Optimization method for thickness of reserved waterproof rock stratum

Abstract

The invention relates to a method for optimizing the thickness of a reserved waterproof rock stratum, and belongs to the technical field of metal mine mining methods. According to the technical scheme, the stability of the stope and the sensitivity degree of the water pressure value are analyzed through numerical simulation; and then stability analysis is conducted on stoping under the conditions of different water pressures and water-resisting layer thicknesses, then the optimal water-resisting rock stratum thickness under each water pressure value is obtained, and the metal mine mining reserved water-resisting rock stratum thickness under the water-rich karst aquifer is reasonably determined. The method has the beneficial effects that the expansion rule of the pressure-bearing mining plastic zone can be better evolved, the change trend of stress, displacement, the plastic zone and a water inrush safety coefficient along with water pressure and a top plate is comprehensively considered, and the method is more in line with the actual mining.

Description

Technical Field

[0001] The invention relates to an optimization method for reserving the thickness of a water-proof rock layer, and belongs to the technical field of metal ore mining methods. Background Art

[0002] Skarn iron ore deposits are usually found in the contact zone between rock mass and carbonate, so most of them are large water deposits. Carbonate surrounding rocks often contain a large number of cavities and pores of different development degrees, which are very likely to cause water inrush accidents during mining, especially when mining broken rock mass with poor rock stability. Broken rock mass often contains a large number of water-conducting fissures, and water prevention and control plans must be formulated in advance. Common treatment plans include: advance exploration and leaving aquicludes. In general, the aquiclude is a rock mass with good integrity, which can effectively maintain the safety of the underlying mining area. However, when the aquiclude is an ore body, as mining proceeds, the aquiclude will serve as the direct roof of the mining chamber. Affected by excavation and blasting, the integrity of the aquiclude is destroyed. Therefore, the choice of the thickness of the aquiclude affects the safety production and economic benefits of the mine. If the thickness of the aquiclude is not selected reasonably, two situations will occur. One is that the thickness of the roof aquiclude is too large, which will cause a waste of resources. Second, the thickness of the water barrier is too small, which cannot effectively prevent the loose excavation area from connecting with the water guide belt, which is prone to accidents such as roof collapse and water inrush. Therefore, optimizing the thickness of the water barrier and analyzing its stability according to different water pressures has very important engineering practical value.

[0003] Regarding the optimization method of the thickness of the impermeable rock layer, the existing technologies mainly include: Hu Shaoping et al. constructed a numerical calculation model of the coal pillar in the mine pressure influence zone and the effective impermeable zone, and optimized the reasonable width of the impermeable coal pillar; Li Zhu et al., through theoretical analysis and FLAC3D, explored the water-blocking capacity and stability evolution law of the impermeable coal pillar under different coal pillar width conditions, and proposed a method for determining the width of the impermeable coal pillar combined with the three zones of "seepage zone + elastic compaction water-blocking zone + plastic zone"; Jin Aibing et al. used numerical simulation to study the distribution law of coal pillar support pressure and the stress and plastic deformation of coal pillars of different widths during mining at the upper and lower sections, and obtained a reasonable width; Huang Hui et al. studied the stress distribution and spatial damage characteristics of the gently inclined floor under mining based on the theory of elastic mechanics and numerical simulation, and proposed a method for evaluating the hazard of water inrush considering the spatial characteristics of floor damage Su Chengzhi et al. used numerical simulation to predict the water-conducting fracture zone and obtained the maximum damage depth of the surrounding rock, thereby optimizing the mining method; Zou Guanghua et al. used numerical simulation to invert the stress changes during mining and concluded that the mining height should be lowered when passing through a group of faults to allow the water-conducting belt to penetrate; Ma Haitao et al. used numerical simulation to use stress and strain as evaluation indicators to conduct a water inrush safety analysis of an iron mine; Cha Chunlei et al. used numerical simulation to obtain the floor damage depth and stress change law during mining, thereby preventing floor water damage; Yao Xuancheng et al. analyzed the crack propagation law from multiple angles such as theory and numerical simulation to explore the formation mechanism of water inrush and sand burst, and obtained a crack propagation prediction model; Meng Wenqing et al. used similar experiments and numerical simulation to study the upper limit of aquifer mining and obtained the safe mining height.

[0004] In summary, the existing technologies are mostly aimed at determining the width of the impermeable coal pillar and calculating the thickness of the impermeable coal seam based on empirical formulas, while the research on the optimization method of the thickness of the reserved impermeable rock layer for metal mining under the water-rich karst aquifer is very limited. Summary of the invention

[0005] The purpose of the present invention is to provide a method for optimizing the thickness of reserved impermeable rock layers, by adopting numerical simulation to analyze the sensitivity of mining field stability to water pressure values; then, stability analysis is performed on mining under different water pressure and impermeable layer thickness conditions, and the optimal thickness of impermeable rock layers under various water pressure values ​​is obtained, and the thickness of impermeable rock layers reserved for metal mining under water-rich karst aquifers is reasonably determined. This method can better evolve the expansion law of the plastic zone of pressurized mining, comprehensively consider the changing trends of stress, displacement, plastic zone and water inrush safety factor with water pressure and roof, is more in line with mining practice, and effectively solves the above-mentioned problems existing in the background technology.

[0006] The technical solution of the present invention is: a method for optimizing the thickness of a reserved water-repellent rock layer, comprising the following steps:

[0007] S1. Monitor the water pressure at typical water outlet points underground and analyze the water pressure monitoring results at the measuring points;

[0008] S2. Use FLac 3D to establish mining models under different initial water pressure conditions and conduct stope stability-water pressure sensitivity analysis;

[0009] S3. Determine the numerical simulation optimization scheme for the thickness of the waterproof layer on the roof of the mining area, and conduct comparative analysis on the stress, displacement and plastic zone of the roof of the mining area by changing the water pressure value and the thickness of the waterproof layer, so as to further determine the reasonable thickness of the waterproof layer on the roof;

[0010] S4. Optimize the thickness of the roof impermeable rock layer under different water pressures, select the maximum principal stress of the roof, the minimum principal stress of the roof, the displacement of the roof and the plastic zone of the mining area as the basis for stability analysis, and conduct the mining area stability analysis under different water pressures and different thicknesses of impermeable rock layers;

[0011] S5. Optimize the thickness of the waterproof layer under different water pressure conditions, study the influence of the thickness of the waterproof layer on various indicators of the roof under different water pressure conditions, compare and analyze the interaction between water pressure and the thickness of the waterproof layer on the damage of the roof, and further optimize the thickness of the waterproof layer under different water pressure conditions.

