Confining-pressure-controlled unsaturated soil matrix suction-temperature-heat conductivity coefficient combined testing device and confining-pressure-controlled unsaturated soil matrix suction-temperature-heat conductivity coefficient combined testing method
By designing a joint test device for suction-temperature-thermal conductivity of unsaturated soil matrix controlled by confining pressure, the problem of difficulty in realizing continuous joint determination of soil thermal conductivity under different conditions in the prior art is solved, and high-precision thermal conductivity is achieved Measurement to meet the needs of complex conditions in energy geotechnical engineering.
Patent Information
- Application Number
- CN202411410162.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-10-10
- Publication Date
- 2025-02-21
AI Technical Summary
The existing soil thermal conductivity measurement methods are difficult to achieve continuous joint measurement under different confining pressures, matrix suction and temperature conditions, and the data accuracy is insufficient, which cannot meet the needs of complex conditions in energy geotechnical engineering.
A joint test device for suction-temperature-thermal conductivity of unsaturated soil matrix with confining pressure control is designed, including confining pressure control, matrix suction control, temperature control and sensor and data acquisition system. By adjusting confining pressure, matrix suction and temperature, Continuous joint determination of thermal conductivity of unsaturated soil is achieved.
The device can accurately measure the thermal conductivity of unsaturated soil under different confining pressures, matrix suction and temperature conditions, improve the accuracy and reliability of measurement, and provide a more reliable test basis.
Smart Images
Figure CN119492776A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of thermal conductivity of unsaturated soil in energy geotechnical engineering, and more specifically to a combined testing device and method for matrix suction-temperature-thermal conductivity of unsaturated soil controlled by confining pressure. Background Art
[0002] As the national economy continues to grow, the demand for energy continues to increase. In the future, while the per capita energy consumption will increase, carbon emissions need to be significantly reduced. Geothermal energy plays an important role in the adjustment of my country's future energy structure. The thermal conductivity controls the steady-state heat transfer process of the soil and is an important physical property parameter in geothermal engineering applications.
[0003] The existing methods for measuring thermal conductivity of soil are divided into steady-state method and transient method. The steady-state method is a method that applies a constant heat flux to the soil and monitors the temperature within a certain distance of the soil heat source when the heat transfer of the soil reaches a steady state. A common steady-state method is the guarded hot plate method (GHP), which clamps two identical specimens between a main heater that provides a constant heat flux to the specimens and two isothermal cold plates that are kept at a fixed temperature by a heat sink. This method has been standardized. Common transient methods include transient line source (TLS) or thermal probe method. The thermal probe method uses a single probe or a dual probe, and heat energy is emitted from a thin wire heat source (such as a needle probe) embedded in the medium. The corresponding temperature response is monitored by a temperature sensor. By analyzing the change in temperature over time, the thermal conductivity of the single probe or dual probe configuration is calculated. In addition, there are transient plane source (TPS) method, modified transient plane source (MTPS) method, etc.
[0004] The above-mentioned GHP technique requires considerable heat input, which may cause water migration in porous materials. This technique is considered problematic for measuring thermal conductivity in unsaturated soils. In addition, the GHP technique is not suitable for highly saturated soils. On the other hand, the transient method is applicable to most unsaturated and saturated soils, but has uncertainties related to disturbances and sensor-soil contact resistance.
[0005] There are many empirical formulas and theoretical models for predicting the thermal conductivity of geotechnical materials, and the influence of factors such as mineral composition, porosity, water content, particle shape, and dry density are taken into account. Such studies are often conducted at room temperature and low stress conditions, and most of them only involve two states: dry and saturated soil. In fact, the field of energy geotechnicals (such as the collection and application of geothermal energy) usually involves the coupling of soil water, heat and force. This more complex engineering condition poses new challenges to traditional thermal conductivity prediction methods. Summary of the invention
[0006] The purpose of the present invention is to develop a combined testing device and method for measuring the thermal conductivity of unsaturated soil under different confining pressures, different matrix suctions (saturations) and different temperatures, which can realize the continuous combined determination of the thermal conductivity of unsaturated soil under different confining pressures, different matrix suctions (saturations) and different temperatures, while ensuring the advantage of data accuracy.
