Soil detection method based on freeze thawing-unsaturated dynamic load hollow cylinder torsional shear apparatus

Abstract

The invention relates to the technical field of soil detection, and particularly discloses a soil detection method based on a freeze thawing-unsaturated dynamic load hollow cylinder torsional shear apparatus, which comprises the following steps: respectively applying a vertical dynamic load, a torsional shear dynamic load and confining pressure through a vertical servo motor, a torsional shear servo motor and a triaxial pressure chamber, and simulating the stress state of a roadbed under the action of a dynamic wheel load; the unsaturated state of the sample is controlled through an axis translation method for controlling pore air pressure and pore water pressure, and the hydraulic state of the roadbed in the actual situation is simulated; the temperature of the sample is further controlled through a low-temperature water bath connected with the upper bottom surface and the lower bottom surface of the sample, and the temperature gradient in the roadbed freezing and thawing process is simulated; therefore, the instrument can introduce the influence of freezing and thawing and unsaturation while simulating the traffic dynamic load, can more accurately reflect the mechanical response of the cold region roadbed under the coupling action of freezing and thawing-unsaturation-traffic load, and provides a more accurate and more scientific basis for the performance evaluation of the cold region roadbed.

Description

Technical Field

[0001] The 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 periodic axial load generated by vehicles has a significant impact on the deformation characteristics of the roadbed and base materials, and is an important factor causing rutting damage to road structures. On the other hand, with the movement of the wheel load, for a fixed point on the top surface of the roadbed, the magnitude of the normal stress and shear stress in the vertical direction changes continuously, which in turn causes the direction of the principal stress to change continuously. This phenomenon is the principal stress axis rotation (PSAR). Studies have shown that the principal stress axis rotation will amplify the vertical permanent strain of the roadbed, so its influence cannot be ignored in road design. In unit tests, the principal stress axis rotation phenomenon of the roadbed can usually be simulated by a hollow cylindrical torsion shear apparatus, and the vertical permanent strain under its influence can be studied.

[0003] The core principle of the hollow cylinder torsion shear apparatus is to simulate the mechanical response of soil or rock under different stress states by applying torsion (shear) and axial pressure to the cylindrical sample. 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 torsion 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 areas is also affected by freeze-thaw.

[0004] Therefore, for the influence of the rotation of the principal stress axis 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 torsion 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 torsion shear instrument to solve the above technical problems.

[0006] The purpose of the present invention can be achieved through the following technical solutions: The soil detection method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument includes the following steps: S1: Weigh the dry soil sample and put it into a soil pan and add water to the optimal moisture content. Cover with plastic wrap to prevent water evaporation and maintain at room temperature for 24 hours to balance the internal moisture content. S2: Place the cured soil sample into the mold and compact it in layers to the specified compaction degree. Take a small amount of the 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, and use a rubber ring to seal the rubber membrane to ensure that independent water and air passages are formed inside the sample; S5: Fill water into the pressure chamber through the confining pressure control valve of the base of the triaxial pressure chamber until it is higher than the specimen cap. After the water filling is completed, close the control valve of the base, and pressurize the triaxial pressure chamber through the exhaust valve of 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, and pause 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, and continue to further 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 the 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 is gradually increased to the specified value inside the sample 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 bottom 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 sample is loaded with compressive shear loads; S11: Monitor and record data through the host computer. After the experiment is completed, all sensor data are output and processed.

[0007] As a further solution of the present invention: in S2, for the roadbed soil, the specified compaction degree is 95%.

[0008] 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.

[0009] As a further solution of the present invention: in the above S6, the effective confining pressure of the sample is maintained at 20 kPa during the further saturation of the sample for 24 hours, and the B value is measured after completion to ensure that the compaction degree reaches more than 96%.

[0010] As a further solution of the present invention: in S7, during the process of applying the confining pressure, the confining pressure takes the maximum value or average value of the transverse normal stress.

[0011] 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: ; ; Among them, 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.

[0012] 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: ; ; Among them, τ is the shear stress, T is the output torque, γ is the shear strain, and Δθ is the rotation angle.

[0013] The beneficial effects of the present invention are as follows: simulating the stress state of the roadbed under the action of the dynamic wheel load; controlling the unsaturated state of the sample by the axis translation method of controlling the pore air pressure and the pore water pressure, simulating the hydraulic state of the roadbed in the actual situation; 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 coupling action of freeze-thaw-unsaturated-traffic load, providing a more accurate and scientific basis for the performance evaluation of the cold region roadbed. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will be further described below in conjunction with the accompanying drawings.

[0015] Figure 1 It is a structural schematic diagram of the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear apparatus of the present invention; Figure 2 It is a schematic diagram of the stress cycle change of a certain point inside the roadbed obtained by calculating the actual moving wheel load of the present invention; Figure 3 Schematic diagram of shear stress distribution of the sample of the present invention; Figure 4 Schematic diagram of shear strain of the sample of the present invention.

