Induction type soil matrix suction measuring device and method based on fiber bragg grating

Abstract

The invention discloses an induction type soil matrix suction measuring device and method based on a fiber bragg grating. The device mainly comprises a shell protection module, a signal excitation module, a contact balance module, a wavelength detection module, an electromagnetic induction module and a signal processing module. Compared with an existing measuring device, the device is easy and convenient to operate and accurate in control, high-precision, continuous, rapid and quasi-distributed measurement of the matrix suction of the soil body can be achieved, and therefore changes of the matrix suction along with external climate conditions such as rainfall, evaporation and the underground water level are effectively monitored. The device has very important significance in the aspects of judging the engineering property of the unsaturated soil and researching the property of the unsaturated soil.

Description

Technical Field

[0001] The present invention relates to the technical field of soil matrix suction measurement, and more specifically, to an inductive soil matrix suction measurement device and method based on fiber Bragg grating. Background Art

[0002] Unsaturated soil is a type of soil that contains pore water and pore gas. The presence of pore gas makes the properties of unsaturated soil more complicated. The water-gas interface (shrinkage film) has surface tension, and the presence of the gas phase significantly affects the mechanical properties, permeability and thermal conductivity of unsaturated soil. Matrix suction is an important parameter for evaluating unsaturated soil, and has an important influence on its strength, permeability and thermal conductivity. When the pore gas pressure is greater than the pore water pressure, the shrinkage film is subjected to this pressure difference, and this difference is the matrix suction. Matrix suction is an important indicator for describing the mechanical properties of unsaturated soil, and the soil-water characteristic curve (SWCC) is a relationship curve between matrix suction and soil moisture content, and is an important tool for studying unsaturated soil. Therefore, accurate measurement of soil matrix suction is of great significance for theoretical research and engineering practice.

[0003] Accurate measurement of matrix suction is crucial for a deep understanding of the mechanical behavior of unsaturated soils. It is a basic requirement for revealing the mechanical mechanism of unsaturated soils. The relationship between matrix suction and soil water content is the core of the mechanical properties of unsaturated soils. This relationship is defined as the soil-water characteristic curve (SWCC), which is a key function for analyzing and interpreting the properties of unsaturated soils. At present, the main methods for measuring matrix suction include: tensiometer method, thermal conductivity sensor method, dielectric constant method, axis translation method, dialysis method, contact filter paper method, salt solution gas phase method and high-speed centrifuge method. These methods have their own advantages and disadvantages, and are suitable for specific research needs and soil types, providing a variety of technical means for the study of unsaturated soil mechanics.

[0004] The tensiometer method can accurately measure the in-situ matrix suction of soil. It uses a high air intake ceramic plate as a measurement system to directly measure the negative pore water pressure in the soil. However, the water in the tensiometer may cavitate and evaporate through the ceramic head, and the range is limited, so it is only suitable for measuring small-scale matrix suction. The traditional thermal sensor method has problems such as slow reading, difficulty in disassembling and replacing parts, and easy aging after long-term use. There is often a lag during calibration, low accuracy, and it cannot be monitored for a long time. The dielectric constant method is greatly affected by the salinity of the soil. Soils with high salinity cannot be tested, cannot be used in long-distance monitoring, and have a small measurement range. The axis translation technology is to place the unsaturated soil sample in a closed pressure chamber and use a saturated high air intake ceramic needle to measure the negative pore water pressure. However, the equipment is large in size and has high requirements for the test environment. Bubble accumulation will occur during use. The air pressure of the pressure plate needs to be manually adjusted. The control accuracy is low and the measurement process is cumbersome. It is mainly used for indoor measurements. The dialysis method is suitable for low-suction parts, but the operation is cumbersome, easy to contaminate, and the measurement time is long. The contact filter paper technique, salt solution gas phase method and high-speed centrifuge method have high requirements for operation technology and poor universality. When the salt solution gas phase method measures the matrix suction below 1500kPa, the accuracy is low and the measurement range is small. When the high-speed centrifuge method is used for measurement, the possibility of water in the soil sample being completely discharged from the soil is low, the equipment is expensive, and the damage to the soil is large. By optimizing existing methods or developing new technologies, the matrix suction of unsaturated soil can be measured more accurately, providing more reliable data support for theoretical research and engineering applications.

[0005] In recent years, some new soil matrix suction measurement methods have emerged. For example, in the invention patent "A quasi-distributed in-situ measurement method and device for soil matrix suction based on fiber Bragg grating" (publication number: ZL201911104712.7) by Zhu Honghu et al., the device is based on the principle of active heating to measure water content, but there is a disadvantage that heating damages the soil structure, especially for special soils such as loess, laterite and expansive soil. The soil structure determines the water holding state of the soil, so it is impossible to monitor the soil samples quickly and accurately, and the resolution and accuracy are low; in the invention patent "A multifunctional in-situ matrix suction measuring instrument for unsaturated soil" (publication number: CN114034733 A) by Yu Ziwang et al., the device measures the thermal diffusivity based on heating the soil, and substitutes it into the thermal conductivity-matrix suction equation to obtain the matrix suction. The measurement method is relatively convenient, but heating the soil will damage the soil structure, so the matrix suction cannot be accurately measured, and it is greatly affected by the environment; in the invention patent "A high-frequency electric field charge induction soil moisture sensor" (publication number: CN202111265407.3) by Guo Huijun et al., the operation is relatively simple, but the technical solution can only measure at a single point, and cannot achieve quasi-distributed measurement, and cannot effectively shield the influence of external electric and magnetic fields on the measurement results, and the measurement accuracy is poor. It can be seen that the current matrix suction measurement device is not perfect and needs further exploration.

[0006] As we all know, fiber Bragg grating measurement technology has been widely used in many measurement fields due to its unique advantages. This technology is not affected by factors such as light source fluctuations, fiber bending loss, connection loss and detector aging, and has strong anti-magnetic field interference capabilities. In addition, fiber Bragg grating sensors have the characteristics of high sensitivity, small size, corrosion resistance, resistance to electromagnetic radiation interference and good long-term stability, which make them very suitable for long-term monitoring. By connecting to a computer (1-1), fiber Bragg grating measurement technology can build a networking system to achieve real-time and quasi-distributed measurement of the target. If fiber Bragg grating technology is applied to the measurement of matrix suction, its superior performance will help improve the accuracy and reliability of the measurement. By adopting a special structural design, the fiber Bragg grating sensor can be further optimized to meet the measurement needs of different types of soils and realize the accurate measurement of matrix suction of all soil types. Such applications not only expand the application scope of fiber Bragg grating technology, but also provide new solutions for the research and engineering monitoring of unsaturated soil mechanics. Summary of the invention

[0007] In order to solve the above problems, the present invention provides an inductive soil matrix suction measurement device and method based on fiber Bragg grating, which solves the problems existing in the prior art.

[0008] The technical solution and steps adopted by the present invention are as follows:

[0009] An inductive soil matrix suction measurement device based on fiber Bragg grating includes a shell protection module, a signal excitation module, a contact balance module, an electromagnetic induction module, a wavelength detection module and a signal processing module;

[0010] A contact balancing module is fixed in the shell protection module, and the contact balancing module includes a clay cylinder, a bentonite layer and a plastic cylinder arranged in sequence from the outside to the inside;

[0011] The signal excitation module includes a signal source, which includes a high-frequency signal source and a low-frequency signal source. The high-frequency signal source is electrically connected to one end of a plurality of groups of transmitting probes through a transmission cable, and each group of transmitting probes includes two transmitting probes; a sensing probe is provided to match the transmitting probe; one end of the sensing probe is electrically connected to the electromagnetic induction module; the other end of the sensing probe and the other end of the transmitting probe pass through a plastic tube and are inserted into the bentonite layer; the two sensing probes are located inside the two transmitting probes;

[0012] The electromagnetic induction module comprises an induction coil and an arc-shaped electrode plate on the same side. Induction electrode plates arranged orthogonally to the induction coil are installed on both sides of the induction coil. The induction electrode plates are respectively electrically connected to the induction probes; the two ends of the induction coil are respectively electrically connected to the two ends of the conductive elastic element to form a closed loop;

[0013] The wavelength detection module includes a strain grating and a temperature compensation grating; the strain grating is pasted and fixed to the elastic element, one end of the strain grating is connected to the signal processing module through an optical fiber line, and the other end is connected to the temperature compensation grating; one end of the arc electrode plate on the same side is electrically connected to the signal processing module, and the other end is electrically connected to the low-frequency signal source; the arc electrode plate on the same side is located in the bentonite layer; the wavelength detection module and the electromagnetic induction module are communicatively connected to the signal processing module.

