Key stratum pre-splitting and risk prevention and control method for face anti-collision under thick and hard rock stratum

Abstract

The application relates to the technical field of coal mine rock burst prevention and control, and discloses a key layer presplitting and risk prevention and control method for rock burst prevention of a working face under a thick hard rock stratum, which comprises the following steps: obtaining physical and geological parameters of a target working face, performing roof structure level division, and measuring and calculating basic breaking step distances before each level is presplit; a parameter mapping mechanism is constructed to calculate target breaking step distances after presplitting, and differential targeted hydraulic presplitting is performed; a parameter solution is used to calculate a spatial activity boundary of overburden strata of a multi-connection working face and perform abnormal deviation correction, and a corrected safety boundary and a comprehensive risk assessment result are output; a rock burst prevention key area is delimited based on the above step distances, boundary and result, and engineering control instructions are issued. Through the establishment of a physical parameter mapping mechanism and a mining superposition effect evaluation model, the rock burst prevention parameter is closed from theoretical space boundary calculation to field entity construction intervention, which helps to solve the boundary calculation deviation problem caused by complex structures and realizes targeted control of thick hard rock stratum breaking.

Description

Technical Field

[0001] This invention relates to the field of coal mine rockburst prevention and control technology, specifically to a method for pre-fracture and risk control of key layers in working faces under thick, hard rock strata to prevent rockburst. Background Technology

[0002] Coal mine rock bursts are dynamic damage phenomena caused by the instantaneous release of elastic deformation energy of the coal and rock mass surrounding the mine face and tunnels during coal mining. When mining under thick, hard rock strata, the thick, hard rock strata can have a large area of ​​suspended roof and accumulate a large amount of elastic energy. Currently, the key layer pre-splitting and risk control method refers to the engineering technology of preventing dynamic disasters by drilling into the thick, hard rock strata and injecting high-pressure fluid to change the original structural characteristics of the rock, shorten the natural fracture step of the thick, hard rock strata, and combining regional risk assessment methods to plan on-site pressure relief support engineering.

[0003] Existing face rockburst prevention technologies rely on engineering experience to set rockburst prevention parameters and plan construction. Based on comprehensive lithological data provided in the mine geological exploration report and combined with conventional mine pressure observation results, engineering technicians formulate unified roof hydraulic fracturing schemes and coal wall physical decompression schemes in the early stages of coal mining. When determining the specifications of fracturing boreholes and pumping fluid pressure parameters, existing face rockburst prevention technologies use fixed empirical values ​​covering the entire construction area. When assessing mining risk areas, existing face rockburst prevention technologies use microseismic energy data and support pressure distribution range recorded during historical mining to delineate dangerous construction boundaries and issue corresponding roadway reinforcement and coal body decompression instructions to the mine operation site.

[0004] Because existing face erosion prevention technologies fail to consider the multi-level physical characteristics of the overburden space and the dynamic evolution of stress during deep multi-face mining, there is a mismatch between the erosion prevention engineering control commands and the actual stress state of the rock strata. Ignoring the internal physical and mechanical differences of the overburden makes it difficult for single pre-splitting parameters to weaken the thick, hard rock strata at different spatial levels, causing the main key layer to deviate from the expected step distance and collapse. In deep multi-face mining scenarios, existing face erosion prevention technologies fail to mechanically superimpose the tectonic stress amplification effect caused by geological structures with the number of square cuts during multi-face mining, resulting in calculation deviations in the calculated overburden space activity boundary. These calculation deviations prevent the mine dispatch center from defining key energy accumulation areas, leading to errors in the delineation of key erosion prevention areas. This can cause engineering control measures such as borehole decompression and coal pillar retention to lose their effectiveness, resulting in large-scale roof collapses and strong dynamic load rockbursts in multi-face mining. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for pre-fracture of key layers and risk control in working faces under thick and hard rock strata. It aims to solve the problems of existing anti-scour technologies, which lack multi-level differentiated pre-fracture of overburden and assessment of the superimposed effects of mining in multiple working faces, resulting in deviations in the calculation of overburden activity boundaries and inaccurate delineation of key anti-scour areas, thus making engineering control commands lose their specificity.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for pre-fracture and risk control of key layers in working faces under thick, hard rock strata to prevent erosion, comprising the following steps: Obtain the physical and geological parameters of the target working face, perform the top plate structure layer division based on the physical and geological parameters, and calculate the pre-splitting foundation failure step distance corresponding to each layer. Based on the pre-fracture breakage step distance and physical geological parameters corresponding to each division level, a physical parameter mapping mechanism is constructed to calculate the target breakage step distance after pre-fracture for each division level, and differentiated targeted hydraulic pre-fracture operations are performed for each division level. The active boundary of the overburden space is calculated using physical geological parameters, and geological structural anomaly deviation correction is performed on the active boundary of the overburden space. The corrected safe active boundary and comprehensive risk assessment results are output. Based on the pre-fracture target fracture step distance, the corrected safe activity boundary, and the comprehensive risk assessment results corresponding to the main critical layer, spatial mapping and association are performed to delineate key anti-scraping areas, and corresponding engineering control instructions are issued to key anti-scraping areas.

[0008] Furthermore, based on physical and geological parameters, the top slab structure is divided into hierarchical levels, and the pre-fracture foundation failure step distance corresponding to each level is calculated, specifically including: Physical geological parameters include geological exploration data and structurally proven data; Based on the depth, lithology and thickness characteristics of the strata, the roof above the coal seam is divided into three independent physical levels from bottom to top: the immediate roof, the subcritical stratum and the main critical stratum, which serve as the dividing levels. This method distinguishes the differences in mechanical properties within the overburden through a three-level spatial division pattern, providing a target area basis for differentiated pre-splitting operations; it extracts the thickness, tensile strength, and control stratum load of each division level; and it inputs the thickness, tensile strength, and control stratum load into a beam fracture mathematical model with fixed supports at both ends to derive the pre-splitting foundation failure step distance corresponding to each division level. The process of deriving the pre-fracture fracture step distance of the foundation corresponding to each division level also includes: Based on the extracted structural exploration data, the fracture angle of each graded rock mass is determined; using the boundary of the mining coal body at the working face as the spatial origin, a spatial projection ray is drawn into the overburden along the determined fracture angle. Combined with the design width of the mining face, the maximum vertical height of the overburden damage wave after the working face is mined is calculated, which is used to obtain the basic fracture height benchmark data for each graded rock mass.

[0009] Furthermore, based on the pre-fracture fracture step distance and physical geological parameters corresponding to each division level, a physical parameter mapping mechanism is constructed to calculate the specific post-fracture target fracture step distance corresponding to each division level, including: The elastic modulus of the rock mass and the degree of development of natural fractures corresponding to each division level in the physical and geological parameters are collected and measured. A comprehensive physical weight mapping relationship is established in combination with the tensile strength in the physical and geological parameters, and the differential pre-splitting influence coefficient corresponding to each division level is calculated and output. The mapping mechanism of the pre-splitting influence coefficient is inversely proportional to the elastic modulus and tensile strength of the rock mass, and directly proportional to the degree of development of natural fractures. It is used to reflect the actual engineering requirements of rock strata for artificial intervention. The pre-fracture breakage step distance of the foundation corresponding to each division level is combined with the differential pre-fracture influence coefficient corresponding to each division level and substituted into the reduction correction equation for correction, and the target fracture breakage step distance after pre-fracture corresponding to each division level is calculated and determined.

