A method for controlling secondary mine pressure of a close-range overlying concentrated coal pillar

Abstract

The application discloses a kind of near distance overlying concentrated coal column secondary mine pressure control method, belong to coal mining technical field, including multiple position pressure relief and reinforcement support, multiple position pressure relief includes, determining optimal mining position, coal pillar borehole pressure relief, high roof directional drilling hydraulic fracturing pressure relief, middle-low roof carbon dioxide presplitting blasting, working face front advanced pre-pressure relief;Reinforcement support includes dividing area, using drilling rig drilling and cleaning rock powder, push into constant resistance energy-absorbing grouting anchor cable body, sectional grouting, anchor cable tensioning after standing, build auxiliary support.The application adopts the above-mentioned near distance overlying concentrated coal column secondary mine pressure control method, by using multiple position pressure relief technology, three levels are collaboratively realized secondary mine pressure all-around prevention and control, on the basis of multiple position pressure relief, grouting constant resistance energy-absorbing anchor cable is used to strengthen roadway stability, constructs anchor injection integrated support system, to form pressure relief support system, realizes the long-term stability of roadway.

Description

Technical Field

[0001] This invention relates to the field of coal mining technology, and in particular to a method for controlling secondary mine pressure caused by concentrated overlying coal pillars at close range. Background Technology

[0002] Coal, as my country's core energy source, has long held a dominant position in the energy system. Currently, despite the rapid development of clean energy, it is unlikely to completely replace coal's role as a pillar energy source in the short term. Furthermore, as single coal seam resources with favorable geological conditions become increasingly depleted, near-shore coal seam mining has become one of the key ways to ensure the continuity of coal production capacity.

[0003] Closely spaced coal seams are combinations of coal seams with small vertical spacing between adjacent coal seams. Under these geological conditions, mining one coal seam can significantly affect the mining of adjacent coal seams. Mining the overlying coal seam creates a complex stress environment, which may trigger a series of problems such as severe secondary mine pressure manifestation and inter-layer rock instability, seriously hindering the improvement of mine safety production levels.

[0004] To address secondary mine pressures generated by overlying coal seams, the industry has developed various control methods. These include reducing the stress impact of overlying coal pillars through staggered mining or adjusting the working face position; rationally designing the distribution of upper coal pillars to disperse stress; drilling stress relief holes in high-stress areas to release concentrated stress; using high-strength support materials to enhance roadway stability; and utilizing microseismic monitoring and other technologies to monitor stress changes in real time and take timely measures. Although these methods are effective to some extent, under conditions of close proximity to thick coal seams or ultra-high mining heights, the increased goaf space and intensified stress concentration in the remaining coal pillars make roadway surrounding rock stability control more difficult, and existing technologies cannot fully meet the requirements.

[0005] Therefore, there is an urgent need to develop more efficient control technologies to further optimize stress distribution and surrounding rock stability in order to meet the requirements of safe mining under complex geological conditions. Summary of the Invention

[0006] The purpose of this invention is to provide a method for controlling secondary mine pressure caused by concentrated coal pillars in close proximity. This method employs multi-position decompression technology, using a group of large-diameter boreholes to decompress the remaining coal pillars and release concentrated stress. Layered roof control is implemented, with high-level hydraulic fracturing creating fracture zones and mid-to-low-level carbon dioxide pre-fracture breaking key layers to block longitudinal stress transmission. Energy is released through pre-decompression boreholes constructed in the working face's advanced zone. This three-level synergy achieves comprehensive control of secondary mine pressure. Furthermore, based on multi-position decompression, grouting constant-resistance energy-absorbing anchor cables are used to enhance roadway stability, constructing an integrated anchoring and grouting support system. This effectively constrains the expansion deformation of the rock mass after decompression, preventing further fracture development. Together with the decompression measures, this forms a decompression support system, achieving long-term roadway stability.

[0007] To achieve the above objectives, the present invention provides a method for controlling secondary mine pressure caused by concentrated overlying coal pillars in close proximity, including multi-position pressure relief and reinforcement support, specifically comprising the following steps:

[0008] S1. Analyze the stress transfer characteristics of the coal pillars left in the overlying coal seam, and determine the optimal mining location of the roadway by combining three-dimensional numerical simulation with field measurement.

[0009] S2. Implement large-diameter borehole decompression for the coal pillars remaining in the overlying coal seam;

[0010] S3. Decompression is achieved by using ground-directed drilling hydraulic fracturing technology for the key layers of the high-level roof slab.

[0011] S4. Use carbon dioxide pre-splitting blasting technology to destroy the integrity of the middle and lower roof slabs.

