Soil testing method based on freeze-thaw-unsaturated dynamic load hollow cylinder torsional shear instrument

Abstract

The present invention relates to the technical field of soil testing, and specifically discloses a soil testing method based on a freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument. A vertical dynamic load, a torsion shear dynamic load, and a confining pressure are applied respectively by a vertical servo motor, a torsion shear servo motor, and a triaxial pressure chamber to simulate the stress state of a roadbed under the action of a moving wheel load. The unsaturated state of the sample is controlled by an axial translation method that controls the pore air pressure and pore water pressure to simulate the hydraulic state of the roadbed in real conditions. Furthermore, the sample temperature is controlled by a low-temperature water bath connected to the upper and lower bottom surfaces of the sample to simulate the temperature gradient during the freeze-thaw process of the roadbed. Therefore, the instrument can introduce the effects of freeze-thaw and unsaturation while simulating traffic dynamic loads, and can more accurately reflect the mechanical response of the cold-region roadbed under the coupled action of freeze-thaw-unsaturated traffic loads, providing a more accurate and scientific basis for cold-region roadbed performance evaluation.

Description

Technical Field

[0001] The present invention relates to the technical field of soil detection, and in particular to a soil detection method based on a freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument. Background Art

[0002] In road engineering, the cyclical axial loads generated by vehicles significantly affect the deformation characteristics of the roadbed and base materials, and are a major factor in causing rutting damage to road structures. Furthermore, as the wheel load shifts, the magnitude of the normal and shear stresses in the vertical direction for a fixed point on the top of the roadbed continuously changes, causing the direction of the principal stresses to continuously shift. This phenomenon is known as principal stress axis rotation (PSAR). Research has shown that principal stress axis rotation amplifies the vertical permanent strain of the roadbed, and its impact cannot be ignored in road design. In unit tests, a hollow cylindrical torsional shear apparatus can often be used to simulate the principal stress axis rotation phenomenon in the roadbed, allowing the vertical permanent strain under its influence to be studied.

[0003] The core principle of the hollow cylinder torsional shear apparatus is to simulate the mechanical response of soil or rock under different stress states by applying torsion (shear) and axial pressure to cylindrical samples. In this device, the sample is in the shape of a hollow cylinder, and the test process simulates different three-dimensional stress states by applying torque (shear force) and axial pressure. This experimental equipment can study the strain, shear strength, liquefaction characteristics, yield behavior, etc. of the soil under a controlled environment. Existing hollow cylinder torsional shear apparatuses are usually only suitable for experiments on saturated soil at room temperature, but the actual road subgrade is in an unsaturated state, and the subgrade in cold regions is also affected by freeze-thaw.

[0004] Therefore, for the influence of the principal stress axis rotation on the permanent strain of the roadbed under the load of moving wheels under the action of freeze-thaw and unsaturation in cold areas, it is necessary to provide a freeze-thaw-unsaturated dynamic load hollow cylindrical torsional shear apparatus and test method to solve the above problems. Summary of the Invention

[0005] The purpose of the present invention is to provide a soil testing method based on a freeze-thaw-unsaturated dynamic load hollow cylindrical torsional shear instrument to solve the above technical problems.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] The soil testing method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument includes the following steps:

[0008] S1: Weigh the dry soil sample and place it in a soil pan. Add water to the optimal moisture content. Cover with plastic wrap to prevent water evaporation. Curing at room temperature for 24 hours to balance the internal moisture content.

[0009] S2: Place the cured soil sample into the mold and compact it layer by layer to the specified compaction degree. Take a small amount of excess soil sample in three groups for drying and measuring the moisture content.

[0010] S3: Transfer the sample to the sample base. If the sample preparation process is carried out on the sample base, remove the mold and put the triaxial test rubber film on the inside and outside of the sample;

[0011] S4: Operate the vertical servo motor to slowly lower the sample cap until it just contacts the top of the sample, with the contact pressure not exceeding 10N. Use a rubber ring to seal the rubber membrane to ensure that independent water and air passages are formed inside the sample.

