An FBG optical fiber sensing optical cable for monitoring internal mechanical behavior of a slope

Abstract

The present application relates to the technical field of DFOS optical fiber sensing, and discloses an FBG optical fiber sensing cable for monitoring internal mechanical behavior of a slope, which comprises an outer tube, an inner tube and optical fibers, wherein the outer tube is sleeved on the inner tube, the inner tube holds three optical fibers adhered by epoxy resin, the three optical fibers can detect three groups of data, and the accuracy of the data is improved; an air layer is arranged between the inner tube and the outer tube to serve as a buffer for the inner tube when external force is applied, and when the external force disappears, the inner tube restores elastically by itself, so that the flexibility of the whole FBG optical fiber sensing cable is ensured, and the accuracy of monitoring the internal mechanical behavior of the slope is not affected. The FBG optical fiber sensing cable for monitoring internal mechanical behavior of a slope can realize real-time and accurate monitoring of the internal mechanical behavior of the slope, and provide a scientific basis for slope stability evaluation and geological disaster warning.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of DFOS optical fiber sensing, and particularly relates to an FBG optical fiber sensing optical cable for monitoring internal mechanical behavior of a slope. BACKGROUND

[0002] In recent decades, global climate disasters have become increasingly severe, and slope protection has received widespread attention. Landslides and slope collapses pose a significant threat to ecosystems and human activities. These threats include soil erosion and debris flow caused by subsidence, earthquakes, sinkholes and collapses. Therefore, to prevent geological disasters of slopes 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 techniques can be mainly divided into two categories: 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 borehole leveling devices, inclinometers and displacement meters, are usually used to collect underground slope displacement and deformation data, thereby addressing 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 stratum and related geological interface deformation characteristics. Therefore, further improvement and refinement of these methods are needed.

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

[0005] Therefore, it is necessary to understand the deformation coordination problem between the FBG optical fiber sensing optical cable and the surrounding soil. After the FBG optical fiber sensing optical cable is used as a distributed strain sensing nerve, the earth becomes sensitive and intelligent, accurately sensing geological processes and early identifying related geological disasters SUMMARY

[0006] To solve the above technical problems, the FBG optical fiber sensing cable for monitoring internal mechanical behavior of slope is provided for obtaining internal mechanical behavior of slope displacement, analyzing change rule of slope displacement strain and shear stress through DFOS optical fiber sensing technology, revealing progressive failure behavior of slope displacement under different conditions through analysis of soil layer distribution characteristics of shear stress, and inversely calculating peak shear stress and residual shear stress of interface through existing theoretical formula, which is of great significance for understanding internal mechanical behavior of slope displacement.

[0007] The FBG optical fiber sensing cable for monitoring internal mechanical behavior of slope provided by the application comprises an outer tube, an inner tube and optical fibers, wherein the outer tube is sleeved on the inner tube, the inner tube holds three optical fibers bonded with epoxy resin, the three optical fibers can detect three groups of data, and the accuracy of data is improved; the space between the inner tube and the outer tube is an air layer, which can buffer the inner tube when external force is applied, and the inner tube restores elastically when the external force disappears, so that the flexibility of the whole FBG optical fiber sensing cable is ensured, and the accuracy of monitoring internal mechanical behavior of slope is not affected.

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

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

[0010] Preferably, the FBG optical fiber sensing cable is provided with a cable head fixator at the end, which has sealing and waterproof and dustproof functions, and ensures the reliability of the cable in harsh environments. The cable fixator (also known as a waterproof cable joint and a 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.

[0011] The application provides a device for monitoring internal mechanical behavior of slope by using the FBG optical fiber sensing cable, which comprises the FBG optical fiber sensing cable, a tension meter, a weighing sensor and a fiber grating demodulator. The FBG optical fiber sensing cable is used for sensing strain information at the cable-slope soil layer interface during the test. The fiber grating demodulator is used for quickly capturing strain measurement values. The weighing sensor can measure the pulling force. The pulling force control center obtains corresponding pulling force and displacement information through the tension meter. Through the application of the test device, the stress deformation characteristics and progressive failure behavior of the FBG optical fiber sensing cable and the slope soil layer interface can be effectively identified. Before the pulling process, the dryness and humidity of the slope soil layer are measured, and it is found that the influence of soil dryness and humidity on the experimental results can be ignored during the slope sliding process.

