Gas-containing coal and rock mass true triaxial indoor test method based on coaxial unequal force

Abstract

The invention provides a coaxial unequal force-based gas-containing coal and rock mass true triaxial indoor test method, which comprises the following steps of: preparing a sample, injecting CH4 into a confining pressure cavity, synchronously starting an acoustic emission device and a binocular miniature high-speed camera, and testing the gas-containing coal and rock mass true triaxial indoor test. Acoustic emission monitoring software and DIC analysis software are used for recording acoustic emission and strain field evolution data, stiffness loading systems in the sigma 1, sigma 2, sigma 3 + and sigma 3-directions are controlled through a controller and a servo valve, confining pressure is loaded in a force control loading mode, and sample data are processed and analyzed; according to the invention, the condition of unequal coaxial stress load in the free face direction in the mining process of an underground working face can be simulated, and the actual condition of a mine can be better reflected; a video probe side pressing plate is arranged, and a binocular miniature high-speed camera is embedded into the clamp, so that the structural change of a test surface in the loading process can be recorded in real time, three-way surface strain field dynamic monitoring is realized, and the change conditions of a crack field and a strain field of a coal sample under the gas-containing condition are better reflected.

Description

Technical Field

[0001] The present invention relates to the field of coal and rock technology testing, and in particular to a true triaxial indoor test method for gas-containing coal and rock masses based on coaxial unequal forces. Background Art

[0002] With the gradual depletion of shallow coal resources in my country, the development and utilization of deep coal resources is imperative. However, deep coal resource development faces complex mining environments such as high geostress and high gas content, which increases the difficulty of mining. During deep mining of gas-bearing coal and rock masses, the interaction between geostress and gas significantly increases the risk of coal rock dynamic hazards such as rock bursts and coal and gas outbursts. Furthermore, the presence of gas within the coal mass can affect the development of cracks and mechanical properties, reducing its strength and weakening its deformation capacity during failure, making it more likely to become unstable and fail. Therefore, studying the mechanical behavior and failure patterns of gas-bearing coal and rock masses under external loads can help reveal the instability mechanism of gas-bearing coal and rock masses and predict coal rock dynamic hazards.

[0003] Indoor testing is an important means of revealing the mechanical behavior and instability mechanism of gas-bearing coal rock. It can study its instability failure mechanism through coal bodies with different gas contents and different stress paths. At present, the instability failure research of gas-bearing coal rock is mainly based on uniaxial, conventional triaxial and true triaxial compression failure tests with the same coaxial stress. However, the equipment and methods currently used for compression failure tests of gas-bearing coal rock do not take into account the coaxial unequal stress state that occurs in the direction of the free face during underground mining. Although a few equipment take into account the existence of the free face during mining, they cannot simulate the coaxial unequal stress state in different directions of the free face after mining, and cannot simulate the gas distribution state inside the coal body, which is significantly different from the actual stress and gas distribution state on site.

[0004] Existing technologies lack research on the unequal stress conditions on the same side of the decompression direction during underground working face mining. This results in an inadequate understanding of the stress, strain evolution, and fracture patterns of gas-bearing coal and rock masses under true triaxial coaxial unequal stress conditions. Therefore, there is an urgent need to develop a technology to address the instability and failure of gas-bearing coal and rock masses under coaxial unequal stress conditions. Summary of the Invention

[0005] In response to the above problems, the present invention provides a true triaxial indoor test method for gas-containing coal rock based on coaxial unequal forces. It can use an airtight device to ensure the airtightness of the sample, load unequal stresses in the coaxial direction, and realize dynamic and real-time monitoring of the entire process of coal sample destruction and internal crack development through a combination of visualization and acoustic emission internal and external monitoring means, effectively revealing the instability and failure mechanism of gas-containing coal rock.

