Method and equipment for predicting surface subsidence of overlying strata during insufficient mining of extremely thick and weakly cemented overlying strata

Abstract

The invention discloses a method and equipment for predicting surface subsidence of super-thick weakly-cemented overlying strata under insufficient mining, and belongs to the field of surface treatment of coal mining subsidence areas. Establishing a physical strata plate theoretical model according to the structure and the property of each strata, determining the size of each strata plate according to the actual mining progress of a working face, actually measured drilling data and mechanical test report parameters, and further establishing an overall model of overlying strata under the strata; the state information of each rock stratum under different advancing distances of the working face is judged, the flexural deformation value of the rock stratum in the advancing process of the working face is calculated, then the flexural value of each point of a rock stratum plate serves as the coal seam mining thickness, the subsidence deformation value of a rock-soil layer above the rock stratum subjected to flexural deformation is calculated, and then the surface subsidence value is predicted. The method has higher adaptability and accuracy for surface subsidence deformation prediction of mining areas with special stratum environments such as the condition of extremely thick and weak cemented overburden strata, calculation is simple, and a scientific theoretical basis is provided for coal mining design under regional structures.

Description

Technical Field

[0001] The present invention relates to a method and device for predicting surface subsidence caused by insufficient mining of extremely thick and weakly cemented overburden, and belongs to the field of surface treatment in coal mining subsidence areas. Technical Background

[0002] In coal mining, when the surface subsidence value of the goaf does not reach the maximum value expected under the geological conditions, the mining operation is called insufficient mining, and the mining carried out during this stage is also called insufficient mining. Because the overburden in the mining area is generally distributed with multiple layers of thick to extremely thick argillaceous weakly cemented sandstone of different particle sizes, this type of rock layer is mainly characterized by low strength, poor cementation, lack of small faults and joints, and high overall stiffness. This leads to differences in the regional surface subsidence pattern and conventional understanding. Especially during the insufficient mining stage, the controlling effect of the extremely thick and weakly cemented overburden structure is more significant, resulting in a significant reduction in surface subsidence.

[0003] Weakly cemented sandstone is a rock that is easily weathered and cemented when exposed to cement. The cement is calcareous and insoluble in water, and the mechanical properties of the cement are similar to those of rock particles. This type of rock has a low degree of cementation, large pores, and a relatively loose structure, resulting in very low compressive and deformation resistance. The compressive strength and elastic modulus of conventional sandstone are 16.5 times and 21.3 times that of weakly cemented sandstone, respectively. When weakly cemented sandstone is distributed in the stratum with a thickness of hundreds of meters, the overall stiffness of the rock stratum is relatively large because cracks, joints, and faults in the rock stratum are almost undeveloped. The surface movement and deformation laws in coal mining are significantly different from conventional understanding.

[0004] The commonly used surface subsidence prediction method, the probability integral method, is more suitable for predicting surface subsidence in mining areas with numerous inter-stratum fissures and poor integrity. However, extremely thick, weakly cemented rock layers are thick, well-integrated, and have high rigidity. Especially during the incomplete mining phase, after coal is extracted, the rock layer retains its original stable structure, resulting in minimal surface subsidence. Therefore, the probability integral method is not suitable for predicting surface subsidence in incompletely mined mining areas with extremely thick, weakly cemented overburden. Therefore, the innovation of this method is to combine mechanical plate theory with stochastic medium theory. The plate theory considers the unique overburden structure and physical properties of extremely thick, weakly cemented overburden from a mechanical perspective to calculate the deflection value of the extremely thick, weakly cemented overburden. Based on this, it is combined with the probability integral method, a traditional surface subsidence prediction method based on stochastic medium theory, to predict surface subsidence values.

[0005] Currently, the most widely used surface subsidence prediction method is the probability integral method, which is based on stochastic medium theory. However, the probability integral method is based on a discontinuous medium model of granular medium mechanics and is more suitable for the conditions of moderately hard and weak overburden in eastern mining areas that are cut by faults and joints. However, for the extremely thick and weakly cemented overburden in western China's mining areas, which lack faults and joints, directly using the probability integral method model will have significant deviations, posing potential risks to the operation of oil and gas pipelines under the influence of mining. Therefore, it is urgent to develop a method for predicting surface subsidence in undermined areas of extremely thick and weakly cemented overburden. Summary of the Invention

[0006] In order to address the shortcomings of the above technologies, a method and equipment for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden is provided. The method and equipment have simple steps, high accuracy, and take into account the structural characteristics of extremely thick and weakly cemented overburden.

[0007] To achieve the above technical objectives, the present invention provides a method for predicting surface subsidence due to insufficient mining of extremely thick, weakly cemented overburden, which comprises the following specific steps:

[0008] Strata with a difference in attribute data between coal seams and surface rock layers less than a preset value are divided into the same rock stratum, and thick, weakly cemented overburden rocks above coal seams are divided into multiple rock stratum plates. The mechanical properties of each rock stratum plate are represented by the average mechanical properties of the collected rock samples.

[0009] If it is determined that there are unbroken strata in the thick, weakly cemented overburden below the current working face advancement distance, the deformation of each unbroken stratum is analyzed from bottom to top, and the coordinating control stratum that coordinates the deformation of all the strata above it is found. The flexural deformation value of the coordinating control stratum in the non-full mining stage is calculated.

