FLAC-PFC coupling-based portal frame type double-row pile stress three-dimensional analysis method

Abstract

The invention discloses a gantry type double-row pile stress three-dimensional analysis method based on FLAC-PFC coupling. The gantry type double-row pile stress three-dimensional analysis method comprises the following steps that 1, a double-row pile supporting structure model is built through FLAC software; step 2, dispersing side slope particles by utilizing PFC (Power Factor Correction) software; 3, based on the double-row pile supporting structure model constructed in the step 1 and the discrete slope particle flow in the step 2, coupling simulation is conducted on soil around the piles; 4, calibrating mesoscopic parameters of the spherical particles; step 5, performing time step control, boundary condition application, monitoring point deployment and control and gravity loading; and 6, extracting the internal force of the pile body for analysis. According to the gantry type double-row pile stress three-dimensional analysis method based on FLAC-PFC coupling, a continuous numerical analysis method and a discrete element method are combined to research pile-soil interaction, the defects of a single method in analysis can be effectively overcome, and the pile-soil interaction is deeply analyzed.

Description

Technical Field

[0001] The invention relates to the field of slope numerical simulation, in particular to a three-dimensional analysis method of the forces of portal double-row piles based on FLAC-PFC coupling. Background Art

[0002] The portal-type double-row anti-slide piles are a spatial support structure system consisting of two rows of parallel reinforced concrete piles, connecting beams and crown beams. The piles, beams and rock and soil between the piles work together to resist the active earth pressure and landslide thrust of slopes and landslides. The supporting capacity is better than that of single-row piles. Compared with double-row piles without connecting beams, the force is more reasonable and the displacement is smaller. It has been widely used in practical projects such as slope protection and landslide control.

[0003] In order to reasonably analyze the internal force and deformation of double-row portal anti-sliding piles, scholars introduced simplified assumptions and established two types of calculation models based on structural mechanics: the first type of model represented by He Yihua, Qian Tonghui and Zhou Cuiying artificially distributes the active earth pressure or landslide thrust to the front and rear rows of piles; the second type of model represented by Zheng Gang, Shen Yongjiang, Yang Guanghua and Cao Jing uses horizontal components or plane trusses to approximate the interaction between the soil between piles and the front and rear rows of piles, avoiding the distribution of earth pressure or landslide thrust. The structural mechanics calculation model can provide analytical means for the analysis of the internal force and deformation of double-row portal piles, and some research results have been adopted by the Technical Code for Building Foundation Pit Support. In recent years, related research is still ongoing. For example, Li Xunchang simplified the double-row portal anti-sliding piles into two single piles disconnected at the zero bending moment of the pile top connecting beam, and established the full pile internal force calculation model of the loaded section and the embedded section respectively; Tan Tianchong expanded the structural mechanics calculation model of double-row piles to three rows of piles.

[0004] However, the calculation model of structural mechanics generally simplifies the spatial effect and regards the double-row piles as a plane rigid frame, which cannot adapt to special positions such as the corners of the foundation pit; when applied to new structures such as non-equal length double-row piles and spaced double-row piles, whether the existing assumptions of the calculation model are in line with the actual engineering needs to be discussed. Numerical simulation analysis is an effective means to overcome the above limitations of the calculation model. Finite element and finite difference methods have been applied to a certain extent. However, the particle interaction between the soil and piles as the force transmission medium of the front and rear rows of piles is complex. Most existing studies have regarded the piles and the rock and soil as continuum simulations, and it is difficult to consider the influence of this factor on the pile-soil interaction. The particle discrete element has the ability to simulate the interaction and particle crushing behavior of bulk particles under external loads. The particle bonding model can also be used to simulate concrete pile beams, but the simulation of the deformation of the double-row anti-sliding piles and slopes by using it alone requires a large number of particles to ensure accuracy, and the calculation efficiency is difficult to guarantee. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a three-dimensional analysis method for the force of double-row gantry piles based on FLAC-PFC coupling, which combines the continuous numerical analysis method with the discrete element method to study the pile-soil interaction, which can not only effectively play their respective advantages, but also effectively make up for the shortcomings of a single method in the analysis. The method can also conduct an in-depth analysis of the pile-soil interaction.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: The three-dimensional analysis method of the double-row portal pile force based on FLAC-PFC coupling includes the following steps: Step 1: Use FLAC software to construct a double-row pile support structure model; Step 2: Discretize slope particles using PFC software; Step 3: Based on the double-row pile support structure model constructed in step 1 and the slope particle flow discretized in step 2, a coupled simulation of the soil around the piles is performed; Step 4: Calibrate the microscopic parameters of the spherical particles; Step 5: Perform time step control, apply boundary conditions, arrange monitoring points, and perform gravity loading; Step 6: Extract the internal force of the pile for analysis.