[0010] Furthermore, the differentiated targeted hydraulic pre-fracturing operations for each tier are specifically as follows: The pre-fracture target fracture step distance corresponding to each division level is used as a rigid constraint condition, and the working space depth and target physical parameters are sent to the downhole hydraulic fracturing system. Based on the stratigraphic characteristics, low-pressure, low-flow hydraulic fracturing operations are performed on the immediate top, medium-pressure hydraulic fracturing operations are performed on the subcritical strata, and high-pressure, high-flow penetrating hydraulic fracturing operations are performed on the main critical strata. The stepwise configuration of fracturing parameters can reduce the overall integrity of the roof at different levels, providing physical conditions for controlling the orderly collapse of the overlying strata. The standard for setting the output pump pressure of hydraulic fracturing equipment is: the fluid pressure is greater than the sum of the original rock stress of the target rock layer and the tensile strength of the target rock layer itself.

[0011] Furthermore, the calculation of the overburden space activity boundary for multiple consecutive working faces using physical geological parameters specifically includes: Extract the strike length, dip width, total overburden thickness, and fracture angle of the working face from the aforementioned physical and geological parameters; The square theory is introduced to calculate the basic geometric boundary of the overburden activity characteristics of the working face, and the activity range of the foundation strike and the activity range of the foundation dip are obtained respectively, and the two are used together as the spatial activity boundary of the overburden. Multiply the length of the working face by the number of square cuts made during the working face mining to obtain the basic strike range. Calculate the tangent of the fracture angle, multiply the total thickness of the overburden by the tangent, and add twice the product to the dip width of the working face to obtain the range of basic dip movement.

[0012] Furthermore, geological structural anomaly deviation correction is performed on the overlying strata spatial activity boundary, and the corrected safe activity boundary specifically includes: Extract the three-dimensional spatial coordinates of geological anomalies around the working face and project them vertically onto the horizontal plane to form a two-dimensional coordinate set; Compare the two-dimensional coordinate set with the two-dimensional topological boundary formed by the basic orientation activity range and the basic dip activity range; When the horizontal coordinates of a geological anomaly fall within the two-dimensional topological boundary, a structural stress amplification factor is introduced and the movement range of the foundation strike and the movement range of the foundation dip are respectively producted for correction, and the corrected safe movement boundary that conforms to the actual stress state of the project is output. This step quantifies the amplification effect of the geological anomaly on the local stress field and solves the calculation deviation problem caused by the geological structure leading to abnormal deviation of the overburden boundary.

[0013] Furthermore, the output of the comprehensive risk assessment results specifically includes: The number of times the working face is mined into squares is used as an iterative variable. The evolution state of the overburden fracture height from the immediate roof, sub-key layer to the main key layer is determined by superimposing the results, and a superposition effect evaluation model is established. The critical stress threshold for coal seam impact and the set threshold for total energy accumulation are obtained. The critical stress threshold for coal seam impact is obtained by performing uniaxial compression and acoustic emission tests on raw coal samples in the laboratory. The set threshold for total energy accumulation is determined based on the statistical regularity of historical rockburst microseismic monitoring data in the mining area. When any of the following conditions are met, the comprehensive risk assessment result of the current successive working face is determined to be a high-risk state: The stress concentration coefficient of the coal body after the superposition of multiple working faces exceeds the critical stress threshold of the coal seam impact at the working face; the calculated total energy accumulation reaches the set threshold of total energy accumulation.

[0014] Furthermore, based on the pre-fracture target fracture step distance, the corrected safe activity boundary, and the comprehensive risk assessment results corresponding to the main critical layer, spatial mapping and correlation are performed to delineate key areas for shock prevention, specifically including: The pre-fracture target fracture step distance corresponding to the main critical layer is used as the basic cycle step distance. The first preset safety distance is extended forward and backward along the working face advancement direction to delineate the fracture influence zone of the main critical layer. The first preset safety distance is determined based on the range of the stress leading to the working face cycle. The revised safety activity boundary is spatially topologically compared with the historical goaf location, the overlapping area is extracted and extended to the solid coal side by a second preset safety distance, and the goaf connection boundary area is delineated. The second preset safety distance is determined based on the influence range of the lateral support pressure in the goaf; The concentration range of the peak value of the pre-stress in the working face opening, the final mining line and the return airway is defined as the key parts of the roadway; Extract the spatial coordinates of surface buildings, combine them with the rock strata movement angle, and project the spatial coordinates of surface buildings onto the underground to form a spatial envelope surface. Delineate the geometric intersection area between the spatial envelope surface and the coal seam plane as the protective boundary area of ​​surface buildings, and obtain the geometric intersection width of the envelope surface.

[0015] Furthermore, issuing corresponding engineering control instructions to key anti-erosion areas specifically includes: Based on the comprehensive risk assessment results, for the main critical layer fracture impact area, a large-diameter borehole decompression instruction was issued for the coal body, and large-diameter boreholes were periodically drilled to release the coal's accumulated elastic energy. For the boundary area connecting the goaf, the construction instructions for the restricted small coal pillar and the pressure relief groove are issued. The size design of the isolation coal pillar is adjusted to that of the restricted small coal pillar, and the pressure relief groove with a depth greater than the plastic failure zone of the surrounding rock of the roadway is constructed in the restricted small coal pillar area to force the coal pillar to yield as a whole and cut off the stress transmission path. For key sections of the roadway, instructions were issued to increase the density of advanced joint support and intensive pressure relief. For the protection boundary zone of surface buildings, an instruction to extend the boundary protection coal pillar is issued, extending the width of the conventional protection coal pillar of the final mining line to a value greater than the geometric intersection width of the envelope surface.

[0016] This invention provides a method for pre-fracture of key layers and risk control in working faces under thick, hard rock strata to prevent erosion. It has the following beneficial effects: 1. This invention performs top structural layering by acquiring physical geological parameters and constructs a physical parameter mapping mechanism to calculate the differentiated pre-fracture influence coefficients corresponding to each layer. The original empirical parameters are converted into control indicators that are dynamically adjusted according to the development degree and mechanical properties of natural rock fractures. Combined with the corrected target fracture step distance, differentiated hydraulic pre-fracture operations are performed on the immediate top, subcritical layer and main critical layer respectively. This helps to achieve targeted control of the fracture step distance of thick and hard rock layers and provides physical conditions for the orderly collapse of the overlying strata.

