Chain type instability evaluation method for coal pillar group-roof system

Abstract

The invention belongs to the technical field of coal mine room and pillar mining, and particularly relates to a coal pillar group-roof system chain type instability evaluation method. Comprising the steps of calculating the effective subordinate bearing area of each coal pillar, the elastic foundation coefficient of an effective supporting area, the borne average stress and the strength of the effective bearing size; calculating the safety coefficient of each coal pillar in the goaf, and judging that the coal pillar with the safety coefficient less than a critical value is unstable and the coal pillar with the safety coefficient greater than the critical value is not unstable; all the unstable coal pillars are deleted, and the adjacent coal pillars of all the unstable coal pillars are calculated again until all the coal pillars with the safety coefficients smaller than the critical value are deleted; delineating a suspended exposed roof range which is not supported by the coal pillar group; calculating the bending moment of the exposed roof, solving the maximum bending moment by using a limit method, and calculating the maximum tensile stress borne by the exposed roof; and judging whether the exposed roof is broken or not. According to the method, the possible chain collapse risk can be predicted by identifying the potential coal rock instability trigger point, and technical support is provided for coal mining under complex geological conditions.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal mine room-and-pillar mining, and particularly relates to a method for evaluating the chain instability of a coal pillar group-roof system. Background Art

[0002] After the formation of a room-type goaf, a large number of remaining coal pillars exist densely to form a coal pillar group, and the coal pillar group and the roof constitute a coal pillar group-roof system. The stability of the remaining coal pillar group and the roof is the decisive factor for the overall stability of the goaf. For a long time, many experts and scholars have carried out a series of research and practices on the stability of the coal pillar group and the roof, and on this basis, many evaluation methods for the instability of the room-type goaf have been given. These rich research results provide an important guarantee for the safety of the room-type goaf. The instability of the coal pillar group-roof system is the inducement and precursor for the occurrence of greater disasters in the mine. However, there is a lack of instability criteria established with the coal pillar group-roof system as an overall object in the existing research results, and it cannot provide a basis for the effective prediction of mine goaf disasters. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention proposes a method for evaluating the chain instability of a coal pillar group-roof system, including the following steps:

[0004] S1: Obtain the geological and mining parameters of the coal pillars in the room-type goaf;

[0005] S2: Determine the rib spalling size of each coal pillar;

[0006] S3: Calculate the effective subordinate bearing area of each coal pillar, calculate the elastic foundation coefficient of the effective support area of each coal pillar, calculate the average stress borne by each coal pillar; calculate the strength of the effective bearing size of each coal pillar;

[0007] S4: Calculate the safety factor of each coal pillar in the goaf, determine that the coal pillar with a safety factor less than the critical value is unstable, and the coal pillar with a safety factor greater than the critical value is stable;

[0008] S5: Delete all unstable coal pillars, and return to S3 to recalculate the adjacent coal pillars of all unstable coal pillars until all coal pillars with a safety factor less than the critical value are deleted;

[0009] S6: Encircle the range of the hanging roof that loses the support of the coal pillar group;

[0010] S7: According to the boundary conditions of the hanging roof, calculate the bending moment of the hanging roof, obtain the maximum bending moment by the limit method, and calculate the maximum tensile stress borne by the hanging roof;

[0011] S8: Judge whether the hanging roof will break. If the maximum tensile stress borne by the hanging roof is less than the allowable stress of the roof, the roof is stable; otherwise, the hanging roof breaks and collapses.

[0012] Preferably, in step S1, the mining parameters include the positions, initial lengths, initial widths, and heights of the coal pillars; a coordinate system is established with the coal pillar length direction as the x-axis and the coal pillar width direction as the y-axis.

[0013] Preferably, in step S2, the rib spalling size of coal pillar i is calculated according to formula (1);

[0014] l pbi = 0.1624h pi 0.8135 t i 0.1865 (1)

[0015] In the formula: l pbi is the rib spalling size of coal pillar i; h pi refers to the height of coal pillar i; t i refers to the formation time of coal pillar i.

[0016] Preferably, in step S3, the elastic foundation coefficient of the effective support area of coal pillar i is calculated using formula (2)

[0017]

[0018] In the formula: k pi is the elastic foundation coefficient of the effective support area of coal pillar i; E pi represents the elastic modulus of coal pillar i; l pxi represents the length of coal pillar i in the x direction; l pyi represents the width of any coal pillar i in the y direction; l fxi represents the length of the coal roadway at coal pillar i in the x direction; l fyi represents the width of the coal roadway at coal pillar i in the y direction.

