Optical fiber soil coupling performance simulation method based on discrete elements

Abstract

The invention discloses an optical fiber soil body coupling performance simulation method based on discrete elements, which comprises the following steps: step 1, determining size parameters of a test box, the diameter of an optical fiber, the minimum radius and the maximum radius of soil particles, and macromechanical parameters of a test soil body, the size parameters of the test box including length, width and height, and the diameter of the optical fiber; the macromechanical parameters of the test soil body comprise density, cohesion and friction angle; 2, simulating through a two-dimensional particle flow direct shear test to obtain material mesoscopic parameters of the test soil body; 3, generating a three-dimensional drawing discrete element model; and 4, pulling the optical fiber, and observing the force-displacement of the optical fiber in the drawing process, the deformation information of the sample soil body and the cementing failure state of the test soil body. According to the optical fiber soil coupling performance simulation method based on the discrete elements, the problem that an existing continuous medium analysis method is not suitable for discontinuous media is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of numerical simulation methods for optical fiber pulling tests, and in particular relates to a method for simulating optical fiber-soil coupling performance based on discrete elements. Background Art

[0002] Distributed fiber optic sensing technology is often used to monitor soil deformation due to its characteristics of continuous distribution, long distance, good durability and high cost performance. The coupling performance between soil and sensing optical cable is a key factor affecting the accuracy of monitoring results. When conducting indoor pull-out tests, it is easily affected by factors such as low confining pressure, loose backfill materials, difficult parameter testing and single working conditions. Numerical simulation methods can well avoid the above limitations and provide a way to better understand the fiber-soil mechanics coupling. Among the many numerical simulation methods, the discrete element method has significant advantages. It is mainly reflected in the ability of the discrete element method to simulate the behavior of soil at the particle level, thereby deeply analyzing the microscopic properties and interactions of materials. The discrete element method is particularly suitable for dealing with non-continuous materials such as soil, gravel and rock, in which the interaction and relative motion between particles have a significant impact on the overall behavior, and traditional methods based on continuous medium models (such as the finite element method) are difficult to effectively capture these characteristics. Another very important advantage of the discrete element method is that it can record the stress-strain of the optical fiber during the pull-out process, and can also more intuitively display the fiber-soil interface failure law and soil deformation information. This provides a reliable basis for recording and analyzing the whole process of soil deformation and progressive failure. Therefore, the discrete element method is an effective tool for evaluating the reliability of distributed optical fiber monitoring of soil deformation. Summary of the invention

[0003] The purpose of the present invention is to provide a method for simulating optical fiber-soil coupling performance based on discrete elements, which solves the problem that the existing continuous medium analysis method is not applicable to non-continuous media.

[0004] The technical solution adopted by the present invention is: a method for simulating optical fiber-soil coupling performance based on discrete elements, comprising the following steps:

[0005] Step 1, determine the test box size parameters, optical fiber diameter, minimum and maximum radius of soil particles, and macroscopic mechanical parameters of the test soil, wherein the test box size parameters include length, width and height, and the macroscopic mechanical parameters of the test soil include density, cohesion and friction angle;

[0006] Step 2: obtain the material microscopic parameters of the test soil through two-dimensional particle flow direct shear test simulation;

[0007] Step 3, generating a three-dimensional drawing discrete element model;

[0008] Step 4: Pull the optical fiber and observe the force-displacement of the optical fiber, the deformation information of the sample soil, and the bonding failure state of the test soil during the pulling process.

[0009] The present invention is also characterized in that:

[0010] Step 2 is specifically as follows: setting the test soil material mesoscopic parameters corresponding to the minimum and maximum radii of the soil particles in step 1, obtaining the shear stress-shear deformation curves under four horizontal normal stresses respectively by sampling, preloading, cementing, applying normal stress and shear monitoring, recording the peak value and normal stress of the shear stress-shear deformation curve, drawing the shear strength and normal stress relationship curve with shear strength as the ordinate and normal stress as the abscissa, and the inclination angle and intercept of the shear strength and normal stress relationship curve are the simulated internal friction angle and cohesion of the test soil, and then comparing the simulated internal friction angle and cohesion of the test soil with the macroscopic mechanical parameters of the test soil in step 1. If the error is within the set error range of 10%, the material mesoscopic parameters corresponding to the macroscopic mechanical parameters of the test soil are obtained; otherwise, the material mesoscopic parameter setting value of the test soil is adjusted so that the simulated internal friction angle and cohesion of the test soil are within the set error range with the macroscopic mechanical parameters of the test soil in step 1.

[0011] The difference between two adjacent horizontal normal stresses in the four horizontal normal stresses is equal, and the difference range is 50kPa to 100kPa.

[0012] The linear contact bonding model is selected as the test soil contact model for bonding.

[0013] The material microscopic parameters of the test soil set include density, porosity, minimum particle radius, maximum particle radius, effective modulus, normal-tangential stiffness ratio, particle friction coefficient, bond tensile strength, bond shear strength, bonding spacing, and local damping factor, where the minimum particle radius is set to the minimum radius of the soil particles in step 1, and the maximum particle radius is set to the maximum radius of the soil particles in step 1.

