Frozen soil frost heaving and thawing test system and test method based on low-field nuclear magnetic resonance

Abstract

The invention discloses a frozen soil frost heaving and freezing thawing test system based on low-field nuclear magnetic resonance. The frozen soil frost heaving and freezing thawing test system comprises a low-field nuclear magnetic scanning device, a circulating refrigeration device, a clamp holder and a soil optical fiber temperature probe, the low-field nuclear magnetic scanning device comprises symmetrically arranged magnetic poles, a constant temperature and humidity chamber, a slide rail, a support, a spectrometer cabinet, a radio frequency cabinet and a gradient cabinet. The circulating refrigerating device comprises a cooling liquid storage device, a refrigerating machine, a water replenishing tank and a temperature sensor interface; the water replenishing water tank is divided into an A independent water tank and a B independent water tank which are used for supplying liquid water and gaseous water respectively; the clamp holder is arranged on the sliding rail, consists of a temperature control sleeve box and a sample loading device, and is divided into a frost heaving clamp holder or a freezing and thawing circulating clamp holder; and the soil mass optical fiber temperature probe is inserted into the temperature probe hole of the sample loader to monitor the temperature of the soil mass. According to the invention, the soil frost heaving amount, the moisture migration amount and the icing condition under the temperature-moisture double gradient and the soil in-situ frost heaving and unfrozen water distribution condition under the freezing and thawing cycle action can be intuitively measured by using multiple observation modes.

Description

Technical Field

[0001] The invention belongs to the technical field of geotechnical engineering frozen soil testing, and specifically relates to a frozen soil frost heave and freeze-thaw testing system and a testing method based on low-field nuclear magnetic resonance. Background Art

[0002] Soil frost heave has always been an important research topic in the field of geotechnical engineering. The soil frost heave test is an important means to study its frost heave mechanism, frost heave sensitivity and other properties. The current consensus in the academic and engineering communities is that frost heave is caused by the migration of external liquid water to the negative temperature zone to form ice lenses due to ice segregation. Therefore, the ice segregation mechanism is one of the most core issues in the study of soil frost heave. The amount and distribution of segregated ice determines the magnitude of soil frost heave.

[0003] There are three basic conditions for frost heave to occur: freezing temperature, external water supply, and frost heave sensitive soil. The direction of external water supply is opposite to that of freezing. Freezing generally occurs from top to bottom, while water needs to be supplied from bottom to top. This involves the study of the driving force of water migration in frozen soil, which is one of the core contents of the study of the mechanism of water migration in frozen soil. Regarding the study of water migration, the academic community has successively proposed a variety of theoretical hypotheses on the driving force of water migration in frozen soil, including capillary force, pore water pressure, soil water potential, osmotic pressure, electroosmotic force, and ice pressure gradient. At present, the more mainstream driving forces of water migration in frozen soil are pore water pressure and soil water potential.

[0004] The current testing technology and instrument development level are limited, and the soil water potential in frozen soil is difficult to measure directly. Most of the existing water potential measurement instruments and methods cannot be used for frozen soil water potential measurement. Therefore, the mainstream method is to use empirical formulas, thermodynamic theories and experimental inversion analysis and apply them to the study of frozen soil moisture migration.

[0005] In mainstream frost heave tests, the amount of water migration is often obtained by the total change in the water level height in a Marsh flask. It is impossible to obtain the water distribution and migration amount in different temperature gradient sections in the soil, nor is it possible to quantify the relationship between freezing time and water migration and ice formation.

[0006] Secondly, how to intuitively reflect the pore water migration channels, explore the pore evolution and ice lens formation during frost heave, etc. are also difficult to respond using existing technologies.

[0007] In addition, the mainstream test equipment on the market has a single function and does not have multi-field coupling function. Some instrument design loopholes directly affect the test results, and problems such as instrument function fixation and poor upgrade potential are becoming increasingly obvious. Summary of the invention

[0008] The main object of the present invention is to provide a freeze-thaw test system for frozen soil based on low-field nuclear magnetic resonance, which can intuitively reflect and record the complete process of the germination of ice lenses and the water migration phenomenon during soil freezing.

[0009] The freeze-thaw test system for frozen soil based on low-field nuclear magnetic resonance provided by the present invention includes a low-field nuclear magnetic resonance scanning device, a circulating refrigeration device, a holder, and a soil fiber optic temperature probe; the low-field nuclear magnetic resonance scanning device includes symmetrically arranged magnetic poles, a temperature and humidity controlled chamber, a slide rail, a support, a spectrometer cabinet, a radio frequency cabinet, and a gradient cabinet; the slide rail is arranged at the bottom of the inner cavity of the temperature and humidity controlled chamber and is driven to slide horizontally by a slide rail motor; the magnetic poles are connected to the spectrometer cabinet, the radio frequency cabinet, and the gradient cabinet for exciting and collecting nuclear magnetic resonance signals; the circulating refrigeration device includes a coolant reservoir, a refrigerator, a water replenishing tank, and a temperature sensor interface; the refrigerator is connected to the coolant reservoir through a pipeline for controlling the temperature of the coolant circulation; the water replenishing tank is divided into two independent tanks, namely tank A and tank B, which respectively provide liquid water and gaseous water replenishment; the holder is arranged on the slide rail and is composed of a temperature controlled sleeve box and a sample loading device, and is divided into a frost heave holder or a freeze-thaw cycle holder; the temperature controlled sleeve box of the frost heave holder is divided into a high-temperature end and a low-temperature end, which are respectively connected to two circulating refrigeration devices to form a test temperature gradient field; the temperature controlled sleeve box of the freeze-thaw cycle holder is provided with a spiral return pipe inside and is connected to a single circulating refrigeration device; the soil fiber optic temperature probe is inserted into the temperature probe hole of the sample loading device to monitor the soil temperature.