[0017] 2. This invention introduces the square theory to calculate the basic geometric boundary of overburden activity characteristics, compares the extracted two-dimensional coordinate set of geological anomalies with the geometric boundary, introduces a tectonic stress amplification factor to perform geological structural anomaly deviation correction, and establishes a superposition effect evaluation model with the number of square mining operations as the iteration variable. This quantifies the evolution of overburden spatial activity and energy superposition state under the continuous mining environment of multiple deep working faces, which helps to solve the problem of deviation in overburden boundary calculation caused by complex geological structures.

[0018] 3. This invention spatially maps and associates the target fracture step distance, the corrected safe activity boundary, and the comprehensive risk assessment results to delineate the main key layer fracture impact area, the goaf connection boundary area, the key parts of the roadway, and the surface building protection boundary area. Based on the characteristics of each area, specific engineering control instructions are issued for the construction of large-diameter boreholes for coal body pressure relief, confined small coal pillars and pressure relief trenches, densification of advanced support, and expansion of boundary protection coal pillars, thereby achieving closed-loop prevention and control from the theoretical calculation boundary of anti-scour to the on-site engineering entity construction intervention. Attached Figure Description

[0019] Figure 1 This is a system framework diagram of an embodiment of the present invention; Figure 2 This is a schematic diagram of the method flow according to an embodiment of the present invention; Figure 3 This is a comparison diagram of the fracture step distance and released energy before and after the pre-cracking of the top plate in a specific application embodiment of the present invention; Figure 4 The graph shows the stress and microseismic evolution curves before and after the implementation of the anti-shock measures in a specific application embodiment of the present invention. Detailed Implementation

[0020] The technical solutions in 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.

[0021] Reference Appendix Figure 1 This invention provides a critical layer pre-fracture and risk control system for working faces under thick, hard rock strata to prevent erosion, comprising: The data acquisition terminal is deployed in the downhole working face and surface area, specifically including drilling core equipment, geophysical logging tools, borehole stress gauges, microseismic monitoring stations, and surface subsidence monitoring equipment. The data acquisition terminal is used to acquire geological exploration data, structural exploration data, and mechanical monitoring signals from the target working face, and transmits the acquired basic data to the data processing server via industrial Ethernet. The data processing server is deployed in the mine dispatch center. The data processing server integrates a processor and a memory. The processor calls the calculation model and physical parameter mapping mechanism that are fixed in the memory, receives the basic data transmitted by the data acquisition terminal, performs the roof structure hierarchical division, foundation failure step distance calculation, safety activity boundary calculation and comprehensive risk assessment, and generates anti-scour engineering control instructions based on the assessment results. The engineering execution end is distributed at the underground operation site, specifically including the hydraulic fracturing system and the borehole decompression equipment. The engineering execution end receives anti-impact engineering control instructions issued by the data processing server and performs differentiated targeted hydraulic pre-fracturing operations and coal body physical decompression operations at the working face.

[0022] See attached document Figure 2 This invention provides a method for pre-fracture and risk control of key layers in working faces under thick and hard rock strata to prevent erosion, comprising the following steps: S1. Obtain the physical and geological parameters of the working face, perform roof structure hierarchical division, and calculate the foundation failure step distance. First, obtain the geological exploration and structural exploration data of the target working face. Based on the occurrence depth, lithology, and thickness characteristics of the rock strata, the roof above the coal seam is divided into three independent physical levels from bottom to top: the immediate roof, the subcritical stratum, and the main critical stratum. Subsequently, the rock physical and mechanical parameters and control strata loads of each level were extracted. The pre-fracture step distance of the foundation before pre-fracture was calculated by using a beam fracture model based on the extracted parameters. At the same time, the fracture angle method was used to determine the foundation fracture height of each level in combination with the on-site geological conditions.

[0023] S2: A physical parameter mapping mechanism is constructed to calculate the target fracture step distance and perform differentiated targeted hydraulic pre-fracture. Based on the tensile strength, elastic modulus, and natural fracture development degree of each rock layer obtained in S1, a preset comprehensive physical weight mapping relationship is constructed, thereby adaptively outputting the differentiated pre-fracture influence coefficient corresponding to each layer; the foundation fracture step distance before pre-fracture is combined with the extracted differentiated pre-fracture influence coefficient to correct the equation and calculate the target fracture step distance after pre-fracture for each layer; Finally, using the target fracture step distance at each level as a rigid constraint, the working space depth and target physical parameters are sent to the downhole hydraulic fracturing system. Based on the formation characteristics, differentiated targeted pre-fracturing operations with different orifice diameters, different pumping pressures and flow gradients are performed on the immediate top, subcritical layer and main critical layer respectively.

[0024] S3 calculates the spatial activity boundary of the overburden in multi-continuous working faces and performs geological structural anomaly deviation correction. For deep multi-continuous working face mining scenarios, combined with the overburden parameters obtained in S1, the square theory is introduced to calculate the basic geometric boundary of the overburden activity characteristics of the working face, and the activity range of the foundation strike and the activity range of the foundation dip are obtained respectively. Further extract the spatial coordinates of geological anomalies such as faults around the working face, and perform a two-dimensional topological comparison with the foundation range. When the geological anomaly falls into the range, introduce a tectonic stress amplification factor to multiply and correct the foundation range, and output the corrected safe activity boundary. On this basis, use the number of mining cuts as the iteration variable to successively determine the evolution state of the overburden fracture height at each level, and output a comprehensive risk assessment result that includes the superposition state of stress and energy.

[0025] S4 delineates key anti-scour areas by relating spatiotemporal calculation boundaries and issues and executes engineering control strategies. It receives the target fracture step distance after pre-fracture of the main critical layer from the S2 calculation output and the corrected safe activity boundary range from the S3 solution output, maps the above calculation parameters to the working face plane coordinate system, thereby spatially defining four key anti-scour areas: the main critical layer fracture influence area, the goaf connection boundary area, key parts of the roadway, and the surface building protection boundary area. Finally, based on the physical mechanisms of impact disasters in different regions, engineering control instructions were issued to the site for rigid large-diameter drilling for coal body decompression, construction of confined small coal pillars and decompression trenches, densification of advanced joint support, and expansion of boundary protection coal pillars, so as to achieve a reliable closed loop from theoretical calculation to on-site construction of anti-impact parameters.

[0026] See attached document Figure 2 Step S1 specifically includes the following sub-steps: S101, Obtain geological exploration and structural exploration data of the target working face, and classify the roof structure into layers based on the depth, lithology and thickness characteristics of the rock strata. Since the overlying rock strata of the mine are a complex mechanical system composed of a variety of lithologies, the rock strata at different levels have different bearing and fracture characteristics.

[0027] In this embodiment, the geological exploration and structural exploration data specifically include overburden columnar data, spatial distribution coordinates of rock strata, and geometric parameters of geological anomalies such as faults, obtained through borehole coring and geophysical logging.

[0028] Based on the data obtained above, the roof above the coal seam is divided into three independent levels from bottom to top: the immediate roof, the subcritical stratum, and the main critical stratum.

