A method for testing stability of bedded roof under mining-induced deflection gradient stress

Abstract

The application discloses a kind of bedding roof stability test methods under the action of mining deflection gradient stress, it is related to coal mining surrounding rock stability analysis and control technical field, for analyzing coal mining stress evolution induced bedding roof delamination, slip, instability damage and support regulation and control, the innovation of the application mainly includes: design variable cross-section trapezoidal prism test piece simulates the gradient stress field of bedding roof along the depth direction of borehole, sample processing design is made into certain angle with the axis of bedding roof simulation field mining principal stress deflection, realizes the synchronous simulation of mining principal stress deflection and gradient stress coupling effect, test loading environment and field actual stress state are highly consistent, test result is more scientific and reliable;Based on the stress data measured in situ, the principal stress deflection angle and stress gradient are calculated, and the trapezoidal test piece size and internal bedding dip angle are designed accordingly. Parameters can be adjusted according to the measured data of different mining conditions, and it has strong applicability. Test operation is simple and the process is standard. The application can provide scientific basis for coal mining roadway surrounding rock stability evaluation and control.

Description

Technical Field

[0001] This invention relates to the field of coal mine surrounding rock stability analysis and control technology, specifically to a test method for the stability of the bedding roof under the action of mining-induced deflection gradient stress. It is applicable to analyzing the law of surrounding rock failure caused by changes in the stress environment of the surrounding rock in the coal mine during mining, and provides a scientific basis for the design of coal mine roadway support and the evaluation of surrounding rock stability. Background Technology

[0002] The redistribution of surrounding rock stress caused by coal mining leads to a non-uniform gradient stress field around the goaf, accompanied by a significant deflection of the principal stress direction. This results in a complex, non-orthogonal angular relationship between the principal stress axis and the sedimentary rock bedding planes. Particularly in the weakly cemented soft rock mining areas of western China, due to the low degree of cementation and well-developed bedding structures, the coupling effect of this principal stress deflection and gradient stress significantly exacerbates the deformation and damage of the surrounding rock. This is a key mechanical factor inducing surrounding rock instability, support structure failure, and roof disasters.

[0003] However, existing rock mechanics testing methods are mostly limited to single stress paths or simple bedding anisotropy studies, lacking systematic simulations of the coupling effects of mining-induced principal stress deflection and gradient stress fields. Due to the lack of unified non-uniform stress loading methods and bedding anisotropic sample preparation standards, conventional tests cannot realistically reproduce the complex stress state of the surrounding rock in the field. This results in significant differences between the mechanical parameters and damage evolution laws obtained from the tests and those in engineering practice, making it difficult to meet the accuracy requirements for surrounding rock stability control in deep and complex geological environments.

[0004] In view of this, this invention aims to construct a standardized test system for the complex stress coupling characteristics of mining-induced surrounding rock. By constructing a gradient stress grading model and a bedding anisotropy matching criterion, an equivalent simulation of complex occurrence environments at the laboratory scale is achieved. Combined with multi-scale damage synchronous monitoring, the evolution law of surrounding rock fracture under coupling effects is quantitatively characterized, thereby overcoming the limitations of existing test methods and improving the accuracy and reliability of surrounding rock stability analysis. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing testing methods and provide a method for testing the stability of bedding roofs under mining-induced deflection gradient stress. By designing a truncated pyramidal specimen with a gradient cross-section containing inclined bedding, the method accurately simulates the gradient stress field of the bedding roof along the borehole depth direction. Furthermore, acoustic emission technology is used to dynamically monitor the entire process of surrounding rock damage and failure, providing a more realistic testing method for understanding the instability mechanism and stability control of surrounding rock in underground engineering. The method for testing the stability of bedding roofs under mining-induced deflection gradient stress includes on-site stress measurement, calculation of principal stresses and deflection angles, design of trapezoidal specimen dimensions, and loading test procedures. The specific steps are as follows:

[0006] (1) On-site drilling and stress testing: Drill holes in the middle of the roadway roof and measure the stress state of the surrounding rock within a depth range of 1.5m to 2.5m in the borehole to obtain the horizontal normal stress. Vertical normal stress and the shear stress τ perpendicular to the horizontal direction x The diameter of the drilled hole is 5cm to 10cm.

