Distributed in-situ testing method for multiple physical property parameters of frozen soil in cold region

Abstract

The invention discloses a distributed in-situ test method for multiple physical property parameters of frozen soil in a cold region, and the method comprises the following steps: burying a plurality of distributed optical cables in the frozen soil, and backfilling with in-situ frozen soil to ensure the coupling between the distributed optical cables and the frozen soil; the distributed optical cable corresponds to a measuring point of the frequency domain reflection probe and is connected to a data demodulation analysis system for acquiring side point data information; calculating shear stiffness of an optical cable-frozen soil interface; and on the basis of the function relationship between the optical cable-frozen soil interface shear stiffness and the multi-physical-property parameters of the frozen soil, calculating to obtain a distribution curve of the in-situ multi-physical-property parameters of the frozen soil along the length direction of the optical cable. According to the distributed in-situ testing method for the multiple physical parameters of the frozen soil in the cold region, in-situ measurement of the multiple physical parameters of the frozen soil is achieved, disturbance to the frozen soil is small, the problem that the parameters are difficult to measure due to unstable properties of the frozen soil is solved, and the distributed in-situ testing method is economical, safe, convenient to operate, high in automation degree, high in anti-jamming capability and reliable in precision.

Description

Technical Field

[0001] The present invention relates to the technical field of in-situ testing of multiple physical property parameters of frozen soil, and in particular to a distributed in-situ testing method for multiple physical property parameters of frozen soil in cold regions. Background Art

[0002] Frozen soil is a complex, multiphase system that is highly sensitive to temperature and exhibits unstable physical properties. Its characteristics are related to a variety of factors, including soil texture, density, and moisture content. Basic physical properties of frozen soil, such as temperature, moisture content, density, frost heave, and thermal conductivity, are crucial information for theoretical research on water-heat-mechanical coupling and engineering practice in permafrost regions. Temperature is the primary determinant of changes in frozen soil properties. As temperature changes, the composition of frozen soil shifts, and the transformation between ice and water within the soil, along with its moisture content, ice content, thermal conductivity, and other processes, undergo dynamic changes. Therefore, accurate measurement of basic physical properties of frozen soil is crucial for theoretical and experimental research in permafrost regions.

[0003] Currently, the methods for measuring the unfrozen water and ice content of frozen soil include the dilatation method, dielectric spectroscopy, heat pulse method and nuclear magnetic resonance (NMR). The basic principle of the dilatation method is to place the soil sample to be tested in a cylindrical container and calculate the volume of ice based on the expansion coefficient of water freezing into ice. The dielectric spectroscopy method indirectly reflects the parameters by measuring the dielectric constant of the frozen soil. The heat pulse method uses heat pulses to measure the thermal conductivity of the frozen soil to calculate the ice content and unfrozen water content of the frozen soil. NMR is based on the principle that oxygen atoms rearrange themselves under the action of an external strong magnetic field to generate voltage. However, existing studies have shown that the above four methods are limited in their application in theoretical and engineering research due to large disturbances, inaccurate measurements, high prices or unsuitability for in-situ measurements.

[0004] Permafrost thermophysical properties, such as thermal conductivity and heat capacity, can be calculated using steady-state and transient measurement methods or theoretical models. Indoor measurement techniques are relatively mature, using probe-type and plate-type analyzers. However, in situ measurements of permafrost thermophysical properties are mostly calculated using theoretical models, lacking direct and accurate measurement technology.

[0005] Frost heave and thaw settlement are unique phenomena in seasonally frozen areas. Currently, predictions of frost heave and thaw settlement in situ frozen soil are mainly based on empirical and theoretical models, indirectly obtained by monitoring moisture content, ice content, density, and other parameters. However, real-time monitoring of multiple physical parameters in situ is not possible. Summary of the Invention

[0006] The purpose of the present invention is to provide a distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions, which can realize in-situ measurement of multiple physical parameters such as basic parameters of frozen soil (unfrozen water content, ice content, dry density), mechanical parameters (frost heave rate, thaw settlement coefficient, shear strength), and thermophysical properties (thermal conductivity, heat capacity). It causes little disturbance to the frozen soil and solves the problem of difficulty in measuring parameters due to unstable frozen soil properties.

