Testing device for testing water-salt migration and shear strength of novel partition layer of salinized soil roadbed under THMC coupling condition and use method of testing device for testing water-salt migration and shear strength of novel partition layer of salinized soil roadbed under THMC coupling condition

Abstract

The invention discloses a test device for testing water-salt migration and shear strength of a novel partition layer of a salinized soil roadbed under THMC coupling conditions and a use method of the test device. The device comprises a water-salt migration system, a temperature control system, a vertical loading system, a horizontal shear system and a data monitoring system. Wherein the temperature system simulates roadbed temperature difference, the water-salt system reproduces internal water-salt migration, the vertical loading simulates an upper load, the horizontal shearing system directly measures the strength of a soil sample, and the data system monitors and records moisture, salt, temperature, stress and settlement changes in the soil sample in real time. The device can simulate a complex THMC coupling environment, reveal water and salt migration, temperature stress distribution and settlement rules, and can execute a direct shear test under the condition of keeping the original state of a soil sample. On the whole, the method has high feasibility, practicability and applicability, and a powerful tool is provided for salinized soil roadbed research.

Description

Technical Field

[0001] The present invention relates to the technical field of water-salt migration detection inside saline soil roadbed, and in particular to a test device and a test method for monitoring water-salt migration and shear strength of a saline soil roadbed fault layer under THMC coupling conditions. Background Art

[0002] Saline soil is a general term for various salinized and alkaline soils. Due to its special physical and chemical properties, saline soil subgrade often leads to poor roadbed stability and road surface damage in road engineering. The migration and uneven distribution of water and salt in saline subgrade soil may cause salt expansion, frost expansion, and collapse, which may aggravate the uneven settlement of roads and shorten the service life of roads.

[0003] At present, the commonly used composite geomembrane barrier technology ("two fabrics and one membrane") is used to block the migration of water and salt inside the roadbed. However, the phenomenon of increased moisture content in the shallow soil caused by the "pot cover effect" cannot be alleviated.

[0004] Using polymer-modified bentonite waterproof liner instead of traditional composite geomembrane as a new type of isolation layer can effectively alleviate the "pot lid effect". Bentonite particles absorb a large amount of water through hydration, which can effectively alleviate the uneven settlement of the road surface caused by water vapor condensation and accumulation under the isolation layer. However, there is still a lack of in-depth experimental research on its service performance and isolation mechanism in the complex environment of THMC (thermal-hydraulic-mechanical-chemical) coupling.

[0005] At present, there is still a lack of specialized test equipment for effectively simulating and testing the water-salt migration process and strength characteristics of this new type of barrier layer in saline soil roadbed. This situation seriously hinders the in-depth understanding of the damage mechanism of saline soil roadbed, limits the optimization of roadbed design and construction technology, and thus affects the improvement of road maintenance efficiency.

[0006] Therefore, developing a test device that can effectively simulate and test the water-salt migration and shear strength of the new barrier layer of saline soil roadbed has important scientific value and engineering significance for in-depth understanding of the destruction mechanism of saline soil roadbed, optimizing roadbed design and construction technology, and improving road maintenance efficiency. Summary of the invention

[0007] In order to achieve the above-mentioned purpose, the present invention provides a test device for testing the water-salt migration and shear strength of a new type of barrier layer in saline soil roadbed under THMC coupling conditions. The device can simulate the water-salt migration of a new type of barrier layer in a saline soil roadbed in the vertical direction under THMC coupling conditions to obtain the water-salt migration law inside the roadbed. At the same time, a direct shear test can be performed on the soil sample after the water-salt migration test without disturbing the soil sample, and the shear strength of the soil sample can be measured, which solves the problem that the current technology lacks a test device for testing the water-salt migration law and shear strength of a new type of barrier layer in a saline soil roadbed under THMC coupling conditions.

