Search Results (45)

Taking a composite goaf in goaf-side entry retaining as our research focus, a kilogram-level spontaneous combustion experiment was carried out, and limit parameters for coal spontaneous combustion characteristics were assessed. Combined with the key parameters of the site, a numerical model of a multi-area composite goaf was constructed, and the distribution features of the dangerous area for coal spontaneous combustion in the lower layer of in goaf-side entry retaining were determined by means of the upper and lower layer composite superposition division method. The results show that at a floating coal thickness in the goaf of 1.9 m, the lower limit of oxygen concentration C_min, upper limit of air leakage intensity, and corresponding seepage velocity are 6%, 0.282 cm⁻³·s⁻¹·cm⁻², and 11.28 × 10⁻³ m/s respectively. The dangerous area regarding residual coal on the intake side is 23~38 m away from the working face, while that on the return air side is concentrated amid the goaf at 23~75 m, and that on the flexible formwork wall is concentrated at 0~121 m. The research results are of crucial practical importance for the prevention and control of coal spontaneous combustion within a composite goaf. Full article

To learn more about the problem of blast weakening in shallow-buried and extra-thick coal seams, Panjin coal mine was used to provide the engineering background for this study. The influence of blast weakening technology on the vibration of ground buildings was investigated. Based on monitoring the vibration data from the final 400 m of the working face, we established the Sadovsky formula for this coal mine through regression. The maximum safe charge of one blast at different distances was obtained. A numerical model was established and compared with field monitoring data to verify its accuracy. This numerical model was used to analyze the influence of blast weakening vibrations on ground buildings during the final mining stage. Finally, the maximum safe charge for one blast at advancing distances from the working face was derived based on numerical calculation results. It was compared with the maximum safe charge obtained from field measurements. The results show that both exhibit significant consistency, and the maximum safe charge of one blast decreases as the working face advances. In addition, the peak vibration velocity at each monitoring point does not exceed 0.2 cm/s for the remaining 400 m of the measured working face, which is lower than the allowable safety value for blasting vibrations. In the numerical simulation of the final mining stage at 200 m, the ground vibration velocity is largest for the district office, second-largest for the chimney, and smallest for the science and technology building. The maximum vibration velocity and effective stress in the three directions of the three buildings are within the allowable range, indicating that the buildings remained in a safe state. Full article

Based on the engineering conditions of the 1303 working face in Zhaoxian Coal Mine, this study investigates the characteristics of mine pressure behavior and the stress-relief mechanism of advanced presplit blasting in a working face with a thick and hard roof in an extra-thick coal seam. Through a combination of numerical simulations and field experiments, the effects of advanced presplit blasting on stress distribution, roadway stability, and microseismic activity are analyzed. Corresponding mitigation measures and optimization strategies are proposed. The results indicate that the primary cause of deformation in the gob-side roadway is the superposition of lateral abutment pressure from the goaf and the front abutment pressure of the advancing working face. Advanced presplit blasting effectively reduces the magnitude of front abutment stress, inhibits its transmission, decreases the hanging area of the goaf roof, and alleviates vertical stress on the roadway side adjacent to the goaf. Furthermore, both the daily average and peak microseismic energy levels decrease as the working face approaches the advanced blasting zone. The implementation of advanced presplit blasting technology in working faces with thick and hard roofs within extra-thick coal seams significantly mitigates rockburst hazards, enhances roadway stability, and improves overall mining safety. Full article

Steeply inclined coal seams, characterized by their significant inclination angles and complex storage conditions, are globally recognized as challenging seams to mine. An orthogonal test was conducted to study the influence of four key factors, including burial depth, inclination angle, lateral pressure coefficient, and maximum horizontal principal stress direction angle, on the force on the top slab of the sharply inclined extra-thick coal seam. The research findings indicate the following: The normal stress in the hollow area above the working face increases with greater burial depth, and the normal stress in the mining hollow area above the working face increases with an increase in the lateral pressure coefficient. Within the range of 4 m from the top edge of the seam, the normal stress distribution is approximately linear, and the influence of each factor on the average value of normal stress is in the following order: inclination angle > depth of burial > angle between the maximum horizontal principal stress and the strike angle of the seam > lateral pressure coefficient; outside the range of 4 m from the top edge of the seam, the distribution of normal stress is approximately linear, and the influence of each factor on the average value of normal stress is in the following order: angle between the maximum horizontal principal stress and the strike of the formation > inclination angle > depth of burial > lateral pressure coefficient. Full article

