CN112504862B - Measurement control method for controlling substrate suction and temperature by adopting dynamic and static triaxial apparatus - Google Patents

Measurement control method for controlling substrate suction and temperature by adopting dynamic and static triaxial apparatus Download PDF

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CN112504862B
CN112504862B CN202011242973.8A CN202011242973A CN112504862B CN 112504862 B CN112504862 B CN 112504862B CN 202011242973 A CN202011242973 A CN 202011242973A CN 112504862 B CN112504862 B CN 112504862B
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temperature
air
test piece
suction
pressure
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CN112504862A (en
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肖源杰
李文奇
王萌
于群丁
王小明
陈晓斌
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Central South University
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    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
    • G01N3/18—Performing tests at high or low temperatures
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0001—Type of application of the stress
    • G01N2203/0003—Steady
    • G—PHYSICS
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    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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    • G01N2203/0019—Compressive
    • G—PHYSICS
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    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/003—Generation of the force
    • G01N2203/0042—Pneumatic or hydraulic means
    • G01N2203/0044—Pneumatic means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
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    • G01N2203/0075—Strain-stress relations or elastic constants
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    • G01N2203/022—Environment of the test
    • G01N2203/0222—Temperature
    • G01N2203/0226—High temperature; Heating means
    • G—PHYSICS
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    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
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    • G01N2203/067—Parameter measured for estimating the property
    • G01N2203/0676—Force, weight, load, energy, speed or acceleration
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    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02—Details not specific for a particular testing method
    • G01N2203/06—Indicating or recording means; Sensing means
    • G01N2203/067—Parameter measured for estimating the property
    • G01N2203/0682—Spatial dimension, e.g. length, area, angle

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Abstract

A measurement control method for controlling the substrate suction and the temperature by adopting a dynamic and static triaxial apparatus comprises a substrate suction control method, a temperature control method and a local strain measurement method; the sound triaxial apparatus includes: the surface of the permeable stone is rough, the air inlet value is low, and the porous air pressure of the test piece is controlled by installing the permeable stone on the top of the test piece; the high air intake value ceramic disc is arranged at the bottom of the pressure chamber, is subjected to water permeation saturation, allows water to enter the test piece but prevents free air from circulating, and further controls the pore water pressure of the test piece; the heating system consists of a constant temperature regulator, a heater and a thermocouple; the constant temperature regulator is used for controlling and regulating the temperature in the whole pressure chamber through the temperature fed back by the thermocouple; the heaters are arranged on the left side and the right side in the pressure chamber, are controlled by the constant temperature regulator and are used for heating the temperature in the pressure chamber. The invention provides equipment support for researching the mechanism of the suction force and temperature of the matrix in the cyclic loading coupling action, and improves the accuracy and reliability of prediction for establishing a model for predicting accumulated plastic strain and rebound modulus.

Description

Measurement control method for controlling substrate suction and temperature by adopting dynamic and static triaxial apparatus
Technical Field
The disclosure relates to the technical field of intelligent equipment, in particular to a measurement control method for controlling substrate suction and temperature by adopting a dynamic and static triaxial apparatus.
Background
A large number of theoretical and experimental researches at home and abroad show that the unsaturated soil behavior is highly dependent on suction and temperature, but most of the researches concern the unsaturated soil behavior under the action of monotonic load, and at present, researches in relevant documents at home and abroad concern the influence of the suction and temperature coupling action on the unsaturated soil repeated load behavior (including rebound modulus and plastic strain accumulation), which is greatly limited by the lack of large-scale dynamic triaxial test equipment capable of controlling the suction and the temperature simultaneously. The coupling mechanism of stress (super-consolidation ratio), substrate suction and temperature under cyclic loading is not completely mastered; most of the currently-developed test researches are concentrated on silt and clay, and no research is paid attention to the influence of suction and temperature coupling action on repeated loading behaviors (including modulus of resilience and plastic strain accumulation) of unsaturated coarse-grained soil roadbed filling materials at home and abroad, and the research is limited by test equipment capable of independently controlling suction and temperature.
In the prior art, a triaxial device capable of applying dynamic bias stress in the vertical direction and the lateral direction is adopted to evaluate the permanent deformation characteristic of a coarse-grained soil material under the action of a moving vehicle load and establish a corresponding accumulated plastic deformation prediction model, so that higher accumulated plastic volume strain and shear strain can be generated under the action of the moving load, and therefore, the conventional dynamic triaxial test can underestimate the accumulation of plastic deformation and cause unsafe design. Foreign scholars also quantitatively research the influence of the rotation of the main stress axis on the accumulated plastic deformation of the foundation soil by adopting an indoor test, and the result shows that the plastic deformation is accumulated at a faster rate due to the rotation of the main stress axis, and the conventional dynamic triaxial test method for evaluating the permanent deformation characteristic of the coarse-grained soil underestimates the accumulation rate. The traditional dynamic triaxial test cannot accurately simulate the dynamic stress state and stress change path borne by soil body units in the roadbed under the action of repeated moving load, so that the resilience modulus of coarse-grained soil roadbed filling is overestimated and the accumulated plastic deformation of the coarse-grained soil roadbed filling is underestimated, the design of the high-speed railway track roadbed which is not safe enough is easily caused, and the importance of accurately mastering the dynamic response rule of the roadbed is further proved.
