Stability of Multi-Dimensional System of Parallelogram Support in Pseudo-Inclined Longwall Fully Mechanized Mining Face of Steeply Dipping Coal Seam

In order to study the prevention and control technology for hard roof type coal burst in the isolated working face of the Chenjiashan coal mine, the 418 isolated working face was selected as the engineering case study. Based on the different impact danger zones and mining areas, a roof breaking blasting pressure relief technical scheme was proposed. The anti-impact effect was verified through hole peeping and infrared radiation data. The research shows: (1) According to the geological conditions of the 418 working face of the Chenjiashan coal mine, the working face is divided into weak impact hazard areas and moderate impact hazard areas. Targeted roof blasting schemes were proposed for the initial square area of the working face, moderate danger area, weak danger area, initial mining and initial caving area, and the strike area of the working face. (2) On-site borehole data show that after blasting, a large number of fractures and delaminations were formed in the roof, and some fractures further developed into delaminations, with local areas showing crushed zones. This proves the formation of a “buffer zone” in the roof and floor, achieving pre-cracking of the thick and hard roof, full development of fractures, significant reduction in stress concentration, and the roof blasting can achieve good pressure relief effect. (3) The temperature monitoring near the blasting point and the infrared radiation temperature shows that within an hour after the implementation of the roof blasting, the coal mass at the breaking position experienced a process of heating up and then cooling down, with the temperature at the monitoring point rising by 0.5–0.7 °C. After the implementation of the roof blasting, the key layer above the working face was destroyed, and the stress was released and transmitted to the corresponding area of the coal mass, the stress of the coal increased, and the infrared radiation temperature increased, proving that the blasting pressure relief achieved the expected effect.

Previous research recognized the threat of faults to safe mining and the importance of identifying fault failure patterns, and began to use theoretical research and numerical simulations to study the activation laws of faults during mining. Conventional wisdom may suggest that the height of the fractured water-conducting zone (FWCZ) of the overburden strata over goaf will be increasingly caused by fault activation, thereby causing roof water inrush, in particular, goaf water existing in the roof of working face. Therefore, the FWCZ in the overburden strata make accurate judgments that are regarded as a key foundation to evaluate the safety of coal mining under water bodies. In view of this problem, the 15,103 working face of Wenzhuang Coal Mine in Shanxi Province were taken as the engineering background, the height of the FWCZ of the adjacent 15,100 working face was observed by drilling fluid leakage method and drilling television method, the observed results provided a reference for judgment of the height of the FWCZ of 15,103 working face. Additionally, the drilling method was adopted to conduct exploration on the terminal location of F6 reverse fault in overburden strata of No. 15 coal seam, the result showed that the disturbance range of F6 reverse fault was located in the FWCZ formed after mining the 15,103 working face. Furthermore, the method of numerical simulation analysis was used to study the failure height of overburden strata after mining the 15,103 working face through F6 reverse fault. The height of the FWCZ of F6 reversed fault was basically equal to that of the upper and lower plates, and F6 reverse fault had no influence on the height of the FWCZ after mining the 15,103 working face. There was a sufficient thick overburden strata between the maximal elevation of the fractured zone and the roof goaf water, and mining through F6 reverse fault under old goaf was safe and reliable. The research results can provide reference for the safe mining of passing through reverse faults under the influence of roof goaf water.

