Protective Coal Seam Research Articles

Permeability Enhancement of Coal Seam by Lower Protective Layer Mining for Gas Outburst Prevention

Risk prediction of dynamic disasters such as rock burst, gas outburst, and water inrush is closely related to the permeability evolution of coal seam. According to the characteristics of the lower protective layer mining, the basic assumption of gas-solid coupling model of the coal was proposed in this paper. The permeability enhancement coefficient of equivalent layer spacing was first put forward. Based on the three-zone-shaped dynamic evolution of the permeability of the overlying protective layer during the lower protective layer mining, the theories of seepage mechanics and damage mechanics were applied to introduce the permeability enhancement coefficient of equivalent layer spacing. A mathematical model of permeability evolution of the protected coal seam in the lower protective layer mining was established. Based on the engineering background of the lower protective layer mining in Changping Coal Mine, the numerical simulation using the proposed mathematical model was performed. The results showed that the stress and permeability of the protected layer in #3 coal seam evolved dynamically with the advancement of the working face of the protective layer in #8 coal seam. When the working face of the protective layer in #8 coal seam advanced to 80 m, the stress reduction rate in the relief area tended to be stable, and the stress in the stress reduction area was about 50% of the original rock stress. When advanced to 80 m, the permeability of the protected layer of the #3 coal seam increased sharply, and the permeability increased by 873 times. With the continuous advancement, the permeability of the protected layer in #3 coal seam tended to increase steadily, and the permeability increased by 1100–1200 times. The calculated magnitude of permeability increment is consistent with that in the engineering practice, indicating that the permeability evolution model is basically reasonable. The research provides a theoretical guidance for the gas drainage field application in the lower protective layer mining for prevention of coal and gas outburst.

Key technologies and engineering practices for soft-rock protective seam mining

Severe gas disasters in deep mining areas are increasing, and traditional protective coal seam mining is facing significant challenges. This paper proposes an innovative technology using soft rock as the protective seam in the absence of an appropriate coal seam. Based on the geological engineering conditions of the new horizontal first mining area of Luling Coal Mine in Huaibei, China, the impacts of different mining parameters of the soft-rock protective seam on the pressure-relief effect of the protected coal seam were analyzed through numerical simulation. The unit stress of the protected coal seam, which was less than half of the primary rock stress, was used as the mining stress pressure-relief index. The optimized interlayer space was found to be 59 m for the first soft-rock working face, with a 2 m mining thickness and 105 m face length. The physicochemical characteristics of the orebody were analyzed, and a device selection framework for the soft-rock protective seam was developed. Optimal equipment for the working face was selected, including the fully-mechanized hydraulic support and coal cutter. A production technology that combined fully-mechanized and blasting-assisted soft-rock mining was developed. Engineering practices demonstrated that normal circulation operation can be achieved on the working face of the soft-rock protective seam, with an average advancement rate of 1.64 m/d. The maximum residual gas pressure and content, which were measured at the cut hole position of the protected coal seams (Nos. 8 and 9), decreased to 0.35 MPa and 4.87 m3/t, respectively. The results suggested that soft-rock protective seam mining can produce a significant gas-control effect.

A protective seam mining method by cutting roof with chainsaw arm machine

Abstract Multiple-layered coal seams widely exist in main coal mining areas of China. When these coal seams are exploited, the pillar mining method is always employed. This leads to many coal pillars left in the upper coal seams as a protective barrier. As a result, these residual pillars will not only cause the loss of coal resources but also could trigger environmental issues and a serious of mine disasters. A theoretical model was built to analyse the effect of the residual pillars. From the theoretical model, it was found that four stress concentration areas were formed by the upper residual coal pillars. To address the issues of the residual coal pillars, Datong Coal Mine Group has developed an innovative technology of the roof cutting with a chainsaw. A new protective coal seam mining method using chainsaw roof-cutting technology is introduced. A numerical model is constructed to analyse the mining pressure distribution law in working face within the lower layer coal seam. From the numerical simulation, the new protective layer mining method could reduce about 15.2% of the advancing stress, which contributes a lot to controlling the mining pressure within the lower layer. The field measurement showed that the hydraulic support utilised at the site was at lower pressure levels, which proves the new protective seam mining method can significantly reduce the working face pressure.

Distribution law of floor stress during mining of the upper protective coal seam.

