Pressure Relief Area Research Articles

Research on Enhancing Gas Extraction Efficiency through CO2 Gas Fracturing in Outburst-Prone Coal Seams.

Given the background of significant Coal Mine gas disasters worldwide, the research and development of efficient gas control technologies have become critical to ensuring mining safety. CO2 gas fracturing (CO2-Frac), an innovative technology for Coal Mine gas control, has demonstrated efficiency in various mining areas across China. However, its application in outburst-prone coal seams is not yet fully understood. This study presents a field-scale CO2-Frac project conducted in an outburst Coal Mine in China, specifically the Pingshu Coal Mine, where the previously employed dense-borehole gas extraction technology failed to efficiently achieve gas extraction and outburst prevention. The CO2-Frac plan was evaluated through fracture propagation experiments utilizing both single-hole and dual-hole configurations, highlighting the advantages of the dual-hole CO2-Frac method. Subsequently, on-site gas extraction tests were conducted to further assess the efficacy of the CO2-Frac plan. The results indicate that (1) In the dual-hole CO2-Frac scheme, the fractured and pressure relief area expanded to approximately 26.82 m2, which is 220% larger than that of the single-hole scheme. (2) The dual-hole CO2-Frac significantly enhanced gas extraction effectiveness, increasing the flow rate from 0.026 to 0.216 m3/min in a 100m borehole, a 7.3-fold improvement. Additionally, the gas extraction period to reach the standard was reduced from 20 to 6 days. These findings conclusively demonstrate that the dual-hole CO2-Frac technique is an effective method for safe excavation in outburst-prone coal seams, providing both theoretical and practical validation for its application in similar geological settings.

Technology and application of through layer borehole ultra-high pressure hydraulic slotting in soft coal seam to pressure relief and permeability enhancement

ABSTRACT It is a vital method that controls outburst disasters through gas drainage to reduce gas pressure. But the permeability of coal seam in China is low. Therefore, technologies that can decrease pressure and increase permeability such as ultra-high pressure hydraulic slotting (UPHS) are widely applied. Based on the technology of UPHS, the principle of coal breaking and mechanism of pressure relief and permeability enhancement (PRPE) were first analyzed. The effects of PRPE between contrast borehole and UPHS were compared through numerical simulation. Finally, combined with the field test, the parameters of UPHS were determined, and difference in gas drainage effect between contrast borehole and UPHS was investigated. It is found that the exposed area of coal increased due to macroscopic slotting formed by hydraulic slotting, which provide space for coal deformation. The stress of coal relieved the permeability enhanced, resulting in the gas drainage efficiency increasing. The simulated results showed the pressure relief area and plastic deformation caused by UPHS are significantly increased compared with contrast borehole. The average concentration and total of gas drainage in UPHS are 1.49 and 3.13 times than that of contrast borehole. And radius of gas drainage of UPHS 1.77 times than that of contrast borehole under reduce the 46% engineering quantities.

Study on overburden rock structure characteristics and surrounding rock control technology of island working face

Based on the background of ZF2822 island working face in Xiagou Coal Mine, the characteristics of overburden fracture structure and surrounding rock control technology of island working face are explored by means of theoretical analysis, numerical simulation and field measurement. The results show that the ten morphological structures of the main roof fracture can be divided into four typical symmetrical structures and six asymmetric structures. According to the stress calculation formula of the key block B to the coal pillar, the stress characteristics of the 10 structures are classified and the risk classification is carried out. Four typical structures (a), (b), (c) and (d) were constructed to simulate the stress and strain characteristics of surrounding rock. The overlying strata subsidence displacement values of (a), (b), (c) and (d) at the roadway are 1.75 m, 1.5 m, 0.8 m and 0.6 m respectively, and the maximum stresses are 26.6 MPa, 19.5 MPa, 15.5 MPa and 10.0 MPa respectively, (a) > (b) > (c) > (d). Therefore, the surrounding rock control technology of gob-side roadway in island working face is put forward, the fracture structure of basic roof is determined and the reasonable applicable type of gob-side roadway is selected. The results after pressure relief show that the average daily microseismic energy of ZF2822 working face after pressure relief is 0.87 × 104 J, which is about 42.38% lower than that of April. The average stress is 1.13 MPa lower than that of the mining before the pressure relief area, and the decrease is 22.42%. It shows that the pressure relief has played a very good effect and provides a reference for the same type of mine.

