Drilling layout method and gas extraction method for controllable blasting of high-gassy coal seam in tunnel

Abstract

This invention discloses a borehole layout method and a gas extraction method for controlled blasting in high-gas coal seams within tunnels. Boreholes are drilled parallel to the tunnel excavation direction within the tunnel outline. At least three concentric rings with increasing radii are distributed outwards from the central borehole: an inner ring, a middle ring, and an outer ring. Boreholes are spaced apart along each ring. The boreholes on the inner and outer rings are hydraulic perforations, while the boreholes on the middle ring and the central borehole are blasting holes. Hydraulic perforations are used for hydraulic flushing, followed by gas extraction. Blasting is performed using the blasting holes to achieve regional loosening and permeability enhancement of the coal seam within the tunnel outline. After blasting, networked gas extraction is performed on all boreholes, achieving controlled blasting-enhanced gas extraction in high-gas coal seam areas within tunnels. This invention effectively avoids damage to the surrounding rock of the tunnel during blasting while achieving enhanced permeability, realizing enhanced gas extraction and shortening the construction period for coal seam exposure and crossing.

Description

Technical Field

[0001] This invention relates to the field of tunnel gas control technology, and more specifically to a borehole layout method and a gas extraction method for controlled blasting of high-gas coal seams in tunnels. Background Technology

[0002] Due to the complex mountainous terrain, the construction of transportation infrastructure in some areas inevitably requires traversing mountains, and some large-section tunnels even need to pass through high-gas, fragmented, and soft coal seams. Fragmented and soft coal seams have a broken, loose physical structure, are easily deformed, and have poor permeability. Conventional permeability enhancement technologies struggle to maintain the fracture network formed within these seams in the long term, resulting in poor permeability enhancement and gas extraction effects. Furthermore, due to the large tunnel cross-section, the disturbance to the coal seam during construction is extensive and intense. Compared to coal seam gas extraction technologies, tunnel gas control is more likely to induce dynamic disasters in gas-bearing coal and rock, leading to the engineering challenges of "difficult coal seam exposure and slow tunneling" in tunnel construction.

[0003] In practical engineering, for some tunnels with a determined excavation direction, the excavation process may involve crossing underlying high-gas coal seams, meaning the tunnel face is located within the roof strata of the coal seam before crossing it. In such cases, if gas drainage or outburst suppression is to be carried out, a downward borehole perpendicular to the coal seam is required. However, during borehole construction in soft, fractured coal seams, the water used for drainage or outburst suppression is difficult to drain quickly, and the softened coal upon contact with water can easily cause accidents such as stuck drill bits, borehole collapse, and blockage. Simultaneously, due to factors such as the weight of the drill bit causing it to sag, the drilling trajectory of the downward borehole is difficult to control precisely, resulting in borehole deviation from the target area, leading to low gas drainage efficiency, poor results, and high risks.

[0004] Blasting permeability enhancement technology often draws on conventional fracturing design concepts in underground coal mines. Although it is less constrained by geological conditions, it does not fully consider the special engineering conditions of large tunnel cross-sections, high requirements for surrounding rock stability, and long retention time. As a result, the energy generated by blasting is not only used to fracturing the coal seam, but also randomly transmitted to the surrounding rock outside the tunnel outline, causing uncontrolled development of surrounding rock fissures and even inducing deformation problems such as arch subsidence. This significantly increases the support difficulty during tunnel excavation and the subsequent operation and maintenance costs. Summary of the Invention

[0005] To overcome the shortcomings of the existing technology, this invention provides a borehole layout method and a gas extraction method for controlled blasting of high-gas coal seams in tunnels. Utilizing various types of boreholes drilled along the excavation direction, hydraulic flushing and blasting permeability enhancement operations are carried out sequentially. By leveraging the guiding effect of the blasting free surface formed by the hydraulic flushing operation within the tunnel outline, blasting permeability enhancement is achieved while avoiding damage to the surrounding rock of the tunnel, thereby shortening the construction period for coal seam exposure and crossing.

