Stability evaluation method and system for composite water-resisting layer in mining under loose aquifer

Abstract

The present invention provides a method for evaluating the stability of a composite water-resisting layer during mining under a loose aquifer, which includes: constructing a geological conceptual model and a mechanical analysis model of the composite water-resisting layer for inclined coal seam mining in a confined aquifer; obtaining the uniform load R acting on region III from region II by using the Winkler elastic foundation principle and the force system balance condition; obtaining the expression of the deflection function coefficient by using the rectangular thin plate theory and the minimum potential energy principle of elastic mechanics; determining the positions where the overlying rock water-resisting layer is most likely to undergo tensile failure and shear failure according to different yield failure criteria; substituting the coordinates of the most vulnerable failure position points into the corresponding yield failure criteria to obtain the ultimate water pressure values when the overlying rock water-resisting layer and the clay water-resisting layer yield and fail, and further obtaining the critical water pressure value when the composite water-resisting layer undergoes yield failure. This solves the technical problem of low safety in coal resource mining under a loose aquifer due to the lack of an analysis method for the breaking and instability mechanism of the composite water-resisting layer.

Description

Technical Field

[0001] The present invention relates to the technical field of water inrush prediction for coal mining under loose aquifers, and particularly relates to a method and system for evaluating the stability of a composite water-resisting layer during coal mining under a loose aquifer. Background Art

[0002] In many mining areas in North China, East China and other regions of China, there are generally loose aquifers directly occurring on the top of bedrock, which are mainly composed of uncemented sand, gravel, etc. There is often a clay layer with good water-resisting performance at the bottom of the loose aquifer. Due to the good water-resisting performance of the clay layer and the rock in the weathered zone of mudstone, it shows local pressure-bearing characteristics. Moreover, the effect of the hydrophobic pressure reduction operation during the mining process of adjacent working faces is very small. During the coal mining process, the safety of the working face is closely related to the water-resisting ability of the "clay-overburden" composite water-resisting layer.

[0003] During the process of coal resource mining under a loose confined aquifer, the determination of the reasonable size of the waterproof coal (rock) pillar is directly related to the safety of the working face mining. If the size of the coal (rock) pillar is too large, a large amount of coal resources will be stagnated and wasted seriously; if the size of the coal (rock) pillar is too small, it will bring huge potential safety hazards to the working face mining. In the past, for the calculation method of the vertical height of the waterproof coal (rock) pillar, parameters such as the hardness of the roof overburden and the mining height were substituted into the corresponding relevant calculation formulas in the "Three-Under" regulations to obtain the vertical height of the waterproof coal (rock) pillar. The safety and stability of this method are often challenged. Therefore, from the mechanical perspective, the stability mechanism of the "clay-overburden" composite structure water-resisting layer under mining conditions is evaluated, so as to provide a basis for the safe mining of coal resources under a loose confined aquifer.

[0004] The invention patent "Determination method of critical parameters of backfill body based on the stability of seepage isolation zone in overlying strata affected by mining" with the application number CN202111347795.X, when the coal seam is divided into several combined mining gateways (8) for skip mining, each combined mining gateway (8) includes s mining gateways and is mined in sequence, that is, the mining includes s mining stages, and the mining stages are matched with the mining gateways. According to the boundary conditions, the subsidence deflection curve of the i-th rock stratum in the s-th mining stage is obtained, the compression rate of the roof before the mining of the mining gateway in each stage is calculated, and the elastic foundation coefficients of the i-th rock stratum above the boundary coal pillar and the i-th rock stratum above the mining gateway in the s-th stage are calculated respectively, so as to obtain the deformation information of the roof overlying strata seepage isolation zone after the mining and filling in the s-th stage. According to the deformation information, the critical mechanical parameters of the backfill body that satisfy the stability of the seepage isolation zone are deduced. At present, for the research on the mechanism and criterion of water inrush and sand bursting under loose aquifers, the research methods mainly include theoretical analysis, laboratory tests, and numerical simulations. In theoretical analysis, the commonly used mechanical models include fixed-end beam models, fixed-end plate models, etc.; in laboratory tests, the research approaches mostly focus on the method of similar simulation tests; the numerical simulation methods used mainly include discrete element, finite element, finite difference, etc. Many scholars have proposed different early warning indicators using the above research methods. However, it is very rare to analyze the stability of the "clay-overlying strata" composite water isolation layer considering the influence of loose aquifers with special confined characteristics, coal seam dip angle, etc., so as to analyze its breaking and instability mechanism. Therefore, a method for evaluating the stability of the composite water isolation layer during mining under loose aquifers needs to be proposed to predict and warn the water inrush and sand bursting of the working face under this geological condition, so as to ensure the safe coal mining of the mine. The existing technology has the technical problem that the safety of coal resource mining under loose aquifers is relatively low due to the lack of an analysis method for the breaking and instability mechanism of the composite water isolation layer. Summary of the Invention

[0005] The technical problem to be solved by the present invention is how to solve the technical problem that the existing technology has a relatively low safety of coal resource mining under loose aquifers due to the lack of an analysis method for the breaking and instability mechanism of the composite water isolation layer.

[0006] The present invention solves the above technical problem by adopting the following technical solutions: A method for evaluating the stability of a composite water isolation layer during mining under a loose aquifer includes:

[0007] S1. Construct a geological concept model for mining under a loose aquifer according to the occurrence of the coal seam and the superposition of the overlying strata structure. Take the caving zone in the geological concept model for mining under the loose aquifer as an elastic foundation body to obtain the elastic support data of the elastic foundation body for the trapezoidal overlying strata. Establish a force analysis model according to the elastic support data and the force system balance condition;

[0008] S2. Obtain the length of the area affected by the caved gangue according to the geometric relationship, and obtain the relationship data of the uniformly distributed load R of the trapezoidal overlying rock and the relationship data of the self-weight G of the trapezoidal overlying rock according to the force system balance condition of the force analysis model;

[0009] S3. Take the overlying aquifuge as a rectangular thin plate with four sides fixed, set the deflection function w of the overlying aquifuge according to the rectangular thin plate with four sides fixed, and obtain the relationship data of the deformation potential energy U of the overlying aquifuge to obtain the relationship data between the deformation potential energy of the overlying aquifuge and the coefficients of the deflection function;

[0010] S4. Obtain the work relationship data of the upper aquifer clay layer, the work relationship data of the body force of the overlying aquifuge, and the reverse work relationship data of the uniformly distributed load R of the trapezoidal overlying rock according to the relationship data between the deformation potential energy and the deflection function of the overlying aquifuge, so as to obtain the relationship data of the total potential energy of the overlying aquifuge, and process to obtain the coefficients of the deflection function of the overlying aquifuge. According to the stress relationship data of the overlying aquifuge and the deflection function w, process to obtain the relationship of the stress components on the upper surface of the overlying aquifuge;

[0011] S5. Perform assignment processing on the relationship of the stress components on the upper surface of the overlying aquifuge to obtain an isogram of the stress components, so as to obtain the distribution law of the stress components;

[0012] S6. Process the stress data of the overlying aquifuge in the isogram of the stress components with a preset elasticity mechanics logic to obtain the stress position relationship data. Use the Griffith yield criterion to judge and obtain the easily tensile failure position according to the stress position relationship data, and process to obtain the critical water pressure for tensile failure of the overlying aquifuge. Use the Mohr-Coulomb yield criterion to judge and obtain the easily shear failure position according to the stress position relationship data, and process to obtain the critical water pressure for shear yield failure of the overlying aquifuge;

[0013] S7. Take the clay layer as the rectangular thin plate with four sides fixed, and use the overlying aquifuge and a partial rectangular body of the trapezoidal overlying rock to jointly form a second combined trapezoid body. Perform the aforementioned steps S3 to S5 on the clay layer to obtain the stress data of the clay layer, and judge and obtain the critical water pressure for shear instability failure of the clay layer according to the Mohr-Coulomb yield criterion;

[0014] S8. Combine the critical water pressure for shear yield failure of the overlying aquifuge and the critical water pressure for shear instability failure of the clay layer to obtain the critical water pressure for fracture and instability of the composite structure aquifuge.

