CN108180841A - A kind of landslide internal displacement monitoring method based on fiber grating - Google Patents

Abstract

The invention discloses a method for monitoring internal displacement of landslides based on optical fiber gratings. This method combines the fiber Bragg grating sensing technology and the traditional inclinometer. The deformation inside the monitored landslide will cause the bending deformation of the inclinometer tube, resulting in the deformation of the fiber grating. The fiber grating sensor and the fiber grating demodulator can pass through the fiber The wavelength change of the grating obtains the corresponding strain value, and then calculates the displacement at the corresponding measuring point through the strain-displacement differential calculation method, and then accurately calculates the disturbance of the inclinometer tube. It is more convenient to implement in the on-site monitoring of the slope, and the deformation inside the slope can be obtained more accurately by processing the monitoring data through the simple composite Simpson integral formula. The method of the invention is simple and easy to implement in operation, can monitor the side slope in real time for a long time, has simple and accurate data processing method, and has strong popularization and practicability.

Description

A monitoring method for internal displacement of landslide based on fiber Bragg grating

technical field

The invention relates to a landslide monitoring method, in particular to a method for monitoring internal displacement of a landslide based on an optical fiber grating.

Background technique

In the safety and stability monitoring of landslides, long-term monitoring of surface displacement and internal displacement of landslides is generally required. In the current monitoring methods, equipment such as levels, total stations, and inclinometers are mostly used for manual monitoring, but the timeliness of data and the convenience of measurement are difficult to meet the complex terrain, poor safety of manual operation, and real-time monitoring. slope requirements. In recent years, fiber grating sensor, as a new type of sensing device, has the characteristics of anti-electromagnetic interference, safety and reliability, corrosion resistance, high stability, and real-time measurement, and has been widely used in the field of engineering monitoring.

The Fiber Bragg Grating sensor is a kind of Fiber Bragg Grating sensor. Based on the change of the external physical quantity, the center wavelength of the Fiber Bragg Grating will change. The relationship between the wavelength drift and the temperature and strain is linear. The relationship is as follows:

In the formula, Δλ is the wavelength change value, λ is the central wavelength value, α and ξ are the thermo-optic coefficients, P _eff is the elasto-optic coefficient, ΔT is the temperature change value, Δε is the strain change value, K _T is the temperature sensitivity coefficient of the fiber Bragg grating , K _ε is the FBG strain sensitivity coefficient.

The invention patent with the application number 201410659677.6 introduces a method of monitoring the sliding surface of foundation pits based on fiber Bragg gratings. This method uses the FBG demodulator to monitor the wavelength change of the fiber Bragg grating on the inclinometer in real time, and calculates the average value on each section of the inclinometer. Strain, according to the change of the strain value, the position of the slip surface of the foundation pit can be judged. However, this method cannot measure the real-time deformation at different depths of the foundation pit, and can only determine the approximate position of the slip surface, which shows the limitation of this method.

At present, there are many in-situ inclinometer sensors based on the principle of fiber grating monitoring, and there are not a few sensors used for internal deformation monitoring of landslides. Most of the improvements are mainly reflected in the calibration and processing of data, and there are also various problems: the accuracy is not high , Many assumptions and so on.

Contents of the invention

Aiming at the deficiencies of the prior art, the technical problem to be solved by the present invention is to provide a method for monitoring internal displacement of landslides based on fiber gratings.

The technical solution of the present invention to solve the technical problem is to provide a method for monitoring internal displacement of landslides based on fiber gratings, which is characterized in that the method includes the following steps:

1) A symmetrical U-shaped fiber grating string is arranged on the surface centerline of the raised guide groove on the outer edge of the inclinometer tube of the inclinometer, and the fiber grating is closely attached to the surface of the inclinometer tube to form a fiber grating strain sensor;

2) Drill a hole at the position where the landslide needs to be monitored, and splice the fiber grating strain sensor into the drill hole one by one, so that the U-shaped plane of the fiber grating is parallel to the sliding direction of the monitored slope; the fiber grating strain sensor and the fiber optic The grating demodulator is connected to record the initial wavelength signal of the fiber grating strain sensor;

3) The FBG demodulator monitors the wavelength change of the FBG in real time, and uses the calibrated FBG strain sensitivity coefficient K _ε to calculate the average axial strain ε(z) at different sections of the inclinometer tube:

In the formula, the bottom of the inclinometer tube is the origin of the coordinates, and the wavelength values of the front and rear symmetrical FBGs at the distance z from the bottom of the inclinometer tube are λ _u and λ _d , and their initial values are λ _u0 and λ _d0 , respectively, and λ _u0 ≈ λ _d0 = λ ₀ ;

4) According to the integral formula of the disturbance curve of the cantilever beam in the bending state in the mechanics of materials, combined with the composite Simpson integral formula, the disturbance w(z _n ) and the average axial strain ε(z) of the inclinometer tube at different positions are established The relationship can be expressed as Equation 10:

In the formula, R is the outer radius of the inclinometer tube, h is the layout spacing of the fiber grating measuring points, l is the total length of the spliced inclinometer tube, z _i =2ih, n is the total number of measuring points;

5) In landslide monitoring, assuming that the internal deformation of the landslide body and the bending deformation of the inclinometer tube are consistent, the internal deformation of the landslide can be judged in real time according to the disturbance measured by the inclinometer tube at different positions.

