A water-resisting coal pillar width design method based on water seepage gradient weakening

Abstract

The application discloses a kind of based on water seepage gradient weakening water-proof coal pillar width design method, belong to coal mine safety and mine pressure control technical field.The method includes: obtaining the uniaxial compressive strength, cohesion and internal friction angle test data of dry state and engineering saturated state coal sample;Weak index field D ( x ) exponential decay model and c ( x ), tan φ ( x ) Space degradation model;Under Mohr-Coulomb criterion, the limit equilibrium equation of strength space variable is established, respectively solving stress recovery scale x σ And strength recovery scale x D ;Calculate double-scale discriminant index Λ=x σ / x D , determine the main control type of the stability of water immersion side coal body;According to main control type, determine the minimum design width of water-proof coal pillar.Through the explicit discrimination of double-scale competition relationship, the design of water-proof coal pillar width is realized before analysis and after checking integration, provides analytical engineering approach for coal pillar design under water seepage condition along gob.

Description

Technical Field

[0001] This invention belongs to the field of coal mine safety and mine pressure control technology, specifically relating to a method for designing the width of a water-resistant coal pillar based on seepage gradient weakening. Background Technology

[0002] In western my country's mining areas, under high-intensity development conditions, there is a dual constraint: the protection and utilization of mine water resources and the safe mining of coal. Due to the continuous recharge from the Jurassic aquifer, adjacent goaf areas are prone to gradual water accumulation after the working face is mined, forming dynamically changing water-filled spaces. Even after the water is drained and reaches a relatively stable state, the long-term soaking and seepage have already caused strength degradation, pore structure damage, and fracture network expansion in adjacent coal seams. Under these conditions, the determination of the appropriate width of the water-blocking coal pillar during subsequent goaf excavation directly affects the stability of the surrounding rock, the prevention and control of water inrush risks, and the coal resource recovery rate.

[0003] Extensive research has been conducted on coal pillar stability and the placement of water-resistant coal pillars, employing theoretical analysis, numerical simulation, physical experiments, and engineering practice. Representative findings often determine reasonable widths based on single dimensions such as the seepage zone range, plastic zone development, water-blocking zone retention, or elastic compaction zone distribution, leading to combined criteria such as "seepage zone + water-blocking zone" and "plastic zone + elastic core." Simultaneously, the development of detection methods such as distributed fiber optic monitoring, acoustic emission localization, and borehole television observation has provided technical support for identifying internal coal pillar damage and evaluating post-placement stability.

[0004] However, existing technologies still have the following shortcomings in determining the width of the water-proof coal pillar in the goaf excavation roadway under the influence of water accumulation in the goaf: First, the characterization of water-induced weakening effects is rather crude. Existing studies mostly simplify the effects of water immersion into overall parameter reduction or a stepped partitioning treatment of "strongly weakened zone / weakly weakened zone / unaffected zone," which limits the analytical characterization of the continuous gradient degradation of coal strength parameters along the width direction caused by water seepage, making it difficult to reflect the physical nature of the seepage-diffusion process.

[0005] Second, there is a lack of a unified characterization of the competitive relationship between the stress field and the strength field. Existing width criteria mostly focus on identifying the geometric range of the plastic zone, seepage zone, or water-blocking zone, separating the stress recovery process from the strength recovery process, and lacking a unified mathematical description and explicit discrimination criterion for the competitive relationship between the stress recovery scale and the strength recovery scale.

[0006] Third, there is insufficient connection between the design and construction phases. Although existing studies have focused on both width design and on-site monitoring, there is a lack of direct correspondence between theoretical predictions in the design phase and actual measurement verification after installation, making it difficult to achieve an integrated closed loop of "pre-design analysis - post-installation verification". Summary of the Invention

[0007] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, the first objective of this invention is to propose a design method for the width of a water-resistant coal pillar based on seepage gradient weakening. By constructing an exponential decay model of the weakening index field and a spatial degradation model of the strength parameters, a dual-scale competitive discrimination mechanism of stress recovery scale and strength recovery scale is established to achieve accurate design and reliability verification of the width of the water-resistant coal pillar.