[0014] Step 3 specifically includes the following steps:

[0015] Step 3.1. Generate six boundary walls according to the test box dimensions in step 1, and estimate the initial particle radius according to the following formula:

[0016] r / D<0.005

[0017] Where r is the initial particle radius, and D is the minimum value in the test chamber size;

[0018] Step 3.2: Generate N test soil particles according to the initial particle radius r in the boundary wall of step 3.1. The default contact model of the N test soil particles is a linear contact model. At the same time, set the initial density, local damping coefficient, effective modulus and normal-tangential stiffness ratio. Run 2000 time steps, clear the kinetic energy every 50 steps, and obtain the pull-out discrete element model a.

[0019] Step 3.3: Generate a cylindrical geometry I with a radius of 3 cm and a length equal to the length of the test box at the center of the pull-out discrete element model a obtained in step 3.2. Delete the test soil particles in the cylindrical geometry I and generate N' particles in the cylindrical geometry. The radius range of the N' particles is evenly distributed between the minimum radius of the soil particles and the maximum radius of the soil particles in step 1. The default contact model of the N' test soil particles is a linear contact model. At the same time, the initial density, local damping coefficient, effective modulus and normal-tangential stiffness ratio are set. The ratio of the average unbalanced force to the sum of the average contact force is less than 1×10 -5 The standard equilibrium particles are obtained to obtain the drawing discrete element model b;

[0020] Step 3.4, generate a cylindrical geometry II with a diameter equal to the fiber diameter in step 1 and a length equal to the length of the test box at the center of the pull-out discrete element model b obtained in step 3.3, delete the test soil particles in the cylindrical geometry II, and use the "zone create" command in the particle flow program PFC to generate a fiber model in the cylindrical geometry II. The fiber length is 1.2 times the length of the test box, and the fiber diameter is the fiber diameter in step 1; generate a skin layer for the surface of the zone unit representing the optical fiber, and then create a wall named "wall" on the skin layer. The zone unit constitutive model adopts a linear elastic model. At the same time, according to the optical fiber specifications, set the initial density, elastic model and Poisson's ratio of the zone unit to obtain the pull-out discrete element model c;

[0021] Step 3.5, after determining the microscopic parameters corresponding to the macroscopic mechanical parameters of the test soil in step 3.3 and the pull-out discrete element model c obtained in step 3.4, the linear contact constitutive model is selected for the microscopic parameters of the coupling contact between the test soil and the optical fiber, and the relationship curves between the pull-out force and the pull-out displacement obtained by the numerical pull-out test and the indoor pull-out test without confining pressure are compared. When the error is within 10%, it is considered that the selected coupling contact model parameters can reflect the coupling mechanical characteristics of the optical fiber and the test soil, and the calibration results are the microscopic parameters of the coupling contact between the test soil and the optical fiber;

[0022] Step 3.6, clear the velocity, displacement, contact force and contact torque of the test soil particles in the pull-out discrete element model c in step 3.4, assign the test soil particles the mesoscopic parameters corresponding to the macroscopic mechanical parameters of the test soil in step 2 and the mesoscopic parameters of the coupling contact between the test soil and the optical fiber obtained in step 3.5, run 5000 time steps, and obtain the pull-out discrete element model d;

[0023] Step 3.7, set the target stress values ​​in the x, y, and z directions to txx, tyy, and tzz, respectively. In the pull-out discrete element model d obtained in step 3.6, the speed of the six boundary walls in step 3.1 is controlled by the fish function servo so that the stress on the boundary wall reaches the target stress value, and then the ratio of the average unbalanced force to the sum of the average contact force is less than 1×10 -5 The standard balanced particles, the optical fiber is closely combined with the test soil, and the pull-out discrete element model e is obtained.

[0024] Step 4 specifically includes the following steps:

[0025] Step 4.1, specify the loading node and monitoring unit, set the 5% fiber length range of the fiber end as the stretching section, and set the 2.5% fiber length range after the stretching section as the monitoring section;

[0026] Step 4.2, initialize an array "jiance" to store the index of the monitoring segment area in step 4.1, traverse all zone units, if a unit belongs to the monitoring segment, store the index of the unit belonging to the monitoring segment into the "jiance" array, count and record the number of "jiance" array indexes as N"; in each time step, get the current time "time", and calculate the displacement according to the stretching segment speed; initialize the variable force', loop through the first N" areas in the array "jiance", get the stress of each area, accumulate the stress value into force', and then calculate the pull-out force according to the cross-sectional area of ​​the optical fiber:

[0027]

[0028] Where F is the pull-out force, force' is the cumulative stress value, D is the fiber diameter, and N" is the index number of the "jiance" array;

[0029] Step 4.3, set the loading speed v of the stretching section, the final pulling displacement s and the test termination time t', calculate the total time according to the displacement and loading speed, that is, the final displacement divided by the loading speed, and get the time required for the loading process; use the "zonegridpoint" command to clear the node displacement of the pulling discrete element model e obtained in step 3.7, initialize the mechanical time of the model to 0 and reset the time count; apply the speed v to the nodes in the stretching section of step 4.1, solve the model, run to the set test termination time t', modify the pulling discrete element model e to the pulling discrete element model f, and realize the simulation of the optical fiber pulling process.