[0012] In step S1, in order to find out the actual water pressure underground, 4-5 typical water outlet points with different flow rates are selected to install YHY type mining water pressure monitors with an error of ≤±1% for long-term monitoring.

[0013] In the step S2,

[0014] A standard hexahedral grid is used, and the grid gradually becomes sparser from the center to the surrounding areas. The minimum grid length is 2m. The x, y and z axes are taken as the strike, dip and height of the ore body respectively. The model is 600m long, 400m wide and 500m high. The model imposes normal displacement constraints on the bottom and sides, and a uniformly distributed load is applied on the top to simulate the mining depth. The unit body adopts the Mohr-Coulomb constitutive model, and the mechanical parameters are obtained according to existing mechanical tests. The water pressure value is assigned according to the pore water pressure. It is assumed that the water-conducting fracture zone is located on the upper part of the direct roof of the mine room, and the upper part of the roof of the mining area is assigned the pore water pressure.

[0015] Determine the numerical simulation scheme. Based on the on-site water pressure monitoring results, select water pressure values ​​of 0MPa, 0.1MPa, 0.2MPa, 0.5MPa, 1MPa, 1.5MPa and 2MPa. Combined with the mining technical scheme, perform excavation calculations based on a mining length of 50m, a mining width of 15m, a mining height of 60m, and a waterproof layer thickness of 0.1m.

[0016] Taking the maximum displacement of the roof as the stability index of the mining area, the changes in the maximum displacement of the roof after excavation stabilization at various water pressure values ​​were obtained and analyzed; combined with the criterion of rock mass limit displacement, typical water pressure values ​​of 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa and 2.0MPa were selected to optimize the thickness of the aquiclude.

[0017] In step S3, when typical water pressure values ​​are selected as 0.1 MPa, 0.5 MPa, 1.0 MPa, 1.5 MPa and 2.0 MPa, and the thickness of the top waterproof layer is 0 m, 2 m, 4 m and 8 m, respectively, the thickness of the waterproof layer is optimized.

[0018] In step S4, the Average judgment method is selected for plastic zone discrimination, that is, when more than 50% of the volume in the unit body is damaged, the unit body is deemed to be damaged; the maximum principal stress, minimum principal stress, displacement and plastic zone cloud map of the roof are obtained when the water pressure is 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa and 2.0MPa and the thickness of the impermeable rock layer is 0.1m, 2m, 4m and 8m respectively, and stability analysis is performed.

[0019] In step S5, the maximum value of the maximum principal stress, the minimum value of the maximum principal stress, the maximum settlement of the roof and the plastic zone failure depth index value of each scheme at the 0.1m slice on the upper part of the stope roof are extracted, and the influence of the thickness of the waterproof layer on each index under different water pressure conditions is studied. The interaction between the water pressure and the thickness of the waterproof layer on the roof failure is compared and analyzed, as follows:

[0020] (1) Analysis of stress concentration changes: obtain the maximum value of the maximum principal stress of each scheme under different water pressure conditions as the thickness of the impermeable layer changes, and analyze the changes in the stress concentration of the mine roof with the change of the thickness of the impermeable layer and water pressure;

[0021] (2) Analysis of stress release changes: obtain the minimum value of the maximum principal stress of each scheme under different water pressure conditions as the thickness of the impermeable layer changes, and analyze the changes in the stress release of the mine roof as the thickness of the impermeable layer and water pressure change;

[0022] (3) Roof displacement analysis: obtain the roof displacement curves of each scheme under different water pressure conditions as a function of the thickness of the impermeable layer, and analyze the changes in the roof displacement of the mining area as the thickness of the impermeable layer and water pressure change;

[0023] (4) Plastic zone analysis: When the mining area is excavated, it will cause plastic failure of the surrounding rock. When the damage zone is connected with the water-bearing fracture zone, water will flow into the mining area through the plastic zone, causing roof water inrush accidents. In order to intuitively display the connection between the damage zone and the fracture zone, the water inrush safety factor S is defined. When S is greater than 2, it indicates that there is basically no risk of water inrush.

[0024] S=HP- w Z

[0025] Where, S is the water inrush safety factor; Pw is the water pressure, MPa; H is the thickness of the water-blocking layer, m; Z is the failure depth of the plastic zone, m;

[0026] The safety factor of water inrush in the mining area under different water pressures and different thicknesses of impermeable rock layers was calculated respectively, and a line graph of the change of the safety factor of water inrush with the thickness of impermeable rock layers under different water pressures was drawn, and its critical thickness was extracted as the preliminary thickness of the impermeable layer under different water pressure conditions;

[0027] (5) The optimal thickness of the waterproof layer under different water pressure conditions is obtained by comprehensively considering the changing trends of stress, displacement, and plastic zone with water pressure and roof.

[0028] The beneficial effects of the present invention are as follows: by adopting numerical simulation to analyze the sensitivity of mining field stability to water pressure value; then carrying out stability analysis on mining under different water pressure and impermeable layer thickness conditions, and then obtaining the optimal impermeable rock layer thickness under each water pressure value, and reasonably determining the reserved impermeable rock layer thickness for metal mining under the water-rich karst aquifer, the expansion law of the plastic zone of pressurized mining can be better evolved, and the changing trends of stress, displacement, plastic zone and water inrush safety factor with water pressure and roof are comprehensively considered, which is more in line with the actual mining. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a flow chart of the present invention;

[0030] Figure 2 It is a numerical model for analyzing the stability and water pressure sensitivity of a stope in an embodiment of the present invention;

[0031] Figure 3 Schematic diagram of the pressure on the roof of a stope in an embodiment of the present invention;

[0032] Figure 4 is a graph showing the variation of top plate displacement at different water pressure values ​​in an embodiment of the present invention;

[0033] Figure 5 is the maximum principal stress cloud diagram of the top plate in Scheme 1 of the embodiment of the present invention (H=0.1);

[0034] Figure 6 is the minimum principal stress cloud diagram of the top plate in Scheme 1 of the embodiment of the present invention (H=0.1);

[0035] Figure 7 It is the vertical displacement cloud diagram of the top plate of Scheme 1 in the embodiment of the present invention (H=0.1);

[0036] Figure 8 It is the cloud map of the plastic zone of the stope in Scheme 1 of the embodiment of the present invention (H=0.1);

[0037] Fig. 9 is the maximum principal stress cloud diagram of the top plate in scheme 2 of the embodiment of the present invention (H=2);

[0038] Fig.10 is the minimum principal stress cloud diagram of the top plate in scheme 2 of the embodiment of the present invention (H=2);

[0039] Fig.11 is the vertical displacement cloud diagram of the top plate of Scheme 2 in the embodiment of the present invention (H=2);