[0008] The technical solution of the present invention is as follows:

[0009] A test monitoring device for water-salt migration and strength of a saline soil roadbed fault layer, the device includes: a water-salt migration system, a temperature control system, a vertical loading system, a horizontal shear system and a data acquisition system. The water-salt migration system includes an insulation model box, a polystyrene foam board, saline soil, a model box base, a water collection beaker, and a water supply Martens flask. The water-salt migration system simulates the water-salt migration inside the saline soil, the polystyrene foam board isolates the heat exchange inside and outside the model box, and the water supply Martens flask is used to provide brine to supplement the bottom water source. The base of the model box is equipped with universal wheels, which is convenient for pushing the model box to a designated position after the filling is completed. The temperature control system includes a refrigerant outlet conduit of thermostatic bath I, a refrigerant inlet conduit of thermostatic bath I, a thermostatic bath I, a thermostatic bath II, a refrigerant outlet conduit of thermostatic bath II, a refrigerant inlet conduit of thermostatic bath II, an upper temperature guide plate, and a lower temperature guide plate. The refrigerant in the thermostatic bath is pumped to the upper and lower temperature guide plates of the model box by a water pump, and then the temperature guide plate is used to control the temperature of the upper and lower ends of the soil body, thereby realizing the simulation of the temperature difference. The vertical loading system includes a loading plate, a vertical drive device, and a reaction frame. The vertical loading system can simulate various types of traffic loads and the upper constant load of the roadbed borne by the roadbed partition layer. The horizontal shear system includes a model box fixing fixture, a shear box, a horizontal drive device, and a slide rail. The horizontal shear system can complete the direct measurement of the strength of the soil sample after the water-salt migration test. The data acquisition system includes a vertical force sensor, a vertical displacement sensor, a soil pressure sensor, a soil temperature, humidity and salt sensor, a horizontal force sensor, a horizontal displacement sensor and a data acquisition instrument. The data acquisition system can record the changes and distribution of moisture, salt, temperature, stress inside the soil sample, the settlement of the soil sample, the shear strength and shear displacement of the soil sample at any time.

[0010] The data acquisition system, vertical loading system and horizontal shearing system are all connected to a control computer, and the loading parameters and acquisition parameters are set by the control computer.

[0011] The insulation model box is composed of four shear boxes, including from top to bottom: a first layer of shear boxes, a second layer of shear boxes, a third layer of shear boxes and a fourth layer of shear boxes. The shear boxes are fixed by bolts to facilitate layered filling and compaction and subsequent direct shear tests. The insulation model box is wrapped with polystyrene foam boards to prevent heat exchange with the external environment. The inner walls of the front and rear walls of the insulation model box are grooved, and a sliding settlement plate is installed in the groove, which can record the settlement changes of soil layers at different depths. The rest of the slide is covered with a folding baffle to prevent soil particles and moisture from entering the sliding groove. Holes are opened on the left and right side walls of the insulation model box to pass the data cable of the sensor, the water supply pipe of the water supply Martens flask and the water collection pipe of the water collection beaker. Universal wheels are installed on the base of the insulation model box to facilitate moving the model box into the shear box after filling.

[0012] The shear box is equipped with a model box fixing fixture. After the insulation model box to be filled is pushed into the shear box, the fixture is used to support the model box wall from both sides, and the tightness of the fixture is adjusted by bolts.

[0013] The bottom of the temperature conducting plate is provided with a refrigerant liquid inlet pipe interface and a refrigerant liquid outlet pipe interface, which are respectively connected to the refrigerant liquid inlet conduit of the thermostatic tank I and the refrigerant liquid outlet conduit of the thermostatic tank I. A flow path is provided inside the temperature conducting plate to ensure that the refrigerant flows through the entire temperature conducting plate and then returns to the thermostatic tank I, thereby realizing the temperature control of the bottom of the soil sample.

[0014] The present invention also provides a method for testing the water-salt migration and shear strength of a new type of barrier layer of a saline soil roadbed using the above-mentioned test device, which is characterized in that it comprises the following steps:

[0015] Water-salt migration test under THMC coupling conditions:

[0016] 1) Prepare test materials, including saline soil, medium-coarse sand, bentonite waterproof liner and sensors;

[0017] 2) Fix the insulation model box to the model box base with bolts and wrap it with a polystyrene foam board; install the lower temperature conducting plate to the bottom of the model box and connect it to the constant temperature bath;

[0018] 3) Fill the saline soil in layers into the insulation model box and compact it, lay medium-coarse sand on the top of the saline soil and compact it, then cover it with a bentonite waterproof pad, and lay another layer of medium-coarse sand on the pad and compact it; install the temperature conduction plate and connect it to the constant temperature tank; finally, connect the Marsh water supply bottle and the water collection beaker to the side walls of the insulation model respectively.

