A test device and a test method for determining a soil water characteristic curve of a non-saturated soil

Abstract

The present application relates to geotechnical engineering test technical field, disclose a kind of test device and test method for determining the soil water characteristic curve of unsaturated soil, the test device for determining the soil water characteristic curve of unsaturated soil includes pressure chamber, pressure cabin, air pressure loading unit, water pressure loading unit and first measuring piece, pressure cabin is located in pressure chamber, pressure cabin has accommodating cavity, the top of accommodating cavity is equipped with opening, the bottom of accommodating cavity is equipped with pottery plate, opening is equipped with hydrophobic membrane, for plugging opening, the accommodating cavity between pottery plate and hydrophobic membrane is used to install test soil sample, air pressure loading unit is communicated with pressure chamber, for applying air pressure to pressure chamber, water pressure loading unit is communicated with accommodating cavity, for applying water pressure to accommodating cavity, first measuring piece is located on pressure cabin.The present application solves the problem that the pressure plate method in the prior art cannot simulate the negative suction force in the soil body, and cannot reflect the change rule of suction and moisture content in the whole cycle of swelling soil moisture absorption-dampness.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering testing technology, and in particular to a test apparatus and test method for determining the soil-water characteristic curve of unsaturated soil. Background Technology

[0002] Currently, the soil-water characteristic curve of unsaturated soil is a constitutive curve that characterizes the relationship between soil matrix suction and water content. The greater the soil matrix suction, the greater the interaction force between soil particles, and the greater the shear strength and stiffness of the soil matrix. Therefore, the soil-water characteristic curve is usually used to reveal the influence of water content changes on soil mechanical properties.

[0003] In existing technologies, the pressure plate method is commonly used to determine soil-water characteristic curves. This method places a saturated clay plate at the bottom of a pressure chamber. The bottom of the pressure chamber has a channel connected to the atmosphere, allowing atmospheric pressure to act directly on the saturated clay plate, ensuring that the water pressure within the saturated clay plate is always at atmospheric pressure. During the test, the test soil sample is placed on the saturated clay plate. Because the pores between the soil particles inside the test soil sample are connected to the water within the saturated clay plate, the pore water pressure within the test soil sample is also at atmospheric pressure. Air pressure is then applied to the pressure chamber, forcing water out of the test soil sample. When the drainage stops, the difference between the applied air pressure and atmospheric pressure is the soil matrix suction. The test soil sample is then dried, and the mass of the sample before and after drying is measured to obtain the moisture content. By applying different air pressures to multiple identical test soil samples and repeating the above pressurization, drainage, drying, and weighing operations, a series of data points corresponding to soil matrix suction and moisture content can be obtained, thus plotting the corresponding soil-water characteristic curve. In the above test method, the applied air pressure is always greater than atmospheric pressure in order to drive the soil sample to drain, and the measured soil sample suction is always positive. Therefore, the above method is only applicable to soil in which the soil matrix suction is always positive.

[0004] However, during water absorption, the expansion of internal minerals in expansive soil generates significant expansion pressure. This pressure acts directly on the pore water, causing the pore water pressure to rise rapidly and exceed the pore air pressure, creating negative suction within the soil. This results in dramatic soil expansion and a sharp drop in strength. Conversely, during drainage, the pore air pressure exceeds the pore water pressure, creating positive suction. The aforementioned pressure plate method cannot simulate the negative suction within expansive soil, nor can it determine the corresponding soil-water characteristic curve. Consequently, it cannot fully reflect the changes in suction and water content throughout the entire moisture absorption-drainage cycle, and therefore cannot provide reliable data support for the design of expansive soil foundations and slopes. Summary of the Invention

[0005] The technical problem this invention aims to solve is that the pressure plate method cannot simulate the negative suction inside expansive soil, cannot measure the soil-water characteristic curve corresponding to the negative suction, and therefore cannot fully reflect the variation law of suction and water content of expansive soil throughout the entire cycle of moisture absorption and release.

[0006] To address the aforementioned technical problems, this invention provides a test apparatus for determining the soil-water characteristic curve of unsaturated soil, comprising: Pressure chamber; A pressure chamber, located inside a pressure room, with an opening at the top; A clay slab was placed at the bottom of the pressure chamber, and a test soil sample was placed on the clay slab. A hydrophobic membrane is installed near the opening of the pressure chamber. The lower side of the hydrophobic membrane contacts the test soil sample, and the hydrophobic membrane seals the opening. The air pressure loading unit is connected to the pressure chamber. The air pressure applied by the air pressure loading unit passes through the hydrophobic membrane and is applied to the test soil sample. The water pressure loading unit is connected to the bottom of the pressure chamber. The air pressure applied by the water pressure loading unit passes through the clay plate and then applies water pressure to the test soil sample. The first measuring instrument, located on the pressure chamber, is used to measure the moisture content of the test soil sample.

[0007] Preferably, the pressure chamber includes a base and a sample chamber; The base is equipped with an installation groove, and the terracotta plate is installed in the installation groove. The bottom wall of the installation groove is equipped with a water inlet, which is connected to the water pressure loading unit. The sample chamber is fixedly connected to the base on the outer side of the mounting groove. The sample chamber has an inner cavity with openings on both the top and bottom sides. The test soil sample is installed in the inner cavity, and the hydrophobic membrane is installed at the top opening of the inner cavity.

[0008] Preferably, the pressure chamber further includes a pressure ring and a vent plate; An annular groove is provided on the top wall of the sample chamber, and a first sealing ring is installed in the annular groove. A hydrophobic membrane covers the top of the sample chamber, and the edge of the hydrophobic membrane covers the first sealing ring. The air-permeable plate and the pressure ring are stacked on the hydrophobic membrane from bottom to top. The edges of the air-permeable plate and the pressure ring extend to the top wall of the sample chamber. The pressure ring is detachably and fixedly connected to the sample chamber.

[0009] Preferably, the water pressure loading unit includes a water supply bottle, a lifting component, a water pressure volume controller, a first water pipe, and a second water pipe; The water supply bottle is mounted on the lifting component, which is used to move the water supply bottle up and down. The first water pipe connects the pressure chamber and the water supply bottle, and a water valve is installed on the first water pipe. The second water pipe connects the receiving cavity and the water pressure volume controller.

