Apparatus and method for measuring unidirectional freezing and thawing permeability coefficient of soil body in different height range

Abstract

The application provides a device and method for measuring the unidirectional freeze-thaw permeability coefficient of soil bodies in different height ranges, and comprises a water storage pool, a sample cylinder, a top cold plate structure, a bottom cold plate structure, a water supply pipe structure and a variable water head pipe.The water storage pool is located above the sample cylinder, and overflow holes are arranged on the side of the water storage pool to control the water level of the water storage pool.The sample cylinder comprises a plurality of soil sample unit cylinders stacked one above another, and the soil sample unit cylinders are filled with soil samples to be measured.Water injection holes are arranged on the upper part of each soil sample unit cylinder, and a drainage hole is arranged on the lower part of the bottom soil sample unit cylinder.The top cold plate structure is installed on the upper end of the sample cylinder, and the bottom cold plate structure is installed on the lower end of the sample cylinder.The application can measure the permeability coefficient of soil bodies after freeze-thaw, and is more convenient and fast;not only can it realize constant water head permeability test, but also can realize variable water head permeability test, and can measure the permeability coefficient of soil bodies in different height ranges after unidirectional freeze-thaw.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical experimental technology, and in particular relates to a test device and method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different heights. Background Technology

[0002] Whether it's subgrade soil in seasonally frozen soil regions or soil layers reinforced by artificial freezing methods, the soil is affected by freeze-thaw cycles. During freeze-thaw, the phase change and redistribution of water within the soil alter its structure, leading to changes in permeability. This, in turn, causes damage and deterioration of the soil's mechanical properties, and can even trigger engineering disasters. Therefore, studying the permeability of soil after freeze-thaw is of great significance, providing important reference for design and construction. The permeability coefficient is a crucial indicator of soil permeability. Existing indoor permeability tests are divided into constant head permeability tests and variable head permeability tests. The constant head permeability test is used for coarse-grained soils, while the variable head permeability test is used for fine-grained soils, and both require different testing equipment. These tests can only measure the permeability coefficient of soil at normal temperatures. To determine the permeability coefficient of soil after freeze-thaw, both freeze-thaw and permeability tests need to be combined. Currently, there is no integrated testing device for determining the permeability coefficient of soil after freeze-thaw. In addition, indoor soil freeze-thaw tests usually use unidirectional freezing and natural thawing to simulate the freeze-thaw process of soil. Due to the influence of temperature gradient, the porosity of soil at different heights will change to different degrees after freeze-thaw, resulting in different permeability coefficients at different heights. Studying the permeability coefficient of soil at different heights can provide an important theoretical basis for revealing the changes in the mechanical properties of soil after freeze-thaw. At present, there is no test method to determine the vertical permeability coefficient of soil at different heights after freeze-thaw. Summary of the Invention

[0003] In response to the problems mentioned in the background art, this invention proposes a test device and method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different height ranges. This invention can not only provide unidirectional freeze-thaw conditions for soil samples, but also measure the vertical permeability coefficient of soil at different height ranges after freeze-thaw.

[0004] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0005] A test apparatus for measuring the unidirectional freeze-thaw permeability coefficient of soil at different heights includes a water storage tank, a sample cylinder, a top cold plate structure, a bottom cold plate structure, a water supply pipe structure, and a variable head pipe. The water storage tank is located above the sample cylinder, and an overflow hole is provided on the side of the water storage tank to control the water level. The sample cylinder consists of several stacked soil sample unit cylinders, each filled with the soil sample to be tested. The soil samples in adjacent soil sample unit cylinders are separated by a permeable structure, which isolates the soil samples while allowing water to pass freely. Each soil sample unit... The upper part of the cylindrical unit is equipped with a water injection hole. The bottom of the water storage tank is connected to the water injection hole of each soil sample unit through a water supply pipe structure. The lower part of the bottom soil sample unit is equipped with a drainage hole. The top cold plate structure is installed on the upper end of the soil sample to be tested and provides cooling. The bottom cold plate structure is installed on the lower end of the soil sample to be tested and provides cooling. The lower part of the bottom soil sample unit is equipped with a bottom water inlet hole. One end of the variable head pipe is connected to the bottom of the water storage tank, and the other end is connected to the water inlet hole. The variable head pipe is also equipped with a variable head pipe water injection end. An external water source can inject water into the variable head pipe through the variable head pipe water injection end.

