Construction and bearing characteristic analysis method of composite support in large-scale tunnel model test

Abstract

The present application belongs to the technical field of geotechnical engineering model test, and relates to a method for construction and bearing capacity analysis of composite support in large-size tunnel model test. The construction method comprises the following steps: S1) making precast test blocks and measuring basic parameters of anchor rods; the specific parameters are ultimate tensile strength and elastic modulus of the anchor rods; S2) filling surrounding rock similar material; S3) burying anchor rods; S4) making a shotcrete layer; and S5) making a secondary lining. The construction method can simply make the composite support structure, the quality of the support structure is good after forming, and the construction method of the shotcrete layer and the secondary lining in the engineering can be restored as much as possible, so that the actual engineering phenomenon can be reproduced well. Through the bearing capacity analysis method, the ultimate bearing capacity of each support structure can be understood conveniently, and the bearing role of each support structure can be clear, so that the bearing characteristics of each support structure can be studied further.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical engineering model testing technology, specifically relating to a method for the construction and bearing characteristic analysis of composite support in large-size tunnel model tests. Background Technology

[0002] To gain a deeper understanding of tunnel failure processes, numerous scholars have conducted model tests. However, several problems remain: First, most current model tests involve small-sized tunnels with cross-sectional dimensions of approximately 20cm to 60cm. Due to the relatively small cross-sectional dimensions, it is difficult to monitor the stress and failure modes of various support components, such as shotcrete, making it challenging to accurately reproduce actual engineering phenomena and conduct in-depth research on their load-bearing characteristics. Large-sized tunnels, on the other hand, can better reproduce failure phenomena and obtain the stress on various support structures during the load-bearing process, thus enabling better analysis of the load-bearing characteristics of composite support systems. Second, the model fabrication process for composite support systems is relatively complex, with anchor bolt installation being difficult. In most existing tests, the anchor bolt bonding performance is weak, and the quality of the shotcrete and secondary lining is relatively poor. The fabrication methods differ significantly from actual engineering practices, making it difficult to accurately reproduce actual engineering phenomena. Summary of the Invention

[0003] The purpose of this invention is to provide a method for constructing and analyzing the load-bearing characteristics of composite support structures in large-scale tunnel model tests. For large-scale tunnels, this construction method allows for relatively simple construction of composite support structures with good quality after completion, and can replicate the construction methods of anchor bolts, shotcrete, and secondary lining in engineering as closely as possible. This load-bearing capacity analysis method allows for a convenient understanding of the ultimate bearing capacity of each support structure and clarifies the load-bearing role of each support structure.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: A method for constructing a composite support system for a large-scale tunnel model test includes the following steps: S1) Fabricate precast test blocks and determine the basic parameters of the anchor bolts; the specific parameters are the ultimate tensile strength and elastic modulus of the anchor bolts; S2) Filling with similar materials to the surrounding rock; S3) Install anchor bolts; S4) Create the spray layer; S5) Construct secondary lining.

[0005] Furthermore, S1 specifically refers to: S11) Use a mold to make a square precast test block with thicknesses of the spray layer and the secondary lining, respectively, and attach strain gauges to both sides of the test block. S12) Conduct indoor tensile tests on the anchor rods used to obtain the stress-strain curve of the anchor rod material; take the slope of the straight segment on the stress-strain curve and the peak stress to obtain the elastic modulus and ultimate tensile strength of the anchor rod respectively.

[0006] Furthermore, S2 specifically refers to: S21) Before filling, draw lines according to the set height; S22) When filling each layer, pour in the surrounding rock similar material in several batches, and spread and compact it. To prevent stratification, scrape the surface after each compaction before pouring in the surrounding rock similar material. S23) When the surrounding rock material is filled to the arch bottom elevation, place a columnar high-strength foam mold that matches the shape of the tunnel cross section. S24) After all the surrounding rock is filled, let it sit for 12 hours, then use a hot air gun to melt the foam mold and the tunnel will be formed.

