A method for grouting and subsidence reduction of multi-layer separation of overburden strata in three-soft thick coal seam mining

Abstract

The application discloses a kind of three soft thick coal seam mining overburden multi-layer separation grouting subsidence reduction methods, comprising the following steps: S1, based on the theory of composite beam, separation grouting layer is identified, and the filling slurry thickness of overburden multi-layer separation grouting is calculated;S2, by elastic thin plate theory, the mechanical model of overburden separation grouting protective layer is established and the safety thickness of protective layer is determined;S3, build filling mining grouting space equivalent model, obtain the volume of grouting filling space.The application can provide theoretical support for the design of overburden separation grouting subsidence reduction in three soft thick coal seam fully mechanized caving;It effectively solves the defects such as low separation grouting filling rate in three soft thick coal seam, and plays an important role in promoting the application and development of overburden separation grouting subsidence reduction technology.

Description

Technical Field

[0001] This invention relates to the field of coal mining, and in particular to a method for reducing subsidence by grouting multiple layers of overburden in the mining of thick, soft coal seams. Background Technology

[0002] Coal mining-induced surface subsidence and ecological environmental problems have always been among the major issues hindering the healthy development of the coal industry. At the same time, the mining and utilization of coal inevitably generates solid waste such as coal gangue, fly ash, and slag, increasing the risk of environmental pollution to water, soil, and air, and threatening public safety and health.

[0003] Overburden separation grouting and backfilling mining, as an important technical means of green mining, has become a hot topic of widespread academic attention. This mining method involves filling the separation space formed at a certain position above the water-conducting fracture zone during mining with grout material to support the overburden and achieve the purpose of reducing surface subsidence. Engineering practice shows that overburden separation grouting and backfilling mining can effectively control surface subsidence and protect surface structures.

[0004] Accurately identifying the location of the overburden separation and timely grouting are crucial for the successful implementation of overburden separation grouting. In response, numerous scholars have conducted research on the migration patterns of mining-induced overburden and the development of separation layers. Based on the composite beam theory, Yang Lun et al. proposed a theoretical calculation method for the location of overburden delamination. This model has been widely recognized and has become an important theoretical basis for the study of overburden delamination development. Xu Chao et al. used 3DEC to study the evolution law of overburden fractures and quantitatively characterized the delamination rate. Wang et al. used UDEC simulation software to analyze the overburden movement law and delamination characteristics. Cui Ximin et al. established a relationship model between overburden and surface equivalent subsidence, quantitatively analyzed the overburden and surface subsidence law, and thus estimated the amount of overburden delamination. Academician Qian Minggao proposed the theory of key strata controlling the rock layer, further clarified the relationship between overburden migration and delamination evolution law, and applied it to overburden delamination grouting mining engineering. Professor Xu Jialin proposed overburden isolation grouting subsidence reduction technology, which has been successfully applied in multiple coal mines. In addition, research has also been carried out on monitoring or observation methods for overburden delamination, including ground borehole observation and microseismic monitoring technology.

[0005] The overburden in the fully mechanized longwall mining of "three soft" thick coal seams has the characteristics of a large interaction rate between soft and hard rocks and a small average thickness. After mining, the overall coordinated deformation of the overburden is large and it is difficult to generate a large-sized delamination space. As a result, it has defects such as a low delamination grouting filling rate, which needs to be addressed. Summary of the Invention

[0006] The purpose of this invention is to address the above-mentioned problems by providing a grouting and settling reduction method for multi-layered excavation layers in the overburden of three-soft, thick coal seams, which improves the grouting filling rate and the safety of coal seam mining.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A method for reducing settlement by grouting in the overlying layers of thick, soft coal seams includes the following steps:

[0009] S1. Based on the composite beam theory, identify the delamination grouting layer and calculate the filling grout thickness of the multi-layer delamination grouting in the overburden.

[0010] S2. Based on the theory of elastic thin plates, establish a mechanical model of the grouting protective layer for overburden separation and determine the safe thickness of the protective layer;

[0011] S3. Construct an equivalent model of the grouting space for filling mining to obtain the volume of the grouting filling space.

[0012] Furthermore, step S1 includes the following steps:

[0013] S11. Determine the height range of the grouting location for overburden separation;

[0014] S12. Determine the spatial location of the grout separation layer;

[0015] S13. Determine the thickness of the filling grout.

[0016] Furthermore, in step S11, the formula for calculating the height range of the overburden separation grouting location is as follows:

[0017]

[0018] In the formula, H is the distance from the grouting location to the coal seam. d H is the height of the water-conducting fracture zone. f M represents the thickness of the waterproof and safe coal-rock pillar, and M represents the thickness of the coal seam.

[0019] Furthermore, step S12 specifically includes the following steps:

[0020] S121. In an overburden containing m layers of rock, when adjacent layers 1 and n can be in close contact and deform synchronously, considering an n-layer composite beam, the load calculation formula for the i-th layer is:

[0021]

[0022] In the formula, E i h i and γ i These are the elastic modulus, thickness, and volumetric weight of each rock layer;

[0023] S122, when the calculated (q) n+1 ) i Less than (q) n ) iWhen this happens, layer (n+1) will separate from layer n; the calculation formula is:

[0024] (q n+1 ) i <(q) n ) i ;

[0025] S123, after simplification:

[0026]

[0027] S124, when (q) m )1=max((q1)1,(q2)1,…,(q n )1), then a separation occurs between the (m+1)th layer and the mth layer; when (q n )1=max((q1)1,(q2)1,…,(q n If )1), then no separation layer will be generated between the first layer and the nth layer.