Floor stress distribution is the main index for evaluating the mining effect of upper protective coal seam. However, parameters currently used in theoretical analysis of mining-induced stress lack practicality. Therefore, in this article, a model for calculating floor stress during mining of upper protective coal seams was established based on the theory of elastic mechanics. Subsequently, the stress induced by the abutment pressure at any point in the five parts of the floor was derived. Moreover, the distribution characteristics of the horizontal, vertical, and shear stresses of the floor during mining of the upper protective coal seam were elaborated. The results show that with the continuous mining of the working face of the protective coal seam, the vertical stress of the floor strata experiences three stages, that is, rapid increase, abrupt stress relaxation, and gradual recovery to the in situ stress. With regard to the morphology, the floor strata recompress or expand in the vertical direction. Vertical and horizontal stress are relieved in the shallow part of the floor in the goaf behind the working face, and there is an abrupt reduction in the increase of concentration degree of vertical and horizontal stress in the floor strata in front of the working face. High shear stress occurs underneath the goaf near the working face. The isoline of the shear stress is distributed in a bubble shape and is oblique to the goaf. These research achievements can provide some theoretical basis for understanding the gas drainage during the mining of the upper protective coal seam.

Physical simulation of upper protective coal layer mining with different coal seam inclinations

AbstractProtective coal layer (PCL) mining is a preferred regional method for preventing coal and gas outbursts; stress‐relief of PCL mining is the prerequisite of pressure‐relief gas drainage. In order to study the stress‐relief effect of PCL mining on protected coal seams and determine the protection scope of PCL mining under different inclinations, this paper uses physical simulations to study the stress‐relief law and deformation characteristics of the strata during upper PCL mining with a gentle inclination (25°), an intermediate inclination (45°), and a steep inclination (65°). The results show that as the coal seam dip increases, the degree of deformation and failure of the overburden strata gradually decreases, the severe subsidence and expansion area of the overburden strata tends to gradually develop in the uphill direction of the working face, and the floor strata underlying the protective seam significantly expand toward the goaf. However, the absolute value of the upward movement of the floor strata is much smaller than that of the roof strata; and the greater the dip angle, the greater the ratio of floor displacement to roof subsidence. The stress‐relief curve of the protected layer is approximately U‐shaped. As the inclination of the coal seam increases, the stress‐relief range decreases gradually. The maximum relief‐stress and the stress release coefficient of the protected layer initially increase and then decrease. The stress concentration coefficient of the protected coal seam initially increases first and then decreases in both uphill and downhill of the working face in the PCL. As the inclination increases, the stress‐relief angle of the downhill boundary initially increases and then decreases, while the stress‐relief angle of the uphill boundary decreases gradually. Our research results provide guidance for the delimitation of the protected scope of PCLs with different dip angles.

A safe mining approach for deep outburst coal seam groups with hard‐thick sandstone roof: Stepwise risk control based on gas diversion and extraction

AbstractProtective seam mining is the preferred method for gas control and safe mining in high‐outburst coal seam groups. However, a hard‐thick sandstone roof (HTSR) triggers new gas problems when it covers directly above the first‐mined protective coal seam. To solve these problems, a typical case study in No. 12 Coal Mine was conducted. The simulation results indicate that the HTSR greatly prevents the plastic deformation of the upper roof formations without affecting the expansive deformation in the underlying protected seams. The pressure‐relief gas in protected seams will flow into the protective seam without continuing to migrate to roof strata, thereby turning the protective seam into a new high‐risk area. Therefore, roof presplitting (RPS) technology was proposed to effectively break the HTSR, create roof fracture passages, and divert the gob gas of the protective seam upward. Meanwhile, gob‐side entry construction (GSEC) was conducted to enhance gas discharge in the protective seam by adopting rapid filling, roof cutting, and active supporting technologies. Through the upper protective seam mining, RPS, and GSEC, gas migration passages were constructed step‐by‐step to divert the gas from high‐risk areas to low‐risk areas. As a key first step, gas diversion not only optimized the distribution of gas sources but also significantly reduced gas hazards. Source‐by‐source extraction was then performed to effectively control the high‐concentration gas in the newly formed gas source, further eliminating the mining risk. By applying the stepwise risk control method combined with gas diversion and extraction, the safe production was successfully achieved with excellent outburst prevention effects, and the gas extraction rate was significantly improved to more than 66%. This practice could be used for deep high‐gassy or high‐outburst coal seam group mining.

