Indoor test device and method for determining buoyancy reduction coefficients in different stratums

Abstract

The invention discloses an indoor test device and method for determining buoyancy reduction coefficients in different stratums. The test device comprises a model box, a model barrel, a movable bottom plate, a balance weight, a displacement monitoring system and a water supply system. The model barrels are located on the inner sides of the model boxes, and soil sample layers are filled between the model boxes and the model barrels; a round hole is formed in the bottom of the model barrel and covered with an elastic film. The movable bottom plate is inverted-T-shaped, the diameter of the bottom of the movable bottom plate is smaller than that of the round hole in the bottom of the model barrel, the diameter of the top of the movable bottom plate is larger than that of the round hole in the bottom of the model barrel, and the movable bottom plate movably covers the round hole in the bottom of the model barrel and is located above the elastic film; the counterweight is arranged at the top of the movable bottom plate; the displacement monitoring system is arranged in the model barrel and is used for monitoring the displacement of the movable bottom plate; the water supply system is communicated with the model box and supplies water to the model box. The device is used for testing the buoyancy reduction coefficient and is simple in structure, convenient to operate, obvious in test phenomenon, high in accuracy and high in applicability.

Description

Technical Field

[0001] The invention relates to a test device and method for studying buoyancy reduction coefficients in different strata. The buoyancy reduction coefficients under different stratum conditions can be studied by simulating different strata and stratum combinations by changing soil samples filled in a model box and adopting the idea of ​​weight reduction and floating. Background Art

[0002] With the acceleration of urbanization and the increasing number of buildings, the development and utilization of urban underground space is the focus of urban construction and development today. However, in this process, the problem of underground structures being damaged by the adverse effects of groundwater buoyancy has become increasingly prominent. A large number of engineering examples have shown that the actual buoyancy of underground structures is not completely equal to the buoyancy value calculated based on Archimedes' principle. The current mainstream method for determining the buoyancy of underground structures is to reduce the value calculated by Archimedes' law. Therefore, how to reasonably determine the reduction coefficient has become a current research hotspot, which is of great significance for ensuring engineering safety, saving engineering costs, and shortening engineering construction time. At present, the main methods for studying the buoyancy reduction coefficient in different strata are: indoor model tests and numerical simulations.

[0003] Numerical simulation is often the result of the most ideal situation, which is quite different from the actual situation. Indoor model test simulates the physical process under real conditions, which can directly observe the actual physical phenomena and provide more intuitive data and results.

[0004] At present, there are two main problems with the mainstream buoyancy reduction test model device: how to measure or eliminate the sidewall friction, and how to accurately judge the floating moment. Regarding the problem of sidewall friction, you can choose to eliminate it through test design or eliminate it during calculation; the method of eliminating sidewall friction during calculation is based on the fact that there is no change in this force in each test, which is difficult to match the actual situation; it is more reasonable to ignore the sidewall friction during the test through test design. Regarding the judgment of the floating moment, the method of using sensors to determine the floating moment is significantly better than the method of using timed photography to determine it. In general, through the design of the test device and the application of sensors, the actual buoyancy can be measured more accurately and a more accurate buoyancy reduction coefficient can be obtained. Summary of the invention

[0005] In order to overcome the problems in existing test models that the side wall friction is difficult to accurately measure or eliminate, and the floating moment is difficult to accurately judge, the present invention designs a test device and method that does not consider the side wall friction and monitors the floating state through sensors.

[0006] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0007] An indoor test device for determining buoyancy reduction coefficients in different strata, comprising a model box, a model barrel, a movable bottom plate, a counterweight, a displacement monitoring system and a water supply system;

[0008] The model barrel is located inside the model box, and a soil sample layer is filled between the model box and the model barrel; a round hole is opened at the bottom of the model barrel, and the round hole is covered with an elastic film to ensure that external water does not penetrate into the model barrel;

[0009] The movable bottom plate is in an inverted convex shape, the bottom diameter of the movable bottom plate is 1.5 to 2.5 mm smaller than the diameter of the round hole at the bottom of the model barrel, the top diameter of the movable bottom plate is larger than the diameter of the round hole at the bottom of the model barrel, the movable bottom plate is movably covered on the round hole at the bottom of the model barrel, and the movable bottom plate is located above the elastic film;

[0010] The counterweight is located on the top of the movable bottom plate;

[0011] The displacement monitoring system is arranged in the model barrel and is used to monitor the displacement of the movable bottom plate;

[0012] The water supply system is connected with the model box and supplies water to the model box.

[0013] The diameter of the circular hole at the bottom of the above-mentioned model barrel is slightly larger than the diameter of the bottom of the movable bottom plate to ensure that the bottom of the movable bottom plate can smoothly pass through the circular hole, while the top is retained around the circular hole. In this way, under the action of buoyancy and gravity, the movable bottom plate can float up and down freely.

[0014] A displacement monitoring system is installed on the above-mentioned movable bottom plate and a counterweight is placed, and the water supply system realizes different water levels required by the test.