[0028] Furthermore, step S13 specifically includes the following steps:

[0029] S131. The geometric relationship between the filling grout thickness and the mining process parameters is obtained through analysis. The calculation formula is as follows:

[0030]

[0031] In the formula, L b W is the length of the flat-bottomed collapse zone of the injection delamination zone; H is the width of the working face; bmax α represents the thickness of the filling grout; L represents the length of the working face; α represents the injection-production ratio.

[0032] S132. The formula for calculating the thickness of the filling grout is:

[0033]

[0034] In the formula, φ b To fully utilize the angle.

[0035] Furthermore, step S2 includes the following steps:

[0036] S21. Establish a mechanical model of the overburden separation grouting protective layer to calculate the deflection of the protective layer;

[0037] S22. Analyze and calculate the safe thickness of the protective layer and determine whether the grouting layer is safe and feasible.

[0038] Furthermore, step S21 specifically includes the following steps:

[0039] S211. The mechanical model of the overburden delamination grouting protective layer is obtained through analysis, and its calculation formula is as follows:

[0040]

[0041] In the formula, G = Eh 3 / 12(1-μ 2 ) represents the bending stiffness of a single thin plate, where E, μ, and h are the elastic modulus, Poisson's ratio, and thickness of the protective rock layer, respectively; q0 represents the uniformly distributed transverse load of the protective rock layer, and the formula for calculating q0 is:

[0042]

[0043] In the formula, λ is the support coefficient of the underlying rock strata of the protective layer, 0≤λ<1, and λ=0 indicates that there is no contact between the protective layer and the underlying rock strata; σ f The pressure at the grouting layer; P k The orifice pressure of the grouting hole gradually increases with grouting time and can be expressed as a function of time, i.e., P. k (Δt); γ f h f These are the unit weight of fly ash slurry and the depth of grouting layer, respectively; γ b For the density of the protective layer;

[0044] S212. Let the formula for calculating the deflection function be:

[0045]

[0046] S213. The formula for calculating the deflection of the protective layer is:

[0047]

[0048] In the formula, a and b are the strike length and dip width of the protective layer, respectively, and their calculation formula is:

[0049]

[0050]

[0051] In the formula, h i φ1 and φ2 are the fracture angles of the rock strata along the working face advancement direction, and φ3 and φ4 are the fracture angles of the rock strata along the working face width direction.

[0052] Furthermore, step S22 specifically includes the following steps:

[0053] S221. The formula for calculating the principal stress distribution of the protective layer rock slab under uniformly distributed load is:

[0054]

[0055] S222. Based on the relationship between principal stress and stress components, the formulas for calculating the maximum and minimum principal stresses on the neutral surface of the protective layer are as follows:

[0056]

[0057]

[0058] S223. Based on the Mohr-Coulomb criterion, the calculation formula for the rock stratum failure criterion is as follows:

[0059]

[0060] In the formula, C and φ represent the cohesion and internal friction angle of the protective rock layer, respectively.

[0061] S224. The calculation formula for the damage and failure criterion of the protective layer rock is as follows:

[0062]

[0063] When F=1, the protective layer breaks;

[0064] S225. The formula for calculating the safe thickness range of the protective layer is:

[0065]

[0066] S226. The formula for determining whether a grouting layer is safe is:

[0067]

[0068] Furthermore, step S3 includes the following steps:

[0069] S31. The formula for calculating the space height between two delamination slabs is:

[0070]

[0071] In the formula, q s and q x G represents the lateral uniformly distributed loads of the upper and lower rock strata of the filling space, respectively. s and G x These are the bending stiffnesses of the upper and lower rock strata of the filling space, respectively.

[0072] S32. Based on the consensus in thin-plate theory that the deflection is greatest at the center of rock strata, the formula for calculating the maximum height of the grouting space is:

[0073]

[0074] S33. The formula for calculating the volume of a certain delamination grouting layer is as follows:

[0075] In the formula, w s w x These represent the deflection at various points on the upper and lower rock slabs, respectively.

[0076] S34. The total volume of the grouting space is the sum of the volumes of all grouting spaces, and its calculation formula is as follows:

[0077] V = ∑V i .

[0078] Compared with the prior art, the advantages and positive effects of this invention are:

[0079] This invention proposes a multi-layer delamination grouting technology for thick coal seams with soft, three-soft overburden. First, it improves the delamination location determination method based on composite beam theory, identifying the filling positions for multi-layer delamination grouting. Next, using elastic thin-plate theory, it establishes a mechanical model of the rock mass structure for overburden delamination grouting protection layer mining, clarifying the reasonable and safe thickness of the protection layer. Then, it constructs an equivalent model of the grouting space for overburden delamination grouting filling, and based on the fracture laws of the upper and lower strata in the grouting space, it gives the minimum and maximum total volume of the overburden delamination grouting filling space. Finally, using probability integrals and combining actual surface monitoring results, it evaluates the effect of multi-layer delamination grouting in thick coal seams with soft, three-soft overburden, providing theoretical support for the design of overburden delamination grouting and subsidence reduction in fully mechanized mining of thick coal seams with soft, three-soft overburden. It effectively solves the defects such as low filling rate of delamination grouting in thick coal seams with soft, three-soft overburden, and plays an important role in promoting the application and development of overburden delamination grouting and subsidence reduction technology. Attached Figure Description

[0080] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0081] Figure 1 Schematic diagram of grouting mining for overburden separation;

[0082] Figure 2 This is a schematic diagram of a composite beam.