[0019] Preferably, in step S3, the average stress borne by coal pillar i is calculated using formula (3)

[0020]

[0021] In the formula: σ pi is the average stress borne by coal pillar i; x i1 and x i2 respectively represent the original starting and ending coordinates of coal pillar i in the x direction, y i1 and y i2 respectively represent the original starting and ending coordinates of coal pillar i in the y direction; ω i is the deflection of the roof at coal pillar i.

[0022] Preferably, in step S3, the strength of the effective bearing size of coal pillar i is calculated using formula (4)

[0023]

[0024] Where: σ qi is the strength of the effective bearing size of coal pillar i; σ mi is the uniaxial compressive strength of coal pillar i; l pi is the length of the original short side of coal pillar i; α is a constant. When l pi / h pi > 5, α takes the value of 1.4. When l pi / h pi < 5, α takes the value of 1.0.

[0025] Preferably, in step S4, the safety factor of each coal pillar i in the goaf is calculated using formula (5)

[0026]

[0027] Where: f pi is the safety factor of coal pillar i.

[0028] Preferably, in step S7, the bending moments of the four-sided fixed and suspended roof and the four-sided simply supported and suspended roof are calculated using formula (6) and formula (7) respectively

[0029]

[0030] Where: M x is the bending moment of the suspended roof in the x direction; M y is the bending moment of the suspended roof in the y direction; q is the overlying rock load; a is the length of the suspended roof; b is the width of the suspended roof; μ r is the Poisson's ratio of the roof;

[0031]

[0032] Preferably, in step S7, the maximum tensile stress borne by the suspended roof is calculated using formula (8)

[0033]

[0034] Where: σ T is the maximum tensile stress borne by the suspended roof; M max is the maximum bending moment of the suspended roof; h r is the roof thickness.

[0035] Preferably, in step S8, it is judged whether the suspended roof will break using formula (9)

[0036]

[0037] Where: [σ] is the allowable tensile strength of the roof.

[0038] Beneficial technical effects: The research on the method for evaluating the chain instability of the coal pillar group-roof system in this invention can identify potential triggering points of coal and rock instability by analyzing the interaction between the coal pillar group and the roof, improve the theoretical system for evaluating the stability of goafs, predict the possible risk of chain collapse, avoid sudden mine disasters, and provide technical support for coal mining under complex geological conditions. Description of the Drawings

[0039] Figure 1 It is the discriminant flow chart for the chain instability of the coal pillar group-roof system in a room-and-pillar goaf. Specific Embodiments

[0040] The following introduces the specific embodiments of the present invention in conjunction with the drawings.

[0041] The chain instability of the coal pillar group-roof system in a room-and-pillar goaf is a dynamic process. When evaluating the instability of this system, it cannot be judged only by a certain component in the system, but the system should be regarded as a whole for dynamic evaluation. In this regard, the present invention proposes a method for evaluating the chain instability of the coal pillar group-roof system, as Figure 1 shown, the evaluation process is as follows:

[0042] The first step: Collect the data of the room-and-pillar goaf in the mine and conduct on-site investigation and research to obtain the positions, initial reserved lengths, widths and heights of all the remaining coal pillars in the goaf, draw a detailed distribution map of the coal pillars in the goaf, establish a rectangular coordinate system for the goaf, with the length direction of the coal pillar as the x-axis direction and the width direction of the coal pillar as the y-axis direction, number the coal pillars, the coal pillar i represents the i-th coal pillar, and determine the initial coordinate positions of each coal pillar; at the same time, obtain the basic mechanical parameters such as the uniaxial compressive strength and elastic modulus of the coal seam.

[0043] The second step: Determine the formation time of all the coal pillars in the goaf, calculate the rib spalling size of each coal pillar according to formula (1), and subtract the rib spalling size from the initial size of each coal pillar to obtain the effective bearing size and the coordinate of the effective bearing range of each coal pillar during the research period.

[0044] l pbi = 0.1624h pi 0.8135 t i 0.1865 (1)

[0045] In the formula: l pbi is the rib spalling size of a single coal pillar i at any position, m; h pi refers to the height of a single coal pillar i at any position, m; t i refers to the formation time of a single coal pillar i at any position, year;

[0046] Step 3: Calculate and determine the effective subordinate bearing area of a single coal pillar i at any position according to the subordinate area theory, calculate the elastic foundation coefficient of the effective support area of a single coal pillar i at any position using formula (2), calculate the average stress borne by a single coal pillar i at any position using formula (3); calculate the strength of the effective bearing size of a single coal pillar i at any position using formula (4).