[0006] To optimize the above technical solution, the specific measures also include:

[0007] Each of the above-mentioned water injection holes is equipped with a water injection hole stop clamp, the drain hole is equipped with a drain hole stop clamp, the bottom water inlet hole is equipped with a bottom water inlet hole stop clamp, and the water injection end of the variable head pipe is equipped with a variable head pipe water injection end stop clamp. Each stop clamp is used to control the opening and closing of the corresponding pipeline.

[0008] The aforementioned water supply pipe structure and the water injection hole of each soil sample unit can be detachably connected.

[0009] The top cold plate structure and the bottom cold plate structure mentioned above are the same, both including a cold cavity, a cold pipe and a cold bath. The cold cavity is attached to the upper end of the top soil sample to be tested or the lower end of the bottom soil sample to be tested. The cold cavity forms a loop with the cold bath through the cold pipe. Cooling liquid flows in this loop, and the cold bath is used to cool the cooling liquid.

[0010] The permeable structure described above is permeable stone.

[0011] A water-stop rubber ring is provided at the edge connection of the adjacent soil sample unit cylinders mentioned above. The water-stop rubber ring is used to prevent water and the soil sample to be tested from seeping out from the edge of the adjacent soil sample unit cylinders.

[0012] The method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different heights utilizes the aforementioned test apparatus for measuring the unidirectional freeze-thaw permeability coefficient of soil at different heights. The specific measurement method includes the following steps:

[0013] Step 1: Determine the filling method of each soil sample unit according to the properties of the soil sample to be tested: If it is coarse-grained soil, take a representative undisturbed soil or prepare a soil sample, and fill it into the soil sample unit using the layered compaction method; if it is fine-grained soil, cut an undisturbed soil sample or prepare a soil sample vertically along the soil sample, and then fill the fine-grained soil sample into the soil sample unit. Stack several soil sample unit units containing the soil sample to be tested on top of each other and fix them in place.

[0014] Step 2: Close all water-stop clamps, adjust the top and bottom cold plate structures to the same temperature >0℃, maintain the constant temperature for a predetermined time, and then adjust the temperature of the top or bottom cold plate structure to <0℃ to freeze the soil sample in the soil sample unit cylinder from top to bottom or from bottom to top; after a certain period of time, close the cold bath of the top and bottom cold plate structures to allow the soil sample to thaw naturally at room temperature;

[0015] Step 3: When the soil sample is coarse-grained soil, open the water injection hole of the top soil sample unit and the drainage hole of the bottom soil sample unit, and close the water injection holes of other soil sample units and the water inlet hole of the bottom soil sample unit.

[0016] Step 4: Adjust the overflow hole of the water storage tank to be 1-2m higher than the top of the top soil sample unit cylinder, and continuously inject water into the water storage tank so that there is always residual water overflowing from the overflow hole of the water storage tank, while at the same time, there is continuous water flow from the drainage hole of the bottom soil sample unit cylinder without air bubbles, so that the soil sample is fully saturated. Then start the seepage test and measure the vertical distance from the water surface of the water storage tank to the drainage hole, which is the head difference; start the stopwatch and use a measuring cylinder to collect the seepage water volume after a certain period of time at the drainage hole; measure the water temperature in the cylinder, and the vertical permeability coefficient is calculated by formulas (1) and (2):

[0017]

[0018]

[0019] In the formula: k T The permeability coefficient of the sample at water temperature T℃;

[0020] Q represents the amount of water that seeps in within time t seconds;

[0021] L is the seepage diameter, which is equal to the height of the soil sample to be tested between the open injection hole and the drainage hole;

[0022] A represents the cross-sectional area of ​​the soil sample to be tested;

[0023] ΔH is the water level difference;

[0024] k 20 The permeability coefficient of the soil sample to be tested is given at a standard temperature of 20℃.

[0025] η TLet T be the dynamic viscosity coefficient of water at temperature T.

