Numerical calculation method for overburden instability of mining caverns under step-by-step mining conditions in shallow coal seams

Abstract

The present invention discloses a numerical calculation method for overburden instability of coal mining caverns under the condition of step-by-step mining in shallow coal seams, establishes a numerical analysis model of shallow coal seams through the discrete element software program UDEC, and establishes an in-situ fluidized step-by-step mining analysis model in shallow coal seams by simulating stratified water jet coal breaking and lifting in shallow coal seams, obtains numerical simulation results of displacement analysis of surrounding rock of coal mining caverns under the condition of step-by-step mining in shallow coal seams, obtains instability characteristics of surrounding rock of in-situ fluidized mining coal caverns in shallow coal seams under the condition of step-by-step mining by comparing the change law of coal mining cavern displacement field before and after mining, and provides theoretical basis and data support for introducing stratified water jet coal breaking and lifting technology into in-situ fluidized mining of shallow coal seams to study the stability of coal mining caverns in shallow coal seams, formulate efficient in-situ fluidized mining scheme for shallow coal seams, and realize safe mining of coal seams.

Description

Technical Field

[0001] The present invention relates to a numerical analysis method, in particular to a numerical calculation method for instability of overburden rock in a coal mining cave under step-by-step mining conditions of a shallow coal seam, and belongs to the technical field of safe coal mining. Background Art

[0002] Shallow coal seams are generally considered to be coal seams with a burial depth of less than 150 meters, a base load ratio jz (the ratio of bedrock to load layer thickness) of less than 1, a roof that reflects the structural characteristics of a single key layer, and a significant dynamic load. The main problems faced in the process of mining shallow coal seams using traditional solid resource mining methods include: 1. The coal body of shallow coal seams is relatively soft, the surrounding rock thickness is generally thin, and most of them are located in areas with active geological structures. The surrounding geological structures are complex and the terrain is changeable. Geological phenomena such as fractures, folds, and rock layer inclusions are relatively common, which makes the mine pressure distribution uneven, the surrounding rock of the coal seam is unevenly stressed, and the coal seam pressure varies greatly; 2. During the mining of shallow coal seams, due to mining disturbances, the surface is prone to collapse and a large number of ground cracks are generated, causing damage to surface facilities and terrain. These cracks may also damage underground aquifers and cause a large loss of surface water resources, which in turn leads to 1. The surface land in the mining area is desertified and threatens the ecological environment. In addition, shallow coal seams usually contain a large amount of gas. If the gas accumulation exceeds a reasonable range during coal mining, it will seriously threaten the safety of workers and may even cause an explosion. 2. The coal rock mass in shallow coal seams is mostly wrinkled and deformed, prone to breakage and loosening, making coal seam mining relatively difficult. 3. Due to the high mine pressure, frequent geological disasters, and easy breakage and deformation of coal rock mass faced in the mining process of shallow coal seams, support technology faces huge challenges. 4. The discharge and treatment of tailings from shallow coal seam mines are strict, and reasonable tailings treatment technology needs to be adopted to protect the environment and personal safety.

[0003] In-situ physical fluidization mining technology is a new mining concept and technical approach. It involves using high-pressure water jets to crush solid coal resources in situ underground to form a coal-water mixture. This mixture is then brought up to the surface or separated in situ underground to recover the coal. In-situ physical fluidization mining can significantly increase coal mining and recovery rates, reducing resource waste. On the one hand, the high transport efficiency of the fluidized material can shorten the mining cycle and improve production efficiency. On the other hand, fluidized mining technology can reduce mine excavation and support work, thereby reducing structural damage to the mine and thus reducing safety hazards.

