In this study, the movement and failure law of working face overburden and the distribution characteristics of mining stress are analyzed using laboratory test and numerical calculation methods to address the problems of large deformation, failure instability, and difficult maintenance of the soft rock roof roadways of deep stope influenced by strong disturbance, the roof gas comprehensive treatment roadway of 17191 (1) working face of Pansan mine of China Huainan Mining Group was considered as the engineering background. The deformation and failure mechanisms of the surrounding rock in soft rock roof roadway are revealed, the surrounding rock control technology of presplitting and roof cutting pressure relief is proposed, and the key parameters of presplitting and roof cutting are systematically studied. According to the results, after mining, the overburden presents the distribution of “upper three zones,” in which the heights of the caving and fracture zones are 7 m and 38 m, respectively, the influence range of lateral mining abutment pressure is 80 m, and the influence height exceeds 42 m. The roadway is located in the same layer as the fracture zone and within the influence range of mining. Under the influence of overburden migration of the working face, the stress field around the roadway, and the mining stress field, the surrounding rock of the roof roadway is significantly damaged, and the floor heave is violent. Based on the stress distribution characteristics of the stope and the deformation mechanism of the roadway, the pressure relief control technology of surrounding rock presplitting roof cutting is proposed. The optimal values of key parameters are determined as the roof cutting height of 49.9 m, the roof cutting angle of 10°, and the blast hole spacing of 10 m. The results of this study have been successfully applied in 17191 (1) working face.

The proportion of China’s coal energy consumption will continue exceeding 50% until 2025 [1], and the energy consumption structure dominated by coal resources still exists. With the depletion of China’s shallow coal resources, the mining of deep coal resources has long been on the agenda. Gas extraction is often performed before mining to reduce the permeability of coal seams and reduce gas accumulation in deep coal mining. The high gas extraction roadway is located above the coal seam, which is the key channel for gas extraction, treatment, and transportation. The stability of the roadway is a necessary prerequisite for safe and efficient mining. Based on existing studies, the surrounding rock deformation of roof roadway in upward mining is primarily floor heave [2], and the roof deformation is small. Therefore, the focus of the surrounding rock stability control of roof roadway should be on improving the floor heave of the surrounding rock.

Currently, studies on the stability of the surrounding rock of a roof roadway primarily focus on the support structure and reasonable layout [3], which reduce the deformation of the surrounding rock of the roadway and realize the stability control of the surrounding rock of the roadway by improving the support scheme and selecting a reasonable layout position. For the roof roadway of the deep stope, owing to the superposition of high stress of the surrounding rock and mining stress, the stress environment of the surrounding rock of the roadway is complex. Controlling the deformation of the surrounding rock by taking measures from the properties of the surrounding rock and support technology is often difficult. Thus, the stress state of the roadway surrounding rock should first be improved using the technical scheme of manual blasting roof cutting and pressure relief under appropriate conditions [46]. The roof cutting and pressure relief technology of deep hole presplitting blasting can change the deep roof structure of the roadway surrounding rock, reduce the stress of roadway surrounding rock, and realize stability control of the roadway [7]. This technology is based on the design of blasting presplitting parameters. The presplitting parameters used in roadways under different geological conditions often differ significantly. The reason is that the cuttability [8, 9] and crack propagation [10, 11] of rock are affected by confining pressure conditions. References [1215] explored the key parameters of automatic roadway formation (retention) in shallow buried thin coal seam by establishing the mechanical model, deriving the theoretical formula of the seam cutting angle and height, and using the numerical calculations. Ma et al. [16] avoided the complicated field test method and used the mathematical analysis method to study the key parameters of blasting. Through a neural network, the three key parameters of hole sealing length, hole spacing, and single hole charge were determined, which significantly simplified the design process of the blasting parameters. Liu et al. [17] established the mechanical model of three-hinged arch structure, studied the relationship between the rotation angle of key blocks, roadway width, and sidewall force, and determined the optimal blasting parameters. He et al. and Ma et al. [18, 19] used the methods of theoretical analysis, numerical simulation, and field measurement to study the key parameters of thick coal seams and composite roof cutting and retaining roadway. They defined the key parameters of a deep high-stress medium-thick coal seam and composite roof cutting and retaining roadway. Chen et al. [20] deduced the formulas of roof cutting height and angle, established the mechanical model of shaped charge blasting, and determined the key parameters of roof cutting in deep goaf-forming blasting, based on rock fragmentation, swelling, and “S-R” structural characteristics. Gong et al. [21] developed the “deep hole+shallow hole” presplitting blasting roof cutting technology for hard rock roofs, combined with numerical simulations and field test methods. They determined the economic and reasonable blasting parameters of retaining roadway along the goaf.