[0012] S5. Implement advance pre-decompression in front of the working face advance direction, and arrange pre-decompression boreholes along the two sides and roof of the roadway.

[0013] S6. Based on the characteristics of the safe zone where the roadway is located, combined with the projection range of the overlying coal pillar and the distribution of residual stress after pressure relief, the support area is divided into the core bearing area, the transition adjustment area, and the stable area.

[0014] S7. Use a drilling rig to drill holes. After completion, use high-pressure air to blow out the rock powder in the hole and push the constant resistance energy absorption grouting anchor cable into the hole.

[0015] S8. Perform segmented progressive grouting into the borehole, and after grouting is completed, allow it to stand still and then tension it.

[0016] S9. Construct auxiliary support, carry out shallow reinforcement of the roof, roadway side reinforcement, and floor heave control.

[0017] Preferably, in S1, when determining the optimal mining location of the roadway, it is necessary to first drill to obtain the geometric parameters of the overlying coal pillar and the physical and mechanical parameters of the coal seam, establish a stress transfer model of "coal pillar-lower coal seam", use software to simulate the stress distribution law under the coal pillar, calculate the stress concentration coefficient, clarify the boundary range of the stress peak area and the attenuation area, divide the high stress danger area, the second highest stress area, and the safe area, and combine the lower coal seam mining design to arrange the roadway in the safe area and keep a distance from the edge of the overlying coal pillar to ensure that the roadway is initially in a low stress environment.

[0018] Preferably, in S2, the boreholes are arranged in a quincunx pattern. During construction, a hydraulic drilling rig is used to drill at an upward angle along the direction of the coal pillar to avoid uneven stress release caused by borehole deviation. After drilling is completed, high-pressure airflow is used to blow away coal dust in the hole to ensure that the hole is unobstructed. Through the stress release effect of the borehole group, the stress concentration coefficient inside the coal pillar is reduced, weakening its strength as a stress source.

[0019] Preferably, in S3, the depressurization of the key layer of the high-level top plate requires the construction of a directional main hole using rotary steerable drilling technology. The drilling trajectory extends along the direction of the key layer of the high-level top plate, and fracturing holes are constructed in the main hole. During the fracturing operation, a segmented hydraulic fracturing process is adopted, and the formation of a continuous fracture network is ensured by monitoring the micro-vibration signals during the fracturing process.

[0020] Preferably, in S4, when using the carbon dioxide pre-splitting blasting technology, pre-splitting holes need to be constructed in the roof along the roadway direction first, so that the pre-splitting holes are arranged in a fan shape. The carbon dioxide fracturing device is arranged in a single-hole double-pipe manner, filled with liquid carbon dioxide, and detonated using millisecond delay detonators. The detonation sequence is that the lower roof is detonated first, followed by the middle roof, to ensure that the fractures expand step by step. After the blasting, sonic detection is used for verification to block the longitudinal stress transmission path.

[0021] Preferably, in S5, the boreholes are arranged in a three-flower pattern and constructed using a pneumatic drilling rig. After drilling, PVC pipes are immediately inserted to protect the boreholes and prevent collapse. The construction progress is advanced by maintaining a safe distance according to the working face advance speed. The stress concentration in front of the working face is released in advance through the boreholes, so that the peak value of the advance support pressure is transferred to the depth. During the pressure relief implementation in S2-S5, stress, displacement and crack monitoring systems are set up. Stress sensors are used to monitor the stress changes of coal pillars and roof, inclinometers are used to monitor roadway deformation, and borehole inspection instruments are used to observe the development of roof cracks.

[0022] Preferably, in S6, the core bearing area is the roadway roof and two sides below the orthographic projection of the overlying coal pillar. This area is significantly affected by the residual stress of the coal pillar, and the thickness of the roof fracture zone is relatively thick, requiring enhanced grouting and anchor cable anchoring strength. The transition adjustment area is outside the core bearing area, where stress disturbance is relatively weak and the degree of roof fracture is moderate, and simplified parameter collaborative support is adopted. The stable area is the area outside the transition adjustment area, where conventional support can meet the requirements.

[0023] Preferably, in S7, the boreholes are arranged in a dense quincunx pattern in the core bearing area and in a tri-flower pattern in the transition adjustment area. The constant resistance energy-absorbing grouting anchor cable adopts an integrated structure of hollow rod body, constant resistance device and grouting channel. The rod body is made of hollow high-strength low-relaxation steel strand. A grouting channel is set in the center of the rod body. The constant resistance device is equipped with ZYX-II type adaptive constant resistance device, with built-in disc spring group and friction plate structure. An annular grouting fluid drain hole is set at the end of the rod body. The outer sleeve is a biodegradable PVC protective sleeve.

