CN120009035A

CN120009035A - A device and method for preparing a specimen for a direct shear test on a concrete-frozen soil contact surface

Info

Publication number: CN120009035A
Application number: CN202510022976.7A
Authority: CN
Inventors: 唐丽云; 张宝东; 唐华明; 邱培勇; 郑建国; 贺晶晶; 黄俊波; 杜晓奇; 于永堂; 贾海梁; 张蕾; 马力瑶; 刘飞岳; 吴承越
Original assignee: Xian University of Science and Technology
Current assignee: Xian University of Science and Technology
Priority date: 2025-01-07
Filing date: 2025-01-07
Publication date: 2025-05-16

Abstract

The present application belongs to the field of frozen soil engineering technology, and specifically relates to a device and method for making a direct shear test specimen of a concrete-frozen soil interface, including a box, a reaction frame, a heating plate and a cooling plate, wherein a walking device is provided on the top of the reaction frame, a pressurizing device is provided at the bottom of the walking device, and a pressure gauge is provided on the pressurizing device; 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. The advantage is that it solves the problem of excessive moisture content at the concrete-frozen soil interface caused by moisture migration due to gravity.

Description

Device and method for manufacturing direct shear test sample of concrete-frozen soil contact surface

Technical Field

The application belongs to the technical field of frozen soil engineering, and particularly relates to a device and a method for manufacturing a direct shear test sample of a concrete-frozen soil contact surface.

Background

More and more engineering structures including important foundation engineering such as Qinghai-Tibet railways and Qinghai-Tibet highways are already or will be built in frozen soil areas, and pile foundations are widely applied to cold region engineering due to the advantages of strong bearing capacity, wide adaptability, stable long-term performance and the like. The upper load transmission form of the structure is mainly contact transmission of the structure and the frozen soil, so that freezing force between a pile foundation and the frozen soil is an important mechanical parameter in the design of friction piles in a frozen soil area for many years, and the acquisition of the parameter mainly comprises two modes of field detection and indoor test. The freezing force obtained by pile foundation field detection is more accurate, but the defects of higher cost, high operation difficulty and the like exist, and the method is less in engineering application. The indoor direct shear test is the earliest test method for measuring the shear strength of soil, and has the advantages of easy operation and control of the test process, repeatable test process, accurate test result and the like, and is widely applied to practical engineering.

The direct shear test can also be used for detecting the shear strength of contact surfaces of different media, such as the contact surfaces of concrete and frozen soil, so that the contact form of the concrete and the frozen soil in the test piece manufacturing process is an important factor for determining mechanical parameters. The contact surface of the concrete and the frozen soil is in two forms in spatial distribution, namely, the contact surface of the concrete and the frozen soil is vertical to the vertical direction, and the contact surface of the concrete and the frozen soil is vertical to the horizontal direction. The two methods described above produce samples that are substantially identical in appearance, but differ greatly in the formation of the contact surface. According to the first method, the water content near the contact surface is overlarge due to the action of gravity, the test result mainly depends on the contact pressure, the water-gel ratio of concrete and the curing temperature, and the test result is less influenced by the ice content and the soil property of frozen soil, and the shearing strength is mainly the freezing force; the second method considers the moisture migration caused by gravity, and has the same contact surface forming mode as the actual engineering, and the result depends on factors such as the concrete molding temperature, the contact pressure, the frozen soil temperature, the ice content and the like; the second method can simulate the contact state of the concrete and the frozen soil at different depths, the water content of the contact surface caused by moisture migration is prevented from being too high, and the shearing strength obtained by the shearing test can represent the freezing strength of the foundation and the frozen soil at different depths. However, the prior art has the following problems that firstly, concrete is directly poured on the upper side of a frozen soil body, and water in the concrete can migrate near a contact surface due to the gravity factor, so that the water content near the contact surface is larger, and the freezing strength is higher. Second, the sample preparation method does not consider the influence of contact pressure on concrete heat invasion, which is not in line with the actual engineering situation.