[0008] To achieve the above objectives, a first aspect of the present invention proposes a method for designing the width of a waterproof coal pillar based on weakening the seepage gradient, comprising the following steps: S1, to obtain test data of uniaxial compressive strength, cohesion and internal friction angle of coal in dry and engineering saturated states, as well as engineering parameters of coal seam mining height, interfacial friction coefficient and equivalent horizontal boundary stress on the water-immersed side; S2, based on the strength parameters of dry state and engineering saturated state, an exponential decay model of the weakened index field is constructed, and a spatial degradation model of the cohesion and internal friction tangent value along the immersion direction under the Mohr-Coulomb criterion is established. S3. Substitute the spatial degradation model of cohesion and internal friction tangent into the limit equilibrium equation to establish the stress equilibrium differential equation with spatially variable strength parameters, and solve for the stress recovery scale and strength recovery scale. S4, calculate the ratio of stress recovery scale to strength recovery scale as a dual-scale discrimination index, and determine the main control type of coal body stability on the water-soaked side based on the relationship between the discrimination index and 1; S5. Determine the value of the width of the water-immersed control zone according to the main control type, and determine the minimum design width of the water-proof coal pillar by combining the minimum width of the elastic core zone and the width of the roadway side plastic zone.

[0009] In addition, the water-resistant coal pillar width design method based on seepage gradient weakening according to the above embodiments of the present invention may also have the following additional technical features: According to one embodiment of the present invention, the uniaxial compressive strength includes dry uniaxial compressive strength and engineered saturated uniaxial compressive strength, the cohesion includes dry cohesion and engineered saturated cohesion, and the internal friction angle tangent includes dry internal friction angle tangent and engineered saturated internal friction angle tangent.

[0010] According to an embodiment of the present invention, in step S2, the expression for weakening the index field is: D ( x )=1- R u ( x ) / R u0 , The exponential decay form of the spatial distribution of the weakening index field is expressed as: D ( x )= D max · e -αx , D max =1- R u,sat / R u0 , in, D ( x To weaken the indicator field, R u ( x () is the distance from the immersion boundary x Uniaxial compressive strength of the coal seam R u0 This represents the dry uniaxial compressive strength. R u,sat For engineering saturated uniaxial compressive strength, D max The maximum weakening value, α The spatial attenuation coefficient has the dimension m. - ¹, x The distance from the water-soaked boundary to the interior of the coal body is in meters.

[0011] According to an embodiment of the present invention, in step S2, the expression of the spatial degradation model is: c ( x )= c 0[1- a c · D ( x ) / D max ], tan f ( x )=tan f 0[1- a φ · D ( x ) / D max ], in, a c The cohesion degradation coefficient is determined by the ratio of dry cohesion to engineering saturated cohesion, tan f ( x ) is the tangent of the internal friction angle, tan f 0 represents the tangent of the dry-state internal friction angle.a φ The internal friction angle degradation coefficient is determined by the ratio of the tangent of the dry internal friction angle to the tangent of the engineering saturated internal friction angle.

[0012] According to an embodiment of the present invention, in step S3, the limiting equilibrium equation is a model considering spatial degradation. c ( x ) and the tangent of the internal friction angle tan f ( x The changing one-dimensional stress equilibrium differential equation, stress recovery scale x σ The strength recovery scale is defined as the distance from the water-immersed boundary from which the vertical stress of the coal body first recovers to the reference vertical stress. x D Defined as the distance from the immersion boundary when the weakened index field drops to a preset reference threshold, derived from... D ( x D )≤ e The solution is obtained. e Small quantity for reference.

