Retreat tunnel surrounding rock stability control method based on plastic zone butterfly expansion theory

Abstract

The present application relates to the field of rock mass mechanics test and engineering stability analysis, and discloses a method for controlling the stability of surrounding rock of a withdrawal channel based on plastic zone butterfly expansion theory, which collects stress data, strength data and section data of the surrounding rock of the withdrawal channel to identify the plastic zone state and its expansion form of the surrounding rock, and determines the basic roof fracture state in combination with the basic roof fracture line, the stop mining line, the withdrawal channel and the working face advancing position data; determines the completion time of the roof cutting operation according to the working face advancing position data and the movement law of the mining high stress zone, and determines the hydraulic fracturing roof cutting parameters in combination with the plastic zone state and the basic roof fracture state; implements the roof cutting according to the parameters and records the construction process data; collects the mine pressure deformation data and checks the surrounding rock stability result in combination with the construction process data. The present application realizes the control effect of blocking the load evolution of the withdrawal channel from the stress environment and the roof structure in two directions, guarantees the safe withdrawal and effectively reduces the deformation of the surrounding rock of the roadway.

Description

Technical Field

[0001] This invention relates to the field of rock mechanics testing and engineering stability analysis, specifically a method for controlling the stability of surrounding rock in retreat channels based on the theory of butterfly-shaped expansion of the plastic zone. Background Technology

[0002] Currently, in the final stage of mining in extra-thick coal seam fully mechanized longwall faces, the advance speed decreases. Operations such as netting, rope laying, and equipment retraction have longer cycles. A linked rock mass structure forms between the working face roof, the overlying strata of the goaf, and the roof of the retraction tunnel. Due to mining operations, the stress field distribution around the retraction tunnel exhibits non-uniform characteristics. The surrounding rock in the roadway undergoes plastic deformation under complex stress. Increased deformation of the surrounding rock or the occurrence of roof subsidence and sidewall bulging can affect the safe withdrawal of equipment from the working face.

[0003] For controlling the surrounding rock of the retreat passage in the final stage of the working face, conventional methods employ passive support and foundation decompression to maintain the roadway cross-section. On-site, this is achieved by increasing the density of anchor cable supports and installing canopies and stacked supports within the passage. Decompression is primarily achieved through roof blasting or conventional hydraulic fracturing. Blasting decompression involves drilling holes in the coal pillar side or roof and detonating explosives, utilizing stress waves to alter the internal structure of the rock mass and reduce localized stress. Conventional hydraulic fracturing involves injecting water into the roof according to preset fixed hole spacing and pressure parameters, using water pressure to open the original fractures in the rock mass and release localized stress.

[0004] Existing methods for controlling surrounding rock do not use the difference between maximum and minimum confining pressure as a criterion. When the plastic zone expands asymmetrically along the direction of maximum principal stress, conventional methods cannot quantify and identify the unstable failure mode of the surrounding rock, failing to obtain a mechanical basis for determining roadway instability. Furthermore, existing depressurization operations lack determination of the relative spatial position of the basic roof fracture line and the retreat channel. Fixed roof-cutting timing cannot match the process of high-stress zones moving towards the roadway during mining, making it difficult to effectively cut off the load transfer path of long cantilever roof structures and weaken the dynamic load impact caused by overburden fracturing. In addition, the depressurization parameter setting and on-site monitoring processes are independent. During construction, data such as the support cycle end resistance, microseismic events, and roadway displacement are not synchronously statistically compared. The fracturing parameters have not been verified and fed back by actual mine pressure fluctuations, resulting in insufficient reliability of the surrounding rock stability control results for the retreat channel.

[0005] Therefore, this invention provides a method for controlling the stability of surrounding rock in a retreat channel based on the theory of butterfly expansion in the plastic zone, in order to overcome the shortcomings of the prior art. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for controlling the stability of surrounding rock in retreat channels based on the theory of butterfly expansion of the plastic zone. This method solves the problems of unclear identification of the expansion state of the plastic zone in the retreat channels of coal mine working faces at the end of the mining stage, lack of criteria for judging the roof fracture structure during roof cutting and pressure relief, and difficulty in closed-loop verification of the control effect, which lead to the instability of the surrounding rock.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] The method for controlling the stability of surrounding rock in the retreat channel based on the butterfly-shaped expansion theory of the plastic zone includes the following steps: Data on the surrounding rock stress, strength, and cross-sectional features of the retreat passage are collected. The plastic zone state of the surrounding rock in the retreat passage is identified based on these data. The fracture state of the basic roof is determined based on the plastic zone state, the location of the basic roof fracture line, the location of the stop-mining line, the retreat passage location, and the working face advancement location. The timing for roof cutting is determined based on the working face advancement location data, and hydraulic fracturing roof cutting parameters are determined based on the plastic zone state and the basic roof fracture state. Hydraulic fracturing roof cutting is carried out according to these parameters, and construction process data is recorded. Mine pressure deformation data is collected, and the surrounding rock stability results are verified based on the construction process data and the mine pressure deformation data.

[0009] By adopting the above technical solution, the stress field distribution of the surrounding rock in the retreat channel is non-uniform due to the influence of mining, resulting in a specific expansion morphology of the plastic zone. By analyzing the difference between the maximum and minimum confining pressure of the surrounding rock, it can be determined whether the plastic zone is in a stable ear-shaped expansion state or a butterfly-shaped expansion state with malignant deformation characteristics. At the same time, the advancement of the working face causes the basic roof fracture line to move forward, and the long cantilever roof structure causes the advance support pressure to increase and transfer to the retreat channel. By combining the expansion state of the plastic zone and the fracture state of the basic roof, the instability conditions of the retreat channel are obtained from two dimensions: the rock strata fracture structure and the yield failure of the surrounding rock. Then, the timing of roof cutting is set by using the movement law of the high stress zone during mining, and hydraulic fracturing parameters are formulated for key rock strata to force the roof to fall and cut off the stress transmission path. Finally, the stability state is checked by using multi-source field monitoring data in a closed loop. Therefore, the control effect of blocking the load evolution of the retreat channel from both the stress environment and the roof structure is achieved, and the deformation of the surrounding rock in the roadway is reduced.

[0010] Preferably, the specific steps for collecting the surrounding rock stress data of the retreat channel are as follows: collecting time series data of measuring points on the top plate of the retreat channel, measuring points on the bottom plate of the retreat channel, measuring points near the working face side and measuring points far from the working face side; collecting the maximum confining pressure, minimum confining pressure, maximum principal stress direction and minimum principal stress direction around the retreat channel; and determining the stress field distribution around the retreat channel based on the time series data of each stress collection point.

[0011] By adopting the above technical solution, the non-uniform stress state distribution around the retreat channel is obtained, providing a stress parameter basis for the identification of the plastic zone characteristics of the surrounding rock.

[0012] Preferably, the specific steps for collecting the surrounding rock strength data and cross-sectional data of the retreat channel are as follows: collecting rock cohesion, rock internal friction angle, rock tensile strength, elastic modulus, Poisson's ratio, and rock layer thickness; collecting the width, height, and excavation outline area of ​​the retreat channel; determining the equivalent radius of the retreat channel based on the excavation outline area, and using the rock cohesion, rock internal friction angle, and equivalent radius of the retreat channel to calculate the boundary of the plastic zone.

[0013] By adopting the above technical solution, the actual excavation section is transformed into an equivalent circular analysis dimension, and the mechanical properties of the surrounding rock mass are obtained, providing a geometric and material parameter basis for the theoretical calculation of the plastic zone extension boundary.

[0014] Preferably, the specific steps for identifying the plastic zone state of the surrounding rock of the retreat channel are as follows: determining the principal stress ratio based on the maximum and minimum confining pressures; the principal stress ratio is the ratio of the maximum to the minimum confining pressure; calculating the plastic zone boundary based on the principal stress ratio, the equivalent radius of the retreat channel, the rock cohesion, and the rock internal friction angle; based on the Mohr-Coulomb failure criterion, constructing an implicit equation including the equivalent radius of the retreat channel, the distance from the plastic zone boundary point to the center of the roadway, the polar angle of the plastic zone boundary point, the principal stress ratio, the rock internal friction angle, the rock cohesion, and the minimum confining pressure, and solving it to obtain the plastic zone boundary; when the plastic zone boundary is mainly distributed along the direction of the minimum principal stress and does not form an outwardly convex butterfly region along the direction of the maximum principal stress, outputting the ear-shaped expansion state; when the plastic zone boundary forms an outwardly convex butterfly along the direction of the maximum principal stress, outputting the butterfly-shaped expansion state.

[0015] By adopting the above technical solution, the equation of the extended boundary of the plastic zone of the surrounding rock under the action of non-uniform stress field is quantified, and the yield mode of the surrounding rock is determined based on the outward convexity of the plastic zone in the direction of the maximum principal stress, so as to provide a mechanical basis for formulating local pressure relief strategies.