[0007] (2) Calculation of principal stress and deflection angle: Based on the measured stress state of the surrounding rock at a certain depth ( , τ x The maximum principal stress at that location was calculated. and minimum principal stress

[0008]

[0009] in, This represents the maximum principal stress at the measured location. The minimum principal stress at the measured location, The horizontal normal stress at the measured location. This represents the normal stress in the vertical direction at the measured location. The shear stress at the measured location is perpendicular to the horizontal direction.

[0010] At the same time, the maximum principal stress deflection angle at the measured location was calculated.

[0011]

[0012] in, This represents the angle of maximum principal stress deflection.

[0013] (3) Calculate the maximum principal stress gradient

[0014]

[0015] in, Δh represents the maximum principal stress gradient, and Δh represents the depth of the measured ground stress location in the borehole.

[0016] (4) Trapezoidal specimen design and fabrication: based on the calculated maximum principal stress deflection angle and maximum principal stress gradient Design and fabricate a trapezoidal cross-section frustum specimen. The specimen must meet the following requirements:

[0017] Stress gradient matching: The stress gradient across the specimen cross section is equal to the calculated maximum principal stress gradient. ;

[0018] Stratification angle matching: The angle between the strata in the specimen and the axis is equal to... When taking samples on site, mark the bedding direction and the azimuth angle of the sample, and the marking error shall not exceed 2°;

[0019] Dimensions: The length of the bottom of the sample is 10cm~30cm, the length of the top is 5cm~25cm, the axial length is 10cm~30cm, the unevenness of the top and bottom is less than 0.05mm, and the error between the layer and the axis angle after processing does not exceed 1°.

[0020] (5) Preparation of test equipment: A triaxial loading test equipment shall be selected. The equipment shall meet the following requirements: the base size shall be greater than the length of the bottom of the trapezoidal specimen; the stress measurement accuracy shall be less than 0.01 MPa; and the displacement range shall be not less than 0.25 times the height of the specimen. An acoustic emission probe shall be attached to the cylinder of the triaxial loading equipment. The acoustic emission threshold shall be set to 50 dB~60 dB to ensure that the acoustic emission signal is synchronized with the loading process and the time synchronization error is less than 10 ms.

[0021] (6) Loading test and acoustic emission monitoring:

[0022] The processed trapezoidal specimen is installed at the center of the base of the triaxial loading device to ensure that the specimen is subjected to uniform force.

[0023] Applying confining pressure: Place the trapezoidal specimen in a triaxial loading apparatus and apply confining pressure until the measured minimum principal stress is reached. Keep it constant, and then apply axial pressure to reach the measured maximum principal stress. When the axial pressure is applied to the measured maximum principal stress, if the specimen does not fail, the loading rate is maintained and the axial pressure is applied until the specimen fails.

[0024] Apply axial pressure: Apply axial pressure at a constant rate, not exceeding 0.1 MPa / s;

[0025] Synchronous monitoring: During the loading process, acoustic emission signals of the sample are collected in real time through acoustic emission probes to record the temporal and spatial characteristics of microcrack generation and propagation. Combined with stress-displacement data, the amount of bedding plane slip and delamination is obtained. Combined with acoustic emission signal characteristics, the tensile, shear or tensile-shear combined failure modes of the bedding top plate are identified.

[0026] The innovation of this invention compared with existing experimental methods is mainly reflected in the following aspects:

[0027] (i) Accurate simulation of the stress environment of the surrounding rock during mining: Traditional rock mechanics tests use specimens with uniform cross sections, which cannot simulate the stress gradient distribution characteristics of the surrounding rock and do not consider the field deflection of the principal stress of the surrounding rock. In contrast, the present invention designs a trapezoidal frustum specimen with a variable cross section to simulate the gradient stress field in the field. The specimen is processed and designed so that the axis and the bedding form a certain angle to simulate the deflection of the principal stress during mining in the field. This realizes the synchronous simulation of the coupling effect of the principal stress deflection during mining and the gradient stress. The test loading environment is highly consistent with the actual stress state in the field, and the test results are more scientific and reliable.

