Optical fiber-based three-way earth pressure sensor

Abstract

This invention provides a three-way earth pressure sensor with strong electromagnetic interference resistance, long-distance transmission capabilities, and a simple data processing method. [Solution] The optical fiber-based three-directional earth pressure sensor includes a sensing housing, a cubic base, a shear strain beam, a fiberglass gauge (FBG), a fixture, and a support column. The cubic base is fixed to the centroid of the sensing housing via the support column, and the shear strain beam is fixed to three mutually perpendicular mounting surfaces on the cubic base. The three surfaces of the sensing housing corresponding to the shear strain beam are test surfaces, and their inner walls are hemispherical concave. The shear strain beam is L-shaped and includes a fixed end and a cantilevered end. The cantilevered end of each beam is in contact with the hemispherical concave inner wall, and circular blind holes are symmetrically provided on both sides. The FBG is attached to the bottom of the blind holes and, after being connected in series, is pulled out through a pre-drilled hole.

Description

Technical Field

[0001] The present invention belongs to the field of foundation works such as geotechnical tunnels and dams. Specifically, it is a sensor that measures the three-dimensional stress state of soil by using an optical fiber.

Background Art

[0002] In foundation works such as geotechnical tunnels and dams, earth pressure is an important monitoring parameter. In three-dimensional space, earth pressure is classified into σ1, σ2, and σ3. σ1 in the natural formation is usually the vertical self-weight stress along the direction of gravity, and σ1, σ2, and σ3 are orthogonal to each other. Usually, σ1>σ2 = σ3, and the coefficient of earth pressure at rest k0 is σ2 / σ1. However, during the excavation of the soil mass, σ2 and σ3 become different, resulting in an active earth pressure state where the active earth pressure coefficient k1 = σ3 / σ1 is smaller than k0, and a passive earth pressure state where the passive earth pressure coefficient k2 = σ2 / σ1 is larger than k0. In the tunnel construction of a soft soil layer rich in water, the distribution and dynamic monitoring of the three-direction earth pressure are directly related to the stability of the excavation face, ground subsidence, and the safety of the lining structure. Soft soil has the characteristics of high compressibility, low strength, and high sensitivity. When combined with the action of groundwater, the vertical stress σ1 in the initial in-situ stress field becomes the total self-weight stress of the overlying soil mass and water, and the horizontal stresses σ2 = σ3≒k0σ1, and k0 is always close to 1 or even greater than 1. After tunnel excavation, the radial stress σ3 of the free surface is rapidly released, the pore water pressure disappears, the effective stress decreases, and an active stress state of σ1>σ2>σ3 (k1 = σ3 / σ1<<k0) is formed, making it easy for the excavation face to become unstable, and soft soil creep and flow to occur. During shield excavation, it is necessary to actively maintain the three-direction stress balance at the face by means of slurry pressure or earth pressure balance method, and restore σ2 and σ3 to a nearly static state. After the lining support, the rheological effect of the water-rich soft soil and the action of external water-soil load continuously accumulate the circumferential stress σ2, gradually forming a passive earth pressure state (k2 = σ2 / σ1>k0), which may have a profound impact on the mechanical performance and durability of the segments. In addition, each factor of simultaneous injection pressure, ground loss, and long-term consolidation perturbs the three-direction effective stress path. Therefore, it is necessary to monitor the water-soil pressure and effective stress conversion in real time, dynamically adjust the support strategy, and prevent soft soil shear failure, segment deformation, or leakage. The earth pressure cell is a basic tool for monitoring in geotechnical tunnel construction and plays an irreplaceable role in ensuring the safety of geotechnical tunnel construction.

[0003] Currently, commonly used earth pressure cells can only measure earth pressure in one direction of the soil, and cannot simultaneously measure earth pressure in three mutually perpendicular directions. This makes it difficult to monitor the safe construction, normal operation, and health status of foundation work for tunnels, dams, and other structures.