[0016] Preferably, the specific steps for determining the fracture state of the basic roof are as follows: acquiring the location data of the basic roof fracture line, the location data of the stop mining line, the location data of the retreat channel, and the working face advancement location data; when the basic roof fracture line is located behind the stop mining line, outputting the fracture state behind the stop mining line; when the basic roof fracture line is located above the retreat channel, outputting the fracture state above the retreat channel; when the basic roof fracture line is located in front of the retreat channel, outputting the fracture state in front of the retreat channel.

[0017] By adopting the above technical solution, the relative spatial relationship between the basic roof fracture location and the retreat channel is determined, and the overhang and bearing structure status of the roof rock blocks above the roadway are classified and judged.

[0018] Preferably, the specific steps for determining the timing of the top cutting operation are as follows: determining the mining stress distribution curves for different advancement stages based on the working face advancement position data; determining the location of the high-stress zone based on the peak position of the mining stress distribution curve, and determining the process of the high-stress zone moving towards the retreat channel side; determining the timing of the top cutting operation as the period before the high-stress zone reaches the retreat channel based on the working face advancement speed, drilling time, fracturing time, and post-fracturing fracture development time; and calculating the pressure transmitted from the overlying strata to the coal pillar after the top cutting operation based on the length of the basic roof block, the hydraulic fracturing top cutting length, the average unit weight of the overlying strata, the thickness of the overlying weak strata, the equivalent intensity of the mining pressure, and the structural angle of the key block fracture and slippage.

[0019] By adopting the above technical solutions, the timing of the advance support pressure migration brought about by the advancement of the working face is matched, ensuring that the roof cutting operation is completed before the stress peak is superimposed on the retreat channel, thus blocking the process of the long cantilever roof applying additional stress to the coal pillar.

[0020] Preferably, the specific steps for determining the hydraulic fracturing top cutting parameters are as follows: when the basic top fracture state is a fracture state above the retreat channel or a fracture state in front of the retreat channel, the hydraulic fracturing top cutting parameter determination process begins; the fracture target layer is determined based on the top plate lithology columnar data, rock layer thickness, rock strength, and key layer discrimination results, wherein the fracture target layer includes high-level key rock layers, low-level key rock layers, and the corresponding layer of the top plate structure at the working face end area; deep holes, shallow holes, and edge holes are determined based on the fracture target layer; and the determination of... The borehole opening position, final borehole position, borehole depth, borehole diameter, borehole spacing, borehole inclination angle, borehole azimuth angle, fracturing pressure, fracturing flow rate, sealing pressure, holding time, and number of fracturing stages for the deep borehole, shallow borehole, and side borehole are specified. The estimated value of the hydraulic fracturing initiation pressure is determined based on the minimum principal stress at the fracturing point, the maximum principal stress at the fracturing point, and the tensile strength of the top rock layer. The angle between the borehole cut and the vertical direction is determined based on the internal friction angle of the rock, the thickness of the basic top rock layer, the subsidence of the rock block, and the length of the basic top rock block.

[0021] By adopting the above technical solution, drilling space layout parameters and pressure control indicators are established based on the location of the target rock strata and the tensile characteristics of the rock. Cutting angles that meet the conditions for roof structure slippage are established to ensure that fractured rock blocks are cut off and slide into the goaf.

[0022] Preferably, the specific steps for implementing hydraulic fracturing top cutting are as follows: constructing deep holes, shallow holes, and side holes according to the hydraulic fracturing top cutting parameters; sending the sealing device and high-pressure pipeline into the predetermined fracturing section, and performing a sealing pressure test on the predetermined fracturing section; after the sealing pressure test is qualified, pressurizing and maintaining the pressure on the predetermined fracturing section, so that the fracture extends along the key rock layer and connects with the adjacent fracturing section or the fracture of the adjacent borehole to form a continuous cut.

[0023] By adopting the above technical solution, high-pressure water is used to directionally modify the internal structure of key rock strata, causing cracks in independent boreholes to overlap and form a continuous roof fracture surface.

[0024] Preferably, the specific steps for recording construction process data are as follows: continuously record the borehole pressure during the sealing and pressure testing process, and record the borehole number, fracturing section number, maximum water injection pressure, cumulative water injection time, cumulative water injection volume, return water location, and pressure change curve; control the on-site water injection pressure to be no less than the estimated value of the hydraulic fracturing initiation pressure; after the current fracturing section is completed, release the borehole pressure and retract the sealing device to the next predetermined fracturing section; repeat the sealing and pressure testing, pressure fracturing, pressure holding and propagation, and pressure release and tube retraction operations until all fracturing sections in the same borehole are completed.

[0025] By adopting the above technical solution, the pressure parameters of single-hole segmented pipe withdrawal fracturing are tracked and recorded throughout the entire process, and the water injection pressure is controlled to reach the fracturing initiation threshold, ensuring that the fracturing segments at each depth meet the requirements for expansion and connection.

[0026] Preferably, the specific steps for verifying the stability of the surrounding rock are as follows: Collect the support cycle-end resistance, coal stress, microseismic events, roof subsidence in the retreat channel, and sidewall displacement in the retreat channel as mine pressure deformation data; save the mine pressure deformation data in segments according to the distance between the working face and the retreat channel to form a control effect comparison dataset, which includes data before fracturing, data after fracturing, data at the final mining approach stage, data at the breakthrough stage, and data after breakthrough; calculate the average support cycle-end resistance and the root mean square deviation of the support cycle-end resistance based on the support cycle-end resistance, and determine the roof pressure criterion value based on the average support cycle-end resistance and the root mean square deviation of the support cycle-end resistance; The root mean square error of the support cycle end resistance is calculated based on the measured number of cycles, the measured cycle end resistance of each cycle, and the average value of the support cycle end resistance. The roof pressure criterion value is determined by the sum of the average value of the support cycle end resistance and the root mean square error of the support cycle end resistance. The set control range of the roof subsidence and the side displacement of the retreat channel is determined based on the control effect comparison dataset. When the support cycle end resistance does not continuously exceed the roof pressure criterion value, the peak coal stress does not continuously increase, the microseismic event energy level does not continuously increase, and the roof subsidence and side displacement of the retreat channel are both within the set control range determined by the control effect comparison dataset, the result of qualified control of the surrounding rock stability of the retreat channel is output.

[0027] By adopting the above technical solution, the working face is used to quantify the statistical fluctuation of the measured parameters of the mine pressure to determine the pressure threshold. The stress, micro-vibration and deformation data before and after the top cutting are compared simultaneously with multiple parameters to verify the actual control status of hydraulic fracturing and pressure relief measures on the stability of the surrounding rock.

[0028] This invention provides a method for controlling the stability of surrounding rock in a retreat channel based on the butterfly-shaped expansion theory of the plastic zone. It has the following beneficial effects: 1. This invention obtains roadway instability conditions from two dimensions: rock fracture structure and surrounding rock yield failure, by collecting surrounding rock stress and cross-sectional data of the retreat channel and calculating the principal stress ratio, combined with the determination of the relative relationship between the basic roof fracture location and the retreat channel. It can accurately identify whether the plastic zone of the surrounding rock exhibits an ear-shaped expansion state or a severely damaged butterfly-shaped expansion state, and distinguish between three working conditions: fracture behind the stop line, fracture above the retreat channel, and fracture in front of the retreat channel. This provides a clear mechanical basis for determining whether active roof cutting and pressure relief are necessary.

[0029] 2. This invention determines the timing of roof cutting based on the law of the movement of high-stress zones towards the retreat channel during mining, and implements hydraulic fracturing using a combination of deep and shallow boreholes. High-pressure water creates continuous cuts within key rock strata. This transforms the long cantilever key block (before fracturing) into a short cantilever structure and a cuttable block, directly cutting off the path of additional stress applied to the coal pillar from the long cantilever roof, and weakening the dynamic load propagation process caused by the fracturing of the overlying strata. This effectively reduces the impact of strong mine pressure in the final mining stage on the retreat channel and surrounding rock.

[0030] 3. This invention constructs a comparative dataset of control effects covering all stages before and after fracturing, including the synchronous acquisition of multi-source mine pressure deformation data such as the end-cycle resistance of the support, coal stress, microseismic events, and surface displacement of the retreat channel. By utilizing the statistical characteristics of measured mine pressure parameters to establish roof pressure criteria, a multi-parameter closed-loop verification of the surrounding rock stability is performed. This achieves integrated operation of hydraulic fracturing parameter design, on-site construction control, and final effect evaluation, improving the reliability of the surrounding rock stability control results in the retreat channel. Attached Figure Description

[0031] Figure 1 This is a framework diagram of the surrounding rock stability control system for the retreat channel based on the butterfly-shaped expansion theory of the plastic zone of the present invention; Figure 2 This is a diagram of the basic top rock structure of the hydraulically fractured rock layer at the stop-mining line location of the present invention; Figure 3 This is a flowchart of the working method of the surrounding rock stability control system for the retreat channel based on the butterfly expansion theory of the plastic zone according to the present invention; Figure 4 This is a schematic diagram of the stress evolution during the working face advancement process of the present invention; Figure 5 This is a flowchart of the process for collecting surrounding rock parameters and identifying the butterfly-shaped expansion state of the plastic zone according to the present invention; Figure 6 This is a flowchart illustrating the process of determining the basic top fracture structure and the timing of top cutting and pressure relief according to the present invention. Figure 7 This is a flowchart illustrating the parameters for determining the top cutting of hydraulic fracturing in shallow and deep holes according to the present invention. Figure 8 This is a flowchart illustrating the data flow of hydraulic fracturing, top cutting, and construction process recording for this invention. Figure 9 This is a flowchart illustrating the process of collecting mine pressure deformation data and verifying the stability of surrounding rock according to the present invention. Figure 10 This is a diagram showing the horizontal displacement curve of the point furthest from the center of the working face in this invention. Figure 11 This is a vertical displacement curve of the center point of the tunnel roof in this invention.

