Method for determining reasonable positions of pre-excavated retracement roadway and open-off cut under double remaining coal pillars

Abstract

The invention belongs to the field of coal mining, and particularly relates to a method for determining reasonable positions of a pre-excavated retracement roadway and an open-off cut under double remaining coal pillars. Comprising the steps of calculating stress of left coal pillars; establishing a stress transfer mechanism of the lower rock stratum of the left coal pillar; calculating overlying strata gravity borne by the pre-excavated retracement roadway or the open-off cut without considering the vertical stress increased by the left coal pillar; calculating stress borne by a pre-dug withdrawing roadway or a cut hole under the double-layer remaining coal pillar; selecting a pre-digging retracement roadway and an open-off cut position; and a withdrawing channel and a cut hole are pre-dug. According to the method, the vertical stress in the lower coal seam is determined by considering the vertical stress increased by the remaining coal pillars in the upper and middle coal layers and the liberation layer effect of the goaf of the upper and middle coal layers, and the pre-excavated retracement roadway and the open-off cut position are determined based on the critical safe tunneling stress and the stability of the fault protection coal pillars of the lower coal seam; and meanwhile, a pre-digging retracement roadway supporting measure is strengthened to cope with the advanced supporting pressure effect. According to the method, coal resources can be recycled to the greatest extent on the premise of ensuring the safety of the pre-excavated retracement roadway and the open-off cut.

Description

Technical Field

[0001] The invention belongs to the field of coal mining, and in particular relates to a method for determining a reasonable position of a pre-excavated retreat tunnel and a cutting eye under double remaining coal pillars. Background Art

[0002] During the mining of the upper coal seam, when a large fault appears that crosses the entire upper coal seam working face, a fault protection coal pillar is generally left in the upper coal seam working face. After the front section of the upper coal seam working face is mined, the fault protection coal pillar is skipped by the skip mining method to continue mining the rear section. Since the fault generally runs through multiple rock strata and coal seams, when the working face is arranged in the lower coal seam to mine the lower coal seam, the lower coal seam working face will also be crossed by the large fault, and it is also necessary to leave a fault protection coal pillar in the lower coal seam working face. After the front section of the lower coal seam working face is mined, the fault protection coal pillar is skipped by the skip mining method to continue mining the rear section.

[0003] Before the jump mining, the retreat tunnel will be pre-dug in the front section of the working face near the side of the safety protection coal seam, and the cut-eye will be pre-dug in the rear section near the side of the fault protection coal seam, so as to facilitate the subsequent withdrawal of mining equipment in the retreat tunnel, arrange the mining equipment in the cut-eye of the rear section as soon as possible, and recover the rear section as soon as possible to reduce the impact of production interruption caused by the safety fault coal pillar. However, the existence of the fault protection coal pillar left in the upper coal seam will bring huge challenges to the mining of the lower coal seam. When arranging the front section retreat tunnel in the lower coal seam working face, it is necessary to consider the stress concentration problem below the fault protection coal pillar left above in the lower coal seam, that is, how to determine the position of the retreat tunnel in the front section of the lower coal seam working face and the cut-eye position in the rear section, so as to maximize the recovery of coal resources under the premise of ensuring safety is a problem that technical personnel in the field urgently need to solve. If there are two working faces affected by the fault above, that is, there are two residual fault protection coal pillars above, the selection of the position of the retreat tunnel in the front section of the lower coal seam working face is more difficult. Summary of the invention

[0004] In view of the above technical problems, the present invention proposes a method for determining the reasonable position of pre-excavation and withdrawal tunnels and cutting holes under double residual coal pillars, comprising the following steps:

[0005] S1: Calculate the stress of the remaining coal pillar

[0006] The residual coal pillar is a fault protection coal pillar that runs through the working face after the upper working face and the middle working face are mined; the force F carried by the fault protection coal pillar includes the self-weight stress F0 of the overlying rock stratum directly above it, the stress F2 transmitted by the overburden above the goaf in the front section, and the stress F1 transmitted by the overburden above the goaf in the rear section:

[0007] F=F0+F1+F2 (2-1)

[0008] Where: F is the total force of the fault protection coal pillar; F1 is the transferred stress of the goaf in the rear section; F2 is the transferred stress of the goaf in the front section; F0 is the gravity of the overburden above the fault protection coal pillar;

[0009] S2: Establish the stress transfer mechanism of the rock layer under the remaining coal pillar

[0010] Assume that the end of the bottom surface of the residual coal pillar close to the goaf of the previous section is the coordinate origin, the side pointing to the goaf of the previous section is the Y direction, the vertical downward direction is the X direction, the Y-axis coordinate of any point M in the rock layer below the residual coal pillar is y, and the X-axis coordinate is x; the width of the residual coal pillar is D, and the vertical stress increased by a point M in the rock layer below the residual coal pillar due to the residual coal pillar is:

[0011]

[0012] S3: Calculate the overburden gravity on the pre-excavated retreat tunnel or cut-off without considering the vertical stress added by the remaining coal pillar

[0013] Without considering the vertical stress increased by the remaining coal pillar, the overburden gravity σ' of the pre-excavation retreat tunnel or cut in the lower coal seam is:

[0014] σ′=ηk′γH d (2-4)

[0015] Where: σ' is the overburden gravity of the pre-excavated retreat tunnel or cut-hole without considering the vertical stress increased by the legacy pillar; η is the stress reduction coefficient of the surrounding rock caused by the mining of the liberated layer. When there is no goaf directly above the pre-excavated retreat tunnel or cut-hole, η is 1; when there are two layers of goaf, η = 0.8 ~ 0.95; when there is one layer of goaf, η = 0.95 ~ 1.0. The closer the pre-excavated retreat tunnel or cut-hole is to the center of the goaf of the coal seam where the legacy coal pillar is located, the smaller the stress reduction coefficient η; k' is the comprehensive stress concentration coefficient affected by dynamic pressure; H d The depth of the lower coal seam;

[0016] S4: Calculate the stress of the pre-excavated retreat tunnel or cut hole under the double-layer coal pillar

[0017] The stress of the pre-excavated retreat tunnel or cut hole under the remaining coal pillar includes the vertical stress σ increased by the remaining coal pillar. x And the overburden gravity σ' on the pre-excavated retreat tunnel or cut-hole without considering the vertical stress increased by the remaining coal pillar;

[0018] The stress σ of the pre-excavated retreat tunnel under the double-layer legacy coal pillar includes the vertical stress σ increased by the protective coal pillar of the upper coal seam fault xu , the vertical stress σ increased by the fault protection coal pillar in the middle coal seam xm And the overburden gravity σ' on the pre-excavation retreat tunnel without considering the vertical stress increased by the remaining coal pillar;

[0019] σ=σ′+σ xu +σ xm (2-5)

[0020] Combining formulas (2-3) to (2-5), the stress in the pre-excavation tunnel or cut in the lower coal seam can be obtained as:

[0021]

[0022] Where: F u is the total force of the coal pillar left in the upper coal seam; u ,y u The coordinates of the lower right corner of the coal pillar protecting the coal seam fault above the pre-excavation retreat tunnel or the cutting eye are the origin; D u F is the width of the coal pillar protecting the upper coal seam fault; m is the total stress of the coal pillar left in the middle coal seam; m ,y m For pre-excavation of retreat tunnels or cutting holes, the coordinates are taken as the origin at the lower right corner of the middle coal seam fault protection pillar; D m The width of the coal pillar for protection of the fault in the middle coal seam;

[0023] S5: Select the pre-excavation retreat tunnel and the cutting position

[0024] Determine the critical safety vertical stress σ during excavation of the pre-excavation retreat tunnel h , let σ=σ h , according to formula (2-6), determine the critical position of the pre-excavated retreat tunnel, the pre-excavated retreat tunnel is located on the right side of the fault, and the pre-excavated retreat tunnel is the retreat tunnel of the front section;