Physical modeling the formation of roof collapse zones in Vorkuta coal mines

Collapse of the roof during mining is a constant threat to the lives of miners and equipment performance. To predict it, different numerical and physical approaches are used. Physical modeling of the massif region from -951 m to -841 m using equivalent materials were undertaken in this investigation. Various options for mining the Vorkuta coal deposit were considered: mining only the upper industrial seam, mining the lower coal seam as a protective layer to the upper one, and assessing the impact on the collapse zone of the complex structure of the upper coal seam roof while maintaining an upward order of mining. The main results were as follows: calculated collapse zone is approximately 30% larger than theoretical; collapse zone decreased when using mining with lower protective coal seam; the largest collapse zone was observed when pinching the rocks in upward mining order. Therefore, the research has shown the need to adjust the project for a larger methane yield. Further large-scale modeling is needed for understanding the characteristics of the collapse zone.

Permeability distribution characteristics of underlying coal seam disturbed by mining activity

ABSTRACTThe permeability distribution law for protected coal seam is the core problem linked to protective coal seam mining. Based on the geological conditions of the Zhongxing coal mine, a numerical protective coal seam mining model was constructed in order to investigate the spatial evolution of stress and the damage characteristics of protected coal seams. During the mining of protective coal seam, the coal body of the protected coal seam undergoes loading followed by three-directional stress unloading. The permeability evolution law of the protected coal seam was derived by combining the relationship between the permeability and axial strain of the coal sample from mechanical-seepage experiments and the swelling and relaxation value of the protected coal seam from numerical simulations. We divided the permeability of the protected coal seam into the original permeability zone, the permeability decreasing zone, the permeability increasing zone, the permeability stable zone, and the permeability re-decreasing zone. The gas drainage data of the protected coal seam during the mining of the protective coal seam were measured in situ. The change laws of the gas drainage volume and gas concentration were essentially consistent with the permeability distribution of the protected coal seam.

Analysis of Spatial–Temporal Evolution of Mining-Induced Fracture Field: A Case Study Using Image Processing in the Shaqu Coal Mine, China

Mining-induced fracture plays a key role in gas drainage for gas burst-prone underground coal mines, especially for closely multilayered coal seams. The layout and location of the designed gas drainage boreholes have a huge influence on the performance of coal and gas extraction. The Shaqu coal mine exploits the #2 and #4 coal seams, which are gas burst-prone coal seams with low concentration of drained gas. The method of borehole camera observation was used in this study to investigate the evolution of fractures field in order to obtain an optimal borehole design. In addition, a method of pixel extraction and gray value calculation for borehole images was developed to obtain 3D gray value distributions for recorded borehole images. Based on the 3D gray value distribution for a borehole image, the evolution of borehole fractures was identified. Based on the analysis, an area of high-density mining-induced fractures within relatively stable overlying strata was found about 25–30 m above the working face of the protected coal seam. Therefore, it is considered that concentration, amount and stability of gas drainage should be improved if boreholes reach the fracture area to enhance gas drainage efficiency and improve mine safety in extraction of coal and gas.

Overlying Strata Movement of Below Protective Seam

ABSTRACT Bai, Y. and Hu, Y., 2018. Overlying strata movement of below protective seam. In: Liu, Z.L. and Mi, C. (eds.), Advances in Sustainable Port and Ocean Engineering. Journal of Coastal Research, Special Issue No. 83, pp. 193–198. Coconut Creek (Florida), ISSN 0749-0208. The characteristic and intensity of the overlying strata movement during the below protective seam mining play an important role to relieve the effect of the protected coal seam. Taking Wangjialing No. 2 mine as the experimental mine, with the method of deep base point, the settlement regularity of the immediate roof, basic roof and hard rock stratum are studied. At the same time, the mechanical model of basic roof is analyzed. Results show that: The immediate roof is falling down layer by layer, lower part of immediate roof is the first to cave, and its upper caving lags behind the lower caving. The immediate roof will fall behind the working face; The settlement pattern of main key stratum is similar to that of the basic roof, but ...