[0007] Furthermore, S3 specifically refers to: S31) Before installing the anchor bolts, attach strain gauges to the head, middle and tail of the anchor bolts to be monitored. S32) Before placing the cylindrical high-strength foam mold in step S2), mark the corresponding position on the foam mold according to the anchor bolt placement position, then place it on the surrounding rock, and insert a steel pipe with a diameter of 2 to 3 times that of the anchor bolt into the foam. S33) When the surrounding rock is filled to the point where the steel pipe is about to be submerged, pull out the steel pipe, pour a small amount of epoxy resin AB glue into the hole, and then insert the anchor rods with the strain gauges attached into the arch top, arch shoulder and arch waist of the anchor rod section respectively, and inject the adhesive material around the anchor rods. S34) After the hot air gun melts the foam mold and forms the tunnel, the anchor plate is attached near the tunnel wall.

[0008] Furthermore, S4 specifically includes: S41) First, spray a ring of plaster about 1 mm thick using a syringe; S42) The precast test block with strain gauges attached to the inside and outside, which was made in step S1), and the earth pressure cell are pasted onto the plaster surface according to the monitoring plan, and a certain distance is maintained between the monitoring section of the earth pressure cell and the monitoring section of the precast test block. S43) Secure the wire mesh, then apply the spray coating material to the specified thickness; (S44) Place for 24 hours to allow the sprayed layer to fully cure.

[0009] Furthermore, S5 specifically includes: S51) Apply butter to a plastic film and adhere the plastic film to the surface of the spray layer to simulate a waterproof layer; S52) Support sheet metal along the tunnel circumferential direction as template, the distance between the template and the sprayed layer is the thickness of the secondary lining, and apply machine oil to the sheet metal; S53) Fill the space between the sprayed layer and the template with a similar secondary lining material from bottom to top, and tamp it down with wooden sticks during filling; S54) After filling, let it sit for 6 hours until the material has cured, then remove the sheet metal template; S55) At the designated location on the inner wall, excavate holes larger than the precast test block and the earth pressure box, with the hole depth being the thickness of the secondary lining. S56) Place the precast test block with strain gauges attached to the inside and outside and the earth pressure cell at the bottom of the excavation hole according to the monitoring plan, then fill the gap with a similar secondary lining material, and smooth the surface, waiting for complete curing.

[0010] This invention also provides a method for analyzing the bearing characteristics of composite support in large-scale tunnel model tests, the method comprising: A step-by-step loading test was conducted on the composite support system for large tunnels. Based on the above experimental analysis, the ultimate bearing capacity and failure location of the secondary lining and shotcrete, the load sharing ratio of the secondary lining and shotcrete, and the effectiveness of the anchor bolts are analyzed.

[0011] Furthermore, the step-by-step loading test specifically includes: Vertical and lateral loads are applied simultaneously, but the vertical and lateral loads are controlled independently. The vertical load increases by 20 kPa at each level, and the lateral load is determined according to the lateral pressure coefficient. The lateral pressure coefficient is determined based on the actual engineering background. The lateral load at each level is calculated by multiplying the vertical load by the lateral pressure coefficient. Each load level is maintained for 20 minutes during loading. If there is no obvious damage or deformation in the tunnel, the load is increased. If the tunnel is deforming rapidly, the current load is maintained until it stabilizes, and then the next load level is increased. Loading continues until the tunnel finally collapses completely. While loading, the data of the earth pressure cell and strain gauge were recorded using a static test strain gauge, and the failure phenomena that occurred during the test were recorded in detail on the test record sheet.

