A method for testing horizontal displacement of a deep foundation pit pile wall top

Abstract

The application provides a kind of deep foundation pit pile wall top horizontal displacement test method, comprising: first fiber grating string 2 and second fiber grating string 3 are installed on the both sides of carrier;First fiber grating string 2 and second fiber grating string 3 include multiple measuring points;Select two points with the distance of a and b from end point on the measured object 1, mark as A and B, and set the initial horizontal displacement of A and B as 0;When the measured object 1 deforms, the center wavelength of the measuring point of first fiber grating string 2 after deformation and the center wavelength of the measuring point of second fiber grating string 3 after deformation are obtained;Get the displacement of A and B after deformation;Calculate the displacement of each measuring point.The beneficial effects of the present application are: the present application sets two groups of fiber grating strings, real-time measures the compression strain and tensile strain of each interface, tests the horizontal displacement of two end points through external reference point, as displacement boundary, so as to calculate the deflection of the measured object, realizes the real-time monitoring of the measured object, and ensures the safety of the project.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of fiber grating sensing technology and foundation pit engineering monitoring technology, more particularly, it relates to a deep foundation pit pile wall top horizontal displacement testing method. BACKGROUND

[0002] During the excavation of the foundation pit, engineering and technical personnel often need to know the size of the deep layer horizontal displacement of the enclosure structure and the horizontal displacement of the pile wall top and its space-time evolution law to ensure the safety of the construction.

[0003] When the deep foundation pit is excavated, the stress adjustment of the enclosure structure, the surrounding vehicles or the temporary load will cause the horizontal displacement of the pile wall top, which reflects the main index of the stability of the deep foundation pit enclosure structure. According to the provisions of GB50911-2013 "Technical Code for Monitoring of Urban Rail Transit Engineering" and GB50497-2019 "Technical Standard for Monitoring of Building Foundation Pit Engineering", the horizontal displacement of the pile wall top of the foundation pit is a must-measured item. In addition, in the implementation of the safety monitoring of the deep foundation pit in soft soil foundation, factors such as insufficient embedded depth of the enclosure structure, when the horizontal displacement of the deep layer of the enclosure structure is calculated on site, the horizontal displacement of the pile wall top is generally tested by using the polar coordinate method and the line of sight method of the high-precision total station from the orifice, which is used to correct and calibrate the horizontal displacement of each point of the deep layer of the enclosure structure. Affected by various factors such as site limitations and visibility, the precision of the total station for testing the horizontal displacement of the pile wall top is not high, and the testing efficiency is low, so it is impossible to accurately obtain the deformation data of the foundation pit, especially the deformation of the toe of the enclosure structure of the foundation pit in soft soil foundation. The traditional method cannot accurately and truly reflect whether the toe of the foundation pit has occurred. SUMMARY

[0004] The purpose of the present application is to overcome the shortcomings of the prior art, and to provide a deep foundation pit pile wall top horizontal displacement testing method which is simple to implement, high in measurement precision and can realize online monitoring.

[0005] In a first aspect, a deep foundation pit pile wall top horizontal displacement testing method is provided, comprising:

[0006] S1, the first fiber grating string 2 and the second fiber grating string 3 are symmetrically installed on both sides of the carrier about the center line of the carrier, and the two fiber grating strings are located on the same horizontal plane; the first fiber grating string 2 and the second fiber grating string 3 each include a plurality of measuring points, and the measuring points on the first fiber grating string 2 and the measuring points on the second fiber grating string 3 are symmetrically arranged about the center line of the carrier;

[0007] S2, the initial center wavelength of the measuring point of the first fiber grating string 2 and the initial center wavelength of the measuring point of the second fiber grating string 3 are obtained;

[0008] S3, select two points A and B on the measured object 1, which are a and b away from the end point respectively, and set the initial horizontal displacement of A and B as 0;

[0009] S4, after the carrier is deformed synchronously with the measured object 1, the center wavelength of the measuring point of the first fiber grating string 2 after deformation and the center wavelength of the measuring point of the second fiber grating string 3 after deformation are obtained;

[0010] S5, an external reference point is introduced, and the horizontal displacement V of A and B after deformation is obtained by total station test a1 , V b1 ;

[0011] S6, the strain change and displacement of each measuring point are calculated.

[0012] Preferably, in S1, the measuring point is a fiber Bragg grating sensor, and the center wavelength of the measuring point is represented by the following formula:

[0013] λ B = 2n eff Λ

[0014] Where λ B represents the center wavelength, n eff is the equivalent refractive index of the core, and Λ is the grating period.

