Testing method and device for frozen soil water migration characteristics under low-temperature nuclear magnetic triaxial seepage

Abstract

The present application discloses a method and device for testing the water migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage, which relates to the field of frozen soil testing technology. In view of the fact that the existing technology cannot achieve compatibility control of freezing and seepage, and it is difficult to dynamically capture the evolution of the whole process of frozen soil water migration characteristics, the proposed scheme steps are as follows: simulate the real confining pressure, axial pressure and seepage velocity of the environment in which the soil sample is located; collect nuclear magnetic resonance images, overall and layered T2 spectra of the soil sample under the action of seepage before freezing; gradually freeze the soil sample based on different temperature gradients, and collect nuclear magnetic resonance images, overall and layered T2 spectra of the soil sample under the coupling of seepage and freezing in real time; collect data based on the whole process of soil sample freezing, and analyze the evolution law of water migration characteristics during soil freezing. The present invention realizes full-process, visual, and non-destructive detection, and can accurately capture the whole-process evolution law of water migration characteristics such as unfrozen water content and pores and cracks in frozen soil under the coupling of seepage and freezing.

Description

Technical Field

[0001] The present application relates to the field of frozen soil testing technology, and in particular to a method and device for testing the moisture migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage. Background Art

[0002] The migration of water associated with artificially frozen soil during the freezing process is the fundamental cause of frost heave or collapse of overlying buildings, significantly affecting the strength and deformation of the soil as well as the safety, stability, and durability of engineering structures. Unfrozen water, the fundamental source of water migration, often acts as a migration channel through pores and fissures, providing a continuous water supply for water migration and causing severe frost heave and deformation of the soil. Artificially frozen projects near cross-river and cross-sea tunnels and surface / groundwater drainage channels may encounter large seepage flows, which can severely hinder the formation time and strength of the frozen wall, further leading to accidents such as collapse in foundation pit projects, seriously hindering project progress. Therefore, it is crucial to capture the evolutionary characteristics of water migration throughout the soil under the coupled effects of freezing and seepage.

[0003] Traditional methods for measuring unfrozen water content, such as time-domain reflectometry and differential scanning calorimetry, suffer from significant errors, potentially inflating results or interfering with the frozen soil temperature field. In contrast, low-field nuclear magnetic resonance (NMR) technology requires no sampling, minimizing disturbance to the soil sample and providing the most realistic data on the pore and crack structure during freezing. This provides a more direct and accurate picture of the distribution and dynamics of water content within the soil, enabling inverse analysis of the evolution of pores and cracks. However, existing testing techniques typically involve removing small samples from a cryogenic chamber after freezing and then placing them in an NMR instrument for testing. This not only presents the issue of ice melting but also overlooks the limitations of small samples, such as the lack of distinct water migration characteristics, which hinders the ability to reflect variations in water content at different heights within the soil. Furthermore, the impact of seepage on frozen soil is not considered. Therefore, a testing method and device that can couple freezing and seepage is urgently needed to visualize and non-destructively monitor the evolution of water migration characteristics within the soil throughout the freezing process, providing reliable technical support for the safety and stability of frozen soil engineering. Summary of the Invention

[0004] The purpose of this application is to provide a method and device for testing the moisture migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage, which can monitor the evolution law of the moisture migration characteristics inside the soil during the freezing process in a full, visual and non-destructive manner.

[0005] To achieve the above objectives, this application provides the following solutions:

[0006] In a first aspect, the present application provides a method for testing the water migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage, comprising:

[0007] S1, the real confining pressure, real axial pressure and real seepage velocity of the simulated soil sample environment;

[0008] S2. Collect nuclear magnetic resonance images, overall and layered T2 spectra of the soil sample before freezing under seepage; overall and layered T2 spectra refer to the overall T2 spectrum of the soil sample and the layered T2 spectra at different heights of the soil sample;

[0009] S3, gradually freezing soil samples at different temperature gradients, and collecting real-time nuclear magnetic resonance images, overall and layered T2 spectra of soil samples under the coupled effects of seepage and freezing;

[0010] S4. Analyze the evolution of water migration characteristics during the freezing process of soil samples based on the nuclear magnetic resonance images, overall and layered T2 spectra before and during freezing. Water migration characteristics include water distribution in soil samples, changes in water migration, changes in unfrozen water content, and changes in pores and cracks.

[0011] In the second aspect, in order to implement the above-mentioned testing method, the present application provides a device for testing the water migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage, comprising: a soil triaxial testing mechanism, a low-temperature freezing mechanism, a nuclear magnetic resonance testing mechanism, and a data acquisition and analysis mechanism;

[0012] The soil triaxial testing mechanism is used to simulate the real confining pressure, real axial pressure and real seepage conditions of the soil sample environment, and to collect the overall T2 spectrum of the soil sample under the seepage effect before freezing and under the seepage and freezing coupling effect during freezing;

[0013] The low-temperature freezing mechanism is used to gradually freeze the soil sample based on different temperature gradients;

[0014] The nuclear magnetic resonance testing mechanism is used to provide a nuclear magnetic resonance magnetic field to the soil sample, and collect nuclear magnetic resonance images of the soil sample under the action of seepage before freezing and under the coupled action of seepage and freezing during freezing, as well as layered T2 spectra at different heights of the soil sample;

[0015] The data acquisition and analysis mechanism is used to obtain nuclear magnetic resonance images, overall T2 spectra, and layered T2 spectra at different heights of the soil sample before and during freezing, and analyze the evolution law of the moisture migration characteristics of the soil sample during the entire freezing process; the moisture migration characteristics include the moisture distribution in the soil sample, the change in moisture migration, the change in unfrozen water content, and the change in pores and cracks.

[0016] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0017] This application provides a method and device for testing the water migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage, which can simultaneously create a testing environment for freezing and seepage, facilitating further analysis of the evolution of the water content and pore and fracture structure within the soil under the simultaneous action of freezing and seepage. This application can collect nuclear magnetic resonance images, overall and layered T2 spectra of soil samples during the entire freezing process under the coupled action of seepage and freezing, and can analyze the water distribution, water migration changes, unfrozen water content changes, and pore and fracture changes in the soil samples based on a combination of qualitative and quantitative methods. It can monitor the evolution of the water migration characteristics within the soil during the freezing process in a full, visual, and non-destructive manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 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.

