Coal Seam Roof Research Articles

Application and Optimization of the Parameters of the High-Level Boreholes in Lateral High Drainage Roadway

The key parameters of high-level boreholes in high drainage roadways affect the gas treatment effect of the working face directly. Therefore, the layout parameters of high-level boreholes in the lateral high drainage roadway (LHDR) are determined and optimized as necessary. Based on the LHDR layout on the 2-603 working face of the Liyazhuang coal mine, the key technological requirements on high-level borehole parameters were analyzed and the distribution characteristics of the gas volume fraction in the coal roof were studied. The gas migration law in the mined-out areas was obtained and the layout locations of high-level boreholes were determined finally. The research demonstrates that the high-level boreholes lag the 2-603 working face distance and the position of the final borehole (the position of the final borehole in this paper refers to the distance between the final borehole and the roof) influence the stability of boreholes and the gas extraction effect. The distribution of the gas volume fraction from the intake airway to the return airway can be divided into the stable stage, slow growth stage, and fast growth stage. Influenced by the flow field in the mined-out areas, the mean volume fraction of the borehole-extracted gas has no obvious relationship with the gas volume fraction at the upper corner. According to the final optimization, the high-level borehole is determined as having a 15 m lag behind the working face and the position of the final borehole is 44 m away from the coal seam roof. These have been applied successfully in engineering practice.

Instability Mechanism and Control Method of Surrounding Rock of Water-Rich Roadway Roof

Surrounding rock properties and occurrence stability of a coal seam roof are prerequisites for ensuring the safe and efficient operation of mines. In this study, the mechanisms and control of weakened water-rich roadway roof slabs were investigated regarding the engineering background of water-rich roadway roof slab destabilization in a coal mine in the western Qingyang mining area. The spatial and temporal evolution law of rock deformation and damage of such roadways during excavation were determined through field measurements. First, we tested the strength of the roof slab surrounding rock in water-rich roadways with different water contents and concluded that the primary and excavation-disturbing fissures of the coal-sedimentary rock body are the external conditions for the occurrence of water–rock interaction in water-rich coal seam roadways. Moreover, the rock mechanical damage phenomenon exhibited by clay minerals in contact with water is the key factor leading to the destabilization of the water-rich roof slab’s surrounding rock. Second, a technical approach for controlling the stability of the surrounding rock by adjusting the form of the roadway section and optimizing the support parameters was proposed, and the distribution law of the surrounding rock stress field and displacement field of each section was revealed via numerical calculation. It is considered that adjustment of the stress and displacement control of the surrounding rock of the roadway is more favorable for the straight wall circular arch section. Based on the results of the sensitive orthogonal numerical simulation test, the technical parameters and scheme of the roadway support optimization were proposed. Finally, the research results were applied in the field, and the deformations of the top and bottom slab and the two ribs of the roadway after optimizing the section and support parameters were calculated as 61% and 34% lower, respectively, than those before optimization, indicating that the proposed approach can effectively control the deformation of the water-rich roadway’s surrounding rock and achieve more economic and effective stability control of this type of roadway. The research results provide new ideas and methods for controlling the surrounding rock of water-rich soft rock roadways in the western mining areas of China, which has broad application value and prospects.

Overlying Strata Movement and Mine-Pressure Weakening Law of High-Efficiency Longwall Paste Backfilling of Thick Coal

This work focused on the serious coal compression under buildings, railways, and water bodies in central and eastern China; the wide range of rock formation damage during the collapse mining process; the high pressure of mines; and difficulties in controlling surface subsidence after mining. The E1302 working face of Shanxi Gaohe Energy was taken as the engineering background in the work. The mechanical properties of gangue paste-filling materials were studied through laboratory tests, and the critical conditions for bending and fracture of the coal seam roof were analyzed. Discrete-element numerical simulation software was used to study the fracturing process of the roof, and the reasonable filling rate to ensure roof stability was determined to be 95%. Meanwhile, overlying stratum movement and mine-pressure weakening law were studied through numerical simulations and field measurement. The results showed that fracture development during the mining process of thick-coal paste filling was divided into the advanced development, re-compaction, and steady-state maintenance of fractures. Fractures advanced in a “sail shape” and developed only in the main-roof rock strata after recompaction. The maximum subsidence angle of the working face was 87.13° after mining, with a subsidence factor of 0.034 and a maximum horizontal movement coefficient of 0.71. The advanced stress value was weakened by 40%, and the influenced area was reduced by 13%. Overlying stratum movement was controlled, and mine pressures were significantly weakened. The work can provide a scientific basis for green backfill mining, roadway support design, and backfill mining equipment selection.