The design of key parameters of blasting roof cutting and pressure relief is the core problem of the pressure relief roadway protection technology, which impacts the stability of the roadway surrounding rock. The above studies on the key parameters of blasting roof cutting and pressure relief focus primarily on the roadway along the goaf. Furthermore, studies on the key parameters of the roof roadway surrounding rock roof cutting and pressure relief control are scarce. Based on the study background of 17191 (1) deep working face roof gas comprehensive treatment roadway in Pansan Coal Mine, Huainan mining area, China, this study systematically investigates the key parameters of the surrounding rock roof cutting and pressure relief control of soft rock roof roadway in deep mine stopes through the combination of material similarity simulations, numerical simulations, and field tests.

The buried depth of 11-2 coal seam in 17191 (1) working face of Pansan Coal Mine is -730~-801 m and is generally monoclinic, high in the north, and low in the south. The occurrence of the coal seam is 180~260°, the dip angle of the coal seam is 2~8°, with an average of 5°, and the coal thickness is 0.1~2.5 m, with an average thickness of 1.7 m. The thickness of 11-1 coal is 0.4~0.8 m, with an average thickness of 0.6 m. The direct roof of the working face is mudstone, gray/dark gray, sandy argillaceous structure, intercalated with a thin layer of carbonaceous mudstone, with plant fossils, porcelain or shell fracture, brittle, loose, and fragile. The east of line IX-X is gradually transformed into fine sandstone, with an average thickness of 4.9 m. The basic top is made up of silty fine sandstone, light gray, medium-thick, mainly composed of quartz and feldspar, containing a small amount of dark minerals, calcareous cementation, with an average thickness of 7.2 m. The direct bottom is made up of mudstone, dark gray, argillaceous structure, containing a small amount of sandy components. Plant fossils can be seen. The fissure sliding surface is developed and fragile, with an average thickness of 4 m. The specific roof and floor can be seen in Figure 1. The 17191 (1) gas comprehensive treatment roadway is located in the mudstone of the lateral roof of the working face. The horizontal and vertical spacing with the transportation trough of 17191 (1) working face is 35 m and 27 m, respectively. The specific layout is shown in Figure 2.

The 17191 (1) working face gas comprehensive treatment roadway is the key roadway of the “one roadway multipurpose, joint treatment, and continuous mining” gas treatment mode [22]. When the roadway deformation and damage is severe, the safe production of the working face is severely influenced, and the development of this new gas treatment mode is restricted, which contradicts the development concept of safe and efficient production in deep mines. Consequently, the author proposes the presplitting blasting technology to relieve the pressure of the roof roadway and realize the stability control of soft rock roof roadway by optimizing the key parameters of pressure relief.

3.1. Evolution Law of Overburden Structure in Deep Mine Stopes

According to geological conditions and the similarity theory, the geometric similarity ratio is determined as 1 : 100. The similarity ratio of bulk density is 1 : 1.67. The stress similarity ratio is 1 : 167. The size of the model is determined as follows: length×width×height is 3000mm×300mm×1600mm, and the mining thickness is 1.7 cm. Meanwhile, boundary coal pillars of 35 cm and 42 cm were reserved on both sides of the model. In the test, fine sand was selected as the aggregate, gypsum powder and lime powder were used as cementitious materials, mica powder was used for layered laying between each rock stratum, and the remaining weight on the upper part of the model was applied using an additional counterweight.

