An efficient method for caving the suspended top coal at the end of a fully mechanized caving working face in an extra-thick coal seam

Abstract

The present invention discloses a highly efficient method for collapsing the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam. The method utilizes hydraulic coal breaking and caving to weaken the top coal at the end of the fully-mechanized caving working face to precisely control the maximum suspended length of the suspended top coal at the end of the caving working face. Modeling and numerical simulation are first performed based on the basic mechanical parameter data, internal porosity data, and fracture distribution data of the coal body and surrounding rock of the target fully-mechanized caving working face and its goaf. After obtaining the optimal hydraulic coal breaking and caving weakening technical parameters that can meet the target requirements for the maximum suspended length of the top coal, hydraulic coal breaking and caving operations are then performed on-site at the target fully-mechanized caving working face in the mine. The present invention is safe and reliable, cost-effective, and has a good weakening effect. It can achieve precise control while effectively weakening the mechanical strength of the top coal and controlling the maximum suspended length of the top coal. It is particularly suitable for weakening and collapsing the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam.

Description

Technical Field

[0001] The present invention relates to a method for caving suspended top coal, specifically a method for efficiently caving suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam, which utilizes hydraulic coal breaking technology to weaken the mechanical strength of the suspended top coal at the end of a fully-mechanized caving working face and significantly reduce the maximum suspended top length of the suspended top coal. The method belongs to the technical field of safe mining in coal mines. Background Art

[0002] Top-coal caving is a mining technique developed specifically for thick and extra-thick coal seams. When using this method, a mining face with a mining height of 2 to 3 meters is located along the floor of the coal seam or within a certain thickness range. Mining is carried out using a comprehensive mechanized method. Using mine pressure or supplemented by loosening blasting, the top coal is released through "coal windows" behind or on the supports and transported out of the face by a scraper conveyor. This method offers many significant advantages, including the ability to mine the entire height of coal seams 5 to 20 meters thick in one go, allowing for the simultaneous extraction of coal that would have previously required multiple layers in thicker seams. However, if the top coal above the mining face is relatively hard, part of the top coal, especially the top coal at the end of the working face, will be difficult to collapse in time and form a suspended roof. When the suspended length of the top coal at the end is large, this part of the top coal is prone to accumulate a large amount of elastic energy. If it suddenly breaks under the disturbance of the mining site, it may induce a roof accident. At the same time, the existence of suspended top coal will also lead to gas accumulation and air leakage in the goaf, inducing gas disaster risks and coal natural hazards. Therefore, the suspended length of the top coal at the end of the top coal caving working face should not be too long, and technical means should be used to ensure that the suspended top coal at the end of the working face collapses in time.

[0003] The conventional method of forced top coal caving in the suspended top coal working face mainly weakens the suspended coal body in advance to shorten the suspended length required for the natural fall of the top coal. Explosive blasting, hydraulic fracturing, carbon dioxide pre-cracking and other means are usually used, but they all have certain shortcomings: explosive blasting has a good weakening effect, but the high energy release generated can easily induce gas explosions, coal dust and other accidents, posing a major safety hazard, and the acquisition and storage of explosives are both very difficult; hydraulic fracturing is pollution-free and safe, but the fracturing effect in the coal seam is weak, and the controllability of the crack propagation direction and length is poor, making it difficult to achieve precise weakening; carbon dioxide pre-cracking technology is relatively new, the cost of a single fracturing is high, and it is currently difficult to promote widely. Summary of the Invention

[0004] In response to the problems existing in the above-mentioned prior art, the present invention provides a method for efficiently collapsing the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam. The method has the characteristics of being safe, reliable, cost-controlled and having a good weakening effect. It can achieve precise control while effectively weakening the mechanical strength of the top coal and controlling the maximum suspended length of the top coal. The method is particularly suitable for the weakening and collapsing treatment of the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam.

