A method for controlling the surrounding rock by cutting the top of small coal pillars and unloading the pressure in hard roof roadway sections

Abstract

The present invention discloses a method for controlling the top and pressure relief of surrounding rocks by layered cutting of small coal pillars in a hard roof tunnel section, which relates to the technical field of safe mining of coal mines and tunnel surrounding rock control. The method comprises the following steps: S1. Determine the periodic pressure step distance of the working face according to the on-site geological conditions and the damage characteristics of the overburden in the mining area; S2. Establish a mechanical model to determine the primary support strength of the tunnel; S3. The tunnel boring machine cuts and supports the tunnel; S4. Cuts off the direct top of the coal pillar along the edge of the coal pillar on the outside; S5. Establish a mechanical model to determine the secondary support strength of the tunnel; S6. Directly cut the top of the basic top arc triangle block; S7. Grouting reinforcement improves the structure and overall strength of the small coal pillar; S8. Real-time dynamic monitoring of the deformation of the tunnel surrounding rock. The present invention improves the recovery rate of coal resources, improves the tunnel support effect, and realizes safe and efficient production of mines.

Description

Technical Field

[0001] The invention relates to the technical field of safe mining of coal mines and surrounding rock control of tunnels, and in particular to a surrounding rock control method for layered top cutting and pressure relief of small coal pillars in a hard roof tunnel section. Background Art

[0002] As an effective measure to isolate goaf and protect underground tunnels, segmental coal pillars are widely used in the field of underground coal mining, and have a very important impact on the safe production of mines. In the past comprehensive mining production process, a certain width of segmental coal pillars needed to be left between two adjacent working faces, with a size of about 25m. In some mines, affected by geological conditions, the size of the coal pillars reached 50m, resulting in a high loss of coal resources in the mines, which was not in line with the national strategic plan for the development of green, efficient and safe mining in mines.

[0003] In response to the above situation, some industry personnel proposed the technology of small coal pillars to protect the roadway, that is, to leave a small coal pillar of less than 10m along the edge of the goaf to dig the lower working face mining roadway, isolating the goaf and protecting the underground roadway while minimizing the waste of coal resources. However, in the actual production process of coal mines, affected by the lateral support pressure of the upper working face, the roadway along the goaf with small coal pillars is often severely deformed and difficult to maintain, which seriously affects the safe and efficient production of the mine. In response to the problem of small coal pillar support, researchers have proposed coal-free mining technologies such as leaving the roadway along the goaf and cutting the top into the roadway. However, due to the constraints of geological conditions, the section coal pillar protection technology still plays an important role in the technical field of the coal industry.

[0004] At present, the surrounding rock control of small coal pillars along the goaf roadway is mainly concentrated in three aspects: first, by selecting a suitable position, the roadway is arranged as much as possible in the lateral support pressure reduction zone to reduce the influence of stress on the roadway support, but this method often leaves a large width of coal pillars, and even if the size of the coal pillars is small, other measures need to be taken to maintain the roadway; second, the method of strengthening support is used to maintain the safety and stability of the roadway, that is, to increase the strength of the support material of the anchor cable, and on this basis, to cooperate with the support and other maintenance, but the allowable deformation of the anchor cable support material is limited, and it is difficult to take into account the application requirements of high-strength support and large deformation of the small coal pillar along the goaf roadway, which will cause the support material to break and fail, and reduce the active support effect of the roadway, and the erection of steel sheds and supports will cause cumbersome processes, increase the cost of roadway support, and reduce the efficiency of roadway excavation; third, the use of top cutting and The grouting reinforcement method maintains the safety of tunnels and small coal pillars. For example, the "Technology of deep cutting and shallow injection of gangue to protect the tunnel in the inclined coal seam through small coal pillars and method for determining the top cutting position" with publication number CN115182729B and the "Construction method of retaining small coal pillars in sections" with publication number CN107060758B can both achieve the purpose of top cutting and grouting reinforcement of coal pillars, but both have the following shortcomings: (1) The top cutting position is limited to the basic top surface, and the impact of the direct top load on the tunnel along the small coal pillar is not fully considered; (2) The top cutting and tunnel support design are independent of each other, and the support strength required for the tunnel under different top cutting conditions is not given. The tunnel support design is relatively blind and lacks scientific basis; (3) There is a lack of effective monitoring means for the support effect of the small coal pillars along the tunnel, and it is impossible to effectively evaluate the tunnel support and the grouting reinforcement effect of the small coal pillars.