[0013] According to an embodiment of the present invention, in step S4, the determination rule for the main control type is as follows: when the discrimination index is greater than 1, the stress recovery scale is greater than the strength recovery scale, and it belongs to the stress recovery control type; when the discrimination index is less than 1, the strength recovery scale is greater than the stress recovery scale, and it belongs to the strength recovery control type; when the discrimination index is equal to 1, stress recovery and strength recovery are synchronized.

[0014] According to one embodiment of the present invention, in step S5, the minimum design width is determined according to the following formula: B min = x w,l + b min + x p,r , in, B min Minimum design width; x w,l Width of the control zone on the immersion side. x p,r The width of the plastic zone on the side of the tunnel. b min This represents the minimum width of the elastic core region; Specifically, when the control zone is determined to be of the strength recovery control type, the width of the control zone on the immersion side is determined according to the strength recovery scale; when the control zone is determined to be of the stress recovery control type, the width of the control zone on the immersion side is determined according to the stress recovery scale. The minimum width of the elastic core zone is twice the coal seam mining height. The width of the plastic zone on the roadway side is calculated analytically based on the limit equilibrium theory combined with the coal seam mining height and the interface friction coefficient.

[0015] According to an embodiment of the present invention, the above method further includes step S6: after the roadway is excavated, boreholes are arranged in the coal pillar for borehole inspection, images of borehole wall cracks are obtained and fractal dimension analysis is performed, the a priori weakening influence depth is compared with the a priori weakening influence depth in the design stage, and roadway surface convergence monitoring is performed to verify the reliability of the design width.

[0016] According to an embodiment of the present invention, in step S1, the engineering saturated state is a stable water-bearing state formed by simulating the long-term soaking of adjacent coal bodies by accumulated water in the goaf. The test data of the dry state and the engineering saturated state are obtained by performing uniaxial compressive strength test and direct shear test on standard coal samples prepared by core sampling of the target coal seam, respectively.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) Analytical characterization of continuous gradient weakening. By constructing an exponential decay model of the weakening index field and a spatial degradation model of cohesion and internal friction angle, the analytical characterization of the continuous gradient degradation of coal strength parameters along the width direction caused by seepage was realized. This overcomes the limitations of traditional overall reduction or step-by-step partitioning treatment and is more in line with the essential characteristics of seepage-diffusion physical processes.

[0018] (2) Explicit discrimination of dual-scale competition relationship. By establishing the limit equilibrium equation of strength space variable parameter under the Mohr-Coulomb criterion and explicitly solving the stress recovery scale and strength recovery scale, a dual-scale discrimination index is proposed, which provides a unified explicit mathematical criterion for determining the main control type of coal body stability on the water-saturated side, and realizes the quantitative characterization of the competition relationship between stress field and strength field.

[0019] (3) Design-verification integrated closed loop. By unifying the width of the water-immersed control zone, the width of the roadway plastic zone, and the minimum width of the elastic core zone into the formula for determining the minimum design width, and introducing a post-verification step based on borehole observation fractal dimension analysis and roadway convergence monitoring, an integrated closed loop of “pre-design analysis-post-setting verification” for the width of the water-proof coal pillar is realized. Attached Figure Description

[0020] Figure 1 A flowchart of the water-resistant coal pillar width design method based on seepage gradient weakening according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the width partitioning design of the waterproof coal pillar according to an embodiment of the present invention. Detailed Implementation

[0021] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0022] The following description, with reference to the accompanying drawings, describes the design method for the width of water-resistant coal pillars based on seepage gradient weakening proposed in this invention.

[0023] like Figure 1 As shown in the figure, the method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to an embodiment of the present invention includes the following steps: S1, obtains test data on uniaxial compressive strength, cohesion and internal friction angle of coal in dry and engineering saturated states, as well as engineering parameters of coal seam mining height, interfacial friction coefficient and equivalent horizontal boundary stress on the water-immersed side.

[0024] According to an embodiment of the present invention, in step S1, the engineering saturated state is a stable water-bearing state formed by simulating the long-term soaking of adjacent coal bodies by accumulated water in the goaf. The test data of the dry state and the engineering saturated state are obtained by performing uniaxial compressive strength test and direct shear test on standard coal samples prepared by core sampling of the target coal seam, respectively.