[0032] Among them, 1. Cutting line; 2. Stopping line; 3. Retreat passage; 4. Uncut basic roof rock block; 5. Short cantilever structural block; 6. Cutting structural block; 7. Rear basic roof rock block; 8. Working face advance direction. Detailed Implementation

[0033] 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.

[0034] See attached document Figure 1 This invention provides a rock stability control system for a retreat channel based on the theory of butterfly expansion in the plastic zone, including a butterfly zone identification module, a roof determination module, a parameter determination module, a fracturing execution module, and an effect verification module.

[0035] The butterfly area identification module receives data on the surrounding rock stress, strength, and cross-sectional area of ​​the retreat tunnel. The surrounding rock stress data includes stress data for the tunnel roof, floor, and areas near and away from the working face. The surrounding rock strength data includes rock cohesion, internal friction angle, tensile strength, elastic modulus, and Poisson's ratio. The retreat tunnel cross-sectional data includes the tunnel width, height, and excavated area.

[0036] The butterfly zone identification module communicates with the borehole stress gauge, the support pressure acquisition device, the surrounding rock parameter input terminal, and the numerical calculation terminal. The borehole stress gauge is used to collect coal stress change data. The support pressure acquisition device is used to collect support cycle end resistance data. The surrounding rock parameter input terminal is used to input rock strata columnar data, rock mechanics test data, and roadway cross-sectional data. The numerical calculation terminal is used to output the stress field, plastic zone distribution, and deformation field calculation results around the retreat channel.

[0037] The butterfly zone identification module determines the boundary of the plastic zone of the surrounding rock of the retreat channel based on the maximum and minimum confining pressures, the direction of the maximum and minimum principal stresses, rock cohesion, rock internal friction angle, and the equivalent radius of the retreat channel. The module classifies the plastic zone state of the surrounding rock of the retreat channel into ear-shaped expansion and butterfly-shaped expansion states. The ear-shaped expansion state is characterized by the plastic zone mainly distributed along the direction of the minimum principal stress. The butterfly-shaped expansion state is characterized by the plastic zone forming outward-convex butterfly-shaped blades along the direction of the maximum principal stress.

[0038] The roof determination module receives the state of the plastic zone of the surrounding rock in the retreat channel from the butterfly zone identification module, and also receives data on the location of the basic roof fracture line, the stop-mining line, the retreat channel, and the working face advancement position. The roof determination module determines the positional relationship between the basic roof fracture line and the retreat channel, and outputs the fracture status behind the stop-mining line, above the retreat channel, or in front of the retreat channel.

[0039] See attached document Figure 2 , Figure 2 The structure includes a top cutting line 1, a stop mining line 2, a retreat passage 3, an uncut basic roof rock block 4, a short cantilever structure block 5, a cut-off structure block 6, and a rear basic roof rock block 7. Figure 2 The downward arrow in the figure indicates the direction of the load on the overlying rock strata. Figure 2 The dashed box in the image represents the range of the corresponding rock block. Figure 2 The black and white dotted areas represent the areas filled by collapsed rock masses in the goaf. Figure 2 The bottom dimension line indicates that the distance between the retreat channel 3 and the stop line 2 is 10m. Figure 2 The hydraulic support in the diagram is located near the retreat channel 3, indicating the support position during the final mining breakthrough and retreat operations.

[0040] Cutting line 1 is the location of the pre-fracturing cut formed by hydraulic fracturing within the basic roof stratum. Stop mining line 2 is the designed position where mining ceases or when breakthrough control is achieved. Retreat passage 3 is the roadway space for the hydraulic supports and mining equipment to withdraw during equipment retreat. Uncut basic roof block 4 is located above and to the right of cutting line 1. Short cantilever structure block 5 is located to the left of cutting line 1 and near and above stop mining line 2. Cut-off structure block 6 is located to the right of cutting line 1 and above the goaf. Rear basic roof block 7 is located to the right of cut-off structure block 6.

[0041] Top plate determination module according to Figure 2 The structural relationships shown are used for determination. Without hydraulic fracturing, the main roof strata continuously span above the retreat channel 3, forming a long cantilever structure. After hydraulic fracturing, the cutting line 1 divides the continuous main roof strata into short cantilever structural blocks 5 and cut-off structural blocks 6. After the working face advances to the stop line 2, cut-off structural block 6 enters the goaf collapse filling area along the cutting line 1, while short cantilever structural block 5 is located above the retreat channel 3 and bears the load of the overlying strata.

[0042] The parameter determination module receives the butterfly-shaped expansion state output by the butterfly zone identification module and the basic top fracture state output by the top plate determination module, and determines the hydraulic fracturing top cutting parameters. These parameters include fracturing layer position, deep hole depth, shallow hole depth, side hole depth, borehole diameter, borehole spacing, borehole inclination angle, borehole azimuth angle, fracturing pressure, fracturing flow rate, sealing pressure, holding time, and number of fracturing stages.

[0043] The parameter determination module categorizes fracturing boreholes into deep holes, shallow holes, and edge holes. Deep holes are used to cut through high-level critical rock strata, shallow holes are used to cut through low-level critical rock strata, and edge holes are used to treat the roof structure at the working face end. Based on the rock columnar data, rock strength, location of critical strata, and location of the retreat channel, the parameter determination module outputs the spatial arrangement parameters for deep holes, shallow holes, and edge holes.

[0044] The fracturing execution module is used to determine the hydraulic fracturing top-cutting parameters output by the module based on the parameters, and to execute drilling, well sealing and pressure testing, pressure fracturing, pressure holding and propagation, pressure relief and tubing withdrawal, and staged fracturing. The fracturing execution module includes a drilling unit, a well sealing and pressure testing unit, a high-pressure water supply unit, a pressure acquisition unit, and a staged control unit. The drilling unit is used to form deep holes, shallow holes, and side holes. The well sealing and pressure testing unit is used to seal the predetermined fracturing stage. The high-pressure water supply unit is used to supply high-pressure water to the predetermined fracturing stage. The pressure acquisition unit is used to collect fracturing pressure data. The staged control unit is used to control the construction sequence of different fracturing stages within the same borehole.

[0045] After sealing the borehole in the designated fracturing section, the fracturing execution module controls the high-pressure water supply unit to pressurize the section. Once the fracturing pressure reaches the initiation condition, fractures form in the rock mass within the fracturing section. The fracturing execution module continues to control the high-pressure water supply unit to maintain pressure, allowing the fractures to extend along key rock strata and connect with adjacent fracturing sections or adjacent borehole fractures. After the current fracturing section is completed, the fracturing execution module controls the sealing and pressure testing unit to depressurize and controls the sealing device to retract to the next fracturing section.

[0046] The effect verification module receives construction process data output from the fracturing execution module, as well as support cycle end resistance data, coal stress data, microseismic event data, retreat tunnel roof subsidence data, and retreat tunnel side displacement data. Based on changes in support pressure, peak coal stress, microseismic event energy levels, retreat tunnel roof subsidence, and retreat tunnel side displacement, the effect verification module outputs the surrounding rock stability control results for the retreat tunnel.

[0047] The effectiveness verification module outputs a qualified control result when the support cycle resistance does not continuously exceed the pressure criterion value, the peak coal stress does not continuously increase, the microseismic event energy level does not continuously increase, the roof subsidence of the retreat channel is within the set control range, and the sidewall displacement of the retreat channel is within the set control range. The module outputs a verification result when any monitoring item exceeds the set control range. The verification result includes the location of supplementary fracturing layers, supplementary fracturing holes, and supplementary support locations.

[0048] See attached document Figure 3 The present invention provides a method for controlling the stability of the surrounding rock of the retreat channel based on the butterfly expansion theory of the plastic zone, which is based on the above-mentioned system operation, including steps S100 to S500.

[0049] Step S100: Collect surrounding rock parameters and identify the butterfly-shaped expansion state of the plastic zone. This is performed through the butterfly zone identification module.

[0050] Step S200: Determine the basic top fracture structure and the timing for top cutting and pressure relief. This is executed through the top plate determination module.

[0051] Step S300: Determine the cutting parameters for hydraulic fracturing at both deep and shallow holes. This is executed via the parameter determination module.

[0052] Step S400: Perform hydraulic fracturing to cut the top and record the construction process data. This is executed through the fracturing execution module.

