Green high-energy fluid caving weakening method for hard roof of goaf

Abstract

The invention discloses a green high-energy fluid caving weakening method for a hard roof of a goaf, which comprises the following steps of: weakening a top plate at the end of a fully mechanized caving face by utilizing an ultrahigh-pressure water jet to directionally form a cave and extend a crack in a suspended roof; and the maximum suspension length of the suspended roof at the end of the roof caving working face is accurately controlled by combining a dynamically optimized numerical model. The green high-energy fluid caving weakening method for the hard roof of the goaf is safe, efficient and convenient to implement, the mechanical strength of the suspended roof can be remarkably weakened, and the maximum length of the suspended roof is greatly shortened; by controlling the length of the suspended roof, potential safety hazards such as gas accumulation and air leakage at the corner of the end, rock burst, gas disasters and spontaneous combustion of residual coal which are easily induced in the production process of the working face can be remarkably reduced, the effect is remarkable, and a top plate of the end of the working face can timely and reliably collapse, so that the coal mining efficiency and the safety guarantee level are improved.

Description

Technical Field

[0001] This invention relates to a method for the collapse of suspended roofs, specifically a green and efficient collapse method that utilizes high-energy fluid jet rock-breaking technology to weaken the mechanical strength of the suspended roof at the end of a fully mechanized longwall mining face and significantly reduce the maximum length of the suspended roof. It belongs to the field of coal mine safety mining technology. Background Technology

[0002] In recent years, high production, high efficiency, and safety have been two major challenges plaguing my country's coal enterprises during thick coal seam mining. Longwall mining technology, with its advantages of high production, high efficiency, and safety, has become the mainstream method for thick coal seam mining. Among these methods, roof caving mining, a key component of longwall mining, effectively relieves mine pressure concentration by actively controlling the collapse of the roof in the goaf, thus ensuring safe production at the working face. However, when the immediate roof strata overlying the coal face are extremely hard, they are difficult to collapse in the goaf in a timely manner, especially at the end of the working face. Due to spatial constraints and stress concentration effects, large areas of overhanging roofs are easily formed. The maximum overhang length of such roofs can reach 15-30 meters, accumulating enormous bending elastic energy. If they suddenly fracture under mining disturbance, it will induce strong rockbursts, leading to serious accidents such as support damage and roadway blockage. Simultaneously, the voids formed by the overhanging roofs will block the normal compaction of the goaf, causing abnormal gas accumulation and expansion of air leakage channels, significantly increasing the risk of gas explosions and coal seam spontaneous combustion. Therefore, the length of the suspended top plate at the end of the working face should not be too long, and technical means should be used to ensure that the suspended top plate at the end of the working face collapses in a timely manner.

[0003] Conventional methods for forced roof caving in suspended coal seams primarily involve pre-weakening the suspended coal seam to shorten the length of suspension required for natural roof collapse. These methods typically employ explosive blasting, hydraulic fracturing, and carbon dioxide pre-fracture, but each has its drawbacks: explosive blasting offers good weakening, but the high energy release can easily trigger gas explosions and coal dust accidents, posing significant safety hazards, and the acquisition and storage of explosives are also difficult; hydraulic fracturing is pollution-free and safe, but its fracturing effect in rock strata is weak, and the controllability of crack propagation direction and length is poor, making precise weakening difficult; carbon dioxide pre-fracture technology is relatively new, but the cost per fracturing operation is high, making widespread adoption currently difficult. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a high-energy fluid green weakening and collapse method for hard roof in goaf areas. This method is safe, efficient, and easy to implement, and can ensure that the roof at the working face end can collapse in a timely and reliable manner, thereby improving coal mining efficiency and safety assurance.

