A method for roof cutting and pressure relief at the end of a coal mining face

Abstract

The present invention relates to the technical field of mine safety devices, and specifically relates to a method for roof cutting and pressure relief at the end of a coal mining face, mainly including: Step A: Core sampling to determine the roof layer thickness according to the core structure; Step B: Drilling grouting holes, drilling a plurality of grouting holes upward from the lower part of the roof near the coal mining face; Step C: Drilling water injection holes and nitrogen charging holes, drilling a plurality of water injection holes and a plurality of nitrogen charging holes upward on the roof on the side of the grouting holes far from the coal mining face; Step D: Grouting, filling the slurry into the rock layer gaps above the coal mining face; Step E: Water injection, injecting water into the rock layer gaps between the roof and the rock layer above the roof; Step F: Liquid nitrogen charging, injecting liquid nitrogen into the rock layer gaps between the roof and the rock layer above the roof. The drilling density in this solution is small, the number of drill holes is small, the construction time is short, the construction is safe, and no pre-support is required during the drilling operation. The fracturing process in this solution is slow and stable, and will not cause gas explosion accidents, with wide applicability.

Description

Technical Field

[0001] The present invention relates to the technical field of mine safety devices, and particularly relates to a method for roof cutting and pressure relief at the end of a coal mining face. Background Art

[0002] In underground coal seam mining, strata control is the core of safe and efficient coal seam mining. Especially when encountering hard roof occurrences, due to the high strength of the hard roof and its difficulty in breaking, large-area hanging roofs of the hard roof cause continuous stress concentration in the working face stope, serious deformation and damage of roadways, etc. After the coal mining face is mined, it is very difficult for the hard roof to collapse in time, and it is easy to form a cantilever structure. After the main roof fractures, it will cause the cantilever structure to rotate and sink, leading to an increase in the stress of the surrounding rock of the adjacent roadway, serious deformation of the roadway near the working face, and dynamic disasters such as violent vibration of the rock mass in the working face, rock burst, and coal and gas outburst.

[0003] In view of the problems of serious surrounding rock deformation and great difficulty in roof control in the gob-side entry driving mentioned above, relying on traditional bolt and cable bolting to strengthen the support of the roadway cannot effectively control the surrounding rock deformation. Only by cutting off the cantilever roof in the gob area and releasing the elastic potential energy accumulated in the roof can the stress concentration continuously generated by the roof on the surrounding rock of the gob-side roadway be fundamentally eliminated, thereby improving the stability of the surrounding rock of the gob-side roadway. The commonly used technical methods at present include blasting roof cutting and pressure relief technology, hydraulic fracturing weakening roof cutting and pressure relief technology, and composite perforation roof cutting and pressure relief technology. However, the blasting roof cutting and pressure relief technology uses explosive blasting, and explosive blasting is only applicable to low-gas mines. High-gas mines require strict usage technical requirements, and roof cutting blasting may cause the roof in the roadway to fall in advance, resulting in difficult roof management. The conventional hydraulic fracturing weakening roof cutting and pressure relief technology weakens the roof structure by creating and expanding cracks inside the roof with high-pressure water bodies, so that the roof collapses under its own weight to achieve the mystery of roof cutting and pressure relief. However, this method requires the equipment to have extremely high pressure output performance to be able to press out cracks in the roof, and has problems such as high requirements for equipment, complex technology, and poor adaptability to the natural conditions of the roadway. There is also a scheme in the prior art that uses the method of dense drilling to cut the roof and relieve pressure. For example, a roof cutting and pressure relief method based on dense drilling disclosed in the prior art with the publication number of CN110966002 B. This scheme destroys the roof structure by densely drilling holes in the roof forming a cantilever, and the roof automatically collapses under the influence of its own weight and mining. However, this method requires too many drill holes, too long drilling time, cannot predict the roof collapse time, and due to the large drilling density, there may be a danger of sudden roof collapse during the drilling process. Based on this, researching a method for roof cutting and pressure relief at the end of a coal mining face is an urgent problem to be solved. Summary of the Invention

[0004] Aiming at the deficiencies of the above-mentioned prior art, the technical problem to be solved by the present invention is: to provide a method for roof cutting and pressure relief at the end of a coal mining face to solve the problems existing in the above-mentioned prior art.