[0007] In order to achieve the above object, the present invention adopts the following technical solutions:
[0008] First aspect:
[0009] The present invention provides a combined test device for measuring the thermal conductivity of unsaturated soil under different confining pressures, different matrix suctions (saturation), and different temperature conditions. The test device is mainly composed of four systems, including confining pressure control, matrix suction control, temperature control, sensors, and data acquisition systems. The test method steps are: first, the whole set of test devices is thermally calibrated; secondly, the water pressure chamber under the ceramic disc and the pressure chamber base is saturated with degassed water and a membrane tube with an impermeable rubber membrane inside is placed on the base; then, the dried soil sample is pressed into the membrane tube layer by layer according to the target porosity, and the sensor and data acquisition system are installed in the soil sample, and then the remaining pipelines and components such as the temperature control system, the confining pressure control system, and the matrix suction control system are installed, and the soil sample is saturated to a B value greater than 0.95; finally, the thermal conductivity is measured jointly by adjusting different confining pressures, matrix suctions, and temperatures, and the relationship between the matrix suction, temperature, and thermal conductivity of the unsaturated soil under different confining pressures can be obtained. The test device has high accuracy and reliability, and can provide a reliable experimental basis for the study of the thermal conductivity of the soil.
[0010] Furthermore, the confining pressure control system includes a pressure chamber, a pipeline with a valve, and a pressure control panel; the pressure chamber is composed of a side wall, a cover plate fixed on the top of the side wall, a bottom plate fixed on the bottom of the side wall, and a base fixed above the bottom plate; the temperature control system includes a U-shaped copper tube, a hose, and a cold bath circulator; the sensor and data acquisition system include a probe temperature sensor, a patch temperature sensor, a humidity sensor, a tensiometer, a differential pressure sensor, a thermal conductivity analyzer, a temperature data collector, a moisture data collector, a suction data collector, a computer, a nylon tube, and a signal line.
[0011] Furthermore, the bottom plate fixed to the bottom of the side wall is provided with a drainage pipe with a valve and an exhaust pipe with a valve; the cover plate is provided with a drainage pipe with a valve and a circular hole and sealing accessories for passing the sensor signal line; the base is connected to the drainage pipe with a valve; the base is provided with a water pressure chamber, and a high air intake value ceramic disc is provided above the water pressure chamber; the water pressure chamber is connected to the drainage pipe with a valve.
[0012] Furthermore, the U-shaped copper tube is placed around the sample in the pressure chamber and connected to the hose; the hose is connected to a cold bath circulator.
[0013] Furthermore, the probe-type temperature sensor, humidity sensor, and tensiometer are located inside the sample, and the patch-type temperature sensor is attached to the outside of the waterproof rubber membrane; the probe-type temperature sensor is vertically inserted into the center of the sample, and the tensiometer is arranged 1.5 cm away from the probe-type temperature sensor; the probe-type temperature sensor and the thermal conductivity analyzer, the patch-type temperature sensor and the temperature data collector, the humidity sensor and the moisture data collector, and the differential pressure sensor and the suction data collector are respectively connected through the signal line; the tensiometer and the differential pressure sensor are connected through the nylon tube; the thermal conductivity analyzer, the temperature data collector, the moisture data collector, and the suction data collector are respectively connected to the computer through the signal line; the nylon tube and the signal line are sealed with sealing accessories when passing through the sample top cap and the pressure chamber cover.
[0014] Furthermore, the cold bath circulator is connected to the computer via the signal line, and the computer can automatically adjust the flow rate of cold and hot circulating water according to the temperature signal provided by the temperature data acquisition instrument to achieve automatic control of the preset temperature.
[0015] Second aspect:
[0016] The present invention provides a combined testing method for measuring the thermal conductivity of unsaturated soil under different confining pressures, different matrix suctions (saturation), and different temperatures, comprising the following steps:
[0017] A. Thermal calibration: Since changes in temperature and pressure in the pressure chamber may cause volume changes in the pressure chamber, degassed water used to maintain the confining pressure, drainage pipelines and ceramic discs, the system needs to be thermally calibrated before the test to capture the actual volume change of the soil sample due to temperature and pressure changes. First, an aluminum cylinder with a known thermal expansion volume coefficient and the same size as the soil sample plus the top cover is selected as a "dummy" sample and placed on the top of the base; after the pressure chamber, ceramic disc and all drainage pipelines are assembled and connected, degassed water is continuously introduced until the bubbles are completely discharged, and then the degassed water is filled to the pressure chamber. The calibration process starts at room temperature (20°C), the confining pressure of the pressure chamber is adjusted by loading and unloading cycles, and then the same loading and unloading cycles are performed after heating and cooling to each target temperature, and the amount of water flowing out or in from the pressure chamber is measured. The volume change of the calibrated instrument due to temperature and confining pressure changes is calculated by formula (1):
[0018] ΔV ins =ΔV read -α A V A ΔT#(1)
[0019] Where: ΔV ins Change in test fixture calibration volume; ΔV read V is the reading of the amount of water flowing out or in from the pressure chamber; A is the volume of the aluminum cylinder; ΔT is the change in temperature.
