A model test device and test method for simulating concentrated load in an existing tunnel

Abstract

The present invention discloses a model test device and a test method for simulating concentrated surcharge in an existing tunnel. The device includes a model box, and a ground loss simulation device and an existing tunnel model are arranged up and down inside the model box; the existing tunnel model includes a pipe body and end caps installed at both ends of the pipe body. A ballast slab, a reference beam, and a closed water collection tank are arranged inside the pipe body. Both ends of the reference beam pass through the end caps at both ends of the pipe body and are fixed on the model box; the closed water collection tank is arranged on the ballast slab, and each closed water collection tank is separately connected to a water delivery pipe. After the water delivery pipe extends out of the model box, a water supply device injects water into the closed water collection tank; a monitoring section is arranged on the existing tunnel model, and a detection device is arranged at the monitoring section. Through the test method of the present invention, the surcharge process in the tunnel can be simulated; it has the advantages of being fast and convenient; especially in the working condition of the existing tunnel bulging caused by the shield tunneling closely above or the excavation of the foundation pit above, it is particularly suitable for studying the anti-floating effect generated by the surcharge weight.

Description

Technical Field

[0001] The invention belongs to the technical field of shield tunnel model tests, and particularly relates to a model test device and test method for simulating concentrated loading in an existing tunnel, which is applicable to the simulation of concentrated loading conditions in an existing tunnel, and is particularly applicable to the study of the anti-floating effect generated by counterweight under the condition of shield tunneling closely above or excavation of a foundation pit above an existing tunnel causing uplift of the existing tunnel. Background Art

[0002] The shield tunneling adjacent to an existing tunnel will cause uplift deformation of the lower existing tunnel, resulting in hazards such as cracks, water leakage or bolt fracture between the segment rings of the existing tunnel, seriously affecting the service function and safety of the operating tunnel. In order to control the uplift deformation of the existing tunnel, concentrated loading is often used for counterpressure in the tunnel.

[0003] The application of using tunnel loading to control the uplift deformation of the tunnel has been applied in many projects. Many scholars at home and abroad have also studied the influence law of tunnel loading. The main research methods include numerical simulation method, on-site measured data analysis method and indoor model test method. Among them, the numerical simulation method depends to a large extent on the selection of simulation conditions, and the accuracy is difficult to control, often with large errors; the on-site measured data analysis method uses actual engineering measurements, and the measurement results are more intuitive and accurate, but there are also disadvantages such as complex on-site stratum shape, complex construction conditions and difficult embedding of test components. As an effective research method, the indoor model test method can simulate engineering practical problems indoors on the basis of ensuring a certain degree of accuracy and intuitiveness, saving a large amount of manpower and material resources. However, there is no indoor model test research on the counterweight in an existing tunnel under the condition of shield tunneling closely above, which needs to be further carried out.

[0004] In summary, the existing research methods generally have problems such as difficult accuracy control, non-intuitive measurement results, complex research conditions, high cost, time-consuming and laborious, etc., and it is urgent to solve these problems by improving technology. Summary of the Invention

[0005] The purpose of the invention is to overcome the deficiencies in the prior art and provide a model test device and test method for simulating concentrated loading in an existing tunnel.

[0006] To achieve the above technical purpose, the invention adopts the following technical solutions:

[0007] A model test device for simulating concentrated load in an existing tunnel, comprising a model box, in which a stratum loss simulation device and an existing tunnel model are arranged up and down; the existing tunnel model includes a pipe body and end covers installed at both ends of the pipe body, and a ballast slab, a reference beam, and a closed water collecting tank are arranged inside the pipe body. The two ends of the reference beam pass through the end covers at both ends of the pipe body and are fixed on the model box; the closed water collecting tank is arranged on the ballast slab, and each closed water collecting tank is individually connected to a water delivery pipe. After the water delivery pipe extends out of the model box, a water supply device injects water into the closed water collecting tank; a monitoring section is arranged on the existing tunnel model, and a detection device is arranged at the monitoring section.

