Soft soil foundation pit monitoring system and monitoring method based on distributed optical fibers

Abstract

The invention discloses a soft soil foundation pit monitoring system and method based on distributed optical fibers, and belongs to the technical field of constructional engineering. The system comprises a distributed optical fiber monitoring system, an optical fiber grating sensor, a wireless optical fiber grating demodulator and a data analysis system, and can realize real-time, continuous and high-precision monitoring of the soft soil foundation pit. Through the fiber bragg grating sensing technology, the problems of data splitting, complex installation, poor anti-interference capability and the like of a traditional monitoring method are solved, and the device has the advantages of real-time monitoring, high-precision measurement, strong anti-interference capability, simple and convenient installation, multi-parameter monitoring, remote monitoring and the like, and is suitable for fine monitoring of the soft soil foundation pit.

Description

Technical Field

[0001] The present invention relates to the technical field of construction engineering, and particularly relates to a soft soil foundation pit monitoring system based on distributed optical fiber. Background Art

[0002] In recent years, with the development of urban construction and the construction of large and medium-sized urban comprehensive systems, intensive construction has saved a large amount of development space for cities, making the connection between the city center and surrounding urban areas closer. However, its location is generally in densely populated areas of the city. Due to the complex adjacent environment and the continuous increase of above-ground buildings with the development of the city, the increase in the volume of adjacent building groups has led to an increase in the load on existing foundation pits, the building itself, and the surrounding soil and strata. In addition, the long-term extrusion of adjacent building groups after completion on existing foundation pits, the building itself, and the surrounding soil and strata will inevitably lead to varying degrees of damage to existing foundation pits and the building itself, and may even cause the structure of existing foundation pits and the building itself to not meet the requirements of its safe use function. Therefore, whether the volume of adjacent building groups increases or remains unchanged, it will have an impact on the surrounding soil of existing foundation pits and the building itself, resulting in soil loosening, uneven surface settlement, and lateral and longitudinal displacements of the strata, ultimately causing deformation and settlement of the surrounding soil of existing foundation pits and buildings, affecting their structural safety.

[0003] Traditional foundation pit monitoring methods have problems such as fragmented monitoring data, complex installation, and poor anti-interference ability, making it difficult to meet the refined monitoring requirements of soft soil foundation pits. Therefore, there is an urgent need for a soft soil foundation pit monitoring system and method that can achieve real-time, continuous, and high-precision monitoring. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems of fragmented monitoring data, complex installation, and poor anti-interference ability in the prior art, and to achieve real-time, continuous, and high-precision monitoring of soft soil foundation pits, and a soft soil foundation pit monitoring system based on distributed optical fiber is proposed.

[0005] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A soft soil foundation pit monitoring system based on distributed optical fiber, comprising:

[0007] A distributed optical fiber monitoring system for collecting strain and deformation data of the foundation pit, the distributed optical fiber monitoring system including a distributed strain sensing optical cable, a plastic-sealed armored communication optical cable, a fiber Bragg grating sensing unit, and a fiber optic monitoring cable;

[0008] Fiber Bragg grating sensors arranged on the outside and inside of the soft soil foundation pit for monitoring the deep soil uplift, support pile deformation, settlement of the upper structure of the foundation pit, displacement of the foundation pit slope, and surcharge pressure;

[0009] A wireless fiber Bragg grating demodulator for receiving signals from fiber Bragg grating sensors and demodulating them;

[0010] A data analysis system for processing the demodulated data to achieve remote monitoring, real-time warning, and remote management.

[0011] Preferably, the distributed strain sensing optical cable includes:

[0012] A fiber Bragg grating soil heave sensing optical cable at the bottom of the foundation pit, arranged inside the foundation pit for monitoring the soil heave at the bottom of the foundation pit;

[0013] A fiber Bragg grating upper slope settlement sensing and monitoring optical cable, arranged outside the foundation pit for monitoring the settlement of the upper part of the foundation pit slope;

[0014] A fiber Bragg grating foundation pit deformation sensing and monitoring optical cable, arranged outside the foundation pit for monitoring the deformation of the foundation pit;

[0015] Among them, a foundation trench is excavated at the position where the optical fiber is laid, and the elevation inside the foundation trench of the foundation pit is undulating in a wave shape to ensure that the distributed strain sensing optical cable is at different depths, so as to comprehensively monitor the changes in the surrounding environment of the foundation pit.