[0040] Fig.12 It is the cloud diagram of the plastic zone of the stope in Scheme 2 of the embodiment of the present invention (H=2);

[0041] Fig.13 is the maximum principal stress cloud diagram of the top plate in scheme 3 of the embodiment of the present invention (H=4);

[0042] Fig.14 It is the minimum principal stress cloud diagram of the top plate in scheme 3 of the embodiment of the present invention (H=4);

[0043] Fig.15 It is the vertical displacement cloud diagram of the top plate of Scheme 3 in the embodiment of the present invention (H=4);

[0044] Fig.16 It is the cloud diagram of the plastic zone of the stope of Scheme 3 in the embodiment of the present invention (H=4);

[0045] Fig.17 is the maximum principal stress cloud diagram of the top plate of Scheme 4 in the embodiment of the present invention (H=8);

[0046] Fig.18 It is the minimum principal stress cloud diagram of the top plate of Scheme 4 in the embodiment of the present invention (H=8);

[0047] Fig.19 It is the vertical displacement cloud diagram of the top plate of Scheme 4 in the embodiment of the present invention (H=8);

[0048] Fig. 20 It is the cloud diagram of the plastic zone of the stope of Scheme 4 in the embodiment of the present invention (H=8);

[0049] Fig.21 is a curve diagram showing the maximum value Max of the maximum principal stress and the thickness of the waterproof layer in an embodiment of the present invention;

[0050] Fig. 22 is a curve diagram showing the change of the minimum value Min of the maximum principal stress and the thickness of the waterproof layer in the embodiment of the present invention;

[0051] Fig.23 is a curve diagram of the top plate displacement and the thickness of the waterproof layer in an embodiment of the present invention;

[0052] Fig.24 is the calculation result of the water inrush safety factor in the embodiment of the present invention;

[0053] Fig.25 It is a line graph showing the variation of water inrush safety factor with thickness in an embodiment of the present invention. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical solutions and advantages of the invention implementation cases clearer, the technical solutions in the implementation cases of the present invention will be clearly and completely described below in conjunction with the drawings in the implementation cases. Obviously, the implementation cases described are only a small part of the implementation cases of the present invention, rather than all the implementation cases. Based on the implementation cases in the present invention, all other implementation cases obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0055] An optimization method for reserving the thickness of a water-repellent rock layer comprises the following steps:

[0056] S1. Monitor the water pressure at typical water outlet points underground and analyze the water pressure monitoring results at the measuring points;

[0057] S2. Use FLac 3D to establish mining models under different initial water pressure conditions and conduct stope stability-water pressure sensitivity analysis;

[0058] S3. Determine the numerical simulation optimization scheme for the thickness of the waterproof layer on the roof of the mining area, and conduct comparative analysis on the stress, displacement and plastic zone of the roof of the mining area by changing the water pressure value and the thickness of the waterproof layer, so as to further determine the reasonable thickness of the waterproof layer on the roof;

[0059] S4. Optimize the thickness of the roof impermeable rock layer under different water pressures, select the maximum principal stress of the roof, the minimum principal stress of the roof, the displacement of the roof and the plastic zone of the mining area as the basis for stability analysis, and conduct the mining area stability analysis under different water pressures and different thicknesses of impermeable rock layers;

[0060] S5. Optimize the thickness of the waterproof layer under different water pressure conditions, study the influence of the thickness of the waterproof layer on various indicators of the roof under different water pressure conditions, compare and analyze the interaction between water pressure and the thickness of the waterproof layer on the damage of the roof, and further optimize the thickness of the waterproof layer under different water pressure conditions.

[0061] In step S1, in order to find out the actual water pressure underground, 4-5 typical water outlet points with different flow rates are selected to install YHY type mining water pressure monitors with an error of ≤±1% for long-term monitoring.

[0062] In the step S2,

[0063] A standard hexahedral grid is used, and the grid gradually becomes sparser from the center to the surrounding areas. The minimum grid length is 2m. The x, y and z axes are taken as the strike, dip and height of the ore body respectively. The model is 600m long, 400m wide and 500m high. The model imposes normal displacement constraints on the bottom and sides, and a uniformly distributed load is applied on the top to simulate the mining depth. The unit body adopts the Mohr-Coulomb constitutive model, and the mechanical parameters are obtained according to existing mechanical tests. The water pressure value is assigned according to the pore water pressure. It is assumed that the water-conducting fracture zone is located on the upper part of the direct roof of the mine room, and the upper part of the roof of the mining area is assigned the pore water pressure.

[0064] Determine the numerical simulation scheme. Based on the on-site water pressure monitoring results, select water pressure values ​​of 0MPa, 0.1MPa, 0.2MPa, 0.5MPa, 1MPa, 1.5MPa and 2MPa. Combined with the mining technical scheme, perform excavation calculations based on a mining length of 50m, a mining width of 15m, a mining height of 60m, and a waterproof layer thickness of 0.1m.

[0065] Taking the maximum displacement of the roof as the stability index of the mining area, the changes in the maximum displacement of the roof after excavation stabilization at various water pressure values ​​were obtained and analyzed; combined with the criterion of rock mass limit displacement, typical water pressure values ​​of 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa and 2.0MPa were selected to optimize the thickness of the aquiclude.

[0066] In step S3, when typical water pressure values ​​are selected as 0.1 MPa, 0.5 MPa, 1.0 MPa, 1.5 MPa and 2.0 MPa, and the thickness of the top waterproof layer is 0 m, 2 m, 4 m and 8 m, respectively, the thickness of the waterproof layer is optimized.

[0067] In step S4, the Average judgment method is selected for plastic zone discrimination, that is, when more than 50% of the volume in the unit body is damaged, the unit body is deemed to be damaged; the maximum principal stress, minimum principal stress, displacement and plastic zone cloud map of the roof are obtained when the water pressure is 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa and 2.0MPa and the thickness of the impermeable rock layer is 0.1m, 2m, 4m and 8m respectively, and stability analysis is performed.