The prior art, as disclosed in Chinese patent: CN104964878A discloses a triaxial test system and method for unsaturated soil multi-field coupling, and specifically discloses the following technical features (refer to paragraph 0072-: the test system comprises a triaxial pressure chamber 1, a confining pressure applying/physical deformation monitoring unit 2, an axial force applying unit 3, a substrate suction applying unit 4, a temperature control unit 5, a chemical solution circulating and permeating unit 6, an axial displacement measuring unit 7 and a data acquisition unit 8; the matrix suction applying unit 4 is connected with the triaxial pressure chamber 1, applies set matrix suction to the soil sample 0, records the flow of a suction or discharge solution in the soil sample, and simultaneously judges whether the soil sample corresponding to the level of matrix suction reaches a balanced state; the triaxial pressure chamber 1 comprises a pressure chamber outer cover 1.1, an exhaust channel 1.2, a confining pressure pore passage 1.3, a chemical solution pore passage 1.4, a clay plate 1.5, a base 1.6, a pore air pore passage 1.7, a pore water pore passage 1.8 and a porous plate 1.9; the connection relation is as follows: the pottery clay plate 1.5 is placed in the middle of the base 1.6, the soil sample 0 is placed on the upper part of the pottery clay plate 1.5, the exhaust channel 1.2 is arranged on the upper part of the pressure chamber outer cover 1.1 and used for exhausting air in the pressure chamber, the pore water pore channel 1.8 is arranged on the center of the base 1.6, and the pore water pore channel 1.7 is arranged on the edge of the base 1.6 and connected with the porous plate 1.9 on the upper part of the soil sample 0 and used for applying air pressure to the soil sample 0 under the pottery clay plate 1.5; the temperature control unit 5 comprises a resistance wire 5.1, a low-temperature constant-temperature cold bath 5.2, a cold bath liquid circulating copper pipe 5.3 and a temperature controller 5.4; the connection relation is as follows: a resistance wire 5.1 and a cold bath liquid circulating copper pipe 5.3 are respectively arranged on the inner wall of the outer cover 1.1 of the triaxial pressure chamber, a low-temperature constant-temperature cold bath 5.2 is connected with the cold bath liquid circulating copper pipe 5.3, the resistance wire 5.1 and the low-temperature constant-temperature cold bath 5.2 are respectively connected with a temperature controller 5.4 to control the temperature; the temperature controller is connected with the data acquisition unit 8 through a signal transmission line and is used for acquiring the temperature of the soil sample 0 in the test process; the function of the temperature control unit 5 is: setting a required temperature and keeping the temperature constant; the working principle of the temperature control unit 5 is as follows: the temperature control unit 5 is used for controlling the temperature of the water in the triaxial pressure chamber 1, so that the temperature control of the soil sample is realized; the temperature to be reached is set through a temperature controller, if the temperature of the water is lower than the set value, the water is fed back to a built-in resistance wire 5.1 to heat the water in the triaxial pressure chamber 1 (which is equivalent to the temperature of a constant temperature regulator of the application which is used for controlling and regulating the temperature in the whole pressure chamber through feeding back the temperature), and if the temperature of the water is higher than the set value, the water is fed back to a low-temperature constant-temperature cold bath 5.2 to be cooled, and the temperature is kept constant; and the test determination of the multi-field coupling characteristic of the soil sample 0 is realized.
Chinese patent publication No.: CN107607374A discloses a hollow cylinder test system suitable for unsaturated soil, and specifically discloses the following technical characteristics (refer to paragraph 0020 and 0031 and FIG. 1 in the specification): a local outer wall displacement sensor (13) and an LVDT displacement sensor are arranged at the downward 1/4 position in the middle of the inner cavity of the hollow cylindrical pressure chamber, and the radial displacement and the axial displacement change of the outer wall of the test piece can be tested; the inner wall of the sample is provided with a Hall effect sensor and an LVDT displacement sensor, and the radial displacement and the axial displacement change of the inner wall of the sample can be tested; the stress state analysis of the test can be carried out according to the radial displacement and the axial displacement change of the inner wall and the outer wall of the test; the top of the sample is connected with a permeable stone (20), the upper part of the permeable stone (20) is connected with a sample cap (34), pore air pressure is connected with a reserved pore passage on the sample cap (34), air applied by an air pressure controller is connected with pore air in the sample, pore air pressure in the soil sample and air volume change in the whole sample system are measured and controlled, so that the matrix suction of the sample is controlled, and an annular high air admission value argil plate (HAEPD) is inlaid in a base of a heart cylinder pressure chamber, so that water in the hollow cylinder soil sample can pass through but applied air cannot pass through.
