Coal Seam Roof Research Articles

Response Characteristics of Coal-Like Material Subjected to Repeated Hydraulic Fracturing: An Evaluation Based on Real-Time Monitoring of Water Injection Pressure and Roof Stress Distribution

Conventional hydraulic fracturing has several disadvantages, including a short effective extraction time and low fracture conductivity during long-term extraction. Aiming at overcoming these shortcomings, a similar simulation test of repeated hydraulic fracturing was conducted in this study, and the evolutionary rules regarding the injection water pressure and stress distribution of the coal seam roof during this repeated hydraulic fracturing were revealed. The research results show that after multiple hydraulic fracturing, the number of cracks in the coal seam and the range of fracturing influence have increased significantly. As the number of fracturing increases, the initial pressure required for cracking decreases. The highest water injection pressure of the first fracturing was 2.8 MPa, while the highest water injection pressures of the second and third fracturing were 2.7 MPa and 2.4 MPa, respectively. As the number of fracturing increases, the area of increased stress will continue to expand. After the first fracturing, the impact radius of fracturing is 100 cm. After the second fracturing, the radius of influence of fracturing expanded to 150 cm. When the third fracturing was over, the radius of influence of the fracturing expanded to approximately 250 cm. It can be seen that, compared with conventional hydraulic fracturing, repeated hydraulic fracturing shows better fracturing effect. The research results can be used as a basis for repeated hydraulic fracturing field tests to increase coal seam permeability.

Evaluation of the Water Yield of Coal Roof Aquifers Based on the FDAHP-Entropy Method: A Case Study in the Donghuantuo Coal Mine, China

The water yield of coal seam roof aquifers is the key factor for evaluating and controlling water disasters in coal seam roofs. To evaluate the water yield of the sandstone aquifer in the roof of the Carboniferous-Permian Damiaozhuang Formation no. 8 coal seam in the Donghuantuo Mine, North China, seven main controlling factors affecting the water yield of sandstone aquifers are determined, including the permeability coefficient, consumption of drilling fluid, core recovery, aquifer thickness, brittle-plastic rock thickness ratio, fault scale index, and fault point density. Further, the fuzzy Delphi analytic hierarchy process (FDAHP) and entropy weight method (EWM) are used to calculate the subjective and objective weights of each main factor, respectively, and a combination weight model (CWM) is proposed based on the least square method to compose the comprehensive weights. Then, an improved water yield property index (IWYPI) model is established, and the water yield zoning map of sandstone aquifers is acquired. Engineering practice shows that the evaluation accuracy of the water yield property index (IWYPI) model based on the CWM is as high as 93.75%, which is 18.75% and 12.5% higher than that of the water yield property index (WYPI) model based on the FDAHP and EWM, respectively. The research results propose a novel method for evaluating the water yield of coal seam roof aquifers and can provide scientific guidance for the prevention and control of water disasters in the no. 8 coal seam roof of the Donghuantuo Coal Mine.

Stability Assessment of Deep Three-Soft Outburst Coal Seam Roof Based on Fuzzy Analytic Hierarchy Process

The stability of deep “three-soft” coal seam roof has always been a key issue in coal mining. There are a lot of factors affecting the stability of deep three-soft coal seam outburst roof. However, there is currently no definite method able to draw an accurate assessment conclusion on roof stability. In order to accurately determine the main influencing factors of the stability of deep three-soft coal seam outburst roof and reduce the loss of coal production, this paper performed three-soft coal seam risk identification on Lugou Mine based on the introduction of the fuzzy analytic hierarchy process theory. 23 main risk factors were identified. Then, it established a hierarchical structure model of coal seam roof stability in accordance with experts’ opinions. The analytic hierarchy process was used to calculate the weights of indicators at all levels. Next, the paper used the fuzzy comprehensive evaluation method and expert scoring to evaluate various risk factors in the indicator system, as well as the overall safety level. The results showed that the deep three-soft coal seam stability of Lugou Mine ranks the third hazard level. The main risk and harmful factors include safety awareness, safety monitoring system, roof weakness, ventilation system, fire-fighting system, and rock bolt quality. In response to the evaluation results, this paper formulated corresponding control measure in terms of ventilation risk, safety monitoring risks, construction personnel risks, and fire protection risk to reduce losses in the mining process, providing a new evaluation method for the stability assessment of deep outburst coal seam roof.