Disclosure of Invention

The invention aims to provide a device for manufacturing a direct shear test sample of a concrete-frozen soil contact surface, which is low in manufacturing cost, convenient to install and simple to operate, and the second technical problem to be solved by the invention is to provide a method for manufacturing a direct shear test sample of a concrete-frozen soil contact surface. The technical proposal is as follows:

The device for manufacturing the direct shear test sample of the concrete-frozen soil contact surface comprises a box body, a reaction frame, a heating plate and a refrigerating plate, wherein a traveling device is arranged at the top of the reaction frame, a pressurizing device is arranged at the bottom of the traveling device, a pressure gauge is arranged on the pressurizing device, a concrete pouring chamber and a soil sample chamber are arranged below the reaction frame, and a partition plate is arranged between the concrete pouring chamber and the soil sample chamber.

Preferably, the walking device is provided with a sliding cavity sleeved on a support frame at the top of the reaction frame, the reaction frame is provided with two limiting blocks and a rotary stop block, the two limiting blocks are respectively and correspondingly arranged above the concrete pouring chamber and the soil sample chamber outer side wall, and the rotary stop block is clamped at the top of the reaction frame when in use.

Preferably, the support frame and the side support frame at the top of the reaction frame are provided with graduated scales.

Preferably, a temperature controller is arranged on the outer wall of the box body, and the heating plate and the refrigerating plate are in communication connection with the temperature controller.

Preferably, the concrete pouring chamber and the soil sample chamber are provided with heat insulation cotton on the outer side surface and the outer bottom surface, the inner wall of the box body is provided with a heat insulation layer, and the sample preparation modules adopted by the concrete pouring chamber and the soil sample chamber have the same size and specification.

A method for manufacturing a direct shear test sample of a concrete-frozen soil contact surface comprises the following steps:

s1, selecting a soil sample according to actual engineering conditions, and preparing concrete;

S2, preparing a sample soil sample of frozen soil, which comprises the following specific steps:

S21, measuring the surface temperature of a soil sample chamber by using a thermometer, filling a soil sample when the surface temperature is less than 5 ℃, filling configured soil in the soil sample chamber, weighing the required soil amount of a sample according to the compactness requirement, uniformly dividing the required soil amount into three parts, filling three layers of layers, filling each layer of soil sample, moving a pressurizing device to the upper part of the soil sample chamber along a counter-force frame by using a traveling device, placing a pressure bearing plate on the soil sample filled in the soil sample chamber, starting the pressurizing device to press the soil sample, measuring the distance between the upper surface of the soil sample and the upper edge of a die in the pressing process, stopping pressurizing when the distance reaches 2h/3, taking out the pressure bearing plate, scraping and roughening the surface of the soil sample, and pressing the rest two parts of soil samples according to the method until the soil sample surface is level with the upper surface of the die;

s22, freezing the filled soil body sample in a model box for 24-48 hours at the temperature of-5 ℃ to ensure that the soil body temperature is uniform, and finally coating heat preservation cotton on the upper surface of the soil sample for standby and taking out a partition plate;

S3, brushing a release agent on the inner surface of the concrete pouring chamber, adding prepared concrete into the concrete pouring chamber, and stopping pouring when the height of the concrete is level with the edge of the concrete chamber;

S4, placing a bearing plate on the upper surface of the poured concrete, moving a pressurizing device to the upper side of the concrete pouring chamber along a counterforce frame through a traveling device, starting the pressurizing device to apply force P to the bearing plate, observing the force P through a pressure gauge, when the force P reaches a preset value, keeping the force constant, wrapping heat-insulating cotton on the outer side of the bearing plate until the concrete reaches initial setting, removing the loading force, and removing the bearing plate;

S5, sample maintenance is carried out in the model box, the maintenance period is 10-20 days, and after the concrete poured in the step S4 meets the strength requirement, the sample preparation mould is disassembled, so that the sample for the direct shear test of the freezing strength of the concrete-frozen soil contact surface is formed.

