A View from the Coal Face

Coal face spalling is a major issue affecting the safety of a large-cutting-height mining face, especially in deep mining. In order to analyze failure mechanisms and propose corresponding stability control measures in a large-cutting-height longwall face, panel 1303, with a mining depth of 860 m, which is arranged and advanced distances of 300 m and over 1000 m along the dip and strike directions of a coal seam, respectively, was selected as the engineering background. In addition to uniaxial compressive strength (UCS) tests, triaxial compression tests under different confining pressures and loading methods were carried out to investigate the deformation characteristics of the coal specimens. A mechanical model, the “coal face support roof”, was established to illustrate the factors affecting the stability of the coal face. Combined with numerical simulation, the dominant factor was obtained, and the stress distribution around the coal face at different advance distances was revealed. Based on the coal face failure mechanism, the pertinent in situ measures of “manila + grouting” reinforcement technology for controlling coal face spalling were proposed. The results showed that the coal face spalling depended mainly on vertical cyclic loading and horizontal unloading in both initial and periodic weighting. In terms of deep mining, the surrounding stress distribution played a vital role in coal face failure and instability. Specifically, two dimensions of loading conditions were found in the front 3 m of the coal face, and the principal stress σxx of the coal body was significantly less than the other two principal stresses in the front 8 m of the coal face. In addition, the horizontal principal stress σyy was greater than the vertical principal stress σzz. Therefore, the horizontal principal stress and strength of the coal body were the prominent influencing factors in the large-cutting-height coal face. The mining height and support system working resistance were also of great importance with respect to the stability of the coal face to some degree. Lastly, “manila + grouting” reinforcement technology proposed in this study resulted in 70–80% reduced potential for the occurrence of coal face spalling and in the degree of failure of the coal face, as well as grouting cost could be saved of 30–40% compared with pure grouting measures.

Transient electromagnetic perspective technology in the ultra-long coal mining face

Outburst prevention and control technology of thin coal zone in island coal face with potential outburst

The gateroad cross-section is large, and the surrounding rock has a low strength in the fully mechanized coal mining face with a large mining height of 6.7 m. Moreover, under extremely high support pressure in the fully mechanized coal mining face, the gateroad deformation is usually considerable, and a series of accidents such as roof falling disaster occurred in the coal face with conventional low-prestressed anchorage support and advanced support utilizing a single hydraulic prop. Large deformation of a gateroad at the excavation and extraction of the fully mechanized coal mining face with a large mining height as well as difficulty in maintenance are the key problems facing the Wangzhuang Coal Mine. We performed numerical simulation of stress evolution and deformation characteristics of surrounding rocks in the gateroad using FLAC3D. The scheme of surrounding rock control that combines high prestressed anchorage structure and the self-motion advanced hydraulic support is proposed. The staged supporting strategy and relevant parameters are presented. Finally, the staged supporting strategy was verified by the field application and effective stability control of the gateroad-surrounding rocks was achieved.

In this paper, a thick coal seam in Shandong Province is used as a prototype, which is located below the upper layer of 6 m residual pillar, and is a typical isolated island coal face based on the mining theory of upper liberation layer. The optimization study on the coal pillar of the island coal pillar working face formed by the ower stratification is carried out. When mining the coal face with stratified island pillar, it is easy to be affected by the upper residual coal pillar and the lateral support stress of the two side goaf. The isolated island surface is easy to form high concentration stress, the roof movement is violent, the impact ground pressure is easy to be induced, and the support press frame is easy to be formed. Mining earthquake and other dynamic disasters seriously threaten the safety of working face. Therefore, the reasonable determination of the width of coal pillar is guaranteed. By using UDEC numerical simulation method, this paper studies the mining optimization of the coal face under the influence of the upper stratified residual pillar, which is the important factor of the safe mining of the separated island working face under the thick coal seam, and the optimization of the mining of the coal face under the influence of the upper stratified residual pillar. This paper analyzes the stress distribution and evolution law of coal pillar in lower layer protecting roadway, and determines that it is appropriate to keep 12 m coal pillar in isolated island working face of lower stratified coal pillar, and the internal dislocation is 6 m in the residual pillar of upper stratification, which can reduce its impact risk at this time. It can ensure the safe mining and high recovery rate of coal.

