Inductive soil matrix suction measurement device and method based on fiber Bragg grating

Abstract

The present invention discloses an inductive soil matrix suction measurement device and method based on fiber Bragg gratings. The device primarily comprises a housing protection module, a signal excitation module, a contact balancing module, a wavelength detection module, an electromagnetic induction module, and a signal processing module. Compared to existing measurement devices, the device of the present invention is simple to operate and offers precise control. It can achieve high-precision, continuous, rapid, and quasi-distributed measurement of soil matrix suction, effectively monitoring changes in matrix suction with external climatic conditions (such as rainfall, evaporation, and groundwater levels). This device is of great significance in determining the engineering properties of unsaturated soils and studying their properties.

Description

Technical Field

[0001] The present invention relates to the technical field of soil matrix suction measurement, and more particularly 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 both pore water and pore gas. The presence of pore gas further complicates the properties of unsaturated soil. The water-air interface (shrinkage membrane) has surface tension, and the presence of the gas phase significantly affects the mechanical, permeability, and thermal conductivity properties of unsaturated soil. Matric suction is a key parameter for evaluating unsaturated soil, significantly influencing its strength, permeability, and thermal conductivity. When the pore gas pressure exceeds the pore water pressure, the shrinkage membrane experiences this pressure difference, which is defined as matric suction. Matric suction is a crucial indicator for describing the mechanical properties of unsaturated soil, and the soil-water characteristic curve (SWCC), a curve that plots the relationship between matric suction and soil moisture content, is a crucial tool for studying unsaturated soils. Therefore, accurately measuring matric suction is of great significance for both theoretical research and engineering practice.

[0003] Accurate measurement of matric suction is crucial for a deeper understanding of the mechanical behavior of unsaturated soils and is a fundamental requirement for uncovering the mechanical mechanisms of unsaturated soils. The relationship between matric suction and soil water content is central to the mechanical properties of unsaturated soils. This relationship, defined as the soil-water characteristic curve (SWCC), is a key function for analyzing and interpreting unsaturated soil properties. Currently, the main methods for measuring matric suction include tensiometers, thermal conductivity sensors, dielectric constants, axis translation techniques, dialysis, contact filter paper techniques, saline solution vapor phase methods, and high-speed centrifuges. Each of these methods has its own advantages and disadvantages, and is suitable for specific research needs and soil types, providing a diverse range of technical approaches for the study of unsaturated soil mechanics.

[0004] The tensiometer method can accurately measure in situ soil matric suction. Using a high-intake ceramic plate as the measurement system, it directly measures negative pore water pressure in the soil. However, water in the tensiometer can cavitate and evaporate through the ceramic tip, and its range is limited, making it suitable only for measuring matric suction over a small range. Traditional thermal sensor methods suffer from slow readings, difficulty disassembling and replacing components, and aging over time. Calibration often exhibits hysteresis, resulting in low accuracy and inability to monitor over extended periods. The dielectric constant method is significantly affected by soil salinity, making it difficult to test soils with high salinity, unsuitable for long-distance monitoring, and limited in measurement range. The axial translation technique involves placing an unsaturated soil sample in a sealed pressure chamber and measuring negative pore water pressure using a saturated high-intake ceramic needle. However, the equipment is bulky and requires a strict testing environment. Bubble accumulation can occur during use, requiring manual adjustment of the pressure plate, resulting in low control accuracy and a cumbersome measurement process. Therefore, it is primarily used for indoor measurements. The dialysis method is suitable for low-suction areas, but it is cumbersome, prone to contamination, and requires a long measurement time. The contact filter paper technique, the salt solution vapor phase method, and the high-speed centrifuge method require high technical skills and are not universally applicable. The salt solution vapor phase method has low accuracy and a limited measurement range when measuring matric suction below 1500 kPa. The high-speed centrifuge method is less likely to completely drain water from the soil sample, is expensive, and causes significant damage to the soil. By optimizing existing methods or developing new technologies, more accurate measurements of matric suction in unsaturated soils can be achieved, providing more reliable data support for theoretical research and engineering applications.