Rockburst, one of the leading types of disaster in mining and rock engineering causing serious injuries and the loss of property, frequently occurs, involving various features and complex evolutionary mechanisms. Compared to rockbursts occurring at mining faces, those occurring in main roadways cause more serious problems for mine production. This paper first analyzes the characteristics of rockbursts in main roadways using two case studies involving the Gaojiapu and Cuimu coal mines. The causes of rockbursts in main roadways were studied using microseismic monitoring, energy density cloud maps, and seismic velocity tomography. During the mining of the 22306 working face in the Cuimu coal mine, targeted measures, such as deep-hole blasting of the roof strata and deep-hole blasting of the coal seam, were implemented to prevent rockbursts in the main roadways. The effectiveness of these measures was verified through long-term analysis of tremor activities. The study found that the influence of mining at two working faces on both sides of main roadways was significantly greater than that from a single-sided working face. The intensity of the tremor activities occurring near the main roadways was correlated with the distance from the working face to the main roadways. The closer the working face was to the main roadways, the stronger the tremor activities were near the main roadways. According to the distribution range of the tremors, the influence area of working face mining exceeded 800 m, with tremors distributed linearly along the main roadways. Even five months after the completion of working face mining, there were still a large number of tremors near the main roadways, which gradually disappeared after another five months. Mining activities were the main reason for the occurrence of main roadway rockbursts and the stress concentration within the main roadways themselves was another reason for the occurrence of rockbursts. The influence of working face mining could be reduced by deep-hole blasting of roof strata and the stress concentration within main roadways themselves could be reduced by large-diameter drilling. Those joint preventive measures effectively prevented the occurrence of rockbursts in main roadways. This study is of important theoretical and practical significance for further studies of rockburst mechanisms and prevention in regard to main roadways in coal mines, and the findings are significant in terms of the enhancement of safety in coal mines.

When a working face is crossing the abandoned roadways, problems such as roof subsidence, rock fracture, and instability will occur, resulting in widespread roof fall and rib spalling, which seriously affect safe and efficient mining on the working face. In this paper, the no. 23 coal pillar working face of Juji coal mine is taken as the engineering background, a mechanical model of crossing the abandoned roadways is constructed aimed at the problem of the working face crossing the abandoned roadway group, the collapse of the abandoned roadway roof is analyzed, a scheme of crossing the abandoned roadways is designed, and the development law of the stress and plastic zone after the reinforcement scheme is stimulated and analyzed. The results show that when the working face advances to the abandoned roadway, key block B crosses the abandoned roadway and the solid coal to form a “cross-roadway long key block.” It is calculated that the minimum support resistance required for the abandoned roadway is 6700 kN. Based on the results of numerical comparison, it is concluded that filling wood pile when the working face passes through the roof abandoned roadway and adding anchor cables for reinforcement support when the working face crosses the coal seam abandoned roadway effectively reduce the stress concentration of surrounding rocks, decrease the development of the plastic zone, and achieve safe and efficient mining when the working face crosses the abandoned roadways.

With the development of society and the progress of science and technology, people’s living standard has been raised gradually. Behind these, it the development and utilization of energy. China is a large coal production country. Coal is the basic energy in China, and will be the major energy in the current decades and even in the coming decades. As coal is a non-renewable resource, plus the irrational exploitation in the first few years, a great deal of coal resources were wasted, which is contrary to the sustainable development strategy of the country. In recent years, to respond to national policies, vigorously promote sustainable development strategy, improve energy efficiency, major mining areas are enhancing the mining of residual coal, especially in isolated island state of coal. At present, there are some effective safety measures for mining coal under the condition of isolated island in China, but most of the safety research focuses on the mining face, but only a few studies focus on the safety of roadway. Through investigation, it is found that there are a series of problems in roadway driving and working face mining, such as strip wall and coal cannon. This seriously restricts the mining speed of isolated island working face, and always threatens the safety of workers. In this paper, the stress distribution characteristics of coal body under isolated island state are analyzed by numerical simulation. On the basis of this, the stress distribution characteristics of roadway surrounding rock and working face mining are analyzed, and the stress variation law is summarized. It is found that there exists a stress concentration zone in the coal pillar under the condition of isolated island. The stress concentration zone is mainly affected by the geological conditions of isolated island. There is a second stress concentration zone 7 m away from the side of the roadway, which is smaller than the stress concentration zone in the protected coal pillar, but the dynamic change is larger and the stress concentration zone moves with the advance of the working face because of the influence of mining in the working face. Therefore, in the aspect of maintaining the stability of roadway surrounding rock, the effective maintenance mode is determined according to the stress distribution characteristics and variation law of roadway surrounding rock under isolated island working face.

