CN110793940A - Soil matrix suction quasi-distributed in-situ measurement method and device based on fiber bragg grating - Google Patents
Soil matrix suction quasi-distributed in-situ measurement method and device based on fiber bragg grating Download PDFInfo
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- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
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Abstract
The invention discloses a fiber bragg grating pulse heating-based soil matrix suction quasi-distributed in-situ measurement method and a device, and the method comprises the following steps: a heating direct current power supply, a high-heat-sensitive insulating tube (internally provided with a resistance wire and a fiber grating), a pottery clay rod, a fiber grating demodulator and a computer for analyzing and processing monitoring data. Embedding the prepared and pre-saturated matrix suction sensor in the soil body to be detected, and standing for 24 hours to enable the argil rod and the soil body to reach water potential balance; connecting a heating power supply to perform pulse heating on a resistance wire arranged in the substrate suction sensor, and diffusing heat into a pottery clay rod wrapping the high-heat-sensitive insulating pipe through the high-heat-sensitive insulating pipe with good heat conductivity; the fiber bragg grating arranged in the high-heat-sensitive insulating tube is sensitive to temperature change, and the temperature change value can be obtained by converting fiber bragg grating wavelength data collected by a fiber bragg grating demodulator; and finally, establishing a functional relation between the matrix suction force of the argil rod and the temperature change value through a calibration test, and then obtaining the matrix suction force of the soil body corresponding to the temperature change value according to water potential balance. According to the measuring method, a temperature change value is obtained through heat pulse heating, and soil matrix suction under the condition of water potential balance of the sensor is obtained according to the calibration relation of the matrix suction and the temperature change value. The invention can realize rapid, continuous, remote, high-precision and quasi-distributed soil matrix suction measurement.
Description
Technical Field
The invention discloses a method and a device for distributed in-situ measurement of soil matrix suction by using fiber Bragg gratings (fiber bragg gratings), and relates to the technical field of soil matrix suction measurement.
Background
The soil body in unsaturated soil consists of soil particles, water and air, i.e. the soil has pore air pressure in addition to pore water pressure. At the pore water and pore gas interface, the water has surface tension. The suction in the soil comprises a matrix suction and an osmotic suction (solute suction), the values of the pore water pressure and the pore air pressure are not equal due to the existence of the meniscus, and when the pore water pressure (u _ w) is less than the pore air pressure (u _ a), the difference value (u _ a-u _ w) is called the matrix suction. For most unsaturated soils, the suction force exists, when the soil is suddenly rained or soaked in water, the soil is easily quickly saturated, and the phenomenon that the suction force of a matrix is quickly reduced or even disappears often appears, which is one of the reasons for sudden instability and damage of certain landslides. Therefore, the method for measuring the soil matrix suction has very important significance for theoretical research and engineering practice.
At present, the soil matrix suction measuring method mainly comprises a tensiometer technical method, a heat conduction sensor method, a dielectric constant method, an axis translation technical method, a dialysis method, a contact filter paper technical method, a salt solution gas phase method and a high-speed centrifuge method. When the tensiometer technology is used for measurement, the problems that water in the tensiometer is likely to generate cavitation phenomenon and is evaporated through a pottery clay head and the like are solved, and the method is only suitable for measuring the substrate suction value in a small range. The traditional heat sensor method has the problems of slow reading, difficulty in disassembling and replacing parts, easiness in aging after long-time use and hysteresis during calibration. The dielectric constant method is greatly influenced by the salinity of the soil to be detected, and the soil with high salinity cannot be tested and cannot be used in long-distance monitoring.
The shaft translation technology has large equipment volume and high requirement on test environment, is usually used for measuring in a laboratory, and has a higher suction value of a measured substrate when bubbles exist in a soil sample; when air passing through the ceramic plate with high air inlet value is diffused, the measured substrate suction value is low, and the error is large. Dialysis measurements are generally applicable to low suction fractions (<2MPa) and require the addition of small amounts of penicillin to the PEG solution to eliminate bacterial attack of the semipermeable membrane in the soil sample, are cumbersome to operate, and are prone to contamination. The contact filter paper technology, the salt solution gas phase method and the high-speed centrifuge method have higher requirements on the skill and the proficiency of operators. In the measurement of the contact type filter paper technology, the requirement on the batch of filter paper is high, the test takes long time, a calibration experiment needs to be carried out on the filter paper of a specific batch before the test, the operation is complex, and the filter paper can not be widely used. In the salt solution gas phase method measurement, when the measured substrate suction is lower than 1500kPa, the accuracy is low. When the high-speed centrifuge method is used for measurement, the possibility that water in a soil sample is completely discharged out of a soil body is low, and the equipment cost is high.
The fiber Bragg grating (fiber bragg grating) is a periodic grating manufactured on a fiber core and the like, when the fiber is pulled along the axial direction or the ambient temperature changes, the fiber deforms along the axial direction, the refractive index of the fiber changes, so that the spectrum of an output signal changes, and the strain (temperature) measurement of a body to be measured can be realized by processing and analyzing the change value. The fiber grating is an intrinsic wavelength modulation type sensor developed based on a Bragg condition based on a photosensitivity phenomenon, has the advantages of small volume (the outer diameter of a bare grating is 125 mu m), strong anti-electromagnetic interference capability, no humidity influence on performance, good stability, corrosion resistance, high sensitivity and the like, and is widely applied to the stability monitoring of bridges, dams and rock-soil structures in recent years.