Study on key parameters of buckling deformation instability and fracture of rock beams and asymmetric distribution law of stope stress.

The moving deformation of the strata and the redistribution of stope stress after mining show asymmetrical characteristics, which do not conform to the symmetrical structural characteristics of the original rock beam fracture. To further analyze the deformation of rock beams and the asymmetry law of stope pressure distribution after strata caving, the detailed process of instability and deformation of composite rock beams before failure was revealed through similar material simulation, theoretical analysis, and numerical simulation. Through similar simulation experiments, the structural characteristics of strata caving were observed. After excavation, the caving angle near the open-off cut side of the model is greater than that on the stop-mining line side. The maximum bending moment of the rock beam is located at the open-off cut side. The rock beam fracture is located on the partial open-off cut side in the middle of the rock beam. The rock beam on the open-off cut side is easy to shear slip and not easy to hinge. The rock beam in front of the advancing direction of the working face is easily hinged. Based on the structural characteristics of strata caving, considering the thickness of the composite rock beam, the two-hinged arch mechanical model for rock beam fracture is established. On this basis, the key parameters of rock beam instability and fracture such as limit load, additional horizontal stress, limit break distance, and break position are analyzed. Based on the deformation characteristics of two hinged arches, the caving structure and the asymmetric distribution mechanism of stress redistribution during the deformation of overburden in stope are explained. Finally, the deformation of rock beam and the asymmetry of stress distribution in stope are verified by numerical calculation. The results show that the concentrated stress value of the coal pillar at the open-off cut side is greater than that in front of the working face. There is a pressure relief area behind the working face, and the pressure relief area has a certain range. The range of stress concentration area, pressure relief area, and stress value tend to be stable, and only the range of the original rock stress zone expands when the working face is advanced to a certain distance. The asymmetric distribution of compaction stress in goaf is related to the buckling deformation of strata.

Research and Application of High Efficiency Pressure Relief Technology for Ultra-high Pressure Hydraulic Slotted Coal Seam

In order to solve the technical problem of efficient pressure relief of the impact coal seam, the principle of ultra-high pressure hydraulic slit relief and anti-impact technology is expounded, and the ultra-high pressure hydraulic slit device with working pressure up to 100MPa is selected as the pressure relief equipment of coal seam. The field test of high-efficiency pressure relief by ultra-high pressure hydraulic slotting in impact coal seam was carried out. The test showed that the slotting radius of No.3 coal in Tangkou Mine could reach 1.5m under the condition of 100MPa slotting pressure. The stress variation in seam area and large diameter drilling area is compared and analyzed. The stress in seam area decreases significantly by 40%. The stress of bolt (cable) and the deformation of surrounding rock in the slit area are investigated. Compared with the large-diameter pressure relief area, the increase value of bolt is lower and the increase value of cable is significant in the hydraulic slit test area. The technical benefits of hydraulic slotting and large-diameter drilling are summarized and compared. The engineering quantity of drilling with ultra-high pressure hydraulic slotting is reduced by 60%, and the coal output is 2.5 times of that of large-diameter drilling. The large-diameter drilling provides limited pressure relief space for coal seam, but the ultra-high pressure hydraulic slotting technology can effectively increase the deformation space in the coal body and reduce the elastic energy stored in the coal pillar, so as to achieve a good pressure relief effect. The field test shows that compared with large diameter drilling, ultra-high pressure hydraulic slotting technology has remarkable effects of pressure relief, shock absorption and shock prevention.

Numerical Investigation of Tree-Type Hydraulic Fracturing for Balanced Permeability Enhancement of Heterogenous Coal Seams Based on the Finite-Discrete Element Method Model.

