Rock burst is a physical explosion associated with enormous damage at a short time. Due to the complicity of mechanics of rock burst in coal mine roadway, the direct use of traditional investigation method applied in tunnel is inappropriate since the components of surrounding rock are much more complex in underground than that of tunnel. In addition, the reliability of the results obtained through these methods (i.e., physical simulation, theoretical analysis, and monitoring in filed application) is still not certain with complex geological conditions. Against this background, present experimental study was first ever conducted at initial site to evaluate the effect of shock wave during the rock burst. TDS-6 microseismic monitoring system was set up in situ to evaluate the propagation of shock wave resulting in microexplosions of roadway surrounding rock. Various parameters including the distance of epicentre and the characteristic of response have been investigated. Detailed test results revealed that(1)the shock wave attenuated exponentially with the increase of the distance to seismic source according to the equation ofE=E0e-ηl; particularly, the amplitude decreased significantly after being 20 m apart from explosive resource and then became very weak after being 30 m apart from the seismic source;(2)the response mechanics are characteristic with large scatter based on the real location of surrounding rock despite being at the same section. That is, the surrounding rock of floor experienced serious damage, followed by ribs, the roof, and the humeral angles. This in situ experimental study also demonstrated that microseismic monitoring system can be effectively used in rock burst through careful setup and data investigation. The proposed in situ monitoring method has provided a new way to predict rock burst due to its simple instalment procedure associated with direct and reasonable experimental results.
A novel method for fracturing coal is presented in this paper. A chemical solution is injected into coal under high pressure, whereby the coal is fractured and subsequently weakened by chemical erosion over time to produce an anti-impact soft structure. In this study, the mechanical properties of coal under chemical erosion were investigated, and the fracturing design parameters were optimized. The uniaxial compression test and the split Hopkinson pressure bar (SHPB) test were used to determine the dynamic and static mechanical properties of coal after 20 days of immersion in different chemical solutions. After chemical solution erosion, the dynamic and static compressive strengths and elastic modulus of the coal decreased according to an exponential power law in the damage variable. The chemical treatment increased the duration of the pore compaction stage and decreased that of the elastic deformation stage, while decreasing the brittleness and increasing the ductility of coal. The acoustic emission (AE) curve of the immersed coal samples consisted of four stages corresponding to those of the stress-strain curve: pore compaction-closure, a slowly rising linear elastic regime, steady-state prepeak crack propagation, and unsteady crack propagation at the peak strain. The increase in the damage variable of the coal sample from chemical erosion led to a lower dissipated energy, a higher fractal dimension, and a more fragmented coal sample. The effect of the investigated chemical solutions on weakening the coal mechanical properties decreased in the following order: alkaline solution > acidic solution > NaCl solution > distilled water. The experimental results provide a reference for weakening fractured coal seams.
A coal-rock (CR) combined body can be used to simulate structures of coal and rock strata, and its impact-induced failure characteristic conforms more close to engineering practice. Exploring the mechanical properties and impact energy in a CR combined body contributes to better predictions of rock bursts in coal mines. In the study, the mechanical properties of CR combined bodies with four different inclinations (0°, 15°, 30°, and 45°) of structural planes were measured, and also their failure mechanism was analysed. Based on the theory of particle mechanics, a calculation model for impact energy in a CR combined body with inclinations was established and then verified by using monitored acoustic emission (AE) data. The test results showed that inclination affected mechanical properties and failure characteristics of the CR combined body, i.e., the larger the inclination, the lower the strength and impact energy in the CR combined body and the lower the level of damage. The proposed calculation model for impact energy revealed the mechanical essence of energy accumulation and release of a CR combined body, providing a reference for investigating rock burst in coal mines.
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