Coal seam long horizontal drilling pressure relief and rush prevention method and parameter optimization method

Abstract

The application discloses a coal seam long horizontal drilling pressure relief and rush-in prevention method and a parameter optimization method, and belongs to the field of mine pressure and stratum control. Long horizontal drilling holes are arranged in parallel along the direction of a roadway, the distance between the axis of the drilling hole and the roadway side is not less than 1.5 times the length of the anchor rod / cable support, and the distance between adjacent drilling holes is the sum of the radius of the plastic zone on both sides and the elastic core width between the holes. The reasonable width interval of the plastic zone and the elastic core is determined through theoretical calculation, and the optimal elastic core width is selected by taking the complete elastic core before mining and the gradual damage after mining as the criterion and combining numerical simulation. The application realizes the initial pre-pressure relief and the secondary dynamic pressure relief during mining, accurately matches the mechanical properties of the coal seam and the mining stress, avoids the damage to the support system caused by pressure relief, eliminates the pressure relief blind area and excessive pressure relief, reduces the drilling engineering quantity while ensuring continuous pressure relief, and significantly improves the rush-in prevention and the surrounding rock stability of the deep high-stress roadway.

Description

Technical Field

[0001] This invention belongs to the field of mine pressure and strata control technology, specifically relating to a long horizontal borehole pressure relief method for preventing rockburst in deep high-stress roadways, and in particular a borehole parameter design method based on the dynamic response of the inter-hole elastic core. Background Technology

[0002] As coal mining depths increase, the pressure in deep working faces along the goaf becomes increasingly severe. During face advancement, the combined effects of advance support pressure and lateral support pressure easily create high-stress concentration zones on both sides of the roadway, inducing large deformations such as roof subsidence, floor heave, and side heave, which can lead to rockburst disasters in severe cases. Drilling large-diameter boreholes in the roadway sides to relieve pressure is one of the key technologies for preventing rockbursts and softening high-stress coal. The principle is that drilling creates fracture and plastic zones within the coal body, transferring peak stress to deeper layers and converting accumulated elastic energy into plastic deformation energy and crack propagation energy, thereby weakening the coal's tendency to impact.

[0003] While conventional large-diameter borehole pressure relief is widely used, its drawbacks are significant. Firstly, boreholes inherently damage the roadway support system: large-diameter boreholes often weaken or destroy the effective range of sidewall anchors / cables, potentially inducing local rock instability while providing pressure relief, creating a conflict between "pressure relief" and "roadway protection." Secondly, its pressure relief effect is limited by its one-time and static nature: the pressure relief space created after borehole construction is fixed and cannot adapt to the dynamic evolution of the peak support pressure position and magnitude during face advancement, potentially leading to insufficient initial pressure relief or rapid attenuation of the pressure relief effect later, requiring secondary or even multiple additional boreholes, increasing construction costs and time. Furthermore, parameter design relies on experience, lacking quantification and precision: key parameters such as borehole spacing and depth are often determined based on engineering analogies or simplified theories, making it difficult to achieve precise matching with the specific mechanical properties and stress field of the coal seam, potentially leading to discontinuous or excessive pressure relief. Therefore, it is often used as an effective "emergency mitigation" measure rather than the optimal "systemic prevention and control" solution.

[0004] To reduce high-level stress in roadways and improve roadway safety, Chinese Patent Publication No. CN111236942A discloses a method for relieving pressure in coal seams prone to rock bursts. This method involves constructing chambers laterally within the mining roadway and drilling several long horizontal boreholes parallel to the mining roadway in the high-stress zone of the chambers to achieve pressure relief. The spacing between adjacent boreholes is determined based on the effective pressure relief radius, generally not exceeding twice the effective pressure relief radius. This method reduces the amount of drilling work to a certain extent and avoids direct damage to the roadway walls caused by drilling. However, this method still has the following shortcomings: First, its pressure relief mechanism is a one-time static pressure relief, and the pressure relief effect is basically fixed after drilling, which cannot adapt to the dynamic evolution of the peak position and magnitude of the support pressure during the working face advancement. The pressure relief effect decays rapidly with the mining process. Second, the determination of the borehole spacing mainly relies on empirical rules (such as the effective pressure relief radius multiple), lacking theoretical calculation and dynamic verification, making it difficult to achieve precise matching with the mechanical properties of the coal seam and the geostress field, which may lead to discontinuity of the pressure relief zone or excessive pressure relief. Third, this method only focuses on the plastic zone range of the borehole itself, without considering the stress state and failure sequence of the elastic core region between adjacent boreholes, and does not utilize mining stress to achieve secondary dynamic pressure relief. Therefore, it is necessary to further develop a long horizontal borehole pressure relief method for coal seams that can achieve dynamic response and quantitative parameter design. Summary of the Invention

[0005] To address the problem that existing coal seam long horizontal borehole pressure relief and anti-scour technologies rely solely on experience-based design and cannot accurately achieve controllable quantitative dynamic pressure relief, this invention provides a coal seam long horizontal borehole pressure relief and anti-scour method based on inter-hole elastic cores and parameter optimization methods.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for stress relief and scour prevention in long horizontal boreholes of coal seams based on inter-hole elastic cores is characterized by the uniform parallel arrangement of long horizontal boreholes along the roadway direction. The distance between the borehole axis and the roadway sidewall is not less than 1.5 times the length of the sidewall anchor / cable support. The spacing between adjacent boreholes is determined by the radius of the borehole's plastic zone and the width of the inter-hole elastic core. The width of the inter-hole elastic core refers to the width of the coal seam area between the outer edges of the plastic zones of two adjacent long horizontal boreholes, which is still under elastic stress. The width of the inter-hole elastic core should meet the following requirements: maintaining a complete elastic state before the working face is mined to bear and transfer stress, achieving initial stress relief and protecting the support structure; and undergoing progressive plastic failure under the action of advanced support pressure during the working face mining, forming a continuous stress relief zone by connecting with the plastic zones of the boreholes on both sides.