Tree-type hydraulic fracturing (TTHF) is a new technology that can enhance the permeability of coal seams in a balanced manner and increase the coalbed methane production rate. However, the heterogeneity of coal seams is a major challenge in achieving balanced permeability enhancement by TTHF. Traditional methods based on digital image processing are difficult to apply in practice. To address these challenges, we proposed a 2D numerical model of coal seams based on the combined finite-discrete element method (FDEM). The elastic modulus of the coal seams obeys a Weibull distribution, and the coal heterogeneity was quantified by an index m. The effects on the fracture initiation pressure, the fracturing influence range, and displacements of TTHF were analyzed from four aspects, including the homogeneity index of coal, the arrangement angle of branch boreholes, the horizontal stress difference, and the injection rate of the fracturing fluid. The results show that TTHF has a significant effect on the balanced permeability enhancement in coal reservoirs, particularly with strong heterogeneity, and the best permeability enhancement for TTHF is achieved when the branch boreholes are arranged at 45°. The branch boreholes are prefabricated in advance to create a pressure relief area around the injection point, and the hydraulic fracture propagation is affected by the horizontal stress difference only when the fracturing influence range exceeds this area. When the horizontal stress difference increases from 0 to 4 MPa, its fracture initiation pressure increases from 8.93 to 10.86 MPa, with an increase of 21.61%. In addition, the initial stage of fluid injection was found to be crucial for achieving balanced permeability enhancement in TTHF, and a higher injection rate can expand the fracturing influence range. The numerical model has profound implications for the field application of TTHF technology.

Gas explosion impact behavior and disaster analysis based on structural failure: Numerical modeling

For gas leakage explosion accidents that occur in high blockage environments, the failure of building structures often leads to the evolution of explosion waves becoming unpredictable, resulting in the amplification of accident consequences. In this paper, a severe gas leakage and explosion accident in Beijing is analyzed using computational fluid dynamics (CFD) method, and the disasters of the accident are evaluated based on the evolution of physical quantities. The laws of propagation and impact damage of explosion waves based on structural failure are also summarized. Based on the numerical analysis results, the explosion of 4.0–4.1 m3 combustible gas is sufficient to destroy a large number of walls with a maximum indoor overpressure of 1.130 bar and create a high-pressure injury area of 10 m and a high-temperature injury area of 14 m outdoors. The impact behaviours of explosion waves are closely related to the building structures and obstacle characteristics. Structures whose spatial direction is the same as the propagation direction of the initial wave suffer less damage, and similar obstacle characteristics on both sides of structures reduce damage. The disorder of gas explosion intensity leads to the unpredictable failure direction and process of building structures, and the collapse direction of component may be opposite to the path of initial explosion wave; Outdoors, the gas flow rate of several hundred meters and the peak dynamic pressure of 0.5 bar caused all the glass of two buildings to break. Therefore, a reasonable pressure relief area and sufficient safety distance become the key measures to prevent and mitigate the disaster of civil explosion accidents. The research results can provide scientific basis for the investigation and loss prevention of explosion accidents, as well as safety assessments in the process industries.

Study on instability mechanism of soft rock roadway and pressure-relief bolt-grouting support technology

Aiming at the engineering problem of roadway deformation and instability of swelling soft rock widely existed in Kailuan mining area, the mineral composition and microstructure of such soft rock were obtained by conducting scanning electron microscopy, X-ray diffraction experiments, uniaxial and conventional triaxial tests, and the law of softening and expanding of such soft rock and the failure mechanism of surrounding rock were identified. The combined support scheme of multi-level anchor bolt, bottom corner pressure relief and fractional grouting is proposed. The roadway supporting parameters are adjusted and optimized by FLAC3D numerical simulation, and three supporting methods of multi-layer anchor bolt, bottom corner pressure relief and fractional grouting are determined and their parameters are optimized. The study results show that: the total amount of clay minerals is 53–75%, pores, fissures, nanoscale and micron layer gaps are developed, providing a penetrating channel for water infiltration to soften the surrounding rock; the three-level anchor pressure-relief and grouting support technology can control the sinking amount of the roof within 170 mm, the bottom drum amount within 210 mm, the bolts of each level is evenly distributed in tension, and the maximum stress and bottom drum displacement in the pressure relief area are significantly reduced; the pressure-relief groove promotes the development of bottom corner cracks, accelerates the secondary distribution of peripheral stress, and weakens the effect of high stress on the shallow area. Using time or displacement as the index, optimizing the grouting time, filling the primary and excavation cracks, blocking the expansion and softening effect of water on the rock mass, realizing the dynamic unity of structural yielding pressure and surrounding rock modification, has guiding significance for the support control of soft rock roadway.