[0025] Determination of critical safety vertical stress σ during pre-cut tunneling q , let σ=σ q , according to formula (2-6), determine the critical position of the pre-digging cut, the pre-digging cut is located on the left side of the fault, and the pre-digging cut is the cut of the rear section;

[0026] Determine the distance S between the critical position of the pre-excavation retreat tunnel and the critical position of the pre-excavation cut-off l , determine the minimum fault protection coal pillar width S of the lower coal seam working face m If S l ≥S m , then the critical position of eye cutting is taken as the eye cutting construction position, and the critical position of pre-excavation and withdrawal tunnel is taken as the pre-excavation and withdrawal tunnel construction position; if S l <S m , then the critical position of eye cutting is taken as the eye cutting construction position, and the distance from the critical position of the pre-excavated retreat tunnel to the side away from the fault is S m -S l The distance is used as the construction position of the pre-excavation retreat tunnel;

[0027] S6: Pre-digging the retracement channel and cutting the eye

[0028] Pre-excavate a withdrawal channel and cut an eye, and support them separately. The support strength of the pre-excavated withdrawal channel needs to be able to resist the damage to the pre-excavated withdrawal channel caused by the advance support pressure during the subsequent mining of the front section of the lower coal seam working face.

[0029] Preferably, in step S1, the buried depth H of the fault protection coal pillar, the strike length L1 of the goaf in the rear section, the strike length L2 of the goaf in the front section, the collapse angle α2 of the goaf in the front section, the collapse angle α1 of the goaf in the rear section, and the average bulk density γ of the overburden are determined, and the total force F of the remaining coal pillar is obtained as:

[0030]

[0031] Preferably, in step S5: the minimum fault protection coal pillar width S of the lower coal seam working face m The determination method is as follows:

[0032] Determine the initial pressure step after mining the rear section of the lower coal seam working face and set it equal to L1, determine the strike length L2 of the front section of the lower coal seam working face, and calculate the total force F of the fault protection coal pillar of the lower coal seam working face according to formula (2-2) d , compared with the strength of the lower coal seam, the minimum fault protection coal pillar width S of the lower coal seam working face is obtained m , at the minimum fault protection coal pillar width S m Under this condition, the fault-protected coal pillar is in a critical state without losing stability.

[0033] The beneficial technical effects of the present invention are as follows: the present invention determines the vertical stress in the lower coal seam by considering the vertical stress increased by the coal pillars left in the upper and middle coal seams and the role of the liberated layer in the upper and middle coal goafs, determines the position of the pre-excavated retreat tunnel and the cut-eye based on the critical safe excavation stress and the stability of the coal pillars protecting the fault of the lower coal seam, and strengthens the support measures of the pre-excavated retreat tunnel to cope with the effect of the advanced support pressure. The present invention can recover coal resources to the greatest extent under the premise of ensuring the safety of the pre-excavated retreat tunnel and the cut-eye. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the relative position relationship between the pre-excavated retreat tunnel and the cut eye under the double remaining coal pillars in an embodiment of the present invention;

[0035] Figure 2 Schematic diagram of force analysis of the strike profile of the remaining coal pillar according to an embodiment of the present invention;