[0007] To achieve the above object, the present invention provides a distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions, comprising the following steps:

[0008] Step S1: burying multiple distributed optical cables in frozen soil and backfilling with in-situ frozen soil to ensure coupling between the distributed optical cables and the frozen soil;

[0009] Step S2: Align the distributed optical cable with the measurement point of the frequency domain reflection probe and connect it to the data demodulation and analysis system to obtain the side point data information;

[0010] Step S3: Analyze and calculate the shear stiffness of the optical cable-frozen soil interface based on the acquired lateral point data information;

[0011] Step S4: Based on the functional relationship between the shear stiffness of the optical cable-frozen soil interface and the multi-physical property parameters of the frozen soil, a distribution curve of the multi-physical property parameters of the frozen soil in situ along the length direction of the optical cable is calculated.

[0012] Preferably, in step S1, multiple distributed optical cables are buried in frozen soil and backfilled with in-situ frozen soil to ensure coupling between the distributed optical cables and the frozen soil. The specific process is as follows:

[0013] Step S11: drilling holes and laying distributed optical cables in the target area;

[0014] Step S12: Use in-situ frozen soil as backfill material. If the frozen soil is loose, backfill in layers and compact lightly. If the frozen soil has a high ice content, use low-temperature grouting to enhance coupling.

[0015] Step S13: Install a stainless steel or PVC empty pipe at the top of the drill hole to protect the exposed section of the optical cable. After the optical cable passes through the empty pipe, use waterproof sealant to seal the pipe opening to prevent moisture from penetrating and affecting the monitoring data.

[0016] Preferably, in step S11, drilling and laying of distributed optical cables are performed in the target area, and the specific process includes:

[0017] First, determine the drilling locations in the target area based on measurement requirements, ensuring that the drilling spacing is reasonable and covers the key areas of frozen soil;

[0018] Then, a geological drill or manual drilling method is used to drill a hole. The diameter of the hole is slightly larger than the outer diameter of the distributed optical cable, and the depth is determined according to the thickness of the frozen soil.

[0019] Finally, place the distributed optical cable vertically or horizontally into the drilled hole, ensuring that the cable fits tightly against the hole wall.

[0020] Preferably, in step S2, the distributed optical cable is aligned with the measurement point of the frequency domain reflection probe and connected to a data demodulation and analysis system to obtain side point data information. The specific process includes:

[0021] Step S21, side point correspondence and space matching;

[0022] Step S22: Connect to a data demodulation and analysis system.

[0023] Preferably, in step S21, the side point correspondence and space matching are performed, and the specific process is as follows:

[0024] Distributed optical cable side site division: Determine the temperature / strain side site location based on the spatial resolution of the optical cable and record the depth coordinates of each side site;

[0025] Frequency domain reflection probe layout: frequency domain reflection probes are laid out in parallel next to key side points of the distributed optical cable;

[0026] The frequency domain reflection probe is buried by drilling or grooving, and the original frozen soil is backfilled.

[0027] Preferably, in step S22, the data demodulation and analysis system includes: an optical fiber demodulation module, a frequency domain reflection probe data acquisition module and a system integration and synchronization module;

[0028] Fiber demodulation module: connect the distributed optical cable to the high-precision fiber demodulator to set the sampling frequency and spatial resolution;

[0029] Frequency domain reflection probe data acquisition module: connect the frequency domain reflection probe to the frequency domain reflection probe interpreter through a shielded cable, calibrate the probe parameters, and set the timestamp synchronized with the optical fiber data;

[0030] System integration and synchronization: Through a data synchronizer or a unified clock signal, the acquisition time of the optical fiber temperature data and the moisture content data of the frequency domain reflectometer is strictly synchronized.