Study and Design of the Mitigation Structure of a Shell PBX Charge under Thermal Stimulation

To study the design method and pressure relief effect of the mitigation structure of a shell under the action of thermal stimulation, a systematic research method of theoretical calculation-simulation-experimental verification of the mitigation structure was established. Taking the shelled PBX charge as the test material, the pressure relief area that can effectively reduce the reaction intensity of the charge is obtained by theoretical calculation. The influence of the pressure relief hole area, distribution mode, and other factors on the pressure relief effect is calculated by simulation. The pressure relief effect of the mitigation structure was verified by the low-melting alloy plug with refined crystal structure for sealing the pressure relief hole and the cook-off test. The research results show that the critical pressure relief area is when the ratio of the area of the pressure relief hole to the surface area of the charge is AV/SB = 0.0189. When the number of openings increases to 6, the required pressure relief coefficient decreases to AV/SB = 0.0110; When the length/diameter ratio is greater than 5, the opening at one end cannot satisfy the reliable pressure relief of the shell. The designed low-melting-point alloy mitigation structure can form an effective pressure relief channel. With the increase in AV/SB from 0.0045 to 0.0180, the reaction intensity of the cook-off bomb is significantly reduced in both fast and slow cook-off, which improves the safety of the charge when subjected to unexpected thermal stimulation.

Genesis of Low CBM Production in Mid-Deep Reservoirs and Methods to Increase Regional Production: A Case Study in the Zhengzhuang Minefield, Qinshui Basin, China.

With the increase of the burial depth of the no. 3 coal seam in the Zhengzhuang minefield of Qinshui Basin, the production of surface coal bed methane (CBM) vertical wells was low. By means of theoretical analysis and numerical calculation, the causes of low production of CBM vertical wells were studied from the aspects of reservoir physical properties, development technology, stress conditions, and desorption characteristics. It was found that the high in situ stress conditions and stress state changes were the main controlling factors of the low production in the field. On this basis, the mechanism of increasing production and reservoir stimulation was explored. An L-type horizontal well was constructed alternately among the existing vertical wells on the surface to initiate a method to increase the regional production of fish-bone-shaped well groups. This method has the advantages of a large fracture extension range and a wide pressure relief area. It could also effectively connect the pre-existing fracture extension area of surface vertical wells, realizing the overall stimulation of the low-yield area and increasing the regional production. Through the optimization of the favorable stimulation area in the minefield, 8 L-type horizontal wells that adopted this method were constructed in the area with high gas content (greater than 18 m3/t), a thick coal seam (thicker than 5 m), and relatively rich groundwater in the north of the minefield. The average production of a single L-type horizontal well reached 6000 m3/d, which was about 30 times that of the surrounding vertical wells. The length of the horizontal section and the original gas content of the coal seam had a significant influence on the production of the L-type horizontal wells. This method for increasing the regional production of fish-bone-shaped well groups was an effective and feasible low-yield well stimulation technology, which provided a reference for increasing the production and efficiently developing CBM under the high-stress conditions in mid-deep high-rank coal seams.

Studying the Favorable Zone for Pressure-Relief Gas Extraction by Combining Numerical Investigation and On-Site Application

Gas extraction in the pressure-relief area using the depressurization effect of seam mining is an effective way to solve coal-mine gas problems. In this paper, we performed numerical modelling of the 204 working face via FLAC 3D, by which the stress variation of the rock mass and associated permeability evolution during the working face advancement were investigated. Furthermore, the favorable gas extraction zone for pressure relief was proposed based on the gas extraction performance of the on-site boreholes. The results show that: (1) gas extraction varies with the evolution of the three zones of the overlying rock and the associated permeability variation, which becomes stable when the height of the three zones remains unchanged; (2) the favorable zone for gas extraction of the 204 working face ranges from 13 m to 19 m of Dv, in which the gas extraction of on-site boreholes are located, presents better performance than other areas; (3) the gas extraction volume of the optimized gas extraction scheme rises to 77.46 m3/min with a 77.26% gas extraction rate, eliminating gas overrun in the working face and upper corner and increasing the advancement speed to 6.4 m/d. The on-site verification confirms the numerical simulation scope, more accurately determining the favorable zone for gas extraction. This may provide a reliable approach for improving the pressure-relief gas extraction performance and reducing the associated engineering costs.