[0068] In step S5, the maximum value of the maximum principal stress, the minimum value of the maximum principal stress, the maximum settlement of the roof and the plastic zone failure depth index value of each scheme at the 0.1m slice on the upper part of the stope roof are extracted, and the influence of the thickness of the waterproof layer on each index under different water pressure conditions is studied. The interaction between the water pressure and the thickness of the waterproof layer on the roof failure is compared and analyzed, as follows:

[0069] (1) Analysis of stress concentration changes: obtain the maximum value of the maximum principal stress of each scheme under different water pressure conditions as the thickness of the impermeable layer changes, and analyze the changes in the stress concentration of the mine roof with the change of the thickness of the impermeable layer and water pressure;

[0070] (2) Analysis of stress release changes: obtain the minimum value of the maximum principal stress of each scheme under different water pressure conditions as the thickness of the impermeable layer changes, and analyze the changes in the stress release of the mine roof as the thickness of the impermeable layer and water pressure change;

[0071] (3) Roof displacement analysis: obtain the roof displacement curves of each scheme under different water pressure conditions as a function of the thickness of the impermeable layer, and analyze the changes in the roof displacement of the mining area as the thickness of the impermeable layer and water pressure change;

[0072] (4) Plastic zone analysis: When the mining area is excavated, it will cause plastic failure of the surrounding rock. When the damage zone is connected with the water-bearing fracture zone, water will flow into the mining area through the plastic zone, causing roof water inrush accidents. In order to intuitively display the connection between the damage zone and the fracture zone, the water inrush safety factor S is defined. When S is greater than 2, it indicates that there is basically no risk of water inrush.

[0073] S= HZ

[0074] P w

[0075] Where, S is the water inrush safety factor; Pw is the water pressure, MPa; H is the thickness of the water-blocking layer, m; Z is the failure depth of the plastic zone, m;

[0076] The safety factor of water inrush in the mining area under different water pressures and different thicknesses of impermeable rock layers was calculated respectively, and a line graph of the change of the safety factor of water inrush with the thickness of impermeable rock layers under different water pressures was drawn, and its critical thickness was extracted as the preliminary thickness of the impermeable layer under different water pressure conditions;

[0077] (5) The optimal thickness of the waterproof layer under different water pressure conditions is obtained by comprehensively considering the changing trends of stress, displacement, and plastic zone with water pressure and roof.

[0078] In practical application, the present invention comprises the following steps:

[0079] The first step is to monitor the water pressure at typical water outlet points underground. In order to find out the actual water pressure underground, 4 to 5 typical water outlet points with different flow rates are selected to install YHY mining water pressure monitors (error ≤ ±1%) for long-term monitoring and analyze the water pressure monitoring results of the measuring points.

[0080] In the second step, FLac 3D was used to establish a mining model under different initial water pressure conditions, and the stability-water pressure sensitivity analysis of the mining area was carried out to provide a basis for determining the optimization scheme of the numerical simulation of the thickness of the waterproof layer of the mining area roof. A standard hexahedral grid was used, and the grid gradually became sparse from the center to the surrounding area, with a minimum grid length of 2m. The x, y, and z axes were taken as the strike, dip, and height of the ore body, respectively. The model was 600m long, 400m wide, and 500m high. The model imposed normal displacement constraints on the bottom and sides, and a uniformly distributed load was applied on the top to simulate the mining depth. The unit body adopted the Mohr-Coulomb constitutive model, and the mechanical parameters were obtained according to the existing mechanical tests. The water pressure value was assigned according to the pore water pressure. It was assumed that the water-conducting fracture zone was located on the upper part of the direct roof of the mine room, and the upper part of the mining area roof was assigned the pore water pressure.

[0081] Determine the numerical simulation scheme. Based on the on-site water pressure monitoring results, the water pressure values ​​were selected as 0MPa, 0.1MPa, 0.2MPa, 0.5MPa, 1MPa, 1.5MPa, and 2MPa, respectively. Combined with the mining technical scheme, the excavation calculation was carried out according to the mining length of 50m, the mining width of 15m, the mining height of 60m, and the thickness of the impermeable layer of 0.1m.

[0082] The rock mass displacement can more intuitively reflect the stability of the roof rock mass. The maximum displacement of the roof is used as the stability index of the mining area. The changes in the maximum displacement of the roof after excavation stabilization at various water pressure values ​​are obtained and analyzed. Combined with the criterion of rock mass limit displacement, typical water pressure values ​​of 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa, and 2.0MPa are selected to optimize the thickness of the aquiclude.

[0083] The third step is to determine the numerical simulation optimization scheme for the thickness of the aquiclude of the mine roof. The stability of the mine roof is not only affected by the water pressure value Pw, but also by the thickness H of the roof aquiclude (the height of the direct roof from the aquifer). By changing the water pressure value and the thickness of the aquiclude, a comparative analysis of the stress, displacement and plastic zone of the mine roof is conducted to further determine the reasonable thickness of the roof aquiclude. When the typical water pressure values ​​are 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa, and 2.0MPa, and the thickness of the roof aquiclude is 0m, 2m, 4m, and 8m, respectively, the thickness of the aquiclude is optimized.

[0084] The fourth step is to optimize the thickness of the roof waterproofing layer under different types of water pressure. The protective effect of the roof waterproofing layer on the mining area is mainly reflected in the stress and displacement of the roof and the prevention of the plastic zone from connecting with the fracture water conduction zone. Therefore, the maximum principal stress of the roof, the minimum principal stress of the roof, the displacement of the roof, and the plastic zone of the mining area are selected as the basis for stability analysis. In order to facilitate the description of the damage depth, the Average discrimination method is selected for the plastic zone discrimination, that is, when more than 50% of the volume in the unit body is damaged, the unit body is deemed to be damaged. Numerical simulation based on measured water pressure and actual mining parameters can better evolve the expansion law of the plastic zone of pressure mining, and then optimize the thickness of the waterproofing layer.

[0085] The stability analysis of the mining site was conducted at different water pressures and different thicknesses of impermeable rock layers. The maximum principal stress, minimum principal stress, displacement, and plastic zone cloud diagrams of the roof with different thicknesses of impermeable rock layers (0.1m, 2m, 4m, and 8m) were obtained when the water pressure was 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa, and 2.0MPa, and the stability analysis was conducted.

[0086] The fifth step is to optimize the thickness of the waterproof layer under different water pressure conditions. In order to more intuitively compare the changes in stress, displacement, and plastic zone of each scheme, the maximum value of the maximum principal stress, the minimum value of the maximum principal stress, the maximum settlement of the roof, the depth of plastic zone damage and other indicators of each scheme at the 0.1m slice on the upper part of the mining site roof are extracted, and the influence of the thickness of the waterproof layer on each indicator under different water pressure conditions is studied. The interaction between water pressure and the thickness of the waterproof layer on the roof damage is compared and analyzed, and the thickness of the waterproof layer under different water pressure conditions is further optimized.

[0087] (1) Analysis of stress concentration changes. Obtain the maximum value of the maximum principal stress of each scheme under different water pressure conditions as a function of the thickness of the impermeable layer, and analyze the changes in the stress concentration of the mine roof as the thickness of the impermeable layer and water pressure change.

[0088] (2) Analysis of stress release changes. Obtain the curve of the minimum value of the maximum principal stress of each scheme under different water pressure conditions as the thickness of the impermeable layer changes, and analyze the changes in the stress release of the mine roof as the thickness of the impermeable layer and water pressure change.