[0016] Preferably, the fiber Bragg grating sensor is connected to the wireless fiber Bragg grating demodulator through a lead optical fiber, and the wireless fiber Bragg grating demodulator transmits the demodulated signal to the data analysis system. The data analysis system includes:

[0017] A strain measurement system for measuring the strain changes of the foundation pit;

[0018] A vertical displacement calculation system for calculating the vertical displacement of the foundation pit;

[0019] A result display interface for visually displaying the monitoring results and supporting remote operation monitoring to achieve automated monitoring.

[0020] A monitoring method for soft soil foundation pits based on distributed optical fibers, including the following steps:

[0021] Arranging fiber Bragg grating sensors outside and inside the foundation pit;

[0022] Excavating a foundation trench along the position where the optical fiber is laid to ensure that the fiber Bragg grating optical cable is at different depths;

[0023] After the fiber Bragg grating sensing and monitoring optical cable is buried, process the joints to prevent the optical cable from being worn or bent at the joints;

[0024] The fiber Bragg grating sensor is connected to the wireless fiber Bragg grating demodulator through a lead optical fiber, and the demodulator transmits the signal to the data analysis system to achieve remote monitoring and real-time warning.

[0025] Preferably, the data analysis system includes a strain measurement system, a vertical displacement calculation system, and a result display interface, enabling remote operation monitoring, fast measurement speed, intuitive display, and realizing automated monitoring.

[0026] Preferably, fiber Bragg grating sensors are arranged outside and inside the soft soil foundation pit. Using fiber Bragg grating (FBG) sensing technology, the optical fiber is affected by external factors along the line, and the sensing signal after the change of the optical wave characterization parameter is detected.

[0027] For the fiber Bragg grating with a period less than 1μm, the fiber Bragg grating FBG uses the photo-induced refractive index change effect generated by the photosensitive optical fiber under ultraviolet light irradiation, making the refractive index of the fiber core show a periodic distribution along the axial direction, forming a spatial phase grating in the fiber core, thereby changing and controlling the propagation of light in it.

[0028] The FBG reflects light of a specific wavelength, and this wavelength satisfies the following conditions:

[0029] λ B =2n eff .Λ

[0030] In the formula: λB---the central wavelength of the FBG;

[0031] n eff ---the effective refractive index of the fiber core;

[0032] Λ---the grating pitch of the FBG.

[0033] When the grating is affected by environmental factors such as strain, both the grating pitch and the effective refractive index n eff will change accordingly, resulting in the drift of the central wavelength of the FBG in the reflection spectrum. Utilizing the characteristic that the FBG can accurately measure the micro-deformation of materials, the FBG is encapsulated and attached to an elastic element. Through the function of the FBG to well determine the micro-deformation of materials, the encapsulated FBG is installed in the device and encapsulated into sensors such as displacement, pressure, and tilt, realizing multi-variable sensing monitoring of the soft soil foundation pit.

[0034] Preferably, when arranging the fiber Bragg grating slope upper settlement sensing monitoring optical cable and the fiber Bragg grating foundation pit deformation sensing monitoring optical cable outside the foundation pit, and arranging the fiber Bragg grating foundation pit bottom soil heave sensing monitoring optical cable inside the foundation pit, first dig a foundation trench along the optical fiber arrangement position, and dig the elevation inside the pit in a wavy and undulating manner to ensure that the fiber Bragg grating optical cable is at different depths, thereby monitoring the changes in the surrounding environment of the foundation pit.

[0035] Preferably, after the fiber Bragg grating sensing and monitoring optical cable is buried, the joint is processed to prevent the optical cable from being worn or bent at the joint, which may affect the monitoring. After the optical cable is fixed, backfill is carefully carried out with graded sand and gravel to ensure the protection of the fiber Bragg grating sensing and monitoring optical cable.