Due to the concealment of the fire source in the gob, the fire prevention and extinguishing work in the gob is facing great difficulties. This study is made in order to realize the real-time monitoring of gob temperature and the accurate positioning of high temperature area, so that the fire prevention and extinguishing work in gob can be targeted. In this study, the previously developed COMBUSS-3D software was used to predict the high temperature area in the gob of 85001 working face of Yangmeiwu Coal Mine and II830 working face of Zhuxianzhuang Coal Mine in China, and the continuous monitoring system of gob temperature was independently developed to realize the real-time monitoring of gob temperature, achieving the purpose of accurate positioning of high temperature area in gob. The results show that the high temperature area of the gob of 85001 working face of Yangmeiwu Coal Mine was in the range of approximate circle centered on the point (36.6, 30), and the maximum temperature was 31.7°C. The high temperature area of the gob of II830 working face in Zhuxianzhuang Coal Mine presented an approximate ellipse centered on (28.2, 25), and the long axis was parallel to the working face, and the maximum temperature was 43.9°C. The research results are expected to provide reference for the early prediction of spontaneous combustion in gob.

This study aims at the problems of the difficulty in controlling the stability of the surrounding rock and the high-impact danger of knife handle-type working face mining. We take the I010206 working face of Kuangou Coal Mine in Xinjiang as the engineering background, establish the mechanical model of roof periodic fracture and the FLAC3D numerical model of a working face, and analyze the evolution characteristics of the surrounding rock stress and energy when the working face is widened, revealing the mechanism of induced impact caused by overburden fracture in the working face, putting forward the technology of hydraulic fracturing to relieve the danger in the roof area, and comparing the pressure relief effect. The research results show the following: (1) After the working face is widened, the overlying strata load is transferred to the coal seam in front of the working face and the upper and lower sides of the working face. after mining; the abutment pressure of the I010408 working face in the B4-1 coal seam is superimposed with the abutment pressure of the I010206 working face in the B2 coal seam, the stress concentration is higher, and the lateral support pressure of the goaf forms a high static load. The large-area roof caving forms a high dynamic load. All of them are more likely to induce rockburst. (2) In knife handle-type working face mining, the peak value of the advanced abutment pressure in working faces first decreases and then increases, and the advanced abutment pressure increases from 10.31 MPa to 14.62 MPa; the peak value and concentration degree of strain energy density increase with the increase in working face width. (3) Measures were proposed to weaken the hydraulic fracturing roof in advance. After using hydraulic fracturing technology, the pressure step distance of the working surface roof was reduced, and the microseismic energy frequency was significantly reduced. These measures reduced the impact risk of the working face and ensured the safe mining of the working face.

Retaining a waterproof coal pillar is an important measure to defend against water inrush accidents in mining areas and guarantee the safe mining of the next working face. In this paper, the mechanical model of the coal pillar is established and the calculation formula of the waterproof coal pillar width is derived. Then, the development of the water-conducting fracture zone of the overlying rock layer under different coal pillar widths is analyzed using numerical simulation and finally, the integrity of the coal pillar is detected using the geophysical survey method. The main conclusions are as follows: (1) According to the mechanical failure characteristics of the coal pillar, it can be divided into the plastic zone, elastic zone, and water pressure damage zone. The mechanical calculation model for each zone was established, and the formula for calculating the width of the waterproof coal pillar was obtained. (2) Numerical simulation was employed to investigate the development condition of the water conducting fracture zone in the overlying rock strata under the actual width of the waterproof coal pillar; the simulation results indicated that the water conducting fracture zone of two working faces was not connected, which can effectively prevent the accumulation of water in the 2303 goaf. (3) On-site geophysical surveys determined that the influence of water-logged goaf on the coal pillar is between 5 to 15 m; the integrity of the waterproof coal pillar is good, which effectively prevents water accumulation in the previous working face goaf and ensures safe mining in the next working face.