[0047]

[0048] In the formula: k pi is the elastic foundation coefficient of the effective support area of a single coal pillar i at any position; E pi represents the elastic modulus of a single coal pillar i at any position, in GPa; l pxi represents the length of a single coal pillar i at any position in the x direction, in m; l pyi represents the width of a single coal pillar i at any position in the y direction, in m; l fxi represents the length of the coal roadway at the position of a single coal pillar i in the x direction (the spacing between coal pillars in the x direction), in m; l fyi represents the width of the coal roadway at the position of a single coal pillar i in the y direction (the spacing between coal pillars in the y direction), in m;

[0049]

[0050] In the formula: σ pi is the average stress borne by a single coal pillar i at any position, in MPa; x i1 and x i2 respectively represent the original starting and ending coordinates of a single coal pillar i in the x direction (before spalling), y i1 and y i2 respectively represent the original starting and ending coordinates of a single coal pillar i in the y direction (before spalling); ω i is the deflection of the roof at the position of a single coal pillar i, in m;

[0051]

[0052] In the formula: σ pi is the strength of the effective bearing size of a single coal pillar i at any position, in MPa; σ mi is the uniaxial compressive strength of a single coal pillar i at any position, in MPa; l pi is the length of the original short side of coal pillar i (before spalling), in m; α is a constant. When l pi / h pi > 5, α takes the value of 1.4. When l pi / h pi < 5, α takes the value of 1.0;

[0053] Step 4: Calculate the safety factors of each coal pillar in the goaf using formula (5), determine that the coal pillars with safety factors less than the critical value are unstable, and the coal pillars with safety factors greater than the critical value are stable;

[0054]

[0055] Where: f pi is the safety factor of a single coal pillar i at any position; F0 is the critical value of the coal pillar safety factor;

[0056] Step 5: Delete all unstable coal pillars, and return to Step 3 to recalculate the adjacent coal pillars of all unstable coal pillars until all coal pillars with safety factors less than the critical value are deleted, that is, all unstable coal pillars are deleted. At this time, the chain instability of the coal pillar group is considered to stop;

[0057] Step 6: Enclose the range of the overhanging roof supported by the coal pillar group, determine the area and boundary conditions of the overhanging roof, and obtain basic parameters such as the roof thickness, Poisson's ratio, allowable tensile strength, and overlying rock load;

[0058] Step 7: According to the boundary conditions of the overhanging roof, calculate the bending moments of the four-sided fixed overhanging roof and the four-sided simply supported overhanging roof using formula (6) and formula (7) respectively, and obtain the maximum bending moment using the limit method. Further, calculate the maximum tensile stress borne by the overhanging roof using formula (8);

[0059]

[0060] Where: M x is the bending moment of the overhanging roof in the x direction, kN · m; M y is the bending moment of the overhanging roof in the y direction, kN · m; q is the overlying rock load, MPa; a is the length of the overhanging roof (in the x direction), m; b is the width of the overhanging roof (in the y direction), m; μ r is the Poisson's ratio of the roof;

[0061]

[0062] Where: σ T is the maximum tensile stress borne by the overhanging roof, MPa; M max is the maximum bending moment of the overhanging roof, kN·m; h r is the roof thickness, m;

[0063] Step 8: Use formula (9) to judge whether the overhanging roof will break. If the maximum tensile stress borne by the overhanging roof is less than the allowable stress of the roof, the roof is stable; otherwise, the overhanging roof will break and collapse;

[0064]

[0065] In the formula: [σ] is the allowable tensile strength of the roof, in MPa.

[0066] The above eight steps are the complete process for evaluating the chain instability of the room-and-pillar goaf coal pillar group-roof system, which can be used to evaluate the stability state after the formation of a certain room-and-pillar goaf or predict the stability state several years after the formation of a certain room-and-pillar goaf.

[0067] In addition, through the foregoing research, it can be obtained that a series of chain instabilities are very likely to be triggered after the initial instability of a single or local coal pillar in the room-and-pillar goaf. Therefore, when proposing the prevention and control technology for the chain instability of the room-and-pillar goaf in the future, it should start from the root cause of the chain instability, that is, it is necessary to focus on and give priority to preventing and controlling the coal pillars that are most likely to be unstable first in the room-and-pillar goaf.

[0068] The present invention is not limited to the above best implementation manner. Any person can obtain various other forms of methods under the inspiration of the present invention. However, any technical solutions that are the same as or similar to the present application fall within the protection scope of the present invention.

