A method for in-situ stress recovery test of deep anisotropic coal and rock mass

Abstract

The present invention discloses an in-situ stress recovery test method for deep anisotropic coal rock mass, comprising the following steps: step 1, preparation of anisotropic coal rock mass sample; step 2, initial stress loading test; step 3, stress recovery test; step 4, construction of an anisotropic mechanical model of deep coal rock mass. The present invention can solve the problem that the anisotropic characteristics of deep coal rock mass and the in-situ stress recovery process cannot be accurately simulated in the prior art. The present invention can accurately restore the mechanical behavior of deep coal rock mass under different stress paths by combining indoor tests and numerical simulation technology, and provide a scientific basis for deep mine engineering design and disaster prevention and control.

Description

Technical Field

[0001] The present invention relates to underground coal mine rock mass exploration technology, and in particular to an experimental method for in-situ stress recovery of deep anisotropic coal rock mass. It can be widely used in deep coal mining, mine support design, disaster prevention and control, gas extraction and other fields, providing important technical support for the safety and efficiency of mine engineering. Background Art

[0002] As the depth of coal mining continues to increase, the mechanical behavior and stress state of deep coal rock mass have put forward higher requirements for safe mining and engineering design of mines. Deep coal rock mass usually exhibits significant anisotropy characteristics, that is, its mechanical properties (such as strength, elastic modulus, permeability, etc.) vary significantly in different directions. This anisotropy not only affects the deformation and destructive behavior of deep coal rock mass, but is also directly related to the stability of mine tunnels, gas extraction efficiency, and prevention and control of disasters such as rock burst.

[0003] At present, stress testing methods for deep coal and rock masses are mainly concentrated on traditional triaxial compression tests, uniaxial compression tests, etc. Although these traditional methods can simulate the mechanical behavior of deep coal and rock masses to a certain extent, the test results often cannot accurately reflect the stress recovery process of deep coal and rock masses in deep mining environments because the anisotropic characteristics and stress path changes of deep coal and rock masses are not fully considered during the test. In addition, the existing test methods usually rely on static loading and do not consider the complex stress change paths experienced by deep coal and rock masses during the mining process, resulting in a large deviation between the test data and the actual engineering conditions.

[0004] At present, stress recovery tests for deep coal and rock masses are mainly concentrated on traditional triaxial compression tests, uniaxial compression tests, etc. However, existing methods usually fail to take into account the anisotropic characteristics of deep coal and rock masses, and cannot accurately simulate the complex stress recovery behavior of deep coal and rock masses during actual mining. In the stress recovery process of deep coal and rock masses, crack expansion, strength degradation and path dependence are factors that cannot be ignored. The present invention provides a more accurate experimental simulation for stress recovery of deep coal and rock masses by establishing a new test method, combining path dependence with nonlinear recovery mechanism.

[0005] In order to more accurately simulate the in-situ stress recovery process of deep coal and rock masses and solve the problem that traditional methods cannot fully reflect the anisotropic characteristics and complex stress path changes of deep coal and rock masses, it is urgent to develop a new in-situ stress recovery test method for deep coal and rock masses that can comprehensively consider these factors, so as to provide theoretical support for the safe mining of deep mines. Summary of the invention

[0006] The present invention aims to provide an in-situ stress recovery test method for deep anisotropic coal rock mass, so as to solve the problem that the existing technology cannot accurately simulate the anisotropic characteristics of deep coal rock mass and the in-situ stress recovery process. By combining indoor tests and numerical simulation technology, the present invention can restore the mechanical behavior of deep coal rock mass under different stress paths, thereby providing a scientific basis for deep mine engineering design, disaster prevention and control, and gas extraction optimization.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] A deep anisotropic coal rock mass in-situ stress recovery test method comprises the following steps:

[0009] Step 1. Preparation of anisotropic coal rock samples: Based on the bedding structure and geological parameters of the underground coal rock mass, deep coal rock mass is selected from typical mining areas. The deep coal rock mass is required to be a sphere with a radius greater than 500 mm in order to prepare standard samples. First, the physical properties of the collected deep coal rock samples are analyzed using advanced technologies such as nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to obtain bedding distribution, pore structure and anisotropic characteristics. For the same deep coal rock sample, standard cylindrical samples are drilled from parallel, vertical, inclined 30° and inclined 45° to the bedding direction of the coal seam. The radius of the standard cylindrical sample is 50 mm and the height is 100 mm. The number of samples in each direction is sufficient to ensure the statistical significance of the data.

[0010] Acquisition of physical and mechanical parameters: Based on sample preparation, uniaxial compression tests were performed on samples in different bedding directions to obtain key mechanical parameters such as compressive strength and elastic modulus. Among them, the maximum load at which samples in each direction were destroyed was measured through the test. F max , the uniaxial compressive strength is s c :

[0011] (1)

[0012] A is the circular area of ​​the bottom surface of the specimen.

[0013] According to the stress-strain curve of the specimen, the elastic modulus in different bedding directions can be obtained using the formula in the linear stage: E :

[0014] (2)

[0015] These parameters provide basic data for subsequent numerical simulation and dynamic adjustment of the model, and clearly reflect the anisotropic characteristics of deep coal and rock masses in parallel, vertical and inclined directions.