[0005] To achieve the above objectives, this high-energy fluid green weakening caving method for hard roof in goaf areas utilizes ultra-high pressure water jets to directionally create cavities and expand fractures in the suspended roof to weaken the roof at the end of the fully mechanized longwall face. Combined with a dynamically optimized numerical model, it aims to precisely control the maximum suspended length of the suspended roof at the end of the longwall face. The specific steps include:

[0006] Step 1: Obtain mine data including geological conditions and mining conditions of the rock strata of the target fully mechanized longwall face, as well as basic mechanical parameters, internal porosity data and fracture distribution data of the rock mass and surrounding rock of the rock strata and goaf of the target fully mechanized longwall face;

[0007] Step 2: Based on the geological conditions and mining conditions of the rock strata of the target fully mechanized longwall face, and on the basis of obtaining the mechanical parameters of the coal and rock mass through experiments, construct a mathematical model of the load-bearing failure of the coal and rock strata of the target fully mechanized longwall face, and conduct numerical simulations of hydraulic rock breaking and cavity creation on the end roof and the collapse of the end roof during the longwall face mining process, to obtain the optimal hydraulic rock breaking and cavity creation technology parameters for weakening the roof that can meet the target requirements for the maximum suspended length of the roof.

[0008] The diameter and depth of the hydraulic rock-breaking cavity satisfy the following relationship:

[0009] L R cosθ + L4 sinθ < H1

[0010] In the formula: L R L1 is the diameter of the hydraulic rock-breaking cavity, L4 is the depth of the hydraulic rock-breaking cavity along the front-to-back direction, H1 is the thickness of the roof above the tunnel, and θ is the drill rod elevation angle.

[0011] Step 3: Based on the optimized hydraulic rock breaking and cavity creation technology parameters obtained in Step 2, inclined drilling is carried out in the suspended roof behind the end of the target fully mechanized longwall face in the mine. High-pressure water jets are sprayed out through the jet nozzle at the end of the drill rod and sprayed radially along the drill rod to impact and break the rock mass. Through the rotation and retraction of the drill rod, hydraulic rock breaking cavities are formed in the suspended roof, completing one hydraulic rock breaking and cavity creation operation.

[0012] The maximum suspended length of the weakened roof is less than the target control length. The maximum length required for the suspended roof to fracture and collapse is related to the weakened mechanical strength as follows:

[0013]

[0014] f1=ρ1gH1

[0015] f2=ρ2gH1

[0016] In the formula: R mf1 is the tensile strength of the top plate, f2 is the weight of the direct top, f2 is the weight of the basic top, ρ1 is the material density of the direct top, ρ2 is the material density of the basic top, and g is the gravitational acceleration.

[0017] Step 4: As the working face continues to advance and the weakened roof collapses behind it, the numerical simulation model and simulation model are continuously verified and optimized based on the real-time monitoring data of the target fully mechanized longwall mining face. Key parameters are then corrected. Hydraulic rock breaking and cavity creation operations are carried out again at the target fully mechanized longwall mining face after the weakened roof collapses behind it.

[0018] Furthermore, in Step 2, when simulating hydraulic rock breaking and cavity creation on the top plate of the working face end in the simulation model, drilling is performed near the junction of the suspended top plate and the top beam of the working face end support equipment to break the rock and create a cavity, and the forward extension range of the hydraulic rock breaking cavity does not exceed the junction of the suspended top plate and the top beam of the working face end support equipment.

[0019] Furthermore, in Step 2, when simulating hydraulic rock breaking and cavity creation at the top plate of the working face in the simulation model, the internal misalignment angle between the drill rod and the roadway space in front of the working face along the front-back direction is 0°.

[0020] Furthermore, in Step 2, when simulating hydraulic rock breaking and cavity creation at the top plate of the working face in the simulation model, the drill pipe elevation angle θ satisfies the following relationship:

[0021] θ = arctan[(H2-H3) / L1]

[0022] In the formula: H2 is the height dimension of the roadway, H3 is the height dimension 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 rock-breaking cavity extends forward to the junction of the suspended roof and the top beam of the working face end support equipment, and the suspended roof does not fall as expected, the current hydraulic rock-breaking cavity creation operation is stopped. Based on the real-time monitoring data of the weakened roof at the mine target fully mechanized longwall face, the drill rod elevation angle θ of the drilling rig and the distance L1 between the drilling rig and the working face end are adjusted, and Step 2 is repeated to carry out hydraulic rock-breaking cavity creation operation again towards the suspended roof behind the working face end.

[0024] Furthermore, after obtaining the stress distribution, deformation characteristics, and periodic roof collapse data of the target fully mechanized longwall face and roadway in Step 2, the initial positions of different hydraulic rock-breaking cavities, drill pipe elevation angle θ, and diameter L of the hydraulic rock-breaking cavities are analyzed. RNumerical simulations were performed in groups to obtain the maximum value of the suspended length L3 of the top plate at the working face under different hydraulic rock-breaking cavity parameters. The main controlling factors were identified, and the optimal range of rock-breaking cavity parameters that can meet the requirements of the suspended length of the top plate was formed.

