CN110608946A - Early thixotropic strength test and device of soft clay based on FBG and full-flow penetration testing - Google Patents

Abstract

The invention belongs to the technical field of civil engineering and provides an early thixotropic strength test and device for soft clay based on FBG and full-flow penetration testing. The device includes base and fixing device, heightening pillar, ball screw linear guide, motor, slider, steering L-shaped rigid joint, force wire, FBG fiber grating, FBG laser incident end, FBG optical fiber output end, ball probe , Soil sample container, fiber grating demodulator and displacement measuring instrument; replace the straight rod with the force wire, simplify the calculation formula of the resistance stress q _r of the measured spherical probe, and simplify the calculation formula of the existing plastic full flow theory. Since the T-type full-flow penetration test is based on the plastic full-flow theory, the plastic theoretical solution is obtained when the upper limit solution is equal to the lower limit solution, so a more credible theoretical solution is obtained. The FBG strain sensor also has many advantages such as small size, light weight, and corrosion resistance, and can more accurately measure the shear strength of the undisturbed soil with as little disturbance as possible.

Description

Early thixotropic strength test and device of soft clay based on FBG and full-flow penetration testing

technical field

The invention belongs to the technical field of civil engineering, and relates to a novel method for testing the thixotropic strength of soft clay using a full-flow penetrating sounding method, and relates to a fiber Bragg grating (FBG) strain test method and related theories of full-flow penetrating sounding.

Background technique

With the continuous advancement of my country's deep-sea strategy, problems surrounding the construction of marine structures continue to emerge. When designing and constructing marine structures in marine engineering (such as deep-sea oil production platforms, submarine transportation pipelines, etc.), the analysis and calculation of the mechanical and physical properties of soft clay on the seabed will be involved. The soft clay in the natural state mostly has the properties of high water content, poor permeability, and low strength, so the engineering properties of soft clay are poor, which is not conducive to the construction of marine engineering.

The part of the marine structure buried in the soft clay of the seabed will be affected by periodic effects such as ocean currents, waves, and temperature changes, as well as irregular disturbances of marine organisms. In the early stage of disturbance, due to the softening and thixotropy of saturated clay, the strength of clay will decrease exponentially and recover gradually, which poses a great threat to the stability of marine structures. Therefore, it is of great engineering significance to accurately measure the initial thixotropic strength of cohesive soil.

For the shear strength test of soft clay, there are usually direct shear test, triaxial shear test, cross shear plate test, flat plate penetration test, and static penetration testing (CPT) commonly used in recent years. Shear strength measurement methods such as direct shear test and triaxial shear test have the characteristics of large measurement range, low precision, and inaccurate parameters, and are not suitable for the determination of the strength of soft clay with low strength. The plate penetration test has the advantages of simple principle and convenient test device, but it cannot control the plate to remain vertical during the shearing process, and has disadvantages such as unstable loading and low accuracy of test results. Aiming at the problems existing in the original test method, the present invention considers using the plastic full flow theory test method to improve the original test method.

According to the plastic full flow theory, the upper limit solution of the probe resistance in the soft clay sample is equal to the lower limit solution, which is an accurate theoretical solution, and the formula for calculating the undrained shear strength of soft clay obtained from this has the characteristics of less parameters and high reliability . The full-flow CPT penetration method based on the plastic full-flow theory mainly includes spherical, T-shaped, and disk-shaped probes. Traditional full-flow penetrating penetration technology uses resistive strain sensors to measure strain, which has disadvantages such as low measurement accuracy, susceptibility to electromagnetic interference, and corrosion resistance. In order to solve this problem and further improve the test accuracy, the present invention considers the introduction of FBG strain measurement device. In addition, due to the low thixotropic strength in the early stage of soft clay, in order to reduce the disturbance, the present invention intends to preset probes in the soil samples. Considering the uncontrollable partial pull problem in the process of pulling out the probes, a spherical probe is specially used.