However, the above prior art cannot establish a prediction model for the roadbed strength, the rebound modulus and the accumulated plastic deformation by using the test data measured by the novel triaxial apparatus capable of independently controlling the matrix suction and the temperature, and can be used for revealing the action mechanism of the matrix suction and the temperature change in the unsaturated roadbed packing layer on the roadbed strength, the rebound modulus and the accumulated plastic deformation and exploring the feasibility of improving the prediction accuracy by introducing the matrix suction as a stress state parameter into the rebound modulus and the accumulated plastic deformation prediction model.
Disclosure of Invention
In view of the above, the embodiments of the present disclosure provide a measurement control method for controlling a matrix suction force and a temperature by using a dynamic and static triaxial apparatus, and a prediction model for roadbed strength, rebound modulus and accumulated plastic deformation is established by using test data measured by the novel triaxial apparatus capable of independently controlling the matrix suction force and the temperature, so as to reveal the mechanism of action of matrix suction force and temperature change in an unsaturated roadbed packing layer on roadbed strength, rebound modulus and accumulated plastic deformation, and explore feasibility of improving prediction accuracy by introducing the matrix suction force as a stress state parameter into the rebound modulus and accumulated plastic deformation prediction model.
The embodiment of the disclosure provides a measurement control method for controlling substrate suction and temperature by adopting a dynamic and static triaxial apparatus, which comprises a substrate suction control method, a temperature control method and a local strain measurement method; it is characterized in that: the matrix suction control method specifically comprises the following steps: controlling the substrate suction, namely the difference between pore air pressure and pore water pressure, by adopting an axis translation technology, and controlling the substrate suction of the soil test piece by respectively and independently controlling the pore air pressure and the pore water pressure; wherein the pore air pressure is controlled by a rough permeable stone with low air inlet value arranged on the top of the test piece, the permeable stone has rough surface and low air inlet value, the pore air pressure of the test piece is controlled by installing the permeable stone on the top of the test piece, and the pore water pressure is controlled by a saturated ceramic disk with high air inlet value sealed on a pressure chamber base of the triaxial apparatus; the saturated, high air intake ceramic disk allows the passage of moisture but prevents the passage of free air as long as the substrate suction is below its air intake; however, the air dissolved in the water passes through the ceramic pan and accumulates in the bottom of the ceramic pan or in the drainage system; in the test, any accumulated bubbles are flushed out and collected by adopting a diffused air quantity indicator every 24 hours, and the collected air quantity is used for correcting the measured water content change of the soil body;
the temperature control method comprises the following steps: applying a temperature load through a heating system consisting of a thermostat, a heater, and a thermocouple; wherein the heater and the thermocouple are arranged in a pressure chamber of the triaxial apparatus, and both are connected with the constant temperature regulator to form a closed loop control and feedback system; in the test process, the constant temperature controller adjusts the output of the heater through the feedback result of the thermocouple; when the energy dissipation is balanced with the output energy of the heater, the air temperature in the dynamic and static triaxial apparatus pressure chamber is balanced, and the assumed soil body temperature is consistent with the air temperature in the dynamic and static triaxial apparatus pressure chamber in a balanced state; in order to improve the uniformity of the air temperature in the dynamic and static triaxial apparatus pressure chamber, two small fans and a hollow cylindrical aluminum sheet are arranged in the dynamic and static triaxial apparatus pressure chamber, the fans can improve the air circulation, and the cylindrical aluminum sheet improves the heat transfer due to the high thermal conductivity of the cylindrical aluminum sheet; in order to test the uniformity of temperature, two thermocouples are arranged at different positions in the pressure chamber, the thermocouples measure the air temperature at a position 5 mm away from a test piece, and when thermal equilibrium is reached, the temperature readings of the two thermocouples are approximately the same and are kept unchanged;
the local strain measurement method comprises the following steps: the method comprises the following steps of (1) externally measuring axial strain by using a linear variable differential transformer, measuring local soil deformation of each test piece at the middle height and measuring pore water pressure by using a radial sensor, monitoring the pore water pressure at the middle height of the test piece in the repeated loading and unloading process by using a suction gauge, and measuring the pore water pressure at the bottom by using a pore water pressure sensor;
when the test material is manually compacted to the maximum dry density near the optimal water content, the compacted test piece is saturated by using water with de-aerated air; the substrate suction at which water begins to drain is defined as the inlet air pressure value; the low air inlet pressure value indicates that the base material is in an unsaturated state in the field, and before testing, the test piece is saturated to discharge the residual matrix suction force caused by compaction in the optimal water content state; the water with air removed is added into the sample through a ceramic plate with high flow and high air inlet value, which is arranged on the bottom plate of the sample; after the sample is saturated, applying suction force by an air suction device in a dehumidification process to achieve a target substrate suction value; reasonably determining the substrate suction level of each compacted test piece according to the collected field test data; during the substrate suction holding process, the confining pressure is maintained at a target set value, and the target substrate suction is confirmed by checking the equilibrium volume of the effluent water; each experiment included three phases: suction balance, temperature balance and repeated loading-unloading; in the suction range of 0-60kPa, the suction balance takes 2-3 weeks; depending on the suction and temperature conditions, temperature equilibration takes 6-10 days; TDR moisture sensors are arranged at different heights of the cylindrical test piece, so that the change of the substrate suction force is monitored in real time, the distribution form of the substrate suction force along with the depth is determined, and the time point of the substrate suction force balance is determined in real time. .