[0014] A further improvement is that the housing protection module comprises an external fixing frame, and the top and bottom of the external fixing frame are respectively provided with an annular top packaging cover and an annular tail packaging cover; a contact balancing module is installed between the top packaging cover and the tail packaging cover; the top packaging cover and the tail packaging cover are mutually connected and fixed by studs; the top packaging cover and the tail packaging cover of the external fixing frame adopt concentric annular packaging covers with grooves, and a grooved sealing sticker is adopted at the lower part of the plastic protective cover between the bentonite layer and the internal device to prevent the pore water of the bentonite layer from flowing into the internal device;

[0015] The clay tube of the contact balance module is in direct contact with the soil to be tested, and the interior of the clay tube is filled with a cylindrical bentonite layer; the signal source of the signal excitation module outputs a high-frequency signal to the sensing probe inserted into the bentonite layer, so that the sensing probe generates an induced current; the sensing probe transmits the induced current to the sensing electrode plate in the electromagnetic induction module through a transmission cable; the change in the electric field causes the sensing electrode plate to undergo electromagnetic induction, thereby realizing electrical signal conversion; the elastic element inside the electromagnetic induction module is deformed under the action of the induced current; the deformation of the elastic element causes the strain grating adhered to the surface of the elastic element to deform at the same time; the wavelength detection module monitors the corresponding wavelength change by detecting the grating pitch change of the strain grating and the temperature compensation grating on the same optical fiber line.

[0016] As a further improvement, the capacitance meter in the signal processing module is connected to the arc electrode plate of the signal excitation module to capture capacitance signals and transmit the signal data to a computer for processing via a transmission cable; one end of the arc electrode plate on the same side directly touches the bentonite layer, while the other end is connected to the signal source via a transmission cable. The signal source transmits a low-frequency signal to the driving electrode on the arc electrode plate on the same side, and the capacitance meter electrically connected to the arc electrode plate on the same side detects the change in the electric field between the driving electrode and the sensing electrode, and measures the capacitance value of the arc electrode plate on the same side accordingly, and then inversely calculates the relative dielectric constant of the bentonite layer from the measured capacitance value.

[0017] As a further improvement, the high-frequency signal source is electrically connected to three groups of transmitting probes through a transmission cable, and the three groups of transmitting probes are evenly distributed along the circumference of the plastic barrel; the arc-shaped electrode plates on the same side are three groups and are evenly distributed along the circumference of the outer side of the plastic barrel; an insulating baffle is provided between the arc-shaped electrode plates on the same side and the transmitting probes; the clay barrel is closely attached to the bentonite layer, and the insulating baffle is located inside the bentonite layer to isolate the magnetic field interference between the probes;

[0018] The strain grating and temperature compensation grating are obtained by engraving gratings on traditional optical fibers; the strain grating and temperature compensation grating are both engraved on the same optical fiber; the temperature compensation grating is mainly used to eliminate the wavelength error caused by the heating of the elastic element, while the strain grating is mainly used to establish the relationship between the deformation of the elastic element and the wavelength change.

[0019] As a further improvement, the arc-shaped electrode plate on the same side is fan-shaped and includes an insulating substrate on which two sensing electrodes are fixed with a gap between them; shielding electrodes are fixed to the bottom of the insulating substrate and to the outer ends of the two sensing electrodes.

[0020] As a further improvement, the groove areas of the top packaging cover and the tail packaging cover are, from outside to inside, a clay tube groove area, a bentonite groove area, and an internal space groove area. A plastic annular baffle is inserted into the annular groove in the transition area between the internal space groove area and the bentonite groove area to block the internal detection device and the external bentonite.

[0021] As a further improvement, the internal detection device adopts the method of laying internal tracks to fix the lines.

[0022] A method for using an inductive soil matrix suction measuring device based on a fiber Bragg grating, using the above-mentioned inductive soil matrix suction measuring device based on a fiber Bragg grating, specifically includes the following using steps:

[0023] Step S1, device assembly:

[0024] The shell protection module, signal excitation module, electromagnetic induction module, contact balance module, wavelength detection module and signal processing module are assembled to form an inductive soil matrix suction measurement device, and the tail packaging cover is connected by nuts and studs, and then the clay cylinder and the internal plastic protective cover are inserted into the ring of the tail packaging cover in sequence, and before the plastic protective cover is inserted, the groove-type sealing sticker is used to seal the notches of all the probes extending outward, and then the bentonite layer area is filled with bentonite, and then the top packaging cover is connected by nuts and studs, and finally the inductive probe and the transmitting probe are extended;

[0025] Step S2: Device embedding:

[0026] Arrange the external fixing frame at the outermost periphery of the clay tube. After the device is assembled and connected, determine the buried position according to the monitoring requirements, mark the corresponding measuring points or marks at the pre-buried measuring point positions, and use a drilling rig to drill holes to the predetermined depth. Take out the in-situ soil at the corresponding depths, insert the sleeve into the borehole to stabilize the hole wall, and then backfill the in-situ soil into the original borehole according to the corresponding compaction degree, pull out the sleeve, and connect each line to build the entire optical fiber monitoring system;

[0027] Step S3, device measurement:

[0028] The high-frequency signal source is started to transmit the high-frequency alternating current to the transmitting probe. The inductive probe generates an induced current after being affected by the alternating current of the transmitting probe. The induced current causes the induced charge to accumulate on the inductive electrode plate, thereby generating an induced current in the internal loop of the induction coil. The induced current causes the elastic element to heat up and deform. The strain grating adhered to the elastic element is deformed at the same time. The deformation of the strain grating causes its grating pitch to change, thereby causing the change of the wavelength of the internal optical signal, and finally generating data of wavelength change; the signal source transmits a low-frequency signal to the driving electrode of the arc electrode plate on the same side, and the capacitance meter electrically connected to the arc electrode plate on the same side measures the capacitance of the arc electrode plate on the same side, and transmits the capacitance signal to the computer; the signal demodulator is connected to the strain grating to collect the wavelength signals of the strain grating and the temperature compensation grating, and transmits the wavelength signal to the computer using a transmission cable;

[0029] Step S4: Data collection:

[0030] The capacitance meter electrically connected to the arc electrode plate on the same side continuously detects the change of the electric field between the driving electrode and the sensing electrode in the power-on time interval [t1, t2], and measures the capacitance value of the arc electrode plate on the same side accordingly;

[0031] The signal demodulator collects and records the wavelength variation data of the strain grating light signal and the wavelength variation data of the temperature compensation grating in the power-on time interval [t1, t2], respectively represented by Δλ total and Δλ T express;

[0032] The computer records and stores the wavelength variation data and capacitance data;

[0033] Step S5: Data processing:

[0034] Substituting the obtained capacitance data into equation (4), the relative dielectric constant ε of bentonite is obtained: r :

[0035]

