Physical model test method for surrounding rock failure mechanism of lamellar counter-inclined slope tunnel

Abstract

The invention discloses a physical model test method for a surrounding rock failure mechanism of a lamellar counter-inclined slope tunnel, and relates to the technical field of rock mass engineering. The invention provides a lamellar counter-tilt slope physical model pouring method, a lamellar rock mass joint weak plane design, a stress path design of a lamellar counter-tilt slope physical model loaded in a three-dimensional physical model biaxial loading testing machine, a slope tunnel excavation means design and a monitoring scheme design. More real and comprehensive test data and a damage process can be obtained, and the reliability of slope stability research under the action of reverse-tilting thin-layer rock slope tunnel excavation is effectively improved. A distributed optical fiber system, a VIC-2D system and an acoustic emission system are adopted to form a multivariate information monitoring system to comprehensively capture information such as surface and internal deformation and internal cracking of surrounding rock in the test process, and an evidence basis is provided for subsequent analysis of a failure mechanism of the surrounding rock of the thin-layer counter-inclined slope tunnel.

Description

Technical Field

[0001] The invention belongs to the technical field of rock mass engineering, and particularly relates to a physical model test method for the failure mechanism of surrounding rock of a tunnel in a thin-layered reverse-inclined slope. Background Technique

[0002] Reverse-inclined slopes are widely distributed and are also relatively common in engineering. In fields such as mining engineering, water conservancy and hydropower engineering, tunnel engineering, and underground oil and gas storage engineering, the surrounding rock mass is mainly composed of layered rocks. The rock mass failure phenomena caused by these engineering activities are closely related to the joint weak planes of the layered rock mass. In most cases, the degree of damage of layered rocks is affected by their structure, and under the action of the disturbance during tunnel excavation and the internal free surface of the slope rock mass generated after excavation, the failure form of layered rocks is more complex and the degree of damage is greater. The bedding dip angle is the main factor affecting the failure of reverse-inclined layered rock mass slopes. Therefore, it is necessary to carry out physical model test studies on layered rock masses with different bedding dip angles to reveal the mutual influence law between the bedding dip angle and the crack initiation and propagation mechanism. The research on the rock failure mechanism and failure process is not only an important basic topic in rock mechanics but also has important guiding significance for the prevention and control of actual rock engineering disasters.

[0003] Chinese Patent CN1437348 discloses a test method for simulating the failure of layered rock masses, which tests the stability of the rock mass by setting the loading scheme of the model on the test bench, but this method does not systematically design the position of the bedding plane, the excavation sequence, and the multi-source monitoring in the model. In addition, Japanese Patent JP2003198761 adopts the method of synchronizing model loading and excavation, but its bedding plane and failure monitoring means are relatively single and cannot fully reflect the influence of the thin-layer structure in complex rock strata, especially the capture of the failure mechanism is limited during the tunnel excavation stage. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the invention provides a physical model test method for the failure mechanism of surrounding rock of a tunnel in a thin-layered reverse-inclined slope, including a physical model casting method for the thin-layered reverse-inclined slope, the design of joint weak planes of the layered rock mass, the stress path design of loading the physical model of the thin-layered reverse-inclined slope on a three-dimensional physical model biaxial loading testing machine, the design of tunnel excavation means for the slope, and the design of a monitoring scheme. This method effectively provides a physical model making method and a loading and monitoring scheme that fully consider the joint weak plane effect of the thin-layered rock mass, can obtain more real and comprehensive test data and failure processes, and effectively improve the reliability of the research on the slope stability under the action of tunnel excavation in a reverse-inclined thin-layered rock slope.

[0005] The technical solution of the invention is as follows:

[0006] A physical model test method for the failure mechanism of surrounding rock of a tunnel in a thin-layered reverse-inclined slope, comprising the following steps:

[0007] Step 1: Fabricate a physical model specimen of a thin-layered overturned slope.

[0008] Step 1.1: Determine the dimensions of the model box, the dimensions of the bedding part in the model box, the dimensions of the tunnel, and the distance between the bottom of the tunnel and the bottom of the bedding part.

