Determination method for descending fracture bridging amount of shallow coal seam mining overburden strata under thick soil layer

Abstract

The invention relates to the technical field of coal seam mining, in particular to a method for determining the descending fracture bridging amount of mining overburden strata of a shallow coal seam under a thick soil layer, which comprises the following steps of: firstly, determining that a research object is the descending fracture bridging amount of the mining overburden strata of the shallow coal seam under the thick soil layer, and the research problem is that the descending fracture bridging amount of the overburden strata cannot be quantified; and finally, determining a research method, wherein the research method comprises the following steps: S1, establishing a soil fracture-pore seepage mechanical model and performing theoretical analysis on soil seepage; s2, the fracture bridging effect of the mining soil body is achieved; s3, determining a soil body fracture bridging amount formula; S4, verifying the soil body fracture bridging amount; s5, drawing a conclusion; the determination method is combined with soil fracture-pore medium seepage characteristics, a theoretical model is established, a mining overlying strata descending fracture bridging amount calculation method is provided, errors caused by objective and subjective factors of a physical similar simulation experiment are avoided, the overlying strata fracture bridging process and mechanism are fully considered, and the method is suitable for large-scale popularization and application. And a theoretical basis is provided for quantitatively researching the development degree of overlying strata fractures mined by a shallow coal seam under a thick soil layer.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal seam mining, and particularly relates to a method for determining a downhole crack healing amount of overburden strata of shallow coal seam under thick soil layer. BACKGROUND

[0002] The Jurassic coalfield in northern Shaanxi of China is located at the junction of the Loess Plateau and the Mu Us Desert, mainly has shallow buried coal seams, and has the geological characteristics of thin bedrock and thick unconsolidated layer above the coal seam, wherein the unconsolidated layer includes aeolian sand layer and thick soil layer composed of viscous Lishi loess and Triodont red clay; the Salawusu Formation aquifer is widely distributed in the region, and the groundwater resource is abundant, with a water level buried depth of about 10 m, and the viscous thick soil layer below is a good aquiclude;

[0003] Under the mining condition of shallow coal seam, the coal seam is shallow, and therefore the coal seam mining produces concentrated tensile stress on the top of the aquiclude, which easily causes the downhole crack on the top of the aquiclude when the concentrated tensile stress exceeds the ultimate strength of the soil layer, thereby causing the water body in the upper aquifer to seep in the downhole crack of the mining soil layer; under the seepage action, the water body penetrates into the soil pores, the soil particles have chemical reaction under the softening action of the soil pore water, and show the characteristics of "expansion" of the soil particles, so that the downhole crack is closed, that is, the crack healing effect; after the downhole crack is healed, the depth of the downhole crack development under the mining is reduced, which is of great significance to the stability of the aquiclude in the region and the water conservation mining research;

[0004] However, the existing research method is affected by the differences of different batches of experimental materials, laboratory humidity, and the model tamping degree, model excavation step distance, and time effect of excavation, etc., causing large error of the downhole crack healing amount of the mining overburden strata in the physical simulation experiment;

[0005] Moreover, the downhole crack healing amount of the mining overburden strata in the physical similar simulation model is small, has a similar ratio, and is difficult to accurately measure, and has large measurement error;

[0006] In addition, the downhole crack healing of the mining overburden strata is a microcosmic effect and is manifested in macrocosm; the present technology lacks the analysis of the crack healing process and mechanism of the mining overburden strata, and does not establish a soil layer crack seepage healing model, and lacks the quantitative analysis of the downhole crack healing amount of the mining overburden strata. SUMMARY

[0007] The problem solved by the present application is to provide a method for determining the downhole crack healing amount of overburden strata of shallow coal seam under thick soil layer, combining the seepage characteristics of soil crack-pore medium, a theoretical model is established, and a calculation method for the downhole crack healing amount of overburden strata is proposed, which avoids the errors caused by objective and subjective factors of physical similar simulation experiment, and fully considers the crack healing process and mechanism of overburden strata, thereby providing a theoretical basis for quantitatively studying the crack development degree of overburden strata of shallow coal seam under thick soil layer.

[0008] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: a method for determining the downhole crack healing amount of overburden strata of shallow coal seam under thick soil layer, comprising the following steps:

[0009] Firstly, the research object is determined as the downhole crack healing amount of overburden strata of shallow coal seam under thick soil layer, and the research problem is that the downhole crack healing amount of overburden strata cannot be quantified, and finally the research method is determined, and the steps adopted by the research method are as follows: S1, establishing a soil crack-pore seepage mechanics model and soil seepage theory analysis; S2, crack healing effect of mining soil; S3, determining the crack healing amount formula of soil; S4, verifying the crack healing amount of soil; S5, drawing a conclusion;

[0010] S1, seepage characteristics of mining soil

[0011] S11, soil pore seepage continuity equation

[0012] The soil pore seepage is different from the soil crack seepage, because the seepage velocity and direction of each point in the soil pore seepage field are different, in order to study the pore seepage characteristics of saturated soil, a hexahedral microelement with a center point m(x, y, z) and edge lengths Δx, Δy and Δz is taken in the seepage soil, as shown in Figure 2

[0013] In Δt time, the water flow mass flowing into the unit body from the upper surface of the unit body is The water flow mass flowing out of the unit body from the lower surface of the unit body is The water flow mass difference flowing into and out of the unit body along the z-axis is Similarly, the water flow mass flowing into and out of the unit body along the x-axis and the y-axis is and Therefore, the total water mass in the unit body per unit time is:

[0014]

[0015] According to the law of conservation of mass, the total water mass in the unit body per unit time is equal to the change of the liquid mass (ρnΔxΔyΔz) in the unit body per unit time, that is:

[0016] ​

[0017] Equation (2) represents the continuity equation of soil pore seepage;

[0018] S12, soil pore seepage flow

[0019] The seepage flow on the surface in the same direction as the coordinate axis per unit time on the micro-unit is the seepage velocity in different directions, that is, q(x, y, z) = v(x, y, z), and according to Darcy's law (v = KJ), the seepage flow in different directions of the micro-unit soil pore is:

[0020]

[0021] In the formula: q is the seepage flow in different directions, K is the seepage coefficient in different directions, and J is the hydraulic gradient in different directions;

[0022] According to Darcy's law, the hydraulic gradient (J) is the ratio of the water head (H) loss along the seepage path to the corresponding seepage path (L). The water head is the mechanical energy possessed by unit weight of liquid, which includes position water head (Z), pressure water head (P) and flow velocity water head (Vp);

[0023] In the soil pore seepage field, any point m(x, y, z) is as shown in Figure 3 The position water head of point m is Z, and the pressure water head (P) is the ratio of the pore water pressure (u) at the point to the water unit weight (γw). Since the soil pore seepage velocity is relatively small, it is ignored, so the water head at point m can be expressed as:

[0024]

[0025] In the formula: H is the total water head, Z is the height of point m relative to the horizontal reference surface, u is the pore water pressure, and γw is the water specific weight (unit weight);

[0026] Therefore, the hydraulic gradient in each direction is:

[0027]

[0028] Substituting equation (5) into equation (3) can obtain the soil pore seepage flow:

[0029]

[0030] Therefore, the total amount of unit soil pore seepage is:

[0031]

[0032] S2, crack healing effect of mining soil

[0033] The water in the aquifer of Salawusu Formation flows into the soil through the fissures caused by mining, and then infiltrates into the soil pores under the seepage action. The soil particles are chemically reacted under the softening action of the water in the soil pores, and show the property of "expansion" of the soil particles. The mechanism and process of the fissure closure of the soil layer are shown in Figs. 4 and 5, respectively. Figure 4

[0034] The saturated soil is a two-phase system composed of solid soil particles and pore water, which is significantly different from the continuous solid material. According to the principle of effective stress proposed by Terzaghi, the total normal stress σ at any point in the saturated soil can be decomposed into the effective stress σ' and the pore water pressure u, i.e.:

[0035] σ = σ' + u (8)

[0036] The contact area between the solid particles in the saturated soil is very small, and the contact between the solids can be regarded as point contact, while the rest of the soil particles is uniformly surrounded by pore water. Therefore, the shear stress on the soil particles only comes from the effective stress between the solid particles, and the pore water pressure has no shear stress. Therefore, the stress tensor at any point in the saturated soil can be expressed as:

[0037]

[0038] A micro-unit is taken at the tip of the fissure of the mined soil, and a space rectangular coordinate system is established, as shown in Fig. 4, where the z-axis is positive downward, and the stress is positive in compression. Figure 6

[0039] It is assumed that the body force on the saturated soil only considers the gravity, and the three-dimensional equilibrium differential equation of the soil unit in the seepage zone can be written as:

[0040]

[0041] The equilibrium differential equation of the micro-unit can be obtained from the stress tensor expression of formula (9) as:

[0042]

[0043] In the formula, γ is the unit weight of the soil, N / m3; The physical meaning of k is the unit permeability in each direction;

[0044] According to the principle of effective stress, the cause of the deformation of the soil skeleton is the change of the action force between the solid particles, which leads to the relative displacement of the soil skeleton. Therefore, the effective stress determines the deformation of the soil skeleton.

[0045] It is assumed that the swelling deformation of the soil is linear elastic deformation, which obeys the generalized Hooke's law. Therefore, [D] is a linear elastic matrix, and its expression is:

[0046] ​​

[0047] E is the elastic modulus; μ is the Poisson's ratio;

[0048] Using the constitutive equation, that is, the physical equation, that is,

[0049] {σ'} = [D] {ε} (13) Each effective stress in equation (11) can be expressed as strain:

[0050]

[0051] In the formula: εv is the volume strain, G is the shear modulus, and λ is the shear strain;

[0052] Then use the spatial geometry equation to express the deformation (ε) component with the displacement (ω) component. The stress and strain are positive in compression and negative in tension. The geometry equation is:

[0053]

[0054] Substitute equation (15) into equation (14) to obtain the effective stress expressed in displacement components. Then substitute this effective stress expression into the microelement balance differential equation (11) to obtain the microelement balance differential equation of the saturated soil seepage field expressed in displacement and pore pressure:

[0055]

[0056] In the formula: ▽ 2 is the Laplace operator,

[0057] When water flows into the fissure, the soil at the tip of the fissure is saturated soil. According to the continuity of saturated soil and the principle of spatial reciprocity, that is, the soil expansion per unit time per unit soil is equal to the change in fluid flow through the soil unit, then:

[0058]

[0059] Equation (17) can be written as:

[0060]

[0061] Equations (16) and (18) are the control equations of the seepage healing mechanism of the mining soil, which can be solved by the finite element method.