[0036] Where, C is the capacitance of the arc electrode plate on the same side; is the arc angle corresponding to the arc electrode plate on the same side; ε0 represents the vacuum dielectric constant; a and b represent the distance between the two substrates of the arc electrode plate on the same side and the length of the substrate itself, respectively; L is the length of the arc electrode plate on the same side, and the inner arc length and the outer arc length of the arc electrode plate on the same side are equal; by the formula ΔT=Δλ T / K T Get the temperature change ΔT, and then use the formula Δε=(Δλ total -Δλ T ) / K ε The strain change of the elastic element after eliminating the temperature effect Δε; where K TIndicates the temperature sensitivity coefficient of the temperature compensation grating; K ε represents the strain sensitivity coefficient of the strain grating;

[0037] The dielectric constant ε of bentonite r The wavelength variation data collected in step S4 is substituted into the relationship (25) between the moisture content of bentonite θ and the wavelength drift of the strain grating and the temperature compensation grating to calculate the moisture content data of bentonite:

[0038]

[0039] Among them, B1, B2, B3, B4 and B5 are adjustment coefficients, expressed as follows:

[0040]

[0041]

[0042] In the formula, ε w Represents the dielectric constant of pore water, which can be obtained by looking up the table based on the temperature; ε a Indicates the dielectric constant of pore gas, which is 1.00053F / m; ε s represents the dielectric constant of soil particles, which is 5F / m; ω represents the angular frequency; E e is the elastic modulus of the elastic element; A e is the cross-sectional area of ​​the elastic element; l is the probe length, the transmitting probe and the sensing probe have the same length; l e is the length of the elastic element; μ0 represents the magnetic permeability; r is the area of ​​the induction coil; R e is the resistance of the elastic element; M is the signal amplitude; ε0 is the dielectric constant in vacuum, which is 8.854187817×10-12F / m; t1 and t2 are the initial and end times of power-on, respectively; Δλ total Indicates the wavelength drift of the strain grating; Δλ T Indicates the wavelength drift of the temperature compensation grating;

[0043] When the bentonite and the soil to be tested reach water potential equilibrium, the matrix suction of the bentonite is equal to the matrix suction of the soil to be tested. Then, the calculated bentonite water content θ and the collected temperature data T, T = ΔT + T0, T0 represents the initial temperature of the measurement, are substituted into the relationship model between the bentonite matrix suction and water content obtained from the calibration test, and finally the matrix suction value of the soil to be tested is obtained:

[0044]

[0045] Where, ψ is the matrix suction of bentonite; θ is the water content of bentonite; γ w is the density of water, take 10kN / m 3 ; a0, β, n, θs and θ r is the experimental data fitting parameter; T is the temperature, which is calculated from the wavelength data reflected by the internal temperature compensation grating measurement.

[0046] As a further improvement, the step S1 is specifically as follows:

[0047] Step S1.1, using a transmission cable to connect the high-frequency signal source and the low-frequency signal source to the high-frequency signal source interface and the low-frequency signal source interface of the external fixing frame respectively;

[0048] Step S1.2, connecting the signal demodulator to the fiber grating interface of the external fixing frame by using an optical fiber lead;

[0049] Step S1.3, connecting the capacitance meter to the arc electrode plate on the same side with a transmission cable, and connecting the signal demodulator and the capacitance meter to a computer;

[0050] Step S1.4: if multiple assembled soil matrix suction measuring devices are connected in series or parallel for multi-point measurement, repeat steps S1.1 to S1.3 to connect each device in sequence.

[0051] As a further improvement, the step S2 is specifically as follows:

[0052] Step S2.1, marking corresponding measuring points or marks at the pre-buried measuring point positions, drilling holes to a predetermined depth using a drilling rig, taking out the in-situ soil at the corresponding depths, and inserting sleeves into the boreholes to stabilize the hole walls;

[0053] Step S2.2: Divide the obtained in-situ soil into three parts, use the quartering method to take representative soil samples, and test the density ρ and specific gravity G of each in-situ soil according to the "Standard for Geotechnical Test Methods". s and the dielectric constant of soil particles ε s , and take the average value of three in-situ soil bodies as soil layer parameters, and convert the in-situ soil density according to the relationship between the three-phase indicators of soil mechanics;

[0054] Step S2.3, insert the assembled device into the borehole to a predetermined depth, lead the transmission wire out of the borehole, backfill the borehole with in-situ soil of the same density, and make the soil matrix suction measurement device in close contact with the surrounding soil to establish an effective hydraulic connection;

[0055] Step S2.4: if the soil matrix suction of a single section or multiple sections is measured, repeat steps S2.1 to S2.3, and number the soil matrix suction measuring devices at different positions in sequence.

[0056] As a further improvement, the step S3 is specifically as follows:

[0057] Step S3.1, turn on the low-frequency signal source switch and the capacitance meter switch, and debug the arc electrode plate on the same side until the capacitance meter of the signal processing module can effectively record the capacitance signal, ensuring that the module is well connected;

[0058] Step S3.2, turn off the low-frequency signal source switch, turn on the high-frequency signal source switch, and debug the signal source and signal demodulator until the wavelength signal can be effectively recorded;

[0059] Step S3.3, turn on the high and low frequency signal source switches at the same time, so that the fiber Bragg grating-based inductive soil matrix suction measurement system works for a period of time, until the computer can generate wavelength signal data and capacitance signal data within a certain period of time, and record and save the data.

[0060] As a further improvement, the step S5 is specifically as follows:

[0061] Step S5.1, substitute the received capacitance data into equation (4) to obtain the relative dielectric constant ε of bentonite r : The wavelength change Δλ through the temperature compensation grating T The internal temperature change ΔT is calculated inversely; the wavelength change Δλ of the strain grating is calculated inversely. total Subtract Δλ from T , the net wavelength change after deducting the temperature effect is obtained, and the net wavelength change is input into the relationship model between soil moisture content θ and wavelength drift Δλ, that is, formula (25), and analyzed and calculated to finally obtain the moisture content data of bentonite;

[0062] Step S5.2, the obtained bentonite moisture content θ data and the collected temperature data T=ΔT+T0, T0 represents the initial measurement temperature, are substituted into the ψ-θ model of the bentonite obtained from the calibration experiment [i.e., formula (31)], and finally the matrix suction value of the bentonite layer is obtained, which is the matrix suction of the soil to be tested.

[0063] Beneficial effects of the present invention:

[0064] 1. The temperature-corrected bentonite soil-water characteristic curve model is used to calculate the matrix suction, which reduces the influence of external temperature on the results and makes the results more accurate.

[0065] 2. The fiber grating technology is used to measure the matrix suction, which has more accurate measurement effect, better anti-magnetic properties, fast response speed, long service life, etc., which is convenient for long-term measurement.

[0066] 3. The system is easy to install, can be installed and laid out on site, and can measure soil at different depths with a wide measurement range.

[0067] 4. The device is a detachable device, which can separate the external device and the internal detection device to facilitate subsequent maintenance and calibration of the device.

[0068] 5. Temperature-compensated gratings are used on optical fiber lines to eliminate wavelength errors caused by heat from elastic elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 It is a system diagram of an inductive soil matrix suction measurement device based on fiber Bragg grating;

[0070] Figure 2 It is a diagram of an inductive soil matrix suction measurement device based on fiber Bragg grating;

[0071] Figure 3 It is a cross-sectional view of an inductive soil matrix suction measurement device based on fiber Bragg grating;

[0072] Figure 4 It is a diagram of the groove connection part of an inductive soil matrix suction measurement device based on fiber Bragg grating;

[0073] Figure 5 It is a cross-sectional view of the tail groove of an inductive soil matrix suction measurement device based on fiber Bragg grating.

[0074] Figure 6 It is a fixed rod diagram of an inductive soil matrix suction measurement device based on fiber Bragg grating;

[0075] Figure 7 It is a front view of the same-side arc electrode plate of an inductive soil matrix suction measurement device based on fiber Bragg grating;

[0076] Figure 8 It is a top view of the same-side arc electrode plates of an inductive soil matrix suction measurement device based on fiber Bragg grating;

[0077] Fig. 9 It is a matrix suction measurement module diagram of an inductive soil matrix suction measurement device based on fiber Bragg grating;

[0078] Fig.10 It is a cross-sectional view of a wavelength detection module of an inductive soil matrix suction measurement device based on a fiber Bragg grating;

[0079] Fig.11 It is a graph of matrix suction at different temperatures in the calibration test of an inductive soil matrix suction measurement device based on fiber Bragg grating.