[0010] In an embodiment of the above system, the high-temperature end and the low-temperature end of the temperature controlled sleeve box of the frost heave holder are transparent U-shaped box bodies, the box walls are of a cavity structure and are covered with a heat-insulating cavity, the high-temperature end is connected to the water outlet interface of the circulating refrigeration device through a water replenishing pipe interface, and the low-temperature end is connected to the refrigerator through a coolant interface; the sample loading device is a detachable transparent shell, with permeable stones and cover plates fixed on both sides and connected through fixing screw holes.

[0011] In an embodiment of the above system, the freeze-thaw cycle holder is of a transparent design, the sample loading device uses an embedded buckle to fix the cover plate, and a slidable sedimentation plate is provided at the bottom; the return pipe inside the temperature controlled sleeve box is spiral, and uniform temperature rise and fall are realized through a single circulating refrigeration device.

[0012] In an embodiment of the above system, the magnetic poles of the low-field nuclear magnetic resonance scanning device are placed inside a magnetic pole temperature controller, the inner wall of the temperature and humidity controlled chamber is provided with a heat-insulating layer, and a humidity sensor and a temperature control module are integrated on the top; the gradient cabinet is used to apply RF pulses combined with a gradient field to achieve layered scanning and nuclear magnetic resonance imaging.

[0013] In an embodiment of the above system, in the water replenishing tank of the circulating refrigeration device, tank A replenishes liquid water through gravity flow, and tank B is connected to a temperature plate to generate controllable humidity gaseous water.

[0014] In one embodiment of the above system, a bracket is provided on the slide rail. The bracket adjusts the angle through a movable rotating port. A fixed interface is provided on the support rod and is connected by a fixing strip to enhance stability. The bracket can be folded and cooperates with the slide rail.

[0015] In one embodiment of the above system, each temperature control sleeve box is provided with a drain port and a blowing valve for discharging residual coolant. A temperature probe hole is opened on the side wall of the sample loader to insert the soil body optical fiber temperature probe.

[0016] In one embodiment of the above system, the slide rail of the low-field nuclear magnetic resonance scanning device can be docked with an external mechanical test device, and a multi-field coupling test is realized by driving through a slide rail motor.

[0017] A method for conducting a frozen soil frost heave test using the above-mentioned frozen soil frost heave and thaw test system is as follows:

[0018] Step 1: Prepare a saturated or unsaturated test soil sample and load it into the sample loader.

[0019] Step 2: Position the frost heave gripper between the magnetic poles through the slide rail and adjust the environmental parameters of the constant temperature and humidity chamber.

[0020] Step 3: Place a standard oil sample in the frost heave gripper, adjust equipment parameters such as pulse width, scan time, and sampling time to calibrate the nuclear magnetic parameters.

[0021] Step 4: Take out the oil sample, replace it with the sample loader containing the test soil sample, dock the sample loader with the temperature control sleeve box, connect the water replenishment pipe interface and the water supply pipe interface, insert the soil body optical fiber temperature probe and seal it, perform a pre-scan, and adjust the corresponding parameters according to the results to achieve the best effect.

[0022] Step 5: Start two circulating refrigeration devices, set the high-temperature end as the positive-temperature water replenishment area and the low-temperature end as the negative-temperature freezing area, check the temperature information on the numerical control display screen and adjust it appropriately, and turn on the circulating refrigeration to form a temperature gradient field.

[0023] Step 6: Select a liquid water or gaseous water replenishment mode through the water replenishment tank, start the nuclear magnetic device, set parameters such as the sampling time of the CPMG sequence, adjust the sequence parameters in combination with the gradient field to achieve layered scanning of the soil sample, and synchronously collect the nuclear magnetic relaxation signal and the frost heave displacement of the laser rangefinder.

[0024] Step 7: According to the segmented temperature data of the optical fiber temperature probe and in combination with the layered scanning results, obtain the moisture migration amount and the ice lens body distribution data in different temperature gradient segments.

[0025] A method for conducting a frozen soil freeze-thaw cycle test using the above-mentioned frozen soil frost heave and thaw test system is as follows:

[0026] Step 1: Prepare a saturated or unsaturated test soil sample and place it in the sample loader of the freeze-thaw cycle holder. Fix the cover plate with the embedded buckle, and adjust the bottom slidable aging plate to the preset position.

[0027] Step 2: Position the freeze-thaw cycle holder between the magnetic poles through the slide rail, and adjust the environmental parameters of the constant temperature and humidity chamber.

[0028] Step 3: Place a standard oil sample in the temperature control sleeve box, and adjust equipment parameters such as pulse width, scanning time, and sampling time for calibration.