With respect to the problem of the anchorage failure of a broken roof in the roadway of extra-thick coal seams by using a traditional unconstrained pushing anchoring agent, a new anchoring agent installation technology with a push–pull device was proposed. Many research methods were adopted to study the mechanism of the efficient control of anchoring agent installation technology with a push–pull device on surrounding rock and the application of the technology. The results indicated that an unconstrained pushing anchoring agent exhibited two main morphological types: bending equilibrium and bending instability. The pushing force for the anchoring agent installed using the integrated push–pull method was calculated to be 13.52 N, which was less than that of the unconstrained pushing anchoring agent. An anchoring agent pushing with the push–pull device was able to smoothly pass through borehole delamination and collapse zones. When the pull-out force reached 160 kN and 180 kN, there was no significant slip or failure in the anchored section of the cable. The support system with the push–pull device for installing the anchoring agent reduced rock deformation by nearly 50%. This demonstrated that this technology significantly enhances the control of surrounding rock deformation. Full article

In this paper, we investigate the evolution characteristics of floor failure during pressured mining in extra-thick coal seams. A mechanical expression relating floor failure depth to seam thickness is established based on soil mechanics and mine pressure theory. The findings reveal a linear relationship between seam thickness and floor failure depth; specifically, as the coal seam thickens, the depth of floor failure increases. To simulate the mining process of extra-thick coal seams, FLAC3D numerical simulation software is utilized. We analyze the failure process, failure depth, and the behavior of water barriers at the coal seam floor under the influence of extra-thick coal seam mining from three perspectives: rock displacement evolution in the floor, stress evolution in the floor, and plastic deformation. Based on geological characteristics observed in the Longwanggou mine field, we establish a main control index system for assessing floor water-inrush risk. This system comprises 11 primary control factors: water abundance, permeability, water pressure, complexity of geological structure, structural intersection points, thickness of both actual and equivalent water barriers, thickness ratio of brittle–plastic rocks to coal seams, as well as depths related to both coal seams and instances of floor failure. Furthermore, drawing upon grey system theory and fuzzy mathematics within uncertainty mathematics frameworks leads us to propose an innovative approach—the interval grey optimal clustering model—designed specifically for risk assessment concerning potential floor water inrush during pressured mining operations involving extra-thick coal seams. This method of mine water inrush risk assessment is applicable for popularization and implementation in mines with analogous conditions, and it holds practical significance for the prevention of mine water damage. Full article

With the popularization of comprehensive mechanized mining methods and the increase in coal mining intensity, production has become more concentrated and efficient, which inevitably leads to Coal seam accumulates a large amount of gas The existence of huge goaf and mining overburden cracks that form behind the working face provides favorable conditions for the migration of gas to the goaf and its subsequent accumulation. The high concentration of gas that accumulates in the goaf gradually flows toward the working face under the action of pressure and concentration gradients, which can easily cause gas overrun accidents at the working face. Therefore, effective relief of the gas pressure in the goaf is important to guarantee safe and efficient mining at the coal mine working face. One of the most used gas drainage methods in such mines is high-level borehole gas drainage. This method can effectively reduce the gas content of coal seams, ensure the safe production of working faces, and reduce carbon emissions. In this study, the mining of a high-gas and low-permeability extra-thick coal seam in the Shanxi mining area is taken as the engineering background. In order to optimize the extraction design and improve the efficiency of gas extraction, according to the dual characteristics of coal seam pores and cracks, the permeability, and migration form of the gas in the coal body are analyzed, and a COMSOL coal seam gas migration model is established. By controlling different gas extraction horizons, pressure, and the number of boreholes and by optimizing the trajectory of the boreholes, the law of gas migration during high-level borehole gas extraction and the variation law with extraction time and pressure are studied. From this, the effective extraction calculation formula is fitted and statistical analyses are carried out. Through on-site extraction and simulation verification, the gas concentration was found to reach a maximum of 86% at a distance of 23 m from the floor. When using similar extraction times, 20 MPa gas extraction was found to have the best effect. The highest gas concentration in the upper corner was only 0.71%, and the extraction efficiency is higher when the high-level borehole trajectory angle is 30 degrees. The research results have important reference value for gas disaster control in the fully mechanized caving face of high-gas low-permeability and extra-thick coal seams. Full article

