An experimental device and method for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions

Abstract

This invention discloses an experimental apparatus and method for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions. The apparatus includes a low-temperature tank with a movable cover plate at the top. A one-dimensional test mold and a three-dimensional test mold are placed inside the low-temperature tank, with at least two of each. The measurement unit includes a temperature sensor, a moisture sensor, and a suction sensor vertically inserted into the soil sample, used to measure the internal temperature, moisture content, and suction value of the soil sample, respectively. It also includes multiple cold needles inserted into the soil sample to provide condensation nuclei. A laser rangefinder is positioned at the center above the cylindrical mold to measure the vertical deformation of the soil sample. Finally, a data acquisition unit is included, connected to each sensor and the laser rangefinder, and used to transmit the collected data to a host computer.

Description

Technical Field

[0001] This invention belongs to the field of frozen soil experimental technology, specifically relating to an experimental apparatus and method for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions. Background Technology

[0002] Permafrost refers to soil that freezes when its temperature is below 0°C. In my country, permafrost is widely distributed, with perennial permafrost and seasonal permafrost accounting for 21.5% and 53.5% of the country's land area, respectively, mainly concentrated in the Northeast, western high mountains, and the Qinghai-Tibet Plateau. Even at the freezing point, unfrozen water remains in permafrost. In the small pore spaces, this unfrozen water is adsorbed on the surface of soil particles in the form of a thin film of water, maintaining a dynamic equilibrium with pore ice. To predict and interpret permafrost behavior, the relationship between unfrozen water content and sub-zero temperatures is called the soil-freezing characteristic curve, also known as the SFCC (soil-freezing characteristic curve).

[0003] A large number of major infrastructure projects are being constructed or planned in permafrost regions (such as the Sichuan-Tibet Railway and the Qinghai-Tibet Expressway), placing extremely high demands on the engineering performance and stability of soil in these areas. The study of soil freezing characteristic curves (SFCC) is crucial for a deeper understanding of water migration, freeze-thaw heave behavior, shear strength, and elastic modulus in permafrost. Furthermore, the constitutive relationships of the water field, temperature field, and mechanical field in permafrost are closely related to the unfrozen water content. Therefore, a thorough understanding of SFCC curves is of great value for simulating the transport mechanisms of water, heat, and solutes in permafrost, as well as for the engineering design and construction of permafrost under climate change, and for ecological environmental protection.

[0004] The measurement of SFCC in frozen soil is one of the hot and difficult issues in frozen soil engineering. The lack of standardization and uniformity in SFCC measurement methods, as well as the inconsistency in the application of temperature boundary conditions, result in poor comparability of SFCC values ​​obtained from different research institutes for various soil samples, which greatly hinders a deeper understanding of the physical and mechanical behavior of frozen soil. Summary of the Invention

[0005] This invention aims to standardize the SFCC (freeze-thaw cycle) measurement method for frozen soil by proposing an experimental apparatus and method for simultaneously measuring the SFCC curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions. Using this apparatus, one-dimensional and three-dimensional freeze-thaw boundary conditions, constant or varying temperature boundary conditions, and different numbers of freeze-thaw cycles can be applied to soil samples simultaneously, and the freezing and thawing curves can be measured concurrently.

[0006] Therefore, the present invention adopts the following technical solution:

[0007] An experimental device for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions includes a low-temperature tank (internal dimensions are 270 mm long, 230 mm wide, and 160 mm deep), a movable cover plate is connected to the top of the low-temperature tank, and a one-dimensional test mold and a three-dimensional test mold are placed inside the low-temperature tank, with at least two of each.

[0008] The one-dimensional testing mold is placed vertically in the low-temperature bath, with its bottom tightly against the bottom surface of the bath. It includes a cylindrical mold (internal dimensions: diameter 61.8 mm, height 120 mm, thickness 5 mm) for holding the soil sample, placed vertically in the low-temperature bath. The bottom of the cylindrical mold is equipped with permeable stones, and the sides of the mold and the bottom of the permeable stones are covered with an insulation layer. After the soil sample is filled into the cylindrical mold, the one-dimensional testing mold is covered with a waterproof membrane. A measuring unit is connected to the soil sample within the one-dimensional testing mold.