[0024] Preferably, in S8, the grouting material is a dual-liquid high-water-speed-setting grouting material. Grouting is performed through the grouting channel on the constant-resistance energy-absorbing grouting anchor cable. First, low pressure is injected into the bottom 1 / 3 of the hole to fill the main fractures. Then, medium pressure is injected into the middle section to penetrate the secondary fractures. Finally, high pressure is injected into the hole opening section to strengthen the integrity of the hole opening anchoring section. When the grout return concentration at the hole opening is consistent with the original grout, the process is stopped and the valve is closed to stabilize the pressure. After the grouting is allowed to stand, staged tensioning is performed. The adaptive characteristics of the constant-resistance device are used to ensure that the anchor cable maintains a constant working resistance when the surrounding rock deforms, thus avoiding support failure.

[0025] Preferably, in S9, the specific operation of shallow roof reinforcement is to arrange fiberglass anchors between constant resistance energy-absorbing grouting anchors and use resin cartridges for anchoring, forming a deep and shallow coordinated anchoring system with the anchors; the specific operation of roadway side reinforcement is to use a combination of threaded steel anchors and W-steel strips for support on both sides to constrain the expansion of the roadway side coal body; the specific operation of floor heave control is to inject high-water-rate quick-setting material into the grouting fluid drain holes in the roadway floor to solidify the loose coal body of the floor and prevent the floor heave from aggravating after pressure relief.

[0026] Therefore, this invention adopts the above-mentioned method for controlling secondary mine pressure from concentrated coal pillars in close proximity. By employing multi-position decompression technology, large-diameter borehole groups are used to decompress the remaining coal pillars, releasing concentrated stress; the roof is controlled in layers, with high-level hydraulic fracturing to create fracture zones and mid-to-low-level carbon dioxide pre-fracture to break key layers, blocking the longitudinal transmission of stress; energy is released through pre-decompression boreholes constructed in the advance zone of the working face. This three-level coordination achieves comprehensive control of secondary mine pressure. At the same time, based on multi-position decompression, grouting constant resistance energy-absorbing anchor cables are used to enhance the stability of the roadway, constructing an integrated anchoring and grouting support system. This effectively constrains the expansion deformation of the rock mass after decompression, prevents further development of fractures, and forms a decompression support system with the decompression measures, achieving long-term stability of the roadway.

[0027] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0028] Figure 1 This is a flowchart of an embodiment of the method for controlling secondary mine pressure from concentrated coal pillars in close proximity according to the present invention;

[0029] Figure 2 This is a cross-sectional view of a multi-position pressure relief working face in an embodiment of a method for controlling secondary mine pressure caused by a concentrated coal pillar at close range according to the present invention;

[0030] Figure 3 This is a schematic diagram of a carbon dioxide fracturing device in an embodiment of a method for controlling secondary mine pressure from a concentrated coal pillar at close range according to the present invention;

[0031] Figure 4 This is a diagram illustrating the effect of carbon dioxide blasting in an embodiment of a method for controlling secondary mine pressure from a concentrated coal pillar at close range according to the present invention.

[0032] Figure 5 This is a schematic diagram of a constant resistance energy-absorbing grouting anchor cable in an embodiment of a method for controlling secondary mine pressure from concentrated coal pillars over close proximity according to the present invention;

[0033] Figure 6 This is a schematic diagram of the internal structure of the grouting fluid drain hole in an embodiment of a method for controlling secondary mine pressure from a concentrated coal pillar at close range according to the present invention;

[0034] Figure 7 This is a schematic diagram of the reinforcement support in an embodiment of the method for controlling secondary mine pressure from a concentrated coal pillar at close range according to the present invention.

[0035] Figure Labels

[0036] 1. Coal seam mining; 2. Low-level roof; 3. Mid-level roof; 4. Key layers of high-level roof; 5. High-pressure pipe fracturing fluid injection; 6. Large-diameter borehole; 7. Coal pillar installation; 8. Collapse zone; 9. Directional fracture; 10. Fracturing tools; 11. Hydraulic fracturing borehole; 12. Blasting hole; 13. Filling valve; 14. Heating device; 15. Main pipe; 16. Sealing gasket; 17. Constant pressure shear plate; 18. Release pipe; 19. Surrounding rock; 20. Shaped charge tube; 21. Carbon dioxide propellant tube; 22. Detonation wave; 23. Two-way shaped charge device; 24. 25. Rock strata; 26. Constant resistance anchor cable; 27. Grouting pressure device; 28. Grouting fluid drain hole; 29. ​​Biodegradable external sleeve; 30. Anchor cable deformation sensor; 31. Fixing nut; 32. Anchor cable tray; 33. Pressure sensor; 34. Friction pad; 35. Grouting drill bit protective shell; 36. PVC biodegradable protective sleeve; 37. Hard roof; 38. Grouting borehole; 39. Constant resistance energy-absorbing grouting anchor cable; 40. Soft lower roof; 41. Faulty roof in roadway; 42. Threaded steel anchor bolt; 43. Bottom plate grouting fluid drain hole; 44. Anchorage. Detailed Implementation