[0005] In recent years, some new methods for measuring soil matrix suction 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 No.: ZL201911104712.7) by Zhu Honghu et al., this device is based on the principle of measuring water content by active heating. However, it has the disadvantage of damaging the soil structure due to heating, especially for special soils such as loess, red soil, and expansive soil. The soil structure determines the water holding state of the soil, so it is impossible to quickly and accurately monitor soil samples, and the resolution and accuracy are not high. In the invention patent "A Multifunctional In-situ Matrix Suction Measuring Instrument for Unsaturated Soils" (Publication No.: CN114034733 A) by Yu Ziwang et al., this device measures the thermal diffusivity of the heated 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 damages the soil structure, making it impossible to accurately measure matric suction, and it is also significantly affected by the environment. Guo Huijun et al.'s invention patent, "A High-Frequency Electric Field Charge Induction Soil Moisture Sensor" (Publication No. CN202111265407.3), is relatively simple to use, but this technical solution only allows single-point measurement, failing to achieve quasi-distributed measurement. It also fails to effectively shield the measurement results from the influence of external electric and magnetic fields, resulting in poor measurement accuracy. This shows that current matric suction measurement devices are still imperfect and urgently need further research.

[0006] As we all know, fiber Bragg grating (FBG) measurement technology has been widely used in many measurement fields due to its unique advantages. This technology is unaffected by factors such as light source fluctuations, fiber bending loss, connection loss, and detector aging, and exhibits strong resistance to magnetic field interference. Furthermore, FBG sensors offer high sensitivity, compact size, corrosion resistance, immunity to electromagnetic radiation interference, and excellent long-term stability, making them ideal for long-term monitoring. By connecting to a computer (1-1), FBG measurement technology can be used to construct a networked system, enabling real-time and quasi-distributed measurement of targets. If FBG technology is applied to the measurement of matrix suction, its superior performance will help improve measurement accuracy and reliability. By adopting special structural designs, FBG sensors can be further optimized to meet the measurement requirements of different soil types, enabling accurate measurement of matrix suction across all soil types. Such applications not only expand the application scope of FBG technology but also provide new solutions for unsaturated soil mechanics research and engineering monitoring. Summary of the Invention

[0007] 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 solutions 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 housing 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 via a transmission cable, and each group of transmitting probes includes two transmitting probes. A sensing probe is provided in conjunction with 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 includes 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 glued and fixed to the elastic element, one end of the strain grating is connected to the signal processing module via an optical fiber line, and the other end is connected to the temperature compensation grating; one end of the arc-shaped 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-shaped 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 includes an external fixing frame, 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 connected and fixed to each other by studs; the top packaging cover and the tail packaging cover of the external fixing frame adopt grooved concentric ring packaging covers, and a grooved sealing sticker is adopted at the bottom of the plastic protective cover between the bentonite layer and the internal device to prevent pore water in the bentonite layer from flowing into the internal device;

[0015] The clay cylinder of the contact balancing module is in direct contact with the soil to be tested, and the interior of the clay cylinder is filled with a cylindrical bentonite layer; the signal source of the signal excitation module outputs a high-frequency signal to the induction probe inserted into the bentonite layer, causing the induction probe to generate an induced current; the induction probe transmits the induced current to the induction electrode plate in the electromagnetic induction module through a transmission cable; the change in the electric field causes the induction 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 pitch change of the strain grating and the temperature compensation grating on the same optical fiber line.

[0016] A further improvement involves a capacitance meter in the signal processing module connected to the arc-shaped electrode plate of the signal excitation module, capturing capacitance signals and transmitting the signal data to a computer via a transmission cable for processing. One end of the arc-shaped electrode plate on the same side directly contacts the bentonite layer, while the other end is connected to a signal source via a transmission cable. The signal source transmits a low-frequency signal to the drive electrode on the arc-shaped electrode plate on the same side. The capacitance meter, electrically connected to the arc-shaped electrode plate on the same side, detects changes in the electric field between the drive electrode and the sensing electrode, and based on this, measures the capacitance value of the arc-shaped electrode plate on the same side. The relative dielectric constant of the bentonite layer is then inversely calculated from the measured capacitance value.

[0017] A further improvement is that the high-frequency signal source is electrically connected to three groups of transmitting probes via 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-compensated grating are obtained by engraving gratings on traditional optical fibers; both the strain grating and the temperature-compensated grating are engraved on the same optical fiber; the temperature-compensated grating is mainly used to eliminate the wavelength error caused by 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 the outer ends of the two sensing electrodes.