FBG (Fiber Bragg Granting) Fiber Bragg Grating, sensing FBG utilizes the ultraviolet photosensitive properties of doped optical fibers, and makes the core refractive index change periodically along the axial direction through ultraviolet exposure. It is a passive reflection type with wavelength selection characteristics. optical instrument. The FBG sensor has the characteristics of small size, high precision, and is not easy to corrode, and has little impact on the measurement system. The strain and stress level can be estimated according to the drift of the central wavelength.

FBG has continued to develop since its appearance in the 1980s, and now FBG fiber Bragg grating sensors can be used to achieve micron-level strain measurement, which further improves the accuracy of stress and strain measurement on the basis of resistive strain sensors.

Introducing the FBG sensor into the test system can measure the strength of soft clay with high precision, especially for the measurement of the tiny stress when the soil sample is initially disturbed, so as to make a further study on the early thixotropic strength and shear dilution of soft clay. Research.

Based on the shortcomings of existing soft clay early thixotropic strength test methods, a new soft clay early thixotropic strength test method and test device were proposed using the plastic full flow theory combined with FBG high-precision strain measurement.

Contents of the invention

The present invention can provide a method for testing the shear strength of seabed soft clay that is relatively complete in theory and more accurate in experiments, comprehensively considering the excessive disturbance of the soft clay by the full-flow penetrating sounding instrument and the softening of the original soft clay after the initial disturbance and thixotropy.

Technical scheme of the present invention:

An early thixotropic strength test device for soft clay based on FBG and full-flow penetration testing, including a base and a fixing device 1, a heightening support 2, a ball screw linear guide 3, a motor 4, a slider 5, and a steering L-shaped rigid joint 6. Stress wire 7, FBG fiber grating 8, FBG laser incident end 9, FBG optical fiber output end 10, spherical probe 11, soil sample container 12, fiber Bragg grating demodulator 13 and displacement measuring instrument 14;

The base and fixing device 1, the heightening pillar 2, the ball screw linear guide 3, the motor 4, the slider 5 and the steering L-shaped rigid joint 6 are the main body bearing parts of the testing device; the base and the fixing device 1 include The base and the fixing device, the fixing device is fixed on the base for supporting and fixing other parts; the heightening pillar 2 is L-shaped, one end of which is fixed on the base, and the other end is fixed on the ball screw linear guide 3, wherein The intermediate part is rigidly connected with the fixing device; the ball screw linear guide 3 is vertically fixedly installed on the upper surface of the fixing device, and it is further connected with the displacement measuring instrument 14 for measuring and recording the displacement of the testing device; the motor 4 is installed on the ball screw The upper surface of the rod linear guide 3 provides power for the test device. The motor 4 is externally connected to the motor controller through the C and D terminals, and is further controlled by a computer; the slider 5 passes through the ball screw, and on the ball screw linear guide 3 sliding; the front end of the slider 5 is installed to turn to the L-shaped rigid joint 6;

The stressed wire 7, FBG fiber grating 8, FBG laser incident end 9, FBG optical fiber output end 10, spherical probe 11, soil sample container 12, fiber grating demodulator 13 and displacement measuring instrument 14 are the components of the testing device. Measuring part; the lower surface of the steering L-shaped rigid joint 6 is fixed with a force wire 7; the force wire 7 is connected to the FBG fiber grating 8, and the end of the FBG fiber Bragg grating 8 is connected to a ball probe 11; the FBG fiber Bragg grating 8 is divided into FBG at the upper end The laser incident end 9 and the FBG optical fiber output end 10 at the lower end are both connected to a fiber grating demodulator 13 to convert the optical signal into an electrical signal and obtain corresponding stress and strain information; the soil sample vessel 12 is placed on the surface of the base to ensure that the spherical shape The probe 11 is located directly above the soil sample container 12 .