[0036] Preferably, when arranging the fiber grating settlement sensing and monitoring optical cable on the upper part of the slope of the fiber grating, drill holes on the foundation pit slope and arrange them regularly. After drilling to the predetermined depth, use clear water to replace the slurry. After replacing the slurry, lift the drill to form the required measurement hole. Backfill 20-40 cm thick cement mortar at the bottom of the measurement hole, and then put the fiber Bragg grating sensing and monitoring optical cable with the bottom end sealed into the measurement hole, and form a certain gap with the bottom end of the measurement hole.

[0037] After the fiber Bragg grating sensing and monitoring optical cable is placed, it is fixed and then slowly backfilled with sand and gravel to ensure that the fiber Bragg grating sensing and monitoring optical cable is vertically placed to better monitor the settlement change of the soft soil mass.

[0038] Preferably, the soft soil foundation pit monitoring system of the distributed optical fiber includes a wireless fiber Bragg grating demodulator and a data analysis system. The fiber Bragg grating sensor is connected to the wireless fiber Bragg grating demodulator through a 5 mm armored lead optical cable, and the fiber Bragg grating sensor transmits the signal to the wireless fiber Bragg grating demodulator.

[0039] All fiber Bragg grating sensors are connected by lead optical fibers. One end of the lead optical fiber is connected to the wireless fiber Bragg grating demodulator. The data analysis system can remotely receive the wireless fiber Bragg grating demodulator and obtain the measured data.

[0040] The fiber Bragg grating demodulator uses a wireless fiber Bragg grating demodulator. The fiber Bragg grating demodulator measures the change in the wavelength of the fiber Bragg grating to accurately monitor the deformation displacement value of the soil body. The data analysis system can remotely control the fiber Bragg grating demodulator.

[0041] The data analysis system includes three parts: a strain measurement system, a vertical displacement calculation system, and a result display interface. It can remotely operate the monitoring, with a fast measurement speed, intuitive display, and can realize automatic monitoring.

[0042] Compared with the prior art, the present invention provides a soft soil foundation pit monitoring system and monitoring method based on distributed optical fiber, having the following beneficial effects:

[0043] A soft soil foundation pit monitoring system and method based on distributed optical fiber described by the present invention aims at the problems existing in the monitoring of soft soil foundation pits with special soil quality. Through a soft soil foundation pit monitoring system and method based on distributed optical fiber, during the monitoring process of the soft soil foundation pit, through the installation of the optical fiber sensing system, the optical fiber can cover a large area, and at the same time, the intensive installation requirements of traditional monitoring equipment are reduced. It makes the layout of sensors within the soft soil foundation pit more flexible and more applicable, and successfully solves the problems of incomplete and untimely monitoring of soft soil foundation pits. After using this construction method, not only can the occurrence of foundation pit displacement and instability be reduced, but also the following aspects are included:

[0044] 1. Real-time monitoring: By adopting the distributed optical fiber grating sensing technology, real-time and continuous monitoring of the soft soil foundation pit is realized, solving the problems of data fragmentation and untimely of traditional monitoring methods.

[0045] 2. High-precision measurement: The optical fiber grating sensing technology has a monitoring accuracy at the microstrain level, and can accurately locate the abnormal areas of the foundation pit.

[0046] 3. Strong anti-interference ability: The optical fiber itself is not affected by environmental factors such as electromagnetic interference and groundwater, and the monitoring data has high reliability.

[0047] 4. Simple installation: The optical fiber has a wide coverage area, and the layout of sensors is more flexible, reducing the intensive installation requirements of traditional monitoring equipment.

[0048] 5. Multi-parameter monitoring: The system can monitor multiple parameters simultaneously, comprehensively judge the health status of the foundation pit, and improve the comprehensiveness and accuracy of monitoring.