[0006] Specifically, the true triaxial indoor test method for gas-containing coal rock mass based on coaxial unequal forces of the present invention comprises the following steps:

[0007] Step S1: Prepare the sample and spray DIC scattered spots on the surface, and wrap the sample with a transparent video probe side pressure plate and a tempered fixture;

[0008] Step S2: After the wrapped sample is sealed, it is transported to the confined pressure chamber via a transport and positioning trolley; CH4 is injected into the confined pressure chamber, the seepage inlet and outlet are connected to the inlet and outlet of the equipment pressure chamber, and the gas injection is stopped after the gas pressure measured by the pressure measuring probe reaches stability;

[0009] Step S3: Start the experiment, synchronously start the acoustic emission sensor and the micro binocular high-speed camera inside the transparent video side pressure plate, and use the acoustic emission monitoring software and DIC analysis software to record the acoustic emission and strain field evolution data;

[0010] Step S4: Control σ1, σ2, σ through the controller and servo valve 3+ and σ 3- The stiffness loading system in the direction of the load adopts the force control loading method to load the confining pressure, and then makes σ1, σ2, σ 3+ and σ 3- Load to the preset value and ensure σ 3+ and σ 3- Equal in size; keep σ2 and σ 3+ In order to simulate the coaxial unequal force situation caused by excavation unloading in the free face direction, the displacement control method is used to unload at a certain rate σ 3- The stress state is between the original rock stress and the free surface boundary stress state, so that the minimum stress σ 3+ and σ 3- After unloading, the maximum principal stress σ1 is applied until the specimen is damaged, and then the stress and strain data of the specimen are collected using a load cell and a displacement sensor.

[0011] Step S5: Process and analyze the sample data to obtain data on the sample's elastic modulus, peak strength, residual strength, acoustic emission counts, and strain field evolution characteristics.

[0012] Furthermore, in step S1, acrylic spray paint is used to spray DIC scattered spots on the surface of the sample and the sample is left to stand for 4 hours; a 260-frame binocular micro high-speed camera is set on the inner side of the video probe side pressure plate. The binocular micro high-speed camera on the inner side of the video probe side pressure plate records the real-time expansion of the crack and the position change process of the scattered spots through the high-definition transparent PAM plate. The video probe side pressure plate is installed on the upper side pressure plate, the left side pressure plate, and the front side pressure plate of the video probe respectively, and the tempered clamp is installed on the other sides. The binocular micro high-speed camera on the inner side of the video probe side pressure plate records the real-time expansion of the crack and the position change process of the scattered spots through the high-definition transparent PAM plate; and interlocking stress-free blank corner clamps are used for connection at the top and bottom, left and right, front and back. The rear pressure plate uses a pressure block slightly smaller than the sample to prevent the sample from damaging the clamp and wearing it.

[0013] In the further step S1, the flatness of the end surface of the sample is within 0.02 mm.

[0014] Furthermore, in step S2, a sealing device is used to seal the packaged sample.

[0015] Furthermore, in step S2, the acoustic emission sensor collects signals and transmits them to the acoustic emission monitoring software to analyze the initiation, expansion and rupture stages of cracks inside the sample; the binocular miniature high-speed camera monitors the changes in the positions of scattered spots on three sides and records the dynamic rupture process of the coal sample, and transmits the data to the computer to use DIC analysis software to analyze its strain field, crack field distribution and compression deformation characteristics, and save the recorded acoustic emission and strain field evolution data.

[0016] Furthermore, the specific steps of step S4 are as follows:

[0017] Step S41: Use the controller and servo valve to load the confining pressure in four directions in a force control manner, so that σ1, σ2, σ 3+ , σ 3- Load at 0.4 MPa / s until the coaxial forces are equal;

[0018] Step S42: Maintain σ2 and σ 3+ Unchanged, unloading at a certain rate σ 3- Between the original rock stress and the free surface boundary stress state;

[0019] Step S43: Apply the maximum principal stress σ1 until the sample is broken, and then use the load sensor and displacement sensor to collect stress and strain data of the sample; transmit the data to the computer via the data line, analyze and save it using stress-strain analysis software;

[0020] Step S44: Unload the confining pressure, remove the sample, replace the sample prepared in step S1, and repeat the above steps S2-S4 until all samples are tested.