Study on Initial Fracture Characteristics of the Main Roof in Fully Mechanized Caving Mining of Inclined Coalbed

In view of the occurrence conditions of inclined coalbed, the deformation and failure characteristics of the main roof will affect the safe production of the working face. Therefore, the study of the deformation and failure characteristics of the main roof in the inclined coalbed has guiding significance for the control of surrounding rock. This paper takes the II1042 working face of Taoyuan Coal Mine as the research background, adopts the methods of theoretical analysis, numerical calculation, and field practice to analyze the evolutionary characteristics of the initial failure of the main roof of the working face under the background of the inclined coalbed, and explores the mechanical behavior characteristics of the working face roof during the mining of inclined coalbed. Based on the elastic thin plate theory, a mechanical model of the overlying rock roof of a large-angle coal seam is established, and the mechanical characteristics of the surrounding rock under the initial failure of the main roof under the unbalanced load are studied. The stress distribution characteristics of the lower surface are summarized, and the evolution law of the initial fracture of the main roof is summarized. According to the actual geological conditions of the II1042 working face of Taoyuan Coal Mine, the failure characteristics of the main roof and the initial breaking step distance are obtained by analysis, and the analysis results are verified by monitoring the mine pressure of each part of the target working face on site. The research results show that: ① Under the unbalanced load of the inclined coalbed, the deflection surface of the main roof of the coal seam is asymmetrical with respect to the arrangement direction of the working face, and the maximum deflection point is located at the upper middle position of the working face, namely (a/2, 1.836 b/π), and the main roof of the working face breaks for the first time when it advances to 35 m. ② With the advancement of the working face, the two long sides of the roof break first. With the deflection and deformation of the roof, the tensile stress in the middle of the main roof reaches the tensile strength of the rock and breaks, and then the two short sides of the roof break under the action of the breaking and turning of the rock, and the upper short side will break before the lower one. ③ According to the monitoring and analysis of the rock pressure at each part of the working face, it is judged that the initial pressure step distance is between 28.2 m and 34.6 m, which is consistent with the theoretical analysis results.

Effect study of heat treatment on tensile properties of coarse sandstone

Brazilian split experiments were carried out on coarse sandstone, obtained from a coal seam roof passed by the Shihuoshan tunnel in Xinjiang, and treated at different temperatures (room temperature 25 °C and high temperature 100 °C ~ 900 °C). The physical and mechanical characteristics of the samples were studied. The results showed that: after heat treatment, the color of the coarse sandstone samples gradually changed from dark gray to brownish red-pink; the higher the treatment temperature was, the darker the sample color. Microcracks and mineral composition changes occured in the coarse sandstone samples after heat treatment, which decreased the longitudinal wave velocity of the samples. The longitudinal wave velocity of the coarse sandstone samples decreased as a quadratic function of the treatment temperature. With the increase in longitudinal wave velocity, the tensile strength of the samples first increased and then decreased, changing as a quadratic function relationship. After heat treatment, the tensile stress–strain curve of the coarse sandstone could be divided into compaction, elasticity, plasticity, and failure stages. The energy was continuously accumulated in the coarse sandstone before its failure, and it was released suddenly after the failure. With increasing treatment temperature, the cumulative energy in the prepeak stages first increased and then decreased, reaching a maximum value at 500 °C. The prepeak energy and tensile strength of the coarse sandstone samples satisfied a linear function fitting relationship, indicating that a higher tensile strength in the coarse sandstone, led to more accumulated energy in the samples.

