Soft soil-optical cable interface strength testing method and device based on optical fiber sensing technology

Abstract

The invention provides a soft soil-optical cable interface strength testing method based on an optical fiber sensing technology, which comprises the following steps: carrying out a pull-out test in soft soil by using a rubber rod containing an optical fiber to obtain the tensile force and the interface friction force of the rubber rod in a soil body, and calculating the interface shear strength through the tensile force and the interface friction force in the soil body; then, the roughness of the rubber rod and the standing time of the rubber rod in the soil body are changed, and interface shear strength data under different roughness and different standing time are obtained; after all the roughness and the interfacial shear strength data under the standing time are obtained, the interfacial shear strength ratio under the corresponding condition is calculated; and finally, establishing a function relational expression of the standing time, the interface friction coefficient and the interface strength ratio after normalization through a mathematical fitting method, thereby realizing rapid prediction of the soft soil-optical cable interface strength characteristic. The invention provides a feasible solution for analyzing the cable-soil interface problem in the soft soil thixotropic process.

Description

Technical Field

[0001] The invention belongs to the field of civil engineering, and in particular relates to a method and a device for testing the soft soil-optical cable interface strength based on optical fiber sensing technology. Background Art

[0002] Optical cable communication plays an important role in the transmission of modern information and the development of information technology, and has always been one of the most important communication infrastructures in the world. Under the influence of complex environments, the position of optical cables will move after being laid, and it is inevitable that they will rub against the soil and cause wear, which will cause changes in the friction of the optical cable surface. And with the increase of time, the structure and strength of the disturbed soil will gradually recover. In geotechnical engineering, the phenomenon that the strength of the disturbed soil gradually recovers with the increase of time is defined as thixotropy. In actual situations, the thixotropy process of the soft soil in contact with the optical cable and the wear of the optical cable occur simultaneously. Therefore, considering the different degrees of wear of the optical cable, it is of great significance to explore the changes in the shear strength of the soft soil-cable interface during the strength recovery process after the soft soil is disturbed, which is of great significance for in-depth understanding of the thixotropic characteristics of soft soil and understanding of the soft soil-cable interface problem.

[0003] At present, the research on the interface problem between soil and components mostly adopts the pull-out test, that is, by pulling the components buried in the soil, the strain change of the components during the pulling process is obtained, and then combined with Hooke's law, the interface force between the components and the soil can be calculated. The optical cable is generally covered with a sheath to protect the internal structure of the optical cable. The optional materials of the sheath generally include steel or equivalent hard materials, soft polymer wear-resistant materials, such as TPE, HDPE, PP, etc. The most commonly used is HDPE sheath. The external load causes the change of the friction degree of the optical cable surface to be reflected in the friction grooves generated on the surface of the sheath. However, because this hard material has no elastic modulus, it cannot be combined with optical fiber for stress-strain analysis. Summary of the invention

[0004] The purpose of the present invention is to provide a method for testing the soft soil-optical cable interface strength based on optical fiber sensing technology.

[0005] The technical solution of the present invention is as follows:

[0006] A method for testing soft soil-optical cable interface strength based on optical fiber sensing technology, the method comprising:

[0007] A pull-out test is carried out in soft soil using a rubber rod containing an optical fiber to obtain the tensile force and interface friction of the rubber rod in the soil, and the interface shear strength is calculated through the tensile force and interface friction in the soil.

[0008] Then the roughness of the rubber rod and the static time in the soil were changed to obtain the interface shear strength data under different roughness and different static time;

[0009] After obtaining the interface shear strength data under all roughness and static time, the interface shear strength ratio under corresponding conditions is calculated;

[0010] Finally, the functional relationship between the normalized standing time, interface friction coefficient and interface strength ratio was established through mathematical fitting method, so as to realize the rapid prediction of the soft soil-optical cable interface strength characteristics.

[0011] Furthermore, the friction force of the rubber rod in the soil is specifically:

[0012] F τ (t)=πDLτ(t)

[0013] Among them, F τ (t) is the rubber-soil interface force, D is the outer diameter of the rubber rod, L is the axial length of the rubber rod in contact with the soil, and τ(t) is the shear strength of the rubber rod-soil interface.

[0014] Furthermore, the tension of the rubber rod in the soil is obtained through the change in the central wavelength of the optical fiber and Hooke's law.