Research on geological structure characteristics of Lubanshan mine field

Mine geological structure is an important geological factor that affects coal mine safety production. Based on detailed underground geological observations and surface geological survey data, starting with the study of structural traces and structural stress field, the Lubanshan mine field’s geological structural characteristics and structural stress field distribution are analyzed. The research results show that the structural profile of the Lubanshan mine field is mainly controlled by the NE-directed Xunsi anticline. The geological structure at the edge of the minefield is more complicated and the internal geological structure is relatively simple. The development of small faults and layer-slip faults, often in groups of coal seams or coal seam roof and floor, has become one of the biggest obstacles restricting coal mine safety production.

Water abundance prediction of sandstone aquifers based on the distance function

To establish a more accurate roof water abundance evaluation model and mitigate the difficulties in roof water abundance prediction and evaluation in the coal mine production process, the 2101 working face of the Yingpanhao coal mine was studied. Combined with the hydrogeological characteristics and drilling data of the study area and based on the lithology and structure, four controlling factors, including the equivalent thickness of sandstone, the lithological coefficient of sandstone, the degree of fracture development, and the sand-mud interlayer coefficient, were selected as evaluation indexes for the prediction of roof water abundance zones of sandstone aquifers. In the case of limited borehole data, the distance function was used to couple the improved analytic hierarchy process with the entropy weight method. The errors caused by subjective and objective factors were effectively reduced, and on this basis, a water abundance prediction model of roof sandstone was built. With the use of geophysical exploration results, the model predictions were combined with geophysical survey data to generate a comprehensive water abundance zone map, which divided the study area into three zones according to the water abundance. The results show that the high water abundance areas were mainly concentrated in the west of the working face, and the water abundance throughout the study area was generally low. The prediction results were verified by the data from drainage holes exposed by drilling, which provided a scientific and practical method for the prediction of the water abundance of coal seam roofs.

Safe feasibility of retaining sand-proof coal rock pillars in full-mechanized caving mining of extra-thick coal seam

To solve the problem of increasing the mining upper limit by reasonably setting sand-proof coal rock pillars in an extra-thick coal seam under a thick loose stratum and thin bedrock of Yangcun Coal Mine in Yanzhou Mining Area, the geological and hydrogeological conditions of working face were systematically analyzed by taking 325 Working Face in No. 3 coal mining area of the mine as the background and combining with the mining experience of its adjacent working faces. The mining grade under the water body was determined to be grade II, consistent with the conditions for retaining sand-proof coal rock pillars on the roof of coal seam. On this basis, to avoid the limitation of a single method, this paper adopts the key stratum theory, empirical formula analogy, and numerical simulation; the heights of the maximum caved zone (CZ) and the water-conducting fractured zone (WCFZ) were determined as 32.8 m and 82.0 m, respectively. The thickness of protective stratum is 16.4 m; the height of sand-proof coal rock pillars in 325 Working Face is 49.2 m; the minimum thickness of bedrock pillar in 325 Working Face is 53.1 m. Thus, full-mechanized caving mining can be carried out according to the reservation of sand-proof coal rock pillars. The comprehensive application of various methods can effectively improve the reliability and accuracy of the calculation results. The research results provide the main reference for this working face and similar working faces in Yanzhou Mining Area in the future to increase the mining upper limit and set up safe coal rock pillars according to roof sand control.

Mechanical characteristics and deformation control of surrounding rock in weakly cemented siltstone

The roof of coal seams are mostly soft rock with weak cementation. To further study the mechanical characteristics and deformation control scheme of the roadway with weakly cemented siltstone as roof, this paper took Linchang coal mine as the research background and adopted field investigation methods, laboratory experiments, and theoretical analysis. Using a scanning electron microscope, it is found that the weakly cemented siltstone is composed of coarse-grained minerals with a high degree of pore development. According to the analysis, the instability factors of weakly cemented siltstone roadway include the late diagenetic age of rock, the low mechanical strength of rock, and the change of surrounding rock properties by pouring water. The selection of grouting reinforcement materials was studied in detail. The test results show that cement-bonded specimens’ strength is lower than that of Marithan polyurethane cement specimens in general. The combined support scheme based on grouting reinforcement is put forward. Field monitoring data show that the designed support scheme can effectively control the surrounding rock deformation of weakly cemented siltstone roadway.