Claims

1. A method for evaluating chain instability of a coal pillar group-roof system, characterized in that: The steps include: S1: Obtain the geological and mining parameters of coal pillars in room-type goaf; S2: Determine the size of the coal slabs of each coal pillar; S3: Calculate the effective subordinate bearing area of ​​each coal pillar, calculate the elastic foundation coefficient of the effective supporting area of ​​each coal pillar, and calculate the average stress borne by each coal pillar; Calculate the strength of each coal pillar's effective bearing size; S4: Calculate the safety factor of each coal pillar in the goaf, and determine that the coal pillar with a safety factor less than the critical value is unstable, and the coal pillar with a safety factor greater than the critical value is not unstable; S5: Delete all unstable coal pillars, and return to S3 to recalculate the adjacent coal pillars of all unstable coal pillars until all coal pillars with safety factors less than the critical value are deleted; S6: Delineate the range of the exposed roof that has lost the support of the coal pillars; S7: According to the boundary conditions of the exposed roof, the bending moment of the exposed roof is calculated, the maximum bending moment is obtained by the limit method, and the maximum tensile stress borne by the exposed roof is calculated; S8: Determine whether the overhanging roof will break. If the maximum tensile stress borne by the overhanging roof is less than the allowable stress of the roof, the roof is stable; otherwise, the overhanging roof will break and collapse.

2. The method for evaluating chain instability of a coal pillar group-roof system according to claim 1, characterized in that: In step S1, the mining parameters include the position, initial length, initial width and height of each coal pillar; a coordinate system is established with the length direction of the coal pillar as the x-axis and the width direction of the coal pillar as the y-axis.

3. The method for evaluating chain instability of a coal pillar group-roof system according to claim 1, characterized in that: In step S2, the spalling size of coal pillar i is calculated according to formula (1); l pbi =0.1624h pi 0.8135 t i 0.1865 (1) Where: l pbi h is the spalling size of coal pillar i; pi Refers to the height of coal pillar i; t i Refers to the time when coal pillar i is formed.

4. The method for evaluating chain instability of a coal pillar group-roof system according to claim 3 is characterized in that: In step S3, the elastic foundation coefficient of the effective support area of ​​coal pillar i is calculated using formula (2): Where: k pi is the elastic foundation coefficient of the effective supporting area of ​​coal pillar i; E pi represents the elastic modulus of coal pillar i; l pxi represents the length of coal pillar i in the x direction; l pyi represents the width of any coal pillar i in the y direction; l fxi represents the length of the coal room at the coal pillar i in the x direction; l fyi Represents the width of the coal room at coal pillar i in the y direction.

5. The method for evaluating chain instability of a coal pillar group-roof system according to claim 4, characterized in that: In step S3, the average stress of coal pillar i is calculated using formula (3): Where: pi is the average stress borne by coal pillar i; x i1 and x i2 They represent the original starting and ending coordinates of coal pillar i in the x direction, and y i1 and i2 Respectively represent the original starting and ending coordinates of coal pillar i in the y direction; ω i is the deflection of the roof at coal pillar i.

6. The method for evaluating chain instability of a coal pillar group-roof system according to claim 5, characterized in that: In step S3, the strength of the effective bearing size of coal pillar i is calculated using formula (4): Where: qi is the strength of the effective bearing size of coal pillar i; σ mi is the uniaxial compressive strength of coal pillar i; l pi is the length of the original short side of coal pillar i; α is a constant. pi / h pi >5, α is 1.4, when l pi / h pi When <5, α is 1.

0.

7. The method for evaluating chain instability of a coal pillar group-roof system according to claim 6, characterized in that: In step S4, the safety factor of each coal pillar i in the goaf is calculated using formula (5): Where: f pi is the safety factor of coal pillar i.

8. The method for evaluating chain instability of a coal pillar group-roof system according to claim 7, characterized in that: In step S7, the bending moments of the four-side fixed-supported exposed top slab and the four-side simply-supported exposed top slab are calculated using formula (6) and formula (7) respectively: Where: M x M is the bending moment of the exposed top plate in the x direction; y is the bending moment of the exposed roof in the y direction; q is the overburden load; a is the length of the exposed roof; b is the width of the exposed roof; μ r is the Poisson’s ratio of the top plate; 9. The method for evaluating chain instability of a coal pillar group-roof system according to claim 8, characterized in that: In step S7, the maximum tensile stress of the exposed top plate is calculated using formula (8): Where: T M is the maximum tensile stress on the exposed roof; max is the maximum bending moment of the exposed top plate; h r is the top plate thickness.

10. The method for evaluating chain instability of a coal pillar group-roof system according to claim 9, characterized in that: In step S8, formula (9) is used to determine whether the exposed top plate will break. Where: [σ] is the allowable tensile strength of the top plate.

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