[0020] B. Sample installation: Before the sample is installed, the ceramic disc is saturated with degassed water, and degassed water is continuously introduced through the drainage pipe under the ceramic disc to make the water pressure chamber in the base saturated with degassed water; in order to shape the sample, a membrane cylinder with an impermeable rubber membrane inside is placed on the base, and filter paper is placed on the top of the base to prevent the loss of fine particles; air is pumped through the side holes of the membrane cylinder to form a vacuum between the inner surface of the membrane cylinder and the impermeable rubber membrane; then a certain number of dried soil samples are divided into 4 equal parts according to the target porosity and carefully pressed into the membrane cylinder in layers. After the second layer is compacted, the tensiometer and humidity sensor are inserted upright into the middle of the sample; the third and fourth layers of soil samples are continued to be filled and compacted according to the target porosity.
[0021] C. Equipment installation: After the sample is formed, moisten the filter paper with the same diameter as the sample and place it on the top of the sample, insert the probe temperature sensor vertically into the middle of the sample, then put the top cap in place and connect the exhaust pipe and the signal line of the data acquisition system; the upper and lower ends of the waterproof rubber membrane are rolled over the top cap and the base and sealed with O-rings; vacuum is applied to the sample through the exhaust pipe on the top cap, the membrane cylinder is removed and the diameter and height of the sample are checked with a caliper; then the patch temperature sensor, signal line and nylon tube are installed, the pressure chamber is assembled, and finally the pressure chamber is filled with degassed water.
[0022] D. Sample saturation: First, evacuate the sample to near vacuum through the exhaust pipe on the top of the sample and maintain it for more than 1 hour. Then, inject degassed water into the sample under a certain water head until the saturation B value is greater than 0.95.
[0023] E, Thermal conductivity measurement: To measure thermal conductivity at different confining pressures, matrix suctions, and temperatures, first select the preset confining pressure, matrix suction, and temperature.
[0024] E1. At room temperature, when the confining pressure is the initial value, the saturated sample (i.e., the matrix suction is 0) is first compressed and consolidated step by step from low to high according to the preset confining pressure. Each step is consolidated until the excess pore water pressure dissipates, and then the confining pressure is unloaded to the initial value.
[0025] E2. After the initial loading and unloading cycle, the sample is heated and cooled according to the preset temperature. The sample is kept warm for 2 to 3 hours at each stage of heating (or cooling). The volume of super-porous water dissipated in the sample is measured. Then, the heated and cooled sample is subjected to the same loading and unloading cycle as in step E1.
[0026] E3. After the saturated sample has gone through a complete hot and cold cycle, the matrix suction is then controlled. Air pressure is applied through the exhaust pipe at the top of the soil sample, and each level of pressurization is maintained for about 1 day until the quality of the water flowing out of the drain pipe at the base of the sample is stable.
[0027] E4. Under different matrix suction and temperature, the unsaturated soil samples were subjected to the same cooling and heating cycles and confining pressure loading and unloading cycles. The thermal conductivity, volumetric water content and matrix suction of the samples were measured when the matrix suction and soil sample drainage were stable.
[0028] E5. After the sample is tested at the maximum matrix suction, the top of the sample is vented to atmospheric pressure. Due to the relatively high matrix suction, the sample will begin to absorb water, and this water absorption process is periodically stopped at intervals of 1 day. The sample is then isolated from the external atmosphere and water supply for 8 hours to reach equilibrium. The sample is subjected to a single loading and unloading cycle within the preset confining pressure range, during which the thermal conductivity, volumetric water content and matrix suction are measured. When the water absorption is close to 0, the entire water absorption process stops.
[0029] E6. Since the specimen cannot reach a completely or nearly dry state (saturation less than 0.1) at the maximum matrix suction, the specimen needs to be exposed to the atmosphere for low-temperature drying until there are no visible bubbles in the water flowing out of the drain pipe at the base of the specimen. This low-temperature dried specimen is tested according to the same loading-unloading, heating-cooling steps as the saturated specimen to measure its thermal conductivity and volumetric water content. Since the tensiometer loses connection with the specimen after the specimen is exposed, the matrix suction is not measured in this step.
[0030] Compared with the prior art, the present invention has the following beneficial effects:
[0031] At present, there are many empirical formulas and theoretical models for predicting the thermal conductivity of geotechnical materials, but such studies are often only conducted at room temperature and low stress conditions, and most of them only involve two states: dry and saturated soil. This device measures the thermal conductivity of soil by adjusting different confining pressures, matrix suction and temperature, and obtains the relationship between matrix suction, temperature and thermal conductivity of unsaturated soil under different confining pressures. Experiments have shown that the device can continuously and jointly measure the thermal conductivity of unsaturated soil under different confining pressures, matrix suction and temperature conditions. The test device and method have high accuracy and reliability, and can provide a reliable experimental basis for the study of soil thermal conductivity. BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Attached Figure 1 The structure of the combined test device for unsaturated soil matrix suction-temperature-thermal conductivity controlled by confining pressure is shown in the figure.