[0012] Preferably, the load sensor is a tensile and compressive force sensor of S type, which is the most common sensor in sensors, mainly used for measuring the tensile force and pressure between solids, commonly known as tensile and compressive force sensor, because its shape is like S shape, so it is also called S type load sensor, the load sensor adopts alloy steel material and does glue sealing protection treatment, easy to install, convenient to use, suitable for electronic force weighing system such as suspension scale, batching scale, machine modification scale and the like.

[0013] The application provides a monitoring method for monitoring internal mechanical behaviors of a slope by using an FBG optical fiber sensing cable.

[0014] The FBG optical fiber sensing cable is buried in the soil layer of the slope in parallel with the slope interface, the head fixing device of the FBG optical fiber sensing cable is connected with a load sensor and a tension meter, the FBG optical fiber sensing cable is connected with an optical fiber grating demodulator, and the surrounding area is backfilled to ensure the deformation coordination at the cable-soil interface.

[0015] When the soil body is settled, the backfill layer is settled, the FBG optical fiber sensing cable moves relative to the soil body, shear stress is generated, the FBG optical fiber sensing cable monitors the shear stress strain information, the optical fiber grating demodulator collects the strain data of the buried section of the FBG optical fiber sensing cable, the load sensor collects the pulling force, and the tension meter collects the pulling force and displacement information.

[0016] The collected data are processed, the axial strain and shear stress of the slope are analyzed, and the change trend of the internal mechanical behaviors of the slope and the influence 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 behaviors of the slope has the following beneficial effects:

[0018] The FBG optical fiber sensing cable for monitoring the internal mechanical behaviors of the slope is coupled with the state analysis of the force and displacement characteristics and the shear stress change characteristics of the slope interface in the pulling process, ideal elastic-plastic models and strain softening models suitable for the FBG optical fiber sensing cable-slope interface are established, the pre-peak data of the pulling force-displacement of the slope are normalized, the pre-peak pulling process is divided into three coupling states, i.e., optimal coupling, good coupling and general coupling, according to the normalized pulling force and displacement after processing, the existing theoretical formula can be used to inversely calculate the peak shear stress and residual shear stress at the interface, the calculation results are compared with the calculation values of the FBG optical fiber sensing cable, and the applicability of the existing theoretical model is researched. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1A structural schematic diagram of an FBG optical fiber sensing cable for monitoring internal mechanical behavior of a slope is provided in the present application;

[0020] Figure 2 A visualized simulation slope device schematic diagram is provided in the present application;

[0021] Figure 3 A schematic diagram of the visualized simulation slope device in a pulling measurement state is provided in the present application. DETAILED DESCRIPTION

[0022] The present application is further described below in combination with the drawings and embodiments.

[0023] The FBG optical fiber sensing cable for monitoring internal mechanical behavior of a slope provided in the present application comprises an outer tube, an inner tube and optical fibers (as shown in the description attached Figure 1 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 has a diameter of 2200um, and the inner tube has a diameter of 2000um; the inner tube holds three optical fibers adhered by epoxy resin, the three optical fibers can detect three groups of data, and the data accuracy is improved; the outermost layer of the optical fiber is a resin coating for strengthening, and has a diameter of 900um; the middle layer is a low-refractive glass cladding layer, and generally has a diameter of 125um; and the space between the inner tube and the outer tube is an air layer, which can buffer the inner tube when an external force is applied, and the inner tube can restore its elasticity when the external force disappears, so that the whole FBG optical fiber sensing cable is flexible, and the accuracy of monitoring the internal mechanical behavior of the slope is not affected;

[0024] The FBG optical fiber sensing cable is provided with a cable head fixer at the end, has sealing and waterproof and dustproof functions, and ensures the reliability of the cable in harsh environments, and the cable fixer (also known as a cable waterproof joint and a cable joint) is widely used in the fixing and protection of wires and cables of mechanical equipment, optical fibers, ship electrical equipment and anti-corrosion equipment.