[0025] Furthermore, after obtaining the stress distribution, deformation characteristics, and roof periodic collapse data of the target fully mechanized longwall face and roadway in Step 2, group numerical simulations are performed based on the setting conditions of the end support equipment of the working face, the stress change records of the roadway support, and the mine pressure manifestation records. The optimal hydraulic rock breaking and cavity creation technology parameters for weakening the roof that can meet the target requirements under the coupled influence of different periodic pressure, support, and mining disturbance are determined.

[0026] Furthermore, in Step 1, when obtaining the basic mechanical parameters, internal porosity data, and fracture distribution data of the coal seam and surrounding rock of the target fully mechanized longwall face and its goaf, rock mass and surrounding rock samples are selected on-site in the target fully mechanized longwall face and its goaf in the coal mine and brought to the surface. Standard samples are prepared in the laboratory, and the basic mechanical parameters, internal porosity data, and fracture distribution data of the standard samples are tested and obtained.

[0027] Compared with existing technologies, the high-energy fluid green weakening caving method for hard roof in this goaf has the following advantages:

[0028] 1. By using ultra-high pressure water jets to directly break up rock masses in suspended roofs to form cavities, and by utilizing the water wedge effect to expand the original fractures, the compressive strength of the target rock strata is significantly reduced, far exceeding that of hydraulic fracturing, thus achieving efficient directional weakening of hard roof rock strata.

[0029] 2. Based on mine geological data and a multi-field coupled numerical model, jet parameters are dynamically optimized to ensure that the weakened zone covers the stress concentration zone of the roof at the end, achieving precise control of the cantilever length. Field measurements show that this method can strictly control the maximum cantilever length within the 8m safety threshold, completely avoiding the risk of rockburst.

[0030] 3. Hydraulic fracturing and hydraulic coal breaking technologies are purely physical rock breaking methods, which are green, efficient, and safe.

[0031] 4. While breaking the rock, the water jet can continuously soften the mineral cementing layer of the rock mass and promote the connection of the fracture network. This can not only accelerate the collapse of the roof, but also significantly increase the fragmentation coefficient of the collapsed rock mass, enhance the filling density of the goaf, and reduce the space for gas accumulation and air leakage channels. Attached Figure Description

[0032] Figure 1This is a schematic diagram of the layout of the end equipment of the fully mechanized longwall mining face and the hydraulic rock breaking and cavity creation operation of the present invention;

[0033] Figure 2 yes Figure 1 Side view;

[0034] Figure 3 yes Figure 1 A top-down, rotating view.

[0035] In the diagram: 1. Target roof strata, 2. Roadway space in front of the working face, 3. High-pressure water pump, 4. Drilling rig, 5. Drill rod, 6. Working face end support equipment, 7. Jet nozzle, 8. Water jet, 9. Fully mechanized longwall face space, 10. Hydraulic rock breaking cavity, 11. Overhanging roof rock at the working face end, 12. Goaf behind the working face.

[0036] H1 is the thickness of the roof above the tunnel; H2 is the height of the tunnel; H3 is the height of the drilling rig; L1 is the distance between the drilling rig and the working face end; L2 is the width of the working face in the front-to-back direction; L3 is the overhang length of the roof in the front-to-back direction; L4 is the depth of the hydraulic rock-breaking cavity in the front-to-back direction; L5 is the upper width of the tunnel; L6 is the lower width of the tunnel; L... R θ represents the diameter of the hydraulic rock-breaking cavity, and θ is the drill pipe elevation angle. Detailed Implementation

[0037] This invention utilizes a high-energy fluid-based green weakening and caving method for hard roofs in goaf areas. It employs ultra-high-pressure water jets to create directional cavities and expand fractures in the suspended roof, weakening the roof at the end of a fully mechanized caving face. Combined with a dynamically optimized numerical model, it achieves precise control over the maximum suspended length of the roof at the end of the caving face. The following description, using a fully mechanized caving face in a coal mine as an example and with accompanying drawings, further illustrates this invention.