[0089] (3) Roof displacement analysis. Obtain the roof displacement curves of each scheme under different water pressure conditions as a function of the thickness of the impermeable layer, and analyze the changes in the roof displacement of the mining area as the thickness of the impermeable layer and water pressure change.

[0090] (4) Plastic zone analysis. When the mining area is excavated, it will cause plastic failure of the surrounding rock. When the damage zone is connected with the water-bearing fracture zone, water will flow into the mining area through the plastic zone, causing a water inrush accident. In order to intuitively display the connection between the damage zone and the fracture zone, the water inrush safety factor S is defined. When S is greater than 2, it indicates that there is basically no risk of water inrush.

[0091]

[0092] Where S is the water inrush safety factor; Pw is the water pressure, MPa; H is the thickness of the waterproof layer, m; Z is the plastic zone failure depth, m.

[0093] The safety factors of water inrush in the mining area under different water pressures and different thicknesses of impermeable rock layers were calculated respectively, and a line graph of the safety factors of water inrush under different water pressures versus the thickness of the impermeable rock layers was drawn, and the critical thickness was extracted as the preliminary thickness of the impermeable layer under different water pressure conditions.

[0094] (5) Comprehensively considering the changing trends of stress, displacement, and plastic zone with water pressure and roof, the final optimal thickness of the waterproof layer under different water pressure conditions is obtained.

[0095] Example:

[0096] (1) An iron mine in Hebei Province is a large water karst mine. The hydrogeological conditions of the ore body are relatively complex. It is a typical large water karst deposit, and the surrounding rock quality grade belongs to grade IV to V broken rock mass. The current mining middle section is at the level of -230m to -170m. In order to ensure safe mining, it is urgent to carry out research on the optimization of the thickness of the roof aquiclude and groundwater control in pressurized underwater mining.

[0097] In order to find out the water pressure at the -170m level of the mine, four typical water outlet points with different flow rates were selected to install YHY type mine water pressure monitors (error ≤ ±1%) for long-term monitoring. The water pressure monitoring results of each measuring point are shown in Table 1. The water pressure is basically between 0.1 and 2MPa.

[0098] Table 1 Water pressure monitoring results

[0099]

[0100] (2) FLac 3D was used to establish mining models under different initial water pressure conditions and conduct a stope stability-water pressure sensitivity analysis.

[0101] A standard hexahedral grid is used, with the grid becoming sparser from the center to the surroundings, and the minimum grid length is 2m. The x, y, and z axes are taken as the strike, dip, and height of the ore body, respectively. The model is 600m long, 400m wide, and 500m high. The model imposes bottom and side normal displacement constraints, and a uniformly distributed load is applied to the top to simulate the mining depth. See the model schematic diagram Figure 2 The unit body adopts the Mohr-Coulomb constitutive model, and the mechanical parameters are obtained according to the existing mechanical tests, see Table 2. The water pressure value is assigned according to the pore water pressure. Assuming that the water-conducting fracture zone is located on the upper part of the direct roof of the mine room, the upper part of the stope roof is assigned the pore water pressure, such as Figure 3 .

[0102] Table 2 Mechanical parameters of ore and rock

[0103]

[0104] Determine the numerical simulation scheme. Based on the on-site water pressure monitoring results, the water pressure values ​​are selected as 0MPa, 0.1MPa, 0.2MPa, 0.5MPa, 1MPa, 1.5MPa, and 2MPa. Combined with the mining technology plan, the excavation calculation is carried out according to the mining length of 50m, the mining width of 15m, the mining height of 60m, and the thickness of the impermeable layer of 0.1m. The rock mass displacement can more intuitively reflect the stability of the roof rock mass. The maximum displacement of the roof is used as the stability index of the mining field. The changes in the maximum displacement of the roof after the excavation of each water pressure value are obtained. Figure 4 As shown. Figure 4It can be seen that the presence of water pressure will significantly increase the maximum displacement of the roof. The displacement of the roof is positively correlated with the water pressure value. The increment of the roof displacement increases with the increase of water pressure, and the increment of the roof displacement when the water pressure value is 0-1.0MPa is much smaller than the increment of the roof displacement when the water pressure value is 1.0-2.0MPa. Combined with the rock mass limit displacement criterion, for this ore body, the presence of water pressure will cause the mining area to be at risk of instability. When the water pressure value is 0.1-1.0MPa, the mining area may be at risk of instability. When the water pressure value is 1.0-2.0MPa, the deformation of the roof is large, and the mining area is unstable and collapsed. Based on this, the typical water pressure values ​​of 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa, and 2.0MPa are selected below to optimize the thickness of the aquiclude.

[0105] (3) Determine the numerical simulation optimization scheme for the thickness of the impermeable layer of the mining area roof. The stability of the mining area roof is not only affected by the water pressure value Pw, but also by the thickness of the roof impermeable layer H (the height of the direct roof from the aquifer). By changing the water pressure value and the thickness of the impermeable layer, the stress, displacement and plastic zone of the mining area roof are compared and analyzed to further determine the reasonable thickness of the roof impermeable layer. When the typical water pressure values ​​are 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa and 2.0MPa, and the thickness of the roof impermeable layer is 0m, 2m, 4m and 8m respectively, the thickness of the impermeable layer is optimized. The 20 simulated technical schemes are detailed in Table 3.

[0106] Table 3 Numerical simulation scheme

[0107]

[0108] Due to space limitations, only the maximum principal stress, minimum principal stress, displacement, and plastic zone cloud diagrams of the roof of the four technical solutions when the water pressure is 0.1MPa and the thickness of the water-repellent rock layer is 0.1m, 2m, 4m, and 8m, respectively, are used for stability analysis. The maximum principal stress, minimum principal stress, displacement, and plastic zone cloud diagrams of the roof of other technical solutions are omitted.

[0109] (4) Optimization of roof impermeable rock thickness under different water pressures. The protective effect of the roof impermeable layer on the mining area is mainly reflected in the stress and displacement of the roof and the prevention of the plastic zone from connecting with the fracture water conduction zone. Therefore, the maximum principal stress of the roof, the minimum principal stress of the roof, the displacement of the roof, and the plastic zone of the mining area are selected as the basis for stability analysis. In order to facilitate the description of the damage depth, the Average judgment method is selected for the plastic zone judgment, that is, when more than 50% of the volume of the unit body is damaged, the unit body is considered to be damaged. Numerical simulation based on measured water pressure and actual mining parameters can better evolve the expansion law of the plastic zone of pressurized mining, and then optimize the thickness of the impermeable layer. The stability analysis of the mining area is carried out under different water pressures and different thicknesses of impermeable rock layers.