The relationship of lithological and thermobarogeochemical factors in fluid-active zones: A casestudy of the Krasnodonetsky district of Eastern Donbass

The distance of natural gas reservoirs from the country’s industrial centers, the irregularity of spreading of the reserves, the increased costs of gas production and supply, all contribute to the involvement of local and nonconventional resources in various-type production. Coal methane extraction in Eastern Donbass is a goal in this regard. The research included the field study of fluid-active methane-rich zones in coal seams in Eastern Donbass, as well as the lithological and thermobarogeochemical analyses of coal samples. The gas saturation of rocks was correlated with the material composition of deposits. The results of the work can be formulated as follows. Terrigenous rock sediments of age С27 feature the increased content of hydrocarbon gases. Most of methane is localized in the roof of commercialthickness coal seams overlaid with thin clayey shales, siltstones and sandstones of the scoured surface. The scoured-surface sandstone in the coal seam roof has good reservoir properties. The increased fluid permeability of rocks is due to cleavage, which is most intense in heterogeneous strata in the axial parts of small folds. The fluid-saturated zones in coal seams in terrigenous rocks of С27 age at a depth of 240–260 and 320–360 m contain signs of hydrothermal veined mineralization represented by pyrite, dickite, quartz and calcite. These fluidization zones feature high content of gases in coal seams (up to 40–42 m3/t) and enclosing rocks (up to 10–12 m3/t), which points at high prospects for the extraction of hydrocarbon gases from these zones.The study was carried out under the state contract with the Southern Scientific Center, Russian Academy of Sciences, State Registration No. 122020100352-6.

Risk Assessment of Water Inrush from Coal Seam Roof Based on Combination Weighting-Set Pair Analysis

When exploiting Jurassic-era coal resources in Northwest China, there are risks of water inrush and sand burst disasters from coal seam roofs. To improve the safety of coal mining, it is imperative to accurately and objectively evaluate the water inrush risk of sandstone aquifers from coal seam roofs and to reasonably and effectively prevent and control water disasters. In this paper, the 221 mining area of the Shilawusu Coal Mine was considered. By combining the basic geological condition data, hydrogeological condition data, and drilling data in the area studied, four main control factors, including the equivalent thickness of sandstone, the lithology coefficient of sandstone, the interbedded coefficient of sand and mud, and the core recovery rate, were selected as evaluation indexes for predicting the water inrush risk from the coal seam roof. A hierarchical prediction and discrimination model of water inrush risk based on combination weighting-set pair analysis was established. The combination weighting method, which is based on the sum of squared deviations, was used to optimize the subjective and objective weight values obtained by the improved analytic hierarchy process and entropy weight methods. By applying set pair analysis theory, the comprehensive connection degree was determined using the set pair connection degree function that was constructed with 31 instances of drilling data in the study area. Then, the risk grade of each drilling data instance was evaluated by the confidence criterion of set pair analysis to calculate the water inrush risk evaluation index. Finally, the obtained index was combined with the borehole pumping test data and the discharging test data to partition the water inrush risk from the coal seam roof. The results indicated that most of the 221 mining area is safe, and the small transitional and dangerous areas are only in the central and northern regions. Based on the combination weighting-set pair analysis method, the water inrush risk from the coal seam roofs in the study area was accurately and objectively classified by a discrimination model.

Study on key technology of identification of mine water inrush source by PSO-LightGBM

Abstract Mine water inrush is a major type of disaster in coal mine production in China. It causes heavy casualties and serious economic losses and threatens coal mine safety. To quickly and accurately identify mine water inrush source, according to the hydrochemical characteristics of different aquifers in the Donghuantuo mining area, this paper systematically analyzes the hydraulic connection of the aquifers in main coal mining areas before and after mining activities. Four types of hydrochemical data were collected: No. 5 coal seam roof water, No. 8 coal seam roof water, No. 122 coal seam floor water, and No. 1214 coal seam aquifer water in the Donghuantuo mining area. In addition, based on the hydrochemical data, the parameter selection of LightGBM was optimized by Particle Swarm Optimization (PSO) and constructed the PSO-LightGBM water inrush source identification model. The recognition accuracy of PSO-LightGBM model was compared with LightGBM model, classification regression tree (CART) model, and random forest (RF) model. The results showed that coal mining activities would have a significant impact on the water quality characteristics of the roof sandstone fissure water of No. 5 coal mine. Mining activities had a certain impact on the accuracy of the identification model. In addition, compared with the four recognition models, PSO-LightGBM model had the highest recognition accuracy of 97.22%. It showed that the model had high accuracy, stability, generalization ability, and important reference value for the identification of mine water inrush source.