Figures 3(a)–3(e) show the fracture form of the overburden of the inclined section of the working face at different distances of the model excavation. As seen from the figure, when the model excavation distance is 55 m, the overburden collapses gradually, the collapsed height is approximately 5 m, the collapsed gangue gradually fills the goaf, and a suspended roof is formed above the goaf. Owing to the small area of the suspended roof, the basic roof is not broken. After the model excavates for 30 m, the overburden collapse intensifies, fissures gradually develop, the suspended roof area above the goaf increases, the basic roof is broken, and a long arm “F” suspended roof structure is formed on the right side of the goaf. This structure is located below the gas comprehensive treatment roadway’s stratum, indicating that the gas comprehensive treatment roadway’s stratum is temporarily not influenced by mining. With an increase in the excavation distance of the model, cracks also develop continuously, and the influence range of the long arm “F” suspension structure formed on the right side of the goaf increases gradually. When the excavation distance is 115 m, the cracks in the roof of the working face develop, and the separation layer with a width of 5 mm and a length of 60 cm appears in the overlying strata. Simultaneously, the long arm “F” suspension structure on the right side of the goaf influences the horizon and the comprehensive gas treatment roadway. The model continues to excavate for 30 m, and the separation layer is gradually closed and transferred upward and finally stabilized in the rock layer above the gas comprehensive treatment roadway. The influence horizon of the long arm “F” suspended roof structure formed on the right side of the goaf gradually exceeds the horizon where the roadway is located. The model continues to excavate to the stopping line, the movement and deformation of the overlying strata gradually intensify, the suspended roof structure on the right side of the goaf changes from long arm “F” to medium and short arm “F”, and finally forms a rock stratum collapse angle of approximately 55°. After the excavation of the working face, the overburden presents “three zones” zoning, in which the height of the caving zone is approximately 7 m, and the height of the water diversion fracture zone is approximately 45 m. The comprehensive gas treatment roadway is located in the rock stratum of the fracture zone, which is easily influenced by the mining of the working face.

3.2. Distribution Characteristics of Mining Stress in a Deep Mine Stope

According to the actual geological conditions of the 17191 (1) working face, a numerical model is established. Coal pillars of 90 m and 120 m were reserved on the left and right sides of the model to eliminate the boundary effect. The total size of the model (length×wide×height) is 450 m ×560 m ×270 m, and the unit quantity of grid division was approximately 1.2 million. The upper boundary of the model is defined as the free boundary. A load of 17 MPa is applied to simulate the weight of the overburden, and the other boundaries are simple. According to the project’s actual situation, the working face advanced in the Y-direction, and the advancing distance of the simulated working face is 300 m. The numerical calculation model and its steps are shown in Figure 4. The physical and mechanical parameters of each rock stratum used in the simulation are shown in Table 1.

Figure 5 shows the distribution of mining stress of overlying strata at different positions from the working face.

As seen in Figures 5(a)–5(h), the overburden stress after the excavation of the working face presents a horizontal “three zone” distribution, the rock layer above the goaf is in the stress reduction area, the lateral rock layer of the goaf roof is in the area of stress increase and the original rock stress area, of which the area of stress increase is within 80 m from the goaf boundary, and the rock layer beyond 80 m is in the original rock stress area. As seen in Figure 2, the horizontal distance between the gas comprehensive treatment roadway and the goaf boundary is 35 m, located in the mining stress rise area. By comparing and analyzing the mining stress distribution of rock strata at different positions above the working face, it can be seen that the rock strata within 47 m above the working face are influenced by mining and the rock strata within 42 m are significantly influenced. The rock strata outside 42 m are less influenced by mining, indicating that the plastic failure degree of rock strata here is small. The rock stratum outside 42 m can be used as the “transfer channel” for mining stress to the roof roadway.