[0038] The specific steps of the green weakening and collapse method for the hard roof of the goaf in this goaf area include:

[0039] Step 1: First, obtain mine data including geological conditions and mining conditions of the rock strata of the target longwall mining face, as well as basic mechanical parameters, internal porosity data and fracture distribution data of the rock mass and surrounding rock of the rock strata and goaf of the target longwall mining face.

[0040] The average burial depth of the fully mechanized roof caving working face in this coal mine is 500m, the average coal seam dip angle is 31°, and the average coal seam thickness is 7m. Based on the comprehensive columnar section of the overlying coal and rock strata (including thickness and lithology parameters), key data such as the layout of the target fully mechanized roof caving working face and the adjacent goaf areas, the roadway support method, and mining parameters are determined. Figures 1 to 3As shown, the target fully mechanized longwall face has a coal mining height of 2.7m, a coal release height of 4.3m, a roof thickness of H1 of 3.3m, a roadway height of H2 of 3.7m, a mining speed of 3.2m / day, a roadway upper width of L5 of 4.4m, a roadway lower width of L6 of 5.4m, a working face width of L2 in the front-to-back direction of 6m, and a working face end support equipment 6 using end hydraulic support.

[0041] To accurately obtain basic mechanical parameters, internal porosity, and fracture distribution data of the rock mass and surrounding rock of the target fully mechanized longwall face and its goaf, relatively intact large-sized block samples of rock mass, gangue, and rock were selected from the target fully mechanized longwall face and its goaf underground and brought to the surface. Standard samples were prepared in the laboratory, and their actual basic mechanical parameters, such as compressive strength, tensile strength, and Poisson's ratio, were tested and obtained. Details are as follows:

[0042] The block samples selected from the well should be approximately cubic in shape, with a volume of not less than 0.1 m³. 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 testing with an MTS triaxial loading tester. The porosity and fracture distribution inside the standard samples were tested by mercury porosimetry and rock CT machine, respectively.

[0043] Step 2: Based on the geological conditions and mining conditions of the rock strata in the target fully mechanized longwall face, and on the basis of obtaining the mechanical parameters of the coal and rock mass through experiments, construct a mathematical model of the load-bearing failure of the coal and rock strata in the target fully mechanized longwall face, and conduct numerical simulations of hydraulic rock breaking and cavity creation on the end roof and the collapse of the end roof during the longwall face mining process, to obtain the optimal hydraulic rock breaking and cavity creation technology parameters for weakening the roof that can meet the target requirements for the maximum suspended length of the roof.

[0044] First, a mathematical model of the load-bearing failure of the coal and rock strata in the target fully mechanized longwall face was constructed using UDEC software. Mine data including geological conditions and mining conditions of the target fully mechanized longwall face, as well as basic mechanical parameters, internal porosity data, and fracture distribution data of the coal seam and its goaf, were imported into the model to conduct numerical simulation of the mining process of the target fully mechanized longwall face. The stress distribution, deformation characteristics, and roof periodic collapse data of the target fully mechanized longwall face and roadway were obtained. The numerical simulation model was verified and its parameters were optimized using data such as roof periodic fracture collapse data, coal and rock strata mining-induced stress data, and roadway surrounding rock deformation and delamination data from the mine site. The optimized model was formed to conform to the actual production conditions and monitoring data of the mine site.

[0045] Secondly, simulated hydraulic rock breaking and cavity creation were performed on the top plate of the working face in the simulation model. The maximum length required for the suspended top plate to fracture and collapse during the forward advancement of the working face was analyzed. This was achieved by changing the initial position of the hydraulic rock breaking cavity 10, the drill rod elevation angle θ, and the diameter L of the hydraulic rock breaking cavity. R The variation law of the maximum length of the overhanging roof at the working face end was analyzed by parameters such as the depth dimension L4 of the hydraulic rock breaking cavity along the front and rear directions, the height dimension H3 of the drilling rig, and the distance dimension L1 between the drilling rig and the working face end. The variation law of the maximum length of the overhanging roof at the end was analyzed under the coupling effect of enhanced roadway support, periodic pressure change and working face advancement. The model was verified and optimized in combination with the mine field observation results.

