Method for analyzing bearing and supporting effect of jet anchor support in large-size tunnel model test

Abstract

This invention belongs to the field of tunnel engineering testing technology and discloses a method for analyzing the load-bearing capacity and support effect of shotcrete-anchored support in large-scale tunnel model tests. The method includes: S1) determining the loading device and its parameters for the model test; S2) fabricating strain brick test blocks, shotcrete test blocks, and conducting tensile tests on the anchor rods; S3) filling the surrounding rock with material and fine river sand; S4) embedding strain brick test blocks and earth pressure cells; S5) pre-embedding the tunnel mold and anchor rods; S6) applying the shotcrete material; S7) installing displacement gauges; S8) applying graded loading until the shotcrete-anchored support material is destroyed; and S9) monitoring the stress and strain of the surrounding rock and observing whether the surrounding rock or shotcrete structure shows signs of failure. This invention allows direct observation and monitoring of the deformation, failure process, and load-bearing capacity of shotcrete-anchored support under different working conditions, enabling more accurate prediction and evaluation of the load-bearing capacity and support effect of shotcrete-anchored support. It contributes to a deeper understanding of the working mechanism and performance of shotcrete-anchored support, improving the accuracy and reliability of engineering design.

Description

Technical Field

[0001] This invention belongs to the field of tunnel engineering testing technology, specifically relating to a method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests. Background Technology

[0002] Shotcrete and shotcrete support is a commonly used support method in tunnel engineering. It combines rock bolts, shotcrete, and surrounding rock into a unified structure. These three elements work together to improve the tunnel's load-bearing capacity and stability, preventing collapse and slippage due to insufficient load-bearing capacity. After applying rock bolts and shotcrete support, a certain thickness of surrounding rock can be transformed into a self-supporting arch, effectively stabilizing the surrounding rock. The anchored section of the rock bolt effectively transfers loads through friction with the surrounding rock, increasing the lateral confinement force of the surrounding rock. The shotcrete protects the tunnel's free face. When the rock mass is relatively fractured, wire mesh can be used to hold back small rock fragments between the rock bolts, enhancing the shotcrete's support effect on the rock mass.

[0003] The load-bearing capacity of shotcrete-anchor support systems has a significant impact on the safety and economy of tunnel engineering. The load-bearing capacity of these systems is closely related to the tensile, shear, and bending mechanical properties of components such as anchor bolts, shotcrete layers, and reinforcing mesh. In practical engineering, insufficient load-bearing capacity in the designed shotcrete-anchor support system can lead to deformation, damage, and collapse during tunnel operation, thus affecting tunnel safety. Conversely, excessive load-bearing capacity significantly increases project costs and causes unnecessary waste. Therefore, there is an urgent need for a method to accurately calculate and analyze the load-bearing capacity of shotcrete-anchor support systems. However, currently, there is a lack of precise quantitative analysis methods for the load-bearing capacity of shotcrete-anchor support systems. Traditional shotcrete-anchor support design often relies on empirical formulas and theoretical calculations, which have limitations in practical applications, such as poor accuracy and high computational complexity, and cannot meet engineering requirements. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-scale tunnel model tests, thereby resolving the issues in the prior art. The technical solution adopted by this invention is as follows:

[0005] Methods for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-scale tunnel model tests, including:

[0006] S1), determine the loading device and its parameters for the model test;

[0007] S2), to prepare strain brick test blocks, sprayed layer test blocks and conduct tensile tests on anchor rods;

[0008] S3), filling material and fine river sand;

[0009] S4), embed strain brick test blocks and earth pressure cells;

[0010] S5), pre-embedded tunnel molds and anchor bolts;

[0011] S6), apply the spray coating material;

[0012] S7), Install the displacement gauge;

[0013] S8), graded loading until the shotcrete support material is destroyed;

[0014] S9) Monitor the stress and strain of the surrounding rock, observe whether the surrounding rock or sprayed layer structure is damaged, record the vertical load when damage occurs, and analyze the stress at the crown of the surrounding rock, the displacement at the crown, the internal force of the bearing arch, the axial force of the anchor bolts, and the stress of the sprayed layer.