The scheme in the embodiment of the disclosure comprises a permeable stone, wherein the permeable stone has a rough surface and a low air inlet value, and the porous air pressure of a test piece is controlled by installing the permeable stone on the top of the test piece; the high air intake value ceramic disc is arranged at the bottom of the pressure chamber, is subjected to water permeation saturation, allows water to enter the test piece but prevents free air from flowing, and further controls the pore water pressure of the test piece; the heating system consists of a constant temperature regulator, a heater and a thermocouple; the constant temperature regulator is used for controlling and regulating the temperature in the whole pressure chamber through the temperature fed back by the thermocouple; the heaters are arranged on the left side and the right side in the pressure chamber, are controlled by the constant temperature regulator and are used for heating the temperature in the pressure chamber; the thermocouple comprises a thermocouple A and a thermocouple B, the thermocouple A is used for measuring the temperature of the upper part of the coarse-grained soil test piece, detecting the uniformity of the temperature and feeding back to the constant temperature regulator, and the thermocouple B is used for measuring the temperature of the lower part of the coarse-grained soil test piece, detecting the uniformity of the temperature and feeding back to the constant temperature regulator; the heater and the thermocouple are arranged in the pressure chamber of the triaxial apparatus and are connected with the constant temperature regulator to form a closed-loop control and feedback system, the constant temperature controller regulates the output of the heater according to the feedback result of the thermocouple, and when the energy dissipation is balanced with the output energy of the heater, the air temperature in the pressure chamber of the triaxial apparatus is balanced. Through the processing scheme disclosed by the invention, for unsaturated soil tests, the pore water pressure at the middle height of a soil sample can be tested, and the special suction gauge can be used for measuring the negative pore water pressure which reaches 480kPa at most and is close to the air inlet value of a ceramic tip. The average initial suction after the test piece is compacted is measured by adopting a high-load suction meter, so that the variation situation of the measured initial suction along the height of each test piece and the difference situation among different test pieces are tested, the initial suction is ensured to be controlled within 2kPa, and the accurate control of the suction of the matrix can be realized. Calibration was performed at each given temperature using a temperature controlled oven, with calibration performed under both load and unload conditions, taking into account the exposure of the strain sensor and the suction gauge to the elevated temperature environment. The calibration result shows that the influence of the temperature effect on the sensitivity, zero offset, hysteresis and the like can be almost ignored, and the temperature insensitivity of the sensors ensures that the accurate measurement of the pore water pressure and the soil deformation can be carried out under the action of temperature load.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the drawings needed to be used in the embodiments will be briefly described below, and it is apparent that the drawings in the following description are only some embodiments of the present disclosure, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic structural view of a dynamic-static triaxial apparatus for controlling the suction force and temperature of a matrix according to an embodiment of the present disclosure.
1. A loading element; an LVDT; 3. a thermostat; 4. a thermocouple A; 5. a heater; 6. a matrix suction gauge; 7. a high air intake value ceramic disc; 8. confining pressure (compressed air); 9. pore water pressure; 10. fans (accelerating air flow); 11. a hollow aluminum cylinder; 12. a permeable stone; 13. coarse-grained soil test pieces; 14. a radial sensor; 15. an axial sensor; 16. a thermocouple B; 17. pore gas pressure; 18. air charging system
Detailed Description
The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.
The embodiments of the present disclosure are described below with specific examples, and other advantages and effects of the present disclosure will be readily apparent to those skilled in the art from the disclosure in the specification. It is to be understood that the described embodiments are merely illustrative of some, and not restrictive, of the embodiments of the disclosure. The disclosure may be embodied or carried out in various other specific embodiments, and various modifications and changes may be made in the details within the description without departing from the spirit of the disclosure. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict. All other embodiments, which can be derived by a person skilled in the art from the embodiments disclosed herein without making any creative effort, shall fall within the protection scope of the present disclosure.