3.3. Deformation and Failure Mechanism of the Surrounding Rock of Soft Rock Roof Roadway in Deep Mine Stope

After excavating the working face, the “upper three zones” and “transverse three zones” are formed in the overburden of the working face. Simultaneously, the lateral roof of the goaf forms a medium and short arm “F” type suspended roof structure, resulting in additional stress in the surrounding rock of the roof roadway, as shown in Figure 6. The rock stratum of the gas comprehensive treatment roadway is located in the fracture zone in the “upper three zones” and in the stress concentration area, which is significantly influenced by the lateral mining stress of the working face. The original concentrated stress of the surrounding rock around the gas comprehensive treatment roadway is superimposed with the lateral mining bearing pressure of the working face, resulting in the continuous expansion of the plastic area of the surrounding rock, severe deformation of the surrounding rock, and gradual failure and instability. Therefore, to realize the stability control of roof gas comprehensive treatment roadway, the influence of a lateral suspended roof structure and lateral mining stress on the roadway surrounding rock must be cut off.

The key parameters of the presplitting roof cutting technology for deep hole blasting mainly include three parts: roof cutting height, roof cutting angle, and blast hole layout spacing. Experts and scholars have conducted in-depth studies on the layout spacing of blast holes. Therefore, the following only analyzes the two key parameters: the top cutting height and the top cutting angle. The key parameters of presplitting top cutting pressure relief are shown in Figure 7.

4.1. Analysis of the Cutting Height

According to the deformation and failure mechanism of the surrounding rock of the roof roadway, to realize the stability control of the surrounding rock of the soft rock roof roadway, it is necessary to cut off the influence of the mining stress of the working face and the suspended roof structure on the surrounding rock of the roadway. Therefore, the top cutting height should not be less than 42 m. According to the roof rock stratum of the working face, five roof cutting height schemes of 0 m (no roof cutting), 43.4 m (cutting off the sandy mudstone layer), 47.3 m (cutting off half layer of the medium and fine sandstone layers), 49.9 m (cutting off the medium and fine sandstone layers), and 54.5 m (cutting off spotted mudstone layers) are selected for simulation analysis. The physical and mechanical parameters of each rock stratum used in the simulation are consistent with the above parameters. The rock stratum is simulated using the Mohr-Coulomb constitutive model, and the goaf and fracture are simulated using the goaf model.

Figure 8 shows the vertical stress distribution characteristics of overburden and the roof roadway surrounding rock under different roof cutting heights. When the top cutting height is 0 m (without top cutting), as shown in Figure 8(a), the vertical stress concentration of the working face is large. After top cutting, the peak value of the vertical stress of the working face is smaller than that before top cutting, as shown in Figures 8(b)–8(e). The variation law of the stress distribution of the two sides of the gas comprehensive treatment roadway with the cutting height is shown in Figure 9.

As seen in Figures 9(a) and 9(b), when the roof cutting height is 0 m (without roof cutting), the peak vertical stress of the roadway slope far from and close to the goaf is 27.99 MPa and 30.08 MPa, respectively. When the cutting height is 43.4 m, the peak values of the vertical stress in the side of the roadway far from and close to the goaf are 25.06 MPa and 27.19 MPa, respectively. When the cutting height is 47.3 m, the peak values of the vertical stress in the side of the roadway far from and close to the goaf are 22.71 MPa and 23.76 MPa, respectively. When the cutting height is 49.9 m, the peak values of the vertical stress in the side of the roadway far from and close to the goaf are 20.28 MPa and 20.09 MPa, respectively. When the cutting height is 54.4 m, the peak values of the vertical stress in the side of the roadway far from and close to the goaf are 19.78 MPa and 19.98 MPa, respectively. Compared with the scheme without roof cutting, when the roof cutting height is 43.4 m, 47.3 m, and 49.9 m, the peak stress reduction rates of the side of the roadway far from the goaf are 10.5%, 18.9%, 27.5%, and 29.3%, respectively. The peak stress reduction rates of the side of the roadway near the goaf are 9.6%, 21%, 33.2%, and 33.6%, respectively. Therefore, according to the above analysis, the roof cutting height of 49.9 m can effectively reduce the stress concentration of the surrounding rock of the roof roadway, and the pressure relief effect is good. When the top cutting height is continuously increased, the pressure relief effect gain becomes insignificant. As regards the economic factors, the optimal value of the presplitting top cutting height is 49.9 m.