[0016] 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

[0017] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0018] like Figure 1 As shown, the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear apparatus comprises: 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.

[0019] 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.

[0020] 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; 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; The unsaturated control module is composed of a sample cap 18 and a sample base 8, and is used to control the matrix suction in the sample using the axis translation method to achieve a specific unsaturated state. The sample cap 18 is connected to the bottom control valve 14 of the triaxial pressure chamber 7 through a pipeline, and the pore pressure value is measured by the pore pressure gauge 9 for feedback to control the pore pressure in the sample. The sample base 8 is embedded with a high air intake value clay plate to ensure that only water but not air is permeable during the pressurization process. The lower part is connected to the bottom control valve 14 of the triaxial pressure chamber 7, and the pore water pressure value is measured by the pore water pressure gauge 10 for feedback to control the pore water pressure in the sample.

[0021] The temperature control module is composed 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.

[0022] 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.

[0023] 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. The sensor feedback data is uploaded to the host computer through the data collector, and the host computer issues operation instructions to form a closed loop to achieve the purpose of accurately controlling the experimental conditions.

[0024] In another preferred embodiment of the present invention, a method for testing a soil sample using the above-mentioned freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear apparatus is provided, which comprises the following steps: S1: Weigh the dry soil sample and put it into a soil pan and add water to the optimal moisture content. Cover with plastic wrap to prevent water evaporation and cure at room temperature for 24 hours to balance the internal moisture content.

[0025] S2: Place the cured soil sample into a mold and compact it layer by layer to a 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.

[0026] S3: The sample is transferred to the sample base 8 of the above-mentioned instrument. If the sample preparation process is carried out on the sample base 8, the mold is removed and the rubber film for triaxial test is put on the inner and outer sides of the sample.

[0027] 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.

[0028] S5: Water is injected into the pressure chamber through the control valve 14 (confining pressure) at the base of the triaxial pressure chamber 6 to 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 a confining pressure. The initial confining pressure is mainly used to maintain the sample shape and generally does not exceed 20 kPa.

[0029] S6: Inject water into the sample through the control valve 14 (pore water pressure) at the base, and keep 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, and continue to further saturate the sample for 24 hours, during which the effective confining pressure of the sample is maintained at 20 kPa. After completion, measure the B value to ensure that it reaches above 0.96.

[0030] 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.

[0031] S8: After consolidation is completed, the pore water pressure is kept constant, and the air pressure is gradually increased to the specified value inside the sample through the control valve 14 (pore air pressure), and the unsaturated process of the axis translation method is carried out until the specified unsaturated state is reached. In particular, if the experiment of the saturated sample is carried out, this step can be omitted.

[0032] S9: The temperature of the upper and lower bottom surfaces of the sample is controlled by circulating in a low-temperature water bath. The temperature setting and the number of freeze-thaw cycles can be set according to the temperature of the engineering site or the freeze-thaw test standard of the soil. In particular, if the sample is tested at room temperature, this step can be omitted.

[0033] S10: Pass Figure 2 Determine the maximum vertical normal stress and maximum shear stress, and apply compression and shear dynamic loads to the specimen. The vertical normal stress and vertical normal strain can be calculated according to the following formula: ; ; Among them, 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.

[0034] The calculation method of shear stress and shear strain is as follows: Figure 3 and Figure 4 As shown, 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: ; ; Among them, τ is the shear stress, T is the output torque, γ is the shear strain, and Δθ is the rotation angle.

[0035] 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.

[0036] The above is a detailed description of an embodiment of the present invention, but the content is only a preferred embodiment of the present invention and cannot be considered to limit the scope of implementation 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 instrument, characterized in that: The following steps are involved: S1: Weigh the dry soil sample and put it into a soil pan and add water to the optimal moisture content. Cover with plastic wrap to prevent water evaporation and maintain at room temperature for 24 hours to balance the internal moisture content. S2: Place the cured soil sample into the mold and compact it in layers to the specified compaction degree. Take a small amount of the 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, and use a rubber ring to seal the rubber membrane to ensure that independent water and air passages are formed inside the sample; S5: Fill water into the pressure chamber through the confining pressure control valve of the base of the triaxial pressure chamber until it is higher than the specimen cap. After the water filling is completed, close the control valve of the base, and pressurize the triaxial pressure chamber through the exhaust valve of 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, and pause 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, and continue to further 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 the 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 is gradually increased to the specified value inside the sample 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 bottom 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 sample is loaded with compressive 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 detection 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 detection method based on the freeze-thaw-unsaturated dynamic load hollow cylindrical torsion shear instrument according to claim 1 is characterized in that: 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.

4. The soil detection 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 effective confining pressure of the sample is maintained at 20 kPa during the further saturation of the sample for 24 hours, and the B value is measured after completion to ensure that the compaction degree reaches more than 96%.

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

6. The soil detection 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, the magnitudes of the vertical normal stress and the vertical normal strain can be calculated according to the following formula: ； ； Among them, 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 detection 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: ； ； Among them, 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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