[0015] The present invention makes corresponding predictions for water inrush and sand bursting in the working face under specific geological conditions. Starting from the mechanical perspective, this prediction method constructs a geological conceptual model and a mechanical analysis model of a composite water-resisting layer for inclined coal seam mining in a confined aquifer. On the basis of reasonable model simplification, the Winkler elastic foundation principle and the force system equilibrium condition are used to obtain the deflection function and uniform load of the overlying rock water-resisting layer, and the yield failure principle is used to obtain the coordinates of the failure position. Furthermore, the critical water pressure value of the "overlying rock - clay" composite water-resisting layer breaking and losing stability is obtained. The present invention is applicable to the application scenario of a composite water-resisting layer for inclined coal seam mining under a loose confined aquifer, ensuring the safety of underground mining operations in the corresponding application scenarios. The present invention can scientifically evaluate the safe mining of inclined coal seams under a loose confined aquifer.

[0016] In a more specific technical solution, the supporting ability of the caving zone for the trapezoidal overlying rock is reflected by the elastic foundation coefficient k.

[0017] In a more specific technical solution, the step S2 includes:

[0018] S21. According to the geometric relationship, the length of the action area of the caving zone is obtained by the following logic:

[0019]

[0020] , where H is the vertical height of the caving zone gangue, α is the coal seam dip angle, θ is the roof caving angle, and b is the length of the rectangular thin plate with four sides fixed;

[0021] S22. According to the force system equilibrium condition in the force analysis model, it can be known that:

[0022] (G + RL y cosα)cosα = kwL y (1 - 1)

[0023] , where G is the self-weight of the trapezoidal overlying rock, k is the elastic foundation coefficient of the caving zone gangue, and w is the deflection function;

[0024] S23. According to the force system equilibrium condition, the relationship data of the uniform load R of the trapezoidal overlying rock are obtained by the following logic:

[0025]

[0026] S24. According to the force system equilibrium condition, the relationship data of the self-weight G of the trapezoidal overlying rock are obtained by the following logic:

[0027]

[0028] , where γ3 is the average unit weight of the rock mass of the trapezoidal overlying rock.

[0029] In a more specific technical solution, step S3 includes:

[0030] S31. Regarding the overlying rock water - resistant layer as the rectangular thin plate with four - side fixed supports;

[0031] S32. Setting the deflection function of the rectangular thin plate with four - side fixed supports according to the following logic:

[0032]

[0033] , so as to obtain that the inclined length of the working face of the overlying rock water - resistant layer is L b =b / cosα; the advancing distance of its working face along the strike direction is L a =a / cosα, where a and b are the width and length of the rectangular thin plate with four - side fixed supports, and A is the coefficient of the deflection function of the overlying rock water - resistant layer;

[0034] S33. For the thin plate without free edges, obtaining the data of the relationship between the deformation potential energy U of the overlying rock water - resistant layer according to the following logic:

[0035]

[0036] In the formula, D is the flexural rigidity of the clay water - resistant layer thin plate;

[0037] S34. Processing the data of the relationship between the deformation potential energy of the overlying rock water - resistant layer and the coefficient of the deflection function according to the deflection function w and the data of the relationship between the deformation potential energy U of the overlying rock water - resistant layer;

[0038]

[0039] In a more specific technical solution, step S4 includes:

[0040] S41. Regarding the uniform load acting on the overlying rock water - resistant layer as the superposition of the water pressure P0 of the aquifer and the self - weight load G1 of the upper clay layer, and obtaining the data of the work - done relationship of the upper aquifer clay layer according to the data of the relationship between the deformation potential energy and the deflection function of the overlying rock water - resistant layer according to the following logic:

[0041] Ω1 = ∫∫[P0 + G1]wdxdy (1 - 7)

[0042]

[0043] In the formula, γ1 is the average unit weight of the clay layer; H1 is the thickness of the clay layer.

[0044] S42. Obtaining the data of the work - done relationship of the body force of the overlying rock water - resistant layer according to the data of the relationship between the deformation potential energy and the deflection function of the overlying rock water - resistant layer according to the following logic:

[0045] Ω2 = ∫∫∫G2wdxdydz (1-9)

[0046]

[0047] Wherein, γ2 is the average unit weight of the overlying water-resisting layer, and H2 is the thickness of the overlying water-resisting layer;

[0048] S43. According to the data of the deflection function relationship of the deformation potential energy of the overlying water-resisting layer, the data of the reverse work relationship of the uniformly distributed load R of the trapezoidal overlying rock are obtained by the following logic processing:

[0049] Ω3 = -∫∫Rwdxdy (1-11)

[0050]

[0051] , wherein, G is the self-weight of the trapezoidal overlying rock, and L y is the length of the area affected by the gangue in the caving zone;

[0052] S44. According to the data of the work done by the upper aquifer clay layer, the data of the body force work of the overlying water-resisting layer, and the data of the reverse work relationship of the uniformly distributed load R of the trapezoidal overlying rock, the data of the total potential energy relationship of the overlying water-resisting layer are obtained by the following logic processing:

[0053] Π = u - Ω1 - Ω2 - Ω3 (1-13)

[0054] S45. According to the principle of minimum potential energy, the variation of the total potential energy Π of the thin plate is taken to obtain the variation relationship data of the total potential energy Π of the thin plate:

[0055]

[0056] By combining the data of the deflection function relationship of the deformation potential energy of the overlying water-resisting layer, the data of the work done by the upper aquifer clay layer, the data of the body force work of the overlying water-resisting layer, the data of the reverse work relationship of the uniformly distributed load R of the trapezoidal overlying rock, the data of the total potential energy relationship of the overlying water-resisting layer, and the variation relationship data of the total potential energy Π of the thin plate, the coefficient A of the deflection function of the overlying water-resisting layer can be obtained as:

[0057]

[0058] S46. According to the following logic, the stress component relationship on the upper surface of the overlying water-resisting layer is obtained by processing the stress relationship data of the overlying water-resisting layer and the deflection function w:

[0059]

[0060] Wherein, A is the coefficient of the deflection function, and μ is the Poisson's ratio of the overlying water-resisting layer, where,

[0061]

[0062] σ x 、σ y respectively represent the vertical stress component and the tangential stress component, and τ xy is the shear stress.

[0063] In a more specific technical solution, the step S5 includes:

[0064] S51. Collect and substitute the mining conditions and geological parameters of the coal mine into the stress component relationship on the upper surface of the overlying aquifuge to obtain stress component relationship data;

[0065] S52. Use a preset drawing tool to draw an isogram according to the stress component relationship data to obtain the distribution law of each stress component on the upper surface (z = -h / 2) of the overlying aquifuge.

[0066] The present invention uses the Winkler elastic foundation principle and the force system equilibrium condition to obtain the uniform load R acting on region III by region II and the uniform load R' acting on region II by region I. On the basis of regarding the overlying aquifuge and the clay aquifuge as a rectangular thin plate with four sides fixed, combined with the rectangular thin plate theory of elastic mechanics and using the principle of minimum potential energy, the expression of the deflection function coefficient is obtained. It avoids the safety challenges brought by the conventional method of substituting parameters such as the hardness of the roof overlying rock and the mining height into the "Three-Under" regulations for calculating the vertical height of the waterproof coal (rock) pillar in the past, and improves the safety and stability of the composite aquifuge mining.