[0036] Figure 3 It is a schematic diagram of a simplified model of the force transmitted to the bottom rock layer by the remaining coal pillar according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0038] Etuokeqianqi Great Wall No. 5 Mining Co., Ltd. is the main mine under the Xinkuang Group Energy. The mine currently has an annual production capacity of 2 million tons. The main coal seams in the mine include 3 coal, 5 coal and 9 coal. Figure 1 As shown in the figure, the 1301N working face in the top 3 coals has been mined, with an average mining depth of 313m; the 1501N working face in the middle 5 coals has been mined, with an average mining depth of 325m; the 1901N working face, which is being connected, is arranged in the 9 coals, with an average mining depth of 382m; the 1501N goaf is located below the 1301N goaf, and the 1901N working face is located below the 1501N goaf; the 1301N working faces, 1501N working faces, and 1901N working faces are all affected by fault DF5, and fault DF5 needs to be left in the middle of the working faces Layer protection coal pillars; the 1301N working face and the 1501N working face have been mined, the position and range of the fault protection coal pillars, i.e. the residual coal pillars, in the two goafs have been determined, and the mining range of the front section and the rear section of the 1301N working face and the 1501N working face have also been determined; affected by the two layers of residual coal pillars (1301N fault protection coal pillars and 1501N fault protection coal pillars), the selection of the position of the retreat tunnel and the cutting eye in the front section of the lower 1901N working face is particularly important. If the stress here is large, it will be unfavorable for the excavation of the retreat tunnel and the cutting eye.

[0039] The front section refers to the side of the working face that is mined first relative to the fault protection coal pillar, such as Figure 1 The rear section refers to the side of the working face mined after the fault protection coal pillar, such as Figure 1 Left side of the middle; the pre-excavation of the withdrawal tunnel in the 1901N working face means that a withdrawal tunnel is excavated in advance in the front section for the purpose of recovering the coal mining equipment, and the coal mining equipment is arranged in the cut eye of the rear section as soon as possible after recovery. Pre-excavation means that the withdrawal tunnel has been excavated before the working face of the front section is mined to the position of the withdrawal tunnel. The pre-excavation of the withdrawal tunnel of the 1901N working face is the end position of the front section, and it is also the starting position of the fault protection coal pillar left by the 1901N working face affected by the fault DF5; the pre-excavation of the 1901N working face is the starting position of the rear section, and it is also the end position of the fault protection coal pillar left by the 1901N working face affected by the fault DF5.

[0040] In order to ensure the safety of the pre-excavation retreat tunnel and the cut-off and facilitate the safe withdrawal work in the later stage, the present invention proposes a method for determining the reasonable position of the pre-excavation retreat tunnel and the cut-off under the double residual coal pillar, which includes the following steps:

[0041] S1: Calculate the stress of the remaining coal pillar

[0042] like Figure 1 As shown in the figure, the remaining coal pillar is the fault protection coal pillar left after the 1301N working face and the 1501N working face were mined; assuming that all coal seams are nearly horizontal coal seams, the stress on the fault protection coal pillar can be simplified as Figure 2 As shown in the figure, the force F carried by the fault protection coal pillar includes the self-weight stress F0 of the overlying rock layer directly above it, the stress F2 transmitted by the overburden above the goaf in the front section, and the stress F1 transmitted by the overburden above the goaf in the rear section:

[0043] F=F0+F1+F2 (3-1)

[0044] Where: F is the total force of the fault protection coal pillar (MPa); F1 is the transferred stress of the goaf in the rear section (MPa); F2 is the transferred stress of the goaf in the front section (MPa); F0 is the gravity of the overburden just above the fault protection coal pillar (MPa);

[0045] like Figure 2 As shown in the figure, the maximum horizontal collapse position of the overburden in the front section goaf is located in the middle of the front section goaf, and the maximum horizontal collapse position of the overburden in the rear section goaf is located in the middle of the rear section goaf; based on this, the distances l1 and l2 between the maximum collapse position of the fault protection coal pillar in the rear section goaf and the front section goaf and the fault protection coal pillar are obtained respectively:

[0046]

[0047] Where: L1 is the strike length of the goaf in the rear section (m); L2 is the strike length of the goaf in the front section (m);

[0048] The collapse angles of the front section goaf and the back section goaf are α2 and α1 respectively. For the front section goaf, the rock layer above it that has not collapsed is evenly shared by the coal pillars on both sides, that is, the fault protection coal pillar bears half of it. Figure 2 The gravity of the trapezoidal area where the middle character F2 is located is borne by the fault protection coal pillar; for the goaf in the rear section, the rock layer above it that has not collapsed is evenly shared by the coal pillars on both sides, that is, the fault protection coal pillar bears half of the gravity, then Figure 2 The gravity of the trapezoidal area where the middle character F1 is located is borne by the remaining coal pillar;