The method is characterized in that the matrix suction in the soil body is measured based on a quasi-distributed Fiber Bragg Grating (FBG), the wavelength reading of the fiber Bragg grating after pulse heating is collected by a fiber Bragg grating demodulator, the wavelength value is converted into a temperature change value, and finally, the functional relation between the matrix suction of the soil body and the temperature change value is established through a calibration test to obtain the matrix suction of the soil body. Compared with the traditional measuring method, the method has the advantages of rapidness, continuity, long distance, high sensitivity, quasi-distributed measurement of the soil matrix suction, and the like, can be used in indoor tests and field monitoring, and solves the problem that the traditional suction testing method has high requirements on the technical performance of personnel. There is no relevant research report for determining the suction force of the matrix in unsaturated soil by applying the technology at home and abroad.
Disclosure of Invention
The invention aims to provide a method and a device for measuring soil matrix suction in situ based on a quasi-distributed fiber grating, which are used for sensing the temperature of a pottery clay rod heated by pulse by using the temperature sensitive characteristic of the fiber grating according to the temperature response principle, and determining the temperature change value through the temperature rise curve of a rod body, thereby measuring the internal saturation of the pottery clay rod. The aim of measuring the soil matrix suction is achieved based on the relationship between the internal saturation of the calibrated argil rod and the matrix suction. The sensor can realize the long-distance, real-time, accurate and in-situ measurement of the substrate suction.
Through the introduction, the invention adopts the following technical scheme: a substrate suction quasi-distributed in-situ measurement method based on fiber bragg grating comprises the following steps:
the method comprises the following steps that firstly, a completely manufactured matrix suction sensor is fully saturated in water and then implanted into a soil body to be detected, wherein the matrix suction sensor comprises a pottery clay rod body (with a soil-water characteristic curve calibrated), an optical fiber, a heating resistance wire and an optical fiber grating;
step two, standing the sensor in the soil for 24 hours to balance the water potential of the soil body and the argil rod;
step three, the substrate suction sensor starts to heat under the action of the current of the heating resistance wire; stopping heating after the density of the diffused heat flow is constant, and starting cooling the rod body;
step four, collecting and recording a heating time interval [ t ] by a fiber grating demodulator1,t2]Wavelength reading of the internal fiber grating, t1As the heat pulse start time, t2The time for starting to cool the bar body is the time;
converting the wavelength data into bar body temperature information by using calculation software through a wavelength and temperature conversion relation formula when the optical fiber leaves a factory; calculating the temperature change value of the rod body, and according to the empirical relationship between the temperature change value of the pottery clay rod and the water saturation: k ═ k1ΔTt+b1Wherein l is the water saturation of the pottery clay rod, Delta TtValue of temperature change, k, measured for a substrate suction sensor1、b1The water saturation and the temperature change value of a plurality of groups of argil rods are determined by rating tests; the temperature change value of the substrate suction sensor is a fiber bragg grating pulse heating time interval [ t ]1,t2]The difference between the temperature value after the heat pulse heating and the initial temperature is measured; the soil matrix suction force is obtained by combining the relation between the temperature change value and the water saturation of the argil rod and the soil-water characteristic curve of the argil rod.
In the first step, the substrate suction sensor is inserted or buried into the soil to be measured.
In the second step, the substrate suction sensor needs to be fully contacted with the surrounding soil body when being buried.
And step three, the heating power of the substrate suction sensor is constant, and the rapid and stable heating of the sensor can be ensured.
Step (ii) ofTime interval [ t ] of four1,t2]The time interval of the heat pulse is defined, and the value is determined according to the material of the argil rod.
The water saturation l of the pottery clay rod is the ratio of the mass of water in the pottery clay rod to the mass of the pottery clay rod, and is determined by balance measurement and test configuration.
A device used in the fiber bragg grating substrate suction quasi-distributed in-situ measurement method comprises a heating power supply, an argil rod sensor, a fiber bragg grating demodulator and an analysis processing monitoring data device, wherein the substrate suction sensor comprises an argil rod, a high-heat-sensitive insulating tube arranged in the argil rod, a resistance wire and a fiber bragg grating are arranged in the tube, a sensor reinforcing bar is made of an organic material with a thermal expansion coefficient close to that of the argil rod, an optical fiber and a heating resistance wire penetrate through the high-heat-sensitive insulating tube, a plurality of fiber bragg gratings are arranged on the optical fiber, the heating resistance wire in the high-heat-conducting tube in the sensor is connected with the heating power supply through a power-on lead, and the optical fiber is connected with the fiber bragg grating demodulator through a fiber lead and is used for collecting and recording wavelength readings after; the analysis processing monitoring data device is connected with the fiber bragg grating demodulator, and the data analysis processing system is used for converting the wavelength data into the temperature information of the high-temperature-sensitive insulating tube body and calculating the temperature change value.