[0012] S5: Fill the pressure chamber with water to a level above the specimen cap through the confining pressure control valve on the base of the triaxial pressure chamber. After the water filling is completed, close the control valve on the base and pressurize the triaxial pressure chamber through the exhaust valve on the top cover to form confining pressure.

[0013] S6: Inject water into the sample through the pore water pressure control valve on the base. Keep the pore air pressure control valve open until water is discharged from the exhaust pipe. Then, use the double negative pressure method to increase the negative pressure inside the sample to -90 kPa. Continue to saturate the sample for 24 hours.

[0014] S7: After saturation is completed, the magnitude of the transverse normal stress is determined according to the stress cycle change at a certain point inside the roadbed and confining pressure is applied, and the pore water pressure control valve is opened for consolidation and drainage;

[0015] S8: After consolidation is completed, the pore water pressure is kept constant, and the air pressure inside the sample is gradually increased to the specified value through the pore air pressure control valve, and the unsaturated process of the axis translation method is carried out until the specified unsaturated state is reached;

[0016] S9: The temperature of the upper and lower surfaces of the sample is controlled by circulating a low-temperature water bath;

[0017] S10: According to the magnitude of the maximum vertical normal stress and the maximum shear stress, the specimen is loaded with compressive and shear loads;

[0018] S11: Monitor and record data through the host computer. After the experiment is completed, all sensor data are output and processed.

[0019] As a further solution of the present invention: in S2, for the roadbed soil, the specified compaction degree is 95%.

[0020] As a further solution of the present invention: in S5, the initial confining pressure is mainly used to maintain the shape of the sample, and the confining pressure does not exceed 20 kPa.

[0021] As a further solution of the present invention: in the above-mentioned S6, the sample is further saturated for 24 hours, and the effective confining pressure of the sample is maintained at 20 kPa. After completion, the B value is measured to ensure that the compaction degree reaches more than 96%.

[0022] As a further solution of the present invention: in said S7, during the process of applying the confining pressure, the confining pressure takes the maximum value or average value of the transverse normal stress.

[0023] As a further solution of the present invention: in the above S10, the magnitudes of the vertical normal stress and the vertical normal strain can be calculated according to the following formula:

[0024] ;

[0025] ;

[0026] Where, σ is the vertical normal stress, F is the vertical axial pressure, r o is the outer radius of the specimen, r i is the inner radius of the specimen, ε is the vertical normal strain, ΔH is the height change of the specimen, and H is the initial height of the specimen.

[0027] As a further solution of the present invention: in the above S10, assuming that the shear stress is uniformly distributed and equal everywhere, the shear strain is calculated using the deformation along the circumferential direction at the average radius of the inner and outer circles, and the following formula can be obtained:

[0028] ;

[0029] ;

[0030] Where τ is the shear stress, T is the output torque, γ is the shear strain, and Δθ is the rotation angle.

[0031] The beneficial effects of the present invention are as follows: simulating the stress state of the roadbed under the action of dynamic wheel load; controlling the unsaturated state of the sample by the axial translation method of controlling the pore air pressure and pore water pressure, simulating the hydraulic state of the roadbed in actual conditions; and further controlling the sample temperature by a low-temperature water bath connected to the upper and lower bottom surfaces of the sample, simulating the temperature gradient of the roadbed during the freezing and thawing process; therefore, the instrument can introduce the effects of freezing, thawing and unsaturation while simulating the dynamic traffic load, and can more accurately reflect the mechanical response of the cold region roadbed under the coupled action of freeze-thaw-unsaturation-traffic load, providing a more accurate and scientific basis for the performance evaluation of the cold region roadbed. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention will be further described below with reference to the accompanying drawings.