[0009] Step 1.2: Fabricate the model box according to the determined dimensions of the model box; the model box is a rectangular metal box with a roofless structure, and the four sides are detachable metal baffles, and each metal baffle is evenly coated with an oily release agent.

[0010] Step 1.3: Determine the similarity material ratio of the studied thin-layered overturned slope, weigh each component material according to the similarity material ratio, first add water to the mixing bucket, and then pour each component material into the mixing bucket in batches and stir, controlling the stirring time within the set range to obtain a mixture.

[0011] Step 1.4: Pour the non-bedding area in the model box according to the dimensions of the bedding part; the non-bedding area is the part in the model box except the bedding part.

[0012] Specifically: Use a baffle to block the bedding part, pour the mixture into the model box that has been pre-coated with an oily release agent in areas, and use a vibrating rod to tamp the mixture until it is dense.

[0013] Step 1.5: Fabricate the bedding part by using the method of pouring in batches and in layers.

[0014] Specifically: According to the dimensions of each thin layer in the bedding part, prepare several wooden boards of different lengths, and cut the two ends at an and angle, and , the cross-section of the wooden board is an isosceles trapezoid, and are the angles between the two waists of the trapezoid and the horizontal direction respectively. Then, fill the bedding part with a mixture in an amount equal to the thickness of one thin layer, use the cut wooden boards to extrude and fix the mixture at the set inclination angle to make it form layers. When the mixture no longer has fluidity, remove the wooden boards, and use a scraper to level the bedding surface, apply a layer of graphite powder to the bedding surface, complete the pouring of one thin layer of the bedding part, and repeat the above steps until all thin layers are poured, and the production of the bedding part is completed.

[0015] Step 1.6: Cure the non-bedding area and the bedding part in the model box for a period of time t, where the period of time t is set according to requirements. After the mechanical properties are stable, demold, and remove the test box to obtain a physical model specimen of a thin-layered overturned slope.

[0016] Step 2: Set an optical fiber in the physical model specimen of the thin-layered reverse-inclined slope and set speckle points and acoustic emission probes on the surface of the physical model specimen of the thin-layered reverse-inclined slope.

[0017] Specifically: According to the tunnel size, scrape out a channel at the set position of the tunnel crown in the physical model specimen of the thin-layered reverse-inclined slope, set an optical fiber in the channel, and then fill the channel with cement; lay the physical model specimen of the thin-layered reverse-inclined slope flat on the ground, polish the tunnel excavation surface smoothly, spray a layer of white paint on the tunnel excavation surface of the physical model specimen of the thin-layered reverse-inclined slope, dry it, use a speckle roller to make black and white, uneven-sized speckle points distributed on the tunnel excavation surface of the physical model specimen of the thin-layered reverse-inclined slope, and use a marking point to coat evenly at the uneven positions of the speckle points; paste an elastic band at the set position on the surface of the physical model specimen of the thin-layered reverse-inclined slope, and place an acoustic emission probe smeared with coupling agent on the elastic band.

[0018] Step 3: Place the physical model specimen of the thin-layered reverse-inclined slope on the three-dimensional physical model biaxial loading testing machine and align it with the bearing plate in the three-dimensional physical model biaxial loading testing machine, adjust the angle of the bearing plate and fix it, and then set up a multi-information monitoring system; the multi-information monitoring system includes an optical fiber system, a non-contact strain measurement system, and an acoustic emission system.

[0019] Setting up the multi-information monitoring system specifically: Connect the optical fiber to the optical fiber system, place the camera in the non-contact strain measurement system directly opposite the speckle points set on the physical model specimen of the thin-layered reverse-inclined slope, and connect the acoustic emission probe to the acoustic emission system.

[0020] Step 4: Use the three-dimensional physical model biaxial loading testing machine to apply stress loading to the physical model specimen of the thin-layered reverse-inclined slope, excavate the tunnel step by step, and use the set multi-information monitoring system to observe and record the excavation process of the physical model specimen of the thin-layered reverse-inclined slope.

[0021] Step 4.1: Start the three-dimensional physical model biaxial loading testing machine to make the horizontal stress σ 1 and the vertical stress σ 2 be loaded simultaneously at a set speed until the set stress is reached and the loading stops.