[0110] 1) Stability analysis of different waterproofing layer thicknesses when the water pressure is 0.1MPa. When the water pressure is 0.1MPa, the maximum principal stress, minimum principal stress, displacement, and plastic zone cloud diagrams of the roof with different waterproofing layer thicknesses are shown in Figures 5 to 20. After the excavation of the mining area, a stress concentration zone of about 28.123 to 28.486MPa is formed at the intersection of the roof and the side walls, and the concentration phenomenon in the width direction is significantly greater than that in the length direction; the direct roof forms a stress release zone of 0.477 to 0.541MPa, and a semicircular stress release zone is formed on the roof of the adjacent mine room in the width direction of the mining area; the displacement of the roof is elliptical, with a size of about 48.220 to 51.748mm, which is between stable and unstable; the maximum damage depth is less than 3m, and the waterproofing layer thickness is 0.1m: the roof mainly shows tensile failure, and the intersection of the roof and the two sides only shows tensile failure. With the increase of the thickness of the waterproofing layer, the number of shear failures is greatly reduced.

[0111] 2) Stability analysis of different aquiclude thicknesses when the water pressure is 0.5 MPa. When the water pressure is 0.5 MPa, the maximum principal stress, minimum principal stress, displacement, and plastic zone cloud diagrams of the roof with different aquiclude thicknesses are omitted. After the excavation of the mining area, a stress concentration zone of 28.173-28.517MPa is formed at the intersection of the roof and the side walls, and the concentration phenomenon in the width direction is significantly greater than that in the length direction; a stress release zone of 0.316-0.547MPa is formed on the direct roof, and a semicircular stress release zone is formed on the roof of the adjacent mine room in the width direction of the mining area; the displacement of the roof is elliptical, with a size of about 48.331-53.853mm, which is between stable and unstable; the thickness of the aquiclude is 0.1m: the roof is mainly tensile, with a maximum damage depth of less than 5m, and there is shear-tensile damage at the intersection of the roof and the two sides; when the thickness of the aquiclude is greater than 2m, the maximum damage depth of the roof is reduced to less than 3m, and the intersection of the roof and the two sides only shows tensile damage. With the increase of the thickness of the aquiclude, the number of shear failures is greatly reduced.

[0112] 3) Stability analysis of different waterproof layer thicknesses when the water pressure is 1.0MPa. When the water pressure is 1.0MPa, the maximum principal stress, minimum principal stress, displacement, and plastic zone cloud diagrams of the roof with different waterproof layer thicknesses are omitted. After the excavation of the mining area, a stress concentration zone of 28.297-28.561MPa is formed at the intersection of the roof and the side walls, and the concentration phenomenon in the width direction is significantly greater than that in the length direction; a stress release zone of 0.205-0.549MPa is formed on the direct roof, and a semicircular stress release zone is formed on the roof of the adjacent mine room in the width direction of the mining area; the displacement of the roof is elliptical, with a size of about 48.573-63.389mm, which is between stable and unstable; the thickness of the waterproof layer is 0.1m: the roof is mainly tensile, with a maximum damage depth of less than 5m, and there is shear-tensile damage at the intersection of the roof and the two sides; the thickness of the waterproof layer is 2m: the maximum damage depth of the roof is less than 5m, and the intersection of the roof and the two sides still shows more shear-tensile damage; the thickness of the waterproof layer is greater than 4m: the maximum damage depth of the roof is reduced to less than 3m, and the intersection of the roof and the two sides only shows tensile damage. With the increase of the thickness of the waterproof layer, the number of shear failures is greatly reduced.

[0113] 4) Stability analysis of different waterproof layer thicknesses when the water pressure is 1.5MPa. When the water pressure is 1.5MPa, the maximum principal stress, minimum principal stress, displacement, and plastic zone cloud diagrams of the roof with different waterproof layer thicknesses are omitted. After the excavation of the mining area, a stress concentration zone of 28.775-28.597MPa is formed at the intersection of the roof and the side walls, and the concentration phenomenon in the width direction is significantly greater than that in the length direction; a stress release zone of 0.167-0.554MPa is formed on the direct roof, and a semicircular stress release zone is formed on the roof of the adjacent mine room in the width direction of the mining area; the displacement of the roof is distributed in an "elliptical" shape, with a size of approximately 48.657-142.25mm, and is between stable and unstable; the thickness of the waterproof layer is 0.1m: the roof mainly shows tensile failure, the maximum failure depth is less than 5m, and there is shear-tensile failure at the intersection of the roof and the two walls; the thickness of the waterproof layer is 2m: the maximum failure depth of the roof is less than 5m, and the intersection of the roof and the two walls still shows more shear-tensile failure; the thickness of the waterproof layer is greater than 4m: the maximum failure depth of the roof is reduced to less than 3m, and the intersection of the roof and the two walls only shows tensile failure. As the thickness of the aquiclude increases, the amount of shear failure decreases significantly.

[0114] 5) Stability analysis of different waterproof layer thicknesses when the water pressure is 2.0MPa. When the water pressure is 2.0MPa, the maximum principal stress, minimum principal stress, displacement, and plastic zone cloud diagrams of the roof with different waterproof layer thicknesses are omitted. After the excavation of the mining area, a stress concentration zone of 28.453-29.027MPa is formed at the intersection of the roof and the side walls, and the concentration phenomenon in the width direction is significantly greater than that in the length direction; a stress release zone of 0.846-0.605MPa is formed in the direct roof, and a semicircular stress release zone is formed in the roof of the adjacent mine room in the width direction of the mining area; the displacement of the roof is elliptical, with a size of about 48.925-∞mm, which is between stable-unstable-instability, and the mining area may be unstable; the thickness of the aquiclude is 0.1m: the roof is mainly characterized by large-scale tensile damage, with a maximum damage depth of less than 5m, and there are a large number of shear-tensile damage zones at the intersection of the roof and the two sides; the thickness of the aquiclude is 2m: the maximum damage depth of the roof increases to less than 7m, but the damage area is greatly reduced; the thickness of the aquiclude is 4-8m: the damage depth is reduced to less than 3m, and the intersection of the roof and the two sides only shows shear damage. With the increase of the thickness of the aquiclude, the number of shear failures is greatly reduced.

[0115] (5) Optimization of the thickness of the waterproof layer under different water pressure conditions. In order to more intuitively compare the changes in stress, displacement, and plastic zone of each scheme, the maximum value of the maximum principal stress, the minimum value of the maximum principal stress, the maximum settlement of the roof, the depth of plastic zone damage and other indicators of each scheme at the 0.1m slice above the mining site roof are extracted, and the influence of the thickness of the waterproof layer on each indicator under different water pressure conditions is studied. The interaction between water pressure and the thickness of the waterproof layer on the roof damage is compared and analyzed, and the thickness of the waterproof layer under different water pressure conditions is further optimized.