[0004] The initial design of the height of conventional in-situ physical fluidization columnar coal mining caves is based on the thickness of the coal seam. The coal mining caves are formed directly from the top to the bottom of the coal seam by high-pressure water jets. The forming speed is fast, and the deformation speed of the surrounding rock is also fast. However, if Figure 4As shown in the figure, it is difficult to control the deformation and damage of the roof when mining shallow coal seams with soft coal texture, thick surface loose layer and thin overburden roof. The use of in-situ physical fluidization mining technology for mining still needs to consider the overall stability of the overburden to prevent the occurrence of roof step sinking. The stability of the coal mining cave is a key issue, but the existing technology lacks the corresponding theoretical analysis basis and data support. Summary of the Invention

[0005] In response to the problems existing in the above-mentioned prior art, the present invention provides a numerical calculation method for the instability of overburden in coal mining caves under the conditions of step-by-step mining of shallow coal seams, which can provide a theoretical basis and data support for introducing the step-by-step mining method into the in-situ physical fluidization mining of shallow coal seams to study the stability of coal mining caves in shallow coal seams, formulate efficient and stable in-situ fluidization mining plans for shallow coal seams, and realize safe mining of shallow coal seams.

[0006] To achieve the above objectives, the numerical calculation method for overburden instability of coal caverns under the condition of step-by-step mining of shallow coal seams specifically includes the following steps:

[0007] Step 1, on-site investigation: obtain relevant design parameters for coal mining, obtain geological data and information related to shallow coal seams, conduct geological exploration, and estimate the mining impact range;

[0008] Step 2: Establish an in-situ fluidized coal mining cave stability analysis model: Based on the geological conditions, mining methods and step-by-step mining characteristics of shallow coal seams, the discrete element simulation software UDEC is used to establish an in-situ fluidized coal mining cave stability analysis model that includes the mechanical properties of the coal seam, roof, floor and surrounding rock mass, as well as the stress and deformation generated during the simulated mining process. The numerical analysis range is determined according to the mining plan and the geometric dimensions of the coal mining cave. The analysis range includes the rock mass around the coal mining cave and the area affected by mining. The coal-rock layer within the analysis range is divided into multiple small units, and the divided coal-rock layer is grouped and meshed. The joints are calculated using the Mohr-Coulomb sliding model. The divided coal-rock layer is assigned parameters separately, and boundary constraints are imposed according to the actual mining situation.

[0009] Step 3, simulated coal seam mining: Based on the in-situ fluidized coal mining cave stability analysis model, numerical simulation training of layered mining of coal seams is carried out. The step-by-step fluidized coal breaking is carried out in a bottom-up order. After the lower coal seam is mined, grouting is simulated immediately. After the slurry solidifies, the upper coal seam is mined at the same thickness until the simulated mining is completed. Multiple layered mining numerical simulation trainings are carried out, gradually reducing the thickness of the mined coal seam until the lower limit of the stable mining thickness of the coal seam is found. The deformation characteristics of the coal mining cave under the conditions of layered coal breaking and lifting by water jet are calculated, and the data are saved and extracted.

[0010] Step 4, data analysis: Extract and analyze the simulation data of the coal caverns under the conditions of coal seam water jet layered coal breaking and lifting, analyze the stress distribution, deformation and failure mode around the coal caverns, and obtain the instability characteristics of the surrounding rock of the coal caverns in the shallow coal seam in-situ fluidized mining under the conditions of step-by-step mining;

[0011] The original coal-rock stress state is σ1=σ2=σ3=σ0, where σ1, σ2, σ3 are the first principal stress, the second principal stress, and the third principal stress, respectively, and σ0 is the original rock stress;

[0012] The elastic stress field of the coal cave after in-situ fluidized mining is calculated as follows:

[0013]

[0014] Where: σ r , σ θ and σ z are the radial, circumferential and axial stresses of the surrounding rock at radius r, respectively; r0 is the radius of the through-layer drilling hole; r is the radius of the coal mining cave; σ0 is the original rock stress;

[0015] As the high-pressure water jet continues to break the coal, the radius of the surrounding rock plastic zone r p The calculation formula is as follows:

[0016]

[0017] Where: r0 is the radius of the through-layer drilling; σ θ is the tangential stress; σ c is the uniaxial compressive strength of rock; n is the exponent in the stress-strain relationship of rock.

