Testing method and device for water migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance

Abstract

The present application discloses a method and device for testing moisture migration during the freezing and thawing process of fiber cement soil based on low-field nuclear magnetic resonance, which relates to the field of cement soil testing engineering. The method includes obtaining the T2 spectrum of the fiber cement soil sample during the entire freezing and thawing process; inverting the three-dimensional moisture distribution map of the fiber cement soil sample during the entire freezing and thawing process based on the T2 spectrum; and determining the moisture migration of the fiber cement soil sample during the entire freezing and thawing process based on the three-dimensional moisture distribution map of the fiber cement soil sample during the entire freezing and thawing process. The present application is based on nuclear magnetic resonance (NMR) signal gradient positioning technology, maps T2 spectrum data to a three-dimensional spatial position, and obtains a pixel-level three-dimensional moisture distribution map of the fiber cement soil sample by performing data inversion on the T2 spectrum, thereby realizing pixel-level visualization of moisture migration during the freezing and thawing process of the fiber cement soil.

Description

Technical Field

[0001] The present application relates to the field of cement soil testing engineering, and in particular to a method and device for testing moisture migration during the frost heave and thaw settlement process of fiber cement soil based on low-field nuclear magnetic resonance. Background Art

[0002] Artificial ground freezing is widely used in the construction of underground projects. However, during the freezing process, soil often experiences frost heave and thaw settlement deformation, which is one of the main freezing responses of soil directly related to freezing construction. It poses a high risk to the safety and stability of underground construction, and also exerts pressure and deformation on structures above or below the ground, potentially causing structural damage or destruction. To reduce frost heave and thaw settlement deformation and enhance soil strength, on-site grouting is used to improve and reinforce the soil with cement to form cement soil before artificial freezing construction. Cement reinforcement can further optimize the freezing effect by suppressing frost heave and thaw settlement deformation and increasing strength, providing greater protection for construction safety. It is widely used in underground engineering construction and complex working conditions.

[0003] With technological advancements, fiber incorporation technology has also shown potential in improving soil mechanical properties and reducing frost heave and thaw settlement deformation. Fiber incorporation can partially offset the effects of cement, reducing cement dosage and significantly reducing the project's carbon footprint and environmental impact, creating a green and environmentally friendly approach. Plant fibers, such as sisal fibers, in particular, possess excellent tensile strength (mechanical properties) and water absorption. Therefore, investigating the thaw settlement deformation characteristics of fiber-cement soils at different temperatures and pressures, and explaining their mechanisms from the perspective of water migration, can provide relevant references for the design and application of freezing methods for soft soils in coastal areas, promoting greener, more economical, and safer underground engineering construction in coastal areas.

[0004] Existing moisture migration tests primarily rely on image recognition or X-ray tomography. X-ray scanning exploits the differential absorption of X-rays by materials of varying densities as they penetrate a sample, reconstructing internal tomographic images through a computer. However, this approach is limited in sample size and time-consuming. Image recognition technology, a recent development, combines computer vision algorithms with cameras to identify and extract moisture. However, its resolution is limited and requires high imaging accuracy and minimal noise interference. Consequently, the accuracy of existing moisture migration analysis results is limited. Summary of the Invention

[0005] The purpose of this application is to provide a method and device for testing moisture migration during the freezing and thawing process of fiber cement soil based on low-field nuclear magnetic resonance, which can analyze the moisture migration of fiber cement soil during the freezing and thawing process in real time and accurately.

[0006] To achieve the above objectives, this application provides the following solutions:

[0007] In a first aspect, the present application provides a method for testing moisture migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance, comprising:

[0008] Obtaining T2 spectra of the fiber cement soil sample during the entire freezing process and the entire melting process; the T2 spectra of the fiber cement soil sample during the entire freezing process and the entire melting process are T2 spectra obtained by low-field nuclear magnetic resonance technology during the freeze-heave and thaw-settlement test of the fiber cement soil sample;

[0009] According to the T2 spectrum of the fiber cement soil sample during the freezing and melting process, the pixel-level three-dimensional moisture distribution map of the fiber cement soil sample during the freezing and melting process is inverted;

[0010] The pixel-level moisture migration of the fiber cement soil sample during the freezing and melting processes was determined based on the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample during the freezing and melting processes.

[0011] In a second aspect, the present application provides a low-field nuclear magnetic resonance-based moisture migration testing device for fiber cement soil during frost heave and thaw settlement, comprising:

[0012] Triaxial compression component, used to simulate the real confining pressure and real axial pressure of fiber cement soil samples under the target environment;

[0013] Seepage component, used to simulate the actual seepage conditions of fiber cement soil samples under the target environment;

[0014] a temperature regulating component for gradually freezing the fiber cement soil sample to a target temperature and gradually thawing the frozen fiber cement soil sample;

[0015] Imaging monitoring component, used to collect T2 spectra of fiber cement soil samples during the entire freezing and melting process in real time based on low-field nuclear magnetic resonance technology;

[0016] The data analysis component is used to perform the above-mentioned low-field nuclear magnetic resonance-based water migration test method during the frost heave and thaw settlement process of fiber cement soil.