Compared with the prior art, the present invention has the beneficial effects of:

1) This method combines the fiber grating sensing technology and the traditional inclinometer, which is more convenient to implement in the field monitoring of the slope. The deformation inside the slope can be obtained more accurately by processing the monitoring data through the simple composite Simpson integral formula. The method of the invention is simple and easy to implement in operation, can monitor the side slope in real time for a long time, has simple and accurate data processing method, and has strong popularization and practicability.

2) In the data processing method of the present invention, the curvature analysis of the strain distribution calculation is utilized, but the random load distribution in the actual cantilever beam is bypassed to avoid the situation that the strain integral cannot be obtained, and the numerical difference method is cleverly used to obtain high precision difference result.

3) The internal deformation of the monitored landslide will cause the bending deformation of the inclinometer tube, resulting in the deformation of the fiber grating. The fiber grating sensor and the fiber grating demodulator can be used to obtain the corresponding real-time and accurate wavelength change (or frequency difference) of the fiber grating. Strain value, but it is not easy to calculate the disturbance of the inclinometer tube (that is, the deformation of the monitoring body) through the strain. This method proposes a strain-displacement differential calculation method, which can first calculate the displacement at the corresponding measuring point, and then accurately calculate the disturbance of the inclinometer pipe.

4) This method can self-compensate the influence of temperature on the wavelength change of the fiber Bragg grating, avoiding various limitations of simply measuring temperature with an external fiber Bragg grating, and is more suitable for landslide monitoring.

Description of drawings

Fig. 1 is the overall structure schematic diagram of a kind of embodiment of the landslide internal displacement monitoring method based on fiber Bragg grating of the present invention;

Fig. 2 is the cross-sectional view of Fig. 1 of the landslide internal displacement monitoring method based on fiber Bragg grating of the present invention;

Fig. 3 is the actual application state diagram in slope monitoring of an embodiment of the method for monitoring internal displacement of landslides based on fiber gratings in the present invention;

Fig. 4 is the depth of the present invention based on the landslide internal displacement monitoring method embodiment 1 of fiber grating and the relation of the measured displacement that calculates displacement value and inclinometer gained; is the fiber grating, 4 is the fiber grating demodulator, 5 is the monitored slope, and 6 is the potential slip surface)

Detailed ways

Specific examples of the present invention are given below. The specific embodiments are only used to further describe the present invention in detail, and do not limit the protection scope of the claims of the present application.

The present invention provides a kind of landslide internal displacement monitoring method (abbreviation method) based on optical fiber grating, it is characterized in that the method comprises the following steps:

1) The inclinometer tube 1 of the inclinometer is selected as the base material for the fiber grating 3 to be pasted, and a symmetrical U-shaped fiber grating string is arranged on the surface center line of the raised guide groove on the outer edge of the inclinometer tube. Closely fit on the surface of the inclinometer tube to form a fiber grating strain sensor;

2) Drill a hole at the position where the landslide needs to be monitored (that is, the potential sliding surface 6 of the monitored slope 5), and splice the fiber grating strain sensors into the drill hole one by one, so that the U-shaped plane of the fiber grating and the monitored edge The sliding direction of the slope 5 is parallel; the fiber grating strain sensor is connected with the fiber grating demodulator 4, and the initial wavelength signal of the fiber grating strain sensor is recorded;

3) The FBG demodulator 4 monitors the wavelength change of the FBG in real time, and calculates the average axial strain ε(z) at different sections of the inclinometer tube by using the strain sensitivity coefficient K _ε of the FBG calibrated in the laboratory:

In the formula, the bottom of the inclinometer tube is the origin of the coordinates, and the wavelength values of the front and rear symmetrical FBGs at the distance z from the bottom of the inclinometer tube are λ _u and λ _d , and their initial values are λ _u0 and λ _d0 , respectively, and λ _u0 ≈ λ _d0 = λ ₀ ;

4) According to the integral formula of the disturbance curve of the cantilever beam in the bending state in the mechanics of materials, combined with the composite Simpson integral formula, the disturbance w(z _n ) and the average axial strain ε(z) of the inclinometer tube at different positions are established The relationship of can be expressed as Equation 10, the average axial strain ε(z) is used to calculate the disturbance, which can avoid the influence of temperature on the calculation:

In the formula, R is the outer radius of the inclinometer tube, h is the layout spacing of the fiber grating measuring points, l is the total length of the spliced inclinometer tube, z _i =2ih, n is the total number of measuring points;

5) In landslide monitoring, assuming that the internal deformation of the landslide body and the bending deformation of the inclinometer tube are consistent, the internal deformation of the landslide can be judged in real time according to the disturbance measured by the inclinometer tube at different positions.