[0025] Specifically, step S1 aims to obtain mechanical test data and key engineering geological parameters of the coal body under dry and engineering saturated conditions, including: S11, Coal Sample Mechanical Test. Core samples were taken from the target coal seam to prepare standard coal samples conforming to the requirements of "Methods for Determination of Physical and Mechanical Properties of Coal and Rock" (GB / T 23561). The coal samples were divided into two groups: one group was naturally dried under constant temperature and humidity conditions until its mass was constant, serving as a dry sample; the other group was vacuum-sealed in a vacuum saturation device and then injected with distilled water, maintaining a constant water head pressure to simulate the long-term immersion effect of water accumulation in the goaf, until its mass was constant and the rate of change of acoustic velocity was less than 1%, serving as an engineering saturated sample.

[0026] Uniaxial compressive strength test and direct shear test were performed on two groups of specimens respectively. The uniaxial compressive strength test used displacement-controlled loading at a loading rate of 0.5 mm / min, and the peak stress was recorded as the uniaxial compressive strength. The direct shear test used constant normal stress and shear displacement control, with 3–5 levels of normal stress. The moiré stress circle envelope was plotted, and linear fitting was used to determine the cohesion and internal friction angle.

[0027] In other words, standard coal samples are prepared by coring the target coal seam, and uniaxial compressive strength and direct shear tests are conducted in both dry and engineered saturated states to obtain the dry uniaxial compressive strength. R u0 Dry cohesion c0. Tangent of dry internal friction angle tan f 0, and the uniaxial compressive strength in the saturated state of the engineering. R u,sat Engineering saturated cohesion c sat tan tangent of the internal friction angle in the saturated state of the engineering f sat .

[0028] For example, taking the No. 22 coal seam of the 42202 working face of a certain mine as an example, the dry uniaxial compressive strength was measured by test. R u0 =10.44 MPa, uniaxial compressive strength in saturated state of engineering R u,sat =6.50 MPa, dry cohesion c 0 = 1.00 MPa, engineering saturated cohesion c sat =0.70 MPa, tan tangent of dry internal friction angle f θ = 0.4452, tangent of the internal friction angle in the saturated state of the engineering process. f sat =0.4245.

[0029] S12, Engineering Parameter Acquisition. Engineering parameters are acquired through geological reports, borehole columnar sections, and field measurements, including at least: coal seam mining height. h =3.8 m, interfacial friction coefficient between coal seam and roof and floor f =0.55, equivalent horizontal boundary stress on the submerged side p b =0.20 MPa. Wherein, the equivalent horizontal boundary stress on the submerged side is... p b Taking into account factors such as hydrostatic pressure of water accumulation in the goaf, stress transmission due to the self-weight of the overlying rock, and lateral pressure coefficient, the results were obtained through on-site stress relief or hydraulic fracturing methods.

[0030] S2, based on the strength parameters of dry state and engineering saturated state, constructs an exponential decay model of the weakening index field, and establishes a spatial degradation model of cohesion and internal friction tangent along the immersion direction under the Mohr-Coulomb criterion.

[0031] Specifically, step S2 aims to construct an exponential decay model of the weakened index field and establish a spatial degradation model of the intensity parameter, which includes: S21, Define the weakening index field D ( x ): D ( x )=1- R u (x ) / R u0 In the formula, R u ( x () is the distance from the immersion boundary x Uniaxial compressive strength of the coal seam x The distance from the water-soaked boundary to the interior of the coal body is in meters. R u0 This represents the dry uniaxial compressive strength. D ( x The physical meaning of ) is: D ( x )=0 indicates dry coal body that has not been affected by water immersion; D ( x )= D max This indicates that the coal body is in a state of engineering saturation, where its strength loss reaches its maximum. Maximum weakening value. D max Depend on R u,sat / R u0 Sure: D max =1- R u,sat / R u0 ,in, R u,sat This refers to the uniaxial compressive strength under saturated conditions in the engineering project. For example, in the above engineering case... D max =1-6.50 / 10.44=0.3774.