[0033] Figure 1 It is a structural schematic diagram of the freeze-thaw-unsaturated dynamic load hollow cylindrical torsional shear apparatus of the present invention;

[0034] Figure 2 This is a schematic diagram of the stress cycle change at a certain point inside the roadbed obtained by calculating the actual moving wheel load in the present invention;

[0035] Figure 3 Schematic diagram of shear stress distribution of the sample of the present invention;

[0036] Figure 4 Schematic diagram of shear strain of the sample of the present invention.

[0037] In the figure: 1. Displacement meter; 2. Axial pressure rod; 3. Exhaust valve; 4. Dynamometer; 5. Top surface temperature sensor; 6. Sample box; 7. Triaxial pressure chamber; 8. Sample base; 9. Pore barometer; 10. Pore water pressure gauge; 11. Bottom surface temperature sensor; 12. Confining pressure gauge; 13. Torsional shear servo motor; 14. Control valve; 15. Bottom surface low-temperature water bath; 16. Top surface low-temperature water bath; 17. Vertical servo motor; 18. Sample cap; 19. Reaction frame. DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0039] like Figure 1 As shown, the freeze-thaw-unsaturated dynamic load hollow cylindrical torsional shear apparatus includes: a triaxial pressure chamber 7, a reaction frame 19, an unsaturated control module, a temperature control module, a dynamic load loading module, a sensor and a data acquisition module.

[0040] The triaxial pressure chamber 7 is placed on a reaction frame 19 to provide the required confining pressure for the experiment. The top and bottom of the chamber have reserved openings for installing the exhaust valve 3, the control valve 14, the temperature sensor, the dynamometer 4, and the axial pressure rod 2.

[0041] It is worth noting that in this embodiment, the control valve 14 is uniformly arranged at the bottom of the triaxial pressure chamber 7, and is divided into a confining pressure control valve, a pore water pressure control valve and a pore air pressure control valve according to different functions; similarly, Figure 2 As shown, according to different positions, the temperature sensor is also divided into a top surface temperature sensor 5 and a bottom surface temperature sensor 11;

[0042] The unsaturated control module, consisting of a sample cap 18 and a sample base 8, is used to control the matric suction within the sample using an axial translation method to achieve a specific unsaturated state. The sample cap 18 is connected to the control valve 14 at the bottom of the triaxial pressure chamber 7 via a pipeline. A pore pressure gauge 9 measures pore pressure and provides feedback to control the pore pressure in the sample. The sample base 8 is embedded with a high-air-permeability clay plate, ensuring that only water but not air is permeable during pressurization. Its lower portion is connected to the control valve 14 at the bottom of the triaxial pressure chamber 7. A pore water pressure gauge 10 measures pore water pressure and provides feedback to control the pore water pressure in the sample.

[0043] The temperature control module consists of a bottom low-temperature water bath 18, a top low-temperature water bath 16 and a silicone tube. The low-temperature water bath is connected to the refrigerant supply passage of the sample cap 18 and the sample base 8 for freeze-thawing the sample.

[0044] The dynamic load loading module is composed of a vertical servo motor 17 and a torsional shear servo motor 13, which are used to apply vertical stress and torsional shear stress. The vertical servo motor 17 is connected to the axial pressure rod 2, and transmits the vertical load to the sample through the sample cap 18. During the loading process, the vertical stress is measured by the dynamometer 4 for feedback to control the vertical dynamic load. The torsional shear servo motor 13 is connected to the sample base 8, and transmits the torsional shear load to the sample through the sample base 8. During the loading process, the torsional shear stress is measured by the torque sensor for feedback to control the torsional shear dynamic load. In order to better transmit the torsional shear load and reduce the relative sliding between the sample and the contact surface between the sample cap 18 and the sample base 8, the sample cap 18 and the sample base 8 are both provided with protruding steel sheets in the diameter direction to engage with the sample.

[0045] The sensor and data acquisition module consists of a temperature sensor, a dynamometer, a torque sensor, a pressure gauge, a displacement meter, a data collector, and a host computer. Sensor feedback data is uploaded to the host computer via the data collector, which then issues operational instructions, forming a closed loop and achieving precise control of experimental conditions.