[0010] Preferably, the water pressure loading unit further includes an air trap, a third water pipe, and a fourth water pipe; The air trap has a first water outlet at its bottom and a first water inlet and a second water outlet on its side wall. The first water pipe connects the first water inlet to the water supply bottle; the second water pipe connects the first water inlet to the water pressure volume controller; the third water pipe connects the first water outlet to the bottom of the pressure chamber; and the fourth water pipe connects the second water outlet to the bottom of the pressure chamber.

[0011] This invention provides a test method for determining the soil-water characteristic curve of unsaturated soil, using the aforementioned test apparatus for determining the soil-water characteristic curve of unsaturated soil, and comprising the following steps: S1. Install test soil samples; S2. Multiple preset moisture absorption and dehumidification conditions are provided. Each moisture absorption condition has a preset negative suction force, and each dehumidification condition has a preset positive suction force. S3. Apply preset air pressure and preset water pressure through the air pressure loading unit and the water pressure loading unit respectively, so that the difference between the preset air pressure and the preset water pressure reaches the preset negative suction force; S4. Once the measurement data of the first measuring piece is stable, record the measurement data of the first measuring piece; S5. Repeat steps S3 to S4 until multiple sets of moisture absorption conditions are completed. S6. Apply preset air pressure and preset water pressure through the air pressure loading unit and the water pressure loading unit respectively, so that the difference between the preset air pressure and the preset water pressure reaches the preset positive suction force; S7. When the measurement data of the first measuring piece is stable, record the measurement data of the first measuring piece; S8. Repeat steps S6 to S7 until multiple dehumidification conditions are completed.

[0012] Preferably, in step S3: The preset air pressure is kept constant at atmospheric pressure. Different preset water pressures are applied so that the difference between the preset air pressure and the preset water pressure reaches the preset negative suction force for each moisture absorption condition. Specifically: When the preset negative suction force is greater than or equal to -15kPa, the lifting component will move the water supply bottle upward to the preset height to apply the preset water pressure; When the preset negative suction force is less than -15kPa, the water pressure volume controller applies the preset water pressure.

[0013] Preferably, in step S6: When the preset positive suction force is less than or equal to 15 kPa, the preset air pressure is constant at atmospheric pressure. The lifting component moves the water supply bottle downward to different preset heights so that the difference between the preset air pressure and the preset water pressure reaches the preset positive suction force for each dehumidification condition. When the preset positive suction force is greater than 15 kPa, the water pressure volume controller applies a constant preset water pressure, and the air pressure loading unit applies multiple different preset air pressures so that the difference between the preset air pressure and the preset water pressure reaches the preset positive suction force for each dehumidification condition.

[0014] Preferably, in step S1, the test soil sample is installed, specifically as follows: S11. Install the base and sample chamber inside the pressure chamber, and install the clay plate on the base; S12. Inject degassing water into the pressure chamber to submerge the sample chamber. S13, The pneumatic loading unit applies negative pressure to the pressure chamber; S14. When no air bubbles are discharged from the sample chamber, drain the airless water from the pressure chamber and install the test soil sample and hydrophobic membrane.

[0015] Preferably, in step S3, before applying the preset air pressure and preset water pressure through the air pressure loading unit and water pressure loading unit respectively, the water valve is opened first so that the water in the water supply bottle flows into the air trap, the water in the air trap flows into the pressure chamber through the third water pipe, and the water in the pressure chamber flows back into the air trap through the fourth water pipe to discharge the gas in the third water pipe and the fourth water pipe.

[0016] Compared with the prior art, the experimental apparatus and method for determining the soil-water characteristic curve of unsaturated soil of the present invention have the following advantages: This invention discloses an experimental apparatus and method for determining the soil-water characteristic curve of unsaturated soil. A pressure chamber is located within the pressure chamber, which is connected to an air pressure loading unit. A water pressure loading unit is connected to the bottom of the pressure chamber. A clay plate is installed at the bottom of the pressure chamber, and a hydrophobic membrane is installed at the top opening of the pressure chamber. The test soil sample is installed between the clay plate and the hydrophobic membrane. The test is divided into two main sequences: moisture absorption and moisture release, with multiple preset suction values ​​applied in a stepped manner in each sequence. Under the moisture absorption condition, the water pressure applied by the water pressure loading unit is greater than the air pressure applied by the air pressure loading unit. Water enters from the bottom of the pressure chamber and passes through the clay plate into the test soil sample. The hydrophobic membrane, with its hydrophobic and air-permeable properties, prevents liquid water from flowing out from the top of the pressure chamber and seals the moisture inside the test soil sample. This ensures that the water pressure applied by the water pressure loading unit is fully applied to the soil sample to form a stable negative suction state within the sample, solving the technical problem that traditional devices cannot simulate negative suction within the soil. The water pressure loading unit and the air pressure loading unit continue to operate until the data of the first measuring piece is constant (the internal water migration of the soil sample reaches equilibrium). Then, the suction-water content data points under this state are recorded. Multiple hygroscopic conditions are completed to obtain the soil-water characteristic curves corresponding to the negative suction range.

[0017] In the dehumidification condition, the water pressure applied by the hydraulic loading unit is less than the air pressure applied by the pneumatic loading unit. Water in the test soil sample flows out through the clay plate. The saturated clay plate only allows water to pass through, thus isolating soil particles and gas from escaping. This not only ensures the integrity of the test soil sample but also ensures that the air pressure applied by the pneumatic loading unit is fully applied to the test soil sample, forming a stable positive suction state within the sample. The hydraulic and pneumatic loading units continue to operate until the data of the first measuring piece becomes constant (the internal water migration of the soil sample reaches equilibrium). Subsequently, the suction-water content data points under this state are recorded. Multiple dehumidification conditions are completed to obtain the soil-water characteristic curves corresponding to the positive suction range.

[0018] In summary, this invention, through the synergistic effect of the clay plate and the hydrophobic membrane, enables the experimental device to precisely control the complete hydraulic path from high positive suction to negative suction. By using the moisture absorption-discharge path test method, data on the entire cycle of soil sample moisture absorption-discharge are obtained. The resulting soil-water characteristic curve can more realistically reflect the changes in the hydraulic state of the soil under actual wet and dry conditions. This solves the technical problem in the prior art that the pressure plate method cannot simulate the negative suction inside expansive soil and cannot fully reflect the changes in suction and water content of expansive soil throughout the entire moisture absorption-discharge cycle. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the experimental apparatus according to an embodiment of the present invention; Figure 2 This is a three-dimensional structural diagram of the pressure chamber according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the installation of the base and sample chamber according to an embodiment of the present invention; Figure 4 This is a cross-sectional view of the pressure chamber according to an embodiment of the present invention; Figure 5 This is a top view of the base according to an embodiment of the present invention.