[0017] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0018] This application provides a method and apparatus for testing moisture migration during the freeze-heave and thaw-settlement process of fiber-cement soil based on low-field nuclear magnetic resonance. The method acquires T2 spectra of fiber-cement soil samples during the entire freezing and thawing process. Based on these T2 spectra, a three-dimensional moisture distribution map of the fiber-cement soil samples during the freezing and thawing process is generated. Water migration during these processes is then determined. This application utilizes low-field nuclear magnetic resonance (LF-NMR) technology to construct moisture distribution images of fiber-cement soil samples at different freeze-thaw stages through inversion calculation of T2 relaxation spectra, thereby determining moisture migration. Unlike traditional image acquisition methods, this application uses NMR signal gradient localization technology to map T2 spectrum data to three-dimensional spatial locations. By inverting the T2 spectrum data, a pixel-level three-dimensional moisture distribution map of the fiber-cement soil sample is generated. This allows for pixel-level visualization of moisture migration during the freezing and thawing processes of fiber-cement soil, enabling high-precision analysis of moisture migration. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 A flow chart of a method for testing moisture migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance according to one embodiment of the present application;

[0021] Figure 2 A schematic diagram of a low-field nuclear magnetic resonance-based device for testing moisture migration during frost heave and thaw settlement in fiber cement soil according to one embodiment of the present application;

[0022] Figure 3 A schematic structural diagram of a triaxial pressurizing assembly provided in one embodiment of the present application;

[0023] Figure 4 A schematic structural diagram of a sample unit provided in one embodiment of the present application;

[0024] Figure 5 A schematic diagram of a test process implemented by introducing specific components provided in one embodiment of the present application;

[0025] Figure 6 A schematic diagram of water migration during the melting process provided in one embodiment of the present application;

[0026] Reference numerals:

[0027] 1. Imaging monitoring assembly, 2. Triaxial pressurization assembly, 3. Confining pressure chamber, 4. Sample unit, 5. Upper ejector pin, 6. Lower ejector pin, 7. Upper plug, 8. Sealing ring, 9. Permeable stone, 10. Filter paper, 11. Latex membrane, 12. Lower plug, 13. Seepage outlet pipe, 14. Seepage inlet pipe, 15. Low-temperature circulation pump, 16. Circulation pipe, 17. Three-way valve, 18. Confining pressure pipe, 19. Axial pressure pipe, 20. Peristaltic pump, 21. Compensation pipe, 22. Triaxial pressure controller, 23. Seepage pipe, 24. Connecting valve. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0029] Nuclear magnetic resonance (NMR) is widely used to examine core pore structure, identify the formation and decomposition of natural gas hydrates, and measure the unfrozen water content in frozen soil. Compared to traditional methods for testing frost heave and thaw settlement characteristics, NMR is the only method that can directly and non-destructively measure water molecule content. Therefore, this application proposes a method and device for testing water migration during the frost heave and thaw settlement of fiber cement soil based on low-field NMR. This method can accurately and dynamically analyze water migration in fiber cement soil during freezing and thawing processes in real time.

[0030] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0031] In an exemplary embodiment of the present application, a method for testing water migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance is proposed. Figure 1 As shown, the following steps are included.

[0032] S1: Obtaining T2 spectra of the fiber cement soil sample during the entire freezing process and the entire melting process; the T2 spectra of the fiber cement soil sample during the entire freezing process and the entire melting process are T2 spectra obtained by low-field nuclear magnetic resonance technology during the freeze-heave and thaw-settlement test of the fiber cement soil sample.

[0033] S2: The pixel-level three-dimensional moisture distribution map of the fiber cement soil sample during the freezing and melting processes is inverted based on the T2 spectrum of the fiber cement soil sample during the freezing and melting processes.

[0034] S3: Determine the pixel-level moisture migration during the entire freezing and thawing processes of the fiber cement soil sample based on the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample during the entire freezing and thawing processes.

[0035] In this application, by combining the real-time and convenience of the frost heave and thaw settlement tests and nuclear magnetic imaging, the T2 spectra are collected in real time at different freeze-thaw stages to reveal the moisture migration law of fiber cement soil. Different from traditional image acquisition methods, the test method of this application is based on the NMR signal gradient positioning technology. By mapping the T2 spectrum data to the three-dimensional spatial position, the visualization of the pixel-level moisture migration process is realized, and the moisture migration during the freezing and thawing processes of fiber cement soil can be analyzed dynamically in real time, and the accuracy and test efficiency of the test results are improved.

[0036] In another exemplary embodiment of this application, in step S2, the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample during the entire freezing and thawing processes are inverted from the T2 spectra of the fiber cement soil sample during the entire freezing and thawing processes, specifically including:

[0037] (2-1) Determine the three-dimensional spatial distribution information of each pixel point of the fiber cement soil sample during the entire freezing and thawing processes according to the three-dimensional gradient magnetic field in low-field nuclear magnetic resonance technology.

[0038] Three-dimensional space filling: The T2 spectral signals of each pixel point in the XYZ three-dimensional space are accurately positioned at the signal source position through the gradient magnetic field, so as to remap the collected initial T2 signal (the time-domain signal that decays with time) into a spatial distribution, and generate a T2 spectrum with three-dimensional spatial distribution information.

[0039] (2-2) Determine the moisture state of each pixel point according to the T2 spectrum with three-dimensional spatial distribution information by applying the Laplace inverse transform method or the BRD regularization inversion algorithm.

[0040] Through the T2 spectrum inversion by the Laplace inverse transform or the BRD regularization inversion algorithm, the T2 relaxation time of each pixel point is obtained, and then different moisture states are obtained. For example:

[0041]  The T2 relaxation time < 1 ms indicates that the water at this position is bound water;

[0042]  1 ms < T2 relaxation time < 10 ms indicates that the water at this position is capillary water;

[0043]  The T2 relaxation time > 10 ms indicates that the water at this position is free water;

[0044] For example, if the T2 relaxation time of the pixel point (x = 5, y = 7, z = 2) is 3 ms, it can be determined that the water at this position is mainly capillary water.

[0045] (2-3) Based on the moisture status of each pixel point in the T2 spectrum, a pixel-level three-dimensional moisture distribution map of the fiber cement soil sample during the freezing and melting processes was constructed.