Theoretical Study and Numerical Simulation Research on the Effect of Coal-Rock Interface on Multistaged Fracturing in the Roof of Outburst Coal Seam

Multistaged fracturing in the roof of outburst coal seam is an efficient and creative technology for coalbed methane (CBM) drainage, which can effectively improve the permeability of coal seam. To reveal its mechanism of permeability enhancement, the effect of coal-rock interface on multistaged fracturing in the roof of outburst coal seam was simulated and discussed in this paper. Firstly, the lithological difference between outburst coal seam and roof was compared, and the concept and significance of multistaged fracturing in the roof of outburst coal seam were explained. Then, the mechanical conditions of multiple fractures in the roof traversing coal-rock interface were analyzed. The effects of mechanical parameters on multiple fractures were numerically simulated. The results indicated that fracturing borehole in adjacent rocks of outburst coal seam is much easier to drill and maintain gas drainage. Considering gas drainage efficiency and avoiding being blocked by coal fines, multistaged fracturing borehole is generally drilled in the stable rock stratum of roof. Whether the multiple fractures in the roof can traverse coal-rock interface is related to mechanical parameters of coal and rock, friction factor of coal-rock interface, angle between horizontal profile and coal-rock interface, cementing strength of coal-rock interface, minimum horizontal stress, and other factors. Higher fracturing fluid pressure contributes to propagating from the reservoir with low elastic modulus to the one with high elastic modulus for hydraulic fracture. Hydraulic fracture is more likely to propagate in the rock stratum with high brittleness index. The research results can improve multistaged fracturing theory and provide technological support for field test.

Evolution Law of Coal Seam Abutment Pressure under the Influence of Shallow Buried Complex Strata: A Case Study

The evolution law of lateral abutment pressure under the condition of fully mechanized mining in shallow coal seam is studied using the change process of coal pillar stress in disturbed section as the research object. The results of physical simulation experiment show that, after coal mining, due to the collapse of coal seam roof, the overlying strata of key layer will disturb the section coal pillar to different degrees, and the sudden change of degrees of abutment pressure near the coal wall reaches the maximum. Affected by the energy released by the fracture of key stratum, the stress mutation area shifts to the coal wall at a deeper level and the range of plastic zone increases. From the perspective of the numerical simulation, according to the change characteristics of coal pillar abutment pressure in the mining process, the dynamic load process of complex roof strata is divided into three stages: the stage not affected by mining, the stage of dynamic load action, and the stage of static load. In the first stage, the lateral abutment pressure is only affected by the roadway mining, causing stress concentration in the coal body. The stress concentration coefficient is small, and the supporting stress is stable. In the second stage, with the advance of the working face, the coal seam load changes continuously owing to the movement of overlying rock in the goaf, and the lateral abutment pressure changes evidently under the influence of dynamic load. In the third stage, the overlying load forms stress concentration in the coal seam and continuously transfers to the coal wall at a deeper level, which increases the limit equilibrium area of coal body. During this period, the range of plastic zone still increases at a certain rate for a period of time and finally tends to be stable.

Reasonable Arrangement of High-Level Orientation Extraction Boreholes of Pressure Relief Gas in Overlying Strata under High-Strength Fully Mechanized Mining in Low-Gas-Thick-Coal Seam