[0005] To achieve the above objectives, this method for efficiently caving the suspended top coal at the end of a fully-mechanized top-coal caving working face in an extremely thick coal seam utilizes hydraulic coal breaking and cavitation to weaken the top coal at the end of the fully-mechanized top-coal caving working face, thereby accurately controlling the maximum suspended length of the suspended top coal at the end of the top-coal caving working face. The method specifically includes the following steps:

[0006] Step 1: Obtain mine data, including basic mechanical parameter data, internal porosity data, and fracture distribution data of the coal body and surrounding rock of the target fully mechanized caving working face coal seam and its goaf;

[0007] Step 2: Based on the data from Step 1, a load-damage mathematical model of the coal strata mined at the target fully-mechanized caving working face is constructed, and a numerical simulation of the mining process of the target fully-mechanized caving working face is performed to obtain the stress distribution, deformation characteristics, and cyclical collapse data of the roof and top coal of the target fully-mechanized caving working face and roadway. The numerical simulation model is verified and parameters are optimized using the cyclical fracture and collapse data of the roof and top coal at the mine site, the coal seam mining stress data, and the roadway surrounding rock deformation and separation data, thereby forming a simulation model that conforms to the actual production conditions and monitoring data at the mine site. In the simulation model, hydraulic coal breaking and cavitation are simulated at the top coal of the working face end to obtain the optimal hydraulic coal breaking and cavitation weakening technical parameters that can meet the target requirement of the maximum suspended length of the top coal.

[0008] The diameter and depth of hydraulic coal breaking holes meet the following requirements:

[0009] L R cosθ+L4 sinθ

[0010] Where: L R is the diameter of the hydraulically broken coal hole, L4 is the depth of the hydraulically broken coal hole in the front-back direction, H1 is the thickness of the top coal above the roadway, and θ is the drill rod elevation angle;

[0011] In step 3, based on the optimized hydraulic coal breaking and cavitation weakening technical parameters obtained in step 2, a hole is drilled obliquely into the suspended top coal behind the end of the target fully-mechanized caving working face in the mine. A high-pressure water jet is ejected from the jet nozzle at the end of the drill pipe and sprayed radially along the drill pipe to impact and crush the coal body. Through the rotation and retraction of the drill pipe, a hydraulic coal breaking cavity is formed in the suspended top coal, completing a hydraulic coal breaking and cavitation operation.

[0012] The maximum suspended length of the weakened top coal is less than the target control length. The relationship between the maximum length required for the suspended top coal to fracture and collapse and the weakened mechanical strength satisfies

[0013]

[0014] f1=ρ1gH1 ​

[0015] f2=ρ2gH1

[0016] Where: R m is the tensile strength of the top coal, f1 is the self-weight of the immediate top, f2 is the self-weight of the top coal, ρ1 is the material density of the immediate top, ρ2 is the material density of the top coal, and g is the acceleration due to gravity;

[0017] Step 4: As the working face continues to advance forward and the top coal fractures and collapses after being weakened at the rear, the numerical simulation model and the simulation model are continuously verified and optimized based on the real-time monitoring data at the target fully-mechanized caving working face of the mine, and key parameters are corrected. Hydraulic coal breaking and cavitation operations are carried out again at the target fully-mechanized caving working face of the mine after the top coal fractures and collapses after being weakened at the rear.

[0018] Furthermore, in Step 2, when simulating hydraulic coal breaking and hole formation in the top coal of the working face end in the simulation model, drilling and coal breaking and hole formation are performed near the junction of the suspended top coal and the top beam of the working face end support equipment, and the forward extension range of the hydraulic coal breaking hole does not exceed the junction of the suspended top coal and the top beam of the working face end support equipment.

[0019] Furthermore, in Step 2, when simulating hydraulic coal breaking and cavitation at the top coal of the working face in the simulation model, the internal staggered angle between the drill rod and the tunnel space in front of the working face in the front-to-back direction is 0°.

[0020] Furthermore, in Step 2, when the hydraulic coal breaking and cavitation are simulated in the top coal of the working face in the simulation model, the drill rod elevation angle θ satisfies

[0021] θ=arctan[(H2-H3) / L1]

[0022] Where: H2 is the height of the tunnel, H3 is the height of the drilling rig, and L1 is the distance between the drilling rig and the end of the working face.

[0023] Furthermore, in Step 3, when the hydraulic coal breaking cavity extends forward to the junction of the suspended top coal and the top beam of the working face end support equipment, and the suspended top coal does not fall as expected, the hydraulic coal breaking cavity making operation is stopped. According to the real-time monitoring data of the weakened top coal at the target fully mechanized caving working face of the mine, the drill rod elevation angle θ of the drill rig and the distance L1 between the drill rig and the working face end are adjusted, and Step 2 is repeated to re-perform the hydraulic coal breaking cavity making operation in the suspended top coal behind the working face end.