[0029] In this embodiment, the immediate roof is located within a distance of 0 to 30m above the top of the coal seam, and the lithology of the immediate roof is mainly low-strength rock layers such as fine-grained sandstone; the subcritical layer is located within a distance of 30m to 200m above the top of the coal seam, and the lithology of the subcritical layer is mainly medium-thickness rock layers such as siltstone. The main key strata are located 200m to 400m above the top of the coal seam, and the lithology of the main key strata is mainly composed of thick and hard rock layers such as conglomerate. The spatial three-level division model isolates the mechanical differences within the overlying strata, forming the basis for the target area of ​​subsequent differentiated pre-splitting operations.

[0030] S102, after completing the spatial hierarchical division, the rock physical and mechanical parameters and control stratum loads corresponding to each division level are extracted to provide basic data input for subsequent mechanical deduction. The extracted rock physical and mechanical parameters specifically include the thickness, tensile strength, and elastic modulus of the core stratum within each level. In this embodiment, the rock physical and mechanical parameters are obtained through standard laboratory rock mechanics tests.

[0031] For the specimen preparation and loading test process of laboratory standard rock mechanics tests, technical personnel in the field refer to the existing national rock mechanics testing specifications for operation. The instrument operation and data recording methods are well-known technologies in the field and will not be elaborated here.

[0032] In addition to the inherent properties of the rock itself, the controlling strata load is determined. The extraction of the controlling strata load is based on the theory of combined strata, calculating the cumulative self-weight of all weak rock layers above each stratum that exert pressure on it. This cumulative weight is used to characterize the overburden pressure borne by each stratum. From a physical mechanism perspective, hard rock layers not only bear their own weight but also the additional load from the synchronous settlement of weak rock layers above them. The cumulative process objectively reflects the stress state of the underground rock strata.

[0033] S103, combining the extracted parameters, the pre-fracture foundation failure step distance for each divided layer is calculated using a beam fracture model. In the initial stage of coal mining, the two ends of the hard rock strata, which have not yet fractured, are firmly embedded by the surrounding solid coal and rock mass. The stress state of the hard rock strata corresponds to the two-end fixed beam model in mechanics of materials. Applying the above principle to the embodiment, the thickness, tensile strength, and control load of each layer are input into the two-end fixed beam fracture mathematical model to derive the natural limit span of each layer under untreated conditions. The specific calculation formula is as follows: ; In the formula, For the first Pre-fracture step distance of the foundation before pre-fracture of the hierarchical rock strata For the first The thickness of the strata, For the first The tensile strength of the layered rock strata For the first Layer-level control of rock strata load; subscript Hierarchical variables, subscripts The value range is set to 1, 2, and 3, which respectively represent the direct top, subkey layer, and primary key layer.

[0034] The independent fracture step parameters of the direct top, subcritical layer and main critical layer are calculated and output using the above formula.

[0035] S104, the fracture angle method was used to determine the foundation fracture height of each layer based on the on-site geological conditions. After the working face was mined, the fracture range of the overlying strata expanded outwards to the outer rock mass at a specific angle, forming the mining influence boundary.

[0036] In this embodiment, the fracture angle of each rock mass is determined based on the extracted geological structural exploration data. Since the size of the fracture angle is controlled by the internal friction angle and the degree of development of natural joints within the rock mass, and combined with the comprehensive lithological data from the field exploration, the value of the fracture angle is limited to the range of 36° to 38°.

[0037] The determined fracture angle is substituted into the geometric space boundary equation. Specifically, the boundary of the coal body mined by the working face is used as the spatial origin. A spatial projection ray is drawn into the overburden along the determined fracture angle. Then, combined with the design width of the mining working face, the maximum vertical height of the overburden failure wave after the working face is mined is calculated based on the spatial geometric relationship of trigonometric functions.

[0038] The above steps are used to obtain the baseline data of the basic fracture height of the immediate top, subcritical layer and main critical layer under the influence of mining. The baseline data of the basic fracture height provides the initial judgment boundary for subsequent assessment of the superimposed state of overburden activity.

[0039] See attached document Figure 2 Step S2 specifically includes the following sub-steps: S201, based on the foundation fracture step distance and foundation geological data obtained in step S1, a physical parameter mapping mechanism is constructed to output differentiated pre-fracture influence coefficients. Simply relying on fixed engineering experience parameters can easily lead to a disconnect between construction operations and actual rock strata conditions.

[0040] In this embodiment, the elastic modulus and natural fracture development degree of the rock mass corresponding to each division level are collected and measured. Combined with the tensile strength extracted in step S1, a comprehensive physical weight mapping relationship is established. From the perspective of engineering rock mechanics principles, the pre-fracture influence coefficient reflects the degree of artificial intervention required for rock strata. The mapping mechanism of the pre-fracture influence coefficient is as follows: The pre-splitting influence coefficient is inversely proportional to the rock mass's elastic modulus and tensile strength, and directly proportional to the development degree of natural fractures, which characterizes the initial weak surface distribution. Technical personnel in the field, combining historical fracturing data from the mining area, conducted multiple regression analysis to determine the specific weight values ​​corresponding to the above physical parameters. Based on the aforementioned comprehensive physical weight mapping mechanism, the differentiated pre-splitting influence coefficients corresponding to each layer were calculated and output. Under engineering condition calibration, the mapping value corresponding to the immediate top was set to 0.4, the mapping value corresponding to the subcritical layer was set to 0.35, and the mapping value corresponding to the main critical layer was set to 0.3.

[0041] The above values ​​are set based on the increasing relationship of the strength of the corresponding rock strata. The anti-erosion physical mechanism of the numerical settings is that the higher the strength of the rock strata, the higher the degree of artificial intervention required, and the smaller the value of the pre-fracture influence coefficient of the corresponding rock strata, thereby retaining a safe and reliable margin for fracture step control.

[0042] S202, after obtaining the pre-fracture influence coefficients for each level, further calculate the target fracture distance after pre-fracture for each level. Substitute the pre-fracture fracture distance calculated in step S1 and the differentiated pre-fracture influence coefficients determined in this embodiment into the correction equation, and calculate the expected fracture length after artificial fracturing intervention by subtracting from the correction equation. The specific calculation formula is as follows: ; In the formula, For the first Target fracture step distance after pre-fracture of layered rock strata For the first Pre-fracture step distance of the foundation before pre-fracture of the hierarchical rock strata For the first Differential pre-fracture influence coefficient of strata; subscript Hierarchical variables, subscripts The value range is set to 1, 2, and 3, which respectively represent the direct top, subkey layer, and primary key layer.

[0043] S203. After clarifying the target fracture step distance as a control indicator, implementing the target fracture step distance in the downhole physical operation is the basis for achieving the anti-impact closed loop.

[0044] In this embodiment, the calculated target fracture step distance at each level is used as a rigid constraint. The working space depth and target physical parameters are sent to the downhole hydraulic fracturing system, and differentiated targeted hydraulic pre-fracturing operations are performed based on the formation characteristics. For different formations with varying rock mechanical properties and spatial distances, this embodiment employs different borehole and pumping parameters to achieve physical weakening.