[0008] Furthermore, the borehole spacing is equal to the width of the elastic core between the boreholes plus the actual plastic zone radius after correction of the two boreholes, which is equal to the sum of the diameter of the single plastic zone after correction and the width of the elastic core between the boreholes.

[0009] To achieve the aforementioned precise pressure relief, the present invention provides a method for optimizing pressure relief and anti-scour parameters in long horizontal boreholes of coal seams based on inter-hole elastic cores, comprising the following steps:

[0010] S1: Collect basic parameters such as coal seam burial depth, coal mechanics, and original rock stress. Based on the above parameters, theoretically calculate the radius of the plastic zone of the long horizontal borehole, and introduce a correction coefficient to determine its actual radius.

[0011] S2: Using the basic parameters obtained from S1 and the actual radius of the plastic zone, determine the reasonable design range of the width of the elastic core between holes; the lower limit of the range is determined by the tangency condition of the plastic zones of the two boreholes, and the upper limit of the range is solved by combining the ultimate bearing state of the elastic core under mining stress.

[0012] S3: Through numerical simulation, based on the characteristics of mining response, the optimal inter-hole elastic core width is determined within the design range of elastic core width.

[0013] Furthermore, in step S1, the actual radius R of the plastic zone of the long horizontal borehole p The calculation formula is: ,in:

[0014]

[0015] In the formula:

[0016] R2 is the theoretical value of the plastic zone radius, in meters (m).

[0017] k is a correction factor, which is determined by combining the measured range of the plastic zone from experimental boreholes with numerical simulation.

[0018] R t The radius of the fractured zone is given by... Determined, m;

[0019] p is the coal body fragmentation coefficient, which is determined through field measurements and numerical simulation inversion, and is usually taken as 1.2~1.5;

[0020] R is the radius of the pressure relief drill bit (determined according to the drill bit diameter, usually 0.05~0.15m), in meters;

[0021] γ is the average unit weight of the overlying strata, in kN / m³. 3 ;

[0022] H is the burial depth of the coal seam, in meters;

[0023] φ is the friction angle of the surrounding rock, in °;

[0024] λ is the lateral pressure coefficient of the original rock;

[0025] c represents the cohesive force of the coal body, in MPa.

[0026] Furthermore, in step S2, the lower limit D of the width range of the inter-pore elastic core... min This refers to the situation where the plastic zones of two adjacent holes are tangent; the upper limit D of the width range of the elastic core between holes. max The mechanism for determining the value is as follows: Based on the design principle of the inter-hole elastic core width described in the above protection method, the inter-hole elastic core needs to maintain a complete elastic state before the working face is mined, bearing and transmitting stress to achieve initial pressure relief and protect the support structure; during mining, it undergoes progressive plastic failure under the action of advanced support pressure, connecting with the plastic zones of the boreholes on both sides to form a continuous pressure relief zone, achieving secondary dynamic pressure relief. Under the guidance of this principle, the critical criterion is that the total stress borne by the elastic core (that is, the total stress at the midpoint of the elastic core) reaches the ultimate bearing strength of the coal body. The total stress borne by the elastic core includes the increase in mining-induced stress Δσ on the elastic core due to mining and the tangential stress applied by the boreholes on both sides due to coupling before mining. The maximum value of the hole spacing L is obtained by back-calculating the total stress borne by the elastic core. max If the borehole spacing exceeds this value, the elastic core will be too wide to break under mining stress, forming a pressure relief blind zone and failing to meet dynamic pressure relief requirements. Based on the relationship that the borehole spacing equals the width of the elastic core between boreholes plus the radius of the plastic zone between the two boreholes, the maximum borehole spacing L... max The upper limit D of the interpore elastic core interval is further obtained. max Based on the above mechanism, the lower limit D of the interpore elastic core interval is... min and upper limit D max The method for determining this is as follows:

[0027] 2.1: Calculate the lower limit D of the width of the elastic core interval between holes. min ;

[0028]

[0029]

[0030] In the formula: l min Minimum hole spacing, in meters (m); D min The lower limit of the width of the elastic core interval between holes, in meters;

[0031] 2.2: Calculate the upper limit D of the width of the elastic core interval between holes. max ;

[0032] 2.2.1: Obtain the additional stress induced in the elastic core region during working face mining, i.e., the mining-induced stress increment Δσ.

[0033] The mining-induced stress increment Δσ is the additional stress caused by the mining of the working face and is an important component of the total stress of the elastic core. Based on the distribution law of the working face support pressure (measured or numerically simulated), the peak pressure coefficient K of the advance support is determined, and the mining-induced stress increment Δσ on the elastic core is taken as:

[0034]

[0035] In the formula: K is the peak pressure coefficient of the advance support (generally 1.5 to 3.0); γ ​​is the average unit weight, kN / m³. 3 H represents the burial depth of the coal seam, in meters.

[0036] 2.2.2: Calculate the tangential stress σ acting at the midpoint of the elastic core between holes from a single hole. θ ;

[0037] This step calculates the tangential stress generated by the inter-pore coupling effect under the original rock stress on the coal body between pores. This stress is part of the stress experienced by the coal body, while the radial contact force σ at the elasto-plastic boundary is... R It is the boundary stress at the interface between the plastic zone and the elastic core of the borehole, and is the tangential stress σ. θ The solution provides the necessary boundary stress conditions. Based on this mechanism, the tangential stress σ θ The calculation formula is:

[0038]

[0039] In the formula: σ R The radial contact force at the single-hole elastoplastic boundary, in MPa, is given by the formula... Calculated;

[0040] l is half the hole spacing, in meters;

[0041] γ is the average bulk density, kN / m³ 3 ;

[0042] H is the burial depth of the coal seam, in meters;