Abutment pressure distribution is the decisive factor affecting the stability of coal pillars, which has an important significance in realizing safe and efficient mining. In recent years, many working faces adopting double-roadway arrangement have appeared in the western mining areas in China, which makes coal pillar abutment pressure complex under repeated mining. In order to reveal the distribution pattern of coal pillar abutment pressure under repeated mining, the evolution of a stress field of surrounding rock was analyzed by a numerical method based on the actual situation of a double-roadway arranged mining roadway in Longde Coal Mine, and the distribution characteristics of coal pillar abutment pressure were revealed. The reasonable width of the section coal pillar was given further. The research results showed that (1) the surrounding rock stress is transferred to the working face coal body and section coal pillar between roadways under the influence of mining advance the working face, while the stress is transferred to the section coal pillar between roadways under the mining influence behind the working face. The presence of goaf in the upper segment makes the stress distribution of surrounding rock more complicated. (2) The distribution of coal pillar abutment pressure shows a staged change. The influence of mining advance the working face on the peak abutment pressure is notable, which highlights the significance of temporary reinforcement support in the advanced section of the working face. (3) After the mining of the upper working face is finished, the yield damage width of the coal pillar on the side of the goaf was about 4.0 m. During the second mining period, the yield damage width of the coal pillar was totally about 7.0 m. (4) Taking into account factors such as safe production and economic rationality, it was finally determined that the reasonable section coal pillar width is 12.5 m for this mine.

The roadway stability has been regarded as the main challenging issue for safety and productivity of deep underground coal mines, particularly where roadways are affected by coal mining activities. This study investigates the −740 m main roadway in the Jining No. 2 Coal Mine to provide a theoretical basis for the stability control of the main deep roadway affected by disturbances of adjacent working activities. Field surveys, theoretical analyses, and numerical simulations are used to reveal mechanisms of the coal mining disturbance. The field survey shows that the deformation of roadway increases when the work face advances near the roadway group. Long working face mining causes the key strata to collapse based on the key strata theory and then disturbs the adjacent roadway group. When the working face is 100 m away from the stop-mining line, the roadway group is affected by the mining face, and the width roadway protection coal pillar is determined to be about 100 m. Flac3D simulations prove the accuracy of the theoretical result. Through reinforcement and support measures for the main roadway, the overall strength of the surrounding rock is enhanced, the stability of the surrounding rock of the roadway is guaranteed, and the safe production of the mine is maintained.

In the process of mining, a large area of hard roof will be exposed above a goaf and may suddenly break. This can easily induce rock burst and has a significant impact on production safety. In this study, based on the engineering background of the hard roof of the 2102 working face in the Balasu coal mine, the spatial and temporal characteristics of the strain energy of the roof during the initial mining process were explored in depth. Based on a theoretical calculation, it is proposed that hydraulic fracturing should be carried out in the medium-grained sandstone layer that is 4.8–22.43 m above the roof, and that the effective fracturing section in the horizontal direction should be within 30.8 m of the cutting hole of the working face. The elastic strain energy fish model was established in FLAC3D to analyze the strain energy accumulation of the roof during the initial mining process. The simulation and elastic strain energy results show that, as the working face advances to 70–80 m, the hard roof undergoes significant bending deformation. The energy gradient increases with the rapid accumulation of strain energy to a peak value of 140.54 kJ/m3. If the first weighting occurs at this moment in time, the sudden fracture of the roof will be accompanied by the release of elastic energy, which will induce rock burst. Therefore, it is necessary to implement roof cutting and pressure relief before reaching the critical step of 77 m. To this end, the comprehensive hydraulic fracturing technology of ‘conventional short drilling + directional long drilling’ is proposed. A field test shows that the hydraulic fracturing technology effectively weakens the integrity of the rock layer. The first weighting interval is 55 m, and it continues until the end of the pressure at the 70 m position. The roof collapses well, and the mining safety is improved. This study provides an important reference for hard roof control.

The air leakage in goaf can easily lead to disasters such as spontaneous combustion process of residual coal and gas accumulation, threatening production safety in underground coal mines. In order to study and master air leakage flow field distribution in goaf, the particle flow numerical simulation software PFC3D is used for the simulation of the collapse of overlying rock strata with the actual situation of the 3308 working face of Liangbaosi Coal Mine in China taken as an example. The quantitative porosity data of goaf are extracted and imported into FLUENT to simulate the air leakage flow field in goaf. The results show that (1) the porosity in the central part and near the working face of goaf is relatively large. With the increase of the length of goaf, the porosity decreases, and with the increase of the height of goaf, the porosity in the two cross headings is first larger than that in the central part and then smaller than that in the central part. (2) The data of air flow along the dip direction of working face obtained through the CFD numerical simulation is consistent with the actual measurement results basically, which validate the simulation. (3) The main air leakage occurs in the range of 0–10 m along the dip direction of working face. In the case of relatively large air supply rate, the residual coal spontaneous combustion area in goaf is far from the working face and the spontaneous combustion area becomes relatively large, resulting in increased risk.

