Method and device for testing pressure-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetism

Abstract

The invention discloses a low-temperature nuclear magnetism-based method and device for testing pressure-thaw evolution of frozen soil under triaxial stress, and relates to the technical field of frozen soil testing. Aiming at the technical bottlenecks that low-temperature freezing-seepage-triaxial stress coupling application is difficult to realize and pore cracks and ice content changes cannot be dynamically captured in the prior art, a low-temperature nuclear magnetic resonance test technology and a soil triaxial test technology are organically combined, low-temperature online accurate freezing is realized in the whole process, and test data is acquired in real time; and analyzing the frozen soil pressure-melting characteristic evolution law. The testing method comprises the following steps: simulating initial confining pressure, axial pressure and seepage velocity of a real environment of a soil sample, freezing the soil sample to a target temperature and keeping the temperature constant; dynamically acquiring a nuclear magnetic resonance image and an overall and layered T2 spectrum in an initial environment; changing the confining pressure and / or the axial pressure, and dynamically acquiring a nuclear magnetic resonance image and an overall and layered T2 spectrum in the change process of the confining pressure and / or the axial pressure; and analyzing the whole-process evolution of the pressure-thaw characteristics of the frozen soil under the action of different stresses.

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 compression-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance. Background Art

[0002] Frozen soil compression and thawing refers to the phenomenon that ice in frozen soil melts under high stress. When the pressure increases, the melting point of ice decreases, causing part of the ice to melt into water, weakening the stability of the frozen soil structure. Frozen soil compression and thawing will cause changes in the soil pore structure and promote the development of cracks, reducing the strength and bearing capacity of frozen soil and increasing the risk of deformation and damage to engineering structures. Therefore, real-time acquisition of changes in ice content, ice melting characteristics, and pore and crack evolution during the compression and thawing process of frozen soil is crucial for a deep understanding of the mechanical compression and thawing characteristics of frozen soil and engineering safety.

[0003] The core feature of frozen soil compression and thawing testing is that it can achieve the coupled application of triaxial stress (confining pressure, axial pressure) and create a seepage environment that may be faced in actual engineering under a constant low temperature environment, and minimize the impact on ice through a fast and non-destructive testing method, and obtain ice content and pore and crack characteristic data in real time and dynamically. However, most of the existing frozen soil compression and thawing testing technologies have significant limitations. For example, many testing methods only place frozen soil samples in a uniaxial compressive strength testing machine for direct testing, lacking the control of the low temperature environment, and unable to distinguish whether the frozen soil compression and thawing is caused by external effects or pressure. In addition, although some testing machines are equipped with low temperature environment chambers, it is still difficult to dynamically capture the evolution of microscopic pores and cracks inside the soil under pressure, and it is impossible to quantify and analyze the amount of ice melted into water under different pressures.

[0004] As an advanced non-destructive testing method, low-field nuclear magnetic resonance (NMR) technology can accurately determine the ice content in the sample (through water inversion analysis) and the evolution law of pores and cracks by measuring and analyzing low-field NMR signals. At present, low-field NMR technology has been successfully applied to triaxial tests of soil at room temperature, and independent triaxial technology of temperature-controlled soil has also made certain progress. However, the existing technology still has obvious shortcomings: it is impossible to dynamically capture the changes in ice content, ice morphology, and pore and crack evolution morphology in the soil under triaxial stress loading under the coupling of confining pressure, axial pressure, seepage loading and low temperature control. Therefore, a complete testing technology system has not yet been formed around the analysis of the evolution characteristics of the whole process of frozen soil compression and melting under stress. Especially under the action of triaxial stress, how to ensure the real-time observation of the evolution of ice inside the soil during the pressurization process and how to collect the evolution law of pores and cracks during the compression and melting process are still technical problems that need to be solved urgently.

[0005] In order to solve the shortcomings of the above problems, the present application proposes a method and device for testing the compression and thawing evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance. Summary of the invention

[0006] The purpose of this application is to provide a method and device for testing the pressure-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance, which can non-destructively and visually analyze the evolution law of ice content in frozen soil samples when ice melts, as well as the melting characteristics of ice in frozen soil samples and the evolution law of pores and cracks.

[0007] To achieve the above objectives, this application provides the following solutions: In the first aspect, the present application provides a method for testing the pressure-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance, comprising: S1, simulate the initial confining pressure, axial pressure and seepage velocity of the real environment of the soil sample, freeze the soil sample to the target temperature and keep it constant; S2. Dynamically collect the nuclear magnetic resonance images, overall and layered T2 spectra of frozen soil samples under the initial environment. The initial environment refers to the real axial pressure, confining pressure and seepage environment of the simulated frozen soil samples. The overall and layered T2 spectra refer to the overall T2 spectra of the soil samples and the layered T2 spectra at different heights of the soil samples. S3, changing different confining pressures and axial pressures, and dynamically collecting nuclear magnetic resonance images, overall and layered T2 spectra of frozen soil samples during the process of changing confining pressure and / or axial pressure; S4. Based on the nuclear magnetic resonance images, overall and layered T2 spectra of frozen soil samples under initial environment and during changes in confining pressure and / or axial pressure, the evolution characteristics of frozen soil pressure-thaw under single confining pressure change, single axial pressure change and simultaneous changes in confining pressure and axial pressure are analyzed respectively; the evolution characteristics of frozen soil pressure-thaw include the evolution law of ice content when ice melts in frozen soil samples, the melting characteristics of ice in frozen soil samples and the evolution law of pores and cracks; the evolution law of pores and cracks is determined based on the changes in pore and crack radius; the melting characteristics of ice are determined by observing the changes in ice morphology during the melting of ice into water at different positions inside the soil sample.