[0009] The three-dimensional testing mold is placed vertically in the low-temperature bath, and its bottom is lifted off the bottom of the low-temperature bath by a support frame; it includes a cylindrical mold for holding soil samples, which is placed vertically in the low-temperature bath; the bottom of the cylindrical mold is equipped with permeable stones; after the soil sample is filled into the cylindrical mold, the three-dimensional testing mold is wrapped with a waterproof membrane; a measurement unit is connected to the soil sample in the three-dimensional testing mold.

[0010] The measurement unit includes a temperature sensor, a moisture sensor, and a suction sensor that are vertically inserted into the soil sample, which are used to measure the temperature, moisture content, and suction value inside the soil sample, respectively.

[0011] It also includes multiple cold needles (140 mm long and 0.2 mm in diameter, with heat-insulating plugs on top to control the depth of insertion) inserted into the soil sample. The cold needles are used to provide condensation nuclei for the soil sample.

[0012] It also includes a laser rangefinder located at the center of the top of the cylindrical mold, which is used to measure the vertical deformation of the soil sample;

[0013] It also includes a data acquisition unit, which is connected to each sensor and the laser rangefinder. The data acquisition unit is used to transmit the collected data to the host computer.

[0014] Furthermore, the cylindrical mold is made of acrylic material.

[0015] Furthermore, the thickness of the permeable stone is 5 mm.

[0016] Furthermore, an L-shaped bracket is connected to the laser rangefinder, with its lower end fixed to the side wall of the cylindrical mold. The L-shaped bracket has a long side of 120mm and a short side of 25mm. The contact area between the bracket and the cylindrical mold features an arc design, matching the curvature of the mold's side wall to ensure tight contact and prevent slippage. The bracket and small-volume laser rangefinder are installed on the side wall of the cylindrical mold to measure the vertical deformation of the soil sample.

[0017] A test method for measuring the freezing characteristic curve of permafrost under one-dimensional and three-dimensional freeze-thaw boundary conditions includes the following steps:

[0018] 1) Soil sampling: Collect test soil, remove impurities, sieve and dry for later use;

[0019] 2) Preparation of wet soil: Weigh the required mass of dried soil for the experiment, and weigh the corresponding mass of distilled water according to the target moisture content; mix the dry soil and distilled water evenly, let it stand for more than 24 hours to allow the moisture in the soil sample to be evenly distributed, measure the moisture content of the soil sample, and adjust the moisture content by adding water or dry soil according to the difference between the measured value and the target moisture content, and then let it stand again; repeat this step until the target moisture content is reached;

[0020] 3) Preparation of experimental soil samples:

[0021] The wet soil particles from step 2) are compacted in layers in a cylindrical mold. During the compaction process, the soil sample is compacted into multiple layers using the volume control method, with each layer having the same height. After compaction, different masses of distilled water are added to the top of the soil sample and it is wrapped and sealed for a period of time to obtain different mass moisture contents so that the freezing characteristic curve can be measured on soil samples with different initial moisture contents.

[0022] After each layer of soil sample is fully compacted, pre-calculated distilled water is added from the top of the soil sample to achieve the target initial moisture content. After adding distilled water, filter paper is placed on top of the soil sample, and a 2kg weight is placed to restrict its vertical deformation. In actual experiments, the soil sample reaches moisture balance in all areas after about a week.

[0023] 4) Sealing soil samples and deploying sensors:

[0024] For one-dimensional freeze-thaw soil samples, a permeable stone is placed at the bottom of the cylindrical mold. The side walls and bottom of the cylindrical mold need to be wrapped with an insulation layer. The soil sample, along with the cylindrical mold and the permeable stone, is sealed in a waterproof membrane. The insulation layer is then wrapped and sealed with another waterproof membrane. A moisture sensor, a temperature sensor, and a suction sensor are vertically inserted into the soil sample, and the connection between the sensor cables and the waterproof membrane is sealed with adhesive to ensure that the sensor sockets do not leak water or soil during the test.

[0025] For soil samples under three-dimensional freeze-thaw conditions, a permeable stone is placed at the bottom of the cylindrical mold, and the soil sample, along with the cylindrical mold and the permeable stone, is sealed in a waterproof membrane; finally, three sensors are installed; a support frame is used at the bottom to prevent the bottom from touching the bottom and causing poor heat conduction.

[0026] Install a laser rangefinder at the same height directly above each cylindrical mold, with the center of the laser rangefinder facing the center of the soil sample. The laser rangefinder has dimensions of 35mm in length, 25mm in width, and 15mm in height, and an operating temperature range of -40 to +60℃, which can withstand the freeze-thaw cycle environment in the experiment.