[0026] η 20 The dynamic viscosity coefficient of water at 20℃;

[0027] Step 5: Adjust the height of the overflow hole in the water storage tank to change the head difference. Repeat step 4 to obtain multiple vertical permeability coefficients. Take the average value of the vertical permeability coefficients as the permeability coefficient of the soil sample in the height range between the top soil sample unit and the bottom soil sample unit.

[0028] Step Six: Close the water injection hole of the top soil sample unit, open the water injection hole of the next soil sample unit, and keep the drainage hole of the bottom soil sample unit open. Close the water injection holes of the other soil sample units and the water inlet hole of the bottom soil sample unit. Obtain the permeability coefficient of the soil sample height range between the next soil sample unit and the bottom soil sample unit according to Steps Four and Five. Continue in this manner to obtain the permeability coefficient of the soil sample height range between each soil sample unit and the bottom soil sample unit.

[0029] Step 7: Based on the permeability coefficient of the soil sample height range between each soil sample unit and the bottom soil sample unit, calculate the permeability coefficient of the soil sample height range between each soil sample unit using formulas (3) and (4).

[0030]

[0031]

[0032] Where: H j Let be the thickness of the soil sample to be measured in the j-th soil sample unit;

[0033] H represents the total thickness of the soil sample to be tested, which consists of soil samples from n soil sample unit cylinders.

[0034] k z Let H be the vertical equivalent permeability coefficient of the soil sample with thickness H to be tested;

[0035] k j Let be the vertical permeability coefficient of the soil sample to be tested in the j-th soil sample unit;

[0036] Step 8: When the soil sample is fine-grained soil, open the water injection hole of the top soil sample unit and the water inlet hole of the bottom soil sample unit, close the water injection holes, drainage holes and water injection end of the variable head pipe of the other soil sample units, and dismantle the water supply pipe structure.

[0037] Step 9: Adjust the overflow hole of the water storage tank to be 1-2m higher than the top of the uppermost soil sample unit. The water in the water storage tank fills the variable head pipe and is injected into the bottom soil sample unit through the variable head pipe. The water overflows from the water injection hole of the top soil sample unit and there are no air bubbles in the water, thus achieving full saturation of the soil sample. Start the seepage test. Empty the water in the water storage tank, open the water injection end of the variable head pipe, fill the variable head pipe with water to the required height, and start measuring and recording the initial water head height and the initial time in the variable head pipe. Measure and record the changes in water head and time at predetermined time intervals, and measure and record the water temperature of the water injection hole of the top soil sample unit. The vertical permeability coefficient is calculated by formulas (2) and (5):

[0038]

[0039] In the formula: f is the cross-sectional area of ​​the variable head pipe;

[0040] H1 represents the initial water head;

[0041] H2 is the head at the end.

[0042] Then raise the water level in the variable head pipe back to the required height, and then conduct a seepage test. Repeat the test several times, and take the average value of the obtained permeability coefficient as the permeability coefficient of the soil sample in the height range between the top soil sample unit and the bottom soil sample unit.

[0043] Step 10: Close the water injection hole of the top soil sample unit and open the water injection hole of the next soil sample unit. Following Step 9, obtain the permeability coefficient of the soil sample height range between the next soil sample unit and the bottom soil sample unit. Continue in this manner to obtain the permeability coefficient of the soil sample height range between each soil sample unit and the bottom soil sample unit.

[0044] Step 11: Based on the permeability coefficient of the soil sample height range between each soil sample unit and the bottom soil sample unit, calculate the permeability coefficient of the soil sample height range between each soil sample unit using formulas (3) and (4).

[0045] Coarse-grained soil is soil with a particle size greater than 0.075 mm, while fine-grained soil is soil with a particle size less than or equal to 0.075 mm.

[0046] The variable head pipe is equipped with graduations.

[0047] The variable head pipe has an inner diameter of no more than 1 cm, a length of more than 1 m, and a scale division of 1 mm.

[0048] The beneficial effects of this invention are:

[0049] 1. This test device integrates a freeze-thaw test device and a permeability test device into one unit, which can measure the permeability coefficient of soil after freeze-thaw, making it more convenient and faster;

[0050] 2. This test apparatus and method can not only perform constant head permeability tests, but also variable head permeability tests, and can measure the permeability coefficient of soil at different heights after unidirectional freeze-thaw cycles. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the structure of the present invention;

[0052] Figure 2 yes Figure 1 A schematic diagram of the top cold plate structure or the bottom cold plate structure.