In order to solve the problem of pressure relief gas control under high-strength fully mechanized top-coal caving in low-gas-thick-coal seams, this paper studies the evolution of overburden structures and the distribution characteristics of fissure fields during the initial and stable period of working face by physical simulation and numerical analysis. The mathematical model of coupling between mining fracture field and pressure relief gas field is established. The results reveal the distribution characteristics of pressure relief gas field that considers mining-induced fissure field. According to the distribution of mining gas accumulation area, the high directional long boreholes have been put forward to control the pressure relief gas in goafs, and the effect has been tested. The results show that the initial pressure and three periodic pressures occurred from the cutting hole to 135 m in the initial mining period of the working face. The height of collapse zone developed to 22 m, and fracture height developed to 75 m. The development height of caving zone is stable at 25∼27 m, and the development height of fissure zone is stable at 75∼95 m. The process and distribution of pressure relief gas flow in goaf are obtained by solving the numerical model of pressure relief gas flow in mining fissure field. The gas accumulation area is located within 25∼55 m from return laneway and 25∼50 m from the roof of coal seam. After the implementation of high directional long drilling gas drainage technology in the initial mining period and the stable mining period, good results have been obtained in the gas control, where the average concentration of gas extraction is 5.8%, the average gas flow rate is 0.71 m3/min, and the gas concentration in upper corner and return air is less than 0.8%. The results can provide a reference for pressure relief gas control under similar conditions.

Evolution Mechanism of Water-Conducting Channel of Collapse Column in Karst Mining Area of Southwest China

There are many karst collapse columns in coal seam roof in the southern coal field in China, which are different from those in coal seam floor in the northern coal field, due to the stratum characteristics. The karst collapse column in coal seam roof tends to reactivate and conduct water and induce the serious water inrush disaster, when the karst collapse column communicates with the overlying aquifer. In order to reveal the evolution mechanism of water-conducting channel of collapse column in karst mining area of southwest China, the aquifers and water inflow rule in 1908 working face in Qianjin coal mine are analyzed. Besides, the particle size distribution and mineral component of collapse column are researched by the X-ray diffraction test and the screening method, which are the basis for researching the water inrush mechanism in karst collapse column. On this basis, the water inrush of roof collapse column under the influence of mining is researched by establishing the numerical calculation model with the UDEC numerical software. The results show that the water flowing into the 1908 working face comes from the Changxing formation aquifer and Yulongshan formation aquifer above the coal seam, and the proportion of coarse particles and fine particles in collapse column is 89.86% and 10.14%, respectively. With the advance of working face, the water-conducting channel connected the working face with the aquifer, or the surface is formed by collapse pits, karst caves, and collapse column. The research results can be treated as an important basis for the water-preserved mining in southern coal field in China.

PECULIARITIES OF LEAD DISTRIBUTION IN COAL SEAMS OF DONETSK-MAKIIVKA GEOLOGICAL AND INDUSTRIAL AREA OF DONBAS

Increasing requirements for monitoring and environmental protection, especially in the coal-mining regions of Ukraine, where a large number of industrial enterprises are concentrated, necessitates new scientifically based methods for predicting the content of toxic and potentially toxic elements in the rock mass that mined from the mines, in the coal wastes. and coal upgrading, as well as the impact of the coal industry and heating enterprises on the ecological state of the environment. For this purpose it is necessary to have data about concentration, character and peculiarities of the distribution of toxic and potentially toxic elements, including lead in the coal seams.The article presents the results of research covering the whole territory of one of the most studied geological and industrial areas of Donbas – Donetsk-Makiivka for the period from 1983 to 2014. The peculiarities of the distribution are considered and the character of lead distribution in 52 coal seams is determined, the weighted average concentrations of this element are calculated both in the coal of the main working seams and in 7 suites of the middle and upper series of coal period.It is proved that the accumulation of lead in the coal of the main working seams of the Donetsk-Makiivka geological and industrial area is polygenic and polychronic. The change in its concentration in section and area is mainly due to tectonic and facies features of coal-bearing strata formation, controlling the petrographic composition of coal, hydrodynamic regime of the peat accumulation basin, lithological-facies composition of the immediate and main roof of coal seams, coal fracturing and rocks that contain this coal.