A method for testing the early thixotropic strength of soft clay based on FBG and full-flow penetration testing, the steps are as follows:

(1) First calculate the resistive stress q _Ball of the ball probe 11 according to the theory of plastic full flow, and then calculate the _undrained shear strength Su of soft clay according to the approximate formula;

(2) Fiber Bragg grating stress conversion

On the premise that the FBG fiber grating 8 reflection center wavelength λ, the grating effective refractive index n _eff and the grating period Λ are known, the axial strain value of the FBG is calculated;

(3) According to Hooke's law, the stress variation of FBG is obtained;

(4) Replace the straight rod with the stressed wire 7, simplify the calculation formula of the measured resistance stress q _r of the ball probe 11, and obtain the simplified formula of the resistance stress q _Ball of the ball probe 11:

q _Ball = q _r

(5) The formula for calculating the strength of soft clay measured by this test device is:

Among them, E _a is the elastic modulus of the fiber material, Δλ is the center wavelength drift variable of the FBG, λ is the center wavelength of the original FBG, P _ε is the elastic-optic coefficient of the fiber material, N _Ball is the resistance coefficient of the ball probe 11, S _u is the undrained shear strength of soft clay.

Beneficial effects of the present invention:

(1) The calculation process is simple. Replacing the straight rod with the stressed wire can simplify the calculation formula of the measured resistance stress q _r of the spherical probe 11, and simplify the calculation formula of the existing plastic full flow theory.

(2) The calculation results are highly reliable. Since the T-type full-flow CPT test technology is based on the plastic full-flow theory, the plastic theoretical solution can be obtained when the upper limit solution is equal to the lower limit solution, so a more credible theoretical solution can be obtained.

(3) The test method has high precision. The FBG strain sensor also has many advantages such as small size, light weight, and corrosion resistance, and can more accurately measure the shear strength of the undisturbed soil with as little disturbance as possible.

Description of drawings

Figure 1 is a schematic diagram of fiber Bragg grating sensing.

Figure 2 is a side view of the shear strength test device for marine soft clay.

Figure 3 is a perspective view of the shear strength test device for marine soft clay.

In the figure: 1 base and fixing device; 2 heightening pillar; 3 ball screw linear guide; 4 motor; 5 slider; 6 steering L-shaped rigid joint; 7 force wire; 8FBG adapter; 10 FBG optical fiber output end; 11 spherical probe; 12 soil sample vessel; 13 fiber grating demodulator; 14 displacement measuring instrument.

Detailed ways

The specific implementation manner of the present invention will be described in detail below in conjunction with the accompanying drawings and technical solutions.

An early thixotropic strength test device for soft clay based on FBG and full-flow penetration testing, including a base and a fixing device 1, a heightening support 2, a ball screw linear guide 3, a motor 4, a slider 5, and a steering L-shaped rigid joint 6. Stress wire 7, FBG fiber grating 8, FBG laser incident end 9, FBG optical fiber output end 10, spherical probe 11, soil sample container 12, fiber Bragg grating demodulator 13 and displacement measuring instrument 14;

The base and fixing device 1, the heightening pillar 2, the ball screw linear guide 3, the motor 4, the slider 5 and the steering L-shaped rigid joint 6 are the main body bearing parts of the testing device; the base and the fixing device 1 include The base and the fixing device, the fixing device is fixed on the base for supporting and fixing other parts; the heightening pillar 2 is L-shaped, one end of which is fixed on the base, and the other end is fixed on the ball screw linear guide 3, wherein The middle part is rigidly connected with the fixing device; the ball screw linear guide 3 is vertically fixedly installed on the upper surface of the fixing device, and it is further connected with the displacement measuring instrument 14 for measuring and recording the displacement of the testing device; the motor 4 is installed on the ball screw The upper surface of the rod linear guide 3 provides power for the test device. The motor 4 is externally connected to the motor controller through the C and D terminals, and is further controlled by a computer; the slider 5 passes through the ball screw, and the ball screw linear guide 3 sliding; the front end of the slider 5 is installed to turn to the L-shaped rigid joint 6;