[0080] Fig.12 This is a diagram of the probe extension device of an inductive soil matrix suction measurement device based on fiber Bragg grating.

[0081] In the figure: 1-1 computer, 1-2 capacitance meter, 1-3 signal source, 1-4 signal demodulator, 1-5 soil matrix suction measuring device, 1-6 sleeve, 2-1 stud mouth, 2-4 probe telescopic button, 3-1 sealing sticker, 3-2 annular groove, 3-3 plastic annular baffle, 3-4 internal space groove area, 3-5 bentonite groove area, 3-6 clay tube groove area, 4-1 nut, 4-2 gasket, 4-3 stud, 5-1 shielding electrode, 5-2 driving electrode, 5-3 insulating substrate, 5-4 sensing electrode, 8-3 transmission cable, 6-6 internal track, 6-8 internal detection device, 7-2 induction electrode plate, 7-3 optical fiber line, 7-4 elastic element, 7-5 strain grating, 7-6 temperature compensation grating, 8-1 fixing bolt, 8-2 induction coil, 8-3 transmission cable, 9-1 external fixing frame, 9-2 transmitting probe, 9-3 induction probe, 9-5 top optical fiber interface, 9-6 top packaging cover, 9-7 tail packaging cover, 10-1 clay cylinder layer, 10-2 bentonite layer, 10-3 arc-shaped electrode plate on the same side, 10-4 insulating baffle. DETAILED DESCRIPTION

[0082] like Figure 1 As shown, a system composed of an inductive soil matrix suction measuring device based on a fiber Bragg grating includes a computer 1-1 for analyzing data, a capacitance meter 1-2, a signal source 1-3, a signal conditioner 1-4, and a soil matrix suction measuring device 1-5. The signal source provides a high-frequency signal source 1-3 and a low-frequency signal source 1-3, and a high-frequency alternating current signal is passed to the outer probe 9-2 to generate an induced current in the middle probe 9-3 and to provide a low-frequency current to the driving electrode 5-2 of the arc electrode plate 10-3 on the same side of the arc for measuring the relative dielectric constant of the soil; the signal demodulator 1-4 is used to provide a band for the fiber Bragg grating and receive the changed wavelength; after receiving the data from the signal demodulator 1-4, the capacitance meter 1-2 and the computer 1-1, the computer 1-1 uses the computer 1-1 to convert the wavelength data into the corresponding soil matrix suction value.

[0083] refer to Figure 2 An inductive soil matrix suction measuring device based on fiber Bragg grating includes, from inside to outside, an optical fiber line 7-3, a transmission cable 8-3, an inductive probe 9-3, an arc-shaped same-side arc-shaped electrode plate 10-3, a bentonite layer 10-2, a clay tube 10-1, and an external fixing frame 9-1.

[0084] like Figure 1 The signal demodulator 1-4 reflects the water content information by detecting the wavelength signal reflected back by the optical fiber line 7-3. In this experiment, the signal demodulator 1-4 is used to demodulate the optical fiber Bragg grating and collect wavelength data.

[0085] Figure 3The figure shows a 3 / 4 cross-sectional view of the measuring device, where the induction probe 9-3 and the arc-shaped electrode plate 10-3 on the same side are buried in the bentonite layer 10-2, mainly used to detect the water content and relative dielectric constant of the bentonite layer 10-2. The outside of the bentonite layer 10-2 is a clay cylinder 10-1, which mainly blocks foreign gas and establishes hydraulic connection between the bentonite layer 10-2 in the device and the external soil layer to be measured.

[0086] Figure 4 The top view of the package cover of the device housing protection module is shown, and the inner side has three concentric grooves of different radii, into which the clay cylinder 10-1, the bentonite layer 10-2 and the internal detection device 6-8 are inserted from the outside to the inside. The outer side of the package cover has 6 circular holes for placing studs to fix the internal detection device 6-8.

[0087] Figure 5 The figure shows a cross-sectional view of the packaging cover of the device housing protection module. A small groove is engraved at the bottom of the groove of the bentonite layer, and a sealing sticker is pasted on the surface of the small groove, which is mainly used to prevent the pore water inside the bentonite layer 10-3 from flowing to the internal detection device 6-8 when the water content of the bentonite layer 10-3 changes.

[0088] Figure 6 The fixing rod of the matrix suction measuring device is mainly composed of a stud 4-3, a nut 4-1 and a gasket 4-2, and is mainly used to connect the packaging cover and fix the internal device.

[0089] Figure 7 and Figure 8 They are respectively a front view and a top view of the arc electrode plate 10-3 on the same side of the matrix suction measurement device; it is composed of a driving electrode 5-2, a sensing electrode 5-4, a shielding electrode 5-1 and an insulating bottom plate 5-3. By passing a low-frequency current through the driving electrode 5-2, an electric field is formed above the arc electrode plate 10-3 on the same side. The change in the relative dielectric constant of the bentonite layer 10-2 will cause a change in the electric field, thereby realizing the measurement of the relative dielectric constant of the bentonite layer 10-2.

[0090] Fig. 9 and Fig.10 They are respectively a matrix suction measurement module diagram and a partial diagram of a matrix suction measurement device; it is composed of an induction electrode plate 5-4, an induction coil 8-2, an elastic element 7-4, and a strain grating 7-5 and a temperature compensation grating 7-6 in a wavelength detection module; it mainly performs tasks such as receiving current, converting current, and energy conversion. The conversion of current causes the strain grating 7-5 adhered to the elastic element 7-4 to change its grating pitch, thereby causing a change in its corresponding wavelength.

[0091] Fig.11This is a graph showing the relationship between matrix suction and bentonite moisture content at different temperatures in the calibration test of the matrix suction measuring device. This graph is a calibration experimental test data curve fitted by the origin software, which is used to calibrate the matrix suction values ​​at different temperatures.

[0092] Fig.12 The diagram of the probe extension device of the matrix suction measuring device; the probe is fixed on the arc plate, and the arc plate is vertically rotated and hinged on the push-pull rod. During installation, a pad is placed in the plastic barrel, and the height of the pad is consistent with the position of the through hole on the plastic barrel for the probe to pass through. Then the probe structure is lowered, and the probe is pressed down by the push-pull rod to gradually expand outward, and then enters the bentonite through the through hole inserted into the plastic barrel.

[0093] The initial temperature T0 needs to be given during the temperature-compensated fiber Bragg grating calculation process of the device, and the initial temperature of the device is set with reference to the reference temperature.

[0094] The clay tube of the contact balance module is in direct contact with the soil to be tested, and the interior of the clay tube is filled with a cylindrical bentonite layer; the signal source of the signal excitation module outputs a high-frequency signal to the sensing probe inserted into the bentonite layer, so that the sensing probe generates an induced current; the sensing probe transmits the induced current to the sensing electrode plate in the electromagnetic induction module through a transmission cable; the change in the electric field causes the sensing electrode plate to undergo electromagnetic induction, thereby realizing electrical signal conversion; the elastic element inside the electromagnetic induction module is deformed under the action of the induced current; the deformation of the elastic element causes the strain grating adhered to the surface of the elastic element to deform at the same time; the wavelength detection module monitors the corresponding wavelength change by detecting the grating pitch change of the strain grating and the temperature compensation grating on the same optical fiber line.

[0095] As a further improvement, the groove areas of the top packaging cover and the tail packaging cover are, from outside to inside, a clay tube groove area, a bentonite groove area, and an internal space groove area. A plastic annular baffle is inserted into the annular groove in the transition area between the internal space groove area and the bentonite groove area to block the internal detection device and the external bentonite.