Effect of Premining on Hard Roof Distress Behavior: A Case Study

Considering the complex geological conditions of mining in the fully mechanized 828 working face of the Qingdong Coal Mine, a mining plan was developed for premining the upper No. 7 coal seam to control the hard roof and gas of the No. 8 coal seam. Failure depth of the No. 7 coal seam floor and the caving interval of the hard roof were analyzed using a proposed model based on the Terzaghi’s principle and rock beam theory, respectively. The movement rules of the overlying strata under premining (No. 7 coal seam) and non-premining conditions were analyzed using physical material similarity simulations experiment. Furthermore, field monitoring was performed to determine the working resistance of the hydraulic supports. The estimated results from the proposed method showed that the failure depth of the floor of the 726 working face was 13.8 m, which is in agreement with that noted during the field observation. The caving interval of the hard roof decreased from 24.9 m to 10.6 m after premining the No. 7 coal seam. In the simulation, the caving interval of the hard roof of the No. 8 coal seam decreased from 20 m to 10 m after premining the No. 7 coal seam. The hydraulic supports were effective in the actual production process, and the initial and periodic weighting intervals were 20.4 m and 10.4 m, respectively. These findings demonstrate that after premining the No. 7 coal seam, the initial and periodic weighting intervals decreased, thereby reducing the pressure intensity of the roof. This method effectively weakens the hard roof of the No. 8 coal seam. The issues pertaining to the hard roof during the mining of the 828 working face could be overcome by premining a protective coal seam of the upper No. 7 coal seam.

A Gas Seepage Modeling Study for Mitigating Gas Accumulation Risk in Upper Protective Coal Seam Mining Process

Protective coal seam mining (PCSM) is one of the most significant mitigation measures of regional outburst in the process of deep coal seam mining, which has high outburst risk in China. During the PCSM process, the phenomenon of methane concentration exceeding the limit usually occurs in the working face. It is vital to understand factors affecting gas emission from the protective seam working face (PSWF) and to obtain an equation for determining the quantity of gas emission. A gas seepage model (GSM) was developed to simulate the gas emission during the process of upper PCSM. In this study, an equation was formulated to determine the quantity of gas desorbed from the protected seam into PSWF. These equations have been developed by using Fick’s second law of diffusion and Darcy’s flow law. The relationship between permeability and stress was described in an elastic-plastic state, and the mechanics of surrounding rock were investigated. It can be concluded from GSM that the initial gas pressure of protected seam, the characteristics of interlayer rocks, and the ventilation pressure of PSWF were the main factors that influenced the desorption of gas emission from the protected seam into PSWF. The developed GSM was tested for calculating gas emission quantity from the PCSM process by utilizing the actual geological condition data of a coal mine, which is located in Hancheng, China. The results have shown great agreement with obtained field measurements, which is done by combining the fitting curve of ventilation air methane quantity for PSWF with an interlayer spacing. A loss coefficient (δ) of 1.012 × 10−3 was obtained in this study.

Rational Boreholes Arrangement of Gas Extraction from Unloaded Coal Seam

In order to enhance gas extraction from unloaded coal seam by drilling borehole in the floor roadway, the mechanism of stress relief improving permeability by protective coal seam mining was analyzed. Based on the multiphysics field theory, the hydraulic-mechanical coupling model of gas extraction in the unloading coal seam was established, and the gas extraction process by drilling borehole in the floor roadway in the overburden of Panyi Coal Mine 1551 (1) panel was simulated. The influence of different drilling arrangements on the gas extraction effect was analyzed. The results show that the permeability of protected coal seams is characterized by zoning and can be divided into the permeability-enhanced zone, the permeability-reduced zone, and the original permeability zone according to the stress state of coal seam. Under the condition of uniform borehole distribution, the gas pressure decreased slowly in the permeability-reduced zone and is still greater than 0.74 MPa after 180 days of extraction, and there is a large extraction blind area in the protected panel. Under the condition of nonuniform borehole distribution arrangement according to the characteristics of permeability zoning, the effective extraction area can almost cover the protected panel, and the blind extraction area is reduced by 91.22% when compared to uniform borehole distribution. These can provide a reference for unloading gas extraction under similar conditions.