[0004] Patent CN106768521A describes the creation of a press-in type three-way earth pressure sensor and its measurement method for loess and soft soil by integrating three test planes angled 60° to each other in the probe press-in section. The corners of the triangular pyramidal structure in this design tend to cause localized extrusion in loess and soft soil, resulting in additional stress deformation in the soil around the probe and causing a discrepancy between the pressure measured by the resistance strain gauge and the true pressure of the soil.

[0005] Patent CN115326269A describes the fabrication of a three-dimensional earth pressure cell using a cubic strained sheet structure and resistance-strain gauges. While this approach offers high precision, mature technology, and a small volume, the resistance-strain sheet sensors are susceptible to electromagnetic interference, resulting in low long-range monitoring accuracy.

[0006] Patent CN105628279A employs a method of constructing an antenna by interposing an organic polymer cylinder with helical liquid metal microchannels between the six stress-receiving and opposing faces of a regular dodecahedron cell, and exciting the microchannels via a coaxial feed line. This method solves the problems of conventional sensors, which are unable to monitor over long distances, are susceptible to electromagnetic interference, and require multi-angle measurements to obtain a complete stress tensor. It can obtain six stress components wirelessly over long distances at once, has strong resistance to electromagnetic interference, and is suitable for long-term monitoring. However, its regular dodecahedron structure and the configuration of multiple cylinders inside result in a complex sensor structure, a large volume, and a complex conversion relationship between the frequency response of the liquid metal antenna and stress, making calibration difficult. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] To solve the above technical problems, the present invention develops a three-directional earth pressure sensor based on optical fiber. To accommodate the three-directional stress collection function, the sensor's test surface side housing is specially designed, and a shear strain beam is inscribed inside. A fiberglass gauge (FBG) capable of high-precision and long-distance transmission is attached to the bottom of the blind hole in the shear strain beam. When a force is applied to the earth pressure sensor, the shear strain beam deforms under pressure applied perpendicularly to the beam top surface at the free end. By measuring the change in the central wavelength of the FBG, the maximum principal stress received by the shear strain beam is reflected, and the earth pressure value is calculated. The three-directional stress state inside the soil can be measured simultaneously. The specific details of the present invention are as follows. [Means for solving the problem]

[0008] The optical fiber-based three-directional earth pressure sensor disclosed in the present invention includes a sensing housing, a cubic base, a shear strain beam, a fiberglass gauge (FBG), a fastener, and a support column, wherein the cubic base is fixed to the centroid of the sensing housing via the support column, and the shear strain beam is fixed to three mutually perpendicular mounting surfaces on the cubic base via the fastener, the surface of the sensing housing fixed to the support column is the bottom surface, the three corresponding surfaces of the shear strain beam are test surfaces, the inner walls of the test surfaces are hemispherical concave surfaces, the shear strain beam is L-shaped and includes a fixed end and a cantilever end, the length of the fixed end is shorter than the cantilever end, and the fastener penetrates the fixed end and connects to the cubic base The shear strain beam is fixed to the cubic base, the cantilevered end is suspended in mid-air, the fixed end of the shear strain beam is aligned with the center of the edge of the cubic base, the cantilevered end is in contact with the hemispherical concave inner wall of the sensing housing, the shear strain beam has circular blind holes symmetrically drilled along its length on both sides near the fixed end, the cross section of the region where the blind holes are located is I-shaped, the FBG is attached to the bottom of the blind holes on both sides of the shear strain beam and is configured to form a 45° angle with the neutral axis of the shear strain beam and is provided antisymmetrically with respect to the neutral axis, all FBGs are connected in series via optical fibers and then pulled out through pre-drilled holes provided on the non-test surface of the sensing housing.