[0018] Furthermore, in Step 2, when the Mohr-Coulomb slip model is used to assign parameters to the divided coal-rock layers, the shear modulus of the elastic properties of UDEC is as follows:

[0019] 2G(1+v)=E

[0020] Where: G is the shear modulus; E is the Young's modulus; v is the Poisson's ratio;

[0021] The normal and shear stiffness of the joint satisfy the following relationship:

[0022]

[0023] Where: E m is the Young's modulus of rock mass; E r is the Young's modulus of rock; k n is the joint normal stiffness; s is the joint spacing; k s is the joint shear stiffness; G mis the shear modulus of rock mass; G r is the rock shear modulus;

[0024] The Mohr-Coulomb criterion is used as the failure criterion:

[0025]

[0026] Where: is the internal friction angle; c is the cohesion.

[0027] Furthermore, in Step 2, the boundary constraints are as follows: normal constraints are used for the left, right, and bottom surfaces, no constraints are used for the top surface, and the maximum balance ratio is set to 1×10 -7 .

[0028] Furthermore, in Step 2, the geometric dimensions of the coal mining cave include the layered mining thickness, the total thickness of the coal seam, the cave height and diameter.

[0029] Furthermore, in Step 3, when simulating grouting filling in the coal mining cave, the drilling fluid column pressure is used as a control parameter. Based on the drilling fluid column pressure and the formation pressure, the design pressure is calculated as a reference standard for the grouting process. The design pressure = (formation pressure - drilling fluid column pressure) × safety factor, and the safety factor is greater than 1.0.

[0030] Furthermore, in Step 3, the step-by-step fluidized coal breaking is divided into N steps of mining in a bottom-up order, and the value of N is equal to the coal seam thickness divided by the height of the single-layer coal mining cave and rounded up.

[0031] Furthermore, in Step 1, the main parameters of the impact area are obtained in the mining impact range estimation, and the main parameters of the impact area include the maximum subsidence value of the main impact radius; the geological related data information of the shallow buried coal seam includes the coal seam burial depth, coal seam thickness, coal seam distribution, coal seam inclination, and the lithology of the coal seam, surrounding rock and overburden.

[0032] Furthermore, Step 1 is as follows:

[0033] Step 1-1, conduct geological survey, draw stratigraphic histogram, and obtain the occurrence of shallow coal seams;

[0034] Step 1-2: Determine the predicted parameters of surface movement based on the coal seam occurrence and estimate the mining impact range of the goaf treated by the caving method;

[0035] Step 1-3, determine the maximum collapse step distance of the roof of the coal mining cave.

[0036] Compared with the existing technology, this numerical calculation method for overburden instability of coal caverns under step-by-step mining conditions in shallow coal seams aims to solve the problem of damage to the surface and rock structure after mining of shallow coal seams in in-situ physical fluidized coal mining technology. A numerical analysis model of shallow coal seams is established using the discrete element software program UDEC, and an in-situ fluidized step-by-step mining analysis model of shallow coal seams is established by simulating stratified water jet coal breaking and lifting in shallow coal seams. The numerical simulation results of the surrounding rock displacement analysis of coal caverns under step-by-step mining conditions in shallow coal seams are obtained. By comparing the change law of the coal cavern displacement field before and after mining, the surrounding rock instability characteristics of the in-situ fluidized mining coal caverns in shallow coal seams with coal seam caverns under step-by-step mining conditions are obtained. This method can provide a theoretical basis and data support for introducing stratified water jet coal breaking and lifting technology into in-situ fluidized mining of shallow coal seams to study the stability of coal caverns in shallow coal seams, formulate efficient in-situ fluidized mining plans for shallow coal seams, and achieve safe mining of coal seams. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a flow chart of the present invention;