[0037] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0038] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0039] Example 1

[0040] like Figure 1 As shown, the present invention provides a method for controlling secondary mine pressure caused by concentrated overlying coal pillars in close proximity, including multi-position pressure relief and reinforcement support.

[0041] Cross-sectional view of the multi-position pressure relief working face as shown in the figure Figure 2 As shown, there is a coal pillar 7 left in the overlying coal seam collapse zone 8. A large-diameter borehole 6 is drilled in the coal pillar 7 to relieve the pressure on the coal pillar 7. Below the collapse zone 8 is the high-level roof key layer 4. There is a hydraulic fracturing borehole 11 in the high-level roof key layer 4. High-pressure fracturing fluid 5 is injected into the hydraulic fracturing borehole 11. The fracturing tool 10 in the hydraulic fracturing borehole 11 is used to create directional fractures 9, thereby relieving the pressure on the high-level roof key layer 4.

[0042] Below the key layer 4 of the high-level roof is the middle-level roof 3, below the middle-level roof 3 is the low-level roof 2, and below the low-level roof 2 is the mining coal seam 1. The blasting hole 12 penetrates the middle-level roof 3 and the low-level roof 2, and the integrity of the middle-level roof 3 and the low-level roof 2 is destroyed by carbon dioxide pre-splitting blasting technology using the blasting hole 12.

[0043] The structure of a carbon dioxide fracturing unit is as follows: Figure 3 As shown, it consists of a shaped charge tube 20 and a carbon dioxide propellant tube 21, and the carbon dioxide fracturing device is installed in the blast hole 12 inside the surrounding rock 19.

[0044] The carbon dioxide dosing tube 21 consists of a filling valve 13, a heating device 14, a main pipe 15, a sealing gasket 16, a constant pressure shear plate 17, and a release tube 18. The head of the tube is the filling valve 13, the rear of the filling valve 13 is connected to the heating device 14, the rear end of the heating device 14 is connected to the main pipe 15, the rear of the main pipe 15 is connected to the constant pressure shear plate 17, and the rear of the constant pressure shear plate 17 is connected to the release tube 18. Sealing gaskets 16 are provided at the connection between the main pipe 15 and the constant pressure shear plate 17, and at the connection between the constant pressure shear plate 17 and the release tube 18.

[0045] Carbon dioxide explosion effect diagram as shown Figure 4 As shown, the carbon dioxide fracturing device in the blast hole 12 is detonated, and the detonation wave 22 impacts the rock layer 24 directionally along the bidirectional energy-concentrating device 23 in the carbon dioxide fracturing device, causing it to generate directional fractures 9.

[0046] Schematic diagram of reinforcement support as follows Figure 7 As shown, the uppermost layer is a hard roof 36, below which is a soft lower roof 39, below which is a faulted roof 40 on the roadway, below which is the mining coal seam 1, threaded steel anchor bolts 41 are installed on both sides of the mining coal seam 1, a bottom grouting fluid drain hole 42 is installed below the mining coal seam 1, a constant resistance energy-absorbing grouting anchor cable 38 is installed above the mining coal seam 1, and a grouting borehole 37 is located at the upper end of the constant resistance energy-absorbing grouting anchor cable 38.

[0047] The structure of constant resistance energy absorption grouting anchor cable 38 is as follows: Figure 5 As shown, the system includes a constant resistance anchor cable 25, with a grouting pressure device 26 connected to the rear of the constant resistance anchor cable 25. An anchor cable deformation sensor 29, a fixing nut 30, an anchor cable tray 31, and a pressure sensor 32 are arranged between the grouting pressure device 26 and the constant resistance anchor cable 25. The anchor cable deformation sensor 29 is surrounded by the bottom of the constant resistance anchor cable 25, the fixing nut 30 is located below the anchor cable deformation sensor 29, the anchor cable tray 31 is located below the fixing nut 30, and the pressure sensor 32 is located above the anchor cable tray 31.

