Calculation method for caving zone height of nearly-horizontal thick coal seam mining

Abstract

The invention discloses a method for calculating the height of a caving zone in mining of a nearly horizontal thick coal seam, and belongs to the technical field of coal seam mining. The thickness and mechanical parameters of an overlying rock stratum and weak plane parameters of an over-thick rock stratum are obtained through a drill hole histogram and an indoor test, and a weak plane correction coefficient is introduced from the immediate roof of the coal seam to optimize a three-hinged arch stability discrimination equation; the free space height is dynamically updated by combining rock stratum breaking expansion characteristics, rock stratum stability is judged layer by layer from bottom to top, and finally the caving zone height is determined. By introducing the weak plane correction coefficient, the weak plane factor of the over-thick rock stratum is brought into theoretical calculation for the first time, the problem of prediction deviation caused by neglecting the weak plane in the existing method is solved, the free space height is dynamically updated in combination with the rock stratum breaking expansion characteristic, and the accuracy of stability judgment is further ensured.

Description

Technical Field

[0001] This invention belongs to the field of coal seam mining technology, specifically relating to a method for calculating the height of the caving zone in near-horizontal thick coal seam mining. Background Technology

[0002] Coal, as a crucial component of China's energy framework, plays a dominant role in primary energy consumption. However, during coal mining, the overlying strata gradually bend, shift, and eventually break down, forming caving zones, fracture zones, and flexural subsidence zones. Among these, the caving zones are closest to the coal seam and suffer the most severe damage. They are the primary target areas for projects such as grouting for subsidence reduction and gas drainage, and also the main channels for water inrush from the mine roof. Accurately predicting the height of caving zones is of great significance for ecological environmental protection and safe production.

[0003] Currently, the main methods for predicting the height of caving zones include empirical formulas, numerical and physical simulations, and machine learning. However, most of these methods do not consider the influence of weak surfaces inside thick rock layers. During the sedimentary diagenesis period, thick rock layers are prone to forming weak surfaces such as horizontal joints or weak cementation surfaces. Under the influence of mining, these weak surfaces will expand, causing the rock layers to separate. The lower layer will collapse while the upper layer will remain hinged and stable. If the weak surface factor is ignored, the stability judgment of the three-hinged arch will be inaccurate, which will lead to a large deviation between the predicted height of the caving zone and the actual height. Summary of the Invention

[0004] The purpose of this invention is to provide a method for calculating the height of the caving zone in near-horizontal thick coal seam mining, so as to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for calculating the height of the caving zone in near-horizontal thick coal seam mining, comprising the following steps:

[0006] S1. Parameter acquisition: Acquire the basic parameters of the overlying rock layer and the weak surface parameters of the excessively thick rock layer on the working surface. The basic parameters include the thickness h of each rock layer, the uniaxial compressive strength σc, the uniaxial tensile strength σt, and the fracture expansion coefficient. The excessively thick rock layer is a rock layer with a thickness greater than 20m. Its weak surface parameters include the weak surface spacing d and the weak surface dip angle θ.

[0007] S2. Determination of weak surface correction coefficient: The weak surface correction coefficient λ is determined based on the weak surface parameters of the excessively thick rock layer. The more developed the weak surface, the larger the value of λ, and the value range is 1.0 to 1.5.

[0008] S3. Initial stability assessment: Starting from the immediate roof of the coal seam, the stability of the three-hinged arch structure formed after the fracturing of each rock layer is assessed sequentially from bottom to top. λ and the basic parameters are substituted into the optimized three-hinged arch stability discrimination equation. If the discrimination equation ≤ 0, the structure is unstable and the rock layer belongs to the collapse zone. If the discrimination equation > 0, the structure is stable.

[0009] S4. Free space height update: When the lower rock layer becomes unstable and collapses, the free space height Si below the upper rock layer is dynamically updated according to the thickness and fracture expansion coefficient of the collapsed rock layer.

[0010] S5. Layer-by-layer stability assessment: Treat the updated Si as the settlement value Δ of the upper rock layer three-hinged arch structure, repeat steps S2 and S3, and assess the stability of each upper rock layer from bottom to top until a stable rock layer is found.