Referring to fig. 1, the dynamic and static triaxial apparatus for controlling the suction force and temperature of a substrate in the embodiment of the present disclosure includes: the ceramic disc comprises a permeable stone, a high air inlet value ceramic disc and a heating system.
The surface of the permeable stone is rough, the air inlet value is low, and the porous air pressure of the test piece is controlled by installing the permeable stone on the top of the test piece;
the high air intake value ceramic disc is arranged at the bottom of the pressure chamber, is subjected to water permeation saturation, allows water to enter the test piece but prevents free air from flowing, and further controls the pore water pressure of the test piece;
the heating system consists of a constant temperature regulator, a heater and a thermocouple;
the constant temperature regulator is used for controlling and regulating the temperature in the whole pressure chamber through the temperature fed back by the thermocouple;
the heaters are arranged on the left side and the right side in the pressure chamber, are controlled by the constant temperature regulator and are used for heating the temperature in the pressure chamber;
the thermocouple comprises a thermocouple A and a thermocouple B, the thermocouple A is used for measuring the temperature of the upper part of the coarse-grained soil test piece, detecting the uniformity of the temperature and feeding back to the constant temperature regulator, and the thermocouple B is used for measuring the temperature of the lower part of the coarse-grained soil test piece, detecting the uniformity of the temperature and feeding back to the constant temperature regulator;
the heater and the thermocouple are arranged in the pressure chamber of the triaxial apparatus and are connected with the constant temperature regulator to form a closed-loop control and feedback system, the constant temperature controller regulates the output of the heater according to the feedback result of the thermocouple, and when the energy dissipation is balanced with the output energy of the heater, the air temperature in the pressure chamber of the triaxial apparatus is balanced.
Through the scheme, the suction force and the temperature of the matrix can be directly controlled, the two parameters can be directly used as test variables as control parameters, conversion is not needed through other indexes, the factors of stress, the suction force of the matrix and the temperature can be directly and accurately considered at the same time, and the influence mechanism of the stress, the suction force of the matrix and the temperature on various mechanical characteristic parameters of the coarse-grained soil roadbed material can be analyzed. And the matrix suction and the temperature are taken as parameters of the stress state and introduced into a prediction model for predicting the rebound modulus and the accumulated plastic deformation, so that the accuracy and the reliability of the prediction are improved.
Matrix suction control scheme
The matrix suction (namely the difference between pore air pressure and pore water pressure) is controlled by adopting an axis translation technology, and the pore air pressure and the pore water pressure are respectively and independently controlled to control the matrix suction of the soil test piece. The pore air pressure is controlled by a coarse, low air inlet value permeable stone placed on the top of the test piece, and the pore water pressure is controlled by a saturated, high air inlet value ceramic disk sealed to the base of the pressure chamber of the triaxial apparatus. Saturated ceramic disks allow moisture to pass through but prevent free air from passing through as long as the substrate suction is below its inlet value; however, air dissolved in the water may pass through the ceramic pan and accumulate at the bottom of the ceramic pan or in the drainage system. In the test, any accumulated air bubbles were flushed out and collected every 24 hours using a diffused air volume indicator, and the amount of air collected was used to correct for the measured changes in soil moisture content.
Temperature control scheme
The new tri-axial device applies a temperature load by adding a heating system consisting of a thermostat, heater, and thermocouple. The heater and thermocouple are disposed in a pressure chamber of the triaxial apparatus, both of which are connected to a thermostat to form a closed loop control and feedback system. During the test, the thermostat controller adjusts the output of the heater through the feedback result of the thermocouple. When the energy dissipation is balanced with the output energy of the heater, the air temperature in the pressure chamber of the triaxial apparatus is balanced, and the soil temperature can be reasonably assumed to be consistent with the air temperature in the pressure chamber of the triaxial apparatus in a balanced state. Depending on the set target temperature value, it takes about several hours to reach a thermal equilibrium state. To improve the uniformity of the air temperature in the triaxial cell, two small fans are placed in the triaxial cell, which improve the air circulation, and a hollow cylindrical aluminum sheet, which improves the heat transfer due to its high thermal conductivity. To verify the temperature uniformity, two thermocouples were also installed at different locations inside the pressure chamber, measuring the air temperature at approximately 5 mm from the test piece, and when thermal equilibrium was reached, the temperature readings of the two thermocouples were approximately the same and remained essentially unchanged (maximum fluctuation 0.5 ℃).
Local strain measurement scheme
In addition to the conventional external measurement of axial strain using an LVDT (Linear Variable Differential Transformer), the new triaxial apparatus is equipped with a radial sensor to measure the local soil deformation of each test piece at mid-height. The local sensitivity of the radial sensor can also be used to measure the pore water pressure, the suction gauge is used to monitor the pore water pressure at the middle height of the test piece during the repeated loading and unloading process, and the traditional pore water pressure sensor is still used to measure the pore water pressure at the bottom.