Study on stress evolution law of upper coal seam in long‐distance advance mining of lower coal seam

AbstractWith the shift of coal resources to deep mining, the occurrence of long‐distance coal seams has increased, and protective layer mining is facing new challenges. This paper attempts to explain the stress evolution law of the upper coal group in the long‐distance mining of the lower coal group in Pingdingshan No. 8 Coal Mine. A simulation model of advance mining of lower‐group coal long‐distance was established. The stress evolution law of the upper coal seam under the influence of advanced mining disturbance of the lower coal seam is studied. The following conclusions were obtained: The advance mining of the lower coal group had a positive or negative impact on the stress distribution of the upper coal seam group. With the recovery of the lower coal group of the F‐21030 working face, the overburden of the F‐21030 goaf finally formed a “Y” type pressure relief area. The pressure relief effect of the E‐21070 working face near the stopping line was obvious. The coal seam of Group E was divided into three areas affected by the advance mining of the lower coal seam. The maximum pressure relief value was 6.6% lower than the initial stress. According to the simulation results, the E‐21070 working face was divided into three regions, namely, the pressure relief region, the stress increase region, and the original stress region. According to the field drainage results of pressure relief gas, the extraction curve could be divided into three parts, namely, the stable area, pressure relief area, and stress recovery area. The maximum pure gas drainage volume could reach seven to eight times of the original area. The pressure relief extraction effect was remarkable, and the optimal extraction range was 22–210 m behind the coal face of the group.

Studies on the Influencing Factors of Gas Emission in a Steeply Inclined and Ultrathick Coal Seam Working Face.

To determine the factors affecting the gas emission in the working face during the horizontal sublevel mining of steeply inclined and ultrathick coal seams (SIUTCSs), the gas emission sources were identified and evaluated using the analytic hierarchy process (AHP), and the gas emission quantity was calculated using a prediction model. Sobol sensitivity analysis was then conducted on the influencing factors involved in the model to determine their influences on the working face gas emission. The AHP analysis found that the pressure relief area in the lower section is the main gas emission source of the working face, and the adjacent coal seams, old goaf, and rear goaf are not. The gas emission prediction model exhibits good accuracy. The calculation results suggest that the gas released from the lower section coal body accounts for 44.16-50.44% of the total gas emission quantity of the working face, and thus, it is the main gas emission source. The Sobol sensitivity analysis reveals that the dip angle of the coal seam has the greatest influence on the absolute gas emission quantity of the working face with a significantly larger sensitivity than those of other factors. The comprehensive sensitivity data analysis also suggests that the lower section coal body is the major contributor to the gas emission of the working face. Our work further puts forward a technical system of ultrahigh pressure hydraulic "drilling-slitting-pressing-draining" integrated antireflection enlarged gas extraction for controlling the gas emission from the coal body at the bottom of the SIUTCS. The engineering test demonstrates that this system can increase the permeability of the coal body and significantly improve the gas extraction efficiency of the coal body in the study area.

Intensive field measurements for characterizing the permeability and methane release with the treatment process of pressure-relief mining

Characterizing the permeability evolution and methane release is of great significance for the safe mining of the high gas outburst seams, as well as coal and gas simultaneous extraction. It contributes to reduce methane emissions from coal mining for greenhouse effect control. Theoretical analysis, laboratory testing, and numerical simulation are widely used methods to characterize the permeability and methane release with the treatment process of pressure-relief mining. However, these methods cannot fully reflect the complexity of filed practice. In this study, we report the effectiveness of protective coal seam (PCS) mining and the pressure-relief area in the protected coal seam (PDCS) based on detailed and integrated field measurements in a Chinese coal mine. To the best of our knowledge, it is the first time to measure the permeability coefficient and gas pressure evolution in the PDCS during the process of PCS longwall mining. The evolution of the permeability coefficient in the pressure-relief area during PCS mining can be divided into four stages: slowly decreasing, sharply increasing, gradually decreasing, and basically stable. The maximum permeability coefficient is 322 times of the initial value and stabilized at 100 times after the goaf compacted. The gas pressure evolution in the PDCS indicates that the strike pressure relief angle is 52.2° at the active longwall face zone, and 59.3° at the installation roadway side. The inclined pressure relief angles at the lower and upper sides of the longwall face are 75° and 78.9°, respectively. The residual gas content and gas pressure of the PDCS in the pressure-relief area are reduced to less than 6 m3/t and within 0.4 MPa, respectively. The field measurements further prove that pressure-relief mining can prevent coal and gas outbursts in PDCSs. The field observations in this paper can serve as benchmark evidence for theoretical analysis and numerical simulations, and also provide insights into realizing safety mining in similar conditions.

