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Coal bumps occur frequently during coal mining, and monitoring of stress variations of coal mass is a prerequisite for coal bump prediction. To investigate a new method to reflect coal stress variation based on the pressure stimulated current (PSC), localized cyclic loading–maintaining–unloading experiments on coal samples were carried out and weak currents were measured. The response characteristics of PSCs with stress variations were analyzed, physical models showing the mechanisms of PSCs were established, and mathematical models explaining the relationships between PSCs and mechanical behaviors were derived. The results show that PSCs (tens of nA) flowing from the stressed volume to the un-stressed one can be observed upon load application. The currents are repeated with cyclic loading, load maintenance and unloading, which indicate that PSCs correspond well to stress variations. During loading, the PSC increases to a peak value at approximately 0.6 MPa and then it fluctuates until the stress stops increasing. When load is kept constant, the current decays exponentially to a stable value. During unloading, the PSC decreases at a lower rate when stress is greater than 0.6 MPa, after which it decreases at a higher rate to a minimum when stress is removed. Fitting results show that PSC increases exponentially with loading rate and that the stable current increases linearly with the maintained stress. Positive holes activated by the fracture of chemical bonds in the side chain of coal macro-molecules are believed to be the carriers, and the PSCs are generated due to the movement of the holes along stress gradients. Deduced formulas relating PSCs with coal mechanical behaviors show that PSC is proportional to strain rate; that is, PSC is inversely proportional to elastic modulus when stress rate is constant, as verified by the experimental results. These research results are expected to provide a new idea for monitoring of stress variations of coals during mining, which is significant for the prediction and early-warning of coal bumps.

The differences between mechanical properties and acoustic emission (AE) and electromagnetic radiation (EMR) characteristics of natural coal samples and saturated coal samples were analyzed by performing indirect tensile experiments. The experimental results show that coal samples go through four stages: compaction, elastic deformation, plastic deformation and failure. There is good correspondence between AE and EMR signals and the damage to coal samples. Under the action of water, tension strength of samples is reduced, while the plasticity is enhanced; also, the softening coefficient of tensile strength becomes 0.65. The saturated coal samples have a longer plastic stage and a more obvious AE quiet period. The damage to natural coal samples is tension damage, while that of saturated coal samples is due to tension and shear damage, which is more sufficient and irregular. EMR is still remarkable when AE is in a quiet period, and EMR is better for the precursor of rupture. Water weakens the generation and propagation of AE and EMR signals, especially in the earlier stage. AE and EMR damage factor D, defined by AE and EMR counts, has a better description of the damage degree in the indirect tensile process. This study is of great significance for research on the damage mechanism of water-bearing coal, the stability monitoring of water-bearing coals in the actual engineering process and the effect evaluation of hydraulic flushing.

A large amount of gas, which will do great damage to the environment as a kind of greenhouse gas, is discharged directly into the air in the process of coal mining in China. Majority of coal mines in China are characterized by low permeability, great gas pressure, and low strength feature. These features make it difficult for the extraction of gas, which increases the risk of coal and gas outbursts. In order to improve the permeability of coal seam, hydraulic flushing (HF) technology is adopted. So, the effect of hydraulic flushing will be necessary to be analyzed before gas extraction. In this paper, an electromagnetic radiation (EMR) experiment system of coal seam during hydraulic flushing was established, electromagnetic radiation (EMR) signals produced by the deformation and fracture of coal were studied during hydraulic flushing in coal seam in the Songhe coal mine, Guizhou province, China. The results showed that abundant EMR signals could be produced because of coal deformation and fracture. EMR signals were positively associated with the degree of coal failure. In different coal seams, EMR signals had different features during hydraulic flushing. The greater the gas pressure in the coal seam, the more dilatation energy was released. Higher rupture velocity could generate stronger magnetic signals. During hydraulic flushing, the more serious the rupture of coal, the higher the count of EMR pulses, and the higher the frequency of EMR. The principal frequencies and the amplitudes of waves increased gradually with gradually increasing ruptures. The change of EMR signals after hydraulic flushing was more dramatic than before hydraulic flushing. So, it is feasible and dependable to evaluate the effect of coal seam hydraulic flushing by EMR.

Electromagnetic radiation (EMR) anomalies preceding or accompanying coal and rock dynamic disasters have been reported for many years, but the mechanism that produces these anomalies is still unclear. To investigate the role that charge separation plays in these electromagnetic (EM) anomalies, uniaxial compression experiments on three different brittle materials: coal, marble, and polymethyl methacrylate were conducted. The results of the experiments indicate that EMRs are observed in piezoelectric and nonpiezoelectric materials but EM anomalies are only observed when abrupt stress drops occur. The EMR energy rate increases with the magnitude of the stress drop, and those two variables are related by a polynomial function. The charge separation responsible for the anomalies occurs on the newly generated cracks when chemical bonds are broken. A charge distribution model has been established for this phenomenon. Based on this model, the coupling relationship between EM energy density[Formula: see text] and charge density [Formula: see text] has been determined by theoretical analysis. According to this analysis, [Formula: see text] is proportional to [Formula: see text]. In addition, the relationship between [Formula: see text] and surface energy has been established, and it indicates that [Formula: see text] increases with (1) the stress to which the surrounding material is subjected and (2) the length of newly generated cracks. Research findings reported should provide a more detailed understanding of the EM anomalies when coal and rock dynamic disasters occur and provide guidance for developing warning strategies for mine safety.

