A device and method for preparing a sample for a direct shear test of a concrete-frozen soil interface

Abstract

The application belongs to the technical field of frozen soil engineering, and particularly relates to a concrete-frozen soil contact surface direct shear test sample manufacturing device and method, which comprises a box body, a counterforce frame, a heating plate and a refrigeration plate. The top of the counterforce frame is provided with a walking device, the bottom of the walking device is provided with a pressurizing device, and the pressurizing device is provided with a pressure gauge. The bottom of the counterforce frame is provided with a concrete pouring chamber and a soil sample chamber, and a partition plate is arranged between the concrete pouring chamber and the soil sample chamber. The device has the advantages that the problem of excessive water content of the concrete-frozen soil interface caused by water migration due to gravity is solved.

Description

Technical Field

[0001] This application belongs to the field of frozen soil engineering technology, specifically relating to a device and method for preparing a direct shear test specimen for a concrete-frozen soil interface. Background Technology

[0002] More and more engineering structures, including important foundation projects such as the Qinghai-Tibet Railway and the Qinghai-Tibet Highway, have been or will be constructed in permafrost regions. Pile foundations, due to their advantages such as high bearing capacity, wide adaptability, and long-term stable performance, are widely used in cold-region engineering. The transfer of loads from the superstructure is mainly through contact between the structure and the permafrost. Therefore, the freezing force between the pile foundation and the permafrost is an important mechanical parameter in the design of friction piles in permafrost regions. This parameter is mainly obtained through two methods: field testing and laboratory testing. Field testing of pile foundations yields relatively accurate freezing forces, but it has disadvantages such as high cost and operational difficulty, and is therefore less commonly used in engineering. The laboratory direct shear test is the earliest method for determining the shear strength of soil. This test method has advantages such as ease of operation and control, repeatability, and accurate results, and is widely used in practical engineering.

[0003] The direct shear test can also be used to test the shear strength of the interface between different media, such as the interface between concrete and frozen soil. Therefore, the contact form between the concrete and frozen soil interface is a crucial factor determining the mechanical parameters during specimen preparation. The spatial distribution of the concrete-frozen soil interface exists in two forms: first, the interface is perpendicular to the vertical direction; second, the interface is perpendicular to the horizontal direction. Although the specimens prepared by these two methods have essentially the same appearance, the differences in the formation of the contact surface are significant. The first method, due to the excessive moisture content near the contact surface caused by gravity, primarily depends on contact pressure, concrete water-cement ratio, and curing temperature, with less influence from frozen soil ice content and soil properties; the shear strength is mainly determined by freezing force. The second method considers moisture migration due to gravity, mirroring the contact surface formation in actual engineering. Its results depend on factors such as concrete pouring temperature, contact pressure, frozen soil temperature, and ice content. The second method can simulate the contact state between concrete and frozen soil at different depths, avoiding excessive moisture content at the contact surface due to moisture migration. The shear strength obtained from the shear test can represent the freezing strength of the foundation and frozen soil at different depths. However, the existing technology has the following problems: First, concrete is poured directly onto the frozen soil surface; due to gravity, moisture in the concrete migrates to the vicinity of the contact surface, leading to an excessively high moisture content and thus an inflated freezing strength. Second, this sampling method does not consider the influence of contact pressure on thermal intrusion of concrete, which does not conform to actual engineering conditions. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a device for preparing direct shear test specimens for concrete-frozen soil contact surfaces that is low in manufacturing cost, easy to install, and simple to operate; the second technical problem to be solved by this invention is to provide a method for preparing direct shear test specimens for concrete-frozen soil contact surfaces. The technical solution is as follows:

[0005] A sample preparation device for direct shear test of concrete-frozen soil contact surface includes a box, a reaction frame, a heating plate and a cooling plate. The top of the reaction frame is equipped with a walking device, and the bottom of the walking device is equipped with a pressurizing device with a pressure gauge. A concrete pouring chamber and a soil sample chamber are provided below the reaction frame, and a partition is provided between the concrete pouring chamber and the soil sample chamber.

