Rock bursts are a common dynamic disaster in coal mines, and they crucially affect the safety, economics, and efficiency of mining operations. The mitigation and control of rock bursts is challenging owing to their violent, unpredictable characteristics. 1,2 Rock bursts are characterized by the sudden release of elastic strain energy in rock and coal during mining or roadway excavation. The mining-induced redistributed high-stress regions around surrounding rocks are crucial for the evaluation of rock bursts risk, particularly when using the longwall mining method. The excavation is surrounded by thick layers of hard, intact rock capable of storing high levels of strain energy. 3 Therefore, the determination of mining-induced stresses is essential for the evaluation of rock bursts risk in coal mines. Several approaches have been employed to assess and calculate the stress distribution around the extraction field, such as empirical and analytical, 4-9 numerical simulation, [10][11][12][13][14][15] and field monitoring [16][17][18][19][20] approaches.
The occurrence of rockburst in coal mines is closely related to the three-dimensional stress in coal and rock mass. Through hydraulic fracturing measurements in roof, the three-dimensional stress in roof before and after hydraulic fracturing as well as during working face advancing was monitored, and the effect of hydraulic fracturing in roof on controlling rockburst was studied. The test results show that (1) after hydraulic fracturing, the three principal stresses in roof decreased remarkably, whose maximum reduction was about 20%, while the elastic strain energy in roof decreased by about 31% as well; (2) as the working face advanced, the three principal stresses in roof in front of the working face would increase continuously until reaching peaks and induce the strata fracture, and the variation of the elastic strain energy in roof was basically consistent with that of the magnitude of three-dimensional stress; (3) with hydraulic fracturing, the position of the peak stress moved from 11 m to 21 m in front of the working face, the peaks of the three principal stresses decreased, whose maximum reduction was about 32%, and the peak of the elastic strain energy in roof also signi cantly decreased by about 51%. The eld investigation shows that hydraulic fracturing in roof can release the stress and elastic strain energy in coal and rock mass instantaneously, and will reduce the risk of rockburst; hydraulic fracturing in roof can be an effective pre-destressing method to reduce the stress and energy concentration in coal and rock mass during mining, and will prevent the risk of rockburst.
Roof rocks in coal mines are subjected to the combination of in situ stresses and dynamic stresses induced by mining activities. Understanding the mechanical properties of roof rocks under static and dynamic loads at medium strain rates is of great significance to revealing the mechanism of rock bursts. In this study, we employ the digital image correlation (DIC) technique to investigate the energy concentration and dissipation behaviors, failure mode, and deformation characteristics of roof rocks under combined static and dynamic loads. Our results show that both the static pre-stress and dynamic loading rate have significant effects on the uniaxial compressive strength of rock specimens. From the energy principle, when the static pre-stress is the same, both elastic strain energy density and dissipated energy density increase with increasing dynamic loading rate. The hazard of rock bursts increases with decreasing static pre-stress and increasing dynamic loading rate. At higher dynamic loading rates, more cracks are generated, and the failure becomes more violent. The crack initiation, propagation and coalescence processes are identified, and the failure mode is closely related to the evolution of the global principal strain field of the rock specimens.
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