Mining Thickness Determination of Upper and Lower Protective Coal Seam in Pressure Relief Mining

The reasonable selection of the protective coal mining (PCS) thickness and the quantitative relationship between the mining thickness and the interlayer spacing directly affect the performance of the pressure relief gas drainage and the safe mining of the PCS. Thus, the evolution of the protected coal seam (PDCS) permeability under the different PCS mining thickness in Huainan and Hancheng mining area is simulated by using the stress-damage-seepage coupling model. The results show that the PDCS permeability presents an S type increasing with the increase of the mining thickness. The logistic function formula for quantitative analysis of the relationship between the PCS mining thickness and PDCS permeability is given. The optimal mining thickness of the PCS in Huainan and Hancheng mining area is 1.73 m and 0.96 m, respectively, and the reasonable mining thickness range is 0.87–4.06 and 0.68–1.32 m. Based on the numerical simulation, the quantitative curve between the mining thickness and the interlayer spacing of the upper and lower PCS mining in Hancheng and Huainan mining area is presented. Finally, the reliability of the numerical simulation is verified by the filed measurement of the fracture development in the roof and the permeability of the PDCS.

Seepage and Damage Evolution Characteristics of Gas-Bearing Coal under Different Cyclic Loading–Unloading Stress Paths

The mechanical properties and seepage characteristics of gas-bearing coal evolve with changes in the loading pattern, which could reveal the evolution of permeability in a protected coal seam and allow gas extraction engineering work to be designed by using the effect of mining multiple protective seams. Tests on gas seepage in raw coal under three paths (stepped-cyclic, stepped-increasing-cyclic, and crossed-cyclic loading and unloading) were carried out with a seepage tester under triaxial stress conditions. The permeability was subjected to the dual influence of stress and damage accumulation. After being subjected to stress unloading and loading, the permeability of coal samples gradually decreased and the permeability did not increase before the stress exceeded the yield stage of the coal samples. The mining-enhanced permeability of the coal samples in the loading stage showed a three-phase increase with the growth of stress and the number of cycles and exhibited an N-shaped increase under the stepped-cyclic loading while it linearly increased under the other two paths in the unloading stage. With the increase of peak stress and the accumulation of damage in coal samples, the sensitivity of the permeability of coal samples to stress gradually declined. The relationship between the damage variable and the number of cycles conformed to the Boltzmann function.

A Numerical Study of Stress Distribution and Fracture Development above a Protective Coal Seam in Longwall Mining

Coal and gas outbursts are serious safety concerns in the Chinese coal industry. Mining of the upper or lower protective coal seams has been widely used to minimize this problem. This paper presents new findings from longwall mining-induced fractures, stress distribution changes in roof strata, strata movement and gas flow dynamics after the lower protective coal seam is extracted in a deep underground coal mine in Jincheng, China. Two Flac3D models with varying gob loading characteristics as a function of face advance were analyzed to assess the effect of gob behavior on stress relief in the protected coal seam. The gob behavior in the models is incorporated by applying variable force to the floor and roof behind the longwall face to simulate gob loading characteristics in the field. The influence of mining height on the stress-relief in protected coal seam is also incorporated. The stress relief coefficient and relief angle were introduced as two essential parameters to evaluate the stress relief effect in different regions of protected coal seam. The results showed that the rock mass above the protective coal seam can be divided into five zones in the horizontal direction, i.e. pre-mining zone, compression zone, expansion zone, recovery zone and re-compacted zone. The volume expansion or the dilation zone with high gas concentration is the best location to drill boreholes for gas drainage in both the protected coal seam and the protective coal seam. The research results are helpful to understand the gas flow mechanism around the coal seam and guide industry people to optimize borehole layouts in order to eliminate the coal and gas outburst hazard. The gas drainage programs are provided in the final section.

Protection scope and gas extraction of the low-protective layer in a thin coal seam: lessons from the DaHe coalfield, China

Aiming at promoting thin coal seam pressure relief gas extraction, as well as delimiting the protection range of the low-protective layer in thin coal seam, in this paper, we simulated the changing process of the protective layer stress field and deformation field during the low-protective layer mining and obtained the characteristics of the displacement as well as the stress varied characteristics of the overlying strata in the low-protective layer mining. According to the protection layer mining stress relief and the deformation protection criterion, the tendency and trend of the protection layer protection scope were determined, and the theoretical calculation results were basically consistent with the direction protection angle of the protected coal seam. Taking the Dahe (DH) coalmine as an example, the floor rock roadway as the gas drainage roadway to extraction gas as well as the scheme of gas extraction and pressure relief in the upper and lower layers of the protective layer were implemented. The effect of gas control scheme showed that the working gas concentration reached the mining requirement. We finally established the low-protective layer in thin coal mining integrated technical scheme of protection scope and pressure relief gas extraction.