[0083] Figure 3 A schematic diagram of grouting mining of multi-layered segregated overburden;

[0084] Figure 4 Schematic diagram of an equivalent model for grouting and backfilling mining of multi-layered excavated overburden;

[0085] Figure 5 A schematic diagram of the mechanical calculation model for the protective layer;

[0086] Figure 6 This is a simplified schematic diagram of the upper and lower rock strata above the delamination.

[0087] Figure 7 A schematic diagram of an equivalent model for filling the grouting space during overburden separation grouting.

[0088] Figure 8 This is a contour map of surface subsidence during normal mining operations.

[0089] Figure 9 A schematic diagram showing the fitting results of surface strike observations for grouting of multiple extremity layers of overburden;

[0090] Figure 10 Contour map of surface subsidence during mining of multi-layered excavated overburden;

[0091] Figure 11 This is a comparison chart of the predicted surface subsidence values ​​for two mining schemes. Detailed Implementation

[0092] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art to all other embodiments obtained without creative effort should be included within the protection scope of the present invention.

[0093] This invention, through multi-faceted research including the identification of delamination location, determination of grouting layer location, and the rationality of protective layer thickness, proposes a multi-layer delamination grouting mining technology for overburden. It establishes a deformation mechanical model of the protective layer in overburden delamination grouting mining and, based on probability integrals and combined with actual surface monitoring results, analyzes the effect of overburden delamination grouting. This provides good technical support for the overburden delamination grouting and subsidence reduction method in fully mechanized longwall mining of "three soft" thick coal seams, and plays an important role in promoting the application and development of overburden delamination grouting and subsidence reduction technology.

[0094] I. Grouting for Overburden Separation

[0095] 1.1 Research Consensus

[0096] During coal mining, the abscission space generally extends along the interface between soft and hard rock strata, developing from lower to higher positions as the working face advances. Grouting can be performed when the abscission develops to the borehole area. The grouting location must not intersect with the water-conducting fracture zone, as such an intersection may cause the injected mud to surge towards the underground working face, threatening safe underground production. Therefore, the grouting location should be a certain distance above the water-conducting fracture zone; this distance is referred to as the waterproof safety coal-rock pillar (or protective layer thickness). The grouting location is typically as follows:

[0097]

[0098] In the formula, H is the distance from the grouting location to the coal seam. d H is the height of the water-conducting fracture zone. f M represents the thickness of the waterproof and safe coal-rock pillar, and M represents the thickness of the coal seam.

[0099] Based on the results of equation (1) and combined with geological borehole data, a relatively thick sandstone layer (or the main surface controlling rock layer) was determined as the grouting location. The overburden separation grouting was carried out on the main surface controlling rock layer and its supporting structure as follows: Figure 1 As shown in Table 1, when the overlying strata of a thick coal seam are soft, medium-hard, hard, or alternating layers, and the fully mechanized top coal caving mining is adopted, the maximum height of the water-conducting fracture zone above the coal seam should be calculated according to the empirical formula given in the "Guidelines for the Retention of Coal Pillars and Coal Mining in Buildings, Water Bodies, Railways and Main Shafts" (as shown in Table 1).

[0100] Table 1. Empirical Formulas for Calculating the Height of Water-Conducting Fractures

[0101]

[0102] Based on the results of equation (1) and combined with geological borehole data, a relatively thick sandstone layer (or the main surface rock layer) was determined as the grouting location. The schematic diagram of overburden separation grouting is shown below. Figure 1 As shown.

[0103] 1.2 Identification of Grouting Layers for Overburden Delamination

[0104] During the development of separation and fracturing between the overlying strata, the soft and hard interbedded overlying strata can be analyzed as a composite rock beam composed of several superimposed strata, assuming that the load on each stratum is uniformly distributed. A schematic diagram of the composite beam is shown below. Figure 2 As shown. According to the composite beam theory, in an overburden containing m layers of rock, when adjacent layers 1 and n can be in close contact and achieve synchronous deformation, considering an n-layer composite beam, the load of the i-th layer (i≤n) can be derived as follows:

[0105]

[0106] In the formula E i hi and γ i (i=1, 2, ..., n, m) are respectively the elastic modulus, thickness and bulk density of each rock stratum.

[0107] As is known from the mine pressure and strata control theory, when the calculated (q n+1 ) i is less than (q n ) i , it is considered that interlayer separation occurs between the (n+1)th stratum and the nth stratum (as shown in Figure 2 ), that is, separation is generated:

[0108] (q n+1 ) i <(q n ) i (3)

[0109] Combining formula (2) and formula (3), a commonly used simplified formula can be obtained:

[0110]

[0111] Since the two formulas of formula (3) and formula (4) only compare the mechanical property differences between adjacent strata, the mechanical effect of the strata above the (n+1)th stratum on the (n+1)th stratum that may be generated is ignored. For example, assuming that the bending stiffness of the (n+2)th stratum is less than the bending stiffness of the (n+1)th stratum, in this case, the (n+2)th stratum may come into contact with the (n+1)th stratum during the settlement process, causing the (n+1)th stratum to be squeezed by the (n+2)th stratum and generate an additional force. In view of this deficiency, a correction is made: if (q m )₁=max((q₁)₁, (q₂)₁, …, (q n )₁), and 1≤m<n, separation will be generated between the m+1th stratum and the mth stratum; if (q n )₁=max((q₁)₁, (q₂)₁, …, (q n )₁), no separation will be generated between the 1st stratum and the nth stratum.