[0053] The labels in the diagram are as follows: 1. Water storage tank; 11. Overflow hole; 2. Sample tube; 21. Soil sample unit tube; 21a. Water injection hole; 21b. Drainage hole; 21c. Bottom water inlet; 22. Permeable structure; 23. Water-stop rubber ring; 3. Top cold plate structure; 31. Cold cavity; 32. Cold pipe; 33. Cold bath; 4. Bottom cold plate structure; 5. Water supply pipe structure; 6. Variable head pipe; 61a. Detailed Implementation

[0054] To make the objectives, technical solutions, and advantages of this application clearer, the application is described and illustrated below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0055] Obviously, the accompanying drawings described below are merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios based on these drawings without any inventive effort. Furthermore, it is understood that although the efforts made in this development process may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0056] In this application, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places in the specification does not necessarily indicate the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this application may be combined with other embodiments without conflict.

[0057] Unless otherwise defined, the technical or scientific terms used in this application shall have the ordinary meaning understood by one of ordinary skill in the art to which this application pertains. The terms “a,” “an,” “an,” “the,” and similar words used in this application do not indicate quantity limitation and may indicate singular or plural. The terms “comprising,” “including,” “having,” and any variations thereof used in this application are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or device that includes a series of steps or units (elements) is not limited to the listed steps or units, but may also include steps or units not listed, or may include other steps or units inherent to these processes, methods, products, or devices. The terms “connected,” “linked,” “coupled,” and similar words used in this application are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. The terms “multiple” / “several” used in this application refer to two or more. “And / or” describes the relationship between related objects, indicating that three relationships may exist; for example, “A and / or B” can indicate: A alone, A and B simultaneously, or B alone. The character " / " generally indicates that the preceding and following objects have an "or" relationship. The terms "first," "second," and "third," etc., used in this application are merely to distinguish similar objects and do not represent a specific ordering of the objects.

[0058] The soil unidirectional freeze-thaw permeability coefficient measurement test device for different height ranges of the present invention includes an plexiglass sample cylinder, a water storage tank 1, a top cold plate structure 3, a bottom cold plate structure 4, permeable stones, a water supply pipe, a water-stop clamp, a variable head pipe 6, etc. Figure 1 As shown.

[0059] The acrylic sample tube consists of four soil sample unit tubes 21. The uppermost soil sample unit tube 21 is the tallest (9-10cm), providing space for the top cold plate structure 3 to move vertically. The remaining soil sample unit tubes 21 are all the same height (5-6cm), and all four soil sample unit tubes 21 have the same inner diameter (6-7cm) and a wall thickness of 2-4cm (to effectively insulate the sides of the soil sample). Adjacent soil sample unit tubes 21 are connected by mortise and tenon joints, combined with a water-stop rubber ring 23 and a fixing bracket, to prevent water from flowing out from the joints. Except for the bottom soil sample unit tube 21, which has three holes on its side wall—a water injection hole 21a, a drainage hole 21b, and a bottom water inlet hole 21c—the other soil sample unit tubes 21 have only one water injection hole 21a on the top of their side walls.

[0060] The water storage tank 1 has one overflow hole 11 on its side and two outlet holes at its bottom. One outlet hole is connected to the water injection hole 21a on the side wall of each soil sample unit 21 via a water supply pipe structure 5, and the other outlet hole is connected to the variable head pipe 6. During constant head permeability tests, this ensures a constant head at both ends of the sample; during variable head permeability tests, it supplies water to the variable head pipe. Each soil sample unit 21 has a filter screen installed in the holes on its side wall to prevent soil particles from being discharged during seepage.

[0061] Both the top cold plate structure 3 and the bottom cold plate structure 4 have one liquid inlet and one liquid outlet, which are connected to a cold bath to form a closed loop. Temperature control of the bottom and top cold plates is achieved by circulating low-temperature cold liquid. The diameter of the cold cavity 31 of the top cold plate structure 3 is slightly smaller than the inner diameter of the sample tube 2, and two water-stop rubber rings are arranged on the side wall to prevent water in the soil from overflowing from the gap between the top cold plate and the sample tube 2.