[0007] In step 1, the double-row pile support structure model is constructed using FLAC software, which specifically includes: (1) Preparation stage: The data collection preparation phase acquires the physical and mechanical properties of the piles and surrounding soil; at the same time, confirms the model size and scope, determines the height, slope, base width and pile arrangement of the slope, including the number, spacing, layout, pile diameter and length of the piles; (2) Geometric modeling: Use Rhino modeling software to build a pile foundation slope model. Based on the basic model of the actual engineering slope, the actual size of the project is restored one-to-one to generate a three-dimensional model of the slope body. According to the design requirements, the pile foundation is located in the three-dimensional model of the slope body, the center point of the pile is inserted at a specific position, the pile body is drawn, and a three-dimensional model of the pile foundation slope is generated. The three-dimensional model of the pile foundation slope is then meshed using the Griddle plug-in. (3) Importing the three-dimensional model of the pile foundation slope: Adjust the geometric details of the slope and pile foundation as needed to ensure that all elements are correct; use the Griddle plug-in to convert the grid divided into unit files that can be recognized by the FLAC software and import them into the FLAC software; (4) Property definition: Assign material properties to the pile and soil, set up different material models in the FLAC software to reflect the behavior of different materials; define the boundary conditions of the three-dimensional model of the pile foundation slope and run the software to perform simulations and analyze the deformation mode and stress distribution of the pile.

[0008] In step 2, the slope particles are discretized using PFC software, which includes: S2.1: Coupling stage: delete some of the rock and soil solid elements, and introduce the wall of PFC software between the continuous area simulated in FLAC software and the area where particles are to be generated to achieve coupling; S2.2: Particle generation: PFC software is used inside the wall to create spherical particles that meet the predetermined gradation, and they are distributed inside the three-dimensional model of the pile foundation slope. Physical and mechanical properties are also given to the spherical particles. Initialization is performed before starting the simulation to ensure that all spherical particles are in a stable state. S2.3: Run simulation: Execute the simulation command to make the ball particles move according to the laws of physics until they reach equilibrium or simulate physical time.

[0009] In step 3, based on the double-row pile support structure model constructed in step 1 and the slope granular flow discretized in step 2, a coupled simulation of the soil around the piles is performed; specifically, the simulation includes: S3.1: Use Rhino modeling software to determine the position of each node on the boundary surface of the pile foundation slope three-dimensional model, use the curve tool to draw the slope contour, extrude the surface, generate the pile foundation slope three-dimensional model, and construct a double-row pile group; then, use the Griddle plug-in to perform detailed grid division on the established pile foundation slope three-dimensional model to generate a unit file that can be recognized by the FLAC software; S3.2: Start the model module in the FLAC software and load the unit file generated by S3.1; on this basis, further partition the three-dimensional model of the pile foundation slope, set the geometric parameters of the three-dimensional model of the pile foundation slope, and save the configured model file; S3.3: Activate the built-in coupling module of the FLAC software and remove the finite element model of the landslide; this stage includes setting the size of the coupling domain and initializing the random number, and activating the large deformation mode to adapt to the deformation needs of the three-dimensional model of the pile foundation slope; S3.4: At the cantilever end and the slide bed of the pile body, the contact surface with the discrete area is constructed and defined as the wall of the PFC software. The walls are also set around and on the top of the three-dimensional model of the pile foundation slope. The command stream function of generating ball particles in the PFC software is used to fill in the predefined ball particles. S3.5: For the spherical particles around the pile body in the three-dimensional model of the pile foundation slope, carry out regional design, including the setting of the size, distribution gradation and porosity of the spherical particles. In addition, the contact parameters between the spherical particles and between the spherical particles and the wall need to be preliminarily set; S3.6: Delete the ball particles beyond the design area, and perform initial zeroing processing on the displacement and velocity parameters in the three-dimensional model of the pile foundation slope, so as to perform initial balance verification of the three-dimensional model of the pile foundation slope; after completing the operation, save the file of the initial balance state to prepare for the subsequent steps; S3.7: Load the saved initial equilibrium state file, use the custom Fish language program to apply the preset load to the three-dimensional model of the pile foundation slope; at the same time, set the calculation equilibrium parameters and calculation termination conditions, and save the final calculation result file after completion; S3.8: Open the saved calculation result file and conduct a preliminary analysis of the results; extract the required stress-strain information, export key data based on the analysis of representative results, and conduct secondary drawing and detailed analysis to gain a deeper understanding of the behavior of the three-dimensional model of the pile foundation slope; S3.9: The establishment of the pile foundation slope model by Rhino modeling software and the writing and improvement of the Fish language program is an iterative process, and steps S3.1 to S3.8 are repeated; the size of the coupling domain is set after activating the built-in coupling module of the FLAC software, and is determined according to the FLAC-PFC coupling domain, and the value of the coupling domain must be larger than the coupling domain; the wall wraps the spherical particles and fits the solid unit; S3.10: During the initial trial run, the size of the spherical particles is adjusted to 3 to 5 times the original size; after ensuring that the Fish language program is correct, the particle size is adjusted to the original size for accurate calculation, and the contact parameters between particles and between particles and the pile body are continuously adjusted to ensure that the simulation results are consistent with the actual experimental data.

[0010] In step 4, the calibration of the microscopic parameters is as follows: A PFC triaxial compression test model was established, and the mesoscopic parameters of the spherical particles were calibrated according to the known macroscopic mechanical parameters. This process included adjusting the mesoscopic parameters by fixing the variables, establishing the stress-strain curve, observing the influence of these parameters on the test results, conducting sensitivity analysis, and determining the required mesoscopic parameters.