[0046] When analyzing the maximum length required for the suspended roof to fracture and collapse during the forward advancement of the working face, the simulation model is subjected to overburden stress and its own weight, and the mechanical strength of hydraulic rock breaking and cavity creation is applied to the suspended part of the roof. As the working face continues to advance in the simulation, the suspended length of the roof continuously increases. Under the action of overburden stress and its own weight, the damage to the suspended part of the roof continues to intensify until it fractures and becomes unstable. The maximum suspended length L3 of the roof at the end of the working face is recorded at this time.

[0047] When analyzing the variation law of the maximum overhang length at the working face end, different initial positions of hydraulic rock-breaking cavities 10, drill pipe elevation angles θ, and diameters L of the hydraulic rock-breaking cavities were considered. R The parameters such as the depth dimension L4 of the hydraulic rock-breaking cavity along the front-back direction, the height dimension H3 of the drilling rig, and the distance dimension L1 between the drilling rig and the working face end were grouped and numerically simulated to obtain the maximum suspended length L3 of the top plate of the working face end under different hydraulic rock-breaking cavity sizes and different hydraulic rock-breaking cavity parameters. The main controlling factors were obtained to form a range of rock-breaking cavity parameters that can meet the requirements of the top plate suspended length.

[0048] When analyzing the variation law of the maximum length of the suspended roof at the end of the working face under the conditions of enhanced roadway support, periodic pressure variation and the coupling effect of working face advancement, the simulation model is based on the application of enhanced support data (such as enhanced support data obtained after increasing the number of support anchors and increasing the support resistance of the support) and additional periodic pressure data for grouped numerical simulation. The maximum value of the suspended roof length at the end of the working face under different enhanced support data and additional periodic pressure data is obtained, forming a range of rock breaking cavity parameters that can meet the requirements of the suspended roof length.

[0049] To improve the rock-breaking effect, the diameter and depth of the hydraulic rock-breaking cavity 10 should meet the following requirements: L R cosθ+L4sinθ R ​L1 is the diameter of the hydraulic rock-breaking cavity, L4 is the depth of the hydraulic rock-breaking cavity along the front-to-back direction, H1 is the thickness of the roof above the roadway, and θ is the drill rod elevation angle. In order to minimize the distance L1 between the drilling rig and the working face end, and to avoid the drill rod 5 contacting the working face end support equipment 6 when drilling into the target mining rock layer 1, θ = arctan[(H2-H3) / L1] must be satisfied, where θ is the drill rod elevation angle, L1 is the distance between the drilling rig and the working face end, H2 is the height of the roadway, and H3 is the height of the drilling rig.

[0050] Considering the layout of electromechanical equipment in the roadway space 2 ahead of the working face, the remaining space in the roadway, and the dimensions of the drilling rig 4 and high-pressure water pump 3, combined with the on-site construction conditions of the mine, the optimal technical parameters for using hydraulic rock breaking and cavity creation to weaken the roof of the target fully mechanized longwall mining face are obtained. Based on the simulation results, such as... Figures 1 to 3 As shown, the drilling rig 4 is positioned in the middle of the roadway space 2 in front of the working face along the left-right direction. When the maximum allowable length of the top plate of the working face overhang L3 is 4m and the height of the drilling rig H3 is 1.8m, the distance L1 between the drilling rig and the working face end can be determined to be 8.7m, the internal offset angle between the drill rod 5 and the roadway space 2 in front of the working face along the front-back direction is 0°, the drill rod elevation angle θ is 12°, and the diameter of the hydraulic rock-breaking cavity L... R The starting position of the hydraulic rock-breaking cavity 10 and the drilling slant length of the drilling rig 4 are 17.6m, and the depth dimension L4 of the hydraulic rock-breaking cavity along the front-to-back direction is 2.2m.

[0051] Step 3: Based on the optimized technical parameters for using hydraulic rock breaking and cavity creation to weaken the roof obtained in Step 2, hydraulic rock breaking construction is carried out on-site at the target fully mechanized longwall face in the mine. High-pressure water pumps 3 and drilling rigs 4, etc., are arranged in the roadway space 2 in front of the working face. The drilling is carried out at an angle into the suspended roof behind the end. High-pressure water is transported through the drill rod 5 by the high-pressure water pump 3. High-pressure water jets are sprayed radially along the drill rod 5 through the jet nozzles 7 (two are set symmetrically) at the end of the drill rod 5. The high-pressure water jets impact and break the coal body. Through the rotation and retraction of the drill rod 5, a complete hydraulic rock breaking cavity 10 with an approximate cylinder is formed in the suspended roof. The broken coal particles and water flow out in a mixed state from the annular space between the drill rod 5 and the borehole. After the hydraulic rock breaking and cavity creation is completed, the hydraulic rock breaking equipment is removed, and one hydraulic rock breaking and cavity creation operation is completed.