[0096] As a further improvement, the internal detection device adopts the method of laying internal tracks to fix the lines.

[0097] A method for using an inductive soil matrix suction measurement device based on a fiber Bragg grating comprises the following steps:

[0098] Step P1, use the stud 4-3 and the nut 4-1 to connect the top packaging cover and the tail packaging cover, and then insert the clay tube 10-1 and the plastic annular baffle 3-3 in sequence at the ring of the tail packaging cover. Before inserting the plastic tube, use the sealing sticker 3-1 to seal all the notches of the probe protrusion.

[0099] Step P2: Add bentonite 10-2 with a certain density configured in the calibration experiment to the filling layer between the clay tube 10-1 and the plastic tube. When the bentonite 10-2 is added to the slot position, stop adding. Start shaking the device to make the bentonite 10-2 inside full.

[0100] Step P3, bury the 10-3 arc-shaped same-side arc-shaped electrode plate 10-3 therein, and then continue to add the remaining 10-2 bentonite to the filling layer. After filling, shake the device again to enrich the internal soil.

[0101] Step P4, load the internal detection device 6-8 into the plastic annular baffle 3-3. After reaching the depth of the extended slot, the buckle is rotated inward to fix the internal detection device 6-8 inside the plastic annular baffle 3-3, and the position probe is just aligned with the extended slot.

[0102] Step P5: After the fixation is completed, the lines required to be connected to the internal device are connected to the top optical fiber interface 9-5 of the top packaging cover. Finally, the top packaging cover is fixed with the stud 4-3 and the nut 4-1, and the top probe retracting button 2-4 is pressed to extend the probe.

[0103] Step P6, device deployment: mark the corresponding measuring points or marks at the pre-buried measuring point positions, and use a drilling rig to drill holes to a predetermined depth, respectively take out the in-situ soil at the corresponding depths, insert sleeves 1-6 into them, and the dug holes must be larger than the size of the soil matrix suction measuring device;

[0104] Step P7: Divide the obtained in-situ soil into three parts, use the quartering method to take representative soil samples, and test the density r and specific gravity G of each in-situ soil according to the "Standard for Geotechnical Test Methods" s and the dielectric constant of soil particles e s , and take the average value of three in-situ soil bodies as soil layer parameters, and convert the in-situ soil density according to the relationship between the three-phase indicators of soil mechanics;

[0105] Step P8: After the device is buried, the borehole is backfilled with in-situ soil of the same density, and the soil matrix suction measuring device 1-5 is in close contact with the surrounding soil to establish an effective hydraulic connection. Each time the backfill reaches a certain height, a section of the sleeve 1-6 is pulled upward until the backfill is completed, and then the sleeve 1-6 is removed;

[0106] Step P9, construction of optical fiber measurement system: connect the optical fiber line 7-3 of the buried device to the signal demodulator 1-4, power supply and capacitance meter 1-2. The above device is connected to the computer 1-1 from the back end to form the entire optical fiber measurement system;

[0107] Step P10, device measurement: start the high-frequency signal source 1-3, transmit high-frequency alternating current to the transmitting probe 9-2, the inductive probe 9-3 is affected by the alternating current of the transmitting probe 9-2, and generates an induced current. The induced current causes the induced charge to accumulate on the inductive electrode plate 7-2, and then generates an induced current in the internal loop of the inductive coil 8-2. The induced current causes the elastic element 7-4 to heat up and deform, and the strain grating 7-5 adhered to the elastic element 7-4 is deformed at the same time. The deformation of the strain grating 7-5 causes its grating pitch to change, which in turn causes the change in the wavelength of the internal optical signal, and finally generates data on the wavelength change; the signal source transmits a low-frequency signal to the driving electrode 5-2 of the arc electrode plate 10-3 on the same side, and the capacitance meter electrically connected to the arc electrode plate on the same side 1-2 measures the capacitance of the arc electrode plate 10-3 on the same side, and transmits the capacitance signal to the computer 1-1; the signal demodulator 1-4 connects to the strain grating 7-5 to collect the wavelength signals of the strain grating 7-5 and the temperature compensation grating 7-6, and transmits the wavelength signal to the computer 1-1 using the transmission cable 8-3; the capacitance meter electrically connected to the arc electrode plate 10-3 on the same side continuously detects the electric field change between the driving electrode 5-2 and the sensing electrode 5-4 in the power-on time interval [t1, t2], and measures the capacitance value of the arc electrode plate 10-3 on the same side accordingly; the signal demodulator 1-4 collects and records the wavelength change data of the optical signal of the strain grating 7-5 and the wavelength change data of the temperature compensation grating 7-6 in the power-on time interval [t1, t2], respectively represented by Δλ total and Δλ T Indicates that the computer 1-1 records and stores wavelength data and capacitance data.

[0108] Step P11, data collection: Substitute the obtained capacitance data into formula (4) to obtain the relative dielectric constant ε of bentonite 10-2 r :

[0109]

[0110] Wherein, C is the capacitance of the arc electrode plate 10-3 on the same side; is the arc angle corresponding to the arc electrode plate 10-3 on the same side; ε0 represents the vacuum dielectric constant; a and b represent the distance between the two substrates of the arc electrode plate 10-3 on the same side and the length of the substrate itself; L is the length of the arc electrode plate 10-3 on the same side; by the formula ΔT=Δλ T / K T and Δε=(Δλ total -Δλ T ) / K ε The internal temperature change and the strain change of the elastic element that eliminates the temperature effect are obtained respectively. T Indicates the temperature sensitivity coefficient of the temperature compensation grating; K εrepresents the strain sensitivity coefficient of the strain grating; ΔT represents the temperature change, and Δε represents the strain change of the elastic element;

[0111] The wavelength variation data collected in step S4 and the dielectric constant ε of the bentonite obtained in the above step are combined. r Substitute the bentonite moisture content θ into the relationship between the strain grating and the wavelength drift of the temperature compensation grating (25), and then calculate the bentonite moisture content data:

[0112]

[0113] Among them, B1, B2, B3, B4 and B5 are adjustment coefficients, expressed as follows:

[0114]

[0115] In the formula, ε w Represents the dielectric constant of pore water, which can be obtained by looking up the table based on the temperature; ε a Indicates the dielectric constant of pore gas, which is 1.00053F / m; ε s represents the dielectric constant of soil particles, which is 5F / m; ω represents the angular frequency; E e is the elastic modulus of the elastic element; A e is the cross-sectional area of ​​the elastic element; l is the probe length, the transmitting probe and the sensing probe have the same length; l e is the length of the elastic element; μ0 represents the magnetic permeability; r is the area of ​​the induction coil; R e is the resistance of the elastic element; M is the signal amplitude; ε0 is the dielectric constant in vacuum, which is 8.854187817×10-12F / m; t1 and t2 are the initial and end times of power-on, respectively; Δλ total Indicates the wavelength drift of the strain grating; Δλ T Indicates the wavelength drift of the temperature compensation grating;

[0116] When the bentonite (10-2) and the soil to be tested reach water potential equilibrium, the matrix suction of the bentonite (10-2) is equal to the matrix suction of the soil to be tested. Then, the calculated water content θ of the bentonite (10-2) and the collected temperature data T (T = ΔT + T0, T0 represents the initial temperature of the measurement) are substituted into the relationship model between the matrix suction and water content of the bentonite obtained from the calibration test, and finally the matrix suction value of the soil to be tested is obtained:

[0117]

[0118] Where, ψ is the matrix suction of bentonite; θ is the water content of bentonite; γ w is the density of water, take 10kN / m 3 ; a0, β, n, θ s and θr are the experimental data fitting parameters, as shown in Table 2; T is the temperature, which is calculated from the wavelength data reflected by the internal temperature compensation grating measurement.

[0119] Table 2 Experimental data fitting parameters

[0120]

[0121] Note: T0 = 25°C.

[0122] For example, when conducting outdoor measurement experiments at different depths, during the above assembly steps, the independent devices are connected in series using the transmission cable 8-3, and the remaining steps are the same as above.