The stressed wire 7, FBG fiber grating 8, FBG laser incident end 9, FBG optical fiber output end 10, spherical probe 11, soil sample container 12, fiber grating demodulator 13 and displacement measuring instrument 14 are the components of the testing device. Measuring part; the lower surface of the steering L-shaped rigid joint 6 is fixed with a force wire 7; the force wire 7 is connected to the FBG fiber grating 8, and the end of the FBG fiber Bragg grating 8 is connected to a ball probe 11; the FBG fiber Bragg grating 8 is divided into FBG at the upper end The laser incident end 9 and the FBG optical fiber output end 10 at the lower end are both connected to a fiber grating demodulator 13 to convert the optical signal into an electrical signal and obtain corresponding stress and strain information; the soil sample vessel 12 is placed on the surface of the base to ensure that the spherical shape The probe 11 is located directly above the soil sample container 12 .

A method for testing the early thixotropic strength of soft clay based on FBG and full-flow penetration testing, the steps are as follows:

Step 1: Prepare soil samples and preset FBG fiber gratings

Prepare a group of cohesive soil samples with the same water content, and bury the connected stress wire 7 and ball probe 11 into the soil samples.

Step 2: Connect the parts of the device

The base, the fixing device 1 and the heightening pillar 2 are combined and fixed to form the foundation of the device, and the ball screw linear guide 3 and the motor 4 are fixed on the foundation of the device as the power part of the device, and the steering L-shaped rigid joint 6 is fixed on the ball with screws. On the slider 5 of the screw linear guide 3.

Step 3: paste the FBG fiber grating 8 and determine the corresponding parameters of the FBG

Paste the FBG fiber grating 8 directly on the stressed wire 7 . The upper end of the FBG fiber grating 8 leads to the FBG laser incident end 9, and the lower end of the FBG fiber grating 8 leads to the FBG fiber output end 10;

Obtain the FBG fiber Bragg grating 8 reflection center wavelength λ, the grating effective refractive index n _eff , and the grating period Λ, and obtain the grating reflection center wavelength λ according to the fiber Bragg grating sensing principle:

λ=2n _eff Λ

Among them, λ is the grating reflection center wavelength, n _eff is the effective refractive index of the grating, and Λ is the grating period.

Step 4: Connect to the computer

The FBG laser incident end 9 and the FBG optical fiber output end 10 are respectively connected to the fiber grating demodulator 13 and connected to the computer. The motor 4 is connected to the motor controller and connected to another computer from the C and D ends respectively, and the ball screw The linear guide rail 3 is connected with the displacement measuring instrument 14 to record the displacement of the device.

Step 5: Calibrate and test the displacement and speed of the guide rail

Control the displacement and speed of the ball screw linear guide 3 for calibration and calibration, so that it is at the initial position of the test.

Step 6: Connect and Test

Elevate the ball probe 11 preset in the soil sample at a certain speed, and obtain its stress situation from the computer.

Step 7: Experimental data processing and calculation

Calculate the central wavelength drift Δλ of the FBG according to the fiber Bragg grating should transform calculation formula:

Among them, Δλ is the central wavelength drift variable of FBG, λ is the central wavelength of the original FBG, P _ε is the elasto-optic coefficient of the fiber material, Δε 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 grating, and ΔT is the variation of the external temperature.

According to the plastic full flow theory, the plastic theoretical solution is obtained by the upper limit solution equal to the lower limit solution of the plastic full flow theory. The resistance of the soil on the spherical probe 11 is mapped to the strength of the soft clay. The calculation formula is as follows:

Among them: S _u is the undrained shear strength of soft clay, q _Ball is the resistance stress of the ball probe 11, N _Ball is the resistance coefficient of the ball probe 11, q _r is the measured resistance stress of the ball probe 11, σ _v0 is the upper Covering soil stress, u ₀ is the static pore water pressure, α is the surface friction coefficient of the ball probe 11, A _c is the cross-sectional area of the connecting shaft, and A _Ball is the cross-sectional area of the ball probe 11;

Since this device cancels the straight rod of the spherical probe 11 and replaces it with a very thin stressed wire 7, the cross-sectional area of the A _c connecting shaft in the above formula approaches 0, and it can be obtained:

q _Ball = q _r

It is generally considered that the surface of the Ball penetrating probe is lightly sandblasted. α means that the surface friction coefficient of Ball is 0.4, and N _Ball means that the resistance coefficient of the Ball probe generally takes a value of 13.1.