[0046] In the pixel-level three-dimensional moisture distribution map of the fiber cement soil sample, the three-dimensional spatial position of each pixel point (or simply each point) of the fiber cement soil sample is determined, and the moisture state at each pixel point is also determined.

[0047] In another exemplary embodiment of the present application, in step S3, determining the pixel-level moisture migration of the fiber cement soil sample during the entire freezing and thawing process based on the pixel-level three-dimensional moisture distribution map of the fiber cement soil sample during the entire freezing and thawing process specifically includes:

[0048] (3-1) The evolution of moisture in the fiber-cement soil sample at the same position was analyzed based on the pixel-level three-dimensional moisture distribution diagram at each sampling moment during the freezing process of the fiber-cement soil sample, and the pixel-level moisture migration inside the fiber-cement soil sample between each sampling moment during the freezing process was obtained.

[0049] Step (3-1) analyzes the evolution of moisture at the same location in the fiber-cement soil sample based on the pixel-level three-dimensional moisture distribution map at each sampling moment during the entire freezing process of the fiber-cement soil sample, and obtains the pixel-level moisture migration inside the fiber-cement soil sample between each sampling moment during the entire freezing process, specifically including:

[0050] 1) An interpolation algorithm is used to interpolate and fill in the moisture status of each pixel in the pixel-level three-dimensional moisture distribution map of the fiber cement soil sample at each collection moment during the entire freezing process.

[0051] 2) The evolution of moisture in the fiber-cement soil sample at the same location was analyzed based on the moisture state after interpolation and filling, and the continuous pixel-level moisture migration inside the fiber-cement soil sample between each acquisition moment during the entire freezing process was obtained.

[0052] (3-2) The evolution of moisture in the fiber-cement soil sample at the same position was analyzed based on the pixel-level three-dimensional moisture distribution diagram at each sampling moment during the entire melting process of the fiber-cement soil sample, and the pixel-level moisture migration inside the fiber-cement soil sample between each sampling moment during the entire melting process was obtained.

[0053] In order to obtain the continuous pixel-level moisture migration inside the fiber-cement soil sample between the various acquisition moments during the entire melting process, an interpolation algorithm can be used to interpolate and fill in the moisture status of each pixel point in the pixel-level three-dimensional moisture distribution map at each acquisition moment during the entire melting process of the fiber-cement soil sample. Based on the moisture status after interpolation and filling, the evolution of moisture in the fiber-cement soil sample at the same position is analyzed to obtain the continuous pixel-level moisture migration inside the fiber-cement soil sample between the various acquisition moments during the entire melting process.

[0054] (3-3) The pixel-level three-dimensional moisture distribution corresponding to each sampling moment during the entire freezing and thawing process was analyzed to obtain the pixel-level moisture migration inside the fiber cement soil sample corresponding to each sampling moment.

[0055] In order to analyze the pixel-level moisture migration inside the fiber-cement soil sample at a certain time between the set collection moments, an interpolation algorithm can be used to interpolate and fill in the moisture status of each pixel point in the pixel-level three-dimensional moisture distribution map at each collection moment during the entire freezing / thawing process of the fiber-cement soil sample, and then analyze the pixel-level moisture migration at a certain time between the set collection moments based on the interpolated and filled moisture status.

[0056] LF-NMR measurements are performed at different freeze-thaw stages. T2 spectral data is combined with spatial position (XYZ coordinates) to construct a three-dimensional moisture distribution image. The image is represented by different grayscale levels, which can be adjusted to pseudo-color images. Periodic scanning records the evolution of moisture status at the same location, allowing analysis of moisture migration paths. The grayscale changes at the same coordinate point during different freeze-thaw stages reflect moisture changes. By scanning and recording the distribution of moisture status at different locations at the same time, the evolution of moisture distribution over time can be analyzed.

[0057] In this application, the T2 spectrum data is analyzed and processed to determine the moisture migration situation. The source of the signal is located using a gradient magnetic field and converted into an image. Different moisture contents and moisture migration states will result in different signal intensities, thus showing different brightness and contrast. By periodically acquiring nuclear magnetic resonance imaging of the sample, the migration trajectory of moisture within the soil is observed. In addition, this application can also analyze the effects of freeze-thaw and fiber incorporation on moisture migration distribution based on the moisture migration path in fiber cement soil samples.

[0058] In another exemplary embodiment of the present application, after executing step S3 of "determining the pixel-level moisture migration of the fiber cement soil sample during the entire freezing process and the entire melting process based on the pixel-level three-dimensional moisture distribution map of the fiber cement soil sample during the entire freezing process and the entire melting process", the moisture migration test method of the fiber cement soil during the frost heave and thaw settlement process based on low-field nuclear magnetic resonance includes:

[0059] (1) Compare the moisture migration of fiber cement soil samples with different fiber content during the freezing process, and analyze the fiber reinforcement mechanism of fiber cement soil samples during the freezing process.

[0060] (2) Compare the moisture migration of fiber cement soil samples with different fiber content during the melting process, and analyze the fiber reinforcement mechanism of fiber cement soil samples during the melting process.

[0061] In another exemplary embodiment of the present application, in step S1, obtaining the T2 spectrum of the fiber cement soil sample during the entire freezing process and the entire melting process specifically includes:

[0062] (1-1) Simulate the real confining pressure, real axial pressure and real seepage conditions of fiber cement soil samples under the target environment.

[0063] Before executing step (1-1) "simulating the actual confining pressure, actual axial pressure, and actual seepage conditions of the fiber cement soil sample under the target environment", the water migration test method for the fiber cement soil during the freeze-heave and thaw settlement process based on low-field nuclear magnetic resonance includes:

[0064] Fiber cement soil samples were prepared according to the commonly used construction scheme of artificial ground freezing method and the designed fiber incorporation ratio, and the fiber cement soil samples were cured according to the standard age.