[0030] The beneficial effects of the present invention are as follows: the optical fiber soil coupling performance simulation method based on discrete elements of the present invention can consider different confining pressures, different backfill material properties, different anchoring methods and different optical fiber types in actual indoor tests by generating a three-dimensional discrete element pull-out model. Indoor pull-out tests often cost a lot in terms of cost-effectiveness and time. The use of discrete element simulation can be free from the limitations of laboratory tests and has flexibility and scalability. Discrete element simulation can supplement some tests, and the optical fiber soil coupling performance simulation method based on discrete elements can obtain data (stress, displacement, etc.) that are not easy to measure in actual tests. Through the analysis of the data, the interaction mechanism between the optical fiber and the soil can be more deeply understood, and the failure law of the optical fiber interface and the soil deformation information during the pull-out process can also be studied. It can be extended to pull-out tests of other geotechnical materials such as geogrids and anchors. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a shear stress-shear displacement curve calibrated with mesoscopic parameters of the test soil material in Example 3 of the present invention;

[0032] Figure 2 is the shear strength-vertical stress curve of the test soil in Example 3 of the present invention;

[0033] Figure 3 It is a comparison curve of pull-out force-pull-out displacement of the numerical pull-out test and the indoor pull-out test without confining pressure in Example 3 of the present invention;

[0034] Figure 4 is the boundary stress curve of the test soil after the model is balanced in Example 3 of the present invention;

[0035] Figure 5 is the pull-out force-pull-out displacement curve of the numerical pull-out test under the vertical stress of 200 kPa in Example 3 of the present invention;

[0036] Figure 6 is the displacement distribution diagram of the soil and optical fiber unit when the pull-out displacement is 1.9 mm in Example 3 of the present invention;

[0037] Figure 7is a displacement distribution diagram of the soil and optical fiber unit when the pull-out displacement is 5.0 mm in Example 3 of the present invention;

[0038] Figure 8 is the displacement distribution diagram of the soil and optical fiber unit when the pull-out displacement is 8.5 mm in Example 3 of the present invention;

[0039] Fig. 9 This is the soil contact bonding fracture distribution diagram when the pull-out displacement is 1.9 mm in Example 3 of the present invention;

[0040] Fig.10 This is the soil contact bonding fracture distribution diagram when the pull-out displacement is 5.0 mm in Example 3 of the present invention;

[0041] Fig.11 This is the soil contact bonding fracture distribution diagram when the pull-out displacement is 8.5 mm in Example 3 of the present invention. DETAILED DESCRIPTION

[0042] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] Example 1

[0044] The present invention provides a method for simulating the coupling performance of optical fiber and soil based on discrete elements. The discrete element method is used to treat the research object as a discrete unit, allowing a large relative displacement between particles, thereby effectively capturing the deformation of the soil and the displacement of the optical fiber during the pulling process, and can better simulate the contact between the optical fiber and the soil particles. It is an important method for studying the fiber-soil coupling performance and deformation coordination problems. The microscopic parameters of the optical fiber and the soil, and the soil and the soil are calibrated through numerical pulling tests and two-dimensional direct shear tests, and a three-dimensional pulling discrete element model is constructed to realize the simulation of the optical fiber pulling process.

[0045] Example 2

[0046] The present invention provides a method for simulating optical fiber-soil coupling performance based on discrete elements, comprising the following steps:

[0047] Step 1: Determine the size parameters of the test box, including length, width and height, determine the optical fiber diameter, the minimum radius of soil particles and the maximum radius of soil particles; determine the macroscopic mechanical parameters of the test soil, including density, cohesion and friction angle.

[0048] Step 2: The material microscopic parameters of the test soil are obtained through two-dimensional particle flow direct shear test simulation.