[0015] This invention offers the following advantages: It relates to a method for analyzing the bearing capacity and support effect of shotcrete-anchored support based on large-scale model tests. Through large-scale model tests, the deformation, failure process, and bearing capacity of shotcrete-anchored support under different working conditions can be directly observed and monitored. Therefore, the bearing capacity and support effect of shotcrete-anchored support can be predicted and evaluated more accurately, contributing to a deeper understanding of its working mechanism and performance, thereby improving the accuracy and reliability of engineering design. Simultaneously, by changing test parameters and working conditions, different design and optimization schemes can be explored to improve the bearing capacity and support effect of shotcrete-anchored support. Furthermore, the results of large-scale model tests can verify and correct existing theoretical calculations and empirical formulas. By comparing with actual test results, the accuracy of theoretical analysis and empirical formulas can be evaluated, and they can be improved and perfected. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the model test setup;

[0017] Figure 2 Stress-strain curve of a 3mm zinc rod;

[0018] Figure 3 Layout diagram for tunnel stress-strain and displacement monitoring;

[0019] Figure 4 Anchor bolt monitoring layout diagram;

[0020] Figure 5 This is a schematic diagram of the test loading.

[0021] Figure 6 The vertical load-radial stress curve is shown at 13cm from the top of the surrounding rock arch.

[0022] Figure 7 The vertical load-tangential stress curve is shown at 13cm from the top of the surrounding rock arch.

[0023] Figure 8 This is a vertical load-displacement curve diagram of the arch crown of the surrounding rock.

[0024] Figure 9 The diagram shows the internal forces and bending moments of the bearing arch when the shotcrete anchor-rock bearing arch reaches its bearing capacity.

[0025] Figure 10 The axial force of each anchor bolt when the shotcrete-anchor rock bearing arch reaches its bearing capacity;

[0026] Figure 11 This is a diagram of the internal forces in the sprayed layer when the anchor-rock bearing arch reaches its bearing capacity. Detailed Implementation

[0027] The following will be based on embodiments of the present invention. Figures 1-11 The technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0028] The model test provided by this invention has an outer profile cross-section width of 1.17m, a height of 0.97m, and an axial depth of 0.45m.

[0029] This invention provides a method for analyzing the bearing capacity and support effect of shotcrete and anchor support based on large-scale model tests. The detailed process of this method is as follows:

[0030] S1) Based on the required working conditions, the model test device and its parameters are determined. The equipment used in this embodiment of the invention mainly consists of a loading platform, a model box, and a hydraulic loading system. The loading platform has dimensions of approximately 4.10m × 0.99m × 3.80m (length × width × height) and contains a model box with dimensions of 2.08m × 0.45m × 1.98m (length × width × height). Figure 1 As shown. The front of the platform is equipped with a removable tunnel model cover plate, and the front is equipped with an plexiglass observation window. The back of the platform is equipped with four removable movable panels to facilitate the construction of filling materials.

[0031] In S1), the load required for the test is provided by a hydraulic loading system. The hydraulic loading system has two hydraulic oil sources: one for the left and right jacks and one for the top jack, allowing for independent servo loading of the left and right jacks and the top jack. The maximum load is 1 MPa, and the strokes of the left and right jacks and the top jack are 15 cm and 63 cm, respectively. During operation, the bottom of the force transmission plate is in close contact with the model body, and the top beam and side beams are connected to the jacks. The load of the jacks is effectively transferred to the test model body through the force transmission plate. A 31.5 MPa force sensor is used to monitor and provide feedback on the jack pressure, with a measurement accuracy of 0.5% FS. (Reference) Figure 1 The loading platform is located on both sides of the model box, while the hydraulic loading system is located above and on both sides of the model box, applying pressure to the model body from the outside in. During the experiment, surrounding rock material, fine sand material, sprayed layer material, and anchor bolt material are filled into the model box to form the model body, which is then embedded in the loading platform. During operation, the hydraulic loading system provides the load and transfers it to the area around the model body, while the loading platform is responsible for bearing the reaction force from the hydraulic loading system. The hydraulic loading system mainly includes an oil source, jacks, and force transmission plates. The stress-strain measurement system mainly consists of monitoring elements such as earth pressure cells and strain bricks.