In a specific application process, the loading element: the device is used for applying axial load, and static load and dynamic load can be applied respectively according to the test requirements.
And (3) LVDT: and the linear variable differential transformer is used for measuring the axial strain outside the test piece.
A thermostatic regulator: the temperature fed back by the thermocouple is used to control and regulate the temperature throughout the interior of the pressure chamber.
And (3) thermocouple A: the temperature measuring device is used for measuring the temperature of the upper part of the coarse-grained soil test piece, detecting the uniformity of the temperature and feeding back to the constant temperature regulator.
A heater: the left side and the right side in the pressure chamber are both provided with heaters which are controlled by a thermostatic regulator and used for heating the temperature in the pressure chamber.
And the matrix suction meter is used for measuring the matrix suction at the middle height position of the test piece.
High air intake ceramic disk: by being arranged at the bottom of the pressure chamber, the water-permeable saturation is carried out, water is allowed to enter the test piece, but free air circulation is prevented, and further, the pore water pressure of the test piece is controlled.
Confining pressure: water is introduced into the interior of the pressure chamber by means of compressed air, and is used to apply different confining pressures to the test piece depending on the requirements of the test.
Pore water pressure: the water is introduced by connecting the high-air-intake ceramic disc, so that the pore water pressure is provided for the test piece, and the pore water pressure at the bottom is measured by the pore water pressure sensor arranged at the bottom.
A fan: two fans are arranged at the top and the bottom of the pressure chamber, and are used for increasing the air flow speed in the pressure chamber, improving the air circulation and keeping the temperature in the pressure chamber uniform.
Hollow aluminum cylinder: used for increasing the temperature conduction rate in the pressure chamber and improving the heat conduction.
Permeable stone: the permeable stone has rough surface and low air inlet value, and the pore air pressure of the test piece is controlled by being arranged at the top of the test piece.
Coarse-grained soil test piece: is used for placing coarse-grained soil samples required to be tested by the test.
A radial sensor: the device is used for measuring the deformation of the coarse-grained soil test piece in the diameter direction of the test piece at the middle position.
Axial sensor: the method is used for measuring the strain of the coarse-grained soil test piece along the height direction of the test piece.
And (3) thermocouple B: the temperature measuring device is used for measuring the temperature of the lower part of a coarse-grained soil sample, detecting the uniformity of the temperature and feeding back to the constant temperature regulator.
Pore gas pressure: the porous air pressure is provided according to the test requirement by connecting the permeable stones arranged on the top of the test piece.
An inflation system: the air quantity collected is used for correcting the change of the measured soil moisture content.
The above-mentioned components and parts cooperation process forms a plurality of subsystems, includes:
suction control system (related components: 6, 7, 9, 12, 17, 18)
The matrix suction (namely the difference between pore air pressure and pore water pressure) is controlled by adopting an axis translation technology, and the pore air pressure and the pore water pressure are respectively and independently controlled to control the matrix suction of the soil test piece. The pore air pressure is controlled by a coarse, low air inlet value permeable stone placed on the top of the test piece, and the pore water pressure is controlled by a saturated, high air inlet value ceramic disk sealed to the base of the pressure chamber of the triaxial apparatus. Saturated ceramic disks allow moisture to pass through but prevent free air from passing through as long as the substrate suction is below its inlet value; however, air dissolved in the water may pass through the ceramic pan and accumulate at the bottom of the ceramic pan or in the drainage system. In the test, any accumulated air bubbles were flushed out and collected every 24 hours using a diffused air volume indicator, and the amount of air collected was used to correct for the measured changes in soil moisture content.
Temperature control system (related components: 3, 4, 5, 10, 11, 16)
The new tri-axial device applies a temperature load by adding a heating system consisting of a thermostat, heater, and thermocouple. The heater and thermocouple are disposed in a pressure chamber of the triaxial apparatus, both of which are connected to a thermostat to form a closed loop control and feedback system. During the test, the thermostat controller adjusts the output of the heater through the feedback result of the thermocouple. When the energy dissipation is balanced with the output energy of the heater, the air temperature in the pressure chamber of the triaxial apparatus is balanced, and the soil temperature can be reasonably assumed to be consistent with the air temperature in the pressure chamber of the triaxial apparatus in a balanced state. Depending on the set target temperature value, it takes about several hours to reach a thermal equilibrium state. To improve the uniformity of the air temperature in the triaxial cell, two small fans are placed in the triaxial cell, which improve the air circulation, and a hollow cylindrical aluminum sheet, which improves the heat transfer due to its high thermal conductivity. To verify the temperature uniformity, two thermocouples were also installed at different locations inside the pressure chamber, measuring the air temperature at approximately 5 mm from the test piece, and when thermal equilibrium was reached, the temperature readings of the two thermocouples were approximately the same and remained essentially unchanged (maximum fluctuation 0.5 ℃).