[0011] The steps to establish the PFC triaxial compression numerical model include: (1) Sample: First, a model specimen of a specified size is constructed. The model specimen is composed of spherical particle units with different radii. The radius of the smallest spherical particle to the largest spherical particle should follow a Gaussian distribution. (2) Preload: Apply target pre-compressive stress to the model specimen to simulate the initial stress state; apply velocity to the wall to generate displacement, and calculate the pre-applied force by multiplying the displacement by the stiffness; (3) Add cement: When the distance between the two PFC particles is less than the set threshold, contact is established and mechanical parameters are assigned to add bonding; the contact model used is the parallel bonding model; (4) Uninstall: This process simulates the journey of rock samples from the collection site to the laboratory by reducing the target pre-compression stress; (5) Add confining pressure: Apply velocity to the left and right walls of the model specimen to generate displacement to achieve the target stress; (6) Loading; The loading is realized by applying speed to the upper and lower walls of the model specimen, and the conditions for stopping the loading are set to evaluate the mechanical behavior of the material and the stability of the structure.

[0012] In step 5, time step control, boundary condition application, and monitoring point layout are performed, including: (1) First, the calculation equilibrium time is set to a suitable time step, and the number of time steps is sufficient to stabilize the stress and displacement changes in the model; (2) Secondly, a continuous boundary is set according to the deformation characteristics of the slope. The boundaries except the top are fixed to simulate the constraints on the slope under natural conditions. The model adopts the slope self-weight stress field loading mode; (3) Finally, by setting up monitoring points and monitoring the equilibrium state of the slope after excavation, the slope displacement and the stress and strain calculation results of the double-row piles are monitored to evaluate the slope stability, predict possible sliding or collapse areas, and evaluate the effectiveness and safety of the anti-slide pile design.

[0013] The gravity loading in step 5 specifically includes: The material characteristics of the slope are preliminarily set, the gravity load of the soil in the model is monotonically increased, and the microscopic internal friction coefficient f and the internal friction angle φ are reduced by the same multiple to ensure that the shear strength remains unchanged. When the slope reaches the critical failure state, the gravity increase multiple at this time is the safety factor of the slope; the improved gravity increase method formula is:

[0014] In the formula, is the friction coefficient of the slope at critical failure state.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1) Combining the continuous numerical analysis method with the discrete element method to study pile-soil interaction can not only effectively play their respective advantages, but also effectively make up for the shortcomings of a single method in analysis. This method can also conduct an in-depth analysis of pile-soil interaction.

[0016] 2) By constructing real three-dimensional terrain, the simplification of stratum levels in traditional calculations can be avoided.

[0017] 3) By establishing a real three-dimensional model and considering the interaction between the pile and the surrounding strata in three-dimensional space during the coupling process, the forces in different directions are analyzed. This can be used for accurate analysis of spatial effects.

[0018] 4) By adopting the parametric modeling method, different types of double-row pile models can be quickly constructed according to the different geometric characteristics of the pile body (such as the length, diameter, arrangement, etc. of the pile). It is suitable for the calculation and analysis of new structures such as non-equal length double-row piles and interval double-row piles.

[0019] 5) The method of combining continuous numerical analysis with discrete element method to study pile-soil interaction can be slightly modified to study the changes in internal forces of different support measures and soil, such as changing support measures such as retaining walls and underground continuous walls, providing a new method for slope engineering design.

[0020] 6) This invention is significantly different from the patent application number "202310396223.3" "Mesoscopic parameter calibration method for rock grouting anchor pull-out test based on FLAC-PFC coupling": this invention focuses on the study of pile-soil interaction and is mainly used in the field of engineering design; while the patent "202310396223.3" focuses on microscopic parameter calibration and uses numerical analysis to simulate and calibrate the rock grouting anchor pull-out test. These two methods have different focuses in terms of research objects and application directions. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described below in conjunction with the accompanying drawings and embodiments: Figure 1 The FLAC based on the boundary control wall of the present invention 3D -PFC 3D Coupling principle.

[0022] Figure 2 Schematic diagram of slope engineering of the embodiment (A, B, C represent three groups of retaining walls respectively, and D represents the existing ground line).

[0023] Figure 3 This is a simplified diagram of the slope engineering of the embodiment.

[0024] Figure 4 It is a schematic diagram of slope engineering simulation of an embodiment.

[0025] Figure 5 It is a schematic diagram of the slope engineering wall and coupling simulation of the embodiment.

[0026] Figure 6 Schematic diagram of the arrangement of monitoring points for slope engineering according to an embodiment.

[0027] Figure 7Schematic diagram of slope engineering coupling simulation in the embodiment (a is when the FLAC software generates the model; b is when the FLAC-PFC coupling generates the model; c is when the FLAC-PFC coupling simulation is running).

[0028] Figure 8 It is the displacement diagram of the monitoring point of the slope engineering of the embodiment.

[0029] FIG. 9( a ) is a displacement cloud diagram of the top surfaces of the connecting beam and the crown beam of the embodiment.

[0030] FIG9( b ) is a stress cloud diagram of the top surfaces of the connecting beam and the crown beam of the embodiment.