[0024] Furthermore, after obtaining the stress distribution, deformation characteristics and periodic collapse data of the target fully mechanized caving working face and roadway in Step 2, the initial position of the hydraulic coal-breaking hole 10, the drill rod elevation angle θ and the diameter size L of the hydraulic coal-breaking hole are adjusted. R, the depth dimension L4 of the hydraulic coal breaking hole along the front-to-back direction, the height dimension H3 of the drilling rig, the distance dimension L1 between the drilling rig and the end of the working face and other parameters are grouped for numerical simulation, and the maximum value of the suspended length L3 of the top coal at the end of the working face under different hydraulic coal breaking hole parameters is obtained. The main controlling factors are obtained to form an optimal coal breaking hole parameter range that can meet the requirements of the top coal suspended length.

[0025] Furthermore, after obtaining the stress distribution, deformation characteristics and periodic collapse data of the target fully mechanized caving working face and roadway in Step 2, group numerical simulation is performed according to the setting conditions of the on-site working face end support equipment, the stress change record data of the roadway support and the mine pressure manifestation record data, to determine the optimal hydraulic coal breaking and cavitation weakening technical parameters that can meet the target requirements for the maximum suspended length of the top coal under the influence of different periodic pressure, support and mining disturbance coupling.

[0026] Furthermore, when Step 1 obtains the basic mechanical parameter data, internal porosity data and fracture distribution data of the coal body and surrounding rock of the target fully-mechanized caving working face coal seam and its goaf, coal body and surrounding rock samples are selected on-site at the target fully-mechanized caving working face and its goaf in the coal mine and brought to the ground, standard samples are prepared in the laboratory, and the basic mechanical parameter data, internal porosity data and fracture distribution data of the standard samples are tested and obtained.

[0027] Compared with the existing technology, this method of efficient caving of suspended top coal at the end of fully-mechanized caving working face in extra-thick coal seams utilizes hydraulic coal breaking and cavitation to weaken the top coal at the end of fully-mechanized caving working face so as to accurately control the maximum suspended length of suspended top coal at the end of top coal caving working face, which can significantly weaken the mechanical strength of suspended top coal and greatly shorten the maximum length of suspended top, with significant effect; at the same time, the construction process of this method of efficient caving of suspended top coal at the end of fully-mechanized caving working face in extra-thick coal seams is relatively refined, the engineering amount is small, and the water jet coal breaking technology itself is relatively cost-effective. The method is mature and has low construction difficulty. In addition, the high-efficiency caving method of suspended top coal at the end of the fully-mechanized caving working face in extra-thick coal seams can significantly reduce the safety hazards such as gas accumulation and air leakage in the corners of the working face, as well as impact ground pressure, gas disasters and spontaneous combustion of residual coal, which are easily induced during the production process of the working face by controlling the length of the suspended top coal. In addition, the water jet coal breaking process can also promote the initiation and expansion of cracks inside the coal body, and water has a softening effect on the coal body, which can further increase the degree of fragmentation and expansion of the top coal after breaking and caving, thereby improving the top coal release rate and the overall coal mining rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the equipment arrangement at the end of the fully mechanized caving working face and the hydraulic coal breaking and cave-making operation;

[0029] Figure 2 yes Figure 1 lateral view of;

[0030] Figure 3 yes Figure 1 A top-down rotation view of .

[0031] Figure: 1. Target coal seam, 2. Roadway space in front of the working face, 3. High-pressure water pump, 4. Drill rig, 5. Drill rod, 6. Support equipment at the working face end, 7. Jet nozzle, 8. Water jet, 9. Space in the fully mechanized caving working face, 10. Hydraulically broken coal cavities, 11. Overhanging top coal at the working face end, 12. Goaf behind the working face;

[0032] H1 is the thickness of the top coal above the roadway, H2 is the height of the roadway, H3 is the height of the drilling rig, L1 is the distance between the drilling rig and the end of the working face, L2 is the width of the working face in the front-to-back direction; L3 is the suspended length of the top coal in the front-to-back direction, L4 is the depth of the hydraulic coal-breaking cavity in the front-to-back direction, L5 is the upper width of the roadway, L6 is the lower width of the roadway, L R is the diameter of the hydraulic coal breaking cavity, and θ is the drill pipe elevation angle. DETAILED DESCRIPTION

[0033] The present invention will be further described below by taking a comprehensive top coal caving mining face of a coal mine as an example with reference to the accompanying drawings.