Rib spalling is a major issue affecting the safety of steeply inclined coal seam. And the failure coal face and support system can be affected with each other to generate a vicious cycle along with inducing large-scale collapse of surrounding rock in steeply inclined coal seam. In order to analyze failure mechanism and propose the corresponding prominent control measures of steeply inclined coal working face, mechanical model based on coal face-support-roof system and mechanical model of coal face failure was established to reveal the disaster mechanism of rib spalling and the sensitive analysis of related factors was performed. Furthermore, taking 3402 working face of Chen-man-zhuang coal mine as engineering background, numerical model by using FLAC3D was built to illustrate the propagation of displacement and stress fields in steeply inclined coal seam and verify the theory analysis as mentioned in this study. The results show that the coal face slide body in steeply inclined working face can be observed as the failure height of upper layer smaller than that of lower layer exhibiting with an irregular quadrilateral pyramid shape. Moreover, the cracks were originated from the upper layer of sliding body and gradually developed to the lower layer causing the final rib spalling. The influence factors on the stability of coal face can be ranked as overlying strata pressure (P) > mechanical parameters of coal body (e.g., cohesion (c), internal fraction angle (𝛟)) > support strength (F) > the support force of protecting piece (F

In Northwest China, rainfall is low, water resources are scarce, and the ecological environment is fragile. For shallow‐buried and close‐spaced coal seams with a thickness of upper coal bed >60∼70 m, the water‐conducting fissures of the overlying rock will not penetrate the water‐isolating layer after the upper coal seam is mined; the internal and external gap angles of the water‐conducting fissures are not generated from the water‐isolating layer. We set out to explore the critical internal and external dislocations for the second significant development of water‐conducting fissures in the overlying rock after coal mining under control. A calculation model for the critical internal and external staggered distances of coal mining face in shallow‐buried and close‐spaced coal seams is established, the calculation formula is given, and the calculation formula for the critical seam mining ratio under the condition of internal staggered mining mode is given. Numerical simulation performed by UDEC methods: taking the overburden strata in the shallow‐buried and close‐spaced coal seam mining area of Shigetai Coal Mine as a prototype, it was verified that the critical internal and external offsets of the coal mining face in shallow‐buried and close‐spaced coal seams have a significant effect on the overlying water flow cracks in the mining of the lower coal seam. For the feasibility of developmental control, according to the engineering geological conditions of Shigetai, through the calculation method of external staggered distance, it is concluded that the distance of the open cut of the lower coal face and the upper coal face is only 21∼27 m, which is much smaller than the water barrier. It does not produce the critical distance of the water‐conducting cracks. Therefore, in the process of mining the lower coal seam, the water‐proof layer will produce water‐conducting cracks, lose its water‐proof performance, and cause water loss. This is also the cause of the water inrush accident in Shigetai Coal Mine.

Numerical investigation of the effects of coal seam dip angle on coal wall stability

Rock pressure evaluation in coal face based on multi-factor decision-making theory

Aiming at the problem of coal face failure of lower coal seam under the influence of repeated mining in close coal seams, with the working face 17,101 as a background, the coal samples mechanics test clarified the strength characteristics of the coal face under repeated mining, through similar simulation experiments, the development of stable roof structure and surrounding rock cracks under repeated mining of close coal seams are further explored. And based on this, establish a coal face failure mechanics model to comprehensively analyze the influence of multiple roof structural instabilities on the stability of the coal face. Finally, numerical simulation is used to further supplement and verify the completeness and rationality of similar simulation experiment and theoretical analysis results. The results show that: affected by repeated mining disturbances, the cracks in the coal face are relatively developed, the strength of the coal body is reduced, and the coal face is more prone to failure under the same roof pressure; During the mining of coal seam 17#, the roofs of different layers above the stope form two kinds of "arch" structures and one kind of “voussoir beam” structure, and there are three different degrees of frequent roof pressure phenomenon, which is easy to cause coal face failure; Under repeated mining of close coal seams, the roof pressure acting on the coal face is not large. The main controlling factor of coal face failure is the strength of the coal body, and the form of coal face failure is mostly the shear failure of soft coal. The research results can provide a theoretical basis for coal face failure under similar conditions.