[0019] 4) During the soil filling process, soil pressure sensors and soil temperature, humidity and salinity sensors are buried layer by layer, and the sensor data lines are extended to the data acquisition instrument outside the box through the holes in the side wall of the model box; after each layer of soil sample is compacted and leveled, the sliding settlement plate is moved to the flat surface of each layer of soil sample, the corresponding height is recorded, and then the next layer of soil sample is filled.

[0020] 5) Push the assembled model box into the shear model box through the universal wheels on the model box base, adjust the model box fixing fixture to fix the insulation model box to prevent movement during the test; cover the loading plate on the upper temperature conducting plate; adjust the position on the shear box slide rail through the horizontal drive device so that the loading plate is aligned with the vertical drive device, and then apply a vertical constant load through the vertical drive device;

[0021] 6) When simulating the freezing process in winter, the upper temperature guide plate is set at a lower temperature, the lower temperature guide plate is set at a higher temperature, and the soil sample freezes from top to bottom; when simulating the melting process, the temperature of the upper temperature guide plate is increased, and the soil sample melts from top to bottom. During the freeze-thaw cycle, water and salt will continue to migrate in the soil sample, accompanied by phase changes such as water freezing and melting and salt crystallization and dissolution, thereby simulating a series of catastrophic phenomena such as frost heave, salt heave, and dissolution in saline soil roadbed. The migration of water and salt causes changes in soil strength. Under vertical loading, the settlement of soil samples is recorded in real time by vertical displacement sensors. During the entire test cycle, if water penetrates the isolation layer, it will be collected by the water collection cylinder; after the experiment, in the process of digging out the soil samples in layers, the height of the sliding settlement plate after settlement is recorded, and the settlement of soil samples at different depths is calculated.

[0022] 7) The data acquisition instrument collects the changes in temperature, moisture content, salt content, soil pressure of the soil sample under THMC coupling conditions over time and the distribution in the vertical direction in real time and records them in the control computer.

[0023] 8) The present invention realizes the testing of the water-salt migration law and shear strength of the new type of barrier layer of saline soil roadbed under THMC coupling conditions; Direct shear test:

[0024] 9) When the water-salt migration experiment is completed, part of the shear box of the insulation model box is disassembled as needed, fixed on the upper shear box fixing bracket, and the soil sample is consolidated by applying normal stress through the vertical loading system, and the vertical deformation is recorded;

[0025] 10) After the vertical deformation of the soil sample is stable, apply horizontal shear force through the horizontal shear system and record the shear force and shear displacement;

[0026] 11) Evaluate the performance of the insulation layer based on the test results.

[0027] The innovative features and beneficial effects of the present invention are as follows:

[0028] 1. The test device for studying the water-salt migration and strength of the saline soil roadbed barrier layer provided by the present invention has a simple structure, is easy to assemble, and has a wide range of application prospects. By filling the model box with different materials, it can simulate various types of saline soil roadbed barrier layer structures including traditional composite geomembrane barrier layers, aeolian sand barrier layers, and new polymer modified bentonite liner barrier layers. The data monitored by multiple sensors arranged at different positions form a horizontal comparison of the water-salt migration barrier capacity of different barrier layers, filling the gap in the water-salt migration test monitoring device for new barrier layers of saline soil roadbeds under complex conditions, and is worthy of promotion.