[0032] In step S1), to compare and analyze the support effect of shotcrete and rock bolt support, tests were conducted on three conditions: a rough tunnel and two different rock bolt densities. The three conditions were compared with each other. In the shotcrete-rock bolt bearing arch and shotcrete-dense rock bolt bearing arch conditions, the circumferential spacing of the rock bolts was 0.176m and 0.113m, respectively, arranged in 3 rows and 4 rows along the tunnel axis, with a total of 36 and 92 rock bolts, respectively. The rock bolts were arranged in a staggered pattern.

[0033] S2), to prepare strain brick test blocks, sprayed layer test blocks, and to conduct tensile tests on anchor bolt materials.

[0034] S21) Strain brick test blocks and sprayed layer test blocks were made in advance using molds to measure the internal strain of the surrounding rock and the strain on the inner and outer sides of the sprayed layer.

[0035] Among them, the strain brick test blocks were made of surrounding rock materials with a mass ratio of barite: river sand: fly ash: saturated rosin alcohol solution: machine oil = 1013:506:380:120:80. The mass ratio of the sprayed layer test block materials was: barite: river sand: gypsum: water: polycarboxylate superplasticizer: hydroxypropyl methylcellulose = 5400:1350:7400:3000:74:37.

[0036] After the strain brick test block and the sprayed layer test block are air-dried and cured, strain gauges of model BE120-3CA are attached to their surfaces.

[0037] S22), a tensile test was conducted on the anchor rod material in advance. A zinc rod with a length of 360 mm and a diameter of 3 mm was selected as the anchor rod material. Before the test, strain gauges were attached to the upper, middle, and lower parts of the zinc rod. The 3 mm zinc rod was then stretched using a universal testing machine to obtain its stress-strain curve. Figure 2 As shown, the elastic modulus, tensile stiffness, yield strength and ultimate tensile strength of the anchor material can be calculated. At the same time, the stress condition of the anchor during the subsequent model test loading process can be known by referring to the stress-strain curve of the zinc rod.

[0038] The physical and mechanical parameters of the surrounding rock, shotcrete, and anchor bolt test materials are shown in Table 1:

[0039] Table 1 Physical and mechanical parameters of the model test materials

[0040]

[0041] S3), filling surrounding rock material and fine river sand.

[0042] S31) Fine river sand is filled on both sides of the model box so that the load applied by the jack can be transferred to the surrounding rock through the fine river sand. In order to leave space for filling sand on both sides of the model box, steel baffles are first used to separate the surrounding rock material from the fine river sand. The surrounding rock material is then filled in layers inside the model box, with a layer height of about 5cm, and is repeatedly leveled and compacted with a tamping hammer.

[0043] S32) After the surrounding rock material is compacted to the design elevation of each layer, the steel baffle is removed, and PET sheets are placed on both sides of the surrounding rock material to reduce the friction between the fine river sand and the surface of the surrounding rock material, so as to ensure that the fine river sand and the surrounding rock material transmit only radial force as much as possible.

[0044] S33), after the PET sheet is placed, fill the two sides of the model box with fine river sand with good force transmission properties, and flatten and compact it.