[0031] Preferably, in step S3, the shear stiffness of the optical cable-frozen soil interface is analyzed and calculated based on the acquired lateral point data information. The specific process is as follows:

[0032] Taking the length direction of the optical cable as the x-axis and the depth direction of the optical cable as the positive direction of the coordinate axis, the relationship between the axial strain and displacement of the optical cable microelement segment is analyzed as follows:

[0033]

[0034] Where, ε(x) is the axial strain of the optical cable, and u(x) is the displacement of the optical cable;

[0035] According to the force balance condition of the infinitesimal segment, we can get

[0036]

[0037] Where D is the cable diameter, F(x) is the cable axial force, with tension as positive; τ(x) is the shear stress at the cable-soil interface; E is the elastic modulus of the cable;

[0038] Before the optical cable and the surrounding soil debond, the relationship between the interface shear stress and shear strain is as follows:

[0039] τ(x)=Gγ(x);

[0040] Where G is the shear stiffness of the cable-soil interface; γ(x) is the shear strain of the interface;

[0041] The relationship between the cable displacement and the interface shear stress is as follows:

[0042]

[0043] Where h is the thickness of the shear layer soil; u(x) is the displacement of the optical cable;

[0044] Based on the above calculation process, combined with the boundary conditions The cable displacement is obtained as follows:

[0045]

[0046] Where, P is the pull-out force; L is the length of the optical cable; G * =2Gh is the shear coefficient of the cable-soil interface;

[0047] Therefore, the optical cable is i The displacement of the position, and the interface shear stiffness G corresponding to the position i Related, as follows:

[0048] u(x i )=g(G i );

[0049] Combined with the measured distribution curve of the axial strain ε(x) of the optical cable along the depth, combined with the relationship between the axial strain and displacement of the optical cable, the optical cable displacement at x is calculated. i The displacement of position is as follows:

[0050]

[0051] Based on the shear stiffness G of the optical cable-frozen soil interface, the distribution of the shear stiffness of the optical cable-frozen soil interface is obtained.

[0052] Preferably, in step S4, based on the functional relationship between the shear stiffness of the optical cable-frozen soil interface and the multi-physical property parameters of the frozen soil, the distribution curve of the multi-physical property parameters of the frozen soil in situ along the length direction of the optical cable is calculated. The specific process is as follows:

[0053] The multi-physical properties of frozen soil include unfrozen water content, ice content, dry density, mechanical parameters, and thermophysical parameters. Mechanical parameters include frost heave rate, thaw settlement coefficient, and shear strength. The thermophysical parameters include thermal conductivity and heat capacity.

[0054] The least squares method is used to fit the functional relationship between the shear stiffness G of the distributed sensing cable-soil interface and the multi-physical parameters, as shown below:

[0055] G=f(w u ,w i ,ρ d ,ξ,λ,C);

[0056] Among them, w u is the unfrozen water content; w i is the ice content; ρ d is the dry density; ξ is the frost heave rate; λ is the thermal conductivity; and C is the heat capacity.

[0057] Preferably, the baseline value of unfrozen water content is measured by the dielectric constant method; the baseline value of ice content is measured by the nuclear magnetic resonance method; the baseline value of dry density is measured by the ring knife method; the frost heave rate and thaw settlement coefficient are calculated by the vertical displacement of frozen soil in indoor model tests; the shear strength is measured by a triaxial shear test; and the thermal conductivity and heat capacity are measured by a flat plate thermal conductivity analyzer.

[0058] Therefore, the present invention adopts the above-mentioned distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions, realizing in-situ measurement of multiple physical parameters such as basic parameters of frozen soil (unfrozen water content, ice content, dry density), mechanical parameters (frost heave rate, thaw settlement coefficient, shear strength), and thermophysical properties (thermal conductivity, heat capacity). It causes little disturbance to the frozen soil and solves the problem of difficulty in measuring parameters due to unstable frozen soil properties. The method proposed in the present invention is economical and safe, easy to operate, highly automated, has strong anti-interference ability, and reliable accuracy.