[0010] The calculated deflection value of each point on the coordinated control rock plate is used as the coal seam mining thickness, and the probability integral method is used to calculate the surface subsidence deformation value, thereby realizing the accurate prediction of surface subsidence deformation of extremely thick weakly cemented overburden under insufficient mining.

[0011] Furthermore, the strata plates are divided into thin plates and thick plates according to the ratio of the short side size to the thickness: h is represented as the thickness of the strata plate, L is the length of the short side of the strata plate, when h / L≤0.125, it is judged as a thin plate, and when h / L>0.125, it is judged as a thick plate; during the mining process of the working face, the deformation of the thick plate is calculated using the Vlasov thick plate theory, and that of the thin plate is calculated using the elastic thin plate theory.

[0012] Furthermore, according to the actual drilling data of the research mining area and the mechanical test report of the indoor rock samples, it is divided into n layers of rock strata from the top of the coal seam to the surface. The rock strata are divided into the first rock strata to the nth rock strata from the bottom to the top. The n rock strata can deform synchronously. The load acting on the first rock strata is (q n )1 means:

[0013]

[0014] In the formula, (q n )1 refers to the load value of the n layers of rock slabs overlying the first layer of rock slab; h1 and E1 are the thickness and elastic modulus of the first layer of rock slab respectively; h i is the thickness of the corresponding rock layer i; i is the gravity density of the i-th rock layer; E i is the elastic modulus of the i-th rock layer.

[0015] Furthermore, the method for determining whether there are unbroken rock strata in the thick and weakly cemented overburden mining area at different advancing distances of the working face is as follows: calculate the maximum tensile stress value of each rock strata at different mining advancing distances of the coal seam from bottom to top, and compare the calculated maximum tensile stress value of each rock strata with the tensile strength of the corresponding rock layer. When the maximum tensile stress value is less than the tensile strength, it is judged that the rock strata is not broken, and its own state is stable and only bending deformation occurs.

[0016] Furthermore, the deformation of each stratum plate during the mining process of the working face is analyzed from the coal seam upwards. Before the stratum plate breaks, it is embedded in the rock mass on all sides and supported by the lower stratum plate.

[0017] Using the formula: Determine the length a and width b of each stratum plate, where l represents the distance of advancement of the working face, k represents the length of the working face inclination, H represents the mining depth, z represents the depth of the stratum, and β1 and β2 represent the main influencing angles of the mining strike and inclination;

[0018] The maximum bending moment at the center of each stratum plate is calculated using the following formula, where the thick plate is calculated based on M h(max) Calculate the corresponding formula, and select M for thin plate b(max) Corresponding formula calculation:

[0019]

[0020] In the above formula, M h(max) 、M b(max) are the maximum bending moments of thick plates and thin plates respectively; q is the uniformly distributed load on the rock plate; υ is the Poisson's ratio of the rock plate; the maximum bending moment value at the center of all rock plates can be obtained;

[0021] Calculate the maximum tensile stress of all rock strata on the coal seam from bottom to top;

[0022]

[0023] Where σ h(max) , σ b(max)are the maximum tensile stress values ​​of the thick plate and thin plate respectively; q is the uniformly distributed load on the rock plate; υ is the Poisson's ratio of the rock plate; h is the thickness of the rock plate; a and b are the length and width of the rock plate, and the thick plate is calculated based on σ h(max) The calculation is performed by the formula, and the thin plate selects σ b(max) The corresponding formula is used to calculate the maximum tensile stress value.

[0024] Furthermore, when the tensile strength of the stratum plate is less than the maximum tensile stress it is subjected to, the stratum plate is determined to be broken, and the deflection value of the stratum plate that has not been broken is calculated using the following formula:

[0025]

[0026] Where w h 、w b are the deflection values ​​of the thick plate and thin plate at their respective x and y coordinate positions; thick and thin strata plates have corresponding bending stiffness and shear deformation modulus, D represents the bending stiffness of the strata plate; G represents the shear deformation modulus of the strata plate, and the corresponding parameters are substituted into the calculation formula.

[0027] Furthermore, due to the situation that the rock strata may not be broken or all the rock strata may be broken during the surface subsidence and deformation, the prediction methods are as follows:

[0028] When a stratum plate undergoes bending deformation, and the bending deformation is smaller than that of the stratum plate above it, it is judged that the stratum plate acts as a coordinated control stratum plate to coordinate the deformation of all the stratum plates above it. When the coordinated control stratum plate is not damaged but only undergoes bending deformation, the bending deformation value W of the unbroken stratum plate under the insufficient mining stage is calculated based on the flexural equation constructed by Vlasov thick plate theory and elastic thin plate theory. y , the flexural deformation space of the rock stratum is regarded as the coal seam mining space, and the bending deformation value is regarded as the coal seam mining thickness. The probability integral method based on the discontinuous medium theory is used to calculate the subsidence deformation value of the surface subsidence:

[0029]

[0030] Where W(x,y) represents the subsidence deformation value of the surface point (x,y); W y (x,y) is the deflection value at point (x,y) on the rock plate; P is the integration area; r is the main influence radius; and μ and ν are the differential quantities in the x and y directions.