[0016] Step 2, initial stress loading test: according to the deep in-situ stress data obtained from actual measurements at the sampling points or from literature review, including the maximum principal stress, the minimum principal stress and its direction, the corresponding confining pressure and axial load of the triaxial loading test machine GCTS are set; according to the actual burial depth and the ground stress gradient, the loading rate is controlled and gradually applied to the corresponding initial target stress level. After loading to the predetermined stress value, the constant stress is maintained for ≥2 hours, and the stress fluctuation range is monitored within ±0.5%; at the same time, the deformation and microcrack activity information of the sample are collected in real time through strain gauges and acoustic emission sensors to determine the stress balance degree and anisotropic response of the sample under the in-situ stress condition.

[0017] The initial stress loading test is as follows: a multi-axial loading test device is used to simulate the in-situ stress state of deep coal and rock mass in deep mines. In order to simulate the stress state of deep coal and rock mass in deep mines, appropriate confining pressure and axial stress need to be applied in the test. First, confining pressure is applied to the deep coal and rock mass samples through a hydraulic loading system, and the confining pressure is gradually increased until the in-situ minimum principal stress is reached. The loading mode of the confining pressure is isobaric loading to simulate the pressure conditions encountered by deep coal and rock mass during mining. Axial stress is applied while keeping the confining pressure constant.

[0018] During the loading process, the stress and strain fluctuations are monitored in real time by the circumferential strain gauge and acoustic emission equipment arranged on the sample. When the circumferential or volumetric strain change rate is less than 0.005% / min for 30 consecutive minutes, and the acoustic emission event rate is less than 1 time / minute, the sample is considered to have reached a stress equilibrium state, and the loading fluctuation is controlled within ±0.5%. During the loading process, close attention should be paid to the fluctuation range of stress and strain to ensure the stability of the loading process. By recording and analyzing the data in real time, it can be determined whether a stable stress state has been reached. If the stress fluctuation during the loading process exceeds the preset range, the loading rate needs to be adjusted or additional control measures need to be applied to ensure the controllability of the test. During the test, it is determined whether the sample has reached a stress equilibrium state. If the change rate of the circumferential strain or volumetric strain is less than 0.005% / min for 30 consecutive minutes, and the acoustic emission event rate is less than 1 time / minute, it can be considered that the deep coal and rock mass sample has reached an in-situ stress equilibrium state.

[0019] Step 3, stress recovery test: simulate the secondary stress evolution of deep coal and rock mass after mining unloading: first partially unload or maintain for a predetermined time, then apply stress step by step to a new target level according to the stress evolution curve of actual monitoring or numerical prediction; record the crack extension, deformation accumulation and acoustic emission activities of the sample during this process, and obtain the differential response of the sample in the anisotropic direction; judge whether the axial stress or lateral stress exceeds the critical value by whether there is a macro crack penetration or a sudden increase in acoustic emission energy in the sample; combine stress-strain curves, acoustic emission time series and image monitoring to judge the deformation and failure mode of deep coal and rock mass, and obtain the difference of deep coal and rock mass in parallel and perpendicular directions. After the initial stress loading is stable, continue to apply stress to the same sample to simulate the stress recovery process of deep coal and rock mass, reflecting the stress redistribution after mining disturbance.

[0020] In this phase, the stress path dependence and nonlinear recovery mechanism model are innovatively introduced to modify key parameters through real-time data to dynamically reflect the impact of crack expansion and damage evolution on the recovery process. During the test, stress, strain and acoustic emission data are continuously collected. If the stress release rate does not match the preset nonlinear recovery model, the loading parameters are adjusted in real time.

[0021] When the initial stress loading is stable, continue to apply stress to simulate the stress recovery process of the deep coal rock mass, so that the sample experiences stress changes that are closer to the actual mine mining process. Specifically, continue to apply confining pressure to simulate the pressure recovery of the deep coal rock mass after mining. After the confining pressure loading is completed, apply axial stress to restore the deep coal rock sample to the stress state before mining. The loading rate of axial stress should match the actual mining speed of the deep coal rock mass. During the loading process, the loading rates of confining pressure and axial stress should be adjusted according to the stress recovery characteristics of the deep coal rock mass to ensure that the deformation of the deep coal rock mass during the test will not exceed expectations. During the loading process, stress fluctuations should be monitored in real time to ensure the smoothness of the loading process and the smooth stress recovery of the deep coal rock mass. During the stress recovery process, the stress and strain of the deep coal rock sample should remain stable. The deformation and failure behavior of the deep coal rock mass are monitored by real-time data acquisition equipment (strain gauges, acoustic emission). When the stress recovers to the predetermined level, the sample should maintain a stable state for at least 2 hours to ensure that the deep coal rock mass is in a state of equilibrium during this period. During this period, monitor the changes in stress and strain and evaluate the stability of deep coal and rock masses. During the stress recovery process, if the deep coal and rock samples show obvious crack expansion or damage, the failure of the sample should be determined in time. The failure judgment criteria include: 1) when the stress peak drops to less than 80% of the post-peak strength; 2) a sudden increase in acoustic emission energy; 3) macro cracks penetrate the sample.