[0022] Step 4.2: Hold the load for a period of time T, where the period of time T is set according to requirements, and wait until the internal stress adjustment of the physical model specimen of the thin-layered reverse-inclined slope is completed.

[0023] Step 4.3: According to the tunnel size and the distance between the bottom of the tunnel and the bottom of the bedding part, draw the position of the tunnel on the tunnel excavation surface, use a flat-headed metal chisel to chisel along the tunnel position, with the excavation depth each time being the set value. After reaching the ideal depth, use a grinding machine to grind the heading face flat. After each single excavation, visually observe whether there is macroscopic damage to the surrounding rock of the tunnel and take photos with a camera for record. Use the multi-information monitoring system to record the excavation process of the physical model of the thin-layered anti-dip slope until the tunnel excavation is completed.

[0024] The recording of the excavation process of the physical model of the thin-layered anti-dip slope by using the multi-information monitoring system is specifically as follows: During the excavation process, capture the change of the rock mass failure displacement at different positions of the optical fiber over time; track the positions of the speckles on the surface of the physical model specimen of the thin-layered anti-dip slope through the camera in the non-contact strain measurement system; use the acoustic emission system to obtain the elastic stress waves released during the generation, propagation and coalescence of internal cracks in the physical model specimen of the thin-layered anti-dip slope under stress.

[0025] Step 5: After the tunnel excavation is completed, use a three-dimensional physical model biaxial loading testing machine to conduct an overloading stage test on the physical model specimen of the thin-layered anti-dip slope, and at the same time use the set multi-information monitoring system to observe and record the overloading stage of the physical model specimen of the thin-layered anti-dip slope.

[0026] Specifically: After the tunnel excavation is completed, keep the load for a period of time T, load the horizontal stress in stages at a set speed until the horizontal stress reaches the set value, and keep the vertical stress unchanged. After each horizontal stress loading, stay for a period of time T to allow the internal stress of the physical model specimen of the thin-layered anti-dip slope to be readjusted. During the loading process, observe and record whether there is macroscopic damage to the physical model specimen of the thin-layered anti-dip slope, and record the overloading stage of the physical model specimen of the thin-layered anti-dip slope in the same way as in Step 4.3 by using the multi-information monitoring system.

[0027] Step 6: When the overloading stage test is completed, stop loading and remove the bearing plate, and observe and record whether the physical model specimen of the thin-layered anti-dip slope has the characteristics of toppling failure towards the free face.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] ① Through the specimen manufacturing method of the present invention, slope rock mass specimens with various bedding dip angles can be manufactured efficiently and accurately, greatly reducing the influence caused by the inhomogeneity of layered rocks, and better simulating the main failure effect of the bedding weak plane in the slope tunnel project.

[0030] ②Through the loading method of the specimen of the present invention on a three-dimensional physical model biaxial loading testing machine, more realistic and comprehensive test data and failure processes can be obtained, and the reliability of the research on the slope stability under the action of tunnel excavation in an overturned thin-layered rock slope can be effectively improved, and the failure characteristics and disaster-causing mechanisms of the surrounding rock of the tunnel in a layered slope can be effectively explored.

[0031] ③A multi-information monitoring system composed of a distributed optical fiber system, a VIC-2D system, and an acoustic emission system is used to comprehensively capture information such as the surface and internal deformation and internal cracking of the surrounding rock during the test process, providing an evidence basis for the subsequent analysis of the failure mechanism of the surrounding rock of the tunnel in a thin-layered overturned slope. Description of the Drawings

[0032] Figure 1 Schematic diagram of the bedding part and non-bedding part of the physical model of the thin-layered overturned slope in the embodiment of the present invention.

[0033] Figure 2 Schematic diagram of the thin-layered overturned slope physical model after the tunnel excavation is completed in the embodiment of the present invention.

[0034] Figure 3 Schematic diagram of the wooden baffle when pouring in the 1st partition, 2nd partition, 3rd partition, and 4th partition of the non-bedding part in the embodiment of the present invention.

[0035] Figure 4 Schematic diagram of the cutting of the two ends of the wooden baffle in the bedding part in the embodiment of the present invention.

