Shield tunnel settlement automatic monitoring device based on distributed optical fiber sensing technology

Abstract

This invention discloses an automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology. The device includes measuring supports, optical fibers, and a distributed optical fiber demodulator. The measuring supports are I-shaped, with multiple supports arranged continuously, end-to-end, according to the tunnel monitoring range. The optical fibers include temperature-sensing fibers and strain-sensing fibers. The strain-sensing fibers include strain-sensing fiber one and strain-sensing fiber two. The temperature-sensing fiber is arranged in a straight line on the surface of the measuring support. Strain-sensing fiber one and strain-sensing fiber two form a triangular structure, and strain-sensing fiber two is also arranged in a straight line on the surface of the measuring support. The optical fibers are connected to the distributed optical fiber demodulator. This invention not only considers the influence of horizontal tension on settlement calculation but also takes into account the differences caused by temperature variations, thus improving monitoring accuracy. By establishing an equation between tunnel settlement and optical fiber strain, automated monitoring of shield tunnel settlement can be achieved, improving monitoring efficiency and reducing monitoring costs.

Description

Technical Field

[0001] This invention belongs to the field of tunnel settlement monitoring technology, specifically relating to an automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology. Background Technology

[0002] Due to construction disturbances and geological variations, uneven settlement is inevitable in tunnel structures during operation, affecting their safety. Currently, settlement monitoring of tunnel structures mainly relies on total stations and hydrostatic levels, but these methods suffer from problems such as scattered monitoring points, low monitoring efficiency, and high costs. To achieve automated monitoring of tunnel structures and improve the efficiency of tunnel settlement monitoring, a more suitable monitoring method is urgently needed.

[0003] Fiber optic sensing technology has many advantages in terms of size, cost, lifespan, resolution, electromagnetic interference resistance, and resistance to harsh working conditions, and has attracted widespread attention from scholars and engineers at home and abroad. At present, there are also corresponding tunnel monitoring methods based on fiber optic sensing technology, such as Chinese invention patent, publication number CN104807414A, which discloses a method for monitoring settlement deformation of subway tunnels based on distributed fiber optic sensing technology. However, this method has the following problems in tunnel settlement monitoring: (1) It does not conform to the assumptions of material mechanics, and the structure cannot be regarded as a linear elastic material; (2) The algorithm requires boundary conditions, but these boundary conditions are not clear in actual engineering; (3) The calculation model is a pure bending model, but in reality, there will be side friction and shear force interference in the calculation results. For example, Chinese invention patent CN103591930A discloses a device and method for monitoring tunnel settlement using distributed optical fiber. However, this method does not consider the influence of tunnel horizontal tension on optical fiber strain test results, resulting in the monitored data being deformation under the combined influence of horizontal tension and vertical settlement. This leads to inaccurate settlement monitoring and prevents early warning based on relevant design values. Furthermore, this method suffers from complex wiring and requires highly skilled installation personnel, hindering its practical application in tunnel monitoring. Another example is Chinese invention patent CN113483731A, which discloses a multi-directional online monitoring system for tunnel structural health based on optical fiber sensing. This system not only fails to consider the influence of tunnel horizontal tension on monitoring results but also excludes the influence of temperature on strain monitoring results. This results in the monitored data completely failing to reflect tunnel settlement deformation. Summary of the Invention

[0004] To address the technical problems existing in the prior art, the purpose of this invention is to provide an automatic monitoring device for settlement of shield tunnels based on distributed optical fiber sensing technology.

[0005] To achieve the above objectives and technical effects, the technical solution adopted by this invention is as follows:

[0006] An automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology includes a measuring support, optical fiber, and a distributed optical fiber demodulator. The optical fiber is laid on the measuring support and connected to the distributed optical fiber demodulator.

[0007] Furthermore, the measuring supports are I-shaped and there are several of them, which are connected end to end and arranged continuously.

[0008] Furthermore, the measuring support includes two parallel telescopic rods, six base plates, and a limiting rod. Each telescopic rod has base plates symmetrically arranged at its two ends, and each telescopic rod has a base plate in its middle. The telescopic rods and base plates are coaxially arranged. The limiting rod is located at the center of the measuring support. The two ends of the limiting rod are respectively connected to the base plate located in the middle of the telescopic rod. The limiting rod is provided with a spring.