[0008] Further, the stratum loss simulation device is composed of an outer pipe and an inner pipe nested inside the outer pipe. The inner end of the inner pipe is fixed on the side wall of the model box, and the outer end of the outer pipe passes through a receiving hole on one side of the model box and is connected to a traction device.

[0009] Further, the detection device includes displacement gauges, earth pressure cells, and strain gauges. Displacement gauges are arranged at the positions of the two sides of the inner arch waist and the arch crown of the existing tunnel model, and the displacement gauges are fixed on the reference beam. Earth pressure cells are arranged at the positions of the two sides of the outer arch waist, the arch crown, and the arch bottom, and strain gauges are arranged at the arch bottom position.

[0010] Further, the water supply device includes a water pump, a valve, a flow meter, and a water delivery pipe connector arranged in sequence.

[0011] Further, the stratum loss simulation device is in the upper space of the model box, with both the height and the crossing angle fixed. The existing tunnel model is in the lower space, and its height and angle can be adjusted. The existing tunnel model and the stratum loss simulation device are arranged at a certain angle, and this angle can be multiple angles such as 30°, 45°, 60°, 90°, etc., for simulating the tunnel approaching construction under different buried depths and different crossing angle conditions.

[0012] Further, the length, width, and height of the model box are 4m×4m×3m respectively. The front is made of tempered glass and marked with scales, and the other sides are welded with steel plates.

[0013] Further, the stratum loss simulation device and the existing tunnel model are made of aluminum alloy hollow pipes, and steel balls are arranged in the gap between the outer pipe and the inner pipe of the stratum loss simulation device.

[0014] Further, the ballast slab is fixed in the chute inside the existing tunnel model, made of a metal plate, and smeared with a certain amount of lubricating oil.

[0015] Further, both ends of the pipe body are sealed by end covers. A lead hole and a plastic hose are arranged on the end cover at one end, and a small hole for the reference beam to pass through is arranged on both end covers. A rubber grommet is arranged along the circumferential direction inside the small hole.

[0016] Further, the water collecting troughs are symmetrically arranged on both sides of the center of the roadbed slab. Each water collecting trough is a closed box-shaped structure with a detachable cover plate on the top. The water collecting trough has the same width as the roadbed slab, a length of 0.3 - 0.5 m, and a height of 10 - 15 cm. Each water collecting trough corresponds to a water delivery pipe and is individually numbered.

[0017] Further, the length of the reference beam is adjustable, and both ends are fixed to the side wall of the model box in a detachable manner.

[0018] Further, the rapid sand discharge opening is located on the rear facade of the model box. A rectangular opening is opened in the rear facade steel plate, and a movable door is installed, which can be manually opened and closed.

[0019] The second object of the present invention is to provide a test method for a model test device for simulating concentrated surcharge in an existing tunnel as described above, including the following steps:

[0020] (1) Fill fine sand into the model box within the following range of the existing tunnel model, fully compact it, and adjust the existing tunnel model according to the test angle.

[0021] (2) Then evenly fill the external fine sand into the model box until the fine sand reaches the specified height, and connect the displacement gauges, earth pressure cells, and strain gauges to the corresponding instruments.

[0022] (3) Start the traction device, pull out the outer pipe, and determine the water injection time, water injection position, and injected water volume according to the test plan during the tunneling process. The displacement gauges, earth pressure cells, and strain gauges monitor the displacement, circumferential earth pressure, and bending moment change data of the existing tunnel model in real time.

[0023] (4) Process the test data of the earth pressure, bending moment, and displacement of the existing tunnel model, and draw relevant curves.

[0024] The beneficial effects of the present invention are:

[0025] 1) It is convenient to simulate the crossing conditions of different crossing angles and burial depths.