[0020] A further improvement is that the grooves of the top and rear packaging covers are arranged, from the outside inward, as follows: a clay tube groove, a bentonite groove, and an internal space groove. A plastic annular baffle is inserted into the annular groove at the transition between the internal space groove and the bentonite groove to isolate the internal detection device from 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 measurement device based on a fiber Bragg grating, using the above-mentioned inductive soil matrix suction measurement device based on a fiber Bragg grating, specifically includes the following steps:

[0023] Step S1, device assembly:

[0024] The inductive soil matrix suction measurement device is assembled by assembling the housing protection module, signal excitation module, electromagnetic induction module, contact balance module, wavelength detection module, and signal processing module. Nuts and studs are used to connect the tail packaging cover. A clay cylinder and a plastic protective sleeve are then inserted into the ring of the tail packaging cover. Before inserting the plastic protective sleeve, all the probe protrusion slots are sealed with a grooved seal. Bentonite is then filled into the bentonite layer. Nuts and studs are then used to connect the top packaging cover. Finally, the inductive probe and the transmitting probe are extended.

[0025] Step S2: Device installation:

[0026] Place the external fixing frame on the outermost edge of the clay tube. After assembling and connecting the device, determine the buried position according to the monitoring requirements, mark the corresponding measuring points or marks on the pre-buried measuring points, and use a drilling rig to drill to the predetermined depth. Remove the in-situ soil at the corresponding depth, 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 the various lines to build the entire fiber optic monitoring system.

[0027] Step S3, device measurement:

[0028] A high-frequency signal source is started to transmit a high-frequency alternating current to the transmitting probe. The inductive probe is affected by the alternating current of the transmitting probe to generate an induced current. 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 wavelength of the internal light signal to change, and ultimately generating data on the wavelength change; the signal source transmits a low-frequency signal to the driving electrode of the arc electrode plate on the same side, and a 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 a computer; a 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 a 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 during 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 change data of the strain grating light signal and the wavelength change data of the temperature compensation grating in the power-on time interval [t1, t2], which are represented by Δλ total and Δλ T express;

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

[0033] Step S5: Data processing:

[0034] Substituting the obtained capacitance data into formula (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 of 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 change data collected in step S4 are substituted into the relationship between the bentonite moisture content θ and the wavelength drift of the strain grating and the temperature compensation grating (25) to calculate the bentonite moisture content data:

[0038]

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

[0040]

[0041]

[0042] Where, ε w Indicates the dielectric constant of pore water, which can be obtained by looking up the table according to 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 represents the signal amplitude; ε0 represents the dielectric constant in vacuum, which is 8.854187817×10-12F / m; t1 and t2 are the start 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-compensated grating;

[0043] When the bentonite and the soil to be tested reach water potential equilibrium, the matric suction of the bentonite is equal to the matric suction of the soil to be tested. Then, the calculated bentonite water content θ and the collected temperature data T, T = ΔT + T0, where T0 represents the initial measurement temperature, are substituted into the relationship model between the bentonite matric suction and water content obtained from the calibration test. Finally, the matric 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 transmission cables 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 Bragg grating interface of the external fixed frame using an optical fiber lead;

[0049] Step S1.3, connecting the capacitance meter to the arc electrode plate on the same side using a transmission cable, and connecting the signal demodulator and 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: Mark the corresponding measuring points or marks at the pre-buried measuring point locations, drill holes to a predetermined depth using a drilling rig, remove the in-situ soil at the corresponding depths, and insert sleeves into the holes to stabilize the hole walls;

[0053] Step S2.2: Divide the in-situ soil into three parts, take representative soil samples using the quartering method, 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 ε of soil particles s , and take the average value of three in-situ soil masses 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 ensure that the soil matrix suction measurement device is in close contact with the surrounding soil to establish an effective hydraulic connection;

[0055] Step S2.4: If the soil matric suction of a single section or multiple sections is to be measured, repeat steps S2.1 to S2.3, and number the soil matric 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 and capacitance meter switches, and adjust 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 properly 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-frequency and low-frequency signal source switches at the same time, and allow the fiber Bragg grating-based inductive soil matrix suction measurement system to operate 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 formula (4) to obtain the relative dielectric constant ε of bentonite r : The wavelength change Δλ through the temperature compensation grating T Inversely calculate the internal temperature change ΔT; the wavelength change Δλ of the strain grating 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] In step S5.2, the obtained bentonite moisture content θ data and the collected temperature data T = ΔT + T0, where 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)] to finally obtain the matrix suction value of the bentonite layer, which is the matrix suction of the soil to be measured.