[0067] In a more specific technical solution, the step S6 includes:

[0068] S61. Process the stress data of the overlying aquifuge in the stress component isogram with the following preset elastic mechanics logic to obtain the stress position relationship data:

[0069]

[0070] S62. Judge the tensile failure according to the following Griffith yield criterion based on the stress position relationship data:

[0071]

[0072] To obtain an isogram of σ1 + 3σ3, where σ1 is the maximum principal stress and σ3 is the minimum principal stress;

[0073] S63. Process the isogram of σ1 + 3σ3 to obtain the easily tensile failure position, analyze and obtain the tensile failure state data with the tensile failure judgment criterion, and process to obtain the critical water pressure p of the tensile failure of the overlying aquifuge 2t ;

[0074] S64. Determine the critical shear failure of the overburden water - resisting layer according to the following Mohr - Coulomb yield criterion:

[0075]

[0076] , where c is the cohesion of the overburden water - resisting layer; is the internal friction angle;

[0077] S65. Preset the function f(x, y):

[0078]

[0079] S66. Use the preset drawing tool to draw the contour map of the function f(x, y), and find the easily shear - failure position according to the contour map of the function f(x, y) to obtain the critical water pressure p2 of the shear yield failure of the overburden water - resisting layer s ;

[0080] S67. According to the critical water pressure of the tensile failure of the overburden water - resisting layer and the critical water pressure of the shear yield failure of the overburden water - resisting layer, process with the following logic to obtain the critical water pressure p2 of the overburden water - resisting layer:

[0081] p2 = min(p 2t , p 2s ) (1 - 22).

[0082] In a more specific technical solution, the step S7 includes:

[0083] S71. Regard the clay layer as the rectangular thin plate with four - sided fixed supports, and jointly form a new combined trapezoid body with a part of the rectangular body of the overburden water - resisting layer and the trapezoidal overburden (3) to support the clay layer, so as to construct a mechanical analysis model of the clay layer;

[0084] S72. Process according to the mechanical analysis model of the clay layer to obtain the expression of the self - weight G L of the new combined trapezoidal overburden rock mass:

[0085]

[0086] S73. Process according to the force - system balance condition in the mechanical analysis model of the clay layer to obtain the expression of the uniform load R1 of the clay layer acting on the new combined trapezoid body:

[0087]

[0088] S74. Process according to the deflection function of the rectangular thin plate with four - sided fixed supports to obtain the work done by the water pressure P0 of the loose aquifer acting on the clay layer above it:

[0089] Ω1 = ∫∫P0wdxdy (1-25) The work done by the body force G1 of the clay layer is obtained by the following logic:

[0090] Ω2 = ∫∫G1wdxdydz (1-26)

[0091] , where the self-weight G1 of the clay layer = γ1, which is the unit weight of the clay layer;

[0092] The work done by the uniform load R1 of the new combined trapezoidal overlying rock mass on the clay layer is obtained by the following logic:

[0093] Ω3 = -∫∫R1wdxdy (1-27)

[0094] S75. According to the principle of minimum potential energy, the deflection function coefficient A1 of the clay layer is obtained:

[0095]

[0096] , where D is the flexural rigidity of the thin plate of the clay aquitard, and the corresponding expression is:

[0097]

[0098] , where E1, H1, and μ1 are respectively: the elastic modulus, average thickness, and Poisson's ratio of the clay aquitard.

[0099] S76. The Mohr-Coulomb yield criterion is adopted for the clay layer, and the position where the clay layer is most likely to undergo shear yield failure is judged by the following logic:

[0100]

[0101] where c1 and are respectively the cohesion and internal friction angle of the clay layer, and σ1 and σ3 are respectively its maximum and minimum principal stresses;

[0102] S78. The critical water pressure p1 for shear instability failure of the clay layer is obtained according to the formula.

[0103] In a more specific technical solution, in step S8, the critical water pressure p for fracture instability of the composite structure aquitard is obtained by combining the critical water pressure for shear yield failure of the overlying rock aquitard and the critical water pressure for shear instability failure of the clay layer by the following logic:

[0104] p = max[p1, min(p 2t , p 2s )] (1-31)

[0105] Based on finding the most vulnerable (tensile, shear) positions, the present invention uses the expressions of stress components and principal stresses of elastic thin plates, combines the Griffith and Mohr-Coulomb yield criteria, obtains the critical water pressure values corresponding to the yield failure of the clay water-resisting layer and the critical water pressure values corresponding to the failure of the overlying rock water-resisting layer, and finally obtains the critical water pressure value when the composite water-resisting layer undergoes yield failure. Using the critical water pressure value to analyze the breaking instability mechanism and the easily broken positions of the composite water-resisting layer, applying the mechanical analysis method to the engineering practice of the composite water-resisting layer in the mining of inclined coal seams under loose aquifers can provide a reference basis for the evaluation of the stability of water-resisting layers under similar geological conditions.

[0106] In a more specific technical solution, a stability evaluation system for a composite water-resisting layer in mining under a loose aquifer includes:

[0107] A force analysis model establishment module, which is used to construct a geological concept model for mining under a loose aquifer according to the occurrence of coal seams and the formation superposition of overlying rock structure characteristics, regard the caving zone in the geological concept model for mining under a loose aquifer as an elastic foundation body, obtain the geometric relationship between the trapezoidal overlying rock and the elastic foundation body, and establish a force analysis model according to the geometric relationship;

[0108] A trapezoidal overlying rock data module, which is used to obtain the length of the area affected by the caved gangue according to the geometric relationship, and obtain the relationship data of the uniform load R of the trapezoidal overlying rock and the relationship data of the self-weight G of the trapezoidal overlying rock according to the force system balance condition of the force analysis model. The trapezoidal overlying rock data module is connected to the force analysis model establishment module;

[0109] A deflection function coefficient module for the overlying rock water-resisting layer, which regards the overlying rock water-resisting layer as a rectangular thin plate with four sides fixed, sets the deflection function w of the overlying rock water-resisting layer according to the rectangular thin plate with four sides fixed, obtains the relationship data of the deformation potential energy U of the overlying rock water-resisting layer, and obtains the relationship data between the deformation potential energy of the overlying rock water-resisting layer and the deflection function coefficient. The deflection function coefficient module for the overlying rock water-resisting layer is connected to the trapezoidal overlying rock data module;

[0110] A stress component processing module, which is used to obtain the work relationship data of the upper aquifer clay layer, the work relationship data of the body force of the overlying rock water-resisting layer, and the reverse work relationship data of the uniform load R of the trapezoidal overlying rock according to the relationship data of the deflection function of the deformation potential energy of the overlying rock water-resisting layer, so as to obtain the relationship data of the total potential energy of the overlying rock water-resisting layer, process to obtain the deflection function coefficient of the overlying rock water-resisting layer, and process the relationship of the stress components on the upper surface of the overlying rock water-resisting layer according to the stress relationship data of the overlying rock water-resisting layer and the deflection function w. The stress component processing module is connected to the deflection function coefficient module for the overlying rock water-resisting layer;

[0111] The component distribution law processing module is used to assign and process the stress component relationship on the upper surface of the overlying rock aquifuge to obtain a stress component isogram, so as to obtain the stress component distribution law. The component distribution law processing module is connected to the overlying rock aquifuge deflection function coefficient module and the stress component processing module;

[0112] The critical water pressure processing module for the failure of the overlying rock aquifuge is used to process the stress data of the overlying rock aquifuge in the stress component isogram according to the preset elastic mechanics logic, to determine and obtain the easily tensile failure position according to the stress position relationship data by the Griffith yield criterion, so as to process and obtain the critical water pressure for the tensile failure of the overlying rock aquifuge, and to determine and obtain the easily shear failure position according to the stress position relationship data by the Mohr-Coulomb yield criterion, so as to process and obtain the critical water pressure for the shear yield failure of the overlying rock aquifuge. The critical water pressure processing module for the failure of the overlying rock aquifuge is connected to the component distribution law processing module;

[0113] The critical water pressure processing module for the clay layer is used to take the clay layer as the rectangular thin plate with four sides fixed, and to jointly form a second combined trapezoid body with a partial rectangular body of the overlying rock aquifuge and the trapezoidal overlying rock, and to execute the foregoing steps S3 to S5 for the clay layer to obtain the stress data of the clay layer, and to determine and obtain the critical water pressure for the shear instability failure of the clay layer according to the Mohr-Coulomb yield criterion;

[0114] The combined processing module for the composite structure aquifuge is used to combine the critical water pressure for the shear yield failure of the overlying rock aquifuge and the critical water pressure for the shear instability failure of the clay layer to obtain the critical water pressure for the fracture and instability of the composite structure aquifuge. The combined processing module for the composite structure aquifuge is connected to the critical water pressure processing module for the failure of the overlying rock aquifuge and the critical water pressure processing module for the clay layer.