[0006] To achieve its objectives, the present invention employs the following technical solution: The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to this invention is characterized by a borehole layout within the tunnel outline, parallel to the tunnel excavation direction. This layout is centered on a central borehole, with multiple rings of increasing radius distributed outwards. There are at least three rings, arranged sequentially from the central borehole: an inner ring, a middle ring, and an outer ring. Boreholes are spaced apart along each ring. The boreholes on the inner and outer rings are hydraulic perforations, while the boreholes on the middle ring and the central borehole are blasting holes. Hydraulic perforations are used for hydraulic flushing, followed by gas extraction. Blasting is performed using the blasting holes to achieve regional loosening and permeability enhancement of the coal seam within the tunnel outline. After blasting, networked gas extraction is conducted on all boreholes, achieving controlled blasting-enhanced gas extraction in high-gas coal seam areas within the tunnel.

[0007] The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to the present invention is characterized in that: according to the size of the tunnel outline, the inner ring line is set as two ring lines that are spaced apart and adjacent to each other, namely a first inner ring line with a radius of 3.0-4.0m located in the outer ring line and a second inner ring line with a radius of 2.5-3.0m located in the inner ring line, and the distance between the first inner ring line and the second inner ring line is 0.5-1.0m.

[0008] The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to the present invention is characterized in that: the outer ring is based on the tunnel outline, offset inward by 0.5m, and the hydraulic holes arranged on the outer ring form the outer ring of hydraulic holes; the blasting holes arranged on the middle ring form the middle ring of blasting holes; the hydraulic holes arranged on the first inner ring form the first inner ring of hydraulic holes; the hydraulic holes arranged on the second inner ring form the second inner ring of hydraulic holes; and the central borehole is used as the central blasting hole.

[0009] The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to the present invention is characterized by the following: the final positions of each borehole are set as follows: the final position of the outer ring hydraulic boreholes is 0.2-0.5m into the coal seam floor; the final position of the middle ring blasting boreholes is 2-3m into the coal seam floor; the final position of the first inner ring hydraulic boreholes is 1-1.5m into the coal seam floor; the final position of the second inner ring hydraulic boreholes is 1-1.5m into the coal seam floor; and the final position of the central blasting borehole is 2-3m into the coal seam floor.

[0010] The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to the present invention is characterized by the following: the boreholes in two adjacent rings are radially staggered, and the borehole spacing of the outer ring hydraulic boreholes is 4-5m; the number of the first inner ring hydraulic boreholes is 10-12; the number of the second inner ring hydraulic boreholes is 6-8; and the initial borehole diameter of all boreholes is set to 94mm to facilitate construction.

[0011] The gas extraction method implemented according to the borehole layout method for controlled blasting of high-gas coal seams in tunnels of the present invention is carried out according to the following steps: Step 1: Using a 0.5m offset from the tunnel outline as the outer ring, arrange inter-layer hydraulic holes at intervals along the line, with the final hole position being 0.2~0.5m into the coal seam floor, forming the outer ring of inter-layer hydraulic holes; use hydraulic flushing to flush all the outer ring of inter-layer hydraulic holes. After the flushing operation is completed, connect all the outer ring of inter-layer hydraulic holes to the network for gas extraction. Step 2: Drill a central blasting hole at the center of the tunnel face, with the final hole location being 2-3m into the bottom of the coal seam; Step 3: Construct the first inner ring hydraulic borehole and the second inner ring hydraulic borehole around the central blast hole. The final borehole position is 1-1.5m into the bottom of the coal seam. Use hydraulic flushing to flush the first inner ring hydraulic borehole and the second inner ring hydraulic borehole. After flushing, connect all the first inner ring hydraulic boreholes and the second inner ring hydraulic boreholes to the network for gas extraction. Step 4: Drill holes in the middle ring are laid at intervals along the middle ring line, with the final hole position being 2-3m into the bottom of the coal seam; Step 5: Fill the central blasting hole and the middle ring blasting hole with explosive charges, and detonate all the explosive charges in groups at the same time. Step 6: Connect all hydraulic boreholes and blasting boreholes across the layer for gas extraction, and achieve controlled blasting-enhanced gas extraction.

[0012] The gas extraction method of this invention is also characterized in that: the drilling operation of each cross-layer hydraulic hole is carried out sequentially from the top of the arch to the bottom arch, the hydraulic drilling pressure is 5~15MPa, and the drilling water volume is 200~500L / min; the charging position of the central blasting hole and the middle ring blasting hole is starting from the 1 / 2 position of the coal seam thickness and extending forward to the rock stratum section 2~3 m into the bottom of the coal seam.