[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., and is convenient for long-term measurement.

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

[0067] 4. The device is detachable and the external device and internal detection device can be separated 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 This is a system diagram of an inductive soil matrix suction measurement device based on fiber Bragg grating;

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

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

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

[0073] Figure 5 This 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 This 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 This 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] Figure 9 This is a diagram of the matrix suction measurement module of an inductive soil matrix suction measurement device based on fiber Bragg grating;

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

[0079] Figure 11 This is a graph of matric suction at different temperatures during the calibration test of an inductive soil matric suction measurement device based on fiber Bragg grating.

[0080] Figure 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 inner 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, 54 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 comprising an inductive soil matric suction measurement device based on a fiber Bragg grating (FBG) comprises a data analysis computer 1-1, a capacitance meter 1-2, a signal source 1-3, a signal conditioner 1-4, and a soil matric suction measurement device 1-5. The signal source provides a high-frequency signal source 1-3 and a low-frequency signal source 1-3, which transmits a high-frequency alternating current signal to the outer probe 9-2, generating an induced current in the middle probe 9-3, and provides a low-frequency current to the driving electrode 5-2 of the arc-shaped 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 wavelength band for the fiber Bragg grating and receive the changed wavelength. After receiving data from the signal demodulator 1-4, the capacitance meter 1-2, and the computer 1-1, the computer 1-1 converts the wavelength data into the corresponding soil matric suction value.

[0083] refer to Figure 2 An inductive soil matrix suction measuring device based on fiber Bragg grating includes, from the inside to the outside, an optical fiber line 7-3, a transmission cable 8-3, an inductive probe 9-3, an arc-shaped ipsilateral 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 by the optical fiber line 7-3. In this experiment, the signal demodulator 1-4 is used to demodulate the fiber Bragg grating and collect wavelength data.

[0085] Figure 3The figure shows a 3 / 4 cross-section of the measuring device. The sensing probe 9-3 and the arc-shaped electrode plate 10-3 on the same side are buried in the bentonite layer 10-2, primarily used to measure the moisture content and relative dielectric constant of the bentonite layer 10-2. Outside the bentonite layer 10-2 is a clay cylinder 10-1, which primarily blocks external air and establishes a hydraulic connection between the bentonite layer 10-2 inside the device and the external soil layer being measured.

[0086] Figure 4 The top view of the device's housing protection module's packaging cover shows three concentric grooves of varying radii on its interior, which, from the outside inward, house the clay cylinder 10-1, bentonite layer 10-2, and internal detection device 6-8. The exterior of the packaging cover also features six circular holes for receiving studs to secure 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 This is a diagram of a fixing rod of a matrix suction measuring device. The fixing rod mainly consists 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 ipsilateral arc electrode plate 10-3 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 base plate 5-3. By passing a low-frequency current through the driving electrode 5-2, an electric field is formed above the ipsilateral arc electrode plate 10-3. Changes in the relative dielectric constant of the bentonite layer 10-2 will cause changes in the electric field, thereby achieving measurement of the relative dielectric constant of the bentonite layer 10-2.

[0090] Figure 9 and Figure 10 Figures 1 and 2 show a schematic diagram and a partial diagram of the matrix suction measurement module of the matrix suction measurement device, respectively. The device comprises 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 within the wavelength detection module. The device primarily performs tasks such as receiving current, converting current, and converting energy. The current conversion causes the strain grating 7-5 attached to the elastic element 7-4 to change its pitch, thereby causing a change in its corresponding wavelength.

[0091] Figure 11This figure shows the relationship between matric suction and bentonite moisture content at different temperatures during a calibration test of the matric suction measurement device. This figure is a curve of calibration experimental test data fitted by Origin software, used to calibrate matric suction values ​​at different temperatures.