[0048] A biodegradable outer sleeve 28 is connected to the rear of the grouting pressure device 26. The biodegradable outer sleeve 28 is equipped with multiple grout drain holes 27, such as... Figure 6 As shown, an anchor 43 is installed inside the grout drain hole 27. The friction plate 33 is connected to the constant resistance anchor cable 25, and the front end of the friction plate 33 is connected to the grouting drill bit protective shell 34. The front end of the grouting drill bit protective shell 34 is provided with a PVC biodegradable protective sleeve 35.

[0049] The method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars, as described in this embodiment, specifically includes the following steps:

[0050] S1. Analyze the stress transmission characteristics of the coal pillars remaining in the overlying coal seam, and use a combination of three-dimensional numerical simulation and field measurement to determine the optimal mining location of the roadway. When determining the optimal mining location of the roadway, it is necessary to first drill to obtain the geometric parameters of the overlying coal pillar and the physical and mechanical parameters of the coal seam, establish a stress "coal pillar-lower coal seam" transmission model, and use software to simulate the stress distribution law under the coal pillar.

[0051] Calculate the stress concentration factor to clarify the boundary between the stress peak area and the attenuation area. In this embodiment, a stress concentration factor ≥2.0 is defined as a high stress danger zone, 1.5-2.0 as a secondary high stress zone, and <1.5 as a safe zone. In conjunction with the design of the lower coal seam mining, the roadway is arranged in the safe zone with a stress concentration factor <1.5, and the horizontal distance between the roadway and the edge of the overlying coal pillar is not less than 1.2 times the width of the coal pillar, to ensure that the roadway is initially in a low stress environment.

[0052] S2. Large-diameter boreholes are used to relieve pressure on the remaining coal pillar in the overlying coal seam. The boreholes are arranged in a staggered pattern, with a spacing of 1.5-2.0m between holes and rows. The borehole diameter is 150-200mm, and the depth penetrates the entire height of the coal pillar and extends 0.5m into the floor. During construction, a hydraulic drilling rig is used to drill along the coal pillar at a 3°-5° elevation angle to avoid uneven stress release caused by borehole deviation. After drilling, high-pressure airflow is used to blow away coal debris from the holes to ensure unobstructed flow. Through the stress release effect of the borehole group, the stress concentration factor inside the coal pillar is reduced to below 1.3, weakening its strength as a stress source.

[0053] S3. For the critical layer of the high-level roof, surface directional drilling hydraulic fracturing technology is used for depressurization. A 216mm diameter directional main borehole is constructed using rotary steerable drilling technology, with the borehole trajectory extending along the strike of the critical layer of the high-level roof. The horizontal section length is no less than 300m. Fracturing holes with a diameter of 152mm are constructed at 50m intervals within the main borehole, with the angle between the branch holes and the main borehole being 30°-45°. This ensures that the fracturing range covers 20m beyond the projection area of ​​the overlying coal pillar. During fracturing operations, a segmented hydraulic fracturing process is adopted, with a single-segment fracturing fluid volume of 80- Construction pressure is controlled at 30-40MPa, and displacement is 8- By monitoring microseismic signals during the fracturing process, a network of interconnected fractures can be formed, thereby reducing the integrity of critical layers in the upper roof.

[0054] S4. Use carbon dioxide pre-splitting blasting technology to destroy the integrity of the middle and lower roof. Construct pre-splitting holes with a diameter of 75mm in the roof along the roadway direction. The hole depth is 15-20m in the middle roof and 8-12m in the lower roof. The hole spacing is 3-5m, arranged in a fan shape, with the angles to the horizontal plane being 45° and 25° respectively. Arrange the Φ38mm carbon dioxide fracturing device in a single-hole double-pipe manner. The filling pressure is 20-25MPa, and the filling amount is determined according to the hole depth, with 0.8-1.2kg of liquid carbon dioxide per meter of hole depth.

[0055] Millisecond delay detonators were used for initiation, with the lower roof section detonated first, followed by the middle roof section, with a delay time of 50-100 ms to ensure the gradual expansion of fractures. Post-blasting acoustic detection verified that the roof rock mass integrity coefficient had decreased to below 0.5, effectively blocking the longitudinal stress transmission path.

[0056] S5. Implement pre-decompression ahead of the working face advance direction. Arrange pre-decompression boreholes along both sides and the roof of the roadway. The borehole diameter for the sidewalls is 90mm, the depth is 12-15m, the angle with the sidewall is 30°, and the spacing is 2-3m. The borehole diameter for the roof is 110mm, the depth is 15-18m, the angle with the roof normal is 15°, and the arrangement is in a three-flower pattern. Use a pneumatic drilling rig for construction. Immediately after drilling, insert PVC pipes to protect the boreholes to prevent collapse. Maintain a safe distance of 30-40m for the advance construction progress according to the working face advance speed. By releasing the stress concentration in front of the working face in advance through drilling, the peak value of the advance support pressure is transferred to the depth ≥10m.

