In recent years, coal and rock dynamic disasters are becoming more and more severe, which seriously threatens the safety of coal mining. It is necessa

The steeply inclined and extremely thick coal seams (SIETCS) under the condition of gob filling frequently suffer from the occurrence of rockbursts. Figuring out the mechanisms of rockbursts under this condition for taking targeted measures to mitigate rockburst hazards in SIETCS is of great significance. Using the typical SIETCS with an average dip angle of 87° at Wudong Coal Mine (WCM) as a case study, a mechanical model and elastic deformation energy (EDE) function of a “steeply inclined suspended roof structure” was developed, and the influence factors were analyzed by theoretical analysis. Simultaneously, the rockburst risk assessment was carried out based on the theory of a rockburst start-up. The pressure relief measures are optimized by comparing the pressure relief effects of three kinds of destress blasting schemes. The results indicate that the damage characteristics of rockburst are mainly floor heave, the sidewall’s inward deformation and roof subsidence. The damage degree of headentry on the roof side is more severe than that of tailentry, and the resultant impacts showed the directionality from the roof side to the coal side. The steeply inclined and suspended roof breakage is one of the main causes for the occurrence of rockbursts. The EDE of the roof increases with an increasing dip angle of the coal seam from 0° to 72.6° and then decreases as the dip angle increases. Furthermore, that increase is accompanied by the decrease of the lateral pressure coefficient and the supporting force coefficient. The EDE stored in the roof is sufficient to cause roof breakage and induce rockburst after the complete roof exceeds a certain length. The mechanism of rockburst in SIETCS for fully mechanized top-coal caving mining under gob filling conditions was proposed, i.e., “high compressive stress concentration plus breakage of the suspended roof-induced stress” rockburst, and this is further verified by ground destruction, microseismic (MS) monitoring and numerical modeling. The results also indicate that alternate deep and shallow hole-blasting modes are more suitable for pressure relief in SIETCS.

The time–frequency characteristics of electromagnetic radiation (EMR) waveform induced by rock fracture are very important to the monitoring and early–warning using the EMR method for the mine rockburst. The empirical wavelet transform (EWT), as a waveform time–frequency analysis method, has the advantages of a clear theoretical basis, convenient calculation, and no modal aliasing. To apply EWT to the field of EMR time–frequency analysis, the operation of Fourier axis segmentation of EWT is improved. In detail, the adaptive selection method for a window width of closing operation and the adaptive determination method of segment number of Fourier axis are proposed for EWT. The Fourier axis obtained by short–time Fourier transform (STFT) is used in the EWT process, rather than that obtained by discrete Fourier transform (DFT), taking a better Fourier axis segmentation effect. The improved EWT together with Hilbert transform (HT) applied to the time–frequency analysis for the EMR waveform of rock fracture, and the time–frequency spectrum obtained by EWT–HT can well describe the time–frequency evolution characteristics. Compared with STFT and Hilbert–Huang transform (HHT), EWT–HT has significant advantages in time–frequency resolution and overcoming modal aliasing, providing a powerful tool for time–frequency analysis for the EMR waveform induced by rock fracture.

A multisignal nanosecond synchronous acquisition system to measure acoustic emission (AE) and electromagnetic radiation (EMR) generated during the process of loading and failure of coal and rock samples is established. The correlation between the energy of the AE and EMR signals and the loading stress of outburst coal-rock samples was studied, and the characteristics of the AE and EMR signals during the process of loading and fracturing the outburst coal and rock samples were analyzed. The results show that (1) before the failure of the outburst coal and rock samples, the fluctuation of the AE and EMR signals is the largest, with the same rising and falling trend, and the intensity is not strictly positively correlated, with the phenomenon of low EMR when the AE intensity is high; (2) the EMR and AE deviation degree and frequency exhibit a good response to coal and rock fracturing. The correlation between EMR and stress drop is stronger than that of AE, and the AE signal is richer than the EMR signal. The results show that it is feasible to develop combined AE and EMR early warning technology to improve the early forecasting accuracy of coal and gas outbursts.

To reveal the discrete characteristics of the instantaneous frequency of the electromagnetic radiation (EMR) waveform induced by coal and rock fracture, the uniaxial compression experiments for coal and rock samples were carried out, and the EMR signals with full waveform were acquainted and stored. The empirical wavelet transform is used to filter and de-noise the EMR waveform, and then the short-time Fourier transform is used to analyze the time-frequency characteristics of the waveform. The discrete characteristics of the instantaneous frequency with a larger amplitude and the relationship between the centroid frequencies and peak-to-peak values (Vpps) of the EMR waveforms are statistically analyzed. The results show that the centroid frequency of 0–100 kHz is negatively correlated with the Vpp, and the relationship between them shows a logarithm function relation. The instantaneous frequency of the EMR waveform of coal and rock fracture has significant discrete characteristics. In detail, for the rock sample, the instantaneous frequencies with relatively large amplitude are mainly 4.5 kHz, 19.5 kHz, 22.0 kHz, and 27.5 kHz; for the coal sample, the instantaneous frequencies are mainly 1.0 kHz, 4.5 kHz, 9.0 kHz, and 74.0 kHz. This discrete characteristic is determined by the natural properties and fracture characteristics of the sample. Compared with the homogeneous rock samples, the internal cracks of the coal samples are well developed and show strong anisotropy, resulting in the discrete characteristics of the instantaneous frequency being relatively weaker. The findings have certain guiding significance for optimizing the design of the EMR monitoring frequency band and improving the pertinence and accuracy of the monitoring and early warning for coal and rock dynamic disasters.