[0053] Step S500: Collect mine pressure deformation data and verify the surrounding rock stability results. This is performed through the effect verification module.

[0054] See attached document Figure 4 Multiple curves represent the stress distribution curves caused by mining at different stages of advancement. The peak position of the curve indicates the location of the high-stress zone caused by mining. The high-stress zone moves towards the retreat channel 3 along the direction of the working face advancement. The layered region represents the coal seam and its overlying strata structure. The oblique lines and dashed lines represent the movement of the roof strata and the changes in the range of stress influence under the influence of mining.

[0055] See attached document Figure 5 Step S100 includes the following sub-steps.

[0056] Sub-step S101: Collect basic data on the stress state of the surrounding rock in the retreat channel.

[0057] The maximum and minimum confining pressures, as well as the directions of the maximum and minimum principal stresses, were collected around retreat channel 3. The maximum and minimum confining pressures were obtained using borehole stress gauges, support pressure acquisition devices, in-situ geostress testing results, or numerical calculations. The directions of the maximum and minimum principal stresses were obtained using geostress testing results or calculations of the surrounding rock stress field.

[0058] Both the maximum and minimum confining pressures are expressed as positive compressive stress values. The maximum confining pressure is the greater compressive stress borne by the coal and rock mass surrounding retreat channel 3. The minimum confining pressure is the lesser compressive stress borne by the coal and rock mass surrounding retreat channel 3. The units for both the maximum and minimum confining pressures are MPa.

[0059] Stress acquisition points are arranged circumferentially along the retreat channel 3. These points include measuring points on the top plate of the retreat channel, the bottom plate of the retreat channel, measuring points near the working face side, and measuring points far from the working face side. Time-series data from each stress acquisition point are used to determine the stress field distribution around retreat channel 3.

[0060] Sub-step S102: Collect data on the strength and cross-sectional dimensions of the surrounding rock in the retreat channel.

[0061] The data collection includes rock cohesion, internal friction angle, tensile strength, elastic modulus, Poisson's ratio, and stratum thickness. Rock cohesion and internal friction angle are used for calculating the plastic zone boundary. Rock tensile strength is used for calculating the hydraulic fracturing initiation pressure. Stratum thickness is used to determine the location of critical strata to be fracturing.

[0062] Rock cohesion was obtained through laboratory rock mechanics tests, in-situ parameter inversion, or engineering geological data. The internal friction angle of the rock was obtained through shear tests or triaxial compression tests. The tensile strength of the rock was obtained through Brazilian splitting tests or engineering geological data. The elastic modulus and Poisson's ratio were used to establish a numerical calculation model for the surrounding rock.

[0063] The excavation width, height, and outline area of ​​retreat passage 3 were collected. An equivalent circular cross-section was determined based on the outline area of ​​retreat passage 3. The area of ​​the equivalent circular cross-section was the same as the actual excavation outline area of ​​retreat passage 3. The equivalent circular cross-section was used to uniformly calculate the boundary of the plastic zone of the surrounding rock of retreat passage 3.

[0064] Sub-step S103: Calculate the boundary of the plastic zone of the pullback channel and output the state.

[0065] The principal stress ratio is calculated based on the maximum and minimum confining pressures. The principal stress ratio is used to represent the degree of stress difference in the surrounding rock of retreat passage 3. The formula for calculating the principal stress ratio is as follows: ; in, The principal stress ratio represents the degree of non-uniform stress in the surrounding rock of the retreat channel. The maximum confining pressure represents the maximum compressive stress outside the pullback channel, and its unit is MPa. Minimum confining pressure, which represents the minimum compressive stress outside the pullback channel, is expressed in MPa.

[0066] The equivalent radius of the retreat channel is calculated based on the excavation outline area of ​​retreat channel 3. The formula for calculating the equivalent radius of the retreat channel is as follows: ; in, The equivalent radius of the retreat passage represents the radius after the cross-section of the retreat passage is equivalent to a circular roadway, and the unit is m; The cross-sectional area of ​​the retreat tunnel represents the area enclosed by the excavation outline of the retreat tunnel, and the unit is m². 2 ; Pi is the ratio of the circumference of a circle to its diameter.

[0067] The boundary of the plastic region is calculated based on the Mohr-Coulomb failure criterion. The implicit equation for the boundary of the plastic region is as follows: ; in, The equivalent radius of the retreat passage represents the radius after the cross-section of the retreat passage is equivalent to a circular roadway, and the unit is m; The distance from the boundary point of the plastic zone to the center of the roadway represents the radius of expansion of the plastic zone in the corresponding direction, in meters. The polar angle of the boundary point of the plastic zone represents the angle between the boundary point of the plastic zone and the direction of the maximum principal stress, in degrees. The principal stress ratio represents the degree of non-uniform stress in the surrounding rock of the retreat channel. is the internal friction angle of the rock, which represents the shear strength parameter of the coal-rock mass, and the unit is °; Rock cohesion represents the shear strength of the cementation between coal and rock particles, and is measured in MPa. Minimum confining pressure, which represents the minimum compressive stress outside the pullback channel, is expressed in MPa.

[0068] When the boundary of the plastic zone forms two convex regions along the direction of the maximum principal stress, and the extension direction of the two convex regions is consistent with the direction of the maximum principal stress, a butterfly-shaped expansion state is output. When the boundary of the plastic zone is mainly located in the direction of the minimum principal stress, and no convex butterfly-shaped regions are formed along the direction of the maximum principal stress, an ear-shaped expansion state is output. The butterfly region identification module simultaneously outputs the maximum depth of the plastic zone, the butterfly-shaped expansion direction, and the surrounding rock location of the corresponding retreat channel 3.

[0069] See attached document Figure 6 Step S200 includes the following sub-steps.

[0070] Sub-step S201: Identify the relative position of the basic top fracture line and the pullback channel.

[0071] The locations of the working face advance, stop line 2, retreat passage 3, and basic roof fracture line are obtained. The working face advance location is obtained through coal mining machine position data, working face advance records, or measurement data. The stop line 2 location is determined based on the final mining design. The retreat passage 3 location is determined based on roadway measurement data. The basic roof fracture line location is determined through roof structure analysis, microseismic event location, support pressure changes, or numerical calculation results.

[0072] When the basic roof fracture line is located behind the stop line 2, the roof determination module outputs the fracture status behind the stop line. When the basic roof fracture line is located above the retreat channel 3, the roof determination module outputs the fracture status above the retreat channel. When the basic roof fracture line is located in front of the retreat channel 3, the roof determination module outputs the fracture status in front of the retreat channel.

[0073] The fracture state behind the stop line corresponds to the short cantilever structure above retreat passage 3. The fracture state above the retreat passage corresponds to the fractured rock block being directly above retreat passage 3. The fracture state in front of the retreat passage corresponds to the fractured rock block being located between retreat passage 3 and the working face. The roof determination module determines the method by which retreat passage 3 bears the roof load based on the above three states.

[0074] Sub-step S202: Determine the structural transformation relationship of the basic top rock block before and after the top is cut.

[0075] according to Figure 2 The structural relationship shown is used to determine the basic roof rock blocks before and after roof cutting. Before roof cutting, the uncut basic roof rock block 4 and the subsequent basic roof rock block 7 are continuously distributed, forming a continuous cantilever structure above the retreat channel 3. This continuous cantilever structure transfers the load of the rock strata above the goaf to the retreat channel 3 and the coal seam in front of it.

[0076] After the roof is cut, the cutting line 1 divides the continuous cantilever structure into short cantilever block 5, cut-down block 6, and rear basic roof block 7. Short cantilever block 5 is located above the retreat passage 3. Cut-down block 6 is located above the goaf. Rear basic roof block 7 is located to the right of cut-down block 6. After cut-down block 6 enters the goaf collapse rock filling area along the cutting line 1, short cantilever block 5 remains above the retreat passage 3.

[0077] When the roof determination module detects a fracture above or in front of the retreat channel, it outputs the result indicating whether the roof needs to be cut and depressurized. When the roof determines a fracture behind the stop line, it outputs the result indicating whether retreat is permissible based on the length of the short cantilever block 5 and the roof deformation data of the retreat channel 3. While outputting the result indicating whether the roof needs to be cut and depressurized, the parameter determination module enters the fracturing parameter determination process.

[0078] Sub-step S203: Determine the timing of the roof cutting operation and calculate the stress on the coal pillar.

[0079] according to Figure 4 The evolution law of mining-induced stress during the working face advancement process is shown to determine the timing of the roof cutting operation. Figure 4 In the middle, the working face advance direction 8 points to the left. The peak positions of multiple mining stress distribution curves move towards the retreat passage 3 along the working face advance direction 8. The timing for completing the roof cutting operation is set before the mining high stress zone reaches the retreat passage 3.

[0080] The timing of top-cutting operations is determined based on the working face advance speed, drilling time, fracturing time, and post-fracturing fracture development time. The working face advance speed is determined by production records. Drilling time is determined by the number of boreholes, borehole length, and drilling rig efficiency. Fracturing time is determined by the number of fracturing stages, the pressure holding time per stage, and the casing removal time. Post-fracturing fracture development time is determined based on the on-site construction plan.