[0052] As the working face continues to advance, the unsupported length of the roof gradually increases, leading to premature fracture and collapse under the combined effects of the roof's own weight and pressure. To effectively control the safety hazard of the unsupported roof, the maximum unsupported length of the weakened roof should be less than the target control length. The relationship between the maximum length required for the unsupported roof to fracture and collapse and the weakened mechanical strength satisfies the following:

[0053]

[0054] f1=ρ1gH1

[0055] f2=ρ2gH1

[0056] In the formula: R m f1 is the tensile strength of the top plate, f2 is the weight of the direct top, f2 is the weight of the basic top, ρ1 is the material density of the direct top, ρ2 is the material density of the basic top, and g is the gravitational acceleration.

[0057] Considering the complexity of the site conditions, and combining numerical simulation and field application results, the above technical parameters were further optimized and adjusted. Under actual site conditions, the rock-breaking technical parameters required to ensure that the length of the suspended roof does not exceed the expected target length were determined. The results show that when the distance L1 between the drilling rig and the working face end is 9m, the drill rod elevation angle θ is 12°, and the diameter L of the hydraulic rock-breaking cavity is... R When the depth dimension L4 of the hydraulic rock-breaking cavity along the front and rear directions is 2.2m, the average length L3 of the suspended top plate at the end of the working face is 3.2m and the maximum value is 3.8m, which is 4m lower than the target value, thus meeting the technical requirements for efficient collapse with weakened suspended top.

[0058] To ensure the rock-breaking effect, drilling should begin at least near the junction of the suspended roof and the top beam of the working face end support equipment 6 to create a rock-breaking cavity. As the drill rod 5 gradually retracts forward, the range of the hydraulic rock-breaking cavity 10 extends forward continuously. The forward extension range of the hydraulic rock-breaking cavity 10 must not exceed the junction of the suspended roof and the top beam of the working face end support equipment 6, so as to control the impact of the rock-breaking cavity on the surrounding rock support of the working face.

[0059] When the hydraulic rock-breaking cavity 10 extends forward to the junction of the suspended roof and the top beam of the working face end support equipment 6, and the suspended roof still fails to collapse as expected, the current hydraulic rock-breaking cavity creation 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 rock-breaking cavity creation operation should be carried out again towards the suspended roof behind the working face end to further weaken the mechanical strength of the roof and make the suspended roof collapse in time.

[0060] Step 4: As the working face continues to advance and the weakened roof collapses behind it, the numerical simulation model and simulation model are continuously verified and optimized based on the real-time monitoring data of the target fully mechanized longwall mining face. Key parameters are then corrected. Hydraulic rock breaking and cavity creation operations are carried out again at the target fully mechanized longwall mining face after the weakened roof collapses behind it.

[0061] As the working face continues to advance, multiple hydraulic rock breaking and cavity creation operations are carried out sequentially from back to front at the target fully mechanized longwall face in the mine. The impact of different pressure states and different roadway support schemes on the maximum suspended length of the roof is analyzed. The process parameters of drilling construction and jet rock breaking are optimized to form a technical method for efficient collapse of the suspended roof at the end of the longwall face suitable for different operating conditions.

[0062] Multiple hydraulic rock-breaking and cavity-creating operations were carried out sequentially from back to front at the target fully mechanized longwall face in the mine, with real-time monitoring. The numerical simulation model and the simulation model were continuously verified and optimized, and key parameters were corrected. The drill rod elevation angle θ, borehole location, and diameter L of the hydraulic rock-breaking cavity were analyzed. R The influence of the depth dimension L4 of the hydraulic rock-breaking cavity along the front-back direction on the maximum length L3 of the suspended roof at the end of the working face was investigated to obtain the optimal technical parameters for using hydraulic rock-breaking cavity creation to weaken the roof of the target fully mechanized longwall mining face in the mine.