[0034] This method is to efficiently collapse the suspended top coal at the end of a fully-mechanized top-coal caving face in an extremely thick coal seam. It uses hydraulic coal breaking and cavitation to weaken the top coal at the end of the fully-mechanized top-coal caving face, thereby accurately controlling the maximum suspended length of the suspended top coal at the end of the top-coal caving face. Specifically, it includes the following steps:

[0035] Step 1: First, obtain the mine data including the geological conditions and mining conditions of the target fully-mechanized caving working face coal seam, as well as the basic mechanical parameter data, internal porosity data and fracture distribution data of the target fully-mechanized caving working face coal seam and its goaf.

[0036] The average burial depth of the comprehensive top coal caving mining face in this coal mine is 500m, the average coal seam inclination is 31°, and the average coal seam thickness is 7m. Key information such as the layout of the target comprehensive top coal caving working face and the surrounding goaf, the roadway support method, and coal mining parameters are determined based on a comprehensive histogram of the coal and rock strata overlying the coal seam (including parameters such as thickness and lithology). The target comprehensive top coal caving working face has a mining height of 2.7m, a coal caving height of 4.3m, a top coal thickness H1 above the roadway of 3.3m, a roadway height H2 of 3.7m, a mining rate of 3.2m / day, an upper roadway width L5 of 4.4m, a lower roadway width L6 of 5.4m, a working face width L2 in the front-to-back direction of 6m, and a working face end support device 6 using end hydraulic support.

[0037] In order to accurately obtain the basic mechanical parameter data, internal porosity data and fracture distribution data of the coal body and surrounding rock of the target fully-mechanized caving working face and its goaf, relatively complete large-scale block samples of coal body, gangue and rock were selected from the target fully-mechanized caving working face and its goaf in the coal mine. They were brought to the surface, and standard samples were prepared in the laboratory. The actual basic mechanical parameters such as compressive strength, tensile strength, Poisson's ratio, etc. of the standard samples were tested and obtained. The details are as follows:

[0038] The block sample selected underground should be approximately cubic in shape and have a volume of no less than 0.1m 3 In the laboratory, the block samples were processed into cylindrical standard samples of different standard sizes such as Ф50×100mm and Ф50×250mm. After the samples were grouped, the basic mechanical parameters of the standard samples were obtained by using the MTS triaxial loading testing machine, and the porosity and crack distribution inside the standard samples were tested using a mercury intrusion meter and a rock CT machine.

[0039] After testing, the compressive strength of the raw coal standard sample was 14.1 MPa, the tensile strength was 1.0 MPa, the Poisson's ratio was 0.31, and the elastic modulus was 1.1 GPa. The porosity of the raw coal standard sample was 7.8%, the average crack opening was 90.1 μm, and the average crack volume was 2.1×10 7 um 3 , the crack degree is 0.83%.

[0040] Step 2: Based on the geological conditions and mining conditions of the target fully-mechanized caving working face coal seam, and on the basis of the mechanical parameters of the coal and rock mass obtained experimentally, a load-damage mathematical model of the target fully-mechanized caving working face coal and rock strata is constructed. The hydraulic coal breaking and cavitation of the end top coal and the numerical simulation of the end top coal collapse during the working face mining process are then carried out to obtain the optimal hydraulic coal breaking and cavitation weakening technical parameters that can meet the target requirements for the maximum overhang length of the top coal:

[0041] Firstly, the UDEC software was used to construct a load-damage mathematical model of the coal strata mined in the target fully-mechanized caving working face. The mine data including the geological conditions and mining conditions of the coal seam of the target fully-mechanized caving working face, as well as the basic mechanical parameter data of the coal body and surrounding rock of the coal seam and its goaf in the target fully-mechanized caving working face, internal porosity data and fracture distribution data were imported into the model to perform numerical simulation of the mining process of the target fully-mechanized caving working face. The stress distribution, deformation characteristics and cyclic collapse data of the roof and top coal of the target fully-mechanized caving working face and roadway were obtained. The numerical simulation model was verified and its parameters were optimized using the cyclic fracture and collapse data of the roof and top coal, coal seam mining stress data, roadway surrounding rock deformation and delamination data at the mine site, and the optimization was performed to form a simulation model that meets the actual production conditions and monitoring data at the mine site.