[0029] 2. Compared with the existing water migration test instruments, the present invention can simulate the water-salt migration of the new barrier layer of saline soil roadbed under different water, salinity, temperature and load coupling conditions. Specifically, the water head and salinity of the brine can be controlled by the water supply Martens flask of the water-salt migration system, and the temperature of the upper and lower ends of the soil sample can be controlled by the upper and lower temperature guide plates of the temperature control system to simulate different temperature difference environments. The vertical loading system controlled by the computer can apply different upper loads, which truly realizes the monitoring of the water-salt migration law and settlement of the barrier layer of saline soil roadbed under THMC coupling conditions.

[0030] 3. The present invention can monitor the moisture, salinity, stress and temperature of the saline soil roadbed barrier layer under THMC coupling conditions in real time. By arranging soil pressure vertical force sensors and soil temperature, humidity and salinity sensors at different positions of soil samples, the distribution and change rules of soil moisture, salinity, stress and temperature under different test conditions can be obtained. By installing a sliding settlement plate in the chute inside the model box, the settlement of soil layers at different depths can be detected. By comparing and analyzing with other measurement parameters, the settlement rules of the soil under the saline soil roadbed barrier layer under THMC coupling conditions are obtained, providing a quantitative reference for roadbed design in saline soil areas.

[0031] 4. After the water-salt migration experiment of the present invention is completed, the uppermost model box is slightly lifted and fixed to the upper shear box fixing bracket with bolts, and the normal stress is applied by the vertical loading system, and then the lower shear box is pushed by the horizontal shear system to shear the sample, and the readings of the horizontal force sensor and the horizontal displacement meter are recorded, so as to measure the shear strength of the soil sample after the water-salt migration test. During the test process, the present invention reduces the disturbance to the soil sample, so that the test results are closer to the actual project. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the water-salt migration and settlement test device of the saline soil roadbed barrier layer of the present invention.

[0033] Figure 2 The structural side view of the temperature guide plate of the water-salt migration system

[0034] Figure 3A top view of the structure of the temperature guide plate of the water-salt migration system

[0035] Figure 4 Top view of the insulation model box for the water-salt migration system

[0036] Figure 5 Side view of the insulation model box for the water-salt migration system

[0037] Figure 6 Schematic diagram of the cross section of the insulation model box 1-1

[0038] Figure 7 Schematic diagram of the cross section of the insulation model box 2-2

[0039] Figure 8 Schematic diagram of the cross section of the insulation model box 3-3

[0040] In the figure: 1. Constant temperature tank I refrigerant liquid outlet conduit, 2. Constant temperature tank I refrigerant liquid inlet conduit, 3. Constant temperature tank I, 4. Constant temperature tank II, 5. Constant temperature tank II refrigerant liquid outlet conduit, 6. Constant temperature tank II refrigerant liquid inlet conduit, 7. Loading plate, 8. Upper temperature conducting plate, 9. Bentonite waterproof pad 10. Medium coarse sand, 11. Insulation model box, 12. Polystyrene foam board, 13. Saline soil, 14. Earth pressure vertical force sensor, 15. Lower temperature conducting plate, 16. Model box base, 17. Vertical drive device, 18. Vertical displacement sensor, 19. Vertical force sensor, 2 0. Upper shear box fixing bracket, 21. Soil temperature, humidity and salinity sensor, 22. Water collecting beaker, 23. Model box fixing fixture, 24. Shear box, 25. Water supply Martens flask, 26. Horizontal drive device, 27 Horizontal force sensor, 28. Slide rail, 29. Horizontal displacement sensor, 30. Reaction frame, 31. Data acquisition instrument, 32. Control computer, 8 / 15-1. Refrigerant inlet pipe interface, 8 / 15-2. Refrigerant outlet pipe interface, 11-1. Bolts, 11-2. Holes, 11-3. Sliding sedimentation plate, 11-4. Folding baffle. DETAILED DESCRIPTION

[0041] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. The described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] like Figure 1 As shown, the present invention provides a test device for studying the water-salt migration and shear strength of a new type of barrier layer of saline soil roadbed under THMC coupling conditions. The whole device includes a water-salt migration system, a temperature control system, a vertical loading system, a horizontal shear system and a data acquisition system.