[0015] Preferably, in S4, when subjected to external temperature and strain, the center wavelength displacement of the measuring point is represented by:

[0016] Δλ B = 2Δn eff Λ+2n eff ΔΛ

[0017] Where Δλ B is the center wavelength displacement of the measuring point, Δn eff is the change of the equivalent refractive index of the core, and ΔΛ is the change of the grating period.

[0018] Preferably, in S4, the drift of the center wavelength of the measuring point is:

[0019]

[0020] Where ε Z is the axial strain, P e is the elasto-optical coefficient, α Λ is the thermal expansion coefficient of the optical fiber, α n represents the thermo-optic coefficient, and ΔT is the change of temperature.

[0021] Preferably, in S4, the difference between the center wavelength drifts of the two fiber grating measuring points is:

[0022]

[0023] wherein, and respectively represent the center wavelength drift of two fiber grating measuring points.

[0024] As preferred, in S6, the calculation formula of the strain change amount of the measuring point is:

[0025]

[0026] wherein, Δε zi represents the strain change amount, λ Bi0 represents the initial center wavelength of the measuring point of the first fiber grating string 2, λ Bi1 represents the center wavelength of the measuring point of the first fiber grating string 2 after deformation; λ' Bi0 represents the initial center wavelength of the measuring point of the second fiber grating string 3, λ' Bi1 represents the center wavelength of the measuring point of the second fiber grating string 3 after deformation.

[0027] As preferred, in S6, the calculation formula of the displacement of the measuring point is:

[0028]

[0029] wherein, y i represents the displacement, Δx i is the distance between the upper and lower measuring sections along the length of the measured object 1, R is half of the distance between the symmetric two strain measuring points on the section of the measured object 1, ε a is the strain of point A, and ε b is the strain of point B.

[0030] In a second aspect, a deep foundation pit pile wall top horizontal displacement testing device is provided for performing the deep foundation pit pile wall top horizontal displacement testing method of any one of the first aspect, and comprises a carrier, a measured object 1, a first fiber grating string 2 and a second fiber grating string 3.

[0031] wherein, the first fiber grating string 2 and the second fiber grating string 3 are symmetrically installed on both sides of the carrier about the center line of the carrier, and the two fiber grating strings are located on the same horizontal plane; the first fiber grating string 2 and the second fiber grating string 3 each comprise a plurality of measuring points, and the measuring points on the first fiber grating string 2 and the measuring points on the second fiber grating string 3 are symmetrically arranged about the center line of the carrier; the elastic modulus of the carrier material is less than or equal to the elastic modulus of the soil body.

[0032] The beneficial effects of the present application are:

[0033] 1. This invention sets up two linearly distributed fiber optic grating strings, arranged symmetrically with the center line as the axis, to measure the compressive and tensile strains of each interface in real time. The horizontal displacement of the two ends is measured through an external reference point as the displacement boundary, thereby calculating the deflection of the object under test. This realizes real-time monitoring of the object under test and ensures the safety of the project.

[0034] 2. This invention utilizes fiber optic grating sensing technology combined with end-level horizontal displacement verification to measure the horizontal displacement at the top of the pile wall in deep foundation pits. This overcomes the shortcomings of traditional displacement measurement methods that require a large amount of manpower, and has the advantages of convenient installation, high measurement accuracy, and low cost. Attached Figure Description

[0035] Figure 1 A schematic diagram of the structure of a horizontal displacement testing device for the top of a deep foundation pit pile wall before deformation;

[0036] Figure 2 A schematic cross-sectional view of a method for testing the horizontal displacement at the top of a deep foundation pit pile wall;

[0037] Figure 3 This is a schematic diagram of the deformed structure after a method for testing the horizontal displacement of the top of a deep foundation pit pile wall.

[0038] Explanation of reference numerals in the attached figures: 1. Measured object; 2. First fiber optic grating string; 3. Second fiber optic grating string. Detailed Implementation

[0039] The present invention will be further described below with reference to embodiments. The description of the embodiments below is only for the purpose of helping to understand the present invention. It should be noted that those skilled in the art can make several modifications to the present invention without departing from the principle of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0040] As one embodiment, this application provides a method for testing the horizontal displacement at the top of a deep foundation pit pile wall, including:

[0041] S1, such as Figure 1 As shown, the first fiber Bragg grating string 2 and the second fiber Bragg grating string 3 are symmetrically installed on both sides of the carrier about the center line of the carrier, and the two fiber Bragg grating strings are located on the same horizontal plane; the carrier is a straight strip with a circular cross-section, and the elastic modulus of the carrier material is required to be less than or equal to the elastic modulus of the soil; both the first fiber Bragg grating string 2 and the second fiber Bragg grating string 3 include multiple measuring points, and the measuring points on the first fiber Bragg grating string 2 and the measuring points on the second fiber Bragg grating string 3 are symmetrically arranged about the center line of the carrier.