[0043] 2.2.3: Calculate the total stress σ of the elastic core bearing capacity z ;

[0044] Based on the upper limit D of the aforementioned interpore elastic core width range. max The mechanism for determining the value shows that the total stress borne by the elastic core mainly consists of two parts: one is the tangential stress at the midpoint of the elastic core under the original rock stress conditions and the coupling effect of the two holes (according to the tangential stress σ of a single hole). θ (Superposition calculation); secondly, the mining-induced stress increment Δσ generated during working face mining. Due to the symmetry of adjacent boreholes, the elastic core center is subjected to twice the σ. θ The function of this is that when the total stress reaches or exceeds its ultimate bearing strength σ1, the elastic core enters a state of plastic failure, achieving secondary stress relief. Therefore:

[0045]

[0046] 2.2.4: Obtain the ultimate bearing strength σ1 of the coal seam;

[0047] The ultimate bearing capacity σ1 of coal refers to the critical stress value at which the coal transitions from an elastic state to a plastic state during mining; that is, the maximum stress that the coal can withstand without failure. In this invention, σ1 is taken as the ultimate compressive strength σ of the coal. c Alternatively, it can be determined by the Mohr-Coulomb strength criterion, i.e., obtained according to the following formula:

[0048]

[0049] In the formula: σ3 is the minimum principal stress, obtained from the geostress test, in MPa.

[0050] 2.2.5: Obtain the maximum width D of the inter-pore elastic core through inverse kinematics. max ;

[0051] Let σ θ临界 σ is the critical tangential stress acting on the midpoint of the elastic core by a single borehole. z临界 For the elastic core to bear the critical total stress, due to the symmetry of the two holes, the center of the elastic core is subjected to twice the tangential stress. Therefore, the critical total stress σ of the elastic core is... z临界 It is the sum of the mining stress increment and twice the critical tangential stress of a single borehole, i.e. When the elastic core bears the critical total stress σ z临界 When the ultimate bearing capacity σ1 of the coal reaches or exceeds the limit, the elastic core enters the plastic failure state, achieving secondary pressure relief.

[0052] Based on the above mechanism, the elastic core bears the critical total stress σ. z临界 Reaching the ultimate bearing capacity σ1 of the coal seam is the critical criterion, that is... Given the conditions, based on the ultimate bearing capacity σ1 of the coal body obtained in step 2.2.4 and the mining stress increment Δσ obtained in step 2.2.1, the critical tangential stress σ is obtained by solving. θ The critical point is reached, and then the tangential stress σ given in step 2.2.2 is applied. θ The critical value of l can be obtained by solving the inverse equation relating l to half the hole spacing l, and then the maximum hole spacing L can be calculated. max Then, the maximum value D of the elastic core width between the holes was obtained. max ,Right now: .

[0053] Furthermore, the specific method for step S3 is as follows:

[0054] A numerical simulation model for pressure relief of horizontal boreholes in the working face, including roadways and different elastic core width schemes between boreholes, was established. Within the range of elastic core widths between boreholes determined by calculation, three candidate spacing values ​​D1, D2, and D3 were selected according to the principle of equal intervals as inputs for subsequent numerical simulation optimization. Numerical simulation methods were used to further analyze the range of elastic cores between boreholes.

[0055] Further, the simulation first simulates the state of drilling before the working face is mined, analyzing the vertical stress and failure characteristics under different elastic core widths, which is to evaluate the static stress relief effect before mining. Secondly, it simulates the state of excavation of the working face based on the existing boreholes, analyzing the vertical stress of the coal body and the failure of the elastic cores between boreholes under different elastic core widths, studying the stress relief effect of different elastic core widths between boreholes under mining conditions, which is to evaluate the dynamic stress relief effect after mining. Finally, based on the simulation results of the two states, the elastic core width that meets the design conditions is selected.

[0056] Furthermore, the design conditions are as follows:

[0057] 1. Before drilling, the elastic core-plastic zone between boreholes was not damaged, and the maximum vertical stress in this zone after drilling was at least 10% lower than the peak vertical stress before drilling.

[0058] 2. After the working face is mined, as the elastic core between the advance holes of the working face is damaged by the advance support pressure, a continuous pressure relief zone is formed between the boreholes. The maximum vertical stress in this area is reduced by more than 20% compared with the peak support pressure when no borehole is drilled.

[0059] 3. The distance by which the peak position of the supporting pressure migrates to the depth of the coal body is ≥ 2 to 3 times the working face height.

[0060] Finally, based on the above principles, the simulation results under each alternative spacing were compared and analyzed to comprehensively determine the optimal inter-hole elastic core width D. z :

[0061]

[0062] In the formula, D z The optimal width of the inter-hole elastic core is given in meters (m).

[0063] The beneficial effects of this invention are as follows:

[0064] 1. The parallel horizontal drilling decompression technology for coal seams proposed in this invention fully considers the effective bearing radius of anchor bolts / cables when designing the distance between the borehole axis and the roadway sidewall, ensuring that the decompression area completely avoids the core support bearing zone. Based on this, a dynamic decompression mechanism of the inter-hole elastic core is considered, and the borehole spacing is precisely designed to form an elastic core between adjacent boreholes. This elastic core maintains its complete elastic state before mining, bearing and transmitting stress to achieve initial pre-decompression and support protection. During mining, driven by the advance support pressure, the elastic core undergoes gradual plastic failure according to the designed sequence, connecting with the plastic zones of the boreholes on both sides to form a continuous decompression zone. Secondary dynamic decompression is achieved using the stress during mining, allowing for the orderly and slow release of stress. This precisely decompresses the high-stress area of ​​the sidewall without weakening or disturbing the original support system, significantly reducing damage to the sidewall support system during decompression construction, thereby improving the overall stability of the roadway surrounding rock. This fundamentally resolves the contradiction between pressure relief and support, where pressure relief reduces support stability and support hinders pressure relief.