[0065] As an example, a test plan was designed based on a common construction method for artificial ground freezing, using an 8% cement and 0.3% fiber content to prepare fiber-cement samples. Air-dried, sieved soil samples were mixed with cement, then dry-mixed with fiber, and then mixed with water until uniform. The mixture was loaded into a mold in three layers, vibrated on a vibrating table after each layer to remove air bubbles. Excess fiber-cement soil was then scraped off the top of the sample, covered with plastic wrap, and allowed to rest for 24 hours before being removed from the mold. The sample was then cured in a curing box for 14 days before preparation.

[0066] (1-2) The fiber cement soil sample is gradually frozen to the target temperature, and the T2 spectrum of the fiber cement soil sample during the entire freezing process is collected in real time.

[0067] Gradually freeze the fiber cement soil sample to the target temperature, including:

[0068] The fiber cement soil sample is frozen according to a freezing temperature gradient, and during the freezing process, the confining pressure on the fiber cement soil sample is maintained at a preset confining pressure value, and the axial pressure on the fiber cement soil sample is maintained at a preset axial pressure value.

[0069] (1-3) The frozen fiber cement soil sample is gradually thawed, and the T2 spectrum of the frozen fiber cement soil sample is collected in real time during the entire thawing process. Scanning can be performed at each temperature node.

[0070] The frozen fiber cement soil sample was gradually thawed, including:

[0071] The fiber-cement soil samples were thawed along a melting temperature gradient. During the thawing process, the confining pressure on the fiber-cement soil samples was maintained at the preset confining pressure value, and the axial pressure on the fiber-cement soil samples was maintained at the preset axial pressure value. Throughout the freeze-thaw process, the axial pressure was maintained at 0.02 kN to ensure that the displacement sensor contact measured the true value.

[0072] In another exemplary embodiment of the present application, before performing the "real-time acquisition of the T2 spectrum of the fiber cement soil sample during the entire freezing process" in step (1-2), the water migration test method for the fiber cement soil during the freezing and thawing process based on low-field nuclear magnetic resonance includes:

[0073] (1) Scan settings: basic imaging sequence selection

[0074] HSE (Hahn Spin Echo sequence) is used to perform radio frequency pulse excitation relaxation signal to improve the signal-to-noise ratio and optimize the short relaxation time (the relaxation time of water inside cement soil is short).

[0075] (2) Parameter settings

[0076] To improve signal quality and accurately locate moisture distribution, the following parameters need to be adjusted:

[0077] a) Correcting frequency offset: Correcting magnetic field uniformity, reducing frequency offset caused by external interference, and improving data reliability.

[0078] b) Phase Encoding Duration: Select a phase encoding duration between 1 ms and 3 ms to optimize the balance between spatial resolution and signal-to-noise ratio.

[0079] c) Sweep Width: Since the relaxation time of water in cement soil is short, the sweep bandwidth should be as small as possible to obtain the best signal quality in order to increase the acquisition sensitivity of short relaxation signals.

[0080] d) Echo Time (TE): Select an appropriate TE value (e.g., 100µs~1ms) to capture the relaxation characteristics of different water states and ensure that bound water, capillary water, and free water can be distinguished in the T2 spectrum.

[0081] (3) Image acquisition mode setting: pre-scan - positioning scan - formal scan

[0082] a) Pre-scan: Optimize signal strength and signal-to-noise ratio.

[0083] The pre-scan collects low-resolution reference signals and automatically adjusts hardware parameters such as RF transmission power and receiving coil sensitivity.

[0084] b) Positioning scanning: The XYZ gradient magnetic field is used to accurately determine the distribution of water signals at different locations within the sample.

[0085] Apply gradient magnetic field in X, Y, and Z directions to make H 1 The difference in resonant frequency produces position information in the XYZ directions.

[0086] c) Formal scanning: multi-layer scanning, signal acquisition pixel by pixel.

[0087] Multi-slice scanning involves continuously acquiring multiple parallel slices along a specific direction (e.g., axial direction) of the sample, collecting signals pixel by pixel. After the formal scan, T2 relaxation information for each pixel is obtained.

[0088] During the formal scan, the spatial position is encoded through the XYZ gradient magnetic field, and the T2 relaxation signal is collected pixel by pixel, ultimately forming the T2 relaxation signal data for each pixel in the three-dimensional space.

[0089] This application enables non-destructive and non-contact testing, without relying on external cameras. Water distribution images are constructed solely through NMR data inversion, allowing for analysis of water migration. Based on XYZ gradient magnetic field positioning, T2 data mapping of individual pixels is achieved, enabling high-precision analysis of water migration (pixel-level analysis). This application can periodically scan to capture moisture trends, track water migration during freeze-thaw cycles, and implement dynamic freeze-thaw monitoring.

[0090] In order to realize the above-mentioned method for testing moisture migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance, another exemplary embodiment of the present application provides a device for testing moisture migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance, see Figures 2 to 4 , including: a sample unit 4, a triaxial pressurizing component 2, a temperature regulating component, a seepage component, an imaging monitoring component 1 and a data analysis component.

[0091] The triaxial compression component 2 is used to simulate the real confining pressure and real axial pressure of the fiber cement soil sample under the target environment.

[0092] Seepage component is used to simulate the actual seepage conditions of fiber cement soil samples under the target environment.

[0093] The temperature regulating component is used for gradually freezing the fiber cement soil sample to a target temperature and gradually melting the frozen fiber cement soil sample.

[0094] The imaging monitoring component 1 is used for real-time acquisition of the T2 spectrum of the fiber cement soil sample during the entire freezing and melting process.

[0095] The data analysis component is used to implement the above-mentioned low-field nuclear magnetic resonance-based water migration test method for fiber cement soil during frost heave and thaw settlement. The data analysis component can be a host computer.