[0115] The present invention has the following advantages compared with the prior art: The present invention predicts the water inrush and sand bursting in the working face under specific geological conditions. Starting from the mechanical perspective, this prediction method constructs a geological concept model and a mechanical analysis model of the composite water-resisting layer for the inclined coal seam mining in the confined aquifer. On the basis of reasonable model simplification, the deflection function and the uniform load of the overlying rock water-resisting layer are obtained by using the Winkler elastic foundation principle and the force system balance condition, and the coordinate of the failure position is obtained by using the yield failure principle. Furthermore, the critical water pressure value of the "overlying rock - clay" composite water-resisting layer breaking and losing stability is obtained. The present invention is applicable to the application scenario of the composite water-resisting layer for the inclined coal seam mining under the loose confined aquifer, ensuring the safety of underground mining operations in the corresponding application scenario. The present invention can scientifically evaluate the safe mining of the inclined coal seam under the loose confined aquifer. The present invention uses the Winkler elastic foundation principle and the force system balance condition to obtain the uniform load R acting on region III by region II and the uniform load R' acting on region II by region I. On the basis of regarding the overlying rock water-resisting layer and the clay water-resisting layer as a rectangular thin plate with four sides fixed, combined with the rectangular thin plate theory of elastic mechanics and using the principle of minimum potential energy, the expression of the deflection function coefficient is obtained. It avoids the safety challenges brought by the conventional method of substituting parameters such as the hardness of the roof overlying rock and the mining height into the "Three-Under" regulations in the calculation process of the vertical height of the waterproof coal (rock) pillar in the past, and improves the safety and stability of the composite water-resisting layer mining. On the basis of obtaining the most easily damaged (tensile, shear) position, by using the expressions of the stress components and the principal stress of the elastic thin plate and combining the Griffith and Mohr-Coulomb yield criteria, the critical water pressure value p1 corresponding to the yield failure of the clay water-resisting layer and the critical water pressure value p2 corresponding to the failure of the overlying rock water-resisting layer are obtained. Finally, the critical water pressure value p of the composite water-resisting layer when it produces yield failure is obtained. The breaking and losing stability mechanism and the easily broken position of the composite water-resisting layer are analyzed by the critical water pressure value p. Applying the mechanical analysis method to the engineering practice of the composite water-resisting layer for the inclined coal seam mining under the loose aquifer can provide a reference basis for the evaluation of the stability of the water-resisting layer under similar geological conditions. It solves the technical problem in the prior art that the mining safety of coal resources under the loose aquifer is relatively low due to the lack of an analysis method for the breaking and losing stability mechanism of the composite water-resisting layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0116] Figure 1 It is a schematic flow chart of the stability evaluation method for the composite water-resisting layer mined under the loose aquifer;

[0117] Figure 2 It is a schematic diagram of the geological concept model;

[0118] Figure 3 It is a schematic diagram of the geometric relationship of the caving zone;

[0119] Figure 4 It is a schematic diagram of the force on region III;

[0120] Figure 5 Schematic diagram of the mechanical model of thin plates in Region II;

[0121] Figure 6 Schematic diagram of the sectional load distribution in Region II;

[0122] Figure 7 Contour map of σx / MPa on the upper plate surface of the overlying rock aquifuge;

[0123] Figure 8 Contour map of σy / MPa on the upper plate surface of the overlying rock aquifuge;

[0124] Figure 9 Contour map of τxy / MPa on the upper plate surface of the overlying rock aquifuge;

[0125] Figure 10 Contour map of σ1 + 3σ3 (MPa) on the upper plate surface of the overlying rock aquifuge;

[0126] Figure 11 Contour map of the function f(x, y);

[0127] Figure 12 Schematic diagram of the force on the new combined trapezoid;

[0128] Figure 13 Schematic diagram of the load distribution of the clay layer. Detailed implementation manners

[0129] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0130] Embodiment 1

[0131] As Figure 1 shown, a method for evaluating the stability of a composite aquifuge under mining in a loose aquifer includes the following steps:

[0132] S1. Construct a geological conceptual model and a mechanical analysis model of a composite aquifuge for mining inclined coal seams in a confined aquifer;

[0133] S2. Use the Winkler elastic foundation principle and the force system balance condition to obtain the uniform load R acting on Region III in Region II;

[0134] S3. Use the rectangular thin plate theory and the principle of minimum potential energy in elasticity mechanics to obtain the expression of the deflection function coefficient;

[0135] S4. Determine the positions where the overlying water-resisting layer of the overburden is most likely to undergo tensile failure and shear failure according to different yield failure criteria;

[0136] S5. Substitute the coordinates of the most vulnerable failure position points into the corresponding yield failure criteria to find the ultimate water pressure values when the overlying water-resisting layer and the clay water-resisting layer of the overburden yield and fail, and then obtain the critical water pressure value when the composite water-resisting layer yields and fails.

[0137] The present invention makes corresponding predictions for water inrush and sand bursting in the working face under this geological condition. Starting from the mechanical perspective, this prediction method constructs a geological conceptual model and a mechanical analysis model of the composite water-resisting layer for the mining of inclined coal seams in a confined aquifer. On the basis of reasonable model simplification, the deflection function and uniform load of the overlying water-resisting layer are obtained by using the Winkler elastic foundation principle and the force system balance condition, and the failure position coordinates are obtained by using the yield failure principle. Furthermore, the critical water pressure value for the breakage and instability of the "overlying rock - clay" composite water-resisting layer is obtained. The present invention has strong practicability in the process of mining the composite water-resisting layer of inclined coal seams under a loose confined aquifer, ensuring the safety of the corresponding underground mining operations. It can scientifically evaluate the safe mining of inclined coal seams under a loose confined aquifer.

[0138] Embodiment 2

[0139] Geological conceptual model of water-resisting layer stability:

[0140] As Figure 2 shown, according to the coal seam occurrence and overlying rock structure characteristics of a certain coal mine mining area, considering the general situation where the overlying water-resisting layer for mining under a loose aquifer is composed of a bedrock and the overlying cohesive soil layer, a geological conceptual model for mining under a loose aquifer in a certain coal mine mining area is constructed.

[0141] In the geological conceptual model, research area Ⅰ and research area Ⅱ together form a composite water-resisting layer structure, where:

[0142] Research area Ⅱ is the bedrock layer, which is mostly hard rock and plays a role of skeleton support in the composite overlying water-resisting layer structure; research area Ⅰ is the clay layer, which has a small bearing capacity. After the bedrock section in area Ⅱ breaks to form fractures, it can play a role in preventing the fractures from continuing to expand and then sealing the water-conducting fractures.