Study on the Bending Effect and Rock Burst Mechanism of Middle Rock Pillars in Extremely Thick Subvertical Coal Seams

Rock bursts occur in nearly vertical coal seam mines at shallow to moderate burial depths, which endangers safe mining. To study the rock burst mechanisms of nearly vertical and extremely thick coal seams, the characteristics of rock bursts were studied via on-site investigation, and a field test of in situ stress was carried out. The mechanical behavior of rock pillars in the middle of the B1+2 and B3+6 coal seams was analyzed using theoretical and numerical simulation methods. The results show that the horizontal maximum principal stress orientation and the nearly vertical coal seam strike were both 82°. The bending of the rock pillars occurred due to the horizontal unbalanced force, and a large amount of bending energy was accumulated within 50 m above the mining level. Rock pillars were bent toward the B1+2 mining goaf and exerted a reverse bending and squeezing effect on the B3+6 coal seam below the mining levels. In addition to the inclination and compression of the B3+6 coal seam roof, stress concentration zones formed in the B3+6 coal seam, where a large amount of elastic energy had accumulated in the coal-rock mass. Consequently, both the rock pillars and the B3+6 coal body at the mining level are in an unstable state undue to mining disturbance. Rock burst energy theory and numerical calculation results showed that in the stress concentration zones of the B3+6 coal seam, the energy density of the coal mass reached or exceeded its critical value before rock burst occurred, and rock bursts were prone to occur under mining disturbances. The in situ microseismic results showed that high-energy microseismic events were mainly concentrated in middle rock pillars around the mining levels and the coal mass in high-stress concentration zones.

Mechanism and Numerical Simulation of Vertical Fracture Propagation in Composite Coal Rock

Vertical fracture propagation mechanism is important to understand the effect of hydraulic fracturing on the roof of outburst coal seams. In this paper, the differences in physical parameters and in situ stress between an outburst coal seam and its roof strata were compared, and influencing factors of roof-fracturing fractures connecting coal seams were vertically analyzed. The impact of different fracturing strata and injection rates on fracture propagation was studied by numerical models. Results show that the horizontal principal stress of an outburst coal seam is less than that of roof strata, and the fracture length of roof fracturing is larger than that of an outburst coal seam. Roof-fracturing fracture of an outburst coal seam has the material conditions to communicate downward with the coal seam. The downward propagation height of roof-fracturing fracture is positively correlated with the minimum horizontal stress difference between an outburst coal seam and its roof strata. Soft coal fracturing cannot form a long fracture dominated by tensile failure, so the coal seam can be communicated by transforming the roof strata of soft coal to form vertical fractures. The injection rate affects the failure location, fracture width, and fracture-propagation path of the numerical model. Construction parameters should be reasonably designed in accordance with the physical parameters of coal and rock, construction displacement, and in situ stress. These research results can provide a theoretical basis and data support for the design, and optimization of construction parameters of roof fracturing in outburst coal seams.

Comprehensive Water Inrush Risk Assessment Method for Coal Seam Roof

In order to prevent coal mine water inrush accidents, it is necessary to appropriately assess the water abundance of coal mines based on drilling and geophysical data. This paper studied a comprehensive risk assessment method of water inrush. First, a water inrush risk index was proposed based on the analytic hierarchy process-entropy method (AHP-EM) and the water-rich structure index was proposed based on the geological data coupled calculation, then weighted two indices above which established the comprehensive water inrush risk assessment method. Secondly, eight factors were chosen as risk control factors of water inrush: core recovery, aquifer thickness, distance from the indirect aquifer to the coal seam, aquiclude thickness, height of water-conducting fracture zone, sand-mud ratio, total layers of aquifer and aquiclude, and the equivalent thickness of sandstone. Finally, the No. 2 coal seam of Dahaize coal mine was taken as the research object, the factors were calculated, and a comprehensive water inrush assessment model was constructed. With site investigation and observation, the water inrush risk assessment model of the No.2 coal seam roof is consistent with the actual mining situation, which verifies the validity of the model. In addition, this method was used to evaluate the water-richness of the weathered bedrock fractured aquifer in the Zhangjiamao coal mine. The practical application of the two mines has verified the generality of the approach. The research could provide scientific assistance for mine water hazard mitigation and mining safety.