[0096] As an optional embodiment, the fiber cement soil moisture migration test device based on low-field nuclear magnetic resonance during frost heave and thaw settlement further includes: a pressure compensation component.

[0097] The pressure compensation component is used to maintain the pressure on the fiber cement soil sample at a preset confining pressure value during the entire freezing and melting process of the fiber cement soil sample.

[0098] As an optional embodiment, the moisture migration test device for fiber cement soil during frost heave and thaw settlement based on low-field nuclear magnetic resonance also includes: a monitoring component for monitoring the pressure signal and temperature signal of the fiber cement soil sample during the entire freezing and thawing process.

[0099] The following is a detailed description of the structure and function of each of the above components:

[0100] The triaxial pressurization component 2 has a confining pressure chamber 3 and can place a sample unit 4. The triaxial pressurization component 2 can apply axial pressure to the sample unit 4 in the confining pressure chamber 3 and can pass circulating fluid into the confining pressure chamber 3 to apply confining pressure to the sample unit 4; the temperature regulating component is connected to the confining pressure chamber 3, and the temperature regulating component can adjust the temperature of the circulating fluid and can circulate circulating fluid with the confining pressure chamber 3 to achieve heat exchange; the pressure compensation component is connected to the confining pressure chamber 3 and can circulate circulating fluid with the confining pressure chamber 3 to maintain the pressure in the confining pressure chamber 3 stable; the seepage component is connected to the sample unit 4, and the seepage component can pass water into the sample unit 4; the monitoring component is arranged in the confining pressure chamber 3 and can obtain pressure signals and temperature signals; the imaging monitoring component 1 is used to monitor the sample unit 4 and obtain image information of the sample unit 4.

[0101] The sample unit 4 is placed in the confining pressure cavity 3 of the triaxial pressure component 2. The triaxial pressure component 2 clamps the sample unit 4 and applies axial pressure at the same time. The temperature regulating component provides circulating fluids of different temperatures into the confining pressure cavity 3 to realize heat exchange and freeze-thaw of the sample unit 4. At the same time, the circulating fluid can also apply confining pressure to the sample unit 4 to simulate the stress state of the engineering soil. That is, the application of confining pressure and temperature exchange are both realized by the circulating fluid in the confining pressure cavity 3. There is no need to arrange additional temperature control components on the confining pressure cavity 3, which saves space. In addition, under the condition of constant pressure, in order to determine the freeze-thaw settlement rate of the sample, it is necessary to perform cooling (freezing) conditions and heating (cooling) conditions. In the warm (melting) working condition, considering that the pressure in the confining pressure chamber 3 fluctuates due to thermal expansion and contraction of the circulating fluid, which affects the accuracy of the test, a pressure compensation component is set up. Under the indication of the pressure signal and temperature signal obtained by the monitoring component, the circulating fluid is circulated between the pressure compensation component and the confining pressure chamber 3 to replenish or extract the circulating fluid, thereby realizing compensation for the floating pressure; the seepage component provides water to the sample unit 4 to facilitate the migration of water in the sample unit 4, and the imaging monitoring component 1 obtains image information of the sample unit 4 to facilitate subsequent research on the melting settlement deformation characteristics and water migration evolution under the stress state of the engineering soil.

[0102] In the optional scheme of this embodiment, it is more preferred that the temperature regulation component includes a low-temperature circulation pump 15 and a circulation pipeline 16. The low-temperature circulation pump 15 is connected to the confined pressure chamber 3 through the circulation pipeline 16. The low-temperature circulation pump 15 can regulate the temperature of the circulating liquid and can circulate the circulating liquid with the confined pressure chamber 3 through the circulation pipeline 16 to achieve heating or cooling of the circulating liquid in the confined pressure chamber 3; a three-way valve 17 that can control on and off is provided on the circulation pipeline 16; the low-temperature circulation pump 15 can regulate the temperature of the circulating liquid and can meet the circulating liquid temperature required by the confined pressure chamber 3, so that the circulating liquid can be passed into the confined pressure chamber 3 to adjust the temperature.

[0103] In the optional scheme of this embodiment, it is more preferred that the triaxial pressurization assembly 2 includes a confining pressure pipeline 18 connected to the confining pressure chamber 3, and the confining pressure pipeline 18 can pass circulating liquid into the confining pressure chamber 3 to apply confining pressure to the sample unit 4; specifically, the confining pressure chamber 3 can be connected to the confining pressure pipeline 18 and the circulation pipeline 16 through the three-way valve 17, that is, the confining pressure pipeline 18 and the circulating liquid in the confining pressure chamber 3 are circulated and the circulation pipeline 16 and the circulating liquid in the confining pressure chamber 3 are circulated in parallel, so that the temperature of the circulating liquid can be regulated by the low-temperature circulation pump 15 under the premise of maintaining the confining pressure of the confining pressure chamber 3.

[0104] Furthermore, the triaxial pressure assembly 2 also includes an axial pressure mechanism, which can extend into the confining pressure cavity 3 to apply axial pressure to the sample unit 4. Specifically, the axial pressure mechanism includes an upper push rod 5 and a lower push rod 6 respectively clamped on the upper and lower sides of the sample unit 4; specifically, the triaxial pressure assembly 2 can adopt a conventional soil triaxial stress loading system, and also includes a triaxial pressure controller 22, which controls the axial pressure of the upper push rod 5 and the lower push rod 6 through the axial pressure pipeline 19. The triaxial pressure controller 22 can also control the drive of the circulating fluid in the confining pressure pipeline 18, that is, control the confining pressure and the water flow of the seepage component.