[0021] Furthermore, one side of the video probe side pressure plate uses PAM material as the side pressure plate. The PAM plate has high transparency and can transmit confining pressure without being damaged. It contains a 260-frame binocular miniature high-speed camera inside, which can realize two-dimensional DIC monitoring of coal samples and real-time evolution of three-dimensional surface cracks and strain field data collection; the upper pressure plate acoustic emission probe is installed inside the fixture and is evenly distributed at 120°, and the right pressure plate acoustic emission probe is evenly distributed, which can well realize the collection of acoustic emission data and spatial positioning of the internal crack surface.

[0022] Furthermore, in step S2, the air pressure measuring probes are evenly distributed and installed on the tempered fixture in at least 5 lengths, and the air pressure signal is transmitted to the computer through the air pressure measuring connector, so that the air pressure value of the sample can be measured more accurately and quickly.

[0023] Furthermore, silicone sealant is applied to the gap outside the fixture and wrapped with sealing tape to ensure airtightness. The present invention first prepares the sample into a standard rectangular sample and sprays DIC scattered spots with acrylic paint. The sample is wrapped with a transparent PAM plate and a tempered fixture. At the same time, to prevent air leakage, sealant and sealing tape are used to strengthen the sealing of the sample. Then, acoustic emission and visualization monitoring technology are used to analyze the internal damage evolution characteristics of gas-containing coal samples and the displacement field and strain field distribution on the coal sample surface, intuitively reflecting surface deformation, local strain concentration and macro crack propagation path. The combination of the two can realize the whole process mechanism of coal samples from micro damage to macro rupture, avoiding the limitations of a single technology.

[0024] Furthermore, 6 actuators are controlled by 4 controllers and servo valves, where σ1 controller controls the vertical actuator, σ2 controller controls the horizontal actuator, and σ3 direction is controlled by independent controllers and servo valves, where σ 3+ Controller and servo valve control front actuator, σ 3- The controller and servo valve control the rear actuator to achieve coaxial unequal force loading.

[0025] At present, most conventional true three-axis σ3 direction stress loading uses a controller and servo valve to control two actuators. Some true three-axis loading technologies have a dual-valve control mode for σ3 direction stress loading and adopt the master-slave control principle, that is, the left side is set to active control and loaded according to the set force / displacement, and the other side is set to slave control, which follows the force value on the left side in real time, thereby achieving coaxial equal force loading. However, they are unable to perform differentiated loading and simulate the coaxial unequal stress state along the free face after mining. The present invention can simulate underground mining conditions by using two independent controllers and servo valves to independently control σ3 direction. 3+ and σ 3- Differentiated loading mode of the directional actuator (σ1>σ2>σ 3+ ≠σ 3-) reflects the influence of coaxial unequal stress on the mechanical properties of coal samples such as deformation and strength, which helps to understand the stress distribution and changes of coal rocks, better reveal the instability mechanism of coal rocks under high stress conditions, and provide a theoretical basis for predicting and preventing coal rock dynamic disasters.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. The present invention provides a true triaxial indoor testing method for gas-containing coal and rock based on coaxial unequal forces, which can be used to conduct true triaxial mechanical tests on coal and rock in a gas-containing environment, providing a useful technical means for mechanical testing of gas-containing coal samples.

[0028] 2. This invention provides a true triaxial indoor test method for gas-bearing coal and rock mass based on coaxial unequal forces. The interfaces of the experimental fixtures are coated with sealant and wrapped with sealing tape to ensure the airtightness of the experimental device, restore the high-gas environment, and better simulate the gas pressure conditions underground.

[0029] 3. A first multi-purpose air hole is provided on one side of the clamp of the present invention, and a second multi-purpose air hole is provided on the other side of the coal sample container shell. During inflation, one seepage inlet and one seepage outlet can ensure more uniform and stable inflation of the coal sample in the coal sample container.