[0029] Step 4: Take out the oil sample, put the test soil sample in the sample loader, dock the sample loader with the temperature control sleeve box, insert the fiber optic temperature probe for the soil body and seal it to ensure that the probe monitors the soil body temperature in segments, perform a pre-scan, and adjust the corresponding parameters according to the results to achieve the best effect.

[0030] Step 5: Start a single-stage cycle refrigeration device, set the cooling sequence and heating sequence according to the test requirements, measure the water content at different freeze-thaw stages, and ensure that the waiting time for each temperature cycle is greater than or equal to 1 hour to ensure the stability of the soil body temperature. The number of temperature sampling times for each group is 10 times to ensure the sampling accuracy.

[0031] Step 6: Start the nuclear magnetic equipment, set parameters such as the sampling time of the CPMG sequence, adjust the sequence parameters in combination with the gradient field to achieve layered scanning of the soil sample, and synchronously collect the nuclear magnetic relaxation signal and the frost heave displacement of the laser rangefinder.

[0032] Step 7: According to the scanning results of multiple freeze-thaw cycles and combined with the data of the temperature probe, obtain the dynamic distribution of unfrozen water in the soil body and the evolution law of freeze-thaw damage.

[0033] The beneficial effects of the present invention are as follows:

[0034] 1. It can realize intuitive measurement of the frost heave amount, water migration amount, ice formation situation of the soil body under the temperature-water double gradient, and the in-situ frost heave and unfrozen water distribution of the soil body under the action of freeze-thaw cycles from multiple angles and at different freezing stages. According to the experimental requirements, functions such as layered scanning and nuclear magnetic resonance imaging can also be realized.

[0035] 2. The bottom of the low-field nuclear magnetic scanning device is integrated with a slide rail, which can be docked with mechanical test equipment such as dynamic load, static load, and triaxial tests for multi-field coupling tests; at the same time, the slide rail is connected to the bracket, simplifying the operation space of the equipment and realizing different test angle requirements from horizontal to vertical.

[0036] 3. While supporting the soil body frost heave test, it can also measure the in-situ freezing and unfrozen water content distribution of the soil body under the action of freeze-thaw cycles, support large-size soil sample tests, and eliminate the errors caused by size effects.

[0037] 4. The temperature control system has multiple insurance designs to avoid temperature control failure caused by temperature disorder during the experiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Schematic diagram of the use state of the frost heaving test for an embodiment of the present invention;

[0039] Figure 2 is Figure 1 Enlarged schematic diagram of another perspective of the low-field nuclear magnetic resonance scanning device;

[0040] Figure 3 is Figure 1 Enlarged schematic diagram of the circulating refrigeration device;

[0041] Figure 4 is Figure 1 Enlarged schematic diagram of the frost heaving gripper;

[0042] Figure 5 is Figure 4 Schematic diagram of the structure of the temperature control jacket box;

[0043] Figure 6 is Figure 4 Schematic diagram of the structure of the sample loader;

[0044] Figure 7 is Figure 1 Enlarged schematic diagram of the bracket;

[0045] Figure 8 is Figure 7 Schematic diagram of the use state of the bracket;

[0046] Figure 9 Schematic diagram of the use state of the freeze-thaw cycle test for another embodiment of the present invention;

[0047] Figure 10 is Figure 9 Enlarged schematic diagram of the freeze-thaw cycle gripper;

[0048] Figure 11 is Figure 10 Schematic diagram of the structure of the temperature control jacket box;

[0049] Figure 12 is Figure 10 Schematic diagram of the structure of the sample loader;

[0050] The reference numerals are as follows:

[0051] 1 - Low-field nuclear magnetic resonance scanning device, 11 - Magnetic pole, 12 - Temperature and humidity chamber, 13 - Slide rail, 14 - Slide rail motor, 15 - Support, 16 - Spectrometer cabinet, 17 - Radio frequency cabinet, 18 - Gradient cabinet, 19 - Power supply;

[0052] 2 - Circulating refrigeration device, 21 - Coolant reservoir, 22 - Exchanger, 23 - Refrigerator, 24 - Water replenishing tank, 25 - Temperature plate, 26 - Numerical control display screen, 27 - Temperature sensor interface, 28 - Refrigerant interface, 29 - Water pipe interface;

[0053] 3 - Temperature control sleeve box, 31 - Thermal insulation cavity, 32 - Coolant interface, 33 - Temperature sensor, 34 - Water replenishing pipe interface, 35 - Permeable stone, 36 - Laser rangefinder, 37 - Support interface, 38 - Drain port, 39 - Blowing valve;

[0054] 4 - Sampler, 41 - Fixed screw hole, 42 - Cover plate, 43 - Temperature probe hole;

[0055] 5 - Soil fiber optic temperature probe;

[0056] 6 - Support, 61 - Support rod, 62 - Movable rotating port, 63 - Fixed interface, 64 - Fixed strip;

[0057] 7 - Freeze - thaw cycle gripper, 71 - Loop pipe, 72 - Buckle; 73 - Specimen plate. Detailed implementation mode

[0058] Next, the related technical solutions will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0059] Embodiment 1. As Figure 1 shown, this frost heave test system for frozen soil based on low - field nuclear magnetic resonance disclosed in this embodiment includes a low - field nuclear magnetic resonance scanning device 1, a circulating refrigeration device 2, a frost heave gripper, a soil fiber optic temperature probe 5, and a support 6.