[0063] The support measures of the working face can be strengthened by increasing the number of anchor bolts and cables, and increasing the initial support force of the supports in the working face and roadway. The variation law of periodic pressure can be analyzed by recording the stress changes of the working face supports and roadway supports and the mining pressure manifestation records. The influence of periodic pressure changes and support scheme changes on the roof control of the working face end can be analyzed, and the technical scheme for weakening the roof of the working face end under the coupled influence of different periodic pressure, support and mining disturbance can be determined.

[0064] By modifying the initial conditions such as the burial depth, dip angle, coal thickness, mechanical strength of the coal and rock mass, and the layout of the working face and roadway, and substituting them into the corrected simulation model, we can study the technical solutions and process parameters for drilling and jet rock breaking that can effectively control the overhang length under different on-site geological conditions and coal mining conditions. Considering the amount of construction and the difficulty of construction, based on the hydraulic rock breaking method, we can form a technical solution for efficient collapse of the overhanging roof at the end of the working face under different complex conditions.

[0065] The high-energy fluid green weakening and caving method for the hard roof in this goaf is safe, efficient, and easy to implement. On the one hand, the construction process is relatively streamlined and the workload is small. The water jet rock breaking technology itself is relatively mature and the construction difficulty is low. On the other hand, by controlling the length of the suspended roof, the safety hazards such as gas accumulation and air leakage at the end corners, rock bursts, gas disasters, and spontaneous combustion of residual coal that are easily induced during the working face production process can be significantly reduced. Furthermore, the water jet rock breaking process can simultaneously promote the initiation and expansion of internal cracks in the rock mass, and water has a softening effect on the rock mass, which can further increase the degree of fragmentation after the roof breaks and collapses, increase the roof release rate, significantly weaken the mechanical strength of the suspended roof, and greatly shorten the maximum length of the suspended roof. The effect is significant, which can ensure that the roof at the end of the working face can collapse in a timely and reliable manner, thereby improving coal mining efficiency and safety assurance level.

Claims

1. A method for green weakening and collapse of a hard roof in a goaf using high-energy fluids, characterized in that, The roof at the end of the fully mechanized caving face is weakened by directional cavity creation and crack expansion in the suspended roof using ultra-high pressure water jets. Combined with a dynamically optimized numerical model, the maximum suspended length of the suspended roof at the end of the caving face is precisely controlled. The specific steps include: Step 1: Obtain mine data including geological conditions and mining conditions of the rock strata of the target fully mechanized longwall face, as well as basic mechanical parameters, internal porosity data and fracture distribution data of the rock mass and surrounding rock of the rock strata and goaf of the target fully mechanized longwall face; Step 2: Based on the geological conditions and mining conditions of the rock strata of the target fully mechanized longwall face, and on the basis of obtaining the mechanical parameters of the coal and rock mass through experiments, construct a mathematical model of the load-bearing failure of the coal and rock strata of the target fully mechanized longwall face, and conduct numerical simulations of hydraulic rock breaking and cavity creation on the end roof and the collapse of the end roof during the longwall face mining process, to obtain the optimal hydraulic rock breaking and cavity creation technology parameters for weakening the roof that can meet the target requirements for the maximum suspended length of the roof. The diameter and depth of the hydraulic rock-breaking cavity satisfy the following relationship: L R cosθ+L4 sinθ<H1 In the formula: L R L1 is the diameter of the hydraulic rock-breaking cavity, L4 is the depth of the hydraulic rock-breaking cavity along the front-to-back direction, H1 is the thickness of the roof above the tunnel, and θ is the drill rod elevation angle. Step 3: Based on the optimized hydraulic rock breaking and cavity creation technology parameters obtained in Step 2, inclined drilling is carried out in the suspended roof behind the end of the target fully mechanized longwall face in the mine. High-pressure water jets are sprayed out through the jet nozzle at the end of the drill rod and sprayed radially along the drill rod to impact and break the rock mass. Through the rotation and retraction of the drill rod, hydraulic rock breaking cavities are formed in the suspended roof, completing one hydraulic rock breaking and cavity creation operation. The maximum suspended length of the weakened roof is less than the target control length. The maximum length required for the suspended roof to fracture and collapse is related to the weakened mechanical strength as follows: f1=ρ1gH1 f2=ρ2gH1 In the formula: R m f1 is the tensile strength of the top plate, f2 is the weight of the direct top, f2 is the weight of the basic top, ρ1 is the material density of the direct top, ρ2 is the material density of the basic top, and g is the gravitational acceleration. Step 4: As the working face continues to advance and the weakened roof collapses behind it, the numerical simulation model and simulation model are continuously verified and optimized based on the real-time monitoring data of the target fully mechanized longwall mining face. Key parameters are then corrected. Hydraulic rock breaking and cavity creation operations are carried out again at the target fully mechanized longwall mining face after the weakened roof collapses behind it.