The argil rod material of the matrix suction sensor has uneven air inlet values, and can achieve water potential balance in soils with different saturation degrees, and the soil-water characteristic curve of the material is shown in an attached figure 5.
The substrate suction sensor is internally provided with a detachable high heat conducting pipe, two drilling holes are formed in the central points of the cross sections at the two ends of the pipe in the extending direction, and the hole diameter is d1、d2Hole length L1The optical fiber grating and the heating resistance wire are respectively packaged in two holes by adopting a sensitivity-enhanced packaging structure, and the optical fiber is laid in a natural relaxation state without tension. Then injecting the non-solidified heat-conducting paste into the small groove.
The matrix suction sensor is characterized in that a heat conduction material is coated between a heating resistance wire inside the matrix suction sensor and a hollow argil rod, and the external diameter of the argil rod is D2Length of L2。
The matrix suction sensors may be used individually, in series or in parallel; the adjacent substrate suction sensors are connected with nuts through screw ports at two ends; and an optical fiber armor protective sleeve is arranged outside the external optical fiber lead of the matrix suction sensor.
Has the advantages that:
1. based on the relation between the saturation of the pottery clay material and the substrate suction, the invention uses the fiber grating self-heating method to realize the measurement of the saturation according to the difference between the thermal conductivity coefficients of pottery clay rods with different saturations, thereby achieving the measurement of the substrate suction.
2. The invention can measure the soil matrix suction in situ, has small disturbance to the soil, and avoids the structure and component change of the soil in the sampling, transporting and storing processes.
3. The invention can realize the quasi-distributed and continuous measurement of the soil matrix suction.
4. The invention can realize the remote and real-time monitoring of the soil matrix suction.
5. The invention has the advantages of economy, safety, convenient operation, strong anti-interference capability, reliable precision and high stability.
6. The invention can be applied to various soil bodies such as clay, sandy soil and the like, and is slightly influenced by the salinity of the soil body.
7. The invention can be applied to experimental research of different scales, namely indoor test and in-situ test, can improve the spatial resolution by adopting a series connection mode, and can realize multi-direction monitoring by adopting parallel connection.
Drawings
FIG. 1 is a schematic diagram of a longitudinal structure of a self-heating device inside a distributed in-situ measurement sensor according to the present invention.
Wherein, 1, optical fiber protective sleeve; 2. packaging the screw; 3. the heat conducting paste is not solidified; 4. a fiber grating; 5. an optical fiber; 6. packaging the end; 7. a power-on wire; 8. a resistance wire; 9. a high thermal conductivity insulating tube body;
D1diameter of the high thermal conductivity tube body outside the sensitization structure, d1Is the pore diameter of the optical fiber, d2Is the hole diameter of the resistance wire.
Fig. 2 is a schematic diagram of the overall structure of the distributed in-situ measurement sensor according to the present invention.
Wherein, 1, optical fiber protective sleeve; 7. a power-on wire; 9. a high thermal conductivity insulating tube body; 10. a pottery clay rod; 11. the optical fiber lead joint; 12. the joint of the electrified lead; 13. sealing the upper part of the argil rod; 14. a pottery clay rod is used for reinforcing the packaging strip; 15. the lower part of the pottery clay rod is provided with a sealing opening.
D2Is the diameter of the pottery clay rod, L2Is the sensor length.
FIG. 3 is an oblique view of the overall structure of the distributed in-situ measurement sensor of the present invention.
Wherein, 10, a pottery clay rod; 11. the optical fiber jumper wire joint; 12. the joint of the electrified lead; 13. sealing the upper part of the argil rod; 14. a pottery clay rod is used for reinforcing the packaging strip; 15. a packaging opening at the lower part of the argil rod; 16. a sensor fiber series port; 17. the sensor is electrified with a wire series port.
Fig. 4 is a schematic diagram of a soil matrix suction quasi-distributed in-situ measurement system according to the invention.
18, a power-on lead; 19. an optical fiber lead; 20. three matrix suction sensors in series; 21. two matrix suction sensors in series, 20 and 21 in parallel relationship; 22. a fiber grating demodulator; 23. a computer for analyzing and processing the monitoring data; 24. a heating power supply.
Fig. 5 is a graph of the time-dependent change in heating current of one heat pulse of the substrate suction sensor of examples 1 and 2.
FIG. 6 is a graph of the calibration results of the clay rod water saturation as a function of substrate suction in the substrate suction sensor of example 1.
FIG. 7 is a plot of temperature change versus water saturation for clay bars in the matrix suction sensor of example 1.
Fig. 8 is a soil-water characteristic curve chart obtained by measuring the target soil mass in example 2.
Detailed Description
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention. The invention is further explained below with reference to the figures and examples.
As shown in fig. 1, 2 and 3, the fiber grating-based internal heating substrate suction sensor for soil substrate suction measurement sequentially comprises a fiber grating, an optical fiber, a heating resistance wire, an uncured heat conducting paste, a high heat conducting pipe, an optical fiber protective sleeve, an electrified conducting wire, a pottery clay rod and a pottery clay rod reinforcing and packaging strip from inside to outside.