[0046] In another preferred embodiment of the present invention, a method for testing a soil sample using the freeze-thaw-unsaturated dynamic load hollow cylindrical torsional shear apparatus is provided, comprising the following steps:

[0047] S1: Weigh the dry soil sample and place it in a soil pan. Add water to the optimal moisture content. Cover with plastic wrap to prevent water evaporation. Curing at room temperature for 24 hours to balance the internal moisture content.

[0048] S2: Place the cured soil sample into a mold and compact it layer by layer to the specified compaction degree, which is generally 95% for roadbed soil. Take a small amount of the excess soil sample in three groups for drying and measuring the moisture content.

[0049] S3: Transfer the sample to the sample base 8 of the above instrument. If the sample preparation process is carried out on the sample base 8, remove the mold and put the triaxial test rubber film on the inside and outside of the sample.

[0050] S4: Operate the vertical servo motor 17 to slowly lower the sample cap 18 until it just contacts the top of the sample, with the contact pressure not exceeding 10N, and use a rubber ring to seal the rubber membrane to ensure that independent water and air passages are formed inside the sample.

[0051] S5: Water is injected into the pressure chamber through the control valve 14 (confining pressure) at the base of the triaxial pressure chamber 6 until the pressure reaches a level higher than the sample cap 18. After the water injection is completed, the control valve 14 at the base is closed, and the triaxial pressure chamber 7 is pressurized through the exhaust valve 3 on the top cover to form confining pressure. The initial confining pressure is mainly used to maintain the sample shape and generally does not exceed 20kPa.

[0052] S6: Inject water into the sample through the control valve 14 (pore water pressure) at the base, keeping the control valve 14 (pore air pressure) open until water is discharged from the exhaust pipe. Use the double negative pressure method to increase the negative pressure inside the sample to -90 kPa. Continue to saturate the sample for 24 hours, maintaining the effective confining pressure of the sample at 20 kPa. After completion, measure the B value to ensure that it reaches above 0.96.

[0053] S7: After saturation is completed, according to Figure 2 Determine the magnitude of the transverse normal stress and apply confining pressure. Since it is difficult to change the confining pressure quickly, take the maximum or average value of the transverse normal stress and open the control valve 14 (pore water pressure) to perform consolidation and drainage.

[0054] S8: After consolidation is complete, maintain the pore water pressure constant and gradually increase the air pressure inside the specimen to a specified value through control valve 14 (pore air pressure). Perform the axial translation method of unsaturation until the specified unsaturated state is reached. This step can be omitted if a saturated specimen is being tested.

[0055] S9: Control the temperature of the upper and lower surfaces of the specimen by circulating in a low-temperature water bath. The temperature and number of freeze-thaw cycles can be set based on the project site temperature or soil freeze-thaw test standards. In particular, if the specimen is tested at room temperature, this step can be omitted.

[0056] S10: Pass Figure 2 Determine the maximum vertical normal stress and maximum shear stress, and apply compression and shear loads to the specimen. The vertical normal stress and vertical normal strain can be calculated according to the following formula:

[0057] ;

[0058] ;

[0059] Where, σ is the vertical normal stress, F is the vertical axial pressure, r o is the outer radius of the specimen, r i is the inner radius of the specimen, ε is the vertical normal strain, ΔH is the height change of the specimen, and H is the initial height of the specimen.

[0060] The calculation method of shear stress and shear strain is as follows Figure 3 and Figure 4 As shown in the figure, assuming that the shear stress is uniformly distributed and equal everywhere, the shear strain is calculated using the deformation along the circumferential direction at the average radius of the inner and outer circles, and the following formula can be obtained:

[0061] ;

[0062] ;

[0063] Where τ is the shear stress, T is the output torque, γ is the shear strain, and Δθ is the rotation angle.