[0081] The formula for calculating the stress on a coal pillar is as follows: ; in, The pressure transmitted to the coal pillar by the overlying strata represents the equivalent load transmitted to the coal pillar after fracturing and roof cutting, and the unit is kN; The length of the basic top rock block represents the length of the key block along the working face advancement direction, in meters. The hydraulic fracturing cut-off length represents the effective cut-off length formed by the cut in the key rock strata, and is expressed in meters (m). The average unit weight of the overlying strata represents the gravity per unit volume of the strata involved in load transfer, expressed in kN / m³. 3 ; The thickness of the overlying weak rock layer represents the height of the weak rock layer above the critical block that participates in load transfer, in meters. The equivalent intensity of mining pressure represents the additional pressure caused by mining at the working face, and its unit is kN / m. 2 ; The critical block fracture and sliding structural angle represents the angle between the direction of force on the block and the horizontal plane, and the unit is °.

[0082] See attached document Figure 7 Step S300 includes the following sub-steps.

[0083] Sub-step S301: Determine the fracturing sites for deep and shallow holes based on the location of key rock strata.

[0084] The fracturing strata are determined based on the roof lithology columnar data, stratum thickness, rock strength, and key stratum identification results. Fracturing strata include thick, hard, key strata that control the high pressure at the end of mining operations. These thick, hard, key strata include coarse sandstone, medium sandstone, conglomerate, or other strata that control roof fracture.

[0085] Deep-hole fracturing zones correspond to high-level primary or subcritical layers. Shallow-hole fracturing zones correspond to low-level subcritical layers or directly above thick, hard rock layers. Side-hole fracturing zones correspond to the roof structure at the working face end. Deep, shallow, and side-hole fracturing zones together form a continuous fracturing hole group distributed along the dip direction of the retreat channel 3.

[0086] After the fracturing strata are determined, the parameter determination module outputs the fracturing borehole layout results. These results include borehole opening location, end borehole location, borehole depth, borehole inclination angle, borehole azimuth angle, and spacing between adjacent boreholes. The fracture propagation ranges of adjacent fracturing boreholes overlap, creating a continuous shear line within the key rock strata.

[0087] Sub-step S302: Determine the crack initiation pressure based on the ground stress and tensile strength.

[0088] The maximum and minimum principal stresses at the fracturing point, as well as the tensile strength of the rock strata above the fracturing point, are obtained. The maximum and minimum principal stresses at the fracturing point are obtained through in-situ stress testing, numerical calculation, or inversion based on field monitoring. The tensile strength of the rock strata above the fracturing point is obtained through rock mechanics testing or engineering geological data.

[0089] The initiation pressure is used to determine the rated pressure of the fracturing equipment and the control value of the on-site water injection pressure. The calculated initiation pressure is less than the allowable operating pressure of the fracturing equipment. When the on-site water injection pressure is greater than or equal to the calculated initiation pressure, initial fractures form in the rock mass within the fracturing section.

[0090] The formula for calculating the hydraulic fracturing initiation pressure is as follows: ; in, This is an estimated value of the initiation pressure required for hydraulic fracturing, which represents the water injection pressure required to generate initial fractures in the top rock layer, in MPa. is the minimum principal stress at the fracturing point, which represents the minimum directional compressive stress at the fracturing location, and its unit is MPa; is the maximum principal stress at the fracturing point, which represents the maximum directional compressive stress at the fracturing location, and its unit is MPa; This represents the tensile strength of the rock strata in the top layer of the fractured rock, which indicates the rock's ability to resist tensile fracture failure. The unit is MPa.

[0091] Sub-step S303: Determine the cutting angle based on the slip condition of the cut structural block.

[0092] The cutting angle is determined based on the thickness of the basic top rock stratum, the internal friction angle of the rock, the subsidence of the rock block, and the length of the basic top rock block. The cutting angle is used to determine the dip angle of the deep hole, the dip angle of the shallow hole, and the pre-splitting direction. The cutting angle meets the requirement that the cut-off structural block 6 enters the goaf collapse rock filling area along the cut-off line 1.

[0093] The cutting angle corresponds to the length of the short cantilever block 5 above the retreat channel 3. After the cutting angle is determined, the length of the short cantilever block 5 and the length of the cut-off block 6 are determined simultaneously. The parameter determination module outputs the drilling inclination angle of the deep hole and the shallow hole based on the cutting angle, and makes the cutting top line 1 pass through the key rock layer to be cut.

[0094] The formula for calculating the kerf angle is as follows: ; in, The angle between the cut and the vertical direction represents the angle by which the deep hole pre-splitting cut deviates from the vertical direction, in degrees. is the internal friction angle of the rock, which represents the shear strength parameter of the coal-rock mass, and the unit is °; The thickness of the basic top rock layer represents the vertical thickness of the critical rock layer that has been cut off, and the unit is meters (m). The value is the rock block subsidence, which represents the displacement of the rock block on the cut side after it rotates and subsides, and the unit is meters (m). The length of the basic top rock block represents the length of the key block along the working face advancement direction, in meters.

[0095] See attached document Figure 8 Step S400 includes the following sub-steps.

[0096] Sub-step S401: Construct deep holes, shallow holes, and side holes according to the fracturing parameters.

[0097] Based on the orifice location, borehole depth, borehole diameter, borehole inclination angle, and borehole azimuth angle determined in step S300, deep holes, shallow holes, and side holes are constructed. After drilling is completed, the borehole depth, borehole diameter, borehole inclination angle, and borehole azimuth angle are checked. Once the check results meet the design parameters, the borehole sealing and pressure testing process begins.

[0098] Deep holes, shallow holes, and side holes are arranged along the dip direction of retreat channel 3. The deep holes terminate within a high-level key rock layer. The shallow holes terminate within a low-level key rock layer. The side holes terminate at the corresponding stratum of the top plate structure at the working face end. The openings of all deep holes, shallow holes, and side holes are located within retreat channel 3.

[0099] After drilling, the borehole is cleaned. Following cleaning, the borehole is inspected for collapse, spalling, and water backflow. Once the borehole meets the sealing requirements, the sealing device and high-pressure pipeline are introduced into the designated fracturing section. The location of the designated fracturing section is determined by the fracturing section layout results output by the parameter determination module.

[0100] Sub-step S402 completes the sealing pressure test and forms a sealed fracturing section.

[0101] After the sealing device is inserted into the predetermined fracturing section, the fracturing execution module controls the sealing and pressure testing unit to perform setting. After setting is completed, water is supplied to the sealing section and the sealing pressure is recorded. When the sealing pressure reaches the set sealing pressure and remains stable, the sealing section is deemed to meet the conditions for pressurized fracturing.

[0102] During the wellhead pressure test, the pressure acquisition unit continuously records the wellhead pressure. When a sustained pressure drop occurs in the wellhead sealing section, the fracturing execution module stops pressurizing and readjusts the wellhead sealer position. After the pressure in the wellhead sealing section stabilizes, the fracturing execution module records the wellhead pressure, sealing time, wellhead backflow status, and wellhead sealing section number.

[0103] The formula for controlling the on-site water injection pressure is as follows: ; in, The injection pressure is the pressure recorded by the orifice pressure acquisition device during the fracturing process, and the unit is MPa. This is an estimated value for the initiation pressure required for hydraulic fracturing, representing the injection pressure required to generate initial fractures in the top rock strata, expressed in MPa.

[0104] Sub-step S403 involves applying and maintaining pressure to form a continuous slit.

[0105] After the borehole pressure test is passed, the fracturing execution module controls the high-pressure water supply unit to pressurize the predetermined fracturing section. Once the on-site water injection pressure reaches the estimated initiation pressure required for hydraulic fracturing, initial fractures occur in the rock mass within the fracturing section. The pressure acquisition unit records the pressure rise, pressure drop, and pressure stabilization process.

[0106] After the initial fracture forms, the fracturing execution module continues to control the high-pressure water supply unit to maintain pressure and supply water. The fracture extends along the interior of the key rock strata. When the fracture extends to the adjacent fracturing section or the influence range of the adjacent borehole, a continuous cut is formed. Continuous cuts and Figure 2 The middle tangent top line corresponds to 1.

[0107] During fracturing, the water return locations of the roof, coal seam, and adjacent boreholes in the retreat channel 3 are recorded. These water return locations are used to determine the direction of fracture propagation and the state of fracture penetration. Once the current fracturing section reaches the designed holding time, the fracturing execution module stops pressurizing and enters the depressurization and pipe withdrawal process.

[0108] Sub-step S404: Complete the depressurization and pipe withdrawal and cyclically construct all fracturing sections.

[0109] After the current fracturing stage is completed, the fracturing execution module shuts down the high-pressure water supply unit and releases the pressure inside the borehole. Once the pressure inside the borehole drops to a safe operating pressure, the sealing device is released from its setting state. After the sealing device retracts to the next predetermined fracturing stage, the processes of sealing and pressure testing, pressurizing fracturing, holding and propagating fracturing, and depressurizing and tubing retraction are repeated.