[0033] Attached Figure 2 This is a schematic diagram of the thermal calibration device structure.
[0034] Attached Figure 3 This is a schematic diagram of the sample saturation device structure.
[0035] Attached Figure 4 The thermal calibration curve of the embodiment of the present invention is
[0036] Attached Figure 5 It is the temperature-saturation-thermal conductivity isoline (σ3=35kPa) tested in the embodiment of the present invention.
[0037] In the figure: pressure chamber side wall 1, cover plate 2, drain valve 3, bottom plate 4, exhaust pipe 5, drain pipe 6a-6e, U-shaped copper tube 7, hose 8, suction data acquisition instrument 9, differential pressure sensor 10, thermal conductivity analyzer 11, moisture data acquisition instrument 12, computer 13, cold bath circulation machine 14, signal line 15a-15d, nylon tube 16, patch temperature sensor 17, temperature data acquisition instrument 18, base 19, water pressure chamber 20, ceramic disc 21, O-ring 22, impermeable rubber membrane 23, tensiometer 24, sample 25, filter paper 26, top cap 27, sealing accessories 28, probe temperature sensor 29, humidity sensor 30, pressure gauge 31, pressure control panel 32, vacuum pump 33a, vacuum pump 33b, upper water storage tank 34, lower water storage tank 35, aluminum cylinder 36, two-way ball valve I-VI, three-way ball valve VII. DETAILED DESCRIPTION
[0038] In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described in more detail below in conjunction with the drawings in the preferred embodiments of the present application. In the drawings, the same or similar reference numerals throughout represent the same or similar parts or parts with the same or similar functions. The described embodiments are part of the embodiments of the present application, not all of the embodiments. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limitations on the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.
[0039] The embodiments of the present application are described in detail below with reference to the accompanying drawings.
[0040] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, or it can be an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.
[0041] In the description of the present application, it should be understood that the terms "upper", "lower", "front", "back", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are orientations or positional relationships based on the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.
[0042] In addition, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, for example, a process, method, system, product, or display that includes a series of steps or elements is not necessarily limited to those steps or elements explicitly listed but may include other steps or elements not explicitly listed or inherent to such process, method, product, or display.
[0043] The following will be combined Figure 1-2 , a non-interface handrail press strip and a forming method involved in the embodiment of the present application are described in detail. It is worth noting that the following embodiments are only used to explain the present application and do not constitute a limitation on the present application.
[0044] Embodiment 1:
[0045] As attached Figure 1-5 As shown, an embodiment of the present invention discloses a combined test device for measuring the thermal conductivity of unsaturated soil under different confining pressures, different matrix suctions (saturation), and different temperature conditions. The test device is mainly composed of four systems, including confining pressure control, matrix suction control, temperature control, sensors, and data acquisition systems. The confining pressure control system includes a pressure chamber, a pipeline with a valve, and a pressure control panel; the pressure chamber includes a pressure chamber side wall 1, a cover plate 2 fixed on the top of the side wall, a bottom plate 4 fixed on the bottom of the side wall, and a base 19 fixed above the bottom plate; the temperature control system includes a U-shaped copper tube 7, a hose 8, and a cold bath circulator 14; the sensor and data acquisition system include a probe temperature sensor 29, a patch temperature sensor 17, a humidity sensor 30, a tensiometer 24, a differential pressure sensor 10, a thermal conductivity analyzer 11, a temperature data acquisition instrument 18, a moisture data acquisition instrument 12, a suction data acquisition instrument 9, a computer 13, a nylon tube 16, and signal lines 15a-15d. The matrix suction control system includes a vacuum pump, a pressure gauge, a pipeline with a valve, and a water storage tank;
[0046] Furthermore, the bottom plate 4 is provided with a drainage pipe 6a with a valve IV and an exhaust pipe 5 with a valve III; the cover plate 2 is provided with a drainage pipe 6e with a valve I and a circular hole and a sealing accessory 28 for passing the signal line 15c and the nylon tube 16; the base 19 is provided with a water pressure chamber 20, and a 300 kPa high air intake value ceramic disc 21 is provided above the water pressure chamber 20; the drainage pipe 6b is connected to the water pressure chamber for measuring the drainage volume; the drainage pipe 6d is connected to the water pressure chamber for replenishing degassed water.
[0047] Furthermore, the U-shaped copper tube 7 is placed around the sample 25 in the pressure chamber and connected to the hose 8; the hose 8 is connected to the cold bath circulator 14; the cold bath circulator 14 is connected to the computer 13 through the signal line 15d, and the computer 13 can automatically adjust the flow rate of hot and cold circulating water in the cold bath circulator 14 according to the temperature signal provided by the temperature data acquisition instrument 18 to achieve automatic control of the preset temperature.