[0015] During the test, the target soil sample is placed in the model box to simulate the stratum (soil sample layer), the model bucket is placed in the middle of the soil sample layer, and a film, a movable bottom plate, a displacement monitoring system, and a counterweight are installed on the model bucket. The water supply system is used to make the specified water level in the model box. The counterweight is quantitatively reduced in batches until the reading of the displacement monitoring system changes, and the remaining counterweight and the total weight of the movable bottom plate are recorded to obtain the measured buoyancy. It should be noted that the trigger pressure of the monitoring system should be included in the measured buoyancy. The theoretical buoyancy is calculated according to the water level in the model box, and the buoyancy reduction coefficient can be obtained by dividing the two.

[0016] The test device of the present invention can carry out buoyancy reduction tests under single or combined formation conditions indoors, thereby providing the buoyancy reduction coefficient of the formation corresponding to a specific project. The model has the advantages of simple operation and obvious test phenomena, and can provide a theoretical reference for the reasonable value of the water level of anti-floating defense in underground projects, and has a good value for promotion and application.

[0017] To facilitate assembly and testing, the water supply system is a trough-type structure with support columns on both sides of the bottom. The model box is located in the water supply system, with both ends of the bottom of the model box falling on the two support columns. The side walls and bottom of the model box are evenly distributed with seepage holes.

[0018] To facilitate testing, the perimeter height of the water supply system was higher than the height of the soil sample layer.

[0019] In order to facilitate the observation of pressure changes in the water supply system, a pressure measuring tube is installed on the water supply system.

[0020] In order to further improve the accuracy of the test, the model box and the water supply system are an integrated structure. The water supply system is a trough structure. Water baffles are arranged in parallel along the width direction at both ends of the water supply system. The bottoms of the two water baffles fall on two supporting columns. The bottoms of the two water baffles are connected together through a connecting bottom plate. The space between the two water baffles and the connecting bottom plate on the water supply system forms a model box, that is, the model box shares the front and rear side walls of part of the length of the water supply system. The water baffles at both ends are respectively connected with the space between the side walls at both ends of the water supply system, and the space between the connecting bottom plate and the bottom of the water supply system to form an effective water supply system; the two water baffles and the connecting bottom plate are provided with seepage holes. The water supply system seeps from the seepage holes into the model barrel, simulating the changes in the stratum profile in the vertical direction due to the buoyancy of groundwater.

[0021] In order to avoid or reduce the soil sample from entering the water supply system, a gauze layer is provided between the model box and the soil sample layer.

[0022] As one of the specific implementation schemes, the model box and the model barrel are both made of 10-15 mm thick organic glass plates bonded together. After testing, they have good sealing and strength and can meet the test requirements.

[0023] The model box is a rectangular parallelepiped structure, the height of the model box is 600-650 mm, and the length and width are both 480-520 mm. The model barrel is a cylindrical structure, the diameter of the model barrel is 230-270 mm, and the height is 380-420 mm. The diameter of the round hole at the bottom of the model barrel is 130-135 mm; the elastic film is fixed to the model barrel by high-strength adhesive tape and completely covers the round hole to ensure that external water does not penetrate into the model barrel.

[0024] For the convenience of preparation, the movable bottom plate is made of two cylindrical glass plates with a thickness of 10 to 15 mm and different diameters bonded concentrically. The diameters of the two cylindrical glass plates are 128 to 132 mm and 178 to 182 mm respectively. The diameter of the smaller cylindrical glass plate should be slightly smaller than the diameter of the circular hole at the bottom of the model barrel. The difference between the two diameters should not be too large or too small, and should be about 1.5 to 2.5 mm (for example, if the diameter of the circular hole is 132 mm, the diameter of the smaller glass plate should be 130 mm) to ensure that the movable bottom plate can float or sink smoothly.

[0025] The movable bottom plate is a smooth structure. During preparation, the movable bottom plate can be polished with sandpaper to make its surface smooth, so as to avoid or reduce the damage to the plastic film caused by the edge of the movable bottom plate, and also reduce the friction resistance between the movable bottom plate and the plastic film.

[0026] In order to facilitate the test, the counterweight system consists of an annular iron block and a counterweight barrel. There are more than two annular iron blocks, all of which are located in the counterweight barrel. The counterweight barrel can hold water. During the test, a syringe is used to quantitatively draw water from the counterweight barrel in batches.

[0027] For ease of use, water pipes and water pumps can be used to add or pump water into the counterweight barrel at regular intervals and in fixed quantities according to test requirements, and automation can be achieved by combining existing automation technology.

[0028] The above displacement monitoring system uses the PM11-1 displacement sensor produced by Zhejiang Taizhou Xiyu Electronics Factory, and an external WXY / C-DC-WX digital display to directly display the displacement data. Because the sensor requires a DC environment to work, the S-120-24 switching power supply produced by Wenzhou Junlin Electric Technology Co., Ltd. is selected to convert the laboratory's 220V AC power into 24V DC power. Connect the switching power supply to the digital display, and then connect it to the laboratory power supply, so that the displacement data monitored by the displacement sensor can be displayed in real time on the digital display. The trigger pressure of the monitoring system is about 3N.