[0009] Preferably, the sensing housing is made of a plate material with excellent sealing and corrosion resistance, and the inner walls of the three mutually perpendicular surfaces that perform the testing function are designed in a hemispherical concave shape to reduce errors due to uneven pressure distribution, and the shear strain beam is in contact with the hemispherical concave bottom of the inner wall of the test surface, thereby facilitating the vertical transmission of pressure on the test surface, reducing interference from other loads to the measurement, increasing the contact area between edges, and preventing housing failure caused by stress concentration due to sudden changes in stiffness.

[0010] Preferably, the outside of the blind hole is chamfered to prevent damage during optical fiber connection.

[0011] Preferably, the temperature sensitivity coefficients of two FBGs attached to the same shear strain beam must match. During measurement, the wavelength change values ​​of the FBGs in blind holes on both sides of the same shear strain beam are subtracted, and in the direction forming a 45° angle with the central axis, there are principal stresses and principal strains of equal magnitude in tension and compression. By subtracting the wavelength, the effect of temperature can be canceled out and sensitivity can be increased.

[0012] Preferably, the relevant dimensions of the shear strain beam are set as follows: The ratio of the height H to the length L of the cantilever end is:

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[0013] Preferably, the relationship between the sensor test range and the maximum shear stress of the shear strain beam is as follows:

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[0014] Preferably, the relationship between the force received by the test surface and the maximum principal strain in the direction forming a 45° angle with the neutral axis is as follows:

Equation

Advantages of the Invention

[0015] Advantages of the present invention: In the shear strain beam of the present invention, circular blind holes are symmetrically opened along the length direction on both sides near the fixed end, and its cross-section is made into an I shape, generating stress concentration in this area, increasing the strain in this area, improving the sensitivity of the sensor, using the web as the main area for receiving shear force, forming a "pure shear strain area" with uniform stress on the web center line, and not being affected by the bending moment changing along the length of the beam. The fiber-optic based triaxial earth pressure sensor disclosed in the present invention has advantages such as small volume, high accuracy and sensitivity, strong electromagnetic interference resistance ability, being able to transmit over long distances, and having a simple data processing method, etc. It utilizes the principal stress and principal strain with equal magnitudes of tension and compression to enhance the sensitivity and remove the influence of temperature change on the test results.

Brief Description of the Drawings

[0016] [Figure 1] It is a schematic structural view of the present invention. [Figure 2] It is a front view of the central cross-section of the structure of the present invention. [Figure 3] It is a side view of the shear strain beam of the present invention. [Figure 4] It is a view of the I-shaped cross-section of the blind hole of the present invention. [Figure 5] It is a schematic view of the three-dimensional dimensions of the shear strain beam of the present invention.

Modes for Carrying Out the Invention

[0017] Hereinafter, the present invention will be described in more detail in conjunction with the accompanying drawings and specific embodiments.

[0018] The optical fiber-based three-directional earth pressure sensor shown in Figures 1 to 5 includes a sensing housing 1, a cubic base 2, three shear strain beams 3, a fiberglass gate 4, a fixing device 5, and a support column 6. The cubic base 2 is fixed to the centroid of the sensing housing 1 via the support column 6, and the surface of the sensing housing 1 fixed to the support column 6 is the bottom surface. The shear strain beams 3 are each fixed to three mutually perpendicular mounting surfaces on the cubic base 2 via the fixing device 5, and the three corresponding surfaces of the shear strain beams 3 on the sensing housing 1 are test surfaces, and the inner walls of the test surfaces are hemispherical concave surfaces. The shear strain beam 3 is L-shaped and includes a fixed end and a cantilever end, the length of which the fixed end is shorter than that of the cantilever end, the fixing device 5 is fixed to the cubic fixing base 2 by passing through the fixed end, the cantilever end is suspended, the fixed end of the shear strain beam 3 is aligned with the center of the edge of the cubic fixing base 2, the cantilever end is in contact with the hemispherical concave inner wall of the sensing housing 1, the shear strain beam 3 has circular blind holes symmetrically drilled along its length on both sides near the fixed end, and the cross section of the region where the blind holes are located is I-shaped. The FBG4 is attached to the bottom of the blind holes on both sides of the shear strain beam 3 and is configured to form a 45° angle with the neutral axis of the shear strain beam 3 and is provided antisymmetrically with respect to the neutral axis, all of the FBG4 are connected in series via optical fibers and then pulled out through a pre-drilled hole provided on the non-test surface of the sensing housing 1.