[0043] The water-salt migration system includes an insulation model box 11, a polystyrene foam board 12, saline soil 13, a model box base 16, a water collecting beaker 21, and a water supply Martens flask 22. The water-salt migration system simulates the water-salt migration inside the saline soil. The polystyrene foam board 12 isolates the heat exchange inside and outside the model box, and the water supply Martens flask 22 is used to provide brine to supplement the bottom water source. The model box base is equipped with universal wheels 16 to facilitate the adjustment of the position of the insulation model box 11.

[0044] The temperature control system includes a refrigerant water outlet conduit 1 of thermostatic bath I, a refrigerant water inlet conduit 2 of thermostatic bath I, a thermostatic bath I 3, a thermostatic bath II 4, a refrigerant water outlet conduit 5 of thermostatic bath II, a refrigerant water inlet conduit 6 of thermostatic bath II, an upper temperature conducting plate 8, and a lower temperature conducting plate 15. The refrigerant in the thermostatic bath is pumped to the upper and lower temperature conducting plates of the model box by a water pump, and then the temperature of the upper and lower ends of the soil is controlled by the temperature conducting plates, thereby realizing the simulation of the temperature difference.

[0045] The vertical loading system comprises a loading plate 7, a vertical driving device 17 and a reaction frame 30. The vertical loading system can simulate various traffic loads borne by the roadbed isolation layer and the dead load on the roadbed.

[0046] The horizontal shear system includes a model box fixing fixture 23, a shear box 24, a horizontal driving device 26 and a slide rail 28. The horizontal shear system can directly measure the strength of the soil sample after the water-salt migration test.

[0047] The data acquisition system includes a soil pressure sensor 14, a vertical force sensor 19, a vertical displacement sensor 18, a horizontal force sensor 27, a horizontal displacement sensor 29, a soil temperature, humidity and salinity sensor 21, a sliding settlement plate 11-3 and a data acquisition instrument 31. The data acquisition system can record the changes and distribution of moisture, salinity, temperature and stress inside the soil sample, the settlement of different positions of the soil sample, the shear strength and shear displacement of the soil sample at any time.

[0048] The data acquisition system, the vertical loading system and the horizontal shearing system are all connected to a control computer 32 , and the loading parameters and the acquisition parameters are set by the control computer 32 .

[0049] like Figure 2-3 As shown, the bottom of the lower temperature conducting plate is provided with a refrigerant liquid inlet pipe interface 8 / 15-1 and a refrigerant liquid outlet pipe interface 8 / 15-2, which are respectively connected to the refrigerant liquid inlet conduit 2 of the thermostatic bath I and the refrigerant liquid outlet conduit 1 of the thermostatic bath I. A flow path is provided inside the temperature conducting plate to ensure that the refrigerant liquid flows through the entire temperature conducting plate and then returns to the thermostatic bath I 3, thereby realizing the temperature control of the bottom of the soil sample.

[0050] like Figure 4-5As shown, the insulation model box 11 is composed of four shear boxes, including, from top to bottom: a first layer of shear boxes, a second layer of shear boxes, a third layer of shear boxes and a fourth layer of shear boxes. There are a total of eight bolt holes on the edge of each shear box. The shear boxes are connected and fixed by bolts 11-1 to facilitate layered filling compaction and direct shear testing. The insulation model box 11 is wrapped with polystyrene foam boards 12 to prevent heat exchange with the external environment. Universal wheels are installed on the base of the insulation model box 11 to facilitate moving the model box into the shear box after filling.

[0051] like Figure 6 As shown, holes 11 - 2 are drilled on the front and rear sides of the insulation model box respectively, for passing the sensor data line, the water collection beaker 22 and the conduit of the water supply Martens flask 25 .

[0052] like Figure 7 As shown, there are chutes on the inner walls of the left and right sides of the insulation model box. There is a sliding settlement plate 11-3 in the chute. During the filling process, the sliding settlement plate 11-3 is moved to different depths of soil layers for burial. By comparing the height changes before and after, the settlement of soil layers at different depths can be calculated. The rest of the chute is covered with a folding baffle to prevent soil particles from entering the chute.

[0053] like Figure 8 As shown, a roller is installed behind the sliding settlement plate 11-3, which can make the sliding settlement plate 11-3 slide freely up and down in the slide chute as each layer of soil sample settles during the experiment.