[0005] To solve the above technical problems, a basic solution adopted by the present invention is a method for roof cutting and pressure relief at the end of a coal mining face, including the following steps:

[0006] Step A: Core sampling. Drill cores upward at multiple points on the roof where drilling is to be carried out, and determine the roof layer thickness according to the core structure to determine the drilling depth.

[0007] Step B: Drill grouting holes. Drill multiple grouting holes upward from the bottom of the roof near the coal mining face. The multiple grouting holes are arranged in sequence along the direction parallel to the coal mining face, and the drilling depth of the grouting holes exceeds the roof thickness.

[0008] Step C: Drill water injection holes and nitrogen charging holes. Drill multiple water injection holes and multiple nitrogen charging holes upward in sequence and alternately along the direction parallel to the coal mining face on the roof on the side of the grouting holes away from the coal mining face. The drilling depth exceeds the roof thickness.

[0009] Step D: Grouting. Use a grouting device to fill the slurry into the rock layer gaps above the coal mining face through the grouting holes; Step E: Water injection. Inject water into the rock layer gaps between the roof and the rock layer above the roof through the water injection holes by a water injection device to enrich the water body in the rock layer gaps between the roof and the rock layer above the roof.

[0010] Step F: Charge liquid nitrogen. Charge liquid nitrogen into the rock layer gaps between the roof and the rock layer above the roof through the nitrogen charging holes by a nitrogen charging device.

[0011] In the above solution, water is injected into the rock layer gaps above the gob-side entry, and then liquid nitrogen is introduced into the rock layer gaps. The vaporization of liquid nitrogen causes the water body in the rock layer gaps to freeze and expand rapidly, squeezing the roof downward so that the roof breaks at the weakening zone formed by the water injection holes and nitrogen charging holes, achieving the purpose of roof cutting and pressure relief. In this solution, compared with forming a roof weakening zone through dense drilling, the water injection holes and nitrogen charging holes in this solution also form a weakening zone on the roof, but the drilling density of this solution is small. Before the water body in the rock layer gaps freezes and expands due to nitrogen charging, the roof will not break, and the construction safety is high. No pre-support is required during drilling operations; in this solution, first grout and fill the rock layer gaps between the roof and the rock layer above the coal mining face, and then inject water into the rock layer gaps above the gob-side entry, which can effectively prevent the water body from entering the rock layer gaps above the coal mining face, forcing the injected water body to gather in the rock layer gaps above the gob-side entry, effectively preventing excessive pressure on the coal pillar when the water body freezes and expands due to nitrogen charging, and reducing the risk of collapse at the edge of the coal pillar.

[0012] Further, the upper end of the grouting hole is inclined 5-15° towards the direction close to the coal mining face.

[0013] Further, the water injection device includes a water injection pipe and a water pump. The water injection pipe is a rigid pipe with one end open. On the pipe wall of the water injection pipe near the closed end, there is a first water outlet that connects the inner cavity of the water injection pipe with the external space of the water injection pipe. The water injection pipe is connected to the water pump through a first pipe. One end of the first pipe is hermetically connected to the open end of the water injection pipe, and the other end of the first pipe is connected to the water pump. The water pump is connected to an external water source. On the outer wall of the water injection pipe near the closed end, there is a first plugging member to prevent the water body from flowing back through the water injection hole.

[0014] Further, the first plugging member includes a first annular airbag sleeved on the outer wall of the water injection pipe. The inner cavity of the first annular airbag is connected to an external first air pump through a first air pipe.

[0015] Further, the nitrogen filling device includes a nitrogen filling pipe for filling liquid nitrogen into the rock formation cracks, a liquid nitrogen tank for storing liquid nitrogen, and a nitrogen filling pump for pumping the liquid nitrogen in the liquid nitrogen tank into the nitrogen filling pipe. The nitrogen filling pipe is a rigid pipe with one end open. On the pipe wall of the nitrogen filling pipe near the closed end, there is a second water outlet that connects the inner cavity of the nitrogen filling pipe with the external space of the nitrogen filling pipe. The nitrogen filling pipe is connected to the nitrogen filling pump through a second pipe. One end of the second pipe is hermetically connected to the open end of the nitrogen filling pipe, and the other end of the second pipe is connected to the nitrogen filling pump. A second valve body is provided on the second pipe. The nitrogen filling pump is connected to the liquid nitrogen tank. On the outer wall of the nitrogen filling pipe near the closed end, there is a second plugging member to prevent the liquid nitrogen from flowing back through the nitrogen filling hole.