[0022] Step 4, construct an anisotropic mechanical model of deep coal rock mass: According to the anisotropic characteristics of the standard pattern and the test data obtained in steps 1, 2, and 3, the mechanical response and anisotropic characteristics of the deep coal rock mass under various stress paths are analyzed, and an anisotropic mechanical model of the deep coal rock mass is constructed. The anisotropic mechanical model of the deep coal rock mass not only considers the traditional elastic and plastic behaviors, but also integrates the path dependence and nonlinear recovery mechanism.

[0023] Furthermore, the present invention also includes:

[0024] Step 5, application of results: applying the anisotropic mechanical model of deep coal and rock mass to mine tunnel support design and / or gas extraction scheme optimization and / or rock burst prediction.

[0025] Preferably, the process of constructing the stress path dependence and nonlinear recovery mechanism model and the deep coal rock mass anisotropic mechanical model is as follows:

[0026] Path dependency modeling: By introducing the stress history factor (SHF), the path dependency of stress recovery can be reflected in the model. This factor takes into account the stress distribution of deep coal and rock mass in the previous loading process and its influence on the current recovery process of deep coal and rock mass. The definition of the path factor is as follows:

[0027] (3)

[0028] in, s ( t ) is the time of deep coal rock mass during loading process t The stress on t 1 is the current time point. This factor can be adjusted to simulate the stress recovery rate and mechanical response of deep coal and rock mass under different loading paths.

[0029] In the experiment, an initial stress load is first applied, and the stress recovery behavior of deep coal rock mass under different loading paths is simulated by adjusting the path-dependent factor SHF. For example, if the deep coal rock mass experiences a large preload, the stress recovery rate may be faster, while the deep coal rock mass that experiences a small or slow load may show a slower recovery process. The stress history factor is used to simulate the complex loading path of deep coal rock mass in the actual mining process, such as the impact of the staged pressure changes in mine mining on the recovery process of deep coal rock mass.

[0030] In addition, traditional stress recovery models usually assume that the stress-strain relationship of deep coal and rock masses during loading and recovery is linear. However, deep coal and rock masses exhibit obvious nonlinear behavior after experiencing high pressure, long-term load and crack expansion, especially in the recovery stage after mining. The nonlinear behavior is mainly manifested as:

[0031] Crack expansion and strength degradation: As the stress of deep coal rock mass is gradually released, the expansion of cracks causes the strength of deep coal rock mass to gradually decay. In this process, the stress recovery of deep coal rock mass shows nonlinear behavior, and this nonlinearity is mainly affected by the expansion degree of cracks and initial stress.

[0032] Nonlinear recovery function: A nonlinear recovery function is introduced to describe the stress recovery process of deep coal and rock masses. Strength attenuation is related to crack expansion, and the stress-strain relationship during the recovery process can be expressed by a nonlinear function. The recovery function is as follows:

[0033] (4)

[0034] in, s recover For the strength after recovery, s 0 is the initial strength of deep coal rock mass, β is the damage coefficient, D is the damage variable (related to crack extension), n is a nonlinear index.

[0035] By continuously loading deep coal and rock mass and recording stress-strain data in the test, the strength attenuation of deep coal and rock mass is fitted using a nonlinear recovery model to help predict the stress recovery characteristics of deep coal and rock mass at different mining stages. When the crack expansion of deep coal and rock mass is more obvious, the recovery process of deep coal and rock mass will show obvious nonlinear characteristics through the nonlinear recovery function model.

[0036] The crack extension of deep coal-rock mass not only affects its strength, but is also closely coupled with the stress recovery process. As the cracks extend, the strength of deep coal-rock mass gradually decreases, and this crack extension is a nonlinear process. In order to describe the mechanical behavior of deep coal-rock mass during the recovery process, the interaction between damage and stress must be considered.

[0037] Damage evolution model: The damage evolution of deep coal rock mass can be described by a nonlinear evolution equation, which reflects the degradation of strength and stiffness of deep coal rock mass during crack propagation. Damage variables D It increases with increasing strain and is usually described by the following formula:

[0038] (5)

[0039] in, α is the damage sensitivity coefficient, e eq The increase of damage variables will lead to the gradual attenuation of the strength of deep coal and rock mass and affect the stress-strain relationship in its recovery process.

[0040] By combining path dependence, nonlinear recovery mechanism and damage evolution model, the following comprehensive stress recovery model is proposed to describe the overall mechanical response of deep coal and rock mass during the stress recovery process after initial loading:

[0041] (6)

[0042] in: s ( t ) is the time during the recovery process t The stress state at s 0 is the initial stress; β is the damage coefficient; D ( t ) is the damage variable, which evolves over time.

[0043] The second term of the formula is the real-time feedback adjustment part, where: e ( t )= M ( t )- P ( t ) is the time t The error, M ( t ) is the actual measured value, P ( t ) is the model prediction value; K 1 and K 2 are proportional and differential feedback gains respectively; the integral term represents the accumulation of the error signal collected in real time during the entire test process, and the dynamic adjustment of the model parameters is achieved through the PD control law.

[0044] Dynamic adjustment and feedback of the model: In the above comprehensive model, the real-time feedback mechanism calculates the error signal by monitoring the stress, strain and acoustic emission data of the sample during loading and recovery. e ( t ), and the following feedback control law is used to correct the model parameters:

[0045] (7)

[0046] in, i ( t) represents the parameters that need to be dynamically adjusted in the model (such as β , n or related parameters in stress path dependence factors), can be expressed as:

[0047] (8)

[0048] This closed-loop feedback control enables the model to be continuously optimized according to real-time data during the test, ensuring that the deviation between the model prediction value and the actual measurement value is minimized, and ultimately obtaining an anisotropic mechanical model of deep coal and rock mass, thereby achieving accurate simulation of the nonlinear recovery behavior of deep coal and rock mass.