Research on Damage and Acoustic Emission Properties of Rock Under Uniaxial Compression

In this paper, the YSSZ-500A rock biaxial rheological tester was used to investigate the acoustic emission characteristics of coal seam roof rock under uniaxial compression, and analyzed the acoustic emission characteristics, fracture mode and energy damage mechanism in the progressive failure process, and study the damage characteristics of rock samples from the perspective of damage mechanics. The results are as follows: (1) The whole stress–strain process of uniaxial compression can be divided into three stages: compaction stage, linear elastic stage, yield stage and failure stage. (2) In the middle of the period, the AE of the sample increases slowly and rapidly in the process of compression. For AE, the logarithm of the corresponding AE cumulative ring count can be divided into rapid increase period, slow increase period and sharp increase period. (3) The damage constitutive model of specimen during uniaxial compression is established, and the relationship between damage variable and axial strain is analyzed. (4) The energy absorbed by the specimen before the peak is mainly in the form of releasable elastic strain energy, and part of the energy is dissipated by the specimen damage propagation, and the energy dissipation is mainly concentrated in the yield section. A large amount of elastic strain energy is released after the peak, and most of the released energy is transformed into the energy dissipated by the propagation of fracture surface through the surface, which leads to the instability of rock samples.

Investigation of overburden failure characteristics due to combined mining: case study, Henan Province, China

The evolution of fractures in overburden is quantitatively investigated to characterize the effect of mining activity. Scale model testing and numerical modeling were used based on the engineering geological and mining environments of Panel 11050 in the Quandian Coalmine in Henan Province, China. The maximum vertical displacement is 62.76 m, which is 140 m from the initial mining in the scale model test. Based on the fractal geometric theory, the fractal dimensions of the fractures in the overburden are calculated and visualized. The results reveal that if two coal seams are mined at the same time, the fractal dimension of the fracture network in the overburden increase with the progression of mining, but the rate of increase gradually slows. The relationships between the fractal dimension and the maximum height of the overburden failure and maximum overburden subsidence are nonlinear. The structural characteristics of the overburden are represented by the network of fractures. With increasing distance from the coal seam roof, the mining stress is gradually transferred upward to the overlying strata, and the scale of this stress transfer gradually reduces. The variation in the vertical stress gradually weakens and shows a delayed change with the mining process. The maximum principal stress is compressive stress and is distributed in an “arch” shape. The stress on both sides of the “arch” is high, and the intermediate stress is low. The stress within the “arch” shows an opposite trend.

Analysis of hydrochemical evolution in main discharge aquifers under mining disturbance and water source identification

To discuss the hydrochemical evolution characteristics of the mining process of Peigou Coal Mine, based on the test results of 43 water samples collected at different times from three main discharge aquifers, namely, Carboniferous Taiyuan Formation limestone water (L7-8 + L5-6 water), Ordovician limestone water (including Taiyuan Formation L1-4), and Permian main mining coal seam roof and floor sandstone water (roof and floor water), a hydrochemical evolution model of the mining disturbances since 2003 has been established. The carbonate and sulphate dissolution and pyrite oxidation in Ordovician limestone water significantly decreased and then increased in 2006, and silicate weathering was weak. The carbonate and sulphate dissolution, silicate weathering and pyrite oxidation of roof and floor sandstone water increased. At the same time, a water source identification model suitable for the Peigou Coal Mine was developed by comparing the Fisher discriminant and the BP (back propagation) neural network discriminant. The accuracy rates of Fisher discriminant and BP neural network discriminant are 81.40% and 83.72% respectively, which indicates that BP neural network is more accurate. Finally, the evolution of hydraulic connection between aquifers is analysed. We speculate that there is a fracture development channel between Ordovician limestone water and roof and floor water aquifers that is affected in 2005 by the mining disturbance. This study has significance for examining the hydrochemical evolution of groundwater in mines and acting as a guideline to prevent and control water inrushes.

Fault Development Characteristics and Their Effects on Current Gas Content and Productivity of No. 3 Coal Seam in the Zhengzhuang Field, Southern Qinshui Basin, North China

