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The geological conditions of coal mines in China are complicated. In recent years, with the continuous increase of intensity and depth of coal mining, coal and rock dynamic disasters are becoming more and more serious, and an important factor threatening the safety of coal mining. The accurate monitoring and early warning of coal and rock dynamic disasters are of great significance for disaster prevention. A large number of experiments conducted on both laboratory and field scales have demonstrated that the energy accumulated in rock material under loading can be released in the forms of acoustic emission (AE), electromagnetic radiation (EMR), etc. Therefore, AE and EMR, as the real-time, dynamic and continuous geophysical monitoring methods, have been widely used and played important roles in the field of monitoring and early-warning of coal and rock dynamic disasters in mines. To further study the time-frequency characteristics of AE and EMR and the relationships between these characteristics and load, and to provide the experimental basis for the monitoring and early warning of coal and rock dynamic disasters, an acoustic-electric full-waveform synchronous acquisition system of coal and rock fracture under loading was constructed in this paper. Using this system, the full-waveforms of AE and EMR of failure processes of coal samples under uniaxial compression were collected. The correlations among AE energy, EMR energy and load drop were studied, and the spectral characteristics of AE and EMR were analyzed. The results show that, (1) obvious acoustic-electric signals are emitted during the process of coal failure under loading. EMR is the paroxysmal signal, only accompanied by load drop and higher intensity AE. (2) Compared with AE, EMR has a higher correlation with load drop. There are high positive correlations among the cumulative values, relating to the cumulative released energy of coal fracture under loading, of AE energy, EMR energy and load drop. (3) The superiority frequency band of EMR is narrower than that of AE, with the former mainly concentrated in 1-25 kHz and the latter mainly in 1-280 kHz. Influenced by the initiation and propagation of the same crack, AE and EMR have similar low frequency components in the spectrum and main-frequency distribution.

In recent years, rockbursts have occurred frequently during the mining of thick coal seams in China. Use of the top-coal caving or slicing mining method to extract these thick seams may result in distinct geomechanical responses in the strata and, in turn, the pattern of rockburst occurrence around longwall layouts. To establish a thorough understanding of which method (caving or slicing) is better when it comes to preventing rockbursts during the extraction process, a suite of in situ rockburst measurements was conducted. Six typical rockburst-prone collieries were monitored during which a total of 110 rockburst events occurred. Numerical modelling was used to help interpret the observations. Here, we focus on the analysis of these field observations and the numerical simulations employed to develop a conceptual model for rockburst occurrence during caving mining of thick coal seams. We find that caving mining has a certain rockburst-reducing effect in that the method significantly decreases, or even avoids, the occurrence of rockbursts at coalfaces. Moreover, it reduces the scope of the damage likely to be incurred and the severity and frequency of rockbursting. According to the conceptual model, the rockburst- prone area during caving mining is generally the roadway section located a certain distance ahead of the coalface. This distance, which is roughly 10-54 m, is consistent with the rockburst damage observed on site (mostly 10-50 m ahead of the coalface). This rockburst pattern arises because caving mining results in reduced stress concentration, less bottom coal being retained, and wider-ranging fracture zones around the mine openings, especially ahead of the coalface. This makes the rock surrounding the mine openings less likely to accumulate high levels of elastic energy that need releasing and hence it is more difficult to induce rockbursting. Therefore, it is recommended that the top-coal caving method should be the preferred method of extraction when mining coal seams of a certain thickness. Key words: rockburst, thick coal seam, top-coal caving, slicing mining, field investigation, numerical simulation. Au cours des dernieres annees, des eboulements ont frequemment eu lieu pendant l'extraction miniere de couches de charbon epaisses en Chine. L'utilisation de la speleologie de charbon superieur ou la methode d'exploitation par tranchage pour extraire ces couches epaisses peut donner lieu a des reponses distinctes dans les strates geomecaniques et, a son tour, la tendance de l'occurrence d'eboulements autour des emplacements de longue paroi. Afin d'etablir une comprehension approfondie de quelle methode (speleo ou tranchage) est mieux quand il s'agit de prevenir les eboulements au cours du processus d'extraction, une suite de mesures d'eboulements in situ a ete effectuee. Six colliers sujets a des eboulements typiques ont ete suivis dont on a connu un total de 110 evenements d'eboulements. La modelisation numerique est utilisee pour aider a interpreter les observations. Ici, nous nous concentrons sur l'analyse de ces observations sur le terrain et les simulations numeriques utilisees pour developper un modele conceptuel pour l'occurrence d'eboulements pendant l'exploitation miniere de speleologie de charbon epais. Nous constatons que la speleologie miniere a un certain effet sur les eboulements et que la methode diminue considerablement, voire eviter, l'apparition d'eboulements sur le terrain. En outre, il reduit l'etendue des degats qui pourraient eventuellement souffrir et la gravite et la frequence d'eboulements. Selon le modele conceptuel, la zone sujette a des eboulements au cours de l'exploration par speleologie est generalement la section de chaussee situee a une certaine distance sur le terrain. Cette distance, qui est a peu pres 10-54 m, est compatible avec les dommages d'eboulements constates sur le site (plus souvent 10-50 m sur le terrain). Ce modele d'eboulements surgit, car l'exploitation par speleologie aboutit a une concentration de contrainte reduite, et de zones de fracture plus elargies, moins de charbon de fond etant conserve, et une plus large zone de fractures autour des ouvertures de la mine, surtout en avant du terrain. Cela rend la roche entourant les ouvertures de la mine moins susceptibles d'accumuler des niveaux eleves d'energie elastique qui doivent se liberer et c'est pourquoi il est plus difficile a induire des eboulements. Par consequent, il est recommande que la methode speleologie de charbon superieur devrait etre la methode preferee d'extraction lors de l'extraction de couches de charbon d'une certaine epaisseur. [Traduit par la Redaction] Mots-cles: eboulement, epaisse couche de charbon, speleologie de charbon superieur, tranchage minier, enquete sur le terrain, simulation numerique., 1. Introduction Rockbursts are a serious threat to the safe extraction of coal from mines. According to incomplete statistics, more than 700 rockbursts have taken place since 2008 in the [...]