[0065] 2. This invention is guided by the predetermined failure sequence of the inter-hole elastic core, precisely designing the borehole spacing. First, the feasible range of the inter-hole elastic core width is defined through theoretical calculations, then optimized through numerical simulations, ultimately obtaining the optimal elastic core width range. This range ensures precise matching of the pressure relief parameters with the coal seam's mechanical properties, geostress field, and mining conditions, enabling pressure relief to be achieved gradually along the designed path under controllable conditions.

[0066] 3. Conventional large-diameter drilling for pressure relief typically employs a dense borehole layout to ensure effective pressure relief and avoid impact hazards. This results in a large number of boreholes, frequent drilling rig relocation, and high construction costs. In contrast, this invention utilizes a long, horizontal borehole arrangement parallel to the roadway. The optimal borehole spacing is precisely calculated using an elastic core between boreholes. While ensuring continuous pressure relief and safety, this allows for a reasonable increase in borehole spacing, reducing the number of drilling rig relocations and lowering construction costs. Attached Figure Description

[0067] Figure 1 This is a borehole layout diagram. Figure 1 (a) is a schematic diagram of the overall layout of the present invention; Figure 1 (b) is Figure 1 (a) Sectional view along direction AA.

[0068] Figure 2 This is a schematic diagram of the tangent phase in the plastic zone of the borehole.

[0069] Figure 3 This is a schematic diagram showing the non-tangential relationship in the plastic zone of the borehole.

[0070] In the diagram: 1-Working face stop line, 2-Long horizontal borehole, 3-Side anchor / cable support, D-Width of elastic core between boreholes, 4-Side anchor / cable support range, 5-Goaf roadway, 6-Mining direction of the working face, 7-Protective coal pillar, 8-Goaf area, 9-Working face cut-off, R-Borehole radius, R t - Radius of the fracture zone, R p - Corrected plastic zone radius, l - half the hole spacing, L - hole spacing, D - width of the elastic core between holes. Detailed Implementation

[0071] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings, so that those skilled in the art can fully understand the technical solution and beneficial effects of the present invention.

[0072] Figure 1 This invention illustrates the overall concept of a long horizontal borehole pressure relief and anti-scour method for coal seams based on inter-hole elastic cores. As shown in the figure, multiple long horizontal boreholes 2 are arranged parallel to the roadway, and the inter-hole elastic core D is located between the outer edges of the plastic zone of adjacent long horizontal boreholes 2. During construction, the distance between the axis of the long horizontal borehole 2 and the roadway sidewall is required to be no less than 1.5 times the length of the sidewall anchor / cable support (e.g., ...). Figure 1 (4) The spacing between adjacent boreholes is equal to the sum of the radius of the plastic zone of the boreholes on both sides and the width of the elastic core between the boreholes. The width of the elastic core D between the boreholes needs to be optimized through numerical simulation. The purpose of optimization is to maintain a complete elastic state before the working face is mined, so as to bear and transfer stress, achieve initial pressure relief and protect the support structure; during the working face mining, it undergoes progressive plastic failure under the action of advanced support pressure, and connects with the plastic zone of the boreholes on both sides to form a continuous pressure relief zone.

[0073] The optimization method for the width D of the inter-pore elastic core is described in detail below. It includes the following steps:

[0074] S1: Based on the mechanical parameters of the coal seam and the in-situ stress conditions, the radius of the plastic zone of the long horizontal borehole is theoretically calculated, and a correction factor is introduced to determine its actual radius;

[0075] 1.1: Acquisition of geomechanical parameters;

[0076] At the target working face in the mine, geological and related mechanical parameters of the coal seam are obtained through methods such as on-site core sampling and in-situ stress testing. By integrating the in-situ stress test results with laboratory experimental data, a database of mechanical parameters of the roof strata of the working face is established, outputting a complete set of parameters including in-situ stress, burial depth H, unit weight γ, cohesion c, and internal friction angle φ. This step ensures that the spatial location, depth, and stratigraphic design of subsequent boreholes have a reliable geological and mechanical basis.

[0077] 1.2: Theoretical calculation of the radius R2 of the plastic zone in a long horizontal borehole;

[0078] Calculate the original plastic zone radius R2 of the long horizontal borehole 2 based on the parameters, and obtain the corrected plastic zone radius R. p ;

[0079] After drilling, the surrounding rock forms three distinct mechanical states from the borehole wall outwards: a fractured zone, a plastic zone, and an elastic zone. Therefore, when calculating the radius of the plastic zone, the radius R of the fractured zone is calculated first. t This is the actual outer boundary of the fractured zone; then, using this actual outer boundary as the starting boundary of the plastic zone, the radius of the plastic zone is calculated using the Kastner formula.

[0080] Based on this mechanism, first calculate the radius R of the fractured zone formed around the borehole. t The radius of the fractured zone R t The calculation formula is:

[0081] (1)

[0082] In the formula, R t is the radius of the crushed zone, in meters; p is the coal body crushing coefficient, taken as 1.2~1.5; R is the radius of the pressure relief drill bit (determined according to the drill bit diameter, usually 0.05~0.15m), in meters.

[0083] Then, based on the Kastner formula, the radius R2 of the original plastic zone of borehole 2 is calculated using formula (2):

[0084] (2)

[0085] In the formula: R2 is the radius of the plastic zone, m; γ is the average unit weight, kN / m³. 3 H is the burial depth of the coal seam, m; φ is the friction angle of the surrounding rock, °; λ is the lateral pressure coefficient; c is the cohesion, MPa.

[0086] 1.3: The original plastic zone radius R2 is corrected using a correction factor to obtain the corrected plastic zone radius R. p ;

[0087] Due to variations in the hardness of coal during actual construction, the impact of the drill bit on the fractured and plastic zones of the coal varies. In actual drilling, the range of the plastic zone is further expanded. A correction factor k is determined through a combination of laboratory experiments and numerical simulations. Introducing this correction factor k yields the corrected radius R of the plastic zone. p :

[0088] (3)

[0089] In the formula: R pis the corrected radius of the plastic zone, in meters; k is the correction coefficient, determined through field measurements and numerical simulation inversion.