[0038] Figure 2 This is a schematic diagram of the elastic stress state of the coal cave after in-situ fluidized mining;

[0039] Figure 3 This is a schematic diagram of the elastic-plastic stress state of the coal cave after in-situ fluidized mining;

[0040] Figure 4 It is a schematic diagram of shallow coal seam;

[0041] Figure 5 It is a schematic diagram of the first layer of step-by-step mining of a shallow coal seam;

[0042] Figure 6 This is a schematic diagram of the step-by-step mining of the first layer of a shallow coal seam followed by backfilling;

[0043] Figure 7 It is a schematic diagram of the second layer of step-by-step mining of a shallow coal seam;

[0044] Figure 8 This is a model diagram for analyzing the stability of an in-situ fluidized coal mining cavern according to the present invention;

[0045] Figure 9 This is a displacement diagram and a partial enlarged diagram of a shallow coal seam thin roof under critical stability conditions according to an embodiment of the present invention;

[0046] Figure 10 It is the displacement under the critical collapse condition of the thin roof of the shallow coal seam in the embodiment of the present invention. DETAILED DESCRIPTION

[0047] like Figure 1As shown in the figure, the numerical calculation method for the instability of overburden rock in the mining cave under the condition of step-by-step mining of shallow coal seams specifically includes the following steps:

[0048] Step 1, actual on-site investigation of the mine: obtain relevant design parameters for coal mining, obtain geological data information related to shallow coal seams, mainly including coal seam depth, coal seam thickness, coal seam distribution, coal seam inclination, coal seam roof, coal seam floor and physical and mechanical parameters of surrounding rock mass, etc., conduct geological exploration and estimate the mining impact range, and obtain the main parameters of the impact area during the mining impact range estimation, including the maximum subsidence value of the main impact radius. Specifically:

[0049] Step 1-1, conduct geological survey, draw stratigraphic histogram, and obtain the occurrence of shallow coal seams;

[0050] Step 1-2: Determine the predicted parameters of surface movement based on the coal seam occurrence and estimate the mining impact range of the goaf treated by the caving method;

[0051] Step 1-3, determine the maximum collapse step distance of the roof of the coal mining cave.

[0052] Step 2: Establish an in-situ fluidized coal mining cave stability analysis model:

[0053] Considering the geological conditions, mining methods, and step-by-step mining characteristics of shallow coal seams, a numerical model for cave stability analysis was first established using the discrete element simulation software UDEC. The model should include the mechanical properties of the coal seam, roof, floor, and surrounding rock mass, as well as the stresses and deformations generated during the mining process. To avoid interference from the previous model, the first command was "new" to clear any interference from previous data. The scope of the numerical analysis was determined based on the mining plan and the geometric dimensions of the coal caverns, which could include the layered mining thickness, total coal seam thickness, cavern height, and diameter.

[0054] Next, the coal-rock strata are divided into multiple small units according to the distribution of shallow coal seams in the model input command stream. The Mohr-Coulomb slip model is used for joints, and the parameters of the divided coal-rock strata are assigned separately, including the physical and mechanical parameters of coal and rock, such as bulk density, Poisson's ratio and elastic modulus.

[0055] When assigning parameters, for the elastic properties of UDEC, the shear modulus is as follows:

[0056] 2G(1+v)=E

[0057] Where: G is the shear modulus; E is the Young's modulus; v is the Poisson's ratio.

[0058] The normal and shear stiffness of the joint satisfy the following relationship:

[0059]

[0060] Where: E m is the Young's modulus of rock mass; E r is the Young's modulus of rock; k n is the joint normal stiffness; s is the joint spacing; k s is the joint shear stiffness; G m is the shear modulus of rock mass; G r is the shear modulus of rock.

[0061] The Mohr-Coulomb criterion is used as the failure criterion:

[0062]

[0063] Where: is the internal friction angle; c is the cohesion.