[0048] Furthermore, the probe temperature sensor 29 , humidity sensor 30 , and tensiometer 24 are located inside the sample 25 , and the patch temperature sensor 17 is attached to the outside of the waterproof rubber membrane 23 ; the probe temperature sensor 29 is vertically inserted into the center of the sample 25 , and the tensiometer 24 is arranged 1.5 cm away from the probe temperature sensor 29 .
[0049] Furthermore, the patch temperature sensor 17 is connected to the temperature data collector 18 through the signal line 15b, the probe temperature sensor 29 is connected to the thermal conductivity analyzer 11, the humidity sensor 30 is connected to the moisture data collector 12 through the signal line 15c, the differential pressure sensor 10 is connected to the suction data collector 9 through the signal line 15d, the suction data collector 9, the thermal conductivity analyzer 11, the moisture data collector 12, and the temperature data collector 18 are all connected to the computer 13 through the signal line 15a, and the tensiometer 24 is connected to the differential pressure sensor 10 through the nylon tube 16; the nylon tube 16 and the signal lines 15c and 15b are sealed using sealing accessories 28 when passing through the cover plate 2, the bottom plate 4, and the top cap 27.
[0050] Furthermore, the temperature control system uses a cold bath circulator 14 to control the heating or cooling of the water bath in the pressure chamber so that the sample 25 reaches the target temperature. In order to minimize the heat exchange between the temperature control system and the outside, the pressure chamber and the hose 8 are wrapped with glass fiber and foam insulation materials.
[0051] Furthermore, the pressure chamber is completely filled with degassed water, and the drain pipe 6e is connected to the pressure control panel 32, which can provide a confining pressure of -85 to 500 kPa by adjusting the amount of water entering and leaving the pressure chamber.
[0052] This embodiment also discloses a combined test method for measuring the thermal conductivity of unsaturated soil under different confining pressures, different matrix suctions (saturation), and different temperatures, comprising the following steps:
[0053] A. Thermal Calibration
[0054] First, an aluminum cylinder 41 with a known thermal expansion volume coefficient and the same size as the soil sample plus the top cover is selected as a "dummy" sample, and the aluminum cylinder 36 is placed on the top of the base 19; the pressure chamber side wall 1, bottom plate 4, cover plate 2, ceramic disc 21, top cap 27, exhaust pipe 5, drain pipe 6a~6e and the temperature control system are installed, and degassed water is continuously introduced until the bubbles are completely discharged, and then the degassed water is filled into the pressure chamber. The calibration process starts at room temperature (20℃), and the pressure chamber confining pressure is adjusted by loading and unloading cycles. Then, the same loading and unloading cycles are performed after heating and cooling to each target temperature, and the amount of water flowing out or in from the pressure chamber is measured. The volume change of the calibrated instrument due to temperature and confining pressure changes is calculated by formula (1):
[0055] ΔV ins =ΔV read -α A V A ΔT#(1)
[0056] Where: ΔV ins Change in test fixture calibration volume; ΔV read V is the reading of the amount of water flowing out or in from the pressure chamber; A is the volume of the aluminum cylinder; ΔT is the change in temperature.
[0057] The structure of the thermal calibration device is as shown in the attached Figure 2 shown.
[0058] B. Sample installation
[0059] SP sand is used in this embodiment, and the index properties of the SP sand are shown in Appendix 1. Before the sample 25 is installed, the ceramic disc 21 is saturated with degassed water, and degassed water is continuously introduced through the drainage pipes 6b and 6d below the ceramic disc 21, so that the water pressure chamber 20 is saturated with degassed water; in order to shape the sample, a membrane cylinder with an impermeable rubber membrane 23 inside is placed on the base 19, and then a filter paper 26 with a diameter of 10 cm is placed on the base 19 to prevent the loss of fine particles in the sample; air is sucked through the side holes of the membrane cylinder to form a vacuum between the inner surface of the membrane cylinder and the impermeable rubber membrane 23; then a certain number of dried soil samples are divided into 4 equal parts according to the target porosity (e=0.65) and carefully pressed into the molding jacket in layers, so that the final sample height is 12 cm. After the second layer is compacted, the tensiometer 24 and the humidity sensor 30 are inserted upright into the middle of the sample; the third and fourth layers of samples are filled and compacted according to the target porosity.