[0060] As Figure 2 shown, the low - field nuclear magnetic resonance scanning device 1 includes magnetic poles 11, a temperature - and humidity - controlled chamber 12, a slide rail 13, a slide rail motor 14, a support 15, a spectrometer cabinet 16, a radio frequency cabinet 17, a gradient cabinet 18, and a power supply 19.

[0061] The magnetic poles 11 are symmetrically arranged beside the two side walls of the temperature - and humidity - controlled chamber 12, and are used to provide a stable static magnetic field required for low - field nuclear magnetic resonance. The magnetic poles are placed in a magnetic pole temperature controller, and the magnetic pole temperature controller is fixedly connected to the side wall of the temperature - and humidity - controlled chamber, and is used to maintain the constant - temperature environment of the magnetic poles 11; the magnetic field temperature in this embodiment is set to 32 °C.

[0062] The center of the box body of the constant temperature and humidity chamber 12 is a sample storage space. The inner wall is provided with a heat preservation layer, and a humidity sensor and a temperature control module are arranged on the top to maintain a constant temperature and humidity inside the box. During the test, the temperature and humidity inside the box are adjusted according to the test requirements to isolate the interference of water condensation in the air to the low-temperature test as much as possible.

[0063] A slide rail 13 is longitudinally laid at the bottom of the inner cavity of the constant temperature and humidity chamber 12. The slide rail is driven by a slide rail motor 14 to support horizontal sliding.

[0064] A support 15 that can move along the slide rail is arranged on the slide rail 13, and a support interface is arranged on the support.

[0065] Below the two magnetic poles 11, a spectrometer cabinet 16, a radio frequency cabinet 17, a gradient cabinet 18 and a power supply 19 are symmetrically arranged from top to bottom.

[0066] The low-field nuclear magnetic resonance scanning device 1 is built-in with a computer and a signal processing module, and is connected to the magnetic pole 11, the spectrometer cabinet 16, the radio frequency cabinet 17 and the gradient cabinet 18 through optical fibers.

[0067] The spectrometer cabinet 16 and the radio frequency cabinet 17 are used to emit radio frequency pulses to apply a secondary magnetic field, and its direction is perpendicular to the main external magnetic field at 90° to excite atomic nuclei and record the free induction decay (FID) generated when the excited atomic nuclei return to the equilibrium direction through the relaxation process.

[0068] The gradient cabinet 18 is used to amplify signals, increase the signal-to-noise ratio, and make the scanning experiment more accurate. In addition, an RF pulse (Sinc envelope) can be applied in combination with a gradient field (Gz) to achieve layer-by-layer scanning. On this basis, a phase encoding gradient and a frequency encoding gradient are applied to achieve nuclear magnetic resonance imaging, and then the water migration phenomenon during soil frost heaving can be monitored more intuitively.

[0069] The power supply 19 provides a regulated power supply for each functional cabinet.

[0070] A storage cabinet is arranged below the constant temperature and humidity chamber 12 for storing standard oil samples, tools and spare parts.

[0071] As Figure 3 shown, the circulating refrigeration device 2 of this embodiment is composed of two devices. The circulating refrigeration device includes a coolant reservoir 21, a switch 22, a refrigerator 23, a water replenishing tank 24, a temperature plate 25, a numerical control display screen 26, a temperature sensor interface 27, a refrigerant interface 28 and a water pipe interface 29.

[0072] The coolant reservoir 21 stores a fluorinated liquid cooling medium and is connected to the switch 22 through a pipeline. The switch is connected to the refrigerator 23 through a bottom pipeline to maintain the circulating pressure and flow rate of the coolant.

[0073] The refrigerating machine 23 can adjust the temperature of the fluorinated liquid by heating or cooling according to the coolant temperature data fed back by the temperature sensor. The temperature-controlled coolant is conveyed to the frost heave gripper 3 through the coolant interface 28.

[0074] Among the two devices, one is equipped with a water replenishing tank 24 and a temperature plate 25.

[0075] The water replenishing tank 24 is divided into two independent tanks, namely Tank A and Tank B. Tank A is used for liquid water replenishment, and the bottom of the tank is horizontally aligned with the water conveying pipe interface 29 to avoid the interference of the water head difference. Tank B is connected to the temperature plate 25 and can vaporize water into gas to simulate the migration of gaseous water. Scale lines and electronic sensors are provided on the side wall of the tank to monitor the water replenishment amount in real time and feed it back to the numerical control display screen 26.

[0076] The temperature sensor interface 27 is connected to the coolant temperature sensor of the frost heave gripper 3 to collect the coolant temperature data in real time.

[0077] The numerical control display screen 26 is integrated on the upper part of the circulating refrigeration device 2, and displays the coolant temperature, water replenishment amount, data of the soil temperature probe 4, and frost heave displacement amount, and supports manual adjustment of the temperature control parameters.

[0078] The water conveying pipe interface 29 is connected to the water replenishing pipe interface of the frost heave gripper 3 through a water conveying pipe to provide external water source replenishment for the test.