[0006] Preferably, the walking device is provided with a sliding cavity and is fitted onto the support frame at the top of the reaction force frame. The reaction force frame is provided with two limiting blocks and a rotation stop block. The two limiting blocks are respectively installed above the outer side wall of the concrete pouring chamber and the soil sample chamber. In use, the rotation stop block is locked onto the top of the reaction force frame.

[0007] Preferably, the top support frame and the side support frame of the reaction frame are both equipped with scales.

[0008] Preferably, a temperature controller is provided on the outer wall of the box, and the heating plate and the cooling plate are communicatively connected to the temperature controller.

[0009] Preferably, the outer side and bottom of the concrete pouring chamber and the soil sample chamber are provided with thermal insulation cotton, and the inner wall of the box is provided with a thermal insulation layer; the sample preparation modulus and size specifications used in the concrete pouring chamber and the soil sample chamber are the same.

[0010] A method for preparing a direct shear test specimen for a concrete-frozen soil interface includes the following steps:

[0011] S1. Select soil samples according to the actual engineering conditions and prepare concrete;

[0012] S2. Prepare frozen soil samples, the specific steps are as follows:

[0013] S21. Use a thermometer to measure the surface temperature of the soil sample chamber. When the surface temperature is less than 5℃, fill the soil sample. Fill the soil sample chamber with the prepared soil. According to the compaction requirements, weigh the amount of soil required for the sample. Divide the required amount of soil evenly into three parts and fill them in three layers. After filling each layer of soil sample, move the pressurizing device to the top of the soil sample chamber along the reaction frame via the walking device. Place the pressure plate on the soil sample filled in the soil sample chamber and turn on the pressurizing device to press the soil sample. During the pressing process, measure the distance between the upper surface of the soil sample and the upper edge of the mold. When the distance reaches 2h / 3, stop pressurizing, remove the pressure plate, and scratch and roughen the surface of the soil sample. Then press the remaining two soil samples in the same way until the surface of the soil sample is flush with the upper surface of the mold. The soil sample preparation is complete.

[0014] S22. Place the filled soil sample in a model box and freeze it at -5℃ for 24 to 48 hours to make the soil temperature uniform. Finally, wrap the upper surface of the soil sample with insulation cotton for later use and remove the partition.

[0015] S3. Apply release agent to the inner surface of the concrete pouring chamber, then add the prepared concrete into the concrete pouring chamber. Stop pouring when the concrete height is level with the edge of the concrete chamber. Take out a small vibrator, insert it into the concrete chamber, and turn on the vibrator. Continue to add concrete to the vibrated concrete pouring chamber, and stop pouring when the concrete is level with the upper surface of the concrete chamber.

[0016] S4. Place the bearing plate on the surface of the poured concrete, move the pressurizing device to the top of the concrete pouring chamber via the walking device along the reaction frame, turn on the pressurizing device to apply force P to the bearing plate, observe the magnitude of force P through the pressure gauge, when the preset magnitude is reached, keep the force constant, wrap the outside of the bearing plate with thermal insulation cotton until the concrete reaches initial setting, then remove the loading force and remove the bearing plate.

[0017] S5. Curing of the specimen in the model box for 10 to 20 days. After the concrete poured in step S4 reaches the required strength, the specimen mold is removed to form the specimen for the direct shear test of the freezing strength of the concrete-frozen soil interface.

[0018] Preferably, in step S1, the specific steps for selecting the soil sample are as follows:

[0019] First, crush the soil sample required for the test, put the sieved soil sample into an oven to dry, and finally take out the dried soil sample and place it in a room temperature environment to air dry the soil sample to room temperature; select the soil moisture content according to the actual engineering conditions, calculate the amount of water required for the soil sample with a specific moisture content, and weigh the required amount of water; divide the soil sample into three equal parts, first spread one part in an iron pan, and evenly sprinkle 1 / 3 of the required amount of water on the surface of the soil layer, and then treat the other two soil samples in the same way, wrap the iron pan with plastic wrap after adding water, and test its moisture content. The moisture content error should be less than 1%.

[0020] Preferably, in step S1, the concrete preparation steps are as follows: according to the sample mold size, weigh the concrete raw materials required for the sample, first mix the coarse aggregate, then add cement for dry mixing, weigh the required mass of water according to the required water content for the test, dissolve an appropriate amount of concrete antifreeze agent in the water, add the water to the dry-mixed mixture, and take out the mixed concrete for later use.