[0062] S3, determine the fissure healing amount of the mining soil

[0063] According to the soil balance differential equation of the seepage area, the sum of the left and right sides of equation (16) in three directions can be obtained:

[0064]

[0065] wherein △ 2 is Laplace operator, and ▽ is divergence (gradient);

[0066] The displacement-volume strain relationship expressed by equation (15) can express equation (19) as:

[0067]

[0068] Thus, the rate of change of volume strain (expansion rate) can be obtained:

[0069]

[0070] According to Equation (21) can be simplified as:

[0071]

[0072] According to equation (17) and equation (21), the expansion amount of the soil in the seepage area is:

[0073]

[0074] wherein ΔV is the expansion amount of the soil around the fissure, m; γ is the bulk density of the soil, Mpa; μ is the Poisson's ratio of the soil; △u is the change amount of the pore pressure; E is the elastic modulus of the saturated soil, Mpa; K is the permeability coefficient, m / d; t is the equivalent time of the soil expansion when meeting water, d; γw is the bulk density of water, Mpa;

[0075] Due to the "water-soil" interaction, the soil around the fissure will expand in the seepage area, and when the soil expands, the lateral direction is limited, and only the expansion in the vertical direction is considered, so only the soil porosity n and the unit height Δz change with the expanded pore pressure, and then the right end of equation (2) representing the seepage continuity equation of the soil around the fissure can be rewritten as:

[0076]

[0077] According to equation (4), the change of the water head (H) will be caused by the change of the pore pressure (u) of the soil around the fissure, and then the equation and can be expressed as:

[0078]

[0079]

[0080] wherein n is the porosity, %; H is the water head, m;

[0081] Substitute formula (25) and (26) into formula (24), we get:

[0082]

[0083] Thus, the continuity equation (2) becomes:

[0084]

[0085] According to the unit soil seepage flow, the seepage velocity of the unit soil in unit time is the seepage flow, so substitute formula (6) into the left end of formula (28), we get:

[0086]

[0087] After simplification, we get:

[0088]

[0089] According to formula (17) and formula (30), we get:

[0090]

[0091] Integrate the time (t) on both sides of formula (31), we get:

[0092] ΔV = ΔH (32)

[0093] Substitute formula (4) and formula (23) into formula (32), we get:

[0094]

[0095] Since the pore water pressure before and after the crack tip closure is 0, formula (33) is expressed as:

[0096]

[0097] In the formula: Z1, Z2 are the position water head of the crack tip before and after the closure of the mining crack, m; the difference between them represents the closure amount of the mining crack;

[0098] Thus, the closure amount of the downward crack of the overburden strata under the shallow buried coal seam under thick soil layer is:

[0099]

[0100] According to the swelling and permeability experiment of the clay aquifuge in Daliuta Coal Mine, the permeability coefficient of Lishi loess is 0.725 m / d, the equivalent swelling time of loess is 1.75 d, the saturated soil bulk density is 0.0232 MPa, the Poisson's ratio is 0.35, and the elastic modulus is 0.35 MPa. Substituting into formula (34), we get ΔZ = 2.52 m.

[0101] S4, Numerical calculation verifies the crack healing amount of the mining soil body

[0102] S41, Model establishment

[0103] The downward crack development depth of the surface is 8.14m when the working face of Daliuta coal mine is mined, a numerical calculation model of the downward crack soil body under mining is established by using FLAC3D, as shown in Fig. 7(a), the model size is length x width x height = 10m x 10m x 10m, the middle "V" shaped part of the model is a simplified soil crack, the height is 8.14m, and the width of the widest part of the crack is 2m, in addition, a model with fluid filled in the "V" shaped crack is established, as shown in Fig. 7(b); the soil mechanical constitutive model adopts the dilatant elastoplasticity criterion, the corresponding program function of the soil skeleton change is constructed by using FISH language, the formula (16) and (18) can be converted into FISH language to describe the soil particle dilatancy and written in, and the fluid seepage adopts the fluid isotropic seepage model;

[0104] S42, crack healing amount and healing degree

[0105] The water in the crack and the loess interact in the seepage process, the loess particles produce dilatancy phenomenon, the soil particles displace and extrude each other, the pore water between the saturated soil particles is extruded out, and the soil body dilates, and the soil mining crack heals, as shown in Fig. 8(a); under the fluid-structure coupling action, the crack gradually heals, so after the water pressure coupling balance under the seepage action, the crack has reached full healing, as shown in Fig. 8(b);

[0106] According to the analysis of the soil body pore seepage healing theory, the soil body pore seepage healing equation is written into the finite element numerical calculation software, the finite element method is used for calculation and solution, the soil crack healing amount in the longitudinal direction is 2.54m and the healing degree is 31.2% on the basis of the final balance result, which is consistent with the actual situation, so the theoretical analysis is reliable;

[0107] S5, Conclusion

[0108] The method for determining the crack healing amount of the thick soil layer under the shallow coal seam mining is feasible.

[0109] Preferably, in step S1, the seepage model is considered as the continuous flow of liquid particles filling the entire seepage area under the condition that the original boundary conditions and the seepage flow of the seepage area are unchanged.

[0110] Preferably, in step S2, according to soil mechanics, the soil with viscosity will dilate under the "water-force" action in the seepage process of the water body in the soil crack-pore medium under the mining of the soil body, which leads to the trend that the secondary crack of the dilated soil gradually closes, that is, the crack healing effect of the mining soil layer.