[0031] Furthermore, if all the rock strata are broken, the probability integral method is used to directly calculate the subsidence deformation value of the ground surface after the rock strata are broken:

[0032]

[0033] Where W(x,y) represents the subsidence deformation value of the surface point (x,y); W0 is the maximum surface subsidence value, which is determined according to the surface measured leveling data; W0 = mQcosα; m is the coal seam mining thickness; Q is the subsidence coefficient; α is the coal seam inclination; P is the integration area; r is the main influence radius; and μ and ν are the differential quantities in the x and y directions.

[0034] A computer device includes a processor and a memory, wherein the processor is electrically connected to the memory, the memory is used to store instructions and data, and the processor is used to execute a method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden.

[0035] Beneficial effects: This method is based on the fact that surface subsidence in the insufficient mining stage is greatly affected by the overburden structure, and combined with the characteristics of good overall integrity and undeveloped joints and faults of the extremely thick and weakly cemented overburden, a prediction method for surface subsidence in the insufficient mining of extremely thick and weakly cemented overburden is established based on field measured data, indoor experiments and theoretical analysis. The calculation process is simple and provides a scientific theoretical basis for solving the potential hidden dangers caused by the operation of oil and gas pipelines under the influence of mining.

[0036] This method first calculates and studies the maximum tensile stress value of the overlying rock strata during the mining process of the working face of the mining area, and compares it with its own tensile strength. If it is less than the maximum tensile stress value, it means that no damage has occurred. If there is no damage, the plate theory is used to calculate the deflection value of the rock stratum. Then, the deflection space is used as the mining coal seam and the probability integral method is used to predict the surface deformation. When all rock strata are damaged, the probability integral method is directly calculated based on the coal seam mining thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a schematic diagram comparing the predicted and measured surface subsidence under different advance distances of actual mining in the working face in an embodiment of the present invention.

[0038] Figure 2 Schematic diagram comparing the predicted results and the measured results of the probability integration method without corrected parameters and the predicted results of the probability integration method after corrected parameters in an embodiment of the present invention. DETAILED DESCRIPTION

[0039] The present invention will be further described in detail below with reference to the figures and specific implementation process:

[0040] The present invention's method for predicting surface subsidence due to inadequate mining of extremely thick and weakly cemented overburden is based on the special sedimentary environment of the Shaanxi-Inner Mongolia mining area, where the overburden structure is significantly different from that of the eastern mining area. Under the strata are multiple layers of thick to extremely thick layered muddy and weakly cemented sandstone, resulting in regional surface subsidence patterns that differ from conventional understandings.

[0041] Determine the rock layer thickness, mechanical properties and coal seam mining thickness based on data and measured data, and determine the rock layer plate size based on actual working face mining advancement speed and other data;

[0042] The strength and stability of a solid and complete rock structure are relatively high. Considering the synergistic effect of its own gravity and the pressure of the overlying rock strata, the uniformly distributed load on the rock stratum is calculated based on the principle of equal synchronous deformation curvature. Now assume that there are n groups of overlying rock strata, and the rock strata from the top of the coal seam to the surface are the first rock layer to the nth rock layer. Then these n groups of rock strata undergo synchronous deformation, and the load q acting on the first rock layer is expressed as:

[0043]

[0044] In the formula, (q n )1 refers to the load value of the overlying n groups borne by the first layer; h1 and E1 are the thickness and elastic modulus of the first layer respectively; h i is the thickness of the corresponding rock layer i; i is the gravity density of the i-th rock layer; E i is the elastic modulus of the ith rock layer.

[0045] According to the mechanical properties and thickness of the extremely thick weakly cemented rock stratum after mining, the deformation of the rock and soil above it is coordinated and controlled by the main key layer during the rock stratum deformation movement. The rock stratum properties and mining conditions are both important factors affecting surface subsidence. According to the actual mining area drilling data and working face mining information, the rock stratum plates under the stratum are divided, and the rock stratum is divided into thick plates and thin plates. When h / L≤0.125, it is a thin plate, and when h / L>0.125, it is a medium-thick plate, where h is the rock stratum thickness and L is the rock stratum size value.

[0046] For thick, weakly cemented rock formations, based on Vlasov's thick plate theory, the mid-surface of the rock formation is used as the reference plane. The following is the entire process from calculating the maximum tensile stress on the thick plate during mining at the working face, determining the state of rock formation damage, calculating the rock formation deflection value, and then calculating the surface subsidence value using the probability integral method using the rock formation deflection value as the coal seam mining thickness:

[0047] First, the extremely thick weakly cemented rock layer is a thick plate. The flexural deformation of the extremely thick weakly cemented rock layer is calculated by Vlasov thick plate theory:

[0048] Based on various assumptions, the internal force and internal moment equations of thick plates are established:

[0049]

[0050] In the above formula, Mx and My are the bending moments of the rock plate around the x and y axes; υ is the Poisson's ratio; w is the deflection of the thick plate; D is the bending stiffness of the rock plate; Mxy is the torque of the rock plate; G is the shear modulus of the rock plate; Ψx and Ψy are the rotation angles of x and y; and Qx and Qy are the transverse shear forces of the rock plate along the x and y directions, respectively. The calculation equations for the bending stiffness D and the shear modulus G are:

[0051]

[0052] Where E is the elastic modulus of the rock stratum; h is the thickness of the rock stratum; and υ is the Poisson's ratio. Thick, weakly cemented rock stratum has almost no cracks. During the deformation of the rock stratum, it is embedded in the rock mass and bends. The differential equilibrium equation of the rock stratum is:

[0053]

[0054] In the above formula, is the Laplace factor; Φ is the stress function; and q(x,y) is the uniformly distributed load acting on the slab. Using double trigonometric functions, we assume that the deflection, rotation, and load of the slab are displacement functions. Under the fixed boundary conditions, the boundary rotation, bending moment, and deflection are all zero.

