FBG optical fiber sensing optical cable for monitoring internal mechanical behaviors of side slope

Abstract

The invention relates to the technical field of DFOS optical fiber sensing, and discloses an FBG optical fiber sensing optical cable for monitoring internal mechanical behaviors of a slope. The FBG optical fiber sensing optical cable for monitoring the internal mechanical behavior of the slope comprises an outer tube, an inner tube and optical fibers, the outer tube sleeves the inner tube, the inner tube wraps three optical fibers adhered by epoxy resin, and the three optical fibers can detect three groups of data, so that the accuracy of the data is enhanced; and an air layer is arranged between the inner tube and the outer tube and is used for buffering the inner tube when external force is applied, and when the external force disappears, the elasticity of the inner tube is recovered, so that the smoothness of the whole FBG optical fiber sensing optical cable is ensured, and the accuracy of monitoring the internal mechanical behavior of the slope is not influenced. According to the FBG optical fiber sensing optical cable for monitoring the internal mechanical behaviors of the side slope, the internal mechanical behaviors of the side slope can be accurately monitored in real time, and a scientific basis is provided for side slope stability evaluation and geological disaster early warning.

Description

Technical Field

[0001] The invention relates to the technical field of DFOS optical fiber sensing, and in particular to an FBG optical fiber sensing cable for monitoring the internal mechanical behavior of a slope. Background Art

[0002] In recent decades, as the global climate disaster problem has intensified, the slope protection problem has received widespread attention. Landslides and slope collapses pose a major threat to ecosystems and human activities. These threats include mudslides, earthquakes, sinkholes and collapses caused by soil erosion and melt sedimentation. Therefore, in order to prevent slope geological disasters and ensure the safety of engineering construction, it is crucial to accurately and real-time monitor the displacement and deformation of slopes.

[0003] Slope displacement and deformation monitoring technologies can be divided into two categories according to the monitoring method: non-contact and contact. Non-contact monitoring methods for slope displacement and deformation mainly include global satellite navigation system (GNSS), synthetic aperture radar (InSAR), airborne laser scanning technology, and digital photogrammetry. These monitoring methods have been widely used in complex surface slope displacement monitoring. Contact monitoring instruments, such as drilling leveling devices, inclinometers, and displacement meters, are usually used to collect underground slope displacement and deformation data, thereby solving the underground measurement constraints of non-contact monitoring methods. However, they have limitations in achieving accurate and real-time monitoring of geological profile deformation, hindering a comprehensive understanding of the deformation characteristics of strata and related geological interfaces. Therefore, these methods need to be further improved and refined.

[0004] Distributed fiber optic sensing technology has been recognized as an advanced geotechnical monitoring method with numerous advantages. In the past few decades, DFOS fiber optic sensing technology has played a vital role in solving various geoengineering challenges, such as slope stability, foundation bearing capacity, tunnel deformation, and embankment settlement. At present, the application of DFOS fiber optic sensing technology in slope displacement and deformation monitoring mainly revolves around two scientific issues: soil settlement and soil migration.

[0005] To this end, it is necessary to understand the deformation coordination problem between the FBG fiber optic sensor cable and the surrounding soil layer. After using the FBG fiber optic sensor cable as a distributed strain sensor nerve, the earth becomes sensitive and intelligent, and can accurately perceive geological processes and identify related geological disasters at an early stage. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a FBG optical fiber sensing cable for monitoring the internal mechanical behavior of the slope, which is used to obtain the internal mechanical behavior of the slope displacement. By using the DFOS optical fiber sensing technology, the variation law of the slope displacement strain and shear stress is analyzed. By analyzing the soil layer distribution characteristics of the shear stress, the progressive failure behavior of the slope displacement under different conditions is revealed. In addition, by using the existing theoretical formula, the peak shear stress and residual shear stress of the interface can be inverted and calculated, which is of great significance for understanding the internal mechanical behavior of the slope displacement.