[0045] During fracturing operations, the standard for setting the pumping pressure output of fracturing equipment is that the fluid pressure must be greater than the sum of the original rock stress of the target rock layer and the tensile strength of the target rock layer itself, in order to overcome the overall resistance of the rock.

[0046] For the direct top located 10m to 30m above the top of the coal seam, a drilling parameter of 90mm is used to perform low-pressure, low-flow hydraulic fracturing operations, which promotes the formation of a shallow artificial fracture network within the fine-grained sandstone layer.

[0047] For the subcritical strata located 50m to 200m above the top of the coal seam, directional drilling parameters of 110mm diameter were used to perform medium-pressure hydraulic fracturing operations, and medium pumping pressure was used to penetrate the main siltstone strata.

[0048] For the massive, key strata located 200m to 400m above the top of the coal seam, given the dense lithology and high strength of the key strata, a deep-hole fixed-point drilling parameter of 130mm diameter was adopted to carry out high-pressure, high-flow-rate penetrating hydraulic fracturing operations, using high-pressure fluid to forcibly induce penetrating macroscopic fractures in the conglomerate layer.

[0049] By utilizing the aforementioned progressively increasing fracturing parameters, the overall integrity of the roof at different levels is reduced, providing physical conditions for controlling the orderly descent of the overlying strata.

[0050] See attached document Figure 2 Step S3 specifically includes the following sub-steps: S301, for deep multi-face mining scenarios, combining the overburden parameters obtained in step S1, the square theory is introduced to calculate the basic geometric boundaries of the overburden activity characteristics of the working face. Deep multi-face mining results in a large area of ​​overhang above the coal seam, and the spatial activity range of the overlying strata expands non-linearly with the increase of the mining area. According to the square theory in mining rock mechanics, when the working face advances to an integer multiple of the working face dip width, the large-area exposure of the roof leads to a significant increase in overburden movement and mine pressure manifestation.

[0051] Based on the aforementioned mechanical evolution laws, the range of activity for the foundation strike and the range of activity for the foundation dip are calculated using the obtained geometric and geological parameters. The specific calculation formulas are as follows: ; ; In the formula, Based on the scope of activities, The length of the working face. The number of times the working face is mined to a square depth is obtained in this embodiment by dividing the actual advancing length of the working face by the dip width of the working face and then rounding down. Based on the scope of activities, For the working face inclination width, The total thickness of the overlying strata. The breaking angle.

[0052] S302, after outputting the basic geometric boundary, performs a geological structural anomaly deviation correction judgment. Geological anomalies such as faults are often accompanied by initial structural stress concentration. When the working face approaches a geological anomaly, it is easy to break the original stress balance, induce stress superposition abrupt change, and cause abnormal deviation of the overburden activity boundary.

[0053] To quantify the above-mentioned abnormal deviation effect, in this embodiment, the three-dimensional spatial coordinates of geological anomalies such as faults around the working face are extracted, and the three-dimensional spatial coordinates of the geological anomalies are projected vertically onto the horizontal plane to form a two-dimensional coordinate set.

[0054] Subsequently, the projected two-dimensional coordinate set is compared with the two-dimensional topological boundary formed by the movement range of the foundation strike and the movement range of the foundation dip. When the horizontal coordinates of the geological anomaly fall within the range of the two-dimensional topological boundary, a tectonic stress amplification factor is introduced to dynamically correct the foundation boundary range through multiplication.

[0055] In this embodiment, the value of the tectonic stress amplification factor is greater than 1. Under normal geological conditions, the value of the tectonic stress amplification factor is between 1.2 and 1.5. The specific value of the tectonic stress amplification factor is obtained by combining in-situ geostress test data with numerical simulation inversion methods, and is used to quantify the amplification effect of geological anomalies on the local stress field. The modified calculation formula is as follows: ; ; In the formula, To adjust the movement to the activity area after correction, Based on the scope of activities, This is the structural stress amplification factor. To correct the tendency of activity range, This determines the baseline tendency range of activity. Through a product correction process, a corrected safe activity boundary that conforms to the actual stress state of the engineering project is output.

[0056] S303, after completing the spatial boundary correction calculation, uses the number of times the working face is mined to determine the evolution state of the overburden fracture height from the immediate top, sub-key layer to the main key layer, and outputs a comprehensive risk assessment result that includes the superimposed state of stress and energy.

[0057] As the number of successive working faces increases, the goaf area expands continuously, and the fracture height of the overlying strata shows a step-like upward transmission trend. In this embodiment, by incorporating the stress concentration coefficient of the coal body and the continuity state of the overlying fractures left after the previous working face is mined into the evaluation system of the current successive working face, a superposition effect evaluation model is established.

[0058] To accurately define dangerous conditions, this embodiment introduces the critical stress threshold for coal seam impact and the total energy accumulation threshold as judgment criteria. The critical stress threshold for coal seam impact is obtained through joint uniaxial compression and acoustic emission tests of raw coal samples in the laboratory. The total energy accumulation threshold is determined based on the statistical regularity of historical rockburst microseismic monitoring data in the mining area.

[0059] When the stress concentration coefficient of the coal seam after multiple working faces are mined and superimposed exceeds the critical stress threshold for coal seam impact, or when the calculated total energy accumulation reaches the set threshold for total energy accumulation, the comprehensive risk assessment result of the current successive working face is determined to be a high-risk state. The comprehensive risk assessment result, which includes the superimposed stress and energy states, provides physical boundary conditions and data support for the subsequent delineation of key areas for rockburst prevention.

[0060] See attached document Figure 2 Step S4 specifically includes the following sub-steps: S401, receive the target fracture step distance after pre-fracture of the main critical layer calculated and output in step S2 and the corrected safe activity boundary range calculated and output in step S3, map the target fracture step distance after pre-fracture of the main critical layer and the corrected safe activity boundary range to the working face plane coordinate system, and spatially delineate four major anti-impact key areas.

[0061] In this embodiment, the four key anti-rock erosion areas specifically include the main critical layer fracture impact zone, the goaf connection boundary zone, key roadway sections, and the surface building protection boundary zone. The logic for delineating the key anti-rock erosion areas is as follows: Based on the target fracture step distance after the pre-fracture of the main critical layer as the basic periodic step distance, the fracture influence zone of the main critical layer is delineated by extending the first preset safety distance forward and backward along the working face advancement direction.

[0062] In order to make the distance parameters conform to the actual stress state of the project, the first preset safety distance in this embodiment is determined based on the stress advance influence range of the working face cycle. The stress advance influence range is obtained by real-time monitoring peak data of the borehole stress gauges buried on site. The value of the first preset safety distance is usually defined within 30m to 50m in front of the coal wall of the working face. The corrected safety activity boundary range is compared with the historical goaf spatial location. The overlapping area after the spatial topology comparison is extracted, and the second preset safety distance is extended from the overlapping area to the solid coal side to define the goaf connection boundary area. The second preset safety distance is determined based on the influence range of the lateral support pressure of the goaf. The value of the second preset safety distance is usually defined within 15m to 25m inside the solid coal. Based on the distribution law of advance support pressure during mining, the concentration range of advance stress peaks in the working face opening, final mining line, and return airway is defined as the key parts of the roadway. The spatial coordinates of surface structures are extracted, and the spatial coordinates of surface structures are projected onto the underground surface in combination with the rock strata movement angle. The geometric intersection area between the spatial envelope surface and the coal seam plane is defined as the protection boundary zone of surface structures. The value of the rock strata movement angle is determined in combination with the historical subsidence observation data of the mining area. Under normal geological conditions, the value of the rock strata movement angle is between 55° and 65°.