[0030] 4. The fixture of the present invention contains an acoustic emission probe, which can reflect the dynamic evolution characteristics of damage inside the coal sample in real time, as well as the expansion of cracks and energy dissipation mechanisms inside the sample in a gas-containing environment;

[0031] 5. The present invention provides a video probe side pressure plate and embeds a binocular miniature high-speed camera inside the fixture, which can record the structural changes of the test surface during the loading process in real time, realize three-dimensional surface strain field dynamic monitoring, and better reflect the changes in the fracture field and strain field of the coal sample under gas-containing conditions;

[0032] 6. The present invention's true triaxial indoor test method for gas-bearing coal and rock mass based on coaxial unequal forces can achieve unequal stress loading in the coaxial direction, simulating the unequal coaxial stress loading on the inner side of the free face during underground working face mining, thus better reflecting the actual conditions of the mine.

[0033] 7. The stress loading system of the true triaxial indoor test loading experimental device for gas-bearing coal and rock mass based on coaxial unequal forces of the present invention can provide stable pressure, display test data in real time and record mechanical parameters;

[0034] 8. The test method of the present invention has simple steps, strong practicality and is easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the test process of the present invention;

[0036] Figure 2 This is a schematic diagram of the confining pressure loading principle of an embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of a sealed sample device according to an embodiment of the present invention;

[0038] Figure 4 This is a schematic diagram of a side pressure plate device of a video probe according to an embodiment of the present invention;

[0039] Figure 5 This is a diagram showing the arrangement of acoustic emission probes according to an embodiment of the present invention;

[0040] Figure 6 This is a schematic diagram of a pressure probe device according to an embodiment of the present invention;

[0041] Among them: 1-computer, 2-σ1 controller, 3-σ2 controller, 4-σ 3+ Controller, 5-σ 3- Controller, 6-vertical actuator, 7-confining pressure chamber, 8-air pressure measurement connector, 9-tempered fixture, 10-sealing tape, 11-acoustic emission probe, 12-seepage inlet, 13-silicone sealant, 14-seepage outlet, 15-video output connector, 16-binocular miniature high-speed camera, 17-transparent PAM plate, 18-air pressure measurement probe, 19-horizontal actuator, 20-front actuator, 21-rear actuator. DETAILED DESCRIPTION

[0042] The present invention can be better understood according to the following examples. However, it is easy for those skilled in the art to understand that the contents described in the examples are only used to illustrate the present invention, and should not and will not limit the present invention described in detail in the claims.

[0043] The technical solution of the present invention is further described below with reference to implementation cases.

[0044] Example

[0045] This embodiment provides a true triaxial indoor test method for gas-containing coal rock mass based on coaxial unequal forces, such as Figure 1 As shown, the following steps are included:

[0046] S1: First, the complete coal sample is polished and cut into a rectangular parallelepiped sample with a length, width, and height of 100 mm × 100 mm × 200 mm. The unevenness of the coal sample end is less than 0.2 mm. DIC scattered spots are sprayed on the surface of the sample using acrylic spray paint and left to stand for 4 hours.

[0047] S2: Wrap the sample described in step S1 with a video probe side pressure plate and a tempered fixture 9. Install the video probe side pressure plate on one side of the three axes of the sample, and install the tempered fixture 9 on the other sides. A binocular miniature high-speed camera 16 on the inner side of the video probe side pressure plate records the real-time expansion of the crack and the position change process of the collected scattered spots through a high-definition transparent PAM plate 17. Interlocking stress-free blank corner clamps are used to connect the upper and lower, left and right, and front and back. A pressure block slightly smaller than the sample is used on the rear pressure plate to prevent the sample from damaging the fixture. Then, the sample is sealed.

[0048] like Figure 4 The side pressure plate of the video probe shown in the figure is made of a transparent PAM board 17 as the side pressure plate. PAM is a special transparent material with ultra-high strength, high toughness, pure transparency and high definition. It has an optical grade, pure transparency and high definition with 90% transparency and can transmit the confining pressure. A 260-frame binocular micro high-speed camera 16 is installed on the inner side of the transparent PAM board 17. The wide angle of 120° can observe the entire surface of the sample.