[0020] In the diagram, 1. Pressure chamber; 2. Pressure chamber; 2a. Receiving cavity; 21. Base; 21a. Mounting groove; 21b. Water inlet; 21c. Water passage; 21d. Second screw hole; 22. Sample chamber; 22a. Inner cavity; 22b. Annular groove; 22c. Second bolt; 22d. First screw hole; 23. Pressure ring; 23a. First bolt; 24. Ventilation plate; 3. Pneumatic loading unit; 31. Air compressor; 32. Gas controller; 33. First gas pipe; 34. Second gas pipe; 4. Hydraulic loading unit; 41. Water supply bottle; 42. Lifting component; 43. Hydraulic volume controller; 4 4. First water pipe; 45. Second water pipe; 46. Air trap; 47. Third water pipe; 48. Fourth water pipe; 5. First measuring piece; 6. Clay slab; 7. Test soil sample; 8. Hydrophobic membrane; 9. Water valve; 10. First sealing ring; 11. Electronic scale; 12. Bracket; 13. Tension sensor; 14. Humidity sensor; 15. Humidity controller; 16. Temperature controller; 17. Exhaust valve; 18. Intake valve; 19. Sealing screw; 20. Top cover; 35. Data acquisition unit; 36. Computer; 37. Water tank; 38. Third sealing ring; 39. Second sealing ring. Detailed Implementation

[0021] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0022] In the description of this invention, it should be understood that the terms "upper", "lower", "vertical", "horizontal", "bottom", "inner", "outer" and other terms used in this invention to indicate the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0023] It should be understood that the terms "first," "second," etc., are used in this invention to describe various types of information, but these terms are not limited to them; they are only used to distinguish information of the same type from one another. For example, without departing from the scope of this invention, "first" information may also be referred to as "second" information, and similarly, "second" information may also be referred to as "first" information.

[0024] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0025] like Figure 1 As shown, a test apparatus for determining the soil-water characteristic curve of unsaturated soil according to a preferred embodiment of the present invention includes a pressure chamber 1, a pressure chamber 2, an air pressure loading unit 3, a water pressure loading unit 4, and a first measuring element 5.

[0026] like Figure 1 As shown, the pressure chamber 1 has a top cover 20, which is fixed by a sealing screw 19. An exhaust valve 17 is provided on the top cover 20. A bracket 12 is provided at the bottom of the pressure chamber 1. The pressure chamber 2 is placed on the bracket 12. A humidity controller 15 and a temperature controller 16 are provided on the side wall of the pressure chamber 1.

[0027] like Figures 1 to 5 As shown, pressure chamber 2 is located inside pressure chamber 1. Pressure chamber 2 has a receiving cavity 2a. The top of pressure chamber 2 has an opening that communicates with the receiving cavity 2a. A clay plate 6 is placed at the bottom of pressure chamber 2, and a test soil sample 7 is placed on the clay plate 6. A hydrophobic membrane 8 is provided near the opening of pressure chamber 2. The lower side of the hydrophobic membrane 8 contacts the test soil sample 7, and the hydrophobic membrane 8 seals the opening. Further, as... Figures 2 to 4 As shown, the pressure chamber 2 includes a base 21, a sample chamber 22, a pressure ring 23, and a permeable plate 24. The base 21 has a mounting groove 21a, and the clay plate 6 is installed within the mounting groove 21a. Further, as... Figure 4 and Figure 5 As shown, a third sealing ring 38 is fitted around the outer periphery of the terracotta plate 6. When the terracotta plate 6 is installed in the mounting groove 21a, the third sealing ring 38 fills the gap between the terracotta plate 6 and the side wall of the mounting groove 21a to prevent the water injected by the water pressure loading unit 4 from leaking out from the gap between the terracotta plate 6 and the side wall of the mounting groove 21a, causing water pressure loss.

[0028] like Figures 2 to 5As shown, the bottom wall of the mounting groove 21a is provided with a water inlet 21b, which is connected to the water pressure loading unit 4. Specifically, the bottom wall of the mounting groove 21a is provided with a serpentine water trough 37 extending radially therein, and there are two water inlets 21b, which are respectively located at both ends of the water trough 37. The two sides of the base 21 are provided with water passages 21c that are connected to the water inlets 21b. The water inlets of the two water passages 21c are respectively connected to the third water pipe 47 and the fourth water pipe 48. After the water pressure loading unit 4 injects water into the third water pipe 47 and the fourth water pipe 48, the water enters each water passage 21c. The water enters the water tank 37 through the water passage 21c. Since the water passage 21c is set in a serpentine shape, the water in the water tank 37 can be evenly distributed to the bottom of the entire terracotta plate 6, so that the water pressure on the bottom surface of the terracotta plate 6 is equal everywhere. This ensures that when water pressure is applied, the water passes through the terracotta plate 6 evenly and smoothly, so as to ensure that the water pressure applied to the test soil sample 7 is uniform and stable, and improves the accuracy of moisture content measurement.

[0029] like Figures 2 to 4 As shown, the sample chamber 22 is fixedly connected to the base 21 on the outer side of the mounting groove 21a. The sample chamber 22 has an inner cavity 22a with openings on both the upper and lower sides, and the test soil sample 7 is installed in the inner cavity 22a. Specifically, as shown... Figures 3 to 5 As shown, the bottom of the sample chamber 22 is provided with an annular groove, and a second sealing ring 39 is installed in the annular groove. The sample chamber 22 is installed above the base 21. The base 21 is provided with a second screw hole 21d. The second bolt 22c passes through the sample chamber 22 and is threaded into the second screw hole 21d, thereby realizing the fixed connection between the sample chamber 22 and the base 21. At the same time, the second sealing ring 39 is located at the joint between the sample chamber 22 and the base 21, which effectively prevents water from flowing out from the joint between the sample chamber 22 and the base 21, causing water pressure loss.