[0019] Figure 1 This is a flow chart of a method for testing the characteristics of frozen soil water migration under low-temperature nuclear magnetic triaxial seepage in one embodiment of the present application;

[0020] Figure 2 A schematic diagram of a nuclear magnetic resonance image of the water migration process in clay under freezing and seepage provided in one embodiment of the present application;

[0021] Figure 3 A schematic diagram of the T2 spectrum of clay under different freezing temperatures and seepage effects provided in one embodiment of the present application;

[0022] Figure 4 A schematic diagram of clay pore changes under different freezing temperatures and seepage effects provided in one embodiment of the present application;

[0023] Figure 5 This is a structural schematic diagram of a device for testing the characteristics of frozen soil water migration under low-temperature nuclear magnetic triaxial seepage in one embodiment of the present application;

[0024] Figure 6 A schematic diagram of the structure of a soil triaxial testing mechanism provided in one embodiment of the present application;

[0025] Figure 7 A detailed structural diagram of a soil triaxial testing mechanism provided in one embodiment of the present application;

[0026] Figure 8 A schematic diagram of the structures of a confining pressure control pump, an axial pressure control pump, and a seepage control pump provided in one embodiment of the present application;

[0027] Figure 9 A schematic structural diagram of a nuclear magnetic resonance testing mechanism provided in one embodiment of the present application.

[0028] Figure 1: 1- soil triaxial test mechanism; 111- nuclear magnetic probe coil; 112- axial pressure loading cylinder; 113- lower push rod; 114- lower base plate; 115- soil sample lower base; 116- soil sample upper loading cap; 117- upper top plate; 118- upper push rod; 119- axial pressure reaction frame; 120- pressure sensor; 121- displacement sensor; 122- axial pressure inlet; 123- confining pressure chamber; 124- confining pressure inlet; 125- confining pressure outlet; 126- seepage inlet pipeline; 127- seepage outlet pipeline; 128- seepage inlet; 129- seepage outlet; 130- mobile test Frame; 131-displacement sensor base; 2-low-temperature freezing mechanism; 21-freezing circulation fluid inlet; 22-freezing circulation fluid outlet; 23-freezing circulation pipeline; 24-freezing circulation pump; 3-nuclear magnetic resonance testing mechanism; 31-low-field nuclear magnetic resonance apparatus; 32-low-field nuclear magnetic resonance imaging module; 33-layered gradient module; 4-data acquisition and analysis mechanism; 5-axial pressure control pump; 6-confining pressure control pump; 7-seepage control pump; 8-liquid storage tank; 9-control switch; 10-confining pressure sensor; 11-axial pressure sensor; 12-seepage flow rate sensor; A-soil sample; B-nuclear magnetic resonance testing area. DETAILED DESCRIPTION

[0029] 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.

[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] As an advanced nondestructive testing technique, low-field nuclear magnetic resonance (NMR) technology has been widely used in recent years in fields such as materials science, agricultural product quality testing, and seed viability assessment. By measuring parameters such as the spin relaxation time of hydrogen protons in a sample, this technology can indirectly reflect the moisture content, distribution state, and dynamic changes within the sample. However, the application of low-field NMR technology to testing soils under the coupled effects of freezing and seepage is still in its infancy, and comprehensive testing methods and equipment have yet to be developed. Especially in the context of triaxial seepage coupling, ensuring the simultaneous realization of freezing and seepage during testing, truly restoring the true stress state of the soil, capturing the evolution of moisture migration during freezing, and effectively acquiring and analyzing NMR signals are all technical challenges that need to be addressed.

[0032] Therefore, the present invention proposes a method and device for testing the moisture migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage, aiming to achieve comprehensive and accurate monitoring and analysis of the dynamic change process of soil under complex environmental conditions by integrating low-field nuclear magnetic resonance technology and triaxial testing technology, providing strong technical support for research and engineering practice in related fields.

[0033] In an exemplary embodiment of the present application, a method for testing the characteristics of frozen soil water migration under low-temperature nuclear magnetic triaxial seepage is provided, such as Figure 1 As shown, the following steps are included.

[0034] S1. Simulate the real confining pressure, real axial pressure and real seepage velocity of the environment in which the soil sample is located.

[0035] S2. Collect nuclear magnetic resonance images, overall and layered T2 spectra of the soil sample under seepage before freezing; the overall and layered T2 spectra refer to the overall T2 spectrum of the soil sample and the layered T2 spectra at different heights of the soil sample.

[0036] S3. Based on the gradual freezing of soil samples at different temperature gradients, the nuclear magnetic resonance images, overall and layered T2 spectra of the soil samples under the coupling of seepage and freezing are collected in real time.

[0037] S4. Analyze the evolution of water migration characteristics during the freezing process of soil samples based on the nuclear magnetic resonance images, overall and layered T2 spectra before and during freezing. Water migration characteristics include water distribution in soil samples, changes in water migration, changes in unfrozen water content, and changes in pores and cracks.

[0038] More specifically, step S4 includes:

[0039] (1) Based on the nuclear magnetic resonance images of soil samples before and during freezing, the water distribution and water migration changes during the freezing process of soil samples were qualitatively analyzed based on data post-processing. Figure 2The figure shows a schematic diagram of the NMR image of the clay water migration process under freezing and seepage. Figure 2 Brighter locations indicate higher water content.

[0040] Nuclear magnetic resonance images are used to qualitatively analyze water migration. For example, the amount of water accumulated in different parts of a soil sample is different. The parts with more water accumulation will have a strong color in the image (the intensity of the color represents the amount of water), while the parts with less accumulation will be basically colorless. This can be used to determine the general pattern of water migration.

[0041] (2) Based on the overall and layered T2 spectra of the soil sample before and during freezing, the changes in the unfrozen water content of the soil sample as a whole and at different height layers of the soil sample and the evolution law of pores and cracks during the freezing process were quantitatively analyzed based on data post-processing; the evolution law of pores and cracks was determined based on the changes in pore and crack radius. Figure 3 Schematic diagram of the overall T2 spectrum of clay under different freezing temperatures and seepage effects. Figure 4 Schematic diagram of clay pore changes under different freezing temperatures and seepage effects.