[0045] S4), install strain gauge bricks and earth pressure cells. In step S3, when the surrounding rock is filled to a certain height, install the tunnel stress-strain and displacement monitoring layout diagram ( Figure 3 Strain gauge bricks and earth pressure cells are embedded at designated elevations to monitor the stress conditions within the surrounding rock. The earth pressure cells can directly monitor the vertical or lateral stress within the surrounding rock, while the strain gauge bricks can detect strain at 0°, 45°, and 90° angles at the monitoring locations. Figure 3 The tunnel stress-strain and displacement monitoring layout diagram shows that a monitoring section is set up at a position of 0.225m along the tunnel axis, with a total of 27 earth pressure cells (triangular) and 12 strain bricks (circular).

[0046] S5), pre-embedded tunnel molds and anchor bolts.

[0047] In step S3 (S51), when the surrounding rock material is filled to the bottom of the arch, a columnar high-strength foam mold with the same cross-sectional shape as the tunnel is pre-embedded to simulate the horseshoe shape of the tunnel. This tunnel mold has a certain resistance to deformation and is easy to cut and excavate, facilitating the subsequent removal of the mold. And according to the anchor bolt monitoring layout diagram (… Figure 4 Mark the position of each anchor rod on the tunnel mold in advance.

[0048] S52), the anchor bolts are arranged in a quincunx pattern, with a total of 3 cross-sections. To accurately monitor the stress on the anchor bolts during loading, the anchor bolt monitoring layout diagram is used ( Figure 4 Six monitoring anchors were installed at the arch crown, arch shoulder, and arch waist on two monitoring sections respectively, and strain gauges of model BE120-3CA were pre-attached to the upper, middle, and lower parts of these six anchors.

[0049] S53), embed anchor material (360mm long, 3mm diameter zinc rod). Before filling the surrounding rock, insert one end of an iron pipe with an outer diameter slightly larger than the outer diameter of the anchor material into the marked point of the tunnel mold. For example, insert one end of an iron pipe with a length of 360mm and an outer diameter of 4mm into the marked point of the tunnel mold, and ensure that each iron pipe is perpendicular to the cut surface at the marked point on the tunnel mold.

[0050] S54) Fill the surrounding rock material around the iron pipe and compact it. When the other end of the iron pipe is about to be buried by the surrounding rock material, slowly pull out the iron pipe to prevent the hole from collapsing and form a hole with the same outline as the anchor rod.

[0051] S55), inject epoxy resin AB glue, the anchor rod adhesive, into the hole, slowly insert the anchor rod into the hole, and bring the adhesive into the hole along with it until the other end of the anchor rod contacts the surface of the tunnel mold.

[0052] S56) After all the surrounding rock has been filled, let it sit for 12 hours, then melt the foam mold with a hot air gun. After the tunnel is formed, attach the anchor bolt trays near the tunnel wall.

[0053] S6), apply the spray coating material;

[0054] S61) To facilitate the application of the spray coating material, a 1mm thick layer of plaster is first sprayed onto the tunnel wall using a syringe.

[0055] S62), after the gypsum has cured and dried, place the sprayed layer test block with strain gauges attached in step S21 and the earth pressure cell according to the tunnel stress-strain and displacement monitoring layout diagram ( Figure 3 They are then attached to the plaster surface.

[0056] S63), fix the wire mesh, and then apply the spray material to the specified thickness, such as 2mm.

[0057] S64), and finally place for 24 hours to allow the sprayed material to completely dry and cure.

[0058] S7) Displacement gauges were installed to capture the tunnel displacement during the model test loading process. YWC-30 strain gauge displacement sensors were used for measurement, with a range of 0-30 mm. Positive values ​​were recorded for deformation of the surrounding rock converging inwards into the tunnel, and negative values ​​for deformation outwards. The displacement gauges were mainly arranged at the arch crown, arch waist, arch bottom, and at a 45° angle to the arch shoulder. Specific monitoring arrangements are as follows... Figure 3 As shown.

[0059] S8), test loading: to simulate the real stress environment, the top and side jacks of the model box are loaded simultaneously. The lateral pressure coefficient is 0.44, that is, the horizontal load is 0.44 times the vertical load, and servo control is used.