[0049] Given the buried depth H of the fault protection coal pillar, the strike length L1 of the back section goaf, the strike length L2 of the front section goaf, the collapse angles of the front section goaf and the back section goaf are α2 and α1 respectively, and the average bulk density of the overburden γ, the total force F of the remaining coal pillar can be obtained according to formulas (3-1) and (3-2):

[0050]

[0051] Among them, when calculating, the unit length is taken along the inclination of the working face, that is, perpendicular to the direction of the working face, and the same applies to the following text;

[0052] S2: Establish the stress transfer mechanism of the rock layer under the remaining coal pillar

[0053] It is assumed that the coal-rock medium at the bottom of the residual coal pillar, i.e. the fault protection coal pillar, is an ideal elastic rock medium that is continuous, homogeneous, and completely elastic. At the same time, the stress of the residual coal pillar will not cause large deformation or damage on the ideal elastic body. According to the basic laws of elastic mechanics, a stress transfer mechanism of the rock layer below the residual coal pillar is established to calculate the stress increased by any point (unit body) M in the rock layer below the residual coal pillar due to the residual coal pillar.

[0054] like Figure 3 As shown, it is assumed that the end of the bottom surface of the remaining coal pillar close to the goaf of the previous section is the coordinate origin, the side pointing to the goaf of the previous section is the Y direction, the vertical downward direction is the X direction, and the Y-axis coordinate of any point M in the rock layer below the remaining coal pillar is y, and the X-axis coordinate is x;

[0055] Referring to the research method of the force on the boundary of a half-plane body in elastic mechanics, a small length dε is taken at the distance of the remaining coal pillar from the coordinate origin ε, and the vertical stress caused by the small concentrated force dP=Fdε of the small length fault protection coal pillar at any point M in the bottom rock layer can be obtained as follows:

[0056]

[0057] Integrate equation (3-4) with the integral interval being the width D of the legacy coal pillar, i.e. from ε=0→ε=D, and the vertical stress increased by the legacy coal pillar at a point M in the rock layer below the legacy coal pillar is:

[0058]

[0059] Where: x is the vertical stress increased by the legacy coal pillar (MPa); x is the vertical distance between the unit body M and the legacy coal pillar (m); y is the horizontal distance between the unit body M and the boundary of the legacy coal pillar (m), which is positive on the side of the goaf in the front section, and positive on the side under the legacy fault protection coal pillar and the goaf in the back section;

[0060] S3: Calculate the overburden gravity on the pre-excavated retreat tunnel or cut-off without considering the vertical stress added by the remaining coal pillar

[0061] If the pre-excavated retreat tunnel or cut is located below the 1301N goaf and / or 1501N goaf, the mining of the liberated layer at the location of the pre-excavated retreat tunnel will produce a certain stress release, the stress concentration will be reduced and a decompression effect will be formed. Therefore, without considering the vertical stress increased by the remaining coal pillar, the overburden gravity σ' of the pre-excavated retreat tunnel or cut is

[0062] σ′=ηk′γH9 (3-6)

[0063] Wherein: σ' is the overburden gravity of the pre-excavated retreat tunnel or cut-hole without considering the vertical stress increased by the legacy pillar (MPa); η is the stress reduction coefficient of the surrounding rock caused by the mining of the liberation layer. In this embodiment, the liberation layer is the 1301N goaf and / or the 1501N goaf. When there is no goaf directly above the pre-excavated retreat tunnel or cut-hole, η is 1, when there are two goafs, η = 0.8 ~ 0.95, when there is one goaf, η = 0.95 ~ 1.0, and the closer the pre-excavated retreat tunnel or cut-hole is to the goaf center of the coal seam where the legacy coal pillar is located, the smaller the stress reduction coefficient η; k' is the comprehensive stress concentration coefficient affected by dynamic pressure such as tunnel excavation stress and mutual interference of mining, and its size can be determined in combination with on-site monitoring and engineering experience. In this embodiment, k' = 1.2; H9 is the burial depth of 9 coal (m);