[0025] The present application provides a device for monitoring internal mechanical behavior of a slope by using an FBG optical fiber sensing cable, which comprises the FBG optical fiber sensing cable described above, and further comprises a tension meter, a weighing sensor and a fiber grating demodulator; the FBG optical fiber sensing cable is used to perceive strain information at the cable-slope soil layer interface during the test; the fiber grating demodulator is used to quickly capture strain measurement values; the weighing sensor can measure the pulling force; and the pulling force control center obtains corresponding pulling force and displacement information through the tension meter. Through the application of the test device, stress deformation characteristics and progressive failure behavior of the FBG optical fiber sensing cable and the slope soil layer interface can be effectively identified. Before the pulling process, the dryness and humidity of the slope soil layer are measured, and it is found that the influence of the dryness and humidity of the soil on the experimental results can be ignored during the slope sliding process.

[0026] It should be noted that the weighing sensor is a device for converting mass signal into measurable electrical signal output. The sensor should be considered in the actual working environment first. This is very important for the correct selection of the weighing sensor, which is related to the normal work of the sensor, its safety and service life, and even the reliability and safety of the entire weighing instrument. There are qualitative differences between the new and old national standards in the basic concepts and evaluation methods of the main technical indicators of the weighing sensor. There are several types such as S type, cantilever type, spoke type, ring type, diaphragm type, bridge type and cylinder type. The weighing sensor used in the present application is S type, which is the most common sensor in the sensor, mainly used for measuring the tension and pressure between solids, and also called tension and pressure sensor. Because its shape is like S shape, it is also called S type weighing sensor. The weighing sensor adopts alloy steel material and does glue sealing protection treatment, is easy to install and use, and is suitable for electronic force measuring weighing system such as suspension scale, batching scale and machine modified scale.

[0027] It should be noted that the FBG optical fiber sensing optical cable utilizes the DFOS optical fiber sensing technology. This technology refers to obtaining the continuous distribution information of the physical parameters outside the optical fiber in space and time by using the optical information inside the optical fiber, and a technical means for realizing distributed monitoring is distributed optical fiber sensing technology. In the DFOS optical fiber sensing technology, the optical fiber can not only be used as a sensing medium to perceive the external physical quantity, but also be used as a transmission channel to transmit optical information. DFOS optical fiber sensing technology uses the one-way transmission characteristics of optical fiber, takes the physical quantity to be monitored as a function of the light transmission position, and senses the distribution and change of the measured parameters along the optical fiber. The sensing optical fiber is arranged inside the geological body, like a neural network that can sense the changes in the geological body and engineering structure, senses the change rule of the physical properties of the measured object in one dimension, two dimensions and three dimensions, overcomes the shortcomings of point-to-point monitoring and easy missed detection of traditional monitoring methods, and improves the monitoring range. DFOS optical fiber sensing technology is more suitable for monitoring parameters such as overall strain and temperature of large or super-large projects, such as long-distance tunnel uneven settlement monitoring, large-area ground crack distribution and deformation monitoring, long-distance oil pipeline leakage monitoring and large-scale slope deformation monitoring. The basic physical quantities obtained by optical fiber sensing technology mainly include strain, temperature, vibration and length, etc. Based on this, through sensing optical fiber packaging technology and conversion principle, pressure, stress, water level, flow rate, moisture, humidity, flow, current, voltage, liquid level, toxic gas and other physical quantities can also be measured, and the application is very wide.

[0028] In the present application, when the DFOS optical fiber sensing technology is applied to the slope displacement deformation monitoring, it is assumed that only axial strain occurs along the interface, and the specific method is to bury the FBG optical fiber sensing cable parallel to the slope interface in the soil, 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 to move relative to the soil, thereby generating shear stress and selecting a microelement for mechanical analysis.

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

[0030] The internal mechanical behavior of the slope monitored by the present application includes shear stress, which extends from the upper part of the soil, and as the pull-off displacement increases, the shear stress gradually extends downward along the slope interface. When the interface is completely penetrated and damaged, the FBG optical fiber sensing cable and the entire slope interface slide relative to each other, the interface shear stress decreases rapidly, and enters the residual stage. In addition, the pull-off displacement related to the peak shear stress is usually referred to as the effective displacement, which is used as the boundary to distinguish between the effective state and the ineffective state during the pull-off process.