Roof Fractures of Near-Vertical and Extremely Thick Coal Seams in Horizontally Grouped Top-Coal Drawing Method Based on the Theory of a Thin Plate

During the mining process of the near-vertical seam, there will be movement and collapse of the “roof side” rock layer and the “overlying coal seam,” as well as the emergence of the “floor side” rock layer roof which is more complicated than the inclined and gently inclined coal seams, which causes problems with slippage or overturning damage. With the increase of the inclination of the coal seam, the impact of the destruction of the immediate roof on the stope and roadway gradually becomes prominent, while the impact of the destruction of the basic roof on the stope and the roadway gradually weakens. The destruction of the immediate roof of the near-vertical coal seam will cause a large area of coal and rock mass to suddenly rush to the working face and the two lanes, resulting in rapid deformation of the roadway, overturning of equipment, overturning of personnel, and even severe rock pressure disaster accidents, all of which pose a serious threat to coal mine safety and production. It is necessary to carry out research on the mechanical response mechanism of the immediate roof of near-upright coal seams, to analyse the weighting process of steeply inclined thick coal seam sub-level mining. A four fixed support plate model and top three clamped edges simply supported plate model for roof stress distribution are established before the first weighting of the roof during the upper and lower level mining process. The bottom three clamped edges simply supported plate model and two adjacent edges clamped on the edge of a simply supported plate model are established for roof stress distribution before periodic weighting of the roof during the upper and lower level mining process. The Galerkin method is used to make an approximate solution of deflection equation under the effect of sheet normal stress, and then roof failure criterion is established based on the maximum tensile stress strength criterion and generalized Hooke law. This paper utilizes FLAC3D finite element numerical simulation software, considering the characteristics of steeply inclined thick coal seam sub-level mining. It undertakes orthogonal numerical simulation experiment in three levels with different depths, coal seam angles, lateral pressure coefficient, and orientation of maximum horizontal principal stress, and translates roof stress of corresponding 9 simulation experiment into steeply inclined roof normal stress. We conclude that the distribution law of normal stress along dip and dip direction of a roof under the circumstance of different advancing distances and different sub-levels. The caving pace of first weight and periodical weight were counted under the effect of the roof uniform normal stress. It can better predict the weighting situation of the working face and ensure the safe, efficient, and sustainable mining of coal mines.

Experimental Study on Hydraulic Fracturing of Laminated Sandstone Combined with Industrial CT and L-NMR

Hydraulic fracturing in the immediate roof is a suitable method to increase the permeability of soft outburst coal seam. In this work, the propagation law of hydraulic fractures in laminated virgin sandstone and the change of microstructure after hydraulic fracturing were revealed. The results show that the hydraulic fracturing pressure of sandstone with different bedding angles is 15.69~22.17 MPa. In the experiments, the hydraulic fractures extend through the bedding planes when the bedding angle is 45°. When the bedding angle is 0°, 15°, 30°, or 60°, the hydraulic fracture is prone to extend along a bedding plane. The hydraulic fracture in sandstone is jointly determined by the internal fractures and the cementation strengths of the bedding planes. The internal fractures may not necessarily guide the extension of the hydraulic fractures. The hydraulic fracturing pressures of the samples affected by the bedding plane and internal fractures are significantly lower than that of samples with uniform strength. Besides, hydraulic fracturing cannot change the connectivity between pores. The hydraulic fractures are the main channel for gas migration. The results can provide some guidance for the design and application of hydraulic fracturing in the immediate roof of the soft outburst coal seam.

Determination and Application of Reasonable Levels for Highly Directional Long Boreholes in Deep Outburst Coal Seams

In order to solve the problem that the gas in the upper corner is easy to exceed the limit in the mining process of the deep outburst coal seam. Based on the theory of “O-ring” and the distribution of stratigraphy in the field, the development law of mining fissures in the overlying rock layer during the workface retrieval was studied by combining theoretical calculation and FLAC3D numerical modeling. The research results show that the range of fissure development zone in 2913 working face is 14.2 to 33 m from the coal seam roof, and the high drilling arrangement can be in the stable rock layer of the roof. For siltstone-fine sandstone interaction stratum, measures such as opening branches to adjust the layer and lowering the casing to protect the hole wall can be taken to ensure the stability of the borehole and the smooth flow of gas extraction channels. Field application shows that the high directional long borehole has the advantages of long extension distance and wide coverage, the average value of extraction mix is 34.3 m3/min, and the average value of extraction pure is 0.4 mm3/min, which helps to prevent the gas overlimit in the upper corner during the workface recovery. The high directional long borehole has obvious economic and safety benefits over the traditional high hole and has important promotion value in coal mine gas management.