[0032] S22, based on the exponential decay characteristics of seepage-diffusion physical processes, will D ( x The spatial distribution of ) is represented in an exponential decay form: D ( x )= D max · e -αx In the formula, α The spatial attenuation coefficient has the dimension m. - ¹, a larger value indicates a shallower range of the water immersion weakening effect. For example, in the above engineering case... α =0.227 m - ¹. Spatial attenuation coefficient αThe value can be determined by fitting the measured uniaxial compressive strength data of coal samples at different distances using the least squares method; it can also be estimated by using the analytical solution of Fick's law based on the porosity, permeability and water diffusion coefficient of the coal body; or it can be determined by referring to the engineering experience of mining areas with similar geological conditions.

[0033] S23, based on the concept of equivalent damage, demonstrates the influence of water immersion on coal strength parameters through... D ( x The linear, proportional reduction method is introduced into the Mohr-Coulomb strength parameters. A cohesion degradation coefficient is defined. a c =1- c sat / c 0, coefficient of degradation of internal friction angle a φ =1-tan f sat / tan f 0. c ( x ) and tan f ( x The spatial degradation model is as follows: c ( x )= c 0[1- a c · D ( x ) / D max ], tan f ( x )=tan f 0[1- a φ · D ( x ) / D max ].Will D ( x Substituting the exponential decay expression of ) into the equation, we get... c ( x ) and tan f ( x )along x The distribution function of the direction.

[0034] S3. Substitute the spatial degradation model of cohesion and internal friction tangent into the limit equilibrium equation to establish the stress equilibrium differential equation with spatially variable strength parameters, and solve for the stress recovery scale and strength recovery scale.

[0035] According to an embodiment of the present invention, in step S3, the limiting equilibrium equation is a model considering spatial degradation. c (x ) and the tangent of the internal friction angle tan f ( x The changing one-dimensional stress equilibrium differential equation, stress recovery scale x σ The strength recovery scale is defined as the distance from the water-immersed boundary from which the vertical stress of the coal body first recovers to the reference vertical stress. x D Defined as the distance from the immersion boundary when the weakened index field drops to a preset reference threshold, derived from... D ( x D )≤ e The solution is obtained. e Small quantity for reference.

[0036] Specifically, step S3 aims to establish the limit equilibrium equations for the strength space variable parameters and solve for the stress recovery scale and the strength recovery scale, which includes: S31, with the water immersion boundary as the origin, and the direction perpendicular to the water immersion boundary pointing into the coal body as... x A limit equilibrium mechanical model of the coal body on the water-soaked side is established along the positive axis. x Considering the directional infinitesimal element under the Mohr-Coulomb criterion c ( x ) and tan f ( x )along x By considering the spatial variation of direction, we establish the stress equilibrium differential equation.

[0037] S32, combined with boundary conditions (in) x The vertical stress at point =0 is equal to the equivalent horizontal boundary stress on the submerged side. p b The vertical component of the transformation; the vertical stress tends to the reference vertical stress at the point inside the coal body far from the water-soaked boundary), solving the limit equilibrium differential equation, yields the vertical stress along... x The distribution expression of direction.

[0038] S33, Defines the stress recovery scale x σ The distance from the water-submerged boundary to the position where the vertical stress of the coal body first recovers to the reference vertical stress. This is solved by setting the expression for vertical stress equal to the reference vertical stress. x σ For example, the calculations in the above engineering case yielded... x σ ≈4.97 m.

[0039] S34, Define the intensity recovery scale x DThe distance from the water-soaked boundary to the point where the coal strength first recovers to the reference level of dry strength. This is determined by setting a small reference value. e ,make D ( x D )≤ e Solving for the results x D For example, in the above engineering case... x D =5.84 m.