[0026] The ground loss simulation device is set at a fixed position in the upper part of the model box, and 3 - 4 buried layers of existing tunnel models are arranged at appropriate heights in the lower space. During the test, the existing tunnel models in the lower part can be fixed in soil layers with different burial depths and can form multiple angles of 30°, 45°, 60°, and 90° with the ground loss simulation device, so as to simulate the adjacent construction of tunnels under different burial depths and different crossing angle conditions.

[0027] 2) It is convenient to simulate the control effect of different surcharge schemes on the uplift of the tunnel.

[0028] Inside the existing tunnel model, independent catch basins are symmetrically arranged on both sides of the center point of the upper and lower tunnel crossings. Each catch basin corresponds to a water delivery pipe leading to the outside of the model box, and each water delivery pipe is numbered separately. During the simulation of the surcharge process, it is only necessary to inject water into the corresponding water delivery pipe and catch basin. By controlling the position of the injected water in the catch basin and the injection volume, the control of the uplift deformation of the tunnel under different surcharge lengths and surcharge weights can be simulated. In addition, by injecting water before, during, and after the new tunnel crossing respectively, the effects of surcharge at different crossing stages can also be compared.

[0029] 3) High degree of mechanization and simple test operation

[0030] The crossing process of the formation loss simulation device is realized by a winch and steel strands. The fine sand in the model box is layered filled and compacted by mechanical sand sprinkling, and the sand discharge can be quickly completed through the reserved quick sand discharge port on the model box. The test process has a high degree of mechanization, saves manpower, and is convenient for the operation and completion of the test.

[0031] 4) For the layout of measuring instruments, accurately reflecting the test results

[0032] Three-direction LVDT displacement gauges are set at the arch waists and the crown of both sides inside the existing tunnel model, which can measure the overall absolute displacement and structural convergence deformation of the tunnel; earth pressure cells set in four directions outside the tunnel can measure the earth pressure distribution of each part of the tunnel; strain gauges installed on the bottom axis of the outer wall of the tunnel are used to monitor and reflect the bending moment changes generated by the existing tunnel model under the influence of the upper crossing. The three jointly monitor and measure the deformation of the existing tunnel model, and fully reflect the displacement and deformation of the existing tunnel model during the test process. Description of the Drawings

[0033] Figure 1 is the schematic diagram of the overall structure of the test device of the present invention;

[0034] Figure 2 is the layout plan of the existing tunnel model;

[0035] Figure 3 is the layout plan of the formation loss simulation device;

[0036] Figure 4 is the schematic cross-sectional view of the internal structure of the existing tunnel model;

[0037] Figure 5 is the schematic longitudinal section view of the internal structure of the existing tunnel model;

[0038] Figure 6 is the partial detail drawing of the internal structure of the existing tunnel model;

[0039] Description of the reference numerals: model box 1; ground loss simulation device 2; existing tunnel model 3; water collecting tank 4; water delivery pipe 5; detachable cover plate 6; port cover 7; reference beam 8; plastic hose 9; water pump 10; valve 11; flowmeter 12; water delivery pipe connector 13; lead hole 14; steel support 15; pull ring 16; steel strand 17; winch 18; displacement meter 19; earth pressure cell 20; strain gauge 21; rapid sand discharge port 22; receiving hole 23; track slab 24; outer pipe 25; inner pipe 26. Detailed implementation manners

[0040] The present invention will be further described below in conjunction with embodiments. The description of the following embodiments is only for helping to understand the present invention. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and modifications can still be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0041] It should be noted that all the directional indications (such as up, down, left, right, front, back...) in the embodiments of the present application are only used to explain the relative position relationship and movement conditions between components in a specific posture (as shown in the drawings). If the specific posture changes, the directional indications will also change accordingly.