[0031] FIG. 10( a ) is a diagram showing the distribution of the bending moment along the pile depth of the embodiment.

[0032] FIG. 10( b ) is a diagram showing the variation of displacement along the pile depth of the embodiment.

[0033] In the figure: double-row portal piles 1, retaining wall 2, silty clay layer 3, gravel soil layer 4, strongly weathered silt sand mudstone layer 5, moderately weathered silt sand mudstone layer 6. DETAILED DESCRIPTION

[0034] Three-dimensional analysis method of double-row portal piles based on FLAC-PFC coupling, principle analysis: 1) Common development background. PFC and FLAC were both developed by ITASCA, which means that they are highly consistent in design concept, user interface and operation logic.

[0035] 2) Unification of programming languages. The programming and customization functions of both software are based on the Fish language. The consistent use of this dedicated scripting language facilitates the development of interactive and customized models, making data transmission and command execution of the two software smoother.

[0036] 3) Similarity of calculation methods. Both use explicit difference method for numerical calculation and use the concept of time step for iterative solution. The similarity of this calculation framework ensures that the information transmission and update can be carried out seamlessly during the coupled calculation process, thereby reducing the transmission difficulties and lag problems that may occur in the calculation.

[0037] 4) Integration of coupling module. Starting from FLAC6.0 version, the coupling module of PFC program has been embedded in the software, as shown in Figure 3, which greatly simplifies the coupling calculation process. Users can integrate the two software more conveniently without tedious manual data transmission and conversion.

[0038] 5) Compatibility, stability and feasibility. The coupling analysis and calculation of FLAC differential grid and particle region in PFC is not only feasible in theory, but also shows good compatibility and stability in practice.

[0039] A three-dimensional analysis method for the stress of double-row gantry piles based on FLAC-PFC coupling is proposed. The double-row pile support structure is constructed using FLAC software. PFC is used to discretize slope particles. The double-row pile support structure constructed in FLAC software is coupled with the particle flow of PFC software to simulate the soil around the piles, and the internal force is extracted for analysis.

[0040] Example 1 The three-dimensional analysis method of the double-row portal pile force based on FLAC-PFC coupling includes the following steps: (1): Taking a certain slope project as an example, the steep slope position from 458.00 elevation to 468.00 elevation of the slope project adopts a portal-type double-row pile support structure. In order to enhance the stability of the slope, the top of the pile is connected with a crown beam to provide additional stability. The pile spacing of the portal-type double-row piles 1 is 3.0m, the pile diameter is 1.0m, the pile row spacing is 3.0m, and the pile length is 30.0m. The portal-type double-row piles 1 are attached with retaining plates. They use C30 prefabricated plates, are 10m high and 300mm thick. Among them, the portal-type double-row piles 1 are 7m deep in the silty clay layer 3 of the slope, 9.5m long in the gravel soil layer 4, 5m long in the strongly weathered silt mudstone layer 5, and 8.5m deep in the moderately weathered silt mudstone layer 6. The double-row piles of the slope project are arranged as follows: Figure 2 shown.

[0041] (2): Simplify the slope engineering as follows Figure 3 As shown in Figure 1, the aim is to establish a three-dimensional analysis model of the portal double-row piles with four layers of rock and soil, three sets of retaining walls and one set of portal double-row piles. The three-dimensional analysis model of the portal double-row piles is shown in Figure 1. Figure 4 As shown in the figure. The upper layer of the slope soil is silty clay and gravel soil, the middle layer is strongly weathered silty mudstone, the first three layers are discrete element particle units, the lower layer is moderately weathered silty mudstone, and the portal double-row piles are embedded in the rock and soil. In order to facilitate calculation and analysis, the coupling area is set between the strongly weathered silty mudstone and the moderately weathered silty mudstone and between the portal double-row piles 1 and the rock and soil.

[0042] (3): The establishment of the three-dimensional analysis model of the portal double-row pile load using Rhino modeling software is divided into the following steps: a: Confirmation of the size and terrain of the three-dimensional analysis model of the portal double-row pile force. To ensure the accuracy of the simulation, the three-dimensional analysis model of the portal double-row pile force is restored to the actual size of the project in a one-to-one ratio. The specific dimensions of the slope are 100m long, 14m wide, and 61m high. The rock and soil body consists of four layers: silty clay, gravel soil, strongly weathered silt mudstone, and moderately weathered silt mudstone. There are also three sets of retaining walls 2 and one set of portal double-row piles 1. The three sets of retaining walls 2 are represented by A, B, and C respectively.

[0043] b: Drawing of the three-dimensional analysis model of the portal double-row piles. The curve tool of the Rhino modeling software is used to draw the outline of the slope and the boundaries of each group. The surface tool is used to create a surface with a plane curve. Then, the surface is stretched to 14m wide through the extrusion function of the solid tool to form the basic shape of the three-dimensional analysis model of the portal double-row piles. The built-in Boolean difference and union operations of the Rhino modeling software are used to remove the overlapping parts of the three-dimensional analysis model of the portal double-row piles, and the groups of the same category are merged to clearly define the various parts of the three-dimensional analysis model of the portal double-row piles.