[0011] S6. Calculation of caving zone height: Determine the vertical distance from the top of the coal seam to the bottom of the stable rock layer found in S5. This distance is the caving zone height H. If the stable rock layer is part of an excessively thick rock layer, then calculate the vertical distance from the top of the coal seam to the top of the collapsed part of the excessively thick rock layer as the caving zone height.

[0012] Preferably, the basic parameters are obtained as follows: the thickness of the rock strata is determined based on the columnar section of the surface boreholes above the working face provided by the mine. If there is historical data on uniaxial compressive strength and uniaxial tensile strength, it is extracted directly. If not, fresh rock cores are drilled to make standard rock samples. σc is determined by indoor uniaxial compressive strength test and σt is determined by Brazilian splitting test.

[0013] Preferably, the weak surface parameters of the excessively thick rock layer are obtained by scanning the excessively thick rock layer using borehole imaging technology or conducting on-site geological surveys, recording the weak surface spacing and dip angle, and selecting 3 to 5 observation points at the location of the excessively thick rock layer exposed in the downhole working face to count the number of weak surfaces per unit length, i.e., the weak surface development density.

[0014] Preferably, the crushing expansion coefficient is obtained by means of indoor crushing test. If the test conditions are limited, the empirical value of similar lithology can be referred to, wherein 1.060 is taken for coarse sandstone and medium sandstone, 1.045 is taken for fine sandstone and siltstone, and 1.025 is taken for mudstone and sandy mudstone.

[0015] Preferably, the specific value rule for the weak surface correction coefficient is as follows: when the weak surface spacing d ≤ 0.5m, θ is between 30° and 60° and the development density is ≥ 5 lines / m, λ is taken as 1.4 to 1.5;

[0016] When 0.5m<d≤1.0m, θ is between 15° and 30° or between 60° and 75° and the development density is 3 to 5 lines / m, λ is taken as 1.2 to 1.3;

[0017] When d > 1.0m, θ < 15° or θ > 75° and the development density < 3 lines / m, λ is taken as 1.1 to 1.15;

[0018] When the density of weak surfaces is less than 1 line / m, λ is taken as 1.0.

[0019] Preferably, the optimized stability criterion equation for the three-hinged arch is derived based on the moment equilibrium equation of the Voussoir beam model, and the expression is as follows: Where δ is the ratio of the settlement value of the three-hinged arch structure to the thickness of the rock stratum, and N is the ratio of the uniaxial compressive strength to the uniaxial tensile strength.

[0020] Preferably, step S3 further includes verifying the initial fracture distance of the rock strata: using an empirical formula. Where q is the self-weight stress of the rock stratum, q=γh, and γ is the unit weight of the rock stratum, taken as 25kN / m³. Calculate the initial fracture distance L. When the deviation between L and the actual advance distance of the working face is ≤10%, it is confirmed that the formation conditions of the three-hinged arch structure are reasonable.

[0021] Preferably, the free space height Where M is the coal seam mining thickness, n is the number of collapsed rock layers, hj is the thickness of the j-th collapsed rock layer, and kj is the fracture expansion coefficient of the j-th collapsed rock layer. This represents the summation of relevant parameters for all collapsed lower rock strata, with the initial value of Si equal to M when the immediate top is first determined.

[0022] Preferably, if multiple rock layers collapse simultaneously in step S3, the impact of each rock layer on the free space height needs to be calculated sequentially according to the collapse order. That is, first calculate the free space height Si1 corresponding to the lowest collapsed rock layer, and then calculate the free space height Si2 corresponding to the next collapsed rock layer based on Si1, until the free space height of all collapsed rock layers is updated.

[0023] Preferably, the method also includes a correction calculation step: when the judgment result of step S3 does not match the actual situation on site, let f(δ) = 0, substitute the current Si as Δ, and calculate the rock layer thickness h' that satisfies the equation. h' is the actual collapse thickness of the rock layer and is included in the collapse zone.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] 1. This invention introduces a weak surface correction coefficient, which for the first time incorporates the weak surface factor of excessively thick rock layers into theoretical calculations, solving the prediction bias problem caused by neglecting weak surfaces in existing methods. Combined with the dynamic updating of free space height based on the rock layer fracture and expansion characteristics, it further ensures the accuracy of stability judgment.