[0008] In the second aspect, the present application provides a frozen soil pressure-thaw evolution test device under triaxial stress based on low-temperature nuclear magnetic resonance, the frozen soil pressure-thaw evolution test device under triaxial stress based on low-temperature nuclear magnetic resonance is used to implement the above-mentioned frozen soil pressure-thaw evolution test method under triaxial stress based on low-temperature nuclear magnetic resonance, the frozen soil pressure-thaw evolution test device under triaxial stress based on low-temperature nuclear magnetic resonance comprises: a soil triaxial test mechanism, a low-temperature freezing mechanism, a nuclear magnetic resonance test 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 environment in which the soil sample is located, change the confining pressure and / or axial pressure applied to the frozen soil sample, and collect the overall T2 spectrum of the frozen soil sample in the initial environment and the overall T2 spectrum of the frozen soil sample during the process of changing the confining pressure and / or axial pressure; the initial environment refers to the real axial pressure, confining pressure and seepage environment in which the frozen soil sample is simulated; The low temperature freezing mechanism is used to freeze the soil sample to a target temperature and keep it constant; The nuclear magnetic resonance testing mechanism is used to provide a nuclear magnetic resonance magnetic field to the frozen soil sample, collect the nuclear magnetic resonance image of the frozen soil sample in the initial environment, the layered T2 spectrum of the frozen soil sample at different heights, and the nuclear magnetic resonance image of the frozen soil sample during the change of confining pressure and / or axial pressure and the layered T2 spectrum of the frozen soil sample at different heights; The data acquisition and analysis mechanism is used to analyze the evolution characteristics of frozen soil pressure-thaw during the change of single confining pressure, single axial pressure and simultaneous change of confining pressure and axial pressure according to the nuclear magnetic resonance images, overall T2 spectra and layered T2 spectra of frozen soil samples under the initial environment and during the change of confining pressure and / or axial pressure; the evolution characteristics of frozen soil pressure-thaw include the evolution law of ice content when ice in the frozen soil sample melts, the melting characteristics of ice in the frozen soil sample and the evolution law of pores and cracks; the evolution law of pores and cracks is determined according to the changes in pore and crack radius; the melting characteristics of ice are determined by observing the changes in the morphology of ice in the process of ice melting into water at different positions inside the soil sample.

[0009] According to the specific embodiments provided in this application, this application discloses the following technical effects: The present application provides a method and device for testing the compression-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance, simulating the initial confining pressure, axial pressure and seepage velocity of the real environment of the soil sample, freezing the soil sample to the target temperature and keeping it constant; dynamically collecting the nuclear magnetic resonance image, overall and layered T2 spectrum of the frozen soil sample under the initial environment; changing the confining pressure and / or axial pressure, dynamically collecting the nuclear magnetic resonance image, overall and layered T2 spectrum of the frozen soil sample during the change of the confining pressure and / or axial pressure; according to the nuclear magnetic resonance image, overall and layered T2 spectrum of the frozen soil sample under the initial environment and during the change of the confining pressure and / or axial pressure, respectively analyze the compression-thaw evolution characteristics in the process of single confining pressure change, single axial pressure change and simultaneous change of confining pressure and axial pressure. The present application can simulate the stress state of frozen soil in the actual environment, more comprehensively understand the compression-thaw evolution law of frozen soil under different stress conditions, and can monitor the changes in the frozen soil in real time under different stresses, including the change process of ice after compression-thaw, pore structure, crack changes, etc., which helps to deeply understand the behavior mechanism of frozen soil under complex stress environment. In addition, there is no need to physically destroy the frozen soil samples, and the integrity of the samples can be maintained, thereby more realistically reflecting the compression and melting characteristics and pore and crack evolution of frozen soil in its natural state, and realizing non-destructive and visual analysis of the unfrozen water, pore structure, and crack evolution laws inside the soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. 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 paying creative work.

[0011] Figure 1 A schematic flow chart of a method for testing the compression-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance provided in one embodiment of the present application; Figure 2 A schematic diagram of the structure of a test device for the compression-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance provided in one embodiment of the present application; Figure 3 A schematic diagram of the structure of a soil triaxial testing mechanism provided in one embodiment of the present application; Figure 4 A detailed structural diagram of a soil triaxial testing mechanism provided in one embodiment of the present application; Figure 5 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; Figure 6 A schematic diagram of the structure of a nuclear magnetic resonance testing mechanism provided in one embodiment of the present application; Figure 7A schematic diagram of changes in the T2 spectrum of clay during the compression and melting process provided in one embodiment of the present application; Figure 8 for Figure 7 Detailed schematic diagram of the middle C area; Fig. 9 A schematic diagram of clay pore changes under pressure melting provided in one embodiment of the present application; Fig.10 A schematic diagram of the change in ice content under pressure melting provided in one embodiment of the present application.

[0012] Figure numerals: 1-soil triaxial test mechanism; 111-nuclear magnetic probe coil; 112-axial pressure loading cylinder; 113-lower push rod; 114-lower bottom 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 instrument; 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

[0013] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0014] In order to make the above-mentioned objects, 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.

[0015] In an exemplary embodiment of the present application, a test method for the compression-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance is proposed, such as Figure 1 As shown, the following steps are included.

[0016] S1. Simulate the initial confining pressure, initial axial pressure and seepage velocity of the real environment of the soil sample, freeze the soil sample to the target temperature and keep it constant, and provide stable low-temperature boundary conditions.

[0017] S2. Dynamically collect the nuclear magnetic resonance image, overall and layered T2 spectrum of the frozen soil sample under the initial environment; the overall and layered T2 spectrum refers to the overall T2 spectrum of the soil sample and the layered T2 spectrum at different heights of the soil sample.