[0049] like Figure 3 The specific sealing treatment shown is to apply silicone sealant 13 at the joints between the clamps and wrap the clamps tightly with sealing tape 10 on the outside of the clamps to ensure that the coal sample gas does not leak to the outside;

[0050] S3: The sealed sample is transported to the center of the main machine confining pressure chamber 7 by the positioning trolley;

[0051] S4: Inject gas into the confining pressure chamber 7, connect the seepage inlet 12 and the seepage outlet 14 to the inlet and outlet of the equipment pressure chamber. To ensure the safety of the test, inject CH4 gas into the seepage inlet until the gas pressure of the sample reaches 1 MPa. Stop injecting gas after the measured gas pressure reaches stability.

[0052] like Figure 3 The tempered fixture 9 shown is provided with multiple air holes on both sides. The seepage inlet 12 of the rear pressure plate tempered fixture is used for air intake, and the seepage outlet 14 of the right tempered fixture is used for air exhaust to ensure uniform internal gas. Figure 3 and Figure 6 The tempered fixture 9 shown here features evenly distributed pressure measuring probes 18 integrated within the pressure pad. At least five pressure measuring probes 18 are located along the length of the test pad. These probes transmit signals to the computer 1 via a pressure measuring connector 8, enabling more accurate and rapid measurement of the specimen's pressure. The pressure measuring probes have an effective measurement range of 0-10 MPa, a force resolution of 1 kPa, and a force accuracy within ±1% of the indicated value.

[0053] S5: Place a fill light around each fixture and run a binocular micro high-speed camera 16. Adjust the parameters of the binocular micro high-speed camera 16 so that the sample is within the field of view and the sample image remains clear. Turn on the fill light and run the DIC software. Set the parameters of the acoustic emission device and run the acoustic emission monitoring software. Set the acoustic emission pre-amplification to 40 dB, the threshold to 40 dB, the sampling interval to 1 μs, and the frequency to 1 MHz. The resolution of the binocular micro high-speed camera is 1280 × 720, and the image acquisition speed is 30 frames / s.

[0054] like Figure 3 and Figure 5 The specific upper pressure plate acoustic emission probes 11 are distributed at 120 degrees inside the tempered fixture 9, and the other side pressure plate acoustic emission probes 11 are evenly distributed inside the tempered fixture 9. The layout of the acoustic emission probes 11 can ensure that there are no stress blank areas when the sample is under pressure and can also accurately measure the acoustic emission signal, so as to monitor in real time whether the internal microcracks of the coal sample are generated, expanded and penetrated during the loading process.

[0055] S6: At the start of the test, the acoustic emission sensor 11 and the binocular miniature high-speed camera 16 are simultaneously activated; the acoustic emission sensor 11 collects signals and transmits them to the acoustic emission monitoring software to analyze the initiation, expansion, and rupture of cracks within the sample. The binocular miniature high-speed camera 16 monitors the changes in the position of scattered spots on the lower, front, and left pressure plates and records the real-time dynamic rupture process of the coal sample. The data is transmitted to the computer 1 via the video output connector 15, where DIC analysis software is used to analyze the sample strain field, crack field distribution, and compression deformation characteristics, and the recorded acoustic emission and DIC data are stored.

[0056] S7: Use the controllers installed in independent control cabinets in each direction to control σ1, σ2, σ 3+ and σ 3- The stiffness loading system in the direction of the load is loaded at 0.4MPa / s by force control, so that σ1, σ2, σ 3+ and σ 3- The loading was stopped after reaching 20MPa, 12MPa, 8MPa, and 8MPa respectively;

[0057] like Figure 2 During the test shown, computer 1 sends commands to σ1 controller 2, σ2 controller 3, σ 3+ Controller 4 and σ 3- The controller 5 transmits the loading signal, and the σ1 controller 2 and the σ2 controller 3 perform synchronous movement in the coaxial direction after receiving the signal; specifically, the vertical actuator 6 in the σ1 direction uses the same σ1 controller 2 and servo valve to control the vertical loading, and the horizontal actuator 19 in the σ2 direction uses the same σ2 controller 3 and servo valve to control the horizontal loading; 3+Controller 4 and σ 3- After the controller 5 receives the signal, σ 3+ The controller 4 controls the front actuator 20 and σ 3- The rear actuator 21 controlled by the controller 5 operates independently, σ 3+ , σ 3- The load size can be freely controlled in the direction, and coaxial unequal force loading can be implemented.