Preferably, in step S1, the concrete steps for selecting the soil sample are as follows:

Firstly crushing a soil sample required by a test, putting the sieved soil sample into a baking oven for baking, finally taking out the baked soil sample, putting the baked soil sample in a room temperature environment, airing the soil sample to the room temperature, selecting the water content of a soil body according to actual engineering conditions, calculating the water content required by the soil sample with specific water content, weighing the required water content, dividing the soil sample into three equal parts, firstly paving one part into iron plates, uniformly spreading 1/3 of the required water content on the surface of the soil layer, then treating the other two parts of soil samples according to the operation, coating a preservative film outside the iron plates after water addition, testing the water content, and ensuring that the water content error is less than 1%.

Preferably, in the step S1, concrete raw materials required by a sample are weighed according to the size of a sample die, coarse aggregate is firstly mixed, cement is then added for dry mixing, water with required mass is weighed according to the water content requirement required by the test, a proper amount of concrete antifreezing agent is dissolved in the water, the water is added into the dry-mixed mixture, and the mixed concrete is taken out for standby.

Preferably, the bearing plate applied force P derivation step is as follows:

When concrete is poured into frozen soil, the lateral pressure of the concrete generated by the frozen soil has a certain promoting effect on the heat invasion of the frozen soil, the lateral pressure is basically equal to the pressure value generated by static fluid, the standard value F of the maximum lateral pressure of the cast-in-place concrete to the template is calculated, the smaller value of the two formulas is taken, and the calculation formula is as follows:

(one);

f2 =γ _c H (two);

Gamma _c is the gravity density of the concrete, T ₀ is the initial setting time of the concrete, T is the temperature of the concrete, beta is the slump correction coefficient of the concrete, V is the casting speed of the concrete, H is the casting height of the concrete, and k ₀ is the static side pressure coefficient;

Step two, calculating H under the condition that the formula (I) is equal to the formula (II), and marking the H _T = H;

Calculating the side pressure by using the formula (II) when the depth is smaller than H _T, and calculating the side pressure by using the formula (I) when the calculated depth is larger than H _T;

step three, calculating an equivalent lateral pressure:

the method for calculating the side pressure of concrete to the frozen soil by adopting the method for calculating the active soil pressure of the retaining wall is adopted, when the load q is uniformly distributed on the surface of the concrete, the load q can be regarded as the self weight of fictitious concrete gamma H, and the total side pressure acted on the frozen soil is as follows:

Step four, calculating uniform load, namely substituting side pressure F1 or F2 at the H position into the step three to obtain a position q, namely:

P=f1 or p=f2;

Fifthly, applying force P to the bearing plate is as follows:

a is the area of the acting surface of the bearing plate.

Compared with the prior art, the application has the following beneficial effects:

1. according to the invention, the contact surface of the concrete and the frozen soil is perpendicular to the horizontal direction, so that the excessive water content near the contact surface caused by the migration of water in the concrete due to the action of gravity is avoided.

2. According to the invention, concrete is pressurized in the test piece manufacturing process, so that the real contact state of the concrete and the frozen soil at different depths can be simulated.

3. According to the invention, concrete pouring is performed in a low-temperature environment, and heat preservation cotton is coated outside the concrete chamber, so that the concrete pouring in the low-temperature environment is simulated, the defect of early dissipation of concrete temperature caused by small size (generally smaller than 10mm in diameter) of a laboratory preparation sample is avoided, and the contact surface form formed by concrete hydration heat is reduced more truly.

4. The invention can finish the preparation of the freezing force test samples of the contact surfaces of various different materials, has the advantages of convenient installation, firmness, reliability, low manufacturing cost, accurate data and the like, can effectively reduce the cost expenditure of a laboratory, and is worth popularizing and using.

Drawings

Figure 1 is a schematic view of the structure of the present application,

Figure 2 is an enlarged view of a in figure 1,

In the figure, 100 parts of a box body, 200 parts of a reaction frame, 300 parts of a heating plate, 400 parts of a pressurizing device, 500 parts of a pressure gauge, 600 parts of a concrete pouring chamber, 700 parts of a sample preparation mold, 800 parts of a traveling device, 900 parts of a bearing plate, 1000 parts of a refrigerating plate, 1100 parts of a temperature controller, 1200 parts of a partition plate, 1300 parts of a soil sample chamber, 1400 parts of a limiting block, 1500 parts of a rotation stop block and A01 parts of a heat preservation layer.