[0064] S4: Calculate the stress of the pre-excavated retreat tunnel or cut hole under the double-layer coal pillar

[0065] The stress of the pre-excavated retreat tunnel or cut hole under the remaining coal pillar includes the vertical stress σ increased by the remaining coal pillar. x And the overburden gravity σ' on the pre-excavation retreat tunnel or cut-hole without considering the vertical stress increased by the remaining coal pillar;

[0066] In this embodiment, the stress σ of the pre-excavation retreat tunnel under the double-layer residual coal pillar includes the vertical stress σ increased by the protective coal pillar of the 3rd coal seam fault. x3 , 5. The vertical stress σ increased by the coal pillar protected by the coal seam fault x5 And the overburden gravity σ' on the pre-excavation retreat tunnel without considering the vertical stress increased by the remaining coal pillar;

[0067] σ=σ′+σ x3 +σ x5 (3-7)

[0068] Where: σ is the stress of the pre-excavated retreat tunnel or cut hole under the double-layer residual coal pillar (MPa); x3 is the additional stress transmitted by the coal pillar of the 3 coal seam fault (MPa); σ x5 is the additional stress transmitted by the coal pillar of the 5th coal seam fault (MPa);

[0069] Combining formulas (3-5) to (3-7), the stress in the pre-excavation tunnel or cut in the 9th coal seam can be obtained as:

[0070]

[0071] In the formula: F3 is the total force of the coal pillar left in the 3rd coal seam; x3 and y3 are the vertical distance and horizontal distance from the pre-excavation and withdrawal tunnel or cutting eye to the lower right corner of the fault protection coal pillar of the 3rd coal seam respectively; D3 is the width of the fault protection coal pillar of the 3rd coal seam; F5 is the total force of the coal pillar left in the 5th coal seam; x5 and y5 are the vertical distance and horizontal distance from the pre-excavation and withdrawal tunnel or cutting eye to the lower right corner of the fault protection coal pillar of the 5th coal seam respectively; D5 is the width of the fault protection coal pillar of the 5th coal seam.

[0072] S5: Select the pre-excavation retreat tunnel and the cutting position

[0073] Determine the critical safety vertical stress σ during excavation of the pre-excavation retreat tunnel h , let σ=σ h , according to formula (3-8), the critical position of the pre-excavated retreat tunnel is determined. The pre-excavated retreat tunnel is located on the right side of the DF5 fault. The pre-excavated retreat tunnel is the retreat tunnel of the front section. The pre-excavated retreat tunnel is set farther away from the fault, the smaller the vertical stress it is subjected to and the safer it is.

[0074] Determination of critical safety vertical stress σ during pre-cut tunneling q , let σ=σ q , according to formula (3-8), the critical position of the pre-dig eye is determined. The pre-dig eye is located on the left side of the DF5 fault, and the pre-dig eye is the eye of the rear section. The farther the pre-dig eye is set from the fault, the smaller the vertical stress and the safer it is.

[0075] Determine the distance S between the critical position of the pre-excavation retreat tunnel and the critical position of the pre-excavation cut-off l , determine the minimum fault protection coal pillar width S of the 1901N working face m If S l ≥S m , then the critical position of eye cutting is taken as the eye cutting construction position, and the critical position of pre-excavation and withdrawal tunnel is taken as the pre-excavation and withdrawal tunnel construction position; if S l <S m , then the critical position of eye cutting is taken as the eye cutting construction position, and the distance from the critical position of the pre-excavated retreat tunnel to the side away from the fault is S m -S l The distance is used as the construction position of the pre-excavation retreat tunnel;

[0076] Among them, the minimum fault protection coal pillar width S of the 1901N working face m The determination method is as follows:

[0077] Determine the initial pressure step after mining in the rear section of the 1901N working face and set it equal to L1, determine the strike length L2 of the front section of the 1901N working face, calculate the total force F9 of the fault protection coal pillar of the 1901N working face according to formula (3-3), and compare it with the strength of coal No. 9 to obtain the minimum width S of the fault protection coal pillar of the 1901N working facem (The larger the width of the fault protection coal pillar, the better the stability). m The fault protection coal pillar is in a critical state of instability. There are several well-known methods in the art for calculating the width of the protection coal pillar, which are applicable and will not be described here. In the present invention, when calculating the width of the 1901N fault protection coal pillar, its front side is the strike length L2 of the front section, and its rear side is the strike length L1 of the initial pressure step of the rear section.

[0078] S5: Pre-digging the retracement channel and cutting the eye

[0079] Pre-excavate the withdrawal channel and cut the eye, and support them separately. The support strength of the pre-excavated withdrawal channel needs to be able to resist the damage to the pre-excavated withdrawal channel caused by the advance support pressure during the subsequent mining of the front section of the 1901N working face.

[0080] Of course, the above description is only a preferred embodiment of the present invention, and the present invention is not limited to the above embodiments. It should be noted that all equivalent substitutions and obvious deformation forms made by any technician familiar with the field under the guidance of this specification fall within the essential scope of this specification and should be protected by the present invention.

Claims

1. Method for determining the reasonable position of pre-excavation retreat tunnel and cutting eye under double residual coal pillars, It is characterized in that The steps include: S1: Calculate the stress of the remaining coal pillar The residual coal pillar is a fault protection coal pillar that penetrates the working face after the upper working face and the middle working face are mined; the force F borne by the fault protection coal pillar includes the self-weight stress F of the overlying rock layer directly above it. 0 and the stress F transmitted by the overburden above the goaf in the front section 2 The stress F transmitted by the overburden above the goaf in the rear section 1 : F=F 0 +F 1 +F 2 (1-1) Where: F is the total force of the fault protection coal pillar; F 1 Transfer stress to the goaf area in the rear section; F 2 Transfer stress to the goaf of the front section; F 0 The gravity of the overburden rock directly above the fault-protecting coal pillar; S2: Establish the stress transfer mechanism of the rock layer under the remaining coal pillar Assume that the end of the bottom surface of the residual coal pillar close to the goaf of the previous section is the coordinate origin, the side pointing to the goaf of the previous section is the Y direction, the vertical downward direction is the X direction, the Y-axis coordinate of any point M in the rock layer below the residual coal pillar is y, and the X-axis coordinate is x; the width of the residual coal pillar is D, and the vertical stress increased by a point M in the rock layer below the residual coal pillar due to the residual coal pillar is: S3: Calculate the overburden gravity on the pre-excavated retreat tunnel or cut-off without considering the vertical stress added by the remaining coal pillar Without considering the vertical stress increased by the remaining coal pillar, the overburden gravity σ' of the pre-excavation retreat tunnel or cut hole in the lower coal seam is: σ′=ηk′γH d (1-3) Where: σ' is the overburden gravity of the pre-excavated retreat tunnel or cut-hole without considering the vertical stress increased by the legacy pillar; η is the stress reduction coefficient of the surrounding rock caused by the mining of the liberated layer. When there is no goaf directly above the pre-excavated retreat tunnel or cut-hole, η is 1; when there are two layers of goaf, η = 0.8 ~ 0.95; when there is one layer of goaf, η = 0.95 ~ 1.