Coupling Mechanism of Coal Body Stress–Seepage around a Water Injection Borehole

In China, coal seam water injection technology is widely used in the prevention and control of mine gas and dust disasters. The evolution law of stress–seepage field in the range of hydraulic influence in the process of coal seam water injection is the most basic theoretical problem in coal seam water injection technology. Therefore, this paper first uses the radial seepage experimental system to test the permeability of raw coal samples, and then numerically inverts the stress–seepage evolution law of the coal body around the water injection hole. The results show that the coal seam permeability decreases exponentially with the increase in the experimental overburden pressure. The dynamic water intrusion causes the pressure relief area and the stress concentration area to transfer to the deep part of the coal body, and abnormal stress occurs near the water injection hole wall. The composite stress area formed by the superposition of stress and pore water pressure reduces the permeability of the hole wall. During the radial development of water injection, the pore water pressure dropped sharply and eventually tended to zero, while Darcy’s velocity decreased significantly (within the pressure relief zone, it rapidly decreased from 7.26 × 10−3 m s−1 to 2.5 × 10−3 m s−1 (by 65.56%)). Near the initiation point, the maximum step-down height of Darcy’s velocity decreased from 6.73 × 10−3 m s−1 to 5.27 × 10−3 m s−1 (by 22.73%). This can make it clear that the seepage velocity presents a non-Darcy seepage phenomenon under the influence of pore pressure.

Numerical Simulation of Gas-Solid Coupling in Pressure-Relief Gas Drainage of Short-Distance and Underprotective Steeply Inclined Coal Seam Mining

The accuracy of the drainage radius plays a vital role in the gas drainage effect, and the establishment of the drainage model and numerical calculation of the model has an essential value for the accurate determination of the drainage radius. Based on the elastoplastic constitutive model of mining coal and rock mass, effective stress equation, gas content equation, gas flow continuity, and control equation established by predecessors, the gas-solid coupling model of coal-rock deformation and pressure-relief gas flow in protective layer mining was established in this paper. The numerical simulation prototype was based on the mining engineering practice of a short-distance and underprotective steep seam mining in a mine in western Henan of China. COMSOL Multiphysics coupling software was used to numerically calculate the coupling model, and the influence radius of different mining pressure-relief area and different drainage time was obtained. The results show that under the same geological conditions and mining conditions, the impact range of drilling in the pressure-relief area of the protected layer is larger than that of the nonrelief area. As the working face of the protection layer advances, the pressure-relief area of the protected layer gradually increases, the influence range of the drainage borehole increases, the drainage borehole has undergone a process of initial stress-stress concentration-stress reduction-stress recovery successively, and the strike drainage radius is larger than the inclined.

Experimental study on displacement of CH4 by liquid CO2 in coal seam

ABSTRACT Mine gas drainage is an important work in high gas outburst coal mines. How to improve the gas drainage concentration and drainage volume has always been the direction of exploration by coal mine technicians. In this paper, the experiment and field test of displacing CH4 with liquid CO2 were carried out. In the process of low temperature liquid CO2 phase transformation, a low stress and high permeability pressure relief area is formed in the coal and rock mass, so as to accelerate the free state of primary gas in coal and realize the intensified gas extraction of low-permeability coal seams. The results show that CH4 concentration and displacement efficiency in the displacement flow decrease at a greater rate and then stabilize; the cumulative CH4 displacement volume increases rapidly and stabilizes after reaching the critical time; the average concentration of gas drainage from single hole increases from 27.2% to 53.2%, and the gas drainage purity increases 2.3 times from 0.018 m3/min to 0.041 m3/min. The technology is simple to operate, does not produce high temperature and open flame, has the advantages of economy, safety, high efficiency and so on, it can be popularized and applied in gas mines.