[0029] For easy installation, the shell of the displacement monitoring system is connected to the inner wall of the model barrel through a fixed fulcrum, and the sensing end of the displacement monitoring system is connected to the top of the movable bottom plate. The fixed fulcrum is made of a plexiglass plate, which is glued to the inner wall of the model barrel. As one of the specific implementation schemes, a clamp is set on the fixed fulcrum, and the shell of the displacement monitoring system is clamped by the clamp. The connection between the sensing end of the displacement monitoring system and the top of the movable bottom plate can be referred to the instructions for use or existing conventional methods.

[0030] In order to improve the accuracy of the test, there are two displacement monitoring systems, which are symmetrically arranged on both sides of the counterweight. The installation method of the two displacement monitoring systems is the same.

[0031] A method for determining buoyancy reduction coefficients in different formations comprises the following steps:

[0032] S1: Lay a 100-mesh fine yarn layer on the bottom and side walls of the model box to prevent or reduce the amount of soil samples entering the water supply system.

[0033] S2: Fill the soil sample in the model box according to the test requirements. It is required to fill in layers and compact and level. The thickness of each layer is 9 to 11 cm. After each layer is filled, compacted and leveled, use a level ruler to check. After confirming that it is leveled, fill the next layer. After the second layer of soil sample is filled, place the model bucket in the middle of the soil sample and continue filling until it is completed. The soil sample between the model box and the model bucket forms a soil sample layer; a round hole is opened at the bottom of the model bucket, and the round hole is covered with an elastic film. The round hole on the elastic film is covered with a movable bottom plate, and a counterweight is placed on the movable bottom plate. At the same time, fill the model bucket with water. The model bucket is filled with water to ensure that the bottom of the movable bottom plate is in close contact with the soil sample, and to avoid deformation of the model bucket caused by the external soil sample squeezing inward;

[0034] S3: After the filling is completed, add water to the water supply system until the water level in the water supply system is higher than the surface of the soil sample layer, and let it stand for three days. During this period, observe at any time and ensure that the water level in the water supply system is always higher than the height of the soil sample layer in the model box (water needs to be added as needed) to make the soil sample layer in the model box fully saturated. Then observe the water level every 1 hour for 6 hours. If the water level in the water supply system does not change, it means that the soil sample layer in the model box is completely saturated; if the water level in the water supply system changes, repeat the above steps until the water level in the water supply system does not change after 6 hours of continuous observation; then drain the water in the model barrel, such as using a water pipe and a water pump to pump out the water in the model barrel. When draining, pay attention to press a heavy object on the upper part of the model barrel (2) to prevent the model barrel (2) from floating upward during the drainage process;

[0035] S4: installing a displacement monitoring system for monitoring the displacement of the movable bottom plate in the model barrel, placing a counterweight barrel equipped with a counterweight block on the movable bottom plate at the bottom of the model barrel, and injecting water into the counterweight barrel;

[0036] S5: Select the stable water head pressure as am, that is, add water to the water supply system so that the water level in the water supply system is always higher than the bottom am of the movable bottom plate. After waiting for 2 hours, observe that the water level in the pressure measuring tube does not change within 30 minutes, which means that the stable water head pressure has been reached;

[0037] S6: Adjust the weight of the counterweight barrel by removing the counterweight block in the counterweight barrel as a rough adjustment and by pumping out the water in the counterweight barrel as a fine adjustment. After one weight adjustment, wait for 15 minutes to observe whether the reading of the displacement monitoring system changes. If so, stop the test and use an electronic scale to weigh the total weight of the remaining counterweight and the movable bottom plate (including the counterweight block, the counterweight barrel, the movable bottom plate and the remaining water in the counterweight barrel); if there is no change, continue to pump out the water in the counterweight barrel until the reading of the displacement monitoring system changes, and record the total mass of the remaining counterweight and the movable bottom plate (including the counterweight block, the counterweight barrel, the movable bottom plate and the remaining water in the counterweight barrel) to complete the first set of tests;

[0038] S7: then put the taken out counterweight block back into the counterweight barrel, and refill the counterweight barrel with water, wait for 15 minutes and then start the test, repeat S6 to complete the second set of tests;

[0039] S8: Repeat S7 until more than three sets of experiments are completed;

[0040] S9: The buoyancy reduction coefficient of each group of tests is obtained by calculating the ratio of the measured buoyancy to the theoretical buoyancy, and the target buoyancy reduction coefficient is obtained by taking the average value. It should be noted that the trigger pressure of the monitoring system should be included in the measured buoyancy.

[0041] In order to improve accuracy, in the above S6 to S8, when taking out the counterweight block and the counterweight barrel, the movable bottom plate should be pressed down in each group of tests to prevent the movable bottom plate from completely floating up and affecting the parallel tests within the group.