[0040] S4, calculate the ratio of stress recovery scale to strength recovery scale as a dual-scale discrimination index, and determine the main control type of coal body stability on the water-soaked side based on the relationship between the discrimination index and 1.

[0041] According to an embodiment of the present invention, in step S4, the determination rule for the main control type is as follows: when the discrimination index is greater than 1, the stress recovery scale is greater than the strength recovery scale, and it belongs to the stress recovery control type; when the discrimination index is less than 1, the strength recovery scale is greater than the stress recovery scale, and it belongs to the strength recovery control type; when the discrimination index is equal to 1, stress recovery and strength recovery are synchronized.

[0042] Specifically, through the formula L = x σ / x D Calculate the dual-scale discriminant index, where, L To determine the indicators, x σ For stress recovery scale, x D This is the strength recovery scale. When... L When the stress recovery scale is greater than the strength recovery scale, the stability of the coal body on the water-soaked side is controlled by stress recovery (stress recovery controlled type); when... L When <1, the strength recovery scale is greater than the stress recovery scale, and the stability of the coal body on the water-soaked side is controlled by strength recovery (strength recovery controlled type); when L When the value is 1, stress recovery and strength recovery occur simultaneously. For example, in the above engineering case... L =4.97 / 5.84≈0.85<1, which belongs to the strength recovery control type, indicating that strength recovery is the main controlling factor limiting the bearing capacity of coal pillars.

[0043] S5. Determine the value of the width of the water-immersed control zone according to the main control type, and determine the minimum design width of the water-proof coal pillar by combining the minimum width of the elastic core zone and the width of the roadway side plastic zone.

[0044] According to one embodiment of the present invention, in step S5, the minimum design width is determined according to the following formula: B min = x w,l + b min + x p,r , in, B min Minimum design width; x w,l Width of the control zone on the immersion side. x p,r The width of the plastic zone on the side of the tunnel. b min This represents the minimum width of the elastic core region; Specifically, when the control zone is determined to be of the strength recovery control type, the width of the control zone on the immersion side is determined according to the strength recovery scale; when the control zone is determined to be of the stress recovery control type, the width of the control zone on the immersion side is determined according to the stress recovery scale. The minimum width of the elastic core zone is twice the coal seam mining height. The width of the plastic zone on the roadway side is calculated analytically based on the limit equilibrium theory combined with the coal seam mining height and the interface friction coefficient.

[0045] Specifically, refer to Figure 2 Step S5 aims to determine the minimum design width of the waterproof coal pillar based on the main control type, specifically including: S51, determine the width of the immersion side control zone according to the main control type. x w,l When it is an intensity recovery control type ( L When <1), x w,l According to the strength recovery scale x D The value of is determined to ensure that the strength of the coal body in this area fully recovers to near-dry state levels; when it is a stress recovery control type ( L When >1), x w,l According to stress recovery scale x σ The value is taken from the size of the variable. For example, in the above engineering case, the value is taken from... x w,l = x D =5.84 m.

[0046] S52, Calculate the width of the plastic zone on the side of the roadway. x p,r Based on the limit equilibrium theory, considering the mining height... h and interfacial friction coefficient f An analytical expression for the width of the plastic zone on the side of the roadway is established and solved. For example, the calculation in the above engineering case yields...x p,r ≈3.96 m.

[0047] S53, Determine the minimum width of the elastic core region. b min The elastic core zone, as the core area for coal pillar load-bearing and water-blocking, should have a minimum width that ensures sufficient load-bearing capacity and water-blocking effect. For example, it can generally be taken as... b min It is twice the maximum mining height.

[0048] S54, Calculate the minimum design width B min = x w,l + b min + x p,r For example, in the above engineering case... B min =5.84 + 7.6 + 3.96 = 17.4 m. The actual width reserved in the project is 18 m, which meets the design requirements.