[0042] The fiber grating can not only sense strain, but also transmit signals. Multiple fiber grating sensors in series can form a fiber grating string for distributed strain observation. The fiber sensing technology has the advantages of light weight, thin diameter, strong anti-electromagnetic interference, corrosion resistance, high temperature resistance, small signal attenuation, integration of information sensing and information transmission, etc., and can realize strain measurement of the measured object.

[0043] It should be noted that the measured object 1 in the embodiment refers to the top of the enclosure structure. The measured object 1 has a large range, and the carrier is attached to the measured object 1. The carrier is embedded in the measured object. Therefore, when the sensing element cannot be directly installed on the measured object 1, the measured object 1 can realize measurement of the measured object by installing the sensing element on the carrier.

[0044] S2, obtaining the initial center wavelengths of the first fiber grating string 2 measuring points and the initial center wavelengths of the second fiber grating string 3 measuring points.

[0045] For example, the initial center wavelengths of the first fiber grating string 2 in the initial state are λ B10 , λ B20 , λ B30 , …, λ Bi0 , …, λ Bm0 , and the initial center wavelengths of the second fiber grating string 3 in the initial state are λ' B10 , λ' B20 , λ' B30 , …, λ' Bi0 , …, λ' Bm0 .

[0046] S3, selecting two points A and B with distances a and b from the end points on the measured object 1, and setting the initial horizontal displacement of A and B as 0.

[0047] S4, obtaining the center wavelengths of the first fiber grating string 2 measuring points after deformation and the center wavelengths of the second fiber grating string 3 measuring points after deformation when the carrier deforms synchronously with the measured object 1.

[0048] For example, the center wavelengths of the first fiber grating string 2 after deformation are λ B11 , λ B21 , λ B31 , …, λ Bi1 , …, λ Bm1 , and the center wavelengths of the second fiber grating string 3 are λ' B11 , λ' B21 , λ' B31 , …, λ' Bi1 , …, λ' Bm1 .

[0049] S5, introduce external reference point, through the total station test to obtain the horizontal displacement V of A and B after deformation a1 、V b1 .

[0050] S6, compare the test value with the initial value, according to the principle of fiber Bragg grating sensing, vertical strain compensation and temperature compensation, calculate the strain change of each measuring point.

[0051] 1, fiber Bragg grating sensing principle

[0052] Fiber Bragg grating sensor is a functional fiber sensor using fiber Bragg grating as a sensitive element. According to the theory of fiber coupling mode, when a wide band light passes through the fiber grating, mode coupling will be produced, when the Bragg condition is met, the grating will act as a mirror, reflecting a narrow band light wave (the rest of the wavelength transmits through the fiber grating and continues to transmit), the center wavelength of the narrow band light wave is the grating Bragg wavelength λ B .

[0053] λ B =2n eff Λ

[0054] Where, λ B represents the center wavelength, n eff is the equivalent refractive index of the core, and Λ is the grating period.

[0055] When the grating is affected by external temperature, strain and other factors, Λ and neff change ΔΛ and Δneff, which leads to the displacement of the reflected wavelength satisfying the Bragg condition Δλ B .

[0056] Δλ B =2Δn eff Λ+2n eff ΔΛ

[0057] Where, Δλ B is the center wavelength displacement of the measuring point, Δn eff is the change of the equivalent refractive index of the core, and ΔΛ is the change of the grating period.

[0058] The above formula is the mechanism of grating sensing monitoring. The displacement Δλ B of the reflected center wavelength satisfying the Bragg condition is monitored to detect the temperature, strain, stress and other external measured signals acting on the fiber grating.

[0059] The drift of the center wavelength of the fiber grating is:

[0060]

[0061] Where, ε ZP is the axial strain e α is the photoelastic coefficient Λ α is the thermal expansion coefficient of the optical fiber n ΔT represents the change in temperature.