[0021] Preferably, the derivation steps for the applied force P on the bearing plate are as follows:

[0022] Step 1: When concrete is poured into frozen soil, the lateral pressure exerted by the concrete on the frozen soil has a certain promoting effect on the "thermal intrusion" of the frozen soil, which is basically equivalent to the pressure value generated by a static fluid. Calculate the standard value F of the maximum lateral pressure of the cast-in-place concrete on the formwork, and take the smaller value of the two formulas. The calculation formula is as follows:

[0023] (one);

[0025] F2=γ c H (II);

[0026] γ c t0 is the density of concrete; t0 is the initial setting time of concrete; T is the temperature of concrete; β is the slump correction factor of concrete; V is the concrete pouring speed; H is the concrete pouring height; k0 is the static lateral pressure coefficient.

[0027] Step 2: Calculate H when formula (I) and formula (II) are equal, and denote it as H. T =H;

[0028]

[0029] When the depth is less than H T When the calculation depth is greater than H, the lateral pressure is calculated using equation (ii). T When the side pressure is calculated, Equation (I) is used.

[0030] Step 3: Calculate the equivalent side pressure:

[0031] The lateral pressure exerted by concrete on frozen soil is calculated using the active earth pressure calculation method for retaining walls. When a uniformly distributed load q acts on the concrete surface, the load q can be considered as the imaginary self-weight of the concrete γH. Therefore, the total lateral pressure acting on the frozen soil is:

[0032]

[0033] Step 4: Calculate the uniformly distributed load: Substitute the lateral pressure F1 or F2 at point H into equation (III) to find q, which gives:

[0034] P = F1 or P = F2;

[0035] Step 5: The force P applied to the bearing plate is:

[0036]

[0037] A represents the area of ​​the working surface of the pressure plate.

[0038] Compared with the prior art, the beneficial effects of this application are as follows:

[0039] 1. The present invention sets the contact surface between concrete and frozen soil to be perpendicular to the horizontal direction, thereby avoiding excessive moisture content near the contact surface due to the migration of moisture in the concrete caused by gravity.

[0040] 2. In the process of specimen preparation, the present invention applies pressure to the concrete, which can simulate the real contact state between concrete and frozen soil at different depths.

[0041] 3. This invention involves pouring concrete in a low-temperature environment and wrapping the concrete with insulation cotton outdoors. This not only simulates concrete pouring in a low-temperature environment, but also avoids the disadvantage of premature heat loss in concrete due to the small size of laboratory-prepared samples (generally less than 10mm in diameter). It more realistically reproduces the contact surface morphology formed by the heat of hydration of concrete.

[0042] 4. This invention can prepare test specimens for freezing force of various material contact surfaces. It has the advantages of convenient installation, strong and reliable construction, low production cost, and accurate data. It can effectively reduce laboratory costs and is worth promoting. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the structure of this application.

[0044] Figure 2 for Figure 1 Enlarged view of A in the middle.

[0045] In the diagram: 100—box body, 200—reaction frame, 300—heating plate, 400—pressurization device, 500—pressure gauge, 600—concrete pouring chamber, 700—sample preparation mold, 800—walking device, 900—pressure plate, 1000—cooling plate, 1100—temperature controller, 1200—partition, 1300—soil sample chamber, 1400—limiting block, 1500—rotation stop block; A01—insulation layer. Detailed Implementation

[0046] The technical solution of this application will be described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that the embodiments and specific features in the embodiments are detailed descriptions of the technical solution of this application, rather than limitations thereof. Specific technical features can be combined with each other.

[0047] Figure 1-2 As shown, a sample preparation device for a direct shear test of a concrete-frozen soil contact surface includes a housing 100, a reaction frame 200, a heating plate 300, and a cooling plate 1000. The reaction frame 200 has a traveling device 800 at its top and a pressurizing device 400 (such as a cylinder) at its bottom, with a pressure gauge 500 mounted on the pressurizing device 400. A concrete pouring chamber 600 and a soil sample chamber 1300 are located below the reaction frame 500, with a partition 1200 between them. A temperature controller 1100 is mounted on the outer wall of the housing 100, and the heating plate 300 and cooling plate 1000 are communicatively connected to the temperature controller 1100. A scale is mounted on the support frame at the top and the side support frames of the reaction frame 200.