[0064] Based on the actual conditions of the coal seam being mined, appropriate boundary conditions and loads are applied to the model boundaries. Boundary conditions can include fixed boundaries, free boundaries, or constrained boundaries. Loads can include gravity loads, stresses and deformations generated during mining, etc. The boundary constraints are as follows: normal constraints are used for the left, right, and bottom surfaces, and no constraints are used for the top surface. The maximum balance ratio is set to 1×10 -7 , after initial equilibrium, reset the model to zero.

[0065] Step 3, simulate coal seam mining: Based on the in-situ fluidized coal mining cave stability analysis model, conduct numerical simulation training on the coal seam layered mining, gradually reduce the coal seam thickness until the lower limit of the coal seam mining stability thickness is found, and the step-by-step fluidized coal breaking is divided into N steps of mining in a bottom-up order. The N value is equal to the coal seam thickness divided by the height of the single-layer coal mining cave and rounded up. After the lower coal seam is mined, simulate grouting immediately. After the slurry solidifies, continue to mine the upper coal seam of the same thickness until the simulated mining is completed. When simulating grouting filling in the coal mining cave, the borehole fluid column pressure is used as the control parameter. According to the borehole fluid column pressure and the formation pressure, the design pressure is calculated as the reference standard for the grouting process. The design pressure = (formation pressure - borehole fluid column pressure) × safety factor, and the safety factor is greater than 1.0. Use the coal seam water jet layered coal breaking and lifting method to simulate and analyze the coal seam cave mining. Calculate the deformation characteristics of the coal mining cave under the conditions of coal seam water jet layered coal breaking and lifting, and save and extract the data;

[0066] Step 4, data analysis: extract the simulation data of coal mining caves under the conditions of coal seam water jet layered coal breaking and lifting, analyze the stress distribution, deformation and possible failure modes around the coal mining caves, and obtain the instability characteristics of the surrounding rock of the in-situ fluidized mining coal mining caves in shallow coal seams under step-by-step mining conditions.

[0067] The original coal-rock stress state is σ1=σ2=σ3=σ0, where σ1, σ2, σ3 are the first principal stress, the second principal stress, and the third principal stress, respectively, and σ0 is the original rock stress.

[0068] The elastic stress state of the coal cave after in-situ fluidized mining is as follows: Figure 2 As shown, the stress field is calculated as follows:

[0069]

[0070] Where: σ r , σ θ and σ z are the radial, circumferential and axial stresses of the surrounding rock at radius r; r0 is the radius of the through-layer drilling hole; r is the radius of the coal mining cave; σ0 is the original rock stress.

[0071] As the high-pressure water jet continues to break the coal, the radius of the coal mining cave continues to increase, and plastic deformation occurs. Figure 3 As shown in the figure, according to the classical elastic-plastic theory, the radius of the plastic zone of the surrounding rock is r p The calculation formula is as follows:

[0072]

[0073] Where: r0 is the radius of the through-layer drilling; σ θ is the tangential stress; σ c is the uniaxial compressive strength of rock; n is the exponent in the stress-strain relationship of rock.

[0074] The following uses a shallow coal seam mining project in a mine as an example to evaluate cave stability using the numerical analysis method described above, and the present invention is further illustrated with reference to the accompanying figures. The coal seam in the mine has a dip angle of 0°, i.e., a shallow horizontal coal seam. The geological data is shown in Table 1 below.