[0112] The corrected mechanical principle for generating separation space can be explained as: if (q m )==max((q₁)₁, (q₂)₁, …, (q n )₁), 1≤m<n, the 1st to mth rock strata can bend synchronously in the form of a composite beam. In addition, if (q m )₁>max((q m+1 )₁, (q m+2 )₁, …, (q n1)) The deformation of each layer between the (m+1)th layer and the nth layer is less than that of the mth layer. Therefore, the interface between the mth layer and the (m+1)th layer can be pulled apart by the mth layer. Based on the calculation results, the closed delamination space that cannot be formed within the water-conducting fracture zone is eliminated, thereby determining the location of the grouting delamination space.

[0113] 1.3 Grouting of Multi-Layer Excavation Layers in Overburden and Its Equivalent Model

[0114] For fully mechanized longwall mining of thick coal seams characterized by "three softnesses" (soft, soft, and soft rock), the interaction rate between the overlying strata and hard rock is high, and the thickness is relatively short. This results in large overall coordinated deformation of the overlying strata after mining, and rapid development, closure, and transmission of delamination, making it difficult to accurately time single-layer grouting. To address this, this invention proposes a multi-layer delamination grouting technology for mining-induced overlying strata. Firstly, by identifying the grouting layers in the overlying strata, the locations of multiple grouting layers are determined, thereby achieving precise grouting of the overlying strata. A schematic diagram of multi-layer delamination grouting mining is shown below. Figure 3 As shown.

[0115] An equivalent model for multi-layer grouting and backfilling mining of overburden layers is established, where the thickness of the high-level grouting and backfilling body (H) is... u When H(max) is 0, it is equivalent to single-layer overburden separation grouting; when H u When max>0, the high-level grouting delamination filling material can be projected onto the low-level grouting delamination zone, making the delamination zone space equivalent to an inverted trapezoid, and the boundaries of the filling material are all located directly above the edge of the working face direction, such as... Figure 4 As shown. Its relationship with the mining process parameters satisfies the following geometric relationship in engineering:

[0116] When H u When max>0, the separation space can be equivalent to an inverted trapezoid, and the boundary of the filling material is located directly above the edge of the working face width. It satisfies the following geometric relationship with the mining process parameters:

[0117]

[0118] In the formula, L b W is the length of the flat-bottomed collapse zone of the injection delamination zone; H is the width of the working face; bmax α is the thickness of the filling grout; L is the length of the working face; α is the injection-production ratio, which is generally taken as 0.35 to 0.45.

[0119] The thickness of the filling grout is then:

[0120]

[0121] In the formula, φ b To ensure full mining, the angle is defined as the angle between the edge of the lowest flat part of the filling body and the line connecting the edge of the coal seam above the working face in the strike direction, and is generally taken as 50° to 60°.

[0122] During grouting, the increased grouting pressure further expands the horizontal and vertical separation range of the grouting layer, leading to a corresponding increase in grouting volume. Simultaneously, the grouting pressure increases with the grouting volume, gradually compacting the goaf below the grouting separation zone and closing the pre-developed separation below it. Similarly, grouting also generates uplift force on the overlying strata; therefore, the settlement of the overlying strata above the high-level grouting separation zone can be considered equal to the surface settlement. Thus, the engineering solution for the surface settlement of the multi-level overlying separation grouting layer is:

[0123]

[0124] In the formula, H hmax The main controlling stratum subsidence; H cmin H represents the minimum residual thickness of the collapsed rock mass in the goaf area. k The height of the landslide zone; K p The residual fragmentation coefficient of the collapsed rock mass.

[0125] II. Deformation Mechanics Analysis of Protective Layer in Delamination Grouting Mining Based on Elastic Thin Plate Theory

[0126] In overburden separation grouting work at medium burial depth and large mining thickness, calculating the reasonable thickness and deformation of the protective layer and determining the stability of the protective layer are necessary conditions to ensure the safety, effectiveness and rationality of the overburden separation grouting layer. Based on the geological characteristics of the overburden, a calculation method is proposed as a necessary condition according to the elastic thin plate theory.

[0127] 2.1 Deflection Mechanical Model of Protective Rock Layer in Separation Grouting Mining

[0128] The protective layer is simplified to a single layer, which sinks, deforms, and comes into contact with the underlying rock strata. Based on the theory of elastic thin plates, a mechanical analysis of the protective layer deformation is performed. The following assumptions are made: the protective layer is in integral contact with the underlying rock strata; the load on the upper part of the protective layer is a uniformly distributed load; and the magnitude of the load is the grouting pressure (σ). f The supporting force under the protective layer is uniformly distributed; the rock strata that make up the protective layer deform in a coordinated manner as a whole, and the interlayer shear effect is ignored; the influence of complex geological structures such as fault weakening zones on the mechanical properties of the protective layer is ignored.

[0129] During the initial development of delamination, the upper and lower rock strata are supported by the surrounding rock strata, with each side rigidly fixed. An elastic composite rock slab with four sides fixed is selected as the mechanical calculation model for rock strata fracture, such as... Figure 5 As shown, the boundary conditions of the model are:

[0130]

[0131] The differential equation for the bending of a thin plate of protective rock strata is:

[0132]

[0133] In the formula, G = Eh 3 / 12(1-μ 2 ) represents the bending stiffness of a single thin plate, where E, μ, and h are the elastic modulus, Poisson's ratio, and thickness of the protective rock layer, respectively; q0 is the uniformly distributed lateral load on the protective rock layer, which can be expressed as:

[0134]

[0135] In the formula, λ is the support coefficient of the underlying rock strata of the protective layer, 0≤λ<1, and λ=0 indicates that there is no contact between the protective layer and the underlying rock strata; σ f The pressure at the grouting layer; P k The orifice pressure of the grouting hole gradually increases with grouting time and can be expressed as a function of time, i.e., P. k (Δt); γ f h f These are the unit weight of fly ash slurry and the depth of grouting layer, respectively; γ b For the bulk density of the protective layer.