[0027] 5) Constant temperature cold bath:

[0028] Place the cylindrical mold in a low-temperature bath filled with antifreeze that covers the top of the mold; connect all sensors to the data acquisition unit, turn on the constant temperature cold bath, and use a step-by-step cooling method for the freeze-thaw process; maintain each temperature level for no less than 12 hours.

[0029] 6) Data processing: Upload the collected experimental data to the host computer for storage and analysis.

[0030] Furthermore, in step 4), each cylindrical mold is filled with at least 4 layers of soil sample, and the height of each layer of soil sample is 25mm.

[0031] Furthermore, in step 5), four cold needles are inserted into each soil sample, and the four cold needles are arranged in a rectangular pattern.

[0032] The beneficial effects of this invention are as follows:

[0033] 1. This invention designs an experimental device for measuring SFCC curves under one-dimensional and three-dimensional freeze-thaw boundary conditions; the entire device adopts a constant temperature cold bath, and the soil sample is immersed in the freezing liquid to achieve uniform and precise control of the temperature around the soil sample; it can simultaneously measure the soil sample under one-dimensional and three-dimensional freeze-thaw boundary conditions, apply constant or changing temperature boundary conditions, and different freeze-thaw cycles, thus realizing the accurate measurement of SFCC (including freezing curve and thawing curve) of soil samples under one-dimensional and three-dimensional freeze-thaw boundary conditions under complex working conditions;

[0034] 2. In previous SFCC curve measurements, the freezing process would result in overcooling, which would affect the experimental results. This invention proposes a cold needle penetration method, in which a specially frozen cold needle is inserted into the soil, providing nucleation conditions for ice crystal formation and growth, thus eliminating the influence of overcooling.

[0035] 3. A non-contact, non-destructive measurement of soil sample freeze-thaw deformation was achieved using a laser rangefinder, which can measure the vertical deformation of soil samples in real time during the freeze-thaw process. This solves the problems of traditional displacement sensors being unsuitable for measurement in narrow, enclosed spaces and causing measurement errors due to contact with the ends of the soil sample. Attached Figure Description

[0036] Figure 1 This is a front view of the experimental apparatus of the present invention;

[0037] Figure 2This is a top view of the experimental apparatus of the present invention;

[0038] Figure 3 These are SFCC melting curves of the test soil under one-dimensional and three-dimensional freeze-thaw conditions;

[0039] In the diagram: 1-Low temperature bath, 2-One-dimensional test mold, 3-Three-dimensional test mold, 4-Cylindrical mold, 5-Permeable stone, 6-Insulation layer, 7-Waterproof membrane, 8-Insulation plug, 9-Bracket, 10-Laser rangefinder, 11-Cold needle, 12-Sensor socket, 13-Cold needle socket, 14-Suction sensor, 15-Moisture sensor, 16-Temperature sensor, 17-Support frame, 18-Cold needle socket. Detailed Implementation

[0040] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0041] This experiment used volcanic soil widely distributed in Hokkaido, Japan, and undisturbed test soil was collected from a volcanic crater near Sapporo. SFCC tests were conducted using soil particles with a diameter less than 2 mm. The dry test soil was light gray, while the moist soil was brown. Its basic properties are shown in Table 1.

[0042] Table 1 - Basic physical properties of the test soil

[0043] Physical properties numerical values proportion 2.50 <![CDATA[Maximum dry density (g / cm 3 ).]]> 1.12 <![CDATA[Minimum dry density (g / cm 3 )]]> 0.76 Natural mass moisture content (%) ≈30 Sand content (%) 64 Particle content (%) 28 Clay content (%) 8 <![CDATA[Curvature coefficient C c > 1.54 <![CDATA[Coefficient of non-uniformity C u > 45

[0044] The specific experimental steps are as follows:

[0045] 1) Soil sample collection: Collect test soil, remove impurities, pass through a 2mm sieve and dry for later use.

[0046] 2) Preparation of wet soil: First, mix the weighed dry test soil with distilled water by hand, then put it into a plastic bag and let it stand for more than 24 hours to make the moisture in the soil sample evenly distributed. Measure the moisture content of the soil sample. According to the difference between the measured value and the natural mass moisture content, add water or dry soil to adjust the moisture content and let it stand again. Repeat this step until the natural mass moisture content is reached.