Chinese coal mines often apply hydraulic flushing technology for pressure relief and permeability increase of difficult-to-extract coal seams In order to study effective detection methods for the impact range of hydraulic flushing on coal seams, we first experimentally investigated the response characteristics of the apparent resistivity of small-scale coal mass to hydraulic flushing, then theoretically analyzed the characteristics of its temporal variation and spatial distribution, and lastly measured the changes in apparent resistivity in the Hydraulic flushing process at Nuodong Coal Mine, Guizhou Province , China, and the effective range of Hydraulic flushing in the coal bed. The experimental results showed that the spatial distribution and variation of coal and rock mass structure could be reflected by apparent resistivity. Under high-pressure water, coal structure was changed (loosen, broken, or out-washed), forming cavities and a network of cracks in the surroundings of the drilling borehole , which reduced the conductivity of coal and caused a significant increase in apparent resistivity. In both laboratory and field experiments, the coal/rock apparent resistivity was relatively consistent, and the apparent resistivity in the hydraulic flushing impact region significantly grew. The hydraulic flushing impact range was gradually increasing, and the response of apparent resistance was closely associated with the role of high-pressure water and the evolution of the fracture field.

During the process of cross-cut coal uncovering, the stress and gas pressure of the coal mass change with time and even lead to coal and gas outburst. This paper establishes a coupled model that includes the equation of coal deformation, gas diffusion , gas seepage and permeability evolution based on the dual poroelastic theory. This model is applied into COMSOL Multiphysics numerical software. The distribution and evolution of the stress and gas pressure in front of the working face can be obtained from the numerical results. The stepped increase mechanism of the combined potential energy in the tunneling process is proposed, which is used to analyze the evolution of elastic potential energy and gas potential energy. The results can be summarized as follows: under natural conditions, an obvious abnormal stress area occurs near the fault, and the stress of the coal seam is greater than that of the rock stratum. Under tunneling conditions, a original stress (OS), stress concentration (SC) and stress reduction (SR) are formed successively in the front of the working face. And correspondingly, the gas accumulates in SC due to the closure of cracks. The distributions of gas pressure p f and p m are approximately the same during tunneling, and they can reach an equilibrium state after 300 days. The coupled effects of stress and gas pressure gradually intensify, so the elastic potential energy and the gas potential energy of the coal continuously accumulate. The combined potential energy of coal increases with tunneling length, and an outburst is very likely to occur when the combined energy exceeds the surface energy of the coal body at the “Key point”. This research is helpful to understand the evolution mechanism of stress and gas pressure of coal seam in the process of cross-cut coal uncovering, and to know the effects of it to the dynamic disaster.

Hydraulic flushing in gas predrainage is widely used, but the hydraulic-flushing effect is evaluated in a traditional way, by determining the desorption volume, moisture content, gas drainage rate and other conventional indices. To verify the rationality and feasibility of the multielectrode resistivity method in the evaluation of coal-seam hydraulic flushing and to research the spatio-temporal evolution of apparent resistivity during hydraulic flushing, a field test was conducted in 17# coal seam at Nuodong Mine, Guizhou. During hydraulic flushing, four stages were defined according to the variation in coal rock resistivity with time, namely, the preparation stage, the sharply decreasing stage, the rapidly increasing stage and the steady stage. The apparent resistivity of the coal rock mass is affected mainly by its own degree of fragmentation and flushing volume. A more serious rupture and a greater flushing volume yield a smaller apparent resistivity during the sharply decreasing stage and a higher resistivity during the stable stage. After three months of gas predrainage, the residual gas content and the gas pressure at different points in the expected affected area decrease below the critical value. Changes in the residual gas content and gas pressure at these points are consistent with the apparent resistivity, which validates the rationality and feasibility of the multielectrode resistivity method in evaluating coal-seam hydraulic flushing.

Based on poroelasticity theory and diffusion-seepage mechanism, the dual porosity and permeability evolution model was established by considering fracture gas pressure and pore gas pressure, respectively. By the solid and PDE modules of COMSOL Multiphysics, the protective seam mining was simulated on the base of the effective coupling of stress and gas fields. The results showed that with the mining of the protective layer, stress relief region (SRR) and stress concentration region (SCR) are formed in the protective seam, which correspond to the gas releasing region (GRR) and gas accumulative region (GAR), respectively, in the protected seam. The porosity and permeability increase in the SRR, whereas they decrease in the SCR. It is notable that the porosity cannot be restored to the original level due to the irreversibility of internal damage in coal mass. With further mining, the surrounding rock is tensioned to form the damage region, in which tensile fractures are formed, which eventually expand and connect with each other. When the destruction area gradually enlarges to link up the protected coal seam and the gob of the protective layer, the methane accumulated in the protected coal seam flows into the gob. The evolution of stress, porosity, and gas pressure is closely related. The variation of stress will cause change in porosity, which influences gas pressure, whereas the desorption of methane affects the stress of coal, all of which together form the coupling relation. This study is helpful to reveal the coupling mechanism of coalbed stress and methane, understand the dynamic porosity and permeability evolution, and learn tensile damage development.