[0062] The inner diameter of the variable head pipe 6 should not exceed 1 cm, the length should be more than 1 m, and the graduation value should be 1 mm.

[0063] Furthermore, to illustrate the experimental apparatus for measuring the unidirectional freeze-thaw permeability coefficient of soil at different height ranges provided by this invention, detailed steps of the experimental process are given:

[0064] (1) Test preparation stage. For coarse-grained soil, take a representative undisturbed soil sample (6 cm high, 6 cm in diameter) or prepare a soil sample. Use a layered compaction method to control the void ratio and water content of the soil. For fine-grained soil, cut a undisturbed soil sample or prepare a soil sample vertically along the soil sample. The height of the sample should be the same as the minimum height of the soil sample unit 21, which is 5 cm. Assemble the four soil sample unit 21s containing the soil samples and install them on the test apparatus, such as... Figure 1 As shown.

[0065] (2) Freeze-thaw stage. Close all water-stop clamps, adjust the two cold baths to the same temperature (5℃), circulate the cold liquid to keep the temperature of the top and bottom cold plates consistent, and keep the sample at a constant temperature for 12 hours; then adjust the temperature of the bottom cold plate to a low temperature (-20℃), and keep the temperature of the top cold plate constant (5℃), so as to freeze the soil from bottom to top for 12 hours; close the cold baths and let the soil sample thaw naturally at room temperature for 24 hours.

[0066] (3) To test the coarse soil sample, open the water injection hole 21a of the top soil sample unit 21 and the drainage hole 21b of the bottom soil sample unit 21, and close the water injection holes 21a of the other soil sample unit 21 and the water inlet hole 21c of the bottom soil sample unit 21.

[0067] (4) Adjust the overflow hole of water storage tank 1 to be 2m above the top of the soil sample, and continuously inject water into water storage tank 1 so that there is always residual water overflowing from the overflow hole of water storage tank 1, and at the same time, there is continuous water flow from the drainage hole of the bottom soil sample unit cylinder 21 without air bubbles, thereby achieving full saturation of the soil sample and keeping the water level at both ends of the water inlet and outlet constant, and then start the seepage test. Measure the vertical distance from the water surface of water storage tank 1 to the drainage hole, which is the head difference; start the stopwatch, and at the same time use a graduated cylinder to collect the seepage water volume at the drainage hole for 10 seconds; record the water temperature in the graduated cylinder. At this time, the vertical permeability coefficient can be calculated by formulas (1) and (2).

[0068]

[0069]

[0070] In the formula: k T The permeability coefficient of the sample at water temperature T℃;

[0071] Q represents the amount of water that seeps in within time t seconds;

[0072] L is the seepage diameter, which is equal to the height of the soil sample to be tested between the open injection hole and the drainage hole;

[0073] A represents the cross-sectional area of ​​the soil sample to be tested;

[0074] ΔH is the water level difference;

[0075] k 20 The permeability coefficient of the soil sample to be tested is given at a standard temperature of 20℃.

[0076] η T Let T be the dynamic viscosity coefficient of water at temperature T.

[0077] η 20 The dynamic viscosity coefficient of water at 20℃;

[0078] Adjust the overflow hole of the water storage tank 1 to be 1.5m above the top of the soil sample to change the water head difference. Repeat the above test process and calculate the permeability coefficient. Take the average value as the permeability coefficient of the soil sample in the range of height between the top soil sample unit 21 and the bottom soil sample unit 21.

[0079] (5) Close the water injection hole 21a of the top soil sample unit 21, open the water injection hole 21a of the next soil sample unit 21, keep the drainage hole 21b of the bottom soil sample unit 21 open, close the water injection holes 21a of the other soil sample unit 21 and the bottom water inlet hole 21c, repeat step (4), and the permeability coefficient of the soil sample height range between the next soil sample unit 21 and the bottom soil sample unit 21 can be calculated; similarly, the permeability coefficient of the soil sample height range between each soil sample unit and the bottom soil sample unit can be calculated respectively.

[0080] (6) Based on the calculation formulas (3) and (4) for the vertical equivalent permeability coefficient of layered soil layers, the permeability coefficient of the soil sample height range between each soil sample unit cylinder can be calculated.