[0049] According to an embodiment of the present invention, the above method further includes step S6: after the roadway is excavated, boreholes are arranged in the coal pillar for borehole inspection, images of borehole wall cracks are obtained and fractal dimension analysis is performed, the a priori weakening influence depth is compared with the a priori weakening influence depth in the design stage, and roadway surface convergence monitoring is performed to verify the reliability of the design width.

[0050] Specifically, to further verify the reliability of the design method, a post-verification check is performed after the tunnel excavation is completed, which includes: S61, Borehole Inspection and Fractal Dimension Analysis. Three to five exploratory boreholes are arranged within the coal pillar along a direction perpendicular to the water immersion boundary. The borehole depth should exceed 1.2 times the theoretical prior weakening influence depth. Images of borehole wall fractures are acquired using borehole television or an in-hole camera system. Geometric parameters of the fracture network are extracted using image processing software, and the fractal dimension is calculated using box counting.

[0051] Fractal dimension is positively correlated with the degree of coal damage: the larger the fractal dimension, the more complex the fracture network and the more severe the coal damage. By plotting the distribution curve of fractal dimension along the borehole depth, the depth at which the fractal dimension drops to the baseline value of dry coal is determined as the posterior weakening influence depth.

[0052] For example, in the above engineering case, the average posterior weakening influence depth is 5.31 m, and the relative deviation from the theoretical prior value of 5.84 m is: d =∣5.84 5.31 | / 5.84 × 100% = 9.1%; The deviation is within the preset tolerance range (±15%), indicating that the theoretical predictions in the design phase are in good agreement with the actual field measurements.

[0053] S62, Roadway Convergence Monitoring. After roadway excavation, surface convergence monitoring sections are set up at intervals of 20–30 m to monitor the relative convergence of the roof and floor, the relative convergence of the sidewalls, and the roof subsidence. Convergence deformation-time curves are plotted to determine whether the surrounding rock has entered a stable period.

[0054] For example, in the above engineering case, the convergence deformation of the surrounding rock of the tunnel entered a stable period about 100 days after excavation, and the cumulative convergence amount was less than the allowable value of the reserved tunnel cross-sectional shrinkage rate, which verified the reliability of the design width.

[0055] S63, Verification and Judgment. If the deviation between the posterior weakening influence depth and the theoretical prior value is within the preset tolerance range, and the roadway surrounding rock convergence deformation is stable, then the design width is deemed reliable; if the deviation exceeds the limit or the surrounding rock deformation continues to increase, the cause should be analyzed, and if necessary, reinforcement support or adjustment of the coal pillar width in subsequent sections should be taken.

[0056] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0057] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0058] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0059] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for designing the width of a waterproof coal pillar based on weakening the seepage gradient, characterized in that, Includes the following steps: S1, to obtain test data of uniaxial compressive strength, cohesion and internal friction angle of coal in dry and engineering saturated states, as well as engineering parameters of coal seam mining height, interfacial friction coefficient and equivalent horizontal boundary stress on the water-immersed side; S2, based on the strength parameters of dry state and engineering saturated state, an exponential decay model of the weakened index field is constructed, and a spatial degradation model of the cohesion and internal friction tangent value along the immersion direction under the Mohr-Coulomb criterion is established. S3, Substitute the spatial degradation model of cohesion and internal friction tangent into the limit equilibrium equation, establish the stress equilibrium differential equation with spatially variable strength parameters, and solve for the stress recovery scale and strength recovery scale; S4, calculate the ratio of stress recovery scale to strength recovery scale as a dual-scale discrimination index, and determine the main control type of coal body stability on the water-soaked side based on the relationship between the discrimination index and 1; S5. Determine the value of the width of the water-immersed control zone according to the main control type, and determine the minimum design width of the water-proof coal pillar by combining the minimum width of the elastic core zone and the width of the roadway side plastic zone.

2. The method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to claim 1, characterized in that, In step S1, the uniaxial compressive strength includes dry uniaxial compressive strength and engineered saturated uniaxial compressive strength, the cohesion includes dry cohesion and engineered saturated cohesion, and the internal friction angle tangent includes dry internal friction angle tangent and engineered saturated internal friction angle tangent.