[0030] like Figures 2 to 5 As shown, the hydrophobic membrane 8 is installed at the top opening of the inner cavity 22a. Specifically, as... Figure 4As shown, the top wall of the sample chamber 22 is provided with an annular groove 22b, and a first sealing ring 10 is installed in the annular groove 22b. A hydrophobic membrane 8 covers the top of the sample chamber 22, and the edge of the hydrophobic membrane 8 covers the first sealing ring 10. A breathable plate 24 and a pressure ring 23 are stacked on the hydrophobic membrane 8 from bottom to top. The edges of the breathable plate 24 and the pressure ring 23 extend to the top wall of the sample chamber 22. A clearance groove is provided at the bottom of the pressure ring 23, and the hydrophobic membrane 8 and the breathable plate 24 are located in the clearance groove. The bottom surface of the hydrophobic membrane 8 is flush with the bottom surface of the pressure ring 23. The top of the sample chamber 22 is provided with multiple first screw holes 22d. A first bolt 23a passes through the pressure ring 23 and is threaded into the first screw hole 22d so that the pressure ring 23 and the sample chamber 22 can be detachably and fixedly connected. The first sealing ring 10 is located between the pressure ring 23, the hydrophobic membrane 8 and the sample chamber 22 to prevent water in the test soil sample 7 from leaking out through the gap between the pressure ring 23, the hydrophobic membrane 8 and the sample chamber 22, causing water pressure loss and making it impossible to achieve the preset suction force.

[0031] like Figure 1 As shown, the pneumatic loading unit 3 is connected to the pressure chamber 1. The air pressure applied by the pneumatic loading unit 3 passes through the hydrophobic membrane 8 and applies pressure to the test soil sample 7. Specifically, the pneumatic loading unit 3 includes an air compressor 31, a pressure controller 32, a first air pipe 33, and a second air pipe 34. The first air pipe 33 connects the air compressor 31 and the pressure controller 32, and the second air pipe 34 connects the pressure controller 32 and the pressure chamber 1. An air inlet valve 18 is provided on the second air pipe. The air compressor 31 serves as the air source, and the compressed air it generates enters the pressure controller 32 through the first air pipe 33. The pressure controller 32 adjusts the air pressure of the compressed air and delivers the gas to the pressure chamber 1. After passing through the hydrophobic membrane 8, the gas acts on the test soil sample 7 to apply pressure to the test soil sample 7.

[0032] like Figure 1 As shown, the water pressure loading unit 4 is connected to the bottom of the pressure chamber 2. The air pressure applied by the water pressure loading unit 4 passes through the clay plate 6 and then applies water pressure to the test soil sample 7. Specifically, the water pressure loading unit 4 includes a water supply bottle 41, a lifting component 42, a water pressure volume controller 43, a first water pipe 44, a second water pipe 45, an air trap 46, a third water pipe 47, and a fourth water pipe 48. The water supply bottle 41 is mounted on the lifting component 42, which is used to move the water supply bottle 41 up and down. The bottom of the air trap 46 has a first water outlet, and the side wall of the air trap 46 has a first water inlet and a second water outlet. The first water pipe 44 connects the first water inlet to the water supply bottle 41, the second water pipe 45 connects the first water inlet to the water pressure volume controller 43, the third water pipe 47 connects the first water outlet to the bottom of the pressure chamber 2, and the fourth water pipe 48 connects the second water outlet to the bottom of the pressure chamber 2.

[0033] Before applying the preset air pressure and preset water pressure through the air pressure loading unit 3 and water pressure loading unit 4 respectively, the water valve 9 is opened to allow water in the water supply bottle 41 to flow into the air trap 46. The water in the air trap 46 flows into the receiving cavity 2a through the third water pipe 47, and the water in the receiving cavity 2a flows back into the air trap 46 through the fourth water pipe 48 to expel the gas in the third water pipe 47 and the fourth water pipe 48, avoiding water pressure fluctuations caused by the compressibility of air bubbles, thereby ensuring that the water pressure applied to the soil sample is stable and precisely controllable. Subsequently, water is injected into the air trap 46 using the water supply bottle 41 or the water pressure volume controller 43. The water entering the air trap 46 submerges the second outlet, and the water in the air trap 46 is pressurized by injecting water into the receiving cavity 2a through the third water pipe 47 and the fourth water pipe 48, thus loading the water pressure loading unit 4.

[0034] In this embodiment of the invention, the water supply bottle 41 is a Marriott bottle, the first water pipe 44 is connected to the outlet of the Marriott bottle, and the lifting component 42 is a lifting frame. The Marriott bottle works using atmospheric pressure and the principle of communicating vessels. The pressure at its outlet depends only on the vertical height difference between the lower end of the air inlet pipe and the outlet, and is independent of the remaining water volume in the bottle. It can automatically provide an absolutely stable inlet pressure for the test soil sample in long-term tests, fundamentally avoiding pressure fluctuations and ensuring the stability and reliability of the test data. The lifting component 42 moves the Marriott bottle up and down, pressurizing the pressure chamber 2 through the head difference. Its pressurization range is limited, but its accuracy is high, suitable for conventional suction range conditions. Moreover, the lifting component 42 changes the head difference by physically raising or lowering the bottle, thereby simulating the rise and fall of the groundwater level. This is completely consistent with the principle of static pore water pressure generation in nature. The simulation process is realistic, and the obtained data can more directly reflect the behavior of the soil under actual hydrogeological conditions. The water pressure volume controller 43 can provide pressures far exceeding those of static water head, thus enabling the measurement of soil properties under high suction conditions, covering a high measurement range that is not achievable with a Marriott bottle.

[0035] Furthermore, an electronic scale 11 is provided on the lifting component 42, and the water supply bottle 41 is placed on the electronic scale 11. During the moisture absorption and desiccation process of the test soil sample 7, under each working condition, when the change in the mass of water in the water supply bottle 41 is less than 0.02g per day, it is considered that the moisture content in the test soil sample 7 has tended to stabilize, and the water pressure and air pressure borne by the test soil sample 7 and its internal suction have reached a state of equilibrium.

[0036] like Figure 1As shown, the first measuring element 5 is mounted on the pressure chamber 2 and is used to measure the moisture content of the test soil sample 7. In this embodiment of the invention, the test device also includes a computer 36 and a data acquisition unit 35. The first measuring element 5 is a moisture sensor. The side wall of the pressure chamber 2 is also equipped with a tension sensor 13 and a humidity sensor 14. The tension sensor 13 is used to measure the tension (soil matrix suction) in the test soil sample 7, and the humidity sensor 14 is used to measure the relative humidity of the pore gas in the test soil sample 7. Specifically, the side wall of the sample chamber 22 is provided with an opening. The first measuring element 5, the tension sensor 13, and the humidity sensor 14 are respectively inserted into the corresponding openings and fixed in the openings with sealant. The first measuring element 5, the tension sensor 13, and the humidity sensor 14 are all electrically connected to the data acquisition unit 35 through transmission wires to transmit the measured data to the data acquisition unit 35. The data acquisition unit 35 is electrically connected to the computer 36 to transmit the measurement data to the computer 36.