[0015] Furthermore, the formula for the change in the central wavelength of the optical fiber is:

[0016]

[0017] Among them, Δλ(t) is the change of the central wavelength of the optical fiber, λ is the central wavelength of the original fiber Bragg grating, P ε is the elastic-optic coefficient of the optical fiber material, and Δε(t) is the axial strain of the FBG.

[0018] Furthermore, the Hooke's law is specifically:

[0019] Δσ(t)=E a Δε(t)

[0020] Among them, E a is the elastic modulus of the rubber material.

[0021] Furthermore, the specific formula of the tension of the rubber rod in the soil is:

[0022]

[0023] Among them, A is the cross-sectional area of ​​the rubber rod, E a is the elastic modulus of the rubber material, Δλ(t) is the change in the central wavelength of the optical fiber, λ is the central wavelength of the original fiber Bragg grating, P ε is the elastic-optical coefficient of the optical fiber material.

[0024] Furthermore, the shear strength relationship between the flexible structure and the soil interface is specifically:

[0025]

[0026] Among them, E a is the elastic modulus of the optical fiber material; A is the cross-sectional area of ​​the optical fiber; Δλ(t) is the central wavelength drift of the optical fiber Bragg grating; λ is the central wavelength of the original optical fiber Bragg grating; P ε is the elastic-optical coefficient of the optical fiber material; D is the outer diameter of the optical fiber; L is the axial length of the optical fiber in contact with the soil

[0027] Furthermore, the functional relationship between the normalized standing time, the interface friction coefficient and the interface strength ratio is specifically:

[0028]

[0029] Where n is the interface friction coefficient, is the normalized time.

[0030] The present invention also provides a soft soil-optical cable interface strength testing device, the device comprising:

[0031] Rubber rod, fiber Bragg grating, soil container box, overburden load, fixture, digital dynamometer, stepper motor and computer;

[0032] The surface of the rubber rod is engraved with grooves of different depths and widths to simulate the surface of optical cables with different roughness;

[0033] The fiber Bragg grating is used to monitor the deformation of the rubber rod in real time during the drawing process, and calculate the strain of the rubber rod in combination with the change in the optical fiber wavelength;

[0034] The stepper motor controls the pulling speed, the digital dynamometer measures the pulling force in real time, and the computer analyzes the interface strength through data acquisition and processing;

[0035] The soil container box is used to carry soft soil samples and simulate the actual situation of optical cables being buried in the soil;

[0036] The overburden load is used to simulate the external load on the optical cable in the soil.

[0037] The clamp is used to fix the rubber rod and apply a pulling force.

[0038] Compared with the prior art, the present invention has the following advantages:

[0039] The present invention uses a rubber material with grooves carved on the surface to simulate an optical cable with friction loss on the surface, and at the same time, combined with an optical fiber, can accurately measure the continuous deformation of the rubber rod during the pulling process. The present invention solves the problem that the optical cable sheath material has no elastic modulus and it is difficult to analyze the interface problem between it and the soil. At the same time, the pulling test adopted is simple and easy to operate. The different stages of the soil in the thixotropic process can be simulated by different static time of the soil in the test, thereby providing a feasible solution for analyzing the cable-soil interface problem in the thixotropic process of soft soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The accompanying drawings generally illustrate various embodiments by way of example and not limitation, and together with the description and claims, serve to illustrate the embodiments of the invention. Where appropriate, the same reference numerals are used throughout the drawings to refer to the same or similar parts. Such embodiments are illustrative and are not intended to be exhaustive or exclusive embodiments of the present apparatus or method.

[0041] Figure 1 is a top view of the rubber rod and optical fiber combination device of the present invention;

[0042] Figure 2 is a side view of the rubber rod and optical fiber combination device of the present invention;

[0043] Figure 3 is a perspective view of a rubber rod and optical fiber combination device in the present invention;

[0044] Figure 4 It is a front view of the optical cable-soil interface characteristic testing device of the present invention;

[0045] Figure 5 This is a perspective view of the optical cable-soil interface characteristic testing device of the present invention.

[0046] Figure 6 It is a schematic diagram of the strength ratio of a certain soil interface - friction coefficient - normalized time;

[0047] The meanings of the marks in the figure are: half rubber rod 1, half rubber rod 2, FBG fiber Bragg grating 3, rubber and optical fiber combination device 4, soil container box 5, overlying load 6, clamp 7, digital dynamometer 8, stepper motor 9, computer 10, fiber Bragg grating regulator 11, drainage hole 12, network cable 13. DETAILED DESCRIPTION

[0048] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0049] The embodiment of the present invention provides a method for testing the soft soil-optical cable interface strength based on optical fiber sensing technology, the method comprising:

[0050] A pull-out test is carried out in soft soil using a rubber rod containing an optical fiber to obtain the tensile force and interface friction of the rubber rod in the soil, and the interface shear strength is calculated through the tensile force and interface friction in the soil.