[0042] In order to further improve the test accuracy, the above selection is different from the stable head pressure in S5, and the test steps S5 to S9 are repeated to obtain the buoyancy reduction coefficient under different stable head pressures. If the buoyancy reduction coefficients under different stable head pressures are relatively consistent, the test is completed.

[0043] The buoyancy reduction coefficients under different stable water head pressures obtained by the present application are relatively consistent, which proves that the test accuracy of the present application is high.

[0044] When conducting the buoyancy reduction test under combined formation conditions, different soil samples are filled in layers in S2 according to the test requirements, and the quality of each layer of test medium is controlled and flattened. The thickness of each layer is required to be 9 to 11 cm. After each layer is filled, compacted and leveled, it is checked with a spirit level. After confirming that it is leveled, the next layer is filled, and then S3 to S9 are carried out in sequence to obtain the buoyancy reduction coefficient under combined formation conditions.

[0045] The technologies not mentioned in the present invention are all referred to the prior art.

[0046] Beneficial effects: Compared with the prior art, the indoor test device for determining the buoyancy reduction coefficient in different strata of the present invention has a simple structure, convenient test operation, and obvious test phenomenon. It only needs to quantitatively reduce the counterweight in batches and observe the change in the displacement meter reading to measure the real buoyancy of the movable bottom plate, and the buoyancy reduction coefficient can be obtained by comparing it with the theoretical buoyancy. The test device only needs to change the filling soil in the model box to obtain the buoyancy reduction coefficient in different strata, and has strong applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 A schematic diagram of the structure of an indoor test device for determining buoyancy reduction coefficients in different formations according to the present invention;

[0048] In the figure, 1 is a model box, 2 is a model barrel, 3 is a movable bottom plate, 4 is a counterweight, 5 is a displacement monitoring system, 6 is a water supply system, 7 is a soil sample layer, and 8 is a support column. DETAILED DESCRIPTION

[0049] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0050] The directional words such as up and down, left and right, horizontal, and vertical in this application are all based on the relative directions or positional relationships shown in the drawings and should not be understood as absolute limitations on this application.

[0051] Example 1

[0052] An indoor test device for determining buoyancy reduction coefficients in different strata, comprising a model box 1, a model barrel 2, a movable bottom plate 3, a counterweight 4, a displacement monitoring system 5 and a water supply system 6;

[0053] The model barrel 2 is located inside the model box 1, and a soil sample layer 7 is filled between the model box 1 and the model barrel 2; a round hole is opened at the bottom of the model barrel 2, and the round hole is covered with an elastic film, which is fixed to the model barrel by a high-strength adhesive tape and completely covers the round hole, so that water in the soil sample layer will not penetrate into the model barrel;

[0054] The movable bottom plate 3 is in an inverted convex shape, the bottom diameter of the movable bottom plate 3 is 2 mm smaller than the diameter of the circular hole at the bottom of the model barrel 2, the top diameter of the movable bottom plate 3 is larger than the diameter of the circular hole at the bottom of the model barrel 2, the movable bottom plate 3 is movably covered on the circular hole at the bottom of the model barrel 2, and the movable bottom plate 3 is located above the elastic film;

[0055] The counterweight 4 is placed on the top of the movable bottom plate 3;

[0056] The displacement monitoring system 5 is arranged in the model barrel 2 and is used to monitor the displacement of the movable bottom plate 3;

[0057] The water supply system 6 is communicated with the model box 1 and supplies water to the model box 1 .

[0058] The diameter of the circular hole at the bottom of the model barrel 2 is slightly larger than the diameter of the bottom of the movable bottom plate, so as to ensure that the bottom of the movable bottom plate can smoothly pass through the circular hole, while the periphery of the top is retained above the circular hole.

[0059] During the test, the target soil sample is placed in the model box to simulate the soil sample layer of the stratum, and the model bucket is placed in the middle of the soil sample layer. A film, a movable bottom plate, a displacement monitoring system, and a counterweight are installed on the model bucket, and the specified water level is reached in the model box through the water supply system. The counterweight is quantitatively reduced in batches until the reading of the displacement monitoring system 5 changes, and the remaining counterweight and the total weight of the movable bottom plate are recorded to obtain the measured buoyancy. The theoretical buoyancy is calculated according to the water level in the model box, and the buoyancy reduction coefficient can be obtained by dividing the two. The above device can carry out buoyancy reduction tests under single or combined stratum conditions indoors, thereby giving the buoyancy reduction coefficient of the stratum corresponding to the specific project.