Numerical Simulation Investigation on Fracture Propagation of Fracturing for Crossing Coal Seam Roof

The fracturing crossing coal seam roof is a technology that fulfills the fracturing of a coal seam through the vertical propagation of fractures. Geological conditions are the key factors determining the effect of this kind of fracturing, but there is hardly any research on this aspect. To determine the favorable geological conditions for through-roof fracturing, based on a 3D fracture propagation model, and considering the interlayer vertical fracture toughness and leak-off heterogeneity, a mathematical model of fracturing through a horizontal well in a coal seam roof was established, and the calculation method of fractures crossing layer propagation was determined. In this method, the effect of fracture communication with the coal seam is evaluated by taking the area and the area ratio of fractures in the coal seam as the objective functions. The effects of parameters such as in situ stress combination profile, coal seam fracture toughness, and fluid loss coefficient on fracturing results were evaluated. The reasonable distance from the horizontal well to the coal seam’s top surface was determined in this work. The study results show that: (i) the fracturing effect is better when the coal seam is lower in in situ stress; (ii) the distance between the horizontal well and the top surface of the coal seam is recommended to be less than 4 m to obtain the ideal fracturing effect; and (iii) the combination of the in situ stress profile is the key factor, and the fracture toughness and fluid loss coefficient of the coal seam, fluid viscosity, and the number of perforations in one cluster are the secondary factors affecting the fracturing effect.

Study on the Mechanism of Weakening Thick and Hard Roof by Deep-Hole Blasting in Deep Coal Mines

In the mining process of deep coal mines with a thick and hard roof, it is easy to form a large overhanging roof and accumulate a lot of strain energy. The sudden breaking of the suspended roof may induce seisms, which seriously threaten the safety of underground miners and equipment. To reveal the mechanism of weakening the thick and hard roof by blasting in deep mining, the four times deep-hole blasting in 63upper06 panel of Dongtan coal mine are considered. First, the temporal and spatial distribution of seismicity events monitored before and after blasting is investigated. Second, the process of deep-hole blasting is simulated using finite element and discrete element methods. After blasting, the small-energy mining-induced seismicities within 20 m of the roof suddenly increase and gradually transfer to the mined-out area and the front of the working face. Meanwhile, the high rock stratum is broken and a large amount of strain energy is released, which causes the range of fall zone enlarge a lot, and the stress in the roof is decreased obviously. With the increase in the number of deep-hole blasting, the pressure relief effect becomes more obvious, and the newly developed fracture is mainly located in the low strata. Microseismic monitoring results agree with the calculated results. Moreover, the rock caving steps are easily formed after deep-hole blasting, and the collapse is more sufficient. Furthermore, blasting cracks are most restrained in the vertical direction of the maximum principal stress. Moreover, the delayed blasting method is more conducive to the expansion and penetration of blasting cracks. Results show that the deep-hole blasting technology has a significant effect on weakening the roof of coal seam and preventing and controlling large-energy mining-induced seismicities.

Floor Disturbance and Failure Characteristics of Super-Large Mining Height Working Face

The failure depth of the working face floor caused by mining is a key parameter to predict water inrush from the working face floor. Extensive research has been conducted on the floor failure zone of the working face. However, in recent years, there have been few studies on the floor failure law of the working face with a super-large mining height (over 20 m) in the western mining area of China. In this paper, the working face with a super-large mining height of 24 m in Longwanggou Coal Mine was studied. Electrical monitoring and numerical simulation of the working face floor were carried out. The measured floor failure depth of the working face is 15 m while the average numerical simulation result is 11 m. When compared with the calculation results of the statistical analysis formulas, the floor failure depth of the working face with a super large mining height obtained from field measurement and numerical simulation is smaller. The reason for the smaller failure zone of the floor of the super large mining height working face is that the falling roof inhibits the pressure relief of the floor, the large mining height reduces the advance pressure, and the shear stress of the floor decreases. The research methods and findings are of great significance to the prediction of floor failure and water inrush in working face with a super-large mining height, the prevention and control of water disasters in coal seam roof and the monitoring of mine pressure.