[0055] w| x＝0,x＝a =0,ψ y | x＝0,x＝a =0,M y | x＝0,x＝a =0

[0056] w| y＝0,y＝b =0,ψ x | y＝0,y＝b =0,M x | y＝0,y＝b =0

[0057] In the above formula, W| x＝0,x＝a 、W| y＝0,y＝b are the deflections of the boundaries x=0, x=a and y=0, y=b, respectively; ψ y | x＝0,x＝a is the rotation angle around the y-axis at the boundary of x=0,x=a; x | y＝0,y＝b is the rotation angle around the x-axis at the boundary of y=0,y=b; y | x＝0,x＝a 、M x | y＝0,y＝b are the bending moments around the x and y axes at the boundary x=0, x=a and the boundary y=0, y=b, respectively; where a and b are the length and width of the plate. Solve for the coefficient A of the double trigonometric function mn ,B mn ,C mn When m=n=1, it still has high convergence and can obtain high accuracy, which is suitable for the application and solves A 11 ,B 11 ,C 11 for:

[0058]

[0059] Now A 11 ,B 11 ,C 11 Combined with the deflection and rotation of the rock stratum plate into the internal force and internal moment function equations of the rock stratum plate, the bending moment expressions of the rock stratum plate around the x and y axes are deduced, and the maximum bending moment value is obtained as follows:

[0060]

[0061] Where M h(max) is the maximum bending moment of the thick plate; q is the uniformly distributed load on the rock layer; υ is the Poisson's ratio of the rock layer; h is the thickness of the rock layer; a and b are the length and width of the rock layer plate. The bending moment is the largest at the center of the rock layer plate, while the thin rock layer is modeled based on the elastic thin plate theory. The maximum bending moment of the thin plate is also the largest at the center of the rock layer plate. The maximum bending moment equation of the thin plate is:

[0062]

[0063] Where M b(max) Refers to the maximum bending moment of the thin plate; q is the uniformly distributed load on the rock layer; υ is the Poisson's ratio of the rock layer;

[0064] The main damage during rock movement and deformation is tensile fracture. The structural state of the rock stratum is an important criterion for this prediction method. In the stage of incomplete mining, the deformation of thick and weakly cemented rock strata is mainly bending deformation, and the deformation is small, which reflects the small subsidence of the surface. The maximum tensile stress value is concentrated in the center of the lower surface of the rock stratum. Combined with the mechanical rock stratum theory, the maximum tensile stress calculation equation is:

[0065]

[0066] Where σ max is the maximum tensile stress value; M max is the maximum bending moment; σ h(max) , σ b(max) where q is the maximum tensile stress for thick and thin rock layers, respectively; υ is the Poisson's ratio; h is the thickness; and a and b are the length and width of the rock slab. When the rock slab's tensile strength is greater than its maximum tensile stress, it can maintain structural stability; otherwise, it will experience tensile failure. Combining the solved functions for the rock slab's deflection, rotation, and load, we obtain the following deflection equation for the rock slab:

[0067]

[0068] In the above formula, w h 、w b are the deflection values ​​of the thick plate and thin plate at the x and y points of the rock plate respectively; D is the bending stiffness of the thick plate; G is the shear deformation modulus of the rock plate; a and b are the length and width of the rock plate.

[0069] When the thick and weakly cemented rock layer is not damaged, the bending deformation of each point on the rock stratum is calculated according to the actual mining progress of the working face. Then, the bending deformation of each point is regarded as the thickness of the coal mined at the current position, and the subsidence deformation of the rock stratum, its overlying rock and soil is calculated by the probability integral method:

[0070]

[0071] Where W(x,y) represents the subsidence deformation value of the surface point (x,y); W y (x,y) is the deflection value at point (x,y) on the rock plate; P is the integration area; r is the main influence radius; and μ and ν are the differential quantities in the x and y directions.

[0072] If all the rock strata are broken, the probability integral method is used to directly calculate the subsidence deformation value of the ground surface after the rock strata are broken:

[0073]

[0074] Where W(x,y) represents the subsidence deformation value of the surface point (x,y); W0 is the maximum surface subsidence value, which is determined according to the surface measured leveling data; W0 = mQcosα; m is the coal seam mining thickness; Q is the subsidence coefficient; α is the coal seam inclination; P is the integration area; r is the main influence radius; and μ and ν are the differential quantities in the x and y directions.

[0075] When the rock stratum is tensilely broken, the surface movement and deformation value after the rock stratum is broken is calculated according to the surface movement and deformation prediction model of the probability integral method, thereby realizing the prediction of surface subsidence and deformation of the extremely thick and weakly cemented overburden rock under insufficient mining in the Shaanxi-Inner Mongolia mining area.