3. The method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to claim 2, characterized in that, In step S2, the expression for the weakening index field is: D ( x )=1- R u ( x ) / R u0 ， The exponential decay form of the spatial distribution of the weakened index field is expressed as follows: D ( x )= D max · e -αx ， D max =1- R u,sat / R u0 ， in, D ( x ) represents the weakened index field. R u ( x () is the distance from the immersion boundary x Uniaxial compressive strength of the coal seam R u0 This represents the dry uniaxial compressive strength. R u,sat For engineering saturated uniaxial compressive strength, D max The maximum weakening value, α The spatial attenuation coefficient has the dimension m. - ¹, x The distance from the water-soaked boundary to the interior of the coal body is in meters.

4. The method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to claim 3, characterized in that, In step S2, the expression for the spatial degradation model is: c ( x )= c 0[1- a c · D ( x ) / D max ],so φ ( x )=tan φ 0[1- a φ · D ( x ) / D max ], in, a c The cohesion degradation coefficient is determined by the ratio of dry cohesion to engineering saturated cohesion, tan φ ( x ) is the tangent of the internal friction angle, tan φ 0 represents the tangent of the dry-state internal friction angle. a φ The internal friction angle degradation coefficient is determined by the ratio of the tangent of the dry internal friction angle to the tangent of the engineering saturated internal friction angle.

5. The method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to claim 1, characterized in that, In step S3, the limiting equilibrium equation is a model considering spatial degradation. c ( x ) and the tangent of the internal friction angle tan φ ( x The changing one-dimensional stress equilibrium differential equation, the stress recovery scale x σ The strength recovery scale is defined as the distance from the water-immersed boundary from which the vertical stress of the coal body first recovers to the reference vertical stress. x D Defined as the distance from the immersion boundary when the weakened index field drops to a preset reference threshold, derived from... D ( x D )≤ ε The solution is obtained. ε Small quantity for reference.

6. The method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to claim 1, characterized in that, In step S4, the determination rule for the main control type is as follows: when the discrimination index is greater than 1, the stress recovery scale is greater than the strength recovery scale, and it belongs to the stress recovery control type; when the discrimination index is less than 1, the strength recovery scale is greater than the stress recovery scale, and it belongs to the strength recovery control type; when the discrimination index is equal to 1, stress recovery and strength recovery are synchronized.

7. The method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to claim 6, characterized in that, In step S5, the minimum design width is determined according to the following formula: B min = x w,l + b min + x p,r ， in, B min This refers to the minimum design width; x w,l Width of the control zone on the immersion side. x p,r The width of the plastic zone on the side of the tunnel. b min This represents the minimum width of the elastic core region; Specifically, when the control zone is determined to be of the strength recovery type, the width of the immersion side control zone is determined according to the strength recovery scale; when the control zone is determined to be of the stress recovery type, the width of the immersion side control zone is determined according to the stress recovery scale. The minimum width of the elastic core zone is twice the coal seam mining height, and the width of the roadway side plastic zone is analytically solved based on the limit equilibrium theory combined with the coal seam mining height and the interface friction coefficient.

8. The method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to claim 1, characterized in that, It also includes step S6: After the roadway is excavated, boreholes are arranged in the coal pillar for borehole inspection, to obtain images of borehole wall cracks and perform fractal dimension analysis, to compare the a priori weakening influence depth with the a priori weakening influence depth in the design stage, and at the same time to monitor the roadway surface convergence to verify the reliability of the design width.

9. The method for designing the width of a waterproof coal pillar based on weakened permeability gradient according to claim 1, characterized in that, In step S1, the engineering saturated state is the stable water-bearing state formed by the long-term soaking of adjacent coal bodies by water accumulation in the simulated goaf. The test data of the dry state and the engineering saturated state are obtained by conducting uniaxial compressive strength test and direct shear test on standard coal samples prepared by core sampling of the target coal seam.

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