[0057] During each decompression step, stress, displacement, and crack monitoring systems are deployed. Stress sensors are used to monitor stress changes in the coal pillar and roof, inclinometers are used to monitor roadway deformation, and borehole inspection instruments are used to observe the development of roof cracks. Through real-time dynamic adjustments, it is ensured that secondary mine pressure remains within a controllable range.

[0058] S6. Based on the roadway layout and stress concentration coefficient, combined with the projection range of the overlying coal pillar and the distribution of residual stress after decompression, the support area is divided into the core bearing area, the transition adjustment area, and the stable area.

[0059] The core bearing area is the roadway roof and two sides within 20m below the orthographic projection of the overlying coal pillar. This area is significantly affected by the residual stress of the coal pillar, and the thickness of the roof fracture zone is relatively thick, requiring enhanced grouting and anchor cable anchoring strength. The transition adjustment area is the area 10-20m outside the core area, where stress disturbance is relatively weak and the degree of roof fracture is moderate, and simplified parameter collaborative support is adopted. The stable area is the area outside the transition area, where only conventional support is required to meet the requirements.

[0060] S7. Drill holes using a drilling rig with a diameter of Φ65mm, which is 10mm larger than traditional anchor cable drilling to meet grouting requirements. The hole depth is two meters thicker than the loose zone of the roof to ensure that the anchoring section penetrates more than 1.5m into the stable rock layer. The core area adopts a dense quincunx pattern with a hole spacing of 1.0m×1.0m and an upward angle of 15° with the roof normal to adapt to the tilting characteristics of the nearby coal seam roof. The transition area adopts a tri-flower pattern with a hole spacing of 1.5m×1.5m and an angle adjusted to 10°. After drilling is completed, blow the hole with high-pressure air for 3 minutes to remove rock powder. Then, slowly push the anchor cable with constant resistance device into the hole to ensure that the grouting channel is unobstructed and the exposed length is controlled at 0.3-0.5m.

[0061] The constant resistance energy-absorbing grouting anchor cable adopts an integrated structure of hollow rod body, constant resistance device, and grouting channel. The rod body is made of Φ21.8mm hollow high-strength low-relaxation steel strand, and an Φ8mm grouting channel is set in the center of the rod body, which can penetrate into the fractured cracks of the roof through high-pressure grout. The constant resistance device is equipped with a ZYX-II type adaptive constant resistance device, with a built-in disc spring group and friction plate structure. When the deformation of the surrounding rock exceeds the threshold, dynamic unloading is achieved through friction energy dissipation to avoid the anchor cable breaking due to excessive instantaneous stress. The end of the rod body is equipped with annular grouting fluid discharge holes with a diameter of 5mm and a spacing of 100mm. The outer sleeve is a biodegradable PVC protective sleeve. During grouting, the grout breaks through the protective sleeve and enters the fractured rock mass. After completion, it automatically degrades without affecting the mechanical properties of the anchor cable.

[0062] S8. Grouting is carried out into the borehole through the hollow channel. When the grout concentration at the borehole opening is consistent with the original grout, the grouting is stopped. The valve is closed and the pressure is stabilized for 5 minutes. The grouting material is a two-component high-water-rate quick-setting grouting material. Component A is water-cement ratio of 1:1.2 + 3% ultrafine cement, and component B is water-cement ratio of 1:1.0 + 2% quick-setting agent. After mixing them at a volume ratio of A:B=1:1, the gel time is controlled at 20-40s to meet the requirements of rapid sealing of the broken roof. The 28-day compressive strength is ≥10MPa.

[0063] The grouting process adopts a segmented progressive approach. First, low pressure of 3-5 MPa is injected into the bottom 1 / 3 of the hole to fill the main fractures. Then, medium pressure of 8-10 MPa is injected into the middle section to penetrate the secondary fractures. Finally, high pressure of 12-15 MPa is injected into the hole opening section to strengthen the integrity of the hole opening anchoring section. The grouting volume of each section is calculated according to "fracture volume × 0.8 porosity" to ensure that the grout diffusion radius is ≥1.5m.

