Broken coal and rock masses are the major part of the goaf. The compaction characteristics of coal and rock masses and the breakage law of whose particles during compaction exert an important influence on various aspects including control of strata motion, prediction of surface subsidence, and backfill mining. In this paper, the triaxial compaction experiment on broken coal‐rock masses with different mixing ratios was carried out. The test results showed that with the increase of stresses, the strain of coal‐rock masses gradually rose while the porosity, bulking factor, and degree of compaction gradually declined. During the compaction of coal‐rock masses, the fitting curves of the strain, porosity, bulking factor, and degree of compaction with stresses of coal samples all appeared as a cubic function of stresses. The breakage behavior of coal particles underwent three stages: structure re‐arrangement and breakage of particles, particle breakage, and compression‐induced deformation of particles. With increasing stress, the crushing amount of particles gradually grew while the increase rate of the crushed particles gradually decreased and the larger the particle strength was, the lower the increase rate of the crushing amount. Additionally, in the compaction process of samples, particle breakage mainly appeared before the stress reached to 8 MP a while the coal and rock particles were hardly crushed after the stress was larger than 8 MP a. With increasing stresses, the particle size gradation of samples gradually became reasonable and the lower the particle strength of samples was, the more reasonable the particle size gradation of compacted samples. The particle size gradation of various compacted and crushed samples showed a favorable fractal characteristic. In the stage with a low stress, the value of fractal dimension D rapidly grew and the fractal dimensions D of various samples tended to be stabilized after the stress reached to a high level.
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