[0049] This paper proposes a stress path dependence and nonlinear recovery mechanism model, which integrates nonlinear mechanics, damage mechanics and path dependence theory, and attempts to describe the behavior of deep coal and rock mass in the stress recovery process after initial loading from a physical perspective. The stress path dependence and nonlinear recovery mechanism model fully considers the influence of deep coal and rock mass crack expansion, damage evolution and stress history on the recovery process during loading, thereby improving the model's prediction accuracy for the nonlinear recovery behavior of the sample.

[0050] After completing the stress recovery test, all the data obtained during the initial loading and stress recovery tests are used to invert and integrate the anisotropic mechanical parameters of the specimens to construct a complete anisotropic mechanical model of deep coal and rock mass. The specific steps are as follows:

[0051] It can summarize the anisotropic elastic parameters (such as elastic modulus in different directions, Poisson's ratio, etc.) obtained from the initial loading test; integrate the nonlinear stress-strain data, damage evolution curve and loading path information (including stress history factor SHF data) recorded in the stress recovery test. β , n , α and anisotropic correction coefficient) for statistical analysis and regression fitting; combined with the dynamic parameters obtained in the feedback adjustment process, the elastic, damage and plastic parts of the anisotropic constitutive model are calibrated. Based on the inversion and calibration, the following staged constitutive relationship is used to construct the overall model:

[0052] Basic assumptions: There are three assumptions:

[0053] (1) Material symmetry assumption: It is assumed that the deep coal rock mass is an orthotropic material with three main directions perpendicular to each other and independent mechanical properties;

[0054] (2) Small deformation assumption: Ignore geometric nonlinearity and focus on material constitutive nonlinearity;

[0055] (3) Damage-seepage-stress coupling: considering the effect of crack extension on permeability and strength.

[0056] The constitutive equation adopts the elastic-damage-plastic coupling model to describe the mechanical behavior in stages, including:

[0057] The anisotropic elastic stage satisfies:

[0058] (9)

[0059] in, s is the stress vector, e is the strain vector, and C is the elastic stiffness matrix of the anisotropic material:

[0060] (10)

[0061] in, C ij are anisotropic elastic parameters, which can be obtained through initial stress loading tests and adjusted in the subsequent model optimization process.

[0062] As loading progresses, the microcracks in the deep coal and rock mass gradually expand, showing anisotropic damage effects. D ij The tensor form is used to describe the damage evolution of deep coal and rock mass in different directions:

[0063] (11)

[0064] in, D ij is the damage variable in each direction, α ij is the anisotropic damage sensitivity coefficient, is the equivalent strain. This relationship shows that with the increase of strain, the damage of deep coal rock in different directions gradually increases, and the damage tends to be stable after reaching a certain critical value. Damage affects the stiffness of the material. To consider the damage effect, the modified stiffness matrix It can be expressed as:

[0065] (12)

[0066] in:

[0067] I is the identity matrix; D( t ) is the damage tensor, which reflects the accumulation of damage in different directions of the material, and its value is between 0 and 1. D When the value is 0, it means that the direction is not damaged; DWhen the value tends to 1, it means that the stiffness in this direction is significantly reduced; C is the original (undamaged) stiffness matrix, which reflects the elastic properties of the material in each direction.

[0068] In practical applications, in order to simplify the analysis, the damage tensor D( t ) is taken as a diagonal form, that is,

[0069] (13)

[0070] in D ii Indicates along the main direction i Correspondingly, the modified stiffness matrix is:

[0071] (14)

[0072] In the plastic yield stage, the anisotropic modified Drucker-Pragerqr yield criterion is used to describe the yield behavior of the material. At the same time, the Lode angle is introduced to modify the yield surface of the material. The specific expression is as follows:

[0073] (15)

[0074] Among them, α reflects the material's average stress I 1 sensitivity; k is the yield strength of the material at zero mean stress, or can be considered as a hardening / cohesion parameter; I 1 is the first stress invariant, reflecting the sensitivity of the material to the mean stress (pressure dependence). For deep coal and rock mass and other fractured materials, as the mean stress increases, the material's ability to resist shear failure will also increase.

[0075] (16)

[0076] J 2 is the second-order invariant of the deviatoric stress tensor:

[0077] (17)

[0078] in, s ij is the deviatoric stress tensor.

[0079] In addition, Lode Point i It reflects the difference in shear modes of materials under three-dimensional stress state. β ( i ) is the anisotropy coefficient related to the Lode angle.

[0080] In addition, the expression of stress recovery is obtained by combining the stress history factor:

[0081] (18)

[0082] in: β is the damage coefficient; n is the nonlinear index; c is the path dependence influencing factor.

[0083] Combined with real-time feedback, the final anisotropic mechanical model of deep coal and rock mass can be expressed as:

[0084] (19)

[0085] The first term after the equal sign describes the stress in the anisotropic elastic stage; the second term describes the effect of stress path dependence on the recovery process; and the third term describes the real-time feedback adjustment, which enables the model to dynamically optimize parameters throughout the experiment.