[0009] Furthermore, the measuring support includes a first fixed support and a second fixed support on the base plate located in the middle of one of the telescopic rods and on the base plates located at opposite ends of the remaining telescopic rod. Only the second fixed support is provided on all the remaining base plates. The first fixed support is connected to a strain sensing fiber 1, and the second fixed support is connected to a temperature sensing fiber 2 and a strain sensing fiber 2.

[0010] Furthermore, the first fixed support includes a first buckle, a first base, a first annular rubber and a first outer shell, and the second fixed support includes a second buckle, a second base, a second annular rubber and a second outer shell. The first base and the second base are respectively provided with screw holes. The first annular rubber and the second annular rubber are respectively coaxially arranged with the first outer shell and the second outer shell. The first fixed support is provided with an arc-shaped groove, and the second fixed support is provided with a linear groove.

[0011] Furthermore, the optical fiber includes a temperature sensing optical fiber and a strain sensing optical fiber. The temperature sensing optical fiber is arranged in a straight line on the surface of the measuring support, and the strain sensing optical fiber is arranged in a triangular shape on the surface of the measuring support.

[0012] Furthermore, the strain sensing fiber includes strain sensing fiber one and strain sensing fiber two, which form a triangular structure, and strain sensing fiber two is laid in a straight line on the surface of the measuring support.

[0013] Furthermore, the temperature sensing fiber is a communication optical cable, and the strain sensing fiber is a distributed tight-packed strain sensing optical cable or a densely distributed strain sensing optical cable.

[0014] The installation method of the automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology includes the following steps:

[0015] Step 1: Determine the location of the measuring supports based on the size of the tunnel segments. Fix the measuring supports to the tunnel segments using rivets or bolts through the screw holes on the base plate.

[0016] Step 2: Lay strain sensing fiber 1 and strain sensing fiber 2 in a triangular and straight pattern respectively. Pre-stretch them during laying. After pre-stretching to the set value, fix strain sensing fiber 1 and strain sensing fiber 2 with the first and second clips respectively. After the strain sensing fiber is laid, lay the temperature sensing fiber in a straight pattern and keep it in a relaxed state. Then fix the temperature sensing fiber to the second fixed support with the second clip. According to the measurement range, complete the laying of all measuring supports and their fibers in sequence.

[0017] Step 3: Connect the deployed strain sensing fiber 1, strain sensing fiber 2, and temperature sensing fiber in series, and connect them to a distributed fiber optic demodulator via fiber optic patch cords. By establishing the equation between tunnel settlement and fiber optic strain, the tunnel settlement can be monitored automatically.

[0018] The monitoring method for an automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology includes the following steps:

[0019] The three adjacent first fixed supports and the strain sensing fiber 1 form a triangular structure. The length of the strain sensing fiber 1 is denoted as L. The horizontal distance between the two first fixed supports on the same horizontal line is denoted as 2D. The vertical distance between the two first fixed supports on the same horizontal line and the remaining first fixed support is denoted as S. The extension and contraction of the telescopic rod is proportional to the length of the tunnel segment.

[0020] When two adjacent tunnel segments experience only relative settlement, the strain generated by the strain sensing fiber between the adjacent first fixed supports is ε1, and the strain generated by the temperature sensing fiber is ε. t Then, using the principle of similar triangles, the settlement of the tunnel can be expressed as:

[0021]

[0022] When adjacent tunnel segments not only experience relative settlement but also horizontal tension, the strains monitored by strain sensing fiber 1 and strain sensing fiber 2 are ε3 and ε2, respectively, and the strain monitored by the temperature sensing fiber is ε. t Due to vertical settlement, the strain generated by the strain sensing fiber is ε. s1 The strain generated by the second strain-sensing fiber is negligible; due to the horizontal tension, the strain generated by the first strain-sensing fiber is ε. h1 The strain generated by the strain sensing fiber is ε.h2 Due to temperature changes, the strain generated by both strain sensing fiber 1 and strain sensing fiber 2 is ε. t ;