[0058] The specific σ1 direction stiffness loading system has a maximum test force of 3000kN, a load loading and unloading rate of 10kN / s, and a displacement control speed of 50mm / min. The displacement measurement and control range is 50mm, with a measurement accuracy of <±0.5% FS, a measurement resolution of 0.001mm, and a displacement control fluctuation of less than 0.003mm.

[0059] The specific σ2 direction stiffness loading system has a maximum test force of 2000kN; a force measurement range of 1% to 100% FS, a force measurement accuracy of within ±1% of the indicated value, and a force control fluctuation of less than 0.1%; a load loading and unloading rate of 10kN / s, and a displacement control speed of 50mm / min.

[0060] Specific σ 3+ Direction and σ 3- The maximum test force of the directional stiffness loading system is 2000kN; the load loading and unloading rate is 100kN / s, and the displacement control speed is 500mm / min. The load loading and unloading rate ranges and displacement loading rate ranges in the four directions have no effect on the test results.

[0061] S8: Maintain σ2, σ 3+ Unchanged, unloading at a certain rate σ 3- To the minimum stress σ between the original rock stress and the free surface boundary stress state 3+ and σ 3- The sizes are different to reproduce the unequal coaxial forces on the coal body near the open face after excavation;

[0062] S9: After unloading, the maximum principal stress σ1 is applied at a rate of 0.002 mm / s using displacement control until the specimen fails. The stress and strain data of the specimen are collected using a load sensor and a displacement sensor. The data are transmitted to the computer stress-strain analysis software via a data line for analysis and storage.

[0063] S10: Unload the confining pressure, remove the sample, replace the sample prepared in step S1, and repeat the above steps until all samples are tested;

[0064] S11: Process and analyze sample data to obtain multiple data such as sample elastic modulus, peak strength, residual strength, acoustic emission counts, strain field evolution characteristics, etc.

[0065] Compared with conventional true triaxial tests, the present invention can create a high-gas enclosed environment, restore the three-dimensional ground stress state, and at the same time present a coaxial unequal force state, truly reproducing the entire stress process of the gas-containing coal rock mass during coal mining, "first bearing the three-dimensional initial ground stress, and then mining and unloading to produce coaxial unequal forces." In addition, due to the invisible enclosed structure of conventional true triaxial instruments, the coal sample fracture process and the dynamic evolution characteristics of surface cracks cannot be observed. The present invention realizes real-time and continuous monitoring of the destruction process of the gas-containing coal rock mass by placing a visual side pressure plate on one side of the triaxial test, and analyzes the stress and strain process and surface strain field and crack field distribution of the sample in three directions. At the same time, combined with acoustic emission technology, it further reveals the real-time evolution characteristics of the deformation field, energy field, and crack field of the gas-containing coal mass. The present invention has practical guiding significance and research value for predicting dynamic disasters in deep mining of coal and rock and ensuring underground safety.