[0036] Figure 5 Schematic diagram of the fiber optic cable burial position at the crown of the tunnel in the embodiment of the present invention.

[0037] Figure 6 Schematic diagram of the acoustic emission probe setting position in the embodiment of the present invention. Detailed Embodiment

[0038] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0039] In this embodiment, a physical model test of a thin-layered overturned slope with a rock layer dip angle of 60° is taken as an example.

[0040] A physical model test method for the failure mechanism of the surrounding rock of a tunnel in a thin-layered overturned slope includes the following steps:

[0041] Step 1: Fabricate a physical model specimen of a thin-layered overturned slope.

[0042] Step 1.1: Determine the size of the model box, the size of the bedding part in the model box, the size of the tunnel, and the distance between the bottom of the tunnel and the bottom of the bedding part.

[0043] In this embodiment, the size of the model box is determined to be 300 mm × 300 mm × 250 mm according to the three-dimensional physical model biaxial loading platform used. As Figure 1 shown, the size of the bedding part is determined to be 120 mm × 180 mm × 200 mm, and the size of the tunnel is as Figure 2 , which is 60 mm × 90 mm, and the distance between the bottom of the tunnel and the bottom of the bedding part is 50 mm.

[0044] Step 1.2: Since the physical model of the thin-layered anti-dipping slope is constructed by layered casting, it needs to be operated in a fixed model box. According to the determined size of the model box, the model box is fabricated. The model box is a cuboid metal box with a roofless structure, and the four sides are detachable metal baffles. Each metal baffle is evenly coated with an oily release agent.

[0045] In this embodiment, each metal baffle of the model box needs to be cleaned with dishwashing liquid and dried in the sun to make each metal baffle clean and dry, and then each metal baffle of the model box is evenly coated with an oily release agent for standby.

[0046] Step 1.3: Determine the similarity material ratio of the studied thin-layered anti-dipping slope, weigh each component material according to the similarity material ratio, first add water to the mixing bucket and then pour each component material into the mixing bucket in batches and stir, avoiding uneven stirring caused by sedimentation of materials when pouring in large quantities, and control the stirring time within the set range to obtain the mixture.

[0047] In this embodiment, the similarity material ratio of the studied thin-layered anti-dipping slope is determined, and each component material is weighed according to the similarity material ratio, that is, the weight ratio of feldspar, quartz, barite powder, 425 cement and high-definition gypsum is 27:25:33:6:9. Weigh each component material and stir evenly. When stirring, add water first and then add materials, and pour them into the mixing bucket in batches to avoid uneven stirring caused by sedimentation of materials when pouring in large quantities. Stop the mixer during stirring, scrape off the mixture sticking to the wall of the mixing bucket with a spatula, and then start the mixer to continue stirring. The single-bucket stirring time is controlled at about 5 minutes.

[0048] Step 1.4: Pour the non-bedding area in the model box according to the size of the bedding part; the non-bedding area is the part in the model box except the bedding part.

[0049] Specifically: Use baffles to block the bedding part, and pour the mixture into the model box that has been pre-coated with an oily release agent in areas, and use a vibrating rod to tamp the mixture densely.

[0050] In this embodiment, in accordance with Figure 3As shown in the area, pour the mixture into the model box which has been spliced ​​and coated with oily release agent in advance. The pouring order is partition 1-partition 2-partition 3-partition 4. Use wooden baffles with a thickness of 10mm to block the areas. After the previous area stops flowing and has a certain strength, place the baffle and pour the next area. When pouring areas 2, 3, and 4, place an object with a certain strength on the side where the mixture is not poured to support the wooden board to prevent the position of the wooden board from changing. After pouring the concrete, use a vibrator to compact the mixture and then use a scraper to smooth the surface.

[0051] Step 1.5: Use batch and layered casting method to make the layered part.