Research on the Movement Law of Roof Structure in Large-Inclined Coal Seam Working Face: A Case Study in Liu.Pan.Shui. Mining Area

In the Liu.Pan.Shui. coal field, Guizhou Province, China, 35°∼60° steeply inclined coal seams are widely distributed, and the coal resources present are of high quality and are mostly coking coal. However, because the roof overlying rock structure is affected by the geological conditions at a large dip angle, the combined cantilever beam structure of the overlying rock near the working face and the overlying rock masonry beam structure in front of the working face have different migration laws from when the condition of the near-level coal seam is generated. The pressure has emerged with new characteristics, posing a threat to coal mine production safety. Therefore, by establishing a mechanical model of roof overlying rock structure in accordance with the characteristics of coal seam inclination, mining height, and goaf overlap in the Liu.Pan.Shui mining area and using the analytical formula of material mechanics to calculate the behavior of rock pressure, guide the roof weakening means, construction technology, and parameters. It is used to quantitatively control the transportation load of the overlying rock on the roof of the large inclination coal seam and realize the safe and efficient mining of the precious coal resources of the Liu.Pan.Shui coal field.

Structural Characteristics of Regenerated Roof and Distribution Law of Overburden Porosity in Downward Mining of a Bifurcated Coal Seam

During downward mining of a bifurcated coal seam, the roof of the lower coal seam is relatively broken and difficult to control due to the mining influence of the upper coal seam. Roof accidents occur frequently during mining of the lower coal seam, reducing mining efficiency. How to ensure safe and efficient mining of the lower coal seam is a significant issue. In this paper, overlying strata migration and fracture characteristics of the lower coal seam, the structure and stability of the regenerated roof, and porosity and permeability characteristics of the overlying strata under the mining influence of the upper coal seam are studied by using similar simulation tests. Results show that the overburden structure of the lower coal seam is altered due to the mining influence of the upper coal seam, and the regenerated roof of the lower coal seam is divided into three structural types from top to bottom, namely: intact rock mass+block fracture rock mass+loose rock mass (type I structure); intact rock mass+block fracture rock mass+loose rock mass+cataclastic rock mass (type II structure); and intact rock mass+block fracture rock mass+loose rock mass+cataclastic rock mass+slab-rent rock mass (type III structure). The stability of each type of rock mass structure is evaluated, and the stability of three types of rock mass structures is III > II > I. The overburden porosity and slurry permeability coefficient are relatively large at the cutting hole and stopping line. The porosity of the caving zone within 70 m of the cut hole and stopping line is greater than 5%, and the permeability coefficient is greater than 0.1 m/s. Based on differences in the surrounding rock porosity and permeability characteristics, the grouting difficulty of overburden is divided into three types of areas: extremely easy grouting areas, easy grouting areas, and difficult grouting areas. The results of this paper can provide reference for the stability evaluation of the regenerated roof and the selection of grouting treatment parameters for the broken roof under similar conditions.

Study on the Synchronous Instability Mechanism of the Coal Wall and Direct Roof of High Soft Coal Seam

The advanced mining stress on a large working face can easily lead to the failure of the roof and coal mass; coal wall spalling and roof caving occur. The degree of coal wall failure and the depth of the plastic fracture zone are closely related to the advanced mining stress. The mechanical analysis model is established, and the stress distribution in the fracture and plastic zone is analyzed by the elastic-plastic theory, and the depth function expressions of two zones are determined. Comparative analysis of the factors affecting mining strength shows that the mining depth, mining height, and strength of coal are the key factors affecting the stability of the coal wall. The A1 coal of Zhangji mine in Huainan is soft and thick, and the direct roof is easily separated by the organic membrane, which is easy to form arc-shaped sliding instability. The increase in the unsupported roof area causes the direct roof caving and reduces its bearing capacity and stiffness. It is unable to provide enough support for the broken block of the overlying key stratum, which makes the broken block reverse rotation and further aggravate the roof fall; the synchronous instability of the coal wall and direct roof is formed.