[0143] As Figure 3As shown in the figure, if the composite structure water-resisting layer composed of Region I and Region II undergoes yield instability, according to the geological conceptual model, the water in the aquifer will enter the caving zone from the boundary, thereby causing water inrush at the working face. Therefore, we regard Region III as the overlying rock in the transition zone, which mainly plays the role of transferring loads in the mechanical analysis. Existing research shows that the gangue in the caving zone also has four stages of elastic deformation, yield, plastic deformation, and plastic failure under triaxial compression, and its stress state shows non-uniform distribution along the width direction of the filling body. Therefore, the caving zone can be regarded as an elastic foundation body, and its supporting capacity for Region III is reflected by the elastic foundation coefficient k. Based on this, on the basis of the geometric relationship between the caving zone and Region III.

[0144] As Figure 4 shown, according to the geometric relationship, the length of the area affected by the gangue in the caving zone can be obtained where H is the vertical height of the gangue in the caving zone, α is the coal seam dip angle, and θ is the roof caving angle.

[0145] Considering the self-weight load of the trapezoidal overlying rock III, according to the force system balance condition:

[0146] (G + RL y cosα)cosα = kwL y (1-1)

[0147] where G is the self-weight of the trapezoidal overlying rock mass III, and k is the elastic foundation coefficient of the caving gangue.

[0148] Then the expression for the uniformly distributed load R is:

[0149]

[0150] From the volume formula of the trapezoidal body III, etc., the expression for the self-weight G of the trapezoidal overlying rock mass III is:

[0151]

[0152] where γ3 is the average unit weight of the rock mass in Region III of the trapezoidal overlying rock.

[0153] Mechanical analysis of the stability of the water-resisting layer:

[0154] As Figure 5 and Figure 6 shown, taking Region II as the research area and regarding it as a rectangular thin plate with four sides fixed (meeting the ratio of the thickness of the thin plate to the minimum dimension of 1 / 80 - 1 / 5), its mechanical model and load distribution schematic diagram.

[0155] For a rectangular thin plate with four sides fixed, its deflection function can be set as:

[0156]

[0157] Then the inclined length L of the working face b = b / cosα; the advancing distance L of the working face along the strike direction a = a / cosα.

[0158] For a thin plate without free edges, the expression of its deformation potential energy is:

[0159]

[0160] From Eqs. (1-4) and (1-5), we can obtain:

[0161]

[0162] The uniformly distributed load acting on the overburden water-resisting layer II is the superposition of the water pressure P0 of the aquifer and the self-weight load H1 of the upper clay layer, and the work done by it on the overburden water-resisting layer II (positive work along the z-axis) is:

[0163] Ω1 = ∫∫[P0 + G1]wdxdy (1-7)

[0164] From Eqs. (1-4) and (1-7), we can obtain:

[0165]

[0166] Where γ1 is the average unit weight of the clay layer; H1 is the thickness of the clay layer.

[0167] Ω2 = ∫∫∫G2wdxdydz (1-9)

[0168] Considering the self-weight of the overburden water-resisting layer, the work done by its body force G2 is:

[0169] From Eqs. (1-4) and (1-9), we can obtain:

[0170]

[0171] Where γ2 is the average unit weight of the overburden water-resisting layer; H2 is its thickness.

[0172] The work done by the uniformly distributed load R on it (negative work against the z-axis) is:

[0173] Ω3 = -∫∫Rwdxdy (1-11)

[0174] From Eqs. (1-4) and (1-11), we can obtain:

[0175]

[0176] Where G is the self-weight of the trapezoidal overlying rock mass, and its expression is as shown in Eq. 1-3; L y is the length of the area affected by the gangue in the caving zone;

[0177] The total potential energy Π of the overlying aquifuge is as follows:

[0178] Π = u - Ω1 - Ω2 - Ω3 (1-13)

[0179] According to the principle of minimum potential energy, the variation of the total potential energy of the thin plate is taken:

[0180]

[0181] Then, by combining equations (1-6), (1-8), (1-10), (1-12), (1-13), and (1-14), the coefficient A of the deflection function can be obtained as:

[0182]

[0183] It can be seen from the stress component expressions that:

[0184]

[0185] The positive and negative values of the tensile stress and compressive stress defined here are opposite to the regulations in elasticity mechanics, that is, the tensile stress is negative and the compressive stress is positive.

[0186] By combining equations (1-4) and (1-16), we can obtain:

[0187]

[0188] In the formula, A is the coefficient of the deflection function (as shown in formula 1-15); μ is the Poisson's ratio of the overlying aquifuge;

[0189] Based on the mining conditions and geological parameters of the 312 working face of a certain coal mine, assignment analysis is carried out. The width a of the thin plate is 237 m, that is, the length of the working face along the coal seam strike is about 241 m, and the length b of the thin plate is 600 m, that is, the advancing length of the working face along the inclined direction is about 628 m; the thickness h1 of the clay layer is 4 m, and its unit weight γ1 is 16 kN / m 3 , the thickness h2 of the overlying strata is 50 m, and its average unit weight γ2 is 24 kN / m 3 , the elastic modulus E is 30 GPa, the Poisson's ratio μ is 0.24, the cohesion C is 15 MPa, the coal seam dip angle α is 12°, the roof caving angle θ is 75°, the corresponding elastic foundation coefficient k is 15 MPa / m, the water pressure P0 of the loose aquifer is 1 MPa, and the vertical height H of the caving zone is 14 m. Substituting the above parameter values into the stress component expression (formula 1-17), and drawing the contour maps of each stress component through the mathematical software Mathematica, the distribution law of each stress component on the upper surface (z = -h / 2) of the overlying aquifuge can be obtained.

[0190] As Figures 7 to 9 shown, tensile stress appears in the boundary region of the overlying rock aquifuge, while compressive stress appears in the middle region of the plate. For σ x , the maximum negative value (the maximum tensile stress region) appears at two positions of (0m, 300m) and (240m, 300m), σ x = -10.8MPa, and the maximum positive value (the maximum compressive stress region) appears at the center position of the plate (120m, 300m), σ x = 11.2MPa; For σ y , the maximum negative value (the maximum tensile stress region) appears at two positions of (0m, 300m) and (240m, 300m), σ y = -2.6MPa, and the maximum positive value (the maximum compressive stress region) appears at the center position of the plate (120m, 300m), σ y = 4.3MPa; For τ xy , the maximum value positions of its positive and negative shear stresses are (60m, 450m), (180m, 150m) and (60m, 150m), (180m, 450m) respectively.

[0191] It can be seen from the above analysis that the overlying rock aquifuge is a tensile stress concentration region at the boundary, and its maximum value is located at the midpoint of the long side; it is a compressive stress concentration region at the middle position of the plate, and the maximum value is located at the middle position of the overlying rock aquifuge. From the property that rock resists compression but not tension, and the fact that the numerical values of tensile stress and compressive stress are similar, it can be known that the overlying rock aquifuge is prone to tensile failure first at the midpoint position of the long side, that is, the midpoint of the long side of the overlying rock aquifuge is a position extremely prone to tensile failure. In elasticity mechanics, the solution formula of the principal stress is shown in Equation (1-18):

[0192]

[0193] For tensile failure, the Griffith yield criterion can be used for judgment, that is:

[0194]

[0195] As Figure 10 shown, in the above value-taking situation, the contour map of σ1 + 3σ3 can be obtained, and σ1 + 3σ3 obtains the maximum negative value -34×10 6Pa indicates that tensile failure occurs at the midpoint of the long side of the overlying aquifuge layer, i.e., at positions (0, b / 2) and (a, b / 2). Therefore, the tensile failure criterion can be used for analysis, that is, the ultimate tensile stress value -σ3 at this position is compared with the unidirectional tensile strength R of the overlying aquifuge layer. When -σ3 > R, tensile failure occurs at this position; when -σ3 < R, tensile failure does not occur at this position; when -σ3 = R, this position is in a critical failure state. Accordingly, we define the water pressure value corresponding to the critical tensile failure state of the overlying aquifuge layer as the tensile critical water pressure p 2t , then by combining equations (1-15), (1-17), (1-18), and (1-19), the analytical solution of p 2t can be obtained.