[0081]

[0082]

[0083] Where: H j Let be the thickness of the soil sample to be measured in the j-th soil sample unit 21;

[0084] H represents the total thickness of the soil sample to be tested, which consists of soil samples from n soil sample unit cylinders 21.

[0085] k z Let H be the vertical equivalent permeability coefficient of the soil sample with thickness H to be tested;

[0086] k j Let be the vertical permeability coefficient of the soil sample to be tested in the j-th soil sample unit 21.

[0087] (7) Test the fine soil sample, open the water injection hole 21a of the top soil sample unit 21 and the water inlet hole 21c of the bottom soil sample unit 21, close the water injection hole 21a, drainage hole 21b and water injection end 61a of the variable head pipe of the other soil sample unit 21, and remove the water supply pipe structure 5.

[0088] (8) Adjust the overflow hole of the water storage tank 1 to be 2m above the top of the soil sample. Fill the variable head pipe 6 with water from the water storage tank 1 and inject it into the sample tube until no air bubbles are found in the water overflowing from the injection hole 21a, thus achieving full saturation of the soil sample. Then, begin the seepage test. Drain the water from the water storage tank 1, open the water injection end 61a of the variable head pipe, fill the variable head pipe 6 with water to the required height, and begin measuring and recording the initial water head height and initial time in the variable head pipe 6. Measure and record the changes in water head and time at predetermined intervals of 10s, and also measure and record the water temperature of the injection hole 21a. At this time, the vertical permeability coefficient can be calculated using formulas (2) and (5).

[0089]

[0090] In the formula: f is the cross-sectional area of ​​the variable head pipe 6;

[0091] H1 represents the initial water head;

[0092] H2 is the head at the end.

[0093] Then, the overflow hole of the water storage tank 1 was adjusted to be 1.5m and 1m higher than the top of the soil sample, and the measurement was recorded twice. Then, the water level of the variable head pipe 6 was raised to the required height, and the measurement was recorded several times. The test was repeated 5-6 times or more, and the permeability coefficient was calculated. The average value was taken as the permeability coefficient of the soil sample in the range of height between the top soil sample unit 21 and the bottom soil sample unit 21.

[0094] (9) Close the water injection hole 21a of the top soil sample unit 21, open the water injection hole 21a of the next soil sample unit 21, and repeat step (8) to calculate the permeability coefficient of the soil sample height range between the next soil sample unit 21 and the bottom soil sample unit 21. Similarly, the permeability coefficient of the soil sample height range between each soil sample unit 21 and the bottom soil sample unit 21 can be calculated. Finally, the permeability coefficient of the soil sample height range between each soil sample unit 21 is calculated by back-calculating according to formulas (3) and (4).