[0013] Compared with existing technologies, the beneficial effects of this invention are reflected in: 1. This invention proposes a drilling layout mode parallel to the tunnel excavation direction, which not only effectively solves the problems of easy hole collapse and difficult slag and drainage in traditional downward drilling, but also shortens the construction length of a single hole. This reduces the risk of the hole deviating from the target area, ensures that the extraction range covers the target coal seam, and reduces the path of gas escaping to the working face after the hole exposes the coal seam, thus achieving a certain degree of unity between drilling efficiency and safety benefits. 2. This invention proposes a "hydraulic perforation and zoned blasting" method for enhanced gas extraction. First, hydraulic perforation is used to construct a blasting permeability-enhancing free surface in the target coal seam. Then, by rationally designing the drilling depth and explosive placement, the efficient distribution and utilization of blasting energy is achieved, allowing the blasting energy to fully act on the coal seam and form a fracture network with good connectivity and wide coverage, thereby achieving regional permeability enhancement of the coal seam and effectively addressing the limitations of gas extraction under a single permeability enhancement method. 3. This invention forms a protection system of "outer layer protection and inner layer energy control". By arranging hydraulic holes and blasting holes at intervals, the energy guiding role of the hydraulic holes is fully utilized, and the energy generated by the explosion is concentrated on the coal body within the tunnel outline. This promotes the connection between the blasting fracture and the free surface of the hydraulic hole, and achieves dual optimization of enhanced extraction and surrounding rock protection. It solves the problem of "difficult coal exposure and slow tunnel crossing" in traditional extraction methods. Attached Figure Description

[0014] Figure 1 This is a front view schematic diagram of the drilling layout in this invention.

[0015] Figure 2 This is a side view of the drilling layout in this invention.

[0016] Figure 3 This is a schematic diagram showing the installation position of the explosive charge in this invention.

[0017] Figure 4 This is a schematic diagram of the energy release during blasting at the central blast hole in this invention.

[0018] Figure 5 This is a schematic diagram of the energy release from the blasting of the central blast hole in this invention.

[0019] Figure 6 This is a schematic front view of the explosion fracture of the present invention.

[0020] Figure 7 This is a side view of the explosion fracture of the present invention.

[0021] The diagram is labeled as follows: 1. Tunnel outline; 2. Outer ring hydraulic boreholes; 3. Middle ring blasting boreholes; 4. First inner ring hydraulic boreholes; 5. Second inner ring hydraulic boreholes; 6. Central blasting borehole; 7. Underlying high-gas coal seam; 8. Coal seam roof strata; 9. Blasting charge; A is the blasting fracture; B is the tunnel excavation direction. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings.

[0023] See Figure 2 This embodiment is for large-section tunnels with a defined excavation direction. Figure 2 The diagram shows the tunnel excavation direction B. During the excavation process, the tunnel passes through the underlying high-gas coal seam 7. Before passing through the underlying high-gas coal seam 7, the tunnel face is always located in the roof stratum 8 of the coal seam.

[0024] In this embodiment, the borehole layout method for controlled blasting of high-gas coal seams in tunnels involves laying out boreholes parallel to the tunnel excavation direction within the tunnel outline. This layout uses a central borehole as the core, with multiple rings of increasing radius distributed outwards. There are at least three rings: an inner ring, a middle ring, and an outer ring, arranged sequentially from the central borehole. Boreholes are spaced apart along each ring. Specifically, the boreholes on the inner and outer rings are hydraulic perforations, while the boreholes on the middle ring and the central borehole are blasting holes. Hydraulic perforations are used for hydraulic flushing, followed by gas extraction. Blasting holes are used for blasting, using blasting energy to achieve regional loosening and permeability enhancement of the coal seam within the tunnel outline. After blasting, networked gas extraction is performed on all boreholes, achieving controlled blasting-enhanced gas extraction in the high-gas coal seam area of ​​the tunnel. The specific implementation involves setting up two adjacent inner rings, spaced apart, based on the size of the tunnel outline. The first inner ring is located on the outer track with a radius of 3.0-4.0m, and the second inner ring is located on the inner track with a radius of 2.5-3.0m. The distance between the first inner ring and the second inner ring is 0.5-1.0m.

[0025] See Figure 1 and Figure 2 In this embodiment, the outer ring is based on the tunnel outline 1, offset by 0.5m. The hydraulic holes that penetrate the layers on the outer ring form the outer ring hydraulic holes 2; the blasting holes that penetrate the layers on the middle ring form the middle ring blasting holes 3; the hydraulic holes that penetrate the layers on the first inner ring form the first inner ring hydraulic holes 4; the hydraulic holes that penetrate the layers on the second inner ring form the second inner ring hydraulic holes 5, and the central borehole is used as the central blasting hole 6.