As shown in fig. 4, a fiber grating-based soil matrix suction quasi-distributed in-situ measurement system includes a heating power supply, a computer for analyzing and processing monitoring data, a fiber grating demodulator, and a fiber grating matrix suction sensor. The heating power supply keeps constant power, so that the resistance wire works under stable current; the substrate suction sensor is a fiber bragg grating substrate suction sensor with an internal heating function; the fiber grating demodulator is connected with an optical fiber with a fiber grating and is used for collecting and recording wavelength readings after the heating tends to be stable; and the computer for analyzing and processing the monitoring data is connected with the fiber grating demodulator, and the data analyzing and processing system is used for converting the wavelength data into the temperature information of the argil rod and calculating the temperature change value of the argil rod.
The fiber grating demodulator detects the Bragg wavelength reflected by the fiber grating to reflect temperature information. In the test, an A-01 fiber grating demodulator produced by Suzhou Nanzhi sensing technology Limited company is adopted to demodulate the fiber grating and collect the wavelength reading. The demodulator sample recording interval was 1 second.
A soil matrix suction quasi-distributed in-situ measurement method based on fiber bragg gratings comprises the following steps:
the method comprises the following steps that firstly, a completely manufactured matrix suction sensor is fully saturated in water and then implanted into a soil body to be detected, wherein the matrix suction sensor comprises a pottery clay rod body (with a soil-water characteristic curve calibrated), an optical fiber, a heating resistance wire and an optical fiber grating;
step two, standing the sensor in the soil for 24 hours to balance the water potential of the soil body and the argil rod;
step three, the substrate suction sensor starts to heat under the action of the current of the heating resistance wire; stopping heating after the density of the diffused heat flow is constant, and starting cooling the rod body;
step four, collecting and recording a heating time interval [ t ] by a fiber grating demodulator1,t2]Wavelength reading of the internal fiber grating, t1As the heat pulse start time, t2The time for starting to cool the bar body is the time;
converting the wavelength data into bar body temperature information by using calculation software through a wavelength and temperature conversion relation formula when the optical fiber leaves a factory; calculating the temperature change value of the rod body, and according to the empirical relationship between the temperature change value of the pottery clay rod and the water saturation: k ═ k1ΔTt+b1Wherein l is the water saturation of the pottery clay rod, Delta TtValue of temperature change, k, measured for a substrate suction sensor1、b1The water saturation and the temperature change value of a plurality of groups of argil rods are determined by rating tests; the temperature change value of the substrate suction sensor is a fiber bragg grating pulse heating time interval [ t ]1,t2]The difference between the temperature value after the heat pulse heating and the initial temperature is measured; the soil matrix suction force is obtained by combining the relation between the temperature change value and the water saturation of the argil rod and the soil-water characteristic curve of the argil rod.
Inserting or burying the substrate suction sensor into soil to be detected; and for the same argil rod, configuring different water-containing states of the argil rod by controlling the water content, and calibrating 16-20 groups of matrix suction of the argil rod under different saturation degrees by combining a filter paper suction test method to draw a soil-water characteristic curve of the argil rod.
In the second step, the substrate suction sensor needs to be fully contacted with the surrounding soil body when being buried.
And step three, the heating power of the substrate suction sensor is constant, and the rapid heating of the sensor is ensured.
Said time interval [ t ] in step four1,t2]The time interval of the heat pulse is defined, and the value is determined according to the material of the argil rod.
The water saturation of the pottery clay rod is the ratio of the mass of water in the pottery clay rod to the mass of water when the pottery clay rod is saturated with water, and is determined by balance measurement and test configuration.
Further, the soil matrix suction quasi-distributed in-situ measurement method based on the fiber bragg grating is characterized in that the calibration test of the water saturation and temperature change value of the argil rod comprises the following steps:
step one, weighing a sensor rod body to obtain the mass of a pottery clay rod;
fully mixing and stabilizing a substrate suction sensor which is manufactured and packaged completely with water, wherein the substrate suction sensor is a fiber grating substrate suction sensor with an internal heating function and consists of a pottery clay rod, an optical fiber, a heating resistance wire and a fiber grating;
and step three, drying the water-saturated argil rods in a natural state, and obtaining 16-20 groups with different saturation states by using a weighing method.
Step four, aiming at each group of different saturation degrees, connecting a power supply, electrifying and heating the matrix suction sensors with different saturation degrees in the step three for a short time, and starting to heat the rod body under the action of current; stopping electrifying and heating after the density of the diffused heat flow around the sensor is constant, and cooling the rod body;
step five, collecting and recording heating time [ t ] by the fiber grating demodulator1,t2]Wavelength reading of the fiber grating within the interval, t1As the heat pulse start time, t2Converting the wavelength data into bar temperature information for the time when the bar starts to cool;
step six, converting the wavelength data into bar body temperature information through the conversion relation between the wavelength and the temperature; calculating the temperature change value of the rod body, and according to the empirical relationship between the temperature change value of the pottery clay rod and the water saturation: k ═ k1ΔTt+b1Where l is the water saturation of the clay rod,. DELTA.TtValue of temperature change, k, measured for a substrate suction sensor1、b1Is constant and is determined by the above groups of experiments; the temperature change value of the substrate suction sensor is a fiber bragg grating pulse heating time interval [ t ]1,t2]Measured temperature value and initial temperature value after heat pulse heatingThe difference in temperature.