[0042] As Figure 1As shown in the figure, the present invention provides a model test device for simulating concentrated surcharge in an existing tunnel, including a model box 1, which is scaled down at a ratio of 15:1, with a length, width, and height of 4m×4m×3m respectively. The interior includes a ground loss simulation device 2 and an existing tunnel model 3. The ground loss simulation device 2 is in the upper space. A steel support 15 is arranged on the right side of the model box 1, and the top surface of the steel support 15 is at the same height as the lowest point of the receiving hole 23, facilitating the placement after the traction device and the outer pipe 25 are pulled out. The left end of the ground loss simulation device 2 is fixed to the left side wall of the model box 1, and the right end passes through the right receiving hole 23. A pull ring 16 is arranged at the right end of the ground loss simulation device 2, and the pull ring 16 is connected to the traction device (since the winch 18 has the advantages of simple mechanical equipment, easy operation, wide application range, etc., the winch 18 is adopted in this embodiment) through a steel strand 17. The outer pipe 2 can be pulled to the right by the operation of the winch 18 to simulate shield tunneling. A quick sand discharge port 22 is arranged at the rear elevation of the model box 1 at a position slightly lower and to the left of the existing tunnel model 3. The quick sand discharge port 22 is tightly closed during the test and is manually opened when it is necessary to remove the upper sand layer, quickly discharging the upper sand layer and reducing the consumption of manual soil unloading. The water delivery pipe 5 inside the existing tunnel model 3 is led out from the inside of the model box 1 through a lead hole 14 and a plastic hose 9, and then is sequentially connected to a water delivery pipe connector 13, a flow meter 12, a valve 11, and a water pump 10. The water delivery pipe connector 13 is used as a connecting device for the water delivery pipe 5 and can be detached or connected to the corresponding numbered water delivery pipe 5 at any time for injecting water into the water collecting tanks 4 at different positions. The flow meter 12 is used to display and control the amount of injected water.

[0043] As Figure 2 shown, 3 - 4 burial depths of the existing tunnel model 3 are selected in the lower space of the ground loss simulation device 2 in the model box 1. At the same burial depth, the existing tunnel model 3 can be fixed at an angle of 90°, 60°, 45°, or 30°. Different shield tunneling conditions can be simulated by adjusting the burial depth and layout angle of the existing tunnel model 3, increasing the research content of the test device. The length of the existing tunnel model 3 remains unchanged, but as the angle changes, the distances between the two ends of the existing tunnel model 3 and the side walls of the model box 1 will change, and the length of the reference beam 8 needs to be adjusted. End covers 7 are arranged at both ends of the existing tunnel model 3, and a lead hole 14 is left on the front end cover 7. The internal water delivery pipe 5 and the measuring line can be led out of the model box 1 through the lead hole 14 and the plastic hose 9.

[0044] As Figure 3As shown in the figure, the ground loss simulation device 2 is arranged horizontally from left to right in the model box 1. The ground loss simulation device 2 consists of an outer pipe 25 and an inner pipe 26. The outer pipe 25 is sleeved outside the inner pipe 26, and the gap between them is filled with steel balls to facilitate the subsequent extraction of the outer pipe 25 more conveniently. The left end of the inner pipe 26 is fixed on the left side wall of the model box 1, and the outer pipe 25 is not fixed to the left side wall. The right ends of both the outer pipe 25 and the inner pipe 26 are placed on the receiving hole 23. A pull ring 16 is welded to the right end of the outer pipe 25. During the test, the winch 18 is used to pull the pull ring 16 through the steel strand 17 to slowly extract the outer pipe 25. Due to the diameter difference between the outer pipe 25 and the inner pipe 26, the existing gap is used to simulate the influence of soil loss caused by the shield tunneling process.

[0045] As Figure 4 shown, a closed water collecting tank 4 is arranged on the ballast slab 24 inside the existing tunnel model 3. A detachable cover plate 6 is provided at the top of each water collecting tank 4, and a water delivery pipe 5 is correspondingly connected. Three-direction LVDT displacement gauges 19 are arranged at the waist and crown of the inner sides of both sides of the existing tunnel model 3. The LVDT displacement gauges 19 are all installed on the reference beam 8 to measure the displacement change of the existing tunnel model 3. Earth pressure cells 20 are pasted in four directions outside the existing tunnel model 3 to measure the circumferential earth pressure change of the existing tunnel model 3.