[0031] The present application uses FBG optical fiber sensing cable to monitor the internal mechanical behavior of the slope in the field simulation, which is described below through the following examples:

[0032] Example 1:

[0033] The present application proposes a device for monitoring the internal mechanical behavior of the slope using FBG optical fiber sensing cable, which simulates the shear sliding failure and sliding state of the rock mass during landslide, and designs a visual simulation slope device (see the attached Figure 2 ) of the specification). The test device is composed of tempered glass, bricks and soil. The heights of brick 1, brick 2 and brick 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 an angle of 45 degrees to the horizontal plane to simulate the soil in the brick slope. First, connect the pull-off meter to the head of the FBG optical fiber sensing cable, bury the cable in the slope soil layer, and start the pull-off measurement (see the attached Figure 3). During the simulation test, the strain data of the FBG optical fiber sensing cable buried section were obtained by using the fiber grating demodulator. In the test, the pullout displacement and axial strain were gradually increasing with the passage of time. The strain distribution along the longitudinal direction of the soil sample was linear, and the linear distribution range of the strain gradually expanded. Finally, the strain distribution was uniform throughout the buried section of the FBG optical fiber sensing cable. When the pullout force was 1 N, the strain distribution curve along the longitudinal direction of the soil sample was linear. However, when the pullout force increased to 2 N, the strain distribution curve along the depth direction of the soil sample showed a nonlinear pattern. During the rapid pullout, a considerable amount of sand and stone in the soil layer had a significant impact on the cable interface characteristics due to the relative friction between the cable and the soil. The pullout force increased significantly when it was greater than 2 N, indicating that the coupling behavior at the interface was significantly enhanced. The interface shear stress decreased with the increase of the pullout force, indicating that the lower the pullout force, the greater the shear stress. With the passage of time, the shear stress at the interface of the FBG optical fiber sensing cable increased continuously with the increase of the pullout displacement until the interface was completely destroyed. Once the interface was penetrated, the shear stress stabilized and remained unchanged. The efficiency of the FBG optical fiber sensing technology in describing the displacement of the foundation soil layer was largely dependent on the coupling relationship between the FBG optical fiber sensing cable and the soil. The pre-peak hardening stage of the pullout force-displacement curve was an effective means to characterize the coupling state of the FBG optical fiber sensing cable and the slope soil interface. In order to analyze the coupling state, the pre-peak pullout force and displacement were normalized. FN is the normalized pullout force, which represents the coupling of the FBG optical fiber sensing cable and the slope soil interface. The pre-peak pullout force is calculated by dividing the peak pullout force by the pre-peak pullout force. The ratio of the pre-peak and peak pullout forces is used. Similarly, ZN represents the normalized pullout displacement, which is obtained by dividing the pre-peak pullout displacement by the peak pullout displacement. The pre-pullout process is divided into three coupling states using a 1:1 line as the boundary in the test simulation device. In the good coupling state, the pullout force can accurately represent the pullout resistance at the interface between the FBG optical fiber sensing cable and the slope soil. For the good coupling state, it is considered that the pullout force in this range basically reflects the pullout resistance at the interface between the FBG optical fiber sensing cable and the slope soil. For the general coupling state, the pullout force in 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 pullout force in each coupling state can be calculated based on the peak force. The mechanical behavior of the interface during the pullout process can be effectively described by the elastic-plastic model and the softening model. These two models accurately capture the relationship between the FBG optical fiber sensing cable and the slope soil interface, providing insights into the deformation and failure mechanisms observed during the pullout process. The pullout force-displacement pre-peak data of the slope soil are normalized, and the pre-peak pullout process is divided into three coupling states: optimal coupling, good coupling, and general coupling, based on the normalized pullout force and displacement after processing.The existing theoretical formula can be used to inversely calculate the peak shear stress and residual shear stress at the interface. The inverse calculation result is compared with the calculated value of the FBG optical fiber sensing, and the applicability of the existing theoretical model is studied.