[0060] Example 2

[0061] On the basis of Example 1, the following improvements are further made: in order to facilitate assembly and testing, the water supply system 6 is a trough-type structure, support columns 8 are provided on both sides of the bottom of the water supply system 6, the model box 1 is located in the water supply system 6, and the two ends of the bottom of the model box 1 fall on the two support columns 8; the side walls and the bottom of the model box 1 are evenly distributed with seepage holes. In order to facilitate testing, the peripheral height of the water supply system 6 is higher than the soil sample layer 7. In order to facilitate the observation of the pressure change in the water supply system, a pressure measuring tube is provided on the water supply system 6, and the pressure measuring tube can measure the water pressure at the bottom of the water supply system 6. In order to avoid or reduce the soil sample from entering the water supply system, a mesh layer is provided between the model box 1 and the soil sample layer 7.

[0062] Example 3

[0063] On the basis of Example 2, the following improvements are further made: the model box 1 and the model barrel 2 are both made of 12 mm thick organic glass plates bonded together, and have good sealing and strength after testing, and can meet the test requirements.

[0064] The model box 1 is a rectangular parallelepiped structure, and the dimensions of the model box 1 are: 500mm wide × 500mm long × 630mm high, and 12mm thick; the model barrel 2 is a cylindrical structure, and the dimensions of the model barrel 2 are: 250mm in diameter, 400mm in height, and 12mm in thickness; the diameter of the circular hole at the bottom of the model barrel 2 is 132mm.

[0065] The movable bottom plate 3 is formed by concentrically bonding two 12 mm thick cylindrical glass plates to form an inverted convex shape. The diameters of the two 12 mm thick cylindrical glass plates are 130 mm and 180 mm respectively. The total weight of the movable bottom plate 3 is 440 g, and the movable bottom plate 3 is polished to a smooth structure.

[0066] Example 4

[0067] On the basis of Example 3, the following improvements are further made: for the convenience of testing, the counterweight 4 is composed of an annular iron block and a counterweight barrel. The number of annular iron blocks is 3, all located in the counterweight barrel, and each annular iron block weighs 500g. The displacement monitoring system 5 uses the PM11-1 displacement sensor produced by Xiyu Electronics Factory in Taizhou, Zhejiang, and an external WXY / C-DC-WX digital display directly displays the displacement data. Because its sensor requires a DC environment to work, the S-120-24 switching power supply produced by Wenzhou Junlin Electric Technology Co., Ltd. is selected to convert the laboratory 220V AC power into 24V DC power. Connect the switching power supply to the digital display, and then connect the laboratory power supply externally, so that the displacement data monitored by the displacement sensor can be displayed in real time on the digital display.

[0068] Example 5

[0069] On the basis of Example 4, the following improvements are further made: for the convenience of installation, the shell of the displacement monitoring system 5 is connected to the inner wall of the model barrel 2 through a fixed fulcrum, and the sensing end of the displacement monitoring system 5 is connected to the top of the movable bottom plate 3. The material used for the fixed fulcrum is a plexiglass plate, which is glued to the inner wall of the model barrel. A clamp is set on the fixed fulcrum, and the shell of the displacement monitoring system 5 is clamped by the clamp. The connection between the sensing end of the displacement monitoring system 5 and the top of the movable bottom plate 3 can be referred to the instructions for use or the existing conventional method. In order to improve the accuracy of the test, there are two displacement monitoring systems 5, and the two displacement monitoring systems 5 are symmetrically arranged on both sides of the counterweight 4. The installation method of the two displacement monitoring systems 5 is the same.

[0070] In this example, the model box (1) and the water supply system (6) are an integrated structure, the water supply system (6) is a trough-type structure, and water baffles arranged parallel to the width direction are provided at both ends of the water supply system (6) (that is, the water baffles penetrate the entire width direction of the water supply system 6, and the length of the water supply system 6 is divided into three sections, such as Figure 1 As shown, from left to right are the water supply system 6, the model box 1 and the water supply system 6. The pressure measuring tubes are arranged in the water supply system 6 on both sides of the model box 1 to test the bottom pressure of the water supply system 6 on both sides). The bottoms of the two water baffles fall on the two support columns (8). The bottoms of the two water baffles are connected together by a connecting bottom plate. The space between the two water baffles and the connecting bottom plate on the water supply system (6) forms the model box (1), that is, the model box (1) is a front and rear side wall of a part of the length of the water supply system (6). The water baffles at both ends are respectively connected with the space between the side walls at both ends of the water supply system (6) and the space between the connecting bottom plate and the bottom of the water supply system (6) to form an effective water supply system (6); the two water baffles and the connecting bottom plate are provided with seepage holes, which are distributed in a matrix. The diameter of the seepage holes is 1 cm, and the center interval between two adjacent seepage holes is 10 cm. Figure 1As shown, the water supply system (6) is a rectangular trough structure, and the dimensions of the water supply system (6) are: 500 mm wide × 700 mm long × 680 mm high, and the support column 8 is 50 mm high; at both ends of the water supply system (6), a water baffle plate is provided parallel to the width direction at a distance of 100 mm from the two end side walls, and the top of the water baffle plate is flush with the edge of the water supply system (6) and the bottom is 50 mm away from the bottom of the water supply system (6). That is, the length of 100 mm at both ends of the model box (1) and the height of 50 mm at the bottom form a through effective water supply system (6).