[0092] Figure 12 Diagram of the probe extension and retraction device for the matrix suction measurement device; the probe is fixed to an arc-shaped plate, which is vertically rotated and hinged on the push-pull rod. During installation, a pad is placed into the plastic barrel. The pad's height is consistent with the position of the through-hole in the plastic barrel for the probe to pass through. Then, the probe structure is lowered. The probe is pressed down by the push-pull rod and gradually expands 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 this device. The initial temperature of this device is set with reference to the reference temperature.

[0094] The clay cylinder of the contact balancing module is in direct contact with the soil to be tested, and the interior of the clay cylinder is filled with a cylindrical bentonite layer; the signal source of the signal excitation module outputs a high-frequency signal to the induction probe inserted into the bentonite layer, causing the induction probe to generate an induced current; the induction probe transmits the induced current to the induction electrode plate in the electromagnetic induction module through a transmission cable; the change in the electric field causes the induction 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 pitch change of the strain grating and the temperature compensation grating on the same optical fiber line.

[0095] A further improvement is that the grooves of the top and rear packaging covers are arranged, from the outside inward, as follows: a clay tube groove, a bentonite groove, and an internal space groove. A plastic annular baffle is inserted into the annular groove at the transition between the internal space groove and the bentonite groove to isolate the internal detection device from 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: Connect the top and tail packaging covers using studs 4-3 and nuts 4-1. Then, insert the clay cylinder 10-1 and plastic annular baffle 3-3 into the ring of the tail packaging cover. Before inserting the plastic cylinder, seal all the notches where the probes extend with sealing tape 3-1.

[0099] Step P2: Add bentonite 10-2 of a certain density, prepared during calibration experiments, to the filling layer between the clay cylinder 10-1 and the plastic cylinder. Stop adding bentonite 10-2 when it reaches the notch. Begin shaking the device to ensure that the bentonite 10-2 is fully filled.

[0100] Step P3: bury the 10-3 arc-shaped same-side arc-shaped electrode plate 10-3, 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 overhanging notch, rotate the buckle inward to fix the internal detection device 6-8 inside the plastic annular baffle 3-3, and at the same time, the probe is exactly aligned with the overhanging notch.

[0102] Step P5: After the fixation is completed, connect the lines required for the internal device to the top optical fiber interface 9-5 of the top packaging cover. Finally, use the stud 4-3 and nut 4-1 to fix the top packaging cover, and press the top probe retracting button 2-4 to extend the probe.

[0103] Step P6, device deployment: Mark the corresponding measuring points or marks at the pre-buried measuring point locations, and drill holes to a predetermined depth using a drilling rig. Remove the in-situ soil at the corresponding depths and insert sleeves 1-6 into them. The holes dug 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, take representative soil samples using the quartering method, 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 masses 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, backfill the borehole with in-situ soil of the same density, ensuring close contact between the soil matrix suction measurement device 1-5 and the surrounding soil to establish effective hydraulic connection. Each time the backfill reaches a certain height, pull out a section of the sleeve 1-6 until the backfill is complete, at which point remove the sleeve 1-6.

[0106] Step P9, Build the Fiber Optic Measurement System: Connect the buried fiber optic line 7-3 to the signal demodulator 1-4, power supply, and capacitance meter 1-2. These devices are connected to the computer 1-1 from the back end to form the entire fiber optic 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, and the inductive probe 9-3 generates an induced current after being affected by the alternating current of the transmitting probe 9-2. 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 wavelength change data; 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 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 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 changes between the driving electrode 5-2 and the sensing electrode 5-4 during the power-on time interval [t1, t2], and measures the capacitance value of the arc electrode plate 10-3 on the same side based on this; 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 during the power-on time interval [t1, t2], which are 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 acquisition: Substitute the obtained capacitance data into formula (4) to obtain the relative dielectric constant ε of bentonite 10-2 r :

[0109]

[0110] Where, 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 steps 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, which are expressed as follows:

[0114]

[0115] Where, ε w Indicates the dielectric constant of pore water, which can be obtained by looking up the table according to 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 represents the signal amplitude; ε0 represents the dielectric constant in vacuum, which is 8.854187817×10-12F / m; t1 and t2 are the start 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-compensated grating;

