A triaxial fiber optic earth pressure sensor

Abstract

This invention discloses a three-dimensional fiber optic earth pressure sensor, comprising a sensing housing, a cubic fixing stage, a shear strain beam, a fiber Bragg grating (FBG), fixing components, and support columns. The cubic fixing stage is fixed to the centroid of the sensing housing via the support columns. The shear strain beam is fixed to three mutually perpendicular mounting surfaces on the cubic fixing stage. The sensing housing has three test surfaces corresponding to the shear strain beams, with its inner wall being spherical. The shear strain beam is L-shaped, including a fixed end and a cantilever end. The cantilever end of each beam is tangent to the inner wall of the spherical bowl. Symmetrical circular blind holes are provided on both sides. The FBG is attached to the bottom of the blind holes and connected in series before being led out through pre-drilled holes. The shear strain beam structure of this invention avoids the influence of bending stress, significantly improving measurement accuracy. This invention has strong anti-electromagnetic interference capability, good stability, high accuracy, supports long-distance data transmission, and is suitable for earth pressure detection and monitoring in geotechnical engineering projects such as tunnels and dams.

Description

Technical Field

[0001] The present invention belongs to the field of foundation engineering such as geotechnical tunnels and dams, and specifically relates to a sensor for measuring the three-dimensional stress state of soil by using optical fibers. Background Art

[0002] In foundation engineering such as geotechnical tunnels and dams, earth pressure is a key monitoring parameter. In three-dimensional space, earth pressure is divided into σ1, σ2, and σ3. In natural strata, σ1 is generally the vertical self-weight stress along the direction of gravity. σ1, σ2, and σ3 are mutually orthogonal. Generally, σ1 > σ2 = σ3, and the coefficient of earth pressure at rest k0 is σ2 / σ1. However, when the soil is excavated, σ2 and σ3 will no longer be equal, resulting in the active earth pressure state: the active earth pressure coefficient k1 = σ3 / σ1 is less than k0, and the passive earth pressure state: the passive earth pressure coefficient k2 = σ2 / σ1 is greater than k0. In tunnel engineering in water-rich soft soil layers, the distribution and dynamic monitoring of three-dimensional earth pressure are directly related to the stability of the excavation face, ground settlement, and the safety of the lining structure. The high compressibility, low strength, and high sensitivity of soft soil, combined with the action of groundwater, make the vertical stress σ1 in the initial in-situ stress field the total self-weight of the overlying soil and water, and the horizontal stresses σ2 = σ3 ≈ k0σ1, and k0 is often close to 1 or even larger. After the tunnel is excavated, the radial stress σ3 on the free face is rapidly released, the pore water pressure dissipates, and the effective stress decreases, forming an active stress state of σ1 > σ2 > σ3 (k1 = σ3 / σ1 << k0), which is likely to lead to the instability of the excavation face, soft soil creep, or even flow. When tunneling with a shield, it is necessary to actively maintain the three-dimensional stress balance of the face through the slurry pressure or earth pressure balance mode to restore σ2 and σ3 to an approximately static state. After the lining support, the rheological effect of water-rich soft soil and the action of external water-soil loads may cause the circumferential stress σ2 to continuously accumulate, gradually forming a passive earth pressure state (k2 = σ2 / σ1 > k0), seriously affecting the mechanical properties and durability of the segments. In addition, the synchronous grouting pressure, ground loss, and long-term consolidation will all disturb the three-dimensional effective stress path. It is necessary to monitor the conversion of water-soil combined pressure and effective stress in real time, dynamically adjust the support strategy, and prevent soft soil shear failure, segment deformation, or leakage. Earth pressure cells are basic tools for monitoring in geotechnical tunnel engineering and play an irreplaceable role in ensuring the safety of geotechnical tunnel engineering.

[0003] Currently, common earth pressure cells can only measure the earth pressure value in one direction of the soil body and cannot measure the earth pressure values in three mutually perpendicular directions simultaneously, which brings difficulties to the safety construction, normal operation, and health status assessment of monitoring foundation engineering such as tunnels and dams.