[0049] For the test soil, four normal stresses at levels A, B, C and D are set and denoted as σ a , σ b , σ c and σ d, the difference of normal stress between two adjacent levels is equal, and the difference range is 50kPa~100kPa. Through the methods of sample forming, preloading, bonding, applying normal stress and shear monitoring, the shear stress-shear deformation curve under normal stress at level A is obtained. Bonding means selecting a suitable contact model for the particles. The contact model determines the mechanical behavior between particles. The linear contact bonding model is often used to characterize soil, especially clay and other media with bonding characteristics. Therefore, the contact model of the test soil body selects the linear contact bonding model. The two-dimensional particle flow direct shear test simulation contact model of the test soil body under different normal stresses is the same. The material microscopic parameters that need to be set are density, porosity, minimum radius of particles, maximum radius of particles, effective modulus, normal-tangential stiffness ratio, particle friction coefficient, bonding tensile strength, bonding shear strength, bonding spacing, and local damping factor. The minimum radius of particles is the minimum radius of soil particles in step 1, and the maximum radius of particles is the maximum radius of soil particles in step 1. The peak value of the shear stress-shear deformation curve under normal stress at level A is the shear strength τ corresponding to the normal stress at level A. a ; Through the methods of sample forming, preloading, bonding, applying normal stress and shear monitoring, the shear stress-shear deformation curve under the normal stress of level B is obtained, and the peak value of the shear stress-shear deformation curve under the normal stress of level B is recorded as the shear strength τ corresponding to the normal stress of level B b ; Through the methods of sample forming, preloading, bonding, applying normal stress and shear monitoring, the shear stress-shear deformation curve under the normal stress of C level is obtained, and the peak value of the shear stress-shear deformation curve under the normal stress of C level is recorded as the shear strength τ corresponding to the normal stress of C level c ; Through sampling, preloading, bonding, applying normal stress and shear monitoring, the shear stress-shear deformation curve under the normal stress of level D is obtained, and the peak value of the shear stress-shear deformation curve under the normal stress of level D is recorded as the shear strength τd corresponding to the normal stress of level D. With the shear strength as the ordinate and the normal stress as the abscissa, the shear strength and normal stress relationship curve is drawn. The inclination angle and intercept of the shear strength and normal stress relationship curve are the internal friction angle and cohesion of the test soil obtained by simulation; compare the cohesion and internal friction angle of the test soil obtained by simulation with the macroscopic mechanical parameters of the test soil in step 1. If the error is greater than 10%, the size of the mesoscopic parameters needs to be adjusted until the obtained cohesion and internal friction angle are within the error range. It is considered that the mesoscopic parameters match the macroscopic mechanical parameters, and the calibration result is the mesoscopic parameters corresponding to the macroscopic mechanical parameters of the test soil.

[0050] Step 3: Generate a three-dimensional drawing discrete element model.

[0051] Step 3.1. Generate six boundary walls according to the test box dimensions in step 1, and estimate the initial particle radius according to the following formula:

[0052] r / D<0.005

[0053] Where r is the initial particle radius and D is the minimum value of the test chamber size.

[0054] Step 3.2: Generate N test soil particles according to the initial particle radius r within the boundary wall of step 3.1. The default contact model of the N test soil particles is a linear contact model. At the same time, set the initial density, local damping coefficient, effective modulus and normal-tangential stiffness ratio. Run 2000 time steps, clear the kinetic energy every 50 steps, and obtain the pull-out discrete element model a.

[0055] Step 3.3: Generate a cylindrical geometry 1 with a radius of 3 cm and a length equal to the length of the test box at the center of the pull-out discrete element model a obtained in step 3.2. Delete the test soil particles in the cylindrical geometry 1 and generate N' particles in the cylindrical geometry. The radius range of the N' particles is evenly distributed between the minimum radius of the soil particles and the maximum radius of the soil particles in step 1. The default contact model of the N' test soil particles is a linear contact model. At the same time, the initial density, local damping coefficient, effective modulus and normal-tangential stiffness ratio are set. The ratio of the average unbalanced force to the sum of the average contact force is less than 1×10 -5 The standard equilibrium particles are used to obtain the drawing discrete element model b.

[0056] Step 3.4: Generate a cylindrical geometry 2 with a diameter equal to the fiber diameter in step 1 and a length equal to the length of the test box at the center of the pull-out discrete element model b obtained in step 3.3. Delete the test soil particles in the cylindrical geometry 2, and use the "zone create" command in the particle flow program PFC to generate a fiber model in the cylindrical geometry 2. The fiber length is 1.2 times the length of the test box, and the fiber diameter is the fiber diameter in step 1. Generate a skin layer for the surface of the zone unit representing the optical fiber, and then create a wall named "wall" on the skin layer. Considering that the optical fiber structure is a continuous medium, the linear elastic model is selected for the zone unit constitutive model. At the same time, according to the optical fiber specifications, set the initial density, elastic model and Poisson's ratio of the zone unit to obtain the pull-out discrete element model c.

[0057] Step 3.5, after determining the microscopic parameters and pull-out discrete element model c corresponding to the macroscopic mechanical parameters of the test soil in step 3.3, for the microscopic parameters of the coupling contact between the test soil and the optical fiber, the linear contact constitutive model is selected, and the relationship curves of the pull-out force and the pull-out displacement obtained by the numerical pull-out test and the indoor pull-out test without confining pressure need to be compared. When the error between the two is within 10%, it is considered that the selected coupling contact model parameters can reflect the coupling mechanical characteristics of the optical fiber and the test soil, and the calibration results are the microscopic parameters of the coupling contact between the test soil and the optical fiber.

[0058] Step 3.6: Clear the velocity, displacement, contact force and contact torque of the test soil particles in the pull-out discrete element model c in step 3.4, assign the test soil particles the mesoscopic parameters corresponding to the macroscopic mechanical parameters of the test soil in step 2 and the mesoscopic parameters of the coupling contact between the test soil and the optical fiber obtained in step 3.5, run for 5000 time steps, and obtain the pull-out discrete element model d.