[0012] Furthermore, the analysis of the ultimate bearing capacity and failure location of the secondary lining specifically includes: The analysis of the ultimate bearing capacity and failure location of the secondary lining first determines the failure location of the secondary lining based on experimental phenomena. Then, the data from the earth pressure cell in the secondary lining are extracted, and the curve of contact stress versus vertical load is plotted. Observing the variation of the curve, the contact stress generally shows a trend of first increasing and then decreasing with the vertical load. Based on the curve, the magnitude of the vertical load at the peak before the contact stress drops can be roughly determined, and the vertical load value at this time is considered to be the bearing capacity under the ultimate state. The analysis of the ultimate bearing capacity and failure location of the sprayed layer begins by extracting data from the earth pressure cell within the sprayed layer and plotting the contact stress versus vertical load curve. The bearing capacity under the ultimate limit state is then determined based on this curve. Next, monitoring data from precast test blocks is used to obtain the strain on the inner and outer sides of the sprayed layer. The axial force of the sprayed layer is then calculated using the following formula. Finally, an axial force diagram under the ultimate limit state is plotted to determine the location of the maximum axial force within the sprayed layer; this location is the point of failure. In the formula: b The unit length is 1; h This refers to the thickness of the spray layer; E This represents the elastic modulus of the sprayed layer.

[0013] Furthermore, the analysis of the load-sharing ratio between the secondary lining and the shotcrete layer, as well as the effectiveness of the anchor bolts, specifically includes: The load sharing ratio of the secondary lining is calculated by using the earth pressure cell data of the shotcrete layer and the secondary lining to obtain the load sharing ratio of the secondary lining. In the formula, η This refers to the load-sharing ratio for the secondary lining. q 1 and q 2 represents the contact stress between the shotcrete layer and the secondary lining, and the contact stress between the surrounding rock and the shotcrete layer, respectively. The load sharing ratio of the sprayed layer is calculated using the following formula; In the formula, λ is the load sharing ratio of the sprayed layer. q 1 and q 2 represents the contact stress between the shotcrete layer and the secondary lining, and the contact stress between the surrounding rock and the shotcrete layer, respectively. The specific analysis method for the effectiveness of the step anchor bolt is as follows: Based on the strain gauge monitoring results, the stress of the anchor rod can be determined from the obtained elastic modulus. Then, by multiplying the cross-sectional area, the magnitude of the axial force of the anchor rod can be obtained. The degree of performance of the anchor bolt under load can be calculated using the following formula; In the formula, κ To the extent to which the anchor bolt performs, N 1 and N 2 represents the axial force of the anchor bolt and the ultimate strength of the anchor bolt, respectively; By plotting the curve of the anchor bolt's effectiveness as a function of vertical load, we can see the proportion of the anchor bolt that can function under different loads during the bearing process, as well as the range of the anchor bolt's effectiveness under this type of support.

[0014] Technical effects of the present invention: This invention belongs to the field of geotechnical engineering model testing technology, and relates to a method for constructing and analyzing the bearing characteristics of composite support in large-scale tunnel model tests. For large-scale tunnels, this construction method allows for the relatively simple fabrication of composite support structures with good quality after completion, and can largely replicate the construction methods of shotcrete and secondary lining in engineering projects, thus better reproducing actual engineering phenomena. Through this bearing capacity analysis method, the ultimate bearing capacity of each support structure can be easily understood, and the bearing role of each support structure can be clarified, facilitating a more in-depth study of the bearing characteristics of each support structure. Attached Figure Description

[0015] The accompanying drawings illustrate various embodiments generally by way of example rather than limitation, and are used, together with the specification and claims, to explain embodiments of the invention. Where appropriate, the same reference numerals are used in all drawings to refer to the same or similar parts. Such embodiments are illustrative and are not intended to be exhaustive or exclusive embodiments of the apparatus or method.

[0016] Figure 1 Schematic diagram of prefabricated test block.

[0017] Figure 2 Anchor bolt monitoring scheme.

[0018] Figure 3 Monitoring scheme for sprayed layer and secondary lining.

[0019] Figure 4 Loading diagram.

[0020] Figure 5 The curve of contact stress versus vertical load.

[0021] Figure 6 Axial force diagram of the sprayed layer.