[0052] Specifically: according to the size of each thin layer in the bedding part, prepare several wooden boards of different lengths and press their ends and Angle cutting, and , the cross section of the board is an isosceles trapezoid, and The angles between the two waists of the trapezoid and the horizontal direction are respectively set, and then a thin layer of mixture is filled in the bedding part. At this time, the mixture is fluid. Use the cut wooden boards to extrude and fix the mixture at the set inclination angle to fix it into layers. When the mixture is no longer fluid, remove the wooden boards, and use a scraper to flatten the bedding surface. Apply a layer of graphite powder on the bedding surface to complete the pouring of a thin layer of the bedding part. Repeat the above steps until all thin layers are poured and the bedding part is completed.

[0053] In this embodiment, for the bedding area, batch and layer casting is adopted, and wooden boards of different lengths and thicknesses of 10 mm are designed, and the angles of the two ends thereof are cut at 30° and 60°, such as Figure 4 As shown, the section of the wooden board is an unequal trapezoid. That is, according to the designed inclination angle and thin layer thickness, apply the mixture and firmly place the wooden board coated with the release agent, squeeze out the excess mixture and clean up the overflowed mixture with a scraper. When the poured mixture is no longer fluid, remove the wooden board, scrape the bedding surface with a scraper, apply a layer of graphite powder on its surface, and repeat the above steps until the model is completely poured.

[0054] Step 1.6: Curing the non-stratified area and the stratified part in the model box at room temperature for a period of time t, wherein the period of time t is set according to the demand, and demoulding is performed after the mechanical properties are stabilized, and the test box is removed to obtain a thin layered reverse slope physical model specimen.

[0055] In this embodiment, the curing period is 28 days.

[0056] Step 2: Set an optical fiber in the physical model specimen of the thin-layered reverse-inclined slope and set speckle points and acoustic emission probes on the surface of the physical model specimen of the thin-layered reverse-inclined slope.

[0057] Specifically: According to the tunnel size, scrape out a channel at the set position of the tunnel crown in the physical model specimen of the thin-layered reverse-inclined slope, set an optical fiber in the channel, and then fill the channel with cement; lay the physical model specimen of the thin-layered reverse-inclined slope flat on the ground, polish the tunnel excavation surface smoothly, spray a layer of white paint on the tunnel excavation surface of the physical model specimen of the thin-layered reverse-inclined slope, let it dry, use a speckle roller to make black and white, uneven-sized speckle points distributed on the tunnel excavation surface of the physical model specimen of the thin-layered reverse-inclined slope, and use a marking pen to coat evenly at the uneven positions of the speckle points; paste elastic bands at the set positions on the surface of the physical model specimen of the thin-layered reverse-inclined slope, and place acoustic emission probes smeared with coupling agent on the elastic bands.

[0058] In this embodiment, at Figure 5 a 10 mm height and 10 mm depth at the tunnel crown as shown, use a screwdriver to scrape out an extremely thin channel and set an optical fiber; at the four corners of the front and back of the physical model specimen of the thin-layered reverse-inclined slope, paste an elastic band smeared with glue on both sides and with a space in the middle for placing an acoustic emission probe with elastic bands and 502 glue respectively, wait for the glue to dry completely, and place an acoustic emission probe smeared with coupling agent. The positions of the acoustic emission probes are as Figure 6 shown.

[0059] Step 3: Place the physical model specimen of the thin-layered reverse-inclined slope on a three-dimensional physical model biaxial loading testing machine and align it with the bearing plate in the three-dimensional physical model biaxial loading testing machine, adjust the angle of the bearing plate and fix it, and then set up a multi-information monitoring system; the multi-information monitoring system includes an optical fiber system, a non-contact strain measurement system, and an acoustic emission system.

[0060] Setting up the multi-information monitoring system specifically includes: connecting the optical fiber to the optical fiber system, placing the camera in the non-contact strain measurement system directly opposite the speckle points set on the physical model specimen of the thin-layered reverse-inclined slope, and connecting the acoustic emission probe to the acoustic emission system.

[0061] In this embodiment, the acoustic emission system used is the PCI-2 type system developed by Physical Acoustics Corporation of the United States, the probe model is G8, and a total of 8 probes are arranged at the four corners of the free face and the back of the physical model specimen of the thin-layered reverse-inclined slope; the optical fiber system is a DFOS system using Rayleigh scattering technology, and the equipment model is Luna Optical Backscatter Reflectometer4600 (OBR 4600); the non-contact strain measurement system is a VIC-2D system.