[0079] Combined Figure 4 、 Figure 5 and Figure 6 It can be seen that the frost heave gripper includes a temperature control sleeve box 3 and a sample loader 4. The temperature control sleeve box is divided into two independent parts, namely the high-temperature end and the low-temperature end. The sample loader is used to hold the soil sample.

[0080] The high-temperature end and the low-temperature end of the temperature control sleeve box 3 are transparent U-shaped box bodies that can be docked left and right. The box walls are all cavities, and a heat insulation cavity 31 is provided around the outside to reduce external heat interference; the sides of both are connected to the coolant interface 28 of the circulating refrigeration device 2 through the coolant interface 32; a valve is also installed at the liquid outlet of the coolant, and the coolant temperature is monitored in real time through the temperature sensor 33; among them, the high-temperature end is also connected to the water conveying pipe interface 29 of the circulating refrigeration device through the water replenishing pipe interface 34.

[0081] A permeable stone 35 is arranged inside the high-temperature end to uniformly absorb the externally replenished water.

[0082] A laser rangefinder 36 is provided inside the low-temperature end, which can measure the frost heave displacement amount of the soil sample at three levels: upper, middle, and lower.

[0083] Four support interfaces 37 are provided on the side walls of the high-temperature end and the low-temperature end of the temperature control sleeve box for fixed connection with the support 5 to support horizontal or vertical test angles.

[0084] The high-temperature end and the low-temperature end of the temperature control box are also provided with a liquid discharge port 38 and a blowing valve 39. The liquid discharge port is used to discharge the residual fluorinated liquid after the test; the blowing valve is used to pressurize or inject water into the cavity to completely discharge the residual fluorinated liquid in the cavity circuit.

[0085] The sampler 4 is a transparent housing divided into two halves, which can be connected through 4 fixing screw holes 41 in the middle and is used to accommodate the soil sample. After the test, the sampler can be disassembled, which is more convenient for disassembling the sample. At the same time, the ice lens body can be directly observed and its volume can be measured, avoiding the situation that the soil sample freezes together with the sampler and causing unnecessary experimental errors.

[0086] A permeable stone 35 is fixed on one side inside the sampler 4, and a detachable cover plate 42 is provided on the other side to reserve a frost heaving space for the soil sample during the test.

[0087] A temperature probe hole 43 is opened on the side wall of the sampler 4 for inserting the soil optical fiber temperature probe 5.

[0088] The high-temperature end and the low-temperature end of the temperature control box 3 are symmetrically fixed on both sides of the sampler through the fixing screw holes 41 to form a closed temperature control environment.

[0089] As Figure 7 and Figure 8 shown, the bracket 6 adopts a foldable truss structure, including a support rod 61, a movable rotating port 62 and a fixed interface 63.

[0090] The bracket 6 is composed of two groups of symmetrically arranged support rods 61 as support arms to form a main frame. An activity rotating port 62 is provided at the intersection of the support rods 61. A rotating shaft is built in the activity rotating port to allow the bracket to freely adjust the angle.

[0091] Fixed interfaces 63 are provided on the support rod 61 at fixed intervals along the length direction.

[0092] After the bracket 6 adjusts the angle, it can be fixed by fixing bars 64 with different lengths; multiple fixing interfaces are opened on the fixing bars, which are docked and fixed with the fixing interfaces on the bracket.

[0093] This bracket has good foldability and strength. When it is idle, it can be folded and placed beside the slide rail. When in use, the fixing bar is taken out and connected to the fixing interface to achieve different support angles and enhance stability. At the same time, this bracket can be used in cooperation with the slide rail to flexibly adjust the bracket spacing and distance to cooperate with other types of grippers. In addition, it can also be combined with triaxial, displacement and other equipment to meet the needs of multi-functional multi-field coupling tests.

[0094] The specific steps for conducting a frost heaving test using the frost heaving test system for frozen soil based on low-field nuclear magnetic resonance in this embodiment are as follows:

[0095] Step 1: Prepare a saturated or unsaturated test soil sample and load it into the sampler;

[0096] Step 2: Position the frost heave gripper between the magnetic poles through the slide rail and adjust the environmental parameters of the thermostatic and humidistatic chamber.

[0097] Step 3: Place a standard oil sample in the frost heave gripper, adjust equipment parameters such as pulse width, scan time, and sampling time to calibrate the nuclear magnetic parameters.

[0098] Step 4: Take out the oil sample, replace the sampler with the test soil sample, dock the sampler with the temperature control jacket box, connect the water replenishment pipe interface and the water delivery pipe interface, insert the soil optical fiber temperature probe and seal it, and perform a pre-scan.

[0099] Step 5: Start two circulating refrigeration devices, set the high-temperature end as the positive-temperature water replenishment area and the low-temperature end as the negative-temperature freezing area, check the temperature information on the numerical control display screen and adjust appropriately, and turn on the circulating refrigeration to form a temperature gradient field.

[0100] Step 6: Select the liquid water or gaseous water replenishment mode through the water replenishment water tank, start the nuclear magnetic equipment, set parameters such as the sampling time of the CPMG sequence, and realize layered scanning of the soil sample in combination with the gradient field, and synchronously collect the nuclear magnetic relaxation signal and the frost heave displacement of the laser rangefinder.