[0022] Figure 7 Curve showing the variation of the load sharing ratio of the secondary lining with the vertical load.

[0023] Figure 8 Anchor bolt performance curve as a function of vertical load.

[0024] Figure 9 Flowchart of surrounding rock filling and anchor bolt installation.

[0025] Figure 10 Flowchart of spray coating application.

[0026] Figure 11 Flowchart of secondary lining construction. Detailed Implementation

[0027] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0028] The model test provided by this invention has a tunnel span of 1.06m and an axial depth of 0.45m. The tunnel composite support structure consists of a three-layer composite structure comprising an anchored rock bearing arch, a shotcrete layer, and a secondary lining.

[0029] Specifically, this invention provides a method for constructing a composite support system of anchor bolts, shotcrete, and secondary lining in a large-scale tunnel model test, comprising the following steps: S1) Fabrication of precast test blocks and determination of basic parameters of each material; S11) A square precast test block was fabricated using a mold. The test block measures 3*3cm in length and width. The thickness of the sprayed layer is 2cm and the thickness of the secondary lining is 4cm. Strain gauges were attached to both sides. A schematic diagram of the sprayed layer test block is shown below. Figure 1 ; Table 1 Material Parameters

[0030] S2) Filling with similar materials for surrounding rock and embedding anchor bolts; S21) Before filling, draw lines at a certain height, with each layer being 5cm high; S22) When filling each layer, pour in the surrounding rock similar material in 3 times, and spread and compact it. To prevent stratification, scrape the surface after each compaction before pouring in the surrounding rock similar material. S23) When the surrounding rock material is filled to the arch bottom elevation, according to the anchor bolt monitoring plan, such as Figure 2 The anchor bolts are arranged in a quincunx pattern, according to the designed anchor bolt spacing. In the diagram, solid lines represent the first row of anchor bolts, dashed lines represent the second row, and thickened lines represent anchor bolts used for strain monitoring, each marked with a corresponding number. The corresponding positions of the anchor bolts are marked on a cylindrical high-strength foam mold that matches the tunnel cross-section shape. This mold is then placed on the surrounding rock. At the marked positions, a steel pipe with a diameter 2-3 times the anchor bolt diameter is inserted into the foam mold for approximately 1-2 cm. S24) After the surrounding rock has been filled until the steel pipe is about to be submerged, pull out the steel pipe and pour a small amount of epoxy resin AB glue into the hole to prevent the hole from collapsing. Then insert the anchor rod and inject the adhesive material around the anchor rod, which will be carried into the hole along with the anchor rod. Insert the anchor rods with the strain gauges attached into the arch, shoulder and waist of the tunnel respectively. S25) After all the surrounding rock is filled, let it sit for 12 hours, then use a hot air gun to melt the foam mold, and the tunnel will be formed. After the tunnel is formed, attach the anchor bolt trays near the tunnel wall.

[0031] S3) Spray coating application; S31) Spray a ring of plaster about 1 mm thick with a syringe to make the wall surface as smooth as possible; S32) According to the specific monitoring plan, the prefabricated test block with strain gauges attached to its inner and outer sides, along with the earth pressure cell, is attached to the plaster surface according to the monitoring plan, such as... Figure 3 The distance between the earth pressure cell monitoring section and the precast test block monitoring section is 0.225m.

[0032] S33) Secure the wire mesh, then apply the spray coating material to a thickness of 2cm; (S34) Place for 24 hours to allow the sprayed layer to fully cure.

[0033] S4) Secondary lining construction; S41) Apply butter to the plastic film and adhere the plastic film to the surface of the spray layer to simulate a waterproof layer; S42) Support sheet metal along the tunnel circumferential direction as template, the distance between the template and the sprayed layer is 4cm of the secondary lining thickness, and apply machine oil to the sheet metal; S43) Fill with similar secondary lining material, and tamp it down with wooden sticks during filling; S44) After filling, let it sit for 6 hours until the material has slightly solidified and lost its fluidity, then remove the sheet metal template; S45) According to the monitoring plan, as follows Figure 3 At the secondary lining monitoring point, holes slightly larger than the precast test block and the earth pressure box were dug, with a hole depth of 4cm. The 4cm thick precast test block and the earth pressure box were placed at the bottom of the excavated hole, and the gaps were filled with a similar material to the secondary lining. The surface was then smoothed and left to cure completely.