[0059] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 This is a flow chart of a distributed in-situ testing method for multiple physical property parameters of frozen soil in cold regions according to the present invention;

[0061] Figure 2 The graph of the relationship between the interface shear stiffness and ice content and the function obtained by experimental fitting of the present invention is as follows;

[0062] Figure 3 The graph of the interface shear stiffness and thermal conductivity obtained by experimental fitting and the functional relationship thereof is shown in FIG.

[0063] Figure 4 It is the interface shear stiffness and frost heave rate curve and function relationship diagram obtained by experimental fitting of the present invention. DETAILED DESCRIPTION

[0064] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0065] like Figure 1 As shown, the present invention provides a distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions, comprising the following steps:

[0066] Step S1: burying multiple distributed optical cables in frozen soil and backfilling with in-situ frozen soil to ensure coupling between the distributed optical cables and the frozen soil;

[0067] Step S2: Align the distributed optical cable with the measurement point of the frequency domain reflection probe and connect it to the data demodulation and analysis system to obtain the side point data information;

[0068] Step S3: Analyze and calculate the shear stiffness of the optical cable-frozen soil interface based on the acquired lateral point data information;

[0069] Step S4: Based on the functional relationship between the shear stiffness of the optical cable-frozen soil interface and the multi-physical property parameters of the frozen soil, a distribution curve of the multi-physical property parameters of the frozen soil in situ along the length direction of the optical cable is calculated.

[0070] Example

[0071] Step S1: burying multiple distributed optical cables in frozen soil and backfilling with in-situ frozen soil to ensure coupling between the distributed optical cables and the frozen soil.

[0072] Step S11: Drill holes and lay distributed optical cables in the target area.

[0073] First, according to the measurement requirements, the drilling locations are determined in the target area to ensure that the drilling spacing is reasonable and covers the key areas of permafrost.

[0074] Then, a geological drill or manual drilling method is used to drill the hole. The diameter of the hole is slightly larger than the outer diameter of the distributed optical cable, and the depth is determined according to the thickness of the frozen soil.

[0075] Finally, place the distributed optical cable vertically or horizontally into the drilled hole, ensuring that the cable fits tightly against the hole wall.

[0076] Step S12: perform backfilling and coupling processing.

[0077] Use in-situ frozen soil as backfill material to avoid introducing foreign substances that affect the natural state of the frozen soil.

[0078] Among them, if the frozen soil is loose, it can be backfilled in layers and lightly compacted to ensure full contact between the optical cable and the soil; if the frozen soil contains a high amount of ice, low-temperature grouting (such as low-temperature cement slurry or frozen soil slurry) can be used to enhance coupling.

[0079] To ensure the coupling between the distributed optical cable and the frozen soil, a coupling agent can be used selectively, such as coating the surface of the optical cable with thermal conductive silicone grease or a special coupling agent for frozen soil to improve the temperature and strain transfer efficiency.

[0080] Step S13: Install a stainless steel or PVC hollow pipe at the top of the drill hole to protect the exposed section of the optical cable. After the optical cable passes through the hollow pipe, seal the pipe opening with waterproof sealant to prevent moisture from penetrating and affecting the monitoring data.

[0081] Step S2: Align the distributed optical cable with the measurement point of the frequency domain reflection probe and connect it to the data demodulation and analysis system to obtain the side point data information.

[0082] Step S21: side point correspondence and spatial matching.

[0083] Distributed optical cable side site division: Determine the temperature / strain side site location based on the spatial resolution of the optical cable (e.g., 1 cm to 1 m), and record the depth coordinates of each side site (e.g., 0 m, 1 m, 2 m, etc.).