[0075] Table 1 Physical and mechanical parameters of coal strata

[0076]

[0077] The average thickness of the coal seam in this mine is about 10m. Mining is carried out along the bottom of the coal seam, and the coal seam is mined at a height of 2m. The mining cave is in the shape of a column hole. The step-by-step fluidized coal breaking is carried out in five steps from bottom to top. According to the site conditions, a three-dimensional geometric model with a length × height = 300m × 100m was constructed. First, a numerical model for cave stability analysis was established using discrete element simulation software. Figure 8As shown, the model should include the mechanical properties of the coal seam, roof, floor, and surrounding rock mass, as well as the stress and deformation generated during the mining process. The first command is new, which removes the interference of previous data. The scope of the numerical analysis is determined according to the mining plan and the geometric dimensions of the cave. Then, the coal-rock layer is divided into multiple small units according to the distribution of shallow coal seams in the model input command stream. The coal / rock layer within the analysis range is divided into multiple small units. The joints use the Mohr-Coulomb sliding model to assign parameters to the divided coal-rock layers. The parameters include bulk density, Poisson's ratio, and elastic modulus in the physical and mechanical parameters of coal and rock. According to the actual situation of coal seam mining, appropriate boundary conditions and loads are applied to the boundaries of the model. Boundary conditions include fixed boundaries, free boundaries, or constrained boundaries. Loads include gravity loads, stresses and deformations generated during mining, etc. The boundary constraints are as follows: normal constraints are used for the left, right, and bottom surfaces, unconstrained for the top surface, and the maximum balance ratio is set to 1×10 -7 , after initial equilibration, reset the model to zero.

[0078] Water jet layered coal breaking and lifting technology was introduced into in-situ fluidized mining of shallow coal seams. Through-drilling was used to create holes, and coal breaking and mining began at the bottom of the drilled coal seam. A numerical analysis model for shallow coal seam distribution was established using discrete element software. Furthermore, an in-situ fluidized step-by-step mining analysis model for shallow coal seams was established by simulating water jet layered coal breaking and lifting in shallow coal seams. Numerical simulation results for the displacement of surrounding rock in mining caverns under step-by-step mining conditions in shallow coal seams were obtained.

[0079] According to the actual investigation and geological data of the mine site, the coal seam is simulated for mining: first, the thickness of the coal seam to be mined each time is determined to be 2m, and then the coal seam is simulated for mining. Based on the in-situ fluidized coal mining cave stability analysis model, the coal seam is subjected to numerical simulation training for layered mining, and the thickness of the mined coal seam is gradually reduced until the lower limit of the stable thickness of the coal seam is found, such as Figures 5 to 7 As shown, the step-by-step fluidized coal breaking is carried out in a bottom-up order. Grouting is immediately performed after the lower coal seam is mined. After the slurry solidifies, the upper coal seam is mined with the same thickness until the mining is completed. When simulating the grouting filling in the coal mining cave, the borehole fluid column pressure is used as the control parameter. According to the borehole fluid column pressure and the formation pressure, the design pressure is calculated as the reference standard for the grouting process. The coal seam cave mining simulation analysis is carried out using the coal seam water jet layered coal breaking and lifting method. The deformation characteristics of the coal mining cave under the coal seam water jet layered coal breaking and lifting are calculated and saved and extracted. The simulation data of the coal mining cave under the coal seam water jet layered coal breaking and lifting are extracted and analyzed. The stress distribution, deformation and possible failure modes around the coal mining cave are analyzed to obtain the surrounding rock instability characteristics of the shallow coal seam in-situ fluidized mining coal mining cave under the step-by-step mining conditions. According to field experience, the roof subsidence in coal seam stability is the most intuitive indicator of collapse. The details are as follows:

[0080] The numerical simulation training of coal seam layer mining is carried out, and the following results are obtained: Figure 9 The displacement of the shallow coal seam thin roof under critical stability conditions is shown in the figure. Under the conditions of step-by-step mining of shallow coal seams and coal cave unloading, when the mining thickness of each layer is 2m, the vertical displacement of the shallow coal seam thin roof under critical stability conditions is less than 2m, indicating that the roof has not collapsed. In the corresponding displacement diagram (such as Figure 9 It can also be clearly seen that the maximum displacement of the shallow coal seam thin roof under critical stability conditions is about 1.96m.