Since the fiber material is within the range of linear elastic deformation, according to Hooke's law, the stress variation of FBG is obtained:

Δσ _＝ Ea·Δε

Among them, E _a is the elastic modulus of the fiber material.

final use formula Calculate the early thixotropic strength of soil samples.

Claims (2)

1. An early stage thixotropic strength testing device for soft clay based on FBG and full-flow penetration testing, characterized in that the early-stage thixotropic strength testing device for soft clay based on FBG and full-flow penetration testing includes a base and a fixture (1 ), heightening pillars (2), ball screw linear guides (3), motors (4), sliders (5), steering L-shaped rigid joints (6), stressed wires (7), FBG fiber gratings (8) , FBG laser incident end (9), FBG optical fiber output end (10), spherical probe (11), soil sample vessel (12), fiber Bragg grating demodulator (13) and displacement measuring instrument (14); The base and fixing device (1), heightening pillar (2), ball screw linear guide (3), motor (4), slider (5) and steering L-shaped rigid joint (6) are the testing device The main body bearing part; the base and the fixing device (1) include the base and the fixing device, the fixing device is fixed on the base for supporting and fixing other parts; the heightening pillar (2) is L-shaped, one end of which is fixed on the base The other end is fixed on the ball screw linear guide (3), and its middle part is rigidly connected with the fixing device; the ball screw linear guide (3) is vertically fixed and installed on the upper surface of the fixing device, and it is further connected with the displacement measurement Connected to the instrument (14) for measuring and recording the displacement of the test device; the motor (4) is installed on the upper surface of the ball screw linear guide (3) to provide power for the test device, and the motor (4) passes through the C and D terminals respectively Externally connected to the motor controller, and further controlled by the computer; the slider (5) passes through the ball screw and slides on the linear guide rail (3) of the ball screw; the front end of the slider (5) is installed with a steering L-shaped rigid joint (6 ); The stressed wire (7), FBG fiber grating (8), FBG laser incident end (9), FBG optical fiber output end (10), spherical probe (11), soil sample container (12), fiber grating solution The adjusting instrument (13) and the displacement measuring instrument (14) are the measuring parts of the test device; the lower surface of the steering L-shaped rigid joint (6) is fixed with a stressed wire (7); the stressed wire (7) is connected to the FBG fiber grating (8 ), the end of FBG fiber grating (8) is connected with ball probe (11); The grating demodulator (13) is connected to convert the optical signal into an electrical signal and obtain the corresponding stress-strain information; the soil sample container (12) is placed on the surface of the base to ensure that the ball probe (11) is positioned on the surface of the soil sample container (12). Directly above. 2. A method for testing the early thixotropic strength of soft clay based on FBG and full-flow penetration testing, characterized in that the steps are as follows: (1) First calculate the resistance stress q _Ball of the spherical probe (11) according to the theory of plastic full flow, and then calculate the _undrained shear strength Su of soft clay according to the approximate formula; (2) Fiber Bragg grating stress conversion Under the premise of known FBG fiber grating (8) reflection center wavelength λ, grating effective refractive index n _eff and grating period Λ, calculate the axial strain value of FBG; (3) According to Hooke's law, the stress variation of FBG is obtained; (4) Replace the straight rod with the stressed wire (7), simplify the calculation formula of the measured resistance stress q _r of the ball probe (11), and obtain the simplified formula of the resistance stress q _Ball of the ball probe (11): q _Ball = q _r (5) The formula for calculating the strength of soft clay measured by this test device is: Among them, E _a is the elastic modulus of the fiber material, Δλ is the center wavelength drift variable of the FBG, λ is the center wavelength of the original FBG, P _ε is the elasto-optic coefficient of the fiber material, and N _Ball is the resistance coefficient of the spherical probe (11) , Su is the _undrained shear strength of soft clay.