[0007] The FBG optical fiber sensing cable for monitoring the internal mechanical behavior of a slope provided by the present invention comprises an outer tube, an inner tube and an optical fiber: wherein the outer tube is sleeved on the inner tube, and the inner tube embraces three optical fibers bonded with epoxy resin, and the three optical fibers can detect three sets of data, thereby enhancing the accuracy of the data; an air layer is provided between the inner tube and the outer tube, so as to buffer the inner tube when an external force is applied, and when the external force disappears, the inner tube recovers its own elasticity, thereby ensuring the flexibility of the entire FBG optical fiber sensing cable and not affecting the accuracy of monitoring the internal mechanical behavior of the slope.

[0008] Preferably, the outer tube is made of polyurethane, the outer tube and the inner tube are 1500 mm long, the outer tube has a diameter of 2200 um, and the inner tube has a diameter of 2000 um.

[0009] Preferably, the outermost layer of the optical fiber is a resin coating for reinforcement, with a diameter of 900um; the middle is a low-refractive index glass cladding, with a diameter of generally 125um.

[0010] Preferably, the FBG optical fiber sensor cable is provided with an optical cable head fixture at the end, which has sealing and waterproof and dustproof functions to ensure the reliability of the optical cable in harsh environments. This type of optical cable fixture (also known as optical cable waterproof connector, optical cable connector) is widely used in the fixation and protection of wires and cables of mechanical equipment electrical, optical fiber, ship electrical, and anti-corrosion equipment.

[0011] The present invention proposes a device for monitoring the internal mechanical behavior of a slope using an FBG optical fiber sensing cable, wherein the device includes the above-mentioned FBG optical fiber sensing cable, and the device also includes a dynamometer, a weighing sensor, and a fiber grating demodulator: wherein the FBG optical fiber sensing cable is used to sense the strain information at the interface between the optical cable and the slope soil layer during the test; the fiber grating demodulator is used to quickly capture the strain measurement value; the weighing sensor can measure the pull-out force; and the pull-out force control center obtains the corresponding pull-out force and displacement information through the dynamometer. Through the application of this test device, the stress deformation characteristics and progressive destructive behavior of the interface between the FBG optical fiber sensing cable and the slope soil layer can be effectively identified. Before the pulling process, the dryness and humidity of the slope soil layer were measured, and it was found that the influence of the soil dryness and humidity on the experimental results during the slope sliding process was negligible.

[0012] Preferably, the weighing sensor is an S-type tension and pressure sensor, which is the most common type of sensor and is mainly used to measure tension and pressure between solids. It is also generally called a tension and pressure sensor. Because its appearance is S-shaped, it is also customarily called an S-type weighing sensor. The weighing sensor is made of alloy steel and is treated with glue sealing protection. It is easy to install and use, and is suitable for electronic force measuring and weighing systems such as crane scales, batching scales, and machine-modified scales.

[0013] The present invention proposes a monitoring method for monitoring the internal mechanical behavior of a slope using an FBG optical fiber sensing cable, the method comprising the steps of:

[0014] The FBG optical fiber sensing cable is buried in the slope soil layer parallel to the slope interface, and the head fixture of the FBG optical fiber sensing cable is connected to the weighing sensor and the tensile gauge, and the FBG optical fiber sensing cable is connected to the fiber grating demodulator, and the surrounding area is backfilled to ensure the deformation coordination at the cable-soil interface;

[0015] When the soil settles, it will cause the settlement of the backfill layer, causing the FBG optical fiber sensing cable and the soil to move relative to each other, thereby generating shear stress. The FBG optical fiber sensing cable monitors the shear stress strain information, and the fiber grating demodulator collects the strain data of the buried section of the FBG optical fiber sensing cable. At the same time, the weighing sensor obtains the pull-out force, and the tensile meter collects the pull-out force and displacement information.

[0016] The collected data are processed to analyze the mechanical behavior of the axial strain and shear stress inside the slope, and the changing trend of the mechanical behavior inside the slope and its impact on the stability of the slope are evaluated.