[0004] Patent CN106768521A integrates three test planes at 60° angles to each other into the probe indentation part, creating an indentation-type triaxial soil pressure sensor and its measurement method for loess and soft soil. In this design, the sharp edges of the triangular pyramid structure easily cause localized compression of the loess and soft soil, leading to additional stress deformation in the soil around the probe, resulting in a deviation between the pressure measured by the resistance strain gauge and the actual soil pressure.

[0005] Patent CN115326269A describes the fabrication of a three-dimensional earth pressure cell using a thinned cubic structure and resistance strain gauges. This approach offers high accuracy, mature technology, and small size; however, the resistance strain gauge sensor is susceptible to electromagnetic interference, and its accuracy is relatively low over long distances.

[0006] Patent CN105628279A employs a scheme where an organic polymer cylinder with spiral liquid metal microchannels is sandwiched between the six stress-bearing surfaces and opposite surfaces of a dodecahedral box, and an antenna is formed by exciting the microchannels through a coaxial feed line. This scheme solves the problems of traditional sensors, such as inability to monitor over long distances, susceptibility to electromagnetic interference, and the need for multi-angle measurements to obtain the complete stress tensor. It can acquire six stress components wirelessly and remotely at once, has strong anti-electromagnetic interference capabilities, and is suitable for long-term monitoring. However, its dodecahedral structure and internal multi-cylinder layout result in a complex sensor structure and large size, and the conversion relationship between the frequency response and stress of the liquid metal antenna is complex, making calibration difficult. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention develops a fiber-optic-based triaxial earth pressure sensor. The sensor's test surface shell is specially designed to handle triaxial stress acquisition, and a shear strain beam is internally embedded within it. A high-precision, long-distance transmission-capable fiber-optic shear strain gauge (FBG) is attached to the bottom of the blind hole in the shear strain beam. When the earth pressure sensor is under stress, the shear strain beam deforms due to pressure from the top surface of the vertical free end of the beam. By measuring the change in the center wavelength of the FBG, the maximum principal stress on the shear strain beam is reflected, and the earth pressure value is calculated. It can simultaneously measure the triaxial stress state within the soil. The specific scheme of this invention is as follows:

[0008] This invention discloses a three-dimensional fiber optic earth pressure sensor, comprising a sensing housing, a cubic fixing platform, a shear strain beam, an FBG (fiber optic cable), fixing components, and a support column. The cubic fixing platform is fixed to the centroid of the sensing housing via the support column. The shear strain beam is fixed to three mutually perpendicular mounting surfaces on the cubic fixing platform via the fixing components. The surface of the sensing housing fixed to the support column is the bottom surface, and the three sides of the shear strain beam are correspondingly arranged as test surfaces. The inner wall of the test surface is spherical. The shear strain beam is L-shaped, including a fixed end and a cantilever end. The fixed end is shorter than the cantilever end, and the fixing component passes through the fixed end and is fixed to the FBG. On the cube-shaped fixed platform, the cantilever end is suspended in the air. The fixed end of the shear strain beam is aligned with the center of the edge of the cube-shaped fixed platform, and the cantilever end is tangent to the spherical inner wall of the sensing housing. The shear strain beam has symmetrically arranged circular blind holes along its length on both sides near the fixed end, making the cross-section of the section where the blind holes are located in the shear strain beam I-shaped. The FBG is attached to the bottom of the blind holes on both sides of the shear strain beam and arranged at a 45° angle to the neutral axis of the shear strain beam, and anti-symmetrical about the neutral axis. All FBGs are connected in series through optical fibers and led out through the reserved holes on the non-test surface of the sensing housing.

[0009] As a preferred embodiment, the sensing housing is composed of a plate with good sealing performance and excellent corrosion resistance. The inner walls of the three mutually perpendicular surfaces that undertake the testing task are designed in a spherical shape to reduce errors caused by uneven pressure distribution. The shear strain beam is tangent to the bottom of the spherical shape of the inner wall of the testing surface to facilitate the vertical transmission of pressure from the testing surface, reduce the interference of other loads on the measurement, increase the contact area between the edges, and prevent stress concentration caused by sudden changes in stiffness, which could lead to housing damage.

