Gob-side entry driving small coal pillar dilatation deformation test system and method under simulation of dynamic and static loads

Abstract

The invention relates to a gob-side entry driving small coal pillar dilatation deformation test system and method under simulation of dynamic and static loads, and belongs to the technical field of coal pillar dilatation deformation test. The system comprises a bearing frame unit, an axial pressure loading unit, a constraint frame unit, an impact loading unit, a monitoring unit and a controller, the axial pressure loading unit is arranged at the bottom of the bearing frame unit, the constraint frame unit is arranged on the upper side of the axial pressure loading unit, and the impact loading unit is arranged at the top of the bearing frame unit. The bearing frame unit is provided with a monitoring unit, the monitoring unit is connected with a data collector, and the data collector, the axial pressure loading unit and the impact loading unit are all connected with a controller. Different rotation angles of the gob-side entry driving lateral top plate are simulated, independent or synchronous application of the gradient load and the impact load on the top of the coal sample is realized in the axial direction, the test operation is simple, and the test accuracy is high.

Description

Technical Field

[0001] The invention relates to a system and method for testing the expansion and deformation of a small coal pillar along a goaf roadway under simulated dynamic and static loads, and belongs to the technical field of coal pillar expansion and deformation testing. Background Art

[0002] Small coal pillars protecting the roadway are of great significance for improving resource recovery rate and achieving safe and efficient mining. It is one of the important tasks of green mining in my country's coal mines. Gob-side roadway excavation refers to the mining roadway of the next working face excavated along the edge of the gob area of ​​the previous longwall working face. A 4-8m small coal pillar is often left between the new roadway and the gob area to isolate the gob area. However, affected by the disturbance of working face mining, the lateral roof above the coal pillar continues to rotate and sink, and the gradient load acting on the coal pillar causes the coal pillar to continue to expand and deform. At the same time, since the length of the small coal pillar is much larger than its width, it is equivalent to imposing a displacement constraint on the length direction of the small coal pillar. Under the action of the above-mentioned gradient load and the displacement constraint in the length direction, the side walls of the coal pillar are severely bulging, and even the overall slip of the coal wall occurs. Especially when the lateral roof or the overlying high-level hard roof suddenly breaks, the coal pillar will be superimposed with a large degree of dynamic load on the basis of the original static load, which will lead to further aggravation of deformation, and even severe dynamic disasters, threatening the safe production of the working face. Therefore, accurately grasping the expansion deformation behavior of the coal pillar under dynamic and static loads is of great significance for guiding the stability control of the coal pillar on the engineering site.

[0003] In the prior art, Chinese patent document CN111208010A discloses a method for simulating roof rock rotation and fracture test. By adopting a roof rock rotation and fracture loading device to load the sample, the effect of lateral roof rotation on small coal pillars along goaf roadway is effectively restored. However, this method does not take into account the displacement constraint of the coal pillar in the length direction, and the roof rock rotation and fracture loading device in this method can only simulate the rotation of the lateral roof along goaf roadway, while in actual engineering, the two states of lateral roof rotation and sinking exist at the same time, which is inconsistent with actual engineering. In addition, this method cannot apply dynamic loads to the coal pillars, and has little guidance on the expansion and deformation characteristics of the coal pillars under dynamic and static loads.

[0004] In addition, Chinese patent document CN111811856A discloses a comprehensive experimental device and experimental method for the cumulative damage evolution of coal pillar dams, and Chinese patent document CN110082227A discloses a test instrument and method for simulating the stability of coal pillar groups in knife-pillar residual mining areas under dynamic loads. Both of the above methods perform combined dynamic and static loading on coal pillars, but the above methods do not take into account the gradient load effect caused by the lateral rotation and sinking of the roof at the top of the coal pillar, and cannot effectively guide the research on the expansion and deformation of small coal pillars in gob-side tunneling.