[0110] After all fracturing sections within the same borehole are completed, the borehole is sealed. The fracturing execution module records the borehole number, fracturing section number, maximum injection pressure, cumulative injection time, cumulative injection volume, return water location, and pressure change curve. This construction process data is then transmitted to the effect verification module.

[0111] If any fracturing segment fails to meet the field injection pressure control formula, supplementary fracturing is performed on that segment. After supplementary fracturing, the pressure change curve and water return status are re-recorded. The supplementary fracturing data and the original fracturing data are jointly incorporated into the effectiveness verification process.

[0112] See attached document Figure 9 Step S500 includes the following sub-steps.

[0113] Sub-step S501: Collect micro-vibration and deformation data of the support under pressure stress.

[0114] After hydraulic fracturing, the following data were collected: support cycle end resistance, coal stress, microseismic events, roof subsidence in the retreat tunnel, and side displacement in the retreat tunnel. Support cycle end resistance was collected using a hydraulic support pressure monitoring system. Coal stress was collected using borehole stress gauges. Microseismic events were collected using a microseismic monitoring system. Roof subsidence and side displacement in the retreat tunnel were collected using measuring lines and points or convergence monitoring devices.

[0115] The resistance at the end of the support cycle is statistically analyzed according to the working face advance cycle. Coal stress is statistically analyzed at different distances in front of retreat channel 3. Microseismic events are statistically analyzed according to event energy level, event location, and event occurrence time. Deformation of the retreat channel is statistically analyzed separately at the roof center point, the side center point far from the working face, and the floor measuring point.

[0116] Monitoring data is stored in segments according to the distance from the working face to retreat channel 3. The segmented data includes pre-fracturing data, post-fracturing data, data at the final stage, data at the breakthrough stage, and data after breakthrough. The segmented data is used to establish a control effect comparison dataset.

[0117] The control range is determined based on the control effect comparison dataset. This dataset includes data before fracturing, data after fracturing, data near the final extraction stage, data during the breakthrough stage, and data after breakthrough. The control range for the roof subsidence of the pullback channel is determined based on data from the post-fracturing stage, breakthrough stage, and post-breakthrough stages, as well as the allowable deformation of the pullback channel design. The control range for the sidewall displacement of the pullback channel is determined based on data from the post-fracturing stage, breakthrough stage, and post-breakthrough stages, as well as the allowable deformation of the support structure.

[0118] Sub-step S502: Calculate the resistance pressure criterion value at the end of the support cycle.

[0119] The average and root mean square deviation of the stent's end-cycle resistance are calculated. The average stent end-cycle resistance characterizes the stress level on the stent within the statistical period. The root mean square deviation of the stent end-cycle resistance characterizes the degree of stress fluctuation on the stent within the statistical period. The pressure criterion value is determined by the average and root mean square deviation of the stent end-cycle resistance.

[0120] The formula for calculating the root mean square error of the stent's end-cycle resistance is as follows: ; in, The mean squared error of the stent end-cycle resistance represents the degree of fluctuation of the stent end-cycle resistance around the average value, and the unit is MPa. The actual number of cycles represents the number of stent cycles included in the statistics; For the first The measured resistance at the end of each cycle represents the pressure borne by the support at the end of a single cycle, in MPa. The average value of the stent end-cycle resistance represents the average level of stent end-cycle resistance within the statistical period, and the unit is MPa. This is the cycle number, and its value ranges from 1 to... .

[0121] The formula for calculating the criterion of pressure from the top plate is as follows: ; in, The pressure threshold for the top plate is the pressure criterion value, which represents the pressure threshold for the judgment cycle, and the unit is MPa; The average value of the stent end-cycle resistance represents the average level of stent end-cycle resistance within the statistical period, and the unit is MPa. The mean squared deviation of the stent end-cycle resistance represents the degree of fluctuation of the stent end-cycle resistance around the average value, and the unit is MPa.

[0122] Sub-step S503: Comprehensively verify the stability control results of the surrounding rock of the retreat channel.

[0123] The support's final cycle resistance is compared with the roof pressure criterion. When the support's final cycle resistance exceeds the roof pressure criterion, and the roof subsidence in the retreat channel increases or the energy level of microseismic events increases during the same time period, the effect verification module outputs the pressure enhancement stage. When the support's final cycle resistance does not continuously exceed the roof pressure criterion, and the roof subsidence in the retreat channel does not suddenly increase, the effect verification module outputs the controlled stage.

[0124] The location of the peak coal stress is correlated with the position of the working face advancement. When the peak coal stress is located in front of the retreat channel 3 and moves towards the retreat channel 3 as the working face advances, the migration process of the advance support pressure is recorded. When the peak coal stress continues to rise in the region of the retreat channel 3, the effect verification module outputs the verification results.

[0125] The energy level and location of microseismic events are correlated with the locations of the roof cutting line 1, the retreat channel 3, and the goaf. When the energy level of the microseismic event does not continuously increase, and the number of microseismic events in the secondary side area of ​​retreat channel 3 does not continuously increase, the effect verification module outputs the energy propagation reduction result. When the support cycle resistance does not continuously exceed the roof pressure criterion value, the peak coal stress does not continuously increase, the energy level of the microseismic event does not continuously increase, and the roof subsidence and side displacement of the retreat channel are both within the set control range, the effect verification module outputs the result that the surrounding rock stability control of the retreat channel is qualified.

[0126] Specific application examples: In a certain mining area, the cumulative thickness of the coal seam and interbedded rock in a fully mechanized longwall face of an extra-thick coal seam is 23.00m, with a recovery rate determined at 93%. During the final mining stage, the face is controlled by thick, hard overlying rock strata. Key strata identification results show that a 6.80m thick coarse sandstone sub-key stratum exists at a vertical height of 35.30m, a 6.60m thick medium sandstone sub-key stratum exists at a vertical height of 46.40m, and a 10.20m thick coarse sandstone main key stratum exists at a vertical height of 73.00m. These key rock strata are selected as target layers for hydraulic fracturing at both shallow and deep boreholes.

[0127] The numerical model has a length of 245.60 m, a vertical height of 137.41 m, and a top depth of 223.33 m above the Earth's surface. The bottom boundary of the model is fixed, the left and right horizontal boundaries are fixed, and the top boundary is a free boundary. A uniformly distributed load of 6.03 MPa is applied to the top of the model, and the horizontal stress is determined by multiplying the lateral pressure coefficient of 1.20 by the vertical stress.

[0128] The minimum confining pressure is taken as the uniformly distributed load at the top of the model, i.e. The maximum confining pressure is determined by multiplying the lateral pressure coefficient of 1.20 by the minimum confining pressure, i.e. The calculation process for the principal stress ratio is as follows: ; The above calculations show that the surrounding rock of retreat passage 3 is under non-uniform stress. Combining the calculation results of the plastic zone boundary and the numerical simulation results of the final mining stage, it is determined that the surrounding rock of retreat passage 3 has a risk of plastic zone expansion. This risk assessment result will serve as input data for the subsequent roof assessment module and parameter determination module.

[0129] During the working face advancement, the influence range of the advance support pressure is 50m in front of the working face. The completion time of the top cutting operation is set before the advance support pressure rise zone reaches the retreat channel 3. Considering that the construction period of retreat channel 3 is 30 days, the hydraulic fracturing operation is completed when the working face is 250m away from retreat channel 3. This time is earlier than the time when the advance support pressure rise zone acts on retreat channel 3.

[0130] The maximum principal stress at the fracturing point is taken as The minimum principal stress at the fracturing point is taken as The tensile strength of coarse sandstone is taken as The calculation process for the estimated initiation pressure required for hydraulic fracturing is as follows: ; The calculations above show that the estimated initiation pressure required for hydraulic fracturing is approximately 34.53 MPa. The on-site injection pressure is set to be no less than 50 MPa, meeting the control requirement that the on-site injection pressure not be less than the estimated initiation pressure required for hydraulic fracturing. The flow rate under maximum fracturing pressure conditions should be no less than 80 L / min.

[0131] The internal friction angle of the rock is taken as The thickness of the basic top rock layer is taken as The amount of rock subsidence was taken. The basic length of the top rock block is taken as The calculation process for the angle between the cut and the vertical direction is as follows: ; As calculated above, the angle between the cut and the vertical direction is not less than 9.60°. The drilling angle on site is determined based on the location of the key rock strata and the spatial conditions of the retreat channel 3, so that the top cutting line 1 penetrates the target key rock strata and the cut-off structural block 6 enters the goaf collapse rock filling area along the top cutting line 1.

[0132] In this specific application embodiment, the deep holes have a depth of 74m, an azimuth of 90°, an inclination of 69°, a hole spacing of 15m, and a two-end hole spacing of 12m. There are 17 deep holes, and the cumulative length of the deep holes is calculated as 17 × 74m = 1258m. The shallow holes have a depth of 51m, an azimuth of 90°, an inclination of 54°, a hole spacing of 15m, and 16 shallow holes. The cumulative length of the shallow holes is calculated as 16 × 51m = 816m. The two side holes have a depth of 51m, an inclination of 80°, and azimuths of 77° and 103° respectively. There are 3 side holes on each side, and the cumulative length of a single side hole is calculated as 3 × 51m = 153m.