[0196] Based on the Griffith yield criterion for tensile failure above, we have obtained the critical water pressure for tensile failure of the overlying aquifuge layer. If the overlying rock layer undergoes shear yield failure under mining-induced stress and water pressure, it can be judged according to the Mohr-Coulomb yield criterion, that is, when the maximum principal stress σ1 and the minimum principal stress σ3 satisfy the relationship of equation (1-20), the overlying aquifuge layer undergoes critical shear failure.

[0197]

[0198] In the formula, c is the cohesion of the overlying aquifuge layer; is the internal friction angle.

[0199] Define the function f(x, y), and its value is:

[0200]

[0201] Substitute the coordinates of points x and y into the function f(x, y). When f(x, y) = 1, it is the position of the critical failure point. Accordingly, under the same parameters, the contour map of the function f(x, y) can be drawn using the Mathematica software, as Figures 1 - 7 shown, and then the positions prone to shear failure can be found.

[0202] As Figure 11 shown, the maximum value points of the function f(x, y) are also at the midpoints of the long sides of the thin plate, that is, at (0, b / 2) and (a, b / 2), indicating that under multi-directional stress, the positions where the overlying aquifuge layer is most prone to shear yield failure are the midpoints of the long sides of the overlying aquifuge layer corresponding to the inclined advancing direction of the working face. We define the water pressure value corresponding to the critical shear yield failure of the overlying aquifuge layer as the critical water pressure p 2s , then by combining equations (15), (17), (18), and (20), the analytical solution of p 2smax can be obtained.

[0203] Therefore, when tensile or shear failure occurs in the overlying aquifuge, the corresponding critical water pressure p2 can be expressed by Equation (1-22):

[0204] p2 = min(p 2t , p 2s ) (1-22)

[0205] As Figure 12 and Figure 13 shown, the area Ⅰ where the clay layer is located can be regarded as a rectangular thin plate with four sides fixed. Then, a new combined frustum is formed by part of the rectangular bodies in area 2 and area 3, which plays a supporting role for area Ⅰ. The mechanical model and the schematic diagram of the load distribution are as follows.

[0206] The self-weight G L of the combined trapezoidal overlying rock mass is expressed as:

[0207]

[0208] According to the force system balance condition, the expression of the uniformly distributed load R1 acting on the combined frustum is:

[0209]

[0210] The deflection function of the rectangular thin plate with four sides fixed is as shown in Equation 1-4. The work done by the water pressure P0 in the loose aquifer above the clay layer on it is:

[0211] Ω1 = ∫∫P0wdxdy (1-25). Considering the self-weight of the clay layer, the work done by its body force G1 is:

[0212] Ω2 = ∫∫∫G1wdxdydz (1-26)

[0213] In the formula, the self-weight of the clay layer G1 = γ1, which is the unit weight of the clay layer.

[0214] The work done by the uniformly distributed load R1 on the clay layer (negative work against the z-axis) is:

[0215] Ω3 = -∫∫R1wdxdy (1-27)

[0216] According to the principle of minimum potential energy, the expression of the coefficient A1 corresponding to the deflection function can be obtained:

[0217]

[0218] In the formula, D is the flexural rigidity of the thin plate of the clay aquifuge, and the corresponding expression is:

[0219]

[0220] In the formula, E1, H1, and μ1 are respectively: the elastic modulus, average thickness, and Poisson's ratio of the clay aquiclude.

[0221] Based on the plastic failure characteristics of the clay layer, the Mohr-Coulomb yield criterion can be used to determine the shear yield failure of the clay layer. The most vulnerable failure position is the same as that of the upper plate surface of the thin plate of the overlying rock aquiclude, which is the midpoint position of the long side of the thin plate.

[0222] Similarly, we can define the function:

[0223]

[0224] In the formula, c1 and are respectively the cohesion and internal friction angle of the clay layer; σ1 and σ3 are respectively its maximum and minimum principal stresses, and their calculation formulas are shown in Equation (1-18). Accordingly, the critical shear failure of the clay layer can be determined according to Equation (1-30), and then the critical water pressure value p1 of the shear instability failure of the clay layer can be obtained by back-solving.

[0225] Based on the above analysis, for the "clay-overlying rock" composite structure aquiclude, the critical water pressure p for its fracture and instability is:

[0226] p = max[p1, min(p 2t , p 2s )] (1-31)

[0227] In summary, the present invention conducts corresponding prediction on water inrush and sand bursting in the working face under specific geological conditions. Starting from the mechanical perspective, this prediction method constructs a geological concept model and a mechanical analysis model of a composite water-resisting layer for inclined coal seam mining in a confined aquifer. On the basis of reasonable model simplification, the Winkler elastic foundation principle and the force system balance condition are used to obtain the deflection function and uniform load of the overlying rock water-resisting layer, and the failure position coordinates are obtained using the yield failure principle. Furthermore, the critical water pressure value for the breakage and instability of the "overlying rock - clay" composite water-resisting layer is obtained through processing. The present invention is applicable to the application scenario of the composite water-resisting layer for inclined coal seam mining under a loose confined aquifer, ensuring the safety of underground mining operations in the corresponding application scenario. The present invention can scientifically evaluate the safe mining of inclined coal seams under a loose confined aquifer. The present invention uses the Winkler elastic foundation principle and the force system balance condition to obtain the uniform load R acting on region III by region II and the uniform load R' acting on region II by region I. On the basis of regarding the overlying rock water-resisting layer and the clay water-resisting layer as a rectangular thin plate with four sides fixed, combined with the rectangular thin plate theory of elastic mechanics and using the principle of minimum potential energy, the expression of the deflection function coefficient is obtained. It avoids the safety challenges brought by the conventional method of substituting parameters such as the hardness of the roof overlying rock and the mining height into the "Three-Under" regulations in the calculation process of the vertical height of the waterproof coal (rock) pillar in the past, and improves the safety and stability of the composite water-resisting layer mining. On the basis of obtaining the most easily damaged (tensile, shear) position, the present invention uses the expressions of the stress components and principal stresses of the elastic thin plate, combined with the Griffith and Mohr-Coulomb yield criteria, to obtain the critical water pressure value p1 corresponding to the yield failure of the clay water-resisting layer and the critical water pressure value p2 corresponding to the failure of the overlying rock water-resisting layer. Finally, the critical water pressure value p for the yield failure of the composite water-resisting layer is obtained. The breakage and instability mechanism and the easily broken position of the composite water-resisting layer are analyzed using the critical water pressure value p. Applying the mechanical analysis method to the engineering practice of the composite water-resisting layer for inclined coal seam mining under a loose aquifer can provide a reference basis for the evaluation of the stability of the water-resisting layer under similar geological conditions. It solves the technical problem in the existing technology that the mining safety of coal resources under a loose aquifer is relatively low due to the lack of an analysis method for the breakage and instability mechanism of the composite water-resisting layer.