[0136] Let the deflection function be:

[0137]

[0138] Given that the deflection function w satisfies the fixed plate boundary conditions, taking the first term satisfies the actual engineering requirements. Therefore, for a single thin plate subjected to grouting pressure and its own weight, the expression for the deflection of the protective rock layer is:

[0139]

[0140] In the formula, a and b are the strike length and dip width of the protective rock layer, respectively, and are expressed as:

[0141]

[0142]

[0143] In the formula, h i φ1 and φ2 are the fracture angles of the rock strata along the working face advancement direction, and φ3 and φ4 are the fracture angles of the rock strata along the working face width direction, generally taken as 70° to 85°.

[0144] 2.2 Criteria for Protective Layer Instability

[0145] The principal stress distribution of the protective layer rock slab under uniformly distributed load is as follows:

[0146]

[0147] Based on the relationship between principal stresses and stress components, the expressions for the maximum and minimum principal stresses of the neutral plane (z = h / 2) of the protective layer can be obtained:

[0148]

[0149]

[0150] Assuming shear failure of the rock strata, based on the Mohr-Coulomb criterion, the failure criterion for the rock strata is:

[0151]

[0152] In the formula, C and φ represent the cohesion and internal friction angle of the protective rock layer, respectively.

[0153] Substituting equation (17) into the Mohr-Coulomb criterion, we obtain the criterion equation for the failure of the protective layer. When F = 1, the protective layer breaks:

[0154]

[0155] To determine whether the thickness of the protective layer is safe and reasonable during mining, it is necessary to establish the allowable thickness range of the protective rock strata, namely:

[0156]

[0157] Therefore, for working faces with medium burial depth, large mining thickness, and "three soft" characteristics, when using fully mechanized caving for overburden separation grouting, the grouting layer should be determined according to the overburden separation grouting layer identification method, and then the safety and feasibility of the grouting layer should be judged according to the following formula:

[0158]

[0159] III. Calculation of the volume of the delamination filling space

[0160] Separation spaces generally exist at the interface between soft and hard rock layers. As the coal seam advances, the vertical deflection of soft and hard rock is inconsistent, and the separation is caused by this difference in deflection. The four sides of the separation are considered to be closed regions, such as... Figure 6 As shown. According to equation (12), the space w between the two plates is... b Height is:

[0161]

[0162] In the formula, q s and q x G represents the lateral uniformly distributed loads of the upper and lower rock strata of the filling space, respectively. s and G x These are the bending stiffnesses of the upper and lower rock strata filling the space, respectively.

[0163] Based on the consensus in thin-plate theory that the deflection is greatest at the center of the rock stratum (a / 2, b / 2), the maximum height of the grouting space is:

[0164]

[0165] The volume of a certain delamination grouting layer can then be expressed as:

[0166]

[0167] In the formula, w s w x These represent the deflection at various points on the upper and lower rock slabs, respectively.

[0168] When multi-layer grouting is used for overburden separation mining, the total volume of the grouting space is the sum of the volumes of each grouting space, which is:

[0169] V = ∑V i (25)

[0170] In engineering practice, based on actual conditions, the grouting pressure will exert an uplift force on the overlying rock strata during the grouting process. For ease of calculation, it is assumed that the subsidence of the main surface rock strata during the filling process is close to zero, combined with... Figure 3 Establish an equivalent model for the grouting space in the overlying delamination grouting filling operation, such as... Figure 7 As shown. Similarly, since the deflection is greatest at the center of the rock stratum (a / 2, b / 2), it can be known that H bmax =w bmax Substituting into equation (12), it can be expressed as:

[0171]

[0172] The engineering solution for filling the delamination space volume V can be calculated as follows:

[0173]

[0174] IV. Engineering Case Analysis

[0175] 4.1 Geological conditions

[0176] Peigou Coal Mine, located 35 km southwest of the city, is one of the main mines of Zhengzhou Coal Group Corporation. It covers an area of ​​approximately 190,000 m². 2The designed production capacity is 1.8 million tons per year. The main coal seam mined is No. 2-1 coal seam, with a simple structure and a thickness of 4–11 meters, averaging 7.5 meters. Its immediate roof is mudstone and sandy mudstone, with an average thickness of 6–7 meters; the main roof is medium-fine to medium-coarse-grained sandstone, with an average thickness of 7 meters; and the immediate floor is mudstone and sandy mudstone, with an average thickness of 10 meters. Based on the classification of the roof and floor of gently dipping coal seams, the No. 2-1 coal seam mined at Peigou Coal Mine is a typical "three-soft" unstable coal seam. The mine adopts a multi-level vertical shaft development method with zonal exhaust ventilation. The entire mining area is located within Laiji Town, with a main topographic feature of high elevations in the north, west, and south, and low elevations in the east, forming a hilly terrain with gentle slopes. Surface gullies are well-developed, and most rivers are seasonal streams. The coal seams in this mining area belong to the Carboniferous and Carboniferous-Permian coal series, and the surface is completely covered by Quaternary loess. The mining area is cut by strike-southeast faults.