[0062] Step 4: Use a three-dimensional physical model biaxial loading testing machine to apply stress loading to the physical model specimen of the thin-layered anti-dip slope, excavate the tunnel step by step, and use the set multi-information monitoring system to observe and record the excavation process of the physical model specimen of the thin-layered anti-dip slope.

[0063] Step 4.1: Start the three-dimensional physical model biaxial loading testing machine, and make the horizontal stress ( σ 1 ) and the vertical stress ( σ 2 ) be loaded simultaneously at a set speed until the set stress is reached and the loading stops.

[0064] In this embodiment, the horizontal stress ( σ 1 ) and the vertical stress ( σ 2 ) are loaded simultaneously at a speed of 0.2 kN / s. When = 1 MPa, = 0.5 MPa, the loading stops.

[0065] Step 4.2: Keep the load for a period of time T, where the period of time T is set according to requirements, and wait for the internal stress of the physical model specimen of the thin-layered anti-dip slope to be adjusted.

[0066] In this embodiment, the load is kept for 5 minutes.

[0067] Step 4.3: According to the tunnel size and the distance between the bottom of the tunnel and the bottom of the bedding part, draw the tunnel position on the tunnel excavation surface, use a flat-headed metal chisel to cut along the tunnel position, and the excavation depth each time is the set value. After reaching the ideal depth, use a grinding machine to smooth the face of the tunnel. After each single excavation, visually observe whether there is macroscopic damage to the tunnel surrounding rock and take pictures using a camera (except for another camera in the non-contact strain measurement system). Use the multi-information monitoring system to record the excavation process of the physical model of the thin-layered anti-dip slope until the tunnel excavation is completed.

[0068] The specific process of using the multi-information monitoring system to record the excavation process of the physical model of the thin-layered anti-dip slope is as follows: During the excavation process, capture the change of the rock mass failure displacement at different positions of the optical fiber over time; track the position of the surface scatter spots of the physical model specimen of the thin-layered anti-dip slope through the camera in the non-contact strain measurement system, and finally analyze the evolution law of the surface deformation field of the physical model specimen of the thin-layered anti-dip slope during the test process through software; use the acoustic emission system to obtain the elastic stress waves released during the generation, propagation, and combination of internal cracks in the physical model specimen of the thin-layered anti-dip slope during the stress application process, which can intuitively reflect the internal damage evolution process of the model during the entire loading stage.

[0069] In this embodiment, the excavation depth for each time is 20 mm, and a total of 9 excavation steps are carried out with a total excavation depth of 180 mm. The time interval between adjacent steps is 5 min.

[0070] Step 5: After the tunnel excavation is completed, use a three-dimensional physical model biaxial loading testing machine to conduct an overloading stage test on the physical model specimen of the thin-layered anti-dipping slope, and at the same time use the set multi-information monitoring system to observe and record the overloading stage of the physical model specimen of the thin-layered anti-dipping slope.

[0071] Specifically: After the tunnel excavation is completed, keep the load for a period of time T, and load the horizontal stress in stages at a set speed until the horizontal stress reaches the set value, and the vertical stress remains unchanged. After each horizontal stress loading, stay for a period of time T to allow the internal stress of the physical model specimen of the thin-layered anti-dipping slope to be readjusted. Observe and record whether there is macroscopic damage to the physical model specimen of the thin-layered anti-dipping slope during the loading process, and use the multi-information monitoring system to record the overloading stage of the physical model specimen of the thin-layered anti-dipping slope in the same way as in step 4.3.

[0072] In this embodiment, after the tunnel excavation, keep the load for 5 minutes. Load the horizontal stress in stages ( σ 1 ), load at a speed of 0.2 kN / s, load 0.5 MPa each time, and a total of 4 levels are loaded until the horizontal stress is loaded to 3 MPa. The vertical loading is loaded to 0.5 MPa in the initial loading stage and then the stress remains unchanged. Observe and record whether there is macroscopic damage to the physical model specimen of the thin-layered anti-dipping slope during the loading process, and stay for 5 minutes after each loading to allow the internal stress of the model to be readjusted.