[0228] The above embodiments are only used to illustrate the technical solutions of the present invention, not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for evaluating the stability of a composite water-resisting layer during mining under a loose aquifer, characterized in that, Including: S1. Construct a geological conceptual model for mining under a loose aquifer according to the occurrence of coal seams and the formation superposition composition characteristics of overlying strata structures. Take the caving zone in the geological conceptual model for mining under a loose aquifer as an elastic foundation body to obtain elastic support data of the elastic foundation body for trapezoidal overlying strata. Establish a force analysis model according to the elastic support data and the force system balance condition; S2. Obtain the length of the area affected by caved gangue according to geometric relationships. Obtain the relationship data of the uniformly distributed load R of the trapezoidal overlying strata and the relationship data of the self-weight G of the trapezoidal overlying strata according to the force system balance condition of the force analysis model; S3. Take the overlying aquifuge as a rectangular thin plate with four sides fixed. Set the deflection function w of the overlying aquifuge according to the rectangular thin plate with four sides fixed to obtain the relationship data of the deformation potential energy U of the overlying aquifuge, so as to obtain the relationship data between the deformation potential energy of the overlying aquifuge and the deflection function coefficient; S4. Obtain the work relationship data of the upper aquifer clay layer, the work relationship data of the body force of the overlying aquifuge, and the reverse work relationship data of the uniformly distributed load R of the trapezoidal overlying strata according to the relationship data of the deflection function of the deformation potential energy of the overlying aquifuge, so as to obtain the relationship data of the total potential energy of the overlying aquifuge. Process to obtain the deflection function coefficient of the overlying aquifuge. Process according to the stress relationship data of the overlying aquifuge and the deflection function w to obtain the relationship of the stress components on the upper surface of the overlying aquifuge; S5. Perform assignment processing on the relationship law of the stress components on the upper surface of the overlying aquifuge to obtain an isogram of stress components, so as to obtain the distribution law of stress components; Among them, step S5 includes: S51. Collect and assign the mining conditions and geological parameters of the coal mine into the relationship of the stress components on the upper surface of the overlying aquifuge to obtain stress component relationship data; S52. Use a preset drawing tool to draw an isogram according to the stress component relationship data to obtain the distribution law of each stress component on the upper surface of the overlying aquifuge; S6. Process the stress data of the overlying aquifuge in the isogram of stress components with a preset elasticity mechanics logic to obtain stress position relationship data. Use the Griffith yield criterion to discriminate and obtain the easily tensile failure position according to the stress position relationship data, and process to obtain the critical water pressure for tensile failure of the overlying aquifuge. Use the Mohr-Coulomb yield criterion to discriminate and obtain the easily shear failure position according to the stress position relationship data, and process to obtain the critical water pressure for shear yield failure of the overlying aquifuge; S7. Take the clay layer as the rectangular thin plate with four sides fixed. Take the overlying aquifuge and a partial rectangular body of the trapezoidal overlying strata to jointly form a second combined trapezoid body. Perform the aforementioned steps S3 to S5 on the clay layer to obtain the stress data of the clay layer. Use the Mohr-Coulomb yield criterion to discriminate and obtain the critical water pressure for shear instability failure of the clay layer; S8. Combine and process the critical water pressure for shear yield failure of the overlying aquifuge and the critical water pressure for shear instability failure of the clay layer to obtain the critical water pressure for breakage and instability of the composite structure aquifuge.

2. The stability evaluation method of a composite water-resisting layer for mining under a loose aquifer according to claim 1, characterized in that The supporting ability of the caving zone to the trapezoidal overlying strata in step S1 is reflected by the elastic foundation coefficient k.

3. A method for evaluating the stability of a composite water-resisting layer during mining under a loose aquifer according to claim 1, characterized in that, Step S2 includes: S21. According to the geometric relationships, obtain the length of the area affected by the caving zone through the following logic: ， Where H is the vertical height of the caved gangue, α is the dip angle of the coal seam, θ is the roof caving angle, and b is the length of the rectangular thin plate with four sides fixed; S22. It can be known from the force system balance condition in the force analysis model: (G + RL y (cosα)cosα = kwL y (1 - 1), In the formula, G is the self-weight of the trapezoidal overlying strata, k is the elastic foundation coefficient of the caved gangue, and w is the deflection function; S23. According to the force system balance condition, the following logic is used to obtain the data of the relationship between the uniformly distributed load R of the trapezoidal overlying strata: S24. According to the force system balance condition, the following logic is used to obtain the data of the relationship between the self-weight G of the trapezoidal overlying strata: ， In the formula, γ3 is the average unit weight of the rock mass of the trapezoidal overlying strata.

4. A method for evaluating the stability of a composite water-resisting layer during mining under a loose aquifer according to claim 1, characterized in that The step S3 includes: S31. Regard the overlying strata water isolation layer as the rectangular thin plate with four sides fixed; S32. Set the deflection function of the rectangular thin plate with four sides fixed according to the following logic: ， To obtain the inclined length of the working face of the overlying water-resisting layer as L b = b / cosα; the advancing distance of the working face along the strike direction is L a = a / cosα, where a and b are the width and length of the rectangular thin plate with four sides fixed, and A is the deflection function coefficient of the overlying water-resisting layer; S33. For the thin plate without free edges, obtain the data of the relationship between the deformation potential energy U of the overlying strata water isolation layer according to the following logic: In the formula, D is the flexural rigidity of the clay water isolation layer thin plate; S34. Process the deflection function w and the data of the relationship between the deformation potential energy U of the overlying strata water isolation layer to obtain the data of the relationship between the deformation potential energy of the overlying strata water isolation layer and the deflection function coefficient:

5. The stability evaluation method for a composite water-resisting layer during mining under a loose aquifer according to claim 1, wherein The step S4 includes: S41. Take the uniformly distributed load acting on the overlying strata water isolation layer as the superposition of the aquifer water pressure P0 and the self-weight load G1 of the upper clay layer. According to the data of the relationship between the deformation potential energy deflection function of the overlying strata water isolation layer, obtain the data of the work done by the upper aquifer clay layer according to the following logic: Ω1 = ∫∫[P0 + G1]wdxdy (1 - 7) In the formula, γ1 is the average unit weight of the clay layer; H1 is the thickness of the clay layer, and a, b are the width and length of the rectangular thin plate with four sides fixed; S42. According to the data of the relationship between the deformation potential energy deflection function of the overlying strata water isolation layer, obtain the data of the work done by the body force of the overlying strata water isolation layer according to the following logic: Ω2 = ∫∫∫G2wdxdydz (1 - 9) In the formula, γ2 is the average unit weight of the overlying strata water isolation layer, and H2 is the thickness of the overlying strata water isolation layer; S43. According to the data of the relationship between the deformation potential energy deflection function of the overlying strata water isolation layer, obtain the data of the reverse work done by the uniformly distributed load R of the trapezoidal overlying strata according to the following logic: Ω3 = -∫∫Rwdxdy (1 - 11) ， where G is the self-weight of the trapezoidal overburden rock, and L y is the length of the area affected by the gangue in the caving zone; S44. According to the data of the work done by the upper aquifer clay layer, the data of the work done by the body force of the overlying strata water isolation layer, and the data of the reverse work done by the uniformly distributed load R of the trapezoidal overlying strata, obtain the data of the total potential energy relationship of the overlying strata water isolation layer according to the following logic: Π = u - Ω1 - Ω2 - Ω3 (1 - 13) S45. According to the principle of minimum potential energy, take the variation of the total potential energy Π of the thin plate to obtain the data of the variation of the total potential energy Π of the thin plate: Combining the data of the relationship between the deformation potential energy deflection function of the overlying strata water isolation layer, the data of the work done by the upper aquifer clay layer, the data of the work done by the body force of the overlying strata water isolation layer, the data of the reverse work done by the uniformly distributed load R of the trapezoidal overlying strata, the data of the total potential energy relationship of the overlying strata water isolation layer, and the data of the variation of the total potential energy Π of the thin plate, the deflection function coefficient A of the overlying strata water isolation layer can be obtained as: S46. Process the stress component relationship of the upper surface of the overlying water - resisting layer according to the stress relationship data of the overlying water - resisting layer and the deflection function w with the following logic: Wherein, A is the deflection function coefficient, μ is the Poisson's ratio of the overlying rock aquifuge, where σ x , σ y represent the vertical stress component and the tangential stress component respectively, and τ xy is the shear stress.