[0023] Therefore, the strain ε generated by the strain sensing fiber due to vertical settlement... s1 Represented as:

[0024] ε s1 =ε3-ε t -ε h1

[0025] in,

[0026] The settlement of the tunnel is expressed as:

[0027]

[0028] The settlement of any tunnel segment is expressed as:

[0029]

[0030] Where, y n-1 This represents the settlement of the adjacent tunnel segment.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] 1) This invention establishes an equation relating tunnel settlement to optical fiber strain, transforming tunnel settlement into optical fiber strain variation. By measuring optical fiber strain, the relative settlement between adjacent tunnel segments can be determined, enabling automated monitoring of shield tunnel settlement, improving the efficiency of shield tunnel settlement monitoring and reducing monitoring costs.

[0033] 2) The present invention designs an adjustable measuring support, which reduces the difficulty of optical cable laying and increases the adjustable range of optical cable sensitivity.

[0034] 3) This invention not only considers the influence of horizontal tension on settlement calculation, but also the differences caused by temperature changes, and can realize temperature self-compensation and horizontal tension compensation, thereby improving the accuracy of tunnel settlement monitoring results. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the structure of the present invention;

[0036] Figure 2 This is a schematic diagram of the measuring support and optical fiber of the present invention;

[0037] Figure 3 This is a schematic diagram of the structure of the first fixed support of the present invention;

[0038] Figure 4 This is a schematic diagram of the structure of the second fixed support of the present invention;

[0039] Figure 5 This is a schematic diagram illustrating the settlement calculation principle of the present invention.

[0040] Figure 6 This is a diagram showing the indoor test results of the present invention; wherein, Figure 6 Figure A shows the results of indoor tests conducted under displacement conditions of (1,2mm), (2,4mm), (3,4mm), (4,4mm), (5,2mm), (6,2mm), (7,4mm), (8,4mm), (9,2mm), (10,0mm), (11,3mm), (12,6mm), (13,6mm), (14,3mm), (15,3mm), (16,6mm), (17,6mm), (18,6mm), and (19,3mm). Figure 6 Figure B shows the results of indoor tests conducted under the following displacement conditions: (1,0mm), (2,-2mm), (3,2mm), (4,-2mm), (5,2mm), (6,0mm), (7,2mm), (8,-2mm), (9,2mm), (10,0mm), (11,0mm), (12,3mm), (13,-3mm), (14,3mm), (15,0mm), (16,3mm), (17,-3mm), (18,3mm), (19,-3mm), (19,0mm).

[0041] Figure 7 This is a diagram showing the field test results of the present invention; wherein, Figure 7 Image A shows the monitoring results in early July 2022. Figure 7 B is a comparison chart of monitoring results in mid-July 2022. Detailed Implementation

[0042] The present invention will now be described in detail so that its advantages and features can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention.

[0043] The following provides a brief overview of one or more aspects to offer a basic understanding of them. This overview is not an exhaustive summary of all conceived aspects, nor is it intended to identify key or decisive elements of all aspects, nor to define the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form to prepare for the more detailed descriptions that follow.

[0044] like Figure 1-7As shown, the automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology includes a measuring support 1, an optical fiber 2, and a distributed optical fiber demodulator 3. The measuring support 1 is I-shaped and consists of several units connected end-to-end in a continuous arrangement. The measuring support 1 is fixed to the tunnel structure using rivets or bolts. The optical fiber 2 is fixed to the fixed support of the measuring support 1 using clips. The optical fiber 2 is tangent to the annular rubber of the measuring support 1. The optical fiber 2 is connected to the distributed optical fiber demodulator 3. The optical fiber 2 includes a temperature sensing optical fiber 21 and a strain sensing optical fiber 22. 21 uses a communication optical cable, which is laid in a straight line on the surface of the measuring support 1. The temperature sensing optical fiber 21 ensures sufficient relaxation, which is greater than the maximum deformation of the tunnel. The strain sensing optical fiber 22 ensures a certain pre-tension, which varies with the monitoring alarm value. The strain sensing optical fiber 22 adopts a distributed tight-packed strain sensing optical cable or a densely distributed strain sensing optical cable, including strain sensing optical fiber 1 221 and strain sensing optical fiber 222. The strain sensing optical fiber 1 221 and strain sensing optical fiber 222 form a triangular structure. The strain sensing optical fiber 222 is laid in a straight line on the surface of the measuring support 1.