[0048] The walking device 800 is equipped with a sliding cavity and is fitted onto the support frame at the top of the reaction force frame 200. The reaction force frame 200 is equipped with two limiting blocks 1400 and a rotation stop block 1500. The two limiting blocks 1400 are respectively installed above the outer walls of the concrete pouring chamber 600 and the soil sampling chamber 1300. The rotation stop block 1500 is locked above the partition plate 1200. (It is removed when the walking device 800 slides, and locked at the fixed scale position above the partition plate 1200 when in use.)

[0049] The outer sides and bottom surfaces of the concrete pouring chamber 600 and the soil sample chamber 1300 are provided with thermal insulation cotton, and the inner wall of the box is provided with a thermal insulation layer; the sample preparation mold 700 used in the concrete pouring chamber 600 and the soil sample chamber 1300 are of the same size and specifications.

[0050] The sample preparation unit includes a soil sample chamber, a concrete pouring chamber, and a partition; the temperature control unit includes a heating plate, a cooling plate, and a temperature controller; the loading mechanism includes a reaction frame, a pressurizing device, a pressure gauge, and a walking mechanism; the insulation unit includes an outdoor insulation layer for sample preparation; the length, width, and height of the concrete-frozen soil sample mold are selected as l×w×h=200mm×100mm×100mm, the mold has a partition in the middle with a size of 100mm×100mm, and the thickness of both the mold and the partition is 5mm. The sample mold is bolted together, and there is a groove in the middle of the mold, into which the partition can be embedded.

[0051] A method for preparing a direct shear test specimen for a concrete-frozen soil interface, comprising the following steps:

[0052] (1) First, turn on the cooling plate 1000 and the temperature controller 1100 to control the temperature in the pouring chamber. When the temperature in the soil sample chamber 1300 is lower than -5℃, turn on the heating plate 300 to keep the temperature in the soil sample chamber at -5.5℃. Cover the outer side and bottom of the concrete pouring chamber 600 and the soil sample chamber 1300 with thermal insulation cotton. Place the mold with thermal insulation cotton in the box 100, insert the partition 1200 in the middle of the mold, and leave it for no less than 2 hours.

[0053] (2) First, crush the soil sample required for the test and pass the crushed soil through a 2mm sieve. Place the sieved soil sample in a 120℃ oven and dry it for 8 hours. Finally, remove the dried soil sample and place it at room temperature. Select the soil moisture content according to the actual engineering conditions and calculate the amount of water required for the soil sample with a specific moisture content. Weigh the required amount of water. Divide the soil sample into three equal parts. First, spread one part in an iron pan and evenly sprinkle 1 / 3 of the required amount of water on the surface of the soil layer. Then, treat the other two parts of the soil sample in the same way. Cover the iron pan with plastic wrap after adding water and let it stand for 12 hours. Test its moisture content. The moisture content error should be less than 1%.

[0054] (3) Weigh the required concrete raw materials according to the sample mold size. First, mix the coarse aggregate, then add cement and dry mix for 1 minute. Weigh the required mass of water according to the required water content for the test. Dissolve an appropriate amount of concrete antifreeze agent in the water, add the water to the dry-mixed mixture, and stir for 2 minutes. Take out the mixed concrete for later use.

[0055] (4) Use a thermometer to measure the surface temperature of the soil sample chamber 1300. When the surface temperature is less than 5℃, fill the soil sample. Fill the soil prepared in step (2) into the soil sample chamber. According to the compaction requirements, weigh the amount of soil required for the sample. Divide the required amount of soil evenly into three parts and fill them in three layers. After filling each layer of soil sample, move the pressurizing device 400 to the top of the soil sample chamber along the reaction frame 200 through the walking device 800. Place the bearing plate 900 on the soil sample filled in the soil sample chamber and turn on the pressurizing device 400 to press the soil sample. During the pressing process, measure the distance between the upper surface of the soil sample and the upper edge of the mold. When the distance reaches 2h / 3, stop the pressurization, remove the bearing plate, and scratch and roughen the surface of the soil sample. Then press the remaining two soil samples in the same way until the surface of the soil sample is flush with the upper surface of the mold. The soil sample preparation is then complete.