4.2. Top Cutting Angle Analysis

The pressure relief control effect differs under the same topping height and different topping angles. To select the optimal topping angle, five topping angle schemes of 0°, 5°, 10°, 15°, and 20° are adopted for simulation, in which the topping height is 49.9 m.

Figures 10(a)–10(e) show the vertical stress distribution characteristics of overburden and the roof roadway surrounding rock under different roof cutting angles. Based on the analysis, with a decrease in the cutting angle, the vertical stress concentration degree of the running slope of the working face decreases/increases. The variation law of the stress distribution at the two sides of the gas comprehensive treatment roadway with the roof cutting angle is shown in Figure 11.

As seen in Figures 11(a) and 11(b), when the roof cutting angle is 0°, the peak vertical stresses of the roadway side far from and close to the goaf are 18.28 MPa and 20.09 MPa, respectively. When the top cutting angle is 5°, the peak vertical stresses of the side slope away from and close to the goaf are 17.03 MPa and 18.00 MPa, respectively. When the top cutting angle is 10°, the peak vertical stresses of the side slope away from and close to the goaf are 16.23 MPa and 16.14 MPa, respectively. When the top cutting angle is 15°, the peak vertical stresses of the side slope away from and close to the goaf are 16.70 MPa and 17.11 MPa, respectively. When the cutting angle is 20°, the peak vertical stresses of the side slope away from and close to the goaf are 17.35 MPa and 18.22 MPa, respectively. Compared with the scheme of 0° roof cutting angle, when the roof cutting angle is 5°, 10°, 15°, and 20°, the peak reduction rates of the vertical stress in the side slope away from the goaf are 6.84%, 11.2%, 8.64%, and 5.09%, respectively. The peak value reduction rates of the vertical stress near the side slope of the goaf are 10.4%, 19.7%, 14.8%, and 9.31%, respectively. Therefore, according to the above analysis, when the roof cutting angle is 10°, the stress concentration of the surrounding rock of the roof roadway can be effectively reduced, and the pressure relief effect is the best. Whether the cutting angle is greater than 10° or less than 10°, the pressure relief effect is weakened and the optimal value of this presplitting cutting angle is determined as 10°.

4.3. Blast Hole Layout Spacing

The distance between roof cutting should be less than the periodic pressure step that can be borne by the stope support. The spacing of roof cutting is related to the lithology, thickness, and fracture development of the roof. Simultaneously, the spacing of roof cutting is related to the crack width formed by blasting. The width of the crack zone formed by blasting roof cutting is large, and the caving is significant; however, the width is too large, which may cause the roof to be seriously broken and difficult to manage during coal mining. Moreover, the workload of drilling and blasting increases exponentially, and the row spacing of blast holes should be less than twice the crack length. During the normal mining period of 17191 (1) working face, the periodic pressure step distance is approximately 20-25 m, and the top cutting spacing of presplitting blasting is preliminary determined as 10 m.

According to the above analysis results, the presplitting blasting holes are constructed on the roof of the advanced working face section in 17191 (1) working face. The height of the holes is 49.9 m, the holes are deflected to the side of the working face by 10°, and the interval distance between the holes is 10 m.

The cross layout method is used to observe the roadway surface displacement, and the observation results of surface displacement of two stations 1 and 2 are selected for analysis. Stations 1 and 2 are located 100 m and 200 m in front of the roof cutting section, respectively, as shown in Figure 12.

As seen in Figures 12(a) and 12(b), with an increase in the distance from the working face and the displacement of the roadway roof, the floor and two sides of 100 m in front of the roof cutting section show a change trend of an initial sharp increase, then slowly increase, and finally stabilized. The maximum displacements of the roof, floor, and two sides after stability are 1043 mm and 470 mm, respectively. The displacement of the roof and floor exceeds 1 m and the deformation is large. The displacement of the roadway roof, floor, and two sides of 200 m in front of the roof cutting section shows a trend of an initial rapid increase, then slowly increase, and finally stabilized. The maximum displacements of the roof, floor, and two sides after stability are 476 mm and 215 mm, respectively, and the deformation is less than 500 mm.