[0042] Secondly, the hydraulic coal breaking and hole making was simulated in the top coal of the working face in the simulation model, and the maximum length required for the suspended top coal to fracture and collapse during the forward advancement of the working face was analyzed. By changing the initial position of the hydraulic coal breaking hole 10, the drill rod elevation angle θ and the diameter size L of the hydraulic coal breaking hole R The changing pattern of the maximum length of the suspended top coal at the end of the working face is analyzed based on parameters such as the depth dimension L4 of the hydraulic coal breaking cavity along the front-to-back direction, the height dimension H3 of the drilling rig, and the distance dimension L1 between the drilling rig and the end of the working face. The changing pattern of the maximum length of the suspended top coal at the end of the working face is analyzed under the coupling effect of strengthened tunnel support, periodic pressure changes and working face advancement, and the model is verified and optimized in combination with the on-site observation results of the mine.

[0043] When analyzing the maximum length required for the suspended top coal to fracture and collapse during the forward advancement of the working face, the overburden stress and its own gravity were applied to the simulation model, and the mechanical strength of hydraulic coal breaking and cavitation was applied to the suspended part of the top coal. As the working face continued to advance forward in simulation, the suspended length of the top coal continued to increase. Under the action of the overburden stress and its own gravity, the damage to the suspended part of the top coal continued to intensify until fracture and instability occurred. The maximum suspended length L3 of the top coal at the end of the working face was recorded at this time.

[0044] When analyzing the variation law of the maximum length of the suspended top at the end of the working face, the initial position of the hydraulic coal breaking hole 10, the drill rod elevation angle θ and the diameter size L of the hydraulic coal breaking hole are analyzed. R , the depth dimension L4 of the hydraulic coal breaking hole in the front-to-back direction, the height dimension H3 of the drilling rig, the distance dimension L1 between the drilling rig and the end of the working face and other parameters are grouped for numerical simulation, and the maximum suspended length of the top coal L3 of the working face end under the conditions of different sizes of hydraulic coal breaking holes 10 and different hydraulic coal breaking hole parameters is obtained. The main controlling factors are obtained, and the coal breaking hole parameter range that can meet the requirements of the top coal suspension length is formed.

[0045] When analyzing the changing law of the maximum length of the suspended top coal at the end under the coupling effects of enhanced tunnel support conditions, periodic pressure changes and working face advancement, the enhanced support data (such as the enhanced support data obtained after increasing the number of support anchors, increasing the support resistance of the bracket, etc.) and additional periodic pressure data were applied on the basis of the simulation model to perform grouped numerical simulation. The maximum value of the suspended top coal length L3 at the working face end under different enhanced support data and additional periodic pressure data conditions was obtained, forming a coal-breaking cavity parameter range that can meet the requirements of the top coal suspended length.

[0046] In order to improve the coal breaking effect, the diameter and depth of the hydraulic coal breaking hole 10 should meet the following requirements: R cosθ+L4sinθ R ​is the diameter of the hydraulically broken coal hole, L4 is the depth of the hydraulically broken coal hole in the fore-aft direction, H1 is the thickness of the top coal above the roadway, and θ is the drill rod elevation angle. To minimize the distance L1 between the drill rig and the working face end and prevent the drill rod 5 from contacting the working face end support equipment 6 when drilling into the target mining coal seam 1, θ = arctan[(H2-H3) / L1] must be satisfied, where θ is the drill rod elevation angle, L1 is the distance between the drill rig and the working face end, H2 is the roadway height, and H3 is the drill rig height.

[0047] Considering the layout of electromechanical equipment in the tunnel space 2 in front of the working face, the remaining space in the tunnel, the dimensions of the drilling rig 4 and the high-pressure water pump 3, and combining the mine site construction conditions, the optimal technical parameters for weakening the top coal by hydraulic coal breaking and cavitation in the target fully mechanized caving working face at the mine site are obtained. According to the simulation results, Figures 1 to 3 As shown in the figure, the drilling rig 4 is set in the middle position of the tunnel space 2 in the left-right direction in front of the working face. When the maximum allowable length L3 of the coal hanging above the working face end is 4m and the height dimension H3 of the drilling rig is 1.8m, the distance dimension L1 between the drilling rig and the working face end can be determined to be 8.7m, the inner staggered angle between the drill rod 5 and the tunnel space 2 in the front-to-back direction is 0°, the drill rod elevation angle θ is 12°, and the diameter dimension L of the hydraulically broken coal hole is 0°. R The starting position of the hydraulic coal-breaking hole 10 and the drilling oblique length of the drilling rig 4 are 17.6 m, and the depth dimension L4 of the hydraulic coal-breaking hole in the front-to-back direction is 2.2 m.