Faulting is one of the most common geological features that impacts current gas content (CGC) of coal seams, which is an important parameter for coalbed methane (CBM/gas) production. However, the detailed faulting effects on CGC and productivity have not been thoroughly investigated, because of inadequate methods for quantitative evaluation of the effects. In this study, we carry out comprehensive analysis of diverse geological coring data and experimental results and production data of exploration wells as well as the three-dimensional fault interpretation in the Zhengzhuang field. We introduce a parameter fault scale (F) to quantify the scale of faults, which is described by fault length, fault throw, and the investigated area. Then we propose a horizontal grid method to determine the faulting influence on CGC. The relationships between the CGC and the fracture density of coal core, the mechanical properties of the roof and floor of coal seam, the average gas production in a faulting system (including the upthrown side, downthrown side, and fault plain) are analyzed. In the study area, the well-developed normal faults have significant influence on CGC, but the locally developed thrust faults have negligible influence. The square grid with length of 1.5 km is probably the most effective grids for analyzing faulting effected distance on CGC. Thus, the normal faults can be divided into small- (F1.5 < 0.05), moderate- (0.05 < F1.5 < 0.15), and large-scale (F1.5 > 0.15). The evident effect of faulting on CGC can be observed up to ∼1.5 km away from the normal fault. When the coal seam is further away from the normal faults (i.e., >1.5 km), the CGC is higher or closer to 20.7 m3/t. In contrast, when the coal seam is <1.5 km from the normal faults, the CGC decreases with increasing value of F. In general, the CGC is higher on the downthrown side, with better gas preservation conditions than the upthrown side. Relatively high CGC (>20 m3/t) is necessary for good gas production of CBM wells. The CBM wells with low ADGP (<500 m3/d) and low CTGP (<105 m3) are commonly located on the fault plane and upthrown side of normal faults. In general, the places far away (>1.5 km) from the normal faults is the most favorable productivity area, and only the small-scale normal faults developed is the secondary favorable productivity area. These places have the average gas production as high as 1000 m3/d and cumulative total gas production as high as 4 × 105 m3.

Innovation and future of mining rock mechanics

The 121 mining method of longwall mining first proposed in England has been widely used around the world. This method requires excavation of two mining roadways and reservation of one coal pillar to mine one working face. Due to considerable excavation of roadway, the mining roadway is generally destroyed during coal mining. The stress concentration in the coal pillar can cause large deformation of surrounding rocks, rockbursts and other disasters, and subsequently a large volume of coal pillar resources will be wasted. To improve the coal recovery rate and reduce excavation of the mining roadway, the 111 mining method of longwall mining was proposed in the former Soviet Union based on the 121 mining method. The 111 mining method requires excavation of one mining roadway and setting one filling body to replace the coal pillar while maintaining another mining roadway to mine one working face. However, because the stress transfer structure of roadway and working face roof has not changed, the problem of stress concentration in the surrounding rocks of roadway has not been well solved. To solve the above problems, the conventional concept utilizing high-strength support to resist the mining pressure for the 121 and 111 mining methods should be updated. The idea is to utilize mining pressure and expansion characteristics of the collapsed rock mass in the goaf to automatically form roadways, avoiding roadway excavation and waste of coal pillar. Based on the basic principles of mining rock mechanics, the “equilibrium mining” theory and the “short cantilever beam” mechanical model are proposed. Key technologies, such as roof directional presplitting technology, negative Poisson's ratio (NPR) high-prestress constant-resistance support technology, and gangue blocking support technology, are developed following the “equilibrium mining” theory. Accordingly, the 110 and N00 mining methods of an automatically formed roadway (AFR) by roof cutting and pressure releasing without pillars are proposed. The mining methods have been applied to a large number of coal mines with different overburdens, coal seam thicknesses, roof types and gases in China, realizing the integrated mode of coal mining and roadway retaining. On this basis, in view of the complex geological conditions and intelligent mining demand of coal mines, an intelligent and unmanned development direction of the “equilibrium mining” method is prospected.

Similarity simulation study on displacement and stress changes response to coal mining in Longdong coal mine, China

The complicated geological conditions, including the Fault Sun, in East No. 2 mining sub-area of the Longdong coal mine will influence the stability of strata during mining, leading to serious geological hazards. To circumvent this issue, a similarity simulation experiment was designed and performed in this study, in which the failure characteristics and evolution of displacement and stress within the strata were investigated, and the optimum width of a waterproof coal pillar was determined. The results showed that, as the working face progressed, the coal seam roof gradually deformed, from initial caving of the immediate roof to complete movement and curved subsidence of the entire roof. Significant changes in displacement and stress within the coal seam roof were recorded, and these increased during continuous mining activity. Displacement and stress difference on either side of the fault gradually increased and reached remarkable values with increase in mining distance. On the basis of the experiment results, water inrush is believed to be caused by the interaction between mining and the fault, and, as calculated from parameters collected in field investigations, a waterproof coal pillar of 50 m width should be established to prevent Fault Sun activation, thereby reducing the risk of water inrush from neighbouring aquifers.