[0060] Table 1 Index properties of SP sand selected in the examples
[0061] Sample name USCS <![CDATA[D 50 (mm)]]> <![CDATA[D 10 (mm)]]> <![CDATA[C u ]]> <![CDATA[C c ]]> <![CDATA[G s ]]> <![CDATA[e min ]]> <![CDATA[e max ]]> <![CDATA[e test ]]> Fines, % SP sand SP 0.22 0.15 1.6 1 2.66 0.51 0.75 0.6 1.1
[0062] C. Equipment Installation
[0063] After the sample is formed, a filter paper 26 with a diameter of 10 cm is moistened and placed on the top of the sample, a probe temperature sensor 29 is vertically inserted into the middle of the sample, and then the top cap 27 is placed in place and connected to the exhaust pipe 5 and the signal line 15c of the data acquisition system; the upper and lower ends of the water-impermeable rubber membrane 23 are rolled over the top cap 27 and the base 19 and sealed with an O-ring 22; a vacuum of -35 kPa is applied to the sample through the exhaust pipe 5, and then the film bearing cylinder is removed and the diameter and height of the sample are checked with a caliper. The average diameter of the sample in this embodiment is 10.06 cm and the average height is 11.9 cm. Considering the volume of the sensor, according to the measured size, the actual porosity ratio of the sample is 0.638. Then install the patch temperature sensor 17, the signal line 15b and the nylon tube 16, assemble the pressure chamber, and finally fill the pressure chamber with degassed water through the drainage pipe 6a.
[0064] D, sample saturation
[0065] Sample saturation device as attached Figure 3As shown. The upper water tank 34 is connected to the bottom of the sample, and the lower water tank 35 is connected to the top of the sample. The elevation difference between the upper and lower water tanks is about 1.5m. The two water tanks are filled with degassed water to half of their capacity, and the sample is evacuated to a vacuum with an initial pressure of -35kPa using a vacuum pump 33b. The confining pressure is set to 0 using the pressure control panel 32. At this time, the initial effective stress of the sample is 35kPa. Then keep valve II closed and open valve III to balance the air pressure in the sample and the lower water tank 35. At this time, the vacuum pressure in the pressure chamber and the two water tanks increases at the same rate at the same time until the confining pressure and the pressure of the two water tanks reach -50kPa and -85kPa respectively, and the effective stress of the sample remains unchanged at 35kPa. Keep the pressure in the sample and the confining pressure for about 1h so that the air pressure in the sample reaches -85kPa uniformly. Then open valve II to allow the degassed water in the upper water tank 34 to penetrate into the sample driven by the head difference between the upper and lower water tanks. The penetration process usually takes 10 minutes to saturate the sample. Finally, close valve III, and then open valve III for 5 minutes to ensure the internal pressure balance of the sample. In order to check the saturation B value, valves II and III remain closed, and the vacuum pressure of the pressure chamber is reduced to -25kPa, so that the sample generates excess pore water pressure due to undrained conditions. The excess pore water pressure is measured by the differential pressure sensor 10, and the B value calculation formula is shown in Formula 2 below:
[0066]
[0067] Where Δu is the change in excess pore water pressure, and Δσ3 is the change in confining pressure.
[0068] After the B value is checked, the vacuum pressure of the upper water storage tank 34 is reduced to -60 kPa, and then the valve II is opened again until the water level of the upper water storage tank 34 does not change significantly and the effective stress in the sample returns to the equilibrium state of 35 kPa. Repeat this process, and increase the confining pressure by 25 kPa until the B value reaches 95%. In this embodiment, when the confining pressure increases from 0 to 25 kPa, the B value reaches 96%. After saturation is completed, the pressure in the sample is reduced to 0, and then the confining pressure is adjusted to 35 kPa.
[0069] E. Thermal conductivity measurement: In order to measure the thermal conductivity under different temperatures, confining pressures and matrix suctions, this embodiment selects preset temperatures (5.5, 20, 47.5, 75.5°C), confining pressures (35, 100, 200, 400 kPa), and matrix suctions (0, 4, 4.5, 5, 5.5, 6, 6.5, 7, 14, 45 kPa).
[0070] E1. At room temperature (20°C) and a confining pressure of 35 kPa, the saturated specimen (i.e., matrix suction is 0) is first compressed and consolidated step by step at the preset confining pressure (100, 200, and 400 kPa). Each step is consolidated until the excess pore water pressure dissipates, and then the saturated specimen is unloaded to 35 kPa.
[0071] E2. After the initial loading and unloading cycle, the sample is heated and cooled at preset temperatures (5.5, 20, 47.5, 75.5℃). The sample is kept warm for 2 to 3 hours at each stage of heating (or cooling), and the volume of pore water dissipated in the sample is measured; then the heated and cooled sample is subjected to the same loading and unloading cycle as in step E1 (35 to 400 kPa).
[0072] E3. After the saturated sample has gone through a complete hot and cold cycle, the matrix suction is then controlled. Air pressure is applied through the exhaust pipe at the top of the soil sample, and each level of pressurization is maintained for about 1 day until the quality of the water flowing out of the drain pipe at the base of the sample is stable.
[0073] E4. Under different matrix suction and temperature, the unsaturated soil samples were subjected to the same cooling and heating cycles and confining pressure loading and unloading cycles. The thermal conductivity, volumetric water content and matrix suction of the samples were measured when the matrix suction and soil sample drainage were stable.