The Dzungaria Basin is located north of Xinjiang and is one of the largest inland basins in China. The eastern coalfields in the Dzungaria Basin contain a large amount of coal resources, and the thickness of the coal seams is significant. Therefore, the aim of this study was to classify the paleovegetation types and develop paleoclimate succession models of the extra-thick coal seams. We conducted the sampling, separation, and extraction of spores and pollen and carried out microscopic observations in the Wucaiwan mining area of the eastern coalfields in the Dzungaria Basin. The vertical vegetation succession in the thick seam (Aalenian Stage) in the study area was divided into three zones using the CONISS clustering method. The results show that the types of spore and pollen fossils belong to twenty families and forty-five genera, including twenty-three fern, twenty gymnosperm, and two bryophyte genera. The types of paleovegetation in the study area were mainly Lycopodiaceae and Selaginellaceae herb plants, Cyatheaceae, Osmundaceae, and Polypodiaceae shrub plants, and Cycadaceae and Pinaceae coniferous broad-leaved trees. The paleoclimate changed from warm–humid to humid–semi-humid and, finally, to the semi-humid–semi-dry type, all within a tropical–subtropical climate zone. The study area was divided into four paleovegetation communities: the nearshore wetland paleovegetation community, lowland cycad and Filicinae plant community, slope broad-leaved and coniferous plant mixed community, and highland coniferous tree community. This indicates that there was a climate warming event during the Middle Jurassic, which led to a large-scale lake transgression and regression in the basin. This resulted in the transfer of the coal-accumulating center from the west and southwest to the central part of the eastern coalfields in the Dzungaria Basin. Full article

The permeability of a coal seam is a crucial factor in coal seam gas extraction. Poor permeability of coal seams can lead to difficulties in over-pumping as well as high gas emissions after mining. This issue is particularly prominent when mining extra-thick coal seams with hard roofs, and it is the major problem that restricts the safe and efficient mining of coal seams. In the context of extra-thick coal seam mining in the Datong mine area, field investigation into the gas emission patterns of the working face reveals that the volume of gas emissions correlates closely with variations in working face pressure, demonstrating a high degree of consistency. The mechanism of irregular gas emission was analyzed, and the influence law of roof breakage on gas emission was obtained. It was found that roof breakage will aggravate gas emission. As a result, an integrated control technology involving “ground fracturing + gas extraction” was innovatively proposed. Based on the characteristics of ground fracture network, the mechanism of pressure relief and permeability enhancement of fractured wells and the characteristics of full time and space extraction were analyzed. Using the 8101 and 8204 working faces of the Tashan Coal Mine as a case study, the results demonstrated that vertical well fracturing of the 8101 working face enabled gas extraction 150 m ahead, with an accelerated increase in gas concentration within a 40 m range. Similarly, the horizontal well of the 8204 working face served as a drainage well after fracturing. Gas concentration at the mining position 50 m away from the horizontal well increased rapidly, and the gas extraction rate stabilized at approximately 30 m³/min. The approach effectively mitigated the problem of uneven gas emission caused by gas accumulation and roof fractures in the working face. Ground fracturing not only reduced the area and intensity of stress concentration in the advanced coal body but also enhanced gas emission. Furthermore, the fracturing well served as a gas drainage well, improving the control and achieving positive application results. Full article