[0054] The specific operation is as follows:

[0055] Water-salt migration test under THMC coupling conditions:

[0056] 1) Fix the heat preservation model box 11 to the model box base 16 by bolts 11-1. Wrap a layer of polystyrene foam board 12 on the outside of the model box. Install the lower temperature conducting plate to the bottom of the model box and connect it to the constant temperature bath I3.

[0057] 2) Fill the saline soil 13 into the model box 11 in layers and compact it. Lay medium-coarse sand 10 on top of the saline soil 13 and compact it, then cover it with bentonite waterproof pad 9, and lay another layer of medium-coarse sand 10 on top of the pad and compact it. Install the temperature conducting plate 8 and connect it to the constant temperature bath II 4. Finally, connect the Marsh water supply bottle 25 and the water collection beaker 22 to the side walls of the model box respectively.

[0058] 3) During the soil filling process, soil pressure sensors 14 and soil temperature, humidity and salinity sensors 21 are buried layer by layer, and the sensor data lines extend through the holes 11-2 on the side walls of the model box to the data acquisition device 31 outside the box. At the same time, after each layer of soil is compacted and leveled, the sliding settlement plate 11-3 is moved to the flat surface of each layer of soil, the corresponding height is recorded, and then the next layer of soil is filled.

[0059] 4) Push the assembled model box 11 into the shear model box through the universal wheels of the model box base 16, adjust the model box fixing fixture 23 to fix the insulation model box 11 to prevent movement during the test. Cover the loading plate 7 on the upper temperature conducting plate 8. Adjust the position of the loading plate on the shear box slide rail 28 through the horizontal drive device 26 so that the loading plate is aligned with the vertical drive device 17, and then apply the vertical constant load through the vertical drive device 17.

[0060] 5) When simulating the freezing process of roadbed soil in winter, the upper temperature conducting plate 8 is set at a lower temperature, the lower temperature conducting plate 5 is set to a higher temperature, and the soil sample is frozen from top to bottom; when simulating the melting process of roadbed soil in spring, the temperature of the upper temperature conducting plate is increased, and the soil sample is melted from top to bottom. During the freeze-thaw cycle, water and salt will continue to migrate in the soil sample, accompanied by phase changes such as water freezing and melting and salt crystallization and dissolution, thereby simulating a series of catastrophic phenomena such as frost heave, salt heave, and dissolution in saline soil roadbed. The migration of water and salt causes the strength of the soil to change. Under vertical loading, the settlement of the soil sample is recorded in real time by the vertical displacement sensor 18. During the entire test cycle, if there is water penetrating the isolation layer, it will be collected by the water collecting cylinder 21. After the experiment, in the process of digging out the soil samples in layers, the height of the sliding settlement plate 11-3 after settlement is recorded, and the settlement of soil samples at different depths is calculated.

[0061] 6) The data acquisition instrument 31 collects the changes in temperature, moisture content, salt content, soil pressure of the soil sample under THMC coupling conditions over time and the distribution in the vertical direction in real time and records them in the control computer 32.

[0062] 7) The present invention realizes the test of the water-salt migration law and shear strength of the new type of barrier layer of saline soil roadbed under THMC coupling conditions. In addition, the roadbed barrier layer under different temperature gradients, chemical solutions, moisture content, and overlying loads can be tested according to needs, and the performance of roadbed barrier layers with different structural forms can also be tested.

[0063] Direct shear test:

[0064] If it is necessary to test the shear strength of the soil sample after the water-salt migration test, then after the water-salt migration experiment is completed, the temperature of the soil sample is controlled at the temperature required for the direct shear test through the temperature control system. Disassemble the insulation model box 11 according to the surface that needs to be sheared. Take the test of the shear strength of the soil sample at the direct shear interface of the first layer shear box and the second layer shear box as an example. First, loosen the fixing bolts 11-1 of the direct shear of the first layer shear box and the second layer shear box. Lift the first shear box slightly and fix it to the upper shear box fixing bracket 20 with bolts so that there is a gap between the first shear box and the second shear box. Apply normal stress through the vertical loading system to consolidate the soil sample, and record the vertical deformation of the soil sample. After the vertical deformation of the soil sample is stable, apply horizontal shear force through the horizontal shear system, and record the shear force and shear displacement during the shearing process. Using this method for testing can avoid moving the soil sample for direct shear test after the test is completed, reduce the error caused by soil sample disturbance, and the test process is convenient and the test data is accurate.