[0037] like Figure 1 As shown, based on the above-described embodiment of the invention, an experimental apparatus for determining the soil-water characteristic curve of unsaturated soil is provided. This invention also provides a method for determining the soil-water characteristic curve of unsaturated soil, comprising the following steps: S1. Install test soil sample 7; In step S1, to avoid the influence of air in the pores inside the clay plate 6 on the test results, the gas in the clay plate 6 must be expelled before the test, so that its interior is filled with water and reaches a saturated state. This includes the following steps: S11, Mounting base 21, sample chamber 22 are placed inside pressure chamber 1, and clay plate 6 is mounted on base 21; Specifically, loosen the sealing screw 19 and remove the top cover 20, assemble the base 21 and sample chamber 22 and place them on the bracket 12, and place the clay plate 6 in the mounting groove 21a.

[0038] S12. Inject degassing water into pressure chamber 1 so that the degassing water submerges sample chamber 22; Specifically, degassing water is added to pressure chamber 1 so that the degassing water level is higher than that of sample chamber 22, and a portion of air is reserved above the degassing water level. Degassing water enters sample chamber 22, and then the top cover 20 is installed through sealing screw 19. The exhaust valve 17 and the air inlet valve 18 are closed to keep the inside of pressure chamber 1 sealed.

[0039] S13, the pneumatic loading unit 3 applies negative pressure to the pressure chamber 1; Specifically, the air intake valve 18 is opened, the air pressure controller 32 is electrically connected to the computer 36, and the computer 36 controls the air pressure controller 32 to deliver air pressure into the pressure chamber 1, with the air pressure being -80kPa.

[0040] S14. When no air bubbles are discharged from the sample chamber 22, drain the airless water from the pressure chamber 1 and install the test soil sample 7 and the hydrophobic membrane 8.

[0041] Specifically, under a suction force of -80 kPa, degassing water enters the clay plate 6 and expels gas from the pores of the clay plate 6. Bubbles continuously emerge from the sample chamber 22. When no more bubbles emerge from the sample chamber 22, the gas in the pores of the clay plate 6 has been completely expelled, and the clay plate 6 is saturated with water. Subsequently, the test soil sample 7, the hydrophobic membrane 8, the permeable plate 24, and the pressure ring 23 are installed. When installing the test soil sample 7 and the hydrophobic membrane 8, the top of the test soil sample 7 should abut against the bottom of the hydrophobic membrane 8.

[0042] S2. Multiple preset moisture absorption and dehumidification conditions are provided. Each moisture absorption condition has a preset negative suction force, and each dehumidification condition has a preset positive suction force. In step S2, multiple sets of moisture absorption and dehumidification conditions are designed. Each moisture absorption condition has a negative suction force, and the absolute value of the negative suction force of multiple moisture absorption conditions increases step by step. Each dehumidification condition has a positive suction force, and the positive suction force value of multiple dehumidification conditions increases step by step.

[0043] S3. Apply preset air pressure and preset water pressure through air pressure loading unit 3 and water pressure loading unit 4 respectively, so that the difference between preset air pressure and preset water pressure reaches preset negative suction force; Furthermore, to avoid the influence of gas in the first water pipe 44, second water pipe 45, third water pipe 47, and fourth water pipe 48 on the preset water pressure when the water pressure loading unit 4 applies water pressure, the water valve 9 is opened before the preset air pressure and preset water pressure are applied by the air pressure loading unit 3 and the water pressure loading unit 4, respectively, so that the water in the water supply bottle 41 flows into the air trap 46. The water flows through the first water pipe 44 into the air trap 46, and the water in the first water pipe 44 also carries the air inside into the air trap 46. The air trap 46 discharges this part of the gas, thereby clearing the air in the first water pipe 44. The water in the air trap 46 flows into the receiving cavity 2a along the third water pipe 47, and the water in the receiving cavity 2a flows back into the air trap 46 through the fourth water pipe 48 to discharge the gas in the third water pipe 47 and the fourth water pipe 48. When the water pressure volume controller 43 is switched to pressurize, the water valve 9 is closed, and the water pressure volume controller 43 is started to supply water to the air trap 46. The water flows through the second water pipe 45 into the air trap 46. The water in the second water pipe 45 also brings the air inside into the air trap 46. The air trap 46 discharges this part of the gas, thus clearing the air in the second water pipe 45.

[0044] Subsequently, water is injected into the air trap 46 using the water supply bottle 41 or the water pressure volume controller 43. The water entering the air trap 46 submerges the second outlet. The water in the air trap 46 is pressurized by injecting water into the receiving cavity 2a through the third water pipe 47 and the fourth water pipe 48, thereby realizing the loading of the water pressure loading unit 4.

[0045] In step S3, under each moisture absorption condition, the preset water pressure is greater than the preset air pressure, and the difference between the two is negative, which is the preset negative suction force. In this embodiment of the invention, the preset air pressure is constant at atmospheric pressure, and different preset water pressures are applied so that the difference between the preset air pressure and the preset water pressure reaches the preset negative suction force for each moisture absorption condition. When the preset negative suction force is greater than or equal to -15 kPa, that is, when the absolute value of the preset negative suction force is less than or equal to 15 kPa, the lifting member 42 drives the water supply bottle 41 to move upward from a position level with the pressure chamber 2 to a preset height, and applies the preset water pressure through the water head difference between the water supply bottle 41 and the pressure chamber 2; when the preset negative suction force is less than -15 kPa, that is, when the absolute value of the preset negative suction force is greater than 15 kPa, the computer 36 controls the water pressure volume controller 43 to apply the preset water pressure.