[0019] The sensing housing 1 is constructed from a plate material with excellent sealing and corrosion resistance, and is pre-embedded in a humid environment to avoid affecting monitoring accuracy. The three test surfaces responsible for the testing function are, for example, the front, right, and bottom surfaces in Figure 1. The inner walls of the three test surfaces are designed in a hemispherical concave shape, thereby reducing stress concentration, avoiding localized fatigue, and reducing errors due to uneven pressure distribution, thereby improving measurement accuracy.

[0020] The shear strain beam 3 is a cantilever beam, with its cantilever end in contact with the hemispherical concave inner wall of the sensing housing 1, thereby facilitating the vertical transmission of pressure to the test surface and effectively reducing interference from lateral and torsional loads during measurement. Two blind holes are symmetrically machined at neutral axis positions on both sides near the fixed end to enhance the strain response of the shear strain beam under pressure. The shear force exhibits a constant distribution along the length of the beam, the stress does not attenuate or fluctuate with respect to location, and the symmetrical flanges balance the moment of inertia of the cross-section and stabilize the neutral axis. The outside of the blind holes is chamfered to prevent damage during optical fiber connection.

[0021] The FBG is attached so as to form a 45° angle with the neutral axis of the shear-strained beam, measuring the maximum principal stress in the pure shear state, eliminating bending stress interference, capturing the principal stress converted from the shear stress, and achieving accurate measurement. As shown in Figure 3, the FBGs attached to the blind holes on both sides are antisymmetric with respect to the neutral axis, and the temperature sensitivity coefficients of the two FBGs attached to the same shear-strained beam must be identical. During measurement, the wavelength change measured by the FBGs on both sides is subtracted, and in the I-section web, there are two principal stresses and principal strains with equal magnitudes in tension and compression in the direction forming a 45° angle with the central axis. Subtracting the wavelength change on both sides increases sensitivity and simultaneously compensates for the influence of ambient temperature changes on the test results, thus providing temperature compensation. The I-section is shown in Figure 4, where parameter a is the width of the flange on the I-section, parameter α is the angle between the line connecting the midpoint of the flange and the web of the I-section and the neutral axis, h is the height of the flange on the I-section, and b is the width of the web of the I-section.

[0022] Finite element simulations will be performed on the shear-strained beam to verify that the range and sensitivity requirements are simultaneously met within the following dimensional range. The ratio of the cantilever end height H to length L is:

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[0023] The relationship between the sensor test range and the maximum shear stress of the shear strain beam is as follows:

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[0024] The initial center wavelength of each FBG must be pre-calibrated before placement. The bottom of the blind hole is polished to remove any burrs, the surface is wiped in one direction with a dust-free cloth dampened with acetone to ensure the bonding area is clean and flat, the solvent is allowed to evaporate completely, and the direction forming a 45° angle with the central axis of the bottom of the blind hole is accurately marked with a marker pen. Before bonding, the optical fiber exit path is planned and temporarily secured with tape, the epoxy resin is mixed in the specified ratio and stirred thoroughly, and the adhesive is applied evenly to the processed blind hole bonding area with a special tool or blade. The adhesive layer should preferably not exceed 3 microns to ensure a rigid connection between the optical fiber sensor and the functional surface. The bonding surface of the FBG is moved parallel to the bonding area and lightly attached, and the FBG is slid outward while lightly pressing the center of the FBG to remove air bubbles and excess adhesive. Finally, it is secured with tape to prevent disturbance to the optical fiber during the curing process and to maintain stable pressure. After complete curing, a small amount of adhesive is applied to the fiber optic lead-out section and the fillet of the blind hole of the FBG to prevent breakage due to bending of the fiber optic cable. Throughout the entire process from bonding to curing, the change in the center wavelength can be observed in real time by connecting to a demodulator. A sudden change in wavelength occurs at the moment of bonding, and during the curing process, the wavelength drifts slowly as the adhesive hardens, stabilizing at a new value after curing is complete. Based on the changes in wavelength, the success of the bonding and the presence or absence of slippage or peeling are verified.

