Liang Chen

Given the lack of systematic research on the movement of overburden strata and damage characteristics of floor residual coal in the process of thick coal seam top coal caving, this study takes the Anping Coal Mine as the engineering background. The spatiotemporal evolution of overburden collapse, force chain transmission, and floor stress redistribution was investigated via the FLAC^3D‒PFC coupled numerical method. Owing to the large mining space of the thick coal seam, the collapse of the roof has the progressive characteristic of “first two ends, then the middle”, with a measured collapse angle of approximately 57°, forming an evident caving band and residual coal accumulation zones. Moreover, the force chain network in the goaf evolves from sparse to dense, gradually forming a stable load-bearing structure. In addition, the porosity of the floor fluctuates from a wide range (0.15–0.43) to a more stabilized level (0.18–0.40) after compaction. Correspondingly, the vertical stress in the center of the residual coal floor increases from nearly 0 MPa to 1–3 MPa as the overburden load is transferred downward. The research results have important reference value for the destabilization mechanism of surrounding rock in thick coal seam mined-out areas and the design of residual coal remining and provide feasible ideas for improving mine productivity and guaranteeing safe production and the sustainable development of coal resources.

Mastering the deformation and air leakage patterns of gob-side retained roadway in fully mechanized mining faces, as well as the distribution characteristics of the gas flow field, is of great significance for the comprehensive prevention and control of gas in the goaf. Taking the 20,107 fully mechanized mining face of Qincheng Coal Mine in Shanxi Province as a case study, this paper employs fracture mechanics and plate-beam theory to analyze the impact of the roof fracture position on stress concentration and deformation failure of the roadway wall. It is found that when the fracture line is located outside the wall, the supporting stress can be effectively transferred and pressure can be relieved. Based on this, a calculation method for the critical position of roof-cutting and pressure-relief is proposed. Through field measurements and statistical analysis, the quantitative characteristics of the air leakage flow field and gas concentration distribution in the gob-side retained roadway goaf area were obtained. Consequently, a comprehensive gas prevention and control technology for gob-side retained roadway was proposed, which primarily includes directional drilling with staged hydraulic fracturing for roof cutting and pressure relief, along with enhanced coordinated gas extraction. Additionally, it incorporates auxiliary measures such as silicate composite material spraying for leakage sealing, optimization of ventilation pipe parameters in retained roadway, and airflow regulation for pressure reduction. The proposed approach was validated through field practice. The research results indicate that: (1) The air leakage flow field in the 20,107 goaf is characterized by positive-pressure leakage from the intake airflow of the belt roadway and high-concentration gas accumulation on the retained roadway side, which then enters the return airflow. The positive-pressure air leakage volume in the inclined section of the working face (0–36 m) is 332.84 m³/min, accounting for 58.17% of the total, while the air leakage volume in the retained roadway section (36–108 m along the strike) is 408.45 m³/min, accounting for 87.09%. (2) After optimization, the average pure gas extraction rate of directional drilling boreholes (fracturing and extraction boreholes) in the fractured zone reaches 7.46 m³/min, while the average gas concentration in the gob-side retained roadway gradually decreases from 0.59 to 0.34%. These findings provide a theoretical and practical basis for controlling the deformation of gob-side retained roadway walls, reducing air leakage in the goaf, and improving gas extraction efficiency, thereby guiding the safe and efficient production of fully mechanized mining faces.

Laser etching is one of the more cutting-edge processing technologies in the field of modern machining. However, the current laser etching technology still has some room for improvement in terms of processing effectiveness and efficiency. To this end, a laser etching mapping algorithm based on GPU and point cloud data is proposed to improve the efficiency and mapping accuracy of industrial processing. The decomposed point cloud data is obtained by first obtaining the mesh data of the 3D surface, then initialising the emission points of the rays and performing intersection detection based on a hierarchical wraparound box algorithm. The surface distance between any two points is obtained by discrete summation of the point cloud data. The selection of processing points is based on the obtained distance and the requirements of the etched texture, etc. To further improve the efficiency of the algorithm, a GPU-based RT core is investigated and optimised for prefixing and parallelisation. The average local error of the algorithm is 0.295mm, the average global error variance is 3.902*105, and the average running time is 2.95s. The algorithm is able to generate mapping points with high efficiency and accuracy and improve processing efficiency.