[0066] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces, characterized in that: The following steps are involved: Step S1: Prepare the sample and spray DIC scattered spots on the surface, and wrap the sample with a transparent video probe side pressure plate and a tempered fixture; Step S2: After the wrapped sample is sealed, it is transported to the confined pressure chamber via a transport positioning trolley; CH4 is injected into the confined pressure chamber, and the gas injection is stopped after the gas pressure measured by the pressure measuring probe reaches a stable state; Step S3: Start the experiment, synchronously start the acoustic emission device and the binocular miniature high-speed camera inside the transparent video side pressure plate, and use the acoustic emission monitoring software and DIC analysis software to record the acoustic emission and strain field evolution data; Step S4: Control σ1, σ2, σ through the controller and servo valve 3+ and σ 3- The stiffness loading system in the direction of the load is loaded with confining pressure by force control loading, and then σ1, σ2, σ 3+ and σ 3- Load to the preset value and ensure σ 3+ and σ 3- Equal in size; keep σ2 and σ 3+ Unchanged, displacement control is used to unload at a certain rate σ 3- To make the minimum stress σ between the original rock stress and the free surface boundary stress state 3+ and σ 3- After unloading, the maximum principal stress σ1 is applied until the specimen is damaged, and then the stress and strain data of the specimen are collected using a load cell and a displacement sensor. Step S5: Process and analyze the sample data to obtain data such as the sample elastic modulus, peak strength, residual strength, acoustic emission counts, and strain field evolution characteristics.

2. The true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces according to claim 1 is characterized in that: In step S1, DIC specks were sprayed on the sample surface using acrylic spray paint and left to stand for 4 hours. A 260-frame binocular miniature high-speed camera was installed on the inner side of the video probe side pressure plate. The video probe side pressure plates were installed on the upper side pressure plate, the left side pressure plate, and the front side pressure plate of the video probe, respectively, and tempered fixtures were installed on the other sides.

3. The true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces according to claim 1 is characterized in that: In step S2, a sealing device is used to seal the packaged sample.

4. The true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces according to claim 1 is characterized in that: In step S3, the acoustic emission sensor collects signals and transmits them to the acoustic emission monitoring software to analyze the initiation, expansion and rupture stages of the cracks inside the sample; A binocular miniature high-speed camera monitors the position changes of scattered spots on three sides and records the dynamic fracture process of coal samples. The data is transmitted to a computer to use DIC analysis software to analyze its strain field, crack field distribution and compression deformation characteristics, and save acoustic emission and strain field evolution data.

5. The true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces according to claim 1 is characterized in that: The specific steps of step S4 are as follows: Step S41: Use the controller and servo valve to load the confining pressure in four directions in a force control manner, so that σ1, σ2, σ 3+ , σ 3- Load at 0.4 MPa / s until the coaxial forces are equal; Step S42: Maintain σ2 and σ 3+ Unchanged, unloading at a certain rate σ 3- Between the original rock stress and the stress state of the free surface boundary; Step S43: Apply the maximum principal stress σ1 until the sample is broken, and then use the load sensor and displacement sensor to collect stress and strain data of the sample; transmit the data to the computer via the data line, analyze and save it using stress-strain analysis software; Step S44: Unload the confining pressure, remove the sample, replace the sample prepared in step S1, and repeat the above steps S2-S4 until all samples are tested.

6. The true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces according to claim 2 is characterized in that: One side of the transparent video probe side pressure plate is made of PAM material as the side pressure plate. The PAM plate has high transparency and can transmit the confining pressure without being damaged. It contains a 260-frame binocular miniature high-speed camera; the upper pressure plate acoustic emission probe is installed inside the tempered fixture and is evenly distributed at 120°, and the other side pressure plate acoustic emission probes are evenly distributed inside the tempered fixture.

7. The true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces according to claim 1 is characterized in that: In step S2, the air pressure measuring probes are evenly distributed and installed on the tempered fixture in at least 5 lengths, and the air pressure signals are transmitted to the computer through the air pressure measuring connector.

8. The true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces according to claim 3 is characterized in that: Apply silicone sealant to the gap outside the clamp and wrap it with sealing tape.

9. The true triaxial indoor test method for gas-containing coal and rock mass based on coaxial unequal forces according to claim 5 is characterized in that: Six actuators are controlled by four controllers and servo valves, of which the σ1 controller controls the vertical up and down actuators, the σ2 controller controls the left and right actuators, and the σ3 direction is controlled by an independent controller and servo valve. 3+ Controller and servo valve control front actuator, σ 3- The controller and servo valve control the rear actuator to achieve coaxial unequal force loading.

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