[0177] The 22151 working face of Peigou Coal Mine was selected as the research area for overburden separation grouting. The original coal seam of the No. 2 coal seam is generally thick and relatively stable, with localized interbedded rock at the bottom of the coal seam near the cutting roadway in the west. The working face has a strike length of approximately 300m and a dip length of approximately 106m. The surface elevation of the working face ranges from +220.1m to +226.4m, and the working face elevation ranges from -60.5m to -97m, with an average burial depth of 302m. The average coal seam thickness is approximately 7.3m, and the average dip angle is approximately 12°. The working face employs a fully mechanized top-coal caving mining method, with natural caving for roof management. The surface area mainly consists of villages, houses, and farmland. The Wangguan Highway runs north-south through the area, with relatively dense buildings on both sides. According to the site survey, most buildings on both sides of the highway are 1-2 story brick-concrete structures, with a few 3-5 story buildings, as well as some important buildings such as gas stations and hospitals.

[0178] 4.2 Feasibility Prediction of Overburden Separation Grouting Coal Mining at the Working Face

[0179] The overlying strata of the 22151 working face in Peigou Coal Mine mainly consist of weak rock layers such as sandy mudstone and mudstone, and hard rock layers such as medium-grained sandstone and fine-grained sandstone. The ratio of weak to hard rock layers in the overlying strata is 1.59:1. Based on the lithology and thickness of each overlying stratum, and referring to the stratified lithology evaluation coefficient table, the comprehensive evaluation coefficient of the overlying strata is calculated to be P = 0.55, and the influence coefficient of the overlying strata lithology is D = 1.73. Therefore, the comprehensive lithology evaluation of the overlying strata of the working face is medium-hard.

[0180] Based on the calculation formula for the height of the water-conducting fracture zone in thick coal seam mining under hard conditions in Table 1, the approximate development height of the water-conducting fracture zone is calculated to be 81.4–152 m. According to the average coal seam thickness of adjacent working faces provided by the mine, which is 8.1 m, the height of the water-conducting fracture zone caused by working face mining is 117.7 m, i.e., the fracture-to-mining ratio is 14.5. Combining the coal seam mining thickness of the 22151 working face (0–13.17 m, average thickness 7.1 m), empirical formulas, and field measurement data, the height of the water-conducting fracture zone in the 22151 working face is determined to be 103–152 m. Since the working face mining along the strike has not reached full mining activity, the midpoint of this range is selected as the final height of the water-conducting fracture zone, which is 127.5 m.

[0181] The modified method was used to determine the location of the top strata delamination, eliminating delamination spaces within the water-conducting fracture zone that could not be sealed. Table 2 shows the delamination results of the rock strata above the water-conducting fracture zone. The grouting layer closest to the top of the water-conducting fracture zone was selected for analysis first.

[0182] Taking rock stratum number 1 as the first layer of the composite beam, the calculation results show that:

[0183] max(q1)1,(q2)1...(q 24 )1=max(q5)1=203.76Kpa;

[0184] Delamination occurs between rock layers 5 and 6. Then, rock layer 6 is used as the first layer of the composite beam for further delamination calculations. The results show that:

[0185] max(q6)6,(q7)6...(q 24 )6=max(q8)6=363.15Kpa;

[0186] A delamination occurs between rock layers No. 8 and No. 9. Further calculations of the delamination are performed using rock layer No. 9 as the first layer of the composite beam. The results show that:

[0187] max(q9)9,(q9)9...(q 24 )9=max(q 24 )9 = 595.35 kPa;

[0188] Based on the calculation results, it was ultimately determined that multi-layer excavation grouting would be carried out from the 9th layer of fine-grained sandstone to the 5th layer of mudstone. If grouting is carried out at this location, the protective layer thickness will be 21.02m.

[0189] Based on the actual on-site grouting conditions, the orifice pressure P k The pressure is 1.0–1.2 MPa, so P is selected. k =1.1MPa; fly ash slurry bulk density γ f Based on a concentration of 70%, γf =15KN / m 3 Grouting layer depth h f =149.8m. The other calculation parameters of the protective layer in Equation (20) are determined based on basic physical and mechanical tests of rock, field monitoring and engineering experience. Table 3 gives the average values ​​of each parameter of the protective layer.

[0190] Substituting the above parameters into equation (20) yields:

[0191]

[0192] Based on the feasibility assessment conditions for overburden separation grouting in working faces with medium burial depth and large mining thickness, it can be predicted that: when the mining thickness of the 22151 working face in Peigou Coal Mine is 7.3m, the protective layer with a thickness of 25.2m can safely implement the overburden separation grouting work.

[0193] 4.3 Estimated Grouting Volume for Overburden Separation at the Working Face

[0194] To verify the accuracy of the delamination location in multi-layer delamination grouting of overburden, additional high-level grouting boreholes were drilled, and the grouting location was identified through borehole inspection. The borehole inspection results showed that fly ash slurry existed beneath the fine-grained sandstone at a distance of 168.6 m from the coal seam, verifying the feasibility of the multi-layer delamination grouting design.