In response to the frequent occurrence of coal face spalling affecting the working face production during the mining of extra-thick hard coal seams, this study investigates the characteristics of typical hard coal face damage. Coal face spalling is categorized into three stages: dynamic load influence, crack propagation, and bending instability phase. Employing the Rayleigh-Ritz method and the principle of stationary potential energy in conjunction with beam plate strength theory and maximum tensile stress strength theory, a displacement formula for coal face spalling has been developed. The formula has been validated using numerical simulation software. Additionally, a three-dimensional similitude modeling experimental platform was utilized to explore the development and failure patterns of spalling. Experimental results confirm the consistency between the theoretical derivation and the observed trajectories and locations of coal face spalling movement. The findings provide a theoretical foundation and technical reference for the management and prevention of spalling in extra-thick hard coal seam faces.

Rockbursts are common dynamic phenomena in coal mining and have become a prominent topic in the mining industry. This study examines the isolated island coal face in an extra-thick, fully mechanized top-coal caving operation in deep burial conditions, using it as a representative case. A comprehensive pressure relief strategy was employed at the coal face, incorporating directional roof blasting and loose roof blasting techniques. These techniques facilitated the pre-cracking and fracturing of the hard roof, addressing the issue of excessive suspended roof areas and high rock pressure affecting the roadway. Furthermore, a multi-source monitoring and evaluation method was developed to assess the effectiveness of the pressure relief measures. This method was based on the evolution of the rock loosening zone, stress distribution, displacement, and microseismic events. The study demonstrates that this approach can accurately evaluate the conditions at the coal mining face, providing valuable data to inform decision-making and enabling timely implementation of prevention and control measures.

Down-hole electromagnetic method for detecting water hazard of coal mine

At present, the prediction method of gas emission in coal mining face has low precision, based on grey system theory system, grey GM (1, 1) gas emission prediction model is constructed, the real-time, dynamic and non-linear prediction of gas emission is realized, and the application test is carried out in a mine of Huainan mining area. The results show that the average relative error between the predicted value and the measured value is 6.14%, the predicted Index C value is 0.48, P value is 0.90, and the prediction accuracy reaches the second level, which may guide the real-time, dynamic and accurate prediction of gas emission in the coal face.

Risk and security are two symmetric descriptions of the uncertainty of the same system. If the risk early warning is carried out in time, the security capability of the system can be improved. A safety early warning model based on fuzzy c-means clustering (FCM) and back-propagation neural network was established, and a genetic algorithm was introduced to optimize the connection weight and other properties of the neural network, so as to construct the safety early warning system of coal mining face. The system was applied in a coal face in Shandong, China, with 46 groups of data as samples. Firstly, the original data were clustered by FCM, the input space was fuzzy divided, and the samples were clustered into three categories. Then, the clustered data was used as the input of the neural network for training and prediction. The back-propagation neural network and genetic algorithm optimization neural network were trained and verified many times. The results show that the early warning model can realize the prediction and early warning of the safety condition of the working face, and the performance of the neural network model optimized by genetic algorithm is better than the traditional back-propagation artificial neural network model, with higher prediction accuracy and convergence speed. The established early warning model and method can provide reference and basis for the prediction, early warning and risk management of coal mine production safety, so as to discover the hidden danger of working face accident as soon as possible, eliminate the hidden danger in time and reduce the accident probability to the maximum extent.