[0051] Then the roughness of the rubber rod and the static time in the soil were changed to obtain the interface shear strength data under different roughness and different static time;

[0052] After obtaining the interface shear strength data under all roughness and static time, the interface shear strength ratio under corresponding conditions is calculated;

[0053] Finally, the functional relationship between the normalized standing time, interface friction coefficient and interface strength ratio was established through mathematical fitting method, so as to realize the rapid prediction of the soft soil-optical cable interface strength characteristics.

[0054] Specifically:

[0055] (1) A rubber rod with different depths and widths is used to simulate the surface of a flexible structure with different roughness. An optical fiber is bonded in the rubber rod. A pull-out test is used to measure the interface strength between the surface of the flexible structure and the soft soil. At a certain roughness and a standing time of t, it is assumed that the pull-out test meets the following two conditions:

[0056] The rubber optical fiber-soil interface force and the tension on the optical fiber satisfy the static equilibrium, that is,

[0057] F τ (t) = F P (t)

[0058] The friction force at the interface between the rubber rod and the soil is evenly distributed on the surface of the rubber rod, that is,

[0059] F τ (t)=πDLτ(t)

[0060] where F τ (t) is the rubber-soil interface force, F P (t) is the tensile force on the rubber rod, D is the outer diameter of the rubber rod, L is the axial length of the rubber rod in contact with the soil, and τ(t) is the shear strength of the rubber rod-soil interface.

[0061] (2) Determine the tensile force F on the rubber rod P (t)

[0062] According to the fiber Bragg grating strain transformation calculation formula, the central wavelength change Δλ(t) of the optical fiber under a certain static time t is calculated:

[0063]

[0064] Among them, Δλ(t) is the change of the central wavelength of FBG, λ is the central wavelength of the original FBG grating, P ε is the elastic-optic coefficient of the optical fiber material, Δε(t) is the axial strain of the FBG, θ is the thermo-optic coefficient of the fiber Bragg grating, is the thermal expansion coefficient of the fiber Bragg grating, and ΔT is the change in the external temperature. Under the condition of keeping the external temperature constant, the strain of the rubber rod under a certain static time is obtained according to the measured change in the optical fiber wavelength, and then according to Hooke's law:

[0065] Δσ(t)=E a Δε(t)

[0066] Among them, E a is the elastic modulus of the rubber material. Similarly, when the external temperature is kept constant, the stress of the rubber rod can be obtained from the strain of the rubber rod, and the tensile force of the rubber rod in the pull-out test under a certain static time can be obtained:

[0067]

[0068] Where A is the cross-sectional area of ​​the rubber rod.

[0069] (3) Determine the interface shear strength τ(t) between the rubber rod and the soil

[0070] According to the mechanical balance and the previously determined rubber rod tension, the expression of the interface shear strength τ(t) between the rubber rod and the soil after a certain static time can be obtained:

[0071]

[0072] Among them, E a is the elastic modulus of the optical fiber material; A is the cross-sectional area of ​​the optical fiber; Δλ(t) is the central wavelength drift of the optical fiber Bragg grating; λ is the central wavelength of the original optical fiber Bragg grating; P ε is the elastic-optical coefficient of the optical fiber material; D is the outer diameter of the optical fiber; L is the axial length of the optical fiber in contact with the soil.

[0073] (4) After obtaining the interface strength data under all roughness and static time, the interface shear strength ratio M under the corresponding conditions is calculated, and the interface shear strength ratio M between the rubber rod and the soil and the interface friction coefficient n and the normalized time are established by mathematical fitting and normalization methods. Functional relationship

[0074]

[0075] Normalized interface friction coefficient n-normalized standing time —Interface shear strength ratio M space coordinates, establish a three-dimensional surface schematic diagram as shown in the figure. In actual engineering, for a certain type of soft soil, such a chart can be established in the early stage to predict the thixotropic time of the soft soil after being disturbed and the degree of wear of the optical cable, and then quickly determine the interface strength characteristics between the optical cable and the soil.