[0195] Based on equations (23), (24), and (25), and Table 2, combined with the geological conditions, the theoretical value of the maximum total filling volume that can be formed within the post-mining grouting layer of working face 22151 can be approximately calculated to be 6.3 × 10⁻⁶. 4 m 3 Where a and b are equivalent to L and W, respectively. According to equation (27), combined with Table 2, the maximum total filling volume that can be formed in the post-mining grouting layer of the 22151 working face is approximately calculated to be 9.8 × 10⁻⁶. 4 m 3 , where H bmax Approximately 5.06m, with H = 152.7m, φ b =60°. The on-site slurry concentration must be greater than 70%, γ f The value is 15 KN / m³, and the fly ash consumption for the two calculation methods is approximately 9.5 × 10⁻⁶ kN / m³. 4 tons and 14.7 × 10 4 The actual grouting volume was approximately 13.3 × 10 tons. 4 The injection-to-production ratio is approximately 0.41. Therefore, based on the predicted grouting space volume derived from the thin-plate theory and the calculation assumptions of the engineering solution, the two can be used as the minimum and maximum total volume of the grouting and filling space for overburden separation in medium-depth, thick, and "three-soft" coal seams.

[0196] Table 2 Location of delamination in the top slab

[0197]

[0198]

[0199] Table 3 Calculation parameters for protective layer thickness

[0200]

[0201]

[0202] 4.4 Effect of Multi-Layer Separation Grouting on Surface Settlement Control

[0203] The probability integral method was used to calculate the surface movement and deformation of the mining area of ​​the 12051 working face. Based on the "Probability Integral Method Predicted Parameters of Major Mines in a Certain City" in "Subsidence Law and Application of Mining in 'Three-Soft' Coal Seams", the probability integral method calculation parameters suitable for the 22151 working face were obtained through comprehensive analysis, as shown in Table 4. Figure 8 This is a contour map of the subsidence. According to the calculation results, if the 22151 working face does not adopt overburden separation grouting, the maximum surface subsidence value is 3378.9 mm, the surface movement and deformation is relatively serious, and the maximum surface movement and deformation value reaches 20 mm / m. Without overburden separation grouting, the maximum damage level of the houses near the working face reaches level IV, and demolition and reconstruction measures are required.

[0204] Table 4. Calculation parameters of the surface movement probability integral method for working face 22151

[0205] 0.78 0 2.2 85° 0.34

[0206] To evaluate the settlement reduction effect of grouting in the multi-layered overburden separation layer, surface displacement was observed three times a week, for a total of 86 observations. Based on the last observation, the settlement coefficient q was determined to be 0.147 using curve fitting in Matlab. Figure 9 As can be seen, the measured surface deformation values ​​and the fitting results have a high degree of fit, indicating that the obtained parameters are relatively accurate. Figure 10 Contour maps of grouting mining in multi-layered overburden separation layers are provided.

[0207] Based on on-site measurements, after multi-layer grouting extraction of the overlying strata, the maximum surface subsidence was 649.8 mm. The maximum horizontal deformation at most residential locations was within the Class I damage range, while the surface deformation at a few residential locations exceeded Class I, but was controlled within Class II damage. A comparison of the predicted surface subsidence values ​​from the two extraction schemes shows (e.g.) Figure 11 As shown in the figure, the surface subsidence reduction rate is approximately 80.77%, which has a significant effect on controlling surface subsidence caused by mining and protecting surface structures.

[0208] V. Conclusion

[0209] (1) Under the geological conditions of large interaction between soft and hard overlying strata and small strata thickness, this invention proposes a multi-layer precision grouting technology for overlying strata in thick coal seams with large overall coordinated deformation, difficulty in generating large-sized delamination spaces, and low delamination grouting filling rate during the fully mechanized mining of thick coal seams with "three soft" conditions.

[0210] (2) Based on the composite beam theory and borehole inspection, this invention determined and verified the grouting location of the multi-layered overburden separation layer; and through the elastic thin plate theory, it clarified the reasonable and safe thickness of the protective layer. The results show that multi-layered overburden separation layer grouting was successfully implemented between the bottom of the medium-grained sandstone at 152.6m from the coal seam and the bottom of the fine-grained sandstone at 168.6m from the coal seam; when using overburden separation layer grouting in the 22151 working face of Peigou Coal Mine, the thickness of the protective layer must be greater than 16.5m.

[0211] (3) The predicted grouting space volume derived from the thin-plate theory and the calculation assumptions of the engineering solution in this invention can be used as the minimum and maximum total volume of grouting and filling space for coal seams with medium burial depth, large mining thickness, and "three soft" overburden separation. The theoretically calculated grouting volume is approximately 9.5 × 10⁻⁶. 4 ~14.7×10 4 The actual grouting volume for multi-layer excavation grouting of overburden is approximately 13.3 × 10⁻⁶ tons. 4 The injection-to-production ratio is approximately 0.41.

[0212] (4) After the multi-layer delamination grouting method of the overlying strata was adopted in this invention, the maximum surface subsidence was 649.8 mm, the subsidence coefficient was 0.147, and the maximum horizontal deformation value of the surface at most residential sites was within the level I damage range. The surface subsidence control effect of multi-layer delamination grouting method of the overlying strata was significant, with a subsidence reduction rate of approximately 80.77%.