[0016] Further, the second plugging member includes a second annular airbag sleeved on the outer wall of the nitrogen filling pipe. The inner cavity of the second annular airbag is connected to an external second air pump through a second air pipe.

[0017] Further, the nitrogen filling device further includes a pressurizing unit for accelerating the rapid expansion of the liquid nitrogen in the nitrogen filling pipe so that the liquid nitrogen in the nitrogen filling pipe can be quickly filled into the rock formation cracks.

[0018] Further, the pressurizing unit includes an electric heating layer attached to the inner wall of the nitrogen filling pipe. The electric heating layer is electrically connected to an external power source through a power cord.

[0019] Compared with the prior art, the present invention has at least the following beneficial effects:

[0020] 1. Compared with forming a roof weakening zone by dense drilling, the water injection holes and nitrogen filling holes in this solution also form a weakening zone on the roof. However, the drilling density in this solution is small. Before the water in the rock formation cracks freezes and expands without nitrogen filling, the roof will not break, and the construction safety is high. There is no need for pre-support during the drilling operation.

[0021] In this solution, the water body injected is restricted in the rock layer cracks above the roof on the side far from the coal pillar through pre-sealing, avoiding the water body and liquid nitrogen injected from entering the rock layer cracks above the coal pillar, preventing excessive pressure on the coal pillar when the water body freezes and expands during nitrogen filling, and reducing the risk of collapse at the edge of the coal pillar.

[0022] 3. Through the fracturing pressure relief of this solution, the fracturing process is slow and stable compared with blasting pressure relief, and will not cause gas explosion.

[0023] 4. Compared with the existing hydraulic fracturing weakening roof cutting pressure relief technology which requires the equipment to have extremely high pressure output, has high requirements for equipment, complex technology, and poor adaptability to roadway natural conditions, the water injection pressure required for injecting water into the rock layer cracks in this solution is relatively small compared with conventional hydraulic fracturing weakening roof cutting pressure relief. It does not require complex high-pressure water injection equipment, has low requirements for equipment, simple technology, and high site adaptability.

[0024] 5. In this solution, water can be injected into the rock layer cracks and nitrogen can be filled step by step according to the deformation and fracture degree of the roof to gradually fracture the roof, and the fracturing and pressure relief of the roof are highly controllable. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The drawings described herein are used to provide a further understanding of the present application and form a part of the present application. The schematic embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation of the present application. In the drawings:

[0026] Figure 1 is a schematic elevation structure diagram of a roof cutting pressure relief method for the end of a coal mining face according to the present invention Figure 1 。

[0027] Figure 2 is a schematic plan structure diagram of a roof cutting pressure relief method for the end of a coal mining face according to the present invention.

[0028] Figure 3 is a schematic elevation structure diagram of a roof cutting pressure relief method for the end of a coal mining face according to the present invention Figure 2 。

[0029] Figure 4 is a schematic diagram of the water injection pipe structure.

[0030] Figure 5 is a schematic diagram of the nitrogen filling pipe structure.

[0031] Figure 6 is a schematic diagram of the nitrogen filling device structure.

[0032] Figure 7 is a flowchart of a roof cutting pressure relief method for the end of a coal mining face according to the present invention.