[0123] like Figure 5 The method for measuring the relative dielectric constant of bentonite 10-2 uses the capacitance of the arc electrode plate 10-3 on the same side to inversely calculate the dielectric constant of bentonite 10-2. The principle is as follows:

[0124] First, the annular area above the electrode plates is converted into a strip area using the logarithmic transformation method, that is,

[0125]

[0126] In the formula, ζ is the logarithmic change; is the curvature of the arc electrode plate.

[0127]

[0128] Where A' is the area after logarithmic change; ζ A and B They are the logarithmic ordinates of point A and point B respectively; R1 is the distance between the electrode and point O (which is the origin of the coordinate system after logarithmic transformation); R2 is the distance between the electrode and point O.

[0129] After logarithmic transformation, the capacitance expression of the arc electrode plate on the same side is:

[0130]

[0131] Where, C is the capacitance of the arc electrode plate 10-3 on the same side; A' is the area of ​​the plates facing each other after the transformation; L is the length of the two plates before the transformation; d' is the distance between the plates after the transformation; ε0, ε r are the relative dielectric constants of vacuum and soil, respectively.

[0132] Based on the capacitance C of the arc electrode plate 10-3 on the same side measured by a capacitance meter, R1=a / 2 and R2=b+a / 2 are substituted into formula (3), and the relative dielectric constant ε of the soil is r Calculated as:

[0133]

[0134] In the formula, a and b represent the distance between the two substrates and the length of the substrate itself, respectively;

[0135] Calculation principle of the relationship between matrix suction and wavelength signal:

[0136] The length and width of the probe are set to l and b respectively, and the spacing between the second and third probes is d1. For the soil to be tested between the first and second probes, according to the three-phase composition and proportional relationship of the soil, it can be regarded as pore gas, pore water and soil particles, where the widths of pore gas, pore water and soil particles are b respectively. a 、b w and b s , and b=b a +b w +b s , then the pore gas V a 、Pore water V w Soil particles V s The volumes can be expressed as:

[0137] V a =ld1b a (5)

[0138] V w =ld1b w (6)

[0139] V s =ld1b s (7)

[0140] Where V a 、V w and V s They represent the volumes of soil pore gas, pore water and soil particles respectively.

[0141] From equations (5) to (7), we know that the soil porosity ratio e can be expressed as:

[0142]

[0143] Similarly, according to the conversion relationship between the three-phase indicators of soil, the soil saturation S r It can be expressed as:

[0144]

[0145] According to (8) to (9), b w and b s The relationship between them is:

[0146]

[0147] According to the propagation speed of electromagnetic waves in soil Two-way travel time The equivalent dielectric constant ε of the soil to be tested can be calculated r for:

[0148]

[0149] Where c0 represents the electromagnetic wave velocity; ε a Indicates the dielectric constant of pore gas, generally taken as 1.00053F / m; ε w Represents the dielectric constant of pore water, which can be obtained by looking up Table 1 according to the ambient temperature; ε s It represents the dielectric constant of soil particles, generally taken as 5F / m.

[0150] Table 1 Dielectric constants of water at different temperatures

[0151]

[0152] Combining equations (5) to (11), the width of the pore water and the width of the soil particles to be tested are expressed as:

[0153]

[0154] A high electric field penetrating the soil to be tested will be generated between the first probe and the fourth probe to which a high-frequency signal is applied. Due to the electrostatic induction phenomenon, induced charges will be generated on the second probe and the third probe located between the first probe and the fourth probe. The charge q(t) is expressed as:

[0155] q(t)=ε r ε0AMsinωt (14)

[0156] Where M represents the signal amplitude; ε0 represents the dielectric constant in vacuum, which is generally 8.854187817×10 -12 F / m; ω is the angular frequency; t is the power-on time; A is the sensing area of ​​all probes.

[0157] From formula (15), we know that the induced current I generated in the soil is expressed as:

[0158]

[0159] Theoretically, the induced current generated in the soil is equal to the sum of the induced currents generated by the pore gas, pore water and soil particles as electrolytes. However, since the induced current generated by the pore gas as the dielectric is much smaller than that of the pore water and soil particles, the total induced current I generated by the soil to be tested is t =I w +I s, then from formula (15), we know that the induced currents generated by pore water and soil particles as dielectrics are respectively expressed as:

[0160] I w =ε w ε0A w Mωcosωt (16)

[0161] I s =ε s ε0A s Mωcosωt (17)

[0162] In the formula, I w and I s Respectively represent the induced current generated by pore water and soil particles as dielectrics; A w and A s Respectively represent the area occupied by pore water and soil particles on the probe, and are calculated using the following formula:

[0163] A w =lb w (18)

[0164] A s =lb s (19)

[0165] According to the total current law ∮ L B q ·dl=μ0∫∫ s J D ·dS calculates magnetic induction intensity B q , which can be expressed as:

[0166]

[0167] In the formula, B q represents the magnetic induction intensity; μ0 represents the magnetic permeability; r is the area of ​​the induction coil; J D is the displacement current,

[0168] The induced electromotive force U generated by the induction coil under the influence of the change of magnetic flux is expressed as:

[0169]

[0170] Where φ is the magnetic flux in the induction coil.

[0171] The elastic element generates heat after the induced current passes through it. The heat Q generated is calculated according to Joule's law as:

[0172]

[0173] In the formula, R eis the resistance of the elastic element; t1 and t2 are the start and end times of the wavelength signal stable segment respectively.

[0174] The strain energy accumulated by the elastic deformation caused by the heat generation of the elastic element is expressed as:

[0175]

[0176] In the formula, E e is the elastic modulus of the elastic element; Δε is the strain change of the elastic element; A e is the cross-sectional area of ​​the elastic element; l e is the length of the elastic element.

[0177] Since the central wavelength of the strain grating changes linearly with the temperature and strain, the wavelength drift of the strain grating caused by temperature and strain is expressed as:

[0178] Δλ total =K ε Δε+Δλ T (twenty four)

[0179] In the formula, Δλ total Indicates the wavelength drift of the strain grating; Δλ T represents the wavelength drift of the temperature-compensated grating; Kε represents the grating strain sensitivity coefficient; Δε represents the strain change of the elastic element.

[0180] According to the law of conservation of energy, i.e., Q=W, by connecting equations (5) to (24) in parallel, the relationship between the moisture content θ of the soil to be measured and the wavelength drift is expressed as equation (25). Referring to relevant literature, the relationship between the moisture content of bentonite and the matrix suction is expressed as equation (31).

[0181] The above example is only a specific implementation of the present invention and is not intended to be a limitation of the present invention. Any simple improvements and replacements made thereto are within the protection scope of the present invention.

Claims

1. An inductive soil matrix suction measurement device based on fiber Bragg grating, characterized in that: It includes a shell protection module, a signal excitation module, a contact balance module, an electromagnetic induction module, a wavelength detection module and a signal processing module; A contact balancing module is fixed inside the shell protection module, and the contact balancing module comprises a clay cylinder (10-1), a bentonite layer (10-2) and a plastic cylinder which are sequentially arranged from the outside to the inside; The signal excitation module comprises a signal source (1-3), the signal source (1-3) comprises a high-frequency signal source and a low-frequency signal source, the high-frequency signal source is electrically connected to one end of a plurality of groups of transmitting probes (9-2) via a transmission cable, each group of transmitting probes comprises two transmitting probes (9-2); An induction probe (9-3) is provided in conjunction with the transmitting probe (9-2); one end of the induction probe (9-3) is electrically connected to an electromagnetic induction module; the other end of the induction probe (9-3) and the other end of the transmitting probe (9-2) pass through a plastic tube and are inserted into the bentonite layer (10-2); the two induction probes (9-3) are located inside the two transmitting probes (9-2); The electromagnetic induction module comprises an induction coil (8-2) and an arc-shaped electrode plate (10-3) on the same side; induction electrode plates (7-2) arranged orthogonally to the induction coil (8-2) are installed on both sides of the induction coil (8-2); the induction electrode plates (7-2) are respectively electrically connected to induction probes (9-3); and the two ends of the induction coil (8-2) are respectively electrically connected to the two ends of a conductive elastic element (7-4) to form a closed loop; The wavelength detection module comprises a strain grating (7-5) and a temperature compensation grating (7-6); the strain grating (7-5) is fixed to the elastic element (7-4) by adhesive bonding; one end of the strain grating (7-5) is connected to the signal processing module via an optical fiber line (7-3), and the other end is connected to the temperature compensation grating (7-6); one end of the arc-shaped electrode plate (10-3) on the same side is electrically connected to the signal processing module, and the other end is electrically connected to the low-frequency signal source; the arc-shaped electrode plate (10-3) on the same side is located in the bentonite layer (10-2); and the wavelength detection module and the electromagnetic induction module are communicatively connected to the signal processing module.