[0005] Chinese patent document CN118443879A discloses a physical similarity simulation experimental device and method for the coal pillar dam of an underground reservoir. The device realizes the dynamic and static combined loading of the coal pillar and the gradient load above the coal pillar. Chinese patent document CN113686704A discloses a multi-stress gradient adjustable speed drop hammer impact testing machine, which realizes the gradient-impact loading of coal and rock samples. However, the gradient load in the above two methods is essentially a segmented load. This method loads the top of the coal pillar through multiple hydraulic cylinders or impact hammers under the vertical loading plate, and is not a continuous gradient load form. In addition, the above methods do not consider the displacement constraints of the small coal pillars along the goaf in the length direction, and cannot accurately obtain the expansion deformation behavior of the small coal pillars. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a test system for the expansion and deformation of small coal pillars in goaf-side tunneling under simulated dynamic and static loads, which simulates different rotation angles of the lateral roof of goaf-side tunneling and realizes independent or synchronous application of gradient load and impact load on the top of the coal sample in the axial direction. The test operation is simple and the test accuracy is high.

[0007] The present invention also provides a test method for the above-mentioned test system for the expansion deformation of small coal pillars along gob-side tunneling under simulated dynamic and static loads.

[0008] The technical solution of the present invention is as follows:

[0009] A test system for the expansion and deformation of small coal pillars in gob-side tunneling under simulated dynamic and static loads comprises a bearing frame unit, an axial pressure loading unit, a constraint frame unit, an impact loading unit, a monitoring unit and a controller, wherein an axial pressure loading unit is arranged at the bottom of the bearing frame unit, a constraint frame unit is arranged on the upper side of the axial pressure loading unit, an impact loading unit is arranged at the top of the bearing frame unit, a monitoring unit is arranged on the bearing frame unit, the monitoring unit is connected to a data collector, and the data collector, the axial pressure loading unit and the impact loading unit are all connected to the controller.

[0010] Axial pressure loading unit, used to apply axial pressure from bottom to top to the coal sample;

[0011] The constraint frame unit is used to impose displacement constraints on the coal sample in the front-to-back direction, and the left-to-right direction is free space;

[0012] An impact loading unit is used to apply an axial impact load from top to bottom to the coal sample;

[0013] The monitoring unit is used to monitor the stress, deformation and damage of the coal sample during loading.

[0014] Preferably according to the present invention, the bearing frame unit includes a crossbeam, a column and a base, the columns are symmetrically arranged on the base, an axial pressure loading unit is arranged in the middle of the base, a crossbeam is arranged on the top layer of the column, and an impact loading unit is arranged on the lower side of the crossbeam.

[0015] Preferably, according to the present invention, the axial pressure loading unit includes an axial pressure loading cylinder and a first pressure head. The bottom of the axial pressure loading cylinder is fixed to the supporting frame unit, and the first pressure head is arranged on the top of the axial pressure loading cylinder. The first pressure head is a cylindrical structure, and a constraint frame unit is fixedly arranged on the top of the first pressure head. The first pressure head and the constraint frame unit can be raised and lowered as the hydraulic oil pressure in the axial pressure loading cylinder changes, thereby applying an axial load to the coal sample.

[0016] Preferably, according to the present invention, the constraint frame unit includes a hollow steel frame and a transparent placement block. The transparent placement block is installed in the hollow steel frame. The hollow steel frame is used to fix the internal transparent placement block and improve its strength. The transparent placement block is a hollow rectangular block with a length × width × height of 170mm × 170mm × 240mm. It is integrally formed without splicing. Through holes are provided on the top surface and two symmetrical side surfaces of the transparent placement block to provide free space for the expansion and deformation of the coal sample.

[0017] Preferably according to the present invention, the impact loading unit includes a storage cylinder, an energy storage cylinder and a pressure column. The storage cylinder is fixed to the crossbeam, the bottom of the storage cylinder is connected to the energy storage cylinder, the bottom output end of the energy storage cylinder is connected to the pressure column, and a gradient control unit is arranged at the bottom of the pressure column.

[0018] According to the present invention, which is further preferred, the gradient control unit includes a lifting cylinder, a lifting pressure head, a sleeve, a motor and a second pressure head. The second pressure head is a rigid seal structure, that is, the upper part is a 3 / 4 spherical structure, and the lower part is a rectangular parallelepiped. The top of the second pressure head contacts a pressure column, and the bottom directly acts on the top of the coal sample. A motor is connected to one side of the second pressure head, and a sleeve is provided on one side of the motor. The sleeve is connected to the lifting cylinder through the lifting pressure head. The lifting cylinder is fixed to the pressure column. The lifting cylinder is used to drive the motor and the second pressure head to move up and down. The motor adjusts the inclination angle of the second pressure head. The lifting cylinder, the lifting pressure head, the sleeve and the motor are all located on one side of the second pressure head.