[0076] The present invention also provides a device for implementing the above method:

[0077] The device comprises: a rubber rod 1, a rubber rod 2, a FBG fiber Bragg grating 3, a simulation optical cable 4; a soil container box 5, an overburden load 6, a clamp 7, a digital dynamometer 8, a stepping motor 9; a computer 10, and a fiber Bragg grating demodulator 11;

[0078] The rubber rod 1, the rubber rod 2, the FBG fiber Bragg grating 3, the simulated optical cable 4, the soil container box 5, the overlying load 6, the clamp 7, the digital display dynamometer 8, and the stepping motor 9 constitute a testing device. Grooves are engraved on the surface of the rubber rods 1 and 2 to simulate the change of the surface roughness of the optical cable, and the FBG fiber Bragg grating 3 is attached between the rubber rods 1 and 2 to form a set of simulated optical cables 4 that can measure continuous deformation; a layer of geotextile and a layer of filter paper are laid at the bottom of the soil container box, and a soil sample is prepared by a mud method. Then the assembled rubber rod is buried in the soil container box 5, and a layer of geotextile and a layer of filter paper are laid on the top of the soil container box 5. Laying the geotextile and filter paper is beneficial to soil consolidation and drainage; an overburden load 6 is placed above the soil container box 5, and a uniform upper load pressure is provided by adding weights; a clamp 7 is installed on the rubber rod extending out of the soil container 5 to facilitate the application of a pulling force to the rubber; then the clamp is connected to a digital dynamometer 8, and the dynamometer can read the change of the pulling force during the pulling process; finally, the digital dynamometer 8 is connected to a stepper motor 9, and the stepper motor is used to provide power to control the pulling speed of the rubber rod.

[0079] The computer 10 and the fiber Bragg grating demodulator 11 constitute a quantitative device for collecting required data; the fiber Bragg grating demodulator 11 is connected to the FBG fiber Bragg grating 3 to demodulate the optical signal and convert it into an electrical signal; the computer 10 is connected to the fiber Bragg grating demodulator 11 to collect the test data.

[0080] The operation method of the device is:

[0081] Step 1: Prepare test supplies

[0082] First, prepare half rubber rods 1 and 2 of appropriate length and radius, carve grooves of different depths and spacings on the surface of the rubber rods, and use a friction coefficient meter to determine the static friction coefficient and dynamic friction coefficient of the surface, thereby obtaining a simulated optical cable with a known surface friction coefficient. Attach the optical fiber 3 to the middle of the half rubber rod 1 and bond the other half rubber rod 2 to it for standby use. Lay geotextile and filter paper in the soil container box 5, prepare a test soil of a predetermined type and moisture content in the soil container box by using a mud method, and bury the rubber and optical fiber combination 4 in the soil container box.

[0083] Step 2: Connect the test equipment

[0084] The soil container box 5 with the embedded rubber and optical fiber combination device 4 is left to stand for a preset time, and then the combination device 4 is connected to the fixture 7, the digital display dynamometer 8, and the stepper motor 9 in sequence. At the beginning of the test, the stepper motor 9 is started to pull the combination device 4 at a constant speed. The test is stopped when the interface between the combination device 4 and the soil is destroyed, the combination device 4 is completely pulled out, or the optical fiber strain value reaches the maximum value of its monitoring range. During the experiment, the fiber grating demodulator 11 records the wavelength changes of the grating area in real time throughout the whole process.

[0085] Step 3: Experimental testing

[0086] (1) Obtaining the rubber optical fiber-soil interface strength

[0087] At a certain static time t, the tensile force value of the rubber rod 4 is read from the tensile meter 8 in the device, and then the tensile force value of the rubber rod 4 is calculated based on the reading of the optical fiber wavelength change. The rubber optical fiber-soil interface strength τ is calculated by combining the two.

[0088]

[0089] (2) Repeat the test

[0090] Change the surface roughness of the rubber rod and the standing time respectively, and repeat the above test

[0091] Step 4: Data processing

[0092] After obtaining the interface strength data under all roughness and static time, the interface shear strength ratio M between the rubber rod and the soil and the interface friction coefficient n, normalized time Functional relationship

[0093]

[0094] Friction coefficient at the interface n-normalized standing time —Interface shear strength ratio M space coordinates, establish a three-dimensional surface diagram as shown in the figure Figure 6In actual engineering, for a certain type of soft soil, such a chart can be established in advance to predict the thixotropic time of the soft soil after being disturbed and the degree of wear of the optical cable, and then the strength characteristics of the interface between the optical cable and the soil can be quickly determined.