[0105] In the optional scheme of this embodiment, it is more preferred that the pressure compensation component includes a peristaltic pump 20 and a compensation pipeline 21. The peristaltic pump 20 is connected to the confining pressure chamber 3 through the compensation pipeline 21, and the peristaltic pump 20 circulates the circulating fluid with the confining pressure chamber 3 through the compensation pipeline 21; specifically, the compensation pipeline 21 is connected to the confining pressure pipeline 18 through the connecting valve 24. When the pressure fluctuation in the confining pressure chamber 3 becomes smaller, the peristaltic pump 20 replenishes the circulating fluid into the confining pressure pipeline 18 to compensate for the floating pressure; when the pressure fluctuation in the confining pressure chamber 3 increases, the peristaltic pump extracts additional circulating fluid from the confining pressure pipeline 18 to compensate for the floating pressure; wherein the peristaltic pump 20 is used for driving, high-precision pressure compensation can be achieved, and the three-axis pressure controller 22 can be used to meet the confining pressure fluctuation caused by the temperature change of the circulating fluid; wherein the connecting valve 24 can control the on-off of the compensation pipeline 21 and the confining pressure pipeline 18.

[0106] Among the optional solutions of this embodiment, it is more preferred to refer to Figure 4The seepage component includes a seepage inlet pipe 14 and a seepage outlet pipe 13. The seepage inlet pipe 14 and the seepage outlet pipe 13 are respectively connected to the water inlet and water outlet of the sample unit 4. The seepage inlet pipe 14 is connected to the seepage pump in the triaxial pressurization component 2 through the seepage pipe 23 so that water can be introduced into the sample unit 4.

[0107] Specifically, see Figure 3 and Figure 4 The sample unit 4 adopts a conventional structure. The sample is covered with a latex film 11, and filter paper 10 and permeable stone 9 are arranged at the upper and lower ends of the sample in sequence. The upper and lower ends of the sample are sealed with an upper plug 7 and a lower plug 12 through a sealing ring 8. The seepage inlet pipe 14 and the seepage outlet pipe 13 are connected to the sample unit 4 through the upper plug 7 and the lower plug 12 respectively; specifically, the seepage inlet pipe 14 can be connected to the fluid channel in the lower push rod 6, and the fluid channel passes through the lower plug 12 to be connected with the sample unit 4.

[0108] In an optional solution of this embodiment, more preferably, the monitoring component includes a pressure sensor and a temperature sensor arranged in the confining pressure chamber 3; specifically, the pressure sensor may include sensors for monitoring the confining pressure and the axial pressure respectively.

[0109] In the optional scheme of this embodiment, it is more preferred that the imaging monitoring component 1 is configured as a low-field nuclear magnetic resonance device, and the confining pressure chamber 3 and the sample unit 4 are placed in the low-field nuclear magnetic resonance device to save overall space so that the low-field nuclear magnetic resonance device can monitor the sample unit 4; the low-field nuclear magnetic resonance device commonly uses conventional devices to collect the signals released by the excitation in the sample, determine the source position of the signal through the gradient magnetic field, and convert it into an image. Different moisture content and moisture migration state will result in different signal intensities, thereby presenting different brightness and contrast in the results. By periodically acquiring nuclear magnetic resonance imaging of the sample and comparing images at different time points, the migration trajectory of moisture inside the soil is observed, providing support for analyzing the mechanical properties of fiber-reinforced cement soil.

[0110] Furthermore, the image signals of the low-field nuclear magnetic resonance device and the signals of the monitoring component can be transmitted to data analysis equipment such as a computer terminal to process the relaxation signals generated by the nuclear magnetic resonance device, inversely analyze the acquired moisture information and deformation data, and generate visual images and signal quantity-relaxation time curves, thereby revealing the moisture migration law and fiber reinforcement effect.

[0111] Among the optional solutions of this embodiment, it is more preferred that the circulating liquid adopts electronic fluorinated liquid, which has good thermal conductivity, provides confining pressure while transmitting the low temperature of the low-temperature heat exchange system, ensures that the sample can be cooled accurately, and at the same time ensures the stability of the confining pressure, and can meet the use requirements of low-field nuclear magnetic resonance equipment.

[0112] Among the optional solutions of this embodiment, it is more preferred that the triaxial pressurization component 2 adopts nuclear magnetic compatible materials, such as titanium alloy, nickel aluminum alloy and ceramics, so as to avoid affecting the use of low-field nuclear magnetic resonance equipment, and at the same time has good thermal conductivity. It can provide confining pressure while transmitting low temperature through the internal fluorinated liquid, ensuring that the sample can be accurately cooled while ensuring the stability of the confining pressure.

[0113] Further preferably, in order to meet the cooling requirements of various heating elements in the test device, such as a pump, a cooling bath is provided to cool the various heating elements, and the coolant may be water or other conventional coolants.

[0114] Based on the above-mentioned low-field nuclear magnetic resonance-based moisture migration test device during the frost heave and thaw settlement of fiber cement soil, Figure 5 As shown in FIG, the complete process of executing the above-mentioned low-field nuclear magnetic resonance-based method for testing moisture migration during frost heave and thaw settlement of fiber cement soil is as follows:

[0115] (1) The triaxial compression component is used to simulate the real confining pressure and real axial pressure of the fiber cement soil sample under the target environment. The seepage component is used to simulate the real seepage of the fiber cement soil sample under the target environment.

[0116] Before performing this step, it is necessary to connect the monitoring component first to confirm the normal response; exhaust the triaxial pressurization component 2 to avoid affecting the accuracy of the test, and complete the installation of the sample unit 4; the confining pressure pump of the triaxial pressurization component 2 replenishes the electronic fluorine liquid in the confining pressure chamber 3 and exhausts the gas in the chamber at the same time.

[0117] Before installing the fiber cement soil sample and before the confining pressure cavity 3, the fiber cement soil sample needs to be placed in a saturator, placed in a vacuum saturation pump device to be vacuumed and saturated with water, so as to more accurately measure the internal nuclear magnetic signal.