[0026] 2. This invention does not rely on empirical data from specific regions. It obtains parameters through conventional methods such as borehole columnar sections and laboratory tests. It is applicable to near-horizontal thick coal seams with different geological conditions in North China and Northwest China, breaking through the regional limitations of existing empirical formulas. At the same time, it sets up special processing rules for complex scenarios such as the layered collapse of excessively thick rock strata and the simultaneous collapse of multiple rock strata, further expanding the applicable scenarios of the method.

[0027] 3. All parameters required by this invention can be obtained through conventional mine drilling and laboratory tests, without the need for expensive numerical simulation software or physical simulation equipment. The calculation process involves only basic mathematical operations, which can be mastered by on-site technicians after 1 to 2 days of training, facilitating its application in various types of mines and significantly reducing engineering application costs. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the process of the present invention;

[0029] Figure 2 This is a schematic diagram of the three-hinged arch structure of the present invention. Detailed Implementation

[0030] 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, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Please see Figure 1 and Figure 2 This invention provides a method for calculating the height of the caving zone in near-horizontal thick coal seam mining, comprising the following steps:

[0032] S1. Parameter Acquisition: Acquire the basic parameters of the overlying strata and the weak surface parameters of the excessively thick strata on the working face. The basic parameters include the thickness h of each stratum, uniaxial compressive strength σc, uniaxial tensile strength σt, and fracture expansion coefficient. The excessively thick strata are those with a thickness greater than 20m, and their weak surface parameters include the weak surface spacing d and the weak surface dip angle θ. The basic parameters are acquired by determining the stratum thickness based on the columnar section of the boreholes above the working face provided by the mine. If historical data on uniaxial compressive and tensile strength is available, it is directly extracted. Otherwise, fresh core samples are drilled to prepare standard rock samples. σc is determined by indoor uniaxial compressive strength testing, and σt is determined by Brazilian splitting tests. The weak surface parameters of the excessively thick strata are acquired by scanning the excessively thick strata using borehole imaging technology or conducting on-site geological surveys, recording the weak surface spacing and dip angle. Simultaneously, at the location of the excessively thick strata exposed in the underground working face, select 3 to 5 observation points and count the number of weak surfaces per unit length, i.e., the weak surface development density. The fracture expansion coefficient is obtained by indoor fracture test. If the test conditions are limited, the empirical value of similar lithology can be referred to. The coefficient is 1.060 for coarse sandstone and medium sandstone, 1.045 for fine sandstone and siltstone, and 1.025 for mudstone and sandy mudstone.

[0033] Further details regarding the acquisition of basic parameters: The rock stratum thickness *h* is determined based on the columnar section of boreholes exposed above the working face provided by the mine. The elevations of the top and bottom plates of each rock stratum must be marked, and the vertical thickness calculated. If borehole data is missing, one or two verification boreholes should be constructed to cover key locations along the strike and dip of the working face to ensure the completeness and accuracy of the thickness data. Uniaxial compressive strength and uniaxial tensile strength: Historical mine test data should be used preferentially. If historical data is unavailable, fresh core samples of the corresponding rock strata should be drilled according to the requirements of "Methods for Determination of Physical and Mechanical Properties of Coal and Rock" (GB / T23561-2022). Standard samples should be prepared, and uniaxial compressive strength tests should be conducted using an MTS electro-hydraulic servo pressure testing machine. The uniaxial compressive strength should be directly read. Uniaxial tensile strength should be calculated using the Brazilian splitting test. Each test group should be conducted in at least three parallel trials, and the average value should be used as the final parameter to reduce experimental errors. The fracture expansion coefficient is determined according to the "Specifications for Hydrogeological, Engineering Geological and Environmental Geological Exploration of Coal Deposits" (GB / T12719-2022). After crushing the aforementioned standard rock sample, the ratio of the loose volume to the original rock volume is measured using a graduated cylinder; this is the fracture expansion coefficient. If field testing conditions are limited, empirical values ​​for similar lithologies can be referenced, but adjustments must be made based on actual measurement results during subsequent post-mining verification to ensure parameter accuracy. Details for obtaining weak surface parameters in excessively thick rock strata: The YCS40 mining ultrasonic imaging logging tool is used to scan the excessively thick rock strata section to obtain the weak surface spacing and dip angle. At the location of the excessively thick rock strata exposed in the underground working face, 3 to 5 observation points are selected, each with a range of 1m × 1m. The number of weak surfaces per unit length is counted, which is the weak surface development density. The obtained weak surface parameters are then screened for validity, eliminating abnormal data caused by borehole interference or observation errors, such as abnormal values ​​with weak surface spacing less than 0.05m, to ensure that the parameters accurately reflect the development of weak surfaces in excessively thick rock strata.