The purpose of this study is to understand the law of gas migration in the goaf and reduce the gas on the working face. Taking the N2105 working face of the coal mining industry as the research object, the mathematical model of gas seepage in the goaf was established based on the percolation theory of porous media, and the model was solved. Using Fluent software to simulate the initial pressure, the working face airflow, and gas concentration distribution, different ventilation modes of gas concentration distribution and migration law with different wind speeds after the initial gas pressure. It is concluded that for the first time, the effect of gas on the working face is insignificant, and the influence of the initial pressure on the working surface is gradually revealed. The influence of airflow speed on the goaf is mainly concentrated in the 20~30 m area near the working face, which is affected by the airflow speed of the working face. The gas concentration in the goaf is low, and the fluctuation is obvious. The types of ventilation directly affect the seepage law of goaf gas. The U + I and U + L type ventilation can reduce the gas concentration in the upper corner and f gas seepages from goaf to the working face. Full article

In order to solve the problem of gas overrun in the fully mechanized caving face and the upper corner of high gas and extra-thick coal seam, the fracture and caving process of the roof in the goaf is analyzed and studied by using the relevant theories of fracture mechanics and seepage mechanics. The mathematical model of fracture and caving of the immediate roof and main roof in the goaf is established. Combined with ANSYS Fluent 6.3.26, the seepage process of gas in coal and rock accumulation in the goaf under different ventilation modes is simulated. The distribution law of gas concentration in the goaf is obtained, and the application scope of different ventilation modes is determined. In addition, the influence of the tail roadway application and the wind speed size on the gas concentration in the goaf and the upper corner of the fully mechanized caving face is also explored. The results show that, affected by wind speed and rock porosity, along the strike of the goaf, about 30 m near the working face, the gas concentration is low and growth is slow. In the range of 30~160 m, the gas concentration increases rapidly and reaches a higher value. After 160 m, the gas concentration tends to be stable. Along with the tendency of the working face, the gas concentration in the goaf increases gradually from the inlet side to the return side, and the gas concentration increases noticeably near the return air roadway. Along the vertical direction of the goaf, the gas concentration gradually increases, and the concentration of the fracture zone basically reaches 100%. Different ventilation modes have different application scopes. The U-type ventilation mode is suitable for the scenario of less desorption gas in the coal seam, while U + I and U + L-type ventilation modes are suitable for the scenario of more desorption gas in coal seam or higher mining intensity. The application of the tail roadway can reduce the gas concentration in the upper corner to a certain extent, but it has limited influence on the overall gas concentration distribution in the goaf. In addition, when the wind speed of the working face should be controlled at 2.0~3.5 m/s, it is more conducive to the discharge of gas, the method of reducing the gas concentration in the upper corner by increasing the wind speed of the working face is more suitable for the case where the absolute gas emission of the fully mechanized caving face is low, and the effect is limited when the absolute gas emission is high. The above conclusions provide a reference for solving the problem of gas overrun in the goaf and the upper corner of a fully mechanized caving face. Full article

In order to determine the reasonable parameters of high-gas and extra-thick coal seam drainage, considering the factors of the coal seam metamorphic degree, stress condition, gas occurrence state, and permeability dynamic change, the gas desorption, diffusion, and transport process of coal seam gas are analyzed. A secondary distribution model of coal around the borehole, a porosity variation model of coal around the borehole, a stress–seepage coupling model, a pore flow model of the pressure-driven transition flow zone, and a free molecular flow zone are established. Taking the gas drainage of Zhangcun Coal Mine of Lu’an Group as the research object, the influence of drilling hole diameter, coal seam permeability, gas original pressure, and other factors on the control range of coal seam drainage drilling is simulated by ANSYS Fluent 6.3.26. The results show that secondary stress distribution occurs in the coal seam drill hole under the action of lead stress, which leads to the change in porosity; the seepage zone, transition zone, molecular flow zone, and original rock stress zone are presented around the drill hole; and the range of influence of the drill hole is mainly based on the seepage zone and the transition zone, supplemented by the molecular flow zone. The control range of the drill hole is in a positive proportional relationship to the diameter of the drill hole, the porosity of the coal seam, and the original pressure of the gas. Full article