[0005] In summary, the existing technology mainly focuses on the basic top cutting or surrounding rock support design of small coal pillar gob-side tunnels, but fails to consider the impact of direct top load, and lacks effective monitoring methods for tunnel maintenance effects. Therefore, it is necessary to propose a small coal pillar layered top cutting and pressure relief surrounding rock control technology in hard roof tunnel sections. Summary of the invention

[0006] In view of the above problems, the present invention discloses a method for controlling the surrounding rock by layered top cutting and pressure relief of small coal pillars in a hard roof tunnel section. The method fully considers the influence of the direct roof and basic roof loads on the tunnel along the small coal pillar, combines top cutting with tunnel support, scientifically and reasonably designs the tunnel support parameters and schemes, and improves the tunnel surrounding rock control effect.

[0007] According to the purpose of the present invention, a method for controlling the top pressure relief of small coal pillars in a hard roof tunnel section by layered cutting comprises the following steps:

[0008] S1. Determine the periodic pressure step length of the working face according to the on-site geological conditions and the damage characteristics of the overburden in the mining area, and calculate the length of the basic top arc triangle block along the inclination direction of the working face according to the periodic pressure step length of the working face and the inclination length of the working face;

[0009] S2. Establish a mechanical analysis model when the basic top arc-shaped triangle block is at the critical point of breaking, establish a force balance equation in the vertical direction according to the established mechanical analysis model parameters to obtain the primary support strength of the tunnel, and select support components according to the obtained primary support strength of the tunnel;

[0010] S3, cutting the tunnel with a roadheader, and carrying out anchor net cable support on the tunnel according to the primary support strength of the tunnel and the selected support materials;

[0011] S4. After a certain distance from the lagging excavation working face, the directional pre-cutting technology is used to cut the roof of the roadway outside the small coal pillar in the section, so that the direct roof outside the coal pillar is cut off along the edge of the coal pillar;

[0012] S5. Establish a mechanical analysis model when the basic top arc triangle block touches the rock, solve the mechanical equilibrium equation when the basic top arc triangle block touches the rock to obtain the secondary support strength of the tunnel, and use high-strength anchor cables to reinforce the tunnel with secondary support;

[0013] S6. After the secondary support reinforcement of the roadway, the directional pre-cutting technology is used to directional cut the top of the basic top arc triangle block so that it is cut down along the edge of the small coal pillar in the section;

[0014] S7. After the secondary top cutting of the tunnel is completed, the grouting reinforcement method is used to improve the structure and overall strength of the small coal pillar;

[0015] S8. Conduct real-time dynamic monitoring of tunnel surrounding rock deformation and optimize tunnel support based on the monitoring results.

[0016] Preferably, in S1, the calculation formula for the periodic pressure step of the working surface is as follows:

[0017]

[0018] Where: H1 is the thickness of the basic top; σ' is the tensile strength of the basic top; P is the unit area load of the overlying strata on the basic top;

[0019] The calculation formula for the length of the basic top arc triangle block along the inclination direction of the working surface is:

[0020]

[0021] Where: s is the inclined length of the working face; s1 is the periodic pressure step of the working face; s2 is the length of the basic top arc triangle block along the inclined direction of the working face.

[0022] Preferably, in S2, when the arc-shaped triangle block of the basic top is at the critical point of breaking, the mechanical analysis model is as follows: the fracture position of the basic top on the solid coal wall side is simplified to a bending moment, the stress of the overlying rock strata acting on the basic top is regarded as a uniformly distributed load along its cross section, the support reaction force given by the solid coal wall to the basic top rock beam is regarded as a nonlinear load along its cross section, the primary support strength of the tunnel is a uniformly distributed load along the cross section of the tunnel, and the support force provided by the coal pillar is simplified to a uniformly distributed load along the cross section of the coal pillar.