In order to study the mechanics and acoustic emission (AE) characteristics of fractured rock under water-rock interaction, dried and saturated sandstone samples with prefabricated double parallel cracks were prepared. Then, uniaxial compression experiments were performed to obtain their AE signals and crack propagation images. The results show that water reduces the strength and fracture toughness of fractured sandstone and enhances plasticity. After saturation, the samples start to crack earlier, the cracks grow slowly, the failure mode is transformed from shear failure along the prefabricated cracks to combined shear and tensile failure, more secondary cracks are produced. The saturated samples release less elastic energy and weaker AE signals in the whole failure process. However, their AE precursor information is more obvious and advanced, and their AE sources are more widely distributed. Compared with dry specimens, the AE frequencies of saturated specimens in the early stage of loading are distributed in a lower frequency domain. Besides, the saturated samples release less complex AE signals which are dominated by small-scale signals with weaker multi-fractal characteristics. After discussion and analysis, it is pointed out that this may be because water makes rock prone to inter-granular fracture rather than trans-granular fracture. The water lubrication also may reduce the amplitude of middle-frequency band signals produced by the friction on the fracture surface. Multi-fractal parameters can provide more abundant precursory information for rock fracture. This is of great significance to the stability of water-bearing fractured rock mass and its monitoring, and is conducive to the safe exploitation of deep energy.

Current coal seams in China are characterized by great depth, low permeability, and strong adsorption features, and thus, the extraction of methane is relatively difficult and gas disaster remains serious. To overcome this limitation, the hydraulic flushing and hydraulic slotting are adopted to improve the permeability of coal seams. A 2D model of gas seepage was established by using the solid mechanics and darcy module in Comsol Multiphysics, version 4.3b. The factors influencing pressure relief, such as the radius of borehole after hydraulic flushing, the initial gas pressure, and the permeability of coal seam, were studied. The coupling effects of these three factors on the effective influence radius were studied through the response surface methodology, and a quadratic polynomial equation between effective influence radius and three factors was derived. The results showed that Gas pressure distribution (e.g. gas full emission region, gas pressure transition region and original gas region) near the borehole, corresponded well with stress distribution (stress relief zone, stress concentrating range and original stress zone). The plastic zone formed around the borehole leaded to stress reduction, and the increasing porosity and permeability resulted in pressure decline. The effective influence radius increased with the borehole radius and the initial permeability, but decreased with original gas pressure. Besides, it was found that an increase in one factor would restrict the role of other two factors on the effective influence radius. This study is helpful to choose the most suitable parameters to achieve high extraction efficiency and lower gas content/pressure.

The acoustic emission (AE) and ultrasonic (UT) simultaneous monitoring program is designed using concrete samples under step loading. The time-varying response characteristics of AE-UT are studied and the cross-correlation analysis between AE-UT parameters is obtained. Moreover, the joint response of UT-AE spatial distribution field is analyzed, and an AE-UT joint monitoring method to detect early-warning signals of a rockburst disaster in a coal seam is proposed. The results show the following. (1) During the loading process, the AE pulses/energy and UT attenuation coefficient first slowly decrease and then increase steadily and finally rapidly increase, while the UT velocity shows a trend of first gradually increasing and then slowly decreasing and finally a sharp decline. (2) AE pulses and energy are significantly or highly correlated with the UT velocity and attenuation coefficient. The AE energy and UT attenuation coefficient can better characterize the damage evolution of concrete under step loading. (3) The UT field evolves ahead of the rupture on the surface, and the long/narrow strip distribution region of UT parameters is consistent with the future failure zone; meanwhile, the AE events can visually reflect the evolution path of internal damage as well as the dynamic migration mechanism of UT field.

In order to explore the causes of acoustic emission (AE) signals during coal failure, the coal samples with original joints were subjected to uniaxial compression experiments, and the AE signals were monitored by AEwin Test for Express-8.0. Based on the multifractal theory, the multifractal characteristics of AE were analyzed. The results showed that the AE counts and accumulative counts change over time corresponded well with the load-time, which reflected the degree of crack evolution and loading. During the initial loading stage, the cracks expanded gradually along the trace of the original cracks, which could induce a few AE events, while with the increase of load, the cracks enlarged gradually and then joined together to form a macroscopic fracture, which would cause much more AE events within a larger value. Multifractal spectrum [[Formula: see text]] of AE was more concentrated in the right side, illustrating that the frequency of small signals was greater than that of the large signals in AE sequences, which revealed cracks expanding and microfracture events dominated during the loading process. The greater the multifractal spectrum width ([Formula: see text] was, the larger the AE signals differences were, which reflected that AE varied more intensely. The more developed the original cracks, the more obvious the multifractal characteristics. This research revealed the causes and percentage of the AE events within small or large signals, which would help us to recognize crack evolution of coal and generation mechanism of AE.