0. The closer the pre-excavated retreat tunnel or cut-hole is to the center of the goaf of the coal seam where the legacy coal pillar is located, the smaller the stress reduction coefficient η; k' is the comprehensive stress concentration coefficient affected by dynamic pressure; H d The depth of the lower coal seam; S4: Calculate the stress of the pre-excavated retreat tunnel or cut hole under the double-layer coal pillar The stress of the pre-excavated retreat tunnel or cut hole under the remaining coal pillar includes the vertical stress σ increased by the remaining coal pillar. x And the overburden gravity σ' on the pre-excavation retreat tunnel or cut-hole without considering the vertical stress increased by the remaining coal pillar; The stress σ of the pre-excavated retreat tunnel under the double-layer legacy coal pillar includes the vertical stress σ increased by the protective coal pillar of the upper coal seam fault xu , the vertical stress σ increased by the fault protection coal pillar in the middle coal seam xm And the overburden gravity σ' on the pre-excavation retreat tunnel without considering the vertical stress increased by the remaining coal pillar; σ=σ′+σ xu +s xm (1-4) Combining formulas (1-2) to (1-4), the stress in the pre-excavation tunnel or cut in the lower coal seam can be obtained as: Where: F u is the total force of the coal pillar left in the upper coal seam; u ,y u The coordinates of the lower right corner of the coal pillar protecting the coal seam fault above the pre-excavation retreat tunnel or the cutting eye are the origin; D u F is the width of the coal pillar protecting the upper coal seam fault; m is the total stress of the coal pillar left in the middle coal seam; m ,y m The coordinates of the pre-excavation retreat tunnel or eye cutting are the lower right corner of the middle coal seam fault protection pillar as the origin; D m The width of the coal pillar for protection of the fault in the middle coal seam; S5: Select the pre-excavation retreat tunnel and the cutting position Determine the critical safety vertical stress σ during excavation of the pre-excavation retreat tunnel h , let σ=σ h , according to formula (1-5), determine the critical position of the pre-excavated retreat tunnel, the pre-excavated retreat tunnel is located on the right side of the fault, and the pre-excavated retreat tunnel is the retreat tunnel of the front section; Determination of critical safety vertical stress σ during pre-cut tunneling q , let σ=σ q , according to formula (1-5), determine the critical position of the pre-digging cut, the pre-digging cut is located on the left side of the fault, and the pre-digging cut is the cut of the rear section; Determine the distance S between the critical position of the pre-excavation retreat tunnel and the critical position of the pre-excavation cut-off l , determine the minimum fault protection coal pillar width S of the lower coal seam working face m If S l ≥S m , then the critical position of eye cutting is taken as the eye cutting construction position, and the critical position of pre-excavation and withdrawal tunnel is taken as the pre-excavation and withdrawal tunnel construction position; if S l <S m , then the critical position of eye cutting is taken as the eye cutting construction position, and the distance from the critical position of the pre-excavated retreat tunnel to the side away from the fault is S m -S l The distance is used as the construction position of the pre-excavation retreat tunnel; S6: Pre-digging the retracement channel and cutting the eye Pre-excavate a withdrawal channel and cut an eye, and support them separately. The support strength of the pre-excavated withdrawal channel needs to be able to resist the damage to the pre-excavated withdrawal channel caused by the advance support pressure during the subsequent mining of the front section of the lower coal seam working face.

2. According to the method for determining the reasonable position of the retreat tunnel and the cut-off hole according to claim 1, It is characterized in that In step S1, the buried depth H of the fault protection coal pillar and the strike length L of the goaf in the rear section are determined. 1 , the strike length of the goaf in the front section L 2 , the collapse angle of the goaf in the front section α 2 , the collapse angle of the goaf in the rear section α 1 , the average bulk density of the overburden γ, the total force F of the remaining coal pillar is obtained as:

3. According to the method for determining the reasonable position of the retreat tunnel and the cut hole according to claim 2, in step S5: the minimum fault protection coal pillar width S of the lower coal seam working face m The determination method is as follows: Determine the initial pressure step distance after mining the rear section of the lower coal seam working face and set it equal to L 1 , determine the strike length L of the section in front of the lower coal seam working face 2 According to formula (1-6), the total force F of the fault protection coal pillar at the lower coal seam working face is calculated d , compared with the strength of the lower coal seam, the minimum fault protection coal pillar width S of the lower coal seam working face is obtained m , at the minimum fault protection coal pillar width S m Under this condition, the fault-protected coal pillar is in a critical state without losing stability.

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