[0023] Preferably, in S2, the calculation formula for the primary support strength of the roadway is:

[0024]

[0025] In the formula: σ1 is the primary support strength of the tunnel; P1 is 4 to 8 times the deadweight of the mining height; P2 is the support strength of the coal pillar; P3 is the support strength of the side wall of the tunnel; H1 is the thickness of the basic roof; γ1 is the bulk density of the basic roof; H2 is the thickness of the immediate roof; γ2 is the bulk density of the immediate roof; H3 is the thickness of the coal seam; γ3 is the bulk density of the coal seam; H4 is the height of the tunnel; a is the distance between the broken position of the basic roof and the coal wall of the tunnel; b is the width of the tunnel; c is the width of the coal pillar; s2 is the length of the arc-shaped triangle block of the basic roof along the inclination direction of the working face.

[0026] Preferably, the calculation formula for the length of the primary support anchor bolt of the tunnel is:

[0027] L=L1+R+L3

[0028] Where: L is the length of the primary support anchor; L1 is the anchoring length of the primary support anchor; L3 is the exposed length of the primary support anchor; R is the loose circle of the surrounding rock;

[0029] The calculation formula of the loose zone of tunnel surrounding rock is:

[0030]

[0031] Where: γ0 is the average volume force of the overlying rock layer; M is the buried depth of the tunnel; P' is the support resistance; R' is the theoretical radius of the tunnel; C is the cohesion of the rock mass; is the internal friction angle; f is the correction coefficient of the mechanical parameters of coal rock mass, 1 / 3~1 / 2.

[0032] Preferably, the ultimate elongation of the primary support anchor cable is calculated according to the suspension theory, and its magnitude is higher than the basic top settlement above the inner side of the small coal pillar in the section before the secondary top cutting of the tunnel. The calculation formula is as follows:

[0033] Δh <L2k2

[0034] Where: Δh is the basic roof settlement above the inner side of the small coal pillar in the section; L2 is the length of the primary support anchor cable; k2 is the ultimate elongation of the primary support anchor cable.

[0035] Preferably, in S5, the mechanical analysis model when the basic top arc-shaped triangular block touches the rock and is in equilibrium is: the fracture position of the basic top is simplified to a hinged support, and the other end is a free end. After the basic top touches the rock, the force of the overlying rock strata at its fracture position is set to be the ground stress. The goaf gives a supporting force to the basic top arc-shaped triangular block, the coal pillar support force is simplified to a uniformly distributed load along its cross section, and the secondary support strength of the tunnel is a uniformly distributed load along its cross section.

[0036] Preferably, in S5, the calculation formula for the secondary support strength of the tunnel is:

[0037]

[0038] Wherein: σ2 is the secondary support strength of the tunnel; P4 is the weight of the overlying strata on the basic roof; P2 is the support strength of the coal pillar; P3 is the support strength of the side wall of the tunnel; H1 is the thickness of the basic roof; γ1 is the bulk density of the basic roof; H2 is the thickness of the immediate roof; γ2 is the bulk density of the immediate roof; H3 is the thickness of the coal seam; γ3 is the bulk density of the coal seam; H4 is the height of the tunnel; a is the distance between the broken position of the basic roof and the coal wall of the tunnel; b is the width of the tunnel; c is the width of the coal pillar; s2 is the length of the arc-shaped triangle block of the basic roof along the inclination direction of the working face.

[0039] Preferably, in S7, the method for grouting reinforcement of small coal pillars is: arrange two rows of grouting drill holes on the surface of the small coal pillars, one row of inclined drill holes, and one row of vertical drill holes, and the two are arranged alternately; the angle between the inclined drill holes and the horizontal line of the small coal pillars is 60-70°, the depth is 4m, the drill hole spacing is 4m-5m, and the distance from the tunnel floor is 2m-2.5m; the vertical drill hole depth is 2m, the drill hole spacing is 4m-5m, and the distance from the tunnel floor is 0.9m-1.1m; the horizontal spacing between the inclined drill holes and the vertical drill holes is 2m-2.5m.