The No. 2₁ coal seam in the Zhengzhou mining area is a soft, three-layer, low-permeability coal seam prone to outbursts. The three-layer structure includes the coal seam and the roof and floor layers, which exhibit high gas contents and poor permeability. The dynamic hazards caused by coal and gas outbursts are very serious. A new permeability-increasing fracturing technique that combines hydraulic perforation and hydraulic fracturing was developed specifically for the geologic conditions of the gas-bearing No. 2₁ coal seam. Numerical simulations were developed to study the influence of the technique on the stress distribution and permeability of the coal around the borehole. In addition, the extracted borehole gas concentrations and extraction amounts at multiple sites were investigated before and after using the technique. The study shows that the permeability-increasing fracturing technique destroys the concentrated stress coal pillars via the development of fractures between boreholes in exposed hydraulically perforated coal. The coal stress within the zone with an effective increase in permeability decreased by 30. Furthermore, the permeability in this zone increased by three times, and the average extracted gas concentration increased by over six times. The gas pressure in the No. 2₁ coal seam decreased from 1.1MPa to 0.4MPa, and the gas content decreased from 15.96m³/t to 5.6m³/t. All outburst prediction indexes measured on site did not exceed their respective limits. The technique not only effectively eliminated the dynamic hazards caused by coal and gas outbursts but also achieved efficient gas extraction in the Zhengzhou mining area.

In order to study the coal deformation and failure mechanism in fully mechanized caving face under the high‐intensity mining, based on the equivalent mechanical model of transversely isotropic cylindrical coal with fractures, the equivalent equations for axial, radial, and volume strains of coal sample loaded in linear elastic and plastic stages were derived in this paper. The equivalent mechanical model shows good reliability through the conventional triaxial experiment. Taking the N1206 workface in Yuwu coal mine of Luan group as the example, we have simulated the stress concentration factor of the coal body ahead of the working face with FLAC and divided three regions according to stress distribution in coal mining. Mathematical equations were derived to express the horizontal and vertical stress, which provide theoretical guidance of the stress paths in triaxial experiment about real mining stress environment simulation. Experimental results show that the volume strain’s value is about 0.4% in the coal mass deformation progress of axial compression increasing slowly area. In axial compression increasing rapidly area, the volume strain’s value varies from 0.41% to 0.27%, and the radical strain changes from compression deformation to expansion deformation gradually. The volume strain of coal sample increases sharply in axial compression releasing rapidly area; meanwhile, there are good linear relationships between Poisson’s ratio and axial strain and radial strain.

The sealing depth of a gas‐drainage borehole is critically important as it directly affects the efficiency of the whole drainage system. In order to determine the shortest reasonable sealing depth, in this paper, a theoretical drainage model using different sealing depths was proposed. Based on theoretical analysis presented, two parts of the fractures system surrounding the drainage borehole were proposed, i.e. the fractures induced by roadway excavation and the fractures induced by borehole drilling. A series of geological in‐situ tests and simulations research were conducted to determine the stress and fracture distributions in the surrounding rock of the borehole. The depths of crushing zones, plastic zones and stress concentration zones were determined as 5 m, 2 m and 12 m, respectively. Meanwhile, stress simulation shows that the depth of the stress concentration zone was 12 m from the roadway wall and the stress peak was located at the depth of 8 m, which can be verified by the results of drilling penetration velocity analysis. To determine the optimum sealing depth, gas drainage holes with different sealing depths were drilled in the field. The field results revealed that the crushing zones were the main area for air leakage, and the stress concentration induced by roadway excavation assisted in the reduction of air leakage. Therefore, the optimized sealing depth should both cover the plastic zone and the stress concentration zone. The research achievements can provide a quantitative method for the determination of optimum sealing depth in cross‐measure drainage boreholes.