[0019] Preferably, according to the present invention, the monitoring unit includes a stress sensor, a displacement sensor, a multi-point laser rangefinder, a strain gauge and a high-speed camera. Stress sensors and displacement sensors are provided in the axial pressure loading cylinder and the energy storage cylinder. The multi-point laser rangefinder is arranged on the column. The multi-point laser rangefinder has multiple laser transmitters and receivers, which can simultaneously emit lasers and receive light signals reflected by the lasers to monitor the full-section deformation on both sides of the coal sample. The strain gauge is arranged on the coal sample, and the strain gauge is connected to a data collector through a dynamic strain gauge. A high-speed camera is arranged on the outside of the bearing frame unit.

[0020] The test method of the small coal pillar expansion deformation test system for gob-side roadway driving under simulated dynamic and static loads is as follows:

[0021] (1) A rectangular coal sample with a length × width × height = 50 mm × 50 mm × 100 mm was prepared, and speckle pattern was sprayed on the front of the coal sample. Four strain gauges were pasted on the top area of ​​the back of the coal sample and evenly distributed along the width direction;

[0022] (2) placing the coal sample in the transparent placement block of the constraint frame unit, adjusting the deflection angle of the second pressure head through the gradient control unit, performing calibration and zeroing, and adjusting the position of the coal sample so that it is located directly below the second pressure head;

[0023] (3) Arrange monitoring units and connect them to data collectors;

[0024] (4) applying axial pressure to the coal sample through an axial pressure loading unit until a set static load value is reached;

[0025] (5) According to the test plan, the impact load size and the change form of the deflection angle of the second pressure head during the impact loading process are set, and the coal sample is impact loaded;

[0026] (6) Stress sensors and displacement sensors monitor the axial stress and impact load during the loading process of the coal sample. The multi-point laser rangefinder monitors the full-section deformation of the coal sample in the lateral direction. The strain gauge monitors the loading characteristics of the top of the coal sample. The gradient load distribution at the top of the coal sample is calculated based on the elastic modulus of the coal sample and the strain gauge monitoring results. The high-speed camera records the changes in the speckle displacement field of the coal sample and the impact damage process.

[0027] According to the present invention, further preferably, in step (6), the gradient load calculation method at the top of the coal sample is as shown in formula (1):

[0028] σ i =Eε i (1)

[0029] In the formula, σ i is the stress at the ith strain gauge, in MPa, E is the elastic modulus of the coal sample, in GPa, ε i is the strain value at the i-th strain gauge.

[0030] The beneficial effects of the present invention are:

[0031] 1) The present invention simulates different rotation angles of the lateral roof of the gob-side tunneling by controlling different deflection angles of the second pressure head, and realizes independent or synchronous application of the gradient load and the impact load on the top of the coal sample in the axial direction, and the test operation is simple.

[0032] 2) The present invention realizes the displacement constraint of coal samples in the front-to-back direction through the hollow steel frame and the internal transparent placement block, truly restores the displacement constraint of small coal pillars in the length direction along the goaf, and can observe the expansion and destruction process of coal pillars in real time, and has strong engineering applicability.

[0033] 3) The present invention can restore the real stress environment of small coal pillars along goaf roadway excavation to a certain extent, effectively simulate the lateral roof rotation and sinking and the independent or synchronous application of impact loads, accurately obtain the expansion behavior of small coal pillars along goaf roadway excavation under dynamic and static loads, and provide more accurate experimental data support for guiding the stability control of small coal pillars. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a structural front view of the present invention;

[0035] Figure 2 It is a front view of the first pressing head structure of the present invention;

[0036] Figure 3 It is a schematic diagram of the structure of the transparent placement block of the present invention;

[0037] Figure 4 It is a side view schematic diagram of the transparent placement block of the present invention;

[0038] Figure 5 It is a top view schematic diagram of the transparent placement block of the present invention;

[0039] Figure 6 It is a schematic diagram of the structure of the gradient control unit of the present invention;

[0040] Figure 7 It is a schematic diagram of the connection between the strain gauge and the dynamic strain gauge of the present invention;

[0041] Figure 8 The experimental principle diagram for simulating the expansion and deformation of small coal pillars in gob-side roadway driving under dynamic and static loads;

[0042] Fig. 9 The present invention is a flow chart of the method.