[0056] (5) Place the soil sample filled in step (4) in box 100 and freeze it at -5℃ for 24 to 48 hours to make the soil temperature uniform. Finally, wrap the upper surface of the soil sample with insulation cotton for later use. Remove partition 1200 and check the compaction of the soil sample. If there is obvious stratification or unevenness in the middle of the soil sample, remove the soil sample and refill it according to steps (2) and (4).

[0057] (6) First, apply a release agent to the inner surface of the concrete pouring chamber 600. Then, add the concrete prepared in step three into the concrete pouring chamber 600. Stop pouring when the concrete height is level with the edge of the concrete chamber. Take out a small vibrator, insert it into the concrete chamber 600, and turn on the vibrator to vibrate for 90 seconds. The diameter of the vibrator should be less than 30mm. Continue to add concrete to the vibrated concrete pouring chamber 600. Stop pouring when the concrete is level with the upper surface of the concrete chamber.

[0058] (7) Place the bearing plate 900 on the concrete surface poured in step (6), move the pressurizing device 400 above the concrete pouring chamber 600 via the walking device 800 along the reaction frame 200, turn on the pressurizing device 400 to apply force P to the bearing plate, and observe the magnitude of force P through the pressure gauge 500. When the preset magnitude is reached, keep the force constant, wrap the outside of the bearing plate with thermal insulation cotton until the concrete reaches initial setting, then remove the loading force and remove the bearing plate. To ensure the specimen size is regular, prepare a small amount of concrete according to step (6) to fill the part that has sunk after the specimen is pressed, and wrap the upper surface of the concrete with thermal insulation cotton.

[0059] (8) The sample is cured in the box 100 for 10 to 20 days. After the concrete poured in step (6) reaches the strength requirement, the sample mold is removed to form the sample for the direct shear test of the freezing strength of the concrete-frozen soil contact surface.

[0060] (9) Place the sample from step (8) in the low-temperature chamber of the low-temperature direct shear tester and perform direct shear test on the sample in accordance with the requirements of the Geotechnical Test Procedure (YS / T5225-2016) to test the interfacial shear strength.

[0061] The order of the above steps is not fixed and can be adjusted according to the actual situation. For example, (3) and (4), (5) and (6) can be adjusted, and it doesn't matter whether the soil sample preparation or the concrete preparation is done first.

[0062] When concrete is poured into frozen soil, the lateral pressure exerted by the concrete on the frozen soil promotes the "thermal intrusion" of the frozen soil, essentially equivalent to the pressure exerted by a static fluid. The standard value F (kN / m²) of the maximum lateral pressure exerted by cast-in-place concrete on the formwork is calculated according to the national standard "Code for Construction of Concrete Structures" (GB 50666-2011). 2 The calculation formula is as follows (take the smaller value of the two formulas):

[0063]

[0064] F2=γ c H (II)

[0065] Wherein: γ cThe density of concrete is taken as 24 kN / m³. 3 ;

[0066] t0 is the initial setting time of the concrete, which can be determined according to the actual situation; when experimental data is lacking, it can be calculated as t0 = 200 / (T+15), where T is the temperature of the concrete (°C). β is the concrete slump correction factor: 0.85 when the slump is greater than 50mm but not greater than 90mm; 0.9 when the slump is greater than 90mm but not greater than 130mm; and 1.0 when the slump is greater than 130mm but not greater than 180mm. V is the concrete pouring speed (m / h), which is the ratio of the concrete pouring height to the pouring time. H is the concrete pouring height (m), the total height from the location where the concrete lateral pressure is calculated to the top surface of the newly poured concrete. k0 is the static lateral pressure coefficient, taken as 0.7.