[0046] The lifting component 42 moves the water supply bottle 41 upwards, pressurizing the pressure chamber 2 through the head difference. While its pressurization range is limited, its accuracy is high, making it suitable for situations with conventional suction ranges. Furthermore, the lifting component 42 alters the head difference by physically raising or lowering the bottle, thus simulating the rise and fall of groundwater levels. This is entirely consistent with the principle of static pore water pressure generation in nature, resulting in a realistic simulation process. The obtained data more directly reflects the behavior of soil under actual hydrogeological conditions. The water pressure volume controller 43 can provide pressure far exceeding the static head, enabling the measurement of soil characteristics under high suction conditions, covering a high measurement range that the water supply bottle 41 cannot achieve.

[0047] S4. When the measurement data of the first measuring component 5 is stable, record the measurement data of the first measuring component 5. In step S4, the lifting component 42 moves the water supply bottle 41 upward and pressurizes the pressure chamber 2 through the water head difference. When the mass change of the water in the water supply bottle 41 is less than 0.02g per day, the preset air pressure, preset water pressure and matrix suction inside the test soil sample 7 are balanced. The data of the first measuring component 5 and the tension sensor 13 are recorded at this time. The first measuring component 5 measures the water content in the test soil sample 7, and the tension sensor 13 measures the tension in the test soil sample 7, that is, the matrix suction of the soil.

[0048] When pressurization is applied using the water pressure volume controller 43, and the water content in the test soil sample 7 measured by the first measuring element 5 is stable, the preset air pressure, preset water pressure and the matrix suction inside the test soil sample 7 are balanced, and the data of the first measuring element 5 and the tension sensor 13 are recorded at this time.

[0049] The difference between the preset air pressure and the preset water pressure is the negative suction force. However, there is a loss during the process of applying the preset air pressure and preset water pressure to the test soil sample 7. Using the difference between the two as the soil matrix suction force in the soil-water characteristic curve will result in an error. In order to improve the accuracy of the test results, the tension sensor 13 is used to measure the actual soil matrix suction force of the test soil sample 7, and its data is used as the soil matrix suction force in the soil-water characteristic curve, so that the test results are more accurate.

[0050] S5. Repeat steps S3 to S4 until multiple sets of moisture absorption conditions are completed. This allows us to obtain multiple sets of preset negative suction-water content data points, and through data processing, we can plot the soil-water characteristic curves for the corresponding negative suction range.

[0051] S6. Apply preset air pressure and preset water pressure through air pressure loading unit 3 and water pressure loading unit 4 respectively, so that the difference between preset air pressure and preset water pressure reaches preset positive suction force; In step S6, under each dehumidification condition, the preset water pressure is less than the preset air pressure, and the difference between the two is a positive value, which is the preset positive suction force. In this embodiment of the invention, when the preset positive suction force is less than or equal to 15 kPa, the preset air pressure is constant at atmospheric pressure. The lifting component 42 drives the water supply bottle 41 to move downward from a position level with the pressure chamber 2 to a preset height. The preset water pressure is applied through the water head difference between the water supply bottle 41 and the pressure chamber 2, so that the difference between the preset air pressure and the preset water pressure reaches the preset positive suction force for each dehumidification condition. When the preset positive suction force is greater than 15 kPa, the water pressure volume controller 43 applies a constant preset water pressure, and the computer 36 controls the air pressure loading unit 3 to apply multiple different preset air pressures, so that the difference between the preset air pressure and the preset water pressure reaches the preset positive suction force for each dehumidification condition.

[0052] S7. When the measurement data of the first measuring component 5 is stable, record the measurement data of the first measuring component 5. In step S7, when water pressure is applied to the water supply bottle 41, if the daily change in the mass of water in the water supply bottle 41 is less than 0.02g, the preset air pressure, preset water pressure, and the matrix suction inside the test soil sample 7 are balanced. The data from the first measuring element 5 and the tension sensor 13 are recorded at this time. When pressurization is applied using the water pressure volume controller 43, if the water content in the test soil sample 7 measured by the first measuring element 5 is stable, the preset air pressure, preset water pressure, and the matrix suction inside the test soil sample 7 are balanced. The data from the first measuring element 5 and the tension sensor 13 are recorded at this time.

[0053] S8. Repeat steps S6 to S7 until multiple dehumidification conditions are completed.

[0054] This allows us to obtain multiple sets of preset positive suction-water content data points, and through data processing, we can plot the soil-water characteristic curves for the corresponding positive suction range.

[0055] By combining the soil-water characteristic curves in the positive suction range and the negative suction range, the variation law of soil matrix suction and water content in the whole cycle of moisture absorption and dissipation can be obtained.

[0056] Furthermore, such as Figure 1 As shown, the suction of unsaturated soil is significantly affected by hydraulic paths. There are four common hydraulic paths in nature: free water infiltration, soil water drainage, condensation of water vapor in the air, and evaporation of soil water. An experimental apparatus for determining the soil-water characteristic curve of unsaturated soil according to an embodiment of the present invention can simulate these four hydraulic paths. The aforementioned hygroscopic condition simulates free water infiltration, and the hygroscopic condition simulates soil water drainage.

[0057] Simulated humidification path of water vapor condensation in air: Loosen the sealing screw 19 and remove the top cover 20. Assemble the base 21 and sample chamber 22 and place them on the support 12. Add the prepared dry test soil sample 7 into the sample chamber 22. The upper surface of the test soil sample 7 is in contact with the air in the pressure chamber 1. Then, add an ice-water mixture into the pressure chamber 1. The water level of the ice-water mixture is 1 cm to 2 cm away from the upper surface of the pressure chamber 2, that is, the ice-water mixture does not enter the interior of the pressure chamber 2. Cover the surface of the ice-water mixture with a certain number of layers of plastic wrap to isolate the ice-water mixture from the air inside the pressure chamber 1 and ensure that there is a certain distance between the plastic wrap and the ice-water mixture. A certain amount of air is present, with only the upper surface of the test soil sample 7 in contact with the air inside the pressure chamber 1. The top cover is then installed using the sealing screw 19 to seal the pressure chamber 1. The temperature controller 16 and humidity controller 15 are turned on to keep the air inside the pressure chamber 1 warm and with high relative humidity (temperature 25℃, humidity 100%). Water vapor in the air enters the interior of the test soil sample 7 and condenses. The values ​​of the first measuring element 5 and the tension sensor 13 are read to obtain the relationship between the matrix suction and water content of the test soil sample 7. The soil-water characteristic curve of the test soil sample 7 under the condensation and humidification state is plotted.