[0018] S3. Change the confining pressure and / or axial pressure, and dynamically collect the nuclear magnetic resonance images, overall and layered T2 spectra of the frozen soil samples during the change of the confining pressure and / or axial pressure.

[0019] S4. Based on the nuclear magnetic resonance images, overall and layered T2 spectra of frozen soil samples under initial environment and during changes in confining pressure and / or axial pressure, the evolution characteristics of frozen soil pressure-thaw under single confining pressure change, single axial pressure change and simultaneous changes in confining pressure and axial pressure are analyzed respectively; the evolution characteristics of frozen soil pressure-thaw include the evolution law of ice content when ice melts in frozen soil samples, the melting characteristics of ice in frozen soil samples and the evolution law of pores and cracks; the evolution law of pores and cracks is determined based on the changes in pore and crack radius; the melting characteristics of ice are determined by observing the changes in ice morphology during the melting of ice into water at different positions inside the soil sample.

[0020] The information on the change of unfrozen water content is because water will gradually disappear during the compression and melting process. According to the overall T2 spectrum of the frozen soil sample and the layered T2 spectrum at different heights, the overall distribution of water in the soil can be observed, and then the disappearance area and amount of ice during the compression and melting process can be analyzed. The evolution of ice-water deformation and pores and cracks during the compression and melting process can also be observed. The nuclear magnetic resonance image of the frozen soil during the compression and melting process is obtained based on the distribution of nuclear magnetic resonance signals. Based on the image information, the shape of the soil itself when frozen and the melting characteristics of ice during the compression and melting process can be observed.

[0021] According to the NMR images of frozen soil samples under the initial environment and during the change of confining pressure and / or axial pressure, the melting characteristics of ice under different stresses are qualitatively analyzed based on data post-processing. It refers to the change in the morphology of ice in the process of melting into water at different positions inside the soil sample under different pressures. Specifically, the gradual change process of the ice morphology is judged by the distribution of water element information in the NMR images of frozen soil samples. When the highlighted area in the image transitions from the middle to the edge of the soil sample, it means that the moisture content at the edge of the soil sample increases, that is, the ice lens gradually melts from the center to the edge; when the non-highlighted area in the image changes from a continuous distribution to a point distribution, it means that the porous ice melts from a continuous network structure to isolated ice crystal particles; when the image is all highlighted areas, it means that the cemented ice at the contact of soil particles completely disappears.

[0022] In another exemplary embodiment of the present application, in step S4, the calculation formulas for pore and crack radius are as follows.

[0023] ; In the formula, T 2 is the collected T 2 The transverse relaxation time of the spectral 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, F Expressed as the crack characteristic factor, F =1; when calculating the pore radius, F Expressed as the pore characteristic factor, spherical pores, F =3; columnar pores, F =2. S is the pore (crack) surface area; V is the pore (crack) volume.

[0024] In another exemplary embodiment of the present application, in step S4, the ice content is calculated based on the inversion of the unfrozen water content. The calculation formula of the ice content is as follows, and its core principle is based on the initial nuclear magnetic signal of the soil under the initial environment (the signal is proportional to the water content, and the more water, the stronger the signal), and then the corresponding unfrozen water content is obtained by inverse calculation based on the signal of the soil under different working conditions. The reduced water is considered to have formed ice.

[0025] ; In the formula, w i is the ice content; w u is the unfrozen water content of frozen soil samples during application of different stresses; w 0 is the total water content of the frozen soil sample under the initial environment; is the total intensity value of the nuclear magnetic resonance signal of the frozen soil sample under the initial environment, and the total intensity value of the nuclear magnetic resonance signal is determined according to the T2 spectrum; is the total intensity value of the NMR signal during the application of different stresses.

[0026] As an optional implementation method, the present application can apply nuclear magnetic resonance testing technology, low-temperature freezing technology and soil triaxial testing technology to the testing method, realize multi-field coupled loading of low-temperature freezing, seepage and triaxial stress, and use low-field nuclear magnetic resonance testing technology for dynamic non-destructive and rapid measurement. The present application effectively ensures that the compression and melting process is always in a stable low-temperature freezing environment, so that the compression and melting process is completely caused by different confining pressures or axial pressures, eliminating external interference. At the same time, the use of nuclear magnetic resonance testing technology realizes rapid and non-destructive detection of the entire compression and melting process, providing reliable technical support for the study of the mechanical behavior of frozen soil.

[0027] In this application, the overall T2 spectrum and layered T2 spectrum in the process of frozen soil compression and thawing under different confining pressures and axial pressures are inverted and analyzed to obtain the changes in unfrozen water content and the characteristics of pore and fissure structure. The core purpose is that during the compression and thawing process of frozen soil, ice will melt into water under the action of external forces, and unfrozen water is the main reason for aggravating the frost heave of the soil, so it is necessary to timely understand the unfrozen water content in frozen soil. The pore structure provides an advantageous place for water storage, providing a reference for the anti-frost heave design and management of frozen soil. Therefore, this application can study the internal changes of frozen soil during the compression and thawing process, including the melting process of ice, water migration, the evolution of pore and fissure structure, etc., so as to reveal the microscopic mechanism of frozen soil compression and thawing. This application can monitor and analyze these changes to provide a scientific basis for the prediction and prevention of geological disasters.

[0028] In order to realize the above-mentioned frozen soil pressure-thaw evolution test method under triaxial stress based on low-temperature nuclear magnetic resonance, another exemplary embodiment of the present application provides a preferred frozen soil pressure-thaw evolution test device under triaxial stress based on low-temperature nuclear magnetic resonance, such as Figure 2 As shown, the frozen soil compression-thawing evolution test device under triaxial stress based on low-temperature nuclear magnetic resonance includes: a soil triaxial test mechanism 1, a low-temperature freezing mechanism 2, a nuclear magnetic resonance test mechanism 3 and a data acquisition and analysis mechanism 4.