Detailed Description

The following detailed description of the technical solutions of the present application will be made by specific embodiments and accompanying drawings, and it should be understood that the embodiments of the present application and specific features in the embodiments are detailed descriptions of the technical solutions of the present application, and not limiting the technical solutions of the present application, and that the specific technical features may be combined with each other.

1-2, A concrete-frozen soil contact surface direct shear test sample manufacturing device comprises a box body 100, a reaction frame 200, a heating plate 300 and a refrigerating plate 1000, wherein a traveling device 800 is arranged at the top of the reaction frame 200, a pressurizing device 400 (such as a cylinder) is arranged at the bottom of the traveling device 800, a pressure gauge 500 is arranged on the pressurizing device 400, a concrete pouring chamber 600 and a soil sample chamber 1300 are arranged below the reaction frame 500, and a partition plate 1200 is arranged between the concrete pouring chamber and the soil sample chamber. The outer wall of the box body 100 is provided with a temperature controller 1100, and the heating plate 300 and the refrigerating plate 1000 are in communication connection with the temperature controller 1100. And graduated scales are arranged on the support frame at the top of the reaction frame 200 and the side support frames.

The running gear 800 is provided with a sliding cavity, and is sleeved on a supporting frame at the top of the reaction force reaction frame 200, the reaction force frame 200 is provided with two limiting blocks 1400 and a rotary stop block 1500, the two limiting blocks 1400 are respectively correspondingly arranged above the outer side walls of the concrete pouring chamber 600 and the soil sample chamber 1300, and the rotary stop block 1500 is clamped above the partition plate 1200. (the running gear 800 is taken away when sliding, and is clamped at the position above the baffle 1200 for fixing the scale when in use)

The concrete pouring chamber 600 and the soil sample chamber 1300 are provided with heat preservation cotton on the outer side surface and the outer bottom surface, the inner wall of the box body is provided with a heat preservation layer, and the size and the specification of the sample preparation mould 700 adopted by the concrete pouring chamber 600 and the soil sample chamber 1300 are the same.

The sample preparation unit comprises a soil sample chamber, a concrete pouring chamber and a partition board, the temperature control unit comprises a heating plate, a refrigerating plate and a temperature controller, the loading mechanism comprises a counter-force frame, a pressurizing device, a pressure gauge and a running mechanism, the heat preservation unit comprises a sample preparation chamber external heat preservation layer, the length, width and height of a concrete-frozen soil sample mould are selected to be l multiplied by w multiplied by h=200 mm multiplied by 100mm, the partition board is arranged in the middle of the mould, the size of the partition board is 100mm multiplied by 100mm, the thickness of the mould and the thickness of the partition board are 5mm, the sample mould is connected by adopting bolts, a groove is formed in the middle of the mould, and the partition board can be embedded into the groove.

The preparation method of the direct shear test sample of the concrete-frozen soil contact surface comprises the following steps:

(1) First, the cooling plate 1000 and the temperature controller 1100 are opened to control the temperature in the casting chamber, and when the temperature in the soil sample chamber 1300 is lower than-5 ℃, the heating plate 300 is opened so that the temperature in the soil sample chamber is maintained at-5.5 ℃. Insulating cotton is attached to the outer sides and the outer bottoms of the concrete pouring chamber 600 and the soil sample chamber 1300, a sample mold for attaching the insulating cotton is placed in the box 100, the partition 1200 is inserted in the middle of the mold, and the placing time is not less than 2 hours.

(2) Firstly crushing a soil sample required by the test, sieving the crushed soil with a 2mm sieve, putting the sieved soil sample into a 120 ℃ oven for drying for 8 hours, finally taking out the dried soil sample, putting the dried soil sample in a room temperature environment, and airing the soil sample to room temperature. And selecting the water content of the soil body according to the actual engineering condition, calculating the water required by the soil sample with the specific water content, and weighing the water required. Dividing the soil sample into three equal parts, firstly paving one part in an iron disc, uniformly scattering 1/3 of the required water amount on the surface of a soil layer, and then treating the other two parts of soil samples according to the operation. And (5) coating a preservative film outside the iron plate with the water, and standing for 12 hours. And the water content is tested, and the error of the water content is less than 1%.

