CN114199679A - Optical fiber drawing-based distributed in-situ testing device and method for frozen soil multi-physical-property parameters - Google Patents
Optical fiber drawing-based distributed in-situ testing device and method for frozen soil multi-physical-property parameters Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/02—Details not specific for a particular testing method
- G01N2203/06—Indicating or recording means; Sensing means
- G01N2203/067—Parameter measured for estimating the property
- G01N2203/0676—Force, weight, load, energy, speed or acceleration
Abstract
The invention discloses a distributed in-situ test device and a method for frozen soil multi-physical-property parameters based on optical fiber drawing, wherein the in-situ test device comprises a distributed sensing optical cable, optical fiber demodulation equipment, a drawing tester, an empty pipe and a clamp; the method comprises the steps of drawing the distributed sensing optical cable through a drawing tester, recording drawing force and drawing displacement, drawing an axial strain distribution curve and an optical cable drawing force-drawing displacement curve along the length direction of the distributed sensing optical cable in the drawing process, obtaining the distribution of the rigidity of the distributed sensing optical cable-frozen soil interface based on the coupling deformation relation of the distributed sensing optical cable-frozen soil interface, obtaining the multi-physical property parameter value of the frozen soil according to the functional relation between the shearing rigidity of the optical cable-frozen soil interface and the multi-physical property parameter, and obtaining the distribution curve of the multi-physical property parameter along the length direction of the optical cable. The invention realizes the in-situ measurement of various physical parameters of the frozen soil, has small disturbance to the frozen soil and solves the problem that the parameters are difficult to measure due to the unstable property of the frozen soil.
Description
Technical Field
The invention relates to a testing device and a testing method in the technical field of optical fiber monitoring of rock-soil body deformation, in particular to a distributed in-situ testing device for frozen soil multi-physical-property parameters based on optical fiber drawing and a distributed in-situ testing method for frozen soil multi-physical-property parameters based on optical fiber drawing.
Background
Frozen soil is a complex multi-phase system which is very sensitive to temperature and unstable in physical properties, and the characteristics of the frozen soil are related to various factors such as soil texture, density, water content and the like. Basic physical property parameters of frozen soil such as temperature, water content, density, frost heaving rate, heat conductivity coefficient and the like are important information required by water-heat-force coupling rationality research and engineering practice in a frozen soil area. The temperature is the most main determinant factor of frozen soil property change, the components of the frozen soil change along with the change of the temperature, the transformation between ice and water in the frozen soil occurs, and the water content, the ice content, the heat conductivity coefficient and the process of the frozen soil are all in the dynamic change process, so that the accurate measurement of the basic physical property parameters of the frozen soil has important significance for theoretical and experimental research in a frozen soil area.
Currently, methods for measuring unfrozen water and ice content of frozen soil include an expansion method, a dielectric spectroscopy method, a thermal pulse method, and a Nuclear Magnetic Resonance (NMR) method. The basic principle of the expansion method is to place a soil sample to be measured in a cylindrical container and calculate the volume of ice according to the expansion coefficient of water frozen into ice. Dielectric spectroscopy indirectly reflects the parameters by determining the dielectric constant of frozen earth. The thermal pulse method measures the thermal conductivity of the frozen soil by means of thermal pulses so as to calculate the ice content and the unfrozen water content of the frozen soil. And (3) the principle determination of the voltage generated by the rearrangement of NMR oxygen atoms under the action of an external reinforced magnetic field. However, the existing research shows that the four methods are all limited in application in theoretical and engineering research due to large disturbance, inaccurate measurement, high price or inapplicability to in-situ measurement.
The frozen soil thermophysical property parameters such as heat conductivity coefficient and heat capacity can be calculated by two measurement methods of steady state and transient state or a theoretical model. Indoor measurement technology is mature, thermophysical property parameter measurement can be carried out through a probe type analyzer, a flat plate type analyzer and the like, thermophysical property parameter measurement of in-situ frozen soil is mostly obtained through theoretical model calculation, and direct and accurate measurement technology is lacked.
Frost heaving and thaw collapse are phenomena specific to seasonal frozen soil regions. At present, frost heaving and thaw collapse of in-situ frozen soil are mainly predicted through experience and theoretical models and are indirectly obtained through monitoring water content, ice content, density and other parameters, and real-time monitoring of in-situ multiple physical parameters cannot be realized.
Disclosure of Invention
The purpose of the invention is as follows: aiming at the defects in the prior art, the invention provides a distributed in-situ test device for frozen soil multi-physical-property parameters based on optical fiber drawing and also provides a distributed in-situ test method for frozen soil multi-physical-property parameters based on optical fiber drawing.
The technical scheme is as follows: the invention relates to a distributed in-situ testing device for frozen soil multi-physical-property parameters based on optical fiber drawing, which comprises a distributed sensing optical cable, optical fiber demodulation equipment, a drawing tester, an empty pipe and a clamp;
the drawing tester clamps and fixes the distributed sensing optical cable through a clamp, controls the drawing rate and displacement, and records the axial drawing force and displacement of the distributed sensing optical cable in the drawing process;
the optical fiber demodulation equipment measures strain and temperature distribution of frozen soil in the drawing process of the distributed sensing optical cable.