The value of the average axial strain ε(z) is the mean value of the strain induced by the front and back symmetrical fiber grating at the distance z from the bottom of the inclinometer tube.

The derivation process of formula 10 is as follows:

It is assumed that the landslide and the inclinometer tube are coordinating deformation, that is, it is assumed that the internal deformation of the landslide body and the bending deformation of the inclinometer tube are in harmony; the deformation characteristics of the inclinometer tube can be analyzed as equivalent to the bending deformation structure of a cantilever beam:

The relationship between the cantilever beam’s disturbance and the radius of curvature is shown in Equation 1:

In the formula, w(z) is the disturbance of the inclinometer tube, ρ(z) is the radius of curvature, and z is the coordinate along the axis of the inclinometer tube;

The relationship between the radius of curvature of the inclinometer tube and the average axial strain is shown in Equation 2:

In the formula, R is the outer radius of the inclinometer tube, ε(z) is the average axial strain of the inclinometer tube;

Combining formula 1 and formula 2, formula 3 can be obtained:

By double integrating the left and right sides of Equation 3, the relationship 4 between the disturbance of the inclinometer tube and the average axial strain can be obtained as:

A number of fiber grating measuring points are arranged at different positions of the inclinometer tube. The bottom of the inclinometer tube is the origin of the coordinate axis, and the measuring points along the axis are recorded as zi _{and zi+1/2} , expressed as formula 5:

In the formula, n is the total number of measuring points; h is the layout spacing of fiber grating measuring points; l is the total length of spliced inclinometer tubes;

Combining Equations 4 and 5, based on the composite Simpson integral formula, the relationship between the disturbance of the inclinometer tube and the average axial strain can be expressed as Equation 6:

in,

According to Equations 7, 8 and 9, the relationship between the disturbance of the inclinometer tube and the average axial strain can be expressed as Equation 10:

The derivation process of formula 11 is as follows:

1) The relationship between the wavelength drift of the front and back symmetrical fiber gratings in the inclinometer tube and the temperature and strain is Equation 12 and Equation 13:

In the formula, ΔT is the temperature change value, the strains induced by the symmetrical FBG at the distance z from the bottom of the inclinometer tube are Δε _u and Δε _d respectively, K _T is the temperature sensitivity coefficient of the FBG, and K _ε is the strain sensitivity of the FBG coefficient;

2) When the cantilever beam is bent in linear elasticity, according to the assumption of plane section, the front and rear surface strain values of the cantilever beam satisfy Equation 14:

Δε _u = -Δε _d (14)

3) According to formulas 12, 13 and 14, and λ _u0 ≈ λ _d0 = λ ₀ , the relationship between the average axial strain and wavelength at different sections of the inclinometer can be calculated as formula 11:

Example 1

This embodiment is applied to landslide monitoring as shown in Figures 1 and 2. The fiber grating 3 is pasted on the center line of the surface of the raised guide groove on the outer edge of the inclinometer tube 1, and is arranged in a symmetrical U-shape to form a fiber grating strain sensor; The sensor is connected with a fiber grating demodulator 4 through an optical fiber lead wire 2 .

When in use, as shown in Figure 3, the drilled hole passes through the potential sliding surface 6 of the monitored slope 5, and carefully puts the inclinometer tube 1 into it. The sliding direction is parallel. The optical fiber grating demodulator 4 is connected to monitor the wavelength change of the optical fiber grating in real time, so as to calculate the corresponding displacement changes at different positions.

The fiber grating strain sensor was placed in a certain slope for monitoring, and the data was collected regularly. A total of 3 data were collected. The displacement calculated by this method was compared with the displacement calculated by the traditional inclinometer tube. The results are shown in Figure 4. It can be seen from Figure 4 that the displacement value (calculated value) calculated by this method is very close to the displacement value (measured value) calculated by the traditional inclinometer tube, and the judgment of the position of the slip surface is the same. The drilling position is 12m below the ground, and this method can monitor the internal displacement of the slope in real time over a long period of time and at a long distance, and has high precision at the same time.

What is not mentioned in the present invention is applicable to the prior art.