[0043] In the figure: 1—crossbeam, 2—column, 3—base, 4—coal sample, 5—controller, 6—data collector, 7—axial pressure loading cylinder, 8—first pressure head, 9—hollow steel frame, 10—transparent placement block, 11—storage cylinder, 12—energy storage cylinder, 13—pressure column, 14—lifting cylinder, 15—lifting pressure head, 16—sleeve, 17—motor, 18—second pressure head, 19—multi-point laser rangefinder, 20—strain gauge, 21—dynamic strain gauge, 22—high-speed camera, 23—small coal pillar, 24—roof rock beam. DETAILED DESCRIPTION

[0044] The present invention will be further described below by way of embodiments in conjunction with the accompanying drawings, but is not limited thereto.

[0045] Embodiment 1:

[0046] like Figure 1-9 As shown, this embodiment provides a test system for the expansion and deformation of small coal pillars along goaf tunneling under simulated dynamic and static loads, including a bearing frame unit, an axial pressure loading unit, a constraint frame unit, an impact loading unit, a monitoring unit and a controller, wherein an axial pressure loading unit is arranged at the bottom of the bearing frame unit, a constraint frame unit is arranged on the upper side of the axial pressure loading unit, an impact loading unit is arranged on the top of the bearing frame unit, a monitoring unit is arranged on the bearing frame unit, the monitoring unit is connected to a data collector 6, and the data collector 6, the axial pressure loading unit and the impact loading unit are all connected to a controller 5.

[0047] Axial pressure loading unit, used to apply axial pressure from bottom to top to the coal sample;

[0048] The constraint frame unit is used to impose displacement constraints on the coal sample in the front-to-back direction, and the left-to-right direction is free space;

[0049] An impact loading unit is used to apply an axial impact load from top to bottom to the coal sample;

[0050] The monitoring unit is used to monitor the stress, deformation and damage of the coal sample during loading.

[0051] The load-bearing frame unit includes a crossbeam 1, a column 2 and a base 3. The columns 2 are symmetrically arranged on the base 3, an axial pressure loading unit is arranged in the middle of the base 3, the crossbeam 1 is arranged on the top layer of the column 2, and an impact loading unit is arranged on the lower side of the crossbeam 1.

[0052] The axial pressure loading unit includes an axial pressure loading cylinder 7 and a first pressure head 8. The bottom of the axial pressure loading cylinder 7 is fixed to the bearing frame unit, and the top of the axial pressure loading cylinder 7 is provided with the first pressure head 8. The first pressure head 8 is a cylindrical structure, and a constraint frame unit is fixedly provided on the top of the first pressure head 8. The first pressure head and the constraint frame unit can be raised and lowered as the hydraulic oil pressure in the axial pressure loading cylinder changes, thereby applying an axial load to the coal sample.

[0053] The constraint frame unit includes a hollow steel frame 9 and a transparent placement block 10. The hollow steel frame 9 is equipped with a transparent placement block. The hollow steel frame is used to fix the internal transparent placement block and improve its strength. The transparent placement block 10 is a hollow rectangular block with a length × width × height of 170mm × 170mm × 240mm. It is integrally formed without splicing. The top surface and two symmetrical side surfaces of the transparent placement block 10 are provided with through holes to provide free space for the expansion and deformation of the coal sample.

[0054] The impact loading unit includes a storage cylinder 11, an energy storage cylinder 12 and a pressure column 13. The storage cylinder 11 is fixed to the beam 1. The bottom of the storage cylinder 11 is connected to the energy storage cylinder 12. The bottom output end of the energy storage cylinder 12 is connected to the pressure column 13. A gradient control unit is arranged at the bottom of the pressure column 13.