ОЦІНКА СТІЙКОСТІ ПОКРІВЛІ У ВУГЛЕПОРОДНОМУ МАСИВІ, ЩО ВМІЩАЄ ВИРОБКИ ПРИ РОЗРОБЦІ КРУТИХ ВУГІЛЬНИХ ПЛАСТІВ

Purpose. Estimation of stability of a roof of a steep coal seam in the coal massif for maintenance of an operational condition of mine workings. Methods. To achieve this goal, analytical studies were performed using the basic principles of classical mechanics, the theory of elasticity and resistance of materials, when the roof of the coal seam was studied in the form of a model of a beam with a support. Results. As a result of the performed researches it is established that for an inclined single-girder beam with supports, at any regularity of distribution of loading on a surface the maximum deflection around the middle of span within (0,48 – 0,579) length of a beam is fixed. The value of the maximum deflection of the deformed load-bearing element depends on the angle of inclination and the ratio of the linear dimensions of the simulated beam. The curved axis is not perfectly symmetrical about the middle of the beam. The deviation from the symmetry depends on the linear dimensions of the model, the angle of inclination and is determined by the direction of the vector of total displacement of the beam. The deviation from the symmetry depends on the linear dimensions of the model, the angle of inclination and is determined by the direction of the vector of total displacement of the beam. The area in which the loss of stability of the load-bearing element is possible, occupies about 10% of the length of the inclined beam. In the presence of an asymmetrical deflection, to avoid loss of stability of the model, the placement of the system of supporting supports is advisable in this area of the beam. Scientific novelty. For the roof of a steep coal seam in its modeling in the form of an inclined single-girder beam with supports, when the external load is distributed on the surface naturally, the direction of the vector of complete displacement of the loaded model determines the contour of the curved axis of the deformed load-bearing element. Practical significance. The stability of the lateral rocks of the steep coal seam, which determines the operational condition of the mine workings in the coal massif, is provided by a system of supporting supports, the deformation characteristics of which are determined by the predicted deflection of the roof in the produced space. Keywords: stability, lateral rocks, coal massif, mining, deflection, supporting support.

Numerical Simulation and Theoretical Study of the Effect of Fracturing Strata on Multi-Stage Fracturing in the Roof of an Outburst Coal Seam

ABSTRACT Multi-stage fracturing is one of the key techniques that use boreholes instead of tunnels and can effectively transform a coalbed methane (CBM) reservoir and increase its permeability. However, due to the lack of systematic and in-depth research on the use of multi-stage fracturing for gas drainage, the development of multi-stage fracturing is slow, and there are many problems, which to some extent restrict the application of multi-stage fracturing in low permeable outburst coal seams. In this study, the No. 21 coal seam in the Zhongmacun Coal Mine was taken as the research object, and the lithologies of the roof and floor of the No. 21 coal seam and the in-situ stress were tested. Then, three types of fracturing strata were numerically simulated to study the effect of fracturing strata on the fracture initiation, propagation and morphological characteristics induced by multi-stage fracturing in the roof of an outburst coal seam. The influence of the brittleness index of fractured strata on the formation of a fracture network was analyzed theoretically. The results indicate that the sequence in decreasing order of the fracturing range and failure mode is the medium sandstone model, the sandy mudstone model, and the mudstone model. It is useful to form a fracture network when the fractured strata in the roof of the outburst coal seam is a medium sandstone or siltstone. By comparing the brittleness index of the roof and floor lithologies of the No.21 coal seam in the Zhongmacun Coal Mine, the proper strata for the multi-stage fracturing in the roof of the outburst coal seam was selected. The results of this study provide a theoretical basis and data support for selecting proper strata for multi-stage fracturing in the roof of an outburst coal seam.