[0010] As a preferred embodiment, the blind hole has an external chamfer to prevent damage during fiber optic connection.

[0011] As a preferred option, the temperature sensitivity coefficients of the two FBGs attached to the same shear strain beam should be consistent. During the test, the wavelength change values ​​of the FBGs in the blind holes on both sides of the same shear strain beam are subtracted. Since there are principal stresses and principal strains of equal magnitude in the tensile and compressive directions at 45° to the central axis, the wavelength subtraction can offset the temperature effect and enhance the sensitivity.

[0012] As a priority, 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 beam satisfies the following formula: The ratio of the distance d1 from the center of the blind hole to the edge of the cantilever beam to the length L of the cantilever beam satisfies the following formula: The ratio of the blind hole diameter R to the cantilever beam height H satisfies the following formula: The ratio of the blind hole depth D to the cantilever beam width B satisfies the following formula: .

[0013] As a priority, the relationship between the sensor's measurement range and the maximum shear stress of the shear strain beam is as follows: In the formula, The test surface pressure is the maximum range of the sensor (the maximum range is determined by the usage requirements). The width of the web of the I-shaped cross-section. The flange height is the height of the I-shaped section. The angle between the line connecting the midpoint of the flange and the web of the I-shaped section and the neutral axis is... The maximum shear stress of the shear strain beam. The width of the upper flange of the I-shaped cross section.

[0014] As a priority, the relationship between the force on the test surface and the maximum principal strain at a 45° angle to the neutral axis is as follows: In the formula, The elastic modulus of the material, For the material's Poisson's ratio, The maximum principal strain is at a 45° angle to the neutral axis.

[0015] Advantages of this invention: The shear strain beam of this invention has symmetrically formed circular blind holes on both sides near the fixed end along the length direction, making its cross-section I-shaped. This creates stress concentration in this area, increasing the strain in this region and improving the sensitivity of the sensor. The web, as the main shear force bearing area, forms a uniformly stressed "pure shear strain zone" at the central axis of the web, unaffected by the bending moment that varies along the beam length. The triaxial fiber optic earth pressure sensor disclosed in this invention has advantages such as small size, high accuracy and sensitivity, strong anti-electromagnetic interference capability, long-distance transmission capability, and simple data processing method. It utilizes principal stresses and principal strains of equal magnitude for tension and compression to enhance sensitivity and eliminate the influence of temperature changes on the test results. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of the present invention.

[0017] Figure 2 This is a front view of the central cross-section of the structure of the present invention.

[0018] Figure 3 This is a side view of the shear strain beam of the present invention.

[0019] Figure 4 This is a cross-sectional view of the I-shaped section at the blind hole of the present invention.

[0020] Figure 5 This is a three-dimensional dimension diagram of the shear strain beam of the present invention.

[0021] In the figure: 1-sensor housing, 2-cubic fixed stage, 3-shear strain beam, 4-FBG, 5-fixed component, 6-support column. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0023] like Figures 1 to 5 A triaxial fiber optic earth pressure sensor is shown, comprising a sensing housing 1, a cubic mounting platform 2, three shear strain beams 3, an FBG 4, a fixing element 5, and a support column 6. The cubic mounting platform 2 is fixed to the centroid of the sensing housing 1 by the support column 6, with the surface of the sensing housing 1 fixed to the support column 6 being the bottom surface. The shear strain beams 3 are respectively fixed to three mutually perpendicular mounting surfaces on the cubic mounting platform 2 by the fixing element 5. The sensing housing 1 is provided with three sides corresponding to the shear strain beams 3 as test surfaces, and the inner wall of the test surface is spherical. The shear strain beam 3 is L-shaped, including a fixed end and a cantilever end. The fixed end is shorter than the cantilever end. The fixing member 5 passes through the fixed end and is fixed to the cube-shaped fixing platform 2, so that the cantilever end is in a suspended state. The fixed end of the shear strain beam 3 is aligned with the center of the edge of the cube-shaped fixing platform 2, and the cantilever end is tangent to the spherical inner wall of the sensing housing 1. The shear strain beam 3 has symmetrically arranged circular blind holes on both sides near the fixed end along its length, so that the cross-section of the section 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 arranged at a 45° angle to the neutral axis of the shear strain beam 3, and is anti-symmetrical about the neutral axis. All FBG4 are connected in series by optical fibers and led out through the reserved holes on the non-test surface of the sensing housing 1.