[0081] The numerical simulation training of coal seam layer mining is carried out, and the following results are obtained: Figure 10 The displacement under the shallow coal seam thin roof collapse condition is shown in the figure. Under the conditions of step-by-step mining of shallow coal seams and the shallow coal seam thin roof collapse condition, when the mining thickness of each layer is 2m, the vertical displacement under the shallow coal seam thin roof collapse condition is greater than 2m, indicating that the roof has collapsed. In the corresponding displacement diagram (such as Figure 10 (As shown) the collapse of the thin roof of the shallow coal seam can also be clearly seen.

[0082] This numerical calculation method for the instability of overburden in coal caverns under the condition of step-by-step mining in shallow coal seams aims to solve the problem of damage to the surface and rock structure after mining of shallow coal seams in in-situ physical fluidized coal mining technology. A numerical analysis model of shallow coal seams is established through a discrete element software program, and an in-situ fluidized step-by-step mining analysis model of shallow coal seams is established by simulating the stratified water jet coal breaking and lifting in shallow coal seams. The numerical simulation results of the displacement analysis of the surrounding rock of the coal caverns under the condition of step-by-step mining in shallow coal seams are obtained. By comparing the change law of the coal cavern displacement field before and after mining, the instability characteristics of the surrounding rock of the in-situ fluidized mining coal caverns in shallow coal seams with coal seam cave formation under step-by-step mining conditions are obtained. This can provide a theoretical basis and data support for introducing the stratified water jet coal breaking and lifting technology into the in-situ fluidized mining of shallow coal seams to study the stability of coal caverns in shallow coal seams, formulate an efficient in-situ fluidized mining plan for shallow coal seams, and achieve safe mining of coal seams.

Claims

1. A numerical calculation method for overburden instability of coal mining caverns under step-by-step mining conditions of shallow coal seams, characterized in that: The specific steps include: Step 1, on-site investigation: obtain relevant design parameters for coal mining, obtain geological data and information related to shallow coal seams, conduct geological exploration, and estimate the mining impact range; Step 2: Establish an in-situ fluidized coal mining cave stability analysis model: Based on the geological conditions, mining methods and step-by-step mining characteristics of shallow coal seams, the discrete element simulation software UDEC is used to establish an in-situ fluidized coal mining cave stability analysis model that includes the mechanical properties of the coal seam, roof, floor and surrounding rock mass, as well as the stress and deformation generated during the simulated mining process. The numerical analysis range is determined according to the mining plan and the geometric dimensions of the coal mining cave. The analysis range includes the rock mass around the coal mining cave and the area affected by mining. The coal-rock layer within the analysis range is divided into multiple small units, and the divided coal-rock layer is grouped and meshed. The joints are calculated using the Mohr-Coulomb sliding model. The divided coal-rock layer is assigned parameters separately, and boundary constraints are imposed according to the actual mining situation. Step 3, simulated coal seam mining: Based on the in-situ fluidized coal mining cave stability analysis model, numerical simulation training of layered mining of coal seams is carried out. The step-by-step fluidized coal breaking is carried out in a bottom-up order. After the lower coal seam is mined, grouting is simulated immediately. After the slurry solidifies, the upper coal seam is mined at the same thickness until the simulated mining is completed. Multiple layered mining numerical simulation trainings are carried out, gradually reducing the thickness of the mined coal seam until the lower limit of the stable mining thickness of the coal seam is found. The deformation characteristics of the coal mining cave under the conditions of layered coal breaking and lifting by water jet are calculated, and the data are saved and extracted. Step 4, data analysis: Extract and analyze the simulation data of the coal caverns under the conditions of coal seam water jet layered coal breaking and lifting, analyze the stress distribution, deformation and failure mode around the coal caverns, and obtain the instability characteristics of the surrounding rock of the coal caverns in the shallow coal seam in-situ fluidized mining under the conditions of step-by-step mining; The original coal-rock stress state is σ1=σ2=σ3=σ0, where σ1, σ2, σ3 are the first principal stress, the second principal stress, and the third principal stress, respectively, and σ0 is the original rock stress; The elastic stress field of the coal cave after in-situ fluidized mining is calculated as follows: Where: σ r , σ θ and σ z are the radial, circumferential and axial stresses of the surrounding rock at radius r, respectively; r0 is the radius of the through-layer drilling hole; r is the radius of the coal mining cave; σ0 is the original rock stress; As the high-pressure water jet continues to break the coal, the radius of the surrounding rock plastic zone r p The calculation formula is as follows: Where: r0 is the radius of the through-layer drilling; σ θ is the tangential stress; σ c is the uniaxial compressive strength of rock; n is the exponent in the stress-strain relationship of rock.