[0073] Step 6: The loading plate has a certain fixing effect on the physical model specimen of the thin-layered anti-dipping slope. When the overloading stage test is completed, retract the loading device in the three-dimensional physical model biaxial loading testing machine, stop the oil pump valve, remove the loading plate, and observe and record whether the physical model specimen of the thin-layered anti-dipping slope has the characteristics of toppling failure towards the free face.

Claims

1. A physical model test method for the failure mechanism of surrounding rock of a thin layered reverse slope tunnel, characterized in that: The following steps are involved: Step 1: Prepare a thin layered reverse slope physical model specimen; Step 2: an optical fiber is arranged in the thin-layered reverse-dip slope physical model sample and a speckle and acoustic emission probe are arranged on the surface of the thin-layered reverse-dip slope physical model sample; Step 3: Place the thin layered reverse slope physical model specimen on the three-dimensional physical model biaxial loading test machine and align it with the pressure plate in the three-dimensional physical model biaxial loading test machine, adjust the angle of the pressure plate and fix it, and then set up the multi-information monitoring system; Step 4: Use a three-dimensional physical model biaxial loading test machine to stress load the thin-layered anti-dip slope physical model sample, excavate the tunnel step by step, and use the set multi-information monitoring system to observe and record the excavation process of the thin-layered anti-dip slope physical model sample; Step 5: After the tunnel excavation is completed, the overload stage test is carried out on the thin-layered anti-dip slope physical model sample using a three-dimensional physical model biaxial loading test machine, and the overload stage of the thin-layered anti-dip slope physical model sample is observed and recorded using the set multi-information monitoring system; Step 6: After the overload stage test is completed, stop loading and remove the bearing plate, observe and record whether the thin layered reverse slope physical model specimen has the characteristics of tipping over and failing toward the free surface.

2. A physical model test method for the failure mechanism of surrounding rock of a thin layered reverse slope tunnel according to claim 1, characterized in that: The multi-information monitoring system described in step 3 includes an optical fiber system, a non-contact strain measurement system and an acoustic emission system.

3. The physical model test method for the failure mechanism of thin layered reverse slope tunnel surrounding rock according to claim 1 is characterized in that: Step 1 specifically includes: Step 1.1: Determine the size of the model box, the size of the bedding part in the model box, the size of the tunnel, and the distance between the bottom edge of the tunnel and the bottom edge of the bedding part; Step 1.2: According to the determined model box size, a model box is made; the model box is a rectangular metal box with a topless structure and detachable metal baffles on all sides, and each metal baffle is evenly coated with an oily release agent; Step 1.3: Determine the similar material ratio of the thin layered reverse slope under study, and weigh each component material according to the similar material ratio. First add water to the mixing barrel, then pour each component material into the mixing barrel in batches and stir, and control the stirring time within the set range to obtain a mixture; Step 1.4: Casting a non-bedding area in the model box according to the size of the bedding part; the non-bedding area is the part of the model box other than the bedding part; Specifically: use a baffle to block the layered part, and pour the mixture into the model box with oil release agent applied in advance in different areas, and use a vibrating rod to compact the mixture; Step 1.5: Using batch and layer casting method to make the layered part; Step 1.6: Curing the non-stratified area and the stratified part in the model box for a period of time t, wherein the period of time t is set according to the demand, and demoulding is performed after the mechanical properties are stabilized, and the test box is removed to obtain a thin layered reverse slope physical model specimen.

4. A physical model test method for the failure mechanism of surrounding rock of a thin layered reverse slope tunnel according to claim 3, characterized in that: Step 1.5 is as follows: Prepare several wooden boards of different lengths according to the size of each thin layer in the bedding part, and press the ends of the boards together. and Angle cutting, and , the cross section of the board is an isosceles trapezoid, and The angles between the two waists of the trapezoid and the horizontal direction are respectively set, and then the bedding part is filled with a mixture of a thin layer thickness. The cut wooden boards are used to extrude and fix the mixture at the set inclination angle to fix it into layers. When the mixture is no longer fluid, the wooden boards are removed, and the bedding surface is scraped flat with a scraper. A layer of graphite powder is applied to the bedding surface to complete the pouring of a thin layer of the bedding part. The above steps are repeated until the pouring of all thin layers is completed and the bedding part is completed.