[0044] c: Refinement and meshing of the three-dimensional analysis model of the double-row portal piles. The three-dimensional analysis model of the double-row portal piles was exploded, duplicate faces were deleted, non-manifolds were merged, and meshes were divided. The mesh density of the double-row portal piles was set to 0.2m to improve the calculation accuracy. The mesh sizes of other groups were set in the range of 0.5m to 4m, and a total of 845,078 units were generated.

[0045] d: Mesh conversion and FLAC import. Use the Griddle plug-in to convert the mesh of the three-dimensional analysis model of double-row gantry piles constructed by the Rhino modeling software into a unit file recognizable by the FLAC software, and import the converted mesh file into the FLAC software to complete the construction process of the three-dimensional analysis model of double-row gantry piles.

[0046] (4): FLAC software was used as a platform for coupling calculations. The Load PFC function was used to activate the coupling switch system. A FLAC-PFC continuous discrete coupling portal double-row pile stress three-dimensional analysis model consisting of three key modules: rock mass, portal double-row piles and retaining wall was successfully constructed. The FLAC-PFC continuous discrete coupling portal double-row pile stress three-dimensional analysis model simulates the interaction of the three modules in a relatively fine manner. The double-row pile structure was simulated in the FLAC-PFC continuous discrete coupling portal double-row pile stress three-dimensional analysis model by the solid unit of the FLAC software. The solid unit is based on the finite difference numerical principle and can accurately obtain macroscopic stress and deformation information, truly reflecting the actual dynamic behavior of the material. In addition, the lower rock mass is moderately weathered silt mudstone, which also uses the FLAC software solid unit, which provides a basis for simulating the actual behavior of the material.

[0047] At the same time, the soil around the double-row gantry piles was simulated using the discrete element structure of the PFC software. This method is not limited by deformation and can more accurately handle non-continuous medium mechanics problems, which is consistent with the actual soil particle mechanics properties. Through this continuous-discrete coupling modeling method, the upper soil model (including retaining walls, gravel soil, silty clay and highly weathered silt mudstone) is realized with the particle units of parallel bonded contact of the PFC software and the lower model (including double-row piles and moderately weathered silt mudstone) is realized with the Mohr-Coulomb calculation model in the FLAC software.

[0048] (5): In addition, according to the parameter calibration principle, the radius range of the spherical particles around the piles is finally determined, with a minimum value of 0.18m and a maximum value of 0.27m. The maximum-minimum particle radius ratio of the spherical particles is 1.66. For the newly added retaining wall C and retaining wall A and retaining wall B, the PFC discrete element structure is used, and the spherical particles with a radius of 0.1m are arranged regularly. The wall unit is generated between the double-row gantry piles and the rock and soil around the piles. The wall model and the schematic diagram of the coupled simulation are shown in the figure. Figure 5 shown.

[0049] Through this comprehensive coupling simulation method, the slope model finally constructed includes 174,728 particle units and 310,462 structural units, which not only ensures the accuracy of the FLAC-PFC continuous discrete coupling gantry double-row pile stress three-dimensional analysis model, but also meets the requirements of calculation speed.

[0050] (6): For the simulation of geotechnical engineering problems using the FLAC-PFC continuous discrete coupling portal double-row pile load three-dimensional analysis model, the setting of macroscopic parameters and microscopic parameters is crucial.

[0051] Macro parameters reflect the overall mechanical properties of rock and soil mass and structural surface, such as elastic modulus, Poisson's ratio, internal friction angle, cohesion, etc., which are obtained through geological survey reports and engineering experience. Since the direct quantitative relationship between macro parameters and micro parameters has not yet been established, the calibration of micro parameters mainly depends on experience and repeated debugging. PFC software is used to adjust micro parameters, and its simulation results are compared with known macro mechanical parameters until the simulation results match the actual macro mechanical properties.

[0052] Among them, the various macroscopic physical and mechanical parameters of the slope rock and soil mass and the structural surface are finally adopted as shown in Table 1. The microscopic parameters are based on the macroscopic parameters and material characteristics, and the aforementioned parameter calibration method is adopted. The macro and microscopic parameters are repeatedly debugged and calibrated using the PFC software. The values ​​of the microscopic contact parameters of soil and rock particles are shown in Table 2. The physical and mechanical parameters of the pile parameters are shown in Table 3.

[0053] Table 1 Physical and mechanical parameters of PFC rock and soil

[0054] The deformation modulus E is converted as follows:

[0055] Where: is the compression modulus of soil; is Poisson's ratio.

[0056] Table 2 PFC model calculation parameters

[0057] Table 3 FLAC differential grid pile parameters

[0058] (7): Time step control is a key factor in coupled simulation. By setting the time step appropriately, the calculation process of the model can be ensured to be both accurate and efficient. First, the calculation equilibrium time is set at 1 million steps. This number of time steps is sufficient to stabilize the stress and displacement changes in the model, so as to observe the long-term stability performance of the slope after excavation. Secondly, according to the deformation characteristics of the slope, a continuous boundary is set, and the boundary except the top is fixed to simulate the constraints of the slope under natural conditions. The model adopts the slope self-weight stress field loading mode to ensure the authenticity of the simulation results. Finally, the main monitoring period is the equilibrium state after the slope is excavated. By monitoring the slope displacement and the stress and strain calculation results of the double-row piles, the slope stability can be evaluated, the possible sliding or collapse areas can be predicted, and the effectiveness and safety of the double-row pile design can be evaluated.