[0062] 2. Temperature compensation principle

[0063] The multi-point displacement testing device adopts a two-fiber grating string design, as shown in Figure 1 and Figure 2 The carrier is a circular tubular material, FBG1 and FBG2 are located on the two sides of the circular cross section symmetrically relative to the center of the circle, and the temperature self-compensation of FBG2 to FBG1 can be achieved. The thermal expansion coefficients and the thermo-optic coefficients of FBG1 and FBG2 are the same, so the temperature sensitivity coefficients of the two are also the same. As can be seen from the structural design, the drift of the center wavelength is independent of the temperature and only changes with the strain. The difference between the center wavelength drifts of the two fiber gratings is:

[0064]

[0065] wherein, and respectively represent the center wavelength drifts of the two fiber grating measuring points.

[0066] From the above, the strain change amount Δεzi of each point in the embodiment can be expressed by the following formula:

[0067]

[0068] wherein, Δε zi represents the strain change amount, λ Bi0 represents the initial center wavelength of the measuring point of the first fiber grating string 2, λ Bi1 represents the center wavelength of the measuring point of the first fiber grating string 2 after deformation; λ' Bi0 represents the initial center wavelength of the measuring point of the second fiber grating string 3, λ' Bi1 represents the center wavelength of the measuring point of the second fiber grating string 3 after deformation.

[0069] In S6, according to the strain change amount of each point, the displacement amount of the two end points, and the material mechanics theory, the displacement y i of each point is obtained.

[0070] According to the material mechanics theory, it is known that:

[0071]

[0072] The relationship between the rotation angle θ i of a certain cross section of the measured object 1 relative to the adjacent cross section and the strain difference Δε i of the two symmetric points on the cross section is:

[0073]

[0074] where Δx i is the distance between the two measuring sections along the length of the measured object 1, and R is half the distance between the two symmetrically located strain measuring points on the cross section of the measured object 1.

[0075] The displacement of the measuring point is then calculated as:

[0076]

[0077] where y i is the displacement, ε a is the strain at point A, and ε b is the strain at point B.

Claims

1. A method for testing horizontal displacement of a top portion of a deep foundation pit pile wall, characterized in that, The application relates to a method for measuring the deformation of a measured object (1) by using two fiber grating strings. S1, the first fiber grating string (2) and the second fiber grating string (3) are symmetrically arranged on both sides of a carrier about a carrier center line, and the two fiber grating strings are located on the same horizontal plane; the first fiber grating string (2) and the second fiber grating string (3) each comprise a plurality of measuring points, and the measuring points on the first fiber grating string (2) and the measuring points on the second fiber grating string (3) are symmetrically arranged about the carrier center line; The measuring point is a fiber Bragg grating sensor, and the central wavelength of the measuring point is expressed by the following formula: ； wherein, denotes the center wavelength, is the core equivalent refractive index, is the grating period; S2, the initial central wavelengths of the measuring points of the first fiber grating string (2) and the second fiber grating string (3) are obtained; S3, two points A and B with distances a and b from the end points are selected on the measured object (1), and the initial horizontal displacement of A and B is set as 0; S4, the central wavelengths of the measuring points of the first fiber grating string (2) and the second fiber grating string (3) after deformation are obtained after the carrier is deformed synchronously with the measured object (1); When affected by external temperature and strain, the central wavelength displacement of the measuring point is expressed by the following formula: ； wherein, is the center wavelength shift of the measurement point, is the change in the effective refractive index of the fiber core, is the change in the grating period; The drift of the central wavelength of the measuring point is expressed by the following formula: ； wherein is the axial strain, is the photoelastic coefficient, is the thermal expansion coefficient of the optical fiber, denotes the thermo-optic coefficient, is the change in temperature; The difference between the central wavelength drifts of the two fiber grating measuring points is expressed by the following formula: ； wherein, and respectively represent the center wavelength drifts of the two fiber grating measurement points; S5, introduce external reference point, obtain the horizontal displacement V of A and B after deformation by total station test a1 , V b1 ; S6, the strain change amount and the displacement of each measuring point are calculated; The calculation formula of the strain change amount of the measuring point is expressed by the following formula: ； wherein, represents a strain change amount, represents an initial center wavelength of the measuring point of the first fiber grating string (2), represents a center wavelength of the measuring point of the first fiber grating string (2) after deformation; represents an initial center wavelength of the measuring point of the second fiber grating string (3), represents a center wavelength of the measuring point of the second fiber grating string (3) after deformation; The calculation formula of the displacement of the measuring point is expressed by the following formula: ； wherein, represents displacement, is the distance between the two measuring sections along the length of the object (1), and R is half the distance between the two symmetrically located strain measuring points on the cross section of the object (1), is the strain at point A, is the strain at point B.

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