[0090] S2: Determine the design range of the width of the inter-hole elastic core;

[0091] 2.1: Determine the lower limit value D of the width range of the inter-hole elastic core. min (See also) Figure 2 );

[0092] Substituting the relevant parameters into equation (3) above, we can obtain the radius R of the plastic zone. p When the drilled plastic zone is tangent (see...) Figure 2 When the elastic core width is at its minimum, the minimum hole spacing and the elastic core width are:

[0093] (4)

[0094] (5)

[0095] In the formula: L min Minimum hole spacing, in meters (m); D min The minimum elastic core width is m.

[0096] The calculation principle of formulas (4) and (5) is: In theory, when the plastic zones of adjacent boreholes are tangent, the width of the elastic core between the holes is the smallest, and the distance between the holes is also the smallest, which is equal to the sum of the radii of the two plastic zones. The minimum width of the elastic core is 0. In actual construction, the width of the elastic core is usually taken as a value greater than 0.

[0097] 2.2: Determine the upper limit value D of the elastic core width range. max (See also) Figure 3 );

[0098] The design concept of this step is as follows: the upper limit of the elastic core width range is the width corresponding to the elastic core when it bears the maximum stress superimposed by two holes, and the limit state of the superimposed stress of the two holes is when the coal body reaches its own ultimate bearing strength. Based on this mechanism, the total stress borne by the elastic core under the theoretical state is first calculated. This stress is formed by the superposition of the mining-induced stress increment and twice the tangential stress generated by a single hole at the midpoint of the elastic core. When the total stress equals the ultimate bearing strength of the coal body, the critical tangential stress of a single hole at the midpoint of the elastic core is obtained by inverse calculation; then, combined with the relationship between tangential stress and hole spacing, the hole spacing corresponding to the critical stress is solved, and finally the upper limit of the elastic core width is calculated. The specific steps are as follows:

[0099] 2.2.1: Obtain the mining-induced stress increment Δσ;

[0100] Mining-induced stress increment Δσ: Based on the distribution law of the working face support pressure (measured or numerically simulated), determine the peak pressure coefficient K of the advance support. Take the mining-induced stress increment at the elastic core location as:

[0101] (6)

[0102] In the formula: Δσ is the mining stress increment; K is the peak pressure coefficient of the advance support (generally 1 to 3).

[0103] Based on similar engineering experience, for safety design, Δσ=0.2γH is taken as the critical trigger value, that is, when the vertical stress exceeds 20% of the original rock stress (i.e. K≥1.2), the elastic core begins to enter the plastic state.

[0104] 2.2.2: Radial contact force σ based on single-hole elastoplastic boundary R Solve for the tangential stress σ at the midpoint of the elastic core under the action of a single hole. θ ;

[0105] After the borehole layout, the plastic zone R of adjacent boreholes p The two boreholes interact, forming a rock mass region between them that has not yet undergone plastic failure, called the inter-bore elastic core D. This elastic core will undergo further plastic failure under the pre-mining support pressure, thus providing secondary pressure relief. To reasonably determine the width of the inter-bore elastic core, the radial contact force on the inter-bore elastic core D under the original rock stress state must first be calculated.

[0106] According to the Mohr-Coulomb yield criterion in elastoplastic mechanics, the radial contact force σ at the single-hole elastoplastic boundary is... R It can be determined by the following formula:

[0107] (7)

[0108] This formula reflects the characteristic value of radial stress when the rock mass at the boundary of the plastic zone transitions from an elastic state to a plastic state, and is the basis for subsequent calculation of the stress distribution between holes.

[0109] Under the condition of parallel arrangement of two or more boreholes, the tangential stress at any point within the elastic core is affected by the superposition of the plastic zones of adjacent boreholes. Based on the thick-walled cylinder theory in elasticity mechanics, and considering the stress transfer at the boundary of the plastic zone, the tangential stress σ acting on the midpoint of the elastic core from a single borehole is... θ It can be calculated using the following formula:

[0110] (8)

[0111] In the formula, l is half of the hole spacing, in meters (m).

[0112] This formula is based on the thick-walled cylinder theory in elasticity, and the radial contact force σ at the boundary of the plastic zone is... R As an inner boundary condition, combined with the original rock stress field, the tangential stress σ at the midpoint of the elastic core is obtained by solving. θThis tangential stress is an important component of the total stress on the coal body and will be used in subsequent analysis of the elastic core bearing capacity.

[0113] First, calculate σ from H, γ, c, and φ obtained in step S1. R Then calculate σ θ The expression is used as the basis for the subsequent elastic core ultimate bearing analysis.

[0114] 2.2.3: Calculate the total stress σ of the elastic core bearing capacity z ;

[0115] σ θ The stress is a single pore acting on the tangential stress at the midpoint of the elastic core. Because the two pores are symmetrical, the center of the elastic core is subjected to twice the σ stress. θ The role of the elastic core, according to the design concept, is to bear the total stress σ. z Calculated by the following formula:

[0116] (9)

[0117] 2.2.4: Calculate the ultimate bearing strength σ1 of the coal seam;

[0118] The ultimate compressive strength σ of the coal body is obtained through the uniaxial or triaxial compression test in step one. c Under actual confining pressure, we take σ1 = σ c Alternatively, the ultimate bearing strength σ1 of the coal seam can be calculated using the Mohr-Coulomb criterion:

[0119] (10)

[0120] In the formula: σ1 is the ultimate bearing capacity of the coal body, MPa; σ3 is the minimum principal stress (valued based on in-situ stress test), MPa.