[0111] Preferably, in step S3, the mechanism of fissure healing of the soil layer is: mining fissure of the soil layer is generated → water body seepage and clay interact → soil particles swell → soil skeleton deforms → soil pore becomes small → pore water pressure changes → water head, hydraulic gradient and flow size change → fissure gradually heals.

[0112] Preferably, in step S4, the swelling elastic-plastic criterion refers to a series of criteria and models describing the mechanical behavior of materials in the process of swelling and plastic deformation, which includes yield criterion, flow criterion and strengthening criterion.

[0113] The beneficial effects of the present application are: by establishing a soil seepage mechanics model, combining the fissure-pore medium seepage characteristics of the soil, a calculation method for the healing amount of the downward fissure of the overburden strata under mining is proposed, which avoids the errors caused by objective and subjective factors of physical similar simulation experiments, and fully considers the fissure healing process and mechanism of the overburden strata, thereby providing a theoretical basis for quantitatively studying the fissure development degree of the overburden strata of the shallow coal seam under thick soil layer mining. BRIEF DESCRIPTION OF DRAWINGS

[0114] Figure 1 It is the flow chart for determining the healing amount of the downward fissure of the overburden strata under mining of the present application;

[0115] Figure 2 It is the structural schematic diagram of the unit body of the seepage zone of the soil body under mining of the present application;

[0116] Figure 3 It is the structural schematic diagram of the water head of any point in the saturated soil body of the present application;

[0117] Figure 4 It is the flow chart of the fissure healing mechanism of the soil body of the present application;

[0118] Figure 5a It is the saturated soil body schematic diagram of the fissure healing process of the soil layer under mining of the present application;

[0119] Figure 5b It is the soil particle swelling stage schematic diagram of the fissure healing process of the soil layer under mining of the present application;

[0120] Figure 5c It is the soil particle swelling stable stage schematic diagram of the fissure healing process of the soil layer under mining of the present application;

[0121] Figure 6 It is the structural schematic diagram of the three-dimensional space stress state of the soil unit of the seepage zone of the present application;

[0122] Figure 7a It is the loess fissure tip model schematic diagram in the loess fissure seepage model of the present application;

[0123] Figure 7bThe figure shows the size of the loess crack tip model in the loess crack seepage model of the loess layer under mining of the present application;

[0124] Figure 8a The figure shows the structure of the preliminary closure of the soil body of the present application;

[0125] Figure 8b The figure shows the structure of the full closure of the soil body of the present application. DETAILED DESCRIPTION

[0126] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0127] The specific embodiments are given below.

[0128] Reference is made to Figure 1 Fig. 8 is a method for determining the closure amount of the downward crack of the overburden strata of the shallow coal seam under thick soil layer mining, which comprises the following steps:

[0129] Firstly, the research object is the closure amount of the downward crack of the overburden strata of the shallow coal seam under thick soil layer mining, and the research problem is that the closure amount of the downward crack of the overburden strata cannot be quantified. Finally, the research method is determined, and the steps adopted by the research method are as follows: S1, establishing the soil crack-pore seepage mechanics model and the theoretical analysis of soil seepage; S2, the closure effect of the mining soil crack; S3, determining the soil crack closure amount formula; S4, verifying the soil crack closure amount; and S5, drawing a conclusion.

[0130] S1, seepage characteristics of the mining soil

[0131] S11, soil pore seepage continuity equation

[0132] The soil pore seepage is different from the soil crack seepage, because the seepage velocity and direction of each point in the soil pore seepage field are different. In order to study the characteristics of the saturated soil pore seepage, a hexahedral micro unit with a center point m(x, y, z) and edge lengths of Δx, Δy and Δz is taken in the seepage soil, as shown in Fig. 1. Figure 2

[0133] The water flow mass flowing into the unit body from the upper surface of the unit body in Δt time is The water flow mass flowing out of the unit body from the lower surface of the unit body is The water flow mass difference flowing into and out of the unit body along the z axis is Similarly, the water flow mass flowing into and out of the unit body along the x axis and the y axis is and​ Therefore, the total water mass in the unit volume per unit time is:

[0134]

[0135] According to the law of conservation of mass, the total water mass in the unit volume per unit time is equal to the change in liquid mass in the unit volume per unit time (pnΔxΔyΔz), that is:

[0136]

[0137] Equation (2) represents the soil pore seepage continuity equation;

[0138] S12, soil pore seepage flow

[0139] The seepage flow on the surface of the micro-unit in the same direction as the coordinate axis per unit time is the seepage velocity in different directions, that is, q(x, y, z) = v(x, y, z), and according to Darcy's law (v = KJ), the seepage flow in different directions of the micro-unit soil pore is:

[0140]

[0141] In the formula: q is the seepage flow in different directions, K is the seepage coefficient in different directions, and J is the hydraulic gradient in different directions;

[0142] According to Darcy's law, the hydraulic gradient (J) is the ratio of the water head (H) loss along the seepage path to the corresponding seepage path (L). The water head is the mechanical energy possessed by unit weight of liquid, which includes position water head (Z), pressure water head (P) and flow velocity water head (Vp);

[0143] In the soil pore seepage field, any point m(x, y, z) is as shown in Figure 3 The position water head at point m is Z, and the pressure water head (P) is the ratio of the pore water pressure (u) at the point to the water unit weight (γw). Since the soil pore seepage velocity is relatively small, it is ignored, so the water head at point m can be represented as:

[0144]

[0145] In the formula: H is the total water head, Z is the height of point m relative to the horizontal reference surface, u is the pore water pressure, and γw is the water specific weight (unit weight);

[0146] Therefore, the hydraulic gradient in each direction is:

[0147]

[0148] Substituting equation (5) into equation (3) gives the soil pore seepage flow:

[0149]

[0150] Therefore, the total amount of unit soil body pore seepage is:

[0151]

[0152] S2, fissure healing effect of mining soil body

[0153] The water body in the aquifer of Salawusu Formation flows into the soil body through the fissure of the mining soil body, and under the seepage effect, it is immersed in the soil body pores. The soil particles undergo chemical reactions under the softening effect of the soil body pore water, showing the property of "expansion" of the soil particles. The mechanism and process of soil layer fissure healing are shown in Figure 4 and Fig. 5;

[0154] The saturated soil body is a two-phase system composed of solid soil particles and pore water, which is significantly different from continuous solid materials. According to the effective stress principle proposed by Terzaghi, the total normal stress σ at any point in the saturated soil body can be decomposed into two parts, the effective stress σ' and the pore water pressure u, that is:

[0155] σ = σ' + u (8)

[0156] The contact area between solid particles in the saturated soil body is very small, and the contact between solids can be regarded as point contact, while the rest around the soil particles is uniformly surrounded by pore water. Therefore, the shear stress on the soil particles only comes from the effective stress between solid particles, and the pore water pressure has no shear stress. Therefore, the stress tensor at any point in the saturated soil body can be expressed as:

[0157]

[0158] A micro-unit is taken at the tip of the fissure of the mining soil body, and a space rectangular coordinate system is established, as shown in Figure 6 The z-axis is positive downward, and the stress is positive in compression;

[0159] It is assumed that the body force on the saturated soil body only considers the gravity, and the three-dimensional equilibrium differential equation of the soil body unit in the seepage zone can be written as:

[0160]

[0161] The equilibrium differential equation of the micro-unit can be obtained from the stress tensor expression of formula (9) as:

[0162]

[0163] In the formula: γ is the unit weight of the soil body, N / m3; The physical meaning of is the unit seepage force in each direction;

[0164] According to the effective stress principle, the reason causing the skeleton deformation of the soil is that the relative displacement of the soil skeleton is caused by the change of the action force between the solid particles, and thus the effective stress determines the skeleton deformation of the soil;

[0165] Supposing that the swelling deformation of the soil is linear elastic deformation and obeys the generalized Hooke's law, [D] is a linear elastic matrix, and the expression thereof is as follows:

[0166]

[0167] In the formula, E is the elastic modulus, and μ is the Poisson's ratio;

[0168] The constitutive equation, that is, the physical equation, is as follows:

[0169] {σ′}=[D]{ε} (13) The effective stresses in the formula (11) can be expressed by the strains as follows:

[0170]

[0171] In the formula, εv is the volume strain, G is the shear modulus, and λ is the shear strain;

[0172] The deformation (ε) components are expressed by the displacement (ω) components by using the spatial geometry equation, and the stress and strain are positive in compression and negative in tension, and the geometry equation is as follows:

[0173]

[0174] The effective stress expressed by the displacement components is obtained by substituting the formula (15) into the formula (14), and the micro-unit balance differential equation of the saturated soil seepage field expressed by the displacement and the pore pressure is obtained by substituting the effective stress expression into the micro-unit balance differential equation (11) of the micro-unit:

[0175]

[0176] In the formula, ▽ 2 is the Laplace operator,

[0177] When the water flow is filled in the fissure, the soil at the fissure tip is the saturated soil, according to the continuity of the saturated soil and the spatial mutual equality principle, that is, the swelling amount of the unit soil per unit time is equal to the change amount of the fluid flow through the soil unit, and thus the following formula is obtained:

[0178]

[0179] The formula (17) can be written as:

[0180]

[0181] Integrating equation (16) and equation (18) is the control equation of the seepage closure mechanism of the mining soil mass, which can be solved by the finite element method.

[0182] S3, determining the crack closure amount of the mining soil mass

[0183] According to the soil balance differential equation of the seepage area, the sum of the three direction differential equations of equation (16) can be obtained:

[0184]

[0185] In the formula:▽ 2 is the Laplace operator, and▽is the divergence (gradient);

[0186] The relationship between displacement and volume strain represented by equation (15) can be represented as:

[0187]

[0188] Therefore, the change rate (expansion rate) of the volume strain can be obtained:

[0189]

[0190] According to Equation (21) can be simplified as:

[0191]

[0192] According to equation (17) and equation (21), the expansion amount of the soil mass in the seepage area is:

[0193]

[0194] In the formula: ΔV is the expansion amount of the soil mass around the crack, m; γ is the unit weight of the soil, Mpa; μ is the Poisson's ratio of the soil;▽u is the change amount of the pore pressure; E is the elastic modulus of the saturated soil, Mpa; K is the permeability coefficient, m / d; t is the equivalent time of the soil expansion when encountering water, d; γw is the unit weight of water, Mpa;