[0040] Preferably, in S8, the method for real-time dynamic monitoring of tunnel surrounding rock deformation is: installing mine pressure monitoring instruments and mine anchor non-destructive testing instruments in the test tunnel, monitoring the displacement of the tunnel surrounding rock and the anchor cable load, analyzing the stress change characteristics of the anchor, and using geological radar and infrared radiation methods to perform non-destructive testing on the stability of small coal pillars and the support conditions of the tunnel surrounding rock, and optimizing and reinforcing the tunnel support based on the monitoring results.

[0041] Compared with the prior art, the method for controlling the pressure relief of surrounding rock by layered cutting of small coal pillars in a hard roof tunnel section disclosed in the present invention has the following advantages:

[0042] 1. The present invention fully considers the influence of direct roof and basic roof loads on the gob-side tunnels with small coal pillars, combines roof cutting with tunnel support, and scientifically and reasonably designs tunnel support parameters and schemes to avoid the increase in support costs caused by excessive tunnel support strength and the serious tunnel deformation caused by too low support strength, thereby improving the tunnel surrounding rock control effect, increasing the service life of the mine, improving the safety of coal mine construction, and at the same time improving the coal recovery rate.

[0043] 2. The present invention installs a variety of monitoring equipment in the tunnel to effectively monitor the stability of small coal pillars and the supporting conditions of tunnel surrounding rocks, which can effectively evaluate the tunnel support and small coal pillar grouting reinforcement effects, and can optimize and reinforce the tunnel support according to the monitoring results. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0045] Figure 1 This is a schematic diagram of the layout of small coal pillar mining tunnels and coal mining working faces.

[0046] Figure 2 This is a schematic diagram of the primary support and roof cutting of the small coal pillar mining tunnel.

[0047] Figure 3 It is a schematic diagram of the basic top arc triangle block contact rock balance.

[0048] Figure 4 This is a schematic diagram of the secondary cutting and support reinforcement of the roof of the small coal pillar mining tunnel.

[0049] Figure 5 This is a schematic diagram of the drilling arrangement for grouting reinforcement of small coal pillars.

[0050] In the figure: 1- goaf; 2- basic roof; 3- direct roof; 4- small coal pillar; 5- tunnel; 6- working face; 7- primary support anchor rod; 8- primary support anchor cable; 9- secondary support anchor cable; 10- vertical drilling hole; 11- inclined drilling hole. DETAILED DESCRIPTION

[0051] The specific implementation of the present invention is briefly described below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0052] Figure 1-Figure 5 The preferred embodiments of the present invention are shown and analyzed in detail.

[0053] The present invention discloses a method for controlling the small coal pillars in a hard roof tunnel section by layered top cutting and pressure relief of surrounding rocks. The size of the small coal pillars 4 in the mining face 6 section is 7m, and the method comprises the following steps:

[0054] S1. Determine the 6-cycle pressure step distance of the working face according to the on-site geological conditions and the damage characteristics of the overburden in the mining area. Calculate the length of the basic top 2 arc triangle block along the 6-dip direction of the working face according to the 6-cycle pressure step distance of the working face and the 6-dip length of the working face. The calculation formula for the 6-cycle pressure step distance of the working face is:

[0055]

[0056] Where: H1 is the thickness of the basic top 2; σ' is the tensile strength of the basic top 2; P is the unit area load of the overlying rock layer on the basic top 2.

[0057] The calculation formula for the length of the basic top 2 arc-shaped triangular block along the inclination direction of the working surface 6 is:

[0058]

[0059] In the formula: s is the inclination length of working surface 6; s1 is the pressure step distance of working surface 6 period; s2 is the length of the basic top 2 arc-shaped triangle block along the inclination direction of working surface 6.