[0059] (8): Monitoring points 1 to 6 are set from left to right on the PFC simulation slope surface of the slope engineering, namely: monitoring point ①, monitoring point ②, monitoring point ③, monitoring point ④, monitoring point ⑤, and monitoring point ⑥. Strongly weathered silty mudstone has almost no effect on the landslide process, so particle monitoring is arranged on the slope surface. The layout of the monitoring points is shown in the figure below. Figure 6 This method can not only reveal the instability process of landslides, but also help evaluate the stability of slopes and possible dangerous areas, providing a scientific basis for slope management and disaster prevention and mitigation.

[0060] (9): The improved gravity increase method is used for the slope. Based on this method, the required safety factor of 1.3 is achieved. That is, the gravity is increased to 1.3 times, and the microscopic internal friction coefficient f and internal friction angle φ of the particles are reduced by the same multiple. The displacement of the soil particles in the slope changes significantly, and the gradual change process of slope failure can be clearly observed, such as Figure 7 Schematic diagram of slope engineering coupling simulation.

[0061] Figure 8 The displacement diagram of the monitoring point of the slope engineering of the embodiment is shown in FIG. Figure 8 It can be seen that when the calculation balance time is set at 1 million steps, the stress and displacement changes in the slope model can be stabilized, so that the long-term stability of the slope engineering can be observed. As the gravity increases, the slope soil first reaches its shear strength limit, causing the bond between particles to begin to break, and the particles from the foot of the slope to the top of the slope produce relative displacement, eventually forming a slip surface, causing the entire landslide to slide downward. This process will cause the slope stability to gradually decrease. Observe the displacement of the monitoring point, such as Figure 8 ,The displacement of the monitoring point fluctuates greatly before 650,000 time steps. After 650,000 time steps, the displacement tends to balance and reaches a relatively stable state.

[0062] Figure 9 (a) and Figure 9 (b) are displacement and stress cloud diagrams of the top surface of the connecting beam and the crown beam of the embodiment. Displacement and stress distribution at the top surface of the connecting beam and the crown beam. The displacement of the front and rear rows of crown beams is basically the same, and after magnification, it shows obvious convex features. The crown beam at the pile row ③ has the largest lateral displacement. As the safety factor increases, the displacement of the crown beam increases. When the safety factor is 1.10, the maximum displacements of the front and rear rows of crown beams are 6.94mm and 6.78mm, respectively. When the safety factor increases to 1.30, the maximum displacements are 9.57mm and 9.24mm, respectively. In general, the displacement of the front row of crown beams is slightly larger than that of the rear row of crown beams, so the connecting beam at the pile row ③ is in a tensile state. Ignoring the stress concentration phenomenon at the boundary caused by the model constraint conditions, it can be found that the rear row of crown beams is mainly subjected to compressive stress, while the front row of crown beams is mainly subjected to tensile stress. This force form shows that the connecting beam and the crown beam exert structural resistance as a reliable bonded whole.

[0063] Figure 10 (a) shows the distribution of bending moment along the pile depth. In Figure 10 (a), the bending moment distribution of the front and rear rows of piles presents an obvious "S" curve: the bending moment of the pile near the bottom of the pile is close to zero; the pile first bears positive bending moment along the pile depth, which gradually increases to a peak value and then gradually decreases; after passing the inflection point, the pile bears negative bending moment, which still gradually increases to a peak value and then gradually decreases; the rear row of piles at the top of the pile bears positive bending moment, while the bending moment of the front row of piles is close to zero. With the increase of the required safety factor, the positive and negative bending moments borne by the piles increase significantly, resulting in an increase in the degree of concavity of the curve. When the safety factor is 1.10, the peak positive bending moments of the front and rear piles are 394kN.m and 545kN.m, and the peak negative bending moments are -745kN.m and -785kN.m respectively; when the safety factor is 1.20, the peak positive bending moments of the front and rear piles are 587kN.m and 654kN.m, and the peak negative bending moments are -1174kN.m and -894kN.m respectively; when the safety factor is 1.30, the peak positive bending moments of the front and rear piles are 1420kN.m and 1703kN.m, and the peak negative bending moments are -1760kN.m and -1040kN.m respectively. As the safety factor increases, the position of the inflection point of the front piles moves up significantly, while there is no clear pattern for the rear piles. When the safety factor is 1.10, the inflection points of the front and rear piles are located at a depth of about 8.5m and 12m respectively; when the safety factor is 1.20, the inflection points are located at a depth of about 7m and 14.5m respectively; when the safety factor is 1.30, the inflection points are located at a depth of about 6.5m and 8.5m respectively. In general, as the required safety factor increases, the front and rear piles are subjected to greater bending moments and more intense bending moment changes, and the inflection points of the front piles move upward.