(3) According to the size of a sample mould, weighing concrete raw materials required by the sample, mixing coarse aggregate, adding cement, carrying out dry mixing for 1 minute, weighing water with required mass according to the water content requirement required by the test, dissolving a proper amount of concrete antifreezing agent in the water, adding the water into the dry-mixed mixture, and stirring for 2 minutes. Taking out the mixed concrete for standby;

(4) The surface temperature of the soil sample chamber 1300 was measured using a thermometer, and when the surface temperature was less than 5 ℃, soil sample loading was performed. And (3) loading the soil body prepared in the step (2) into a soil sample chamber, weighing the soil quantity required by a sample according to the compactness requirement, uniformly dividing the required soil quantity into three parts, filling the soil sample into layers and three layers, moving a pressurizing device 400 to the upper part of the soil sample chamber along a counter-force frame 200 through a traveling device 800 after filling each layer of the soil sample, placing a bearing plate 900 on the soil sample filled in the soil sample chamber, starting the pressurizing device 400 to press the soil sample, measuring the distance between the upper surface of the soil sample and the upper edge of a die in the pressing process, stopping pressurizing when the distance reaches 2h/3, taking out the bearing plate, and scraping and roughening the surface of the soil sample. And pressing the remaining two soil samples according to the method until the surface of the soil sample is level with the upper surface of the die, and finishing the preparation of the soil sample.

(5) Freezing the soil body sample filled in the step (4) in the box body 100 for 24-48 hours at the temperature of minus 5 ℃ to ensure that the soil body temperature is uniform, and finally coating heat preservation cotton on the upper surface of the soil sample for later use. Taking out the partition 1200, checking the compaction degree of the soil sample, and taking out the soil sample if the soil sample has obvious layering or obvious uneven phenomenon in the middle, and reloading according to the steps (2) and (4).

(6) Firstly, brushing a release agent on the inner surface of a concrete pouring chamber 600, then adding the concrete prepared in the step three into the concrete pouring chamber 600, and stopping pouring when the height of the concrete is level with the edge of the concrete chamber. The small-sized shaking rod is taken out and inserted into the concrete chamber 600, and the shaking rod is started to shake for 90 seconds, and the diameter of the shaking rod is smaller than 30mm. And (3) continuously adding concrete into the concrete pouring chamber 600 after vibration, and stopping pouring when the concrete is level with the upper surface of the concrete chamber.

(7) Placing a bearing plate 900 on the upper surface of the concrete poured in the step (6), moving the pressurizing device 400 to the position above the concrete pouring chamber 600 along the counter-force frame 200 through the traveling device 800, starting the pressurizing device 400 to apply force P to the bearing plate, observing the force P through the pressure gauge 500, when the force P reaches the preset force, keeping the force unchanged, wrapping heat-insulating cotton on the outer side of the bearing plate until the concrete reaches initial setting, removing the loading force, and removing the bearing plate. In order to ensure the regular size of the sample, a small amount of concrete is prepared according to the step (6), the descending part of the pressed sample is filled up, and heat preservation cotton is coated on the upper surface of the concrete.

(8) And (3) carrying out sample maintenance in the box body 100 for 10-20 days, and disassembling the sample preparation mould after the concrete poured in the step (6) meets the strength requirement, so as to form a sample for the direct shear test of the freezing strength of the concrete-frozen soil contact surface.

(9) And (3) placing the sample in the step (8) in a low-temperature chamber of a low Wen Zhijian instrument, and performing direct shear on the sample according to the requirements of geotechnical test procedure (YS/T5225-2016) to test the shear strength of an interface.

The sequence of the steps is not uniform, and can be adjusted according to practical situations, such as (3) and (4), (5) and (6), and the preparation of soil samples and concrete can be performed before.