[0026] Figure 2The final positions of each borehole are shown as follows: the outer ring hydraulic borehole 2 ends at a depth of 0.2-0.5m into the coal seam floor; the middle ring blasting borehole 3 ends at a depth of 2-3m into the coal seam floor; the first inner ring hydraulic borehole 4 ends at a depth of 1-1.5m into the coal seam floor; the second inner ring hydraulic borehole 5 ends at a depth of 1-1.5m into the coal seam floor; and the central blasting borehole 6 ends at a depth of 2-3m into the coal seam floor.

[0027] Figure 1 The diagram shows the radially staggered distribution of boreholes in two adjacent rings, with the borehole spacing of the outer ring hydraulic boreholes 2 being 4-5m; the number of boreholes of the first inner ring hydraulic boreholes 4 being 10-12; and the number of boreholes of the second inner ring hydraulic boreholes 5 being 6-8; the initial borehole diameter of all boreholes is set to 94mm to facilitate construction.

[0028] Gas extraction, implemented according to the borehole layout method for controlled blasting of high-gas coal seams in this embodiment, is carried out in the following steps: Step 1: Using the tunnel outline 1 offset by 0.5m as the outer ring, arrange hydraulic boreholes at intervals along the line, with the final borehole position being 0.2~0.5m into the coal seam floor, forming the outer ring of hydraulic boreholes 2. Use hydraulic perforation to perforate all the outer ring of hydraulic boreholes 2. After the perforation is completed, connect all the outer ring of hydraulic boreholes to the network for gas extraction, extracting gas from the coal seam in the area where the tunnel outline is located in advance, creating a stable construction environment for subsequent drilling, blasting and other operations. When the gas concentration at the borehole opening drops significantly and there is no obvious fluctuation, the subsequent operations can begin.

[0029] Step 2: Drill a central blasting hole 6 at the center of the tunnel face. The final hole position is 2-3m into the coal seam floor to better fracture the coal seam floor strata, form a connected fracture network, and provide more good migration channels for gas.

[0030] Step 3: Construct the first inner ring hydraulic borehole 4 and the second inner ring hydraulic borehole 5 around the central blast hole 6. The final borehole positions are both 1-1.5m into the coal seam floor. Use hydraulic perforation to perforate the first inner ring hydraulic borehole 4 and the second inner ring hydraulic borehole 5. After perforation, connect all the first inner ring hydraulic borehole 4 and the second inner ring hydraulic borehole 5 to the network for gas extraction. On the basis of constructing a pressure relief extraction system and expanding the range of tunnel gas pre-extraction, absorb the initial energy released by the central blast hole 6, induce the energy released by the middle ring blast hole 3 to diffuse centripetally, and further fracture the coal body within the tunnel outline. When the gas concentration at the borehole opening drops significantly and there is no obvious fluctuation, start the subsequent operation.

[0031] Step 4: Drill holes 3 in the middle ring are arranged at intervals along the middle ring line. The final hole position is 2-3m into the bottom of the coal seam, so as to better fracture the bottom rock strata of the coal seam, form a connected fracture network, and provide more good migration channels for gas.

[0032] Step 5: Fill the central blasting hole 6 and the middle ring blasting hole 3 with explosive charges, and detonate all the explosive charges in a group at the same time to give full play to the superposition effect of blasting energy, form a continuous and stable gas extraction channel, and avoid problems such as uneven fracture development or abnormal gas outburst caused by single hole or delayed detonation.

[0033] Step 6: Connect all hydraulic boreholes and blasting boreholes across the layer for gas extraction, and achieve controlled blasting-enhanced gas extraction.

[0034] During construction, the drilling of each hydraulic borehole was carried out sequentially from the arch crown to the bottom arch, with a hydraulic drilling pressure of 5-15 MPa and a drilling water flow rate of 200-500 L / min. [Further details are needed for accurate translation.] Figure 2 and Figure 3 As shown: The charging positions of the central blasting hole 6 and the middle ring blasting hole 3 are starting from the position of 1 / 2 of the coal seam thickness and extending forward to the rock strata section 2-3 m into the bottom of the coal seam, forming a blasting column 9.