[0064] Figure 10 (b) is a graph showing the displacement variation along the pile depth of the embodiment. As can be seen from Figure 10 (b), the lateral displacement of the front and rear rows of piles is close to zero at the bottom of the pile, but the variation trend with the pile depth is slightly different. The lateral displacement of the rear row of piles gradually increases upward along the pile depth and reaches a peak at the pile top; the lateral displacement of the front row of piles first gradually increases upward along the pile depth, and then shows a decreasing trend when approaching the pile top, resulting in a convex phenomenon in the lateral displacement curve near the pile top. The lateral displacement of the front row of piles is generally greater than that of the rear row of piles, but the difference is very small at the pile top, indicating that the connecting beam and the crown beam effectively constrain the lateral displacement difference between the front and rear row of piles at the pile top. With the increase of the safety factor, the peak values ​​of the lateral displacement of the front and rear row of piles increase accordingly. When the safety factor is 1.10, the peak lateral displacements of the front and rear rows of piles are 16.4 mm and 10.2 mm respectively; when the safety factor is increased to 1.20, the peak lateral displacements are 19.9 mm and 17.2 mm respectively; when the safety factor is increased to 1.30, the peak lateral displacements are 20.6 mm and 18.5 mm respectively. Therefore, the lateral displacements of the front and rear rows of piles show a nonlinear growth with the increase of the safety factor.

Claims

1. A three-dimensional analysis method for the load of double-row portal piles based on FLAC-PFC coupling includes the following steps: Step 1: Use FLAC software to construct a double-row pile support structure model; Step 2: Discretize slope particles using PFC software; Step 3: Based on the double-row pile support structure model constructed in step 1 and the slope granular flow discretized in step 2, a coupled simulation of the soil around the piles is performed; Step 4: Calibrate the microscopic parameters of the spherical particles; Step 5: Perform time step control, apply boundary conditions, arrange monitoring points, and perform gravity loading; Step 6: Extract the internal force of the pile for analysis.

2. The three-dimensional analysis method of the gantry double-row pile force based on FLAC-PFC coupling according to claim 1 is characterized by: In step 1, the double-row pile support structure model is constructed using FLAC software, which specifically includes: (1) Preparation stage: The data collection preparation phase acquires the physical and mechanical properties of the piles and surrounding soil; at the same time, confirms the model size and scope, determines the height, slope, base width and pile arrangement of the slope, including the number, spacing, layout, pile diameter and length of the piles; (2) Geometric modeling: Use Rhino modeling software to build a pile foundation slope model. Based on the basic model of the actual engineering slope, the actual size of the project is restored one-to-one to generate a three-dimensional model of the slope body. According to the design requirements, the pile foundation is located in the three-dimensional model of the slope body, the center point of the pile is inserted, the pile body is drawn, and a three-dimensional model of the pile foundation slope is generated. The three-dimensional model of the pile foundation slope is then meshed using the Griddle plug-in. (3) Importing the three-dimensional model of the pile foundation slope: Adjust the geometric details of the slope and pile foundation as needed to ensure that all elements are correct; use the Griddle plug-in to convert the grid divided into unit files that can be recognized by the FLAC software and import them into the FLAC software; (4) Property definition: Assign material properties to the pile and soil, set up different material models in the FLAC software to reflect the behavior of different materials; define the boundary conditions of the three-dimensional model of the pile foundation slope and run the software to perform simulations and analyze the deformation mode and stress distribution of the pile.

3. The three-dimensional analysis method of the gantry double-row pile force based on FLAC-PFC coupling according to claim 2 is characterized by: In step 2, the slope particles are discretized using PFC software, which includes: S2.1: Coupling stage: delete some of the rock and soil solid elements, and introduce the wall of PFC software between the continuous area simulated in FLAC software and the area where particles are to be generated to achieve coupling; S2.2: Particle generation: PFC software is used inside the wall to create spherical particles that meet the predetermined gradation, and they are distributed inside the three-dimensional model of the pile foundation slope. Physical and mechanical properties are also given to the spherical particles. Initialization is performed before starting the simulation to ensure that all spherical particles are in a stable state. S2.3: Run simulation: Execute the simulation command to make the ball particles move according to the laws of physics until they reach equilibrium or simulate physical time.