[0063] S402 After completing the geometric calibration of the above-mentioned spatial area, it is necessary to issue rigid engineering control instructions for the delineated critical layer failure impact area and the boundary area connecting the goaf, based on the disaster-causing physical mechanism corresponding to the key anti-scour area.

[0064] For the critical stratum fracture zone, due to the high energy release accompanying the instantaneous fracture of the thick, hard rock strata, this embodiment deploys microseismic monitoring stations to acquire precursory microseismic signals of rock strata fracture at a high-frequency sampling rate. Capturing high-frequency energy events generated by micro-fractures in the rock strata requires high data acquisition accuracy; therefore, the high-frequency sampling rate threshold for the microseismic monitoring stations is set to no less than 1000Hz.

[0065] While monitoring, a large-diameter borehole pressure relief instruction was issued to the site. The borehole diameter setting benchmark for pressure relief must meet the requirements of destroying the overall internal structure of the coal body. The borehole diameter value is defined in the range of 130mm to 150mm. By periodically constructing large-diameter boreholes in the main key layer fracture influence zone, the elastic energy accumulated inside the coal body is forcibly released and the high concentrated stress in front of the coal wall of the working face is transferred.

[0066] For the boundary area connecting the goaf, the lateral overhang structure is prone to multi-directional stress superposition.

[0067] In this embodiment, construction instructions for the confined small coal pillar and pressure relief trough are issued to the site. The specific physical control method is to adjust the traditional large-size isolation coal pillar to a confined small coal pillar design. The width of the confined small coal pillar is set based on being less than the horizontal distance between the peak position of the lateral support pressure in the goaf connection boundary area and the coal wall, so that the coal pillar as a whole is forced into a plastic yielding state to avoid excessive accumulation of elastic energy.

[0068] In addition to reducing the size of the coal pillar, pressure relief trenches of a specific depth are constructed in confined small coal pillar areas. The specific depth of the pressure relief trench is set to be greater than the depth of the plastic failure zone of the surrounding rock of the roadway, usually set to 1.5m to 2.5m. The physical fracture surface of the pressure relief trench is used to cut off the transmission path of horizontal stress between the coal seams.

[0069] S403, the anti-scour engineering system not only requires pressure relief at the source, but also reinforcement of the load-bearing channel. For key parts of the roadway and the protection boundary zone of surface buildings, instructions for advanced support densification and protection boundary expansion are issued to complete the closed loop from theoretical calculation of anti-scour parameters to on-site engineering construction.

[0070] Key sections of the roadway are most severely affected by pre-mining stress. In this embodiment, instructions for intensified and densely depressurized pre-support were issued. The pre-support structure comprises a combination of high-strength anchor cables and I-beams. By shortening the spacing between anchor cables and I-beams, the support resistance of the roadway surface against large deformations is increased. Combined with dense, large-diameter depressurization boreholes covering the entire cross-section of the roadway, the mechanical coordination of deep depressurization and shallow strong support within the roadway surrounding rock is achieved. For the protection boundary zone of surface buildings, the core protection objective is to prevent the strong dynamic loads generated by the failure of the main critical layer from causing structural damage to surface buildings.

[0071] In this embodiment, the width of the conventional protective coal pillar at the final mining line is extended beyond the geometric intersection width of the envelope surface defined in step S401. This wide-band coal pillar blocks the propagation path of strong dynamic loads towards surface structures, and surface settlement and deformation monitoring equipment acquires surface deformation data in real time. Step S4 converts all theoretical parameters derived from steps S1 to S3 into executable physical operation boundaries, achieving closed-loop risk control in the context of multi-face mining of thick, hard rock strata.

[0072] Specific application examples: This embodiment takes the first mining face with a thick and hard roof in a deep, high-stress mining area and the subsequent mining face facing a serious risk of rock bursts as the application objects, to demonstrate the targeted pre-fracture process and rock burst prevention effect verification of thick and hard rock strata in multiple working faces.

[0073] Step 1: Obtain geological parameters and calculate the core fracture step distance.

[0074] First, the physical and geological parameters of the working face to be optimized were obtained. The roof of the first mining face contains a thick, hard, composite key stratum, located 200m to 400m from the coal seam. The lithology of the key stratum is conglomerate as its core, with an average thickness of 70m and an average tensile strength of 5.5MPa. The subsequent working face is the third mining face in the panel, with an average mining depth of 950m. Large areas of goaf exist in the surrounding area, exhibiting a strong tendency for impact.

[0075] The roof strata were divided based on structural characteristics. The immediate roof is located within 30m of the top of the coal seam, with fine-grained sandstone as the main lithology, a thickness of 4.44m, and a tensile strength of 0.67MPa. The subcritical strata are located between 30m and 200m of the top of the coal seam, with siltstone as the main lithology, a thickness of 10m, and a tensile strength of 1.42MPa. The main critical strata are the thick conglomerate layers located between 200m and 400m. A control stratum load of 25MPa was set, and a beam-type fracture mathematical model with both ends fixed was adopted based on the obtained parameters of each stratum. Theoretical deduction was performed. Calculations showed that the foundation failure steps before pre-splitting were approximately 17m for the direct top layer, 70m for the subcritical layer, and 220m for the main critical layer. Further spatial geometric projection calculations were performed using a 37° failure angle at the site, and the foundation failure heights of the direct top layer, subcritical layer, and main critical layer were determined to be 30m, 200m, and 400m, respectively.

[0076] Step 2: Set the pre-splitting influence coefficient and perform differentiated targeted pre-splitting.

[0077] Based on the rock mass mechanical properties of each key layer, differentiated pre-splitting influence coefficients were set. In this embodiment, the pre-splitting influence coefficient corresponding to the immediate roof was set to 0.4, the sub-key layer to 0.35, and the main key layer to 0.3. This was based on the reduction and correction equation. The calculated target fracture steps after pre-splitting were 10.2m for the direct top, 45.5m for the subcritical layer, and 154m for the main critical layer.

[0078] The constraint parameters are sent to the downhole hydraulic fracturing system to perform differentiated, targeted fracturing. For the immediate roof, the system arranges fracturing boreholes with a spacing of 8m and a diameter of 90mm within a range of 10m to 30m from the top of the coal seam, and performs low-pressure, low-flow fracturing to create a shallow fracture network.