Mechanical testing system (related components: 1, 2, 6, 8, 9, 14, 15, 17)
In addition to the conventional external measurement of axial strain using LVDT, the new triaxial apparatus is equipped with a radial sensor to measure the local soil deformation of each test piece at mid-height. The suction gauge is used to monitor the pore water pressure at the mid-height of the test piece during repeated loading and unloading, and conventional pore water pressure sensors are still used to measure the pore water pressure at the bottom.
The test material was manually compacted to maximum dry density near the optimum moisture content. The compacted test pieces were saturated with deaerated water. The substrate suction at which water begins to drain is defined as the value of the inlet air pressure. A low inlet pressure value indicates that the substrate material is typically unsaturated in the field. Prior to testing, the test specimens were saturated to remove residual matrix suction due to compaction at optimum moisture content conditions. The deaerated water was added to the sample through a high flow, high air intake ceramic plate (mounted on the specimen base plate). After the specimen is saturated, suction is applied by an air aspirator during dehumidification to achieve the target substrate suction value. The matrix suction level for each compacted test specimen is reasonably determined from the collected field test data. During the substrate suction hold, the confining pressure is maintained at the target set point, and the target substrate suction is confirmed by examining the equilibrium volume of the effluent water. Each experiment included three phases: suction equalization, temperature equalization, and repeated load-unload. In the suction range of 0-60kPa, the suction balance takes approximately 2-3 weeks; depending on the suction and temperature conditions, the temperature equilibration takes approximately 6-10 days. TDR moisture sensors are arranged at different heights of the cylindrical test piece, so that the change of the substrate suction force is monitored in real time, the distribution form of the substrate suction force along with the depth is determined, and the time point of the substrate suction force balance is determined in real time.
Has the advantages that:
substrate suction control
For unsaturated soil tests, the pore water pressure at the middle height of the soil sample can be tested, and a special suction meter can be used for measuring the negative pore water pressure of 480kPa at most, which is close to the air inlet value of the ceramic tip. The average initial suction after the test piece is compacted is measured by adopting a high-load suction meter, so that the variation situation of the measured initial suction along the height of each test piece and the difference situation among different test pieces are tested, the initial suction is ensured to be controlled within 2kPa, and the accurate control of the suction of the matrix can be realized.
Temperature control
Calibration was performed at each given temperature using a temperature controlled oven, with calibration performed under both load and unload conditions, taking into account the exposure of the strain sensor and the suction gauge to the elevated temperature environment. The calibration result shows that the influence of the temperature effect on the sensitivity, zero offset, hysteresis and the like can be almost ignored, and the temperature insensitivity of the sensors ensures that the accurate measurement of the pore water pressure and the soil deformation can be carried out under the action of temperature load.
According to one specific implementation of the disclosed embodiment, two fans are provided within the pressure chamber of the triaxial apparatus for improved air circulation and a hollow cylindrical aluminum sheet is provided for enhanced heat transfer. To verify the temperature uniformity, two thermocouples were also installed at different locations inside the pressure cell, measuring the air temperature at approximately 5 mm from the test piece, and when thermal equilibrium was reached, the temperature readings of the two thermocouples were approximately the same and remained essentially the same.
According to a specific implementation manner of the embodiment of the present disclosure, the method further includes:
a linear variable differential transformer for externally measuring axial strain;
the radial sensor is used for measuring the local soil deformation of each test piece at the middle height, and the suction meter is used for monitoring the pore water pressure at the middle height of the test piece in the repeated loading and unloading process.
According to a specific implementation manner of the embodiment of the present disclosure, the method further includes:
and the loading element is used for applying axial load and respectively applying static load and dynamic load according to the test requirements.
According to a specific implementation manner of the embodiment of the present disclosure, the method further includes:
and the matrix suction meter is used for measuring the matrix suction at the middle height position of the test piece.
According to a specific implementation manner of the embodiment of the present disclosure, the method further includes:
and (4) confining pressure, wherein water is introduced into the pressure chamber through compressed air, and different confining pressures are applied to the test piece according to the test requirements.
According to a concrete implementation mode of the embodiment of the disclosure, the high-air-intake ceramic disc is connected to introduce water, so that the pore water pressure is provided for the test piece, and the pore water pressure at the bottom is measured through the pore water pressure sensor arranged at the bottom.
According to a specific implementation manner of the embodiment of the present disclosure, the method further includes:
and the axial sensor is used for measuring the strain of the coarse-grained soil test piece along the height direction of the test piece.