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

[0035] Compared with the traditional non-contact monitoring, the FBG optical fiber sensing cable for monitoring the internal mechanical behavior of a slope provided by the application has higher accuracy and sensitivity in obtaining the internal shear stress of the slope. The DFOS optical fiber sensing technology is adopted to realize quasi-distributed and full-distributed measurement. One optical fiber can accurately measure the strain, temperature, displacement and seepage and more than ten kinds of multi-parameter information at each point along the optical fiber. If the sensing optical fiber is arranged in a dense manner, the distribution of the measured object in the plane and in the space can be obtained; the continuous measurement and monitoring of the slope of several meters to several hundred kilometers can be realized, meeting the needs of monitoring various large-scale infrastructure projects; the basic physical quantities obtained by the optical fiber sensing technology mainly include strain, temperature, vibration, length and the like, based on which, the sensing optical fiber packaging technology and conversion principle can also be used to measure pressure, stress, water level, flow rate, moisture, humidity, flow, current, voltage, liquid level, toxic gas and other physical quantities, and the application face is very wide.

[0036] The above only describes the embodiments of the application, and does not limit the patent scope of the application, and any equivalent structure or equivalent flow transformation obtained by using the content of the specification and drawings, or direct or indirect application in other related technical fields, are also included in the patent protection scope of the application.

Claims

1. An FBG fiber optic sensing cable for monitoring the internal mechanical behavior of slopes, characterized in that, Includes outer tube, inner tube, and optical fiber: The outer tube is fitted onto the inner tube, and the inner tube holds three optical fibers bonded together with epoxy resin. The three optical fibers can detect three sets of data. An air layer exists between the inner and outer tubes to buffer the inner tube when external force is applied, and the inner tube elastically recovers when the external force is removed. The outer tube is made of polyurethane, and the outer tube and inner tube are 1500mm long, the outer tube diameter is 2200um, and the inner tube diameter is 2000um. The outermost layer of the optical fiber is a resin coating for reinforcement, with a diameter of 900 μm; the middle layer is a low-refractive-index glass cladding, with a diameter of 125 μm. The FBG fiber optic sensing cable is equipped with a cable head retainer at the end for sealing and waterproofing / dustproofing, ensuring the reliability of the cable in harsh environments.

2. A device for monitoring the internal mechanical behavior of a slope using an FBG fiber optic sensing cable, characterized in that, The device includes the FBG fiber optic sensing cable as described in claim 1, and further includes a force gauge, a load cell, and a fiber Bragg grating demodulator: wherein the FBG fiber optic sensing cable is used to sense strain information at the interface between the cable and the slope soil layer during the test; the fiber Bragg grating demodulator is used to quickly capture strain measurement values; the load cell can measure the magnitude of the pull-out force; and the pull-out force control center obtains the corresponding pull-out force and displacement information through the force gauge.

3. The device for monitoring the internal mechanical behavior of a slope using an FBG fiber optic sensing cable as described in claim 2, characterized in that, The weighing sensor is an S-type tensile and compressive sensor made of alloy steel and protected with adhesive sealing.

4. A monitoring method for monitoring the internal mechanical behavior of a slope using an FBG fiber optic sensing cable, characterized in that, The steps include: The FBG fiber sensing cable described in claim 1 is buried in the slope soil layer at the parallel slope interface, and the head fixer of the FBG fiber sensing cable is connected to the weighing sensor and tension gauge described in claim 2. The FBG fiber sensing cable is also connected to the fiber optic demodulator, and the surrounding area is backfilled to ensure the deformation coordination at the cable-soil interface. When the soil settles, it will cause the backfill layer to settle, causing the FBG fiber optic sensing cable to move relative to the soil, thereby generating shear stress. The FBG fiber optic sensing cable monitors the shear stress strain information, the fiber optic grating demodulator collects the strain data of the FBG fiber optic sensing cable buried section, and at the same time, the weighing sensor obtains the pull-out force, and the tension gauge collects the pull-out force and displacement information. The collected data are processed to analyze the mechanical behavior of axial strain and shear stress inside the slope, and the changing trend of the internal mechanical behavior of the slope and its impact on slope stability are assessed.

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