Therefore, through the steps, the relation between the water saturation and the temperature change value of the pottery clay rod can be determined.
A soil matrix suction quasi-distributed in-situ measuring device based on fiber bragg grating pulse heating comprises a heating power supply, a matrix suction sensor, a fiber bragg grating demodulator and a computer for analyzing, processing and monitoring data. The heating power supply keeps constant power, so that the resistance wire works under stable current; the matrix suction sensor comprises a pottery clay rod and a high-heat-sensitive insulating tube, wherein a resistance wire and a fiber grating are arranged in the tube, a sensor reinforcing strip is made of stainless steel materials, an optical fiber and a heating resistance wire penetrate through the high-heat-sensitive insulating tube, a plurality of fiber gratings are arranged on the optical fiber, the heating resistance wire in the high-heat-conductivity insulating tube in the sensor is connected with a heating power supply through a power-on lead, and the optical fiber is connected with a fiber grating demodulator through a fiber lead and is used for collecting and recording wavelength readings after heating tends to be stable; the analysis processing monitoring data device is connected with the fiber grating demodulator, and the data analysis processing system is used for converting the wavelength data into the temperature information of the high-temperature-sensitive insulating tube body and calculating the internal temperature change value. The fiber grating demodulator is connected with the optical fiber with the fiber grating and used for collecting and recording the wavelength reading after the heating tends to be stable.
Further, according to the soil matrix suction quasi-distributed in-situ measuring device, clay rod materials of the matrix suction sensor are provided with pores which are different in diameter and are uniformly distributed. The surface of the pottery clay rod can be corrugated to increase the relative contact area with the soil body and increase the water potential balance speed of the pottery clay rod and the soil body to be detected. The argil rod material of the matrix suction sensor has uneven air inlet values, and can achieve water potential balance in soils with different saturation degrees, and the soil-water characteristic curve of the material is shown in an attached figure 6.
Furthermore, according to the soil matrix suction quasi-distributed in-situ measurement device, two pore channels are formed in a high heat conduction pipe body in the fiber bragg grating matrix suction sensor, two holes are drilled from one end to the inside of the pipe in the extension direction, the positions of the holes are near the circle center when viewed from the circular cross section, one of the holes is provided with a heating resistance wire, and the other pore channel is provided with an optical fiber with a fiber bragg grating.
Further, the soil matrix suction quasi-distributed in-situ measuring device is based on the characteristic that the fiber bragg grating simultaneously responds to temperature and strain and the characteristic that the optical fiber is easy to break, the fiber bragg grating improves the temperature sensitivity coefficient through sensitivity enhancement packaging, and a high heat conduction pipe body in the sensor can be detachably replaced. The inside of the sensitization high heat conduction pipe has two diameters d1、d2The semi-circular cylinder cavity penetrates through the cavity, and the length of the cavity is L1. Respectively packaging the fiber grating and the heating resistance wire in two holes, adopting a sensitivity-enhanced packaging structure, and laying the optical fiber at the radius d1In the small groove, two ends are in a natural relaxation state without being pulled, and the heating wires are laid at the radius d2In the small grooves, the uncured heat-conducting paste is injected into the two grooves to accelerate the heat conduction speed, and meanwhile, the optical fiber strain is buffered and even eliminated. And fixing and packaging the two ends of the heat conduction pipe by epoxy glue and a corrosion-resistant material. The tube body material of the high-heat-conductivity insulating tube is made of high-heat-sensitivity insulating non-metallic materials, and can be made of carbon fiber or corundum materials.
Further, the matrix suction sensor of the soil body matrix suction quasi-distributed in-situ measuring device can be used independently, in series or in parallel; a plurality of fiber gratings are connected in series in the substrate suction sensor; the multiple matrix suction sensors are connected in series, so that quasi-distributed in-situ measurement of the soil matrix suction can be realized, and reading and recording can be carried out at multiple points. The adjacent substrate suction sensors are connected with nuts through screw ports at two ends. The matrix suction sensor is characterized in that a heat conduction material is coated between a heating resistance wire inside the matrix suction sensor and a hollow argil rod, and the external diameter of the argil rod is D2Length of L2。
Furthermore, according to the soil matrix suction quasi-distributed in-situ measuring device, an external optical fiber lead of the matrix suction sensor is packaged in a protective mode through an optical fiber armored protective sleeve, and the side wall of a clay rod is provided with a partial small-area reinforcing strip.
The top and the tail of the soil matrix suction quasi-distributed in-situ measurement sensor of the fiber bragg grating are respectively provided with a positive interface and a negative interface, so that the head and the tail of a plurality of sensors can be connected with each other, and the series connection can be realized. In addition, matrix suction sensors connected by different fiber optic pathways may achieve a parallel relationship between the sensors.