[0121] 2.2.5: Obtain the maximum width D of the inter-pore elastic core through inverse kinematics. max ;

[0122] According to the design concept, when the elastic core bears the total stress σ z When the ultimate bearing strength σ1 of the coal body is reached or exceeded, the elastic core undergoes plastic failure. The width of the elastic core at this point is the upper limit value D of the design. max Based on this, the critical condition for formula (11) is established:

[0123] (11)

[0124] In the formula: σ z临界 The critical total stress under the action of two holes on the elastic core is expressed in MPa; σ θ临界 The critical tangential stress acting on the midpoint of the elastic core through a single hole, in MPa;

[0125] Substituting the coal body ultimate bearing strength σ1 obtained from formula (10) and the mining stress increment Δσ calculated from formula (6) into formula (11), σ can be obtained. θ临界 Then, the tangential stress σ acting at the midpoint of the elastic core by a single hole is given by formula (8). θ The maximum hole spacing L can be obtained by solving the relationship between the hole spacing and half the hole spacing l. max Then, based on the relationship between the width of the elastic core between holes and the hole spacing, the maximum value D of the width of the elastic core between holes is obtained. max :

[0126] (12)

[0127] The final determined width of the inter-pore elastic core should not be greater than D. max This ensures that the elastic core between holes can undergo plastic fracture under the pressure of the advance support during mining, thereby achieving secondary pressure relief and effectively reducing the pressure concentration at the working face.

[0128] Therefore, theoretically, the width D of the interpore elastic core ranges as follows:

[0129] (13)

[0130] The theoretically calculated interval defines the "controllable pressure relief window" for the inter-pore elastic core. Its mechanical implications lie in: when D... <D min At this time, the plastic zones are interconnected, leading to loosening and instability of the coal body in the sidewalls (excessive pressure relief); when D>D max At this time, the elastic core is too wide to be triggered and destroyed by mining stress, forming a high-stress island (insufficient pressure relief).

[0131] S3: Determine the optimal width D of the inter-pore elastic core. z ;

[0132] As can be seen from step S2, the core task of numerical simulation is not simply to select values, but to verify whether the elastic core satisfies the dynamic evolution mechanism of "complete energy storage before mining and energy release through destruction during mining".

[0133] Within the aforementioned theoretical range, to accurately capture the critical behavior of the elastic core transitioning from "elastic" to "plastic," dynamic verification through numerical simulation is necessary. Based on engineering experience and drilling rig parameters, representative candidate values ​​are selected within the range, with three candidate spacing values ​​chosen at equal intervals, denoted as D1, D2, and D3. These candidate values ​​will serve as input parameters for subsequent numerical simulation optimization. Simulation analysis will be conducted to examine the stress path and failure characteristics of the elastic core under different widths throughout the entire mining process.

[0134] During the simulation, a numerical simulation model of the stress relief of the horizontal borehole 2 in the working face, including the roadway and different elastic core width schemes between boreholes, was established. The model simulated and analyzed the stress distribution and failure characteristics of the elastic core between boreholes under the original stress state and the influence of mining, ultimately determining the optimal elastic core width between boreholes. The specific method is as follows:

[0135] 3.1: Numerical model establishment;

[0136] Based on the actual geological and mining conditions of the working face (coal seam thickness, roof strata combination, burial depth, in-situ stress, etc.), establish a three-dimensional or plane strain numerical model (FLAC can be used). 3D (Use software such as COMSOL, ABAQUS, or similar software) to set up three sets of models, where only the width of the elastic core between the holes is changed in each set of models.

[0137] To verify and optimize the width of the inter-pore elastic core, FLAC was used. 3D A three-dimensional numerical model was established. The model dimensions were 200m × 150m × 80m. Displacement constraints were applied to the bottom and surrounding boundaries, and vertical stress with an equivalent burial depth was applied to the top. The mesh around the borehole was refined to 0.1m. The long horizontal borehole 2, with a length of 100m, could not exceed the working face stop line 1 and was simulated using a direct excavation unit (nullmodel). The borehole was arranged parallel to the goaf roadway 5, avoiding the protection range 4 of the side anchor / cable support. The side anchor / cable support 3 was designed first, and the width of the protective coal pillar 7 was retained according to the design. Then, the adjacent working face goaf area 8, the goaf roadway 5, the working face cut-out 9, and the long horizontal borehole 2 were excavated.

[0138] The mining proceeded along the designed working face direction 6, employing a step-by-step excavation method, with each step measuring 5 meters. An empty element model was used, and excavation continued until the working face stop line 1 was reached. The coal and rock mass were modeled using a Mohr-Coulomb constitutive model, with material parameters derived from the laboratory tests conducted in step one. By monitoring the vertical stress curve and plastic zone distribution ahead of the working face, three stress relief indices were compared under different elastic core widths.

[0139] 3.2: Simulation analysis under original stress conditions (unmined);

[0140] Simulated stress redistribution in the surrounding rock after borehole drilling during the unmined stage of the working face:

[0141] Analyze the vertical stress distribution and plastic zone development morphology of coal under different inter-pore elastic core widths D;

[0142] Investigate the width and stress state of the inter-hole elastic core D (the region where plastic failure has not occurred);

[0143] Calculate the maximum vertical stress in the area after drilling and compare it with the un-drilled scheme. The stress reduction should be ≥10% (as one of the subsequent screening criteria).

[0144] 3.3: Simulation analysis under mining conditions (working face excavation);

[0145] Based on the established borehole model, the gradual advancement process of the working face is simulated:

[0146] The stress evolution and plastic failure process of the inter-hole elastic core under the action of advance support pressure were analyzed under different inter-hole elastic core widths as the working face advanced; the timing and range of elastic core failure were monitored to determine whether a continuous pressure relief zone was formed; the vertical stress distribution curve in front of the working face was extracted to determine the peak position of the support pressure and the distance it migrated to the deeper part of the coal body.