[0116] 1) Analysis of stress concentration changes. Obtain the maximum value of the maximum principal stress of each scheme under different water pressure conditions as the thickness of the waterproof layer changes. Fig.21 As shown in the figure, the stress concentration of the roof of the mining area is analyzed with the change of the thickness of the impermeable layer and the water pressure. As can be seen from the figure, the maximum principal stress is affected by the joint action of rock and water pressure, and is positively correlated with both when the mining area is in a stable state. When the water pressure value is 0.1-0.5MPa, the increase of water pressure has little effect on the stability of the roof, and the stress change mainly comes from the surrounding rock at this time; when the water pressure value is 0.5-2.0MPa, the stress concentration increases with the increase of the thickness of the impermeable layer. When the thickness increases from 0 to 4m, the influence of water pressure on stress concentration gradually decreases, indicating that the stress change at this time mainly comes from water pressure, and increasing the thickness of the roof is conducive to the stability of the roof; when the thickness increases from 4 to 8m, the change rate of stress concentration with the increase of water pressure value has no obvious change, indicating that the stress change comes from the surrounding rock at this time, and the water pressure has little effect on the stability of the roof.

[0117] The comprehensive changes of stress concentration with the thickness of the waterproof layer and water pressure show that: when the water pressure is high, the stress concentration increases with the thickness of the waterproof layer and there is a minimum value of the waterproof layer thickness, which is between 2 and 6m; when the thickness of the waterproof layer is small, the stress concentration increases with the water pressure, and the positive and negative slopes of the curves have a critical water pressure value, which is between 1.0 and 1.5MPa. From the perspective of stress concentration, technical solutions 2-4, 6-8, 10-12, 15, 16, 19, and 20 have better stability.

[0118] 2) Analysis of stress release changes. The minimum value of the maximum principal stress of each scheme under different water pressure conditions varies with the thickness of the waterproof layer. Fig. 22 , the changes of stress release of the roof of the mining area with the change of the thickness of the aquiclude and the water pressure are analyzed. As can be seen from the figure, the stress release gradually changes from a negative correlation with the water pressure to a positive correlation with the increase of thickness. When the thickness of the aquiclude is 0-2m, the degree of stress release is negatively correlated with the water pressure value, and the stress release change rate increases with the increase of water pressure. When the thickness of the aquiclude is 2-4m, as the thickness increases, the stress and water pressure values ​​gradually change from negative correlation to positive correlation. When the thickness of the aquiclude increases to 6-8m, although the stress value increases, the change is not obvious, and the stress and water pressure values ​​maintain a positive correlation. Therefore, when the thickness of the aquiclude is small, the degree of stress release is mainly affected by the water pressure value, which hinders the unloading of the roof and is not conducive to the stability of the mining area. When the thickness of the aquiclude is large, the direct effect of water pressure on the roof is weakened and the impact is small. From the perspective of stress release, technical solutions 1-12, 15, 16, 19, and 20 have good stability.

[0119] 3) Analysis of roof displacement. Obtain the roof displacement curves of each scheme under different water pressure conditions as a function of the thickness of the waterproof layer. Fig.23 , the change of the displacement of the mining area roof with the change of the thickness of the impermeable layer and the water pressure is analyzed. On the whole, the maximum settlement of the roof is negatively correlated with the thickness of the impermeable layer, and the displacement reduction decreases with the increase of thickness. The maximum settlement of the roof is positively correlated with the water pressure, and the stress increment increases with the increase of water pressure. According to the rock mass limit displacement criterion (50mm), technical solutions 2-4, 6-8, 10-12, 15, 16, 19, and 20 have good stability.

[0120] 4) Plastic zone analysis. When the mining area is excavated, it will cause plastic failure of the surrounding rock. When the damage zone is connected with the water-bearing fracture zone, water will flow into the mining area through the plastic zone, causing roof water inrush accidents. In order to intuitively display the connection between the damage zone and the fracture zone, the water inrush safety factor S is defined. When S is greater than 2, it indicates that there is basically no risk of water inrush.

[0121]

[0122] Where S is the water inrush safety factor; Pw is the water pressure, MPa; H is the thickness of the waterproof layer, m; Z is the plastic zone failure depth, m.

[0123] The safety factor of water inrush in the mining area under different water pressures and different thickness of impermeable rock layers is calculated respectively. The results are as follows: Fig.24 In order to obtain a more accurate thickness of the impermeable layer, assuming that there is a linear relationship between adjacent thicknesses, a line graph of the water inrush safety factor under different water pressures versus the thickness of the impermeable rock layer is drawn as shown in the figure below: Fig.25 , extract its critical thickness. Water pressure is 0.1MPa: when the thickness of the aquiclude is less than 1.8m, the depth of the plastic zone is greater than the thickness of the aquiclude, and there is a risk of water inrush in the mining area. When the thickness of the aquiclude is greater than 1.8m, the depth of the plastic zone is less than the thickness of the aquiclude, the stability of the mining area is good, and the thickness of the aquiclude is not less than 1.8m. Similarly, when the water pressure is 0.5MPa, the thickness of the aquiclude must be no less than 3m; when the water pressure is 1.0MPa, the thickness of the aquiclude must be no less than 4m; when the water pressure is 1.5MPa, the thickness of the aquiclude must be no less than 5.6m; when the water pressure is 2.0MPa, the thickness of the aquiclude must be no less than 7.1m.

[0124] 5) Based on the changing trends of stress, displacement and plastic zone with water pressure and roof, the final optimal thickness of the waterproof layer under different water pressure conditions is as follows: when the water pressure is 0.1MPa, the thickness of the waterproof layer must be no less than 1.8m; when the water pressure is 0.5MPa, the thickness of the waterproof layer must be no less than 3m; when the water pressure is 1.0MPa, the thickness of the waterproof layer must be no less than 4m; when the water pressure is 1.5MPa, the thickness of the waterproof layer must be no less than 5.6m; when the water pressure is 2.0MPa, the thickness of the waterproof layer must be no less than 7.1m.

[0125] The present invention firstly establishes a mining model based on the measured water pressure, conducts a mining field stability-water pressure sensitivity analysis, and proposes a numerical simulation optimization scheme for the thickness of the mining field roof waterproof rock layer on the basis of the analysis results. The numerical simulation based on the measured water pressure and actual mining parameters can better evolve the expansion law of the plastic zone of pressurized mining; and optimizes the calculation formula of the safety factor of water inrush in metal mines; and further optimizes the thickness of the waterproof layer on the basis of the comprehensive stress, displacement, and plastic zone changing trends with water pressure and roof. The optimization result is more in line with the actual underground mining and has good engineering guidance significance.