[0060] S2. Establish a mechanical analysis model when the basic top 2 arc-shaped triangle block is at the critical point of breaking. According to the established mechanical analysis model parameters, establish the force balance equation in the vertical direction to obtain the primary support strength of tunnel 5. Select support components such as anchor rods and anchor cables based on the obtained primary support strength of tunnel 5. The mechanical performance parameters of coal rock layer and anchor rod cable are obtained by laboratory tests.

[0061] The mechanical analysis model when the arc-shaped triangular block of the basic roof 2 is at the critical point of fracture is as follows: the fracture position of the basic roof 2 on the solid coal side is simplified to a bending moment, the stress of the overlying rock strata acting on the basic roof 2 is regarded as a uniformly distributed load along its cross section, the support reaction force of the solid coal side given to the basic roof 2 rock beam is regarded as a nonlinear load along its cross section, the primary support strength of tunnel 5 is a uniformly distributed load along the cross section of tunnel 5, and the support force provided by the coal pillar is simplified to a uniformly distributed load along the cross section of the coal pillar.

[0062] The calculation formula for the primary support strength of tunnel 5 is:

[0063]

[0064] Wherein: σ1 is the primary support strength of tunnel 5; P1 is 4 to 8 times the deadweight of the mining height; P2 is the support strength of the coal pillar; P3 is the support strength of the side wall of tunnel 5; H1 is the thickness of the basic roof 2; γ1 is the bulk density of the basic roof 2; H2 is the thickness of the immediate roof 3; γ2 is the bulk density of the immediate roof 3; H3 is the thickness of the coal seam; γ3 is the bulk density of the coal seam; H4 is the height of tunnel 5; a is the distance between the broken position of the basic roof 2 and the coal wall of tunnel 5; b is the width of tunnel 5; c is the width of the coal pillar; s2 is the length of the arc-shaped triangle block of the basic roof 2 along the inclination direction of the working face 6.

[0065] The calculation formula for the length of the primary support anchor bolt 7 of tunnel 5 is:

[0066] L=L1+R+L3

[0067] In the formula: L is the length of the primary support anchor 7; L1 is the anchoring length of the primary support anchor 7; L3 is the exposed length of the primary support anchor 7; R is the loose circle of the surrounding rock.

[0068] The calculation formula of the surrounding rock loosening zone of tunnel 5 is:

[0069]

[0070] Where: γ0 is the average volume force of the overlying rock layer; M is the buried depth of tunnel 5; P' is the support resistance; R' is the theoretical radius of tunnel 5; C is the cohesion of the rock mass; is the internal friction angle; f is the correction coefficient of the mechanical parameters of coal rock mass, 1 / 3~1 / 2.

[0071] The ultimate elongation of the primary support anchor cable 8 is calculated based on the suspension theory, which is higher than the settlement of the basic roof 2 above the inner side of the small coal pillar 4 in the section before the secondary top cutting of the tunnel 5. The calculation formula is as follows:

[0072] Δh <L2k2

[0073] Wherein: Δh is the settlement of the basic roof 2 above the inner side of the small coal pillar 4 in the section; L2 is the length of the primary support anchor cable 8; k2 is the ultimate elongation of the primary support anchor cable 8.

[0074] S3. The tunnel 5 is cut by a roadheader, and anchor mesh cable support is performed on the tunnel 5 according to the primary support strength of the tunnel 5 and the selected support materials.

[0075] S4, the delayed excavation working face 6 20m adopts the directional pre-cutting technology to cut the roof of the tunnel 5 outside the small coal pillar 4 in the section, so that the direct roof 3 outside the coal pillar is cut off along the edge of the coal pillar.

[0076] S5. Establish a mechanical analysis model when the basic top 2 arc-shaped triangular block touches the rock in equilibrium, solve the mechanical equilibrium equation when the basic top 2 arc-shaped triangular block touches the rock to obtain the secondary support strength of tunnel 5, and use secondary support anchor cable 9 to reinforce tunnel 5 for secondary support. The high-strength anchor cable should be anchored in the rock layer of the top plate of tunnel 5, and the anchoring length should not be less than 1m.