[0058] Simulating the evaporation path of water in soil: After performing steps S1 to S5 of the test method for determining the soil-water characteristic curve of unsaturated soil according to the above embodiment of the invention, the test soil sample 7 has completed moisture absorption and reached a saturated state. The water pressure volume controller 43 and the air pressure controller 32 are turned off, the pressure ring 23, the air permeable plate 24 and the hydrophobic membrane 8 are removed, so that the upper surface of the test soil sample 7 is in contact with the air. The humidity controller 15 is turned on to maintain the humidity in the pressure chamber 1 at a low humidity state to accelerate the evaporation of water in the test soil sample 7. The values ​​of the first measuring element 5 and the tension sensor 13 are read to obtain the relationship between the matrix suction and the water content of the test soil sample 7, and the soil-water characteristic curve of the test soil sample 7 in the evaporation state is plotted.

[0059] An experimental device for determining the soil-water characteristic curve of unsaturated soil according to an embodiment of the present invention can simulate four hydraulic paths, achieving a complete reproduction of the four basic water transport paths in nature. Researchers can accurately simulate the full-cycle hydraulic process of soil from drying (evaporation, drainage) to wetting (infiltration, condensation) in a controlled laboratory, thereby more realistically reflecting the hydraulic state evolution of soil under complex environments such as actual climate alternation and groundwater level fluctuations. It can accurately measure the soil-water characteristic curve under the corresponding path, providing irreplaceable experimental data support for quantitatively revealing the degradation mechanism of strength and deformation characteristics of special soils under different hydrological conditions and establishing more accurate disaster prediction models.

[0060] In summary, the present invention provides an experimental apparatus and method for determining the soil-water characteristic curve of unsaturated soil. A pressure chamber 2 is located within a pressure chamber 1, which is connected to a pneumatic loading unit 3. A hydraulic loading unit 4 is connected to the bottom of the pressure chamber 2. A clay plate 6 is installed at the bottom of the receiving cavity 2a of the pressure chamber 2, and a hydrophobic membrane 8 is installed at the top opening of the receiving cavity 2a. The test soil sample 7 is installed between the clay plate 6 and the hydrophobic membrane 8. The test is divided into two main sequences: moisture absorption and moisture release, with multiple preset suction values ​​applied in a stepped manner in each sequence. Under hygroscopic conditions, the water pressure applied by the water pressure loading unit 4 is greater than the air pressure applied by the air pressure loading unit 3. Water enters from the bottom of the receiving cavity 2a and passes through the clay plate 6 into the test soil sample 7. The hydrophobic membrane 8 has hydrophobic and air-permeable properties, which can prevent liquid water from flowing out from the top of the receiving cavity 2a and seal the water inside the test soil sample 7, ensuring that the water pressure applied by the water pressure loading unit 4 is fully applied to the soil sample to form a stable negative suction state within the test soil sample 7. This solves the technical problem that traditional devices cannot simulate negative suction inside the soil. The water pressure loading unit 4 and the air pressure loading unit 3 continue to act until the data of the first measuring element 5 is constant (the water migration inside the soil sample reaches equilibrium). Then, the suction-water content data points under this state are recorded. Multiple hygroscopic conditions are completed to obtain the soil-water characteristic curves corresponding to the negative suction range.

[0061] In the dehumidification condition, the water pressure applied by the water pressure loading unit 4 is less than the air pressure applied by the air pressure loading unit 3. The water in the test soil sample 7 flows out through the clay plate 6. After the clay plate 6 is saturated with water, it only allows water to pass through, thus isolating soil particles and gas from flowing out. This not only ensures the integrity of the test soil sample 7, but also ensures that the air pressure applied by the air pressure loading unit 3 is fully applied to the test soil sample 7, so as to form a stable positive suction state within the test soil sample 7. The water pressure loading unit 4 and the air pressure loading unit 3 continue to operate until the data of the first measuring piece 5 is constant (the internal water migration of the soil sample reaches equilibrium). Then, the suction-water content data points under this state are recorded. Multiple dehumidification conditions are completed to obtain the soil-water characteristic curves corresponding to the positive suction range.

[0062] Moreover, the embodiments of this invention can simulate four hydraulic paths, achieving a complete reproduction of the four basic water transport paths in nature. Researchers can accurately simulate the full-cycle hydraulic process of soil from drying (evaporation, drainage) to wetting (infiltration, condensation) in a controlled laboratory, thereby more realistically reflecting the hydraulic state evolution of soil under complex environments such as actual climate alternation and groundwater level fluctuations. It can accurately measure the soil-water characteristic curves under the corresponding paths, providing irreplaceable experimental data support for quantitatively revealing the degradation mechanism of strength and deformation characteristics of special soils under different hydrological conditions and establishing more accurate disaster prediction models.

[0063] In summary, the embodiments of the present invention, through the synergistic effect of the clay plate 6 and the hydrophobic membrane 8, enable the experimental device to precisely control the complete hydraulic path from high positive suction to negative suction. By using the moisture absorption-discharge path test method, data of the entire moisture absorption-discharge cycle of the soil sample are obtained. The resulting soil-water characteristic curve can more realistically reflect the changes in the hydraulic state of the soil under actual wet and dry conditions. This solves the technical problem in the prior art that the pressure plate method cannot simulate the negative suction inside expansive soil and cannot fully reflect the changes in suction and water content of expansive soil throughout the entire moisture absorption-discharge cycle.

[0064] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A test apparatus for determining the soil-water characteristic curve of unsaturated soil, characterized in that, include: Pressure chamber (1); Pressure chamber (2) is located inside pressure chamber (1), and the top of pressure chamber (2) has an opening; A clay plate (6) is placed at the bottom of the pressure chamber (2), and a test soil sample (7) is placed on the clay plate (6). The pressure chamber (2) is provided with a hydrophobic membrane (8) near the opening. The lower side of the hydrophobic membrane (8) contacts the test soil sample (7), and the hydrophobic membrane (8) seals the opening. The air pressure loading unit (3) is connected to the pressure chamber (1). The air pressure applied by the air pressure loading unit (3) passes through the hydrophobic membrane (8) and then applies air pressure to the test soil sample (7). The water pressure loading unit (4) is connected to the bottom of the pressure chamber (2). The air pressure applied by the water pressure loading unit (4) passes through the clay plate (6) and then applies water pressure to the test soil sample (7). The first measuring element (5) is set on the pressure chamber (2) and is used to measure the moisture content of the test soil sample (7).