[0060] The experimental loading diagram is as follows Figure 5 As shown, the vertical load was applied starting at 50 kPa, increasing by 10 kPa in each increment; the lateral load was applied starting at 22 kPa, increasing by approximately 4.4 kPa in each increment. Each load increment was maintained for 20 minutes. During the loading process, the failure of the surrounding rock and the shotcrete support structure was closely observed, and the failure process was recorded using a camera. Detailed records of the failure phenomena occurring during the loading process were also kept on the test record sheet. The tunnel was considered to have reached its bearing capacity when the surrounding rock could no longer maintain its existing cross-sectional shape and failed; when a section of the shotcrete structure first showed signs of crushing, cracking, or spalling; or when the tunnel collapsed. The test ended when the tunnel completely collapsed.

[0061] S9), the method for analyzing the bearing capacity and support effect of shotcrete and anchor support based on large-scale model tests mainly includes the following aspects:

[0062] S91) Determine the bearing capacity of the shotcrete and anchor support. During the loading process, focus on observing the damage to the shotcrete support structure. The vertical load corresponding to the first obvious damage phenomenon such as crushing, cracking or spalling of the shotcrete structure during the loading process is taken as the bearing capacity of the shotcrete and anchor support. Although the tunnel may not have collapsed at this time, it has lost its basic function. Therefore, it is considered that the bearing capacity of the shotcrete and anchor support has been reached at this time.

[0063] In this embodiment, for the shotcrete-anchored rock bearing arch condition, when the vertical load reaches 290 kPa, a large area of ​​the shotcrete layer on the left arch shoulder experiences spalling, exposing the inner steel mesh. Therefore, the bearing capacity of the shotcrete-anchored rock bearing arch is determined to be 290 kPa. For the shotcrete-dense anchored rock bearing arch condition, when the vertical load reaches 330 kPa, a large area of ​​the shotcrete layer on the right arch shoulder experiences spalling, accompanied by extensive exposure of the wire mesh, which is severely twisted. This is determined to be crushing failure, and the bearing capacity of the shotcrete-anchored rock bearing arch is determined to be 330 kPa. Furthermore, comparison shows that shotcrete-anchored support can significantly improve the tunnel's bearing capacity. Additionally, after densifying the anchor bolts, the bearing capacity of the shotcrete-anchored support also shows a certain improvement.

[0064] S92), Stress analysis of the surrounding rock at the arch crown: Using the strain at 0°, 45° and 90° monitored by the strain bricks embedded at the arch crown location in step S4), the tangential stress (maximum principal stress) and radial stress (minimum principal stress) in the surrounding rock at the embedment location can be calculated according to formula (1). Vertical load-radial stress variation curves and vertical load-tangential stress variation curves are plotted respectively.

[0065]

[0066] In equation (1), σ1 and σ2 represent the maximum and minimum principal stresses, respectively, E is the elastic modulus of the surrounding rock material, μ is the Poisson's ratio of the surrounding rock material, ε0 is the strain at 0° (horizontal), and ε 45 For a strain of 45°, ε 90 For 90° (vertical) strain.

[0067] In this embodiment, the radial stress and tangential stress at 13cm from the crown of the surrounding rock are respectively as follows: Figure 6 , Figure 7 As shown, with the increase of vertical load, radial stress first increases and then decreases, while tangential stress shows a monotonically increasing trend. Under the same load, both radial and tangential stresses exhibit the following order: loose rock < shotcrete and anchor support < dense shotcrete and anchor support. In particular, the radial stress of the dense shotcrete and anchor support combination increases by 1.4 to 5.6 times compared to the shotcrete and anchor support combination throughout the loading process. Radial stress characterizes the lateral confinement force of the surrounding rock. This indicates that applying anchor bolts can significantly increase the lateral confinement force of the surrounding rock, adjust the stress state of the surrounding rock, and thus improve the compressive strength and bearing capacity of the surrounding rock itself.