[0034] This invention also provides a method for analyzing the bearing characteristics of a composite support system consisting of anchor bolts, shotcrete, and secondary lining in a large-scale tunnel model test. The method includes: Test loading; The loading is performed in stages, with vertical and lateral loads applied simultaneously, but the vertical and lateral loads are controlled independently. Each stage of the vertical load increases by 20 kPa, with a lateral pressure coefficient of 0.44. Each stage of the lateral load increases by approximately 8.8 kPa. A loading diagram is shown below. Figure 4 Each load level is maintained for 20 minutes, and loading continues until the tunnel finally collapses completely. Load-bearing characteristic analysis; Based on the experimental phenomena, the failure location of the secondary lining is at the arch shoulder. Data from the earth pressure cell in the secondary lining were extracted, and the curve of contact stress versus vertical load was plotted, yielding an ultimate bearing capacity of 390 kPa. Figure 5 As shown; Data from the earth pressure cell in the sprayed layer were extracted, and the curve of contact stress versus vertical load was plotted, yielding an ultimate bearing capacity of 390 kPa. Figure 5 As shown, based on the monitoring results of the precast test block, the strains on the inner and outer sides of the sprayed layer were obtained, and the axial force was calculated according to formula (1). The axial force diagram under the ultimate state was drawn, and it can be seen that the axial force is the largest at the shoulder of the sprayed layer, and the failure occurs at the shoulder. Figure 6 As shown; (1) In the formula: b The unit length is 1; h This refers to the thickness of the spray layer; E This represents the elastic modulus of the sprayed layer.

[0035] Calculate the load sharing ratio of the secondary lining according to formula (2), such as Figure 7 As shown, the results indicate that the secondary lining bears a smaller load at the arch waist and arch foot, approximately 37% to 50% and 3% to 20% respectively, while the secondary lining bears a larger load at the arch crown and arch shoulder, with distribution ranges of approximately 29% to 83% and 8% to 88% respectively. (2) In the formula, η This refers to the load-sharing ratio for the secondary lining. q 1 and q 2 represents the contact stress between the shotcrete layer and the secondary lining, and the contact stress between the surrounding rock and the shotcrete layer, respectively.

[0036] According to formula (3), the load sharing ratio of the sprayed layer is calculated. The sprayed layer shares a larger load at the arch waist and arch foot, which is about 50% ~ 63% and 80% ~ 97% respectively. The secondary lining shares a relatively smaller load at the arch crown and arch shoulder, which is about 17% ~ 71% and 12% ~ 92% respectively. When the load is small, the sprayed layer is the main load-bearing component. As the load increases, the secondary lining gradually becomes the main load-bearing component. (3) In the formula, λ This refers to the load-sharing ratio of the sprayed layer. q 1 and q 2 represents the contact stress between the shotcrete layer and the secondary lining, and the contact stress between the surrounding rock and the shotcrete layer, respectively.

[0037] The anchor's performance under load is calculated using formula (4). A curve showing the anchor's performance versus vertical load is plotted. As the load increases, the anchor's performance gradually increases, and the anchor can perform up to approximately 75% to 80% of its ultimate strength. Figure 8 As shown; (4) In the formula, κ To the extent to which the anchor bolt performs, N 1 and N2 represents the axial force of the anchor bolt and the ultimate strength of the anchor bolt, respectively.