[0133] The total number of boreholes is calculated as 17 + 16 + 3 + 3 = 39. The cumulative borehole length is calculated as 1258m + 816m + 153m + 153m = 2380m. The borehole diameter is 75mm. The above borehole layout is used to form a combination of deep holes, shallow holes, and edge holes for fracturing.

[0134] The sealing pressure was 18 MPa to 20 MPa. The pressure holding time for each section was 25 to 30 minutes. Fracturing was carried out using a segmented retraction method. After the current fracturing section was completed, the pressure inside the borehole was released, and the sealing device was retracted to the next fracturing section. After all fracturing sections were completed, the fracturing borehole was inspected by drilling. The drilling inspection results showed that multiple fracturing sections formed radial macroscopic fractures and axial fractures, which extended within the target critical rock strata.

[0135] See attached document Figure 10 , Figure 10 The horizontal axis represents the Stage Number, indicating the stage number. Figure 10 The vertical axis is Absolute Horizontal Displacement m, which represents the absolute horizontal displacement in meters. Figure 10 The broken line in the figure is formed by connecting multiple triangular data points, representing the horizontal displacement change of the pullback channel 3 away from the center point of the working face under different stage numbers. Figure 10The horizontal displacement gradually increases from 0m in the initial stage, reaching approximately 0.141m when the stage number approaches 39.

[0136] Figure 10 The data shows that the horizontal displacement of the retreat channel 3, away from the center point of the working face, exhibits two accelerated growth stages. The first accelerated growth stage occurs during the stress adjustment phase after the formation of retreat channel 3, with the horizontal displacement reaching approximately 0.064m. The second accelerated growth stage occurs during the final mining phase of the working face, starting near stage number 26, with the horizontal displacement increasing from approximately 0.095m to approximately 0.141m after completion. The incremental horizontal displacement during the second accelerated growth stage is 0.141m - 0.095m = 0.046m. Figure 10 The curve in the middle did not show a drop section throughout, indicating that the side wall far from the working face continued to undergo horizontal deformation into the roadway space during the final mining process.

[0137] Figure 10 Used to verify the deformation control results of the third side of the pullback channel. The effect verification module will... Figure 10 The corresponding horizontal displacement curves were correlated with the fracturing operation time, working face advancement position, and support pressure data to determine the relationship between the bulging deformation of the 3rd sidewall of the retreat channel and the final mining advancement stage. Figure 10 The final value of the horizontal displacement is approximately 0.141m, which is used to determine the deformation control range of the retreat channel 3 away from the working face side.

[0138] See attached document Figure 11 , Figure 11 The horizontal axis represents the Stage Number, indicating the stage number. Figure 11 The vertical axis is Absolute Vertical Displacement m, which represents the absolute vertical displacement in meters. Figure 11 The broken line in the diagram is formed by connecting multiple triangular data points, representing the vertical displacement change of the center point of the top plate of the pullback channel 3 under different stage numbers. Figure 11 The vertical displacement gradually increases from 0m in the initial stage, reaching approximately 0.211m when the stage number approaches 39.

[0139] Figure 11 The data shows that the vertical displacement of the center point of the roof in the pullback passage 3 exhibits an accelerated growth phase. This accelerated growth phase occurs during the stress adjustment phase after the formation of pullback passage 3, with the vertical displacement reaching approximately 0.105m. Subsequently, the vertical displacement continues to increase with the final mining advance, reaching approximately 0.211m after the breakthrough. The increase in vertical displacement during the breakthrough phase relative to the stress adjustment phase is 0.211m - 0.105m = 0.106m. Figure 11The curve in the figure shows an increasing trend throughout, indicating that the roof subsidence is affected by the stress adjustment after the excavation of the retreat channel 3 and the final mining advance of the working face.

[0140] Figure 11 Used to verify the deformation control results of the top plate of the retraction channel 3. The effect verification module will... Figure 11 The corresponding vertical displacement curve and Figure 10 Compare the corresponding horizontal displacement curves. Figure 11 The final vertical displacement is approximately 0.211m, which is greater than... Figure 10 The final horizontal displacement is approximately 0.141m, and the difference between the two is 0.211m - 0.141m = 0.070m. The ratio between the two is 0.211 ÷ 0.141 ≈ 1.50. The above comparison results are used to determine the top plate area of ​​the retreat channel 3 as the key verification area.

[0141] Experimental verification and effect comparison: After the excavation of retreat passage 3, the plastic zone was mainly located in the roof and the sidewalls far from the working face. The maximum depth of the plastic zone in the roof was 3.40m, and the maximum depth of the plastic zone in the horizontal direction was 5.70m. In the final stage of mining, the range of the plastic zone continued to expand. At the time of breakthrough, the maximum depth of the plastic zone in the roof reached 18.40m, and the maximum depth of the plastic zone far from the working face reached 19.40m.

[0142] The calculation process for the maximum depth magnification factor of the plastic zone in the roof is 18.40 ÷ 3.40 = 5.41. The calculation process for the maximum depth magnification factor of the plastic zone in the sidewalls far from the working face is 19.40 ÷ 5.70 = 3.40. The above calculation results show that the depth of the plastic zone in the roof during the final breakthrough stage is 5.41 times the depth of the plastic zone in the roof after the excavation of retreat passage 3, and the depth of the plastic zone in the sidewalls far from the working face is 3.40 times the depth of the horizontal plastic zone after the excavation of retreat passage 3.

[0143] Vertical stress was compared under different mining thicknesses. At mining thicknesses of 3.80m, 4.30m, and 7.00m, the maximum vertical stress at measuring point A was 17.72MPa, 17.66MPa, and 18.45MPa, respectively; at measuring point B, the maximum vertical stress was 20.57MPa, 20.43MPa, and 19.55MPa; at measuring point C, the maximum vertical stress was 7.99MPa, 8.06MPa, and 5.93MPa; and at measuring point D, the maximum tensile stress was 0.63MPa, 0.96MPa, and 0.35MPa. At mining thicknesses of 3.80m and 4.30m, the remaining top coal provided support for the collapsed rock strata. At a mining thickness of 7.00m, the roof subsidence increased. Based on the mining thickness conditions, the adaptability of the retreat equipment, and the roof subsidence results, the final mining height was determined to be 4.30m. Compared to a mining height of 3.80m, a mining height of 4.30m increases the amount of coal extracted by more than 8,000 tons.

[0144] A total of 193,907 data points on the working resistance of the support were obtained during the final stage of hydraulic fracturing. The proportion of support working resistance exceeding 20 MPa was 93.97%, exceeding 25 MPa was 85.91%, exceeding 30 MPa was 54.54%, exceeding 35 MPa was 30.81%, exceeding 40 MPa was 18.75%, and exceeding 45 MPa was 6.45%. The average pressure of the front support column was 31.70 MPa, and the average pressure of the rear support column was 6.81 MPa. The difference between the average pressure of the front and rear columns was 31.70 MPa - 6.81 MPa = 24.89 MPa. The ratio of the average pressure of the front and rear columns was 31.70 ÷ 6.81 ≈ 4.65. These support pressure data were used by the effect verification module to calculate the roof pressure criterion value and to evaluate the pressure distribution state of the working face during the final stage of hydraulic fracturing and roof cutting.

[0145] Stress monitoring of the retreat channel 3 and auxiliary transport roadway showed that the stress at each measuring point generally reached its peak at a distance of 14m to 17m from the coal face, and the stress change was related to... Figure 4 The diagram corresponds to the migration process of the high-stress zone during mining. Microseismic monitoring showed that microseismic events with energy levels of 38562 J and 41587 J occurred when the working face was 140 m away from the retreat channel 3. The energy difference between the two microseismic events was 41587 J - 38562 J = 3025 J. No higher-energy microseismic events were recorded when the working face continued to advance to 78 m away from the retreat channel 3 and during the breakthrough stage. The number of microseismic events in the secondary side area of ​​the retreat channel 3 was lower than the number of microseismic events near the roof cutting line 1 and on the goaf side.

[0146] After the working face was completed, the top slab and support structure of the retreat passage 3 remained intact, and the side-mounted supports and hydraulic supports of the working face did not experience any damage. The maximum subsidence of the top and bottom slabs of the retreat passage 3 was 600mm, or 0.600m. This maximum subsidence is greater than... Figure 11 The final vertical displacement of the center point of the roof was 0.211m, and the difference between the two values ​​was 0.600m - 0.211m = 0.389m. The subsidence in the central region was greater than that at the ends, and no significant displacement was observed in the coal pillar sides. The above results are consistent with... Figure 10 and Figure 11 The deformation evolution data correspond to the data, indicating that after adopting the method of the present invention, the deformation of the surrounding rock of the retreat channel 3 is within the monitoring and verification range.