[0064] Twenty-four hours after grouting, staged tensioning is adopted: 0→30% of the design tension → 60%→100%, with each stage held for 5 minutes. Finally, the tension is over-tensioned to 110%, stabilized for 3 minutes, and then unloaded to the design value. The adaptive characteristics of the constant resistance device are used to ensure that the anchor cable maintains a constant working resistance when the surrounding rock deforms, thus avoiding support failure.

[0065] S9. Construct auxiliary support, carry out shallow reinforcement of the roof, roadway side reinforcement, and floor heave control.

[0066] Shallow reinforcement of the top slab: Φ22mm fiberglass anchor rods with a length of 3.5m are arranged between constant resistance anchor cables, with a spacing of 0.8m×0.8m. Resin cartridges are used for anchoring, forming a "deep and shallow coordination" anchoring system with the anchor cables.

[0067] Roadway side reinforcement: Φ18mm threaded steel anchor bolts, 2.5m in length and W steel strips, 5mm thick, are used for the two sides to support and constrain the expansion of the coal seam in the roadway side;

[0068] Bottom heave control: Φ50mm grout drain holes are constructed in the roadway floor, with a hole depth of 2.0m and a spacing of 2.0m×2.0m. High-water-content and fast-setting material is injected to solidify the loose coal body in the floor and prevent the bottom heave from worsening after pressure relief.

[0069] The data described above are based on the specific circumstances of this embodiment. The specific parameters such as borehole diameter, spacing, depth, grouting pressure, and angle need to be set according to the actual geological conditions.

[0070] Therefore, this invention adopts the above-mentioned method for controlling secondary mine pressure from concentrated coal pillars in close proximity. By employing multi-position decompression technology, large-diameter borehole groups are used to decompress the remaining coal pillars, releasing concentrated stress; the roof is controlled in layers, with high-level hydraulic fracturing to create fracture zones and mid-to-low-level carbon dioxide pre-fracture to break key layers, blocking the longitudinal transmission of stress; energy is released through pre-decompression boreholes constructed in the advance zone of the working face. This three-level coordination achieves comprehensive control of secondary mine pressure. At the same time, based on multi-position decompression, grouting constant resistance energy-absorbing anchor cables are used to enhance the stability of the roadway, constructing an integrated anchoring and grouting support system. This effectively constrains the expansion deformation of the rock mass after decompression, prevents further development of fractures, and forms a decompression support system with the decompression measures, achieving long-term stability of the roadway.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for controlling secondary mine pressure caused by concentrated overlying coal pillars at close range, characterized in that: This includes multiple pressure relief and reinforcement supports, specifically comprising the following steps: S1. Analyze the stress transfer characteristics of the coal pillars left in the overlying coal seam, and determine the optimal mining location of the roadway by combining three-dimensional numerical simulation with field measurement. S2. Implement large-diameter borehole decompression for the coal pillars remaining in the overlying coal seam; S3. Decompression is achieved by using ground-directed drilling hydraulic fracturing technology for the key layers of the high-level roof slab. S4. Use carbon dioxide pre-splitting blasting technology to destroy the integrity of the middle and lower roof slabs. S5. Implement advance pre-decompression in front of the working face advance direction, and arrange pre-decompression boreholes along the two sides and roof of the roadway. S6. Based on the characteristics of the safe zone where the roadway is located, combined with the projection range of the overlying coal pillar and the distribution of residual stress after pressure relief, the support area is divided into the core bearing area, the transition adjustment area, and the stable area. S7. Use a drilling rig to drill holes. After completion, use high-pressure air to blow out the rock powder in the hole and push the constant resistance energy absorption grouting anchor cable into the hole. S8. Perform segmented progressive grouting into the borehole, and after grouting is completed, allow it to stand still and then tension it. S9. Construct auxiliary support, carry out shallow reinforcement of the roof, roadway side reinforcement, and floor heave control.

2. The method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 1, characterized in that: In S1, when determining the optimal mining location of the roadway, it is necessary to first drill to obtain the geometric parameters of the overlying coal pillar and the physical and mechanical parameters of the coal seam, establish a stress transfer model of "coal pillar-lower coal seam", use software to simulate the stress distribution law under the coal pillar, calculate the stress concentration coefficient, clarify the boundary range of the stress peak area and the attenuation area, divide the high stress danger zone, the second highest stress zone, and the safe zone, and combine the lower coal seam mining design to arrange the roadway in the safe zone and keep a distance from the edge of the overlying coal pillar to ensure that the roadway is initially in a low stress environment.

3. The method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 2, characterized in that: In S2, the boreholes are arranged in a quincunx pattern. During construction, a hydraulic drilling rig is used to drill at an upward angle along the direction of the coal pillar to avoid uneven stress release caused by borehole deviation. After drilling is completed, high-pressure airflow is used to blow away coal dust in the hole to ensure that the borehole is unobstructed. Through the stress release effect of the borehole group, the stress concentration coefficient inside the coal pillar is reduced, weakening its strength as a stress source.