[0067] Calculation parameter values:

[0068] Calculation parameters <![CDATA[γ c ]]> <![CDATA[t0]]> β <![CDATA[k0]]> V Value 24 3 1 0.7 10

[0069] but

[0070] When using equation (ii), it is necessary to multiply by the static lateral pressure coefficient. Furthermore, since the calculation depth and lateral pressure have a linear relationship, when F1 = F2, i.e., γ... c Hk0 = 66.5, and H is calculated to be 3.96m. This indicates that when the calculation depth is less than 3.96m, Equation (II) is used to calculate the lateral pressure. When the calculation depth is greater than 3.96m, the lateral pressure can be taken as 66.5kPa.

[0071] Taking calculation depths of 1m, 2m, 3m, and 4m as examples, the pressure of concrete on the sidewall is calculated, and the results are as follows:

[0072] Calculate depth H / m 1 2 3 4 Lateral pressure F / kPa 16.8 33.6 50.4 66.5

[0073] The lateral pressure of concrete on frozen soil is calculated using the active earth pressure calculation method for retaining walls. When a uniformly distributed load q (kPa) acts on the concrete surface, the load q can be considered as the imaginary self-weight of the concrete γH. Therefore, the total lateral pressure acting on the frozen soil is:

[0074] ;

[0075] Wherein: for concrete, k0 is taken as 0.7; since the mold size is 100mm×100mm×200mm, H is taken as 0.1; and γ is taken as 24. The calculation results of uniformly distributed load q and normal force P at different depths are shown in the following table:

[0076] Calculate depth / m 1 2 3 4 Uniformly distributed load q / kPa 238.8 478.8 718.8 948.8 Normal force P / kN 2.388 4.788 7.188 9.488

[0077] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A device for preparing a direct shear test specimen for a concrete-frozen soil contact surface, characterized in that, It includes a box body, a reaction frame, a heating plate, and a cooling plate. The top of the reaction frame is equipped with a walking device, and the bottom of the walking device is equipped with a pressurizing device with a pressure gauge. Below the reaction frame are a concrete pouring chamber and a soil sampling chamber, and a partition is provided between the concrete pouring chamber and the soil sampling chamber. The walking device is equipped with a sliding cavity and is fitted onto the support frame at the top of the reaction frame. The reaction frame is equipped with two limiting blocks and a rotation stop block. The two limiting blocks are respectively installed above the outer side wall of the concrete pouring chamber and the soil sample chamber. In use, the rotation stop block is locked onto the top of the reaction frame.

2. The apparatus for preparing direct shear test specimens for concrete-frozen soil contact surfaces according to claim 1, characterized in that, The top support frame and the side support frame of the reaction frame are both equipped with scales.

3. The apparatus for preparing direct shear test specimens for concrete-frozen soil contact surfaces according to claim 1, characterized in that, A temperature controller is provided on the outer wall of the box, and the heating plate and the cooling plate are communicatively connected to the temperature controller.

4. The apparatus for preparing direct shear test specimens for concrete-frozen soil contact surfaces according to claim 1, characterized in that, The outer sides and bottom surfaces of the concrete pouring chamber and the soil sample chamber are provided with thermal insulation cotton, and the inner wall of the box is provided with a thermal insulation layer; the sample preparation modulus and size specifications used in the concrete pouring chamber and the soil sample chamber are the same.