[0068] S93), displacement analysis of the arch crown in the surrounding rock, extracting the monitoring data from the arch crown displacement gauge, and plotting the vertical load-arch crown displacement variation curve, as shown. Figure 8 As shown, with the increase of vertical load, the displacement of the arch increases. Under the same load, the displacement of the burr hole is the largest, and the smaller the spacing of the anchor bolts, the smaller the displacement of the tunnel. This indicates that shotcrete-anchor support can effectively improve the tunnel's ability to resist deformation.

[0069] S94), internal force analysis of the bearing arch: In step S92), the tangential stress at different positions of the arch crown has been obtained. Further, the axial force and bending moment at the center of the bearing arch are calculated based on the plane section assumption. The calculation of axial force and bending moment at the arch shoulder and arch waist of the bearing arch is similar. First, the 0°, 45° and 90° strains obtained by the strain monitoring bricks embedded at the arch shoulder and arch waist are converted into tangential stress (maximum principal stress) according to formula (1). Then, the axial force and bending moment at the center of the bearing arch are calculated based on the plane section assumption, such as Figure 9 As shown in the figure, the axial force is the largest at the arch waist and the smallest at the arch crown. Under the combined action of axial force and negative bending moment, the inner surrounding rock at the arch shoulder first shows crushing and cone-shaped failure, which is consistent with the experimental phenomenon.

[0070] S95), Anchor Bolt Axial Force Analysis: First, extract the strain data of six monitoring anchor bolts embedded in the arch crown, arch shoulder, and arch waist. Compare these data with the stress-strain curves of the anchor bolts obtained from the tensile test in step S22). The stress of each anchor bolt during loading can be obtained. Multiplying the stress by the cross-sectional area of ​​the anchor bolt material yields the anchor bolt axial force. Then, plot the axial force of each anchor bolt when the shotcrete-anchor rock bearing arch reaches its ultimate bearing capacity. Figure 10 ), and calculate the ultimate strength of each anchor rod according to formula (2). Figure 10 It can be seen that when the bearing capacity is reached, the maximum axial force on each anchor rod is approximately 194N to 294N, based on the aforementioned tensile test results of the anchor rod material ( Figure 2 It can be seen that the yield strength of the anchor rod is 30MPa, corresponding to an axial force of approximately 212N, and the ultimate strength is 50.3MPa, corresponding to an axial force of approximately 355N. It can be seen that the anchor rod has basically reached the yield strength and has achieved 54.6% to 82.8% of the ultimate strength.

[0071]

[0072] In the formula, κ represents the anchor rod's performance level, and N1 and N2 represent the maximum axial force and ultimate strength of each anchor rod when the shotcrete-anchor rock bearing arch reaches its ultimate bearing capacity, respectively.

[0073] S96), internal force analysis of the sprayed layer, extract the strain data of the inner and outer sides of the sprayed layer test block, calculate the axial force and bending moment of the sprayed layer according to formulas (3) and (4), and draw the axial force and bending moment diagram of the sprayed layer when the anchor-rock bearing arch of the sprayed layer reaches the bearing capacity. Figure 11 ).from Figure 11 As can be seen from the data, the bending moment of the sprayed layer is small, and the axial force is mainly axial compression. The sprayed layer is under small eccentric compression, which can give full play to the material strength and load-bearing capacity. When the sprayed layer fails, it exhibits crushing failure, which is consistent with the experimental phenomenon.

[0074]

[0075]

[0076] In equations (3) and (4): E is the elastic modulus of the sprayed layer; ε 内 ε represents the strain inside the sprayed layer specimen. 外 denoted as σb, where σb is the strain on the outer side of the sprayed layer specimen; b is the unit length, taken as 1 unit; h is the thickness of the sprayed layer.