[0017] Compared with the related art, the FBG optical fiber sensing cable for monitoring the internal mechanical behavior of the slope provided by the present invention has the following beneficial effects:

[0018] The FBG optical fiber sensing cable provided by the present invention for monitoring the internal mechanical behavior of the slope performs coupling state analysis through force and displacement characteristics and the change characteristics of the shear stress of the slope interface during the pulling process, and establishes an ideal elastic-plastic model and a strain softening model suitable for the FBG optical fiber sensing cable-slope interface; the pull-out force-displacement pre-peak phase data of the slope is normalized, and the pre-peak pulling process is divided into three coupling states according to the normalized pull-out force and displacement after processing: excellent coupling, good coupling, and general coupling. The peak shear stress and residual shear stress at the interface can be inversely calculated using the existing theoretical formula. The inverse calculation results are compared with the calculated values ​​of the FBG optical fiber sensing cable, and the applicability of the existing theoretical model is studied. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1A schematic diagram of the structure of an FBG optical fiber sensing cable for monitoring the internal mechanical behavior of a slope provided by the present invention;

[0020] Figure 2 A schematic diagram of a visualized slope simulation device provided by the present invention;

[0021] Figure 3 A schematic diagram of the visualized slope simulation device provided by the present invention in a pull-out measurement state. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with the accompanying drawings and implementation modes.

[0023] The FBG optical fiber sensing cable for monitoring the internal mechanical behavior of the slope provided by the present invention comprises an outer tube, an inner tube and an optical fiber (as shown in the attached manual). Figure 1 As shown): wherein, the outer tube is sleeved on the inner tube, the outer tube is made of polyurethane, the outer tube and the inner tube are 1500mm long, the outer tube diameter is 2200um, and the inner tube diameter is 2000um; the inner tube embraces three optical fibers bonded with epoxy resin, the three optical fibers can detect three sets of data, and the accuracy of the data is enhanced, the outermost layer of the optical fiber is a resin coating for reinforcement, the diameter is 900um; the middle is a low-refractive index glass cladding, the diameter is generally 125um; there is an air layer between the inner tube and the outer tube, which can buffer the inner tube when an external force is applied. When the external force disappears, the inner tube recovers its own elasticity, ensuring the flexibility of the entire FBG optical fiber sensor cable, and does not affect the accuracy of monitoring the internal mechanical behavior of the slope;

[0024] The FBG optical fiber sensor cable is provided with an optical cable head fixture at the end, which has sealing and waterproof and dustproof functions to ensure the reliability of the optical cable in harsh environments. This type of optical cable fixture (also known as optical cable waterproof joint, optical cable joint) is widely used in the fixation and protection of wires and cables of mechanical equipment electrical, optical fiber, ship electrical, and anti-corrosion equipment.

[0025] The present invention proposes a device for monitoring the internal mechanical behavior of a slope using an FBG optical fiber sensing cable, wherein the device includes the above-mentioned FBG optical fiber sensing cable, and the device also includes a dynamometer, a weighing sensor, and a fiber grating demodulator: wherein the FBG optical fiber sensing cable is used to sense the strain information at the interface between the optical cable and the slope soil layer during the test; the fiber grating demodulator is used to quickly capture the strain measurement value; the weighing sensor can measure the pull-out force; and the pull-out force control center obtains the corresponding pull-out force and displacement information through the dynamometer. Through the application of this test device, the stress deformation characteristics and progressive destructive behavior of the interface between the FBG optical fiber sensing cable and the slope soil layer can be effectively identified. Before the pulling process, the dryness and humidity of the slope soil layer were measured, and it was found that the influence of the soil dryness and humidity on the experimental results during the slope sliding process was negligible.

[0026] It should be noted that a weighing sensor is a device that converts a mass signal into a measurable electrical signal output. When using a sensor, the actual working environment of the sensor should be considered first. This is crucial to the correct selection of a weighing sensor. It is related to whether the sensor can work normally, its safety and service life, and even the reliability and safety of the entire weighing instrument. In the basic concepts and evaluation methods of the main technical indicators of weighing sensors, the old and new national standards have qualitative differences. There are mainly several styles such as S-type, cantilever type, spoke type, plate ring type, diaphragm box type, bridge type, column type, etc. The weighing sensor used in the present invention is S-type, which is the most common type of sensor. It is mainly used to measure the tension and pressure between solids. It is also commonly called a tension and pressure sensor. Because its appearance is like an S shape, it is also customarily called an S-type weighing sensor. The weighing sensor is made of alloy steel and is treated with glue sealing protection. It is easy to install and easy to use. It is suitable for electronic force measurement and weighing systems such as crane scales, batching scales, and machine-modified scales.