[0046] As Figure 5 shown, monitoring sections are arranged at regular intervals inside and outside the existing tunnel model 3. LVDT displacement gauges 19 are arranged at the waist and crown positions on both sides of the inner part to measure the displacement change of the existing tunnel model 3. Earth pressure cells 20 are arranged at the waist, crown and bottom positions on both sides of the outer part to measure the circumferential earth pressure change of the existing tunnel model 3. Strain gauges 21 are arranged at the bottom position to measure the bending moment change generated outside. Six water collecting tanks 4 are arranged symmetrically along the center on the ballast slab 24. Each water collecting tank 4 has the same width as the ballast slab 24, a length of 0.5 m, and a height of 15 cm. Each water collecting tank 4 corresponds to a water delivery pipe 5 and is numbered separately. Port covers 7 are installed at both ends of the existing tunnel model 3. The left port cover 7 is provided with a lead hole 14 and a plastic hose 9 to lead out the internal water delivery pipe 5 and the measuring line from the model box 1. The reference beam 8 is used to install the LVDT displacement gauges 19 and is located at the center of the existing tunnel model 3. Due to the small holes reserved for circumferentially arranged rubber gaskets on both ends of the port covers 7, the reference beam 8 can be easily passed through the small holes and fixed on the side wall of the model box 1. And because the rubber gasket has a certain elasticity along the radial direction, while blocking the sand from entering the inside of the existing tunnel model 3, it allows the existing tunnel model 3 to have a certain range of settlement and heave displacement.

[0047] As Figure 6As shown, the port cover 7 can be opened, and the roadbed slab 24 can be withdrawn along the chute preset on the inner wall of the existing tunnel model 3, facilitating the installation and replacement of internal devices and instruments. At the same time, the detachable cover plate 6 on the top of the water collecting tank 4 can be lifted to facilitate the removal of the accumulated water injected inside.

[0048] In addition, a specific case will be described in detail.

[0049] The model size involved in the present invention can be flexibly adjusted according to the selected reduction scale ratio and the actual engineering situation to be simulated. The present invention selects a ratio of 15:1. The outer diameter of the existing tunnel model 3 is 41.3 cm (the actual diameter is selected as 6.2 m). The length of one water collecting tank 4 is 0.5 m, the width is 0.3 m, and the height is 15 cm. There are 6 water collecting tanks 4 on the roadbed slab 24. Therefore, the total length covered by the water collecting tanks 4 is 3 m. Since the surcharge in the tunnel often occurs symmetrically along the center of the surcharge, if water is injected into the two middlemost water collecting tanks 4, the actual surcharge length that can be simulated is 15 m; if water is injected into the four middle water collecting tanks 4, the actual surcharge length that can be simulated is 30 m; if water is injected into all six water collecting tanks 4 at the same time, the actual surcharge length that can be simulated is 45 m. The maximum water injection volume of a single water collecting tank 4 is 22.5 L. If the density of water is 1 kg / L, the maximum water injection mass of a single water collecting tank 4 is 22.5 kg, and the maximum water injection mass of six water collecting tanks 4 is 135 kg. The maximum surcharge amount in the simulated actual project is 675 kg / m. Since the amount of water injection can be flexibly controlled, in summary, the test device of the present invention can simulate the actual engineering surcharge ranges of 15 m, 30 m, and 45 m at three levels, and can simulate the concentrated surcharge amount in the tunnel of 0 - 675 kg / m.

[0050] It should be noted that the structure of the ground loss simulation device 3 involved in the present invention is made with reference to the utility model patent "A device for simulating ground loss caused by the propulsion of a shield tunnel in a similar rectangular shape" (Application No.: 201720664274X). For the detailed structure and function introduction, please refer to the original text.