The section span of the withdrawal space of fully mechanized top coal caving in an extra-thick coal seam is large, and with the gradual withdrawal of the hydraulic support, a series of strong dynamic pressure disasters occur in the withdrawal space, and the difficulty of surrounding rock support control increases sharply. In order to study the control mechanism of surrounding rock in the final mining withdrawal space in detail and put forward a reasonable support technology scheme, taking the large-section withdrawal space of an 8309 fully mechanized caving face in an extra-thick coal seam of a mine as the research object—through the theoretical investigation of whether the key blocks of the main roof are stably hinged under varied stopping coal caving distances and fracture locations of the main roof—the reasonable and optimal stopping coal caving distances and roadway formation time are determined. Using numerical simulation and similar simulation methods, the vertical stress and the maximum shear stress research indicators were introduced to verify the accuracy of the theoretical analysis results. The results show the following: (1) The reasonable stopping coal caving span is 1~2 times the cycle weighting interval, the best stopping coal caving distance in this geological condition is 30 m, and the best fracture position of the main roof is located above the goaf. (2) The migration of overlying strata in the withdrawal space has obvious zoning characteristics, and the zoning is as follows: a stopping coal caving area, support area of the hydraulic support, withdrawal channel area, and stopping coal pillar area. (3) According to the zoning characteristics of overlying strata movement, the asymmetric zoning support control scheme of the withdrawal space is proposed. The field monitoring results show that the maximum roof subsidence in the withdrawal space is 151 mm, the maximum internal squeezing amount of the stopping coal pillar is 82 mm, and the supporting and anchoring effect of each partition in the withdrawal space is good. The set of partition asymmetric support control schemes has been successfully applied to field practice. Full article

Channel wave seismic activity often occurs with thin and medium-thick coal seams being the main target layer. To address the problem of channel wave applicability detection in extremely thick coal seams, the propagation and identification characteristics of channel waves remain the focus of research. Therefore, this paper takes the in-seam wave exploration of a 27 m extremely thick coal seam as an example and uses the staggered mesh finite difference method to construct a three-dimensional medium model for numerical simulation. An analysis of the physical parameters of coal and rock, along with the dispersion characteristics of channel waves in extra-thick coal seams, is utilized, through the Zoeppritz equation and the total reflection propagation method, to calculate the imaging. We found the following: (1) The dispersion areas and weak dispersion areas along the detection direction are extremely thick coal seams. (2) There are apparent channel waves in extra-thick coal seams, with a waveform similar to body waves; the length of the wave train is shorter than that of the conventional channel wave, and the arrival time can be estimated accurately. The amplitude of the apparent channel wave is affected by the degree of dispersion, with lower attenuation and higher resolution. The characteristic of total reflection in extremely thick coal seams is that the incident angle is equal to the critical angle, and the dispersion characteristics are weak. (3) The channel waves with weak dispersion characteristics in extra-thick coal seams are mainly Love-type waves. Full article

An improved method for rockbolt fracture is proposed in this paper to determine the exact fracture position of rockbolts simulated using cable structural elements (cableSELs) in FLAC3D. This method employs the total elongation of the free segment of the rockbolt as the fracture criterion. The maximum deformation position is identified by comparing the length of each cableSEL in the free segment, leading to the fracture. The simulation results validated through a rockbolt tensile test closely match actual conditions. The proposed method was used to optimize the roadway support in deep extra-thick coal seams (DECSs). Optimized parameters were obtained by simulating and analyzing different lengths and spacings of rockbolts and anchor cables. The field implementation conducted shows that the optimized deformation and support strength of the roadway meet safety needs. Full article