The positive interface and the negative interface of the soil matrix suction quasi-distributed in-situ measurement sensor for the fiber bragg grating comprise an optical fiber lead interface and a resistance wire lead interface, the two interfaces are respectively provided with a waterproof and insulating protective cap, and the protective caps can be opened to realize end-to-end connection.
The principle of the invention is as follows: the soil matrix suction quasi-distributed in-situ measurement method based on the fiber bragg grating has the basic principle that the soil matrix suction is measured by utilizing the change relationship between the temperature change value in the temperature rise process measured by the fiber bragg grating and the water saturation of the argil rod and combining the change relationship between the water saturation of the argil rod and the matrix suction. It can be further explained as: the heat conductivity of the pottery clay rod is determined by the pottery clay solid particle material, gas and water, wherein the air heat conductivity coefficient is 0.024W/(m.K), and the water heat conductivity coefficient is 0.60W/(m.K). Various characteristics of the solid of the argil rod are kept unchanged in the measurement process, the gas heat conductivity coefficient is far smaller than that of water, so that the gas heat conductivity coefficient can be ignored, the total saturation of the argil rod is kept constant, and the heat conductivity of the argil rod is determined by the water saturation of the argil rod. Since the thermal conductivity of water is 25 times that of air, the higher the saturation, the stronger the thermal conductivity of the soil. The matrix suction sensor heated by fiber bragg grating pulses is implanted into a soil body to be measured, the temperature of the high thermosensitive insulating tube rises after the matrix suction sensor is electrified, the temperature difference is formed between the matrix suction sensor and the argil rod, the higher the saturation of the matrix suction sensor is, the stronger the heat transfer capacity in the argil rod is, the more the total energy generated by constant power of a heating power supply is fixed, the more the energy is diffused into the argil rod with the higher saturation, the less the energy is used for heating the tube body of the high thermosensitive insulating tube, and the lower the temperature change value of the tube body is. Therefore, the saturation of the pottery clay rod is obtained by measuring the temperature change value obtained after the high-heat-sensitive insulating tube is heated for a certain time. According to the transformation relation between the saturation of the argil rod and the matrix suction, the matrix suction of the soil body can be obtained.
Example 1
Indoor calibration experiments using the matrix suction sensor of the method and apparatus of the invention. The method comprises the following specific steps:
step one, weighing a sensor rod body to obtain the mass of a pottery clay rod;
step two, for the same argil rod, configuring different water-containing states of the argil rod by controlling the water content, calibrating 16-20 groups of matrix suction forces of the argil rod under different saturation degrees by combining a filter paper suction force test method, and fitting a relation between the matrix suction force and the water saturation change of the argil rod to obtain a soil-water characteristic curve of the argil rod;
fully mixing and stabilizing a substrate suction sensor which is manufactured and packaged completely with water, wherein the substrate suction sensor is based on fiber bragg grating pulse heating and consists of a pottery clay rod, an optical fiber, a heating resistance wire and a fiber bragg grating;
and step four, drying the water-saturated argil rods in a natural state, and obtaining 8-15 groups with different saturation states by using a weighing method.
Connecting a power supply according to each group of different saturation degrees, electrifying and heating the matrix suction sensors with different saturation degrees in the step three for a short time, and starting to heat the rod body under the action of current; stopping electrifying and heating after the density of the diffused heat flow around the sensor is constant, and cooling the rod body;
sixthly, collecting and recording heating time [ t ] by the fiber grating demodulator1,t2]Wavelength reading of the fiber grating within the interval, t1As the heat pulse start time, t2Converting the wavelength data into bar temperature information for the time when the bar starts to cool;
step seven, converting the wavelength data into bar body temperature information through the conversion relation between the wavelength and the temperature; calculating the temperature change value of the rod body, and according to the empirical relationship between the temperature change value of the pottery clay rod and the water saturation: k ═ k1ΔTt+b1Where l is the water saturation of the clay rod,. DELTA.TtValue of temperature change, k, measured for a substrate suction sensor1、b1Is constant and is determined by the above groups of experiments; the temperature change value of the substrate suction sensor is a fiber bragg grating pulse heating time interval [ t ]1,t2]The difference between the measured temperature value after heating by the heat pulse and the initial temperature. And seventhly, obtaining the relation between the water saturation of the argil rod and the temperature change value.
Example 2
The method and the device are applied to carry out a distributed in-situ test for measuring the suction force of the soil matrix, and the ice content of soil at different depths is monitored. The test site selects soil body soil of a certain clay region of Nanjing.
Firstly, carrying out a calibration test on the fiber bragg grating substrate suction force sensor according to the method and the steps of embodiment 1, and fitting to obtain a change relation between a temperature change value and the substrate suction force;
and step two, standing the sensor in water for 24 hours to saturate the pores of the argil rod. The clay is tightly attached to and wrapped around the sensor, so that the clay rod is ensured to be fully contacted with the soil sample to be detected and is embedded in the soil sample to be detected;
step three, after standing for 24 hours, the argil rod and the soil body reach water potential balance, and the matrix suction sensor is connected to a power supply with stable power and the fiber bragg grating; connecting a power supply, electrifying and heating the fiber bragg grating substrate suction sensor in the second step, and starting heating the rod body under the action of current; stopping electrifying and heating after the density of the diffused heat flow around the sensor is constant, and cooling the rod body;
automatically acquiring and recording wavelength readings by the fiber grating demodulator every 1 second, and converting the wavelength data into temperature information of the pottery clay rod;
and fifthly, calculating the temperature change value sensed by each fiber grating by using a data analysis processing system, substituting the obtained temperature change value into a curve determined by a calibration test to obtain the internal saturation of the argil rod, and obtaining the soil matrix suction force according to the relation between the argil rod saturation and the matrix suction force. By combining the test with the conventional soil saturation testing method, a curve of the change of the soil matrix suction force along with the soil saturation can be obtained, as shown in fig. 8.