[0048] Step 3, based on the optimal technical parameters for weakening the top coal by hydraulic coal breaking and forming a cavity obtained in Step 2, hydraulic coal breaking construction is carried out on site at the target fully mechanized caving working face of the mine, and hydraulic coal breaking equipment such as a high-pressure water pump 3 and a drilling rig 4 are arranged in the tunnel space 2 in front of the working face. A hole is drilled obliquely into the suspended top coal behind the end, and high-pressure water is transported by the high-pressure water pump 3 through the drill pipe 5. The high-pressure water jet is sprayed radially along the drill pipe 5 through the jet nozzle 7 (two are symmetrically arranged) at the end of the drill pipe 5. The coal body is crushed by the impact of the high-pressure water jet, and a complete hydraulic coal breaking cavity 10 of approximately cylindrical shape is formed in the suspended top coal through the rotation and retreat of the drill pipe 5. The crushed coal particles and water flow out from the annular space between the drill pipe 5 and the borehole in a mixed state; after the hydraulic coal breaking and cavity forming are completed, the hydraulic coal breaking equipment is removed, and a hydraulic coal breaking and cavity forming operation is completed.

[0049] As the working face continues to advance, the suspended length of the top coal gradually increases, and under the action of the top coal's own weight and the roof pressure, it breaks and collapses in advance. In order to effectively control the safety hazards of the suspended top coal, the maximum suspended length of the weakened top coal should be less than the target control length. The relationship between the maximum length required for the suspended top coal to break and collapse and the weakened mechanical strength satisfies

[0050]

[0051] f1=ρ1gH1

[0052] f2=ρ2gH1

[0053] Where: R m is the tensile strength of the top coal, f1 is the self-weight of the immediate roof, f2 is the self-weight of the top coal, ρ1 is the material density of the immediate roof, ρ2 is the material density of the top coal, and g is the acceleration due to gravity.

[0054] Taking into account the complexity of the site conditions, combined with the numerical simulation and field application results, the above technical parameters were optimized and adjusted again. Under the actual site conditions, the coal breaking technical parameters required to ensure that the length of the suspended top coal is not greater than the expected target length were determined. The results show that when the distance L1 between the drill rig and the end of the working face is 9m, the drill rod elevation angle θ is 12°, and the diameter of the hydraulic coal breaking hole L is 12°, the hydraulic coal breaking hole diameter L is 12°. R When the depth dimension L4 of the hydraulic coal-breaking cavity in the front-to-back direction is 3m and 2.2m, the suspended length L3 of the top coal at the end of the on-site working face is 3.2m on average and 3.8m on maximum, which is 4m lower than the target value, meeting the technical requirements for efficient collapse of the suspended roof weakened.

[0055] In order to ensure the coal breaking effect, drilling and coal breaking and hole creation should be carried out at least from the vicinity of the junction of the suspended top coal and the top beam of the working face end support equipment 6; as the drill rod 5 gradually retreats forward, the range of the hydraulic coal breaking hole 10 continues to extend forward, and the forward extension range of the hydraulic coal breaking hole 10 shall not exceed the junction of the suspended top coal and the top beam of the working face end support equipment 6, so as to control the impact of the coal breaking hole on the working face surrounding rock support.

[0056] When the hydraulic coal breaking hole 10 extends forward to the junction of the suspended top coal and the top beam of the working face end support equipment 6, and the suspended top coal has not fallen as expected, the hydraulic coal breaking and hole making operation should be stopped immediately, the drill rod elevation angle θ of the drilling rig 4 and the distance L1 between the drilling rig and the working face end should be adjusted, and the hydraulic coal breaking and hole making operation should be carried out again in the suspended top coal behind the working face end to further weaken the mechanical strength of the top coal and make the suspended top coal collapse in time.

