Method for combined grouting reinforcement of goaf before lower slice mining in thick coal seam slice mining

Abstract

The application discloses a method for combined grouting reinforcement of goaf before lower layer mining in thick coal seam separate layer mining, which comprises the following steps: determining the treatment area of the influence of the upper layer goaf on the lower layer mining; dividing the treatment area into a main grouting area of the floor drainage gallery and a supplementary grouting area of the two roadways of the working face; for the main grouting area of the floor drainage gallery, drilling a through-layer grouting borehole upward from the floor drainage gallery, so that the through-layer grouting borehole passes through the floor stratum and the lower layer coal seam in sequence, and the terminal hole position enters the caving zone or the broken zone of the upper layer goaf; for the supplementary grouting area of the two roadways, drilling a supplementary grouting borehole from the air inlet and / or the air return entry of the lower layer working face, so that the supplementary grouting borehole passes through the coal seam and enters the caving zone or the broken zone of the boundary of the upper layer goaf; the method has the beneficial effects that the problems of excessively long borehole depth, difficult deflection control, insufficient grouting coverage and low construction efficiency in drilling boreholes from the floor drainage gallery to the two ends or the boundary area of the working face are solved.

Description

Technical Field

[0001] This invention relates to the field of coal seam mining technology, and more particularly to a method for combined grouting reinforcement of goaf areas before layered mining in thick coal seams. Background Technology

[0002] In thick coal seam mining, a layered mining process is often adopted, where the upper layer is mined first, and the lower layer is mined only after the upper layer is completed. After the upper layer is mined, the roof strata in the goaf collapse, break, and undergo uneven compaction under the pressure of the overlying strata, forming a caving zone with a loose structure, well-developed voids, and poor overall stability. When the lower layer is subsequently mined, the upper goaf and the caving and broken rock mass within it directly overlay the lower coal seam. Due to the uneven distribution of broken rock blocks within the caving zone, there are numerous cavities and fissures, which often leads to roof breakage, coal body spalling, uneven support loading, premature roof coal collapse, and abnormal pre-collision pressure during the lower layer mining process, seriously affecting the safe and efficient advancement of the working face.

[0003] Grouting reinforcement is one of the commonly used techniques for treating goaf areas and fractured surrounding rock. Its basic principle is to inject cement grout, fly ash grout, cement and water glass two-liquid grout, or other chemical grouts into the fractured rock mass or goaf fissures through construction boreholes. The grout diffuses, fills, and cements the fissures, cavities, and fractured rock blocks, thereby improving the integrity and bearing capacity of the fractured rock mass.

[0004] However, directly drilling grouting boreholes within the overlying goaf faces severe technical challenges when the goaf has already collapsed. The goaf zone consists of loose, fractured rock blocks and gangue, exhibiting extremely unstable structure. During drilling, problems such as borehole collapse, stuck drill bits, drill bit burial, and borehole wall instability are highly likely to occur. Simultaneously, drill cuttings and rock debris are difficult to remove normally from the loose, fractured strata, leading to difficulties in borehole formation, low drilling efficiency, and even preventing the borehole from reaching the designed grouting target area. Therefore, conventional direct drilling grouting methods within the goaf are ineffective under these conditions, necessitating the exploration of novel borehole layouts and construction schemes adapted to grouting reinforcement of the overlying goaf goaf under the conditions of layered mining of thick coal seams.

[0005] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0006] The technical problem to be solved by this invention is to address the low borehole formation rate during grouting drilling in overlying goaf areas.

[0007] The present invention solves the above-mentioned technical problems through the following technical means: This invention claims protection for a method for combined grouting reinforcement of the goaf before the lower layer mining in thick coal seam layered mining, applicable to the following scenario: in thick coal seam layered mining, after the upper layer mining is completed, no filling or grouting reinforcement measures have been taken for the goaf, and a bottom drainage roadway is provided on the coal seam floor, located in the rock strata of the lower layer coal seam floor; intake airway and return airway are arranged on both sides of the lower layer working face, including the following steps: Identify the treatment areas where the upper-layer goaf affects the lower-layer mining; The treatment area is divided into the main grouting area of ​​the bottom extraction roadway and the supplementary grouting area of ​​the two roadways of the working face. For the main grouting area of ​​the bottom drainage roadway, cross-layer grouting boreholes are drilled upwards from the bottom drainage roadway, so that the cross-layer grouting boreholes pass through the bottom rock layer and the lower coal seam in sequence, and the final hole position enters the collapse zone or fracture zone of the upper goaf. For the supplementary grouting area of ​​the two roadways, supplementary grouting boreholes are constructed by the intake airway and / or return airway of the lower working face, so that the supplementary grouting boreholes penetrate the coal seam and enter the collapse zone of the upper goaf or the broken zone at the boundary of the goaf. The supplementary grouting boreholes of the two roadways and the cross-layer boreholes of the bottom drainage roadway are spatially connected. Grout is injected into the upper-layer goaf caving zone through cross-layer grouting boreholes and supplementary grouting boreholes until at least one termination condition is met. The grout fills and cements the fissures, cavities and broken rock masses in the goaf caving zone, thereby forming a grouting solidified body located above the lower coal seam.

[0008] This invention is primarily aimed at the special scenario in thick coal seam mining where the upper layers have been mined, drilling and grouting are required, and a gas control bottom drainage roadway has been installed on the coal seam floor. It abandons the conventional approach of drilling from top to bottom through the caving zone in existing technologies, and instead utilizes the existing bottom drainage roadway for gas control to drill upwards through the layers. This allows the main section of the borehole to be within stable rock strata, with only the final section entering the caving zone. This avoids long-distance drilling directly within the loose caving zone, solves the problems of borehole collapse, stuck drill, and difficulty in slag removal, and improves the borehole success rate. In addition, two supplementary boreholes are set up in the working face to compensate for the problems of excessive hole depth, difficulty in controlling deviation, insufficient grouting coverage, or low construction efficiency when drilling from the bottom extraction roadway to the two ends or boundary areas of the working face. The two sets of boreholes work together, with the bottom extraction roadway borehole controlling the central main control area and the two roadway boreholes controlling the two ends and boundary areas. The grouting ranges of the two sets of boreholes overlap to form a full-coverage reinforcement system without blind spots, and jointly solve the problems of excessive hole depth, difficulty in controlling deviation, insufficient grouting coverage, or low construction efficiency when drilling from the bottom extraction roadway to the two ends or boundary areas of the working face.

[0009] Preferably, the treatment area includes: The upper-layer goaf collapse zone located above the lower-layer working face; The fractured rock mass area located at the bottom of the upper-layered goaf and directly overlying the lower-layered coal seam; The boundary areas at both ends of the lower working face and near the two roadways that are significantly affected by the overlying goaf. Key areas where roof breakage, abnormal support load, coal wall spalling, or roof collapse may occur during the lower-level mining process.

[0010] Preferably, the boreholes in the bottom extraction tunnel can be arranged in a fan shape, a grid, or a group.

[0011] Preferably, the method further includes the following steps: after drilling is completed, casing protection and sealing treatment are carried out on the borehole opening and the stable rock stratum section.

[0012] Preferably, during grouting, the main grouting area of ​​the bottom extraction tunnel is grouted first, and then the supplementary grouting areas of the two tunnels are supplemented; or the grouting sequence of boundary first and then middle, deep first and then shallow, or key areas first and then general areas is adopted.

[0013] Preferably, during grouting, a low-concentration, low-viscosity grout is used in the initial stage of grouting; the grout concentration is gradually increased or a fast-setting grout is used in the middle and later stages of grouting.

[0014] Preferably, during grouting, the grouting pressure is determined based on the depth of the goaf, the strength of the coal and rock strata, the location of the borehole, and the stability of the surrounding rock of the roadway, and is controlled by a step-by-step pressure increase method from low to high.

[0015] Preferably, single-component grout or two-component grout is used during grouting.

[0016] After drilling is completed, grouting is carried out in sequence and in stages. First, low-viscosity grout penetrates the cracks, and then high-concentration grout fills the voids. This can make the grouting effect better than single grout or single pressure grouting.

[0017] Preferably, the method further includes the following steps: after grouting is completed, the grouting effect is tested and reinforced.

[0018] Preferably, the grouting termination conditions include: The grouting pressure reaches the designed final pressure and stabilizes for a certain period of time; The grouting volume per hole reached the designed grouting volume; Effective slurry return occurs in adjacent boreholes or goaf fractures; Grouting flow rate decreased significantly and tended to stabilize; Monitoring results show that the filling effect of the reinforced area meets the design requirements.

[0019] The advantages of this invention are: I. Breaking away from the conventional approach of drilling downwards from the caving zone in existing technologies, this method utilizes existing bottom drainage roadways used for gas control to drill upwards through layers. This allows the main borehole section to remain within stable rock strata, with only the final section entering the caving zone. This avoids long-distance drilling directly within loose caving zones, solving problems such as borehole collapse, drill bit jamming, and difficulties in cuttings removal, thus improving the borehole success rate. With the main borehole section operating within stable rock strata and coal seams, drill cuttings can be normally removed, and only the final section enters the caving zone. This solves the problems of borehole collapse, drill bit jamming, and drill bit burial encountered when drilling directly into the caving body of loose goaf areas.

[0020] 2. Large grouting coverage area with no blind spots. Boreholes in the bottom extraction tunnel cover the middle of the working face, while boreholes in the two tunnels cover both ends and boundary areas of the working face. The grouting areas of the two sets of boreholes overlap to form a continuous reinforcement zone.

[0021] III. Highly targeted and effective grouting. The sequential grouting method, which first injects low-viscosity grout into cracks and then fills voids with high-concentration grout, ensures that cracks and voids of different sizes can be effectively filled.

[0022] IV. Reduced workload and low cost. Utilizing existing bottom grouting tunnels and two side tunnels as construction space, there is no need to excavate dedicated grouting tunnels; the entire grouting operation can be completed within the existing tunnel system. Attached Figure Description

[0023] Figure 1 This is a flowchart of the combined grouting reinforcement method for the goaf before layered mining in thick coal seams in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of a thick coal seam cross-section in Embodiment 1 of the present invention; 1. Supplementary grouting boreholes; 2. Collapse zone in goaf; 3. Return airway; 4. Upward cross-layer boreholes; 5. Bottom drainage roadway; 6. Overlying strata; 8. Lower coal seam; 9. Intake airway; 10. Floor strata. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 See Figure 1 and Figure 2 This embodiment requires a combined grouting reinforcement method for the goaf before layered mining of thick coal seams, mainly targeting coal mines that meet the following conditions: Scenario 1: In thick coal seam mining, when layered mining is adopted, the upper layer has been mined out, but no backfilling or grouting reinforcement measures have been taken for the goaf. In this case, after the roof of the upper layer's goaf collapses, a fractured caving zone is formed, which directly overlays the lower coal seam 8. Before the lower working face is mined out, it is expected to be affected by the overlying strata 6, with risks of roof fracture, abnormal support load, and coal wall spalling.

[0026] Scenario 2: A gas control bottom drainage roadway 5 has been installed on the coal seam floor. Bottom drainage roadway 5 is located in the rock stratum 10 on the floor of the lower coal seam 8. Intake airway 9 and return airway 3 are arranged on both sides of the lower working face.

[0027] For the aforementioned coal mine, the grouting reinforcement method includes the following steps: S1. Before mining the lower layer working face, based on the upper layer mining data, goaf range, coal seam thickness, layer spacing, roof and floor lithology, geological structure, mine pressure manifestation, and lower layer working face layout, determine the treatment area where the upper layer goaf affects the lower layer mining.

[0028] Specifically, on the plane, the overlapping area where the boundary of the upper goaf and the boundary of the lower working face intersect is taken as the treatment area. When the boundary of the upper goaf completely covers the lower working face, the treatment area is the entire caving zone directly above the lower working face; when the boundary of the upper goaf only covers part of the lower working face, only the covered area is grouted and reinforced, and the uncovered area is not included in the treatment scope.

[0029] Vertically upwards, from the bottom of the upper caving zone to the roof of the lower coal seam 8. If the interlayer spacing between the upper and lower layers is large and the caving zone does not directly overlay the roof of the lower layer, then the fractured rock mass in this space is still the target for grouting; if the caving zone has been compacted or is in contact with the roof of the lower layer, then the target for grouting is the fractured rock mass near the contact zone.

[0030] For areas where the boundary of the goaf does not completely overlap with the boundary of the working face, a certain range is extended outside the boundary of the overlapping area as a transition treatment zone to prevent insufficient slurry diffusion at the boundary from causing discontinuous reinforcement.

[0031] Therefore, the governance area includes: The upper-layer goaf collapse zone 2 is located above the lower-layer working face; The fractured rock mass area located at the bottom of the upper-layered goaf and directly overlying the lower-layered coal seam; The boundary areas at both ends of the lower working face and near the two roadways that are significantly affected by the overlying goaf. Key areas where roof breakage, abnormal support load, coal wall spalling, or roof collapse may occur during the lower-level mining process.

[0032] S2. Based on the spatial relationship between the bottom grouting tunnel 5 and the lower working face, the treatment area is divided into the main grouting area of ​​the bottom grouting tunnel 5 and the supplementary grouting area of ​​the two tunnels.

[0033] Specifically, the bottom grouting tunnel 5 is located in the bottom stratum 10 directly below the middle of the lower working face. When drilling through the stratum from the bottom grouting tunnel 5 upwards, the area with the optimal drilling angle, hole depth and slag discharge conditions corresponds to the middle area of ​​the working face. This area is designated as the main control grouting area of ​​the bottom grouting tunnel 5.

[0034] For the sidewall areas near the intake air roadway 9 and return air roadway 3, as well as the two ends of the working face direction, due to the increased horizontal distance from the bottom extraction roadway 5, the borehole inclination angle becomes gentler, the borehole depth increases, drill cuttings are difficult to remove, borehole deviation is difficult to control, and the grouting coverage effect decreases. Therefore, these areas are designated as supplementary grouting areas for the two roadways, and supplementary grouting is carried out using the two roadways of the lower working face as construction space.

[0035] S3. For the main grouting zone of bottom drainage tunnel 5, borehole 4 is drilled upwards from bottom drainage tunnel 5.

[0036] Specifically, a drilling site or drilling point is set up in the bottom drainage roadway 5, and cross-layer drilling is carried out from the bottom drainage roadway 5 upwards. The drilling passes through the bottom rock layer 10, the lower coal seam 8, and the interbedded gangue or coal-rock layer between the upper and lower layers in sequence. The final hole position enters the bottom of the upper goaf caving zone 2, the fractured rock mass area, or the fractured area.

[0037] Furthermore, the boreholes in the bottom drainage roadway 5 can be arranged in a fan shape, a grid pattern, or in groups. The borehole inclination angle, azimuth angle, and depth are determined based on the spatial relationship between the bottom drainage roadway 5 and the goaf, so that multiple boreholes form a continuous or nearly continuous grouting coverage area within the collapse zone 2 of the upper goaf.

[0038] During drilling, the main borehole section is located in the relatively intact bottom rock strata 10 and coal seam. Rock cuttings and coal dust during the drilling process can be discharged through conventional slag removal methods. The borehole only enters the goaf caving zone 2 or the fractured zone in the final section, thereby avoiding long-distance drilling in the loose caving body.

[0039] S4. For the supplementary grouting areas of the two roadways, supplementary grouting boreholes 1 shall be constructed by the intake airway 9 and / or return airway 3 of the lower working face.

[0040] Specifically, when drilling from the bottom extraction roadway 5 towards both ends or boundary areas of the working face results in problems such as excessive hole depth, difficulty in controlling deviation, insufficient grouting coverage, or low construction efficiency, supplementary grouting boreholes 1 are arranged in the intake roadway 9 and return roadway 3 of the lower working face.

[0041] Furthermore, the supplementary grouting borehole 1 starts from the intake airway 9 and / or the return airway 3, passes through the lower layered coal seam 8 or the coal-rock interface, and extends upward to the goaf caving zone 2, the broken zone at the boundary of the goaf, or the overburden zone at both ends of the goaf.

[0042] The supplementary grouting borehole 1 in the two lanes and the upward through-layer borehole in the bottom extraction lane 5 are spatially interconnected, forming a continuous reinforcement zone between the main control grouting zone of the bottom extraction lane 5 and the supplementary grouting zone in the two lanes, reducing grouting blind spots.

[0043] S5. After drilling is completed, casing and sealing treatment are carried out on the borehole opening and the stable rock strata section.

[0044] Specifically, for the cross-layer drilling in the bottom drainage tunnel 5 construction, a casing can be installed from the borehole opening to the stable rock stratum section, and the borehole can be sealed with cement grout, polyurethane material or other sealing materials to ensure that the grouting pressure can be effectively transmitted to the final borehole section.

[0045] For the supplementary grouting borehole 1 in the construction of the intake air roadway 9 and return air roadway 3 of the lower layer working face, a borehole pipe, grout stop plug or segmented sealing device can be set according to the degree of coal seam fracture and borehole wall stability to prevent grout from short-circuiting back along the borehole or coal wall.

[0046] S6. Through cross-layer drilling and supplementary grouting drilling 1, grouting is carried out sequentially in the upper layered goaf caving zone 2 until at least one termination condition is reached, so that the grout fills and cements the cracks, cavities and broken rock mass in the goaf caving zone 2, thereby forming a grouting solidified body above the lower layered coal seam 8.

[0047] During grouting, it is preferable to first grout the main control grouting area of ​​the bottom extraction roadway 5, and then supplement the grouting areas of the two roadways. Alternatively, depending on the degree of fragmentation of the goaf and the grouting situation of the boreholes, the grouting sequence can be adopted, such as first the boundary and then the middle, first the deep and then the shallow, or first the key areas and then the general areas.

[0048] During grouting, single-component or two-component grouts can be used. Grouts include, but are not limited to, cement grout, cement-fly ash grout, cement-water glass grout, chemically reinforced grout, or combinations thereof.

[0049] Furthermore, in the initial stage of grouting, a low-concentration, low-viscosity grout is used to allow it to penetrate into small cracks and loose, fragmented materials. In the middle and later stages of grouting, the grout concentration is gradually increased or a fast-setting grout is used to fill and cement larger voids and gaps between broken rock blocks. The grouting pressure is determined based on the depth of the goaf, the strength of the coal and rock strata, the borehole location, and the stability of the surrounding rock in the roadway, and is controlled by a gradual increase in pressure from low to high.

[0050] It should be noted that the grouting termination conditions include: The grouting pressure reaches the designed final pressure and stabilizes for a certain period of time; The grouting volume per hole reached the designed grouting volume; Effective slurry return occurs in adjacent boreholes or goaf fractures; Grouting flow rate decreased significantly and tended to stabilize; Monitoring results show that the filling effect of the reinforced area meets the design requirements.

[0051] S7. Inspect and reinforce the grouting effect.

[0052] Specifically, after grouting is completed, the reinforcement effect is tested. Testing methods include core drilling, borehole inspection, grouting volume statistics, pressure-flow curve analysis, geophysical exploration, roadway surrounding rock deformation monitoring, and mine pressure monitoring during working face advancement.

[0053] If insufficient grouting volume, inadequate grout diffusion, or the presence of broken or voided areas are found in localized areas, secondary grouting will be carried out by rearranging reinforcement boreholes in the bottom extraction roadway 5 or the two roadways of the working face.

[0054] S8. Once the grouting effect meets the preset requirements, the next working face will be mined.

[0055] In this embodiment, by pre-reinforcing the upper layer goaf caving zone 2 and its bottom fracture zone, the risks of roof breakage, coal wall spalling, uneven support loading, and roof collapse during the lower layer mining are reduced, thereby improving the stability of the working face advance.

[0056] Example 2 Based on Implementation 1, this embodiment provides a specific application of a combined grouting reinforcement method for the goaf before layered mining in a thick coal seam in a certain coal mine. Specifically: First, based on the extent of the upper-layer goaf and the layout of the lower-layer working face, the range of the goaf collapse zone 2 that requires grouting reinforcement is determined. The middle part of the lower-layer working face is designated as the main grouting area of ​​the bottom extraction roadway 5; the two ends of the lower-layer working face in the strike direction, as well as the side wall boundaries on both sides near the intake roadway 9 and the return air roadway 3 in the dip direction, are designated as the supplementary grouting areas of the two roadways.

[0057] Furthermore, for the main grouting area of ​​bottom drainage roadway 5, drilling sites are arranged at intervals along the strike of the lower working face within bottom drainage roadway 5, with multiple sets of fan-shaped cross-layer boreholes drilled upwards within each drilling site. Specifically, the borehole starts from bottom drainage roadway 5, passes through the bottom stratum 10, the lower coal seam 8, and the inter-stratum coal and rock mass in sequence, and finally enters the bottom of the upper goaf caving zone 2.

[0058] It should be noted that during the drilling process, most of the borehole is located within stable coal and rock bodies, allowing the drilling cuttings to be discharged smoothly, thus avoiding the difficulties in removing cuttings caused by drilling directly into the goaf collapse.

[0059] Furthermore, for the supplementary grouting areas in the two roadways, supplementary boreholes are arranged in the intake airway 9 and return airway 3, respectively, for the areas at both ends of the working face and the boundary areas of the side walls of the two roadways, due to the long distance and long borehole path of the drilling from the bottom extraction roadway 5. Specifically, the boreholes are drilled from the two roadways upwards and towards the goaf, passing through the lower layered coal seam 8 and entering the caving zone at the boundary of the upper layered goaf.

[0060] It should be noted that the grouting range of the supplementary boreholes in the two roadways overlaps with the grouting range of borehole 5 in the bottom extraction roadway.

[0061] Then, after completing the supplementary drilling in the two roadways and the drilling in the bottom extraction roadway 5, casings were installed at the borehole openings and the boreholes were sealed. Specifically, low-concentration cement grout was first used to permeate and grout the fractures and broken rock mass in the goaf, followed by filling and cementing with higher-concentration cement-fly ash grout or cement-water glass grout. During the grouting process, the grouting pressure, flow rate, and grouting volume per borehole were recorded in real time. Grouting in a borehole was stopped when the grouting pressure reached the designed final pressure and the flow rate decreased significantly, or when grout returned to an adjacent borehole.

[0062] Finally, after grouting is completed, the reinforcement effect is assessed through borehole inspection, grout volume analysis, and geophysical detection. For areas with insufficient grouting, secondary grouting is performed using short boreholes in the bottom extraction roadway 5 or both roadways. Once the reinforcement effect meets the requirements, the next working face is then mined.

[0063] In this embodiment, the fractured rock mass at the bottom of the collapse zone 2 of the upper layer goaf is filled and cemented by grout to form a solidified body with a certain integrity and bearing capacity, effectively reducing the impact of the overlying fractured goaf on the mining of the lower layer.

[0064] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for combined grouting reinforcement of the goaf before lower-level mining in thick coal seams, applicable to the following scenarios: In thick coal seam layered mining, after the upper-level mining is completed, no filling or grouting reinforcement measures have been taken for the goaf, and a bottom drainage roadway is provided on the coal seam floor, located in the rock strata of the lower-level coal seam floor; intake and return airways are arranged on both sides of the lower-level working face, characterized in that... Includes the following steps: Identify the treatment areas where the upper-layer goaf affects the lower-layer mining; The treatment area is divided into the main grouting area of ​​the bottom extraction roadway and the supplementary grouting area of ​​the two roadways of the working face. For the main grouting area of ​​the bottom drainage roadway, cross-layer grouting boreholes are drilled upwards from the bottom drainage roadway, so that the cross-layer grouting boreholes pass through the bottom rock layer and the lower coal seam in sequence, and the final borehole position enters the collapse zone or fracture zone of the upper goaf. For the supplementary grouting area of ​​the two roadways, supplementary grouting boreholes are constructed by the intake airway and / or return airway of the lower working face, so that the supplementary grouting boreholes penetrate the coal seam and enter the collapse zone of the upper goaf or the broken zone at the boundary of the goaf. The supplementary grouting boreholes of the two roadways and the cross-layer boreholes of the bottom drainage roadway are spatially connected. Grout is injected into the upper-layer goaf collapse zone through cross-layer grouting boreholes and supplementary grouting boreholes until at least one termination condition is met, so that the grout fills and cements the fissures, cavities and broken rock masses in the goaf collapse zone, thereby forming a grouting solidified body located above the lower-layer coal seam.

2. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, The areas to be governed include: The upper-layer goaf collapse zone located above the lower-layer working face; The fractured rock mass area located at the bottom of the upper-layered goaf and directly overlying the lower-layered coal seam; The boundary areas at both ends of the lower working face and near the two roadways that are significantly affected by the overlying goaf. Key areas where roof breakage, abnormal support load, coal wall spalling, or roof collapse may occur during the lower-level mining process.

3. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, Boreholes in bottom drainage tunnels can be arranged in a fan-shaped, grid-like, or grouped pattern.

4. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, It also includes the following steps: After drilling is completed, casing and sealing treatment are carried out on the borehole opening and the stable rock strata section.

5. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, During grouting, the main grouting area of ​​the bottom extraction roadway is grouted first, and then the supplementary grouting areas of the two roadways are supplemented; or the grouting sequence of boundary first and then middle, deep first and then shallow, or key areas first and then general areas is adopted.

6. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, During grouting, a low-concentration, low-viscosity grout is used in the initial stage; the grout concentration is gradually increased or a fast-setting grout is used in the middle and later stages of grouting.

7. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, During grouting, the grouting pressure is determined based on the depth of the goaf, the strength of the coal and rock strata, the location of the borehole, and the stability of the surrounding rock of the roadway, and is controlled by a step-by-step pressure increase method from low to high.

8. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, During grouting, single-component grout or two-component grout is used.

9. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, It also includes the following steps: After grouting is completed, the grouting effect is tested and reinforced.

10. The method for combined grouting reinforcement of the goaf before layered mining in thick coal seams according to claim 1, characterized in that, Grouting termination conditions include: The grouting pressure reaches the designed final pressure and stabilizes for a certain period of time; The grouting volume per hole reached the designed grouting volume; Effective slurry return occurs in adjacent boreholes or goaf fractures; Grouting flow rate decreased significantly and tended to stabilize; Monitoring results show that the filling effect of the reinforced area meets the design requirements.

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