[0047] 3) Preparation of experimental soil samples:

[0048] The wet soil particles from step 2) were compacted in layers in cylindrical mold 4. During the compaction process, the volume control method was used to compact the soil in four layers (each layer was about 25 mm thick). After compaction, different masses of distilled water were added to the top of the soil sample and sealed for a period of time to obtain different mass moisture contents so as to measure the freezing characteristic curve on soil samples with different initial moisture contents. Since the test soil is a non-plastic coarse-grained soil, there is almost no volume change during the moisture absorption process.

[0049] 4) Sealing soil samples and deploying sensors:

[0050] For soil samples under one-dimensional freeze-thaw conditions, a permeable stone 5 is placed at the bottom of the cylindrical mold 4. The side walls and bottom of the cylindrical mold 4 need to be wrapped with an insulation layer 6. The soil sample, along with the cylindrical mold 4 and the permeable stone 5, is sealed in a waterproof membrane 7. The insulation layer 6 is then wrapped and sealed with another waterproof membrane 7. A temperature sensor 16 and a moisture sensor 15 are vertically inserted into the soil sample, and the connection between the sensor cables and the waterproof membrane 7 is sealed with adhesive to ensure that the sensor sockets do not leak water or soil during the test.

[0051] For soil samples under three-dimensional freeze-thaw conditions, a permeable stone 5 is placed at the bottom of the cylindrical mold 4, and the soil sample, along with the cylindrical mold 4 and the permeable stone 5, is sealed in a waterproof membrane 7; finally, two sensors are installed; the bottom is supported by a support frame 17 to prevent it from touching the bottom and causing poor heat conduction at the bottom.

[0052] 5) Constant temperature cold bath:

[0053] Pour antifreeze into the low-temperature bath 1, ensuring the antifreeze covers the top of the cylindrical mold 4. Connect various sensors to the data acquisition unit and turn on the constant temperature cold bath. The soil sample is initially in an unfrozen state. During the freeze-thaw (FT) cycle, the temperature is directly set to -10℃, and the SFCC freezing curve is measured. Then, gradually increase the controlled temperature to the target sub-zero temperature and measure the SFCC thawing curve.

[0054] 6) Data processing: Upload the collected experimental data to the host computer (set to upload data every 15 minutes), and store and analyze the data.

[0055] The experimental data analysis and conclusions are as follows:

[0056] The average unfrozen water content at each temperature was used for construction. Figure 3 The melting curves of SFCC are shown in Tables 2 and 3.

[0057] Table 2. Data on freeze-thaw cycles of experimental soil under one-dimensional freeze-thaw boundary conditions.

[0058] Table 3. Data on freeze-thaw cycles of the test soil under three-dimensional freeze-thaw boundary conditions.

[0059]

[0060] It can be seen that under one-dimensional freeze-thaw boundary conditions, the effect of FT cycles on SFCC is not significant, similar to the three-dimensional condition. Furthermore, there is no significant difference in SFCC measured under one-dimensional and three-dimensional FT conditions, although the time required to reach steady state is much longer in the one-dimensional case.

[0061] The temperature distribution of the specimen differs under one-dimensional and three-dimensional freeze-thaw conditions. For example, under one-dimensional freeze-thaw conditions, the internal temperature of the specimen is non-uniform; a temperature gradient exists within the specimen, while under one-dimensional freezing conditions, the temperature distribution can be considered linear.

[0062] In summary, for this test soil, the effect of FT cycles was not significant under one-dimensional and three-dimensional conditions. Furthermore, the measured SFCC curves showed little difference between one-dimensional and three-dimensional freeze-thaw conditions.

Claims

1. An experimental apparatus for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions, characterized in that, It includes a low-temperature bath, with a movable cover plate connected to the top of the low-temperature bath. A one-dimensional test mold and a three-dimensional test mold are placed inside the low-temperature bath, and at least two of each are provided. The one-dimensional testing mold is placed vertically in the low-temperature bath, with its bottom tightly against the bottom surface of the low-temperature bath; it includes a cylindrical mold for holding soil samples, which is placed vertically in the low-temperature bath; the bottom of the cylindrical mold is equipped with permeable stones, and the sides of the cylindrical mold and the bottom of the permeable stones are covered with an insulation layer; after the soil sample is filled into the cylindrical mold, the one-dimensional testing mold is covered with a waterproof membrane; a measuring unit is connected inside the soil sample of the one-dimensional testing mold; The three-dimensional testing mold is placed vertically in the low-temperature bath, and its bottom is lifted off the bottom of the low-temperature bath by a support frame; it includes a cylindrical mold for holding soil samples, which is placed vertically in the low-temperature bath; the bottom of the cylindrical mold is equipped with permeable stones; after the soil sample is filled into the cylindrical mold, the three-dimensional testing mold is wrapped with a waterproof membrane; a measurement unit is connected to the soil sample in the three-dimensional testing mold. The measurement unit includes a temperature sensor, a moisture sensor, and a suction sensor that are vertically inserted into the soil sample, which are used to measure the temperature, moisture content, and suction value inside the soil sample, respectively. It also includes multiple cold needles inserted into the soil sample, which are used to provide condensation nuclei for the soil sample; It also includes a laser rangefinder located at the center of the top of the cylindrical mold, which is used to measure the vertical deformation of the soil sample; It also includes a data acquisition unit, which is connected to each sensor and the laser rangefinder. The data acquisition unit is used to transmit the collected data to the host computer.