[0195] Due to the "water-soil" interaction, the soil mass around the crack in the seepage area will cause expansion. When the soil mass expands, the lateral direction is limited, and only the vertical expansion is considered. Therefore, only the soil porosity n and the unit height Δz change with the expanded pore pressure, and the right end of equation (2) representing the seepage continuity equation of the soil mass around the crack can be rewritten as:

[0196]

[0197] According to equation (4), the change of the pore pressure (u) of the soil mass around the crack will cause the change of the water head (H), and then the equation can be rewritten as: and is expressed as:

[0198]

[0199]

[0200] wherein: n is porosity, %; H is water head, m;

[0201] Substitute formula (25) and (26) into formula (24), we get:

[0202]

[0203] Thus, the continuity equation (2) becomes:

[0204]

[0205] According to the unit soil seepage flow, the seepage velocity of the unit soil in unit time is the seepage flow, so substitute formula (6) into the left end of formula (28), we get:

[0206]

[0207] After simplification, we get:

[0208]

[0209] According to formula (17) and formula (30), we get:

[0210]

[0211] Integrate the time (t) on both sides of formula (31), we get:

[0212] ΔV = ΔH (32)

[0213] Substitute formula (4) and formula (23) into formula (32), we get:

[0214]

[0215] Since the pore water pressure before and after the crack tip closure is 0, formula (33) is expressed as:

[0216]

[0217] wherein: Z1, Z2 are the position water head of the crack tip before and after the mining crack closure, m; the difference between them represents the closure amount of the mining crack;

[0218] Thus, the closure amount of the descending crack of the overburden strata under the shallow buried coal seam under the thick soil layer is:

[0219]

[0220] According to the swelling and permeability experiment of clay aquiclude in Daliuta coal mine, the permeability coefficient of Lishi loess is 0.725 m / d, the equivalent swelling time of loess is 1.75 d, the saturated soil bulk density is 0.0232 MPa, the Poisson's ratio is 0.35, and the elastic modulus is 0.35 MPa. Substituting into equation (34) can obtain ΔZ = 2.52 m;

[0221] S4, numerical calculation verifies the crack healing amount of mining soil

[0222] S41, model establishment

[0223] The downward crack development depth of the surface is 8.14 m when the working face of Daliuta coal mine is mined. The numerical calculation model of downward crack soil under mining is established by using FLAC3D, as shown in Figure 7(a). The model size is length x width x height = 10 m x 10 m x 10 m. The middle "V" shape part of the model is a simplified soil crack, the height is 8.14 m, and the width of the widest part of the crack is 2 m. In addition, the model filled with fluid in the "V" shape crack is established, as shown in Figure 7(b). The soil mechanical constitutive model adopts the expansion elastoplasticity criterion. The corresponding program function of soil skeleton change is constructed by using FISH language. The formula (16) and (18) can be converted into FISH language to describe the soil particle expansion amount and written in. The fluid seepage adopts the fluid isotropic seepage model.

[0224] S42, crack healing amount and healing degree

[0225] In the seepage process, the water in the crack reacts with the loess, the loess particles produce swelling phenomenon, the soil particles displace and extrude each other, the pore water between the saturated soil particles is extruded out, and the soil produces swelling after the soil mining crack healing phenomenon, as shown in Figure 8(a). When the soil is under the action of fluid-structure coupling, the crack gradually heals, so after reaching the hydraulic coupling balance under the action of seepage, the crack has reached full healing, as shown in Figure 8(b).

[0226] According to the theoretical analysis of soil pore seepage healing, the soil pore seepage healing equation is written into the finite element numerical calculation software, and the finite element method is used to calculate and solve. On the final equilibrium result, the soil crack healing amount in the longitudinal direction is 2.54 m, and the healing degree is 31.2%, which is consistent with the actual situation. Therefore, the theoretical analysis is reliable.

[0227] S5, conclusion

[0228] The method for determining the crack healing amount of shallow coal seam under thick soil layer under mining is feasible;

[0229] From the above steps, the method can avoid the error caused by objective and subjective factors of physical similar simulation experiment by establishing soil seepage mechanics model, combining with soil fracture-pore medium seepage characteristics, and putting forward a calculation method of descending fracture healing amount of overburden rock under mining, and fully considers the fracture healing process and mechanism of overburden rock, which provides a theoretical basis for quantitatively studying the fracture development degree of overburden rock under shallow coal seam mining in thick soil layer.

[0230] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art can make equivalent replacement or change according to the technical solution and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