[0033] The meanings of the reference numerals in the drawings are as follows:

[0034] Coal mining face - 10; Roof - 11; Goaf - 12; Gob-side roadway - 13; Rock fissure - 14; Coal pillar - 15;

[0035] Water injection hole - 21; Nitrogen filling hole - 22; Grouting hole - 23;

[0036] Water injection pipe - 31; Outlet 1 - 311; Pipe 1 - 32; Annular airbag 1 - 312; Air pipe 1 - 33;

[0037] Nitrogen filling pipe - 41; Outlet 2 - 411; Annular airbag 2 - 412; Liquid nitrogen tank - 42; Nitrogen filling pump - 43; Pipe 2 - 44; Valve body 2 - 441; Air pipe 2 - 45; Electric heating layer - 46; Power cord - 47. Specific implementation mode

[0038] To make the objectives, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be clearly and completely described below in conjunction with specific embodiments of the present application and the corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present application.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments, and are not intended to limit the present invention.

[0040] As Figure 7 shown, a method for roof cutting and pressure relief at the end of a coal mining face in this solution mainly includes the following steps:

[0041] Step A: Core sampling. Drill cores upward at multiple points on the roof 11 where drilling is to be carried out, and determine the thickness of the roof 11 based on the core structure to determine the drilling depth;

[0042] Step B: Drill grouting holes 23. Drill multiple grouting holes 23 upward from the lower part of the roof 11 near the coal mining face 10. The multiple grouting holes 23 are arranged in sequence in a direction parallel to the coal mining face 10, and the drilling depth of the grouting holes 23 exceeds the thickness of the roof 11;

[0043] Step C: Drill water injection holes 21 and nitrogen filling holes 22. Drill multiple water injection holes 21 and multiple nitrogen filling holes 22 upward in sequence and alternately along a direction parallel to the coal mining face 10 on the roof 11 on the side of the grouting holes 23 away from the coal mining face 10, and the drilling depth exceeds the thickness of the roof 11;

[0044] Step D: Grouting. Use grouting equipment to fill the slurry into the rock fissures 14 above the coal mining face 10 through the grouting holes 23.

[0045] Step E: Water injection. Use a water injection device to inject water into the rock fissures 14 between the roof 11 and the rock layer above the roof 11 through the water injection holes 21, so that the water body is enriched in the rock fissures 14 between the roof 11 and the rock layer above the roof 11.

[0046] Step F: Liquid nitrogen filling. Use a nitrogen filling device to fill liquid nitrogen into the rock fissures 14 between the roof 11 and the rock layer above the roof 11 through the nitrogen filling holes 22.

[0047] In step A of this solution, by taking multi-point core samples in the pre-drilling area, and then analyzing and determining the thickness of the roof 11 above the gob-side entry 13 according to the structure of the sampled cores, further determining the specific position of the rock fissures 14 between the roof 11 and the rock layer above the roof 11, and further determining the depth of the upward drilling, so as to accurately inject water and liquid nitrogen into the rock fissures 14. Specifically, conventional core sampling equipment can be used to reasonably select multiple sampling points above the areas of the grouting holes 23, water injection holes 21, and nitrogen filling holes 22 on the roof 11 to drill cores upward. Conventional core sampling tools can be used. After drilling the cores, observe the sampled rock columns. The position where obvious stratification and fracture occur on the rock columns is the position of the rock fissures 14 on the rock columns. The distance from the end of the sampled rock column to the obvious stratification on the rock column is the thickness of the roof 11 at this sampling point. This thickness is also the depth of the rock fissures 14 at this sampling point from the lower surface of the roof 11. After obtaining multiple depth data through multi-point sampling and measurement, the trend of the rock fissures 14 in the drilling area and the distance from the lower surface of the roof 11 at this location can be initially determined. When drilling the grouting holes 23, water injection holes 21, and nitrogen filling holes 22 subsequently, the drilling depth should be appropriately greater than the initially determined depth distance to ensure that grouting, water injection, and nitrogen filling can be effectively carried out into the rock fissures 14 above the grouting holes 23, water injection holes 21, and nitrogen filling holes 22 at this position.

[0048] Combined with Figure 1 、 Figure 2 As shown, during the process of drilling the grouting holes 23 in step B of this solution, the upper end of the grouting hole 23 is inclined 5-15 degrees, preferably 10°, towards the coal mining face 10, so that the filled slurry is located in the rock fissures 14 above the coal pillar 15, preventing the later injected water body from entering the rock fissures 14 above the coal pillar 15 and making the rock fissures 14 above the gob-side entry 13 be filled with the injected water body as much as possible.