2. The experimental apparatus for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions according to claim 1, characterized in that, The cylindrical mold is made of acrylic material.

3. The experimental apparatus for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions as described in claim 1, characterized in that, The thickness of the permeable stone is 5mm.

4. The experimental apparatus for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions according to claim 1, characterized in that, The laser rangefinder is connected to an L-shaped bracket, the lower end of which is fixed to the side wall of the cylindrical mold.

5. A test method for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions, comprising the experimental apparatus described in any one of claims 1-4, characterized in that, The testing method includes the following steps: 1) Soil sampling: Collect test soil, remove impurities, sieve and dry for later use; 2) Preparation of wet soil: Weigh the required mass of dried soil for the experiment, and weigh the corresponding mass of distilled water according to the target moisture content; mix the dry soil and distilled water evenly, let it stand for more than 24 hours to allow the moisture in the soil sample to be evenly distributed, measure the moisture content of the soil sample, and adjust the moisture content by adding water or dry soil according to the difference between the measured value and the target moisture content, and then let it stand again; repeat this step until the target moisture content is reached; 3) Preparation of experimental soil samples: The wet soil particles from step 2) are compacted in layers in a cylindrical mold. During the compaction process, the soil sample is compacted into multiple layers using a volume control method, with each layer having the same height. After compaction, different masses of distilled water are added to the top of the soil sample to obtain different mass moisture contents, so as to measure the freezing characteristic curve on soil samples with different initial moisture contents. 4) Sealing soil samples and deploying sensors: For one-dimensional freeze-thaw soil samples, a permeable stone is placed at the bottom of the cylindrical mold. The side walls and bottom of the cylindrical mold need to be wrapped with an insulation layer. The soil sample, along with the cylindrical mold and the permeable stone, is sealed in a waterproof membrane. The insulation layer is then wrapped and sealed with another waterproof membrane. A moisture sensor, a temperature sensor, and a suction sensor are vertically inserted into the soil sample, and the connection between the sensor cables and the waterproof membrane is sealed with adhesive to ensure that the sensor sockets do not leak water or soil during the test. For soil samples under three-dimensional freeze-thaw conditions, a permeable stone is placed at the bottom of the cylindrical mold, and the soil sample, along with the cylindrical mold and the permeable stone, is sealed in a waterproof membrane; finally, three sensors are installed; a support frame is used at the bottom to prevent the bottom from touching the bottom and causing poor heat conduction. Install laser rangefinders at the same height directly above each cylindrical mold, with the center of the laser rangefinder facing the center of the soil sample. 5) Constant temperature cold bath: The cylindrical mold is placed in a low-temperature bath filled with antifreeze that covers the top of the mold. Various sensors are connected to the data acquisition unit via cables. The constant-temperature cold bath is turned on, and the freeze-thaw process adopts a stepped cooling method. Each step temperature is maintained for no less than 12 hours. The surface of the cold needle is pretreated with water mist and then frozen. When the soil temperature reaches -0.5℃ during the freezing process, the cold needle is removed from the freezing equipment and quickly inserted into the soil sample through the insertion hole. 6) Data processing: Upload the collected experimental data to the host computer for storage and analysis.

6. The test method for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions according to claim 5, characterized in that, In step 4), each cylindrical mold is filled with at least 4 layers of soil sample, and the height of each layer of soil sample is 25mm.

7. The test method for measuring the freezing characteristic curve of frozen soil under one-dimensional and three-dimensional freeze-thaw boundary conditions according to claim 5, characterized in that, In step 5), four cold needles are inserted into each soil sample, and the four cold needles are arranged in a rectangular pattern.

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