[0034] S2. Determination of Weak Surface Correction Coefficient: The weak surface correction coefficient λ is determined based on the weak surface parameters of excessively thick rock layers. The more developed the weak surface, the larger the value of λ, ranging from 1.0 to 1.5. The specific rules for determining the weak surface correction coefficient are as follows: When the weak surface spacing d ≤ 0.5m, θ is between 30° and 60°, and the development density is ≥ 5 surfaces / m, λ is 1.4 to 1.5. When 0.5m < d ≤ 1.0m, θ is between 15° and 30° or between 60° and 75°, and the development density is 3 to 5 surfaces / m, λ is 1.2 to 1.3. When d > 1.0m, θ < 15° or θ > 75°, and the development density is < 3 surfaces / m, λ is 1.1 to 1.15. When the weak surface development density is < 1 surface / m, λ is 1.0.

[0035] Furthermore, when determining the value, it is necessary to comprehensively judge based on the actual geological characteristics of the excessively thick rock layer. For example, if the weak surface spacing d=0.4m, θ=45°, and development density of 6 surfaces / m in an excessively thick rock layer, and field observations show that the weak surfaces are mostly continuous joints, then λ should be taken as the upper limit of 1.5 to fully consider the weakening effect of the weak surfaces. If the weak surface is a non-continuous cemented surface, the value of λ can be appropriately reduced.

[0036] S3. Preliminary Stability Assessment: Starting from the immediate roof of the coal seam, the stability of the three-hinged arch structure formed after the fracturing of each rock stratum is assessed sequentially from bottom to top. λ and the foundation parameters are substituted into the optimized stability discrimination equation for the three-hinged arch. If the discrimination equation ≤ 0, the structure is unstable, and the rock stratum belongs to the caving zone. If the discrimination equation > 0, the structure is stable. The optimized stability discrimination equation for the three-hinged arch is derived based on the moment equilibrium equation of the Voussoir beam model, and its expression is: Where δ is the ratio of the settlement value of the three-hinged arch structure to the thickness of the rock stratum, and N is the ratio of the uniaxial compressive strength to the uniaxial tensile strength.

[0037] It also includes verification of the initial fracture distance of the rock strata: through empirical formulas. Where q is the self-weight stress of the rock stratum, q=γh, and γ is the unit weight of the rock stratum, taken as 25kN / m³. Calculate the initial fracture distance L. When the deviation between L and the actual advance distance of the working face is ≤10%, it is confirmed that the formation conditions of the three-hinged arch structure are reasonable.

[0038] If multiple rock layers collapse simultaneously, the impact of each rock layer on the free space height must be calculated sequentially according to the order of collapse. That is, first calculate the free space height Si1 corresponding to the lowest collapsed rock layer, then calculate the free space height Si2 corresponding to the next collapsed rock layer based on Si1, until the free space height of all collapsed rock layers is updated.

[0039] When the judgment result does not match the actual situation on site, let f(δ) = 0, substitute the current Si as Δ, and calculate the rock layer thickness h' that satisfies the equation. h' is the actual collapse thickness of the rock layer, which is included in the collapse zone.

[0040] Furthermore, the formation principle of the three-hinged arch structure is as follows: After the coal seam is mined out, the immediate roof stratum loses its lower support, forming a suspended beam structure. When the span of the beam reaches the initial breakage distance, the beam breaks under the action of bending moment, and the two ends form hinge points with the intact rock strata around the goaf, and the middle forms a hinge point, ultimately forming a three-hinged arch structure. This structure can transfer loads through its own hinges and has temporary bearing capacity.