Claims (3)

1. A landslide internal displacement monitoring method based on fiber bragg grating is characterized by comprising the following steps:

1) symmetrical U-shaped fiber bragg grating strings are arranged on the surface center line of a raised guide groove at the outer edge of an inclinometer, and the fiber bragg gratings are tightly attached to the surface of the inclinometer to form a fiber bragg grating strain sensor;

2) drilling a hole at a position to be monitored on a landslide, splicing the fiber bragg grating strain sensors section by section and placing the spliced fiber bragg grating strain sensors into the hole, so that the U-shaped plane of the laid fiber bragg grating is parallel to the sliding direction of the monitored side slope; connecting the fiber grating strain sensor with a fiber grating demodulator, and recording an initial wavelength signal of the fiber grating strain sensor;

3) the fiber grating demodulator monitors the wavelength change of the fiber grating in real time and utilizes the calibrated strain sensitivity coefficient K of the fiber grating_εAnd calculating the average axial strain epsilon (z) of the inclinometer at different sections:

in the formula, the bottom of the inclinometer is a coordinate origin, and the front and back symmetrical fiber bragg grating wavelength values at the position z away from the bottom of the inclinometer are respectively lambda_uAnd λ_dWith initial values of λ respectively_u0And λ_d0，λ_u0≈λ_d0＝λ₀；

4) According to the disturbance curve integral formula of the cantilever beam in a bending state in material mechanics, combining a composite Simpson integral formula, establishing the disturbance w (z) of the inclinometer at different positions_n) And the average axial strain ε (z) can be expressed as equation 10:

wherein R is the outer radius of the inclinometer, h is the arrangement distance of the fiber grating measuring points, l is the total length of the spliced inclinometer, and z is_i＝2ih，z_i+1/2＝z_i+hn is the total number of measuring points;

5) in landslide monitoring, the internal deformation condition of the landslide can be judged in real time according to the disturbance degree measured by the inclinometer at different positions on the assumption that the internal deformation of the landslide body and the bending deformation of the inclinometer are coordinated and consistent.

2. The fiber grating-based landslide internal displacement monitoring method according to claim 1, wherein the derivation process of equation 10 is as follows:

the landslide and the inclinometer are assumed to be in coordinated deformation, namely the internal deformation of the landslide body and the bending deformation of the inclinometer are assumed to be in coordination; the deformation characteristics of the inclinometer can be equivalently analyzed as a cantilever beam bending deformation structure:

the relation between the deflection and the curvature radius of the cantilever beam is shown as the formula 1:

wherein w (z) is the deflection of the inclinometer, ρ (z) is the radius of curvature, and z is the coordinate along the axial direction of the inclinometer;

the relationship between the radius of curvature and the average axial strain of the inclinometer is shown as formula 2:

wherein R is the outer radius of the inclinometer and ε (z) is the average axial strain of the inclinometer;

combining formula 1 and formula 2, formula 3 can be obtained:

the double integral is carried out on the left side and the right side of the formula 3, and the relation 4 of the deflection and the average axial strain of the inclinometer can be obtained as follows:

arranging a plurality of fiber bragg grating measuring points at different positions of the inclinometer, wherein the bottom of the inclinometer is a coordinate axis origin, and the measuring points along the axis are marked as z_iAnd z_i+1/2Expressed by formula 5:

wherein n is the total number of measuring points; h is the layout space of the fiber bragg grating measuring points; l is the total length of the spliced inclinometer pipe;

combining equations 4 and 5, based on the composite simpson integral formula, the relationship between the deflection and the average axial strain of the inclinometer can be expressed as equation 6:

wherein,

according to equations 7, 8 and 9, the relationship between the deflection and the average axial strain of the inclinometer can be expressed as equation 10:

3. the fiber grating-based landslide internal displacement monitoring method according to claim 1, wherein the derivation process of formula 11 is as follows:

1) the relationship between the shift of the wavelength of the fiber grating which is symmetrical back and forth in the inclinometer and the temperature and the strain is shown as an equation 12 and an equation 13:

in the formula,. DELTA.TIs the temperature variation value, and the strain induced by the symmetrical fiber bragg gratings at the position of z away from the bottom of the inclinometer is delta epsilon_uAnd Δ ε_d，K_TIs the temperature sensitivity coefficient, K, of the fiber grating_εIs the fiber grating strain sensitivity coefficient;

2) when the cantilever beam is in linear elastic inner bending, the strain values of the front surface and the rear surface of the cantilever beam satisfy the formula 14 according to the assumption of a flat section:

Δε_u＝-Δε_d(14)

3) according to formulae 12, 13 and 14, with λ_u0≈λ_d0＝λ₀The relationship between the average axial strain and the wavelength at different sections of the inclinometer can be calculated as the following formula 11: