A method for pre-fracture of top coal in steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas

Abstract

This invention discloses a method for pre-fracture of top coal in steeply inclined, extra-thick coal seams based on long-term water injection and creep pre-weakening in the goaf. It relates to the field of coal mining technology. Utilizing the temporal characteristics of steeply inclined horizontal segmented mining, the upper segmented goaf serves as a water injection and storage space. The fractures formed by the disturbance during mining in the upper segment act as water channels. Controlled long-term water injection is implemented during the upper segment's mining cycle, allowing water to continuously infiltrate to the top coal in the lower segment. Under the coupling effect of water, coal, and stress, a water-bearing creep pre-weakening zone is formed. Subsequently, the pre-fracture blasting scheme is optimized based on the pre-weakening state, allowing the blasted fractures to expand and connect along the pre-weakening fracture field. This invention can improve the uniformity and caving properties of top coal fragmentation, reduce blasting intensity and construction interference, and is suitable for horizontal segmented fully mechanized longwall mining of steeply inclined, extra-thick coal seams.

Description

Technical Field

[0001] This invention relates to the field of coal mining technology, and more specifically to a method for pre-fracture of the top coal of steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas. Background Technology

[0002] Steeply dipping coal seams are an important component of my country's coal resources. For steeply dipping thick and extra-thick coal seams, the horizontal segmented fully mechanized longwall mining method is often adopted. This involves dividing the coal seam into several horizontal segments along its vertical or dip direction and mining each segment sequentially from top to bottom. In the horizontal segmented fully mechanized longwall mining of steeply dipping extra-thick coal seams, there is usually a thick roof coal above the lower segment working face. Due to the large coal seam thickness, high roof coal height, complex coal body structure, and uneven fracture development, the roof coal is difficult to collapse and fully break in a timely manner, easily leading to problems such as large pieces of roof coal, local overhang, discontinuous coal release, coal flow arching, large amounts of residual coal at the end face, and low roof coal recovery rate. From the perspective of roof coal space destruction characteristics, the upper part of the lower segment roof coal is adjacent to the upper segment goaf. Affected by the disturbance of the upper segment mining, goaf collapse, and inter-layer fracture expansion, the integrity of the coal body is relatively reduced, forming a certain range of mining-induced damage zone, fracture development zone, and structurally weakened zone. This area is not only significantly affected by the mining disturbances of the upper section, but also serves as a crucial water-conducting channel for downward water infiltration after the goaf is injected. The lower part of the top coal in the lower section, close to the working face of this section, is also prone to fracturing, loosening, and collapse due to the mining stress, support pressure disturbances, and coal release space. In contrast, the middle part of the top coal in the lower section is far from the strongly disturbed area of ​​the upper goaf and has not yet been fully affected by the mining damage of the working face of this section; it is typically a relatively intact area with insufficient influence from both upper and lower mining disturbances. This area exhibits low fissure development, good structural integrity, and high strength, making it the most difficult part of the lower section's top coal to break, collapse, and be fully released.

[0003] Currently, for steeply inclined, extra-thick coal seams with large top coal layers, methods such as pre-fracturing blasting, high-pressure hydraulic fracturing, and in-face water injection for softening are commonly used. While pre-fracturing blasting has a direct effect, for thick, intact top coal seams, the blasting fractures are often concentrated near the blast holes, making it difficult to form a continuous, interconnected fracture network over a large area. This often results in fractures near the blast holes while areas between or far from the blast holes remain intact. Simply increasing the blasting intensity, reducing the blast hole spacing, or increasing the number of blasts to improve the coal breaking effect may lead to increased blasting disturbance, intensified surrounding rock deformation, damage to the support system, and increased safety risks. High-pressure hydraulic fracturing can create fractures in the coal seam, but it requires large equipment investment, has high requirements for hole sealing, and the direction and extent of fracture propagation are significantly controlled by ground stress, bedding structure, and weak coal and rock surfaces, resulting in problems such as high-pressure water channeling, surrounding rock damage, and high construction costs. While water injection in traditional working faces can reduce the strength of coal to some extent, it usually requires drilling water injection holes in the lower section mining space, which affects the normal mining continuity. Moreover, the water injection time is relatively short, making it difficult to form a sufficient, continuous and uniform pre-weakening effect on the middle of thick top coal.

[0004] Steeply inclined horizontal segmented fully mechanized longwall mining is characterized by a "first mining of the upper segment, then the lower segment" sequence. The upper segment has a longer mining cycle. During this process, a goaf gradually forms behind the working face. Simultaneously, the mining disturbance in the upper segment damages the upper top coal and inter-layer coal and rock mass of the lower segment, creating fissures, weak surfaces, and locally fractured areas. These fissures and damaged areas formed by mining disturbance can serve as natural water-conducting channels for downward water infiltration. If controlled water injection can be carried out into the goaf during the upper segment mining, allowing water to continuously infiltrate the lower segment's top coal through these mining fissures and damaged areas, the lower segment's top coal to be treated can undergo a prolonged period of water-bearing softening, fissure development, and creep aging damage before the lower segment mining and pre-splitting blasting.

[0005] Therefore, the existing technology lacks a collaborative coal breaking method that fully utilizes the water-conducting conditions of the fractures formed by the disturbance of the upper segment mining and the long mining cycle of the upper segment, performs long-term water injection creep pre-weakening on the thick top coal before the advanced pre-fracture blasting of the lower segment, and improves the subsequent fracture expansion effect by pre-weakening the fracture field. Summary of the Invention

[0006] In view of this, the present invention provides a method for pre-fracture of top coal in steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, in order to solve the problems existing in the background art.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for pre-fracture of the top coal of steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas includes: S1. Based on the coal seam occurrence state, segmented mining layout, location of upper segment working face, location of lower segment working face, range of upper segment goaf formation, range of mining disturbance impact, and difficulty of top coal crushing, determine the area of ​​top coal to be processed in the lower segment, and at the same time identify the area of ​​fracture water-conducting channels formed by the upper segment mining disturbance in the upper top coal of the lower segment. S2, as the upper section working face advances in mining, the upper section goaf is gradually formed behind the working face; the mining stress disturbance of the upper section damages the upper top coal and inter-layer coal and rock mass of the lower section, forming fissures, weak surfaces, local fractured areas and seepage channels, which constitute water guiding channels for water to seep into the deeper parts of the lower section; S3, using the upper segment goaf as the water injection space and water storage space, construct a water injection system that is hydraulically connected to the upper segment goaf; S4. During the complete mining cycle of the upper segment, controlled water injection is carried out in the upper segment goaf. After the injected water is retained and diffused in the goaf, it continues to infiltrate the lower segment top coal area to be treated along the water guide channel. Under the combined effect of the upper segment mining disturbance and the controlled water injection in the upper segment goaf, a long-term water-bearing creep pre-weakening zone is formed in the lower segment top coal area to be treated. S5, before implementing advanced pre-splitting and crushing in the lower segment, monitor the pre-weakening state and safety state of the top coal area to be treated in the lower segment, and proceed to S6 after determining that the conditions for advanced pre-splitting operation are met. S6. Based on the location, range, fracture development status, coal softening degree, creep damage degree, water-conducting range of mining fractures and target crushing effect of the top coal area to be treated in the lower segment, optimize the advanced pre-splitting crushing scheme of the lower segment, and implement advanced pre-splitting blasting according to the optimized scheme. S7. After the lower segment advanced pre-splitting blasting is completed and safety is confirmed, the water injection process and pre-splitting parameters of the subsequent mining area are dynamically adjusted based on the coal release effect and on-site feedback.

[0008] Preferably, the lower segment of the top coal area to be treated and the fracture water-conducting channel area are determined by at least one of the following methods: borehole inspection, ground-penetrating radar, acoustic detection, on-site exposure, top coal block size statistics, working face mine pressure manifestation, mining-induced fracture observation, or numerical simulation.

[0009] Preferably, the water injection system includes at least one of the following: water injection borehole, directional water guide hole, water injection pipeline, control valve, flow control device, pressure monitoring device, and sealing structure.

[0010] Preferably, the construction location of the water injection system is selected from at least one of the following: upper section roadway, sidewall of goaf, adjacent drilling site, overlying roadway, and surface borehole.

[0011] Preferably, the water injection system is arranged to meet the following requirements: it can guide water into the upper segment goaf or its collapse fracture zone, and create conditions for water storage, retention and diffusion within the goaf; it can enable water to move along the mining-induced fractures and interlayer weak surfaces of the upper top coal of the lower segment to the area of ​​the top coal to be treated; it does not affect the normal mining, ventilation, transportation, drainage and safe production systems of the upper segment working face; and it can adjust the water injection parameters according to the on-site water inflow, drainage and surrounding rock stability.

[0012] Preferably, the formation process of the long-term water-bearing creep pre-weakening zone is as follows: 1) The disturbance caused by the mining of the upper section causes cracks and localized fracture zones in the upper top coal of the lower section; 2) After water is injected into the upper section of the goaf, it forms a water storage and retention state; 3) Water seeps and diffuses downwards along mining-induced fractures, weak interlayer surfaces, and primary fractures into the top coal to be treated in segments; 4) After absorbing water, the coal body becomes wetted and softened, and its structural strength is initially reduced; 5) Under the long-term interaction of water and coal rock, and the coupled effect of original rock stress and mining stress, the coal body undergoes creep aging damage; 6) The internal fissures of the top coal continue to expand, connect and develop, and the structural integrity of the difficult-to-break top coal in the middle gradually decreases.

[0013] Preferably, in S6, the arrangement of blasting holes needs to match and coordinate the location of the top coal area to be treated in the lower segment, the spatial relationship between the upper segment goaf and the lower segment top coal, the distribution of the water-conducting zone of the mining-induced fractures, the distribution of the pre-weakened fractures formed by long-term water injection, the range of the water-bearing creep damage weakened zone, the dip and dip angle of the coal seam, the direction of the primary fractures and the interlayer weak surface, the space for subsequent mining and coal release, and the safety production requirements of the working face.

[0014] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a method for pre-fracture of the top coal of steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, with the following beneficial effects: (1) Make full use of the water-conducting conditions of the fractures formed by the disturbance of the upper segment mining. This invention does not simply use the goaf as a water channel, but rather uses the goaf as a water injection and storage space. The cracks, fractured areas, and weak surfaces formed by the disturbance of the upper section mining and damage to the upper top coal of the lower section are used as the main water channels for water to seep downward, so that the water injection can be more effectively transmitted to the top coal area to be treated in the lower section.

[0015] (2) Make full use of the longer mining cycle of the upper segment to form creep aging weakening

[0016] This invention utilizes the longer mining cycle of the upper section to allow water to act on the top coal of the lower section continuously or in stages over a longer period. Under the combined effects of water content, original rock stress, and mining-induced stress, the top coal undergoes creep and aging damage, causing fractures to gradually expand and connect, and the integrity of the coal body structure to continuously decrease, thus creating conditions conducive to subsequent crushing.

[0017] (3) Improve the effect of pre-splitting crack propagation

[0018] This invention involves controlled water injection into the goaf during the upper segment mining process, causing the top coal to be treated in the lower segment to become water-bearing, softened, fractured, and creep-damaged before blasting. During subsequent mining, the water-injected weakened fractures, creep-damaged fractures, mining-induced fractures, and inter-layer weak surfaces that have already formed will expand and connect, increasing the range and degree of fracture expansion during blasting.

[0019] (4) Improve the breaking effect of difficult-to-break top coal in the middle.

[0020] The middle section of the top coal in the lower segment is usually a difficult-to-break area due to insufficient disturbance from both the upper and lower mining operations. This invention utilizes the long mining cycle of the upper segment and the water-conducting characteristics of the mining fractures to allow water to act on the top coal in the middle of the lower segment for a longer period of time, thereby reducing the integrity of the top coal in the middle and improving the subsequent breaking effect.

[0021] (5) Avoid safety risks caused by blasting intensity

[0022] This invention reduces the difficulty of coal fracturing by water injection creep pre-weakening. If subsequent auxiliary blasting and crushing will not result in excessive blasting intensity, it will help reduce blasting disturbance, reduce surrounding rock deformation and the risk of support damage.

[0023] (6) Reduce water injection construction interference in the lower segment mining space

[0024] This invention mainly utilizes the upper section goaf as a water injection and storage space, and uses mining-induced fractures as infiltration channels, which can reduce the need to construct a large number of water injection holes in the lower section mining space and reduce the impact on the preparation and construction of the lower section mining.

[0025] (7) Form a synergistic control mechanism for mining disturbance and water injection creep pre-weakening.

[0026] This invention is not a simple water injection weakening, but rather forms a synergistic control mechanism of "upper segment mining disturbance and cracking - goaf water injection and storage - mining-induced fracture water infiltration - lower segment top coal water-bearing creep pre-weakening - advanced pre-fracture fracture expansion - mining-induced coal release". Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0028] Figure 1 A flowchart of the method steps provided by the present invention; Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and 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.

[0030] This invention discloses a method for pre-fracture of the top coal of steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, such as... Figure 1 As shown, it includes: S1. Based on the coal seam occurrence state, segmented mining layout, location of upper segment working face, location of lower segment working face, range of upper segment goaf formation, range of mining disturbance impact, and difficulty of top coal crushing, determine the area of ​​top coal to be processed in the lower segment, and at the same time identify the area of ​​fracture water-conducting channels formed by the upper segment mining disturbance in the upper top coal of the lower segment. Under the conditions of horizontal segmented fully mechanized longwall mining of steeply inclined extra-thick coal seams, the area of ​​top coal to be processed in the lower segment is determined based on the coal seam occurrence state, segmented mining layout, location of the upper segment working face, location of the lower segment working face, range of upper segment goaf formation, range of mining disturbance impact, and difficulty of top coal crushing.

[0031] The top coal area to be processed in the lower section is preferably the difficult-to-break area in the middle of the top coal. This area is usually characterized by insufficient disturbance from the upper section goaf, insufficient impact from the mining in this section, good coal integrity, low degree of fracture development, and poor effect of natural collapse and simple blasting.

[0032] Simultaneously, it is necessary to identify the fracture development zone, broken zone, and structurally weak surface in the upper top coal of the lower section, formed by the disturbance during the upper section's mining. This area serves as the main water-conducting channel for water to seep into the deeper top coal of the lower section after water injection into the goaf, and is a key transitional area connecting the water injection space of the upper section's goaf with the top coal area to be treated in the lower section.

[0033] The areas of top coal to be treated and the areas where water channels are developed can be determined comprehensively based on borehole inspection, ground-penetrating radar, sonic detection, on-site exposure, top coal block size statistics, working face mining pressure manifestation, mining-induced fracture observation, or numerical simulation results.

[0034] S2, as the upper section working face advances in mining, the upper section goaf is gradually formed behind the working face; the mining stress disturbance of the upper section damages the upper top coal and inter-layer coal and rock mass of the lower section, forming fissures, weak surfaces, local fractured areas and seepage channels, which constitute water guiding channels for water to seep into the deeper parts of the lower section; During the mining process of the upper section working face, as the working face advances, a goaf gradually forms behind the upper section mining area. At the same time, the redistribution of mining stress caused by the mining of the upper section will disturb and damage the upper top coal and inter-layer coal and rock mass of the lower section.

[0035] Under the influence of this mining disturbance, fissures, weak surfaces, locally fractured zones, and seepage channels are formed in the upper top coal and inter-layer coal and rock mass of the lower section. These mining-induced fissures and damaged zones reduce the structural integrity of the upper top coal of the lower section on the one hand, and provide water-conducting conditions for subsequent water injection and downward seepage into the goaf on the other.

[0036] This invention utilizes the water-conducting conditions of the fractures formed by the mining disturbance to enable the water injected into the upper segment goaf to migrate downwards along the upper top coal failure fractures, mining fractures, interlayer weak surfaces, and primary fractures of the lower segment to the segmented top coal area to be treated.

[0037] S3, the above segmented goaf area serves as the water injection space and water storage space, constructing a water injection system that is hydraulically connected to the upper segmented goaf area; During the mining process of the upper section working face, as the working face advances, an upper section goaf is formed behind the working face. The upper section goaf is used as a water injection and storage space.

[0038] The pores of fallen gangue, the goaf spaces, local fissures, and low-lying areas within the upper section of the goaf can accommodate and temporarily store injected water, creating conditions for water storage, retention, and diffusion within the goaf. Subsequently, under the influence of gravity, goaf water pressure, seepage from mining-induced fissures, and water conduction from weak coal and rock surfaces, the water gradually seeps into the lower section's untreated top coal area along the mining-induced fractures in the upper top coal.

[0039] This invention does not involve temporarily injecting water into the top coal before the lower section blasting, but rather, during the upper section mining process, water injection into the goaf is carried out simultaneously or in stages, taking advantage of the gradual formation of the goaf. This allows the upper section mining process, the goaf formation process, the mining-induced fracture development process, the goaf water injection process, and the pre-weakening process of water-bearing creep in the lower section top coal to overlap in time.

[0040] During the upper section mining, a water injection system connected to the upper section goaf is constructed using the upper section roadway, the sidewall of the goaf, the adjacent drilling site, the overlying roadway, the surface borehole, or other locations where construction conditions are available.

[0041] The water injection system may include water injection boreholes, directional water guide holes, water injection pipelines, control valves, flow control devices, pressure monitoring devices, sealing structures, or other facilities capable of introducing water into the upper segment goaf.

[0042] The layout of the water injection system should meet the following requirements: (1) It enables water to enter the upper segment goaf area or its collapse fracture zone; (2) It enables water bodies to form conditions for water storage, retention or diffusion within the goaf; (3) It enables water to migrate downward through the top coal mining failure fractures, interlayer weak surfaces and coal body fractures in the lower segment to the top coal area to be treated; (4) It does not affect the normal mining and safety production systems such as ventilation, transportation, and drainage of the upper section working face; (5) It can regulate water injection based on the on-site water inflow, drainage and surrounding rock stability; (6) Avoid the risk of uncontrolled water inrush, cross-flow, water accumulation in roadways, or water hazards in goaf areas.

[0043] S4, during the complete mining cycle of the upper section, controlled water injection is carried out in the upper section goaf; after the injected water is retained and diffused in the goaf, it continues to infiltrate the lower section top coal area to be treated along the water guide channel. Under the combined effect of the upper section mining disturbance and the upper section goaf controlled water injection, a long-term water-bearing creep pre-weakening zone is formed in the lower section top coal area to be treated. During the upper section mining cycle, based on the advancing position of the upper section working face, the range of goaf formation, the degree of development of mining-induced fractures, the pre-weakening requirements of the top coal to be treated in the lower section, and the mine water hazard safety conditions, continuous or intermittent controlled water injection is carried out in the upper section goaf.

[0044] Water injection methods can include continuous water injection, intermittent water injection, staged water injection, zoned rotation water injection, or multi-channel coordinated water injection. The water injection area can be dynamically adjusted as the upper section working face advances, and localized enhanced water injection can also be carried out in key, difficult-to-break top coal areas of the lower section.

[0045] During the water injection process, the water first enters the upper section goaf, where it is temporarily stored, diffused, and redistributed. Subsequently, the water gradually infiltrates the upper section top coal area to be treated, utilizing the destructive fractures, mining fractures, interlayer weak surfaces, and primary fractures formed by the disturbance of the upper section top coal.

[0046] The purpose of this step is not to completely saturate the top coal of the lower segment, nor is it primarily aimed at high-pressure hydraulic fracturing. Rather, it is to allow the top coal area to undergo prolonged water softening, fracture development, and creep aging damage before the advanced pre-fracturing blasting of the lower segment, thereby creating favorable conditions for subsequent blasting fracture expansion.

[0047] Under the combined effects of disturbance from the upper section mining and controlled water injection into the goaf, the water injected into the upper section goaf seeps into the top coal of the lower section along the mining-induced fractures and weak surfaces between layers, and gradually forms a long-term water-bearing creep pre-weakening zone in the top coal area to be treated in the lower section.

[0048] The formation process of this long-term water-bearing creep pre-weakening zone includes: (1) The disturbance caused by the mining of the upper section causes the top coal of the lower section to form cracks and local fracture zones; (2) Water injection into the goaf area allows water to enter the upper section of the goaf area and form a water storage or stagnant state; (3) Water seeps downward along the top coal mining fractures, weak interlayer surfaces and primary fractures in the upper part of the lower section; (4) When water enters the top coal of the lower section to be treated, it causes the coal body to be wetted, softened and weakened in structure; (5) Under the combined influence of long-term water-coal-rock interaction, original rock stress and mining stress, the coal body undergoes creep aging damage; (6) The internal fractures of the top coal further expand, connect, and develop; (7) The structural integrity of the difficult-to-break top coal in the middle of the lower section is reduced; (8) Before blasting, the coal body forms a pre-weakened structure that is conducive to the propagation of fractures.

[0049] The long-term moisture-bearing creep pre-weakening zone is not the final coal breaking zone, but rather the basis for blasting efficiency enhancement in subsequent sub-segment advanced pre-splitting blasting.

[0050] S5, before implementing advanced pre-splitting and crushing in the lower segment, monitor the pre-weakening state and safety state of the top coal area to be treated in the lower segment, and proceed to S6 after determining that the conditions for advanced pre-splitting operation are met. Before implementing advanced pre-splitting blasting in the next segment, the pre-weakening state of the top coal area to be treated is monitored and assessed.

[0051] The monitoring and judgment content includes, but is not limited to: (1) Moisture content of the top coal in the lower section; (2) The degree of development of fractures in the top coal; (3) Changes in the acoustic wave velocity of top coal; (4) Images of the fracture seen through borehole drilling; (5) Pore water pressure state; (6) Water inrush situation in the lower section roadway; (7) Deformation and stability of surrounding rock; (8) Changes in the integrity of the top coal structure; (9) Safety conditions for water accumulation and drainage in the upper section of the goaf; (10) Changes in the water-conducting range of mining-induced fractures; (11) Characteristics of creep deformation or aging damage of top coal.

[0052] When the top coal area to be treated exhibits water softening, enhanced fracture development, reduced structural integrity, and accumulated damage over time, and there is no obvious risk of water inrush, water seepage, mud formation, roadway water accumulation, or surrounding rock instability, it is determined that it meets the conditions for advanced pre-splitting blasting in the next stage.

[0053] If the pre-weakening is insufficient, water injection can continue through the goaf, or the water-bearing creep aging process can be extended. If excessive water content, abnormal water inflow, abnormal surrounding rock deformation, or other safety risks occur, water injection should be suspended or the water injection method adjusted, and drainage, pressure relief, sealing, or safety monitoring measures should be taken.

[0054] S6. Based on the location, range, fracture development status, coal softening degree, creep damage degree, water-conducting range of mining fractures and target crushing effect of the top coal area to be treated in the lower segment, optimize the advanced pre-splitting crushing scheme of the lower segment, and implement advanced pre-splitting blasting according to the optimized scheme. After the top coal of the lower segment reaches the pre-weakening requirement, the advanced pre-fracture scheme of the lower segment is optimized according to the location, range, fracture development status, coal softening degree, creep damage degree, water-conducting range of mining fractures and target fracturing effect of the top coal area to be treated.

[0055] The following example uses explosive breaking: Blasting holes should be preferentially arranged in the middle of the lower segment, in areas where fractures are difficult to break, in areas with insufficient fracture development, in the boundary area of ​​the pre-weakening zone, or in local areas where stronger breaking is required.

[0056] The arrangement of blasting holes should be coordinated with the following factors: (1) Location of the top coal area to be processed in the lower segment; (2) Spatial relationship between the upper segment goaf and the lower segment top coal; (3) Water-conducting fracture zone formed by the disturbance of the upper segment mining; (4) Distribution of pre-weakened fractures formed by long-term water injection; (5) Damage and weakening zone formed by water-containing creep; (6) Coal seam dip and dip angle; (7) Direction of primary fractures, mining-induced fractures and weak interlayer surfaces; (8) Subsequent coal release space; (9) Safety requirements for working face.

[0057] The focus of this invention is not on limiting specific types of explosives, charge amounts, borehole spacing, or blasting parameters, but rather on improving the coal body structure conditions before the advanced pre-splitting blasting in the lower segment by utilizing the water-conducting conditions of the fractures formed by the disturbance of the upper segment mining, the long-term water injection effect in the goaf, and the pre-weakening state formed by the water-bearing creep damage of the top coal, and thereby optimizing the blasting range and the blasting fracture propagation path.

[0058] According to the optimized blasting plan, advanced pre-splitting blasting was carried out on the lower section of the top coal area to be treated, which had been pre-weakened by water injection and creep in the long-term goaf.

[0059] When blasting occurs, the blasting stress wave and blasting gas act on the coal body that has already undergone water softening, fracture development and creep damage, causing the blasting fractures to extend and connect along water-injection weakened fractures, creep-damaged fractures, mining-induced fractures, primary fractures, bedding planes and weak interlayer planes.

[0060] Because the coal body has already undergone fracturing caused by upper segment mining, long-term pre-weakening by water injection in the goaf, and water-bearing creep damage before blasting, the blasting fractures are no longer limited to the vicinity of the blast holes, but are more likely to extend along the pre-weakening fracture field, so that a more continuous fracture network is formed between blast holes, between different fracture systems, and inside the difficult-to-crush area in the middle, thereby improving the overall pre-fracture and crushing effect of thick top coal.

[0061] After blasting, the expansion of top coal fissures, the size of broken pieces, large residual pieces, local overhang, stability of the surrounding rock in the roadway, and the risk of water hazards are inspected. For areas with insufficient local breaking, supplementary water injection, extended aging time, localized supplementary blasting, or other auxiliary coal breaking measures can be taken based on the inspection results.

[0062] S7. After the lower segment advanced pre-splitting blasting is completed and safety is confirmed, the water injection process and pre-splitting parameters of the subsequent mining area are dynamically adjusted based on the coal release effect and on-site feedback.

[0063] After the pre-splitting blasting of the lower segment is completed and safety is confirmed, normal mining and top coal caving operations will be carried out in the lower segment working face.

[0064] During the coal mining process, the timeliness of top coal collapse, the continuity of coal release, the size of coal blocks, the blockage of the coal release port, the mixing of coal and gangue, and the residual top coal are observed and evaluated.

[0065] Based on the coal release effect and on-site feedback, adjustments were made to the water injection location, injection method, water injection duration, water conduction range assessment of mining-induced fractures, creep aging weakening process, layout of pre-splitting blasting holes, and blasting range in subsequent areas. This resulted in a continuously optimized process of "upper segment mining disturbance and fracture creation—controlled water injection and storage in the goaf—water seepage along mining-induced fractures—long-term water-bearing creep pre-weakening of the lower segment top coal—pre-splitting blasting—coal release and recovery."

[0066] The water injection location in this invention is not limited to the upper part of the upper segment goaf, but can also be the upper segment transport roadway, return air roadway, connecting roadway, goaf sidewall, adjacent drilling site, surface borehole, overlying roadway, or other location that can form a hydraulic connection with the upper segment goaf.

[0067] The water injection channel in this invention is not limited to ordinary boreholes, but can also be directional boreholes, water guide holes, water injection pipelines, goaf connecting fissures, zoned water guide channels, or other channels that can guide water into the upper segment of the goaf.

[0068] The water injection method in this invention is not limited to continuous water injection, but can also be intermittent water injection, staged water injection, zoned rotation water injection, multi-channel synchronous water injection, or a controlled water injection method adjusted according to on-site monitoring feedback.

[0069] The water injection medium in this invention is not limited to clean water, but can also be mine treatment water, aqueous solution containing wetting agent, low concentration inorganic salt solution, or other liquid media that meet the requirements of coal mine safety, environmental protection and equipment adaptability.

[0070] The method for judging the pre-weakening effect in this invention is not limited to moisture content monitoring. It can also be achieved by pore water pressure, acoustic wave velocity, borehole inspection, borehole coring, ground-penetrating radar, acoustic CT, resistivity detection, microseismic monitoring, surrounding rock deformation monitoring, water inflow monitoring, creep deformation monitoring, or coal discharge effect feedback.

[0071] The blasting method in this invention is not limited to conventional pre-splitting blasting, but can also be deep-hole pre-splitting blasting, segmented pre-splitting blasting, directional pre-splitting blasting, local supplementary blasting, or other blasting methods suitable for top coal crushing in coal mines.

[0072] This invention is not only applicable to horizontal segmented longwall top coal caving mining of steeply inclined extra-thick coal seams, but also to other segmented mining or top coal caving mining conditions where there are thick top coal seams, difficult-to-break coal bodies in the middle, and poor simple blasting effects.

[0073] Compared with the prior art, this application differs as follows: 1. Difference from simple advanced pre-splitting blasting Simple pre-splitting blasting is carried out in the original or near-original state of the coal body, and the blasting fractures are easily confined to the vicinity of the blast holes. This invention first utilizes the water-conducting conditions of the fractures formed by the disturbance of the upper segment mining and the pre-weakening effect of long-term water injection creep in the goaf to form a pre-weakened fracture field in the top coal of the lower segment. If the subsequent top coal fracturing state is not good, blasting is then carried out, allowing the blasting fractures to expand and penetrate along the pre-weakened fractures, thereby enhancing the top coal fracturing effect.

[0074] 2. Difference from short-term water injection before blasting

[0075] Pre-blasting water injection typically has a limited duration, making it difficult for the water to fully penetrate the thick top coal seam and induce significant creep damage. This invention involves injecting water into the goaf during the upper section's mining process, utilizing the longer mining cycle of the upper section to create a long-term water-bearing creep weakening process. This allows the water more time to diffuse, soften, and accumulate damage before the lower section's blasting.

[0076] 3. Difference from simple water softening

[0077] Simple water injection softening aims to directly improve the top coal caving resistance through the action of water and mining. This invention, however, uses water injection in the goaf and creep pre-weakening as pre-effect enhancement measures for advanced pre-splitting blasting, ultimately achieving further fracturing and breaking of the thick top coal through blasting.

[0078] 4. Difference from water injection within the working face

[0079] Water injection within the working face typically involves injecting water into the top coal from the lower section mining space, which is significantly affected by the continuity of mining operations. This invention utilizes the goaf formed during the upper section mining as a water injection and storage space, and uses the top coal fractures in the upper part of the lower section formed by the disturbance of the upper section mining as a water guiding channel, thus completing long-term creep pre-weakening before the lower section mining and blasting.

[0080] 5. Differences from high-pressure hydraulic fracturing

[0081] High-pressure hydraulic fracturing primarily aims to split the coal seam with high-pressure water. This invention uses controlled water injection in the goaf, water conduction in mining-induced fractures, water softening, and long-term creep damage as the main pretreatment methods, without requiring high-pressure hydraulic fracturing as a necessary condition.

[0082] The upper segment goaf refers to the goaf area located above the current segment to be mined during the horizontal segmented mining of steeply inclined coal seams, which gradually forms as the upper segment is mined. In this invention, the upper segment goaf mainly serves as a water injection space and a water storage space; the mining-induced fracture water guiding channel refers to the destructive fractures, mining-induced fractures, structural weak surfaces, and locally fractured areas formed in the upper top coal and inter-layer coal and rock mass of the lower segment under the disturbance of the upper segment mining. This channel is the main path for water to seep from the upper segment goaf into the lower segment top coal area to be treated; long-term water injection creep pre-weakening: refers to the continuous or phased injection of water into the upper segment goaf during a long mining cycle of the upper segment, so that the water seeps into the lower segment top coal through the water-conducting channel of mining-induced fractures, and under the combined action of long-term water-coal-rock interaction, original rock stress and mining-induced stress, the water-bearing softening, fracture development, structural weakening and creep-induced damage accumulation of the top coal are caused; lower segment top coal area to be treated: refers to the top coal area that needs to be improved by water injection creep pre-weakening and advanced pre-splitting blasting before the lower segment mining, with priority given to the difficult-to-break area in the middle of the lower segment; advanced pre-splitting crushing: refers to the pre-crushing operation carried out on the top coal area to be treated before the formal mining or coal release of the lower segment, which is used to form fractures, reduce the integrity of the top coal and improve the effect of subsequent caving and release.

[0083] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. The methods disclosed in the embodiments are described simply because they correspond to the methods disclosed in the embodiments; relevant parts can be found in the method section.

[0084] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for pre-fracture of the top coal of steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, characterized in that, include: S1. Based on the coal seam occurrence state, segmented mining layout, location of upper segment working face, location of lower segment working face, range of upper segment goaf formation, range of mining disturbance impact, and difficulty of top coal crushing, determine the area of ​​top coal to be processed in the lower segment, and at the same time identify the area of ​​fracture water-conducting channels formed by the upper segment mining disturbance in the upper top coal of the lower segment. S2, as the upper section working face advances in mining, the upper section goaf is gradually formed behind the working face; the mining stress disturbance of the upper section damages the upper top coal and inter-layer coal and rock mass of the lower section, forming fissures, weak surfaces, local fractured areas and seepage channels, which constitute water guiding channels for water to seep into the deeper parts of the lower section; S3, using the upper segment goaf as the water injection space and water storage space, construct a water injection system that is hydraulically connected to the upper segment goaf; S4, during the complete mining cycle of the upper section, controlled water injection is carried out in the upper section goaf; After the injected water is retained and diffused in the goaf, it continues to infiltrate the top coal areas to be treated in the lower segments along the water guide channel. Under the combined effect of the disturbance of the upper segment mining and the controlled water injection in the upper segment goaf, a long-term water-bearing creep pre-weakening zone is formed in the top coal areas to be treated in the lower segments. S5, before implementing advanced pre-splitting in the lower segment, monitor the pre-weakening and safety status of the top coal area to be treated in the lower segment, and proceed to S6 after determining that the conditions for advanced pre-splitting operation are met. S6. Based on the location, range, fracture development status, coal softening degree, creep damage degree, water-conducting range of mining fractures and target crushing effect of the top coal area to be treated in the lower segment, optimize the advanced pre-splitting crushing scheme of the lower segment, and implement advanced pre-splitting blasting according to the optimized scheme. S7. After the lower segment advanced pre-splitting blasting is completed and safety is confirmed, the water injection process and pre-splitting parameters of the subsequent mining area are dynamically adjusted based on the coal release effect and on-site feedback.

2. The method for pre-fracture of top coal in steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, as described in claim 1, is characterized in that... The lower segment of the untreated top coal area and the fracture water-conducting channel area are determined by at least one of the following methods: borehole inspection, ground-penetrating radar, acoustic detection, on-site exposure, top coal block size statistics, working face mine pressure manifestation, mining-induced fracture observation, or numerical simulation.

3. The method for pre-fracture of top coal in steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, as described in claim 1, is characterized in that... The water injection system includes at least one of the following: water injection borehole, directional water guide hole, water injection pipeline, control valve, flow control device, pressure monitoring device, and sealing structure.

4. The method for pre-fracture of top coal in steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, as described in claim 1, is characterized in that... The construction location of the water injection system is selected from at least one of the following: upper section roadway, sidewall of goaf, adjacent drilling site, overlying roadway, and surface borehole.

5. A method for pre-fracture of top coal in steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, as described in claim 1, is characterized in that... The water injection system is designed to meet the following requirements: it can guide water into the upper segment goaf or its collapse fracture zone, and create conditions for water storage, retention and diffusion within the goaf; it can enable water to move along the mining-induced fractures and interlayer weak surfaces of the upper top coal of the lower segment to the area of ​​the top coal to be treated; it does not affect the normal mining, ventilation, transportation, drainage and safe production systems of the upper segment working face; and it can adjust the water injection parameters according to the on-site water inflow, drainage and surrounding rock stability.

6. A method for pre-fracture of top coal in steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, as described in claim 1, is characterized in that... The formation process of the long-term water-bearing creep pre-weakening zone is as follows: 1) The disturbance caused by the mining of the upper section causes cracks and localized fracture zones in the upper top coal of the lower section; 2) After water is injected into the upper section of the goaf, it forms a water storage and retention state; 3) Water seeps and diffuses downwards along mining-induced fractures, weak interlayer surfaces, and primary fractures into the top coal to be treated in segments; 4) After absorbing water, the coal body becomes wetted and softened, and its structural strength is initially reduced; 5) Under the long-term interaction of water and coal rock, and the coupled effect of original rock stress and mining stress, the coal body undergoes creep aging damage; 6) The internal fissures of the top coal continue to expand, connect and develop, and the structural integrity of the difficult-to-break top coal in the middle gradually decreases.

7. A method for pre-fracture of top coal in steeply inclined extra-thick coal seams based on long-term water injection creep pre-weakening in goaf areas, as described in claim 1, is characterized in that... In S6, the arrangement of blasting holes needs to be matched and coordinated with the location of the top coal area to be treated in the lower segment, the spatial relationship between the upper segment goaf and the lower segment top coal, the distribution of the water-conducting zone of mining-induced fractures, the distribution of pre-weakened fractures formed by long-term water injection, the range of the water-bearing creep damage weakened zone, the dip and dip angle of the coal seam, the direction of the primary fractures and inter-layer weak surfaces, the space for subsequent mining and coal release, and the safety production requirements of the working face.

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