[0059] Step 3.7, set the target stress values ​​in the x, y, and z directions to txx, tyy, and tzz, respectively. In the pull-out discrete element model d obtained in step 3.6, the speed of the six boundary walls in step 3.1 is controlled by the fish function servo so that the stress on the boundary wall reaches the target stress value, and then the ratio of the average unbalanced force to the sum of the average contact force is less than 1×10 -5 The standard balanced particles, the optical fiber is closely combined with the test soil, and the pull-out discrete element model e is obtained.

[0060] Step 4: Pull the optical fiber and observe the force-displacement of the optical fiber, the deformation information of the sample soil, and the bonding failure state of the test soil during the pulling process.

[0061] Step 4.1, specify the loading node and monitoring unit, set the 5% fiber length range of the fiber end as the stretching section, and set the 2.5% fiber length range after the stretching section as the monitoring section.

[0062] Step 4.2, initialize an array "jiance" to store the index of the monitoring segment area in step 4.1, traverse all zone units, if a unit belongs to the monitoring segment, store the index of this unit in the "jiance" array, count and record the number of "jiance" array indexes as N". In each time step, get the current time "time" and calculate the displacement according to the stretching segment speed. Initialize the variable force', loop through the first N" areas in the array "jiance", get the stress of each area, add the stress value to force', and then calculate the pull-out force according to the cross-sectional area of ​​the optical fiber:

[0063]

[0064] Where F is the pull-out force, force' is the cumulative stress value, D is the fiber diameter, and N" is the index number of the "jiance" array.

[0065] Step 4.3, set the loading speed v of the tensile section, the final drawing displacement s and the test termination time t', calculate the total time according to the displacement and loading speed, that is, the final displacement divided by the loading speed, and get the time required for the loading process. Use the "zonegridpoint" command to clear the node displacement of the drawing discrete element model e in step 3.7, and initialize the mechanical time of the model to 0 to reset the time count. Apply velocity v to the nodes in the tensile section of step 4.1, solve the model, run to the set test termination time t', modify the drawing discrete element model e to the drawing discrete element model f, and realize the simulation of the optical fiber drawing process.

[0066] Example 3

[0067] The present invention provides a method for simulating optical fiber-soil coupling performance based on discrete elements, comprising the following steps:

[0068] Step 1: Test box size parameters, including length, width and height, where the length is 500mm, the width is 180mm, and the height is 180mm. The fiber diameter is 2mm, the minimum radius of soil particles is 0.1mm, and the maximum radius of soil particles is 0.275mm.

[0069] Determine the macroscopic mechanical parameters of the test soil, which include density, cohesion and friction angle. The density is 2600kg / m 3 , cohesion 17.94kPa, internal friction angle 10.97°.

[0070] Step 2: The material microscopic parameters of the test soil are obtained through two-dimensional particle flow direct shear test simulation.

[0071] For the test soil, the shear stress-shear deformation curves under four normal stresses of 100 kPa, 200 kPa, 300 kPa and 400 kPa were obtained by sampling, preloading, cementing, applying normal stress and shear monitoring, as shown in Figure 1 The peak value and normal stress of the shear stress-shear deformation curve are recorded, and the shear strength is used as the ordinate and the normal stress is used as the abscissa to draw the shear strength and normal stress relationship curve. The inclination angle and intercept of the shear strength and normal stress relationship curve are the internal friction angle and cohesion of the test soil obtained through simulation, as shown in Figure 2 As shown in the figure, the simulated cohesion (19.15 kPa) and internal friction angle (11.14°) are within 10% of the macroscopic mechanical parameters of the test soil in step 1. It is believed that the calibration results are the microscopic parameters corresponding to the macroscopic mechanical parameters of the test soil, and the linear contact bonding model is selected as the contact model of the test soil. The microscopic parameters are shown in Table 1.

[0072] Table 1 Parameters used in the contact bonding model

[0073]

[0074] Step 3: Generate a three-dimensional drawing discrete element model.

[0075] Step 3.1: To balance calculation efficiency and accuracy, generate six boundary walls according to the test box size in step 1, and estimate the initial particle radius according to the following formula:

[0076] r / D<0.005

[0077] Where r is the initial particle radius, and D is the minimum value of the test chamber size. The minimum test chamber size is 180 mm in width or height, and the initial particle radius is 0.004 mm.

[0078] Step 3.2: Generate 36740 test soil particles with an initial particle radius of 0.004 mm within the boundary wall of step 3.1. The default contact model of the 36740 test soil particles is the linear contact model, and the initial density is set to 2600 kg / m 3 , local damping coefficient 0.7, effective modulus 1E+8Pa and normal-tangential stiffness ratio 1.0, run 2000 time steps, clear the kinetic energy every 50 steps, and obtain the drawing discrete element model a.