[0041] The optimized stability discrimination equation for the three-hinged arch is derived by introducing the weak surface correction coefficient λ based on the moment equilibrium equation of the Voussoir beam model.

[0042] When f(δ)≤0, the three-hinged arch structure cannot withstand its own weight and the upper load, and will become unstable and collapse. This rock layer belongs to the collapse zone.

[0043] When f(δ) > 0, the three-hinged arch structure can transfer the load through the hinge and remain stable, and the rock stratum does not belong to the collapse zone.

[0044] Verification of the initial fracture distance of the rock strata: To ensure the rationality of the formation conditions of the three-hinged arch structure, the initial fracture distance L of the rock strata is calculated using empirical formulas for auxiliary verification. When the deviation between the calculated initial fracture distance L and the actual advance distance of the working face is ≤10%, the formation conditions of the three-hinged arch structure are confirmed to be reasonable, and further judgment can proceed. If the deviation exceeds 10%, the accuracy of the rock strata mechanical parameters needs to be rechecked, or the weak surface correction coefficient needs to be adjusted until the deviation meets the requirements.

[0045] Handling simultaneous collapse of multiple rock layers: If on-site observation or geological analysis reveals simultaneous collapse of multiple rock layers, such as when the immediate top and the 1 to 2 overlying thin rock layers break simultaneously due to weak surface connectivity, the impact of each rock layer on the free space height must be calculated sequentially according to the collapse order. That is, first calculate the free space height corresponding to the lowest collapsed rock layer, and then calculate the free space height corresponding to the next collapsed rock layer based on it, until the free space height of all collapsed rock layers is updated, to avoid calculation deviations caused by ignoring simultaneous collapse of multiple layers.

[0046] When the judgment results contradict the actual situation on site, such as air suction or stuck drill pipe phenomena, increased water seepage from the roof of the working face, or discrepancies between the actual collapse range of the rock strata and the judgment results found in post-mining core observations, a corrective calculation must be performed to ensure the accuracy of the results.

[0047] For example, if a thick rock stratum is judged to be stable, but on-site drilling reveals a 15m collapse zone at the bottom of the stratum, let f(δ) = 0, substitute it into Δ = Si = 6.2m, and solve for Δ = 0.32. Then the actual collapse thickness h' = 6.2 / 0.32 = 19.38m, which is basically consistent with the actual collapse range on site. The height of the collapse zone is corrected accordingly.

[0048] S4. Free Space Height Update: When the underlying rock layer becomes unstable and collapses, the free space height Si below the upper rock layer is dynamically updated based on the thickness and fracture expansion coefficient of the collapsed rock layer. Where M is the coal seam mining thickness, n is the number of collapsed rock layers, hj is the thickness of the j-th collapsed rock layer, and kj is the fracture expansion coefficient of the j-th collapsed rock layer. This represents the summation of relevant parameters for all collapsed lower rock strata, with the initial value of Si equal to M when the immediate top is first determined.

[0049] Furthermore, when the stability of the immediate roof strata was initially assessed, no strata had collapsed, and the initial value of the free space height was equal to the coal seam mining thickness.

[0050] For example: In a certain working face, the coal seam thickness is M=7.3m, and the collapsed rock strata are direct top sandstone with h1=18.56m and k1=1.060. Then the free space height Si=7.3-18.56×(1.060-1)=7.3-1.114=6.186m. This value is the settlement value of the three-hinged arch when judging the stability of the overlying coarse sandstone strata.

[0051] S5. Layer-by-layer stability assessment: Treat the updated Si as the settlement value Δ of the upper rock layer three-hinged arch structure, repeat steps S2 and S3, and assess the stability of each upper rock layer from bottom to top until a stable rock layer is found.

[0052] Furthermore, the updated free space height in step S4 is considered as the settlement value of the upper rock strata's three-hinged arch structure. Steps S2 and S3 are repeated to determine the stability of each upper rock stratum sequentially from bottom to top. If the upper rock stratum is determined to be unstable, it is marked as a collapse zone, and step S4 is executed again to update the free space height, continuing to determine the uppermost rock strata. If the upper rock stratum is determined to be stable, the determination stops, and this rock stratum is the first stable rock stratum above the collapse zone. During the determination process, if an excessively thick rock stratum is encountered, its weak surface correction coefficient needs to be re-determined according to the rules in step S2 to avoid determination errors caused by ignoring the differences in weak surfaces of different excessively thick rock strata.