The invention relates to a distributed in-situ test method of frozen soil multi-physical parameters based on optical fiber drawing, which adopts a distributed in-situ test device of frozen soil multi-physical parameters based on optical fiber drawing to test, and comprises the following steps:
(1) embedding the distributed sensing optical cable in the corresponding position in the in-situ frozen soil to be detected along the direction vertical to the ground surface; installing the hollow pipe at the ground surface along the direction vertical to the ground surface; the distributed sensing optical cable penetrates out of the ground surface from the bottom to the top from the hollow pipe;
(2) connecting the distributed sensing optical cable with optical fiber demodulation equipment, reading temperature information along the length direction of the distributed sensing optical cable by the optical fiber demodulation equipment according to set spatial resolution and acquisition frequency, and obtaining the distribution and the change of the temperature of the in-situ frozen soil along the depth direction;
(3) according to the distribution and change of the temperature of the in-situ frozen soil along the depth direction, determining the freezing depth of the in-situ frozen soil and different freezing stages of the frozen soil;
(4) before drawing, fixing a drawing tester at a corresponding position on the earth surface, positioning a clamp above the hollow pipe, aligning the drawing tester to the center of the hollow pipe, and fixing a distributed sensing optical cable at the center of the clamp;
(5) in different freezing stages of in-situ frozen soil, a drawing tester is adopted to draw the distributed sensing optical cable at a constant speed, the drawing force and the drawing displacement are recorded, and the optical fiber demodulation equipment monitors the axial strain distribution of the distributed sensing optical cable along the length direction in the drawing process in real time;
(6) drawing an axial strain distribution curve and an optical cable drawing force-drawing displacement curve along the length direction of the distributed sensing optical cable in the drawing process, and analyzing the curves according to the coupling deformation relation of the distributed sensing optical cable and the frozen soil interface to obtain the distribution of the shearing rigidity of the distributed sensing optical cable and the frozen soil interface;
(7) and measuring the multiple physical parameters, calculating the multiple physical parameter values of the frozen soil according to the functional relation between the shear stiffness of the optical cable-frozen soil interface and the multiple physical parameters of the frozen soil, and further obtaining the distribution curve of the multiple physical parameters along the length direction of the optical cable.
In the step (1), a plurality of distributed sensing optical cables are installed in the frozen soil to be measured along the direction vertical to the ground surface by adopting a drilling direct burial mode, the in-situ frozen soil is used as a backfill material, and the coupling performance between the distributed sensing optical cables and the frozen soil is ensured. In the step, a plurality of distributed sensing optical cables are arranged in parallel, drawing tests are carried out at different freezing stages of the in-situ frozen soil during monitoring, further, staged and distributed measurement of multiple physical parameters of the in-situ frozen soil is achieved, and the number of the distributed sensing optical cables is determined according to actual situations and monitoring requirements of a site.
In the step (2), the distribution and change of the temperature of the in-situ frozen soil along the depth direction are as follows: (a) the soil temperature rises from the surface to the deep part and is stably distributed in a constant temperature layer; and the ground temperature gradient of the shallow ground temperature at different depths; (b) the change of soil temperature along with the fluctuation of atmospheric temperature, solar radiation and snow cover along with time.
In the step (3), the depth of the position at 0 ℃ is taken as a freezing depth judgment standard, the freezing depth of the in-situ frozen soil is further determined, and the freezing states of the in-situ frozen soil at different stages are judged according to the change of the freezing depth, wherein the different stages of the in-situ frozen soil comprise a non-frozen soil period, a freezing development period, a freezing full period and a freezing and thawing period.
And (4) attaching an elastic polymer cushion layer to the inner surface of the clamp adopted in the step (4), and engraving a sawtooth-shaped groove for fixing the distributed sensing optical cable in the center of the cushion layer.
In the step (6), the distribution of the shear stiffness of the distributed sensing optical cable-frozen soil interface is obtained by the following method:
in the drawing process, the optical cable micro-element section is analyzed by taking the length direction of the optical cable as an x axis and the depth direction of the optical cable as the positive direction of a coordinate axis, and the relationship between the axial strain and the displacement is as follows:
wherein ε (x) is the axial strain of the cable, and u (x) is the displacement of the cable;
according to the stress balance condition of the infinitesimal section, the product is obtained
Wherein D is the diameter of the optical cable, F (x) is the axial force of the optical cable, and the pull is positive; τ (x) is the shear stress at the cable-soil interface;
wherein E is the elastic modulus of the optical cable;
simultaneous front three formulas
The relationship between interfacial shear stress and shear strain before debonding the cable from the surrounding soil may be expressed as
τ(x)＝Gγ(x) (5)
Wherein G is the cable-soil interface shear stiffness and γ (x) is the shear strain of the interface;
assuming that the shear strain of the soil body in the shear layer is linearly reduced along the radial direction, the relationship between the drawing displacement and the interface shear stress is expressed as
Wherein h is the thickness of the shear layer soil body;
vertical type (4) - (6) combined with boundary conditions
Obtaining the displacement of the optical cable:
wherein P is the drawing force, L is the cable length,
G*2G/h is defined as the shear coefficient of the cable-soil interface; thus, the cable is in xiDisplacement of position, interface shear stiffness G corresponding to the positioniIn connection with, i.e. with
u(xi)＝g(Gi) (8)
Combining the measured distribution curve of the axial strain epsilon (x) of the optical cable along the depth, and calculating the optical cable in x by combining the formula (1)iDisplacement of position
And (4) obtaining the shear rigidity G of the optical cable-frozen soil interface of the optical cable by integrating the formula (8) and the formula (9), and obtaining the distribution of the shear rigidity of the optical cable-frozen soil interface by combining an axial strain distribution curve and an optical cable drawing force-drawing displacement curve.
In the step (7), the functional relation between the shear stiffness of the optical cable-frozen soil interface and the multiple physical property parameters of the frozen soil is obtained through a calibration test, and the calibration process is as follows:
(7.1) preparing a plurality of groups of frozen soil samples and filling the frozen soil samples into the die in layers, wherein each group of samples is provided with a plurality of samples with different parameters;
(7.2) installing a distributed sensing optical cable in the frozen soil sample;
(7.3) placing the frozen soil sample in a test box for freezing;
(7.4) carrying out a drawing test at a constant speed by using a drawing tester, recording the axial drawing force and the drawing displacement of the distributed sensing optical cable in real time, and testing the strain distribution of the optical fiber in the drawing process in real time by using optical fiber demodulation equipment;
(7.5) drawing a strain distribution curve and an optical cable drawing force-drawing displacement curve along the length direction of the distributed sensing optical cable in the drawing process, and analyzing the obtained curves according to the optical cable-soil interface coupling deformation relation to obtain a distribution curve of the shear stiffness of the distributed sensing optical cable-frozen soil interface along the length direction of the optical cable;
(7.6) measuring the mass ice content of the frozen soil sample by a nuclear magnetic resonance method; measuring the heat conductivity coefficient of the frozen soil sample by a heat conductivity coefficient analyzer; through the height h of frozen soil sample before and after freezing1,h2Calculating the frost heaving rate
Obtaining the frost heaving rate of each group of frozen soil;
(7.7) establishing a functional relation G (f) between the shear stiffness G of the distributed sensing optical cable-soil interface and the multi-physical-property parameters by applying least square fitting (w)u，wi，ρdξ, λ, C); wherein, wuIs the unfrozen water content, wiIs the ice content, ρdIs the dry density, ξ is the frost heaviness, λ is the thermal conductivity, C is the heat capacity.