Claims

1. A test device for testing the water-salt migration law and shear strength of a new type of barrier layer in saline soil roadbed under THMC coupling conditions, characterized in that: include: Water and salt transport system, temperature control system, vertical loading system, horizontal shear system and data acquisition system; The water-salt transport system is used to simulate the water-salt transport in saline soil, and includes: The thermal insulation model box is composed of at least four layers of shear boxes, which is used to fill the saline soil in layers and simulate the actual roadbed structure; A polystyrene foam board is wrapped around the outside of the thermal insulation model box to isolate the heat exchange between the inside and outside of the model box; The water collection beaker and water supply Martens flask are used to collect infiltration water and replenish the bottom water source, respectively, to simulate the water-salt migration process; The temperature control system comprises at least two thermostatic baths, a refrigerant liquid outlet conduit, a refrigerant liquid inlet conduit, an upper temperature conducting plate, and a lower temperature conducting plate. The thermostatic bath is connected to the upper temperature conducting plate and the lower temperature conducting plate through the refrigerant liquid outlet conduit and the refrigerant liquid inlet conduit. The refrigerant liquid in the thermostatic bath is pumped to the upper and lower temperature conducting plates of the insulation model box through a built-in water pump in the thermostatic bath. The upper and lower temperature conducting plates are used to control the temperature at the upper and lower ends of the soil body to simulate the temperature difference effect. The vertical loading system is used to simulate the vertical load borne by the roadbed isolation layer; The horizontal shear system is used to perform a direct shear test on the soil sample after completing the water-salt migration test; The data acquisition system includes a soil pressure sensor, a vertical force sensor, a vertical displacement sensor, a horizontal force sensor, a horizontal displacement sensor, a soil temperature, humidity and salinity sensor and a data acquisition instrument, which is used to monitor the changes and distribution of moisture, salinity, temperature and stress inside the soil sample, the settlement of the soil sample, the shear strength and shear displacement of the soil sample in real time; The data acquisition system, vertical loading system and horizontal shearing system are respectively connected to a control computer, and the loading parameters and acquisition parameters are set by the control computer.

2. The test device according to claim 1, characterized in that: The heat-insulating model box is placed on a model box base, and universal wheels are arranged below the model box base to facilitate moving the heat-insulating model box into the shear box.

3. The test device according to claim 1 or 2, characterized in that: The thermal insulation model box comprises, from top to bottom, a first layer of shear boxes, a second layer of shear boxes, a third layer of shear boxes and a fourth layer of shear boxes, and the shear boxes are connected and fixed by bolts to facilitate layered filling compaction and direct shear testing.

4. The test device according to claim 3, characterized in that: A slide groove is opened on the inner walls of the front and rear box walls of the insulation model box, and a sliding settlement plate is installed in the slide groove. The sliding settlement plate is equipped with rollers and can slide up and down in the slide groove to record the settlement changes of soil layers at different depths; the rest of the groove wall is covered with a folding baffle to prevent soil particles and moisture from entering the sliding groove.

5. The test device according to claim 3, characterized in that: The left and right side walls of the heat preservation model box are provided with holes for passing the data line of the sensor, the water supply pipe of the water supply Martens flask and the water collection pipe of the water collection beaker.

6. The test device according to claim 1, characterized in that: A model box fixing fixture is installed on the inner side of the shear line to fix the insulation model box during the test. After the insulation model box with soil filling is pushed into the shear box, the fixture is used to support the model box wall from both sides, and the tightness of the fixture is adjusted by bolts.