[0029] 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, change the confining pressure and / or axial pressure applied to the frozen soil sample, and collect the overall T2 spectrum of the frozen soil sample in the initial environment, and the overall T2 spectrum of the frozen soil sample during the confining pressure and / or axial pressure change process; the initial environment refers to the real axial pressure, confining pressure and seepage environment in which the frozen soil sample is simulated.

[0030] The low temperature freezing mechanism 2 is used to freeze the soil sample to a target temperature and keep it constant.

[0031] The NMR testing mechanism 3 is used to provide a NMR magnetic field to the frozen soil sample, collect the NMR image of the frozen soil sample in the initial environment, the layered T2 spectrum of the frozen soil sample at different heights, and the NMR image of the frozen soil sample during the change of confining pressure and / or axial pressure and the layered T2 spectrum of the frozen soil sample at different heights. Figure 2 As shown, the soil triaxial test mechanism 1 is arranged in the nuclear magnetic resonance test area of ​​the nuclear magnetic resonance test mechanism 3. Figure 6 In the figure, B refers to the NMR test area.

[0032] The data acquisition and analysis mechanism 4 is used to analyze the evolution characteristics of frozen soil pressure-thaw during a single confining pressure change, a single axial pressure change, and a simultaneous change of confining pressure and axial pressure according to the nuclear magnetic resonance images, overall T2 spectra, and layered T2 spectra of the frozen soil samples under the initial environment and during the change of confining pressure and / or axial pressure; the evolution characteristics of frozen soil pressure-thaw include the evolution law of ice content when ice melts in the frozen soil samples, the melting characteristics of ice in the frozen soil samples, and the evolution law of pores and cracks; the evolution law of pores and cracks is determined based on the changes in pore and crack radius; the melting characteristics of ice are determined by observing the changes in the morphology of ice during the melting of ice into water at different positions inside the soil sample. Specifically, the evolution law of ice content and the evolution law of pores and cracks in the frozen soil sample during the change of confining pressure and / or axial pressure are analyzed according to the layered T2 spectrum, the overall T2 spectrum, the layered T2 spectrum and the overall T2 spectrum of the frozen soil sample during the change of confining pressure and / or axial pressure under the initial environment, and the evolution law of pores and cracks is determined according to the change of pore or crack radius; and the melting characteristics of ice in the frozen soil sample during the change of confining pressure and / or axial pressure are analyzed according to the nuclear magnetic resonance image under the initial environment and the nuclear magnetic resonance image of the frozen soil sample during the change of confining pressure and / or axial pressure. The data acquisition and analysis mechanism 4 can be a computer. The data analysis process in step S4 can be implemented using the data acquisition and analysis mechanism 4.

[0033] 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.

[0034] The axial pressure simulation module is used to simulate the real axial pressure of the environment in which the frozen soil sample is located and to continue to apply axial pressure to the frozen soil sample on the basis of the initial axial pressure.

[0035] The confining pressure simulation module is used to simulate the real confining pressure of the environment in which the frozen soil sample is located and to continue to apply confining pressure to the frozen soil sample based on the initial confining pressure.

[0036] The seepage simulation module is used to simulate the actual seepage conditions of the environment in which the soil sample is located.

[0037] The nuclear magnetic probe coil 111 is used to excite the resonance of the frozen soil sample and receive the resonance signal of the frozen soil sample to obtain the overall T2 spectrum of the frozen soil sample in the initial environment and during the change of the confining pressure and / or axial pressure. The present application arranges the nuclear magnetic probe coil 111 in the soil triaxial testing mechanism 1 to collect the overall T2 spectrum of the soil body. The resonance signal of the frozen soil sample detected by the nuclear magnetic probe coil 111 is further inverted by the nuclear magnetic matching data processing software (such as the data acquisition and analysis mechanism 4) to obtain the overall T2 spectrum under different stresses.

[0038] The simulation of the initial confining pressure, initial axial pressure and seepage velocity in step S1 of the above test method can be realized by using the axial pressure simulation module, the confining pressure simulation module and the seepage simulation module of the soil triaxial test mechanism 1, and the freezing process of the soil sample is realized by using the low-temperature freezing mechanism 2. The collection of the overall T2 spectrum in step S2 and step S3 is realized by using the nuclear magnetic probe coil 111 of the soil triaxial test mechanism 1. The change of the confining pressure and / or axial pressure applied to the frozen soil sample in step S3 can be realized by using the axial pressure simulation module and the confining pressure simulation module of the soil triaxial test mechanism 1.

[0039] In another exemplary embodiment of the present application, Figure 3 and Figure 4 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.

[0040] The upper end of the axial pressure loading cylinder 112 is connected to the lower end of the soil sample through the lower push rod 113, the lower bottom plate 114, and the soil sample lower base 115 in sequence; the lower push rod 113 penetrates the lower bottom plate 114 and contacts the lower end of the soil sample lower base 115. The lower bottom plate 114 (which can be called the axial loading lower bottom plate) uses water as a driving force to control the axial loading. Figure 4 The "A" in the table refers to the soil sample.

[0041] The lower push rod 113, the lower bottom plate 114 and the soil sample lower base 115 are fixedly connected and integrated.