[0025] After connecting all the optical fibers in series, they are pulled out through a pre-drilled hole on the non-test side of sensing housing 1, connected to the demodulator, and the pre-drilled hole is sealed.

[0026] One end of the support column 6 is welded to the center of the open face of the cube-shaped fixing base, and the other end is welded to the center of the non-open face that does not perform the testing function. The six faces are then sequentially connected and assembled, and the structure is securely sealed.

[0027] The conversion formula between the force acting on the test surface of a three-way earth pressure sensor based on optical fibers and the measured strain is as follows:

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[0028] Calibration of a three-way earth pressure sensor based on optical fiber: First, perform FBG temperature calibration, and then perform sensor pressure calibration.

[0029] Temperature Calibration: The ends of the fiberglass gauge (FBG) are fixed to a lightweight rod, and the rod is placed in a constant temperature bath with a high-precision platinum resistance thermometer positioned nearby. After ensuring that the optical fiber is relaxed and not subjected to external forces, it is connected to the demodulator. The temperature of the constant temperature bath is started at -10°C and increased in 2°C steps up to 40°C. After maintaining the temperature at each temperature point until the wavelength is completely stable, the thermometer reading and the FBG wavelength are recorded. The wavelength change for each FBG is calculated, and linear fitting is performed with the temperature change as the x-coordinate and the wavelength change as the y-coordinate to obtain the temperature sensitivity coefficient for each FBG.

[0030] Pressure Calibration: A three-directional earth pressure sensor based on optical fibers is installed in a three-directional calibration tank, ensuring that its X, Y, and Z axes align with the pressurization direction of the tank body. Unidirectional stepwise pressure and depressurization are performed, and after the pressure stabilizes at each stage, the standard pressure value and FBG wavelength value are recorded. Temperature compensation correction is performed on all FBG wavelength data, and linear fitting is performed with the standard pressure value as the horizontal coordinate and the wavelength change after temperature compensation as the vertical coordinate to obtain one principal sensitivity coefficient and two cross sensitivity coefficients. The wavelength changes in the three directions are then analyzed sequentially to obtain a sensitivity matrix.

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[0031] In the equation, Δλ x、y、z This is the wavelength change in three directions, σ x、y、z is the amount of distortion in three directions, and K is the sensitivity coefficient.

[0032] The specific steps for burying the optical fiber-based three-way earth pressure sensor are as follows: Dig a hole slightly larger than the sensing housing at the designated location, lay about 2 cm of fine sand at the bottom of the hole, level it with a spirit level and compact it to form a flat bottom layer, providing a uniform force-receiving surface and assisting in deformation adjustment. Place the sensor horizontally in the center of the hole with the side from which the optical fiber is drawn facing upwards, correct the horizontal alignment with a spirit level, lightly press the edge of the force-receiving sensing housing to make complete contact with the sand layer, fill the area around the sensing housing with fine sand of similar thickness and compact it horizontally in the same way. Finally, backfill with the original soil at the site, completely enclosing the force-receiving sensing housing, pull the optical fiber out to the surface in a relaxed state, insert it into a protective tube, and collect it in the data acquisition box to record the optical fiber path and prevent subsequent excavation damage. [Explanation of Symbols]

[0033] 1-Sensing Housing 2-Cube fixed stand 3-Shear Strain Beam 4-FBG 5- Fixtures 6-Support column