[0049] During the drilling process of step C of this solution, a plurality of water injection holes 21 and a plurality of nitrogen filling holes 22 are arranged in an alternating manner in sequence along a direction parallel to the coal mining face 10 at a position of the roof 11 close to the coal mining face 10. The depths of the water injection holes 21 and the nitrogen filling holes 22 both exceed the thickness of the roof 11 upward, so that water and liquid nitrogen can be injected into the rock stratum cracks 14 through a water injection device and a nitrogen filling device.

[0050] In step D of this solution, a conventional grouting pump and a grouting pipe are used to inject the slurry into the rock stratum cracks 14. The slurry can be cement slurry or organic grouting materials such as polyurethane and acrylic acid, etc., and cement slurry is preferred.

[0051] After the slurry to be filled solidifies, water is injected into the rock stratum cracks 14 above the gob-side entry through the water injection holes by the water injection device, such as Figure 4 As shown, the water injection device used in step E includes a water injection pipe 31 and a water pump (not shown). The water injection pipe 31 is a rigid pipe with one end open. A water outlet 311 that communicates the inner cavity of the water injection pipe 31 with the external space of the water injection pipe 31 is provided on the wall of the water injection pipe 31 near the closed end of the water injection pipe 31. The water injection pipe 31 is connected to the water pump through a pipe 32. One end of the pipe 32 is hermetically connected to the open end of the water injection pipe 31, and the other end of the pipe 32 is connected to the water pump. In actual setting, a main water outlet pipe is provided at the water outlet of the water pump, and the pipes 32 on each water injection pipe 31 are hermetically connected to the main water outlet pipe. The water body can be synchronously introduced into each water injection pipe 31 by controlling the on-off of the main water outlet pipe. The water pump is connected to an external water source. A plugging member 312 is provided on the outer wall of the water injection pipe 31 near the closed end of the water injection pipe 31 to prevent the water body from flowing out of the water injection hole 21. The plugging member 312 includes an annular airbag 312 sleeved on the outer wall of the water injection pipe 31. Specifically, the annular airbag 312 is located below the water outlet 311. The annular airbag 312 is sleeved on the outer wall of the water injection pipe 31 and fixedly connected to the water injection pipe 31. The inner cavity of the annular airbag 312 is connected to an external air pump (not shown) through an air pipe 33. The water pump and the air pump are both conventional ordinary devices and will not be elaborated here.

[0052] After water is injected through the water injection holes 21, liquid nitrogen is filled into the rock stratum cracks 14 above the gob-side entry through the nitrogen filling holes 22 by the nitrogen filling device, such as Figure 5 、 Figure 6As shown in the figure, the nitrogen filling device used in step F includes a nitrogen filling pipe 41 for filling liquid nitrogen into the rock formation gap 14, a liquid nitrogen tank 42 for storing liquid nitrogen, a nitrogen filling pump 43 for pumping the liquid nitrogen in the liquid nitrogen tank 42 into the nitrogen filling pipe 41, a second plugging member, and a pressurizing unit. The nitrogen filling pipe 41 is a rigid pipe with one end open. On the pipe wall of the nitrogen filling pipe 41 near the closed end, there is a second water outlet 411 that connects the inner cavity of the nitrogen filling pipe 41 with the external space of the nitrogen filling pipe 41. The nitrogen filling pipe 41 is connected to the nitrogen filling pump 43 through a second pipe 44. One end of the second pipe 44 is hermetically connected to the open end of the nitrogen filling pipe 41, and the other end of the second pipe 44 is connected to the nitrogen filling pump 43. A second valve body 441 is provided on the second pipe 44, and the second valve body 441 can independently control the on-off of the corresponding second pipe 44. In actual setting, a second main water outlet is provided at the water outlet of the nitrogen filling pump 43, and the second pipes 44 on each nitrogen filling pipe 41 are hermetically connected to the second main water outlet. The on-off of the second main water outlet can be controlled to simultaneously introduce liquid nitrogen into each nitrogen filling pipe 41. The nitrogen filling pump 43 is connected to the liquid nitrogen tank 42 through a pipe. The second plugging member is provided on the outer wall of the nitrogen filling pipe 41 near the closed end of the nitrogen filling pipe 41 to prevent the liquid nitrogen from flowing out of the nitrogen filling hole 22 outside the nitrogen filling hole 22. Specifically, the second plugging member includes a second annular airbag 412 sleeved on the outer wall of the nitrogen filling pipe 41. The second annular airbag 412 is located below the second water outlet 411 and is fixedly connected to the outer wall of the nitrogen filling pipe 41. The inner cavity of the second annular airbag 412 is connected to an external air pump two (not shown) through an air pipe two 45. The pressurizing unit is used to accelerate the rapid expansion of the liquid nitrogen in the nitrogen filling pipe 41 so that the liquid nitrogen in the nitrogen filling pipe 41 can be quickly filled into the rock formation gap 14. The pressurizing unit includes an electric heating layer 46 attached to the inner wall of the nitrogen filling pipe 41. The electric heating layer 46 can be set as an electric heating sheet attached to the inner wall of the nitrogen pipe 41. The electric heating layer 46 is electrically connected to an external power supply through a power cord 47. The air pump two and the nitrogen filling pump 43 are both conventional ordinary devices and will not be elaborated here.

