No abstract

Some industrial activities in mines, such as underground coal mining, destress blasting for preventing rockburst, and ground blasting for mining, can cause microseismic occurrence. The microseismic waveform contains abundant information on the hypocenter and propagation path, which is valuable to study the microseismic mechanism and propagation. Therefore, this study adopts the multifractal detrended fluctuation analysis (MF-DFA) and the Hilbert–Huang transform (HHT) method to study the nonlinear and time–frequency–energy characteristics of different types of microseismic waveforms. The microseismic waveform induced by mining and destress blasting has a higher dominant frequency (above 100 Hz) and shorter duration (less than 0.5 s) than ground blasting-induced microseismic waveforms (dominant frequency below 25 Hz and duration more than 3 s). Furthermore, for destress blasting-induced microseismic waveforms, the waveform is characterized by rich spectrum, complex energy attenuation, developed coda wave, and clear multifractal characteristics, which indicate that the waveform is more complex and variable. The complex underground geological environment and the superposition effect of blasting stress and mining stress are the main reasons. Moreover, the propagation distance and source energy of microseismic waveforms also greatly affect waveform characteristics. The results show that the waveform information of destress blasting-induced microseismic waveforms can describe the release process of blasting stress and mining stress. Based on this, a blasting efficiency index Be was proposed to evaluate the effect of pressure relief, and the classification system was developed. Then, the evaluation index was successfully applied to 63 rounds of destress blasting in the Yutian coal mine. The research results can provide a certain reference for some work such as the identification of different microseismic, rock dynamic failure process analysis, and evaluation of the destress blasting effect.

This research is aimed at investigating the influence of the coal height ratio on the mechanical properties and damage behavior of rock-coal-rock combined samples (RCRCS) under coupled static and dynamic loads. For this purpose, a uniaxial cyclic dynamic loading experiment with four different coal height ratios of RCRCS was conducted. Mechanical properties, failure modes, and wave velocity evolution of RCRCS were analyzed; the process of rock burst under coupled static and dynamic loads in rock-coal-rock combined structure was discussed. The following research results are obtained. (1) The peak strength of RCRCS under static and dynamic load decreases with the increasing coal height ratio as an inverse proportional function. (2) The loading and unloading modulus remains consistent for the same levels of dynamic load; the coal height ratio of 40% may be the limit for the stable value of modulus. (3) The increase of the coal height in RCRCS leads to a gradual increase of the energy release rate; the cracks develop preferentially in coal and then extend to rock sample. The distribution of AE events and damage is consistent with the distribution of passive wave velocity. The research results provide important scientific bases for the guidance of early warning of rock burst.

With the increase in mining depth, coal bursts have become a major challenge in the safe mining of steeply inclined and extra thick coal seams (SIETCSs). Based on a typical mining induced seismicity triggered coal burst (MSTCB) in SIETCS, a large-scale numerical model was developed using the Universal Distinct Element Code. The numerical model was calibrated and validated by laboratory results and field observations. The stress evolution, crack development and ejection velocity patterns in the MSTCB were analysed, and the effect of mining induced seismicity vibration velocity on the MSTCB was discussed. The results show that a triangular static stress concentration zone is formed in the coal on the roof side. And the high-energy mining induced seismicity leads to high dynamic stresses in the coal at the roof side rib and top of the headentry. Coal bursts occur under the superposition of static and dynamic stresses. The MSTCB results in tensile failure near the headentry surface and shear failure in the depth. The vibration velocity has a significant effect on the roof side rib and top of the headentry, while it has only a slight effect on the working face rib and bottom of the headentry. The dynamic stress and ejection velocity in the roof side rib and top of the headentry are positively correlated with the vibration velocity. Finally, measures for MSTCB prevention were proposed. The findings presented in this study can provide guidance for the prevention and control of MSTCBs in SIETCSs.

Coal bursts occurring in steeply inclined coal seams (SICSs) are increasingly severe. To solve this problem, a mechanical model for the distribution of static stress on coal-rock masses along panels and the distribution of dynamic load induced by the breakage of thick and hard roofs with propagation distance was established. The stress characteristics after a superposition of dynamic and static loads on the roof and floor roadways (Rr and Rf) were determined. In addition, precursory information characteristics and index sensitivities of four indices for dynamic loads and the CT index for static loads based on seismic tomography were separately analyzed. The monitoring and warning indices for SICSs and flat seams were compared. The results showed that the static stress of Rr was significantly higher than that of Rf, which provided a basis for the stress-triggering coal burst behaviors. Three indices for dynamic loads and seismic tomography results exhibited remarkable precursory information and high sensitivity. However, the performance of lack of shock index is poor. The continuous anomaly and the contradiction of indices at Rr and Rf can be considered as precursory information for predicting coal bursts.