[0028] (ii) Quantitative and controllable sample processing parameters: Based on the measured stress data on site, the principal stress deflection angle and stress gradient can be calculated, and the size of the trapezoidal specimen and the internal bedding dip angle can be designed accordingly, ensuring the quantitative accuracy of the specimen design. The parameters can be adjusted according to the measured data of different mining conditions, making it highly applicable.

[0029] (III) Simple operation and standardized process: The relevant tests can be carried out using conventional triaxial testing equipment and acoustic emission monitoring equipment. Simply place the specimen on the loading table, apply the confining pressure first and then the axial pressure, and perform acoustic emission monitoring at the same time to complete the test. The operation is simple and the process is standardized. Attached Figure Description

[0030] Figure 1 This is a schematic diagram showing the deflection of the principal stress direction after tunnel excavation.

[0031] Figure 2 This is a schematic diagram showing the change of principal stress with depth after tunnel excavation.

[0032] Figure 3 This is a schematic diagram of the trapezoidal sample structure;

[0033] Figure 4 This is a flowchart of the method of the present invention;

[0034] The components in the attached diagram are labeled as follows: 1. Horizontal ground stress, 2. Vertical ground stress, 3. Tunnel, 4. Minimum principal stress before tunnel excavation. 5. Maximum principal stress before tunnel excavation 6. Lateral normal stress after tunnel excavation 7. Vertical normal stress after tunnel excavation 8. y-plane shear stress τ y 9.x plane shear stress τ x (τ) y= τ x 10. Maximum principal stress after tunnel excavation 11. Minimum principal stress after tunnel excavation 12. Deflection angle of the maximum principal stress 13. The angle between the maximum principal stress after excavation and the bedding plane. 14. Maximum principal stress distribution curve after excavation; 15. Magnitude of ground stress at the burial depth before tunnel excavation; 16. Minimum principal stress distribution curve after excavation; 17. Bedding; 18. Width of the top section of the variable cross-section frustum; 19. Height of the variable cross-section frustum; 20. Width of the bottom section of the variable cross-section frustum; 21. Thickness of the variable cross-section frustum; 22. Angle between the side and bottom sides of the frustum. 23. Variable cross-section frustum specimen. Specific implementation methods

[0035] The experimental method of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0036] (1) Drill holes in the middle of the roadway roof and measure the stress state of the surrounding rock at a certain depth (range 1.5m~2.5m) on site. , τ x ).

[0037] (2) Stress testing method: Stress testing is conducted using the borehole stress relief method or the borehole strain gauge method. In each borehole, a measuring point is arranged every 0.5 m to 1.0 m to measure the horizontal normal stress at that point. Vertical normal stress and the shear stress τ perpendicular to the horizontal direction x .

[0038] (3) Based on the measured stress state of the surrounding rock at a certain depth ( , τ x The maximum principal stress at this location was calculated. and minimum principal stress

[0039]

[0040] in, This represents the maximum principal stress at the measured location. The minimum principal stress at the measured location, The horizontal normal stress at the measured location. This represents the normal stress in the vertical direction at the measured location. This represents the shear stress at the measured location perpendicular to the horizontal direction.

[0041] (4) The maximum principal stress deflection angle at the measured location was calculated.

[0042]

[0043] in, This represents the maximum principal stress deflection angle.

[0044] (5) Calculate the maximum principal stress gradient

[0045]

[0046] in, Δh represents the maximum principal stress gradient, and Δh represents the depth of the measured ground stress location in the borehole.

[0047] (6) Design the dimensions of the trapezoidal specimen to meet two conditions: first, the stress gradient of the specimen cross section is equal to the calculated principal stress gradient. Secondly, the angle between the strata and the axis in the sample is equal to... .

[0048] (7) Conduct rock mechanics tests on trapezoidal specimens, first applying confining pressure to the measured minimum principal stress. Keep it constant, and then apply axial pressure to reach the measured maximum principal stress. If the sample does not break, continue applying axial pressure until the sample breaks.

[0049] (8) Collect information such as axial load, displacement, confining pressure, number of acoustic emission events, energy, and location throughout the entire process, with a sampling interval of no more than 1 s.