[0042] The upper end of the soil sample is connected to the axial pressure reaction frame 119 through the soil sample upper loading cap 116, the upper top plate 117 and the upper push rod 118 in sequence; the lower end of the upper push rod 118 penetrates 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, the upper top plate 117, the upper push rod 118 and the axial pressure reaction frame 119 are not fixedly connected, but are independent components. When axial pressure is applied, the components are in contact and connected. The material of the axial pressure reaction frame 119 can be selected from aluminum alloy, which has a certain strength.

[0043] A pressure sensor 120 is arranged between the upper push rod 118 and the axial pressure reaction frame 119; the pressure sensor 120 is connected to the data acquisition and analysis mechanism 4 in communication; the pressure sensor 120 is used to measure the magnitude of the axial pressure applied to the soil sample by the axial pressure loading cylinder 112. The pressure sensor 120 monitors the force applied to the upper push rod 118, measures the magnitude of the axial pressure applied to the soil sample by the axial pressure loading cylinder 112, records the change of pressure under the axial pressure, and reflects the magnitude of the applied axial pressure in real time.

[0044] A displacement sensor base 131 is arranged at the upper end of the axial pressure loading cylinder 112, and the displacement sensor base 131 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, and is not fixedly connected. A displacement sensor 121 is arranged on the side of the displacement sensor base 131, and the displacement sensor 121 monitors the axial pressure deformation by monitoring the base deformation caused by the lower push rod 113 acting on the displacement sensor base 131; the displacement sensor 121 is connected to the data acquisition and analysis mechanism 4 in communication. By monitoring the deformation of the displacement sensor base 131, the displacement change value of the frozen soil sample under different stresses is monitored. By using the pressure sensor 120 and the displacement sensor 121, the changes in the confining pressure, axial pressure, and axial displacement during the loading process can be recorded in real time.

[0045] In another exemplary embodiment of the present application, the lower base 115 of the soil sample is made of polytetrafluoroethylene material, which has no nuclear magnetic resonance signal; the upper loading cap 116 of the soil sample is made of ceramic or PEEK polyetheretherketone material, which has little effect on the nuclear magnetic resonance test.

[0046] In another exemplary embodiment of the present application, the confining pressure simulation module includes a confining pressure chamber 123; the soil sample, the lower base 115 of the soil sample and the upper loading cap 116 of the soil sample 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.

[0047] A confining pressure inlet 124 is provided at the lower end of the confining pressure chamber 123, and a confining pressure outlet 125 is provided at the upper end of the confining pressure chamber 123; control valves are provided at both the confining pressure inlet 124 and the confining pressure outlet 125. The confining pressure inlet 124 is connected to the confining pressure control pump 6 through a pipeline to realize pressure loading. The outer side of the confining pressure chamber 123 is made of non-metallic material containing less hydrogen elements, and the internal chamber is a hollow structure. During the test, fluorinated liquid is injected into the confining pressure chamber 123 and the pressure is controlled by the confining pressure control pump 6.

[0048] In another exemplary embodiment of the present application, the seepage simulation module includes a seepage inlet pipeline 126 and a seepage outlet pipeline 127 .

[0049] The seepage inlet pipeline 126 passes through the lower push rod 113 and the soil sample lower base 115 from bottom to top in sequence; the outlet of the seepage inlet pipeline 126 contacts the lower end of the soil sample; and the inlet of the seepage inlet pipeline 126 is arranged on the side wall of the lower push rod 113 .

[0050] The inlet of the seepage outlet pipeline 127 contacts the upper end of the soil sample, and the outlet of the seepage outlet pipeline 127 is located at the lower part of the confining pressure chamber 123; Figure 4 As 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 .

[0051] 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.

[0052] like Figure 5 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, and the liquid storage tank 8 is divided into three chambers, which respectively supply the corresponding action media of the axial pressure, confining pressure and seepage. For the axial pressure and seepage control, the liquid in the axial pressure chamber and the seepage chamber in the liquid storage tank 8 is distilled water. For the confining pressure control, the liquid in the confining pressure chamber in the liquid storage tank 8 can be fluorinated liquid. The pipelines at the outlets of the axial pressure control pump 5, the confining pressure control pump 6 and the seepage control pump 7 are respectively provided with control switches 9. The axial pressure control pump 5, the confining pressure control pump 6 and the seepage control pump 7 are respectively provided with confining pressure sensors 10, axial pressure sensors 11 and seepage flow rate sensors 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 velocity 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 values ​​of the actually loaded confining pressure, axial pressure and seepage flow rate.

[0053] In order to realize the support of soil triaxial testing mechanism 1, Figure 3 As shown, the soil triaxial test 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 on the support rod of the axial pressure reaction frame 119 to realize that the confining pressure chamber 123 is located at the center of the nuclear magnetic probe coil 111.

[0054] like Figure 4As 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.

[0055] The freezing circulation liquid inlet 21 and the freezing circulation liquid outlet 22 are connected to the freezing circulation pump 24 through the freezing circulation pipeline 23; a temperature sensor is provided in the inlet and outlet of the freezing circulation pipeline 23; the temperature sensor is connected to the data acquisition and analysis mechanism 4 in communication; the temperature sensor is used to measure the temperature of the freezing circulation liquid at the inlet and outlet of the freezing circulation pipeline 23 in real time. The actual response temperature of the soil sample can be determined by measuring the temperature of the inlet and outlet circulation liquid. For example, before the test, the freezing temperature of the soil body is set, and three temperature sensors are used, two of which are at the inlet and outlet of the freezing circulation liquid, and the other is inserted into the soil body. By recording the soil body temperature and the average inlet and outlet temperature of the circulation liquid, a temperature difference can be obtained. In the actual test, the actual temperature value of the soil sample is obtained by adding and subtracting the temperature difference from the known inlet and outlet average temperature. In the actual test, the measurement of the actual temperature value of the soil sample is calibrated and calculated by using the reading of the temperature sensor inserted into the soil body before the test and the difference between the average inlet and outlet temperature of the circulation liquid, so as to minimize the influence of the temperature sensor on the soil body during the test. The actual temperature value of the soil body will be displayed in the data acquisition and analysis mechanism 4.