[0079] For the subcritical strata, the system deploys directional boreholes with a spacing of 15m and a diameter of 110mm in the range of 50m to 200m to perform medium-pressure fracturing to penetrate the main siltstone strata. For the main critical strata, the system deploys deep-hole fixed-point boreholes with a spacing of 20m and a diameter of 130mm in the conglomerate strata from 200m to 400m to forcibly generate penetrating macroscopic fractures through high pressure and high flow rate, reducing the overall integrity of the thick conglomerate strata by more than 30%.

[0080] Step 3: Quantification of overburden activity characteristics and risk superposition assessment of multiple working faces.

[0081] For the working face that serves as the third mining area, the strike length is input as 2160m and the dip width as 154m. The strike range calculation formula is based on the square theory. The number of times the working face was mined With a value of 3, the calculated range of the working face is as high as 6480m.

[0082] A comprehensive assessment was made based on the superimposed evolution of multiple working faces. After the first working face was mined, only the roof fractured directly, resulting in a fracture height of 60m. After the second working face was mined, the subcritical stratum experienced local fracture, with the fracture height extending to 90m. When the goaf of the subsequent working face formed a three-sided effect, the overburden fracture height surged to 135m, and the main critical stratum subsequently began to fracture violently.

[0083] Superimposed effect analysis shows that the stress concentration factor of the coal seam remained between 1.3 and 1.5 after the first two working faces were mined. However, after the subsequent working face was mined, the stress concentration factor of the coal seam soared to over 2.0, directly exceeding the critical stress threshold of 18 MPa for coal seam impact. Simultaneously, the total cumulative energy from the three working faces reached 3.8 × 10⁻⁶. 7 J. The system determines, based on its parameters, that it is currently in a high-impact-risk state.

[0084] Step 4: Delineation of key areas and implementation of engineering anti-erosion instructions.

[0085] Based on spatial boundaries and fracture distance, four key anti-scour zones were clearly defined and control instructions were issued. For the fracture impact zone of the main critical layer, the fracture impact zone was defined as the 50m range before and after the working face advanced to 220m. The system increased the microseismic monitoring frequency in the fracture impact zone to once every 5 seconds and instructed the drilling of a large-diameter pressure relief borehole with a depth of 20m and a diameter of 150mm every 10m of advance in the fracture impact zone.

[0086] For the boundary area connecting goaf areas, the boundary area is defined as the 200m range adjacent to the boundary of the adjacent goaf area. On the construction site, confined small coal pillars with a width of only 4m are used to replace traditional large-sized coal pillars, and continuous stress relief trenches with a width of 1m and a depth of 2m are constructed in the area of ​​the confined small coal pillars to cut off stress transmission.

[0087] For critical sections of the roadway, the coverage area includes the area 75m ahead of the opening and return airway face. In these critical sections, 15m long high-strength anchor cables spaced 1m apart are used in conjunction with I-beams for support, along with dense drilling to eliminate deep, high-concentration stress.

[0088] For the protection boundary zone of surface buildings, the system issues an instruction to forcibly extend the protective coal pillar of the final mining line from the conventional 100m to a wide coal pillar of 400m, effectively blocking the path of strong dynamic load to surface buildings through the wide coal pillar.

[0089] Field measurement verification and effect evaluation of on-site anti-impact control parameters: To verify the accuracy of the optimized parameters and the reliability of the anti-scour measures, microseismic monitoring stations and borehole stress gauges were used to collect data on the optimized working face throughout the entire cycle, and the results were compared with those of the traditional mining mode that did not adopt the invented solution.

[0090] Key layer fracture characteristics and energy release verification: The fracture and microseismic energy data of each top slab structural level were extracted, and the relevant measured data are summarized in Table 1.

[0091] Table 1. Comparison of fracture and energy data before and after pre-fracture in key layers of the target working face.

[0092] As shown in the detailed measured data in Table 1, under the traditional uninterrupted condition, the average span of the main critical layer reaches 220m, and the energy released in a single burst at the moment of failure of the main critical layer is as high as 1.28 × 10⁻⁶ m. 7 J possesses a strong risk of destructive dynamic impact. After pre-splitting guided by the invented scheme, the measured fracture step distance of the main critical layer was controlled at 155.8m, highly consistent with the target calculated value of 154m. Simultaneously, with the weakening of the overall rock stratum, the single fracture energy plummeted to 3.2 × 10⁻⁶. 6 J. The energy release cycle changed from an instantaneous burst to a gradual release of tens of seconds, and no large-scale rooftop disaster occurred.

[0093] Verification of superimposed stress and comprehensive impact resistance in deep multi-working-face areas: During the mining of high-risk follow-up working faces, the engineering indicators before and after the implementation of the invention were dynamically compared, and the specific evaluation data are summarized in Table 2.

[0094] Table 2. Comparison of Engineering Indicators Before and After Implementation of Anti-scour Measures for the Working Face

[0095] Combining the indicator data in Table 2 and Figure 4 Analysis of the evolution curves reveals that if traditional large-sized coal pillars and conventional supports are used, under the superimposed compression of multiple working faces, the stress concentration coefficient of the coal body increases in a stepwise manner as the working face advances, eventually deteriorating to 2.14, accompanied by the outbreak of dense micro-seismic high-energy events. After introducing the strategy of restricted small coal pillar roof cutting, dense large-diameter pressure relief, and anti-rockburst support calculated by the invention, the stress concentration coefficient transmitted to the periphery of the working face is stabilized below the peak value of 1.65, far below the critical disaster threshold. The frequency of high-energy events is reduced to zero throughout the entire continuous mining cycle, no rockburst dynamic disasters occur, the maximum deformation of the surrounding rock in the roadway is significantly reduced by 85.8%, and the overall working face advancement efficiency is increased by 20.5% against the trend. Detailed data verify the excellent performance of the invention in the field of rockburst prevention in deep well multi-continuous working faces.

Claims

1. A method for pre-fracture and risk control of key layers in working faces under thick, hard rock strata to prevent erosion, characterized in that, Includes the following steps: Obtain the physical and geological parameters of the target working face, perform the top plate structure layer division based on the physical and geological parameters, and calculate the pre-fracture step distance of the foundation corresponding to each layer. Based on the pre-fracture fracture step distance corresponding to each division level and the physical geological parameters, a physical parameter mapping mechanism is constructed to calculate the post-fracture target fracture step distance corresponding to each division level, and differentiated targeted hydraulic pre-fracture operations are performed for each division level. The physical and geological parameters are used to calculate the overburden space activity boundary of the multi-continuous working face, and the geological structural anomaly deviation correction is performed on the overburden space activity boundary. The corrected safe activity boundary and comprehensive risk assessment results are output. Based on the pre-fracture target fracture step distance corresponding to the main critical layer, the corrected safe activity boundary, and the comprehensive risk assessment results, spatial mapping and association are performed to delineate key anti-cracking areas, and corresponding engineering control instructions are issued to the key anti-cracking areas.

2. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 1, characterized in that, The process of dividing the roof structure into hierarchical levels based on the aforementioned physical and geological parameters, and calculating the pre-fracture foundation failure step distance corresponding to each level, specifically includes: The physical and geological parameters include geological exploration data and structural exploration data; Based on the depth, lithology and thickness characteristics of the rock strata, the roof above the coal seam is divided into three independent physical levels from bottom to top: the immediate roof, the subcritical stratum and the main critical stratum, which are referred to as the respective division levels. Extract the thickness, tensile strength, and control rock load of each of the defined layers; By inputting the thickness, tensile strength, and control rock layer load into a beam fracture mathematical model with fixed supports at both ends, the pre-fracture step distance corresponding to each division level is derived.

3. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 2, characterized in that, The process of deriving the pre-fracture step distance of the foundation corresponding to each of the aforementioned division levels also includes: The fracture angle of each graded rock mass is determined based on the extracted structural exploration data; Using the boundary of the coal face as the spatial origin, a spatial projection ray is drawn into the overburden along the determined fracture angle. Combined with the design width of the mining face, the maximum vertical height of the overburden damage wave after the mining face is recovered is calculated, thereby obtaining the basic fracture height benchmark data for each division level.

4. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 1, characterized in that, The calculation of the target fracture step distance after pre-fracture corresponding to each division level, based on the pre-fracture foundation fracture step distance corresponding to each division level and the physical geological parameters, specifically includes the following: The elastic modulus of the rock mass and the degree of development of natural fractures corresponding to each division level in the physical and geological parameters are collected and measured. A comprehensive physical weight mapping relationship is established in combination with the tensile strength in the physical and geological parameters, and the differential pre-splitting influence coefficient corresponding to each division level is calculated and output. The pre-fracture breakage step distance corresponding to each division level is combined with the differential pre-fracture influence coefficient corresponding to each division level and substituted into the reduction correction equation for correction, and the target fracture breakage step distance corresponding to each division level after pre-fracture is calculated and determined.

5. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 2, characterized in that, The specific implementation of differentiated targeted hydraulic pre-fracturing operations for each of the defined hierarchical levels is as follows: The pre-fracture target fracture step distance corresponding to each division level is used as a rigid constraint condition, and the working space depth and target physical parameters are sent to the downhole hydraulic fracturing system. Based on the stratigraphic characteristics, low-pressure, low-flow hydraulic fracturing operations are performed on the immediate top; medium-pressure hydraulic fracturing operations are performed on the subcritical strata; and high-pressure, high-flow penetrating hydraulic fracturing operations are performed on the main critical strata. The standard for setting the output pump pressure of hydraulic fracturing equipment is: the fluid pressure is greater than the sum of the original rock stress of the target rock layer and the tensile strength of the target rock layer itself.

6. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 1, characterized in that, The calculation of the overburden space activity boundary of multiple consecutive working faces using the aforementioned physical and geological parameters specifically includes: Extract the strike length, dip width, total overburden thickness, and fracture angle of the working face from the aforementioned physical and geological parameters; The square theory is introduced to calculate the basic geometric boundary of the overburden activity characteristics of the working face, and the activity range of the foundation strike and the activity range of the foundation dip are obtained respectively, and the two are used together as the activity boundary of the overburden space. Multiply the working face length by the number of square cuts made during working face mining to obtain the basic strike range; Calculate the tangent of the fracture angle, multiply the total thickness of the overburden by the tangent, and add twice the product to the dip width of the working face to obtain the range of basic dip movement.

7. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 6, characterized in that, The process of performing geological structural anomaly deviation correction on the active boundary of the overlying space and outputting the corrected safe active boundary specifically includes: Extract the three-dimensional spatial coordinates of geological anomalies around the working face and project them vertically onto the horizontal plane to form a two-dimensional coordinate set; Compare the two-dimensional coordinate set with the two-dimensional topological boundary formed by the basic orientation activity range and the basic tendency activity range; When the horizontal coordinates of the geological anomaly fall within the two-dimensional topological boundary range, a structural stress amplification factor is introduced and the foundation strike range and foundation dip range are respectively multiplied for correction, and a corrected safe activity boundary that conforms to the actual stress state of the project is output.

8. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 1, characterized in that, The output of the comprehensive risk assessment results specifically includes: The number of times the working face is mined into squares is used as an iterative variable. The evolution state of the overburden fracture height from the immediate roof, sub-key layer to the main key layer is determined by superimposing the results, and a superposition effect evaluation model is established. The critical stress threshold for coal seam impact and the set threshold for total energy accumulation were obtained. The critical stress threshold for coal seam impact was obtained by performing uniaxial compression and acoustic emission tests on raw coal samples in the laboratory. The set threshold for total energy accumulation was determined based on the statistical regularity of historical rockburst microseismic monitoring data in the mining area. The overall risk assessment result of the current working face is determined to be high-risk when any of the following conditions are met: The stress concentration factor of the coal body after multiple working faces mining exceeds the critical impact stress threshold of the coal seam at the working face. The calculated total energy accumulation reaches the set threshold of the total energy accumulation.

9. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 1, characterized in that, The spatial mapping and correlation based on the pre-fracture target fracture step distance corresponding to the main key layer, the corrected safe activity boundary, and the comprehensive risk assessment results for delineating key areas for erosion prevention specifically includes: The pre-fracture target fracture step distance corresponding to the main critical layer is used as the basic cycle step distance. The first preset safety distance is extended forward and backward along the working face advancement direction to delineate the fracture influence zone of the main critical layer. The first preset safety distance is determined based on the range of the stress leading to the working face period. The corrected safety activity boundary is compared with the historical goaf spatial location using spatial topology, the overlapping area is extracted and extended to the solid coal side by a second preset safety distance, and the goaf connection boundary area is delineated; the second preset safety distance is determined based on the influence range of the lateral support pressure of the goaf. The concentration range of the peak value of the pre-stress in the working face opening, the final mining line and the return airway is defined as the key parts of the roadway; Extract the spatial coordinates of surface buildings, combine them with the rock strata movement angle, and project the spatial coordinates of the surface buildings onto the underground to form a spatial envelope surface. Delineate the geometric intersection area between the spatial envelope surface and the coal seam plane as the surface building protection boundary area, and obtain the geometric intersection width of the envelope surface.

10. The method for pre-fracture and risk control of key layers in the working face under thick and hard rock strata as described in claim 9, characterized in that, The specific steps of issuing corresponding engineering control commands to the key anti-collision areas include: Based on the comprehensive risk assessment results, for the main critical layer fracture impact zone, a coal seam large-diameter borehole decompression instruction is issued, and large-diameter boreholes are periodically drilled. For the boundary area connecting the goaf, a construction instruction for restricted small coal pillars and pressure relief trenches is issued, the size design of the isolation coal pillar is adjusted to that of restricted small coal pillars, and pressure relief trenches with a depth greater than the depth of the plastic failure zone of the surrounding rock of the roadway are constructed in the restricted small coal pillar area. For the key parts of the roadway, instructions for advanced joint support and intensive pressure relief were issued; For the surface building protection boundary area, a boundary protection coal pillar extension command is issued to extend the width of the conventional protection coal pillar of the final mining line to a value greater than the geometric intersection width of the envelope surface.

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