[0051] Using the above-mentioned simulation device for concentrated surcharge in an existing tunnel to conduct a concentrated surcharge model test, including the following steps:

[0052] 1) Making the model: Using aluminum alloy hollow pipes to make the ground loss simulation device 2 and the existing tunnel model 3 according to a ratio of 15:1. Weld a metal pull ring 16 at one end of the outer pipe 25. Set two chutes at the installation position of the roadbed slab 24 inside the existing tunnel model 3. Install the water collecting tanks 4 symmetrically on both sides of the midpoint of the roadbed slab 24, connect the water delivery pipes 5, and number them respectively.

[0053] 2) Install the instruments: Temporarily fix the reference beam 8 at both ends of the existing tunnel model 3. Install the LVDT displacement gauge 19 on the reference beam 8. Paste the earth pressure cell 20 and the strain gauge 21 on the outside of the existing tunnel model 3. Number all the measuring lines and bundle them up to avoid the problems of messy and unidentifiable measuring lines during the test;

[0054] 3) Install the existing tunnel model 3: Install the track slab 24 into the existing tunnel model 3 along the chute. Install the port covers 7 at both ends of the existing tunnel model 3. During the installation process, pass the measuring line of the LVDT displacement gauge 19 and the water delivery pipe 5 through the lead hole 14 and the plastic hose 9, and pass both ends of the reference beam 8 through the small holes. After the installation is completed, place it aside for standby. Then fill the model box 1 with fine sand in the following range of the existing tunnel model 3, compact it fully, place the existing tunnel model 3 in the model box 1 according to the test angle, fix the reference beam 8 on the side wall of the model box 1 in a detachable manner, connect and fix the plastic hose 9 on the small holes reserved in the model box 1, and lead the water delivery pipe 5 and the measuring line of the LVDT displacement gauge 19 out of the model box 1;

[0055] 4) Install the ground loss simulation device 2: Fix one end of the inner pipe 26 on the side wall of the model box 1, and place the other end on the receiving hole 23. Apply lubricant on the surface of the inner pipe 26 and wrap it with a PE film. Then, use the receiving hole 23 to sleeved the outer pipe 25 from the outside. Add steel balls to the gap between the inner pipe 26 and the outer pipe 25. Apply lubricant on the surface of the outer pipe 25 and put on a smooth PE film;

[0056] 5) Fill with fine sand and install the instruments: Use the sand sprinkling system to evenly fill the model box 1 with external fine sand. Compact it once every 100 mm of filling, and repeat the operation until the fine sand is filled to the specified height. Lead the measuring lines of the earth pressure cell 20 and the strain gauge 21 out of the sand layer along the inner wall of the model box 1 and connect them to the corresponding instruments. Connect the water delivery pipe 5 led out from the inside of the existing tunnel model 3 to the water delivery pipe connector 13, the flowmeter 12, the valve 11 and the water pump 10 in sequence, and connect the measuring line of the LVDT displacement gauge 19 to the corresponding instrument;

[0057] 6) Conduct a single-group test: Connect the pull ring 16 at one end of the outer pipe 25 to the winch 18 through the steel strand 17. Start the winch 18 and use the steel strand 17 to pull the outer pipe 25 to move to the right. During the tunneling process, determine the water injection time, water injection position and the amount of water injected according to the test plan. The relevant measuring elements monitor the settlement and deformation data of the existing tunnel model 3 in real time;

[0058] 7) More subsequent groups of tests: After completing the single-group crossing test, open the quick sand discharge port 22 on the model box 1 to quickly remove the sand layer above the existing tunnel model 3. Remove the reference beam 8 fixed at both ends, open the port cover 7, pull out the track slab 24, lift the detachable cover plate on the catch basin 4 to remove the internal accumulated water. Then reassemble and readjust the embedding angle or depth of the existing tunnel model 3 according to the test plan, clean the surface of the inner pipe 26, reapply lubricant, and sleeve the outer pipe 25 into the receiving hole 23 again, apply lubricant. After the operation is completed, repeat the above sand filling process to conduct the next group of tests;

[0059] 8) Post-processing: Process the test data of the earth pressure, bending moment and displacement of the existing tunnel model 3, draw relevant curves, and study the displacement and deformation laws of the existing tunnel model 3 under the combined action of crossing and surcharge loading.