[0074] E5. After the sample is tested at the maximum matrix suction (45 kPa), the top of the sample is vented to atmospheric pressure. Due to the relatively high matrix suction, the sample will begin to absorb water, and this water absorption process is periodically stopped at intervals of 1 day. The sample is then isolated from the external atmosphere and water supply for 8 hours to reach equilibrium. The sample is subjected to a single loading and unloading cycle between 35 and 400 kPa, during which the thermal conductivity, volumetric water content, and matrix suction are measured. When the water absorption approaches zero, the entire water absorption process stops.
[0075] E6. Since the specimen cannot reach a completely or nearly dry state (saturation below 0.1) at the maximum matrix suction (45 kPa), the specimen needs to be exposed to the atmosphere and heated at 75.5°C until there are no visible bubbles in the water flowing out of the drain pipe at the base of the specimen. This heat-dried specimen is tested according to the same loading-unloading, heating-cooling steps as the saturated specimen to measure its thermal conductivity and volumetric water content. It should be noted that since the tensiometer loses connection with the specimen during this step, matrix suction is not measured during this period.
[0076] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.
Claims
1. A combined test device for unsaturated soil matrix suction-temperature-thermal conductivity controlled by confining pressure, characterized in that: It includes confining pressure control system, matrix suction control system, temperature control system, sensors and data acquisition system; The confining pressure control system includes a pressure chamber, a pressure gauge, a pipeline with a valve, and a pressure control panel; The pressure chamber is composed of a side wall, a cover plate fixedly mounted on the top of the side wall, a bottom plate fixedly mounted on the bottom of the side wall, and a base fixedly mounted above the bottom plate; The matrix suction control system includes a vacuum pump, a pressure gauge, a pipeline with a valve, and a water storage tank; The temperature control system includes a U-shaped copper tube, a hose, and a cold bath circulation machine; The sensor and data acquisition system include a probe temperature sensor, a patch temperature sensor, a humidity sensor, a tensiometer, a differential pressure sensor, a watertight rubber membrane, a thermal conductivity analyzer, a temperature data acquisition instrument, a moisture data acquisition instrument, a suction data acquisition instrument, a computer, a nylon tube and a signal line.
2. The unsaturated soil matrix suction-temperature-thermal conductivity combined testing device controlled by confining pressure according to claim 1 is characterized by: The bottom plate fixed to the bottom of the side wall is provided with a drainage pipe with a valve and an exhaust pipe with a valve; the cover plate is provided with a drainage pipe with a valve and a circular hole and sealing accessories for passing the sensor signal line; the base is connected to the drainage pipe with a valve; the base is provided with a water pressure chamber, and a high air intake value ceramic disc is provided above the water pressure chamber; the water pressure chamber is connected to the drainage pipe with a valve.
3. The combined testing device for unsaturated soil matrix suction-temperature-thermal conductivity controlled by confining pressure according to claim 1 is characterized by: The U-shaped copper tube is placed around the sample in the pressure chamber and connected to the hose; the hose is connected to a cold bath circulator.
4. The unsaturated soil matrix suction-temperature-thermal conductivity combined testing device controlled by confining pressure according to claim 1 is characterized by: The probe-type temperature sensor, humidity sensor and tensiometer are located inside the sample, and the patch-type temperature sensor is attached to the outside of the waterproof rubber membrane; the probe-type temperature sensor is vertically inserted into the center of the sample, and the tensiometer is arranged beside the probe-type temperature sensor.
5. The combined testing device for unsaturated soil matrix suction-temperature-thermal conductivity controlled by confining pressure according to claim 1, characterized in that: The probe temperature sensor and the thermal conductivity analyzer, the patch temperature sensor and the temperature data collector, the humidity sensor and the moisture data collector, the differential pressure sensor and the suction data collector are all connected via the signal line respectively; the tensiometer and the differential pressure sensor are connected via the nylon tube; the thermal conductivity analyzer, the temperature data collector, the moisture data collector, and the suction data collector are all connected to the computer via the signal line respectively.
6. The unsaturated soil matrix suction-temperature-thermal conductivity combined testing device controlled by confining pressure according to claim 1, characterized in that: The nylon tube and the signal line are sealed by using sealing accessories when passing through the sample top cap and the pressure chamber cover.
7. The combined testing device for unsaturated soil matrix suction, temperature and thermal conductivity controlled by confining pressure according to claim 5, characterized in that: The cold bath circulation machine is connected to the computer via the signal line, and the computer can automatically adjust the flow rate of cold and hot circulating water according to the temperature signal provided by the temperature data acquisition instrument to realize automatic control of the preset temperature.