[0084] Frequency Domain Reflectometer (FDR) placement: FDR probes should be placed parallel to key side locations of the distributed optical cable (e.g., near the freeze-thaw front and in areas affected by engineering structures). Ensure that the horizontal / vertical spacing between the FDR and the optical cable measurement point is ≤10 cm to reduce spatial errors.

[0085] The FDR is buried by drilling or trenching, and the original frozen soil is backfilled to ensure environmental consistency.

[0086] Step S22: Connect to a data demodulation and analysis system.

[0087] The data demodulation and analysis system includes: a fiber demodulation module, a frequency domain reflectometry (FDR) data acquisition module and a system integration and synchronization module.

[0088] Fiber demodulation module: Connect the distributed optical cable (strain / temperature sensing fiber) to a high-precision fiber demodulator (such as OFDR or BOTDR equipment) and set the sampling frequency (such as 1 Hz) and spatial resolution (such as 1 cm).

[0089] FDR data acquisition module: Connect the FDR probe to the FDR interpreter via a shielded cable, calibrate the probe parameters, and set the timestamp synchronized with the fiber optic data.

[0090] System integration and synchronization: Through a data synchronizer or a unified clock signal (such as GPS timing), the acquisition time of the optical fiber temperature data and the FDR moisture content data is ensured to be strictly synchronized (error <1ms).

[0091] Step S3: Analyze and calculate the shear stiffness of the optical cable-frozen soil interface based on the acquired lateral point data information.

[0092] Taking the length direction of the optical cable as the x-axis and the depth direction of the optical cable as the positive direction of the coordinate axis, the relationship between the axial strain and displacement of the optical cable microelement segment is analyzed as follows:

[0093]

[0094] Where ε(x) is the axial strain of the optical cable, and u(x) is the displacement of the optical cable.

[0095] According to the force balance condition of the infinitesimal segment, we can get

[0096]

[0097] Where D is the cable diameter, F(x) is the cable axial force, with tension as the positive value, τ(x) is the shear stress at the cable-soil interface, and E is the elastic modulus of the cable.

[0098] The ideal elastic-plastic model is a commonly used model for the progressive failure of optical cable interfaces. Before the optical cable and the surrounding soil debond, there is good coordinated deformation. The relationship between the interfacial shear stress and shear strain is as follows:

[0099] τ(x)=Gγ(x);

[0100] Where G is the shear stiffness of the cable-soil interface; γ(x) is the shear strain of the interface.

[0101] Assuming that the shear strain of the soil in the shear layer decreases linearly along the radial direction, the relationship between the pull-out displacement and the interface shear stress is as follows:

[0102]

[0103] Where h is the thickness of the shear layer soil; u(x) is the displacement of the optical cable.

[0104] Based on the above calculation process, combined with the boundary conditions The cable displacement is obtained as follows:

[0105]

[0106] Where, P is the pull-out force; L is the length of the optical cable; G * =2Gh is the shear coefficient of the cable-soil interface.

[0107] Therefore, the optical cable is i The displacement of the position, and the interface shear stiffness G corresponding to the position i Related, as follows:

[0108] u(x i )=g(G i );

[0109] Combined with the measured distribution curve of the axial strain ε(x) of the optical cable along the depth, the calculation of the optical cable at x is obtained by combining formula (1): i The displacement of position is as follows:

[0110]

[0111] Based on the shear stiffness G of the optical cable-frozen soil interface, the distribution of the shear stiffness of the optical cable-frozen soil interface is obtained.

[0112] Step S4: Based on the functional relationship between the shear stiffness of the optical cable-frozen soil interface and the multi-physical property parameters of the frozen soil, a distribution curve of the multi-physical property parameters of the frozen soil in situ along the length direction of the optical cable is calculated.