[0038] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for analyzing the bearing characteristics of composite support in large-scale tunnel model tests, characterized in that, The method includes: A step-by-step loading test was conducted on the composite support of a large-scale tunnel model. Based on the above experimental analysis, the ultimate bearing capacity and failure location of the secondary lining and shotcrete, the load sharing ratio of the secondary lining and shotcrete, and the effectiveness of the anchor bolts are analyzed. The step-by-step loading test specifically refers to: Vertical and lateral loads are applied simultaneously, but the vertical and lateral loads are controlled independently. The vertical load increases by 20 kPa at each level, and the lateral load is determined according to the lateral pressure coefficient. The lateral pressure coefficient is determined based on the actual engineering background. The lateral load at each level is calculated by multiplying the vertical load by the lateral pressure coefficient. Each load level is maintained for 20 minutes during loading. If there is no obvious damage or deformation in the tunnel, the load is increased. If the tunnel is deforming rapidly, the current load is maintained until it stabilizes, and then the next load level is increased. Loading continues until the tunnel finally collapses completely. While loading, the data of the earth pressure cell and strain gauge were recorded using a static test strain gauge, and the failure phenomena that occurred during the test were recorded in detail on the test record sheet; The analysis of the load sharing ratio between the secondary lining and the shotcrete layer, as well as the effectiveness of the anchor bolts, is as follows: The load sharing ratio of the secondary lining is calculated by using the earth pressure cell data of the shotcrete layer and the secondary lining to obtain the load sharing ratio of the secondary lining. In the formula, η This refers to the load-sharing ratio for the secondary lining. q 1 and q 2 represents the contact stress between the shotcrete layer and the secondary lining, and the contact stress between the surrounding rock and the shotcrete layer, respectively; The load sharing ratio of the sprayed layer is calculated according to the following formula; In the formula, λ is the load sharing ratio of the sprayed layer. q 1 and q 2 represents the contact stress between the shotcrete layer and the secondary lining, and the contact stress between the surrounding rock and the shotcrete layer, respectively; The specific analysis method for the effectiveness of the anchor bolt is as follows: The strain of the anchor rod is obtained from the strain gauge monitoring results. The stress of the anchor rod can be known from the obtained elastic modulus. Then, the axial force of the anchor rod can be obtained by multiplying it by the cross-sectional area. The degree of performance of the anchor bolt under load can be calculated using the following formula; In the formula, κ To the extent to which the anchor bolt performs, N 1 and N 2 represents the axial force of the anchor bolt and the ultimate tensile strength of the anchor bolt, respectively; By plotting the curve of the anchor bolt's effectiveness as a function of the vertical load, we can see the proportion of the anchor bolt that can function under different loads during the bearing process, as well as the range of the anchor bolt's effectiveness under this type of support.

2. The method for analyzing the bearing characteristics of composite support in large-size tunnel model tests according to claim 1, characterized in that, The analysis of the ultimate bearing capacity and failure location of the secondary lining is as follows: The analysis of the ultimate bearing capacity and failure location of the secondary lining first determines the failure location of the secondary lining based on experimental phenomena. Then, the data from the earth pressure cell in the secondary lining are extracted, and the curve of contact stress versus vertical load is plotted. Observing the variation of the curve, the contact stress generally shows a trend of first increasing and then decreasing with the vertical load. Based on the curve, the magnitude of the vertical load at the peak before the contact stress drops can be roughly determined, and the vertical load value at this time is considered to be the ultimate bearing capacity of the secondary lining. The analysis of the ultimate bearing capacity and failure location of the sprayed layer first extracts the data of the earth pressure cell in the sprayed layer, plots the curve of contact stress with vertical load, and judges the ultimate bearing capacity of the sprayed layer based on the curve of contact stress. Based on the monitoring data of the precast test block, the strain on the inner and outer sides of the sprayed layer is obtained, and then the axial force of the sprayed layer is calculated. Draw the axial force diagram under extreme conditions to determine which location of the sprayed layer experiences the greatest axial force. The location of the greatest axial force is the location where failure occurs.

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