2. The inductive soil matrix suction measuring device based on fiber Bragg grating according to claim 1 is characterized in that: The housing protection module comprises an external fixing frame (9-1), and an annular top packaging cover (9-6) and an annular tail packaging cover (9-7) are respectively arranged on the top and bottom of the external fixing frame (9-1); a contact balancing module is installed between the top packaging cover and the tail packaging cover; The top packaging cover and the tail packaging cover are connected and fixed to each other via studs (4-3); the top packaging cover (9-6) and the tail packaging cover (9-7) of the external fixing frame (9-1) are grooved concentric ring packaging covers, and a grooved sealing sticker (3-1) is used at the lower part of the plastic protective cover (3-3) between the bentonite layer (10-2) and the internal device to prevent pore water in the bentonite layer (10-2) from flowing into the internal device; The clay cylinder (10-1) of the contact balance module is in direct contact with the soil body to be tested, and the clay cylinder (10-1) is filled with a cylindrical bentonite layer (10-2); the signal source (1-3) of the signal excitation module outputs a high-frequency signal to the sensing probe inserted into the bentonite layer (10-2), so that the sensing probe (9-3) generates an induced current; the sensing probe (9-3) transmits the induced current to the sensing electrode plate (7-2) in the electromagnetic induction module through the transmission cable (8-3); the change of the electric field causes the sensing electrode plate (7-2) to generate electromagnetic induction phenomenon, thereby realizing electrical signal conversion; the elastic element (7-4) inside the electromagnetic induction module is deformed under the action of the induced current; the deformation of the elastic element (7-4) causes the strain grating (7-5) adhered to the surface of the elastic element (7-4) to be deformed at the same time; The wavelength detection module detects the grating pitch change of the strain grating (7-5) and the temperature compensation grating (7-6) on the same optical fiber line, thereby monitoring the corresponding wavelength change.

3. The inductive soil matrix suction measuring device based on fiber Bragg grating according to claim 1, characterized in that: The capacitance meter (1-2) in the signal processing module is connected to the arc electrode plate (10-3) of the signal excitation module to capture capacitance signals and transmit the signal data to the computer (1-1) for processing via a transmission cable (8-3); one end of the arc electrode plate (10-3) on the same side directly touches the bentonite layer (10-2), while the other end is connected to the signal source (1-3) via the transmission cable (8-3); the signal source (1-3) transmits a low-frequency signal to the driving electrode (5-2) on the arc electrode plate (10-3) on the same side, and the capacitance meter (1-2) electrically connected to the arc electrode plate (10-3) on the same side detects the electric field change between the driving electrode (5-2) and the sensing electrode (5-4), and measures the capacitance value of the arc electrode plate (10-3) on the same side accordingly, and then inversely calculates the relative dielectric constant of the bentonite layer (10-2) from the measured capacitance value.

4. The inductive soil matrix suction measuring device based on fiber Bragg grating according to claim 1, characterized in that: The high-frequency signal source (1-3) is electrically connected to three groups of transmitting probes (9-2) via a transmission cable, and the three groups of transmitting probes (9-2) are evenly distributed along the circumference of the plastic tube; the arc-shaped electrode plates (10-3) on the same side are three groups and are evenly distributed along the circumference of the outer side of the plastic tube; an insulating baffle (10-4) is provided between the arc-shaped electrode plates (10-3) on the same side and the transmitting probes (9-2); the clay tube (10-1) is closely attached to the bentonite layer (10-2), and the insulating baffle (10-4) is located inside the bentonite layer to isolate the magnetic field interference between the probes; The strain grating (7-5) and the temperature compensation grating (7-6) are obtained by engraving a grating on a conventional optical fiber; the strain grating (7-5) and the temperature compensation grating (7-6) are both engraved on the same optical fiber; the temperature compensation grating (7-6) is mainly used to eliminate the wavelength error caused by the heating of the elastic element, and the strain grating (7-5) is mainly used to establish the relationship between the deformation of the elastic element (7-4) and the wavelength change.

5. The inductive soil matrix suction measuring device based on fiber Bragg grating according to claim 1, characterized in that: The arc-shaped electrode plate (10-3) on the same side is in a fan-shaped ring shape and comprises an insulating substrate (5-3) on which two sensing electrodes (5-4) are fixed, with a gap between the two sensing electrodes (5-4); shielding electrodes (5-1) are fixed to the bottom of the insulating substrate (5-3) and to the outer ends of the two sensing electrodes (5-4).