According to a specific implementation manner of the embodiment of the present disclosure, the method further includes:
and the air flushing system is used for flushing the bubbles accumulated at the bottom of the ceramic disc out of the pressure chamber and collecting the bubbles by adopting a diffused air quantity indicator every 24 hours, and the collected air quantity is used for correcting the change of the measured water content of the soil body. According to a specific implementation manner of the embodiment of the disclosure, the pore air pressure is provided according to the test requirement by connecting the permeable stone arranged at the top of the test piece.
The above description is only for the specific embodiments of the present disclosure, but the scope of the present disclosure is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present disclosure should be covered within the scope of the present disclosure. Therefore, the protection scope of the present disclosure shall be subject to the protection scope of the claims.

Claims (1)

1. A measurement control method for controlling the substrate suction and the temperature by adopting a dynamic and static triaxial apparatus comprises a substrate suction control method, a temperature control method and a local strain measurement method; it is characterized in that:
the matrix suction control method specifically comprises the following steps: controlling the substrate suction, namely the difference between pore air pressure and pore water pressure, by adopting an axis translation technology, and controlling the substrate suction of the soil test piece by respectively and independently controlling the pore air pressure and the pore water pressure; wherein the pore air pressure is controlled by a rough permeable stone with low air inlet value arranged on the top of the test piece, the permeable stone has rough surface and low air inlet value, the pore air pressure of the test piece is controlled by installing the permeable stone on the top of the test piece, and the pore water pressure is controlled by a saturated ceramic disk with high air inlet value sealed on a pressure chamber base of the triaxial apparatus; the saturated, high air intake ceramic disk allows the passage of moisture but prevents the passage of free air as long as the substrate suction is below its air intake; however, the air dissolved in the water passes through the ceramic pan and accumulates in the bottom of the ceramic pan or in the drainage system; in the test, any accumulated bubbles are flushed out and collected by adopting a diffused air quantity indicator every 24 hours, and the collected air quantity is used for correcting the measured water content change of the soil body;
the temperature control method comprises the following steps: applying a temperature load through a heating system consisting of a thermostat, a heater, and a thermocouple; wherein the heater and the thermocouple are arranged in a pressure chamber of the triaxial apparatus, and both are connected with the constant temperature regulator to form a closed loop control and feedback system; in the test process, the constant temperature controller adjusts the output of the heater through the feedback result of the thermocouple; when the energy dissipation is balanced with the output energy of the heater, the air temperature in the dynamic and static triaxial apparatus pressure chamber is balanced, and the assumed soil body temperature is consistent with the air temperature in the dynamic and static triaxial apparatus pressure chamber in a balanced state; in order to improve the uniformity of the air temperature in the dynamic and static triaxial apparatus pressure chamber, two small fans and a hollow cylindrical aluminum sheet are arranged in the dynamic and static triaxial apparatus pressure chamber, the fans can improve the air circulation, and the cylindrical aluminum sheet improves the heat transfer due to the high thermal conductivity of the cylindrical aluminum sheet; in order to test the uniformity of temperature, two thermocouples are arranged at different positions in the pressure chamber, the thermocouples measure the air temperature at a position 5 mm away from a test piece, and when thermal equilibrium is reached, the temperature readings of the two thermocouples are approximately the same and are kept unchanged;
the local strain measurement method comprises the following steps: the method comprises the following steps of (1) externally measuring axial strain by using a linear variable differential transformer, measuring local soil deformation of each test piece at the middle height and measuring pore water pressure by using a radial sensor, monitoring the pore water pressure at the middle height of the test piece in the repeated loading and unloading process by using a suction gauge, and measuring the pore water pressure at the bottom by using a pore water pressure sensor;
when the test material is manually compacted to the maximum dry density near the optimal water content, the compacted test piece is saturated by using water with de-aerated air; the substrate suction at which water begins to drain is defined as the inlet air pressure value; the low air inlet pressure value indicates that the base material is in an unsaturated state in the field, and before testing, the test piece is saturated to discharge the residual matrix suction force caused by compaction in the optimal water content state; the water with air removed is added into the sample through a ceramic plate with high flow and high air inlet value, which is arranged on the bottom plate of the sample; after the sample is saturated, applying suction force by an air suction device in a dehumidification process to achieve a target substrate suction value; reasonably determining the substrate suction level of each compacted test piece according to the collected field test data; during the substrate suction holding process, the confining pressure is maintained at a target set value, and the target substrate suction is confirmed by checking the equilibrium volume of the effluent water; each experiment included three phases: suction balance, temperature balance and repeated loading-unloading; in the suction range of 0-60kPa, the suction balance takes 2-3 weeks; depending on the suction and temperature conditions, temperature equilibration takes 6-10 days; TDR moisture sensors are arranged at different heights of the cylindrical test piece, so that the change of the substrate suction force is monitored in real time, the distribution form of the substrate suction force along with the depth is determined, and the time point of the substrate suction force balance is determined in real time.
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