[0035] Figure 4 and Figure 5 The diagram illustrates the direction of blasting energy release in this invention. When the central blast hole 6 is detonated, the adjacent first inner ring hydraulic holes 4 and second inner ring hydraulic holes 5 form an energy barrier, absorbing the initial energy released by the central blast hole 6. Furthermore, due to the staggered arrangement of the adjacent inner ring hydraulic holes, energy is difficult to leak from the gaps between adjacent boreholes. When the middle ring blast hole 3 is detonated, the two inner ring hydraulic holes surrounding the central blast hole 6 more easily guide the energy released by the middle ring blast hole 3 to diffuse centripetally, and superimpose with the central blasting energy to further break up the coal seam. The outer ring hydraulic holes 2 act as an external barrier, effectively absorbing the remaining energy from the middle ring blast hole 3 and the central blast hole 6, thus achieving energy diversion into the coal seam.

[0036] Figure 6 This diagram shows a frontal view of the blasting fracture A formed after the blasting is completed, with a9 as the reference point. # a10 # a11 # a12 # a18 # a19 # a28 # a29 # a30 # and a31 # Hydraulic perforations across layers, and a13 # a41# a42 # and a43 # Taking the blasting holes as an example; for the central blasting hole 6, when it is detonated, most of the cracks extend to the adjacent inner ring hydraulic hole area due to the energy guidance of the through-layer hydraulic hole, forming a radial crack network; for the middle ring blasting hole 3, when it is detonated, its centripetal propagation energy is superimposed with the energy of the central blasting hole, which further expands the radial crack network, while the outward propagation energy is more likely to connect the cracks generated after the drilling operation of the outer ring hydraulic hole 2.

[0037] Figure 7 This diagram shows a side view of the blasting fracture A formed after the blasting is completed, with a1 as the reference numeral in the diagram. # a11 # a14 # a18 # a20 # and a30 # Hydraulic perforations across layers, and a13 # a32 # and a42 # Taking blasting holes as an example. The core objective of the outer ring hydraulic borehole 2 is to achieve pre-decompression of the coal seam. For the underlying high-gas coal seam 7, a shorter borehole length in the arch direction into the bottom plate helps to avoid excessive damage to the surrounding rock along the outer contour line of this area, while a shorter borehole length in the bottom arch direction into the bottom plate helps to induce fractures to develop towards the relatively upper coal seam. Therefore, the final position of the outer ring hydraulic borehole 2 is set at 0.2~0.5m below the coal seam bottom plate. The core objective of the first inner ring hydraulic borehole 4 and the second inner ring hydraulic borehole 5 is to construct a sufficient blasting permeability enhancement free surface within the coal seam within the tunnel contour line 1, while the relatively longer... Boreholes drilled in the bottom plate are better able to guide energy, concentrating the energy generated by blasting onto the coal seam within the outline. Therefore, the final position of the inner ring hydraulic boreholes is set at 1-1.5m on the bottom plate of the coal seam. The core objective of the middle ring blasting hole 3 and the central blasting hole 6 is to effectively break the coal seam and the adjacent bottom rock strata through the energy generated by the explosive explosion, forming a blasting funnel. This promotes the expansion of fractures into the upper coal seam and bottom rock strata, constructing a through-type fracture network from the coal seam to the bottom plate, and significantly increasing the gas seepage channels. Therefore, the final position of the blasting hole in this method is the longest, at 2-3m on the bottom plate of the coal seam.

[0038] The foregoing has shown and described the basic principles, main features, and beneficial effects of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the operational details described above. The sequence of fracturing, the specific arrangement of boreholes, the borehole arrangement parameters, and the fracturing construction parameters can all be adjusted according to actual geological and engineering conditions, and these changes all fall within the scope of protection of the claims of this invention.

Claims

1. A borehole layout method for controlled blasting of high-gas coal seams in tunnels, characterized in that... Within the tunnel outline, boreholes are laid out parallel to the tunnel excavation direction, with a central borehole as the core and multiple rings of increasing radius distributed outwards. There are at least three such rings, numbered from the central borehole outwards as: an inner ring, a middle ring, and an outer ring, with boreholes spaced apart along each ring. The boreholes on the inner and outer rings are hydraulic perforations, while the boreholes on the middle ring and the central borehole are blasting holes. Hydraulic perforations are used for hydraulic flushing, followed by gas extraction. Blasting holes are used for blasting, using blasting energy to achieve regional loosening and permeability enhancement of the coal seam within the tunnel outline. After blasting, networked gas extraction is implemented for all boreholes, achieving controlled blasting-enhanced gas extraction in high-gas coal seam areas within the tunnel.