[0053] In the above solution, after determining the drilling position and depth through core sampling, the grouting hole 23 is drilled upward, and the water injection hole 21 and the nitrogen charging hole 22 are drilled in sequence. After grouting the rock formation gap 14 above the coal pillar 15 through the grouting hole 23, the water injection pipe 31 and the nitrogen charging pipe 41 are respectively inserted into the corresponding water injection hole 21 and nitrogen charging hole 22, so that the water outlet one 311 on the water injection pipe 31 and the water outlet two 411 on the nitrogen charging pipe 41 are both located at the rock formation gap 14. The annular airbag one 312 is inflated through the air pump one to make the annular airbag one 312 expand and press against the inner wall of the water injection hole 21, preventing the water body above the annular airbag one 312 from flowing downward out of the water injection hole 21. The annular airbag two 412 is inflated through the air pump two to make the annular airbag two 412 expand and press against the inner wall of the nitrogen charging hole 22, preventing the liquid nitrogen above the annular airbag two 412 from flowing downward out of the nitrogen charging hole 22.

[0054] First, the water body is injected into the rock formation gap 14 through the main water outlet pipe one and then through each water injection pipe 31. After closing the main water outlet pipe one, the nitrogen charging pump 43 is operated to inject liquid nitrogen into each nitrogen charging pipe 41 respectively. After the inner cavity of the nitrogen charging pipe 41 is filled with liquid nitrogen, the main water outlet pipe two is closed, and the electric heating layer 46 is connected to the external power supply. The electric heating layer 46 heats the liquid nitrogen in the nitrogen charging pipe 41. The liquid nitrogen close to the electric heating layer 46 quickly vaporizes and the volume rapidly expands to generate high pressure, squeezing the remaining liquid nitrogen into the rock formation gap 14 where it vaporizes and absorbs heat. As Figure 3 shown, the water body in the rock formation gap 14 is frozen and the volume expands after being vaporized by the liquid nitrogen absorbing heat, squeezing the roof 11 downward, causing the roof 11 to break at the weakening zone formed by the water injection hole 21 and the nitrogen charging hole 22. In this solution, water can be injected into the rock formation gap 14 and high-pressure nitrogen can be charged multiple times until the roof 11 is squeezed and broken.

[0055] The above are only the embodiments of the present invention. Common general knowledge such as the specific structures and characteristics in the solution are not described in detail here. It should be noted that for those skilled in the art, without departing from the structure of the present invention, several deformations and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to interpret the content of the claims.