[0077] The mechanical analysis model when the arc-shaped triangular block of the basic roof 2 touches the rock is balanced as follows: the fracture position of the basic roof 2 is simplified to a hinged support, and the other end is a free end. After the basic roof 2 touches the rock, the overlying rock force at its fracture position is set to be the ground stress. The goaf 1 gives a supporting force to the arc-shaped triangular block of the basic roof 2. The coal pillar support force is simplified to a uniformly distributed load along its cross section, and the secondary support strength of the tunnel 5 is a uniformly distributed load along its cross section.

[0078] Calculation formula for secondary support strength of tunnel 5:

[0079]

[0080] Wherein: σ2 is the secondary support strength of tunnel 5; P4 is the weight of the overlying strata of the basic roof 2; P2 is the support strength of the coal pillar; P3 is the support strength of the side wall of tunnel 5; H1 is the thickness of the basic roof 2; γ1 is the bulk density of the basic roof 2; H2 is the thickness of the immediate roof 3; γ2 is the bulk density of the immediate roof 3; H3 is the thickness of the coal seam; γ3 is the bulk density of the coal seam; H4 is the height of tunnel 5; a is the distance between the broken position of the basic roof 2 and the coal wall of tunnel 5; b is the width of tunnel 5; c is the width of the coal pillar; s2 is the length of the arc-shaped triangle block of the basic roof 2 along the inclination direction of the working face 6.

[0081] After the secondary support reinforcement of S6 and tunnel 5, the directional pre-cutting technology is used to carry out directional cutting of the arc-shaped triangular block of the basic top 2 so that it is cut down along the edge of the small coal pillar 4 in the section.

[0082] S7. After the secondary top cutting of tunnel 5 is completed, the grouting reinforcement method is used to improve the structure and overall strength of the small coal pillar 4. The specific method is: two rows of grouting holes are arranged on the surface of the small coal pillar 4, one row of inclined holes 11, and one row of vertical holes 10, which are arranged alternately; the angle between the inclined holes 11 and the horizontal line of the small coal pillar 4 is 60°, the depth is 4m, the borehole spacing is 4m, and the distance from the bottom plate of tunnel 5 is 2m; the vertical borehole 10 is 2m deep, the borehole spacing is 4m, and the distance from the bottom plate of tunnel 5 is 1m; the horizontal spacing between the inclined borehole 11 and the vertical borehole 10 is 2m.

[0083] S8. Conduct real-time dynamic monitoring of the deformation of the surrounding rock of Tunnel 5, and optimize the support of Tunnel 5 based on the monitoring results. The specific method is: install mine pressure monitoring instruments and mine anchor non-destructive testing instruments in the test tunnel 5 to monitor the displacement of the surrounding rock of Tunnel 5 and the load of the anchor cable, analyze the stress change characteristics of the anchor, and use geological radar and infrared radiation methods to conduct non-destructive testing on the stability of the small coal pillar 4 and the surrounding rock support status of Tunnel 5, and optimize and reinforce the support of Tunnel 5 based on the monitoring results.

[0084] The above description of the disclosed embodiments enables those skilled in the art to implement and use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit and scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest range consistent with the principles and novel features disclosed herein.