[0045] The measuring support 1 includes telescopic rods 11, base plates 12, and limiting rods 13. Two telescopic rods 11 are provided, arranged parallel and spaced apart. Base plates 12 are symmetrically arranged at opposite ends of the telescopic rods 11, with one base plate 12 located in the middle of each telescopic rod 11. The telescopic rods 11 and base plates 12 are coaxially connected. A total of six base plates 12 are provided. One limiting rod 13 is provided, positioned at the center of the measuring support 1. The two opposite ends of the limiting rod 13 are symmetrically connected to the base plates 12. A spring is provided on the limiting rod 13, which can be stretched to change the angle of the "triangular" fiber optic arrangement, increasing the sensitivity of the fiber optic sensing of vertical settlement. A first fixed support 121 and a second fixed support 122 are provided on the base plate 12 located in the middle of one of the telescopic rods 11 and on the base plates 12 located at opposite ends of the remaining telescopic rod 11. Only the second fixed support 122 is provided on all the remaining base plates 12. The first fixed support 121 is connected to... A strain sensing fiber 221 is connected to a first fixed support 122, and a temperature sensing fiber 21 and a strain sensing fiber 222 are connected to a second fixed support 122. The first fixed support 121 is composed of a first buckle 1211, a first base 1212, a first annular rubber 1213, and a first outer shell 1214. The second fixed support 122 is composed of a second buckle 1221, a second base 1222, a second annular rubber 1223, and a second outer shell 1224. The first base 1212 and the second base 1222 are provided with screw holes. The first annular rubber 1213 and the second annular rubber 1223 are respectively coaxially arranged with the first outer shell 1214 and the second outer shell 1224. The first fixed support 121 is provided with an arc-shaped groove, which can avoid the problem of light loss caused by bending when the strain sensing fiber 221 is arranged in a "triangular" shape, resulting in the inability to measure or inaccurate measurement results. The second fixed support 122 is provided with a linear groove.

[0046] The installation method of the automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology includes the following steps:

[0047] Step 1: Determine the placement of the measuring support 1 according to the size of the tube segment 4, and fix the measuring support 1 to the tube segment 4 using rivets or bolts through the screw holes on the base plate 12.

[0048] Step 2: Lay strain sensing fiber 1 221 and strain sensing fiber 222 in a triangular and straight pattern respectively. Pre-stretch them during laying. After pre-stretching to the set value, fix strain sensing fiber 1 221 and strain sensing fiber 222 with the first clip 1211 and the second clip 1221 respectively. After the strain sensing fiber 22 is laid, lay temperature sensing fiber 21 in a straight pattern and keep it in a relaxed state. Then fix temperature sensing fiber 21 to the second fixed support 122 with the second clip 1221. According to the measurement range, complete the laying of all supports and the fiber 2 on them in sequence. The laying of the measuring support 1 and the corresponding fiber can be completed in the factory.

[0049] Step 3: Connect the laid strain sensing fiber 1 221, strain sensing fiber 2 222, and temperature sensing fiber 21 in series, and connect them to the distributed fiber demodulator 3 through fiber optic patch cords.

[0050] The monitoring method for an automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology includes the following steps:

[0051] The three adjacent first fixed supports 121 and the strain sensing fiber 221 form a triangular structure. The length of the strain sensing fiber 221 is denoted as L. The horizontal distance between two first fixed supports 121 on the same horizontal line is denoted as 2D. The vertical distance between two first fixed supports 121 on the same horizontal line and the remaining first fixed support 121 is denoted as S. The extension and retraction of the telescopic rod 11 is proportional to the length of the tunnel segment 4.