[0113] The multi-physical properties of frozen soil include unfrozen water content, ice content, dry density, mechanical parameters (frost heave rate, thaw settlement coefficient, and shear strength), and thermophysical parameters (thermal conductivity and heat capacity).

[0114] The least squares method is used to fit the functional relationship between the shear stiffness G of the distributed sensing cable-soil interface and the multi-physical parameters, as shown below:

[0115] G=f(w u ,w i ,ρ d ,ξ,λ,C);

[0116] Among them, w u is the unfrozen water content; w i is the ice content; ρ d is the dry density; ξ is the frost heave rate; λ is the thermal conductivity; and C is the heat capacity.

[0117] The benchmark value of unfrozen water content was measured by the dielectric constant method, the benchmark value of ice content was measured by the nuclear magnetic resonance method, the benchmark value of dry density was measured by the ring knife method, the frost heave rate and thaw settlement coefficient were calculated by calculating the vertical displacement of frozen soil in indoor model tests, the shear strength was measured by triaxial shear tests, and the thermal conductivity and heat capacity were measured by a flat plate thermal conductivity analyzer.

[0118] like Figure 2 As shown, the shear stiffness G is related to the ice content w i The functional relationship between them is as follows:

[0119] G=1.714w i +8.317;

[0120] like Figure 3 As shown in Figure 2, the functional relationship between shear stiffness G and ice content λ is as follows:

[0121] G=3.67 λ +17.34;

[0122] like Figure 4 As shown in Figure 2, the functional relationship between shear stiffness G and ice content ξ is as follows:

[0123] G=3.86ξ 2 +5.17ξ+7.28.

[0124] The freezing process in frozen soil causes water migration within the soil, which freezes into ice, causing the soil to expand. As the temperature rises, the ice crystals melt, reducing the structural strength of the frozen soil and causing it to settle under its own weight. This in turn alters the soil's structure and physical and mechanical properties. Therefore, water migration during the freezing process is the primary factor causing frost heave. Frost heave force, a derivative of frost heave, is positively correlated with the initial moisture content, ice content, and dry density of the frozen soil.

[0125] Distributed fiber optic sensing technology leverages the environmental influences of scattered light to perform its monitoring function. Optical frequency domain reflectometry (OFDR) offers relatively high spatial resolution and signal-to-noise ratio, achieving a spatial resolution of 1 mm over a sensing distance of 100 meters and a strain sensing accuracy of ±1.0 με. Distributed temperature sensing (DTS) technology can obtain distributed temperature measurements with high precision, high temporal and spatial resolution, and can reach sensing distances of up to 10,000 meters. Distributed sensing, with its advantages of long range, high precision, and resistance to electromagnetic interference, meets the requirements for refined monitoring of frozen soil deformation.

[0126] The present invention is based on the coupled deformation relationship of the distributed sensing optical cable-frozen soil interface, and realizes parameter measurement by calculating the correlation between the shear stiffness of the optical cable-frozen soil interface and the ice content, dry density, thermal conductivity, frost heave rate and other physical parameters.

[0127] The forces exerted by frozen soil on distributed optical cables are divided into two components: freezing force and frost heave force. The freezing force increases with decreasing temperature and increasing ice content, thereby increasing the shear stiffness of the cable-frozen soil interface. The frost heave force of frozen soil is controlled by the changes in moisture and ice content during the freezing process. It is positively correlated with moisture content and ice content: under the same freezing conditions, the greater the initial moisture content, the greater the frost heave force; and under the same initial moisture content, the greater the ice content, the greater the frost heave force. The frost heave force acts as a normal stress on vertically buried distributed sensor optical cables, and the shear strength and interface shear stiffness of the cable-frozen soil interface are directly proportional to the frost heave force.

[0128] Therefore, the shear stiffness of the distributed optical cable-frozen soil interface is functionally correlated with ice content and unfrozen water content. Furthermore, the mechanical and thermophysical properties of frozen soil are determined by its structure, moisture content, and ice content. Therefore, by establishing a relationship between the shear stiffness of the distributed sensing optical cable-frozen soil interface and multiple physical parameters, in-situ frozen soil multi-parameter measurement can be achieved.