The prevention and control of stress relief gas have crucial influence on safety of coal mine, it is not only the inevitable requirement of safe production, but also an effective way to realize reasonable utilization of gas. Taking the 1103 working face of the Weijiadi coal mine as the background, the mining stress field was analysed by means of numerical simulation, theoretical analysis and field practice, and the control technology of stress relief gas was studied. The results showed that scope of stress relief zone drops gradually associated with an increase of distance from the roof (or floor) to the working face. Additionally, the shape of the stress relief body exhibited a ring-shaped distribution, while four corners of the goaf roof and floor underwent high permeability zones due to a deep stress relief body, where the permeability of two corners near the transportation roadway of the floor was higher. The results provided good information for W-shaped ventilation mode with two inlets and one return which was adopted in the working face. More importantly, the optimized layout of boreholes was put forward, which eventually were useful for solving the gas overrun of the working face. The technology used in 1103 working face has an attractive and practical background with other extensive applications for the prevention and control of relief gas.

Based on the production conditions of the 10103 excavation working face of the Baozigou coal mine, this paper analyzes the potential sources of H2S and the expected emission concentrations of H2S in the working face. Considering the previous engineering practice for controlling H2S disasters in coal mine working faces, numerical simulations were conducted to investigate air flow and H2S migration and diffusion in the tunnel in the excavation working face. The migration and distribution of H2S in the coal seam mining face were studied, and the effects of outlet wind speed, duct location, and duct diameter on the H2S concentration distribution were explored. The higher the outlet wind speed, the more conducive to the emission of H2S gas, but too high a wind speed will be detrimental to the concentrated extraction and purification absorption of H2S; the closer the outlet position of the air duct is to the end of the working surface, the lower the H2S concentration in the vortex area at the corner; the air duct If the diameter is too small, the harmful gases released from hard-to-break coal cannot be entrained and taken away. When the diameter of the air duct is too large, the entrainment volume during the jet process will be expanded. To verify the field distribution of H2S concentration at the bottom, middle, and top of the boring machine, a CD4-type portable H2S instrument was used to analyze the distribution of H2S near the excavation working face.

Coal-mining areas are widely distributed in Northern China, but are under threat from confined water in the mining operation, resulting in a series of floor water- inrush hazards. Therefore, it is significant to effectively evaluate the floor water inrush to ensure safe and efficient coal mining. The 182602 working face of the Wutongzhuang coal mine served as the background for our research. The concept of “pre-mining microseisms” was proposed, and based on this, microseismic monitoring equipment was arranged on site. The correlation between microseismic events and the water abundance of an aquifer was analyzed, and a floor water inrush evaluation method was constructed based on the three elements of an aquifer and pre-mining microseisms. The main results are as follows: first, the microseismic events were excited by artificial disturbances before the mining of the working face including slurry diffusion and neighboring mining, which had the characteristics of sporadicity, clustering, and periodicity. Second, the regional distribution of water abundance was determined by taking the water inflow, water pressure, and grouting volume as the outward performance characteristics of water abundance in the Shanvuqing aquifer. Furthermore, the correlation coefficient between the pre-mining microseisms and the three elements of the aquifer (water inflow, water pressure, and grouting volume) was larger than 0.7. On this basis, an evaluation method associated with the water inrush risk along the strike of the working face was established based on pre-mining microseisms, dividing the working face into dangerous zones, suspected dangerous zones, and safe zones. Furthermore, pre-mining microseisms, water abundance, and structures were introduced as risk-evaluation indices, and the complete weight was calculated using an analytic hierarchy process and entropy-weight technique, before a vulnerability index model of floor water inrush was built. Finally, targeted treatment procedures were efficiently implemented to ensure the safe mining of working face 182602 due to the successful prediction of potential water risk zones. The research results provide scientific and technological support for pre-mining microseisms combined with water abundance as a technical method to prevent floor water inrush.