4. The method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 3, characterized in that: In S3, to relieve pressure on the key layer of the high-level roof, a directional main hole must first be constructed using rotary steerable drilling technology. The drilling trajectory extends along the direction of the key layer of the high-level roof. Fracturing holes are constructed in the main hole. During the fracturing operation, a segmented hydraulic fracturing process is adopted. By monitoring the micro-vibration signals during the fracturing process, it is ensured that a continuous fracture network is formed.

5. The method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 4, characterized in that: In S4, when using carbon dioxide pre-splitting blasting technology, pre-splitting holes need to be constructed in the roof along the roadway direction to make the pre-splitting holes arranged in a fan shape. The carbon dioxide fracturing device is arranged in a single-hole double-tube manner, filled with liquid carbon dioxide, and detonated using millisecond delay detonators. The detonation sequence is that the lower roof is detonated first, followed by the middle roof, to ensure that the fractures expand step by step. After the blasting, sonic detection is used for verification to block the longitudinal transmission path of stress.

6. The method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 5, characterized in that: In S5, the boreholes are arranged in a three-flower pattern and are constructed using pneumatic drilling rigs. After drilling, PVC pipes are immediately inserted to protect the boreholes and prevent collapse. The construction progress is advanced by maintaining a safe distance according to the working face advance speed. The stress concentration in front of the working face is released in advance through drilling, so that the peak of the advance support pressure is transferred to the depth. During the decompression implementation of S2-S5, stress, displacement and crack monitoring systems are set up. Stress sensors are used to monitor the stress changes of coal pillars and roof, inclinometers are used to monitor roadway deformation, and borehole inspection instruments are used to observe the development of roof cracks.

7. The method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 6, characterized in that: In S6, the core bearing area is the roadway roof and two sides below the orthographic projection of the overlying coal pillar. This area is significantly affected by the residual stress of the coal pillar, and the thickness of the roof fracture zone is relatively thick, requiring enhanced grouting and anchor cable anchoring strength. The transition adjustment zone is outside the core bearing area, where stress disturbance is relatively weak and the degree of roof fracture is moderate, and simplified parameter collaborative support is adopted. The stable zone is the area outside the transition adjustment zone, where conventional support can meet the requirements.

8. A method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 7, characterized in that: In S7, the boreholes are arranged in a dense quincunx pattern in the core bearing area and in a tri-flower pattern in the transition adjustment area. The constant resistance energy-absorbing grouting anchor cable adopts an integrated structure of hollow rod body, constant resistance device and grouting channel. The rod body is made of hollow high-strength low-relaxation steel strand. A grouting channel is set in the center of the rod body. The constant resistance device is equipped with ZYX-II type adaptive constant resistance device, with built-in disc spring group and friction plate structure. An annular grouting fluid drain hole is set at the end of the rod body. The outer sleeve is a biodegradable PVC protective sleeve.

9. A method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 8, characterized in that: In S8, a dual-liquid high-water-speed-setting grouting material is selected. Grouting is carried out through the grouting channel on the constant-resistance energy-absorbing grouting anchor cable. First, low pressure is injected into the bottom 1 / 3 of the hole to fill the main fractures. Then, medium pressure is injected into the middle section to penetrate the secondary fractures. Finally, high pressure is injected into the hole opening section to strengthen the integrity of the hole opening anchoring section. When the grout return concentration at the hole opening is consistent with the original grout, the process is stopped and the valve is closed to stabilize the pressure. After the grouting is allowed to stand, staged tensioning is carried out. The adaptive characteristics of the constant-resistance device are used to ensure that the anchor cable maintains a constant working resistance when the surrounding rock deforms, thus avoiding support failure.

10. A method for controlling secondary mine pressure from closely spaced overlying concentrated coal pillars according to claim 9, characterized in that: In S9, the specific operation for shallow roof reinforcement is to arrange fiberglass anchors between constant resistance energy-absorbing grouting anchors and use resin cartridges for anchoring, forming a deep and shallow coordinated anchoring system with the anchors; the specific operation for roadway side reinforcement is to use a combination of threaded steel anchors and W-steel strips for support on both sides to constrain the expansion of the roadway side coal body; the specific operation for floor heave control is to inject high-water-rate quick-setting material into the drainage holes of the roadway floor grouting fluid to solidify the loose coal body of the floor and prevent the floor heave from worsening after pressure relief.

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