[0079] Step 3.3: Generate a cylindrical geometry 1 with a radius of 3 cm and a length equal to the length of the test box at the center of the pull-out discrete element model a obtained in step 3.2. Delete the test soil particles in the cylindrical geometry 1 and generate 85144 particles in the cylindrical geometry. The radius range of the 85144 particles is evenly distributed between the minimum radius and the maximum radius of the soil particles in step 1. The default contact model of the 85144 test soil particles is the same as that in step 3.2. The ratio of the average unbalanced force to the sum of the average contact force is less than 1×10 -5 The standard equilibrium particles are used to obtain the drawing discrete element model b.

[0080] Step 3.4. Generate a cylindrical geometry 2 at the center of the pull-out discrete element model b obtained in step 3.3. The diameter of the cylindrical geometry 2 is the diameter of the optical fiber in step 1 and the length is the length of the test box. Delete the test soil particles in the cylindrical geometry 2, and use the "zone create" command in the particle flow program PFC to generate a fiber model in the cylindrical geometry 2. The fiber length is 1.25 times the length of the test box, which is 62.5 mm, and the fiber diameter is 2 mm. Generate a skin layer for the surface of the zone unit representing the optical fiber, and then create a wall named "wall" on the skin layer. The zone unit constitutive model takes into account that the optical fiber structure is a continuous medium, so a linear elastic model is chosen. At the same time, according to the optical fiber specifications, the initial density of the zone unit is set to 1000 kg / m 3, elastic modulus 5E+08Pa and Poisson's ratio 0.25, and the drawing discrete element model c is obtained.

[0081] Step 3.5: After determining the microscopic parameters and pull-out discrete element model c corresponding to the macroscopic mechanical parameters of the test soil in step 3.3, for the microscopic parameters of the coupled contact between the test soil and the optical fiber, the linear contact constitutive model is selected. It is necessary to compare the relationship curves between the pull-out force and the pull-out displacement obtained by the numerical pull-out test and the indoor pull-out test without confining pressure, such as Figure 3 As shown in the figure, the peak value of the pull-out force and pull-out displacement relationship curve obtained by the numerical pull-out test is within 10% of the peak value of the indoor pull-out test curve. It is believed that the selected coupling contact model parameters can reflect the coupled mechanical properties of the optical fiber-test soil. The calibration results are the coupling contact microscopic parameters between the test soil and the optical fiber, specifically the effective modulus 1E+06Pa, the normal-tangential stiffness ratio 1.5, and the friction coefficient 0.2.

[0082] Step 3.6: Clear the velocity, displacement, contact force and contact torque of the test soil particles in the pull-out discrete element model c in step 3.4, assign the test soil particles the mesoscopic parameters corresponding to the macroscopic mechanical parameters of the test soil in step 2 and the mesoscopic parameters of the coupling contact between the test soil and the optical fiber obtained in step 3.5, run for 5000 time steps, and obtain the pull-out discrete element model d.

[0083] Step 3.7: The static lateral pressure coefficient of the test soil under a normal stress of 200 kPa is 0.37, so the target stress values ​​in the x, y, and z directions are set to 7.4E+04Pa, 7.4E+04Pa, and 2E+05Pa, respectively. In the pull-out discrete element model d obtained in step 3.6, the speed of the six boundary walls in step 3.1 is controlled by the fish function servo so that the stress on the boundary wall reaches the target stress value, as shown in Figure 4 As shown, the ratio of the average unbalanced force to the average contact force is less than 1×10 -5 The standard balanced particles, the optical fiber is closely combined with the test soil, and the pull-out discrete element model e is obtained.

[0084] Step 4: Pull the optical fiber and observe the force-displacement of the optical fiber, the deformation information of the sample soil, and the bonding failure state of the test soil during the pulling process.

[0085] Step 4.1, specify the loading node and monitoring unit, set the 5% fiber length range of the fiber end (-0.3, -0.27) as the stretching section, and set the 2.5% fiber length range (-0.27, -0.255) after the stretching section as the monitoring section.

[0086] Step 4.2, initialize an array "jiance" to store the index of the monitoring segment area in step 4.1, traverse all zone units, if a unit belongs to the monitoring segment, store the index of this unit in the "jiance" array, count and record the number of "jiance" array indexes as 144. In each time step, get the current time "time" and calculate the displacement according to the stretching segment speed. Initialize the variable force', loop through the first 144 areas in the array "jiance", get the stress of each area, add the stress value to force', and then calculate the pull-out force according to the cross-sectional area of ​​the optical fiber:

[0087]

[0088] Where F is the pull-out force, force' is the cumulative stress value, D is the fiber diameter, and N" is the index number of the "jiance" array.

[0089] Step 4.3, set the loading speed v of the tensile section to 0.5mm / s, and the final drawing displacement s to 10mm. Calculate the total time based on the final drawing displacement and loading speed, that is, the final displacement divided by the loading speed, and the time required for the loading process is 20s. Use the "zone gridpoint" command to clear the node displacement of the drawing discrete element model e in step 3.7, and initialize the mechanical time of the model to 0 to reset the time count. Apply the node speed v, solve the model, run to the set test end time (20s), modify the drawing discrete element model e to the drawing discrete element model f, and realize the simulation of the fiber drawing process.