[0086] Preferably, the drilled samples are prepared in parallel bedding, 30° inclined bedding, 45° inclined bedding, and vertical bedding manners.

[0087] Preferably, in step 3, the stress recovery test includes reloading stress to the standard sample, and the reloading stress to the standard sample includes:

[0088] Confining pressure loading: applying lateral confining pressure at a rate of 3.0 MPa / min until a first target value is reached, which corresponds to the deep in-situ minimum principal stress;

[0089] Axial stress loading: Under the condition of keeping the confining pressure constant, the axial stress is applied at a rate of 30 kN / min to a second target value, and the second target value corresponds to the deep in-situ maximum principal stress;

[0090] Stress balance maintenance: Maintain the confining pressure and axial stress constant, and stabilize them for ≥2 hours through the stress control mode. During this period, monitor the stress fluctuation range and the adjustment range is within ±0.5%.

[0091] Furthermore, the present invention also includes:

[0092] Strain stability analysis: Draw the strain-time curve: Draw the curve with circumferential strain or volume strain as the ordinate and load holding time as the abscissa;

[0093] Stability judgment: When the circumferential strain or volume strain change rate is ≤0.005% / min within 30 consecutive minutes, and the acoustic emission event rate is ≤1 time / minute, the sample is judged to have reached the in-situ stress equilibrium state;

[0094] Anisotropy feature mark: record the strain difference in different directions. When the volume strain / circumferential strain>1.5, it is determined that there is significant anisotropy;

[0095] Strain controlled loading until specimen failure

[0096] Loading mode switching: switch from stress control to circumferential strain rate control mode, and set the circumferential strain rate to 0.02mm / min;

[0097] The failure judgment criteria are that the stress peak drops to 80% of the post-peak strength, the cumulative value of acoustic emission energy increases suddenly, and macro cracks penetrate the specimen.

[0098] Preferably, the test data is acquired in real time by means of stress sensors, strain gauges and acoustic emission equipment.

[0099] Preferably, steps 2 and 3 are implemented using a multi-axis loading test device, which includes a hydraulic loading system, a stress sensor, a strain gauge, an acoustic emission device and a data acquisition system.

[0100] Further pre-selected, the multi-axis loading test device comprises a vertical loading frame and a horizontal loading frame, the bottom of the vertical loading frame is connected to a vertical pressure head, the horizontal loading frame is provided with a relatively arranged horizontal pressure head, and the vertical loading frame and the horizontal loading frame are driven by a hydraulic device;

[0101] It also includes a base, a cushion block is arranged on the top surface of the base, and the cushion block is used to place the sample;

[0102] The vertical pressure head is arranged along the axial direction of the sample and is used to apply vertical pressure to the sample;

[0103] The horizontal pressure head is arranged along the circumference of the sample and is used to apply confining pressure to the sample.

[0104] The beneficial effects of the present invention are as follows:

[0105] 1. Accurately simulate deep in-situ stress state: By using a multi-axial loading test device to perform initial stress loading on deep coal and rock samples, the real stress field borne by deep coal and rock in deep mines is accurately restored, and stable loading of deep coal and rock under in-situ conditions is achieved. Real-time monitoring of stress and strain data ensures that the sample is always in a balanced state during loading, effectively reducing experimental errors.

[0106] 2. Reflecting the anisotropic characteristics of deep coal rock mass: During the sample preparation stage, the present invention collects samples from parallel, vertical and different inclination angles, and accurately obtains the physical and mechanical parameters in various directions through uniaxial compression tests, thereby comprehensively reflecting the anisotropic characteristics of deep coal rock mass such as bedding structure and crack distribution, laying a solid foundation for subsequent modeling and data analysis.

[0107] 3. Dynamic adjustment and nonlinear recovery mechanism improve the accuracy of stress recovery: The invention proposes a stress path dependence and nonlinear recovery mechanism model. By introducing stress history factors and nonlinear recovery functions, it can dynamically reflect the damage evolution and strength attenuation of deep coal and rock mass during loading and recovery. The real-time data feedback mechanism enables the model parameters to be dynamically optimized according to actual test data, greatly improving the accuracy and applicability of stress recovery process simulation.

[0108] 4. Comprehensive modeling of multi-stage constitutive relations: By constructing staged elastic, damage and plastic mechanical constitutive relations and combining the orthogonal anisotropy assumption, a theoretical model that fully reflects the mechanical response of deep coal and rock masses is invented and established. This theoretical model that fully reflects the mechanical response of deep coal and rock masses not only accurately describes the linear behavior of deep coal and rock masses in the elastic stage, but also fully reveals the nonlinear recovery characteristics caused by crack expansion and damage evolution, providing a reliable theoretical basis for mine engineering design.

[0109] 5. Broad engineering application prospects: The invented test method and theoretical model can be widely used in the fields of mine tunnel support design, gas extraction optimization and rock burst prediction, which will help improve the safety and economy of mine mining. Through the organic combination of experimental data and numerical simulation, it can provide more scientific and accurate support for the prediction of mechanical behavior and risk assessment of deep coal and rock mass in engineering practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0110] Figure 1 The figure is a flow chart of the in-situ stress recovery method for anisotropic deep coal and rock masses.

[0111] Figure 2 It is the strain-time curve.