[0056] 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 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 heat exchanged through the heat exchanger. The stability of the circulating liquid temperature is controlled by the secondary temperature control to control the stability of the soil temperature and reduce 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 real confining pressure of the environment in which the soil sample is located, and at the same time uses the same liquid medium as the low-temperature freezing mechanism 2 to provide pressure supplement, thereby compensating for the pressure loss caused by temperature changes during the low-temperature freezing test.

[0057] The freezing temperature can be set by the data acquisition and analysis unit 4 to realize freezing simulation at different freezing temperatures, thereby preparing frozen soil samples at different freezing temperatures.

[0058] In another exemplary embodiment of the present application, Figure 6As shown, the NMR testing mechanism 3 specifically includes: a low-field NMR instrument 31, a low-field NMR imaging module 32 and a layered gradient module 33; the low-field NMR instrument 31, the low-field NMR imaging module 32 and the layered gradient module 33 are all connected to the data acquisition and analysis mechanism 4 in communication; during the test, the soil triaxial testing mechanism 1 is located in the NMR testing area (i.e., the soil sample testing area) of the NMR testing mechanism 3. The magnetic field length of the NMR testing mechanism 3 is 0.3T±0.03T, and the shortest echo time is 60μs, so that micro-nano pore relaxation signals can be collected.

[0059] The low-field nuclear magnetic resonance instrument 31 is mainly responsible for providing a uniform and stable magnetic field; the principle of the nuclear magnetic resonance testing mechanism 3 to test unfrozen water is mainly based on the nuclear magnetic resonance phenomenon, that is, after the atomic nucleus is excited by the radio frequency pulse in the magnetic field, the energy level transition will occur and a resonance signal will be generated. For the water in the soil, due to the presence of hydrogen nuclei, the low-field nuclear magnetic resonance technology can be used to detect the pressure melting characteristics of the frozen soil sample and the changes in the pore and crack structure.

[0060] The low-field nuclear magnetic resonance imaging module 32 is mainly used to image the frozen soil samples during the test to observe the internal moisture distribution phenomenon of the frozen soil samples. 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 frozen soil samples 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 frozen soil samples is drawn in real time.

[0061] The layered gradient module 33 can collect the T2 spectrum of each layer by non-destructively slicing and stratifying the frozen soil sample according to a certain thickness, so as to analyze the water content of each layer and the distribution of pore and crack structures by analyzing the nuclear magnetic resonance signal information at different positions along the height of the soil column. Specifically, the frozen soil sample is sliced ​​according to a certain thickness by the layered gradient module 33 to obtain the nuclear magnetic resonance data within a certain thickness, so as to observe the changes in unfrozen water content, pores, cracks, etc. within different thicknesses of the frozen soil sample, so as to comprehensively analyze the distribution of water content and pore and crack structures of each layer.

[0062] In the above test method, the nuclear magnetic resonance image and layered T2 spectrum in step S2 and step S3 can be achieved by the nuclear magnetic resonance test mechanism 3 .

[0063] In this application, the nuclear magnetic resonance equipment is deeply combined with the soil triaxial test system. While achieving stable low-temperature freezing (precisely controlling the temperature boundary), the multi-field coupling of low-temperature freezing, seepage, confining pressure and axial pressure is achieved by applying varying confining pressure and axial pressure. The nuclear magnetic resonance test technology is then used to accurately capture the pressure-melting characteristics and pore-crack evolution characteristics of frozen soil under different axial pressures and confining pressures in a constant low-temperature environment, providing a new solution for in-depth research on the pressure-melting process of frozen soil. Existing test devices cannot 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 loading and low-temperature control in the limited space of the triaxial device. The test device designed in this application can solve the above-mentioned problems.

[0064] Based on the above-disclosed low-temperature nuclear magnetic resonance-based triaxial stress test device for frozen soil compression and thawing evolution, the test process from the preparation of frozen soil samples to the compression and thawing of frozen soil samples is explained.

[0065] (1) Sample preparation: Taking clay as an example, take the saturated original clay and use a soil cutter to roughly and finely cut it to make a standard triaxial sample with a diameter of 3.91 cm and a height of 8 cm. Then put the sample into the triaxial clamp, and wrap the rubber ring to fix the upper and lower ends of the latex film and the clamp to ensure that fluoride liquid will not enter the soil sample.

[0066] (2) Connection lines: Connect the lines of the low-temperature cooling circulating fluid inlet and outlet and the axial pressure loading port, push the clamp into the low-field nuclear magnetic resonance test mechanism 3, and then connect the displacement sensor 121, the pressure sensor 120 and the data line and performance line of the nuclear magnetic resonance itself.

[0067] (3) Simulation of initial environment and data collection: Use soil triaxial test device 1 to simulate the real confining pressure, real axial pressure and real seepage conditions of the environment in which the soil sample is located. For example, set the confining pressure to 70 kPa, the axial pressure to 100 kPa, and the seepage velocity to 0.5 m / d, and use soil triaxial test device 1 to load to the specified value.

[0068] According to the test requirements, the freezing temperature was set at -30°C, and the soil began to freeze. The low-temperature freezing mechanism was used to freeze the soil sample to the target temperature and keep it constant. When the temperature fluctuated slightly (the change per hour did not exceed 0.1°C), it was considered stable.