[0101] Step 7: According to the segmented temperature data of the optical fiber temperature probe in the soil, combined with the layered scanning results, obtain the water migration amount and the ice lens body distribution data in different temperature gradient segments.

[0102] When using this embodiment for the frost heave test, preferably select the number and serial number of the soil temperature probes used in the test. The temperature probes in the soil only display the temperatures of different parts of the soil and do not control the program for the temperature sequence. The two circulating refrigerators independently control the temperatures of the two ends of the temperature control jacket box. The water replenishment end is the high-temperature end and the other end is the low-temperature end, and the test temperature is set to form a gradient field.

[0103] If the test needs to explore the frost heave amount and water migration amount of the soil at different angles, use the bottom bracket to connect with the temperature control jacket box to adjust the test angle. Adjust the temperature in the thermostatic and humidistatic chamber to meet the test requirements, and check the coolant liquid level and the water tank liquid level. Select the test measurement time interval to obtain the water migration amount, frost heave amount, and the formation of ice lens bodies at different freezing times or freezing degrees during the soil freezing process. Segmentally scan the soil according to the number of soil temperature probes inserted to obtain the water migration amount in different temperature gradient segments.

[0104] The advantages of using this test system are as follows:

[0105] 1. Set a temperature sensor at the liquid outlet of the jacket box to form an independent temperature measurement channel with the soil optical fiber temperature probe. Avoid the heat released during the ice-water phase change process when the soil freezes from disturbing the temperature probe in the soil, resulting in unstable temperature sequence and inaccurate temperature measurement.

[0106] 2. Set the cavity of the temperature control box. After being temperature-controlled by the refrigerator, the coolant flows into the cavity, and the effective contact area is increased through the loop to keep the sample at the temperature required for the experiment. The numerical control display screen can display the temperature of the soil temperature probe, the temperature of the coolant in the refrigerator, the temperature of the coolant in the temperature control box, and the frost heave displacement of the soil in real time, and the real-time display of the test situation facilitates the operator to adjust.

[0107] 3. The water replenishing water tank adopts an independent compartment structure. The A water tank supplies liquid water by gravity advection, and the B water tank is connected to the temperature plate to generate controllable humidity gaseous water. This design can simulate various actual working conditions such as groundwater recharge and atmospheric condensation alone or in combination.

[0108] Example 2. As Figure 9 shown, the frozen soil freeze-thaw test system based on low-field nuclear magnetic resonance disclosed in this embodiment is different from Embodiment 1 in that the gripper is a freeze-thaw cycle gripper, which has a different structure from the frost heave gripper in Embodiment 1.

[0109] As Figure 10 、 Figure 11 and Figure 12 shown, the temperature control box of the freeze-thaw cycle gripper 7 is only connected to a single-cycle refrigeration device, and a spiral loop pipe 71 is arranged inside the cavity of the temperature control box. The contact area of the coolant is increased through the loop pipe to improve the temperature transfer efficiency of large-size soil samples.

[0110] While Embodiment 1 adopts a connection structure of dual refrigerators with independent high-temperature and low-temperature ends on the left and right. Because the freeze-thaw cycle test needs to realize the overall uniform temperature rise and fall of the soil body, rather than the temperature gradient field control requirement in Embodiment 1.

[0111] The sample loader of the freeze-thaw cycle gripper 7 uses an embedded buckle 72 to fix the cover plate, and a slidable sedimentation plate 73 is added at the bottom. The quick opening and closing are realized through the buckle design, which is convenient for directly taking out the frozen soil sample after the test without disassembling the shell; the sedimentation plate can move along the chute, which is used to precisely adjust the initial position of the soil sample and support soil samples of different heights.

[0112] The freeze-thaw cycle gripper 7 supports soil bodies of various diameter sizes respectively, and different numbers of temperature probes can be inserted according to the size of the soil sample, and each probe measures temperature independently. Only when the probe temperatures are the same is it considered that the soil body temperature is stable, and after measurement, it enters the next temperature sequence.

[0113] The specific steps for conducting a freeze-thaw cycle test using the frozen soil frost heave test system based on low-field nuclear magnetic resonance in this embodiment are as follows:

[0114] Step 1. Prepare a saturated or unsaturated test soil sample and load it into the sample loader of the freeze-thaw cycle gripper, fix the cover plate through the embedded buckle, and adjust the slidable sedimentation plate at the bottom to the preset position;

[0115] Step 2: Position the freeze-thaw cycle clamp between the magnetic poles through the slide rail and adjust the environmental parameters of the thermo-hygrostat chamber.

[0116] Step 3: Place a standard oil sample in the temperature-controlled jacket box, and adjust equipment parameters such as pulse width, scan time, and sampling time for calibration.

[0117] Step 4: Take out the oil sample, put the test soil sample in the sampler, connect the sampler to the temperature-controlled jacket box, insert the soil fiber optic temperature probe and seal it to ensure that the probe monitors the soil temperature in segments, conduct a pre-scan, and adjust the corresponding parameters according to the results to achieve the best effect.

[0118] Step 5: Start a single-stage cycle refrigeration device, set the cooling sequence and heating sequence according to the test requirements, measure the water content at different freeze-thaw stages, wait for at least 1 hour for each temperature cycle to ensure the stability of the soil temperature, and sample 10 times for each temperature to ensure the sampling accuracy.