Claims

1. A method for controlling the surrounding rock by layered cutting of small coal pillars in a hard roof tunnel section, characterized in that: The following steps are involved: S1. Determine the periodic pressure step length of the working face (6) according to the on-site geological conditions and the damage characteristics of the overburden in the mining area, and calculate the length of the arc-shaped triangle block of the basic top (2) along the inclination direction of the working face (6) from the periodic pressure step length of the working face (6) and the inclination length of the working face (6); S2, establishing a mechanical analysis model when the arc-shaped triangle block of the basic top (2) is at a critical breaking point, establishing a force balance equation in the vertical direction according to the established mechanical analysis model parameters to obtain the primary support strength of the tunnel (5), and selecting support components according to the obtained primary support strength of the tunnel (5); S3, cutting the tunnel (5) by a tunnel boring machine, and performing anchor net cable support on the tunnel (5) according to the primary support strength of the tunnel (5) and the selected support material; S4, the lagging excavation working face (6) is used to cut the roof of the roadway (5) outside the small coal pillar (4) of the section by using the directional pre-cutting technology at a certain distance, so that the direct roof (3) outside the coal pillar is cut off along the edge of the coal pillar; S5, establishing a mechanical analysis model when the arc-shaped triangular block of the basic top (2) touches the rock in equilibrium, solving the mechanical equilibrium equation when the arc-shaped triangular block of the basic top (2) touches the rock to obtain the secondary support strength of the tunnel (5), and using high-strength anchor cables to reinforce the tunnel (5) with secondary support; The mechanical analysis model when the arc-shaped triangular block of the basic roof (2) touches the rock is as follows: the fracture position of the basic roof (2) is simplified to a hinged support, and the other end is a free end. After the basic roof (2) touches the rock, the overburden force at the fracture position is set as the ground stress. The goaf (1) gives the arc-shaped triangular block of the basic roof (2) a supporting force. The support force of the coal pillar is simplified to a uniformly distributed load along its cross section. The secondary support strength of the roadway (5) is a uniformly distributed load along its cross section. The calculation formula of secondary support strength of tunnel (5) is: Where: σ2 is the secondary support strength of the tunnel (5); P4 is the weight of the overlying strata of the basic roof (2); P2 is the support strength of the coal pillar; P3 is the support strength of the side wall of the tunnel (5); H1 is the thickness of the basic roof (2); γ1 is the bulk density of the basic roof (2); H2 is the thickness of the immediate roof (3); γ2 is the bulk density of the immediate roof (3); H3 is the thickness of the coal seam; γ3 is the bulk density of the coal seam; H4 is the height of the tunnel (5); a is the distance between the fracture position of the basic roof (2) and the coal wall of the tunnel (5); b is the width of the tunnel (5); c is the width of the coal pillar; s2 is the length of the arc triangle block of the basic roof (2) along the inclination direction of the working face (6); S6. After the secondary support reinforcement of the tunnel (5), the directional pre-cutting technology is used to directional cut the arc-shaped triangular block of the basic roof (2) so that it is cut along the edge of the small coal pillar (4) of the section; S7. After the secondary top cutting of the tunnel (5) is completed, the grouting reinforcement method is used to improve the structure and overall strength of the small coal pillar (4); S8. Conduct real-time dynamic monitoring of the surrounding rock deformation of the tunnel (5), and optimize the support of the tunnel (5) based on the monitoring results.

2. The method for controlling the pressure relief of surrounding rock by layered cutting of small coal pillars in a hard roof tunnel section according to claim 1 is characterized in that: In S1, the calculation formula for the periodic pressure step distance of the working surface (6) is as follows: Where: H1 is the thickness of the basic top (2); σ' is the tensile strength of the basic top (2); P is the unit area load of the overlying rock layer on the basic top (2); The calculation formula for the length of the arc-shaped triangular block of the basic top (2) along the inclination direction of the working surface (6) is: Wherein: s is the inclined length of the working surface (6); s1 is the periodic pressure step of the working surface (6); s2 is the length of the arc-shaped triangle block of the basic top (2) along the inclined direction of the working surface (6).

3. The method for controlling the pressure relief of surrounding rock by layered cutting of small coal pillars in a hard roof tunnel section according to claim 1 is characterized in that: In S2, when the arc-shaped triangle block of the basic roof (2) is at the critical point of fracture, the mechanical analysis model is as follows: the fracture position of the basic roof (2) on the solid coal side is simplified to a bending moment, the stress of the overlying rock strata acting on the basic roof (2) is regarded as a uniformly distributed load along its cross section, the support reaction force of the solid coal side on the rock beam of the basic roof (2) is regarded as a nonlinear load along its cross section, the primary support strength of the tunnel (5) is a uniformly distributed load along the cross section of the tunnel (5), and the support force provided by the coal pillar is simplified to a uniformly distributed load along the cross section of the coal pillar.