When concrete is poured in the frozen soil, the lateral pressure of the concrete generated by the frozen soil has a certain promotion effect on the 'heat invasion' of the frozen soil, which is basically equivalent to the pressure value generated by static fluid. According to the national standard 'concrete structure engineering construction Specification' (GB 50666-2011), calculating a standard value F (KN/m ²) of the maximum lateral pressure of cast-in-place concrete to a template, wherein the calculation formula is as follows (smaller value in the two formulas):

f2 =γ _c H (two);

wherein gamma _c is the gravity density of the concrete, and 24KN/m ³ is taken;

T ₀ is the initial setting time of the concrete, which can be measured according to practical conditions, and when the test data is absent, T ₀ =200/(t+15) can be used for calculation, and T is the temperature (°c) of the concrete. Beta is a slump correction coefficient, 0.85 is taken when the slump is larger than 50mm and not larger than 90mm, 0.9 is taken when the slump is larger than 90mm and not larger than 130mm, 1.0 is taken when the slump is larger than 130mm and not larger than 180mm, V is a concrete casting speed (m/H), the ratio of the concrete casting height to the casting time is taken, H is the concrete casting height (m), and the side pressure of the concrete is calculated to reach the total height of the top surface of the newly cast concrete. k ₀ is the resting side pressure coefficient, taking 0.7.

Calculating the value of the parameter:

calculating parameters	γ_c	t₀	β	k₀	V
						Value taking	24	3	1	0.7	10

Then

When the equation (II) is used, the stationary side pressure coefficient is multiplied, and when f1=f2, that is, γ _cHk₀ =66.5, is calculated, the calculated H is 3.96m, which means that when the calculated depth is less than 3.96m, the equation (II) is used to calculate the side pressure, and when the calculated depth is greater than 3.96m, the side pressure can take a value of 66.5kPa.

Taking the calculated depths of 1m, 2m, 3m and 4m as examples, the pressure of concrete on the side wall is calculated, and the calculation result is as follows:

calculate depth H/m	1	2	3	4
					Side pressure F/kPa	16.8	33.6	50.4	66.5

The lateral pressure of concrete to the frozen soil is calculated by adopting a retaining wall active soil pressure calculation method, when the uniform load q (kPa) acts on the surface of the concrete, the load q can be regarded as the dead weight generation of fictitious concrete gamma H, and the total lateral pressure acted on the frozen soil is as follows:

;

Wherein, for the concrete k ₀, 0.7 is taken, and since the size of the mould is 100mm×100mm×200mm, 0.1 is taken for H and 24 is taken for gamma. The calculation results of the uniform load q and the normal force P at different depths are shown in the following table:

calculating depth/m	1	2	3	4
					Uniform load q/kPa	238.8	478.8	718.8	948.8
Normal force P/kN	2.388	4.788	7.188	9.488

The embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge of one of ordinary skill in the art without departing from the spirit of the present invention.

Claims

1. A device for making direct shear test specimens on a concrete-frozen soil interface, characterized in that it comprises a box, a reaction frame, a heating plate and a cooling plate, wherein a walking device is provided on the top of the reaction frame, a pressurizing device is provided at the bottom of the walking device, and a pressure gauge is provided on the pressurizing device; 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.

2. The direct shear test specimen preparation device for the concrete-frozen soil contact surface according to claim 1 is characterized in that the walking device is provided with a sliding cavity, which is mounted on the support frame on the top of the reaction frame, and the reaction frame is provided with two limit blocks and a rotation stop block, and the two limit blocks are respectively installed above the outer wall of the concrete pouring chamber and the soil sample chamber; when in use, the rotation stop block is clamped on the top of the reaction frame.

3. The device for making specimens for direct shear test on the concrete-frozen soil interface according to claim 1 is characterized in that a scale is provided on the support frame at the top of the reaction frame and the side support frame.

4. The device for preparing a specimen for a direct shear test on a concrete-frozen soil interface according to claim 1, wherein a temperature controller is provided on the outer wall of the box body, and the heating plate and the cooling plate are communicatively connected to the temperature controller.

5. The device for making direct shear test specimens of the concrete-frozen soil interface according to claim 1 is characterized in that the outer side and outer 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 making modulus size specifications used in the concrete pouring chamber and the soil sample chamber are the same.