Claims

1. A method for controlling the stability of surrounding rock in a retreat channel based on the butterfly-shaped expansion theory of the plastic zone, characterized in that, Includes the following steps: Data on the surrounding rock stress, strength, and cross-sectional dimensions of the retreat tunnel are collected. The plastic zone state of the surrounding rock is identified based on these data. Specifically, the principal stress ratio is determined based on the maximum and minimum confining pressures. An implicit equation is constructed based on the Mohr-Coulomb failure criterion, including the equivalent radius of the retreat tunnel, the distance from the plastic zone boundary point to the tunnel center, the polar angle of the plastic zone boundary point, the principal stress ratio, the rock internal friction angle, the rock cohesion, and the minimum confining pressure. The plastic zone boundary is obtained by solving this equation. When the plastic zone boundary is mainly distributed along the direction of the minimum principal stress and does not form an outwardly convex butterfly-shaped region along the direction of the maximum principal stress, an ear-shaped expansion state is output. When the plastic zone boundary forms an outwardly convex butterfly-shaped region along the direction of the maximum principal stress, a butterfly-shaped expansion state is output. The basic roof fracture status is determined based on the plastic zone status of the surrounding rock in the retreat channel, the location data of the basic roof fracture line, the location data of the stop mining line, the location data of the retreat channel, and the working face advancement location data. Based on the working face advance position data, determine the mining stress distribution curves for different advance stages, determine the location of the mining high stress zone based on the peak position of the mining stress distribution curves, and determine the process of the mining high stress zone moving towards the retreat channel side; based on the working face advance speed, drilling construction time, fracturing construction time, and post-fracturing fracture development time, determine the time period before the mining high stress zone reaches the retreat channel as the timing for the completion of the top cutting operation. When the basic top fracture state is either above the retreat channel or in front of the retreat channel, the process of determining hydraulic fracturing top cutting parameters begins. Based on the top lithological columnar data, stratum thickness, rock strength, and key layer identification results, the layers to be fractured are determined. These layers include high-level key strata, low-level key strata, and the corresponding strata in the top structure of the working face end area. Based on the layers to be fractured, deep holes, shallow holes, and edge holes are determined. The borehole opening positions and final borehole positions for deep holes, shallow holes, and edge holes are then determined. The borehole depth, borehole diameter, borehole spacing, borehole inclination angle, borehole azimuth angle, fracturing pressure, fracturing flow rate, sealing pressure, holding time, and number of fracturing stages are used as parameters for hydraulic fracturing top cutting. The angle between the borehole cut and the vertical direction is calculated and determined based on the internal friction angle of the rock, the thickness of the basic top stratum, the subsidence of the rock block, and the length of the basic top rock block. This angle satisfies the requirement that the cut-off structural block enters the goaf collapse rock filling area along the top cutting line. The formula for calculating the angle between the borehole cut and the vertical direction is: in The angle between the cut and the vertical direction. The internal friction angle of the rock. This represents the thickness of the basic top rock layer. This represents the amount of rock subsidence. The length of the basic top rock block; The estimated initiation pressure required for hydraulic fracturing is determined based on the minimum principal stress at the fracturing point, the maximum principal stress at the fracturing point, and the tensile strength of the rock strata above the fracturing point. The formula for calculating the estimated initiation pressure required for hydraulic fracturing is as follows: in This is an estimated value for the initiation pressure required for hydraulic fracturing. The minimum principal stress at the fracturing point, The maximum principal stress at the fracturing point, The tensile strength of the top rock layer after fracturing; Perform hydraulic fracturing and top cutting according to the aforementioned hydraulic fracturing and top cutting parameters, and record the construction process data; Mine pressure deformation data were collected, and the surrounding rock stability results were verified based on the construction process data and the mine pressure deformation data. Specifically, the following data were collected: support circulation resistance, coal stress, microseismic events, roof subsidence in the retreat channel, and sidewall displacement in the retreat channel. The average and root mean square deviation of the support circulation resistance were calculated based on the support circulation resistance, and the roof pressure criterion value was determined based on the average and root mean square deviation of the support circulation resistance. The formula for calculating the roof pressure criterion value is as follows: in The criterion value for pressure on the top plate. This represents the average resistance value of the stent during circulation. The root mean square of the stent circulation without resistance; When the resistance of the support circulation does not continuously exceed the roof pressure criterion value, the peak stress of the coal body does not continuously increase, the energy level of the microseismic event does not continuously increase, and the roof subsidence and side displacement of the retreat channel are both within the set control range determined by the control effect comparison dataset, the result of qualified control of the surrounding rock stability of the retreat channel is output.

2. The method for controlling the stability of surrounding rock in a retreat channel based on the butterfly-shaped expansion theory of the plastic zone according to claim 1, characterized in that, The specific steps for collecting the surrounding rock stress data of the retreat channel are as follows: Time series data were collected from measuring points on the top plate of the retreat channel, the bottom plate of the retreat channel, measuring points near the working face, and measuring points far from the working face. Collect the maximum and minimum confining pressures, the direction of the maximum principal stress, and the direction of the minimum principal stress around the retreat channel; The stress field distribution around the pullback channel was determined based on the time series data of each stress acquisition point.

3. The method for controlling the stability of surrounding rock in a retreat channel based on the butterfly-shaped expansion theory of the plastic zone according to claim 1, characterized in that, The specific steps for collecting the surrounding rock strength data and cross-sectional data of the retreat tunnel are as follows: Collect rock cohesion, rock internal friction angle, rock tensile strength, elastic modulus, Poisson's ratio, and rock layer thickness; Collect data on the width, height, and excavated area of ​​the retreat tunnel. The equivalent radius of the retreat channel is determined based on the excavation outline area of ​​the retreat channel, and the rock cohesion, the rock internal friction angle, and the equivalent radius of the retreat channel are used to calculate the boundary of the plastic zone.

4. The method for controlling the stability of surrounding rock in a retreat channel based on the butterfly-shaped expansion theory of the plastic zone according to claim 1, characterized in that, The specific steps for determining the basic top fracture state are as follows: Acquire basic top fracture line location data, stop mining line location data, retreat channel location data, and working face advance location data; When the basic top fracture line is located behind the stop mining line, output the fracture state behind the stop mining line; When the basic top break line is located above the retracement channel, output the break state above the retracement channel. When the basic top fracture line is located in front of the retracement channel, output the fracture status in front of the retracement channel.

5. The method for controlling the stability of surrounding rock in a retreat channel based on the butterfly-shaped expansion theory of the plastic zone according to claim 1, characterized in that, The specific steps for performing hydraulic fracturing and top cutting are as follows: Construct deep holes, shallow holes, and side holes according to the hydraulic fracturing top cutting parameters described above; The sealing device and high-pressure pipeline are sent into the predetermined fracturing section, and the predetermined fracturing section is subjected to sealing pressure test. After the borehole is sealed and pressure is tested, pressure is applied to the predetermined fracturing section and maintained, so that the fracture extends along the key rock layer and connects with the adjacent fracturing section or the fracture of the adjacent borehole to form a continuous cut.

6. The method for controlling the stability of surrounding rock in a retreat channel based on the butterfly-shaped expansion theory of the plastic zone according to claim 1, characterized in that, The specific steps for recording construction process data are as follows: During the sealing and pressure testing process, the borehole pressure was continuously recorded, along with the borehole number, fracturing section number, maximum water injection pressure, cumulative water injection time, cumulative water injection volume, return water location, and pressure change curve. After the current fracturing section is completed, release the pressure inside the hole and retract the sealing device to the next predetermined fracturing section; Repeat the sealing and pressure testing, pressure fracturing, pressure holding and propagation, and pressure release and pipe withdrawal operations until all fracturing sections in the same borehole are completed.

7. The method for controlling the stability of surrounding rock in a retreat channel based on the butterfly-shaped expansion theory of the plastic zone according to claim 1, characterized in that, The specific steps for verifying the stability results of the surrounding rock are as follows: Collect the support cycle end resistance, coal stress, micro-seismic events, roof subsidence of the retreat channel and side displacement of the retreat channel as mine pressure deformation data; The mine pressure deformation data is segmented and saved according to the distance between the working face and the retreat channel to form a control effect comparison dataset. The control effect comparison dataset includes data before fracturing, data after fracturing, data at the final mining approach stage, data at the breakthrough stage, and data after breakthrough. The average value and the root mean square deviation of the support cycle end resistance are calculated based on the support cycle end resistance, and the top plate pressure criterion value is determined based on the average value and the root mean square deviation of the support cycle end resistance. Based on the control effect comparison dataset, determine the set control range for the top plate subsidence and side displacement of the retreat channel; When the resistance at the end of the support cycle does not continuously exceed the roof pressure criterion value, the peak stress of the coal body does not continuously increase, the energy level of the micro-seismic event does not continuously increase, and the roof subsidence and side displacement of the retreat channel are both within the set control range determined by the control effect comparison dataset, the result of qualified control of the surrounding rock stability of the retreat channel is output.

Images