[0071] Select the soil sample to be tested as the test medium and conduct the buoyancy reduction test. The specific test steps are as follows:

[0072] S1: Figure 1 As shown, a fine sand mesh layer with a mesh number of 100 is laid at the bottom and left and right sides of the model box 1 to prevent or reduce the soil sample from entering the water supply system 6.

[0073] S2: Figure 1 As shown, fill the model box 1 with soil samples according to the test requirements. The soil samples are standard sand with uniform particles, clean and free of impurities. It is required to fill in layers, control the quality of each layer of soil samples and compact and level them. The thickness of each layer is required to be 10 cm. After each layer is filled, compacted and leveled, use a level ruler to check. After confirming the leveling, fill the next layer. After the second layer of soil samples is filled, place the model bucket 2 in the middle of the soil samples, and continue filling until it is completed. The soil samples between the model box 1 and the model bucket 2 form a soil sample layer 7; put a counterweight 4 on the movable bottom plate, and fill the model bucket 2 with water to ensure that the bottom of the model bucket 2 and the bottom of the movable bottom plate are in close contact with the soil samples, and at the same time, the deformation of the model bucket 2 can be reduced.

[0074] S3: Figure 1 As shown, after the filling is completed, water is added to the water supply system 6 until the water level in the water supply system 6 is higher than the surface of the soil sample layer 7, and it is left to stand for three days. During this period, it is observed at any time, and it is ensured that the water level in the water supply system 6 is always higher than the height of the soil sample layer 7 in the model box 1, so that the soil sample layer 7 in the model box 1 is fully saturated. Then the water level is observed every 1 hour, and the continuous observation for 6 hours. If the water level in the water supply system 6 does not change, it means that the soil sample layer 7 in the model box 1 is completely saturated, and the water in the model barrel 2 is drained, such as using a water pipe and a water pump to pump out the water in the model barrel 2. When draining, it should be noted that a heavy object should be pressed on the upper part of the model barrel 2 to prevent the model barrel 2 from floating upward during the drainage process.

[0075] S4: Install the displacement monitoring system 5 , place the counterweight block and the counterweight barrel 4 on the movable bottom plate 3 at the bottom of the model barrel 2 , and inject water into the counterweight barrel 4 .

[0076] S5: First, a stable water head pressure of 0.1 m is selected, that is, water is injected into the water supply system 6 so that the water level in the water supply system 6 is always 0.1 m higher than the bottom of the movable bottom plate 3. After standing for 2 hours, the water level in the pressure measuring tube is observed to have no change within 30 minutes, which means that the stable water head pressure has been reached.

[0077] S6: Use a syringe to extract about 20g of water from the counterweight barrel 4 each time. After each extraction, wait for 15 minutes to observe whether the reading of the displacement meter 5 changes. If it changes, stop the test and use an electronic scale to weigh the total weight of the counterweight system (including the counterweight block, the counterweight barrel, the movable bottom plate and the remaining water in the counterweight barrel); if there is no change, continue to extract water from the counterweight barrel 4 until the reading of the displacement meter 5 changes, and record the total weight of the remaining counterweight and the movable bottom plate 3 (including the counterweight block, the counterweight barrel, the movable bottom plate and the remaining water in the counterweight barrel) to complete a set of tests. Refill the counterweight barrel 4 with water, put back the counterweight block and the counterweight barrel, wait for 15 minutes before starting the test, repeat the above steps, and complete a set of parallel tests. Under a stable water level, a total of five sets of parallel tests need to be completed. The test data are recorded as shown in Table 1. The buoyancy reduction coefficient is obtained by calculating the ratio of the measured buoyancy to the theoretical buoyancy. At this time, it should be noted that the trigger pressure 3N of the monitoring system should be included in the measured buoyancy. When taking out the counterweight barrel and the counterweight 4 on the movable bottom plate 3, be careful to place heavy objects on the movable bottom plate 3 to prevent the movable bottom plate 3 from completely floating and affecting the parallel tests within the group.

[0078] Table 1 Single formation buoyancy reduction test data record and calculation table

[0079]

[0080] S7: Continue to select stable water head pressures of 0.2m and 0.3m, repeat test steps S5 and S6, and complete the test; the buoyancy reduction coefficients under different pressures in the sandy soil layer measured by the above device are basically consistent, as shown in Table 1, which proves that the test is highly accurate.

[0081] Example 6

[0082] Different from Example 5, a buoyancy reduction test is conducted under combined formation conditions. In S2, different soil samples are filled in layers according to the test requirements. The first layer is filled with sand, which is standard sand with uniform, clean particles and no impurities; the second layer is filled with clay, which has been treated to remove impurities, has clean particles, and basically does not contain impurities such as gravel and organic matter. The quality of each layer of the test medium is controlled and flattened. The thickness of each layer is required to be 10 cm. After each layer is filled, compacted and leveled, it is checked with a level ruler. After confirming the leveling, the next layer is filled. One layer of sand is filled with a total thickness of 10 cm; 5 layers of clay are filled with a total thickness of 50 cm. After filling, S3 to S7 are carried out in sequence. The buoyancy reduction test records and calculation results under combined formation conditions are shown in Table 2. At this time, it should be noted that the trigger pressure of the monitoring system should be included in the measured buoyancy.