Claims

1. A method for determining the amount of downward crack closure of overburden rock in shallow coal seam mining under thick soil layer, characterized in that, Comprising the following steps: First, the research object is the closure of the downhole fissure of the overburden strata of the shallow coal seam under thick soil layer, and the research problem is that the closure of the downhole fissure of the overburden strata cannot be quantified, and finally the research method is determined, and the steps of the research method are: S1, the establishment of soil fissure-pore seepage mechanics model and soil seepage theory analysis; S2, the closure effect of mining soil fissure; S3, determine the soil fissure closure formula, S4, verify the soil fissure closure; S5, draw a conclusion; S1, the seepage characteristics of the mining soil S11, the continuity equation of soil pore seepage Soil pore seepage is different from soil fissure seepage because the seepage velocity and direction of each point in the soil pore seepage field are different. In order to study the characteristics of saturated soil pore seepage, a hexahedral microelement with each edge length of Δx, Δy and Δz is taken as the center of point m(x, y, z) in the seepage soil, as shown in Figure 2; The water flow mass flowing into the unit from the upper surface of the unit in Δt is The water flow mass flowing out of the unit from the lower surface of the unit is The water flow mass difference flowing into and out of the unit along the z axis is Similarly, the water flow masses flowing into and out of the unit along the x axis and the y axis are respectively and Therefore, the total water flow mass in the unit per unit time is According to the law of conservation of mass, the total water mass in the unit body per unit time is equal to the change of liquid mass in the unit body per unit time (ρnΔxΔyΔz), that is: Equation (2) represents the continuity equation of soil pore seepage; S12, soil pore seepage flow The seepage flow on the surface with the same direction of the coordinate axis per unit time of the microelement is the seepage velocity in different directions, that is, q(x, y, z) = v(x, y, z), and according to Darcy's law (v = KJ), the seepage flow in different directions of the microelement soil pore is: In the formula: q is the seepage flow in different directions, K is the seepage coefficient in different directions, and J is the hydraulic gradient in different directions; According to Darcy's law, the hydraulic gradient (J) is the ratio of the water head (H) loss along the seepage path to the corresponding seepage path (L). The water head is the mechanical energy possessed by unit weight of liquid, which includes position water head (Z), pressure water head (P) and flow velocity water head (Vp). In the soil pore seepage field, any point m(x, y, z) is shown in Figure 3. The position water head of point m is Z, and the pressure water head (P) is the ratio of the pore water pressure (u) at the point to the water unit weight (γw). Since the soil pore seepage velocity is relatively small, it is ignored. Therefore, the water head at point m can be expressed as: In the formula: H is the total water head, Z is the height of point m relative to the horizontal reference surface, u is the pore water pressure, and γw is the water specific weight (unit weight). Therefore, the hydraulic gradient in each direction is: Substituting equation (5) into equation (3) gives the soil pore seepage flow as: Therefore, the total amount of unit soil pore seepage is: S2, the closure effect of mining soil fissure The water in the Salawusu Formation aquifer flows into the soil through the fissures of the mining soil, and under the action of seepage, it is immersed in the soil pores. The soil particles undergo chemical reactions under the softening action of soil pore water, showing the property of "expansion" of soil particles. The mechanism and process of soil layer fissure closure are shown in Figures 4 and 5, respectively. Saturated soil is a two-phase system composed of solid soil particles and pore water, which is significantly different from continuous solid materials. According to the effective stress principle proposed by Terzaghi, the total normal stress σ at any point in the saturated soil can be decomposed into effective stress σ' and pore water pressure u, that is: σ = σ' + u (8) The contact area between solid particles in saturated soil is very small, and the contact between solids can be regarded as point contact, while the rest of the soil particles is surrounded by pore water, so the shear stress on the soil particles only comes from the effective stress between solid particles, and the pore water pressure has no shear stress, so the stress tensor at any point in the saturated soil can be expressed as: A micro-unit is taken at the crack tip of the mining soil, and a space rectangular coordinate system is established as shown in Fig. 6, with the z-axis pointing downward as positive, and the stress as positive pressure; Assuming that the volume force on the saturated soil only considers gravity, the three-dimensional equilibrium differential equation of the soil unit in the seepage area can be written as: The equilibrium differential equation of the micro-unit can be obtained from the stress tensor expression of equation (9) as: wherein: γ is the bulk density of the soil, N / m3; The physical meaning of k is the unit permeability in each direction. According to the effective stress principle, the change in the action force between solid particles causes the deformation of the soil skeleton, so the effective stress determines the deformation of the soil skeleton; Assuming that the swelling deformation of the soil is linear elastic deformation, which obeys the generalized Hooke's law, then [D] is a linear elastic matrix, and its expression is: In the formula: E is the elastic modulus; μ is the Poisson's ratio; Using the constitutive equation, that is, the physical equation, that is: {σ′}=[D]{ε} (13) The effective stress in equation (11) can be expressed in terms of strain as: wherein: εv is the volumetric strain, G is the shear modulus, and λ is the shear strain; Then use the space geometry equation to express the deformation (ε) component with the displacement (ω) component, and the stress and strain are positive pressure and negative tension, and the geometry equation is: Substitute equation (15) into equation (14) to obtain the effective stress expressed in terms of displacement components, and then substitute this effective stress expression into the equilibrium differential equation (11) of the micro-unit to obtain the equilibrium differential equation of the micro-unit of the seepage field of the saturated soil expressed in terms of displacement and pore pressure: In the formulae: is the Laplacian operator, When water flows into the crack, the soil at the crack tip is saturated soil. According to the continuity of saturated soil, the space mutual equality principle, that is, the swelling amount of unit soil per unit time is equal to the change amount of fluid flow through the soil unit, then: Equation (17) can be written as: Equations (16) and (18) are the control equations of the seepage healing mechanism of the mining soil, which can be solved by the finite element method; S3, determine the crack healing amount of the mining soil According to the soil balance differential equation of the seepage area, the sum of the three direction differential equations of equation (16) can be obtained as: wherein: is the Laplacian, is the divergence (gradient); The relationship between displacement and volume strain expressed by equation (15) can be expressed as: Therefore, the volume strain rate (swelling rate) is: According to Equation (21) can be simplified as: According to equations (17) and (21), the swelling amount of the soil in the seepage area is: In the formula, ΔV is the swelling amount of soil around the crack, m; γ is the bulk density of soil, MPa; μ is the Poisson's ratio of soil; is the change amount of pore pressure; E is the elastic modulus of saturated soil, MPa; K is the permeability coefficient, m / d; t is the equivalent time of soil swelling when meeting water, d; γw is the bulk density of water, MPa; Due to the "water-soil" interaction, the seepage area around the crack will cause the soil to swell. When the soil swells, the lateral direction is limited, and only the vertical direction is considered. Only the soil porosity n and the unit height Δz change with the swelling pore pressure, so the right side of equation (2) representing the seepage continuity equation around the crack can be rewritten as: From equation (4), it can be seen that the change of pore pressure (u) around the crack will cause the change of water head (H). Thus, equation (4) can be rewritten as and is expressed as: In the formula: n is the porosity, %; H is the water head, m; Substitute equations (25) and (26) into equation (24) to obtain: Therefore, the continuity equation (2) becomes: According to the unit soil seepage flow, the seepage velocity of the unit soil in unit time is the seepage flow, so formula (6) is brought into the left end of formula (28), and the following is obtained: After simplification, the following is obtained: According to formula (17) and formula (30), the following is obtained: The integral of time (t) on both sides of formula (31) is obtained: ΔV = ΔH (32) Formula (4) and formula (23) are brought into formula (32): Since the pore water pressure before and after the crack tip closes is 0, formula (33) is expressed as: In the formula: Z1, Z2 are the position water heads of the crack tip before and after the closure of the mining crack, m; the difference between them represents the closure amount of the mining crack; Thus, the closing amount of the descending fissure of the overburden strata of the shallow coal seam under the thick soil layer is: According to the swelling and permeability experiments of the clay aquiclude in Daliuta Coal Mine, the permeability coefficient of Lishi loess is 0.725 m / d, the equivalent swelling time of loess is 1.75 d, the saturated soil bulk density is 0.0232 MPa, the Poisson's ratio is 0.35, and the elastic modulus is 0.35 MPa. Substituting formula (34) can obtain ΔZ = 2.52 m. S4, numerical calculation verifies the closure amount of the mining soil crack S41, model establishment When the working face of Daliuta Coal Mine is mined, the development depth of the downward crack on the ground is 8.14 m. The numerical calculation model of the downward crack soil under mining is established by using FLAC3D, as shown in Figure 7(a). The model size is length × width × height = 10 m × 10 m × 10 m. The "V" shaped part in the middle of the model is a simplified soil crack, the height is 8.14 m, and the width of the widest part of the crack is 2 m. In addition, the model in which the "V" shaped crack is filled with fluid is established, as shown in Figure 7(b). The soil mechanical constitutive model adopts the swelling elastoplasticity criterion. The corresponding program function of the soil skeleton change is constructed by using FISH language. Formulas (16) and (18) can be converted into FISH language to describe the soil particle swelling amount and written in. The fluid seepage adopts the fluid isotropic seepage model. S42, crack closure amount and closure degree During the seepage process, the water in the crack reacts with loess, the loess particles produce swelling phenomenon, the soil particles displace and extrude each other, the pore water between the saturated soil particles is extruded out, and after the swelling of the soil, the soil mining crack is closed, as shown in Figure 8(a). When the soil is under the fluid-structure coupling action, the crack gradually closes, so after reaching the hydraulic coupling balance under the seepage action, the crack has reached full closure, as shown in Figure 8(b). According to the theoretical analysis of the soil pore seepage closure, the soil pore seepage closure equation is written into the finite element numerical calculation software, the finite element method is used for calculation and solution, the final equilibrium result is directly measured, the longitudinal closure amount of the soil crack is 2.54 m, and the closure degree is 31.2%, which is consistent with the actual situation, so the theoretical analysis is reliable. S5, conclusion The method for determining the closure amount of the mining crack under the thick loess layer and shallow buried coal seam is feasible.