5. The physical model test method for the failure mechanism of thin layered reverse slope tunnel surrounding rock according to claim 1 is characterized in that: Step 2 is specifically as follows: according to the tunnel size, a channel is scraped out at the set position of the tunnel vault in the thin-layered anti-dip slope physical model sample, and an optical fiber is set in the channel, and then the channel is filled with cement; the thin-layered anti-dip slope physical model sample is placed flat on the ground, the tunnel excavation surface is polished flat, a layer of white paint is sprayed on the tunnel excavation surface of the thin-layered anti-dip slope physical model sample, and dried, and a speckle roller is used to distribute black and white scattered spots of different sizes on the tunnel excavation surface of the thin-layered anti-dip slope physical model sample, and the uneven scattered spots are evenly painted with markers; an elastic band is pasted at the set position on the surface of the thin-layered anti-dip slope physical model sample, and an acoustic emission probe coated with a coupling agent is placed on the elastic band.

6. The physical model test method for the failure mechanism of surrounding rock of a thin layered reverse slope tunnel according to claim 2 is characterized in that: The setting of the multi-information monitoring system described in step 3 is specifically as follows: connecting the optical fiber to the optical fiber system, placing the camera in the non-contact strain measurement system directly opposite to the scattered spots set in the thin layered reverse slope physical model specimen, and connecting the acoustic emission probe to the acoustic emission system.

7. The physical model test method for the failure mechanism of thin layered reverse slope tunnel surrounding rock according to claim 2 is characterized in that: Step 4 specifically includes: Step 4.1: Start the three-dimensional physical model biaxial loading test machine to make the horizontal stress σ 1 and vertical stress σ 2 Load at the set speed at the same time until the set stress is reached and then stop loading; Step 4.2: Maintain the load for a period of time T, which is set according to the demand, until the internal stress adjustment of the thin layered reverse slope physical model specimen is completed; Step 4.3: According to the tunnel size and the distance between the bottom edge of the tunnel and the bottom edge of the bedding part, draw the tunnel position on the tunnel excavation surface, and use a flat-head metal chisel to excavate along the tunnel position. The excavation depth each time is the set value. After reaching the ideal depth, use a grinder to grind the heading surface flat. After a single excavation is completed, observe whether there is macroscopic damage to the tunnel surrounding rock with the naked eye and take photos with a camera to record it. Use a multi-information monitoring system to record the excavation process of the thin-layered anti-dip slope physical model until the tunnel excavation is completed.

8. A physical model test method for the failure mechanism of surrounding rock of a thin layered reverse slope tunnel according to claim 7, characterized in that: The excavation process of the thin-layered anti-dip slope physical model is recorded by using the multi-information monitoring system as described in step 4.

3. Specifically, the rock failure displacement changes over time at different positions of the optical fiber are captured during the excavation process; the positions of scattered spots on the surface of the thin-layered anti-dip slope physical model specimen are tracked by the camera in the non-contact strain measurement system; and the elastic stress waves released when the internal cracks of the thin-layered anti-dip slope physical model specimen are generated, expanded and combined during the stress process of the thin-layered anti-dip slope physical model specimen are obtained by using the acoustic emission system.

9. A physical model test method for the failure mechanism of surrounding rock of a thin layered reverse slope tunnel according to claim 7, characterized in that: Step 5 is as follows: after the tunnel excavation is completed, the load is maintained for a period of time T, and the horizontal stress is loaded in stages according to the set speed until the horizontal stress reaches the set value, and the vertical stress remains unchanged. After each horizontal stress loading, it is kept for a period of time T to readjust the internal stress of the thin-layered anti-dip slope physical model specimen. During the loading process, it is observed and recorded whether the thin-layered anti-dip slope physical model specimen has macroscopic damage, and the overload stage of the thin-layered anti-dip slope physical model is recorded using the multivariate information monitoring system in the same way as in step 4.3.

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