[0057] Step 4: As the working face continues to advance, multiple hydraulic coal breaking and caving operations are carried out from the back to the front at the target fully mechanized caving working face in the mine. The effects of different periodic pressure conditions and different roadway support schemes on the maximum suspended length of the top coal are analyzed, and the process parameters of drilling construction and jet coal breaking are optimized to develop a technical method for efficient caving of the suspended top coal at the end of the top coal caving working face that is suitable for different operating conditions. Specifically:

[0058] At the target fully mechanized caving face of the mine, multiple hydraulic coal breaking and cavitation operations were carried out from back to front and monitored in real time. The numerical simulation model and the simulation model were continuously verified and optimized, and key parameters were corrected. The drill rod elevation angle θ, the drilling position, and the diameter L of the hydraulic coal breaking cavity were analyzed. R , the influence of the depth dimension L4 of the hydraulic coal breaking cavity in the front-to-back direction on the maximum length of the suspended length L3 of the top coal at the end of the working face, and the optimal technical parameters for weakening the top coal by using hydraulic coal breaking to create cavities in the target fully mechanized caving working face at the mine site are obtained.

[0059] The support measures of the working face can be strengthened by increasing the number of anchor bolts and anchor cables, increasing the initial support force of the working face and tunnel supports, etc.; the changing pattern of periodic pressure can be analyzed by recording the stress changes of the on-site working face supports and tunnel supports and the mine pressure manifestation; the influence of the periodic pressure changes and the support scheme changes on the control of the hanging top of the working face end is analyzed, and the technical scheme for weakening the top coal of the working face end under the influence of different periodic pressures, support and mining disturbance coupling is determined.

[0060] By changing the initial conditions such as coal seam burial depth, inclination and coal thickness, mechanical strength of coal rock mass, working face and tunnel layout and substituting them into the revised simulation model, the technical schemes and process parameters of drilling construction and jet coal breaking that can effectively control the suspended top length under different on-site geological conditions and coal mining conditions can be studied; considering the construction volume and construction difficulty, based on the hydraulic coal breaking method, a technical scheme for efficient collapse of the suspended top coal at the end of the top coal caving working face that is suitable for different complex conditions is formed.

Claims

1. A highly efficient method for caving the suspended top coal at the end of a fully mechanized caving working face in an extra-thick coal seam, characterized in that: The top coal at the end of the fully-mechanized caving face is weakened by hydraulic coal breaking and caving to achieve precise control of the maximum overhanging length of the top coal at the end of the top-coal caving face. The specific steps include: Step 1: Obtain mine data, including basic mechanical parameter data, internal porosity data, and fracture distribution data of the coal body and surrounding rock of the target fully mechanized caving working face coal seam and its goaf; Step 2: Based on the data from Step 1, a load-damage mathematical model of the coal strata mined at the target fully-mechanized caving working face is constructed, and a numerical simulation of the mining process of the target fully-mechanized caving working face is performed to obtain the stress distribution, deformation characteristics, and periodic collapse data of the roof and top coal of the target fully-mechanized caving working face and roadway. The numerical simulation results are verified and parameters are optimized using the periodic fracture and collapse data of the roof and top coal at the mine site, the coal seam mining stress data, and the roadway surrounding rock deformation and separation data, thereby forming a simulation model that conforms to the actual production conditions and monitoring data at the mine site. In the simulation model, hydraulic coal breaking and cavitation are simulated at the top coal of the working face end to obtain the optimal hydraulic coal breaking and cavitation weakening technical parameters that can meet the target requirement of the maximum suspended length of the top coal. The diameter and depth of the hydraulic coal-breaking hole (10) meet the following requirements: L R cosθ+L4 sinθ<H1 Where: L R is the diameter of the hydraulically broken coal hole, L4 is the depth of the hydraulically broken coal hole in the front-back direction, H1 is the thickness of the top coal above the roadway, and θ is the drill rod elevation angle; Step 3, based on the optimal hydraulic coal breaking and cavitation weakening top coal technical parameters obtained in Step 2, a drilling hole is constructed at an angle in the suspended top coal behind the end of the target fully mechanized caving working face of the mine, and a high-pressure water jet is ejected from the jet nozzle (7) at the end of the drill rod (5) and ejected radially along the drill rod (5) to impact and crush the coal body, and a hydraulic coal breaking cavity (10) is formed in the suspended top coal through the rotation and retreat movement of the drill rod (5), completing a hydraulic coal breaking and cavitation operation; The maximum suspended length of the top coal after weakening is less than the target control length. The relationship between the suspended length L3 of the top coal at the working face end and the weakened mechanical strength satisfies f1=ρ1gH1 f2=ρ2gH1 Where: R m is the tensile strength of the top coal, f1 is the self-weight of the immediate top, f2 is the self-weight of the top coal, ρ1 is the material density of the immediate top, ρ2 is the material density of the top coal, and g is the acceleration due to gravity; Step 4: As the working face continues to advance forward and the top coal fractures and collapses after being weakened at the rear, the numerical simulation model and the simulation model are continuously verified and optimized based on the real-time monitoring data at the target fully-mechanized caving working face of the mine, and key parameters are corrected. Hydraulic coal breaking and cavitation operations are carried out again at the target fully-mechanized caving working face of the mine after the top coal fractures and collapses after being weakened at the rear.