[0095] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0096] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different heights, using a test apparatus for measuring the unidirectional freeze-thaw permeability coefficient of soil at different heights, characterized by: The test apparatus includes a water storage tank (1), a sample tube (2), a top cold plate structure (3), a bottom cold plate structure (4), a water supply pipe structure (5), and a variable head pipe (6). The water storage tank (1) is located above the sample tube (2). An overflow hole (11) is provided on the side of the water storage tank (1). The overflow hole (11) is used to control the water level in the water storage tank (1). The sample tube (2) includes several soil sample unit tubes (21) stacked vertically. 21) The soil sample unit (21) is filled with soil samples to be tested. The soil samples to be tested in adjacent soil sample unit tubes (21) are separated by a permeable structure (22). The permeable structure (22) isolates the soil samples to be tested while allowing water to pass freely. Each soil sample unit tube (21) is provided with a water injection hole (21a) at the top. The bottom of the water storage tank (1) is connected to the water injection hole (21a) of each soil sample unit tube (21) through a water supply pipe structure (5). The bottom soil sample unit tube (21) is provided with a drainage hole (21a) at the bottom. 1b) The top cold plate structure (3) is installed on the upper part of the top soil sample to be tested and provides cooling. The bottom cold plate structure (4) is installed on the lower part of the bottom soil sample to be tested and provides cooling. The bottom soil sample unit cylinder (21) has a bottom water inlet hole (21c) at the bottom. One end of the variable head pipe (6) is connected to the bottom of the water storage tank (1), and the other end is connected to the water inlet hole (21c). The variable head pipe (6) also has a variable head pipe water injection end (61a) so that an external water source can inject water through the variable head pipe. Water is injected into the variable head pipe (6) from end (61a); a water injection hole stop clamp is installed on each water injection hole (21a), a drainage hole stop clamp is installed on the drainage hole (21b), a bottom water inlet hole stop clamp is installed on the bottom water inlet hole (21c), and a variable head pipe water injection end stop clamp is installed on the water injection end (61a) of the variable head pipe. Each stop clamp is used to control the opening and closing of the corresponding pipeline; the water supply pipe structure (5) and the water injection hole (21a) of each soil sample unit cylinder (21) can be detachably connected. The measurement method includes the following steps: Step 1: Determine the filling method of each soil sample unit (21) according to the properties of the soil sample to be tested: If it is coarse-grained soil, take a representative undisturbed soil or prepare a soil sample, and fill it into the soil sample unit (21) by layered compaction; if it is fine-grained soil, cut the undisturbed soil sample or prepare a soil sample along the vertical direction of the soil sample, and then fill the fine-grained soil sample into the soil sample unit (21), and stack several soil sample unit (21) containing the soil sample to be tested on top of each other and fix them. Step 2: Close all water-stop clamps, adjust the top cold plate structure (3) and the bottom cold plate structure (4) to the same temperature >0ºC, maintain the constant temperature for a predetermined time, and then adjust the temperature of the top cold plate structure (3) or the bottom cold plate structure (4) to a temperature <0ºC to achieve freezing of the soil sample to be tested in the soil sample unit cylinder (21) from top to bottom or from bottom to top; after a certain time, close the cold bath of the top cold plate structure (3) and the bottom cold plate structure (4) to allow the soil sample to be tested to thaw naturally at room temperature; Step 3: When the soil sample is coarse-grained soil, open the water injection hole (21a) of the top soil sample unit (21) and the drainage hole (21b) of the bottom soil sample unit (21), and close the water injection holes (21a) of the other soil sample unit (21) and the water inlet hole (21c) of the bottom soil sample unit. Step 4: Adjust the overflow hole (11) of the water storage tank (1) to be 1-2m higher than the top of the top soil sample unit cylinder (21), and continuously inject water into the water storage tank (1) so that there is always residual water overflowing from the overflow hole (11) of the water storage tank (1), while the drainage hole (21b) of the bottom soil sample unit cylinder (21) has a continuous flow of water without air bubbles, so that the soil sample is fully saturated. Then start the seepage test and measure the vertical distance from the water surface of the water storage tank (1) to the drainage hole (21b), which is the head difference; start the stopwatch and use a measuring cylinder to collect the seepage water volume after a certain period of time at the drainage hole (21b); measure the water temperature in the cylinder, and the vertical permeability coefficient is calculated by formulas (1) and (2): （1） （2） In the formula: k T The permeability coefficient of the sample at water temperature TºC; Q The amount of water that seeps in within time t seconds; L The seepage diameter is equal to the height of the soil sample to be tested between the open injection and drainage holes; A The cross-sectional area of ​​the soil sample to be tested; Δ H The difference in water level; k 20 The permeability coefficient of the soil sample to be tested is given at a standard temperature of 20ºC. η T The dynamic viscosity coefficient of water at TºC; η 20 The dynamic viscosity coefficient of water at 20ºC; Step 5: Adjust the height of the overflow hole (11) of the water storage tank (1) to change the head difference. Repeat step 4 to obtain multiple vertical permeability coefficients. Take the average value of the vertical permeability coefficients as the permeability coefficient of the soil sample height range between the top soil sample unit (21) and the bottom soil sample unit (21). Step 6: Close the water injection hole (21a) of the top soil sample unit (21), open the water injection hole (21a) of the next soil sample unit (21), and keep the drainage hole (21b) of the bottom soil sample unit (21) open. Close the water injection holes (21a) of the other soil sample units (21) and the water inlet hole (21c) of the bottom soil sample unit. According to steps 4 and 5, obtain the permeability coefficient of the soil sample height range to be tested between the next soil sample unit (21) and the bottom soil sample unit (21); and so on, obtain the permeability coefficient of the soil sample height range to be tested between each soil sample unit (21) and the bottom soil sample unit (21). Step 7: Based on the permeability coefficient of the soil sample height range between each soil sample unit (21) and the bottom soil sample unit (21), the permeability coefficient of the soil sample height range between each soil sample unit (21) is calculated by formulas (3) and (4). （3） （4） In the formula: H j Let be the thickness of the soil sample to be measured in the j-th soil sample unit (21); H The total thickness of the soil sample to be tested is the total thickness of the soil sample to be tested, which is composed of soil samples to be tested in n soil sample unit cylinders (21); k z Let H be the vertical equivalent permeability coefficient of the soil sample with thickness H to be tested; k j Let be the vertical permeability coefficient of the soil sample to be tested in the j-th soil sample unit (21); Step 8: When the soil sample is fine-grained soil, open the water injection hole (21a) of the top soil sample unit (21) and the water inlet hole (21c) of the bottom soil sample unit (21), close the water injection hole (21a), drainage hole (21b) and water injection end (61a) of the other soil sample unit (21), and dismantle the water supply pipe structure (5). Step 9: Adjust the overflow hole (11) of the water storage tank (1) to be 1-2m higher than the top of the top soil sample unit cylinder (21). The water in the water storage tank (1) fills the variable head pipe (6) and is injected into the bottom soil sample unit cylinder (21) through the variable head pipe (6). The water overflows from the water injection hole (21a) of the top soil sample unit cylinder (21), and there are no air bubbles in the water, thus achieving full saturation of the soil sample. Start the seepage test measurement. Empty the water in the water storage tank (1), open the water injection end (61a) of the variable head pipe, fill the variable head pipe (6) with water to the required height, and start to record the initial water head height and the initial time in the variable head pipe (6). Record the changes in water head and time at predetermined time intervals, and record the water temperature of the water injection hole (21a) of the top soil sample unit cylinder (21). The vertical permeability coefficient is calculated by formulas (2) and (5): （5） In the formula: The cross-sectional area of ​​the variable head pipe (6); H 1 represents the initial water head; H 2 represents the head at the end; Then raise the water level of the variable head pipe (6) back to the required height, and then conduct a seepage test. Repeat the test several times, and take the average value of the obtained permeability coefficient as the permeability coefficient of the soil sample height range between the top soil sample unit (21) and the bottom soil sample unit (21). Step 10: Close the water injection hole (21a) of the top soil sample unit (21), open the water injection hole (21a) of the next soil sample unit (21), and obtain the permeability coefficient of the soil sample height range between the next soil sample unit (21) and the bottom soil sample unit (21) according to Step 9; and so on, to obtain the permeability coefficient of the soil sample height range between each soil sample unit (21) and the bottom soil sample unit (21). Step 11: Based on the permeability coefficient of the soil sample height range between each soil sample unit (21) and the bottom soil sample unit (21), the permeability coefficient of the soil sample height range between each soil sample unit (21) is calculated by formulas (3) and (4).