[0083] Table 2 Combined formation buoyancy reduction test data record and calculation table

[0084]

[0085] The indoor test devices for determining the buoyancy reduction coefficient in different strata in the above examples have the advantages of simple operation and obvious test phenomena. They can provide theoretical references for the reasonable selection of water levels for anti-floating defense in underground projects and have great promotion and application value.

Claims

1. An indoor test device for determining buoyancy reduction coefficients in different formations, characterized in that: It comprises a model box (1), a model barrel (2), a movable bottom plate (3), a counterweight (4), a displacement monitoring system (5) and a water supply system (6); The model barrel (2) is located inside the model box (1), and a soil sample layer (7) is filled between the model box (1) and the model barrel (2); a round hole is opened at the bottom of the model barrel (2), and the round hole is covered with an elastic film to ensure that external water does not penetrate into the model barrel; The movable bottom plate (3) is in an inverted convex shape, the bottom diameter of the movable bottom plate (3) is 1.5 to 2.5 mm smaller than the diameter of the circular hole at the bottom of the model barrel (2), the top diameter of the movable bottom plate (3) is larger than the diameter of the circular hole at the bottom of the model barrel (2), the movable bottom plate (3) is movably covered on the circular hole at the bottom of the model barrel (2), and the movable bottom plate (3) is located above the elastic film; The counterweight (4) is placed on the top of the movable bottom plate (3); A displacement monitoring system (5) is arranged in the model barrel (2) and is used to monitor the displacement of the movable bottom plate (3); The water supply system (6) is connected to the model box (1) and supplies water to the model box (1).

2. The indoor test device for determining buoyancy reduction coefficients in different strata according to claim 1, characterized in that: The water supply system (6) is a trough-shaped structure. Support columns (8) are provided on both sides of the bottom of the water supply system (6). The model box (1) is located in the water supply system (6). Both ends of the bottom of the model box (1) fall on the two support columns (8). The side walls and the bottom of the model box (1) are evenly distributed with water seepage holes.

3. The indoor test device for determining buoyancy reduction coefficients in different strata according to claim 1, characterized in that: The model box (1) and the water supply system (6) are an integrated structure. The water supply system (6) is a trough-shaped structure. Water baffles are arranged in parallel along the width direction at both ends of the water supply system (6). The bottoms of the two water baffles fall on two support columns (8). The bottoms of the two water baffles are connected together through a connecting bottom plate. The space between the two water baffles and the connecting bottom plate on the water supply system (6) forms the model box (1). The two water baffles and the connecting bottom plate are both provided with water seepage holes.

4. The indoor test device for determining the buoyancy reduction coefficient in different strata according to claim 2 or 3, characterized in that: A pressure measuring pipe is provided on the water supply system (6); the peripheral height of the water supply system (6) is higher than the height of the soil sample layer (7).

5. The indoor test device for determining the buoyancy reduction coefficient in different strata according to any one of claims 1 to 4, characterized in that: A gauze layer is provided between the model box (1) and the soil sample layer (7); The model box (1) is formed by gluing together organic glass plates with a thickness of 10 to 15 mm; the model box (1) is a rectangular parallelepiped structure, the height of the model box (1) is 600 to 650 mm, and the length and width are both 480 to 520 mm; The model barrel (2) is formed by gluing organic glass plates with a thickness of 10 to 15 mm; the model barrel (2) is a cylindrical structure with a diameter of 230 to 270 mm and a height of 380 to 420 mm.

6. The indoor test device for determining the buoyancy reduction coefficient in different strata according to any one of claims 1 to 4, characterized in that: The diameter of the round hole at the bottom of the model barrel (2) is 130 to 135 mm; the elastic film is adhered to the bottom of the model barrel (2) by means of an adhesive tape and completely covers the round hole; The movable bottom plate (3) is formed by concentrically bonding two cylindrical glass plates with a thickness of 10 to 15 mm and different diameters. The diameters of the two cylindrical glass plates are 128 to 132 mm and 178 to 182 mm respectively. The diameter of the smaller cylindrical glass plate should be 1.5 to 2.5 mm smaller than the diameter of the circular hole at the bottom of the model barrel (2). The movable bottom plate (3) is a smooth structure.