[0024] The sensor housing 1 is composed of sheet metal with good sealing and corrosion resistance to avoid affecting monitoring accuracy when embedded in a humid environment. The three test surfaces responsible for the testing tasks are as follows: Figure 1 The inner walls of the three test surfaces—front, right, and bottom—are designed in a spherical shape to reduce stress concentration, avoid local fatigue, reduce errors caused by uneven pressure distribution, and improve measurement accuracy.

[0025] The shear strain beam 3 is a cantilever beam, with the cantilever end tangent to the spherical inner wall of the sensing housing 1. This facilitates the vertical transmission of pressure across the test surface and effectively reduces the interference of lateral and torsional loads on the measurement. Two blind holes are symmetrically machined at the neutral axis positions near the fixed ends to enhance the strain response of the shear strain beam under pressure. The shear force is constantly distributed along the beam length, and the stress does not decrease or fluctuate with position. The symmetrical flange equilibrium section moment of inertia stabilizes the neutral axis. The blind holes are chamfered to prevent damage during fiber optic connection.

[0026] The FBG (Bulk Frame Gear) is bonded at 45° to the neutral axis of the shear strain beam. This allows for the measurement of the maximum principal stress under pure shear conditions, eliminating bending stress interference and capturing the principal stress transformed from shear stress, thus achieving accurate measurement. The FBGs bonded in the blind holes on both sides are anti-symmetrical about the neutral axis, and the temperature sensitivity coefficients of the two FBGs bonded to the same shear strain beam should be consistent. Figure 3 During measurement, the wavelength changes measured by the two FBGs are subtracted. The web of the I-shaped cross-section has two principal stresses and strains of equal magnitude, one tensile and one compressive, in a direction at 45° to the central axis. Subtracting the wavelength changes on both sides improves sensitivity and also offsets the influence of external temperature changes on the test results, thus providing temperature compensation. The I-shaped cross-section is as follows: Figure 4 Parameter 'a' is the width of the upper flange of the I-shaped section. The angle between the line connecting the midpoint of the flange and the web of the I-shaped section and the neutral axis is... The flange height is the height of the I-shaped section. The width of the web of the I-shaped cross section.

[0027] Finite element analysis was performed on a shear strain beam to verify that the range and sensitivity requirements can be met simultaneously within the following dimensional range: the ratio of the height H to the length L of the cantilever beam satisfies the following formula: The ratio of the distance d1 from the center of the blind hole to the edge of the cantilever beam to the length L of the cantilever beam satisfies the following formula: The ratio of the blind hole diameter R to the cantilever beam height H satisfies the following formula: The ratio of the blind hole depth D to the cantilever beam width B satisfies the following formula: Specifically, as follows Figure 5 Parameter H is the height of the cantilever beam, parameter B is the width of the cantilever beam, parameter L is the length of the cantilever beam, parameter R is the radius of the blind hole, parameter D is the depth of the blind hole, and parameter d1 is the distance from the center of the blind hole to the edge of the cantilever beam.

[0028] The relationship between the sensor's measurement range and the maximum shear stress of the shear strain beam is as follows: In the formula, The test surface pressure is the maximum range of the sensor (the maximum range is determined by the usage requirements). The width of the web of the I-shaped cross-section. The flange height is the height of the I-shaped section. The angle between the line connecting the midpoint of the flange and the web of the I-shaped section and the neutral axis is... The maximum shear stress of the shear strain beam. The width of the upper flange of the I-shaped cross section.