[0052] When two adjacent tunnel segments 4 experience only relative settlement, the strain generated by the strain sensing fiber 221 between adjacent first fixed supports 121 is ε1, and the strain generated by the temperature sensing fiber 21 is ε. t Therefore, using the principle of similar triangles, the settlement of the tunnel can be expressed as:

[0053]

[0054] In practical applications, tunnel structures experience not only vertical settlement but also horizontal tension. Therefore, when adjacent tunnel segments 4 experience both relative settlement and horizontal tension, the strains monitored by strain sensing fiber 1 221 and strain sensing fiber 222 are ε3 and ε2, respectively, while the strain monitored by temperature sensing fiber 21 is ε. t Due to vertical settlement, the strain generated by the strain sensing fiber 221 is ε. s1 The strain generated by the strain sensing fiber 222 is negligible. Due to the horizontal tension and contraction, the strain generated by the strain sensing fiber 221 is ε. h1 The strain generated by the strain sensing fiber 222 is ε. h2 Due to temperature changes, the strain generated by both strain sensing fiber 1 221 and strain sensing fiber 2 222 is ε. t Therefore, the strain ε produced by the strain-sensing fiber 221 solely due to sedimentation is... s1 , represented as ε s1 =ε3-ε t -ε h1 ,in, Substituting this into formula (1), the settlement of the tunnel can be expressed as:

[0055]

[0056] The settlement of any tunnel segment 4 can be expressed as:

[0057]

[0058] Where, y n-1 This represents the settlement of the adjacent tunnel segment 4.

[0059] Figure 6 The indoor test results diagram of this invention is shown in the figure. The dashed line in the figure represents the settlement data tested by the distributed fiber optic demodulator 3 in this device, and the solid line represents the actual displacement. Figure 6 A was completed under the following displacement conditions: (1,2mm), (2,4mm), (3,4mm), (4,4mm), (5,2mm), (6,2mm), (7,4mm), (8,4mm), (9,2mm), (10,0mm), (11,3mm), (12,6mm), (13,6mm), (14,3mm), (15,3mm), (16,6mm), (17,6mm), (18,6mm), (19,3mm). The first number in parentheses represents the segment number, and the second number represents the actual displacement. Figure 6B was performed under the following displacement conditions: (1,0mm), (2,-2mm), (3,2mm), (4,-2mm), (5,2mm), (6,0mm), (7,2mm), (8,-2mm), (9,2mm), (10,0mm), (11,0mm), (12,3mm), (13,-3mm), (14,3mm), (15,0mm), (16,3mm), (17,-3mm), (18,3mm), (19,-3mm), (19,0mm). The first number in parentheses represents the segment number, and the second number represents the actual displacement.

[0060] Figure 7 The figure shows the field test results of this invention. The dashed lines represent the results monitored using a total station, and the solid lines represent the settlement data measured by the distributed fiber optic demodulator 3 in this device. Figure 7 A is a monitoring result image from early July 2022. Figure 7 B is a comparison chart of monitoring results from mid-July 2022. The difference between the two charts is due to the continuous deformation of the tunnel structure.

[0061] Any parts or structures not specifically described in this invention can be made using existing technologies or products, and will not be elaborated upon here.