[0049] 6. Remote monitoring: Based on the long-distance transmission ability of the distributed optical fiber sensing system, remote monitoring, real-time early warning and remote management are realized, improving the efficiency and real-time of foundation pit monitoring. Description of the Drawings

[0050] Figure 1 It is a three-dimensional schematic diagram of the upper settlement sensing monitoring of a slope based on distributed optical fiber grating of the present invention;

[0051] Figure 2 It is a three-dimensional schematic diagram of the layout of a soft soil foundation pit monitoring system based on distributed optical fiber proposed by the present invention;

[0052] Figure 3 It is a structural schematic diagram of the distributed optical fiber monitoring system in a soft soil foundation pit monitoring system based on distributed optical fiber proposed by the present invention;

[0053] In the figure: 1. Distributed strain sensing optical cable; 2. Plastic encapsulated armored communication optical cable; 3. Fiber Bragg grating sensing unit; 4. Fiber Bragg grating sensing unit; 5. Foundation pit; 6. Fiber Bragg grating foundation pit bottom soil heave sensing and monitoring optical cable; 7. Fiber Bragg grating slope upper settlement sensing and monitoring optical cable; 8. Fiber Bragg grating foundation pit deformation sensing and monitoring optical cable; 9. Lead optical fiber. Specific implementation mode

[0054] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0055] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention.

[0056] Embodiment 1

[0057] As Figures 1 to 3 shown, a soft soil foundation pit 5 monitoring system based on distributed optical fiber includes:

[0058] A distributed optical fiber monitoring system for collecting strain and deformation data of the foundation pit 5, the distributed optical fiber monitoring system includes a distributed strain sensing optical cable 1, a plastic encapsulated armored communication optical cable 2, a fiber Bragg grating sensing unit 3 and a fiber Bragg grating sensing unit 4;

[0059] Fiber Bragg grating sensors are arranged on the outside and inside of the soft soil foundation pit 5 for monitoring the deep soil heave of the foundation pit 5, the deformation of the retaining pile, the settlement of the upper structure of the foundation pit 5, the displacement of the foundation pit 5 slope and the surcharge pressure;

[0060] A wireless fiber Bragg grating demodulator for receiving the signals of the fiber Bragg grating sensors and demodulating them;

[0061] A data analysis system for processing the demodulated data to achieve remote monitoring, real-time warning and remote management.

[0062] The distributed strain sensing optical cable 1 includes:

[0063] A fiber Bragg grating foundation pit bottom soil heave sensing optical cable arranged inside the foundation pit 5 for monitoring the heave of the bottom soil of the foundation pit 5;

[0064] The fiber optic grating settlement sensing and monitoring optical cable 7 for the upper part of the slope is arranged outside the foundation pit 5 and is used to monitor the settlement of the upper part of the slope of the foundation pit 5;

[0065] The fiber optic grating deformation sensing and monitoring optical cable 8 for the foundation pit is arranged outside the foundation pit 5 and is used to monitor the deformation of the foundation pit 5;

[0066] Among them, a foundation trench is excavated at the position where the optical fiber is laid, and the elevation inside the foundation trench and the foundation pit 5 is undulating in a wave shape to ensure that the distributed strain sensing optical cable 1 is at different depths, so as to be able to comprehensively monitor the changes in the surrounding environment of the foundation pit 5.

[0067] Embodiment 2

[0068] As Figures 1 to 3 shown, the fiber optic grating sensor is connected to the wireless fiber optic grating demodulator through the lead optical fiber 9, and the wireless fiber optic grating demodulator transmits the demodulated signal to the data analysis system, and the data analysis system includes:

[0069] A strain measurement system for measuring the strain change of the foundation pit 5;

[0070] A vertical displacement calculation system for calculating the vertical displacement of the foundation pit 5;

[0071] A result display interface for visually displaying the monitoring results and supporting remote operation monitoring to achieve automatic monitoring.

[0072] Embodiment 3

[0073] As Figures 1 to 3 shown, a monitoring method for a soft soil foundation pit based on distributed optical fiber includes the following steps:

[0074] Arranging fiber optic grating sensors outside and inside the foundation pit 5;

[0075] Excavating a foundation trench along the position where the optical fiber is laid to ensure that the fiber optic grating optical cable is at different depths;

[0076] After the fiber optic grating sensing and monitoring optical cable is buried, process the joint to prevent the optical cable from being worn or bent at the joint;

[0077] The fiber optic grating sensor is connected to the wireless fiber optic grating demodulator through the lead optical fiber 9, and the demodulator transmits the signal to the data analysis system to achieve remote monitoring and real-time warning.