2. The numerical calculation method for overburden instability of coal caverns under step-by-step mining conditions of shallow coal seams according to claim 1 is characterized in that: In Step 2, when the Mohr-Coulomb slip model is used to assign parameters to the divided coal-rock layers, the elastic properties and shear modulus of UDEC are as follows: 2G(1+v)=E Where: G is the shear modulus; E is the Young's modulus; v is the Poisson's ratio; The normal and shear stiffness of the joint satisfy the following relationship: Where: E m is the Young's modulus of rock mass; E r is the Young's modulus of rock; k n is the joint normal stiffness; s is the joint spacing; k s is the joint shear stiffness; G m is the shear modulus of rock mass; G r is the rock shear modulus; The Mohr-Coulomb criterion is used as the failure criterion: Where: is the internal friction angle; c is the cohesion.

3. The numerical calculation method for overburden instability of coal caverns under step-by-step mining conditions of shallow coal seams according to claim 2 is characterized in that: In Step 2, the boundary constraints are as follows: normal constraints are used for the left, right, and bottom surfaces, no constraints are used for the top surface, and the maximum balance ratio is set to 1×10 -7 .

4. The numerical calculation method for overburden instability of coal caverns under step-by-step mining conditions of shallow coal seams according to claim 1 is characterized in that: In Step 2, the geometric dimensions of the coal mining cave include the layered mining thickness, the total thickness of the coal seam, the cave height and diameter.

5. The numerical calculation method for overburden instability of coal caverns under step-by-step mining conditions of shallow coal seams according to claim 1 is characterized in that: In Step 3, during the grouting filling simulation in the coal mining cave, the drilling fluid column pressure is used as the control parameter. Based on the drilling fluid column pressure and the formation pressure, the design pressure is calculated as the reference standard for the grouting process. The design pressure = (formation pressure - drilling fluid column pressure) × safety factor, and the safety factor is greater than 1.

0.

6. The numerical calculation method for overburden instability of coal caverns under step-by-step mining conditions of shallow coal seams according to claim 1 is characterized in that: In Step 3, the step-by-step fluidized coal breaking is divided into N steps of mining in a bottom-up order, and the value of N is equal to the coal seam thickness divided by the height of the single-layer mining cave and rounded up.

7. The numerical calculation method for overburden instability of coal caverns under step-by-step mining conditions of shallow coal seams according to claim 1 is characterized in that: In Step 1, the main parameters of the impact area are obtained in the mining impact range estimation. The main parameters of the impact area include the maximum subsidence value of the main impact radius; the geological data information of the shallow coal seam includes the coal seam burial depth, coal seam thickness, coal seam distribution, coal seam inclination, and the lithology of the coal seam, surrounding rock and overburden.

8. The numerical calculation method for overburden instability of coal caverns under step-by-step mining conditions of shallow coal seams according to claim 1 is characterized in that: Step 1 is as follows: Step 1-1, conduct geological survey, draw stratigraphic histogram, and obtain the occurrence of shallow coal seams; Step 1-2: Determine the predicted parameters of surface movement based on the coal seam occurrence and estimate the mining impact range of the goaf treated by the caving method; Step 1-3, determine the maximum collapse step distance of the roof of the coal mining cave.

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