[0050] (9) After the test, extract the spatiotemporal distribution map of acoustic emission events, stress-strain curves, strength parameters, etc., and analyze the failure mode and damage evolution law of the surrounding rock in combination with the gradient stress field and principal stress deflection angle; obtain the amount of slip and delamination of the bedding plane, and identify the tensile, shear or tensile-shear combined failure mode of the bedding top plate in combination with the characteristics of acoustic emission signals.

Claims

1. A method for testing the stability of a top slab under mining-induced deflection gradient stress, characterized in that... A gradient cross-section frustum specimen was designed to simulate the gradient stress field along the borehole depth of the bedding plate. The specimen was fabricated with its axis at a certain angle to the bedding to simulate the deflection of principal stresses during on-site mining. The specific experimental steps are as follows: (a) Drill a hole in the middle of the tunnel roof and measure the stress state of the surrounding rock at a certain depth (range 1.5m~2.5m) on site. , τ x ); (b) Based on the measured stress state of the surrounding rock at a certain depth ( , τ x The maximum principal stress at that location was calculated. and minimum principal stress in, This represents the maximum principal stress at the measured location. The minimum principal stress at the measured location, The horizontal normal stress at the measured location. This represents the normal stress in the vertical direction at the measured location. The shear stress at the measured location is perpendicular to the horizontal direction. (c) The maximum principal stress deflection angle at the measured location is calculated. in, The maximum principal stress deflection angle; (d) Calculate the maximum principal stress gradient in, Δh represents the maximum principal stress gradient, and Δh represents the depth of the measured ground stress location in the borehole. (e) Design the dimensions of the trapezoidal specimen to satisfy two conditions: first, the stress gradient across the specimen cross-section must be equal to the calculated principal stress gradient. Secondly, the angle between the strata and the axis in the sample is equal to... ; (f) Conduct rock mechanics tests on trapezoidal specimens, first applying confining pressure to the measured minimum principal stress. Keep it constant, and then apply axial pressure to reach the measured maximum principal stress. If the sample does not break, axial pressure is continued until the sample breaks. At the same time, micro-vibration is used to monitor the evolution of microcracks and the damage and failure pattern of the sample.

2. The method for testing the stability of the top plate under mining-induced deflection gradient stress according to claim 1, characterized in that, The measured depth of the ground stress borehole ranges from 1.5m to 2.5m, and the borehole diameter ranges from 5cm to 10cm.

3. The method for testing the stability of the top plate under mining-induced deflection gradient stress according to claim 1, characterized in that, When taking samples on site, the bedding direction and the azimuth angle of the sample must be marked, with an error not exceeding 2°.

4. The method for testing the stability of the top plate under mining-induced deflection gradient stress according to claim 1, characterized in that, The length of the lower base of the trapezoidal specimen is 10cm~30cm, the length of the upper base is 5cm~25cm, the axial length is 10cm~30cm, the unevenness of the upper and lower bases of the specimen is less than 0.05mm, and the error between the processing layer and the axis angle does not exceed 1°.

5. The method for testing the stability of the top plate under mining-induced deflection gradient stress according to claim 1, characterized in that, The dimensions of the test equipment base must be greater than the length of the bottom of the trapezoidal specimen, the stress measurement accuracy must be less than 0.01 MPa, and the displacement range must be no less than 0.25 times the height of the specimen.

6. The method for testing the stability of the top plate under mining-induced deflection gradient stress according to claim 1, characterized in that, The confining pressure was applied as the average of the minimum principal stress measured in boreholes at different depths with the same azimuth angle. The axial pressure was applied at a uniform loading rate, which did not exceed 0.1 MPa / s.

7. The method for testing the stability of the top plate under mining-induced deflection gradient stress according to claim 1, characterized in that, The acoustic emission probe is attached to the hydraulic cylinder of the triaxial loading device. The acoustic emission signal is synchronized with the loading, with a time synchronization error of less than 10ms and an acoustic emission threshold of 50dB~60dB.

8. The method for testing the stability of the top plate under mining-induced deflection gradient stress according to claim 1, characterized in that, During the experiment, digital image correlation technology was used to monitor the displacement field of the bedding planes on the sample surface, obtain the amount of bedding plane slip and delamination, and combine the acoustic emission signal characteristics to identify the tensile, shear or tensile-shear combined failure modes of the bedding top plate.

Images