6. The stability evaluation method for a composite water-resisting layer during mining under a loose aquifer as described in claim 1, characterized in that The step S6 includes: S61. Process the stress data of the overlying water - resisting layer in the stress component isogram with the preset elasticity mechanics logic to obtain the stress position relationship data: S62. Judge the tensile failure according to the following Griffith yield criterion based on the stress position relationship data: To obtain the isogram of σ1 + 3σ3, where σ1 is the maximum principal stress and σ3 is the minimum principal stress; S63. The easily stretchable failure position is obtained by processing the contour map of σ1 + 3σ3, and the tensile failure state data is obtained through analysis using the tensile failure judgment criterion, so as to obtain the critical water pressure p of the overlying rock water - resistant layer after processing 2t ; S64. Judge the critical shear failure of the overlying water - resisting layer according to the following Mohr - Coulomb yield criterion: ， Where c is the cohesion of the overburden water-resisting layer; is the angle of internal friction; S65. Preset the function f(x, y): S66. Draw the contour map of the function f(x, y) using the preset drawing tool, and find the easily shear failure position according to the contour map of the function f(x, y) to obtain the critical water pressure p of the shear yield failure of the overlying aquifuge 2s ; S67. Process the critical water pressure p2 of the overlying water - resisting layer according to the critical water pressure of the tensile failure of the overlying water - resisting layer and the critical water pressure of the shear yield failure of the overlying water - resisting layer with the following logic: p2 = min(p 2t , p 2s ) (1-22).

7. The stability evaluation method for a composite water-resisting layer during mining under a loose aquifer according to claim 1, wherein The step S7 includes: S71. Regard the clay layer as a rectangular thin plate with four - side fixed supports, and jointly form a new combined trapezoid body with a part of the rectangular body of the overlying water - resisting layer and the trapezoidal overlying rock to support the clay layer, so as to construct a stress analysis model of the clay layer; S72. Obtain the expression of the self-weight \(G\) of the new combined trapezoidal overlying rock mass according to the stress analysis model of the clay layer L as follows: S73. Obtain the expression of the uniform load R1 of the clay layer acting on the new combined trapezoid body according to the force system balance condition in the stress analysis model of the clay layer: S74. Process the work done by the water pressure P0 of the loose aquifer acting on the clay layer according to the deflection function of the rectangular thin plate with four - side fixed supports: Ω1=∫∫P0wdxdy (1 - 25) Process the work done by the body force G1 of the clay layer with the following logic: Ω2=∫∫∫G1wdxdydz (1 - 26), In the formula, the self - weight G1 of the clay layer = γ1, which is the unit weight of the clay layer; Process the work done by the uniform load R1 of the new combined trapezoidal overlying rock mass on the clay layer with the following logic: Ω3=-∫∫R1wdxdy (1 - 27) S75. Obtain the coefficient A1 of the deflection function of the clay layer according to the principle of minimum potential energy: ， In the formula, D is the flexural rigidity of the clay - water - resisting layer thin plate, and the corresponding expression is: ， In the formula, E1, H1, and μ1 are respectively: the elastic modulus, average thickness, and Poisson's ratio of the clay - water - resisting layer; S76. Adopt the Mohr - Coulomb yield criterion for the clay layer, and judge the position where the clay layer is most likely to occur shear yield failure with the following logic: where, c1 and are the cohesion and internal friction angle of the clay layer respectively, and σ1 and σ3 are its maximum and minimum principal stresses respectively; S78. Obtain the critical water pressure p1 of the shear instability failure of the clay layer according to the formula.

8. A method for evaluating the stability of a composite water-resisting layer during mining under a loose aquifer, as described in claim 1, wherein In the step S8, combine the critical water pressure of the shear yield failure of the overlying water - resisting layer and the critical water pressure of the shear instability failure of the clay layer with the following logic to obtain the critical water pressure p of the break - down and instability of the composite structure water - resisting layer: p = max[p1, min(p 2t , p 2s )] (1-31).

9. A stability evaluation system for a composite water-resisting layer during mining under a loose aquifer, which is used to execute the stability evaluation method for a composite water-resisting layer during mining under a loose aquifer according to any one of the preceding claims 1 to 8, and is characterized in that, Include: A stress analysis model establishment module, used to construct a geological concept model for mining under a loose aquifer according to the occurrence of coal seams and the stratigraphic superposition composition characteristics of the overlying rock structure, regard the caving zone in the geological concept model for mining under a loose aquifer as an elastic foundation body to obtain the elastic support data of the elastic foundation body for the trapezoidal overlying rock, and establish a stress analysis model according to the elastic support data and the force system balance condition; Trapezoidal overburden data module, which is used to obtain the length of the area affected by the caved gangue according to the geometric relationship, and obtain the relationship data of the uniform load R of the trapezoidal overburden and the relationship data of the self-weight G of the trapezoidal overburden according to the force system balance condition of the stress analysis model. The trapezoidal overburden data module is connected to the stress analysis model establishment module; Overburden aquifuge deflection function coefficient module, which is used to regard the overburden aquifuge as a rectangular thin plate with four sides fixed, set the deflection function w of the overburden aquifuge according to the rectangular thin plate with four sides fixed, and obtain the relationship data of the deformation potential energy U of the overburden aquifuge, so as to obtain the relationship data between the deformation potential energy of the overburden aquifuge and the deflection function coefficient. The overburden aquifuge deflection function coefficient module is connected to the trapezoidal overburden data module; Stress component processing module, which is used to obtain the work relationship data of the upper aquifer clay layer, the work relationship data of the overburden aquifuge body force, and the reverse work relationship data of the trapezoidal overburden uniform load R according to the relationship data between the deformation potential energy deflection function of the overburden aquifuge, so as to obtain the relationship data of the total potential energy of the overburden aquifuge, and process to obtain the deflection function coefficient of the overburden aquifuge. According to the stress relationship data of the overburden aquifuge and the deflection function w, the stress component relationship on the upper surface of the overburden aquifuge is processed. The stress component processing module is connected to the overburden aquifuge deflection function coefficient module; Component distribution law processing module, which is used to assign values to the stress component relationship law on the upper surface of the overburden aquifuge to obtain a stress component isogram, so as to obtain the stress component distribution law. The component distribution law processing module is connected to the overburden aquifuge deflection function coefficient module and the stress component processing module; Overburden aquifuge failure critical water pressure processing module, which is used to process the overburden aquifuge stress data in the stress component isogram with a preset elastic mechanics logic to obtain stress position relationship data, and use the Griffith yield criterion to determine and obtain the easily tensile failure position according to the stress position relationship data, so as to process and obtain the critical water pressure for tensile failure of the overburden aquifuge. Use the Mohr-Coulomb yield criterion to determine and obtain the easily shear failure position according to the stress position relationship data, so as to process and obtain the critical water pressure for shear yield failure of the overburden aquifuge. The overburden aquifuge failure critical water pressure processing module is connected to the component distribution law processing module; Clay layer critical water pressure processing module, which is used to regard the clay layer as the rectangular thin plate with four sides fixed, and use the part of the rectangular body of the overburden aquifuge and the trapezoidal overburden to jointly form a second combined trapezoid body, and perform the foregoing steps S3 to S5 for the clay layer to obtain the clay layer stress data, and determine and obtain the critical water pressure for shear instability failure of the clay layer according to the Mohr-Coulomb yield criterion; Composite structure aquifuge combination processing module, which is used to combine the critical water pressure for shear yield failure of the overburden aquifuge and the critical water pressure for shear instability failure of the clay layer to obtain the critical water pressure for breakage and instability of the composite structure aquifuge. The composite structure aquifuge combination processing module is connected to the overburden aquifuge failure critical water pressure processing module and the clay layer critical water pressure processing module.

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