[0027] It should be noted that the FBG fiber optic sensor cable uses DFOS fiber optic sensing technology, which refers to the use of optical information inside the optical fiber to obtain the continuous distribution information of the external physical parameters of the optical fiber in space and time. One of the technical means to achieve distributed monitoring is distributed fiber optic sensing technology. In DFOS fiber optic sensing technology, optical fiber can not only be used as a sensing medium to sense external physical quantities, but also as a transmission channel to transmit optical information. DFOS fiber optic sensing technology uses the characteristics of unidirectional transmission of optical fiber, takes the physical quantity to be monitored as a function of the light transmission position, and senses the distribution and changes of the measured parameters along the optical fiber. Placing the sensing optical fiber inside the geological body is like planting a neural network that can sense external changes in the geological body and engineering structure, sensing the changes in the physical properties of the measured object in one, two and three dimensions, overcoming the disadvantages of traditional monitoring methods that can only monitor point-to-point and are easy to miss, and increasing the scope of monitoring. DFOS fiber optic sensing technology is more applicable in monitoring parameters such as overall strain and temperature in large or super-large projects, such as uneven settlement monitoring of long-distance tunnels, large-area ground fissure distribution and deformation monitoring, long-distance oil pipeline leakage monitoring, and large-scale slope deformation monitoring. The basic physical quantities obtained by fiber optic sensing technology are mainly strain, temperature, vibration, and length. Based on this, through sensing fiber optic packaging technology and conversion principles, it can also measure physical quantities such as pressure, stress, water level, flow rate, moisture, humidity, flow, current, voltage, liquid level, and toxic gas, and has a wide range of applications.

[0028] In the present invention, when DFOS optical fiber sensing technology is applied to slope displacement and deformation monitoring, it is assumed that only axial strain occurs along the interface. The specific method is to bury the FBG optical fiber sensing cable in the soil parallel to the slope interface and backfill the surrounding area to ensure the deformation coordination at the cable-soil interface. When the soil settles, it will cause the settlement of the backfill layer, causing the FBG optical fiber sensing cable and the soil to move relative to each other, thereby generating shear stress and selecting micro units for mechanical analysis.

[0029] The efficiency of FBG fiber optic sensing technology in describing the displacement of slope soil layers depends largely on the coupling relationship between the FBG fiber optic sensing cable and the soil. The pre-peak hardening stage of the pull-out force-displacement curve is an effective means to characterize the coupling state between the FBG fiber-cable and the soil interface. In order to analyze the coupling state, the pre-peak pull-out force and displacement are normalized respectively. FN is the normalized pull-out force, which characterizes the coupling between the FBG fiber and the soil interface. The pre-peak pull-out force is calculated by dividing the peak pull-out force by the pre-peak pull-out force. The ratio of the pre-peak to peak pull-out forces is used. Similarly, ZN represents the normalized pull-out displacement, which is obtained by dividing the pre-peak pull-out displacement by the peak pull-out displacement.

[0030] The internal mechanical behavior of the slope monitored by the present invention includes shear stress. The extension of shear stress starts from the upper part of the soil. As the pull-out displacement increases, the shear stress gradually extends downward along the slope interface. When the interface is completely penetrated and destroyed, the FBG optical fiber sensing cable slides relative to the entire slope interface, and the interface shear stress decreases rapidly and enters the residual stage. In addition, the pull-out displacement associated with the peak shear stress is usually recorded as the effective displacement, which serves as the boundary to distinguish the two working states of the effective state and the invalid state during the pull-out process.

[0031] The following examples illustrate the use of FBG optical fiber sensing cables to simulate monitoring of the internal mechanical behavior of the slope in the field:

[0032] Embodiment 1:

[0033] The present invention proposes a device for on-site simulation monitoring of the internal mechanical behavior of a slope using FBG optical fiber sensing cables. The device simulates the shear sliding damage and sliding state of the rock mass during the landslide process. A visual simulation slope device is designed (see the attached manual). Figure 2 ). The test device consists of tempered glass, bricks and soil. The heights of bricks 1, 2 and 3 are 300mm, 200mm and 100mm respectively, and the widths are 300mm, 200mm and 100mm, which are used to simulate the bedrock inside the slope. Above the bricks is the soil layer, which is at a 45-degree angle to the horizontal plane to simulate the soil in the brick slope. First, connect the tensile meter to the head holder of the FBG fiber optic sensor cable, bury the cable in the slope soil layer, and start the pull-out measurement (see the attached manual). Figure 3). During the simulation test, the strain data of the buried section of the FBG optical fiber sensing cable was obtained by using a fiber optic Bragg grating demodulator. In the test, as time passed, the pull-out displacement and axial strain gradually increased. The strain was linearly distributed along the depth direction of the soil sample, and the linear distribution range of the strain gradually expanded. Finally, the strain was distributed in the entire buried section of the FBG optical fiber sensing cable. This shows that as the pull-out test proceeds, the strain of the buried section of the FBG optical fiber sensing cable is uniformly distributed. When the pull-out force is 1N, the strain distribution curve is linearly distributed along the depth direction of the soil sample. However, when the pull-out force increases to 2N, the strain distribution curve presents a nonlinear pattern along the depth direction of the soil sample. During rapid pulling, a considerable amount of sand and stone in the soil layer has relative friction with the optical cable, which has a significant effect on the cable-soil interface characteristics. It increases significantly when the pull-out force is greater than 2N, indicating that the coupling behavior at the interface is significantly enhanced. The interface shear stress decreases with the increase of the pull-out force, indicating that the lower the pull-out force, the greater the shear stress. As time goes by, the shear stress at the interface of the FBG fiber optic sensing cable continues to increase with the increase of the pull-out displacement until the interface is completely destroyed. Once the interface is penetrated and destroyed, the shear stress stabilizes and remains unchanged. The efficiency of FBG fiber optic sensing technology in describing the displacement of the foundation soil layer depends largely on the coupling relationship between the FBG fiber optic sensing cable and the soil. The pre-peak hardening stage of the pull-out force-displacement curve is an effective means to characterize the coupling state of the interface between the FBG fiber optic sensing cable and the slope soil. In order to analyze the coupling state, the pre-peak pull-out force and displacement are normalized respectively. FN is the normalized pull-out force, which characterizes the coupling between the FBG fiber optic sensing cable and the slope soil interface. The pre-peak pull-out force is calculated by dividing the peak pull-out force by the pre-peak pull-out force. The ratio of the pre-peak to peak pull-out force is used. Similarly, ZN represents the normalized pull-out displacement, which is obtained by dividing the pre-peak pull-out displacement by the peak pull-out displacement. This test simulation device uses the 1:1 line as the boundary and divides the pre-peak pull-out process into three coupling states. In a good coupling state, the pull-out force can accurately characterize the pull-out resistance at the interface between the FBG optical fiber sensing cable and the slope soil. For a good coupling state, it is believed that the pull-out force within this range basically reflects the pull-out resistance at the interface between the FBG optical fiber sensing cable and the slope soil. For a general coupling state, the pull-out force within this range is not equal to the interface resistance. When FN exceeds 1, it enters the decoupling state, indicating that the coupling between the cable and the slope soil interface has been significantly weakened. The pull-out force under each coupling state can be calculated based on the peak force. The mechanical behavior of the interface during the pull-out process can be effectively described by two models: the elastic-plastic model and the softening model. These two models accurately capture the relationship between the interface between the FBG optical fiber sensing cable and the slope soil, and provide insights into the deformation and failure mechanism observed during the pull-out process. The pre-peak phase data of the uplift force-displacement of the slope soil are normalized, and the pre-peak uplift process is divided into three coupling states according to the normalized uplift force and displacement after processing: excellent coupling, good coupling, and general coupling.The peak shear stress and residual shear stress at the interface can be calculated by using the existing theoretical formula. The calculated results are compared with the calculated values ​​of FBG optical fiber sensing to study the applicability of the existing theoretical model.