[0060] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be similarly included in the patent protection scope of the present invention.

Claims

1. A model test device for simulating concentrated surcharge in an existing tunnel, characterized in that, it includes a model box, and a ground loss simulation device and an existing tunnel model are arranged up and down inside the model box; the existing tunnel model includes a pipe body and end covers installed at both ends of the pipe body. A ballast slab, a reference beam, and a closed water collection tank are arranged inside the pipe body. The two ends of the reference beam pass through the end covers at both ends of the pipe body and are fixed on the model box; the closed water collection tank is arranged on the ballast slab, and each closed water collection tank is separately connected to a water delivery pipe. After the water delivery pipe extends out of the model box, a water supply device injects water into the closed water collection tank; a monitoring section is arranged on the existing tunnel model, and a detection device is arranged at the monitoring section; both ends of the pipe body are sealed by end covers. A lead hole and a plastic hose are arranged on the end cover at one end, and a small hole for the reference beam to pass through is arranged on both end covers. A rubber grommet is arranged along the circumferential direction inside the small hole; the tunnel model is composed of an outer pipe and an inner pipe nested inside the outer pipe. The inner end of the inner pipe is fixed on the side wall of the model box, and the outer end of the outer pipe passes through a receiving hole on one side of the model box and is connected to a traction device; the detection device includes displacement gauges, earth pressure cells, and strain gauges. Displacement gauges are arranged at the positions of the arch waists and the crown inside both sides of the existing tunnel model, and the displacement gauges are fixed on the reference beam. Earth pressure cells are arranged at the positions of the outer arch waists, the crown, and the arch bottom, and strain gauges are arranged at the arch bottom position; the water collection tanks are symmetrically arranged on both sides of the center of the ballast slab. Each water collection tank is a closed box-shaped structure, and a detachable cover plate is arranged on the top. Each water collection tank corresponds to a water delivery pipe and is numbered separately.

2. The model test device for simulating concentrated surcharge in an existing tunnel according to claim 1, characterized in that, the water supply device includes a water pump, a valve, a flow meter, and a water pipe connector arranged in sequence.

3. The model test device for simulating concentrated surcharge in an existing tunnel according to claim 1, characterized in that, the front of the model box is made of tempered glass and marked with scales, and the other sides are welded with steel plates.

4. The model test device for simulating concentrated surcharge in an existing tunnel according to claim 1, characterized in that, the length of the reference beam is adjustable, and both ends are fixed on the side wall of the model box in a detachable manner.

5. The model test device for simulating concentrated surcharge in an existing tunnel according to claim 1, characterized in that, it further includes a sand discharge port, which is located on the rear facade of the model box. A rectangular opening is opened on the rear facade steel plate, and a movable door is installed.

6. A test method for the model test device for simulating concentrated surcharge in an existing tunnel according to any one of claims 1-5, characterized in that, it includes the following steps: (1) Fill fine sand into the model box within the range below the existing tunnel model, compact it fully, and adjust the existing tunnel model according to the test angle; (2) Then evenly fill the external fine sand into the model box until the fine sand is filled to the specified height, and connect the displacement gauges, earth pressure cells, and strain gauges to the corresponding instruments; (3)Turn on the traction device, extract the outer pipe, and determine the water injection time, water injection location, and injected water volume according to the test plan during tunneling. The displacement meter, earth pressure cell, and strain gauge monitor the displacement, circumferential earth pressure, and bending moment change data of the existing tunnel model in real time; (4)Process the test data of the earth pressure, bending moment, and displacement of the existing tunnel model, and draw relevant curves.

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