6. A method for using an inductive soil matrix suction measurement device based on fiber Bragg grating, characterized in that: The inductive soil matrix suction measuring device based on fiber Bragg grating according to any one of claims 1 to 5 is used, and specifically comprises the following steps: Step S1, device assembly: The shell protection module, the signal excitation module, the electromagnetic induction module, the contact balance module, the wavelength detection module and the signal processing module are assembled to form an inductive soil matrix suction measurement device, the tail packaging cover (9-7) is connected by a nut (4-1) and a stud (4-3), and then the clay tube (10-1) and the internal plastic protective sleeve (3-3) are inserted into the ring of the tail packaging cover (9-7) in sequence, and before the plastic protective sleeve (3-3) is inserted, the grooved sealing sticker (3-1) is used to seal the notches of all the probes extending outward, and then the bentonite layer area is filled with bentonite (3-5), and then the top packaging cover (9-6) is connected by a nut (4-1) and a stud (4-3), and finally the inductive probe (9-2) and the transmitting probe (9-3) are extended; Step S2: Device embedding: The external fixing frame (9-1) is arranged at the outermost periphery of the clay tube (10-1), and after the device is assembled and connected, the buried position is determined according to the monitoring requirements, and the corresponding measuring points or marks are marked at the pre-buried measuring point positions. A drilling rig is used to drill holes to a predetermined depth, and the in-situ soil bodies at the corresponding depths are respectively taken out, and the sleeves (1-6) are inserted into the boreholes to stabilize the hole walls. The in-situ soil bodies are then backfilled into the original boreholes according to the corresponding compaction degree, and the sleeves (1-6) are pulled out, and the various lines are connected to construct the entire optical fiber monitoring system; Step S3, device measurement: The high-frequency signal source (1-3) is started to transmit a high-frequency alternating current to the transmitting probe (9-2). The induction probe (9-3) is affected by the alternating current of the transmitting probe (9-2) to generate an induced current. The induced current causes the induced charge to accumulate on the induction electrode plate (7-2), thereby generating an induced current in the internal loop of the induction coil (8-2). The induced current causes the elastic element (7-4) to generate heat and deformation. The strain grating (7-5) adhered to the elastic element (7-4) is deformed at the same time. The deformation of the strain grating (7-5) causes the grating pitch to change, thereby causing the wavelength of the internal optical signal to change, and finally generating data of wavelength change. The signal source transmits a low-frequency signal to the driving electrode (5-2) of the arc-shaped electrode plate (10-3) on the same side, and the capacitance meter (1-2) electrically connected to the arc-shaped electrode plate on the same side measures the capacitance of the arc-shaped electrode plate (10-3) on the same side, and transmits the capacitance signal to the computer (1-1); the signal demodulator (1-4) is connected to the strain grating (7-5) to collect the wavelength signals of the strain grating (7-5) and the temperature compensation grating (7-6), and transmits the wavelength signal to the computer (1-1) using a transmission cable; Step S4: Data collection: A capacitance meter electrically connected to the arc-shaped electrode plate (10-3) on the same side continuously detects changes in the electric field between the driving electrode (5-2) and the sensing electrode (5-4) within a power-on time interval [t1, t2], and measures the capacitance value of the arc-shaped electrode plate (10-3) on the same side accordingly; The signal demodulator (1-4) collects and records the wavelength variation data of the optical signal of the strain grating (7-5) and the wavelength variation data of the temperature compensation grating (7-6) within the power-on time interval [t1, t2], respectively represented by Δλ total and Δλ T express; The computer (1-1) records and stores the wavelength variation data and the capacitance data; Step S5: Data processing: Substituting the obtained capacitance data into equation (4), the relative dielectric constant ε of bentonite is obtained: r : Where, C is the capacitance of the arc electrode plate (10-3) on the same side; is the arc angle corresponding to the arc-shaped electrode plate (10-3) on the same side; ε0 represents the vacuum dielectric constant; a and b represent the distance between the two substrates of the arc-shaped electrode plate (10-3) on the same side and the length of the substrate itself, respectively; L is the length of the arc-shaped electrode plate (10-3) on the same side, and the inner arc length and the outer arc length of the arc-shaped electrode plate (10-3) on the same side are equal; by the formula ΔT=Δλ T / K T Get the temperature change ΔT, and then use the formula Δε=(Δλ total -Δλ T ) / KεThe strain change Δε of the elastic element that eliminates the temperature effect; where K T represents the temperature sensitivity coefficient of the temperature compensation grating; Kε represents the strain sensitivity coefficient of the strain grating; The dielectric constant ε of bentonite r The wavelength variation data collected in step S4 is substituted into the relationship (25) between the moisture content of bentonite θ and the wavelength drift of the strain grating and the temperature compensation grating to calculate the moisture content data of bentonite: Among them, B1, B2, B3, B4 and B5 are adjustment coefficients, expressed as follows: In the formula, ε w Represents the dielectric constant of pore water, which can be obtained by looking up the table based on the temperature; ε a Indicates the dielectric constant of pore gas, which is 1.00053F / m; ε s represents the dielectric constant of soil particles, which is 5F / m; ω represents the angular frequency; E e is the elastic modulus of the elastic element; A e is the cross-sectional area of ​​the elastic element; l is the probe length, the transmitting probe and the sensing probe have the same length; l e is the length of the elastic element; μ0 represents the magnetic permeability; r is the area of ​​the induction coil; R e is the resistance of the elastic element; M is the signal amplitude; ε0 is the dielectric constant in vacuum, which is 8.854187817×10-12F / m; t1 and t2 are the initial and end times of power-on, respectively; Δλ total Indicates the wavelength drift of the strain grating; Δλ T Indicates the wavelength drift of the temperature compensation grating; When the bentonite and the soil to be tested reach water potential equilibrium, the matrix suction of the bentonite is equal to the matrix suction of the soil to be tested. Then, the calculated bentonite water content θ and the collected temperature data T, T = ΔT + T0, T0 represents the initial temperature of the measurement, are substituted into the relationship model between the bentonite matrix suction and water content obtained from the calibration test, and finally the matrix suction value of the soil to be tested is obtained: Where, ψ is the matrix suction of bentonite; θ is the water content of bentonite; γ w is the density of water, take 10kN / m 3 ; a0, β, n, θ s and θ r is the experimental data fitting parameter; T is the temperature, which is calculated from the wavelength data reflected by the internal temperature compensation grating measurement.

7. The method for using the soil matrix suction measurement device based on fiber Bragg grating according to claim 6, characterized in that: The step S1 is specifically as follows: Step S1.1, using a transmission cable to connect the high-frequency signal source (1-3) and the low-frequency signal source (1-3) to the high-frequency signal source interface and the low-frequency signal source interface of the external fixing frame (9-1) respectively; Step S1.2, using an optical fiber lead to connect the signal demodulator (1-4) to the optical fiber grating interface (9-5) of the external fixing frame (9-1); Step S1.3, using a transmission cable to connect the capacitance meter (1-2) to the arc electrode plate (10-3) on the same side, and connecting the signal demodulator (1-4), the capacitance meter (1-2) and the computer (1-1); Step S1.4: if multiple assembled soil matrix suction measuring devices are connected in series or parallel for multi-point measurement, repeat steps S1.1 to S1.3 to connect each device in sequence.

8. The method for using the soil matrix suction measurement device based on fiber Bragg grating according to claim 6, characterized in that: The step S2 is specifically as follows: Step S2.1, marking corresponding measuring points or marks at the pre-buried measuring point positions, and drilling holes to a predetermined depth using a drilling rig, respectively taking out the in-situ soil at the corresponding depths, and inserting sleeves (1-6) into the boreholes to stabilize the hole walls; Step S2.2: Divide the obtained in-situ soil into three parts, use the quartering method to take representative soil samples, and test the density ρ and specific gravity G of each in-situ soil according to the "Standard for Geotechnical Test Methods". s and the dielectric constant of soil particles ε s , and take the average value of three in-situ soil bodies as soil layer parameters, and convert the in-situ soil density according to the relationship between the three-phase indicators of soil mechanics; Step S2.3, insert the assembled device into the borehole to a predetermined depth, lead the transmission wire out of the borehole, backfill the borehole with in-situ soil of the same density, and make the soil matrix suction measurement device in close contact with the surrounding soil to establish an effective hydraulic connection; Step S2.4: if the soil matrix suction of a single section or multiple sections is measured, repeat steps S2.1 to S2.3, and number the soil matrix suction measuring devices at different positions in sequence.

9. The method for using the soil matrix suction measurement device based on fiber Bragg grating according to claim 6, characterized in that: The step S3 is specifically as follows: Step S3.1, turn on the low-frequency signal source (1-3) switch and the capacitance meter (1-2) switch, and debug the arc electrode plate on the same side until the capacitance meter (1-2) of the signal processing module can effectively record the capacitance signal, ensuring that the module is well connected; Step S3.2, turn off the low-frequency signal source (1-3) switch, turn on the high-frequency signal source (1-3) switch, and debug the signal source (1-2) and the signal demodulator (1-4) until the wavelength signal can be effectively recorded; Step S3.3, turn on the high and low frequency signal sources (1-3) at the same time, so that the fiber Bragg grating-based inductive soil matrix suction measurement system works for a period of time, until the computer (1-1) can generate wavelength signal data and capacitance signal data within a certain period of time, and record and save the data.

10. The method for using the soil matrix suction measurement device based on fiber Bragg grating according to claim 6, characterized in that: The step S5 is specifically as follows: Step S5.1, substitute the received capacitance data into equation (4) to obtain the relative dielectric constant ε of bentonite r : The wavelength change Δλ through the temperature compensation grating T The internal temperature change ΔT is calculated inversely; the wavelength change Δλ of the strain grating is calculated inversely. total Subtract Δλ from T , the net wavelength change after deducting the temperature effect is obtained, and the net wavelength change is input into the relationship model between soil moisture content θ and wavelength drift Δλ, that is, formula (25), and analyzed and calculated to finally obtain the moisture content data of bentonite; Step S5.2, the obtained bentonite moisture content θ data and the collected temperature data T=ΔT+T0, T0 represents the initial measurement temperature, are substituted into the ψ-θ model of the bentonite obtained from the calibration experiment, i.e., formula (31), and finally the matrix suction value of the bentonite layer is obtained, which is the matrix suction of the soil to be tested.

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