4. The method for controlling surrounding rock by layered top cutting and pressure relief of small coal pillars in a hard roof tunnel section according to claim 1 is characterized in that: In S2, the calculation formula for the primary support strength of tunnel (5) is: Wherein: σ1 is the primary support strength of the tunnel (5); P1 is 4 to 8 times the deadweight of the mining height; P2 is the support strength of the coal pillar; P3 is the support strength of the side wall of the tunnel (5); H1 is the thickness of the basic roof (2); γ1 is the bulk density of the basic roof (2); H2 is the thickness of the immediate roof (3); γ2 is the bulk density of the immediate roof (3); H3 is the thickness of the coal seam; γ3 is the bulk density of the coal seam; H4 is the height of the tunnel (5); a is the distance between the broken position of the basic roof (2) and the coal wall of the tunnel (5); b is the width of the tunnel (5); c is the width of the coal pillar; s2 is the length of the arc triangle block of the basic roof (2) along the inclination direction of the working face (6).

5. The method for controlling the pressure relief of surrounding rock by layered cutting of small coal pillars in a hard roof tunnel section according to claim 4 is characterized in that: The calculation formula for the length of the primary support anchor rod (7) of the tunnel (5) is: L=L1+R+L3 Wherein: L is the length of the primary support anchor rod (7); L1 is the anchoring length of the primary support anchor rod (7); L3 is the exposed length of the primary support anchor rod (7); R is the loose circle of the surrounding rock; The calculation formula of the surrounding rock loosening zone of tunnel (5) is: Where: γ0 is the average volume force of the overlying rock layer; M is the buried depth of the tunnel (5); P' is the support resistance; R' is the theoretical radius of the tunnel (5); C is the rock mass cohesion; is the internal friction angle; f is the correction coefficient of the mechanical parameters of coal rock mass, 1 / 3~1 / 2.

6. The method for controlling the pressure relief of surrounding rock by layered cutting of small coal pillars in a hard roof tunnel section according to claim 5 is characterized in that: The ultimate elongation of the primary support anchor cable (8) is calculated based on the suspension theory, and its magnitude is higher than the settlement of the basic roof (2) above the inner side of the small coal pillar (4) in the section before the secondary top cutting of the tunnel (5). The calculation formula is as follows: Δh<L2k2 Wherein: Δh is the settlement of the basic roof (2) above the inner side of the small coal pillar (4) of the section; L2 is the length of the primary support anchor cable (8); k2 is the ultimate elongation of the primary support anchor cable (8).

7. The method for controlling surrounding rock by layered top cutting and pressure relief of small coal pillars in a hard roof tunnel section according to claim 1, characterized in that: In S7, the method for grouting reinforcement of the small coal pillar (4) is as follows: two rows of grouting holes are arranged on the surface of the small coal pillar (4), one row of inclined holes (11) and one row of vertical holes (10), and the two are arranged alternately; the angle between the inclined holes (11) and the horizontal line of the small coal pillar (4) is 60-70°, the depth is 4m, the spacing between the holes is 4m-5m, and the distance from the bottom plate of the tunnel (5) is 2m-2.5m; the vertical holes (10) are 2m deep, the spacing between the holes is 4m-5m, and the distance from the bottom plate of the tunnel (5) is 0.9m-1.1m; the horizontal spacing between the inclined holes (11) and the vertical holes (10) is 2m-2.5m.

8. The method for controlling surrounding rock by layered top cutting and pressure relief of small coal pillars in a hard roof tunnel section according to claim 1, characterized in that: In S8, the method for real-time dynamic monitoring of the deformation of the surrounding rock of the tunnel (5) is as follows: a mine pressure monitoring instrument and a mine anchor non-destructive testing instrument are installed in the test tunnel (5) to monitor the displacement of the surrounding rock of the tunnel (5) and the load of the anchor cable, analyze the stress change characteristics of the anchor, and use geological radar and infrared radiation methods to perform non-destructive testing on the stability of the small coal pillar (4) and the surrounding rock support condition of the tunnel (5), and optimize and strengthen the support of the tunnel (5) based on the monitoring results.

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