[0062] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. An automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology, characterized in that, It includes a measuring support, an optical fiber, and a distributed optical fiber demodulator. The optical fiber is laid on the measuring support and connected to the distributed optical fiber demodulator. The measuring support is I-shaped and there are several of them. The measuring supports are connected end to end and arranged continuously. The measuring support includes two parallel telescopic rods, six base plates and a limiting rod. The base plates are symmetrically arranged at opposite ends of each telescopic rod, and a base plate is arranged in the middle of each telescopic rod. The telescopic rod and the base plate are coaxially arranged. The limiting rod is located at the center of the measuring support. The opposite ends of the limiting rod are respectively connected to the base plate located in the middle of the telescopic rod. The limiting rod is provided with a spring. The measuring support is provided with a first fixed support and a second fixed support on the base plate located in the middle of one of the telescopic rods and on the base plates located at opposite ends of the remaining telescopic rods. Only the second fixed support is provided on all the remaining base plates. The first fixed support is connected to a strain sensing fiber 1, and the second fixed support is connected to a temperature sensing fiber 2 and a strain sensing fiber 2. The optical fiber includes a temperature sensing optical fiber and a strain sensing optical fiber. The temperature sensing optical fiber is arranged in a straight line on the surface of the measuring support, and the strain sensing optical fiber is arranged in a triangular shape on the surface of the measuring support. The strain sensing fiber includes strain sensing fiber one and strain sensing fiber two, which form a triangular structure, and strain sensing fiber two is laid in a straight line on the surface of the measuring support. The monitoring method of the shield tunnel settlement automatic monitoring device based on distributed optical fiber sensing technology includes the following steps: The three adjacent first fixed supports and the strain sensing fiber 1 form a triangular structure. The length of the strain sensing fiber 1 is denoted as L. The horizontal distance between the two first fixed supports on the same horizontal line is denoted as 2D. The vertical distance between the two first fixed supports on the same horizontal line and the remaining first fixed support is denoted as S. The extension and contraction of the telescopic rod is proportional to the length of the tunnel segment. When only relative settlement occurs between two adjacent tunnel segments, the strain generated by the strain sensing fiber between the two adjacent first fixed supports is ε1, and the strain generated by the temperature sensing fiber is ε2 t Therefore, according to the principle of similar triangles, the settlement of the tunnel is represented as: ； When adjacent tunnel segments not only experience relative settlement but also horizontal tension, the strains monitored by strain sensing fiber 1 and strain sensing fiber 2 are ε3 and ε2, respectively, and the strain monitored by the temperature sensing fiber is ε. t Due to vertical settlement, the strain generated by the strain sensing fiber is ε. s1 The strain generated by the second strain-sensing fiber is negligible; due to the horizontal tension, the strain generated by the first strain-sensing fiber is ε. h1 The strain generated by the strain sensing fiber is ε. h2 Due to temperature changes, the strain generated by both strain sensing fiber 1 and strain sensing fiber 2 is ε. t ; Therefore, the strain ε generated by the strain sensing fiber due to vertical settlement... s1 Represented as: e s1 =ε3-ε t -e h1 ; in, ; The settlement of the tunnel is expressed as: ； The settlement of any tunnel segment is expressed as: ； Where, in the formula y n-1 This represents the settlement of the adjacent tunnel segment.

2. The automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology according to claim 1, characterized in that, The first fixed support includes a first buckle, a first base, a first annular rubber and a first outer shell. The second fixed support includes a second buckle, a second base, a second annular rubber and a second outer shell. The first base and the second base are respectively provided with screw holes. The first annular rubber and the second annular rubber are respectively coaxially arranged with the first outer shell and the second outer shell. The first fixed support is provided with an arc-shaped groove and the second fixed support is provided with a linear groove.

3. The automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology according to claim 1, characterized in that, The temperature sensing fiber is a communication optical cable, and the strain sensing fiber is a distributed tight-packed strain sensing optical cable or a densely distributed strain sensing optical cable.

4. The installation method of the automatic settlement monitoring device for shield tunnels based on distributed optical fiber sensing technology according to any one of claims 1-3, characterized in that, Includes the following steps: Step 1: Determine the location of the measuring supports based on the size of the tunnel segments. Fix the measuring supports to the tunnel segments using rivets or bolts through the screw holes on the base plate. Step 2: Lay strain sensing fiber 1 and strain sensing fiber 2 in a triangular and straight pattern respectively. Pre-stretch them during laying. After pre-stretching to the set value, fix strain sensing fiber 1 and strain sensing fiber 2 with the first and second clips respectively. After the strain sensing fiber is laid, lay the temperature sensing fiber in a straight pattern and keep it in a relaxed state. Then fix the temperature sensing fiber to the second fixed support with the second clip. According to the measurement range, complete the laying of all measuring supports and their fibers in sequence. Step 3: Connect the deployed strain sensing fiber 1, strain sensing fiber 2, and temperature sensing fiber in series, and connect them to a distributed fiber optic demodulator via fiber optic patch cords. By establishing the equation between tunnel settlement and fiber optic strain, the tunnel settlement can be monitored automatically.

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