[0147] 3.4: Optimal interpore elastic core width D z Selection criteria;

[0148] Based on the simulation results of the three sets of simulations combined with the original stress and mining conditions, the optimal inter-hole elastic core width was selected according to the following three principles:

[0149] Pre-decompression effect: Before mining, the elastic core between holes remains intact; compared with the non-drilled scheme, the maximum vertical stress in this area is reduced by ≥10% after drilling, indicating that the drilling has achieved effective stress transfer.

[0150] The effect of mining on the plastic zone: After the working face is mined, as the working face advances, the elastic core between the holes undergoes plastic failure under the action of the advance support pressure, and a continuous pressure relief zone is formed between adjacent boreholes. The vertical stress is reduced by ≥20% (relative to the peak stress when no borehole is drilled), ensuring that the roof is weakened in time.

[0151] Deep transfer effect: The distance at which the peak position of the supporting pressure migrates to the depth of the coal body is ≥ 2 to 3 times the working face height, so as to reduce the load concentration of the near-field coal wall and support, and reduce the risk of spalling and support collapse.

[0152] 3.5: Determine the optimal width of the inter-pore elastic core comprehensively;

[0153] Based on the above three criteria, the simulation results of the candidate hole-to-hole elastic core widths D1, D2, and D3 are compared and analyzed. The spacing that simultaneously meets or is closest to the requirements of the three objectives is selected as the optimal hole-to-hole elastic core width.

[0154] (14)

[0155] In the formula, D z The optimal width of the inter-hole elastic core is given in meters (m).

[0156] The actual inter-hole elastic core width corresponding to this width is the designed inter-hole elastic core width of the long horizontal decompression borehole in the coal seam.

[0157] The above are merely specific embodiments of the present invention and are not intended to limit the technical solutions of the present invention. Any modifications and refinements made to the technical solutions of the present invention without creative effort shall be within the scope of protection of the present invention, and the specific scope of protection shall be determined by the claims.

Claims

1. A method for pressure relief and scour prevention in long horizontal boreholes of coal seams based on inter-hole elastic cores, characterized in that long horizontal boreholes are uniformly and parallelly arranged along the roadway direction, and that... The distance between the borehole axis and the roadway sidewall is not less than 1.5 times the length of the sidewall anchor / cable support; the spacing between adjacent boreholes is determined by the radius of the borehole plastic zone and the width of the elastic core between boreholes; the width of the elastic core between boreholes refers to the width of the coal body area between the outer edges of the plastic zones of two adjacent long horizontal boreholes, which is still in an elastic stress state. The width of the elastic core between boreholes should meet the following requirements: before the working face is mined, it should maintain a complete elastic state to bear and transfer stress, achieve initial pressure relief and protect the support structure; during the working face mining, it should undergo progressive plastic failure under the action of advanced support pressure, and connect with the plastic zones of the boreholes on both sides to form a continuous pressure relief zone.

2. The method for pressure relief and anti-scour of long horizontal boreholes in coal seams based on inter-hole elastic cores as described in claim 1, characterized in that, The spacing between the boreholes is equal to the width of the elastic core between the boreholes plus the radius of the plastic zone after correction of the two boreholes, which is the actual radius of the plastic zone.

3. A method for optimizing pressure relief and anti-scour parameters in long horizontal coal seams based on inter-hole elastic cores, characterized in that, The optimization steps are as follows: S1: Collect basic parameters such as coal seam burial depth, coal mechanics, and original rock stress. Based on the above parameters, theoretically calculate the radius of the plastic zone of the long horizontal borehole, and introduce a correction coefficient to determine its actual radius. S2: Using the basic parameters obtained from S1 and the actual radius of the plastic zone, determine the reasonable design range of the width of the elastic core between holes; The lower limit of the interval is determined by the tangency condition of the plastic zones of the two boreholes, and the upper limit of the interval is solved by combining the ultimate bearing state of the elastic core under mining stress. S3: Through numerical simulation, based on the characteristics of mining response, the optimal inter-hole elastic core width is determined within the design range of elastic core width.

4. The method for optimizing pressure relief and anti-scour parameters of long horizontal boreholes in coal seams based on inter-hole elastic cores as described in claim 1, characterized in that, In step S1, the actual radius of the plastic zone of the long horizontal borehole R p The calculation formula is: ,in: In the formula: R 2 is the theoretical value of the radius of the plastic zone, in meters; The correction factor was determined by combining the measured range of the plastic zone from experimental boreholes with numerical simulation. R t The radius of the fractured zone is given by... Determined, m; p The coefficient for coal mass fragmentation is taken as 1.2~1.5; R The radius of the pressure relief drill bit is in meters. γ The average unit weight of the overlying rock strata is kN / m³. 3 ; H The depth of the coal seam is in meters (m). φ Let be the friction angle of the surrounding rock, in °; λ is the lateral pressure coefficient of the original rock; c ρ represents the cohesive force of the coal body, in MPa.

5. The method for optimizing pressure relief and anti-scour parameters of long horizontal boreholes in coal seams based on inter-hole elastic cores as described in claim 1, characterized in that, In step S2, the lower limit of the width range of the inter-pore elastic core. D min This refers to the situation where the plastic zones of two adjacent holes are tangent; Upper limit of the width range of the interpore elastic core D max The method for determining the value is as follows: the critical criterion is that the total bearing stress of the elastic core reaches the ultimate bearing strength of the coal body. The total bearing stress of the elastic core includes the increase in mining-induced stress Δ experienced by the elastic core due to mining. σ In addition, the maximum hole spacing is obtained by back-calculating the tangential stress applied by the coupling of the two boreholes before mining through the total stress borne by the elastic core. L max ; Based on the relationship that the borehole spacing equals the width of the elastic core between the boreholes plus the actual radius of the plastic zone of the two boreholes, the maximum borehole spacing is used to determine the relationship between the two boreholes. L max The upper limit of the interpore elastic core interval was further obtained. D max .