[0112] Figure 3 Schematic diagram of the direction of heavy loading stress.

[0113] Figure 4 Schematic diagram of the structure of the multi-axial loading test device.

[0114] Among them: 1-vertical loading frame, 2-box cover, 3-horizontal loading frame, 4-specimen, 5-vertical pressure head, 6-horizontal pressure head, 7-pad, 8-base. DETAILED DESCRIPTION

[0115] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings.

[0116] Example 1

[0117] like Figure 1As shown, this embodiment discloses a deep anisotropic coal rock mass in-situ stress recovery test method, comprising:

[0118] Preparation of anisotropic coal-rock samples: Based on the bedding structure and geological parameters of the underground coal-rock mass, deep coal-rock mass was selected from typical mining areas. The deep coal-rock mass was required to be a sphere with a radius greater than 500 mm in order to prepare standard samples. First, the physical properties of the collected deep coal-rock samples were analyzed using advanced technologies such as nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to obtain bedding distribution, pore structure, and anisotropic characteristics. For the same deep coal-rock sample, standard cylindrical samples were drilled from parallel, vertical, 30°, and 45° to the bedding direction of the coal seam. The radius of the standard cylindrical sample was 50 mm and the height was 100 mm. The number of samples in each direction was sufficient to ensure the statistical significance of the data.

[0119] In-situ stress recovery test

[0120] Initial stress loading test: According to the actual stress state of deep coal and rock mass, initial stress is applied to the standard specimen to simulate the in-situ stress environment.

[0121] Stress recovery test: Reload stress on the standard sample to simulate the stress recovery process of deep coal and rock mass after mining, and record the deformation and failure characteristics of the standard sample.

[0122] like Figure 2 , Figure 3 As shown in Figure 2, the specific process of reloading stress is as follows:

[0123] (1) Confining pressure σ 3 load

[0124] Equipment requirements: Use a true triaxial testing machine with independent control of axial σ 1 With lateral σ 2 , σ 3 Stress capacity.

[0125] Loading path: Confining pressure application: Apply lateral confining pressure at a rate of 3.0MPa / min 3 , until the target value σ is reached 3 =σ h , corresponding to the deep in-situ minimum principal stress; axial stress σ 1 Loading: While maintaining σ 3 Under constant conditions, the axial stress σ is applied at a rate of 30 kN / min. 1 To target value σ 1 =σ - H, corresponds to the maximum principal stress in deep situ; stress balance is maintained: maintain σ 1 and σ 3Stable, stable for ≥2 hours in stress control mode, during which the stress fluctuation range is monitored (needed to be ≤±0.5%).

[0126] (2) Strain stability analysis

[0127] Plot the strain-time curve: circumferential strain ε_circumferential=ε 2 +ε 3 Or volume strain ε_vol=ε 1 +ε 2 +ε 3 The vertical axis is the load holding time, and the horizontal axis is the load holding time.

[0128] Stability criterion: When the change rate of circumferential strain or volume strain is ≤0.005% / min for 30 consecutive minutes, and the acoustic emission event rate is ≤1 time / minute, the sample is judged to have reached the in-situ stress equilibrium state.

[0129] Anisotropic feature markers: record the strain differences in different directions, such as |ε 1 / ε 3 When |>1.5, it is judged that there is significant anisotropy.

[0130] (3) Strain-controlled loading to failure

[0131] Loading mode switching: switch from stress control to circumferential strain rate control mode, and set the circumferential strain rate to 0.02mm / min (needs to be converted to strain rate according to the specimen diameter, such as ε_rate≈0.0004min⁻¹ for a 50mm diameter specimen).

[0132] Failure judgment criteria: the stress peak drops to 80% of the post-peak strength; the accumulated value of acoustic emission energy increases suddenly; macro cracks penetrate the specimen.

[0133] According to the test results, the standard for anisotropic mechanical stress recovery of deep coal and rock mass is established as follows:

[0134]

[0135] Stress sensors and strain gauges are used to collect test data in real time, and combined with numerical simulation technology, the mechanical response and anisotropic characteristics of deep coal and rock mass under different stress paths are analyzed. The details are as follows:

[0136] Basic assumptions: Material symmetry assumption: It is assumed that the deep coal rock mass is an orthotropic material, whose three main directions are mutually perpendicular and the mechanical properties are independent; Small deformation assumption: Ignore geometric nonlinearity and focus on material constitutive nonlinearity; Damage-seepage-stress coupling: Consider the influence of crack expansion on permeability and strength.

[0137] The constitutive equation adopts the elastic-damage-plastic coupling model to describe the mechanical behavior in stages, including:

[0138] The anisotropic elastic stage satisfies:

[0139] (9)

[0140] in, s is the stress vector, e is the strain vector, and C is the elastic stiffness matrix of the anisotropic material:

[0141] (10)

[0142] in, C ij are anisotropic elastic parameters, which can be obtained through initial stress loading tests and adjusted in the subsequent model optimization process.