[0116] When the bentonite (10-2) and the soil to be tested reach water potential equilibrium, the matric suction of the bentonite (10-2) is equal to the matric 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 measurement temperature) are substituted into the relationship model between the matric suction and water content of the bentonite obtained from the calibration test, and the matric suction value of the soil to be tested is finally 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 fitting parameters of the experimental data, 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 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 logarithmic transformation method is used to transform the annular area above the electrode plates into a strip area, that is,

[0125]

[0126] Where 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 ζ B are the logarithmic ordinates of point A and point B respectively; R1 is the distance between the nearest end of the plate and point O (this point is the origin of the coordinate after logarithmic transformation); R2 is the distance between the farthest end of the plate 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 permittivities 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] Where 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] Let the length and width of the probe be l and b respectively, and the distance between the second and third probes be d1. For the soil to be measured between the first and second probes, according to the three-phase composition and proportion 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 and 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), 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 based on the relationship between 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 Indicates the dielectric constant of soil particles, generally taken as 5F / m.

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

[0151]

[0152] The combined equations (5) to (11) can be expressed as follows:

[0153]

[0154] A high electric field penetrating the soil to be measured will be generated between the first and fourth probes to which a high-frequency signal is applied. Due to the electrostatic induction phenomenon, induced charges will be generated on the second and third probes located between the first and fourth probes. 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 can be 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), the induced currents generated by pore water and soil particles as dielectrics are expressed as:

[0160] I w =ε w ε0A w Mωcosωt (16)

[0161] I s =ε s ε0A s Mωcosωt (17)

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

[0163] A w =lb w (18)

[0164] A s =lb s (19)

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

[0166]

[0167] Where 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 in 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 flows. According to Joule's law, the heat Q generated is calculated as:

[0172]

[0173] Where R eis the resistance of the elastic element; t1 and t2 are the start and end times of the wavelength signal stability segment respectively.

[0174] The strain energy accumulated by the elastic deformation caused by heat generation in the elastic element can be expressed as:

[0175]

[0176] Where, 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] Where 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, and by combining equations (5) to (24), the relationship between the water 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 water content of bentonite and the matrix suction is expressed as equation (31).

[0181] The above example is only a specific embodiment of the present invention and is not intended to limit the present invention. Any simple improvements and replacements thereto are within the scope of protection 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 in the shell protection module, and the contact balancing module comprises a clay cylinder (10-1), a bentonite layer (10-2), and a plastic cylinder arranged in sequence from the outside to the inside; The signal excitation module includes a signal source (1-3), the signal source (1-3) 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 (9-2) via a transmission cable, and each group of transmitting probes includes 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 the 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 a 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, 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 provided 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) adopt grooved concentric ring packaging covers, and a grooved sealing sticker (3-1) is adopted 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 balancing module is in direct contact with the soil to be measured, and the interior of 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), causing the sensing probe (9-3) to generate an induced current; the sensing probe (9-3) transmits the induced current to the sensing electrode plate (7-2) in the electromagnetic induction module via a transmission cable (8-3); the change in the electric field causes the sensing electrode plate (7-2) to undergo electromagnetic induction, thereby achieving 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 deform simultaneously; The wavelength detection module monitors the corresponding wavelength change by detecting the grating pitch change of the strain grating (7-5) and the temperature compensation grating (7-6) on the same optical fiber line.