[0078] The data analysis system includes a strain measurement system, a vertical displacement calculation system and a result display interface, which can be remotely operated for monitoring, has a fast measurement speed, visually displays, and realizes automatic monitoring.

[0079] Embodiment 4

[0080] On the basis of Embodiment 3, fiber Bragg grating sensors are arranged outside and inside the soft soil foundation pit 5. Using fiber Bragg grating (FBG) sensing technology, the fiber is sensed to be affected by external factors along the line, and the sensing signal after the change of the optical wave characterization parameter is detected;

[0081] For the fiber Bragg grating with a period less than 1 μm, the fiber Bragg grating FBG uses the photoinduced refractive index change effect generated by the photosensitive fiber under ultraviolet light irradiation, so that the refractive index of the fiber core shows a periodic distribution along the axial direction, forming a spatial phase grating in the fiber core, thereby changing and controlling the propagation of light therein;

[0082] The FBG reflects light of a specific wavelength, and this wavelength satisfies the following conditions:

[0083] λ B = 2n eff .Λ

[0084] In the formula: λB---the central wavelength of the FBG;

[0085] n eff ---the effective refractive index of the fiber core;

[0086] Λ---the grating pitch of the FBG.

[0087] When the grating is affected by environmental factors such as strain, the grating pitch and the effective refractive index n eff will both change accordingly, so that the central wavelength of the FBG in the reflection spectrum drifts. Utilizing the characteristic that the FBG can accurately measure the micro-deformation of materials, the FBG is encapsulated and attached to an elastic element. Through the function of the FBG to well determine the micro-deformation of materials, the encapsulated FBG is installed in the device and encapsulated into sensors such as displacement, pressure, and tilt, realizing multi-variable sensing monitoring of the soft soil foundation pit 5.

[0088] When arranging the fiber Bragg grating upper slope settlement sensing monitoring optical cable 7 and the fiber Bragg grating foundation pit deformation sensing monitoring optical cable 8 outside the foundation pit 5, and arranging the fiber Bragg grating foundation pit bottom soil heave sensing monitoring optical cable 6 inside the foundation pit 5, first dig a foundation trench along the fiber arrangement position, and the elevation inside the pit is dug in a wavy and undulating manner to ensure that the fiber Bragg grating optical cable is at different depths, so as to monitor the changes in the surrounding environment of the foundation pit 5.

[0089] After the fiber Bragg grating sensing monitoring optical cable is buried, the joint is processed to prevent the optical cable from being worn or bent at the joint and affecting the monitoring; after the optical cable is fixed, backfill is carefully carried out with graded sand and gravel, etc., to ensure the protection of the fiber Bragg grating sensing monitoring optical cable.

[0090] When arranging the optical fiber grating slope upper settlement sensing monitoring optical cable 7, drill holes on the slope of the foundation pit 5 in a regular pattern. After drilling to the predetermined depth, use clear water to replace the slurry. After replacing the slurry, lift the drill to form the required measurement holes. Backfill the bottom of the measurement holes with 20 - 40 cm thick cement mortar, and then place the optical fiber grating sensing monitoring optical cable with the bottom end sealed into the measurement holes, and form a certain gap with the bottom end of the measurement holes.

[0091] After the optical fiber grating sensing monitoring optical cable is placed, fix it and then slowly backfill with sand and gravel to ensure that the optical fiber grating sensing monitoring optical cable is vertically placed to better monitor the settlement change of the soft soil mass.

[0092] The soft soil foundation pit 5 monitoring system of the distributed optical fiber includes a wireless optical fiber grating demodulator and a data analysis system; the optical fiber grating sensor is connected to the wireless optical fiber grating demodulator through a 5 mm armored lead optical cable, and the optical fiber grating sensor transmits the signal to the wireless optical fiber grating demodulator.