2. The test apparatus for determining the soil-water characteristic curve of unsaturated soil according to claim 1, characterized in that, The pressure chamber (2) includes a base (21) and a sample chamber (22); The base (21) is provided with an installation groove (21a), the clay plate (6) is installed in the installation groove (21a), and the bottom wall of the installation groove (21a) is provided with a water inlet (21b), which is connected to the water pressure loading unit (4). The sample chamber (22) is fixedly connected to the base (21) on the outer side of the mounting groove (21a). The sample chamber (22) has an inner cavity (22a) with openings on both the upper and lower sides. The test soil sample (7) is installed in the inner cavity (22a). The hydrophobic membrane (8) is installed at the top opening of the inner cavity (22a).

3. The test apparatus for determining the soil-water characteristic curve of unsaturated soil according to claim 2, characterized in that, The pressure chamber (2) also includes a pressure ring (23) and a vent plate (24); The top wall of the sample chamber (22) is provided with an annular groove (22b), and a first sealing ring (10) is installed in the annular groove (22b). The hydrophobic membrane (8) covers the top of the sample chamber (22), and the edge of the hydrophobic membrane (8) covers the first sealing ring (10). The breathable plate (24) and the pressure ring (23) are stacked on the hydrophobic membrane (8) from bottom to top. The edges of the breathable plate (24) and the pressure ring (23) extend to the top wall of the sample chamber (22). The pressure ring (23) is detachably and fixedly connected to the sample chamber (22).

4. The test apparatus for determining the soil-water characteristic curve of unsaturated soil according to claim 2, characterized in that, The water pressure loading unit (4) includes a water supply bottle (41), a lifting component (42), a water pressure volume controller (43), a first water pipe (44), and a second water pipe (45). The water supply bottle (41) is mounted on the lifting member (42), which is used to drive the water supply bottle (41) to move up and down. The first water pipe (44) connects the pressure chamber (2) and the water supply bottle (41). A water valve (9) is provided on the first water pipe (44). The second water pipe (45) connects the pressure chamber (2) and the water pressure volume controller (43).

5. The test apparatus for determining the soil-water characteristic curve of unsaturated soil according to claim 4, characterized in that, The water pressure loading unit (4) also includes an air trap (46), a third water pipe (47) and a fourth water pipe (48). The air trap (46) has a first water outlet at its bottom and a first water inlet and a second water outlet on its side wall. The first water pipe (44) connects the first water inlet to the water supply bottle (41), the second water pipe (45) connects the first water inlet to the water pressure volume controller (43), the third water pipe (47) connects the first water outlet to the bottom of the pressure chamber (2), and the fourth water pipe (48) connects the second water outlet to the bottom of the pressure chamber (2).

6. A test method for determining the soil-water characteristic curve of unsaturated soil, characterized in that, The test apparatus for determining the soil-water characteristic curve of unsaturated soil according to any one of claims 1 to 5 includes the following steps: S1. Install test soil samples (7); S2. Multiple sets of moisture absorption and dehumidification conditions are preset. Each moisture absorption condition has a preset negative suction force, and each dehumidification condition has a preset positive suction force. S3. Apply a preset air pressure and a preset water pressure through the air pressure loading unit (3) and the water pressure loading unit (4) respectively, so that the difference between the preset air pressure and the preset water pressure reaches the preset negative suction force; S4. When the measurement data of the first measuring element (5) is stable, record the measurement data of the first measuring element (5); S5. Repeat steps S3 to S4 until multiple sets of the moisture absorption conditions are completed; S6. Apply the preset air pressure and the preset water pressure through the air pressure loading unit (3) and the water pressure loading unit (4) respectively, so that the difference between the preset air pressure and the preset water pressure reaches the preset positive suction force; S7. When the measurement data of the first measuring element (5) is stable, record the measurement data of the first measuring element (5); S8. Repeat steps S6 to S7 until multiple sets of the dehumidification conditions are completed.

7. The test method for determining the soil-water characteristic curve of unsaturated soil according to claim 6, characterized in that, In step S3: The preset air pressure is kept constant at atmospheric pressure. Different preset water pressures are applied so that the difference between the preset air pressure and the preset water pressure reaches the preset negative suction force for each of the moisture absorption conditions. Specifically: When the preset negative suction force is greater than or equal to -15kPa, the lifting member (42) drives the water supply bottle (41) to move upward to the preset height in order to apply the preset water pressure; When the preset negative suction force is less than -15kPa, the water pressure volume controller (43) applies the preset water pressure.

8. The test method for determining the soil-water characteristic curve of unsaturated soil according to claim 6, characterized in that, In step S6: When the preset positive suction force is less than or equal to 15 kPa, the preset air pressure is constant at atmospheric pressure. The lifting component (42) drives the water supply bottle (41) to move downward to different preset heights so that the difference between the preset air pressure and the preset water pressure reaches the preset positive suction force of each of the dehumidification conditions. When the preset positive suction force is greater than 15 kPa, the water pressure volume controller (43) applies a constant preset water pressure, and the air pressure loading unit (3) applies multiple different preset air pressures so that the difference between the preset air pressure and the preset water pressure reaches the preset positive suction force of each dehumidification condition.

9. The test method for determining the soil-water characteristic curve of unsaturated soil according to claim 6, characterized in that, In step S1, the test soil sample (7) is installed, specifically as follows: S11. Install the base (21) and the sample chamber (22) in the pressure chamber (1), and install the clay plate (6) on the base (21); S12. Inject degassing water into the pressure chamber (1) so that the degassing water submerges the sample chamber (22). S13, The pneumatic loading unit (3) applies negative pressure to the pressure chamber (1); S14. When no air bubbles are discharged from the sample chamber (22), the airless water in the pressure chamber (1) is discharged, and the test soil sample (7) and the hydrophobic membrane (8) are installed.

10. The test method for determining the soil-water characteristic curve of unsaturated soil according to claim 6, characterized in that, In step S3, before applying the preset air pressure and preset water pressure through the air pressure loading unit (3) and water pressure loading unit (4) respectively, the water valve (9) is opened first so that the water in the water supply bottle (41) flows into the air trap (46), the water in the air trap (46) flows into the pressure chamber (2) along the third water pipe (47), and the water in the pressure chamber (2) flows back into the air trap (46) through the fourth water pipe (48) to discharge the gas in the third water pipe (47) and the fourth water pipe (48).

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