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Abstract To study the influence of different geological and mining factors on the decompression effect of protective layer mining, a numerical simulation was carried out using FLAC3D numerical software. Taking the Hulusu coal mine as the engineering background, numerical simulation studies were carried out under different mining heights, working face lengths, interlayer lithologies and layer spacing by using FLAC3D numerical software, and the effects of different geological and mining factors on the degree of decompression and the scope of decompression were quantitatively investigated through the control of single‐factor variables. The results show that: the stress at the critical depth of impact ground pressure is 16.44 MPa, and the coefficient of unloading degree C = 0.5 is the indicator of sufficient unloading; the unloading effect of the protected layer decreases with the increase of layer spacing, lithological strength, and length of the working face, and increases with the increase of mining height; the geological and mining parameters of the protected layer show a functional relationship with the critical depth of the unloading, and the order of the influence of pressure relief effect is layer spacing > mining height > interlayer lithology > working face length. The results of the study are very important for the determination of the mining parameters of the protective layer, the estimation of the protective effect, and the design of the management programme of impact pressure.

Abstract A single load compression test of rock-coal-rock assemblages (RCRs) containing coal bodies with different inclinations was carried out with the research background of mining deep, large-steep coal seams. It was found that the larger the inclination angle of the coal body in the RCR samples, the larger the compaction stage of the samples and the smaller the uniaxial compressive strength value. In addition, both the maximum acoustic emission (AE) energy of the samples and the cumulative AE energy at the moment of destruction decreased with the increase of the inclination angle of the coal body in the form of exponential relativities. Also, the input energy and ultimate elastic energy of the samples at the peak stress moment decreased with increased inclination angle of the coal body. Furthermore, both of them changed with the inclination angle of the coal body in accordance with the exponential relationship; meanwhile, when the applied load exceeded the peak strength, the dissipative energy of the RCR samples increased rapidly due to the loss of load-bearing capacity. At the peak moment, the percentage of dissipated energy of the samples increased exponentially with the increase of the inclination angle of the coal body. This study has certain reference significance for the prevention and control of dynamic disasters during the mining of large angle coal seams.

Abstract In order to safeguard the surface structures from mining damage while optimizing the liberation of coal resources under the dense surface buildings of the Cedi River coal mine. Considering that the analysis of the structure and type of surface buildings and the geological mining conditions of the mine, a wide strip mining design with a retention width of 70 m and a mining width of 50 m was finally determined by using the pressure arch theory and Wilson’s theory, combined with the actual layout of working faces 51,002, 51,004 and 51,006 at the site.The strip mining design is verified by probability integral method and FLAC3D numerical simulation calculation respectively, The findings indicate that the highest value of earth surface subsidence created by the mining of the wide strip is 210 mm, the surface horizontal deformation value is 1.0 to − 1.4 mm/m, the damage to surface buildings is less than Level I, which satisfying the prerequisites of the surface building protection level, and can realize the continuous advancement of mine 51,002, 51,004 and 51,006 working faces, The coal pillars of the retained strip have sufficient support strength and long-term consistency, and the movement and deformation of the overburden after mining will not cause undulating subsidence of the surface, which effectively solve the mine's technical difficulties in safely coal mining under surface buildings.

Abstract The macro petrographic compositions and its pore characteristics of coal reservoir play critical role in the accumulation and development of coalbed methane (CBM). In this paper, the pore characteristics of vitrain and durain were analyzed through the experiment and fractal theory. The results indicated that the micropores and microfractures develop in vitrain, and that transitional pores develop in durain. The pore volume and specific surface area (SSA) of vitrain are larger than those of durain, with the micropore SSA of vitrain being 35% higher than that of durain. The threshold pressure and tortuosity of vitrain are greater than that of durain, but the mean pore size of vitrain is smaller than that of durain. The fractal dimension D 1 of vitrain is greater than that of durain, while the fractal dimension D 2 is opposite, indicating that the pore surface of vitrain is coarser, and the pore structure of durain is more complex. The fractal dimension D k of vitrain is larger than that of durain, the mean fractal dimension D s of vitrain is smaller than that of durain, which shows that the diffusivity of vitrain is weak but the seepage capacity is strong due to the developed fractures. The difference in material composition and pore characteristics between vitrain and durain provides a new understanding for the development of CBM in low rank coal.