[0143] As loading progresses, the microcracks in the deep coal and rock mass gradually expand, showing anisotropic damage effects. D ij The tensor form is used to describe the damage evolution of deep coal and rock mass in different directions:

[0144] (11)

[0145] in, D ij is the damage variable in each direction, α ij is the anisotropic damage sensitivity coefficient, is the equivalent strain. This relationship shows that with the increase of strain, the damage of deep coal rock in different directions gradually increases, and the damage tends to be stable after reaching a certain critical value. Damage affects the stiffness of the material. To consider the damage effect, the modified stiffness matrix It can be expressed as:

[0146] (12)

[0147] in:

[0148] I is the identity matrix; D( t ) is the damage tensor, which reflects the accumulation of damage in different directions of the material, and its value is between 0 and 1. D When the value is 0, it means that the direction is not damaged; D When the value tends to 1, it means that the stiffness in this direction is significantly reduced; C is the original (undamaged) stiffness matrix, which reflects the elastic properties of the material in each direction.

[0149] In practical applications, in order to simplify the analysis, the damage tensor D( t ) is taken as a diagonal form, that is,

[0150] (13)

[0151] in D ii Indicates along the main direction i Correspondingly, the modified stiffness matrix is:

[0152] (14)

[0153] In the plastic yield stage, the anisotropic modified Drucker-Pragerqr yield criterion is used to describe the yield behavior of the material. At the same time, the Lode angle is introduced to modify the yield surface of the material. The specific expression is as follows:

[0154] (15)

[0155] Among them, α reflects the material's average stress I 1 sensitivity; k is the yield strength of the material at zero mean stress, or can be considered as a hardening / cohesion parameter; I 1 is the first stress invariant, reflecting the sensitivity of the material to the mean stress (pressure dependence). For deep coal and rock mass and other fractured materials, as the mean stress increases, the material's ability to resist shear failure will also increase.

[0156] (16)

[0157] J 2 is the second-order invariant of the deviatoric stress tensor:

[0158] (17)

[0159] in, s ij is the deviatoric stress tensor.

[0160] In addition, Lode Point i It reflects the difference in shear modes of materials under three-dimensional stress state. β ( i ) is the anisotropy coefficient related to the Lode angle.

[0161] In addition, the expression of stress recovery is obtained by combining the stress history factor:

[0162] (18)

[0163] in: β is the damage coefficient; n is the nonlinear index; c is the path dependence influencing factor.

[0164] Combined with real-time feedback, the final anisotropic mechanical model of deep coal and rock mass can be expressed as:

[0165] (19)

[0166] The first term describes the stresses during the anisotropic elastic phase; the second term describes the impact of stress path dependence on the recovery process; and the third term describes the real-time feedback adjustment, which enables the model to dynamically optimize parameters throughout the experiment.

[0167] Application of results

[0168] This final model can be applied to:

[0169] 1. Mine tunnel support design: predict the deformation and damage of deep coal and rock mass in different directions and optimize support parameters.

[0170] 2. Rock burst prediction: Use the model to simulate the crack expansion during the stress recovery process and provide early warning of rock burst risks.

[0171] 3. Gas extraction optimization: Based on the impact of stress recovery process on coal seam permeability, optimize gas extraction strategy to improve recovery rate and safety.

[0172] Example 2

[0173] like Figure 4 As shown, the specific structure of the multi-axis loading test device disclosed in this embodiment is as follows:

[0174] The multi-axis loading test device comprises a vertical loading frame 1 and a horizontal loading frame 3. The bottom of the vertical loading frame 1 is connected to a vertical pressure head 5. The horizontal loading frame 3 is provided with a relatively arranged horizontal pressure head 6. The horizontal loading frame 3 is provided with a box cover 2. The vertical loading frame 1 and the horizontal loading frame 3 are both driven by a hydraulic device.

[0175] The multi-axial loading test device includes a base 8, a cushion block 7 is arranged on the top surface of the base 8, and the cushion block 7 is used to place the sample 4; the vertical pressure head 5 is arranged along the axial direction of the sample 4, and is used to load the vertical pressure on the sample 4; the horizontal pressure head 6 is arranged along the circumferential direction of the sample 4, and is used to load the confining pressure on the sample 4.

[0176] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, technicians familiar with the field may make various corresponding changes and deformations based on the present invention, but these corresponding changes and deformations should all fall within the scope of protection of the claims attached to the present invention.