[0069] Set the NMR acquisition parameter values ​​(echo time 0.2ms, number of echoes 5000, accumulation times 8 times, waiting time 3000ms). Use the NMR testing mechanism to provide the frozen soil sample with a NMR magnetic field, collect the NMR image of the frozen soil sample in the initial environment and the layered T2 spectrum of the frozen soil sample at different heights. In addition, use the soil triaxial testing mechanism 1 to collect the overall T2 spectrum of the frozen soil sample in the initial environment.

[0070] (4) Changing the confining pressure and axial pressure and the corresponding data collection: The confining pressure is set to 200 kPa and the axial pressure is set to 100 kPa (condition 1), the confining pressure is set to 500 kPa and the axial pressure is set to 300 kPa (condition 2), and the confining pressure is set to 1000 kPa and the axial pressure is set to 600 kPa (condition 3). The soil triaxial test device 1 changes the confining pressure and / or axial pressure applied to the frozen soil sample to achieve the above working condition requirements. At the same time, the soil triaxial test device 1 is used to collect the overall T2 spectrum of the frozen soil sample under each working condition in real time. In addition, the nuclear magnetic resonance test device 3 is used to collect the nuclear magnetic resonance images of the frozen soil samples under each working condition and the layered T2 spectra at different heights of the frozen soil samples.

[0071] (5) Data analysis: The relevant characteristics of the freeze-thaw process were further analyzed based on the MRI images, overall and layered T2 spectrum data at different stages. Figure 7 and Figure 8 The schematic diagram shows the change of T2 spectrum of clay during the compression and melting process. Fig. 9 The schematic diagram of clay pore changes under pressure melting is shown; Fig. 9 The change of clay porosity in the soil is mainly obtained by the calculation formula of the pore and crack radius mentioned above to obtain the distribution of different pore radius sizes, and then according to the existing known clay large pore (i.e. large pore) and small pore (i.e. small pore) radius division points (for example, silty clay in a certain area: less than 2μm is small pore, greater than 2μm is large pore), the proportion of large pores or small pores in the total pore (large pores + small pores) composition under the same group of working conditions is divided. The proportion of large pores or small pores refers to the proportion of small pores or large pores in the total pore composition of the soil sample under a certain working condition. Fig.10 The evolution of ice content under different working conditions is shown; Fig.10 The ice content in is calculated based on the aforementioned ice content calculation formula.

[0072] This application has the following technical effects: 1) This application effectively ensures that the frozen soil is always in a stable low-temperature freezing environment during the compression and thawing process, so that the compression and thawing process is completely caused by different confining pressures or axial pressures, eliminating external interference.

[0073] 2) This application realizes the coupled application of low-temperature freezing-seepage-confining pressure and axial pressure. While ensuring a constant low-temperature environment, the seepage rate is controlled to be constant. Nuclear magnetic resonance testing technology is used to achieve rapid and non-destructive detection of the entire pressure-thawing process, thereby more accurately reflecting the pressure-thawing characteristics and pore and crack evolution of frozen soil in a real environment.

[0074] 3) This application is suitable for frozen soil testing under different temperature and stress conditions, has strong environmental adaptability, and can meet the needs of frozen soil research in various complex environments.

[0075] 4) This application can reproduce the melting characteristics of ice in different regions of the soil body, the distribution of water, and the evolution of pores and cracks during the process of frozen soil compression and thawing under different stresses in all aspects (T2 spectrum of the whole soil sample and at different heights) and with high visualization (nuclear magnetic resonance images). It provides real-time and accurate data support for a deeper understanding of the mechanical properties and physical characteristics of frozen soil or artificial frozen soil, helps promote research progress in the field of frozen soil science, and can also provide important technical support and theoretical basis for cold region engineering, geological disaster prevention and control, and other fields.

[0076] The technical features of the above embodiments may be arbitrarily combined. 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.

[0077] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will 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 compression-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance, characterized in that: include: S1, simulate the initial confining pressure, initial axial pressure and seepage velocity of the real environment of the soil sample, freeze the soil sample to the target temperature and keep it constant; S2. Dynamically collect the nuclear magnetic resonance images, overall and layered T2 spectra of frozen soil samples under the initial environment. The initial environment refers to the real axial pressure, confining pressure and seepage environment of the simulated frozen soil samples. The overall and layered T2 spectra refer to the overall T2 spectra of the soil samples and the layered T2 spectra at different heights of the soil samples. S3, changing the confining pressure and / or axial pressure, and dynamically collecting the nuclear magnetic resonance images, overall and layered T2 spectra of the frozen soil samples during the process of changing the confining pressure and / or axial pressure; S4. Based on the nuclear magnetic resonance images, overall and layered T2 spectra of frozen soil samples under initial environment and during changes in confining pressure and / or axial pressure, the evolution characteristics of frozen soil pressure-thaw under single confining pressure change, single axial pressure change and simultaneous changes in confining pressure and axial pressure are analyzed respectively; the evolution characteristics of frozen soil pressure-thaw include the evolution law of ice content when ice melts in frozen soil samples, the melting characteristics of ice in frozen soil samples and the evolution law of pores and cracks; the evolution law of pores and cracks is determined based on the changes in pore and crack radius; the melting characteristics of ice are determined by observing the changes in ice morphology during the melting of ice into water at different positions inside the soil sample.

2. The method for testing the compression-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance according to claim 1 is characterized in that: Based on the NMR images, overall and layered T2 spectra of frozen soil samples under the initial environment and during the change of confining pressure and / or axial pressure, the evolution characteristics of frozen soil compression and thawing under the change of single confining pressure, single axial pressure and simultaneous change of confining pressure and axial pressure are analyzed respectively, including: Based on the NMR images of frozen soil samples under initial conditions, confining pressure and / or axial pressure changes, the melting characteristics of ice under different stresses are qualitatively analyzed based on data post-processing; According to the overall and layered T2 spectra of frozen soil samples under initial environment and during the change of confining pressure and / or axial pressure, the evolution of ice content and pore cracks of the overall and layered frozen soil samples under different stresses is quantitatively analyzed based on data post-processing.