And (7) calculating to obtain a distribution curve of the multi-physical-property parameters of the frozen soil in situ along the length direction of the optical cable according to the functional relation between the shear stiffness G of the optical cable-frozen soil interface and the multi-physical-property parameters and the distribution curve of the shear stiffness G of the optical cable-frozen soil interface along the length direction of the distributed sensing optical cable obtained in the step (6).
The multiple physical parameters of the frozen soil comprise unfrozen water content, ice content, dry density, mechanical parameters and thermophysical property parameters; the mechanical parameters comprise frost heaving rate, thaw settlement coefficient and shear strength; thermophysical property parameters include thermal conductivity, and heat capacity. The reference value of the unfrozen water content is measured by a dielectric constant method, the reference value of the ice content is measured by a nuclear magnetic resonance method, the reference value of the dry density is measured by a ring cutter method, the frost heaving rate and the thaw coefficient are obtained by calculating the vertical displacement of frozen soil in an indoor model test, the shear strength is measured by a triaxial shear test, and the heat conductivity coefficient and the heat capacity are measured by a flat-plate heat conductivity analyzer.
The working principle is as follows: the freezing action in the frozen soil causes the water in the soil body to migrate, and the water is frozen into ice, so that the volume of the soil body is expanded; the ice crystals melt along with the temperature rise, the structural strength of the frozen soil is reduced, and the frozen soil is settled under the self-weight stress. The structure and physical and mechanical properties of the soil body are further changed. Therefore, moisture migration occurring during freezing is the most dominant factor causing frost heaving. The frost heaving force is a frost heaving derivative and has a positive correlation with the initial water content, ice content and dry density of the frozen soil.
The distributed optical fiber sensing technology performs the monitoring function according to the characteristic that scattered light is influenced by the external environment, wherein the OFDR technology based on optical frequency domain reflection has relatively high spatial resolution and signal-to-noise ratio, the spatial resolution of 1mm is realized within a hundred-meter-level sensing length, and the strain sensing precision reaches +/-1.0 mu epsilon; the distributed temperature sensing DTS technology can obtain distributed measurement values of temperature with high precision, high time and spatial resolution, and the sensing distance reaches ten thousand meters. The distributed sensing measurement has the advantages of long distance, high precision, electromagnetic interference resistance and the like, and can meet the requirement of fine monitoring of frozen soil deformation.
The invention realizes parameter measurement through the correlation among the shear rigidity of the optical cable-frozen soil interface, the ice content, the dry density, the thermal conductivity, the frost heaving rate and other physical parameters in the drawing process based on the coupling deformation relation of the distributed sensing optical cable-frozen soil interface. The principle is further explained as: the acting force of the frozen soil acting on the distributed optical cable is divided into freezing force and frost heaving force. The freezing force increases with the decrease of temperature and the increase of ice content, so that the shear rigidity of the optical cable-frozen soil interface is increased. The frost heaving force of the frozen soil is controlled by the change of water and ice in the freezing process, and the frost heaving force has a positive correlation with the water content and the ice content: under the same freezing condition, the larger the initial water content is, the larger the frost heaving force is; when the initial water content is the same, the greater the ice content, the greater the frost heaviness. The frost heaving force acts on the vertically buried distributed sensing optical cable in a normal stress mode, and the shear strength and the interface shear rigidity of the optical cable-frozen soil interface are in direct proportion to the frost heaving force. Therefore, during drawing, the shear stiffness of the distributed optical cable-frozen soil interface has a function correlation with the ice content and the unfrozen water content. And the mechanical parameters and the thermophysical parameters of the frozen soil are determined by the structure, the water content and the ice content of the frozen soil, so that the in-situ measurement of the multiple parameters of the frozen soil is realized by establishing the relationship between the shear rigidity of a distributed sensing optical cable-frozen soil interface and the multiple physical parameters.
Has the advantages that: compared with the prior art, the invention has the following advantages:
(1) the invention realizes the in-situ measurement of various physical property parameters such as the basic parameters (unfrozen water content, ice content and dry density), mechanical parameters (frost heaving rate, thaw collapse coefficient and shear strength), thermal physical property parameters (heat conductivity coefficient and heat capacity) of the frozen soil, has small disturbance to the frozen soil and solves the problem that the parameters are difficult to measure due to the unstable property of the frozen soil.
(2) The invention realizes high-precision and staged monitoring of in-situ frozen soil multi-physical-property parameters based on a high-precision distributed optical fiber sensing technology.
(3) The invention realizes the distributed measurement of multiple physical parameters of deep frozen soil through the distributed sensing optical cable.
(4) According to the method, the in-situ frozen soil moisture migration, the internal soil temperature transfer rule and the mechanical deformation characteristic are monitored through the frozen soil temperature, the ice content and the frost heaving rate data;
(5) the invention has the advantages of economy, safety, convenient operation, high automation degree, strong anti-interference capability and reliable precision.