7. The test device according to claim 1, characterized in that: A refrigerant liquid inlet pipe interface and a refrigerant liquid outlet pipe interface are provided at the bottom of the lower temperature conducting plate, which are respectively connected to the refrigerant liquid inlet conduit and the refrigerant liquid outlet conduit of the constant temperature tank I; a diversion path is provided inside the temperature conducting plate to ensure that the refrigerant liquid flows through the entire temperature conducting plate and then returns to the constant temperature tank I, thereby realizing temperature control of the bottom of the soil sample.

8. The test device according to claim 1, characterized in that: It also includes an upper shear box fixing bracket, which is used to disassemble the insulation model box according to the surface that needs to be sheared after the water-salt migration test, and fix part of the shear box on the upper shear box fixing bracket to carry out a direct shear test.

9. A method for testing water-salt migration and shear strength of a new type of barrier layer in saline soil roadbed using the test device as described in any one of claims 1 to 8, characterized in that: The following steps are involved: Water-salt migration test under THMC coupling conditions: 1) Prepare test materials, including saline soil, medium-coarse sand, bentonite waterproof liner and sensors; 2) Fix the insulation model box to the model box base with bolts and wrap it with a polystyrene foam board; install the lower temperature conducting plate to the bottom of the model box and connect it to the constant temperature bath; 3) Fill the saline soil in layers into the insulation model box and compact it, lay medium-coarse sand on the top of the saline soil and compact it, then cover it with a bentonite waterproof pad, and lay another layer of medium-coarse sand on the pad and compact it; install the temperature conduction plate and connect it to the constant temperature tank; finally, connect the Marsh water supply bottle and the water collection beaker to the side walls of the insulation model respectively. 4) During the soil filling process, soil pressure sensors and soil temperature, humidity and salinity sensors are buried layer by layer, and the sensor data lines are extended to the data acquisition instrument outside the box through the holes in the side wall of the model box; after each layer of soil sample is compacted and leveled, the sliding settlement plate is moved to the flat surface of each layer of soil sample, the corresponding height is recorded, and then the next layer of soil sample is filled. 5) Push the assembled model box into the shear model box through the universal wheels on the model box base, adjust the model box fixing fixture to fix the insulation model box to prevent movement during the test; cover the loading plate on the upper temperature conducting plate; adjust the position on the shear box slide rail through the horizontal drive device so that the loading plate is aligned with the vertical drive device, and then apply a vertical constant load through the vertical drive device; 6) When simulating the freezing process in winter, the upper temperature guide plate is set at a lower temperature, the lower temperature guide plate is set at a higher temperature, and the soil sample freezes from top to bottom; when simulating the melting process, the temperature of the upper temperature guide plate is increased, and the soil sample melts from top to bottom. During the freeze-thaw cycle, water and salt will continue to migrate in the soil sample, accompanied by phase changes such as water freezing and melting and salt crystallization and dissolution, thereby simulating a series of catastrophic phenomena such as frost heave, salt heave, and dissolution in saline soil roadbed. The migration of water and salt causes changes in soil strength. Under vertical loading, the settlement of soil samples is recorded in real time by vertical displacement sensors. During the entire test cycle, if water penetrates the isolation layer, it will be collected by the water collection cylinder; after the experiment, in the process of digging out the soil samples in layers, the height of the sliding settlement plate after settlement is recorded, and the settlement of soil samples at different depths is calculated. 7) The data acquisition instrument collects the changes in temperature, moisture content, salt content, soil pressure of the soil sample under THMC coupling conditions over time and the distribution in the vertical direction in real time and records them in the control computer. 8) The present invention realizes the testing of the water-salt migration law and shear strength of the new type of barrier layer of saline soil roadbed under THMC coupling conditions; Direct shear test: 9) When the water-salt migration experiment is completed, part of the shear box of the insulation model box is disassembled as needed, fixed on the upper shear box fixing bracket, and the soil sample is consolidated by applying normal stress through the vertical loading system, and the vertical deformation is recorded; 10) After the vertical deformation of the soil sample is stable, apply horizontal shear force through the horizontal shear system and record the shear force and shear displacement; 11) Evaluate the performance of the insulation layer based on the test results.

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