[0076] This method uses plate theory to consider the special overburden structure and physical properties of the rock strata in the extremely thick and weakly cemented overburden mining area from a mechanical perspective, and calculates the rock stratum deflection that coordinates the deformation of the overburden rock and soil. Based on this, it is combined with the traditional probability integral method to predict the subsidence deformation of the overburden rock and soil, that is, to achieve surface subsidence prediction. Compared with the probability integral method for predicting surface subsidence directly starting from the coal seam, this method realizes the prediction of non-full mining surface subsidence from a mechanical perspective by considering the control effect of extremely thick and weakly cemented rock strata.

[0077] Because rock strata movement and deformation are complex processes, the phenomenon caused by rock deformation being transmitted to the surface is known as surface subsidence. Traditional surface prediction methods are not suitable for predicting regional subsidence in mining areas with thick, weakly cemented overburden, and the predicted results differ significantly from actual results. Surface prediction in mining areas with thick, weakly cemented overburden requires consideration of the mechanical properties of the overburden. Especially during the incomplete mining phase, surface subsidence is significantly affected by the overburden structure and physical properties. This prediction method, based on the actual geological environment of the mining area, divides the underlying rock slabs. Based on the advance distance of the mining face, the deformation and failure processes of each slab are analyzed from bottom to top. First, the maximum tensile stress of the first slab is calculated and compared with its own tensile strength. When the maximum tensile stress exceeds its own tensile strength, the rock stratum breaks. When the maximum tensile stress is less than its own tensile strength, the rock stratum remains stable and undergoes bending deformation. The bending deformation values ​​of the next higher stratum are then calculated. The status of each rock layer is analyzed in this way, and the range of the water-conducting fracture zone and the sinking bending zone is divided according to whether the rock layer is damaged. When the rock layer is not completely destroyed, the sinking bending zone should be the main analysis object before predicting the surface subsidence, and the rock layer that plays a coordinating and controlling role in the rock layer movement should be analyzed. Then, the space formed by the bending deformation of the rock layer is regarded as the coal seam mining thickness, and the probability integral method is used to predict the surface subsidence value based on the coordinated rock layer bending deformation. When all the overlying rock layers are broken, the probability integral method is used to predict the surface subsidence according to the coal seam mining thickness.

[0078] Implementation Cases

[0079] Step 1: First, use the measured drilling data in the mining area to determine the stratigraphic conditions in the mining area, obtain the thickness information of each rock layer, and divide it into each rock layer. At the same time, confirm its physical and mechanical properties and analyze the overburden structure. The stratigraphic conditions include the geological mining conditions in the mining area and the mechanical properties of the rock layer.

[0080] The 1208 working face of the 12th panel area of ​​Shilawusu coal mine in Ordos City, which has a thick weakly cemented rock layer, was taken as the research object. According to the drilling data of the mining area, the strata developed from old to new are: Yan'an Formation (J1-2y) of the Middle and Lower Jurassic, Zhiluo Formation (J2z) of the Middle Jurassic, Anding Formation (J2a) of the Middle Jurassic, Zhidan Group (K1zh) of the Lower Cretaceous, Holocene of the Quaternary (Q4eol), and the coal-bearing strata are the Middle and Lower Jurassic. The thick weakly cemented overburden in this stratigraphic environment is distributed in the Zhidan Group of the Lower Cretaceous System. The average thickness of the Holocene System of the Quaternary System (Q4eol) is 20 meters, the average thickness of the Zhidan Group of the Lower Cretaceous System is 330 meters, the average thickness of the Anding Formation (J2a) of the Middle Jurassic System is 80 meters, the average thickness of the Zhiluo Formation (J2z) of the Middle Jurassic System is 180 meters, and the average thickness of the rock stratum from the top boundary of the Yan'an Formation (J1-2y) of the Middle and Lower Jurassic System to the top of the coal seam is 70 meters.

[0081] The stratigraphic information and mechanical parameters of the Shilawusu coal mine area obtained from the drilling data and drilling rock mechanics test report are shown in the following table.

[0082] Table 1 Distribution of rock strata and rock stratum mechanical parameters in the Shilawusu coal mine area

[0083]

[0084]

[0085] Step 2: Determine the state change of the overburden structure in the area during the working face advancement project based on the physical and mechanical properties and the internal force and internal moment equations and equilibrium differential equations of the rock strata; calculate the maximum bending moment of the rock strata using the formula:

[0086]

[0087] According to the maximum bending moment and rock thickness, use the formula: σ max =6M max / h 2 Calculate the maximum tensile stress of the rock layer and then judge the fracture situation of the rock layer. h is the thickness of the rock layer. Figure 1 As shown, based on the measured data, the maximum tensile stress values ​​of each rock layer when the working face advances 350m, 530m, 750m, 900m, 1320m and 1500m are calculated. The calculation results are shown in the following table.

[0088] Table 2 Maximum tensile stress values ​​of various thick rock layers at different advancement distances

[0089]

[0090] During the advancement of the working face, the tensile stress on the thick and weakly cemented overburden is less than its own tensile strength, which can maintain the stability of its own structure. Moreover, the tensile stress gradually increases with the increase of the advancement distance.