[0077] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, alterations, substitutions, or variations made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention shall fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-scale tunnel model tests, characterized in that... include: S1), determine the loading device and its parameters for the model test; S2), to prepare strain brick test blocks, sprayed layer test blocks, and to conduct tensile tests on anchor bolts; S3), filling material for surrounding rock and fine river sand; S4), embed strain brick test blocks and earth pressure cells; S5), pre-embedded tunnel molds and anchor bolts; S6), apply the spray coating material; S7), install the displacement gauge; S8), graded loading until the shotcrete support material is destroyed; S9) Monitor the stress and strain of the surrounding rock, observe whether the surrounding rock or the sprayed layer structure is damaged, record the vertical load when damage occurs, and analyze the stress at the crown of the surrounding rock, the displacement at the crown, the internal force of the bearing arch, the axial force of the anchor bolts, and the stress of the sprayed layer. Step S9) includes: S91), determine the bearing capacity, and determine the vertical load corresponding to the first failure of the sprayed layer during the loading process as the bearing capacity of the shotcrete anchor support; S92), stress analysis of the surrounding rock at the arch crown, using the following formulas to calculate the tangential and radial stresses in the surrounding rock at the installation location. （1） （2） and These represent the maximum and minimum principal stresses, respectively. E The elastic modulus of the surrounding rock material. Poisson's ratio of the surrounding rock material. For 0° horizontal strain, For a strain of 45°, For 90° vertical strain; Plot the vertical load-tangential stress variation curves and the vertical load-radial stress variation curves respectively, and analyze the changes in radial and tangential stresses at the crown as the vertical load increases. S93), the displacement analysis of the arch crown of the surrounding rock was carried out. The data monitored by the arch crown displacement gauge was extracted, and the vertical load-arch crown displacement change curve was plotted to analyze the change of arch crown displacement as the vertical load increases. S94), the internal force analysis of the bearing arch is performed. The tangential stress is obtained by converting according to formula (1). The axial force and bending moment at the center of the bearing arch are calculated according to the plane section assumption. Finally, the internal force and bending moment diagram of the bearing arch when the shotcrete-anchor rock bearing arch reaches the bearing capacity is drawn to analyze the stress situation inside the bearing arch. S95), axial force analysis of anchor bolts, to determine the axial force of each anchor bolt when the shotcrete-anchor rock bearing arch reaches its bearing capacity, and to calculate the ultimate strength of each anchor bolt when the shotcrete-anchor rock bearing arch reaches its bearing capacity; S96), internal force analysis of the sprayed layer, extracting the strain data of the inner and outer sides of the sprayed layer test block, calculating the axial force and bending moment of the sprayed layer, and plotting the axial force and bending moment diagram of the sprayed layer when the anchor-rock bearing arch of the sprayed layer reaches its bearing capacity.

2. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, In step S1), the model test loading device includes a loading platform, a model box, and a hydraulic loading system.

3. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, Step S2) includes: S21), using molds to make strain brick test blocks and sprayed layer test blocks respectively; S22) Tensile tests were conducted on the selected anchor material. Before the test, strain gauges were attached to the upper, middle and lower parts of the selected anchor material. The selected anchor material was stretched using a universal testing machine to obtain the stress-strain curve of the anchor material. At the same time, the elastic modulus, tensile stiffness and ultimate tensile strength of the anchor material were calculated based on the stress-strain curve.

4. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, Step S3) includes: S31), using steel baffles to separate the surrounding rock material from the fine river sand, filling the surrounding rock material in layers in the middle of the model box, and repeatedly leveling and compacting it with a tamping hammer; S32), once the surrounding rock material has been compacted to the design elevation of each layer, remove the steel baffle and place PET sheets on both sides of the surrounding rock material; (S33) After the PET sheet is placed, fill the sides of the model box with fine river sand, and then flatten and compact it.

5. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, In step S4), when the surrounding rock is filled to a specified height, strain bricks and earth pressure cells are buried at a specified elevation to monitor the stress inside the surrounding rock.

6. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, Step S5) includes: S51) When the surrounding rock material is filled to the bottom of the arch, a columnar foam mold with the same shape as the tunnel cross section is pre-embedded, and the position of each anchor rod is marked on the tunnel mold. According to the anchor bolt arrangement scheme, a total of 6 monitoring anchor bolts are set at the arch crown, arch shoulder and arch waist, and strain gauges are attached to the upper, middle and lower parts of the monitoring anchor bolts. S53) Before filling the surrounding rock, insert one end of an iron pipe with an outer diameter larger than that of the anchor material into the marked point of the tunnel mold, and ensure that the iron pipe is perpendicular to the cut surface at the marked point on the tunnel mold. S54), fill and compact the surrounding rock material around the iron pipe. When the other end of the iron pipe is about to be buried by the surrounding rock material, pull out the iron pipe to form a hole with an outline that matches the outline of the anchor rod. S55), inject anchor bolt adhesive at the opening, insert the anchor bolt into the hole, and bring the adhesive into the hole along with the bolt until the other end of the anchor bolt contacts the surface of the tunnel mold; (S56) After all the surrounding rock has been filled, let it sit for 12 hours, then melt the foam mold with a hot air gun. After the tunnel is formed, attach the anchor bolt trays near the tunnel wall.

7. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, Step S6) includes: S61), using a syringe to spray a layer of plaster onto the tunnel wall; S62), after the gypsum has cured and dried, the sprayed test block with strain gauges and the earth pressure cell are respectively attached to the gypsum surface according to the monitoring layout plan; S63), fix the wire mesh, and then apply the spray coating material to the specified thickness; (S64) Finally, let it stand and wait for the sprayed material to completely dry and cure.

8. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, In step S7), displacement gauges are arranged at the top, waist, bottom, and shoulders of the arch in a 45° direction to capture the displacement of the tunnel during the model test loading process.

9. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, In step S8), the top and side jacks of the model box are loaded simultaneously. Each load level is maintained for 10 minutes. If there is no obvious damage to the tunnel or the deformation is not obvious, the load is increased. If the tunnel is continuously and rapidly deforming, the current load is maintained until it stabilizes before increasing the next load level. During the loading process, the damage to the surrounding rock and the shotcrete support structure was recorded using a camera, and the damage phenomena that occurred during the test were recorded in detail on the test record sheet. When the surrounding rock cannot maintain its existing cross-sectional shape and is damaged, when a certain section of the sprayed layer structure first shows signs of failure, or when the tunnel collapses, it is considered that the tunnel has reached its bearing capacity, until the tunnel completely collapses and the loading ends.

10. The method for analyzing the load-bearing capacity and support effect of shotcrete and anchor support in large-size tunnel model tests according to claim 1, characterized in that, S95) includes: anchor bolt axial force analysis, extracting strain data of 6 monitoring anchor bolts embedded in the arch crown, arch shoulder, and arch waist, comparing with the anchor bolt stress-strain curves obtained from the tensile test, obtaining the stress of each anchor bolt during loading, multiplying the stress by the cross-sectional area of ​​the anchor bolt material to obtain the anchor bolt axial force, plotting the axial force of each anchor bolt when the shotcrete-anchor rock bearing arch reaches its bearing capacity, and calculating the ultimate strength of each anchor bolt when the shotcrete-anchor rock bearing arch reaches its bearing capacity using the following formula: （2） κ To the extent to which the anchor bolt performs, N 1 and N 2 represents the maximum axial force and ultimate strength of each anchor bolt when the shotcrete-anchor rock bearing arch reaches its ultimate bearing capacity; In S96), the axial force and bending moment of the sprayed layer are calculated according to formulas (3) and (4) respectively: （3） （4） in, E The elastic modulus of the sprayed layer; The strain is the inner side strain of the sprayed layer test block; Strain on the outer side of the sprayed layer specimen ；b The unit length is 1; h This refers to the thickness of the spray layer.

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