2. The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to claim 1, characterized in that: Based on the size of the tunnel outline, the inner ring road is set as two ring roads that are spaced apart and adjacent to each other. The first inner ring road is located in the outer lane and has a radius of 3.0-4.0m, and the second inner ring road is located in the inner lane and has a radius of 2.5-3.0m. The distance between the first inner ring road and the second inner ring road is 0.5-1.0m.

3. The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to claim 2, characterized in that: The outer ring is based on the tunnel outline, offset by 0.5m, and the hydraulic holes that penetrate the layers on the outer ring form the outer ring hydraulic holes (2). The blasting holes arranged on the middle ring line form the middle ring blasting holes (3); The hydraulic holes that are laid on the first inner ring form the first inner ring hydraulic holes (4). The hydraulic holes that are laid on the second inner ring form the second inner ring hydraulic holes (5). The central borehole was used as the central blasting hole (6).

4. The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to claim 3, characterized in that: The final hole positions for each borehole are set as follows: The final position of the outer ring through-layer hydraulic hole (2) is 0.2~0.5m into the bottom of the coal seam; The final position of the middle ring blast hole (3) is 2-3m into the bottom of the coal seam; The final position of the first inner ring through-layer hydraulic hole (4) is 1~1.5m into the bottom of the coal seam; The final position of the second inner ring through-layer hydraulic hole (5) is 1~1.5m into the bottom of the coal seam; The final position of the central blasting hole (6) is 2-3m into the bottom of the coal seam.

5. The borehole layout method for controlled blasting of high-gas coal seams in tunnels according to claim 3, characterized in that: The boreholes in two adjacent rings are radially staggered, and: The drilling spacing of the outer ring through-layer hydraulic holes (2) is 4-5m; The number of holes in the first inner ring through-layer hydraulic holes (4) is 10-12; The number of holes in the second inner ring through-layer hydraulic holes (5) is 6-8; The initial borehole diameter for all holes was set to 94 mm to facilitate construction.

6. The gas extraction method implemented according to the borehole layout method for controlled blasting of high-gas coal seams in tunnels as described in claim 2, characterized in that: Follow these steps: Step 1: Using the tunnel outline line offset by 0.5m as the outer ring line, arrange the cross-layer hydraulic holes at intervals along the line, and the final hole position is 0.2~0.5m into the bottom plate of the coal seam to form the outer ring cross-layer hydraulic holes (2); use hydraulic punching to punch all the outer ring cross-layer hydraulic holes. After the punching operation is completed, connect all the outer ring cross-layer hydraulic holes to the network for gas extraction. Step 2: Drill a central blasting hole (6) at the center of the tunnel face, with the final hole position being 2-3m into the bottom of the coal seam; Step 3: Construct the first inner ring hydraulic borehole (4) and the second inner ring hydraulic borehole (5) around the central blasting hole (6). The final hole position is 1-1.5m into the bottom of the coal seam. Use hydraulic flushing to flush the first inner ring hydraulic borehole (4) and the second inner ring hydraulic borehole (5). After flushing, connect all the first inner ring hydraulic borehole (4) and the second inner ring hydraulic borehole (5) to the network for gas extraction. Step 4: Drill holes (3) are laid out at intervals along the middle ring line in the middle ring, with the final hole position being 2-3m into the bottom of the coal seam; Step 5: Fill the central blasting hole (6) and the middle ring blasting hole (3) with explosive charges, and detonate all the explosive charges in groups at the same time; Step 6: Connect all hydraulic boreholes and blasting boreholes across the layer for gas extraction, and achieve controlled blasting-enhanced gas extraction.

7. The gas extraction method according to claim 6, characterized in that: The drilling operations of each hydraulic hole are carried out sequentially from the top of the arch to the bottom of the arch. The hydraulic drilling pressure is 5~15MPa and the drilling water volume is 200~500L / min. The charging positions of the central blasting hole (6) and the middle ring blasting hole (3) are from the position of 1 / 2 of the coal seam thickness, extending forward to the rock stratum section 2~3 m into the bottom of the coal seam.

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