[0090] The above simulation method can break through the limitations of indoor test equipment, sample preparation difficulties and single consideration of indoor tests, and can record the information of particles (particle displacement, stress, energy, etc.), which provides a reliable basis for analyzing the process of soil deformation and progressive failure.

[0091] Figure 5 is the pull-out force-pull-out displacement curve under vertical stress of 200 kPa. Figure 5 It can be seen that in the initial stage of pulling, there is no relative sliding in the friction force at the fiber-soil interface, so the pulling force increases slowly with the increase of pulling displacement. With the increase of pulling displacement, partial sliding occurs at the fiber-soil interface, and the interface friction force increases slowly with the increase of pulling displacement, and remains relatively stable in the last stage. Figure 6 to Figure 8 The displacement distribution of soil particles and optical fibers at pull-out displacements of 1.9 mm, 5 mm, and 8.5 mm, respectively. During the pull-out process, the optical cable interacts with the surrounding soil particles, causing the soil to move horizontally with the movement of the optical cable. The displacement of the soil follows the law of gradually extending toward the tail end as the pull-out displacement increases, showing a progressive failure characteristic.

[0092] The movement of the optical fiber will cause changes in the contact force of the surrounding soil. For the contact bonding model, when the contact force between soil particles exceeds the bonding strength limit, the bonding is destroyed. At this time, the contact model degenerates into a linear model. The bond fracture of the soil around the optical cable is manifested as relative sliding of the interface, and its maximum depth can be defined as the slip depth. Figures 9 to 11 The distribution of contact bonding fracture at pull-out displacements of 1.9 mm, 5.0 mm, and 8.5 mm, respectively. The inter-particle contact is an important indicator of the soil-fiber coupling characteristics. It is easy to conclude that the cable slippage depth gradually increases with the increase of pull-out displacement.

Claims

1. A method for simulating the coupling performance of optical fiber and soil based on discrete elements, characterized in that: The following steps are involved: Step 1, determine the test box size parameters, optical fiber diameter, minimum and maximum radius of soil particles, and macroscopic mechanical parameters of the test soil, wherein the test box size parameters include length, width and height, and the macroscopic mechanical parameters of the test soil include density, cohesion and friction angle; Step 2: obtain the material microscopic parameters of the test soil through two-dimensional particle flow direct shear test simulation; Step 3, generating a three-dimensional drawing discrete element model; Step 4: Pull the optical fiber and observe the force-displacement of the optical fiber, the deformation information of the sample soil, and the bonding failure state of the test soil during the pulling process.

2. The method for simulating optical fiber-soil coupling performance based on discrete element as claimed in claim 1, characterized in that: The step 2 specifically comprises: setting the test soil material mesoscopic parameters corresponding to the minimum and maximum radii of the soil particles in step 1, obtaining the shear stress-shear deformation curves under four horizontal normal stresses respectively by sampling, preloading, bonding, applying normal stress and shear monitoring, recording the peak value and normal stress of the shear stress-shear deformation curve, drawing the shear strength and normal stress relationship curve with the shear strength as the ordinate and the normal stress as the abscissa, the inclination angle and intercept of the shear strength and normal stress relationship curve are the simulated internal friction angle and cohesion of the test soil, and then comparing the simulated internal friction angle and cohesion of the test soil with the macroscopic mechanical parameters of the test soil in step 1, if the error is within the set error range of 10%, the material mesoscopic parameters corresponding to the macroscopic mechanical parameters of the test soil are obtained; otherwise, adjusting the setting value of the material mesoscopic parameters of the test soil so that the simulated internal friction angle and cohesion of the test soil are within the set error range with the macroscopic mechanical parameters of the test soil in step 1.

3. The method for simulating optical fiber-soil coupling performance based on discrete element as claimed in claim 2, characterized in that: The difference between two adjacent horizontal normal stresses in the four horizontal normal stresses is equal, and the difference range is 50kPa to 100kPa.

4. The method for simulating optical fiber-soil coupling performance based on discrete element as claimed in claim 2, characterized in that: The bonding method is to select a linear contact bonding model as the test soil contact model.

5. The method for simulating optical fiber-soil coupling performance based on discrete element as claimed in claim 2, characterized in that: The material microscopic parameters of the test soil body include density, porosity, minimum particle radius, maximum particle radius, effective modulus, normal-tangential stiffness ratio, particle friction coefficient, bond tensile strength, bond shear strength, bonding spacing, and local damping factor, wherein the minimum particle radius is set to the minimum radius of the soil particles in step 1, and the maximum particle radius is set to the maximum radius of the soil particles in step 1.

6. The method for simulating optical fiber-soil coupling performance based on discrete element as claimed in claim 2, characterized in that: The step 3 specifically comprises the following steps: Step 3.