[0053] S6. Calculation of caving zone height: Determine the vertical distance from the top of the coal seam to the bottom of the stable rock layer found in S5. This distance is the caving zone height H. If the stable rock layer is part of an excessively thick rock layer, then calculate the vertical distance from the top of the coal seam to the top of the collapsed part of the excessively thick rock layer as the caving zone height.

[0054] Furthermore, based on the location of the stable rock strata found in step S5, and combined with the borehole columnar section, the height H of the collapse zone is determined, specifically in two cases:

[0055] Under normal circumstances (stable rock strata are complete and not excessively thick): Read the elevation H of the coal seam roof. 煤顶 Elevation H of the bottom surface of the stable rock strata 稳底 The height of the landslide zone = H 稳底 -H 煤顶 .

[0056] Special case (stable rock strata are part of an excessively thick rock stratum): If the excessively thick rock stratum has collapsed in layers, then the bottom surface of the stable rock stratum is the top surface of the collapsed part of the excessively thick rock stratum, and the elevation H of this top surface is read. 垮顶 The height of the landslide zone = H 垮顶 -H 煤顶 .

[0057] The working principle and usage process of this invention: The core working principle is based on the stability of the three-hinged arch structure formed after the rock strata break. The weakening effect of the weak surface of the excessively thick rock strata is quantified by the weak surface correction coefficient. Combined with the dynamic update of the free space height by the rock strata fragmentation and swelling, the stability of the rock strata is judged layer by layer from bottom to top, and finally the height of the collapse zone is determined.

[0058] The specific usage process is as follows:

[0059] Preliminary preparations: Collect borehole columnar diagrams and historical mechanical test data from the mine working face; prepare core drilling equipment, indoor testing equipment such as pressure testing machines and measuring cylinders, and borehole imaging instruments.

[0060] Parameter acquisition: Obtain the basic parameters and weak surface parameters of the excessively thick rock layer according to step S1 to ensure that the data is accurate and complete.

[0061] Coefficient determination and stability judgment: Determine the weak surface correction coefficient according to step S2, and perform initial stability judgment and initial fracture distance verification according to step S3. If there is a contradiction, perform correction calculation.

[0062] Free space update and layer-by-layer judgment: Update the free space height according to step S4, and judge the stability of the upper rock layer from bottom to top according to step S5 to find the stable rock layer.

[0063] Height calculation and result verification: Calculate the height of the caving zone according to step S6, and verify the result by drilling after mining. If the error exceeds 10%, adjust the parameters and recalculate to form a closed-loop optimization.

[0064] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to the above embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for calculating the height of the caving zone in near-horizontal thick coal seam mining, characterized in that, Includes the following steps: S1. Parameter acquisition: Acquire the basic parameters of the overlying rock layer and the weak surface parameters of the excessively thick rock layer on the working surface. The basic parameters include the thickness h of each rock layer, the uniaxial compressive strength σc, the uniaxial tensile strength σt, and the fracture expansion coefficient. The excessively thick rock layer is a rock layer with a thickness greater than 20m. Its weak surface parameters include the weak surface spacing d and the weak surface dip angle θ. S2. Determination of weak surface correction coefficient: The weak surface correction coefficient λ is determined based on the weak surface parameters of the excessively thick rock layer. The more developed the weak surface, the larger the value of λ, and the value range is 1.0 to 1.

5. S3. Initial stability assessment: Starting from the immediate roof of the coal seam, the stability of the three-hinged arch structure formed after the fracturing of each rock layer is assessed sequentially from bottom to top. λ and the basic parameters are substituted into the optimized three-hinged arch stability discrimination equation. If the discrimination equation ≤ 0, the structure is unstable and the rock layer belongs to the collapse zone. If the discrimination equation > 0, the structure is stable. S4. Free space height update: When the lower rock layer becomes unstable and collapses, the free space height Si below the upper rock layer is dynamically updated according to the thickness and fracture expansion coefficient of the collapsed rock layer. S5. Layer-by-layer stability assessment: Treat the updated Si as the settlement value Δ of the upper rock layer three-hinged arch structure, repeat steps S2 and S3, and assess the stability of each upper rock layer from bottom to top until a stable rock layer is found. S6. Calculation of caving zone height: Determine the vertical distance from the top of the coal seam to the bottom of the stable rock layer found in S5. This distance is the caving zone height H. If the stable rock layer is part of an excessively thick rock layer, then calculate the vertical distance from the top of the coal seam to the top of the collapsed part of the excessively thick rock layer as the caving zone height.

2. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 1, characterized in that, The basic parameters are obtained as follows: the thickness of the rock strata is determined based on the columnar section of the surface boreholes above the working face provided by the mine. If there is historical data on uniaxial compressive strength and uniaxial tensile strength, it is extracted directly. If not, fresh rock cores are drilled to make standard rock samples. σc is determined by indoor uniaxial compressive strength test and σt is determined by Brazilian splitting test.

3. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 1, characterized in that, The parameters of weak surfaces in the excessively thick rock strata are obtained by scanning the excessively thick rock strata using borehole imaging technology or by conducting on-site geological surveys, recording the spacing and dip angle of the weak surfaces, and selecting 3 to 5 observation points at the location of the excessively thick rock strata exposed in the downhole working face to count the number of weak surfaces per unit length, i.e., the weak surface development density.

4. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 1, characterized in that, The crushing expansion coefficient is obtained by indoor crushing test. If the test conditions are limited, the empirical value of similar lithology can be referred to. The coefficient is 1.060 for coarse sandstone and medium sandstone, 1.045 for fine sandstone and siltstone, and 1.025 for mudstone and sandy mudstone.

5. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 3, characterized in that, The specific rules for determining the weak surface correction coefficient are as follows: when the weak surface spacing d ≤ 0.5m, θ is between 30° and 60° and the development density is ≥ 5 lines / m, λ is taken as 1.4 to 1.

5. When 0.5m<d≤1.0m, θ is between 15° and 30° or between 60° and 75° and the development density is 3 to 5 lines / m, λ is taken as 1.2 to 1.3; When d > 1.0m, θ < 15° or θ > 75° and the development density < 3 lines / m, λ is taken as 1.1 to 1.15; When the density of weak surfaces is less than 1 line / m, λ is taken as 1.

0.

6. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 1, characterized in that, The optimized stability criterion equation for the three-hinged arch is derived based on the moment equilibrium equation of the Voussoir beam model, and its expression is: Where δ is the ratio of the settlement value of the three-hinged arch structure to the thickness of the rock stratum, and N is the ratio of the uniaxial compressive strength to the uniaxial tensile strength.

7. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 1, characterized in that, Step S3 also includes verification of the initial fracture distance of the rock strata: using empirical formulas. Where q is the self-weight stress of the rock stratum, q=γh, and γ is the unit weight of the rock stratum, taken as 25kN / m³. Calculate the initial fracture distance L. When the deviation between L and the actual advance distance of the working face is ≤10%, it is confirmed that the formation conditions of the three-hinged arch structure are reasonable.

8. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 1, characterized in that, The height of free space Where M is the coal seam mining thickness, n is the number of collapsed rock layers, hj is the thickness of the j-th collapsed rock layer, and kj is the fracture expansion coefficient of the j-th collapsed rock layer. This represents the summation of relevant parameters for all collapsed lower rock strata, with the initial value of Si equal to M when the immediate top is first determined.

9. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 1, characterized in that, If multiple rock layers collapse simultaneously in step S3, the impact of each rock layer on the free space height must be calculated sequentially according to the order of rock layer collapse. That is, first calculate the free space height Si1 corresponding to the lowest collapsed rock layer, and then calculate the free space height Si2 corresponding to the next collapsed rock layer based on Si1, until the free space height of all collapsed rock layers is updated.

10. The method for calculating the height of the caving zone in near-horizontal thick coal seam mining according to claim 1, characterized in that, It also includes a correction calculation step: when the judgment result of step S3 does not match the actual situation on site, let f(δ) = 0, substitute the current Si as Δ, and calculate the rock layer thickness h' that satisfies the equation. h' is the actual collapse thickness of the rock layer and is included in the collapse zone.

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