Claims

1. A method for optimizing the thickness of reserved impermeable rock layer, characterized in that The following steps are involved: S1. Monitor the water pressure at typical water outlet points in the well and analyze the water pressure monitoring results at the measuring points; S2. Use FLac 3D to establish mining models under different initial water pressure conditions and conduct stope stability-water pressure sensitivity analysis; S3. Determine the numerical simulation optimization scheme for the thickness of the waterproof layer on the roof of the mining area, and conduct comparative analysis on the stress, displacement and plastic zone of the roof of the mining area by changing the water pressure value and the thickness of the waterproof layer, so as to further determine the reasonable thickness of the waterproof layer on the roof; S4. Optimize the thickness of the roof impermeable rock layer under different water pressures, select the maximum principal stress of the roof, the minimum principal stress of the roof, the displacement of the roof and the plastic zone of the mining area as the basis for stability analysis, and conduct the mining area stability analysis under different water pressures and different thicknesses of impermeable rock layers; S5. Optimize the thickness of the waterproof layer under different water pressure conditions, study the influence of the thickness of the waterproof layer on various indicators of the roof under different water pressure conditions, compare and analyze the interaction between water pressure and the thickness of the waterproof layer on the damage of the roof, and further optimize the thickness of the waterproof layer under different water pressure conditions.

2. The method for optimizing the thickness of a reserved water-repellent rock layer according to claim 1, characterized in that: In step S1, in order to find out the actual water pressure underground, 4-5 typical water outlet points with different flow rates are selected to install YHY type mining water pressure monitors with an error of ≤±1% for long-term monitoring.

3. The method for optimizing the thickness of a reserved impermeable rock layer according to claim 1, characterized in that: In the step S2, A standard hexahedral grid is used, and the grid gradually becomes sparser from the center to the surrounding areas. The minimum grid length is 2m. The x, y and z axes are taken as the strike, dip and height of the ore body respectively. The model is 600m long, 400m wide and 500m high. The model imposes normal displacement constraints on the bottom and sides, and a uniformly distributed load is applied on the top to simulate the mining depth. The unit body adopts the Mohr-Coulomb constitutive model, and the mechanical parameters are obtained according to existing mechanical tests. The water pressure value is assigned according to the pore water pressure. It is assumed that the water-conducting fracture zone is located on the upper part of the direct roof of the mine room, and the upper part of the roof of the mining area is assigned the pore water pressure. Determine the numerical simulation scheme. Based on the on-site water pressure monitoring results, select water pressure values ​​of 0MPa, 0.1MPa, 0.2MPa, 0.5MPa, 1MPa, 1.5MPa and 2MPa. Combined with the mining technical scheme, perform excavation calculations based on a mining length of 50m, a mining width of 15m, a mining height of 60m, and a waterproof layer thickness of 0.1m. Taking the maximum displacement of the roof as the stability index of the mining area, the changes in the maximum displacement of the roof after excavation stabilization at various water pressure values ​​were obtained and analyzed; combined with the criterion of rock mass limit displacement, typical water pressure values ​​of 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa and 2.0MPa were selected to optimize the thickness of the aquiclude.

4. The method for optimizing the thickness of a reserved impermeable rock layer according to claim 1, characterized in that: In step S3, when typical water pressure values ​​are selected as 0.1 MPa, 0.5 MPa, 1.0 MPa, 1.5 MPa and 2.0 MPa, and the thickness of the top waterproof layer is 0 m, 2 m, 4 m and 8 m, respectively, the thickness of the waterproof layer is optimized.

5. The method for optimizing the thickness of a reserved water-repellent rock layer according to claim 1, characterized in that: In the step S4, the Average judgment method is selected for plastic zone discrimination, that is, when more than 50% of the volume in the unit body is damaged, the unit body is deemed to be damaged; the maximum principal stress, minimum principal stress, displacement and plastic zone cloud map of the roof are obtained when the water pressure is 0.1MPa, 0.5MPa, 1.0MPa, 1.5MPa and 2.0MPa and the thickness of the impermeable rock layer is 0.1m, 2m, 4m and 8m respectively, and the stability analysis is performed.

6. The method for optimizing the thickness of a reserved water-repellent rock layer according to claim 1, characterized in that: In step S5, the maximum value of the maximum principal stress, the minimum value of the maximum principal stress, the maximum settlement of the roof and the plastic zone failure depth index value of each scheme at the 0.1m slice on the upper part of the stope roof are extracted, and the influence of the thickness of the waterproof layer on each index under different water pressure conditions is studied. The interaction between the water pressure and the thickness of the waterproof layer on the roof failure is compared and analyzed, as follows: (1) Analysis of stress concentration changes: obtain the maximum value of the maximum principal stress of each scheme under different water pressure conditions as the thickness of the impermeable layer changes, and analyze the changes in the stress concentration of the mine roof with the change of the thickness of the impermeable layer and water pressure; (2) Analysis of stress release changes: obtain the curve of the minimum value of the maximum principal stress of each scheme under different water pressure conditions as the thickness of the impermeable layer changes, and analyze the changes in the stress release of the mine roof as the thickness of the impermeable layer and water pressure change; (3) Roof displacement analysis: obtain the roof displacement curves of each scheme under different water pressure conditions as a function of the thickness of the impermeable layer, and analyze the changes in the roof displacement of the mining area as the thickness of the impermeable layer and water pressure change; (4) Plastic zone analysis: When the mining area is excavated, it will cause plastic failure of the surrounding rock. When the damage zone is connected with the water-bearing fracture zone, water will flow into the mining area through the plastic zone, causing roof water inrush accidents. In order to intuitively display the connection between the damage zone and the fracture zone, the water inrush safety factor S is defined. When S is greater than 2, it indicates that there is basically no risk of water inrush. ； Where, S is the water inrush safety factor; Pw is the water pressure, MPa; H is the thickness of the water-blocking layer, m; Z is the failure depth of the plastic zone, m; The safety factor of water inrush in the mining area under different water pressures and different thicknesses of impermeable rock layers was calculated respectively, and a line graph of the change of the safety factor of water inrush with the thickness of impermeable rock layers under different water pressures was drawn, and its critical thickness was extracted as the preliminary thickness of the impermeable layer under different water pressure conditions; (5) The optimal thickness of the aquiclude under different water pressure conditions is obtained by comprehensively analyzing the changing trends of stress, displacement, and plastic zone with water pressure and roof.

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