2. The method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different height ranges according to claim 1, characterized in that: The top cold plate structure (3) and the bottom cold plate structure (4) have the same structure, both including a cold cavity (31), a cold pipe (32) and a cold bath (33). The cold cavity (31) is attached to the upper end of the top soil sample to be tested or the lower end of the bottom soil sample to be tested. The cold cavity (31) forms a loop with the cold bath (33) through the cold pipe (32). Cooling liquid flows in the loop. The cold bath (33) is used to cool the cooling liquid.

3. The method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different height ranges according to claim 2, characterized in that: A water-stop rubber ring (23) is provided at the edge connection of the adjacent soil sample unit cylinder (21). The water-stop rubber ring (23) is used to prevent water and the soil sample to be tested from seeping out from the edge of the adjacent soil sample unit cylinder (21).

4. The method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different height ranges according to claim 1, characterized in that: The coarse-grained soil is soil with a particle size greater than 0.075 mm, and the fine-grained soil is soil with a particle size less than or equal to 0.075 mm.

5. The method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different height ranges according to claim 1, characterized in that: The variable head pipe (6) is equipped with a scale.

6. The method for measuring the unidirectional freeze-thaw permeability coefficient of soil at different height ranges according to claim 5, characterized in that: The variable head pipe (6) has an inner diameter of no more than 1 cm, a length of more than 1 m, and a scale division of 1 mm.

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