Research of roadway deformation induced by mining disturbances and the use of subsection control technology

AbstractTo address the problem of instability and failure of the 11,514 return air roadway caused by dynamic pressure in Jinhuagong Coal Mine, this study uses theoretical analysis, numerical simulation, and field test methods. The disturbance deformation mechanism and the surrounding rock control technology for the 11,514 return air roadway during face‐to‐face mining and excavation are analyzed. The results show that the causes of regional asymmetric deformation and failure of the surrounding rock of the 11,514 return air roadway are multiple superpositions of lateral bearing pressure of the 11,812 mined‐out area, mining stress in the 8814 working face, and advanced stress of the 11,514 return air roadway. When mining and excavation faces meet, the 11,514 return air roadway is affected by the superimposition of the mining stress. At the same time, the solid coal side of the roadway is in the mining pressure relief area in the 8814 working face in the upper coal seam, and the asymmetric distribution of the principal stress on both sides of the 11,514 return air roadway causes deformation and failure of the surrounding rock. Based on the differences in superimposed stress disturbance in different parts of the roadway, a subsection control technology is proposed. This technology is focused on supporting secondary reinforcement in the range of 40 m before and after the mining overlap position on the 11,514 roadway. Field measurement data confirm the effectiveness of this support technology.

Test research on advance abutment pressure caused by coal mining based on optical fiber monitoring

The strata movement and deformation caused by underground mining activities is a typical “black box” problem, it is difficult to grasp the structural characteristics of complete overburden and the law of ground pressure. Abutment pressure is the vertical supporting force acting on coal and rock mass due to the redistribution of surrounding rock stress caused by underground engineering excavation. The distributed optical fiber and fiber Bragg grating are implanted into the physical similar model to monitor the distribution characteristics of abutment pressure in front of the working face during the whole excavation process. The results show that FBG monitoring to reach the peak strain needs to meet two conditions: the working face passes under it, and the cantilever beam structure above the working face is unstable and collapses, and the grating is located at the end of the suspended fixed beam. When FBG is located in front of the working face, the whole process is divided into four stages: 150-81 m from the grating is the weak influence area; 81-49.5 m is the significant influence area; 49.5-21 m from the grating is the severe influence area; 21-0 m from the grating is the influence recovery area. Based on this, the abutment pressure distribution model of FBG sensing detection is given. The influence range is within 81 m, the peak value is 21 m in front of the working face, and the pressure relief area is directly above the working face. The distributed optical fiber monitoring results show that the upward compressive stress value of the abutment pressure affected area decreases rapidly, and enters into the high roof tensile stress affected area. The tensile stress distribution presents a single peak shape, and gradually decreases after reaching the peak until it returns to the elastic area. The research results provide a new research idea for abutment pressure monitoring caused by mining.

Research on Optimization of Coal Pressure Relief Borehole Parameters under High-Stress Conditions

Determining the parameters of boreholes drilled for relieving pressure in coal seams is the key preventing and controlling rock bursts in boreholes of large diameter. In this study, theoretical analysis, numerical simulation, literature research, and other analysis methods are applied to study the angles of elastic energy dissipation and stress transfer, the distribution law of the pressure relief area, and the areas of stress concentration, energy, stress, displacement, and plastic behavior of large-diameter pressure relief boreholes in coal seams under high-stress conditions. The results are then used to evaluate the relationship between large-diameter pressure relief boreholes and the borehole arrangement. The following results are obtained. (1) A large-diameter results in a large amount of elastic energy released by the surrounding coal, low residual elastic energy density, strong interaction between boreholes, large pressure relief range of the borehole, and high pressure relief efficiency. (2) The main evaluation factor of the borehole pressure relief effect is its thickness and stress concentration area; secondary evaluation is based on the areas of energy, displacement, stress, and plastic behavior. (3) Six evaluation index systems are established to evaluate the effects of borehole pressure relief, which are found to be the thicknesses of the borehole pressure relief area and stress concentration area, reduction degree of energy density, percentage of stress reduction, displacement, and penetration degree of the plastic area. (4) It is determined that when the diameters of the pressure relief boreholes are 100, 120, 180, and 200 mm, a single-row borehole arrangement is adopted; a three-pattern borehole arrangement is adopted with diameters of 140 and 160 mm. These research results can provide theoretical support in selecting reasonable borehole arrangements for pressure relief boreholes of different diameters.