8. A combined test method for unsaturated soil matrix suction-temperature-thermal conductivity controlled by confining pressure, characterized in that: The device according to any one of claims 1 to 7 is implemented, comprising the following steps: A. Thermal calibration: Select an aluminum cylinder with a known thermal expansion volume coefficient that is exactly the same size as the soil sample plus the top cover as a "dummy" sample, and place the aluminum cylinder on top of the base; assemble and connect the pressure chamber, ceramic disc and all drainage pipes, and continuously pass degassed water until the bubbles are completely discharged; fill the pressure chamber with degassed water; B. Sample installation: Before sample installation, saturate the ceramic disc with degassed water, and continuously introduce degassed water through the drainage pipe under the ceramic disc so that the water pressure chamber in the base is saturated with degassed water; place a membrane cylinder with an impermeable rubber membrane inside on the base, and place filter paper on the top of the base. Pump air through the side holes of the membrane cylinder to form a vacuum between the inner surface of the membrane cylinder and the impermeable rubber membrane; divide a certain number of dried soil samples into 4 equal parts according to the target porosity and press them into the membrane cylinder in layers. After the second layer is compacted, insert the tensiometer and humidity sensor upright into the middle of the sample; continue to fill the third and fourth layers of soil samples and compact them according to the target porosity; C. Equipment installation: After the sample is formed, wet the filter paper with the same diameter as the sample and place it on the top of the sample, insert the probe temperature sensor vertically into the middle of the sample, put the top cap in place and connect the exhaust pipe and the signal line of the data acquisition system; roll the upper and lower ends of the waterproof rubber membrane over the top cap and the base and seal; apply vacuum to the sample through the exhaust pipe on the top cap and remove the membrane cylinder; install the patch temperature sensor, signal line and nylon tube, assemble the pressure chamber, and fill the pressure chamber with degassed water; D. Sample saturation: First, evacuate the sample to near vacuum through the exhaust pipe on the top of the sample and maintain it for more than 1 hour. Then, inject degassed water into the sample under a certain water head until the saturation value B is greater than 0.95; E, Thermal conductivity measurement: Select preset confining pressure, matrix suction, and temperature.
9. The method according to claim 8, characterized in that In the thermal calibration, the calibration process starts at room temperature (20°C), the pressure chamber confining pressure is adjusted by loading and unloading cycles, and then the same loading and unloading cycles are performed after heating and cooling to each target temperature, and the amount of water flowing out of or into the pressure chamber is measured; the volume change of the calibrated instrument due to temperature and confining pressure changes is calculated by formula (1): ΔV ins =ΔV read -a A V A ΔT#(1) Where: ΔV ins Change in test fixture calibration volume; ΔV read V is the reading of the amount of water flowing out or in from the pressure chamber; A is the volume of the aluminum cylinder; ΔT is the change in temperature.
10. The method according to claim 8, characterized in that E. Thermal conductivity measurement: Select preset confining pressure, matrix suction and temperature, including: E1. At room temperature, when the confining pressure is the initial value, the saturated sample is first drained and consolidated step by step from low to high according to the preset confining pressure. Each step is consolidated until the excess pore water pressure dissipates, and then the confining pressure is unloaded to the initial value. E2. After the initial loading and unloading cycle, the sample is heated and cooled according to the preset temperature. The sample is kept warm for 2 to 3 hours at each stage of heating or cooling. The volume of super-porous water dissipated in the sample is measured. The heated and cooled sample is subjected to the same loading and unloading cycle as in step E1. E3. After the saturated sample has gone through a complete cooling and heating cycle, the matrix suction is controlled; air pressure is applied through the exhaust pipe on the top of the soil sample, and each level of pressurization is maintained for about 1 day until the quality of the water flowing out of the drainage pipe at the base of the sample is stable; E4. Under different matrix suction and temperature, the unsaturated soil samples were subjected to the same cooling and heating cycle and confining pressure loading and unloading cycle; the thermal conductivity, volumetric water content and matrix suction of the samples were measured when the matrix suction and soil sample drainage were stable; E5. After the sample is tested under the maximum matrix suction, the top of the sample is vented to atmospheric pressure; due to the relatively high matrix suction, the sample will begin to absorb water, and this water absorption process is periodically stopped at intervals of 1 day; the sample is then isolated from the external atmosphere and water supply for 8 hours to reach equilibrium; the sample is subjected to a single loading and unloading cycle within the preset confining pressure range, during which the thermal conductivity, volumetric water content and matrix suction are measured; when the water absorption is close to 0, the entire water absorption process stops; E6. Low-temperature drying is performed by exposing the specimen to the atmosphere until no bubbles are visible in the water flowing out of the drain pipe at the base of the specimen; the low-temperature dried specimen is tested according to the same loading-unloading, heating-cooling steps as the saturated specimen to measure its thermal conductivity and volumetric water content.