[0093] All the optical fiber grating sensors are connected by lead optical fibers 9. One end of the lead optical fiber 9 is connected to the wireless optical fiber grating demodulator, and the data analysis system can remotely receive the wireless optical fiber grating demodulator and obtain the measured data.

[0094] The optical fiber grating demodulator uses a wireless optical fiber grating demodulator. The wireless optical fiber grating demodulator measures the change in the wavelength of the optical fiber grating to accurately monitor the deformation displacement value of the soil body. The data analysis system can remotely control the optical fiber grating demodulator.

[0095] The data analysis system includes three parts: a strain measurement system, a vertical displacement calculation system, and a result display interface, which can remotely operate the monitoring, with a fast measurement speed, intuitive display, and can realize automatic monitoring.

[0096] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent replacements or changes, and all should be covered by the protection scope of the present invention.

Claims

1. A soft soil foundation pit monitoring system based on distributed optical fiber, characterized in that: include: A distributed optical fiber monitoring system for collecting strain and deformation data of foundation pits, wherein the distributed optical fiber monitoring system comprises a distributed strain sensing optical cable, a plastic-encapsulated armored communication optical cable, an optical fiber Bragg grating sensing unit and an optical fiber monitoring cable; Fiber Bragg grating sensors are arranged outside and inside the soft soil foundation pit to monitor the deep soil uplift, support pile deformation, foundation pit superstructure settlement, foundation pit slope displacement and pile load pressure; A wireless fiber Bragg grating demodulator, used for receiving and demodulating signals from a fiber Bragg grating sensor; The data analysis system is used to process the demodulated data to achieve remote monitoring, real-time warning and remote management.

2. The soft soil foundation pit monitoring system based on distributed optical fiber according to claim 1 is characterized in that: The distributed strain sensing optical cable comprises: Fiber Bragg grating foundation pit bottom soil uplift sensor cable is arranged inside the foundation pit to monitor the uplift of the soil at the bottom of the foundation pit; Fiber grating slope upper part settlement sensing monitoring optical cable is arranged outside the foundation pit to monitor the settlement of the upper part of the foundation pit slope; Fiber Bragg grating foundation pit deformation sensing monitoring optical cable is arranged outside the foundation pit to monitor the deformation of the foundation pit; Among them, a foundation trench is excavated at the location where the optical fiber is laid, and the elevation inside the foundation pit fluctuates in a wave-like manner, ensuring that the distributed strain sensing optical cable is at different depths, so as to comprehensively monitor changes in the environment around the foundation pit.

3. The soft soil foundation pit monitoring system based on distributed optical fiber according to claim 1 is characterized in that: The fiber grating sensor is connected to a wireless fiber grating demodulator via a lead optical fiber, and the wireless fiber grating demodulator transmits the demodulated signal to a data analysis system, and the data analysis system includes: Strain measurement system, used to measure the strain changes of the foundation pit; Vertical displacement calculation system, used to calculate the vertical displacement of the foundation pit; The result display interface is used to intuitively display the monitoring results, and supports remote operation monitoring to achieve automated monitoring.

4. A monitoring method for soft soil foundation pit based on distributed optical fiber, characterized in that: The following steps are involved: Arrange fiber grating sensors outside and inside the foundation pit; Dig trenches along the fiber layout to ensure that the fiber grating cables are at different depths; After the fiber optic Bragg grating sensing monitoring cable is buried, the joints are processed to prevent the cable from being worn or bent at the joints; The fiber grating sensor is connected to the wireless fiber grating demodulator through the lead optical fiber. The demodulator transmits the signal to the data analysis system to achieve remote monitoring and real-time early warning.

5. The soft soil foundation pit monitoring method based on distributed optical fiber according to claim 4 is characterized in that: The data analysis system includes a strain measurement system, a vertical displacement calculation system and a result display interface, and can be remotely operated for monitoring, has a fast measurement speed, and can display intuitively, thereby realizing automated monitoring.