[0119] Step 6: Start the nuclear magnetic resonance equipment, set parameters such as the sampling time of the CPMG sequence, adjust the sequence parameters in combination with the gradient field to achieve layered scanning of the soil sample, and simultaneously collect the nuclear magnetic relaxation signal and the frost heave displacement of the laser rangefinder.

[0120] Step 7: Based on the scanning results of multiple freeze-thaw cycles and combined with the temperature probe data, obtain the dynamic distribution of unfrozen water in the soil and the evolution law of freeze-thaw damage.

[0121] When conducting the freeze-thaw cycle test, the soil temperature is considered stable only when the temperature error amplitude of multiple temperature probes in the soil does not exceed ±0.2 °C, and nuclear magnetic resonance scanning can be carried out. After the scanning is completed, the next temperature control is performed.

[0122] If the temperature of the soil probe is disordered, the program takes the temperature of the fluorinated liquid as the standard to keep the temperature stable, avoiding temperature loss or overcooling caused by temperature fluctuations and thus avoiding experimental errors.

[0123] If only one soil temperature probe is used and the temperature fluctuates violently, the temperature of the fluorinated liquid is taken as the standard, and the temperature is not additionally controlled to rise or fall until the temperature is stable and then the temperature measured by the temperature probe is taken as the standard.

[0124] If it is necessary to explore the influence of the coupling of static and dynamic loads and other conditions on the freeze-thaw characteristics of the soil, the bracket or the freeze-thaw cycle clamp can be connected to the slide rail in the preparation stage. The other end of the slide rail is connected to the matching triaxial or vibration instrument. The slide rail supports manual or motor control, and the motor control can automatically realize the nuclear magnetic resonance measurement of multi-field coupling.

[0125] Multi-field coupling requires separate program control and supporting equipment. This equipment only provides freeze-thaw cycle tests and reserved motor slide rail interfaces, and supports the combined use of a series of supporting test facilities.

[0126] The significance of using the test system of this embodiment lies in:

[0127] The frost heave test focuses on the research of moisture migration and the formation mechanism of ice lenses under the temperature gradient field. The freeze-thaw cycle test focuses on the evolution of freeze-thaw damage, the dynamic distribution of unfrozen water and the multi-field coupling effect in the uniform temperature field. Through the structural differential design of the two, it forms a functional complement with Embodiment 1, and jointly constructs a comprehensive test platform covering the whole process of "frost heave-freeze-thaw" of frozen soil, forming a complete test system.

[0128] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the above embodiments have been described in detail, those skilled in the art can still modify the technical solutions recorded in the above embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A frost heaving and freeze-thaw test system for frozen soil based on low-field nuclear magnetic resonance, characterized in that: It includes a low-field nuclear magnetic resonance scanning device, a circulating refrigeration device, a holder, and a soil fiber temperature probe; The low-field nuclear magnetic resonance scanning device includes symmetrically arranged magnetic poles, a temperature and humidity controlled box, a slide rail, a support, a spectrometer cabinet, a radio frequency cabinet, and a gradient cabinet; the slide rail is arranged at the bottom of the inner cavity of the temperature and humidity controlled box and is driven to slide horizontally by a slide rail motor; the magnetic poles are connected to the spectrometer cabinet, the radio frequency cabinet, and the gradient cabinet for excitation and acquisition of nuclear magnetic resonance signals; The circulating refrigeration device includes a coolant reservoir, a refrigerator, a water replenishing tank, and a temperature sensor interface; the refrigerator is connected to the coolant reservoir through a pipeline for controlling the temperature of the coolant circulation; the water replenishing tank is divided into two independent tanks, A and B, which respectively provide liquid water and gaseous water replenishment; The holder is arranged on the slide rail and is composed of a temperature controlled sleeve box and a sample loading device, and is divided into a frost heave holder or a freeze-thaw cycle holder; the temperature controlled sleeve box of the frost heave holder is divided into a high-temperature end and a low-temperature end, which are respectively connected to two circulating refrigeration devices to form a test temperature gradient field; the temperature controlled sleeve box of the freeze-thaw cycle holder is provided with a spiral loop pipe inside and is connected to a single circulating refrigeration device; The soil fiber temperature probe is inserted into the temperature probe hole of the sample loading device to monitor the soil temperature.

2. The frost heaving and freeze-thaw test system for frozen soil based on low-field nuclear magnetic resonance according to claim 1, characterized in that: The high-temperature end and the low-temperature end of the temperature controlled sleeve box of the frost heave holder are transparent U-shaped box bodies, the box walls are of a cavity structure and are covered with a heat preservation and isolation cavity outside, the high-temperature end is connected to the water delivery pipe interface of the circulating refrigeration device through a water replenishing pipe interface, and the low-temperature end is connected to the refrigerator through a coolant interface; the sample loading device is a detachable transparent shell, with permeable stones and cover plates fixed on both sides and connected through fixing screw holes.

3. The frost heaving and freeze-thaw test system for frozen soil based on low-field nuclear magnetic resonance according to claim 1, wherein: The freeze-thaw cycle holder is of a transparent design, the sample loading device uses an embedded buckle to fix the cover plate, and a slidable sedimentation plate is provided at the bottom; the loop pipe inside the temperature controlled sleeve box is spiral, and uniform temperature rise and fall are realized through a single circulating refrigeration device.