Claims

1. A three-way earth pressure sensor based on optical fiber, comprising a sensing housing (1), a cubic fixing base (2), a shear strain beam (3), an FBG (4), a fixing device (5), and a support column (6), The cube-shaped mounting base (2) is fixed to the centroid of the sensing housing (1) via a support column (6), and the shear strain beams (3) are each fixed to three mutually perpendicular mounting surfaces on the cube-shaped mounting base (2) via fasteners (5), the surface of the sensing housing (1) fixed to the support column (6) is the bottom surface, the three surfaces provided corresponding to the shear strain beams (3) are test surfaces, and the inner walls of the test surfaces are The shear strain beam (3) is hemispherical and concave, and is L-shaped, including a fixed end and a cantilevered end, the length of the fixed end being shorter than the cantilevered end, the fixing device (5) is fixed to the cubic fixing base (2) by passing through the fixed end, the cantilevered end is suspended in mid-air, the fixed end of the shear strain beam (3) is aligned with the center of the edge of the cubic fixing base (2), and the cantilevered end is in contact with the hemispherical concave inner wall of the sensing housing (1). The shear strain beam (3) has circular blind holes symmetrically arranged along its length on both sides near the fixed end, and an I-shaped cross section is formed in the region where the blind holes are located. The I-shaped cross section consists of a web and flanges located on both sides of the web, the web is located between the two circular blind holes, and a pure shear strain region is formed along the central axis of the web. The maximum range of the sensor test surface pressure P and the maximum shear stress τ of the shear strain beam (3) ｍａｘ This means that the following relationship is satisfied, [Number 14] In the formula, P is the pressure on the test surface at the maximum range of the sensor, b is the width of the I-section web, h is the height of the flange of the I-section, α is the angle between the line connecting the midpoint of the flange and web of the I-section and the neutral axis, and τ ｍａｘ is the maximum shear stress of the shear strain beam, and a is the width of the flange on the I-section. The FBG (4) is attached to the bottom of the blind holes on both sides of the shear strain beam (3), and is configured to form a 45° angle with the neutral axis of the shear strain beam (3), and is provided antisymmetrically with respect to the neutral axis. A three-way earth pressure sensor based on optical fibers, characterized in that all FBGs (4) are connected in series via optical fibers and then drawn out through pre-drilled holes provided on the non-test surface of the sensing housing (1).

2. The optical fiber-based three-way earth pressure sensor according to claim 1, characterized in that the sensing housing (1) is made of a plate material with excellent sealing and corrosion resistance, and the geometric vertex of its hemispherical concave inner wall is in contact with the cantilevered end of the shear strain beam (3).

3. The optical fiber-based three-way earth pressure sensor according to claim 1, characterized in that the outside of the blind hole is chamfered.

4. The optical fiber-based three-directional earth pressure sensor according to claim 1, characterized in that the temperature sensitivity coefficients of two FBGs (4) attached to the same shear strain beam are the same, and during measurement, the amount of change in the central wavelength of the two FBGs (4) on the same shear strain beam is subtracted to remove the effect of bending strain and extract a pure shear strain signal.

5. The ratio of the height H to the length L of the cantilever end is, [Number 15] The condition is satisfied, and the distance d between the center of the blind hole and the edge of the cantilever end is satisfied. １ The ratio of the length of the cantilevered end L is, [Number 16] The following conditions are met, and the ratio of the hole diameter R of the blind hole to the height H of the cantilever end is: [Number 17] The following conditions are met, and the ratio of the depth D of the blind hole to the width B of the cantilever end is: [Number 18] A three-way earth pressure sensor based on an optical fiber as described in claim 1, characterized in that it satisfies the requirements.

6. The relationship between the force acting on the test surface and the maximum principal strain in the direction making a 45° angle with the neutral axis is as follows: [Number 19] In the formula, E is the elastic modulus of the material, μ is the Poisson's ratio of the material, and ε ４５° The optical fiber-based three-directional earth pressure sensor according to claim 1, characterized in that is the maximum principal strain in the direction making a 45° angle with the neutral axis.

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