[0091] Table 3 Maximum tensile stress values ​​of thin rock layers at different advancement distances

[0092]

[0093] Comparing the calculated results in the table with the tensile strength obtained in the laboratory, it can be seen that except for the extremely thick and weakly cemented rock layers that remain stable, other rock layers are broken during the advancement of the working face.

[0094] Step 3: Use the flexural equation to calculate the bending deformation value of the unbroken thick and weakly cemented overburden. Then, consider the flexural value as the coal seam thickness and use the probability integral method to calculate the subsidence deformation value of the thick and weakly cemented overburden:

[0095]

[0096] The predicted surface subsidence values ​​when the working face advances 350m, 530m, 750m, 900m, 1320m and 1500m are compared with the actual surface observation data and plotted into a graph, such as Figure 2 shown.

[0097] like Figure 2 As shown in Table 4, the predicted value curve of the prediction model established based on the overburden structure angle is highly consistent with the measured value curve. The prediction errors at different advancement distances are shown in Table 4.

[0098] Table 4 Error between predicted and measured values ​​of surface subsidence at different advancement distances

[0099]

[0100] To compare the prediction accuracy of this method with that of the traditional probability integral model, surface subsidence prediction was performed using the established method, the conventional probability integral method, and the probability product method with modified prediction parameters based on the measured leveling data, using the measured leveling data at a working face advance of 1800 m as a baseline. The prediction parameters for the conventional probability integral method were: subsidence coefficient q of 0.65, tangent of the main influence angle of 1.34, mining propagation angle of 90°, inflection point offset of 0.09H, and horizontal movement coefficient of 0.34. The modified prediction parameters based on the measured data were: subsidence coefficient q of 0.22, tangent of the main influence angle of 1.34, mining propagation angle of 90°, inflection point offset of 0.1H, and horizontal movement coefficient of 0.34.

[0101] The root mean square error between the predicted subsidence value and the measured value when the working face advanced 1800m using the probability integral method without parameter correction reached 1128.44mm. After parameter correction, the root mean square error between the predicted subsidence value and the measured value at each point by the probability integral method reached 83.84mm. When the method was used to predict the subsidence value at 1800m of working face advancement, the root mean square error between the predicted value and the measured value was only 12.20mm. The accuracy and reliability of the predictions made by this method were verified using measured leveling data, providing new technical support for ensuring the safety of oil and gas pipelines under the influence of regional mining.

[0102] Surface subsidence is the subsidence phenomenon caused by the reflection of rock movement on the surface. In the overburden of Shaanxi and Inner Mongolia mining areas, multiple layers of thick muddy weakly cemented sandstone of different particle sizes are commonly distributed. The special stratigraphic sedimentary environment causes the surface subsidence law to be quite different from the conventional understanding. Especially in the stage of non-full mining, the surface subsidence is greatly affected by the structure and integrity of the rock layer. Taking the stratum assumed in claim 1 as an example, the thickness of the extremely thick weakly cemented sandstone overburden layer is nearly 280 meters, and the faults and joints are basically undeveloped, and the structure between the rock layers is tight. When the coal seam is mined, the extremely thick weakly cemented rock layer can effectively resist the stress concentration generated during the mining process. Due to the mechanical properties and thickness of the extremely thick weakly cemented rock layer itself, the surface subsidence is seriously small. In the stage of insufficient mining, the overburden of extremely thick and weakly cemented rock has an important influence on the surface subsidence. By solving the deflection value of each point on the rock slab and then regarding the deflection value of each point as the mining thickness of the coal seam at that point, the settlement deformation of the overburden on the curved rock slab is calculated by the probability integral method, and the surface subsidence prediction in the stage of insufficient mining of extremely thick and weakly cemented overburden is realized.

[0103] According to the classification criteria of h as the thickness of the rock plate and L as the length of the short side of the rock plate, when h / L≤0.125, it is a thin plate, and when h / L>0.125, it is a medium-thick plate. The extremely thick weakly cemented overburden is a thick plate.

[0104] Assume that the overlying rock layers on a certain working face are divided into 7 groups. The extremely thick and weakly cemented overlying rock layer is the 6th rock layer, and the surface soil layer above it is the 7th rock layer. The extremely thick and weakly cemented rock layer is subjected to the pressure of the overlying rock layer and its own gravity. Now (q7)6 refers to the load value borne by the 6th rock layer, i.e., the extremely thick and weakly cemented rock layer; h6 and E6 are the thickness and elastic modulus of the extremely thick and weakly cemented rock layer, respectively; h i is the thickness of the corresponding rock layer i; i is the gravity density of the i-th rock layer; E i is the elastic modulus of the i-th rock layer. Considering the synergistic effect of its own gravity and the pressure of the overlying rock layer, the uniformly distributed load on the rock layer is calculated based on the principle of equal synchronous deformation curvature. The calculation method for the uniformly distributed load on the thick and weakly cemented rock layer is:

[0105]

[0106] In order to obtain the bending deformation of the extremely thick and weakly cemented rock layer, it is necessary to first determine the structural stability of the extremely thick and weakly cemented overburden during coal seam mining. By using the Vlasov thick plate theory to calculate the maximum tensile stress value of the extremely thick and weakly cemented rock layer at different mining advance distances, the maximum tensile stress value is compared with its tensile strength. When the maximum tensile stress value is less than the tensile strength, the extremely thick and weakly cemented rock layer has not broken, and the bending deformation of the thick plate can be calculated.