[0118] After all components are completed, the underground stress environment in actual engineering projects is simulated based on the set confining pressure, axial pressure, and seepage conditions. Specifically, the triaxial pressure component 2 introduces circulating fluid into the confining pressure chamber 3, applying confining pressure to the sample unit 4 and cooling and freezing the sample unit 4. The triaxial pressure component 2 can also apply axial pressure to the sample unit 4; the seepage component supplies water to the sample unit 4 to simulate real seepage conditions. During the loading process of the triaxial pressure component 2, the confining pressure is first set and loaded, and then the axial pressure is gradually applied to simulate the underground pressure environment in actual engineering projects.

[0119] (2) Using a temperature regulating component, the fiber cement soil sample is gradually frozen to a target temperature, and the frozen fiber cement soil sample is gradually melted.

[0120] The temperature regulating component regulates the temperature of the circulating liquid in the confining pressure chamber 3 so that the soil sample in the sample unit 4 begins to gradually freeze to the target temperature. Specifically, the heater of the low-temperature circulating pump 15 is turned on to cryogenically freeze the sample through the circulating liquid; the sample is cooled to a predetermined low-temperature condition, and heat exchange is performed through the circulating fluorinated liquid to accurately control and maintain the target temperature of the sample.

[0121] Once the sample reaches the set temperature and maintains that temperature, the temperature setting of the cryogenic circulating pump 15 is adjusted to regulate the circulating fluid temperature, causing the soil sample to begin thawing. During the freezing and thawing process, the load is maintained, and the displacement sensor is in contact with the sample base (axial pressure is applied from bottom to top). This displacement sensor can be used to measure displacement data in real time.

[0122] (3) The imaging monitoring component is used to collect the T2 spectrum of the fiber cement soil sample during the entire freezing and melting process in real time.

[0123] (4) Use the data analysis component to analyze the moisture migration of the fiber cement soil sample based on the T2 spectrum of the fiber cement soil sample during the freezing and melting process. Figure 6 As shown, Figure 6 Brighter colors indicate higher water content.

[0124] The relaxation signal generated by the nuclear magnetic resonance device is spatially located using a host computer to generate a visual image containing moisture distribution information, thereby revealing the moisture migration pattern and fiber reinforcement effect.

[0125] The above test process has the following technical effects:

[0126] (1) In this application, the sample unit is placed in the confining pressure cavity of the triaxial pressure assembly. The triaxial pressure assembly clamps the sample unit and applies axial pressure and introduces circulating fluid into the confining pressure cavity to apply confining pressure to the sample unit, simulating the stress state of the engineering soil. The temperature regulating assembly provides circulating fluid of different temperatures into the confining pressure cavity to realize heat exchange of the sample unit to achieve freeze-thaw, that is, the application of confining pressure and temperature exchange are both achieved through the circulating fluid in the confining pressure cavity, without the need to arrange additional temperature control components on the confining pressure cavity, thus saving space.

[0127] (2) Under constant pressure, in order to determine the freeze-thaw settlement rate of the sample, cooling (freezing) and heating (thawing) conditions are required. Considering that the pressure in the confining pressure chamber fluctuates due to thermal expansion and contraction of the circulating fluid, which affects the accuracy of the test, a pressure compensation component is set up. Under the indication of the pressure signal and temperature signal obtained by the monitoring component, the circulating fluid is circulated between the pressure compensation component and the confining pressure chamber to replenish or extract the circulating fluid, thereby compensating for the floating pressure.

[0128] (3) This application uses imaging monitoring components and data analysis components to obtain the relaxation signal of the sample unit and invert and locate it into image information. By periodically obtaining nuclear magnetic resonance imaging of the sample, the evolution of moisture in the soil is observed, achieving the effect of visualizing the testing of the thawing deformation characteristics and the evolution of moisture migration while reducing the stress state of the soil.

[0129] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0130] This document uses specific examples to illustrate the principles and implementation methods of this application. The description of the above examples is only intended to help understand the method and core concept of this application. At the same time, for those skilled in the art, based on the concept of this application, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A method for testing water migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance, characterized in that: Including: Obtaining the T2 spectra of the fiber cement soil sample during the whole freezing process and the whole melting process; The T2 spectra of the fiber cement soil sample during the whole freezing process and the whole melting process are the T2 spectra obtained based on the low-field nuclear magnetic resonance technology during the frost heaving and thaw settlement tests of the fiber cement soil sample; Inverting the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample during the whole freezing process and the whole melting process from the T2 spectra of the fiber cement soil sample during the whole freezing process and the whole melting process; Determining the pixel-level moisture migration situation of the fiber cement soil sample during the whole freezing process and the whole melting process according to the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample during the whole freezing process and the whole melting process; Inverting the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample during the whole freezing process and the whole melting process from the T2 spectra of the fiber cement soil sample during the whole freezing process and the whole melting process, specifically including: Determining the three-dimensional spatial distribution information of each pixel point of the fiber cement soil sample during the whole freezing process and the whole melting process according to the three-dimensional gradient magnetic field in the low-field nuclear magnetic resonance technology, and obtaining the T2 spectrum with three-dimensional spatial distribution information; Determining the moisture state of each pixel point by applying the inverse Laplace transform method or the BRD regularization inversion algorithm according to the T2 spectrum with three-dimensional spatial distribution information; Constructing the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample during the whole freezing process and the whole melting process according to the moisture state of each pixel point in the T2 spectrum; Determining the moisture state of each pixel point by applying the inverse Laplace transform method or the BRD regularization inversion algorithm according to the T2 spectrum with three-dimensional spatial distribution information, specifically including: When the T2 relaxation time < 1 ms, it indicates that this pixel point is bound water; When 1 ms < T2 relaxation time < 10 ms, it indicates that this pixel point is capillary water; When the T2 relaxation time > 10 ms, it indicates that this pixel point is free water; Analyzing the evolution of the moisture in the fiber cement soil sample at the same position according to the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample at each acquisition moment during the whole freezing process, and obtaining the pixel-level moisture migration situation inside the fiber cement soil sample between each acquisition moment during the whole freezing process, specifically including: Using the interpolation algorithm to interpolate and fill the moisture state of each pixel point in the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample at each acquisition moment during the whole freezing process; Analyzing the evolution of the moisture in the fiber cement soil sample at the same position according to the interpolated and filled moisture state, and obtaining the continuous pixel-level moisture migration situation inside the fiber cement soil sample between each acquisition moment during the whole freezing process.