[0129] Therefore, the present invention adopts the above-mentioned distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions, realizing in-situ measurement of multiple physical parameters such as basic parameters of frozen soil (unfrozen water content, ice content, dry density), mechanical parameters (frost heave rate, thaw settlement coefficient, shear strength), and thermophysical properties (thermal conductivity, heat capacity). It causes little disturbance to the frozen soil and solves the problem of difficulty in measuring parameters due to unstable frozen soil properties. The method proposed in the present invention is economical and safe, easy to operate, highly automated, has strong anti-interference ability, and reliable accuracy.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions, characterized in that: The following steps are involved: Step S1: burying multiple distributed optical cables in frozen soil and backfilling with in-situ frozen soil to ensure coupling between the distributed optical cables and the frozen soil; Step S2: Align the distributed optical cable with the measurement point of the frequency domain reflection probe and connect it to the data demodulation and analysis system to obtain the side point data information; Step S3: Analyze and calculate the shear stiffness of the optical cable-frozen soil interface based on the acquired lateral point data information; Step S4: Based on the functional relationship between the shear stiffness of the optical cable-frozen soil interface and the multi-physical property parameters of the frozen soil, a distribution curve of the multi-physical property parameters of the frozen soil in situ along the length direction of the optical cable is calculated.

2. The distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions according to claim 1 is characterized in that: In step S1, multiple distributed optical cables are buried in frozen soil and backfilled with in-situ frozen soil to ensure the coupling between the distributed optical cables and the frozen soil. The specific process is as follows: Step S11: drilling holes and laying distributed optical cables in the target area; Step S12: Use in-situ frozen soil as backfill material. If the frozen soil is loose, backfill in layers and compact lightly. If the frozen soil has a high ice content, use low-temperature grouting to enhance coupling. Step S13: Install a stainless steel or PVC empty pipe at the top of the drill hole to protect the exposed section of the optical cable. After the optical cable passes through the empty pipe, use waterproof sealant to seal the pipe opening to prevent moisture from penetrating and affecting the monitoring data.

3. The distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions according to claim 2 is characterized in that: In step S11, drilling and laying of distributed optical cables are performed in the target area. The specific process includes: First, determine the drilling locations in the target area based on measurement requirements, ensuring that the drilling spacing is reasonable and covers the key areas of frozen soil; Then, a geological drill or manual drilling method is used to drill a hole. The diameter of the hole is slightly larger than the outer diameter of the distributed optical cable, and the depth is determined according to the thickness of the frozen soil. Finally, place the distributed optical cable vertically or horizontally into the drilled hole, ensuring that the cable fits tightly against the hole wall.

4. The distributed in-situ testing method for multiple physical property parameters of frozen soil in cold regions according to claim 1 is characterized in that: In step S2, the distributed optical cable is aligned with the measurement point of the frequency domain reflection probe and connected to the data demodulation and analysis system to obtain the side point data information. The specific process includes: Step S21, side point correspondence and space matching; Step S22: Connect to a data demodulation and analysis system.

5. The distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions according to claim 4 is characterized in that: In step S21, the side point correspondence and space matching are performed, and the specific process is as follows: Distributed optical cable side site division: Determine the temperature / strain side site location based on the spatial resolution of the optical cable and record the depth coordinates of each side site; Frequency domain reflection probe layout: frequency domain reflection probes are laid out in parallel next to key side points of the distributed optical cable; The frequency domain reflection probe is buried by drilling or grooving, and the original frozen soil is backfilled.