6. The method for optimizing pressure relief and anti-scour parameters of long horizontal boreholes in coal seams based on inter-hole elastic cores as described in claim 5, characterized in that, The specific method for step S2 is as follows: 2.1: Calculate the lower limit of the width of the elastic core interval between holes D min ; In the formula: l min Minimum hole spacing, in meters (m). D min The lower limit of the width of the elastic core interval between holes, in meters; 2.2: Calculate the upper limit of the width of the elastic core interval between holes. D max ; 2.2.1: Obtain the additional stress induced in the elastic core region during working face mining, i.e., the mining-induced stress increment. Δσ ; Based on the distribution law of support pressure at the working face, determine the peak pressure coefficient of the advance support. K Take the incremental stress of the elastic core. Δσ for: , In the formula: The peak pressure coefficient for the advanced support is taken as 1.5 to 3.0; γ The average bulk density is expressed in kN / m³. 3 ; H The depth of the coal seam is in meters (m). 2.2.2: Calculate the tangential stress acting on the midpoint of the elastic core between the holes from a single hole. σ θ ; Tangential stress σ θ The calculation formula is: In the formula: σ R The radial contact force at the single-hole elastoplastic boundary, in MPa, is given by the formula... Calculated; l It is half the hole spacing, in meters (m). γ The average bulk density is expressed in kN / m³. 3 ; H The depth of the coal seam is in meters (m). 2.2.3: Calculate the total stress of the elastic core bearing capacity. σ z ; The total stress borne by the elastic core mainly consists of two parts: one is the tangential stress at the midpoint of the elastic core under the original rock stress conditions and the effect of dual-hole coupling, calculated as the tangential stress of a single hole. σ θ The calculation includes two aspects: superposition calculation and the increase in mining-induced stress generated during the longwall mining process. Δσ Because the two adjacent holes are symmetrical, the center of the elastic core is subjected to twice the force. σ θ The effect is that when the total stress reaches or exceeds its ultimate bearing strength. σ At time 1, the elastic core enters a state of plastic failure, achieving secondary decompression, therefore: ; 2.2.4: Obtaining the ultimate bearing strength of the coal seam σ 1; ultimate bearing strength of coal σ 1 refers to the critical stress value at which the coal body transitions from an elastic state to a plastic state during mining; that is, the maximum stress that the coal body can withstand without failure. σ 1. Ultimate compressive strength of coal seam σ c Alternatively, it can be determined by the Mohr-Coulomb strength criterion, i.e., obtained according to the following formula: The calculation formula is: , In the formula: σ 3 represents the minimum principal stress, obtained from geostress testing, in MPa. 2.2.5 The maximum width of the inter-pore elastic core is obtained by inverse kinematics. D max ; set up σ θ临界 This represents the critical tangential stress acting on the midpoint of the elastic core from a single borehole. σ z临界 The elastic core bears the critical total stress. σ z临界 Reaching the ultimate bearing strength of the coal body σ 1 is the critical criterion, that is, based on As a condition, based on the ultimate bearing capacity of the coal seam obtained in step 2.2.4 σ 1. The mining stress increment Δ obtained in step 2.2.1 σ The critical tangential stress is obtained by solving. σ θ临界 Then, through the tangential stress given in step 2.2.2... σ θ Half the distance between the holes l Inverse solution of the relation l The critical value is used to calculate the maximum hole spacing. L max Then, the maximum value of the elastic core width between the holes was obtained. D max ,Right now: .

7. The method for optimizing pressure relief and anti-scour parameters of long horizontal boreholes in coal seams based on inter-hole elastic cores as described in claim 1, characterized in that, The specific method of step S3 is as follows: establish a numerical simulation model of horizontal borehole pressure relief for working face length, including roadway and different borehole elastic core width schemes. Within the range of the calculated borehole elastic core width, select three candidate spacing values ​​D1, D2, and D3 according to the principle of equal intervals as input for subsequent numerical simulation optimization. Use numerical simulation methods to further analyze the range of the borehole elastic core.

8. The method for optimizing pressure relief and anti-scour parameters of long horizontal boreholes in coal seams based on inter-hole elastic cores as described in claim 7, characterized in that, During the simulation, the state of drilling before the working face is mined is simulated first, and the vertical stress and failure characteristics under different elastic core widths are analyzed. This is to evaluate the static stress relief effect before mining. Second, the state of excavation of the working face based on the existing boreholes is simulated, and the vertical stress of the coal body and the failure of the elastic core between the boreholes are analyzed under different elastic core widths. The stress relief effect of different elastic core widths between the boreholes under mining conditions is studied, which is to evaluate the dynamic stress relief effect after mining. Finally, the elastic core width that meets the design conditions is selected based on the simulation results of the two states.

9. The method for optimizing pressure relief and anti-scour parameters of long horizontal boreholes in coal seams based on inter-hole elastic cores as described in claim 8, characterized in that, The design conditions include: (1) The elastic core-plastic zone between holes was not damaged before the working face was mined. Compared with the peak vertical stress in this zone after drilling, it was reduced by at least 10%; (2) After the working face is mined, as the elastic core between the advance holes of the working face is damaged by the advance support pressure, a continuous pressure relief zone is formed between the boreholes. The maximum vertical stress in this area is reduced by more than 20% compared with the peak support pressure when no borehole is drilled. (3) The distance from the peak position of the supporting pressure to the depth of the coal body is ≥ 2 to 3 times the working face mining height; Based on the above three criteria, the simulation results of candidate hole-to-hole elastic core widths D1, D2, and D3 are compared and analyzed. The spacing that simultaneously meets or is closest to the requirements of the three objectives is selected as the optimal hole-to-hole elastic core width. D z : , In the formula, D z The optimal width of the inter-hole elastic core is given in meters (m).

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