The theoretical derivation process of the functional relationship between the temperature variation value of the matrix suction sensor and the soil matrix suction in this embodiment is described as follows:
the soil body to be measured is assumed to have uniformity, and the rod body is located in an infinite soil layer with consistent initial temperature. The heat transfer in such soil is simplified into a one-dimensional problem. Taking a unit area on the surface of the rod body, according to ohm's law, the energy generated in the unit area per unit time is as follows:
Q1＝I2R (1)
in the formula Q1Is the energy produced on the unit area of the resistance wire, I is the current, and R is the resistance of the resistance wire. I. R are all known constants, Q1And is therefore also constant.
According to the conservation of energy, the energy per unit time for heating the rod is expressed as:
Q2＝Cm(T-T0)＝CmΔTt(2)
in the formula Q2Is the energy used to heat the rod; cmThe average specific heat capacity of the heating wire and the heat conduction pipe where the heating wire is located; t is0The temperature of the heat conduction pipe where the heating wire is positioned before heating; the measured temperature of the heated heat conduction pipe is the average temperature of the heat conduction pipe body after the density of the diffused heat flow around the heat conduction pipe is constant; delta TtDefined as the value of the temperature change.
Heat dissipated per unit area of the heating wire in unit time Q:
Q＝Q1-Q2＝I2R-CmΔTt(3)
and if the heat source is regarded as a wireless long-line heat source, the temperature field of the heating pipe satisfies the following conditions:
wherein r is the radius taking the heating wire as the center of a circle; t is heat exchange time; t (r, T) is the temperature of the pottery clay rod body at the position of the radius r from the center of the pipe body at the moment T; q is the heat exchange quantity of the rod body; l is the length of the rod body; lambda is the coefficient of thermal conductivity of the pottery clay rod; a is the thermal diffusivity of the pottery clay rodβ is a calculation process parameter T∞Is the temperature of the rock-soil mass at infinity from the center of the rod (i.e., the initial temperature); cSIs the unit volume heat capacity of the argil rod; and pi is the circumferential ratio.
Assuming steady-state heat conduction of the resistance wire, r is the radius r of the heating pipe of the resistance wirewAnd then the measured temperature T after heating is:
based on the wireless long-line heat source model, a linear derivation method is used, and the logarithm of T and the logarithm of time T are known to be in a functional relation according to the formula (6). Can be simplified as follows:
T＝klnt+m (7)
wherein T is the average temperature after the resistance wire is heated and stabilized; p is the thermal resistance of the rod body; taking 0.577216 when gamma is Euler constant; k and m are the relationship between the temperature measured by the fiber grating and the time, and the slope and the intercept of the straight line are obtained based on least square fitting.
The heat dissipation Q in combination with the temperature response can yield a thermal conductivity:
the pottery clay rod body consists of a pottery clay skeleton, gas and water, so that the thermal conductivity of the soil body consists of four parts
λ＝λss+λgg+λll (11)
Wherein s, g and l are the ratio of the clay skeleton, the gas mass and the total mass of water and soil in the clay rod. The gas content g is very small and can be ignored, and the heat conductivity coefficient lambda of the pottery clay framework and the waters、λlAnd the content s of the solid particles of the argil skeleton can be relatedExperiments and data have been obtained since the thermal conductivity of clay rods is a function of saturation.
The combined type (3), (10) and (11) can obtain the ice content and the temperature change value delta TtFunctional relationship of (a):
can be further simplified to:
l＝k1ΔTt+b1(14)
in the formula
k1、b1Are all constants.
From equation (14), it can be seen that the temperature change of the matrix suction sensor is a function of the saturation of the sensor's clay rod, as shown in FIG. 7.
And then, according to the calibration relation between the saturation of the argil rod and the substrate suction (namely, the soil-water characteristic curve of the argil rod, see the embodiment 1), the purpose of monitoring the soil substrate suction is achieved, and the soil-water characteristic curve of the soil to be detected can be obtained by combining the traditional soil saturation monitoring method, which is shown in a figure 8.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations are also included in the protection scope of the present invention.