2. The method for determining the closing amount of the descending fracture in the overburden strata of the shallow coal seam under thick overburden strata according to claim 1, characterized in that, In step S1, the seepage model is regarded as the continuous flow of liquid particles filling the entire seepage area under the condition of keeping the original boundary condition and seepage flow of the seepage area unchanged.

3. The method for determining the closing amount of the descending fissure of the overburden strata of the shallow coal seam under thick strata mining according to claim 1, characterized in that, In step S2, according to soil mechanics, the water body in the aquifer has the property of viscosity in the seepage process of the mining soil fissure-pore medium, and the soil body with the property of viscosity will swell under the action of "water force", resulting in soil body swelling, and the secondary fissure has the trend of gradually closing, that is, the fissure healing effect of the mining soil layer.

4. The method for determining the closing amount of the descending fissure of the overburden strata of the shallow coal seam under thick strata mining according to claim 1, characterized in that, In step S3, the fissure healing mechanism of the soil layer is: fissure generation of the mining soil layer → interaction of water seepage and clay → swelling of soil particles → deformation of soil skeleton → decrease of soil pore → change of pore water pressure → change of water head, hydraulic gradient and flow size → gradual healing of fissure.

5. The method for determining the closing amount of the descending fissure of the overburden strata of the shallow coal seam under thick strata mining according to claim 1, characterized in that, In step S4, the swelling elastic-plastic criterion refers to a series of criteria and models describing the mechanical behavior of materials in the process of swelling and plastic deformation, and these criteria include yield criterion, flow criterion and strengthening criterion.

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