Claims

1. A method for reducing settlement by grouting multiple layers of overburden in the mining of thick, soft coal seams, characterized in that: Includes the following steps: S1. Based on the composite beam theory, identify the delamination grouting layer and calculate the filling grout thickness of the multi-layer delamination grouting in the overburden. S2. Using the theory of elastic thin plates, establish a mechanical model of the grouting protective layer for overburden separation and determine the safe thickness of the protective layer; S3. Construct an equivalent model of the grouting space for backfilling mining to obtain the volume of the grouting space; Step S2 includes the following steps: S21. Establish a mechanical model of the overburden separation grouting protective layer to calculate the deflection of the protective layer; S22. Analyze and calculate the safe thickness of the protective layer and determine whether the grouting layer is safe and feasible; Step S21 specifically includes the following steps: S211. The mechanical model of the overburden delamination grouting protective layer is obtained through analysis, and its calculation formula is as follows: ； In the formula, G=Eh 3 / 12(1-μ 2 ) represents the bending stiffness of a single thin plate, where E, μ, and h are the elastic modulus, Poisson's ratio, and thickness of the protective rock layer, respectively; q0 is the uniformly distributed transverse load of the protective rock layer, and the formula for calculating q0 is: ； In the formula, λ is the support coefficient of the underlying rock strata, 0≤λ<1, and λ=0 indicates that there is no contact between the protective rock strata and the underlying rock strata; σ f The pressure at the grouting layer; P k The orifice pressure of the grouting hole gradually increases with grouting time and can be expressed as a function of time, i.e., P. k (Δt); γ f h f These are the unit weight of fly ash slurry and the depth of grouting layer, respectively; γ b For the density of the protective layer; S212. Let the formula for calculating the deflection function be: ； S213. The formula for calculating the deflection of the protective layer is: ； In the formula, a and b are the strike length and dip width of the protective layer, respectively, and their calculation formula is: ； ； In the formula, h i The thickness of the i-th rock layer; the n-th layer is the protective rock layer; The fracture angle of the rock strata along the direction of the working face advance. and It is the fracture angle of the rock strata along the width of the working face.

2. The method for reducing settlement by grouting multiple layers of overburden in the mining of three-soft, thick coal seams as described in claim 1, characterized in that: Step S1 includes the following steps: S11. Determine the height range of the grouting location for overburden separation; S12. Determine the spatial location of the grout separation layer; S13. Determine the thickness of the filling grout.

3. The method for reducing settlement by grouting in the overlying multi-layer separation layer of a three-soft, thick coal seam as described in claim 2, characterized in that: In step S11, the formula for calculating the height range of the overburden separation grouting location is as follows: ； In the formula, H is the distance from the grouting location to the coal seam. d H is the height of the water-conducting fracture zone. f M represents the thickness of the waterproof and safe coal-rock pillar, and M represents the thickness of the coal seam.

4. The method for reducing settlement by grouting in the overlying multi-layer separation layer of a three-soft, thick coal seam as described in claim 3, characterized in that: Step S12 specifically includes the following steps: S121. In an overburden containing m layers of rock, when adjacent layers 1 and n can be in close contact and deform synchronously, considering an n-layer composite beam, the load calculation formula for the i-th layer is: ； In the formula, E i h i and γ i These are the elastic modulus, thickness, and volumetric weight of each rock layer; S122, when the calculated (q) n+1 ) i Less than (q) n ) i When this happens, layer (n+1) will separate from layer n; the calculation formula is: ； S123, after simplification: ； S124, when (q) m )1=max((q1)1,(q2)1,…,(q n )1), then a separation occurs between the (m+1)th layer and the mth layer; when (q n )1=max((q1)1,(q2)1,…,(q n If 1), then no separation layer will be generated between the 1st layer and the nth layer.

5. The method for reducing settlement by grouting in the overlying multi-layer separation layer of a three-soft, thick coal seam as described in claim 4, characterized in that: Step S13 specifically includes the following steps: S131. The geometric relationship between the filling grout thickness and the mining process parameters is obtained through analysis. The calculation formula is as follows: ； In the formula, L b W is the length of the flat-bottomed collapse zone of the injection delamination zone; H is the width of the working face; bmax L represents the thickness of the filling grout; L represents the length of the working face. For injection-production ratio; S132. The formula for calculating the thickness of the filling grout is: ； In the formula, To fully utilize the angle.

6. The method for reducing settlement by grouting multiple layers of overburden in the mining of three-soft, thick coal seams as described in claim 1, characterized in that: Step S22 specifically includes the following steps: S221. The formula for calculating the principal stress distribution of the protective layer rock slab under uniformly distributed load is: ； S222. Based on the relationship between principal stress and stress components, the formulas for calculating the maximum and minimum principal stresses on the neutral surface of the protective layer are as follows: ； ； S223. Based on the Mohr-Coulomb criterion, the calculation formula for the rock stratum failure criterion is as follows: ； In the formula, C, These are the cohesion and internal friction angle of the protective rock layer, respectively. S224. The calculation formula for the damage and failure criterion of the protective layer rock is as follows: ； When F=1, the protective layer breaks; S225. The formula for calculating the safe thickness range of the protective layer is: ； S226. The formula for determining whether a grouting layer is safe is: 。 7. The method for reducing settlement by grouting multiple layers of overburden in the mining of three-soft, thick coal seams as described in claim 1, characterized in that: Step S3 includes the following steps: S31. The formula for calculating the space height between two delamination slabs is: ； In the formula, q s and q x G represents the lateral uniformly distributed loads of the upper and lower rock strata of the filling space, respectively. s and G x These are the bending stiffnesses of the upper and lower rock strata of the filling space, respectively. S32. Based on the consensus in thin-plate theory that the deflection is greatest at the center of rock strata, the formula for calculating the maximum height of the grouting space is: ； S33. The formula for calculating the volume of a certain delamination grouting layer is as follows: ； In the formula, w s w x These represent the deflection at various points on the upper and lower rock slabs, respectively. S34. The total volume of the grouting space is the sum of the volumes of all grouting spaces, and its calculation formula is as follows: 。

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