[0029] Before deploying each FBG, the initial center wavelength needs to be calibrated in advance. Grind the bottom of the blind via to remove any burrs. Wipe the surface in one direction with a lint-free cloth dampened with acetone to ensure the bonding area is clean and flat. After the solvent has completely evaporated, use a marker to accurately mark the bottom of the blind via at a 45° angle to the central axis. Plan the fiber optic lead-out path before bonding and temporarily fix it with tape. Mix the epoxy resin according to the ratio and stir thoroughly. Use a special tool or scraper to evenly apply the adhesive to the treated blind via bonding area. The adhesive layer should not exceed 3 micrometers to ensure a rigid connection between the fiber optic sensor and the working surface. Gently press the bonding surface of the FBG onto the bonding area in a parallel moving manner. Squeeze the center of the FBG and slide it outward to remove air bubbles and excess adhesive. Finally, fix it with tape. Do not disturb the fiber during the curing process and maintain stable pressure. After complete curing, apply a small amount of adhesive to the FBG fiber lead-out point and the rounded corners of the blind via to prevent the fiber from breaking due to bending. Throughout the bonding and curing process, a demodulator can be connected to observe the changes in the center wavelength in real time: there is a jump at the moment of bonding, and during the curing process, there will be a slow drift due to the curing of the adhesive, stabilizing at a new value after curing is complete. The wavelength change can be used to verify whether the bonding was successful and whether there is slippage or detachment.

[0030] After all the optical fibers are connected in series, they are led out from the reserved hole on the non-test surface of the sensor housing 1, connected to the demodulator, and sealed at the reserved hole.

[0031] One end of the support column 6 is welded to the center of a blank face of the cube fixing platform, and the other end is welded to the center of a non-open face that does not undertake the testing task. The six faces are connected and assembled in sequence, and then sealed.

[0032] The conversion formula between force and measured strain on the test surface of a three-dimensional fiber optic earth pressure sensor is as follows: In the formula, The elastic modulus of the material, For the material's Poisson's ratio, The maximum principal strain (i.e., the measured strain) is at a 45° angle to the neutral axis.

[0033] Calibration of the three-dimensional fiber optic earth pressure sensor: First, perform FBG temperature calibration, then perform sensor pressure calibration.

[0034] Temperature Calibration: Fix both ends of the FBG to a lightweight rod, then place the rod inside a constant temperature chamber. Place a high-precision platinum thermometer close to the rod, ensuring the fiber is relaxed and unstressed. Connect the demodulator. Starting from -10℃, increase the chamber temperature in 2℃ increments, up to 40℃. Hold at each temperature until the wavelength is completely stable, then record the thermometer reading and the FBG wavelength value. Calculate the wavelength change for each FBG. Plot the temperature change on the x-axis and the wavelength change on the y-axis, and perform a linear fit to obtain the temperature sensitivity coefficient for each FBG.

[0035] Pressure calibration: Install the triaxial fiber optic earth pressure sensor in the triaxial calibration tank, ensuring that its X, Y, and Z axes are aligned with the tank's pressurization direction. Perform unidirectional graded loading and unloading. After each pressure stage stabilizes, record the standard pressure value and FBG wavelength value. Perform temperature compensation correction on all FBG wavelength data. Plot the standard pressure value on the x-axis and the wavelength change after temperature compensation on the y-axis for linear fitting to obtain a principal sensitivity coefficient and two cross sensitivity coefficients. Analyze the wavelength changes in the three directions sequentially to obtain the sensitivity matrix.

[0036] In the formula, These represent the wavelength changes in three directions. For the dependent variable in three directions, This is the sensitivity coefficient.

[0037] The specific steps for installing a three-dimensional fiber optic earth pressure sensor are as follows: Excavate a pit slightly larger than the sensor housing at the predetermined location. Lay a layer of medium-fine sand about 2cm thick at the bottom of the pit, level and compact it with a leveling rod to form a flat pad, providing a uniform stress surface and helping to coordinate deformation. Place the sensor with the fiber optic cable facing upwards, horizontally in the center of the pit, and level it with a level. Gently press the edge of the stress-bearing housing to ensure it is in complete contact with the sand layer. Fill the perimeter of the housing with medium-fine sand of approximately the same thickness and compact it horizontally using the same method. Finally, backfill with the original soil on site to completely enclose the stress-bearing housing. Loosen the fiber optic cable to the ground surface, pass through a protective pipe, and then merge it into the data acquisition box. Record the fiber optic cable path to prevent damage during subsequent excavation.