2. The method for efficiently caving the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam according to claim 1 is characterized in that: Step 2: When simulating hydraulic coal breaking and hole formation in the top coal of the working face end in the simulation model, drilling is performed near the junction of the suspended top coal and the top beam of the working face end support equipment (6), and the forward extension range of the hydraulic coal breaking hole (10) does not exceed the junction of the suspended top coal and the top beam of the working face end support equipment (6).

3. The method for efficiently caving the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam according to claim 2 is characterized in that: Step 2: When simulating hydraulic coal breaking and cave formation at the top coal of the working face in the simulation model, the internal staggered angle between the drill rod (5) and the tunnel space (2) in front of the working face in the front-back direction is 0°.

4. The method for efficiently caving the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam according to claim 2 is characterized in that: Step 2: When the hydraulic coal breaking and cavitation is simulated at the top coal of the working face in the simulation model, the drill rod elevation angle θ satisfies θ=arctan[(H2-H3) / L1] Where: H2 is the height of the tunnel, H3 is the height of the drilling rig, and L1 is the distance between the drilling rig and the end of the working face.

5. The method for efficiently caving the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam according to claim 1 is characterized in that: In Step 3, when the hydraulic coal breaking cavity (10) extends forward to the junction of the suspended top coal and the top beam of the working face end support equipment (6), and the suspended top coal does not fall as expected, the hydraulic coal breaking cavity making operation is stopped. According to the real-time monitoring data of the weakened top coal at the target fully mechanized caving working face of the mine, the drill rod elevation angle θ of the drill rig (4) and the distance L1 between the drill rig and the working face end are adjusted, and Step 2 is repeated to re-perform the hydraulic coal breaking cavity making operation in the suspended top coal behind the working face end.

6. The method for efficiently caving the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam according to claim 1 is characterized in that: After obtaining the stress distribution, deformation characteristics and periodic collapse data of the target fully mechanized caving working face and roadway in Step 2, the initial position of the hydraulic coal-breaking hole (10), the drill rod elevation angle θ and the diameter size L of the hydraulic coal-breaking hole are used to determine the optimal hydraulic coal-breaking hole. R , the depth dimension L4 of the hydraulic coal breaking cavity along the front-to-back direction, the height dimension H3 of the drilling rig, and the distance dimension L1 between the drilling rig and the end of the working face are grouped for numerical simulation to obtain the maximum value of the top coal hanging length L3 at the end of the working face under different hydraulic coal breaking cavity parameter conditions, obtain the main controlling factors, and form an optimal coal breaking cavity parameter range that can meet the requirements of the top coal hanging length.

7. The method for efficiently caving the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam according to claim 1 is characterized in that: After obtaining the stress distribution, deformation characteristics and periodic collapse data of the target fully mechanized caving working face and roadway in Step 2, a group numerical simulation is performed based on the setting conditions of the on-site working face end support equipment (6) and the stress change record data of the roadway support and the mine pressure manifestation record data to determine the optimal hydraulic coal breaking and cavitation weakening technical parameter range that can meet the target requirements for the maximum suspended length of the top coal under the influence of different periodic pressure, support and mining disturbance coupling.

8. The method for efficiently caving the suspended top coal at the end of a fully-mechanized caving working face in an extra-thick coal seam according to claim 1 is characterized in that: Step 1: When obtaining the basic mechanical parameter data, internal porosity data and fracture distribution data of the coal body and surrounding rock of the target fully-mechanized caving working face and its goaf, coal body and surrounding rock samples are selected on-site at the target fully-mechanized caving working face and its goaf in the coal mine and brought to the ground. Standard samples are prepared in the laboratory, and the basic mechanical parameter data, internal porosity data and fracture distribution data of the standard samples are tested and obtained.

Images