[0055] The gradient control unit includes a lifting cylinder 14, a lifting pressure head 15, a sleeve 16, a motor 17 and a second pressure head 18. The second pressure head 18 is a rigid seal structure, that is, the upper part is a 3 / 4 spherical structure, and the lower part is a rectangular parallelepiped. The top of the second pressure head 18 contacts the pressure column 13, and the bottom directly acts on the top of the coal sample. The second pressure head 18 is connected to a motor 17 on one side, and a sleeve 16 is provided on one side of the motor 17. The sleeve 16 is connected to the lifting cylinder 14 through the lifting pressure head 15. The lifting cylinder 14 is fixed to the pressure column 13. The lifting cylinder is used to drive the motor and the second pressure head to move up and down. The motor adjusts the inclination angle of the second pressure head. The lifting cylinder, the lifting pressure head, the sleeve and the motor are all located on one side of the second pressure head.

[0056] The monitoring unit includes a stress sensor, a displacement sensor, a multi-point laser rangefinder 19, a strain gauge 20 and a high-speed camera 22. The axial pressure loading cylinder 7 and the energy storage cylinder 12 are both provided with stress sensors and displacement sensors. The multi-point laser rangefinder 19 is arranged on the column 2. The multi-point laser rangefinder has multiple laser transmitters and receivers, which can simultaneously emit lasers and receive light signals reflected by the lasers to monitor the full-section deformation on both sides of the coal sample. The strain gauge 20 is arranged on the coal sample 4. The strain gauge 20 is connected to the data acquisition device 6 through the dynamic strain gauge 21. A high-speed camera 22 is arranged on the outside of the bearing frame unit.

[0057] The test method of the small coal pillar expansion deformation test system for gob-side roadway driving under simulated dynamic and static loads is as follows:

[0058] (1) A rectangular coal sample 4 with a length × width × height = 50 mm × 50 mm × 100 mm is prepared, and a speckle pattern is sprayed on the front of the coal sample 4. Four strain gauges 20 are pasted on the top area of ​​the back of the coal sample and are evenly distributed along the width direction;

[0059] (2) placing the coal sample in the transparent placement block 10 of the constraint frame unit, adjusting the deflection angle of the second pressure head 18 through the gradient control unit, simulating the rotation angle θ of the roof rock beam 24, performing calibration and zeroing, and adjusting the position of the coal sample so that it is located directly below the second pressure head 18;

[0060] (3) Arrange the monitoring unit and connect it to the data collector 6;

[0061] (4) applying axial pressure to the coal sample 4 through the axial pressure loading unit until the set static load value is reached;

[0062] (5) According to the test plan, the impact load size and the change form of the deflection angle of the second pressure head during the impact loading process are set, and the coal sample is impact loaded;

[0063] (6) Stress sensors and displacement sensors monitor the axial stress and impact load during the loading process of the coal sample. The multi-point laser rangefinder 19 monitors the full-section deformation of the coal sample in the lateral direction. The strain gauge monitors the loading characteristics of the top of the coal sample 4. The gradient load distribution form at the top of the coal sample is calculated based on the elastic modulus of the coal sample and the monitoring results of the strain gauge. The high-speed camera 22 records the changes in the speckle displacement field of the coal sample and the impact damage process.

[0064] The calculation method of the gradient load on the top of the coal sample is shown in formula (1):

[0065] σ i =Eε i (1)

[0066] In the formula, σ i is the stress at the ith strain gauge, in MPa, E is the elastic modulus of the coal sample, in GPa, ε i is the strain value at the i-th strain gauge.

[0067] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A test system for the expansion and deformation of small coal pillars in gob-side tunneling under simulated dynamic and static loads, characterized in that: It includes a bearing frame unit, an axial pressure loading unit, a constraint frame unit, an impact loading unit, a monitoring unit and a controller, wherein an axial pressure loading unit is arranged at the bottom of the bearing frame unit, a constraint frame unit is arranged on the upper side of the axial pressure loading unit, an impact loading unit is arranged on the top of the bearing frame unit, a monitoring unit is arranged on the bearing frame unit, the monitoring unit is connected to a data collector, and the data collector, the axial pressure loading unit and the impact loading unit are all connected to the controller.

2. The small coal pillar expansion deformation test system for gob-side roadway driving under simulated dynamic and static loads as claimed in claim 1 is characterized in that: The load-bearing frame unit includes a crossbeam, a column and a base. The columns are symmetrically arranged on the base, an axial pressure loading unit is arranged in the middle of the base, a crossbeam is arranged on the top layer of the column, and an impact loading unit is arranged on the lower side of the crossbeam.