4. The three-dimensional analysis method of the gantry double-row pile force based on FLAC-PFC coupling according to claim 3 is characterized by: In step 3, based on the double-row pile support structure model constructed in step 1 and the slope granular flow discretized in step 2, a coupled simulation of the soil around the piles is performed; specifically, the simulation includes: S3.1: Use Rhino modeling software to determine the position of each node on the boundary surface of the pile foundation slope three-dimensional model, use the curve tool to draw the slope contour, extrude the surface, generate the pile foundation slope three-dimensional model, and construct a double-row pile group; then, use the Griddle plug-in to perform detailed grid division on the established pile foundation slope three-dimensional model to generate a unit file that can be recognized by the FLAC software; S3.2: Start the model module in the FLAC software and load the unit file generated by S3.1; on this basis, further partition the three-dimensional model of the pile foundation slope, set the geometric parameters of the three-dimensional model of the pile foundation slope, and save the configured model file; S3.3: Activate the built-in coupling module of the FLAC software and remove the finite element model of the landslide; this stage includes setting the size of the coupling domain and initializing the random number, and activating the large deformation mode to adapt to the deformation needs of the three-dimensional model of the pile foundation slope; S3.4: At the cantilever end and the slide bed of the pile body, the contact surface with the discrete area is constructed and defined as the wall of the PFC software. The walls are also set around and on the top of the three-dimensional model of the pile foundation slope. The command stream function of generating ball particles in the PFC software is used to fill in the predefined ball particles. S3.5: For the spherical particles around the pile body in the three-dimensional model of the pile foundation slope, carry out regional design, including the setting of the size, distribution gradation and porosity of the spherical particles. In addition, the contact parameters between the spherical particles and between the spherical particles and the wall need to be preliminarily set; S3.6: Delete the ball particles beyond the design area, and perform initial zeroing processing on the displacement and velocity parameters in the three-dimensional model of the pile foundation slope, so as to perform initial balance verification of the three-dimensional model of the pile foundation slope; after completing the operation, save the file of the initial balance state to prepare for the subsequent steps; S3.7: Load the saved initial equilibrium state file, use the custom Fish language program to apply the preset load to the three-dimensional model of the pile foundation slope; at the same time, set the calculation equilibrium parameters and calculation termination conditions, and save the final calculation result file after completion; S3.8: Open the saved calculation result file and conduct a preliminary analysis of the results; extract the required stress-strain information, export key data based on the analysis of representative results, and conduct secondary drawing and detailed analysis to gain a deeper understanding of the behavior of the three-dimensional model of the pile foundation slope; S3.9: The establishment of the pile foundation slope model by Rhino modeling software and the writing and improvement of the Fish language program is an iterative process, and steps S3.1 to S3.8 are repeated; the size of the coupling domain is set after activating the built-in coupling module of the FLAC software, and is determined according to the FLAC-PFC coupling domain, and the value of the coupling domain must be larger than the coupling domain; the wall wraps the spherical particles and fits the solid unit; S3.10: During the initial trial run, the size of the spherical particles is adjusted to 3 to 5 times the original size; after ensuring that the Fish language program is correct, the particle size is adjusted to the original size for accurate calculation, and the contact parameters between particles and between particles and the pile body are continuously adjusted to ensure that the simulation results are consistent with the actual experimental data.

5. The three-dimensional analysis method of the gantry double-row pile force based on FLAC-PFC coupling according to claim 1 is characterized by: In step 4, the calibration of the microscopic parameters is as follows: A PFC triaxial compression test model was established, and the mesoscopic parameters of the spherical particles were calibrated according to the known macroscopic mechanical parameters. This process included adjusting the mesoscopic parameters by fixing the variables, establishing the stress-strain curve, observing the influence of these parameters on the test results, conducting sensitivity analysis, and determining the required mesoscopic parameters.

6. The three-dimensional analysis method of the gantry double-row pile force based on FLAC-PFC coupling according to claim 5 is characterized by: The steps to establish the PFC triaxial compression numerical model include: (1) Sample: First, a model specimen of a specified size is constructed. The model specimen is composed of spherical particle units with different radii. The radius of the smallest spherical particle to the largest spherical particle should follow a Gaussian distribution. (2) Preload: Apply target pre-compressive stress to the model specimen to simulate the initial stress state; apply velocity to the wall to generate displacement, and calculate the pre-applied force by multiplying the displacement by the stiffness; (3) Add cement: When the distance between the two PFC particles is less than the set threshold, contact is established and mechanical parameters are assigned to add bonding; the contact model used is the parallel bonding model; (4) Uninstall: This process simulates the journey of rock samples from the collection site to the laboratory by reducing the target pre-compression stress; (5) Add confining pressure: Apply velocity to the left and right walls of the model specimen to generate displacement to achieve the target stress; (6) Loading; The speed is applied to the upper and lower walls of the model specimen to achieve loading, and the conditions for stopping loading are set to evaluate the mechanical behavior of the material and the stability of the structure.

7. The three-dimensional analysis method of double-row gantry piles based on FLAC-PFC coupling according to claim 1 is characterized by: In step 5, time step control, boundary condition application, and monitoring point layout are performed, including: (1) First, the calculation equilibrium time is set to a suitable time step, and the number of time steps is sufficient to stabilize the stress and displacement changes in the model; (2) Secondly, a continuous boundary is set according to the deformation characteristics of the slope. The boundaries except the top are fixed to simulate the constraints on the slope under natural conditions. The model adopts the slope self-weight stress field loading mode; (3) Finally, by setting up monitoring points and monitoring the equilibrium state of the slope after excavation, the slope displacement and the stress and strain calculation results of the double-row piles are monitored to evaluate the slope stability, predict possible sliding or collapse areas, and evaluate the effectiveness and safety of the anti-slide pile design.

8. The three-dimensional analysis method of the gantry double-row pile force based on FLAC-PFC coupling according to claim 1 is characterized by: The gravity loading in step 5 specifically includes: The material characteristics of the slope are preliminarily set, the gravity load of the soil in the model is monotonically increased, and the microscopic internal friction coefficient f and the internal friction angle φ are reduced by the same multiple to ensure that the shear strength remains unchanged. When the slope reaches the critical failure state, the gravity increase multiple at this time is the safety factor of the slope; the improved gravity increase method formula is: In the formula, is the friction coefficient of the slope at critical failure state.

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