5. A method for preparing a direct shear test specimen for a concrete-frozen soil interface, characterized in that, Includes the following steps: S1. Select soil samples according to the actual engineering conditions and prepare concrete; S2. Prepare frozen soil samples, the specific steps are as follows: S21. Use a thermometer to measure the surface temperature of the soil sample chamber. When the surface temperature is less than 5℃, fill the soil sample. Fill the soil sample chamber with the prepared soil. According to the compaction requirements, weigh the amount of soil required for the sample. Divide the required amount of soil evenly into three parts and fill them in three layers. After filling each layer of soil sample, move the pressurizing device to the top of the soil sample chamber along the reaction frame via the walking device. Place the pressure plate on the soil sample filled in the soil sample chamber and turn on the pressurizing device to press the soil sample. During the pressing process, measure the distance between the upper surface of the soil sample and the upper edge of the mold. When the distance reaches 2h / 3, stop pressurizing, remove the pressure plate, and scratch and roughen the surface of the soil sample. Then press the remaining two soil samples in the same way until the surface of the soil sample is flush with the upper surface of the mold. The soil sample preparation is complete. S22. Place the filled soil sample in a model box and freeze it at -5℃ for 24 to 48 hours to make the soil temperature uniform. Finally, wrap the upper surface of the soil sample with insulation cotton for later use and remove the partition. S3. Apply release agent to the inner surface of the concrete pouring chamber, then add the prepared concrete into the concrete pouring chamber. When the concrete height is level with the edge of the concrete chamber, stop pouring; take out a small vibrator, insert it into the concrete chamber, and turn on the vibrator to vibrate. Continue adding concrete to the concrete pouring chamber after vibration, and stop pouring when the concrete is level with the upper surface of the concrete chamber. S4. Place the bearing plate on the surface of the poured concrete, move the pressurizing device to the top of the concrete pouring chamber via the walking device along the reaction frame, turn on the pressurizing device to apply force P to the bearing plate, observe the magnitude of force P through the pressure gauge, when the preset magnitude is reached, keep the force constant, wrap the outside of the bearing plate with thermal insulation cotton until the concrete reaches initial setting, then remove the loading force and remove the bearing plate. S5. Curing the sample in the model box for 10 to 20 days. After the concrete poured in step S4 reaches the strength requirement, the sample mold is removed to form the sample for the direct shear test of the freezing strength of the concrete-frozen soil contact surface. Applying force to the bearing plate The derivation steps are as follows: Step 1: When concrete is poured into frozen soil, the lateral pressure exerted by the concrete on the frozen soil has a certain promoting effect on the "thermal intrusion" of the frozen soil, which is basically equivalent to the pressure value generated by a static fluid. Calculate the standard value F of the maximum lateral pressure of the cast-in-place concrete on the formwork, and take the smaller value of the two formulas. The calculation formula is as follows: (one); (two); This refers to the gravitational density of concrete. This refers to the initial setting time of the concrete. This is the correction factor for concrete slump. For concrete pouring speed; This refers to the height of the concrete pouring. This is the coefficient of pressure on the stationary side; Step 2: Calculate the case where Formula (I) and Formula (II) are equal. H , recorded as H T =H ; = ； When the depth is less than H T When the calculation depth is greater than 1000 meters, the lateral pressure is calculated using equation (ii). H T When the side pressure is calculated, Equation (I) is used. Step 3: Calculate the equivalent side pressure: The lateral pressure of concrete on frozen soil is calculated using the active earth pressure calculation method for retaining walls, when a uniformly distributed load is applied to the concrete surface. At that time, the load can be If we consider the self-weight of the imaginary concrete as the source, then the total lateral pressure acting on the frozen soil is: ； Step 4: Calculate the uniformly distributed load: H Lateral pressure at the location F1 or F2 Substitute into equation (iii) to find That is: = or = 2; Step 5: Apply force to the bearing plate for: = ； A represents the area of ​​the working surface of the pressure plate.

6. The method for preparing a direct shear test specimen for the concrete-frozen soil contact surface according to claim 5, characterized in that, In step S1, the specific steps for selecting the soil sample are as follows: First, crush the soil sample required for the test, put the sieved soil sample into an oven to dry, and finally take out the dried soil sample and place it in a room temperature environment to air dry the soil sample to room temperature; select the soil moisture content according to the actual engineering conditions, calculate the amount of water required for the soil sample with a specific moisture content, and weigh the required amount of water; divide the soil sample into three equal parts, first spread one part in an iron pan, and evenly sprinkle 1 / 3 of the required amount of water on the surface of the soil layer, and then treat the other two soil samples in the same way, wrap the iron pan with plastic wrap after adding water, and test its moisture content. The moisture content error should be less than 1%.

7. The method for preparing a direct shear test specimen for the concrete-frozen soil contact surface according to claim 5, characterized in that, In step S1, the specific steps for preparing concrete are as follows: according to the size of the sample mold, weigh the raw materials required for the concrete sample, first mix the coarse aggregate, then add cement and dry mix, weigh the required mass of water according to the required water content for the test, dissolve an appropriate amount of concrete antifreeze agent in the water, add the water to the dry-mixed mixture, and take out the mixed concrete for later use.

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