Claims

1. A triaxial fiber optic earth pressure sensor, characterized in that: The system includes a sensing housing (1), a cube-shaped fixing platform (2), a shear strain beam (3), an FBG (4), a fixing element (5), and a support column (6). The cube-shaped fixing platform (2) is fixed to the centroid of the sensing housing (1) by the support column (6). The shear strain beam (3) is fixed to three mutually perpendicular mounting surfaces on the cube-shaped fixing platform (2) by the fixing element (5). The surface where the sensing housing (1) is fixed to the support column (6) is the bottom surface, and the three sides of the shear strain beam (3) are correspondingly set as test surfaces. The inner wall of the test surface is spherical. The shear strain beam (3) is L-shaped, including a fixed end and a cantilever end. The length of the fixed end is shorter than that of the cantilever end. The component (5) is fixed to the cube-shaped fixed platform (2) through the fixed end, so that the cantilever end is in a cantilevered state. The fixed end of the shear strain beam (3) is aligned with the center of the edge of the cube-shaped fixed platform (2), and the cantilever end is tangent to the spherical inner wall of the sensor housing (1). The shear strain beam (3) has circular blind holes symmetrically opened on both sides near the fixed end along the length direction, so that the shear strain beam (3) forms an I-shaped cross section in the section where the blind holes are located. The I-shaped cross section is composed of a web and flanges located on both sides of the web. The web is located between the two circular blind holes and forms a pure shear strain zone at the central axis of the web. Among them, the sensor test surface pressure The maximum range and the maximum shear stress of the shear strain beam (3) Satisfy the following relationship: In the formula, The test surface pressure is measured at the sensor's maximum range. The width of the web of the I-shaped cross-section. The flange height is the height of the I-shaped section. The angle between the line connecting the midpoint of the flange and the web of the I-shaped section and the neutral axis is... The maximum shear stress of the shear strain beam. The width of the flange on the I-shaped cross section; the FBG (4) is attached to the bottom of the blind holes on both sides of the shear strain beam (3) and arranged at a 45° angle with the neutral axis of the shear strain beam (3), and is anti-symmetrical about the neutral axis; after all FBG (4) are connected in series by optical fibers, they are led out through the reserved holes set on the non-test surface of the sensing housing (1).

2. The triaxial fiber optic earth pressure sensor according to claim 1, characterized in that: The sensing housing (1) is made of a plate with good sealing and corrosion resistance, and the geometric vertex of its spherical inner wall is tangent to the cantilever end of the shear strain beam (3).

3. A triaxial fiber optic earth pressure sensor according to claim 1, characterized in that: The external chamfer of the blind hole.

4. A triaxial fiber optic earth pressure sensor according to claim 1, characterized in that: The temperature sensitivity coefficients of the two FBGs (4) attached to the same shear strain beam are consistent. During measurement, the change in the center wavelength of the two FBGs (4) on the same shear strain beam is subtracted to eliminate the influence of bending strain and extract the pure shear strain signal.

5. A triaxial fiber optic earth pressure sensor according to claim 1, characterized in that: The ratio of the height H to the length L of the cantilever end satisfies the following formula: The ratio of the distance d1 from the center of the blind hole to the edge of the cantilever end to the length L of the cantilever end satisfies the following formula: The ratio of the blind hole diameter R to the cantilever end height H satisfies the following formula: The ratio of the blind hole depth D to the cantilever end width B satisfies the following formula: .

6. A triaxial fiber optic earth pressure sensor according to claim 1, characterized in that: The relationship between the force on the test surface and the maximum principal strain at a 45° angle to the neutral axis is as follows: In the formula, The elastic modulus of the material, For the material's Poisson's ratio, This represents the maximum principal strain in the direction at 45° to the neutral axis.

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