3. The method for testing the frozen soil pressure-thaw evolution under triaxial stress based on low-temperature nuclear magnetic resonance according to claim 2 is characterized in that: The ice content is calculated based on the inversion of the unfrozen water content. The ice content calculation formula is: ； In the formula, w i is the ice content; w u is the unfrozen water content of frozen soil samples during application of different stresses; w 0 is the total water content of the frozen soil sample under the initial environment; is the total intensity value of the nuclear magnetic resonance signal of the frozen soil sample under the initial environment, and the total intensity value of the nuclear magnetic resonance signal is determined according to the T2 spectrum; is the total intensity value of the NMR signal during the application of different stresses.

4. The method for testing the compression-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance according to claim 1 is characterized in that: The calculation formula for pore and crack radius is: ； In the formula, T 2 is the transverse relaxation time; F is the pore characteristic factor or the fracture characteristic factor; is the surface relaxation rate; R is the pore radius or crack radius.

5. A test device for the compression and thawing evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance, characterized in that: The test device for the pressure-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance is used to implement the test method for the pressure-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance as described in any one of claims 1 to 4, and the test device for the pressure-thaw evolution of frozen soil under triaxial stress based on low-temperature nuclear magnetic resonance 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 environment in which the soil sample is located, change the confining pressure and / or axial pressure applied to the frozen soil sample, and collect the overall T2 spectrum of the frozen soil sample in the initial environment and the overall T2 spectrum of the frozen soil sample during the process of changing the confining pressure and / or axial pressure; the initial environment refers to the real axial pressure, confining pressure and seepage environment in which the frozen soil sample is simulated; The low temperature freezing mechanism is used to freeze the soil sample to a target temperature and keep it constant; The nuclear magnetic resonance testing mechanism is used to provide a nuclear magnetic resonance magnetic field to the frozen soil sample, collect the nuclear magnetic resonance image of the frozen soil sample in the initial environment, the layered T2 spectrum of the frozen soil sample at different heights, and the nuclear magnetic resonance image of the frozen soil sample during the change of confining pressure and / or axial pressure and the layered T2 spectrum of the frozen soil sample at different heights; The data acquisition and analysis mechanism is used to analyze the evolution characteristics of frozen soil pressure-thaw during the change of single confining pressure, single axial pressure and simultaneous change of confining pressure and axial pressure according to the nuclear magnetic resonance images, overall T2 spectra and layered T2 spectra of frozen soil samples under the initial environment and during the change of confining pressure and / or axial pressure; the evolution characteristics of frozen soil pressure-thaw include the evolution law of ice content when ice in the frozen soil sample melts, the melting characteristics of ice in the frozen soil sample and the evolution law of pores and cracks; the evolution law of pores and cracks is determined according to the changes in pore and crack radius; the melting characteristics of ice are determined by observing the changes in the morphology of ice in the process of ice melting into water at different positions inside the soil sample.

6. The frozen soil compression-thaw evolution test device under triaxial stress based on low-temperature nuclear magnetic resonance according to claim 5 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 real axial pressure of the environment in which the frozen soil sample is located and to continue to apply axial pressure to the frozen soil sample on the basis of the initial axial pressure; The confining pressure simulation module is used to simulate the real confining pressure of the environment in which the frozen soil sample is located and to continue to apply confining pressure to the frozen soil sample on the basis of the initial confining pressure; 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 frozen soil sample to resonate and receive the frozen soil sample resonance signal to obtain the overall T2 spectrum of the frozen soil sample in the initial environment and during the change of confining pressure and / or axial pressure.

7. The frozen soil compression-thaw evolution test device under triaxial stress based on low-temperature nuclear magnetic resonance according to claim 6 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 penetrates 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 penetrates 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 for communication; the pressure sensor is used to measure the magnitude of the axial pressure exerted by the axial pressure loading cylinder on 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 with the data acquisition and analysis mechanism.

8. The test device for frozen soil compression and thawing evolution under triaxial stress based on low-temperature nuclear magnetic resonance according to claim 7 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; A confining pressure inlet is arranged at the lower end of the confining pressure chamber, and a confining pressure outlet is arranged at the upper end of the confining pressure chamber; control valves are arranged at both the confining pressure inlet and the confining pressure outlet.

9. The frozen soil pressure-thaw evolution test device under triaxial stress based on low-temperature nuclear magnetic resonance according to claim 8 is characterized in that: The seepage simulation module includes a seepage inlet pipeline and a seepage outlet pipeline; The seepage inlet pipeline passes through the lower top rod and the lower base of the soil sample from bottom to top in sequence; the outlet of the seepage inlet pipeline contacts the lower end of the soil sample; the inlet of the seepage inlet pipeline is arranged on the side wall of the lower top 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.

10. The frozen soil compression-thaw evolution testing device under triaxial stress based on low-temperature nuclear magnetic resonance according to claim 8 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 liquid inlet of the low-temperature freezing mechanism is the confining pressure inlet; the freezing circulating liquid outlet of the low-temperature freezing mechanism is the confining pressure outlet; The freezing circulation liquid inlet and the freezing circulation liquid outlet are connected to the freezing circulation pump through the freezing circulation pipeline; the inlet and outlet of the freezing circulation pipeline are provided with temperature sensors; the temperature sensors are connected to the data acquisition and analysis mechanism in communication; the temperature sensors are used to measure the temperature of the freezing circulation liquid 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.

Images