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, and is used 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 based on the measured capacitance value, 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 cylinder; the arc-shaped electrode plates (10-3) on the same side are three groups and evenly distributed along the circumference of the outer side of the plastic cylinder; 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 cylinder (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 magnetic field interference between the probes; The strain grating (7-5) and the temperature compensation grating (7-6) are obtained by engraving gratings 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, while 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 same-side arc-shaped electrode plate (10-3) is in a fan-shaped ring shape and comprises an insulating substrate (5-3). Two sensing electrodes (5-4) are fixed on the insulating substrate (5-3), and a gap is provided between the two sensing electrodes (5-4). Shielding electrodes (5-1) are fixed to the bottom of the insulating substrate (5-3) and 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 specifically includes the following steps: Step S1, device assembly: 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. The tail packaging cover (9-7) is connected with a nut (4-1) and a stud (4-3). Then, a clay cylinder (10-1) and an internal plastic protective sleeve (3-3) are sequentially inserted into the ring of the tail packaging cover (9-7). Before the plastic protective sleeve (3-3) is inserted, a groove-type sealing sticker (3-1) is used to seal all the notches of the probe extension. Then, bentonite (3-5) is filled into the bentonite layer area. Then, a nut (4-1) and a stud (4-3) are used to connect the top packaging cover (9-6). Finally, the sensing probe (9-3) and the transmitting probe (9-2) are extended. Step S2: Device installation: The external fixing frame (9-1) is arranged at the outermost periphery of the clay cylinder (10-1). After the device is assembled and connected, the buried position is determined according to the monitoring requirements, the corresponding measuring points or marks are marked at the pre-buried measuring point positions, and a drilling rig is used to drill holes to a predetermined depth. The in-situ soil at the corresponding depths is removed, and the sleeve (1-6) is inserted into the borehole to stabilize the hole wall. The in-situ soil is then backfilled into the original borehole according to the corresponding compaction degree, the sleeve (1-6) is pulled out, and the various lines are connected to construct the entire optical fiber monitoring system. Step S3, device measurement: A high-frequency signal source (1-3) is started to transmit a high-frequency alternating current to the transmitting probe (9-2). The sensing probe (9-3) is affected by the alternating current of the transmitting probe (9-2) and generates an induced current. The induced current causes induced charges to accumulate on the sensing 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 heat up and deform. The strain grating (7-5) adhered to the elastic element (7-4) is also deformed. The deformation of the strain grating (7-5) causes its grating pitch to change, thereby causing the wavelength of the internal optical signal to change, and ultimately generating data of the wavelength change. A signal source transmits a low-frequency signal to a driving electrode (5-2) of the arc-shaped electrode plate (10-3) on the same side; a 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 a computer (1-1); a signal demodulator (1-4) is connected to the strain grating (7-5) to collect 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) during the 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 formula (4), the relative dielectric constant ε of bentonite is obtained r : Where C is the capacitance of the arc electrode plate on the same side (10-3); 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 change data collected in step S4 are substituted into the relationship between the bentonite moisture content θ and the wavelength drift of the strain grating and the temperature compensation grating (25) to calculate the bentonite moisture content data: Among them, B1, B2, B3, B4 and B5 are adjustment coefficients, which are expressed as follows: Where, ε w Indicates the dielectric constant of pore water, which can be obtained by looking up the table according to 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 represents the signal amplitude; ε0 represents the dielectric constant in vacuum, which is 8.854187817×10-12F / m; t1 and t2 are the start 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-compensated grating; When the bentonite and the soil to be tested reach water potential equilibrium, the matric suction of the bentonite is equal to the matric suction of the soil to be tested. Then, the calculated bentonite water content θ and the collected temperature data T, T = ΔT + T0, where T0 represents the initial measurement temperature, are substituted into the relationship model between the bentonite matric suction and water content obtained from the calibration test. Finally, the matric 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 transmission cables 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 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: Mark the corresponding measuring points or marks at the pre-buried measuring point positions, and drill holes to a predetermined depth using a drilling rig. Remove the in-situ soil at the corresponding depths, and insert sleeves (1-6) into the holes to stabilize the hole walls. Step S2.2: Divide the in-situ soil into three parts, take representative soil samples using the quartering method, 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 ε of soil particles s , and take the average value of three in-situ soil masses 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 ensure that the soil matrix suction measurement device is in close contact with the surrounding soil to establish an effective hydraulic connection; Step S2.4: If the soil matric suction of a single section or multiple sections is to be measured, repeat steps S2.1 to S2.3, and number the soil matric 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) and the capacitance meter (1-2), and adjust 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 properly 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-frequency and low-frequency signal source (1-3) switches at the same time, so that the fiber Bragg grating-based inductive soil matrix suction measurement system operates 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 formula (4) to obtain the relative dielectric constant ε of bentonite r : The wavelength change Δλ through the temperature compensation grating T Inversely calculate the internal temperature change ΔT; the wavelength change Δλ of the strain grating 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; In step S5.2, the obtained bentonite moisture content θ data and the collected temperature data T = ΔT + T0, where 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 measured.

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