[0064] S11: During the experiment, the host computer is used to monitor and record data. After the experiment is completed, all sensor data are output and processed.

[0065] The above is a detailed description of an embodiment of the present invention. However, the content described is only a preferred embodiment of the present invention and should not be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A soil testing method based on a freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear apparatus, characterized in that: The following steps are involved: S1: Weigh the dry soil sample and place it in a soil pan. Add water to the optimal moisture content. Cover with plastic wrap to prevent water evaporation. Curing at room temperature for 24 hours to balance the internal moisture content. S2: Place the cured soil sample into the mold and compact it layer by layer to the specified compaction degree. Take a small amount of excess soil sample in three groups for drying and measuring the moisture content. S3: Transfer the sample to the sample base. If the sample preparation process is carried out on the sample base, remove the mold and put the triaxial test rubber film on the inside and outside of the sample; S4: Operate the vertical servo motor to slowly lower the sample cap until it just contacts the top of the sample, with the contact pressure not exceeding 10N. Use a rubber ring to seal the rubber membrane to ensure that independent water and air passages are formed inside the sample. S5: Fill the pressure chamber with water to a level above the specimen cap through the confining pressure control valve on the base of the triaxial pressure chamber. After the water filling is completed, close the control valve on the base and pressurize the triaxial pressure chamber through the exhaust valve on the top cover to form confining pressure. S6: Inject water into the sample through the pore water pressure control valve on the base. Keep the pore air pressure control valve open until water is discharged from the exhaust pipe. Then, use the double negative pressure method to increase the negative pressure inside the sample to -90 kPa. Continue to saturate the sample for 24 hours. S7: After saturation is completed, the magnitude of the transverse normal stress is determined according to the stress cycle change at a certain point inside the roadbed and confining pressure is applied, and the pore water pressure control valve is opened for consolidation and drainage; S8: After consolidation is completed, the pore water pressure is kept constant, and the air pressure inside the sample is gradually increased to the specified value through the pore air pressure control valve, and the unsaturated process of the axis translation method is carried out until the specified unsaturated state is reached; S9: The temperature of the upper and lower surfaces of the sample is controlled by circulating a low-temperature water bath; S10: According to the magnitude of the maximum vertical normal stress and the maximum shear stress, the specimen is loaded with compressive and shear loads; S11: Monitor and record data through the host computer. After the experiment is completed, all sensor data are output and processed.

2. The soil testing method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument according to claim 1 is characterized in that: In S2, for the roadbed soil, the specified compaction degree is 95%.

3. The soil testing method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsional shear instrument according to claim 1 is characterized in that: In the above-mentioned S5, the initial confining pressure is mainly used to maintain the shape of the sample, and the confining pressure does not exceed 20 kPa.

4. The soil testing method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument according to claim 1 is characterized in that: In the above-mentioned S6, the sample is further saturated for 24 hours while maintaining an effective confining pressure of 20 kPa. After completion, the B value is measured to ensure that the compaction degree reaches more than 96%.

5. The soil testing method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument according to claim 1 is characterized in that: In the step S7 , during the application of the confining pressure, the confining pressure is taken as the maximum value or average value of the transverse normal stress.

6. The soil testing method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument according to claim 1 is characterized in that: In the aforementioned S10, the magnitudes of the vertical normal stress and the vertical normal strain can be calculated according to the following formula: ； ； Where, σ is the vertical normal stress, F is the vertical axial pressure, r o is the outer radius of the specimen, r i is the inner radius of the specimen, ε is the vertical normal strain, ΔH is the height change of the specimen, and H is the initial height of the specimen.

7. The soil testing method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument according to claim 1 is characterized in that: In the above S10, it is assumed that the shear stress is uniformly distributed and equal everywhere, and the shear strain is calculated using the deformation along the circumferential direction at the average radius of the inner and outer circles, and the following formula can be obtained: ； ； Where τ is the shear stress, T is the output torque, γ is the shear strain, and Δθ is the rotation angle.

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