3. The small coal pillar expansion deformation test system for gob-side roadway driving under simulated dynamic and static loads as claimed in claim 2 is characterized in that: The axial pressure loading unit includes an axial pressure loading cylinder and a first pressure head. The bottom of the axial pressure loading cylinder is fixed to the bearing frame unit, and the top of the axial pressure loading cylinder is provided with the first pressure head. The first pressure head is a cylindrical structure, and a constraint frame unit is fixedly provided on the top of the first pressure head.

4. The small coal pillar expansion deformation test system for gob-side roadway driving under simulated dynamic and static loads as claimed in claim 3 is characterized in that: The constraint frame unit comprises a hollow steel frame and a transparent placement block. The hollow steel frame is provided with a transparent placement block. The transparent placement block is a hollow cuboid. The top surface and two symmetrical side surfaces of the transparent placement block are provided with through holes.

5. The small coal pillar expansion deformation test system for gob-side roadway driving under simulated dynamic and static loads as claimed in claim 4, characterized in that: The impact loading unit includes a storage cylinder, an energy storage cylinder and a pressure column. The storage cylinder is fixed to the crossbeam. The bottom of the storage cylinder is connected to the energy storage cylinder. The output end of the bottom of the energy storage cylinder is connected to the pressure column. A gradient control unit is arranged at the bottom of the pressure column.

6. The small coal pillar expansion deformation test system for gob-side roadway driving under simulated dynamic and static loads as claimed in claim 5, characterized in that: The gradient control unit includes a lifting cylinder, a lifting pressure head, a sleeve, a motor and a second pressure head. The second pressure head is a rigid seal structure. The top of the second pressure head contacts a pressure column. One side of the second pressure head is connected to a motor. A sleeve is provided on one side of the motor. The sleeve is connected to the lifting cylinder through the lifting pressure head. The lifting cylinder is fixed to the pressure column.

7. The small coal pillar expansion deformation test system for gob-side roadway driving under simulated dynamic and static loads as claimed in claim 6, characterized in that: The monitoring unit includes stress sensors, displacement sensors, multi-point laser rangefinders, strain gauges and high-speed cameras. Stress sensors and displacement sensors are installed in the axial pressure loading cylinder and the energy storage cylinder. The multi-point laser rangefinder is installed on the column, the strain gauge is installed on the coal sample, the strain gauge is connected to the data acquisition device through the dynamic strain gauge, and a high-speed camera is installed on the outside of the bearing frame unit.

8. The test method of the test system for the expansion deformation of small coal pillars in gob-side roadway driving under simulated dynamic and static loads as claimed in claim 7, characterized in that: Here are the steps: (1) A rectangular coal sample was prepared, and speckle spraying was applied to the front of the coal sample. Four strain gauges were pasted on the top area of ​​the back of the coal sample and evenly distributed along the width direction; (2) placing the coal sample in the transparent placement block of the constraint frame unit, adjusting the deflection angle of the second pressure head through the gradient control unit, performing calibration and zeroing, and adjusting the position of the coal sample so that it is located directly below the second pressure head; (3) Arrange monitoring units and connect them to data collectors; (4) applying axial pressure to the coal sample through an axial pressure loading unit until a set static load value is reached; (5) According to the test plan, the impact load size and the change form of the deflection angle of the second pressure head during the impact loading process are set, and the coal sample is impact loaded; (6) Stress sensors and displacement sensors monitor the axial stress and impact load during the loading process of the coal sample. The multi-point laser rangefinder monitors the full-section deformation of the coal sample in the lateral direction. The strain gauge monitors the loading characteristics of the top of the coal sample. The gradient load distribution at the top of the coal sample is calculated based on the elastic modulus of the coal sample and the strain gauge monitoring results. The high-speed camera records the changes in the speckle displacement field of the coal sample and the impact damage process.

9. The test method of the test system for the expansion deformation of small coal pillars in gob-side roadway driving under simulated dynamic and static loads as claimed in claim 8, characterized in that: In step (6), the gradient load calculation method at the top of the coal sample is shown in formula (1): s i =Ee i (1) In the formula, σ i is the stress at the ith strain gauge, in MPa, E is the elastic modulus of the coal sample, in GPa, ε i is the strain value at the i-th strain gauge.

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