[0034] Compared with the related art, the FBG optical fiber sensing cable for monitoring the internal mechanical behavior of the slope provided by the present invention has the following beneficial effects:

[0035] Compared with traditional non-contact monitoring, the FBG fiber optic sensing cable provided by the present invention for monitoring the internal mechanical behavior of the slope has higher accuracy and sensitivity in obtaining the internal shear stress of the slope. The present invention adopts DFOS fiber optic sensing technology to achieve quasi-distributed and fully distributed measurements. One optical fiber can accurately measure more than ten kinds of multi-parameter information such as strain, temperature, displacement and seepage at each point along the optical fiber. If the sensing optical fiber is arranged in a dense state, the distribution of the measured object in the plane and three dimensions can be obtained; continuous measurement and monitoring of slopes from several meters to hundreds of kilometers can be achieved to meet the needs of monitoring various large-scale infrastructure facilities; the basic physical quantities obtained by optical fiber sensing technology are mainly strain, temperature, vibration and length, etc. Based on this, through the sensing optical fiber packaging technology and conversion principle, it can also measure physical quantities such as pressure, stress, water level, flow rate, moisture, humidity, flow, current, voltage, liquid level, toxic gas, etc., and the application is very wide.

[0036] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. An FBG optical fiber sensing cable for monitoring the internal mechanical behavior of a slope, characterized in that: Including outer tube, inner tube and optical fiber: The outer tube is placed on the inner tube, and the inner tube is wrapped with three optical fibers bonded with epoxy resin. The three optical fibers can detect three sets of data. There is an air layer between the inner tube and the outer tube, which is used to buffer the inner tube when external force is applied. When the external force disappears, the inner tube recovers its elasticity.

2. The FBG optical fiber sensing cable for monitoring the internal mechanical behavior of a slope according to claim 1, characterized in that: The outer tube is made of polyurethane, the outer tube and the inner tube are 1500 mm long, the outer tube has a diameter of 2200 um, and the inner tube has a diameter of 2000 um.

3. The FBG optical fiber sensing cable for monitoring the internal mechanical behavior of a slope according to claim 1, characterized in that: The outermost layer of the optical fiber is a resin coating for reinforcement, with a diameter of 900um; the middle is a low-refractive index glass cladding, with a diameter of generally 125um.

4. The FBG optical fiber sensing cable for monitoring the internal mechanical behavior of a slope according to claim 1, characterized in that: The FBG optical fiber sensor cable is provided with an optical cable head fixture at the end thereof, which is used for sealing and waterproofing and dustproofing, thereby ensuring the reliability of the optical cable in harsh environments.

5. A device for monitoring the internal mechanical behavior of a slope using FBG optical fiber sensing cable, characterized in that: The device includes the FBG optical fiber sensing cable as described in any one of claims 1 to 4 above, and also includes a dynamometer, a weighing sensor, and a fiber grating demodulator: wherein the FBG optical fiber sensing cable is used to sense the strain information at the interface between the optical cable and the slope soil layer during the test; the fiber grating demodulator is used to quickly capture the strain measurement value; the weighing sensor can measure the pull-out force; and the pull-out force control center obtains the corresponding pull-out force and displacement information through the dynamometer.

6. The device for monitoring the internal mechanical behavior of a slope using a FBG optical fiber sensing cable as claimed in claim 5, characterized in that: The weighing sensor is an S-shaped tension and pressure sensor, made of alloy steel and treated with rubber sealing protection.

7. A monitoring method for monitoring the internal mechanical behavior of a slope using FBG optical fiber sensing cable, characterized in that: The steps include: The FBG optical fiber sensing cable described in claims 1 to 4 is buried in the slope soil layer parallel to the slope interface, and the head fixture of the FBG optical fiber sensing cable is connected to the weighing sensor and the tensile gauge described in claim 5, and the FBG optical fiber sensing cable is connected to the optical fiber Bragg grating demodulator, and the surrounding area is backfilled to ensure deformation coordination at the cable-soil interface; When the soil settles, it will cause the settlement of the backfill layer, causing the FBG optical fiber sensing cable and the soil to move relative to each other, thereby generating shear stress. The FBG optical fiber sensing cable monitors the shear stress strain information, and the fiber grating demodulator collects the strain data of the buried section of the FBG optical fiber sensing cable. At the same time, the weighing sensor obtains the pull-out force, and the tensile meter collects the pull-out force and displacement information. The collected data are processed to analyze the mechanical behavior of the axial strain and shear stress inside the slope, and the changing trend of the mechanical behavior inside the slope and its impact on the stability of the slope are evaluated.

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