[0042] The process of soil freezing water migration mainly involves observing the changes in the unfrozen water content at different heights in the soil sample before and after freezing. The test process mainly uses the nuclear magnetic resonance test results to measure the nuclear magnetic resonance signals of the soil sample before and after freezing, obtain the transverse relaxation time (T2) spectrum of the soil sample before and after freezing, and then based on the nuclear magnetic resonance signal value of the soil sample at room temperature (before freezing), the corresponding unfrozen water content is inverted and calculated according to the nuclear magnetic resonance signals at different freezing temperatures and different heights of the soil sample, so that the entire process of water migration can be quantitatively analyzed. Among them, the calculation formula for the unfrozen water content is:

[0043] ;

[0044] Where, w u is the unfrozen water content of the soil sample during freezing (%); w 0 is the total water content of the soil sample before freezing (%); is the total intensity value of the nuclear magnetic resonance signal before the soil sample is frozen. The total intensity value of the nuclear magnetic resonance signal is determined according to the T2 spectrum and is the sum of the amplitude intensities of the nuclear magnetic resonance signals of each data point in the T2 spectrum before freezing. is the total intensity value of the nuclear magnetic resonance signal during the freezing process of the soil sample, which is the sum of the nuclear magnetic resonance signal amplitude intensities of each data point in the T2 spectrum at a certain low temperature during the freezing process.

[0045] T2 spectrum quantitatively analyzes moisture information. For example, layered T2 spectrum can reveal the moisture content at a specific location. The moisture content at different locations at different times can be used to determine the pattern of moisture migration.

[0046] The observation of frozen pores and cracks in soil under triaxial seepage coupling based on low-field nuclear magnetic resonance is mainly based on the inversion analysis of the nuclear magnetic resonance transverse relaxation time (T2) spectrum to obtain the pore and crack structure characteristics of the soil sample. Pores and cracks are the storage places of water, the favorable channels for promoting water migration, and are an important part of the water migration characteristics. The transverse relaxation time T2 is related to the pore or crack radius. R The relationship can be expressed as the following formula.

[0047] .

[0048] Where T2 is the transverse relaxation time of the collected T2 spectrum data, in ms; is the surface relaxation rate, in units of ; R is the crack or pore radius; the crack radius calculation formula is the same as the pore radius calculation formula. When calculating the crack radius, Fs Expressed as the crack characteristic factor, Fs =1; when calculating the pore radius, Fs Expressed as pore characteristic factor, spherical pores, Fs =3; columnar pores, Fs =2. S is the pore (crack) surface area; V is the pore (crack) volume.

[0049] In another exemplary embodiment of the present application, in order to realize the above-mentioned method for testing the water migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage, the present application provides a preferred testing device for realizing the above-mentioned testing method, namely, a testing device for testing the water migration characteristics of frozen soil under low-temperature nuclear magnetic triaxial seepage, such as Figure 5 As shown, it includes: a soil triaxial testing mechanism 1, a low-temperature freezing mechanism 2, a nuclear magnetic resonance testing mechanism 3 and a data acquisition and analysis mechanism 4.

[0050] The soil triaxial testing mechanism 1 is used to simulate the real confining pressure, real axial pressure and real seepage conditions of the environment in which the soil sample is located, and to collect the overall T2 spectrum of the soil sample under the seepage effect before freezing and under the seepage and freezing coupling effect during freezing. The overall T2 spectrum is obtained by exciting the soil sample resonance and receiving the soil sample resonance signal; the soil sample resonance signal is used to obtain the overall T2 spectrum of the soil before and during freezing of the soil sample under the action of seepage. Figure 5 As shown, the soil triaxial testing mechanism 1 is located in the nuclear magnetic resonance testing area of ​​the nuclear magnetic resonance testing mechanism 3. Figure 9 In the figure, B refers to the NMR test area.

[0051] The low temperature freezing mechanism 2 is used to gradually freeze the soil sample based on different temperature gradients. The freezing temperature during the freezing process of the soil sample can be continuously changed by regulation.

[0052] The nuclear magnetic resonance testing mechanism 3 is used to provide a nuclear magnetic resonance magnetic field to the soil sample, and collect nuclear magnetic resonance images of the soil sample under the action of seepage before freezing and the coupled action of seepage and freezing during freezing, as well as layered T2 spectra at different heights of the soil sample.

[0053] The data acquisition and analysis unit 4 is configured to acquire nuclear magnetic resonance images, overall T2 spectra, and layered T2 spectra at different heights of the soil sample before and during freezing, and analyze the evolution of moisture migration characteristics throughout the freezing process. Moisture migration characteristics include moisture distribution within the soil sample, changes in moisture migration, changes in unfrozen water content, and changes in pores and fissures. The data acquisition and analysis unit 4 can be a computer. The data analysis process in step S4 of the aforementioned testing method can be implemented using the data acquisition and analysis unit 4.

[0054] The low-field NMR technique, which uses triaxial seepage coupling to determine soil freezing cracks, is based on NMR images. Because cracks are natural seepage channels filled with water, and NMR imaging primarily captures water signals, NMR images can visually visualize water content in the soil, providing real-time visualization of crack evolution during freezing and seepage. Since clay forms cracks during freezing, this technique effectively captures these changes.

[0055] In another exemplary embodiment of the present application, the soil triaxial testing mechanism 1 specifically includes: an axial pressure simulation module, a confining pressure simulation module, a seepage simulation module and a nuclear magnetic probe coil 111.

[0056] The axial pressure simulation module is used to simulate the actual axial pressure of the environment in which the soil sample is located.

[0057] The confining pressure simulation module is used to simulate the actual confining pressure of the environment in which the soil sample is located.

[0058] The seepage simulation module is used to simulate the actual seepage conditions of the environment in which the soil sample is located.

[0059] The NMR probe coil 111 employs an integrated receiver design, used to excite soil sample resonance and receive soil sample resonance signals to determine the overall T2 spectrum of the soil sample before and during freezing. This application utilizes the NMR probe coil 111 within the soil triaxial testing mechanism 1 to collect the overall T2 spectrum of the soil. The soil sample resonance signals detected by the NMR probe coil 111 are then inverted using NMR data processing software (e.g., data acquisition and analysis mechanism 4) to obtain the overall T2 spectrum of the entire freezing process.