[0107] The differential equilibrium equation for thick slate:

[0108]

[0109] Where, is the Laplace factor; Φ is the stress function; q(x,y) is the uniformly distributed load acting on the rock stratum; D is the bending stiffness; and G is the shear deformation modulus.

[0110] Thick and weakly cemented rock strata have almost no cracks. During the deformation process of the rock strata, the surrounding areas are embedded in the rock mass and deform, and the rock strata are mainly damaged by tensile fracture. The maximum bending moment at the center of the rock strata is calculated based on the physical rock strata theory. The maximum tensile stress on the rock strata is also concentrated at the center of the lower surface of the rock strata. The calculation method of the maximum tensile stress is:

[0111]

[0112] Where σ h(max) , σ b(max) are the maximum tensile stress values ​​of the thick plate and thin plate respectively; q is the uniformly distributed load on the rock layer; υ is the Poisson's ratio of the rock layer; h is the thickness of the rock layer; a and b are the length and width of the rock layer respectively.

[0113] When the maximum tensile stress is greater than its own tensile strength, the rock layer breaks. At this time, the bending value of the rock layer cannot be calculated. If the entire rock layer breaks, only the traditional probability integral method is used for prediction.

[0114]

[0115] Where W(x,y) represents the subsidence deformation value of the surface point (x,y); w max is the maximum surface subsidence value; P is the integration area; r is the main influence radius; and μ and ν are the differential quantities in the x and y directions.

[0116] When the maximum tensile stress is less than its own tensile strength, the rock layer remains stable and bends. The deflection equation of the rock layer is:

[0117]

[0118] In the above formula, W y (x, y) is the deflection value of the rock plate at point (x, y); D is the bending stiffness of the rock plate; G is the shear deformation modulus of the rock plate; a and b are the length and width of the rock plate.

[0119] After determining the bending deformation of the rock stratum, the deflection value of each point is regarded as the thickness of the coal seam mined at the current point. The probability integral method is used to predict the subsidence deformation value of the overlying rock and soil in the curved space, that is, the surface subsidence value. The calculation formula is:

[0120]

[0121] Where W(x,y) is the predicted subsidence value of the surface point (x,y), W y (x,y) is the deflection value at point (x,y) on the rock plate; P is the integration area; r is the main influence radius; and μ and ν are the differential quantities in the x and y directions.

[0122] The movement of rock strata in the non-full mining stage is greatly affected by the overburden rock structure and rock strata properties, and the probability integral method is no longer applicable due to the particularity of the extremely thick and weakly cemented rock strata themselves. The new prediction method proposed here first uses the rock stratum plate theory to calculate the flexural deformation value of the rock stratum during the mining process, and regards the flexural space of the rock stratum bending deformation as the mining space. Then, the probability integral method is used to predict the subsidence deformation value of the upper surface soil layer, and then the predicted value of the surface subsidence is obtained.

[0123] This method determines the state of the overlying rock strata before prediction, calculates the tensile stress values ​​from the rock strata above the coal seam upwards, and determines the stability information of each rock stratum during coal seam mining after comparing it with its own tensile strength. When all rock strata are broken, the probability integral method is used to predict surface subsidence. For rock strata that have not been broken, the broken rock strata are separated from the unbroken rock strata, and only the bending deformation value of the unbroken rock strata is calculated. The probability integral method is then used on this basis to predict surface subsidence deformation. This method fully considers the characteristics of the non-full mining stage and the rock properties of the extremely thick and weakly cemented rock strata, and provides technical support for solving the coal compression problem of underground oil and gas pipelines in mining areas with similar stratum environments.

Claims

1. A method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden, characterized by: The specific steps are as follows: Strata with a difference in attribute data between coal seams and surface rock layers less than a preset value are divided into the same rock stratum, and thick, weakly cemented overburden rocks above coal seams are divided into multiple rock stratum plates. The mechanical properties of each rock stratum plate are represented by the average mechanical properties of the collected rock samples. If it is determined that there are unbroken strata in the thick, weakly cemented overburden below the current working face advancement distance, the deformation of each unbroken stratum is analyzed from bottom to top, and the coordinating control stratum that coordinates the deformation of all the strata above it is found. The flexural deformation value of the coordinating control stratum in the non-full mining stage is calculated. The calculated deflection value of each point on the coordinated control rock plate is used as the coal seam mining thickness, and the probability integral method is used to calculate the surface subsidence deformation value, thereby realizing the accurate prediction of surface subsidence deformation of extremely thick weakly cemented overburden under insufficient mining.

2. The method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden according to claim 1 is characterized in that: The strata are divided into thin plates and thick plates according to the ratio of the short side size to the thickness: h is represented as the thickness of the strata and L is the length of the short side of the strata. When h / L ≤ 0.125, it is judged as a thin plate, and when h / L > 0.125, it is judged as a thick plate. During the mining process of the working face, the deformation of the thick plate is calculated using the Vlasov thick plate theory, and that of the thin plate is calculated using the elastic thin plate theory.