6. The distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions according to claim 4 is characterized in that: In step S22, the data demodulation and analysis system includes: an optical fiber demodulation module, a frequency domain reflection probe data acquisition module, and a system integration and synchronization module; Fiber demodulation module: connect the distributed optical cable to the high-precision fiber demodulator to set the sampling frequency and spatial resolution; Frequency domain reflection probe data acquisition module: connect the frequency domain reflection probe to the frequency domain reflection probe interpreter through a shielded cable, calibrate the probe parameters, and set the timestamp synchronized with the optical fiber data; System integration and synchronization: Through a data synchronizer or a unified clock signal, the acquisition time of the optical fiber temperature data and the moisture content data of the frequency domain reflectometer is strictly synchronized.

7. The distributed in-situ testing method for multiple physical property parameters of frozen soil in cold regions according to claim 1 is characterized in that: In step S3, the shear stiffness of the optical cable-frozen soil interface is analyzed and calculated based on the acquired lateral point data information. The specific process is as follows: Taking the length direction of the optical cable as the x-axis and the depth direction of the optical cable as the positive direction of the coordinate axis, the relationship between the axial strain and displacement of the optical cable microelement segment is analyzed as follows: Where, ε(x) is the axial strain of the optical cable, and u(x) is the displacement of the optical cable; According to the force balance condition of the infinitesimal segment, we can get Where D is the cable diameter, F(x) is the cable axial force, with tension as positive; τ(x) is the shear stress at the cable-soil interface; E is the elastic modulus of the cable; Before the optical cable and the surrounding soil debond, the relationship between the interface shear stress and shear strain is as follows: τ(x)=Gγ(x); Where G is the shear stiffness of the cable-soil interface; γ(x) is the shear strain of the interface; The relationship between the cable displacement and the interface shear stress is as follows: Where h is the thickness of the shear layer soil; u(x) is the displacement of the optical cable; Based on the above calculation process, combined with the boundary conditions The cable displacement is obtained as follows: Where, P is the pull-out force; L is the length of the optical cable; G * =2Gh is the shear coefficient of the cable-soil interface; Therefore, the optical cable is i The displacement of the position, and the interface shear stiffness G corresponding to the position i Related, as follows: u(x i )=g(G i ); Combined with the measured distribution curve of the axial strain ε(x) of the optical cable along the depth, combined with the relationship between the axial strain and displacement of the optical cable, the optical cable displacement at x is calculated. i The displacement of position is as follows: Based on the shear stiffness G of the optical cable-frozen soil interface, the distribution of the shear stiffness of the optical cable-frozen soil interface is obtained.

8. The distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions according to claim 1 is characterized in that: In step S4, based on the functional relationship between the shear stiffness of the optical cable-frozen soil interface and the multi-physical parameters of the frozen soil, the distribution curve of the multi-physical parameters of the frozen soil in situ along the length of the optical cable is calculated. The specific process is as follows: The multi-physical properties of frozen soil include unfrozen water content, ice content, dry density, mechanical parameters, and thermophysical parameters. Mechanical parameters include frost heave rate, thaw settlement coefficient, and shear strength. The thermophysical parameters include thermal conductivity and heat capacity. The least squares method is used to fit the functional relationship between the shear stiffness G of the distributed sensing cable-soil interface and the multi-physical parameters, as shown below: G=f(w u ,w i ,r d ,ξ,λ,C); Among them, w u is the unfrozen water content; w i is the ice content; ρ d is the dry density; ξ is the frost heave rate; λ is the thermal conductivity; and C is the heat capacity.

9. A distributed in-situ testing method for multiple physical parameters of frozen soil in cold regions according to claim 8, characterized in that: The benchmark value of unfrozen water content was measured by the dielectric constant method; the benchmark value of ice content was measured by the nuclear magnetic resonance method; the benchmark value of dry density was measured by the ring knife method; the frost heave rate and thaw settlement coefficient were calculated by the vertical displacement of frozen soil in indoor model tests; and the shear strength was measured by triaxial shear tests. The thermal conductivity and heat capacity were measured by a flat-plate thermal conductivity analyzer.

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