In order to study the effect of rock strength on the overall instability of coal and rock, different coal and rock composite samples were tested under uniaxial compression condition, and the failure mode, stress-strain characteristics, uniaxial compression strength and acoustic emission characteristics during the uniaxial compression process were researched, besides, the effect of rock strength on mechanical behavior of composite samples was analyzed. The results show that the stress-strain curve of composite sample situates between that of the rock and the coal but closer with the coal stress-strain curve. With the increasing of rock strength, the rate of the combination sample from yield to peak is faster and faster. When the rock strength is low, the crack propagation in coal body expands to the rock, and tensile failure happens to the rock. With the same coal in the composite samples, when the rock strength is high, rupture occurs mainly in the coal. The ratio of yield point stress and peak stress of composite samples vary quite little with each other, the ratio of yield point strain and peak strain increases with the rising of the rock strength, and the ratio has linear relationship with rock strength. The energy and pulse of acoustic emission signals at the peak strength point of the combination sample increase linearly with rock strength rising. [ABSTRACT FROM AUTHOR]

In order to study the surface electric potential signals during the coal adsorbing gas, designed the experiment system of the coal adsorbing gas and tested the electric potential signal generated during the coal adsorbing gas under different gas pressure. Experiment results show that surface potential signals can be generated in the process of adsorbing gas, and they increases gradually along with the growth of adsorption time. For the same coal samples, different gas pressure from the coal surface potential signal to the first adsorption gas pressure of coal potential signal has a memory effect in the process of absorption. When the adsorption gas pressure is higher than the first adsorption gas pressure, the coal induces stronger surface potential signals. And when the adsorption gas pressure is lower than the first adsorption gas pressure, the coal induce weaker surface potential signals. The main reasons why surface potential signals can be generated from coal adsorbing gas are that the gas seepage in coal pores produces flow potential and free charge is generated when the gas damages micro cracks and crack of coal. [ABSTRACT FROM AUTHOR]

The article developed a set of coal and gas outburst simulation equipments by ourselves. Real-time observation of the coal and gas outburst process was obtained by using a high-speed video camera, and the coal rupture evolution law of coal and gas outburst process were analyzed. The author analyzed 12 tests and the results show that coal and gas outburst is a mechanical process of development. It has gone through a birth, start, development and stop four stages. Because of the effects of gas pressure and the radial stress of holes in the wall, coal and gas outburst experiences multiple intermittent development; during the experimental conditions, coal and gas outburst starts, it consumes less than 0.1 s from the development to the end; coal in different parts of the test apparatus has different trajectory during the moment of outburst. After outburst, the author analyzed the crack distribution of the coal, and found that cracks emerged almost parallely, the crack angle was between 65° and 70 °. The crack was like approximate arc and the center position was in a prominent mouth. The test of experiment also shows that there is a critical value between 0.481 MPa and 0.764 MPa gas pressure. It can highlight the succession, which makes a big outburst possible. [ABSTRACT FROM AUTHOR]