6. A method for monitoring soft soil foundation pit based on fiber Bragg grating, characterized in that: The following steps are involved: Fiber Bragg grating sensors are arranged outside and inside the soft soil foundation pit. The fiber Bragg grating (FBG) sensing technology is used to sense the influence of external factors along the optical fiber and detect the perception signal after the light wave characteristic parameters are changed. Fiber Bragg Grating (FBG) uses the photoinduced refractive index change effect of photosensitive optical fiber under ultraviolet light to make the refractive index of the fiber core present a periodic distribution along the axial direction, forming a spatial phase grating in the fiber core, thereby changing and controlling the propagation of light in it. FBG reflects light of a specific wavelength that satisfies the following conditions: l B =2n eff .L Where: λB---the central wavelength of FBG; n eff ---Effective refractive index of the fiber core; Λ---FBG pitch. When the grating is affected by environmental factors such as strain, the grating pitch and effective refractive index n eff Will change accordingly, causing the central wavelength of FBG in the reflection spectrum to drift. Utilizing the property of FBG that can accurately measure the micro-deformation of materials, the FBG is packaged and attached to the elastic element. The function of the micro-deformation of the material can be well determined through FBG. The packaged FBG is installed in the device and packaged into displacement, pressure, tilt and other sensors to realize multivariable sensing monitoring of soft soil foundation pits.

7. The method for monitoring soft soil foundation pit based on fiber Bragg grating according to claim 6, characterized in that: When Fiber Bragg Grating (FBG) cables for sensing and monitoring the upper settlement of the slope and the deformation of the pit are arranged outside the foundation pit, and Fiber Bragg Grating (FBG) cables for sensing and monitoring the soil uplift at the bottom of the pit are arranged inside the foundation pit, the foundation trench is first excavated along the fiber optic layout position, and the elevation in the pit is excavated along the wave-like ups and downs to ensure that the Fiber Bragg Grating (FBG) cables are at different depths, thereby monitoring changes in the surrounding environment of the foundation pit.

8. The method for monitoring soft soil foundation pit based on fiber Bragg grating according to claim 7, characterized in that: After the Fiber Bragg Grating sensor monitoring cable is buried, the joints are processed to prevent the cable from being worn or bent at the joints and affecting the monitoring; after the cable is fixed, the backfill is carefully carried out using extremely well-matched sand and gravel to ensure the protection of the Fiber Bragg Grating sensor monitoring cable.

9. The method for monitoring soft soil foundation pit based on fiber Bragg grating according to claim 8, characterized in that: When arranging the fiber Bragg grating slope upper settlement sensing monitoring cable, holes are drilled on the foundation pit slope and arranged regularly. After drilling to the predetermined depth, the slurry is replaced with clean water. After the slurry is replaced, the drill is lifted to form the required measuring hole; 20-40cm thick cement mortar is backfilled to the bottom of the measuring hole, and then the fiber Bragg grating sensing monitoring cable with the bottom closed is placed into the measuring hole, and a certain gap is formed with the bottom of the measuring hole; After the fiber optic Bragg grating sensing monitoring cable is placed in, it is fixed and then slowly backfilled with sand and gravel to ensure that the fiber optic Bragg grating sensing monitoring cable is placed vertically, so as to better monitor the settlement changes of soft soil.

10. The method for monitoring soft soil foundation pit based on fiber Bragg grating according to claim 9, characterized in that: The distributed optical fiber soft soil foundation pit monitoring system comprises a wireless fiber grating demodulator and a data analysis system; the fiber grating sensor is connected to the fiber grating demodulator through a 5mm armored lead cable, and the fiber grating sensor transmits the signal to the fiber grating demodulator; The fiber grating sensors are all connected by lead optical fiber, one end of which is connected to the wireless fiber grating demodulator. The data analysis system can remotely receive the wireless fiber grating demodulator and obtain the measured data. The fiber Bragg grating demodulator adopts a wireless fiber Bragg grating demodulator, which measures the change of the fiber Bragg grating wavelength to accurately monitor the deformation displacement value of the soil body. The data analysis system can remotely control the fiber Bragg grating demodulator; The data analysis system consists of three parts: strain measurement system, vertical displacement calculation system and result display interface. It can be remotely operated and monitored, has fast measurement speed, intuitive display, and can realize automatic monitoring.

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