2. The method for testing water migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance according to claim 1, characterized in that: Determining the pixel-level moisture migration situation of the fiber cement soil sample during the whole freezing process and the whole melting process according to the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample during the whole freezing process and the whole melting process, specifically including: Analyzing the evolution of the moisture in the fiber cement soil sample at the same position according to the pixel-level three-dimensional moisture distribution maps of the fiber cement soil sample at each acquisition moment during the whole freezing process, and obtaining the pixel-level moisture migration situation inside the fiber cement soil sample between each acquisition moment during the whole freezing process; The evolution of moisture in the fiber-cement soil sample at the same position was analyzed based on the pixel-level three-dimensional moisture distribution map at each sampling moment during the entire melting process, and the pixel-level moisture migration inside the fiber-cement soil sample between each sampling moment during the entire melting process was obtained. The pixel-level three-dimensional moisture distribution maps corresponding to each sampling moment during the entire freezing and melting processes were analyzed to obtain the pixel-level moisture migration situation inside the fiber cement soil sample corresponding to each sampling moment.

3. The method for testing water migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance according to claim 1, characterized in that: After executing the step of "determining the pixel-level moisture migration of the fiber-cement soil sample during the entire freezing and thawing process based on the pixel-level three-dimensional moisture distribution map of the fiber-cement soil sample during the entire freezing and thawing process," the moisture migration test method for the fiber-cement soil during frost heave and thaw settlement based on low-field nuclear magnetic resonance includes: Compare the water migration of fiber cement soil samples with different fiber content during the freezing process, and analyze the fiber reinforcement mechanism of fiber cement soil samples during the freezing process; The moisture migration of fiber cement soil samples with different fiber content during the melting process was compared, and the fiber reinforcement mechanism of fiber cement soil samples during the melting process was analyzed.

4. The method for testing water migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance according to claim 1, characterized in that: Obtain T2 spectra of the fiber cement soil sample during the entire freezing and melting process, including: Simulate the real confining pressure, real axial pressure and real seepage conditions of fiber cement soil samples under target environment; The fiber cement soil sample was gradually frozen to the target temperature, and the T2 spectrum of the fiber cement soil sample during the entire freezing process was collected in real time based on low-field nuclear magnetic resonance technology. The frozen fiber cement soil sample was gradually melted, and the T2 spectrum of the entire melting process of the frozen fiber cement soil sample was collected in real time based on low-field nuclear magnetic resonance technology.

5. The method for testing water migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance according to claim 4, characterized in that: Gradually freeze the fiber cement soil sample to the target temperature, including: Freezing the fiber cement soil sample according to a freezing temperature gradient, and during the freezing process, maintaining the confining pressure on the fiber cement soil sample at a preset confining pressure value, and maintaining the axial pressure on the fiber cement soil sample at a preset axial pressure value; The frozen fiber cement soil sample was gradually thawed, including: The fiber cement soil sample is melted according to a melting temperature gradient, and during the melting process, the confining pressure on the fiber cement soil sample is maintained at a preset confining pressure value, and the axial pressure on the fiber cement soil sample is maintained at a preset axial pressure value.

6. The method for testing water migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance according to claim 1, characterized in that: Before executing the step of "real-time acquisition of the T2 spectrum of the fiber cement soil sample during the entire freezing process", the water migration test method for the fiber cement soil during the freezing and thawing process based on low-field nuclear magnetic resonance includes: The Hahn spin echo sequence was used to generate radiofrequency pulses for relaxation signal excitation; Setting T2 spectrum acquisition parameters; the T2 spectrum acquisition parameters include correction frequency offset, phase encoding duration, scanning bandwidth and echo time; Set the T2 spectrum acquisition mode.

7. A low-field nuclear magnetic resonance-based device for testing water migration during frost heave and thaw settlement of fiber cement soil, characterized in that: include: Triaxial compression component, used to simulate the real confining pressure and real axial pressure of fiber cement soil samples under the target environment; Seepage component, used to simulate the actual seepage conditions of fiber cement soil samples under the target environment; a temperature regulating component for gradually freezing the fiber cement soil sample to a target temperature and gradually thawing the frozen fiber cement soil sample; Imaging monitoring component, used to collect T2 spectra of fiber cement soil samples during the entire freezing and melting process in real time based on low-field nuclear magnetic resonance technology; A data analysis component for executing the water migration test method for fiber cement soil during frost heave and thaw settlement based on low-field nuclear magnetic resonance as described in any one of claims 1 to 6.

8. The device for testing moisture migration during frost heave and thaw settlement of fiber cement soil based on low-field nuclear magnetic resonance according to claim 7, characterized in that: The fiber cement soil moisture migration test device based on low-field nuclear magnetic resonance during frost heave and thaw settlement further includes: a pressure compensation component; The pressure compensation component is used to maintain the confining pressure on the fiber cement soil sample at a preset confining pressure value during the entire freezing and melting process of the fiber cement soil sample.

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