Abstract The characteristics of floor failure and stress changes during the mining process of protective layers are crucial for determining the effectiveness of pressure relief. Three boreholes were designed in the 21104 fully mechanized mining face of Hulusu Coal Mine to implant optical fibers into the floor of the working face. A fiber optic monitoring system was established to monitor the dynamic evolution of stress in the floor rock mass at different mining distances. Based on the information entropy in information theory, the monitoring results in the fiber optic monitoring system are calculated to obtain the stress information entropy at different mining distances. A quantitative dynamic analysis is conducted on the stress change process of the mining floor rock layer, and the stress change law of the protective layer after mining is verified through numerical calculation and similar simulation experiments. The results indicate that the evolution of stress information entropy can be divided into four stages, namely the original rock stress stage, stress concentration stage, stress release stage, and stress recovery stage. The stress information entropy shows a fluctuating upward trend, indicating that coal seam mining leads to a decrease in the orderliness of the overlying rock system and an increase in disorder. In different spatial evolution processes, there are also significant differences in stress information entropy. In the vertical direction, the entropy value of shallow rock layers changes greatly, while the entropy value of deep rock layers changes slightly. Mining leads to a decrease in the orderliness of the entire overlying rock system, an increase in stress information entropy, and a fluctuating upward trend in stress information entropy. The information entropy of overlying rock deformation and re compaction increases, but the degree of change of the former is greater than that of the latter. The Brillouin fiber optic sensing technology provides a new method for monitoring the stress changes in the protective layer mining floor, achieving quantitative analysis of floor rock failure.

Optical fiber sensing technology provides a new method for the deformation monitoring of engineering structures. To verify the reliability and accuracy of distributed optical fiber sensing technology based on Brillouin scattering, i.e., Brillouin optical time domain analysis (BOTDA) and fiber Bragg grating (FBG) technologies for deformation monitoring, the strain test performances of BOTDA and FBG technologies were investigated by carrying out calibration tests on a uniform-strength beam and uniaxial compression tests on standard rock specimens. The performances of a strain gauge, three-dimensional digital speckle measurement based on a digital image correlation (DIC) matching technique, and distributed optical fiber and FBG sensors were comparatively analyzed, and the strain measurement performance, monitoring stability, system error, and strain coefficients of distributed optical fibers with different diameters were obtained. Auxiliary verification was carried out by ANSYS numerical simulation. The measurement accuracy of the distributed optical fibers was found to be ±25 µe. When the frequency shift of an optical fiber was positive, the fiber was under tensile stress, and when the frequency shift was negative, the fiber was under compressive stress. The fluctuation of DIC measurement data was greater than that obtained with the FBG sensor, which indicates that FBG measurement is more stable. The maximum fluctuation rates of the strain gauge data and FBG data were 0.092 and 0.031, respectively. The stability of FBG technology for strain measurement is clearly better than that of traditional resistance strain gauges. A thicker packaging material increased the contact area between the optical fiber and the object under test, made the stress transfer more efficient, and improved the sensitivity of the optical fiber. The strain coefficients calculated by numerical simulation were larger than the calibration test results. By comprehensively considering the two sets of results, the strain coefficients of optical fibers with diameters of 0.1, 0.7, and 0.9 mm were found to be 0.0455, 0.0485, and 0.0520 MHz/µe, respectively. Research on the basic test performance of optical fiber sensing technology is of great significance for promoting its more effective application in engineering fields. [ABSTRACT FROM AUTHOR]

Aquifer breakage is easy to occur in the mining process of shallow coal seam. In this study, the safe water protection mining of shallow coal seam in Western China was studied. The numerical model of coal seam was established by FLAAC3D software. Coal seam mining started after two working conditions, fully caving mining and pillar support mining, were established. The damage of coal seam was analyzed by software. The results showed that there were many shear cracks in fully caving mining, and finally water flowing fractures appeared. In pillar supported mining, there were many tension cracks above the pillar and in the water-resisting clay layer, and the maximum subsidence value of the surface increased with the advance of the working face. Some measures were put forward to ensure the safe water protection exploitation according to the experimental results, which provides some theoretical supports for realization of the win-win situation of coal resources exploitation and water resources protection in the western region. [ABSTRACT FROM AUTHOR]