Claims

1. A deep anisotropic coal rock mass in-situ stress recovery test method, characterized in that: The following steps are involved: Step 1, preparation of anisotropic coal rock sample: based on the underground coal rock bedding structure and geological parameters, a deep coal rock in a typical mining area is selected, and the deep coal rock is a sphere with a radius greater than 500 mm; samples are drilled in the parallel direction, vertical direction and inclined direction of the deep coal rock bedding, and the samples are standard cylinders with a radius of 50 mm and a height of 100 mm; Step 2, initial stress loading test: according to the deep in-situ stress data obtained by actual measurement at the sampling point or by literature review, the deep in-situ stress data includes the maximum principal stress, the minimum principal stress and its direction, and the corresponding confining pressure and axial load of the triaxial loading test machine GCTS are set; according to the actual burial depth and the ground stress gradient, the loading rate is controlled and gradually applied to the corresponding initial target stress level. After loading to the predetermined stress value, the constant stress is maintained for ≥2 hours, and the monitoring stress fluctuation range is within ±0.5%; At the same time, the deformation and microcrack activity information of the sample are collected in real time through strain gauges and acoustic emission sensors to determine the stress balance degree and anisotropic response of the sample under the in-situ stress condition; Step 3, stress recovery test: simulate the secondary stress evolution of deep coal and rock mass after mining unloading: first partially unload or maintain for a predetermined time, then apply stress step by step to a new target level according to the stress evolution curve of actual monitoring or numerical prediction; record the crack extension, deformation accumulation and acoustic emission activities of the sample during this process, and obtain the differential response of the sample in the anisotropic direction; judge whether the axial stress or lateral stress exceeds the critical value by whether there is a macro crack penetration or a sudden increase in acoustic emission energy in the sample; Combining stress-strain curves, acoustic emission time series and image monitoring, the deformation and failure modes of deep coal and rock masses are determined, and the differences in the parallel and perpendicular bedding directions of deep coal and rock masses are obtained; In the stress recovery test, the constructed stress path dependence and nonlinear recovery mechanism model is introduced, and the key parameters of the stress path dependence and nonlinear recovery mechanism model are corrected through real-time data to dynamically reflect the influence of crack extension and damage evolution on the recovery process. Stress, strain and acoustic emission data are continuously collected. If the stress release rate does not conform to the preset nonlinear recovery model, the loading parameters are adjusted in real time. Step 4, construct an anisotropic mechanical model of deep coal rock mass: According to the anisotropic characteristics of the standard pattern and the test data obtained in steps 1, 2, and 3, the mechanical response and anisotropic characteristics of the deep coal rock mass under various stress paths are analyzed, and an anisotropic mechanical model of the deep coal rock mass is constructed. The anisotropic mechanical model of the deep coal rock mass not only considers the traditional elastic and plastic behaviors, but also integrates the path dependence and nonlinear recovery mechanism.

2. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 1, characterized in that: Also includes: Step 5, application of results: applying the anisotropic mechanical model of deep coal and rock mass to mine tunnel support design and / or gas extraction scheme optimization and / or rock burst prediction.

3. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 1 or 2, characterized in that: In step 3, the steps of constructing the stress path dependence and nonlinear recovery mechanism model include: path dependence modeling, crack extension and strength degradation, establishment of damage evolution model, and dynamic adjustment and feedback of the model.

4. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 3, characterized in that: In step 4, the construction of the anisotropic mechanical model of deep coal and rock mass includes: The anisotropic elastic parameters obtained from the initial loading test were summarized, the nonlinear stress-strain data, damage evolution curves and loading path information recorded in the stress recovery test were integrated, and the stress path dependence and nonlinear recovery mechanism models were combined. On the basis of inversion and calibration, the staged constitutive relationship was used to construct the overall model, thus obtaining the anisotropic mechanical model of deep coal and rock mass.

5. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 1, characterized in that: The drilled samples are prepared in parallel bedding, 30° inclined bedding, 45° inclined bedding, and vertical bedding methods.

6. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 1, characterized in that: In step 3, the stress recovery test includes reloading stress to the standard sample, and the reloading stress to the standard sample includes: Confining pressure loading: applying lateral confining pressure at a rate of 3.0 MPa / min until a first target value is reached, wherein the first target value corresponds to the deep in-situ minimum principal stress; Axial stress loading: Under the condition of keeping the confining pressure constant, the axial stress is applied at a rate of 30 kN / min to a second target value, and the second target value corresponds to the deep in-situ maximum principal stress; Stress balance maintenance: Maintain the confining pressure and axial stress constant, and stabilize them for ≥2 hours through the stress control mode. During this period, monitor the stress fluctuation range and the adjustment range is within ±0.5%.

7. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 5, characterized in that: Also includes: Strain stability analysis, Draw the strain-time curve: draw the curve with the circumferential strain or volume strain as the ordinate and the load holding time as the abscissa; Stability judgment: When the circumferential strain or volume strain change rate is ≤0.005% / min within 30 consecutive minutes, and the acoustic emission event rate is ≤1 time / minute, the sample is judged to have reached the in-situ stress equilibrium state; Anisotropy feature mark: record the strain difference in different directions. When the volume strain / circumferential strain>1.5, it is determined that there is significant anisotropy; Strain controlled loading until specimen failure. Loading mode switching: switch from stress control to circumferential strain rate control mode, and set the circumferential strain rate to 0.02mm / min; The failure judgment criteria are that the stress peak drops to 80% of the post-peak strength, the cumulative value of acoustic emission energy increases suddenly, and macro cracks penetrate the specimen.

8. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 1, characterized in that: The test data is acquired in real time through stress sensors and strain gauges.

9. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 8, characterized in that: Steps 2 and 3 are implemented using a multi-axis loading test device, which includes a hydraulic loading system, a stress sensor, a strain gauge and a data acquisition system.

10. The deep anisotropic coal rock mass in-situ stress recovery test method according to claim 9, characterized in that: The multi-axis loading test device comprises a vertical loading frame and a horizontal loading frame. The bottom of the vertical loading frame is connected to a vertical pressure head, and the horizontal loading frame is provided with a relatively arranged horizontal pressure head. The vertical loading frame and the horizontal loading frame are both driven by a hydraulic device. It also includes a base, a cushion block is arranged on the top surface of the base, and the cushion block is used to place the sample; The vertical pressure head is arranged along the axial direction of the sample and is used to apply vertical pressure to the sample; The horizontal pressure head is arranged along the circumference of the sample and is used to apply confining pressure to the sample.

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