7. The indoor test device for determining the buoyancy reduction coefficient in different strata according to any one of claims 1 to 4, characterized in that: The counterweight (4) is composed of an annular iron block and a counterweight barrel. The number of the annular iron blocks is more than two and both are located in the counterweight barrel. The displacement monitoring system (5) is a PM11-1 type displacement sensor; the shell of the displacement monitoring system (5) is connected to the inner wall of the model barrel (2) through a fixed fulcrum, and the sensing end of the displacement monitoring system (5) is connected to the top of the movable bottom plate (3); there are two displacement monitoring systems (5), and the two displacement monitoring systems (5) are symmetrically arranged on both sides of the counterweight (4).

8. A method for determining buoyancy reduction coefficients in different strata, using the indoor test device for determining buoyancy reduction coefficients in different strata according to any one of claims 1 to 7, characterized in that: The steps include: S1: A fine yarn mesh layer is laid on the bottom and side walls of the model box (1) to prevent or reduce the amount of soil sample from entering the water supply system (6). S2: Fill the model box (1) with soil samples according to the test requirements. The filling is required to be layered and compacted and leveled. The thickness of each layer is 9 to 11 cm. After the second layer of soil samples is filled, the model bucket (2) is placed in the middle of the soil samples and the filling is continued until the filling is completed. The soil samples between the model box (1) and the model bucket (2) form a soil sample layer (7). At this time, a heavy object should be placed on the movable bottom plate (3) and the model bucket (2) is filled with water. The model bucket (2) is filled with water to ensure that the bottom of the movable bottom plate (3) is in close contact with the soil sample and to prevent the model bucket (2) from being deformed due to the inward squeezing of the external soil sample. S3: After the filling is completed, water is added to the water supply system (6) until the water level in the water supply system (6) is higher than the surface of the soil sample layer (7). The water is left to stand for three days, and observation is performed at any time during the period to ensure that the water level in the water supply system (6) is always higher than the height of the soil sample layer (7) in the model box (1), so that the soil sample layer (7) in the model box (1) is fully saturated. Then the water level is observed every 1 hour, and the observation is continued for 6 hours. If the water level in the water supply system (6) does not change, it means that the soil sample layer (7) in the model box (1) is fully saturated; if the water level in the water supply system (6) changes, the above steps are repeated until the water level in the water supply system (6) does not change; the water in the model barrel (2) is drained. When draining, it should be noted that a heavy object should be pressed on the upper part of the model barrel (2) to prevent the model barrel (2) from floating upward during the drainage process; S4: installing a displacement monitoring system (5) for monitoring the displacement of the movable bottom plate (3) in the model barrel (2), placing a counterweight barrel (4) equipped with a counterweight block on the movable bottom plate (3) at the bottom of the model barrel (2), and injecting water into the counterweight barrel (4); S5: Select a stable water head pressure of am, that is, inject water into the water supply system (6) so that the water level in the water supply system (6) is always higher than the bottom am of the movable bottom plate (3). After waiting for 2 hours, observe that the water level in the pressure measuring tube does not change within 30 minutes, which means that the stable water head pressure has been reached; S6: The weight of the counterweight barrel (4) is adjusted by removing the counterweight block in the counterweight barrel (4) as a rough adjustment and by pumping out the water in the counterweight barrel (4) as a fine adjustment. After one weight adjustment is completed, wait for 15 minutes to observe whether the reading of the displacement monitoring system (5) changes. If so, stop the test immediately and use an electronic scale to weigh the total weight of the remaining counterweight and the movable bottom plate (3). If there is no change, continue to pump out water in the counterweight barrel (4) until the reading of the displacement monitoring system (5) changes, and record the total weight of the remaining counterweight and the movable bottom plate (3) to complete the first set of tests. S7: then put the taken out counterweight block back into the counterweight barrel (4), and refill the counterweight barrel (4) with water, wait for 15 minutes before starting the test, and repeat S6 to complete the second set of tests; S8: Repeat S7 until more than three sets of experiments are completed; S9: The buoyancy reduction coefficient of each group of tests is obtained by calculating the ratio of the measured buoyancy to the theoretical buoyancy, and the target buoyancy reduction coefficient is obtained by taking the average value. It should be noted that the trigger pressure of the monitoring system should be included in the measured buoyancy when recording.

9. The method for determining buoyancy reduction coefficients in different strata according to claim 8, characterized in that: In S6 to S8, when taking out the counterweight block and the counterweight barrel (4) in each group of tests, the movable bottom plate (3) should be pressed down to prevent the movable bottom plate (3) from completely floating up and affecting the parallel tests within the group.

10. The method for determining buoyancy reduction coefficients in different strata according to claim 8 or 9, characterized in that: Select a stable water head pressure different from that in S5, and repeat test steps S5 to S9 to obtain the buoyancy reduction coefficients under different stable water head pressures. If the buoyancy reduction coefficients under different stable water head pressures are relatively consistent, the test is completed; When conducting the buoyancy reduction test under combined formation conditions, different soil samples are filled in layers in S2 according to the test requirements, the quality of each layer of test medium is controlled and flattened, and the thickness of each layer is required to be 9 to 11 cm. S3 to S9 are then carried out in sequence to obtain the buoyancy reduction coefficient under combined formation conditions.

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