3. The method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden according to claim 2 is characterized in that: According to the actual drilling data of the research mining area and the mechanical test report of the indoor rock samples, there are n layers of rock plates from the coal seam to the surface. The rock plates are from the first layer to the nth layer from the bottom to the top. The n layers of rock plates can deform synchronously. The load acting on the first layer of rock plate is (q n )1 means: In the formula, (q n )1 refers to the load value of the n layers of rock slabs overlying the first layer of rock slab; h1 and E1 are the thickness and elastic modulus of the first layer of rock slab respectively; h i is the thickness of the corresponding rock layer i; i is the gravity density of the i-th rock layer; E i is the elastic modulus of the i-th rock layer.

4. The method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden according to claim 2 is characterized in that: The method for determining whether there are unbroken strata in the thick and weakly cemented overburden mining area at different working face advancement distances is as follows: From bottom to top, the maximum tensile stress value of each rock plate under different mining advancement distances of the coal seam is calculated in sequence, and the calculated maximum tensile stress value of each rock plate is compared with the tensile strength of the corresponding rock layer. When the maximum tensile stress value is less than the tensile strength, it is judged that the rock plate does not break, its own state is stable, and only bending deformation occurs.

5. The method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden according to claim 4 is characterized in that: Analyzing the deformation of each stratum plate during the mining process from the coal seam upwards, before the stratum plate breaks, it is embedded in the rock mass on all sides and supported by the lower stratum plate; Using the formula: Determine the length a and width b of each stratum plate, where l represents the distance of advancement of the working face, k represents the length of the working face inclination, H represents the mining depth, z represents the depth of the stratum, and β1 and β2 represent the main influencing angles of the mining strike and inclination; The maximum bending moment at the center of each stratum plate is calculated using the following formula, where the thick plate is calculated based on M h(max) Calculate the corresponding formula, and select M for thin plate b(max) Corresponding formula calculation: In the above formula, M h(max) 、M b(max) are the maximum bending moments of thick plates and thin plates respectively; q is the uniformly distributed load on the rock plate; υ is the Poisson's ratio of the rock plate; the maximum bending moment value at the center of all rock plates can be obtained; Calculate the maximum tensile stress of all rock strata on the coal seam from bottom to top; Where σ h(max) , σ b(max) are the maximum tensile stress values ​​of the thick plate and thin plate respectively; q is the uniformly distributed load on the rock plate; υ is the Poisson's ratio of the rock plate; h is the thickness of the rock plate; a and b are the length and width of the rock plate, and the thick plate is calculated based on σ h(max) The calculation is performed by the formula, and the thin plate selects σ b(max) The corresponding formula is used to calculate the maximum tensile stress value.

6. The method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden according to claim 5 is characterized in that: When the tensile strength of the stratum plate is less than the maximum tensile stress it is subjected to, the stratum plate is judged to be broken, and the deflection value of the stratum plate that has not been broken is calculated using the following formula: Where w h 、w b are the deflection values ​​of the thick plate and thin plate at their respective x and y coordinate positions; thick and thin strata plates have corresponding bending stiffness and shear deformation modulus, D represents the bending stiffness of the strata plate; G represents the shear deformation modulus of the strata plate, and the corresponding parameters are substituted into the calculation formula.

7. The method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden according to claim 6 is characterized in that: Since there are cases where the rock strata are not broken or all the rock strata are broken during surface subsidence and deformation, the prediction methods are as follows: When a stratum plate undergoes bending deformation, and the bending deformation is smaller than that of the stratum plate above it, it is judged that the stratum plate acts as a coordinated control stratum plate to coordinate the deformation of all the stratum plates above it. When the coordinated control stratum plate is not damaged but only undergoes bending deformation, the bending deformation value W of the unbroken stratum plate under the insufficient mining stage is calculated based on the flexural equation constructed by Vlasov thick plate theory and elastic thin plate theory. y , the flexural deformation space of the rock stratum is regarded as the coal seam mining space, and the bending deformation value is regarded as the coal seam mining thickness. The probability integral method based on the discontinuous medium theory is used to calculate the subsidence deformation value of the surface subsidence: Where W(x,y) represents the subsidence deformation value of the surface point (x,y); W y (x,y) is the deflection value at point (x,y) on the rock plate; P is the integration area; r is the main influence radius; and μ and ν are the differential quantities in the x and y directions.

8. The method for predicting surface subsidence due to insufficient mining of extremely thick and weakly cemented overburden according to claim 7 is characterized in that: If all the rock strata are broken, the probability integral method is used to directly calculate the subsidence deformation value of the ground surface after the rock strata are broken: Where W(x,y) represents the subsidence deformation value of the surface point (x,y); W0 is the maximum surface subsidence value, which is determined according to the surface measured leveling data; W0 = mQcosα; m is the coal seam mining thickness; Q is the subsidence coefficient; α is the coal seam inclination; P is the integration area; r is the main influence radius; and μ and ν are the differential quantities in the x and y directions.

9. A computer device, characterized in that: The method comprises a processor and a memory, wherein the processor is electrically connected to the memory, the memory is used to store instructions and data, and the processor is used to execute the surface subsidence prediction method due to insufficient mining of extremely thick weakly cemented overburden rock as described in any one of claims 1-8.

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