2. The method for green weakening and collapse of hard roof in goaf areas using high-energy fluids according to claim 1, characterized in that, Step 2: When simulating hydraulic rock breaking and cavity creation on the top plate of the working face in the simulation model, drilling is carried out near the junction of the suspended top plate and the top beam of the working face end support equipment to break the rock and create a cavity. The forward extension range of the hydraulic rock breaking cavity does not exceed the junction of the suspended top plate and the top beam of the working face end support equipment.

3. The method for green weakening and collapse of hard roof in goaf areas using high-energy fluids according to claim 2, characterized in that, Step 2: When simulating hydraulic rock breaking and cavity creation at the top plate of the working face in the simulation model, the internal misalignment angle between the drill rod and the roadway space in front of the working face along the front-back direction is 0°.

4. The method for green weakening and collapse of hard roof in goaf areas using high-energy fluids according to claim 1, characterized in that, Step 2: When simulating hydraulic rock breaking and cavity creation at the top plate of the working face in the simulation model, the drill pipe elevation angle θ satisfies the following relationship: θ = arctan[(H2-H3) / L1] In the formula: H2 is the height dimension of the roadway, H3 is the height dimension of the drilling rig, and L1 is the distance between the drilling rig and the end of the working face.

5. The method for green weakening and collapse of hard roof in goaf areas using high-energy fluids according to claim 1, characterized in that, In Step 3, when the hydraulic rock-breaking cavity extends forward to the junction of the suspended roof and the top beam of the working face end support equipment, and the suspended roof does not fall as expected, the current hydraulic rock-breaking cavity creation operation is stopped. Based on the real-time monitoring data of the weakened roof at the target fully mechanized longwall face of the mine, the drill rod elevation angle θ of the drilling rig and the distance L1 between the drilling rig and the working face end are adjusted, and Step 2 is repeated to carry out hydraulic rock-breaking cavity creation operation again towards the suspended roof behind the working face end.

6. The method for green weakening and collapse of hard roof in goaf areas using high-energy fluids according to claim 1, characterized in that, After obtaining the stress distribution, deformation characteristics, and periodic roof collapse data of the target fully mechanized longwall face and roadway in Step 2, the initial position of different hydraulic rock-breaking cavities, drill pipe elevation angle θ, and diameter L of the hydraulic rock-breaking cavities are analyzed. R Numerical simulations were performed in groups to obtain the maximum value of the suspended length L3 of the top plate at the working face under different hydraulic rock-breaking cavity parameters. The main controlling factors were identified, and the optimal range of rock-breaking cavity parameters that can meet the requirements of the suspended length of the top plate was formed.

7. The method for green weakening and collapse of hard roof in goaf areas using high-energy fluids according to claim 1, characterized in that, After obtaining the stress distribution, deformation characteristics, and roof periodic collapse data of the target fully mechanized longwall face and roadway in Step 2, group numerical simulations are performed based on the setting conditions of the end support equipment of the working face, the stress change records of the roadway support, and the mine pressure manifestation records. The optimal hydraulic rock breaking and cavity creation technology parameters for weakening the roof that can meet the target requirements under the coupled influence of different periodic pressure, support, and mining disturbance are determined.

8. The method for green weakening and collapse of hard roof in goaf areas using high-energy fluids according to claim 1, characterized in that, Step 1 involves obtaining basic mechanical parameters, internal porosity data, and fracture distribution data of the coal seam and surrounding rock of the target fully mechanized longwall face and its goaf. Rock samples are selected from the target fully mechanized longwall face and its goaf in the coal mine and brought to the surface. Standard samples are prepared in the laboratory, and the basic mechanical parameters, internal porosity data, and fracture distribution data of the standard samples are tested and obtained.

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