6. A method for preparing a direct shear test specimen of a concrete-frozen soil interface, characterized in that it comprises the following steps:

S1. Select soil samples according to actual engineering conditions and prepare concrete;

S2. Prepare a frozen soil sample. 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°C, fill the soil sample; fill the soil sample chamber with the prepared soil. According to the density requirements, weigh the soil required for the sample, divide the required soil into three parts, and fill it in three layers. After filling each layer of soil sample, move the pressure device to the top of the soil sample chamber through the walking device along the reaction frame, put the pressure plate on the soil sample filled in the soil sample chamber, turn on the pressure device to press the soil sample, measure the distance between the upper surface of the soil sample and the upper edge of the mold during the pressing process, stop pressing, take out the pressure plate, and scrape and roughen the surface of the soil sample; then press the remaining two soil samples according to the above method respectively until the surface of the soil sample is flush with the upper surface of the mold, and the soil sample preparation is completed;

S22. Freeze the filled soil sample in a model box at -5°C 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 the partition.

S3. Brush the release agent on the surface of the concrete pouring chamber, and then add the prepared concrete to the concrete pouring chamber. When the height of the concrete is flush with the edge of the concrete chamber, stop pouring; remove the small vibrator, insert it into the concrete chamber, and turn on the vibrator to vibrate; continue to add concrete to the concrete pouring chamber after vibration, and stop pouring when the concrete is flush with the upper surface of the concrete chamber;

S4. Place a pressure plate on the upper surface of the poured concrete, move the pressure device along the reaction frame through the walking device to the top of the concrete pouring chamber, turn on the pressure device to apply force P to the pressure plate, observe the magnitude of force P through the pressure gauge, and when the preset magnitude is reached, keep the force unchanged, wrap the outer side of the pressure plate with insulation cotton, and remove the loading force and the pressure plate after the concrete reaches initial setting;

S5. Curing the sample in the model box for a period of 10 to 20 days. After the concrete poured in step S4 reaches the strength requirement, the sample preparation mold is disassembled to form a sample for the direct shear test of the freezing strength of the concrete-frozen soil contact surface.

7. The method for preparing a direct shear test specimen for a concrete-frozen soil interface according to claim 6, characterized in that in step S1, the specific steps of selecting a 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 at room temperature, and let the soil sample dry to room temperature; select the soil moisture content according to the actual engineering situation, and 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 lay one of them on an iron plate, and evenly sprinkle 1/3 of the required water on the surface of the soil layer, and then process the other two soil samples according to the above operations, wrap the iron plate with water with plastic wrap, and test its moisture content. The moisture content error should be less than 1%.

8. The method for preparing a direct shear test specimen for a concrete-frozen soil interface according to claim 6 is characterized in that, in step S1, the concrete is prepared by weighing the concrete raw materials required for the sample according to the size of the sample mold, first mixing the coarse aggregate, then adding cement for dry mixing, weighing the required mass of water according to the water content required for the test, dissolving an appropriate amount of concrete antifreeze in water, adding water to the dry-mixed mixture, and taking out the mixed concrete for standby use.

9. The method for preparing a direct shear test specimen of a concrete-frozen soil interface according to claim 6, characterized in that the steps of deriving the force P applied by the pressure plate are as follows:

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

F2＝γ _c H (II);

γ _c is the gravity density of concrete; t ₀ is the initial setting time of concrete; T is the temperature of concrete; β is the slump correction coefficient of concrete; V is the concrete pouring speed; H is the concrete pouring height; k ₀ is the static side pressure coefficient;

Step 2: Calculate H when formula (1) is equal to formula (2), and record it as H _T =H;

When the depth is less than _HT , the lateral pressure is calculated using formula (II); when the calculated depth is greater than _HT , the lateral pressure is calculated using formula (I);

Step 3: Calculate the equivalent lateral pressure:

The lateral pressure of concrete on frozen soil is calculated by the calculation method of active earth pressure of retaining wall. When the uniformly distributed load q acts on the concrete surface, the load q can be regarded as the self-weight of the fictitious concrete γH. Then the total lateral pressure acting on frozen soil is:

Step 4: Calculate the uniformly distributed load: Substitute the lateral pressure F1 or F2 at point H into formula (3) and calculate q, which is:

P＝F1 or P＝F2;

Step 5: The force P applied by the pressure plate is:

A is the area of the effective surface of the pressure plate.