Claims (10)
1. A soil matrix suction quasi-distributed in-situ measurement method based on fiber bragg grating pulse heating comprises the following steps:
the method comprises the following steps that firstly, a completely manufactured matrix suction sensor is fully saturated in water and then implanted into a soil body to be detected, wherein the matrix suction sensor comprises a pottery clay rod body (with a soil-water characteristic curve calibrated), an optical fiber, a heating resistance wire and an optical fiber grating;
step two, standing the sensor in the soil for 24 hours to balance the water potential of the soil body and the argil rod;
step three, the substrate suction sensor starts to heat under the action of the current of the heating resistance wire; stopping heating after the density of the diffused heat flow is constant, and starting cooling the rod body;
step four, collecting and recording a heating time interval [ t ] by a fiber grating demodulator1,t2]Wavelength reading of the internal fiber grating, t1As the heat pulse start time, t2The time for starting to cool the bar body is the time;
converting the wavelength data into bar body temperature information by using calculation software through a wavelength and temperature conversion relation formula when the optical fiber leaves a factory; calculating the temperature change value of the rod body, and according to the empirical relationship between the temperature change value of the pottery clay rod and the water saturation: k ═ k1ΔTt+b1Wherein l is the water saturation of the pottery clay rod, Delta TtValue of temperature change, k, measured for a substrate suction sensor1、b1The water saturation and the temperature change value of a plurality of groups of argil rods are determined by rating tests; the temperature change value of the substrate suction sensor is a fiber bragg grating pulse heating time interval [ t ]1,t2]The difference between the temperature value after the heat pulse heating and the initial temperature is measured; the soil matrix suction force is obtained by combining the relation between the temperature change value and the water saturation of the argil rod and the soil-water characteristic curve of the argil rod.
2. The fiber bragg grating pulse heating-based soil matrix suction quasi-distributed in-situ measurement method according to claim 1, wherein in the first step, a matrix suction sensor is directly inserted or pre-embedded into a soil body to be monitored; and for the same argil rod, configuring different water-containing states of the argil rod by controlling the water content, and calibrating 16-20 groups of matrix suction of the argil rod under different saturation degrees by combining a filter paper suction test method to draw a soil-water characteristic curve of the argil rod.
3. The fiber bragg grating pulse heating-based soil matrix suction quasi-distributed in-situ measurement method according to claim 1, wherein in the second step, the matrix suction sensor is constant in heating power and is a current square wave, so that rapid heating of the sensor is guaranteed; the time interval in the fourth step is 5-10 seconds.
4. The fiber grating-based soil matrix suction quasi-distributed in-situ measurement method according to claim 1, wherein the time interval [ t ] in step three1,t2]Defined as the heat pulse time interval.
5. The fiber grating-based soil matrix suction quasi-distributed in-situ measurement method according to claim 1, wherein the moisture content in the argil rod of the sensor is measured by a fiber grating pulse heating method, and the matrix suction of the measured soil is obtained under the condition of water potential balance by calibrating the established relation between the matrix suction and the moisture content of the argil rod; the soil-water characteristic curve of the argil material is determined by combining a filter paper suction test method according to different water-containing states of the argil material.
6. The device for the fiber bragg grating soil matrix suction quasi-distributed in-situ measurement method is characterized by comprising a heating power supply, a matrix suction sensor, a fiber bragg grating demodulator and an analysis processing monitoring data device; the matrix suction sensor main body is composed of a pottery clay rod and a high-heat-sensitive insulating tube arranged in the pottery clay rod, wherein a resistance wire and an optical fiber grating are arranged in the high-heat-sensitive insulating tube and are reinforced by organic materials with thermal expansion coefficients close to that of the pottery clay rod, the heating resistance wire and an optical fiber penetrate through the high-heat-sensitive insulating tube, the resistance wire is connected with a heating power supply through a power-on lead, a plurality of optical fiber gratings are carved on the optical fiber, and the optical fiber gratings are connected with an optical fiber grating demodulator through optical fiber leads and are used for collecting and recording wavelength readings after heating tends to be stable; the analysis processing monitoring data device is connected with the fiber grating demodulator, converts wavelength data into temperature information of the high thermosensitive insulating tube body through the conversion relation between wavelength and temperature, and calculates the internal temperature change value.
7. The device of claim 6, wherein the tube body of the high heat conducting tube inside the substrate suction sensor is made of a high heat-sensitive insulating non-metallic material, and has a thermal conductivity of 1.13-1.20W/(m-K).
8. The apparatus of claim 6, wherein the substrate suction sensor is a sensitivity-enhanced package structure, and comprises two semi-cylinders with a diameter of R, a circular small groove with a diameter of R is formed in the middle of the cross section of one of the semi-cylinders, the optical fiber is laid in the small groove, and the two ends of the optical fiber are in a natural loose state and are in an unstressed state; injecting non-solidified heat-conducting paste into the small groove; and fixing and packaging the two semi-cylinders by using epoxy glue, and arranging a clamp outside the rod body at intervals of D intervals for fixing the rod body.
9. The device for the fiber bragg grating soil mass matrix suction quasi-distributed in-situ measurement method according to claim 6, wherein the matrix suction sensors can be used individually, in series or in parallel; the adjacent substrate suction sensors are connected with the nuts through screw ports at two ends of the adjacent substrate suction sensors.
10. The device of claim 6, wherein the fiber armored protective sleeve is arranged outside the external fiber lead of the matrix suction sensor and connected with the fiber bragg grating along the outer wall of the sensor.
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