[0095] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for testing the soft soil-optical cable interface strength based on optical fiber sensing technology, characterized in that: The method comprises: A pull-out test is carried out in soft soil using a rubber rod containing optical fiber to obtain the tensile force and interface friction of the rubber rod in the soil, and the interface shear strength is calculated through the tensile force and interface friction in the soil. Then the roughness of the rubber rod and the static time in the soil were changed to obtain the interface shear strength data under different roughness and different static time; After obtaining the interface shear strength data under all roughness and static time, the interface shear strength ratio under corresponding conditions is calculated; Finally, the functional relationship between the normalized standing time, interface friction coefficient and interface strength ratio was established through mathematical fitting method, so as to realize the rapid prediction of the soft soil-optical cable interface strength characteristics.

2. The soft soil-optical cable interface strength testing method based on optical fiber sensing technology according to claim 1 is characterized in that: The friction force of the rubber rod in the soil is specifically: F τ (t)=πDLτ(t) Among them, F τ (t) is the rubber-soil interface force, D is the outer diameter of the rubber rod, L is the axial length of the rubber rod in contact with the soil, and τ(t) is the shear strength of the rubber rod-soil interface.

3. The soft soil-optical cable interface strength testing method based on optical fiber sensing technology according to claim 1 is characterized in that: The tension of the rubber rod in the soil is obtained through the change in the central wavelength of the optical fiber and Hooke's law.

4. The soft soil-optical cable interface strength testing method based on optical fiber sensing technology according to claim 3 is characterized in that: The formula for the change in the central wavelength of the optical fiber is: Among them, Δλ(t) is the change of the central wavelength of the optical fiber, λ is the central wavelength of the original fiber Bragg grating, P ε is the elastic-optic coefficient of the optical fiber material, and Δε(t) is the axial strain of the FBG.

5. The soft soil-optical cable interface strength testing method based on optical fiber sensing technology according to claim 3 is characterized in that: The Hooke's law is specifically: Δσ(t)=E a No Among them, E a is the elastic modulus of the rubber material.

6. The soft soil-optical cable interface strength testing method based on optical fiber sensing technology according to claim 1 is characterized in that: The specific formula of the tension of the rubber rod in the soil is: Where A is the cross-sectional area of ​​the rubber rod, E a is the elastic modulus of the rubber material, Δλ(t) is the change in the central wavelength of the optical fiber, λ is the central wavelength of the original fiber Bragg grating, P ε is the elastic-optical coefficient of the optical fiber material.

7. The soft soil-optical cable interface strength testing method based on optical fiber sensing technology according to claim 1 is characterized in that: The relationship between the shear strength of the interface between the flexible structure and the soil is specifically: Among them, E a is the elastic modulus of the optical fiber material; A is the cross-sectional area of ​​the optical fiber; Δλ(t) is the central wavelength drift of the optical fiber Bragg grating; λ is the central wavelength of the original optical fiber Bragg grating; P ε is the elastic-optical coefficient of the optical fiber material; D is the outer diameter of the optical fiber; and L is the axial length of the optical fiber in contact with the soil.

8. The soft soil-optical cable interface strength testing method based on optical fiber sensing technology according to claim 1 is characterized in that: The functional relationship between the normalized standing time, the interface friction coefficient and the interface strength ratio is specifically: Where n is the interface friction coefficient, is the normalized time.

9. A soft soil-optical cable interface strength testing device for implementing the method described in any one of claims 1 to 8, characterized in that: The device comprises: Rubber rod, fiber Bragg grating, soil container box, overburden load, fixture, digital dynamometer, stepper motor and computer; The surface of the rubber rod is engraved with grooves of different depths and widths to simulate the surface of optical cables with different roughness; The fiber Bragg grating is used to monitor the deformation of the rubber rod in real time during the drawing process, and calculate the strain of the rubber rod in combination with the change in the optical fiber wavelength; The stepper motor controls the pulling speed, the digital dynamometer measures the pulling force in real time, and the computer analyzes the interface strength through data acquisition and processing; The soil container box is used to carry soft soil samples and simulate the actual situation of optical cables being buried in the soil; The overburden load is used to simulate the external load on the optical cable in the soil. The clamp is used to fix the rubber rod and apply a pulling force.

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