[0060] The axial pressure simulation module, confining pressure simulation module, and seepage simulation module in the soil triaxial testing mechanism 1 can be used to simulate the confining pressure, axial pressure, and seepage velocity in step S1 of the above-mentioned testing method, as well as triaxial loading and seepage control. The acquisition of the overall T2 spectrum of the soil sample in step S2 and step S3 of the above-mentioned testing method can be achieved using the nuclear magnetic resonance probe coil 111 in the soil triaxial testing mechanism 1.

[0061] like Figure 6 and Figure 7 As shown, the axial pressure simulation module includes an axial pressure loading cylinder 112, a lower push rod 113, a lower base plate 114, a soil sample lower base 115, a soil sample upper loading cap 116, an upper top plate 117 (which can be called an axial loading upper top plate), an upper push rod 118 and an axial pressure reaction frame 119.

[0062] The upper end of the axial loading cylinder 112 is connected to the lower end of the soil sample via a lower push rod 113, a lower base plate 114, and a soil sample lower base 115. Lower push rod 113 penetrates lower base plate 114 and contacts the lower end of soil sample lower base 115. Lower base plate 114 (also known as the axial loading lower base plate) controls axial loading using water as a driving force. Figure 7 The “A” in the table refers to the soil sample.

[0063] The lower push rod 113, the lower bottom plate 114 and the soil sample lower base 115 are fixedly connected and are integrated.

[0064] The upper end of the soil sample is connected to the axial compression reaction frame 119 through the soil sample upper loading cap 116, upper top plate 117, and upper push rod 118. The lower end of the upper push rod 118 passes through the upper top plate 117 and contacts the upper end of the soil sample upper loading cap 116. The soil sample upper loading cap 116, upper top plate 117, upper push rod 118, and axial compression reaction frame 119 are not fixedly connected but are independent components. When axial compression is applied, these components come into contact with each other. The axial compression reaction frame 119 can be made of aluminum alloy, which provides a certain degree of strength.

[0065] A pressure sensor 120 is disposed between the upper push rod 118 and the axial pressure reaction frame 119. The pressure sensor 120 is in communication with the data acquisition and analysis unit 4 and is used to measure the magnitude of the axial pressure applied by the axial pressure loading cylinder 112 to the soil sample. By monitoring the force applied to the upper push rod 118, the pressure sensor 120 measures the magnitude of the axial pressure applied by the axial pressure loading cylinder 112 to the soil sample, records pressure changes during triaxial freezing and seepage, and provides real-time information on the magnitude of the applied axial pressure.

[0066] A displacement sensor base 131 is disposed at the upper end of the axial pressure loading cylinder 112, which is fixedly connected to the axial pressure loading cylinder 112. The upper end of the displacement sensor base 131 contacts the lower end of the lower push rod 113, but is not fixedly connected. A displacement sensor 121 is disposed on the side of the displacement sensor base 131. The displacement sensor 121 monitors the deformation caused by the lower push rod 113 acting on the displacement sensor base 131 to monitor the deformation caused by the axial pressure. The displacement sensor 121 is in communication with the data acquisition and analysis unit 4. By monitoring the deformation of the displacement sensor base 131, the displacement change value during the triaxial freezing and seepage process of the soil can be monitored, and the frost heave rate and thaw settlement coefficient can be obtained. The axial pressure inlet 122 of the axial pressure loading cylinder 112 is connected to the axial pressure control pump 5 via a pipeline to achieve pressure loading.

[0067] In another exemplary embodiment of the present application, the soil sample lower base 115 is made of polytetrafluoroethylene material, which has no nuclear magnetic resonance signal; the soil sample upper loading cap 116 is made of ceramic or PEEK polyetheretherketone material, which has little effect on nuclear magnetic resonance testing.

[0068] In another exemplary embodiment of the present application, the confining pressure simulation module includes a confining pressure chamber 123; the soil sample, the soil sample lower base 115 and the soil sample upper loading cap 116 are placed in the confining pressure chamber 123; the upper end of the confining pressure chamber 123 is connected to the upper top plate 117, and the lower end of the confining pressure chamber 123 is connected to the lower bottom plate 114; the nuclear magnetic probe coil 111 is located outside the confining pressure chamber 123, for example, it is wound around the outside of the confining pressure chamber 123.

[0069] A confined pressure inlet 124 is located at the lower end of the confined pressure chamber 123, and a confined pressure outlet 125 is located at the upper end. Both the confined pressure inlet 124 and the confined pressure outlet 125 are equipped with control valves. The confined pressure inlet 124 is connected to the confined pressure control pump 6 via a pipeline to apply pressure. The outer surface of the confined pressure chamber 123 is made of a non-metallic material containing a low amount of hydrogen, and the interior chamber is hollow. During the test, fluorinated liquid is injected into the confined pressure chamber 123, and the pressure is controlled by the confined pressure control pump 6.

[0070] In another exemplary embodiment of the present application, the seepage simulation module includes a seepage inlet pipeline 126 and a seepage outlet pipeline 127 .

[0071] The seepage inlet pipe 126 passes through the lower push rod 113 and the soil sample lower base 115 from bottom to top; the outlet of the seepage inlet pipe 126 contacts the lower end of the soil sample; and the inlet of the seepage inlet pipe 126 is provided on the side wall of the lower push rod 113 .

[0072] The inlet of the seepage outlet pipe 127 contacts the upper end of the soil sample, and the outlet of the seepage outlet pipe 127 is located at the lower end of the confining pressure chamber 123; Figure 7As shown, the inlet of the seepage inlet pipeline 126 is the seepage inlet 128 ; the outlet of the seepage outlet pipeline 127 is the seepage outlet 129 .

[0073] The seepage inlet 128 is connected to the seepage control pump 7 through a pipeline. The seepage control pump 7 can control the seepage speed, seepage inlet and outlet pressure and volume.

[0074] like Figure 8 As shown, the inlets of the axial pressure control pump 5, the confining pressure control pump 6, and the seepage control pump 7 are respectively connected to the corresponding liquid storage tank 8. For axial pressure control, the liquid in the corresponding liquid storage tank 8 is distilled water, and for confining pressure control, the liquid in the corresponding liquid storage tank 8 can be a fluorinated liquid. Control switches 9 are respectively installed on the pipelines at the outlets of the axial pressure control pump 5, the confining pressure control pump 6, and the seepage control pump 7. The axial pressure control pump 5, the confining pressure control pump 6, and the seepage control pump 7 are respectively equipped with a confining pressure sensor 10, an axial pressure sensor 11, and a seepage flow rate sensor 12. The axial pressure control pump 5, the confining pressure control pump 6, and the seepage control pump 7 mainly control the confining pressure, axial pressure, and seepage flow rate values, and can carry out unconsolidated undrained shear, consolidated undrained shear, and consolidated drained shear tests. The confining pressure sensor 10, the axial pressure sensor 11, and the seepage flow rate sensor 12 mainly measure the actual loaded confining pressure, axial pressure, and seepage flow rate values.

[0075] In order to support the soil triaxial testing mechanism 1, as shown in Figure 6 As shown, the soil triaxial testing mechanism 1 is arranged on a mobile test frame 130, wherein the axial pressure reaction frame 119 is fixedly connected to the mobile test frame 130 through its support rod, and the axial pressure loading cylinder 112 is also arranged on the mobile test frame 130. In order to fix the nuclear magnetic probe coil 111, the nuclear magnetic probe coil 111 is fixed to the support rod of the axial pressure reaction frame 119 through a fixing device to realize that the confining pressure cavity 123 is located at the center of the nuclear magnetic probe coil 111.

[0076] In another exemplary embodiment of the present application, Figure 7 As shown, the structure of the low-temperature freezing mechanism 2 is the same as that of the confining pressure simulation module; the freezing circulating liquid inlet 21 of the low-temperature freezing mechanism 2 is the confining pressure inlet 124; the freezing circulating liquid outlet 22 of the low-temperature freezing mechanism 2 is the confining pressure outlet 125, so that the confining pressure inlet and outlet are also used to apply liquid circulation to allow the sample to undergo a variable temperature freezing experiment.

[0077] The low-temperature freezing mechanism 2 can be used to perform the operation of gradually freezing the soil sample using different temperature gradients in step S3.

[0078] The freezing circulating fluid inlet 21 and outlet 22 are connected to a low-temperature freezing circulating pump 24 via a freezing circulating pipe 23. Temperature sensors are located within the inlet and outlet of the freezing circulating pipe 23. These temperature sensors are in communication with the data acquisition and analysis unit 4 and are used to measure the temperature of the freezing circulating fluid at the inlet and outlet of the freezing circulating pipe 23 in real time. The actual response temperature of the soil sample can be determined by measuring the temperatures of the circulating fluid at the inlet and outlet. Before the test, the freezing temperature of the soil sample is set. Using three temperature sensors—two at the inlet and outlet of the freezing circulating fluid and one inserted into the soil—the soil temperature and the average inlet and outlet temperature of the circulating fluid are recorded to obtain a temperature difference. In the actual test, the actual soil sample temperature is calculated by adding or subtracting the temperature difference from the known inlet and outlet average temperature. The actual soil sample temperature is calibrated using the difference between the reading of the temperature sensor inserted into the soil before the test and the average inlet and outlet temperature of the circulating fluid, minimizing the impact of the temperature sensor on the soil during the test. This actual soil temperature is displayed on the data acquisition and analysis unit 4.

[0079] Among them, the operating temperature range of the low-temperature freezing mechanism 2 is 65℃~-95℃, the control accuracy is ±0.1℃, and the circulating liquid medium is fluorinated liquid. The freezing circulating liquid inlet 21 and the freezing circulating liquid outlet 22 are connected to the freezing circulating pump 24 through pipelines to achieve gradual freezing of the water in the soil sample. The freezing circulating pump 24 includes a heat exchanger, and the temperature of the circulating liquid in the freezing circulating pump 24 can be exchanged through the heat exchanger. The secondary temperature control is used to control the stability of the circulating liquid temperature and then control the stability of the soil temperature, reducing temperature fluctuations. The freezing circulation pipeline 23 is an insulated pipeline with an operating temperature range of 65℃~-95℃. The circulating liquid is mainly injected into the hollow pressure chamber and shares a chamber with the confining pressure. The confining pressure simulation module simulates the actual confining pressure of the environment in which the soil sample is located. At the same time, it uses the same liquid medium as the low-temperature freezing mechanism 2 to provide pressure supplementation, thereby compensating for the pressure loss caused by temperature changes during the low-temperature freezing test.

[0080] The freezing temperature can be set by the data acquisition and analysis unit 4 to achieve freezing simulation at different freezing temperatures.

[0081] In another exemplary embodiment of the present application, Figure 9 As shown, the nuclear magnetic resonance testing mechanism 3 specifically includes: a low-field nuclear magnetic resonance instrument 31, a low-field nuclear magnetic resonance imaging module 32 and a layered gradient module 33; the low-field nuclear magnetic resonance instrument 31, the low-field nuclear magnetic resonance imaging module 32 and the layered gradient module 33 are all communicated with the data acquisition and analysis mechanism 4; during the test, the soil triaxial testing mechanism 1 is located in the nuclear magnetic resonance testing area (i.e., the soil sample testing area) of the nuclear magnetic resonance testing mechanism 3.

[0082] The low-field nuclear magnetic resonance instrument 31 is primarily responsible for providing a uniform and stable magnetic field. The principle of testing unfrozen water by the nuclear magnetic resonance testing unit 3 is based on the nuclear magnetic resonance phenomenon, whereby atomic nuclei undergo energy level transitions and generate resonance signals when excited by radio frequency pulses in a magnetic field. Low-field nuclear magnetic resonance technology can be used to detect the distribution and migration of water in soil, as well as changes in pore and fracture structures, due to the presence of hydrogen nuclei.

[0083] The low-field nuclear magnetic resonance imaging module 32 is mainly used to image the soil during the test to observe the internal moisture distribution phenomenon of the soil. The principle is mainly based on low-field nuclear magnetic resonance technology. Through operations such as magnetization, excitation, relaxation and signal detection, the internal structure and composition of the soil sample are detected (based on the signal information of water). Then, through data post-processing and pseudo-color enhancement, the nuclear magnetic resonance image of the internal moisture migration change process of the soil sample is drawn in real time.

[0084] The layered gradient module 33 non-destructively slices and layers the soil sample at specific thicknesses, collecting T2 spectra from each layer. This allows analysis of the NMR signal information at different locations along the soil sample's height to determine the water content and distribution of pore and fracture structures within each layer. Specifically, the layered gradient module 33 slices the soil sample at specific thicknesses, generating NMR data within a specific thickness. This allows observation of moisture, pores, and fractures within the soil sample at different thicknesses, leading to a comprehensive analysis of the water content and pore and fracture structure distribution within each layer.

[0085] The nuclear magnetic resonance testing mechanism 3 can be used to collect the nuclear magnetic resonance images in step S2 and step S3 of the above-mentioned testing method and the layered T2 spectra of the soil sample at different heights.

[0086] Existing testing devices are unable to simulate low temperature and seepage environments under real working conditions, and it is difficult to simultaneously achieve the coupling of confining pressure, axial pressure, seepage simulation and low temperature control within the limited space of the triaxial equipment. The testing device designed in this application can solve the above-mentioned problems.

[0087] Based on the test device given above, a specific process of testing implemented by using the test device is given, and the specific steps are as follows.

[0088] Step 1: Use the soil triaxial test device 1 to perform a triaxial test on the soil sample to simulate the real confining pressure, real axial pressure and seepage velocity under actual working conditions of the environment in which the soil sample is located.

[0089] First, the confining pressure simulation module of the soil triaxial testing mechanism 1 is controlled to apply confining pressure to the soil sample according to the actual confining pressure of the soil sample's environment. Before implementing confining pressure loading, some preparation is required. Prepare a soil sample (e.g., 3.91 cm in diameter and 8 cm in height). Before placing the soil sample in the soil triaxial testing mechanism 1, evacuate the air from the seepage inlet 128 and seepage outlet 129 pipes. Once the air is evacuated, place the soil sample on the soil sample lower base 115. Secure the sample's upper top and lower bottom, the soil sample upper loading cap 116, and the soil sample lower base 115. Then, install the triaxial instrument housing, which comprises the confining pressure chamber 123 and the nuclear magnetic resonance probe coil 111. Connect the axial pressure inlet 122, confining pressure inlet 124, confining pressure outlet 125, seepage inlet 128, and seepage outlet 129 to the corresponding pipelines. After the connections are completed, move the soil triaxial testing device 1 to the NMR testing area of ​​the NMR testing device 3. Operate the confining pressure control pump 6 to continuously inject fluorinated liquid into the confining pressure chamber 123. Use the reading of the confining pressure sensor to determine whether the applied confining pressure reaches the target confining pressure value.

[0090] Then, the axial pressure simulation module of the soil triaxial testing mechanism 1 is controlled to apply axial pressure to the soil sample according to the actual axial pressure of the environment in which the soil sample is located. The reading of the axial pressure sensor is used to determine whether the loaded axial pressure reaches the target axial pressure value.

[0091] Finally, the seepage simulation module of the soil triaxial testing mechanism 1 is controlled to simulate the seepage velocity of the soil sample according to the actual working conditions. The seepage velocity is set according to the actual working conditions, and the reading of the seepage velocity sensor 12 is used to determine whether the soil sample has completely seeped. When the flow rate at the seepage outlet 129 is monitored to change and is close to the flow rate at the seepage inlet 128, the soil sample is considered to have achieved complete seepage, that is, the seepage channel is unobstructed.

[0092] Step 2: Use the nuclear magnetic resonance testing device 3 to collect the nuclear magnetic resonance image of the soil sample under the action of seepage before freezing and the layered T2 spectrum of the soil sample at different heights, and use the soil triaxial testing device 1 to collect the overall T2 spectrum of the soil sample under the action of seepage before freezing.

[0093] First, the nuclear magnetic resonance testing mechanism 3 is controlled to provide a nuclear magnetic resonance magnetic field to the soil sample, and the nuclear magnetic resonance imaging parameters (magnet type, radio frequency pulse frequency range, radio frequency frequency control accuracy, radio frequency pulse accuracy, radio frequency transmission power, maximum sampling bandwidth, etc.) are adjusted. Then, the nuclear magnetic resonance image of the soil sample under the action of normal temperature seepage and the layered T2 spectrum at different heights of the soil sample are collected.

[0094] Secondly, the nuclear magnetic probe coil 111 loaded in the soil triaxial testing mechanism 1 is controlled to collect the overall T2 spectrum of the soil sample before the soil sample is frozen.

[0095] Step 3: Use the low-temperature freezing mechanism to gradually freeze the soil sample based on different temperature gradients, use the nuclear magnetic resonance testing mechanism 3 to collect the nuclear magnetic resonance image of the soil sample under the coupling of seepage and freezing and the layered T2 spectrum at different heights in real time, and use the soil triaxial testing mechanism 1 to collect the overall T2 spectrum of the soil sample under the coupling of seepage and freezing in real time.

[0096] First, the freezing circulating liquid inlet 21 and the freezing circulating liquid outlet 22 are connected to the freezing circulating pump 24 through pipelines to achieve gradual freezing of the water in the soil sample. During the freezing process, axial pressure and seepage always exist.

[0097] Secondly, record the readings of the displacement sensor 121, the pressure sensor 120, and the temperature sensor, set the freezing temperature of the pump body according to research needs, turn on the freezing circulation pump 24, and at the same time turn on the control switch 9 of the seepage channel to achieve freezing and seepage at the same time.

[0098] Finally, data is collected in real time, including the nuclear magnetic resonance image of the soil sample under the coupling of seepage and freezing, the overall T2 spectrum, and the layered T2 spectrum at different heights. The test is terminated after all the test condition data are collected, and the data analysis is carried out subsequently.

[0099] Step 4: Analyze the data to derive the evolution of the soil sample's moisture migration characteristics throughout the freezing process. Using the data acquisition and analysis mechanism, based on the NMR images of the soil sample before and during freezing, the data post-processing quantitatively analyzes the moisture distribution and changes in moisture migration during the freezing process. Based on the overall T2 spectrum of the soil sample before and during freezing, as well as the layered T2 spectra at different heights, the data post-processing quantitatively analyzes the changes in unfrozen water content and the evolution of pores and cracks in the soil sample as a whole and at different heights during freezing. Figure 2 This is a schematic diagram of the NMR image of the water migration process in clay under freezing and seepage. Figure 2 Brighter locations indicate higher water content. Figure 3 Schematic diagram of the overall T2 spectrum of clay under different freezing temperatures and seepage effects. Figure 4 Schematic diagram of clay pore changes under different freezing temperatures and seepage effects.

[0100] Data analysis is performed using various images, T2 spectra, or sensor readings collected during the freezing and seepage processes. For example, displacement sensor readings and frost heave deformation caused by freezing can be analyzed to determine the overall frost heave deformation of the soil sample.

[0101] This application has the following technical effects.

[0102] 1) The proposed low-field nuclear magnetic resonance-based triaxial seepage coupling test technology for soil freezing water migration evolution characteristics can simultaneously create a freezing and seepage test environment, facilitating further analysis of the evolution of soil water migration characteristics under the simultaneous effects of freezing and seepage.

[0103] 2) This application enables high-precision, non-destructive, real-time monitoring of moisture migration during soil freezing. Compared to traditional methods such as time-domain reflectometry and differential scanning calorimetry, low-field nuclear magnetic resonance technology can more directly and accurately reflect the distribution and dynamic changes of moisture within the soil, improving the accuracy and reliability of test data.

[0104] 3) This application enables non-invasive, real-time acquisition of changes in pores and cracks during soil freezing. Compared to traditional methods, low-field nuclear magnetic resonance technology eliminates the need for sampling, reduces soil sample disturbance, and provides the most realistic data on pores and cracks during freezing.

[0105] 4) This application proposes a technique for testing the evolution of soil freeze-water migration under triaxial seepage coupling using low-field nuclear magnetic resonance (NMR), significantly improving testing efficiency and reducing testing costs. Traditional testing methods often require extensive sample collection and long testing cycles. This application, through an integrated testing device and automated testing process, significantly improves testing efficiency while reducing sample consumption and labor costs.

[0106] 5) The test technology for the evolution characteristics of soil freezing water migration under triaxial seepage coupling based on low-field nuclear magnetic resonance proposed in this application enhances the repeatability and comparability of data. The use of standardized test equipment and corresponding test methods makes the test results of different batches and under different conditions more repeatable and comparable, which is conducive to the in-depth scientific research and the wide application of the results.

[0107] In summary, this application can provide technical support for studying the evolution of water migration and pore fractures in soils under the coupled effects of freezing and seepage, effectively revealing the hydrothermal coupling mechanism of frozen soils. By constructing a test device for triaxial seepage coupling and combining appropriate testing methods to simulate the soil freezing process in a real environment, this will help promote the development and improvement of permafrost theory, further optimize the parameter settings and construction process of artificial freezing technology, improve freezing effectiveness and construction efficiency, and reduce project costs.

[0108] 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.

[0109] 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 the characteristics of frozen soil water migration under low-temperature nuclear magnetic triaxial seepage, characterized in that: include: S1, the real confining pressure, real axial pressure and real seepage velocity of the simulated soil sample environment; The seepage velocity is set according to the actual working conditions, and the reading of the seepage velocity sensor is used to determine whether the soil sample has completely seeped. When the flow rate at the seepage outlet is monitored to change and is close to the flow rate at the seepage inlet, the soil sample is considered to have achieved complete seepage. S2. Collect nuclear magnetic resonance images, overall and layered T2 spectra of the soil sample before freezing under seepage; overall and layered T2 spectra refer to the overall T2 spectrum of the soil sample and the layered T2 spectra at different heights of the soil sample; S3, gradually freezing soil samples at different temperature gradients, and collecting real-time nuclear magnetic resonance images, overall and layered T2 spectra of soil samples under the coupled effects of seepage and freezing; S4. Analyze the evolution of water migration characteristics during the freezing process of soil samples based on nuclear magnetic resonance images, overall and layered T2 spectra before and during freezing. Water migration characteristics include water distribution in soil samples, changes in water migration, changes in unfrozen water content, and changes in pores and cracks. Based on the nuclear magnetic resonance images, overall and layered T2 spectra of soil samples before and during freezing, the evolution of water migration characteristics during the freezing process of soil samples was analyzed, including: 1) Based on the NMR images of soil samples before and during freezing, the water distribution and water migration changes during the freezing process of the soil samples were qualitatively analyzed through data post-processing; Nuclear magnetic resonance images are used to qualitatively analyze water migration. The amount of water accumulated in different parts of the soil sample is different. The color intensity of the nuclear magnetic resonance image represents the amount of water and can be used to determine the general pattern of water migration. 2) Based on the overall and layered T2 spectra of the soil sample before and during freezing, data post-processing was used to quantitatively analyze the changes in unfrozen water content and the evolution of pores and cracks in the soil sample as a whole and at different height layers during freezing. The evolution of pores and cracks was determined based on the changes in pore and crack radius. The process of soil freezing water migration mainly involves observing the changes in the unfrozen water content at different heights in the soil sample before and after freezing. By inverting and calculating the corresponding unfrozen water content based on the T2 spectrum at different freezing temperatures and different heights of the soil sample, the entire water migration process can be quantitatively analyzed. The T2 spectrum quantitatively analyzes water information, and the unfrozen water content at specific locations can be determined through layered T2 spectra. The water migration pattern can then be determined based on the unfrozen water content at different locations and at different times. Soil freezing cracks are obtained based on nuclear magnetic resonance images. Cracks are natural seepage channels filled with water. The principle of nuclear magnetic resonance imaging is to image the signal information of water. Therefore, the information of water in the soil can be intuitively seen through nuclear magnetic resonance images. The evolution of cracks during freezing and seepage is presented in real time, effectively capturing the changes in cracks. The calculation formula for unfrozen water content is: ； Where, w u is the unfrozen water content of the soil sample during freezing; w 0 is the total water content of the soil sample before freezing; is the total intensity value of the nuclear magnetic resonance signal before the soil sample is frozen, and the total intensity value of the nuclear magnetic resonance signal is determined based on the T2 spectrum; is the total intensity value of the nuclear magnetic resonance signal during the freezing process of the soil sample; The calculation formula for pore and crack radius is: ； Where T2 is the transverse relaxation time; Fs is the pore characteristic factor or the fracture characteristic factor; is the surface relaxation rate; R is the pore radius or crack radius.

2. A device for testing the characteristics of frozen soil water migration under low-temperature nuclear magnetic triaxial seepage, characterized in that: The device for testing the characteristics of frozen soil water migration under low-temperature nuclear magnetic triaxial seepage is used to implement the method for testing the characteristics of frozen soil water migration under low-temperature nuclear magnetic triaxial seepage according to claim 1, and the device for testing the characteristics of frozen soil water migration under low-temperature nuclear magnetic triaxial seepage comprises: a soil triaxial testing mechanism, a low-temperature freezing mechanism, a nuclear magnetic resonance testing mechanism, and a data acquisition and analysis mechanism; The soil triaxial testing mechanism is used to simulate the real confining pressure, real axial pressure and real seepage conditions of the soil sample environment, and to collect the overall T2 spectrum of the soil sample under the seepage effect before freezing and under the seepage and freezing coupling effect during freezing; The low-temperature freezing mechanism is used to gradually freeze the soil sample based on different temperature gradients; The nuclear magnetic resonance testing mechanism is used to provide a nuclear magnetic resonance magnetic field to the soil sample, and collect nuclear magnetic resonance images of the soil sample under the action of seepage before freezing and under the coupled action of seepage and freezing during freezing, as well as layered T2 spectra at different heights of the soil sample; The data acquisition and analysis mechanism is used to obtain nuclear magnetic resonance images, overall T2 spectra, and layered T2 spectra at different heights of the soil sample before and during freezing, and analyze the evolution law of the moisture migration characteristics of the soil sample during the entire freezing process; the moisture migration characteristics include the moisture distribution in the soil sample, the change in moisture migration, the change in unfrozen water content, and the change in pores and cracks.

3. The device for testing frozen soil water migration characteristics under low-temperature nuclear magnetic triaxial seepage according to claim 2 is characterized in that: The soil triaxial testing mechanism specifically includes: an axial pressure simulation module, a confining pressure simulation module, a seepage simulation module and a nuclear magnetic probe coil; The axial pressure simulation module is used to simulate the actual axial pressure of the environment in which the soil sample is located; The confining pressure simulation module is used to simulate the actual confining pressure of the environment in which the soil sample is located; The seepage simulation module is used to simulate the actual seepage conditions of the environment in which the soil sample is located; The nuclear magnetic probe coil is used to excite the soil sample to resonate and receive the soil sample resonance signal to obtain the overall T2 spectrum of the soil sample before and during freezing.

4. The device for testing frozen soil water migration characteristics under low-temperature nuclear magnetic triaxial seepage according to claim 3 is characterized in that: The axial pressure simulation module includes an axial pressure loading cylinder, a lower push rod, a lower base plate, a soil sample lower base, a soil sample upper loading cap, an upper push plate, an upper push rod and an axial pressure reaction frame; The upper end of the axial pressure loading cylinder is connected to the lower end of the soil sample through the lower push rod, the lower bottom plate, and the lower base of the soil sample in sequence; the lower push rod passes through the lower bottom plate and contacts the lower end of the lower base of the soil sample; The upper end of the soil sample is connected to the axial pressure reaction frame through the soil sample upper loading cap, the upper top plate and the upper push rod in sequence; the lower end of the upper push rod passes through the upper top plate and contacts the upper end of the soil sample upper loading cap; A pressure sensor is arranged between the upper push rod and the axial pressure reaction frame; the pressure sensor is connected to the data acquisition and analysis mechanism; the pressure sensor is used to measure the axial pressure applied by the axial pressure loading cylinder to the soil sample; A displacement sensor base is arranged at the upper end of the axial pressure loading cylinder, the upper end of the displacement sensor base is in contact with the lower push rod, and a displacement sensor is arranged on the side of the displacement sensor base. The displacement sensor is used to monitor the axial pressure deformation by monitoring the deformation of the base caused by the lower push rod acting on the displacement sensor base; the displacement sensor is communicatively connected to the data acquisition and analysis mechanism.

5. The device for testing frozen soil water migration characteristics under low-temperature nuclear magnetic triaxial seepage according to claim 4 is characterized in that: The confining pressure simulation module includes a confining pressure chamber; a soil sample, a lower base of the soil sample, and an upper loading cap of the soil sample are placed in the confining pressure chamber; the upper end of the confining pressure chamber is connected to the upper top plate, and the lower end of the confining pressure chamber is connected to the lower bottom plate; the nuclear magnetic probe coil is located outside the confining pressure chamber; The lower end of the confining pressure chamber is provided with a confining pressure inlet, and the upper end of the confining pressure chamber is provided with a confining pressure outlet; control valves are provided at the confining pressure inlet and the confining pressure outlet.

6. The device for testing frozen soil water migration characteristics under low-temperature nuclear magnetic triaxial seepage according to claim 5 is characterized in that: The seepage simulation module includes a seepage inlet pipeline and a seepage outlet pipeline; The seepage inlet pipe passes through the lower push rod and the lower base of the soil sample from bottom to top; the outlet of the seepage inlet pipe contacts the lower end of the soil sample; the inlet of the seepage inlet pipe is set on the side wall of the lower push rod; The inlet of the seepage outlet pipeline contacts the upper end of the soil sample, and the outlet of the seepage outlet pipeline is located at the lower end of the confining pressure cavity.

7. The device for testing frozen soil water migration characteristics under low-temperature nuclear magnetic triaxial seepage according to claim 5 is characterized in that: The structure of the low-temperature freezing mechanism is the same as that of the confining pressure simulation module; the freezing circulating fluid inlet of the low-temperature freezing mechanism is the confining pressure inlet; the freezing circulating fluid outlet of the low-temperature freezing mechanism is the confining pressure outlet; The freezing circulation fluid inlet and the freezing circulation fluid outlet are connected to the freezing circulation pump through a freezing circulation pipeline; a temperature sensor is provided in the inlet and outlet of the freezing circulation pipeline; the temperature sensor is communicatively connected to the data acquisition and analysis unit; the temperature sensor is used to measure the temperature of the freezing circulation fluid at the inlet and outlet of the freezing circulation pipeline in real time; The freezing circulation pump includes a heat exchanger; the heat exchanger is used to perform heat exchange treatment on the freezing circulation fluid in the freezing circulation pump.

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