Drawings
FIG. 1 is a schematic diagram of a distributed in-situ testing device for frozen soil multi-physical parameters based on optical fiber drawing according to the present invention;
FIG. 2 is a cross-sectional view of a distributed sensing optical cable in the distributed in-situ testing device for frozen soil multi-physical property parameters based on optical fiber drawing of the invention;
FIG. 3 is a schematic view of a fixture of the distributed in-situ testing device for frozen soil multi-physical property parameters based on optical fiber drawing according to the present invention;
FIG. 4 is a graph of the relationship between the interfacial shear stiffness and ice content curve and a function obtained by calibration test fitting in the embodiment of the present invention;
FIG. 5 is a graph of the relationship between the interfacial shear stiffness and thermal conductivity curve and the function thereof obtained by calibration test fitting in the embodiment of the present invention;
FIG. 6 is a graph of the relationship between the interfacial shear stiffness and the frost heaving ratio obtained by calibration test fitting in the embodiment of the present invention;
FIG. 7 is a graph showing the distribution of ice content of in situ frozen soil along the depth in an embodiment of the present invention;
FIG. 8 is a graph of the depth profile of the in situ frozen soil thermal conductivity in an embodiment of the present invention;
FIG. 9 is a graph of in situ frozen soil frost heaving rate along a depth distribution in an embodiment of the present invention;
fig. 10 is a schematic diagram of a coupling deformation relationship of a distributed sensing optical cable-frozen soil interface in a drawing process in the embodiment of the present invention.
Detailed Description
Example (b):
as shown in fig. 1, the distributed in-situ testing device for frozen soil multi-physical parameters based on optical fiber drawing comprises a distributed sensing optical cable 4, an optical fiber demodulation device 10 and a high-precision drawing tester 7, wherein the high-precision drawing tester 7 clamps and fixes the distributed sensing optical cable 4 through a clamp 6 and a bolt 20, and is used for controlling the drawing rate and displacement, and measuring and recording the axial drawing force and the drawing displacement of the distributed sensing optical cable 4 in the drawing process. The high-precision drawing tester 7 is connected with a computer 8 through a data transmission lead 9 for program control and data acquisition. The distributed sensing optical cable 4 is connected with an optical fiber demodulation device 10, and further strain and temperature distribution of the frozen soil sample in the drawing process are measured. The optical fiber demodulation device 10 is connected with the computer 8 for data acquisition, processing and analysis. The distributed sensing optical cable 4 is sleeved with a stainless steel hollow pipe 5, and the distributed sensing optical cable 4 penetrates through the stainless steel hollow pipe 5 and then is fixed through a clamp 6.
As shown in fig. 2, the distributed sensing optical cable 4 adopted by the present invention has a dual-core structure, and the distributed sensing optical cable 4 includes a strain sensing optical cable and a temperature sensing optical cable. The strain sensing optical cable consists of a strain bare fiber 11, a coating layer 12, an elastic polymer sheath 13 and a sheath 17, and has high tensile strength and good strain transmissibility. The temperature sensing optical cable consists of a temperature sensing optical fiber 14, a waterproof heat-conducting protective layer 15 and a loose sleeve sheath 16, is packaged into a loose sleeve structure, is not influenced by strain, and has multiple functions of ground temperature monitoring, deep freezing judgment and automatic temperature compensation of strain reading.
As shown in figure 3, the clamp 6 used by the high-precision drawing tester 7 is provided with a high-elasticity polymer cushion layer 18 attached to the inner surface of the clamp 6, a sawtooth-shaped groove 19 is carved in the center of the cushion layer, the diameter of the groove is equal to that of the distributed sensing optical cable 4, so that the phenomenon of slip is avoided in the drawing process, and the optical path damage caused by over-tight clamping is prevented.
As shown in fig. 10, the coupling deformation relationship of the distributed sensing cable 4 at the distributed sensing cable-frozen soil interface during the drawing process is represented by: the unfrozen water 26 is changed into ice 27 in the freezing process of the frozen soil, and the ice has a cementing effect on the soil particles 25 and the distributed sensing optical cable 4, so that after the distributed sensing optical cable 4 is pulled, the frozen soil is stressed to form a sheared frozen soil layer 21, an optical cable-frozen soil bonding area 24 can be generated on an optical cable-frozen soil interface due to frost heaving force 22 and shear stress 23, the property of the frozen soil bonding area 24 is influenced by the ice content, the water content, the heat conductivity coefficient and the frost heaving rate of the frozen soil, and the shearing rigidity of the distributed sensing optical cable-frozen soil interface is influenced in the drawing process.
The invention discloses a distributed in-situ test method of frozen soil multi-physical property parameters based on optical fiber drawing, which comprises the following steps:
(1) a plurality of distributed sensing optical cables 4 are buried in corresponding positions in the frozen soil to be detected along the direction vertical to the earth surface, in the embodiment, the frozen soil to be detected is respectively a first frozen soil layer 1 to be detected, a second frozen soil layer 2 to be detected and a third frozen soil layer 3 to be detected from top to bottom according to the soil type and the soil property; installing a stainless steel hollow pipe 5 on the ground surface along a direction vertical to the ground surface, wherein the burial depth is half of the length of the stainless steel hollow pipe 5; the distributed sensing optical cable 4 passes through the stainless steel hollow pipe 5 from the bottom to the ground surface.
(2) Connect distributed sensing optical cable 4 with optical fiber demodulation equipment 10, optical fiber demodulation equipment 10 reads along the temperature information of distributed sensing optical cable 4 length direction with the spatial resolution who sets for and collection frequency, obtains winter normal position frozen soil temperature and rises to stable distribution along the direction of depth gradually to and the change law of frozen soil along with temperature fluctuation, the undulant change of sun laying, this distribution and change include: 1) in winter, the soil temperature gradually rises from the ground surface to the deep part and reaches a stable spatial distribution rule in a constant temperature layer, and particularly, the ground temperature gradient at different depths with the shallow ground temperature is concerned; 2) the change rule of the soil temperature along with the atmospheric temperature fluctuation, the solar radiation and the snow cover influence, and the change rate of the soil temperature along with the time.
(3) According to the characteristic that the temperature of the in-situ frozen soil gradually rises along the depth direction and reaches stable distribution, the frozen state of the soil is judged by taking the temperature equal to zero as a standard, and the position of the zero temperature as a freezing depth judgment standard, so that the freezing depth of the frozen soil and the freezing states at different freezing stages are determined.
(4) Before drawing, the base of the high-precision drawing tester 7 is fixed at the corresponding position of the earth surface, the clamp 6 of the high-precision drawing tester 7 is positioned above the stainless steel hollow pipe 5, the center of the clamp 6 is aligned to the center of the stainless steel hollow pipe 5, and the distributed sensing optical cable 4 is fixed at the central groove of the clamp 6 and is clamped and fixed through a bolt of a rotary clamp.
(5) In different freezing stages of in-situ frozen soil, a high-precision drawing tester 7 is adopted to draw the distributed sensing optical cable 4 at a constant speed, the drawing force and the drawing displacement are automatically recorded, and the optical fiber demodulation equipment 10 monitors the axial strain distribution of the distributed sensing optical cable 4 along the length direction in the drawing process in real time.
(6) And drawing an axial strain distribution curve and an optical cable drawing force-drawing displacement curve along the length direction of the distributed sensing optical cable 4 in the drawing process, and analyzing the curves according to the coupling deformation relation of the distributed sensing optical cable and the frozen soil interface to obtain a distribution curve of the shear stiffness of the distributed sensing optical cable-the frozen soil interface along the length direction of the distributed sensing optical cable.
(7) And calculating the multi-physical-property parameter values of the frozen soil according to the functional relation between the shear stiffness of the distributed sensing optical cable-frozen soil interface and the multi-physical-property parameters of the frozen soil, and further obtaining the distribution curve of the multi-physical-property parameters along the length direction of the optical cable.
Wherein the multiple physical parameters of the frozen soil comprise unfrozen water content, ice content, dry density, mechanical parameters and thermophysical property parameters; the mechanical parameters comprise frost heaving rate, thaw settlement coefficient and shear strength; thermophysical property parameters include thermal conductivity, and heat capacity.
In the step (1) of this embodiment, a drilling direct burial mode is adopted to install the distributed sensing optical cable in the frozen soil to be measured along the direction perpendicular to the earth surface, the in-situ frozen soil is used as a backfill material, and the coupling between the distributed sensing optical cable and the frozen soil is ensured.
And (2) in the step (1), a plurality of distributed sensing optical cables are arranged in parallel, drawing tests are carried out at different freezing stages of the in-situ frozen soil during monitoring, so that staged and distributed measurement of multiple physical parameters of the in-situ frozen soil is realized, and the number of the distributed sensing optical cables is determined according to the actual situation of a site and the monitoring requirement.
In this embodiment, the inner diameter of the stainless steel hollow tube in step (1) is the same as the outer diameter of the distributed sensing optical cable, and the stainless steel hollow tube and the distributed sensing optical cable are located on the same vertical line to ensure that no axial offset occurs.
And (3) determining the freezing depth of the in-situ frozen soil by taking the depth of the position at 0 ℃ as a judgment standard, so as to judge whether the frozen state of the in-situ frozen soil is a non-frozen soil period, a frozen development period, a frozen full period or a frozen thawing period according to the change rule of the freezing depth.
And (4) attaching a high-elasticity polymer cushion layer to the inner surface of the clamp 6 used in the step (4), engraving a serrated groove in the center of the cushion layer, wherein the diameter of the groove is equal to that of the distributed sensing optical cable, so that slipping is avoided in the drawing process, and the optical path damage caused by over-tight clamping is prevented.
And (3) in the step (7), the functional relation between the shear stiffness of the optical cable-soil interface and various physical parameters such as unfrozen water content, dry density and the like is obtained through a calibration test, the reference value of the unfrozen water content is measured through a dielectric constant method, the reference value of the ice content is measured through a nuclear magnetic resonance method, the reference value of the dry density is measured through a ring cutter method, the frost heaving rate and the thaw coefficient are obtained through calculation of the vertical displacement of the frozen soil in an indoor model test, the shear strength is measured through a triaxial shear test, and the heat conductivity and the heat capacity are measured through a flat plate heat conduction analyzer.
In step (7) of the optical fiber drawing-based distributed in-situ test method for multiple physical parameters of frozen soil, the process of measuring the multiple physical parameters such as ice content, heat conductivity coefficient and frost heaving rate of the in-situ frozen soil in a certain cold region is as follows:
and (3) performing a function relation calibration test of ice content, thermal conductivity, frost heaving rate and shear stiffness of a distributed sensing optical cable-frozen soil interface:
(1) collecting frozen soil samples in the area by drilling, preparing 3 groups of known frozen soil samples, filling the known frozen soil samples into a mold with the diameter of 20cm and the height of 30cm in layers, and setting 6 samples with different parameters for each group of samples;
(2) installing the distributed sensing optical cable 4 at a corresponding position of the frozen soil sample;
(3) placing the sample in a low-temperature test box to be frozen for 12 hours;
(4) the high-precision drawing tester 7 performs a drawing test at a speed v of 1mm/min, records the axial drawing force and the drawing displacement of the distributed sensing optical cable in real time, and the optical fiber demodulator tests the strain distribution of the optical fiber in the drawing process in real time, wherein the scanning time interval of the optical fiber demodulator is t of 15 s;
(5) and drawing a strain distribution curve and an optical cable drawing force-drawing displacement curve along the length direction of the distributed sensing optical cable 4 in the drawing process, and analyzing the obtained curves according to the optical cable-soil interface coupling deformation mechanical relationship to obtain a distribution curve of the shear rigidity of the distributed sensing optical cable-frozen soil interface along the length direction of the optical cable.
(6) The mass ice content of the frozen soil sample is respectively 8%, 10.6%, 14%, 17.5%, 19% and 22% by nuclear magnetic resonance method; the thermal conductivity of the frozen soil sample is respectively 0.76, 0.87, 0.98, 1.31, 1.75 and 1.93W/m.K measured by a thermal conductivity analyzer; through the height h of frozen soil sample before and after freezing1,h2Calculating the frost heaving rate
The frost heaviness ratios were 0.54%, 0.71%, 0.92%, 1.13%, 1.43%, and 1.86%, respectively.
(7) According to the relation between the shear rigidity of the distributed sensing optical cable-frozen soil interface and the ice content, the heat conductivity coefficient and the ice content of the frozen soil, the least square method is applied to fit and establish the shear rigidity G and the ice content w of the distributed sensing optical cable-soil interfaceiThe functional relationship between G ═ f (w)u，wi，ρdξ, λ, C): g ═ 1.714wi +8.317, the fitted curve is shown in fig. 4; the functional relationship between the shear stiffness G of the cable-soil interface and the thermal conductivity lambda is as follows: g-3.67λ+17.34, the fitted curve is shown in fig. 5; the functional relation between the shear rigidity G of the optical cable-soil interface and the frost heaving rate xi is as follows: g3.86 xi2+5.17 ξ +7.28, the fit curve is shown in FIG. 6.
In this embodiment, the optical fiber drawing-based distributed in-situ test device for multiple physical parameters of frozen soil is applied to perform a frozen soil drawing in-situ test, and the specific test method is as follows:
(1) a plurality of distributed sensing optical cables 4 are buried in the in-situ frozen soil to be detected along the vertical earth surface direction in a drilling mode, and the drilling depth is 8 m; the stainless steel hollow pipe 5 is arranged on the ground surface along the direction vertical to the ground surface, and the burial depth is half of the length of the stainless steel hollow pipe 5; the distributed sensing optical cable 4 penetrates out of the ground surface from the bottom upwards from the stainless steel hollow pipe 5;
(2) the distributed sensing optical cable 4 is connected with an optical fiber demodulation device 10, the optical fiber demodulation device 10 reads temperature information along the length direction of the optical cable with certain spatial resolution and acquisition frequency, and temperature distribution and a time-space change rule of the in-situ frozen soil along the depth direction are obtained;
(3) determining the freezing depth and different freezing stages of the frozen soil according to the distribution of the temperature of the in-situ frozen soil along the depth direction;
(4) before drawing, fixing a base of a high-precision drawing tester 7 at a corresponding position on the earth surface, positioning a clamp 6 of the high-precision drawing tester 7 above a stainless steel hollow pipe 5, aligning the center of the clamp 6 with the center of the stainless steel hollow pipe 5, fixing a distributed sensing optical cable 4 at a central groove of the clamp 5, and clamping and fixing the distributed sensing optical cable through a bolt of a rotary clamp;
(5) in different freezing stages of in-situ frozen soil, a high-precision drawing tester is adopted to draw the distributed sensing optical cable at a constant speed v of 1mm/min, the drawing force and the drawing displacement are automatically recorded, and the optical fiber demodulation equipment monitors the axial strain distribution of the distributed sensing optical cable along the length direction in the drawing process in real time;
(6) and drawing an axial strain distribution curve and an optical cable drawing force-drawing displacement curve along the length direction of the distributed sensing optical cable 4 in the drawing process, and analyzing the monitoring curve according to the coupling deformation mechanical relationship of the distributed sensing optical cable-frozen soil interface to obtain a distribution curve of the shear stiffness of the optical cable-frozen soil interface along the length direction of the distributed sensing optical cable.
(7) And calculating the physical index of the frozen soil to be measured according to the established interface strength, the optical cable-frozen soil interface shear stiffness and the multiple physical parameters of the frozen soil, such as the functional relation among the unfrozen water content, the ice content, the dry density, the mechanical parameters (frost heaving rate, melt settlement coefficient and shear strength) and the thermophysical property parameters (heat conductivity coefficient and heat capacity), as shown in FIGS. 7-9. And respectively carrying out the in-situ drawing process at different times to obtain the space-time distribution of each physical property parameter of the frozen soil.
In the test, an OSI series optical frequency domain reflection OFDR demodulator produced by Wuhan Haoheng science and technology limited company and a portable distributed optical fiber temperature demodulator NZS-DTS-A03 produced by Suzhou Nanzhi sensing science and technology limited company are adopted.
The theoretical derivation process of the functional relationship between the shear stiffness of the optical cable-frozen soil interface and various physical parameters such as ice content, thermal conductivity, frost heaving rate and the like in the embodiment is explained as follows:
in the drawing process, the length direction of the optical cable is taken as an x axis, and the depth direction of the optical cable is taken as the positive direction of a coordinate axis. Analyzing the optical cable micro-element section, wherein the relation between the axial strain and the displacement is as follows:
where ε (x) is the axial strain of the cable and u (x) is the displacement of the cable.
According to the stress balance condition of the infinitesimal section, the product is obtained
Wherein D is the diameter of the optical cable, F (x) is the axial force of the optical cable (with the pulling as positive), and tau (x) is the shear stress of the optical cable-soil interface.
Wherein E is the elastic modulus of the optical cable.
Simultaneous front three formulas
The ideal elastic-plastic model is a common model for progressive destruction of the optical cable interface, has good coordinated deformation before the optical cable and the surrounding soil body are debonded, and the relation between the interface shear stress and the shear strain is expressed as
τ(x)＝Gγ(x) (5)
Where G is the cable-soil interface shear stiffness and γ (x) is the shear strain of the interface.
Assuming that the shear strain of the soil body in the shear layer is linearly reduced along the radial direction, the relationship between the drawing displacement and the interface shear stress is expressed as
Wherein h is the thickness of the shear layer soil mass.
Vertical type (4) - (6) combined with boundary conditions
Obtaining the solution of the optical cable displacement:
wherein P is the drawing force, L is the cable length,
G*2G/h is defined as the shear coefficient of the cable-soil interface. Thus, the cable is in xiDisplacement of position, interface shear stiffness G corresponding to the positioniIn connection with, i.e. with
u(xi)＝g(Gi) (8)
Combining the measured distribution curve of the axial strain epsilon (x) of the optical cable along the depth, and calculating the optical cable in x by combining the formula (1)iDisplacement of position
And (8) integrating the (8) and the (9) to obtain the interface shear rigidity G of the optical cable. And obtaining a strain distribution curve and an optical cable drawing force-drawing displacement curve in an in-situ test, and further obtaining the distribution of the shear stiffness of the optical cable-frozen soil interface according to the formulas (8) and (9).
The existing research on frozen soil shows that the frost heaving force of the frozen soil has a positive correlation with the ice content and the dry density and acts on the distributed sensing optical cable 4 in a normal stress mode, so according to the Mokolun criterion, the greater the ice content and the dry density, the greater the frost heaving force, and the better the interface shear resistance; the optical cable and the surrounding frozen soil are bonded together due to freezing action, the bonding strength is called freezing force, the freezing force is increased along with the reduction of temperature, the increase of ice content and the reduction of unfrozen water content, and the shearing resistance of the optical fiber-soil interface is better. Therefore, under the combined action of the frost heaving force and the freezing force, the shearing resistance of the optical cable-soil interface is in positive correlation with the ice content and the dry density. The thermal and mechanical parameters of the frozen soil, such as thermal conductivity coefficient, frost heaving rate and the like, have functional relations with the basic properties of the frozen soil (ice content, unfrozen water content, dry density and other parameters), so the shear stiffness G of the optical cable-frozen soil interface has one-to-one corresponding functional relation with the thermal and mechanical parameters of the frozen soil, namely
G＝f(wu，wi，ρd，ξ，λ，C) (10)
Wherein, wuIs the unfrozen water content, wiIs the ice content, ρdIs the dry density, ξ is the frost heaviness, λ is the thermal conductivity, C is the heat capacity.
Based on the formula (10) and a distribution curve of shear stiffness of the distributed optical cable-frozen soil interface obtained by field test measurement along the length direction of the distributed sensing optical cable, the distribution curve of ice content, heat conductivity coefficient, frost heaving rate and other physical parameters of the frozen soil in situ along the length direction of the distributed sensing optical cable is obtained through calculation.
Claims (10)
1. The utility model provides a many rerum natura parameters of frozen soil distributing type normal position testing arrangement based on optical fiber drawing which characterized in that: the distributed in-situ testing device comprises a distributed sensing optical cable (4), optical fiber demodulation equipment (10), a drawing tester (7), a hollow pipe (5) and a clamp (6);
the drawing tester (7) clamps and fixes the distributed sensing optical cable (4) through the clamp (6), controls the drawing speed and displacement, and records the axial drawing force and the drawing displacement of the distributed sensing optical cable in the drawing process;
the optical fiber demodulation equipment (10) measures strain and temperature distribution of frozen soil in the drawing process of the distributed sensing optical cable (4).
2. A distributed in-situ test method for frozen soil multi-physical-property parameters based on optical fiber drawing is characterized by comprising the following steps: the optical fiber drawing-based distributed in-situ testing device for the frozen soil multi-physical parameters comprises the following steps of:
(1) embedding the distributed sensing optical cable (4) in the in-situ frozen soil to be detected along the direction vertical to the ground surface; the hollow pipe (5) is vertically arranged on the ground surface, and the distributed sensing optical cable (4) penetrates out of the ground surface upwards through the hollow pipe (5);
(2) connecting the distributed sensing optical cable (4) with an optical fiber demodulation device (10), wherein the optical fiber demodulation device (10) reads temperature information of the distributed sensing optical cable in the length direction to obtain the distribution and the change of the temperature of the in-situ frozen soil along the depth direction;
(3) according to the distribution and change of the temperature of the in-situ frozen soil along the depth direction, determining the freezing depth of the in-situ frozen soil and different freezing stages of the frozen soil;
(4) before drawing, fixing a drawing tester (7) on the ground surface, positioning a clamp (6) above an empty pipe (5), aligning the drawing tester (7) to the center of the empty pipe (5), and fixing a distributed sensing optical cable (4) in the center of the clamp (6);
(5) in different freezing stages of in-situ frozen soil, a drawing tester (7) is adopted to draw the distributed sensing optical cable (4) at a constant speed, the drawing force and the drawing displacement are recorded, and the optical fiber demodulation equipment (10) monitors the axial strain distribution of the distributed sensing optical cable (4) along the length direction in the drawing process in real time;
(6) drawing an axial strain distribution curve and an optical cable drawing force-drawing displacement curve along the length direction of the distributed sensing optical cable (4) in the drawing process, and analyzing the curves according to the coupling deformation relation of the distributed sensing optical cable and the frozen soil interface to obtain the distribution of the shearing rigidity of the distributed sensing optical cable and the frozen soil interface;
(7) and determining multiple physical property parameter values, calculating the multiple physical property parameter values of the frozen soil according to the functional relation between the shear stiffness of the optical cable-frozen soil interface and the multiple physical property parameters of the frozen soil, and further obtaining the distribution of the multiple physical property parameters along the length direction of the optical cable.
3. The optical fiber drawing-based distributed in-situ test method for the frozen soil multi-physical parameters is characterized by comprising the following steps of: in the step (1), a drilling direct burial mode is adopted to bury a plurality of distributed sensing optical cables in the in-situ frozen soil to be detected along the direction vertical to the ground surface, the in-situ frozen soil is used as a backfill material, and drawing tests are carried out at different freezing stages of the in-situ frozen soil.
4. The optical fiber drawing-based distributed in-situ test method for the frozen soil multi-physical parameters is characterized by comprising the following steps of: in the step (2), the distribution and change of the temperature of the in-situ frozen soil along the depth direction are reflected in that: (a) the soil temperature rises from the surface to the deep part and is kept in stable spatial distribution in a constant temperature layer; and the ground temperature gradient of the shallow ground temperature at different depths; (b) the change of soil temperature along with the fluctuation of atmospheric temperature, solar radiation and snow cover along with time.
5. The optical fiber drawing-based distributed in-situ test method for the frozen soil multi-physical parameters is characterized by comprising the following steps of: and (3) determining the freezing depth of the in-situ frozen soil by taking the depth of the position at 0 ℃ as a judgment standard, and judging the freezing states of the in-situ frozen soil at different freezing stages according to the change of the freezing depth.
6. The optical fiber drawing-based distributed in-situ test method for the frozen soil multi-physical parameters is characterized by comprising the following steps of: an elastic polymer cushion layer is attached to the inner surface of the clamp (6) adopted in the step (4), and a sawtooth-shaped groove for fixing the distributed sensing optical cable (4) is carved on the cushion layer.
7. The optical fiber drawing-based distributed in-situ test method for the frozen soil multi-physical parameters is characterized by comprising the following steps of: in the step (6), the distribution of the shear stiffness of the distributed sensing optical cable-frozen soil interface is obtained by the following method:
in the drawing process, the optical cable micro-element section is analyzed by taking the length direction of the optical cable as an x axis and the depth direction of the optical cable as the positive direction of a coordinate axis, and the relationship between the axial strain and the displacement is as follows:
wherein ε (x) is the axial strain of the cable, and u (x) is the displacement of the cable;
according to the stress balance condition of the infinitesimal section, the product is obtained
Wherein D is the diameter of the optical cable, F (x) is the axial force of the optical cable, and the pull is positive; τ (x) is the shear stress at the cable-soil interface;
wherein E is the elastic modulus of the optical cable;
simultaneous front three formulas
The relationship between interfacial shear stress and shear strain before debonding the cable from the surrounding soil may be expressed as
τ(x)＝Gγ(x) (5)
Wherein G is the cable-soil interface shear stiffness and γ (x) is the shear strain of the interface;
assuming that the shear strain of the soil body in the shear layer is linearly reduced along the radial direction, the relationship between the drawing displacement and the interface shear stress is expressed as
Wherein h is the thickness of the shear layer soil body;
vertical type (4) - (6) combined with boundary conditions
Obtaining the displacement of the optical cable:
wherein P is the drawing force, L is the cable length,
g ═ 2G/h is defined as the shear coefficient of the cable-soil interface; thus, the cable is in xiDisplacement of position, interface shear stiffness G corresponding to the positioniIn connection with, i.e. with
u(xi)＝g(Gi) (8)
Combining the measured distribution curve of the axial strain epsilon (x) of the optical cable along the depth, and calculating the optical cable in x by combining the formula (1)iDisplacement of position
And (4) obtaining the shear rigidity G of the optical cable-frozen soil interface of the optical cable by integrating the formula (8) and the formula (9), and obtaining the distribution of the shear rigidity of the optical cable-frozen soil interface by combining an axial strain distribution curve and an optical cable drawing force-drawing displacement curve.
8. The optical fiber drawing-based distributed in-situ test method for the frozen soil multi-physical parameters is characterized by comprising the following steps of: in the step (7), the functional relation between the shear stiffness of the optical cable-frozen soil interface and the multiple physical property parameters of the frozen soil is obtained through a calibration test, and the calibration process is as follows:
(7.1) preparing a plurality of groups of frozen soil samples and filling the frozen soil samples into the die in layers, wherein each group of samples is provided with a plurality of samples with different parameters;
(7.2) installing a distributed sensing optical cable (4) in the frozen soil sample;
(7.3) placing the frozen soil sample in a test box for freezing;
(7.4) performing a drawing test at a constant speed by using a drawing tester (7), recording the axial drawing force and the drawing displacement of the distributed sensing optical cable in real time, and testing the strain distribution of the optical fiber in the drawing process in real time by using an optical fiber demodulation device (10);
(7.5) drawing a strain distribution curve and an optical cable drawing force-drawing displacement curve along the length direction of the distributed sensing optical cable (4) in the drawing process, and analyzing the obtained curves according to the optical cable-soil interface coupling deformation relation to obtain a distribution curve of the shear stiffness of the distributed sensing optical cable-frozen soil interface along the length direction of the optical cable;
(7.6) measuring the mass ice content of the frozen soil sample by a nuclear magnetic resonance method; measuring the heat conductivity coefficient of the frozen soil sample by a heat conductivity coefficient analyzer; through the height h of frozen soil sample before and after freezing1，h2Calculating the frost heaving rate
Obtaining the frost heaving rate of each group of frozen soil;
(7.7) applying least square method to fit and establishing function between distributed sensing optical cable-soil interface shear stiffness G and multi-physical-property parametersThe numerical relationship G ═ f (w)u，wi，ρdξ, λ, C); wherein, wuIs the unfrozen water content, wiIs the ice content, ρdIs the dry density, ξ is the frost heaviness, λ is the thermal conductivity, C is the heat capacity.
9. The optical fiber drawing-based distributed in-situ test method for the frozen soil multi-physical parameters is characterized by comprising the following steps of: and (7) calculating to obtain a distribution curve of the multi-physical-property parameters of the frozen soil in situ along the length direction of the optical cable according to the functional relation between the shear stiffness G of the optical cable-frozen soil interface and the multi-physical-property parameters and the distribution curve of the shear stiffness G of the optical cable-frozen soil interface along the length direction of the distributed sensing optical cable obtained in the step (6).
10. The optical fiber drawing-based distributed in-situ test method for the frozen soil multi-physical parameters is characterized by comprising the following steps of: in the step (7), the multiple physical parameters of the frozen soil comprise dry density, unfrozen water content, ice content, mechanical parameters and thermophysical property parameters; the mechanical parameters comprise frost heaving rate, thaw settlement coefficient and shear strength; the thermophysical property parameters include thermal conductivity and heat capacity.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN117366481A (en) * | 2023-12-08 | 2024-01-09 | 中国科学院西北生态环境资源研究院 | Device and method for monitoring moving distance of buried heating oil pipeline in permafrost region |
| CN117366481B (en) * | 2023-12-08 | 2024-02-06 | 中国科学院西北生态环境资源研究院 | Device and method for monitoring moving distance of buried heating oil pipeline in permafrost region |
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