According to the field observation data and field pictures, the effect of deep hole roof cutting is remarkable. The height of the top and bottom of the initial roof cutting section of the roadway is only 1.6-1.8 m, and the height of the two sides is 3.0-3.2 m. The height of the roof and floor of the roadway, 100 m in front of the roof cutting section, is 2.3-2.8 m, and the width of the two sides is 3.4-3.6 m. The deformation of the roadway gradually decreases, and the state of the surrounding rock of the roadway gradually improves, indicating that after the roof cutting, the surrounding rock beyond 100 m in front of the top cutting section achieved the effect of far-field pressure relief and stress blocking. Based on the test results of the roadway, 200 m in front of the roof cutting section, the height of the top and bottom is 2.9-3.0 m, and the height of the two sides is 3.6-3.8 m. No severe deformation and damage occurred. The effect of roof cutting and pressure relief roadway protection is significant, and the roadway maintenance is in good condition, as shown in Figure 13.

In summary, adopting deep hole roof cutting under the optimal parameters can effectively reduce the transfer of mining stress to the surrounding rock of a lateral roof roadway and reduce roadway deformation. After implementing the roof cutting blasting technical measures, there is a certain lag on the pressure relief protection effect of the gas comprehensive treatment roadway; however, with an increase in roof cutting range, the roadway deformation will gradually decrease, and the stability of the surrounding rock will gradually improve.

  • (1)

    The temporal and spatial evolution and stress distribution characteristics of the overburden structure in the stope are as follows: a medium and short arm “F” type suspended roof structure was formed on the right side of the goaf, and the overburden presents “upper three zones” zoning after mining, in which the height of the caving zone was approximately 7 m. The height of the fracture zone was approximately 38 m. The overburden stress presented “horizontal three zones” distribution along the coal seam tendency. The influence range of the stress rise zone was approximately 80 m, and the influence height exceeded 42 m

  • (2)

    The existence of medium and short arm “F” type suspended roof structure led to additional stress in the surrounding rock of the roof roadway. Concurrently, the original concentrated stress of the surrounding rock around the gas comprehensive treatment roadway was superimposed with the lateral mining support pressure of the working face, which further led to the continuous expansion of the plastic area of the surrounding rock, severe deformation of the surrounding rock, and gradual failure and instability

  • (3)

    Through numerical simulation tests, the optimal value of the key parameters of the surrounding rock roof cutting and pressure relief control of the soft rock roof roadway in the deep mine stope was 49.9 m high and 10°. When this parameter was adopted, the stress concentration degree of the surrounding rock of the roof roadway was effectively reduced, and the transmission of the lateral mining support pressure to the roof roadway was blocked

  • (4)

    The results were tested in 17191 (1) working face of Pansan mine of Huainan Mining Group. Based on the application results, roof cutting effectively reduced the transfer of mining stress to the surrounding rock of a lateral roof roadway and reduced roadway deformation. The stability of the roadway surrounding rock improved significantly, the section was maintained in good condition, and its height fully met the requirements of normal ventilation

The experimental test data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that there are no conflicts of interest regarding the publication of this article.

Jucai Chang conceptualized the study, wrote, reviewed, and edited the manuscript, and was responsible for funding acquisition. Tenggen Xiong was responsible for the conceptualization and methodology and wrote the original draft. Chao Qi carried out formal analysis. Chuanming Li was responsible for the resources. Dongdong Pang was responsible for visualization.

This work was supported by the National Natural Science Foundation of China (nos. 52174105 and 52174103), Key Research and Development Projects in Anhui Province (no. 202004a07020045), Anhui Provincial Natural Science Foundation (no. 2008085ME147), and Collaborative Innovation Project of Colleges and Universities in Anhui Province (no. GXXT-2019-029).

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