New insights into the permeability-increasing area of overlying coal seams disturbed by the mining of coal

Many researchers have concluded that the permeability evolution of coal and rock is closely related to the stress change path. However, in the protective coal seam mining practices, the stress change path of the overlying coal seams is ignored, per the related technical criterion and regulations, the traditional permeability-increasing area in the overlying protected coal seam is generally delimited smaller than the mined-out area of the protective coal seam, which wastes a large quantity of coal resources. This study investigates the actual permeability-increasing area of the overlying protected coal seam through theories analysis, FLAC3D numerical simulation and field test. The permeability distribution of the overlying protected coal seam near the mined-out boundary in the strike direction (MBSD) is quantitatively divided into four zones, which are the original permeability zone, permeability decrease zone, permeability insufficient increase zone, and permeability sufficient increase zone. It is suggested that the actual permeability-increasing area is larger than the mined-out area of the protected coal seam. According the permeability distribution of the protected coal seam, optimized cross-measure boreholes were designs for gas drainage. The field tests showed that the outburst risk of the protected coal seam inside the MBSD was eliminated two months after the protective coal seam mining. The theories analysis, numerical simulation and field tests showed that the actual permeability-increasing area is larger than the traditional permeability-increasing area, the protected area in the protected coal seam can be enlarged by the optimized cross-measure boreholes gas drainage.

Gas Permeability Evolution Mechanism and Comprehensive Gas Drainage Technology for Thin Coal Seam Mining

A thin coal seam mined as a protective coal seam above a gas outburst coal seam plays a central role in decreasing the degree of stress placed on a protected seam, thus increasing gas permeability levels and desorption capacities to dramatically eliminate gas outburst risk for the protected seam. However, when multiple layers of coal seams are present, stress-relieved gas from adjacent coal seams can cause a gas explosion. Thus, the post-drainage of gas from fractured and de-stressed strata should be applied. Comprehensive studies of gas permeability evolution mechanisms and gas seepage rules of protected seams close to protective seams that occur during protective seam mining must be carried out. Based on the case of the LongWall (LW) 23209 working face in the Hancheng coal mine, Shaanxi Province, this paper presents a seepage model developed through the FLAC3D software program (version 5.0, Itasca Consulting Group, Inc., Minneapolis, MI, USA) from which gas flow characteristics can be reflected by changes in rock mass permeability. A method involving theoretical analysis and numerical simulation was used to analyze stress relief and gas permeability evolution mechanisms present during broken rock mass compaction in a goaf. This process occurs over a reasonable amount of extraction time and in appropriate locations for comprehensive gas extraction technologies. In using this comprehensive gas drainage technological tool, the safe and efficient co-extraction of thin coal seams and gas resources can be realized, thus creating a favorable environment for the safe mining of coal and gas outburst seams.

Stress distribution characteristic analysis and control of coal and gas outburst disaster in a pressure-relief boundary area in protective layer mining

Coal and gas outburst disasters in coal seams are becoming more serious as coal mines extend deeper underground in China. Furthermore, the protective coal seam mining technology featured by economic efficiency has been proven to be the most effective and widely applied method for the prevention of coal and gas outburst disasters. However, the determinations of the protective area coal and gas outburst prevention in a pressure-relief boundary area are fundamental issues that research should be focused on. The technical method for determining stress distribution in pressure-relief boundary area during protective coal seam mining is put forward in this paper. The method is based on a stress-seepage coupled relationship within a gas-containing coal seam. The method includes complex lab experiments and on-site measurements at the Qingdong Coal Mine. The final data illustrate that the permeability and vertical stress in the pressure-relief boundary area of the coal sample form a negative exponential function relationship. Additionally, the permeability of the coal sample within the abovementioned area is significantly different compared with that located at the center of the pressure-relief area. In the pressure-relief boundary area, the gas pressure distribution gradient is 0.0375 MPa/m, while the vertical stress distribution gradient registers 0.56 MPa/m. Under this condition, coal and gas outburst disasters are prone to be triggered. Therefore, effective precautions against coal and gas outburst disasters can be put forward in accordance with stress distribution characteristics within the abovementioned “boundary area.”