Claims

1. A method for relieving pressure by cutting off the top of a coal mining face, characterized in that: The following steps are involved: Step A: core sampling, drill cores upward from multiple points on the roof where holes are to be drilled, and determine the thickness of the roof layer based on the core structure to determine the drilling depth; Step B: drilling grouting holes, drilling multiple grouting holes from the bottom of the roof upward near the coal mining working face, the multiple grouting holes are arranged in sequence in a direction parallel to the coal mining working face, the drilling depth of the grouting holes exceeds the thickness of the roof, and the upper ends of the grouting holes are inclined 5 to 15 degrees toward the direction close to the coal mining working face; Step C: drilling water injection holes and nitrogen filling holes, and drilling a plurality of water injection holes and a plurality of nitrogen filling holes in an alternating arrangement upward in a direction parallel to the coal mining working face on the roof plate on the side of the grouting hole away from the coal mining working face, and the drilling depth exceeds the thickness of the roof plate; Step D: Grouting, using grouting equipment to fill slurry into the rock stratum cracks above the coal mining face through grouting holes to prevent water from entering the rock stratum cracks above the coal mining face, forcing the injected water to gather in the rock stratum cracks above the gob-side tunnel; Step E: injecting water through the water injection device into the gap between the top plate and the rock layer above the top plate through the water injection hole so that the water is enriched in the gap between the top plate and the rock layer above the top plate; Step F: Filling with liquid nitrogen: Liquid nitrogen is filled into the rock stratum gap between the top plate and the rock stratum above the top plate through the nitrogen filling hole through the nitrogen filling device.

2. The method for relieving pressure by cutting off the top of the coal mining face according to claim 1, characterized in that: The water injection device includes a water injection pipe and a water pump. The water injection pipe is a hard pipe with one end open. A water outlet that connects the inner cavity of the water injection pipe with the external space of the water injection pipe is arranged on the wall of the water injection pipe near the closed end of the water injection pipe. The water injection pipe is connected to the water pump through a pipe. One end of the pipe is connected to the open sealed end of the water injection pipe, and the other end of the pipe is connected to the water pump. The water pump is connected to an external water source. A sealing component that prevents water from flowing back from the water injection hole is arranged on the outer wall of the water injection pipe near the closed end of the water injection pipe.

3. The method for relieving pressure by cutting off the top of the coal mining face according to claim 2, characterized in that: The blocking member 1 includes an annular air bag 1 sleeved on the outer wall of the water injection pipe, and the inner cavity of the annular air bag 1 is connected with an external air pump 1 through an air pipe 1.

4. The method for relieving pressure by cutting off the top of the coal mining face according to claim 1, characterized in that: The nitrogen charging device comprises a nitrogen charging pipe for charging liquid nitrogen into the rock formation gap, a liquid nitrogen tank for storing liquid nitrogen and a nitrogen charging pump for pumping the nitrogen in the liquid nitrogen tank into the nitrogen charging pipe. The nitrogen charging pipe is a hard pipe with one end open. A water outlet 2 for connecting the inner cavity of the nitrogen charging pipe with the external space of the nitrogen charging pipe is arranged on the wall of the nitrogen charging pipe near the closed end of the nitrogen charging pipe. The nitrogen charging pipe is connected with the nitrogen charging pump through a pipeline 2. One end of the pipeline 2 is connected with the open sealed end of the nitrogen charging pipe, and the other end of the pipeline 2 is connected with the nitrogen charging pump. A valve body 2 is arranged on the pipeline 2. The nitrogen charging pump is connected with the liquid nitrogen tank. A plugging member 2 for preventing liquid nitrogen from flowing back from the nitrogen charging hole is arranged on the outer wall of the nitrogen charging pipe near the closed end of the nitrogen charging pipe.

5. The method for relieving pressure by cutting off the top of the coal mining face according to claim 4, characterized in that: The second blocking member comprises a second annular airbag sleeved on the outer wall of the nitrogen filling tube, and the inner cavity of the second annular airbag is connected with a second external air pump through a second air pipe.

6. The method for relieving pressure by cutting off the top of the coal mining face according to claim 5, characterized in that: The nitrogen filling device also includes a pressurizing unit for accelerating the rapid expansion of the liquid nitrogen in the nitrogen filling pipe so that the liquid nitrogen in the nitrogen filling pipe can be quickly filled into the cracks in the rock formation.

7. The method for relieving pressure by cutting off the top of the coal mining face according to claim 6, characterized in that: The pressurizing unit comprises an electric heating layer attached to the inner wall of the nitrogen filling tube, and the electric heating layer is electrically connected to a power source of an external device through a power line.

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