4. The frost heaving and freeze-thaw test system for frozen soil based on low-field nuclear magnetic resonance according to claim 1, wherein: The magnetic poles of the low-field nuclear magnetic resonance scanning device are placed inside a magnetic pole temperature controller, the inner wall of the temperature and humidity controlled box is provided with a heat preservation layer, and a humidity sensor and a temperature control module are integrated on the top; the gradient cabinet is used to apply RF pulses combined with a gradient field to realize layered scanning and nuclear magnetic resonance imaging.

5. The frost heaving and freezing-thawing test system for frozen soil based on low-field nuclear magnetic resonance according to claim 1, characterized in that: In the water replenishing tank of the circulating refrigeration device, tank A replenishes liquid water by gravity advection, and tank B is connected to a temperature plate to generate controllable humidity gaseous water.

6. The frost heaving and freezing-thawing test system for frozen soil based on low-field nuclear magnetic resonance according to claim 1, wherein: A bracket is arranged on the slide rail, the angle of the bracket is adjusted through a movable rotating port, a fixed interface is provided on the support rod, and the support rod is connected through a fixing strip to enhance stability; the bracket can be folded and cooperates with the slide rail.

7. The low-field nuclear magnetic resonance-based frost heaving and freeze-thaw test system according to claim 2 or 3, characterized in that: The temperature controlled sleeve boxes are all provided with a drain port and a blowing valve for discharging residual coolant; temperature probe holes are opened on the side walls of the sample loading device for inserting the soil fiber temperature probe.

8. The frost heaving and freezing-thawing test system for frozen soil based on low-field nuclear magnetic resonance according to claim 1, characterized in that: The slide rail of the low-field nuclear magnetic resonance scanning device can be docked with external mechanical test equipment, and a multi-field coupling test is realized through the drive of the slide rail motor.

9. A method for conducting a frost heave test on frozen soil using the frozen soil frost heave and freeze-thaw test system according to any one of claims 1, 2, 4-8, the specific steps are as follows: Step 1: Prepare a saturated or unsaturated test soil sample and load it into the sample loading device; Step 2: Position the frost heave holder between the magnetic poles through the slide rail and adjust the environmental parameters of the temperature and humidity controlled box; Step 3: Place a standard oil sample in the frost heave holder, adjust equipment parameters such as pulse width, scanning time, sampling time, etc. to correct the nuclear magnetic parameters; Step 4: Take out the oil sample, replace the sampler for placing the test soil sample, dock the sampler with the temperature control jacket box, connect the water replenishment pipe interface with the water delivery pipe interface, insert the soil fiber temperature probe and seal it, and perform a pre-scan; Step 5: Start two circulating refrigeration devices, set the high-temperature end as the positive-temperature water replenishment area and the low-temperature end as the negative-temperature freezing area, check the temperature information on the numerical control display screen and adjust appropriately, and turn on the circulating refrigeration to form a temperature gradient field; Step 6: Select the liquid water or gaseous water replenishment mode through the water replenishment water tank, start the nuclear magnetic equipment, set parameters such as the sampling time of the CPMG sequence, and realize layer-by-layer scanning of the soil sample in combination with the gradient field, and synchronously collect the nuclear magnetic relaxation signal and the frost heave displacement of the laser rangefinder; Step 7: According to the segmented temperature data of the fiber temperature probe and combined with the layer-by-layer scanning results, obtain the moisture migration amount and the ice lens body distribution data in different temperature gradient segments.

10. A method for conducting a frozen soil freeze-thaw cycle test using the frozen soil frost heave and thaw test system according to any one of claims 1, 3-8, the specific steps are as follows: Step 1: Prepare a saturated or unsaturated test soil sample and place it in the sampler of the freeze-thaw cycle holder, fix the cover plate through the embedded buckle, and adjust the bottom slidable sedimentation plate to the preset position; Step 2: Position the freeze-thaw cycle holder between the magnetic poles through the slide rail and adjust the environmental parameters of the constant temperature and humidity chamber; Step 3: Place a standard oil sample in the temperature control jacket box, adjust equipment parameters such as the pulse width, scanning time, and sampling time for calibration; Step 4: Take out the oil sample, put the test soil sample in the sampler, dock the sampler with the temperature control jacket box, insert the soil fiber temperature probe and seal it, ensure that the probe monitors the soil temperature in segments, and perform a pre-scan; Step 5: Start a single circulating refrigeration device, set the cooling sequence and heating sequence according to the test requirements, measure the moisture content in different freeze-thaw stages, the waiting time for each temperature cycle is greater than or equal to 1 hour to ensure the stability of the soil temperature, and the number of temperature samplings for each group is 10 times to ensure the sampling accuracy; Step 6: Start the nuclear magnetic equipment, set parameters such as the sampling time of the CPMG sequence, and realize layer-by-layer scanning of the soil sample in combination with the gradient field, and synchronously collect the nuclear magnetic relaxation signal and the frost heave displacement of the laser rangefinder; Step 7: According to the scanning results of multiple freeze-thaw cycles and combined with the temperature probe data, obtain the dynamic distribution of unfrozen water in the soil and the evolution law of freeze-thaw damage.

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