1. Generate six boundary walls according to the test box dimensions in step 1, and estimate the initial particle radius according to the following formula: r / D<0.005 Where r is the initial particle radius, and D is the minimum value in the test chamber size; Step 3.2: Generate N test soil particles according to the initial particle radius r in the boundary wall of step 3.

1. The default contact model of the N test soil particles is a linear contact model. At the same time, set the initial density, local damping coefficient, effective modulus and normal-tangential stiffness ratio. Run 2000 time steps, clear the kinetic energy every 50 steps, and obtain the pull-out discrete element model a. Step 3.3: Generate a cylindrical geometry I with a radius of 3 cm and a length equal to the length of the test box at the center of the pull-out discrete element model a obtained in step 3.

2. Delete the test soil particles in the cylindrical geometry I and generate N' particles in the cylindrical geometry. The radius range of the N' particles is evenly distributed between the minimum radius of the soil particles and the maximum radius of the soil particles in step 1. The default contact model of the N' test soil particles is a linear contact model. At the same time, the initial density, local damping coefficient, effective modulus and normal-tangential stiffness ratio are set. The ratio of the average unbalanced force to the sum of the average contact force is less than 1×10 -5 The standard equilibrium particles are obtained to obtain the drawing discrete element model b; Step 3.4, generate a cylindrical geometry II with a diameter equal to the fiber diameter in step 1 and a length equal to the length of the test box at the center of the pull-out discrete element model b obtained in step 3.3, delete the test soil particles in the cylindrical geometry II, and use the "zone create" command in the particle flow program PFC to generate a fiber model in the cylindrical geometry II. The fiber length is 1.2 times the length of the test box, and the fiber diameter is the fiber diameter in step 1; generate a skin layer for the surface of the zone unit representing the fiber, and then create a wall named "wall" on the skin layer. The zone unit constitutive model adopts a linear elastic model. At the same time, according to the fiber specifications, set the initial density, elastic model and Poisson's ratio of the zone unit to obtain the pull-out discrete element model c; Step 3.5, after determining the microscopic parameters corresponding to the macroscopic mechanical parameters of the test soil in step 3.3 and the pull-out discrete element model c obtained in step 3.4, the linear contact constitutive model is selected for the microscopic parameters of the coupling contact between the test soil and the optical fiber, and the relationship curves between the pull-out force and the pull-out displacement obtained by the numerical pull-out test and the indoor pull-out test without confining pressure are compared. When the error is within 10%, it is considered that the selected coupling contact model parameters can reflect the coupling mechanical characteristics of the optical fiber and the test soil, and the calibration results are the microscopic parameters of the coupling contact between the test soil and the optical fiber; Step 3.6, clear the velocity, displacement, contact force and contact torque of the test soil particles in the pull-out discrete element model c in step 3.4, assign the test soil particles the mesoscopic parameters corresponding to the macroscopic mechanical parameters of the test soil in step 2 and the mesoscopic parameters of the coupling contact between the test soil and the optical fiber obtained in step 3.5, run 5000 time steps, and obtain the pull-out discrete element model d; Step 3.7, set the target stress values ​​in the x, y, and z directions to txx, tyy, and tzz, respectively. In the pull-out discrete element model d obtained in step 3.6, the speed of the six boundary walls in step 3.1 is controlled by the fish function servo so that the stress on the boundary wall reaches the target stress value, and then the ratio of the average unbalanced force to the sum of the average contact force is less than 1×10 -5 The standard balanced particles, the optical fiber is closely combined with the test soil, and the pull-out discrete element model e is obtained.

7. The method for simulating optical fiber-soil coupling performance based on discrete element as claimed in claim 6, characterized in that: The step 4 specifically comprises the following steps: Step 4.1, specify the loading node and monitoring unit, set the 5% fiber length range of the fiber end as the stretching section, and set the 2.5% fiber length range after the stretching section as the monitoring section; Step 4.2, initialize an array "jiance" to store the index of the monitoring segment area in step 4.1, traverse all zone units, if a unit belongs to the monitoring segment, store the index of the unit belonging to the monitoring segment into the "jiance" array, count and record the number of "jiance" array indexes as N"; in each time step, get the current time "time", and calculate the displacement according to the stretching segment speed; initialize the variable force', loop through the first N" areas in the array "jiance", get the stress of each area, accumulate the stress value into force', and then calculate the pull-out force according to the cross-sectional area of ​​the optical fiber: Where F is the pull-out force, force' is the cumulative stress value, D is the fiber diameter, and N" is the index number of the "jiance" array; Step 4.3, set the loading speed v of the stretching section, the final pulling displacement s and the test termination time t', calculate the total time according to the displacement and loading speed, that is, the final displacement divided by the loading speed, and get the time required for the loading process; use the "zonegridpoint" command to clear the node displacement of the pulling discrete element model e obtained in step 3.7, initialize the mechanical time of the model to 0 and reset the time count; apply the speed v to the nodes in the stretching section of step 4.1, solve the model, run to the set test termination time t', modify the pulling discrete element model e to the pulling discrete element model f, and realize the simulation of the optical fiber pulling process.

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