A method for analyzing the horizontal bearing performance of prestressed hollow square piles

Abstract

The present invention discloses a method for analyzing the horizontal bearing performance of a prestressed hollow square pile, and specifically relates to the technical field of civil engineering. The present invention constructs a prestressed hollow square pile model including a soil model and a pile model based on finite element software, sets the contact mode and contact properties between the soil model and the pile model and performs three-dimensional space grid division, then performs ground stress balance on the prestressed hollow square pile model, applies prestress to the prestressed hollow square pile model, sets the load loading and unloading mode and the stop loading condition of the prestressed hollow square pile model, and then uses the prestressed hollow square pile model to simulate and analyze the influence of soil conditions on the displacement of the pile model, and then uses the prestressed hollow square pile model to simulate and analyze the influence of external loads on the displacement of the pile model, and comprehensively analyzes the horizontal bearing performance of the prestressed hollow square pile. The present invention realizes the accurate analysis of the horizontal bearing performance of the prestressed hollow square pile, which is beneficial to the study of the mechanical response characteristics of the prestressed hollow square pile.

Description

Technical Field

[0001] The invention relates to the technical field of civil engineering, and in particular to a method for analyzing the horizontal bearing performance of a prestressed hollow square pile. Background Art

[0002] At present, the pile types on the market are mainly divided into cast-in-place piles and precast piles according to their preparation locations. Cast-in-place piles have the characteristics of mature technology and good integrity. However, cast-in-place piles have low bearing capacity, high requirements for the construction environment and slow pile formation speed, which makes it difficult to meet all engineering conditions. Compared with cast-in-place piles, precast piles are prefabricated in the factory and can be used after the piles are hoisted and sunk at the construction site. Under the same working conditions, their bearing capacity is higher than that of cast-in-place piles, and they have been widely used in actual projects.

[0003] Considering that different pile types are used in different projects, the difference in pile types directly affects the bearing performance of pile foundations, so it is necessary to study the bearing capacity and influencing factors of different types of pile foundations. Since the bearing mode of the pile foundation is friction piles or end-bearing friction piles, its side area determines the bearing capacity of the pile body. Compared with concrete pipe piles, prestressed hollow square piles have a large side area and have a greater bearing capacity while keeping the cross-sectional area unchanged. At the same time, prestressed hollow square piles are made by centrifugal method, which reduces the deadweight of the pile body and reduces the waste of material strength at the center of the pile, and the cost is lower. Therefore, prestressed hollow square piles have the advantages of saving materials, strong bending resistance, large side friction resistance, good seismic performance, and low foundation cost compared to prestressed hollow pipe piles. However, the research on prestressed concrete square piles is still in its infancy, and the research on the force characteristics of prestressed concrete square piles is relatively lacking.

[0004] Therefore, it is urgent to propose a method for analyzing the horizontal bearing performance of prestressed hollow square piles. By analyzing the influencing factors of the horizontal bearing performance of prestressed hollow square piles, a basis is provided for the study of the mechanical response characteristics of prestressed hollow square piles. Summary of the invention

[0005] The present invention aims to accurately obtain the mechanical response characteristics of prestressed hollow square piles, and proposes a method for analyzing the horizontal bearing performance of prestressed hollow square piles. By analyzing the influence of soil conditions and external loads on the displacement of the prestressed hollow square pile body, the accurate analysis of the horizontal bearing performance of the prestressed hollow square piles is achieved, which is beneficial to the study of the mechanical response characteristics of prestressed hollow square piles.

[0006] The present invention specifically adopts the following technical solutions:

[0007] A method for analyzing the horizontal bearing performance of prestressed hollow square piles, specifically comprising the following steps:

[0008] Step 1, based on the soil test data at the construction site and the design scheme of the prestressed hollow square pile, a prestressed hollow square pile model including a soil model and a pile model is constructed based on finite element software;

[0009] Step 2, setting the contact mode and contact properties between the soil model and the pile model in the prestressed hollow square pile model;

[0010] Step 3, performing three-dimensional space meshing on the prestressed hollow square pile model to obtain a meshed prestressed hollow square pile model;

[0011] Step 4, performing ground stress balance on the prestressed hollow square pile model, and analyzing the ground stress distribution and displacement of the soil model in the prestressed hollow square pile model before and after the ground stress balance;

[0012] Step 5, applying prestress to the prestressed hollow square pile model based on a cooling method;

[0013] Step 6, setting the load loading and unloading mode and the loading stop condition of the prestressed hollow square pile model;

[0014] Step 7, conducting a soil mechanical response test on the prestressed hollow square pile model, analyzing the influence of soil conditions on the displacement of the pile model, obtaining a curve of the relationship between the internal friction angle and the displacement of the top surface of the pile model and a curve of the displacement of the top surface of the pile model changing with the Poisson's ratio, and determining the influence of the internal friction angle and the Poisson's ratio of the soil around the top surface of the pile on the horizontal displacement of the prestressed hollow square pile;

[0015] Step 8, applying external load to the prestressed hollow square pile model, analyzing the influence of the external load on the displacement of the prestressed hollow square pile body, and determining the influence of downward pressure, upward pull, horizontal load direction and pile side angle on the displacement of the prestressed hollow square pile body;

[0016] Step 9, comprehensively considering the influence of soil conditions on the displacement of the pile model and the influence of external loads on the displacement of the prestressed hollow square pile body, and obtaining the horizontal bearing capacity of the prestressed hollow square pile.

[0017] Preferably, the soil model is constructed based on soil test data at the construction site, wherein the soil test data includes soil layer numbers, layer bottom burial depths, soil types, densities, Young's modulus, Poisson's ratio, friction angles, and cohesion of all soil layers;

[0018] A pile body model is constructed according to a design scheme of a prestressed hollow square pile, wherein the pile body model includes a concrete model and a steel bar model, and the design scheme includes the pile body size, hollow size, material, elastic modulus, Poisson's ratio of the prestressed hollow square pile, and material, elastic model, Poisson's ratio, and layout position of the concrete and the steel bar.

[0019] Preferably, the soil model is a Mohr-Coulomb elastoplastic model, and the relationship between the internal friction angle, soil cohesion and shear stress in the soil model is:

[0020]

[0021] Where τ is the shear stress at any cross section; is the friction angle of soil; c is the cohesion of soil;

[0022] The stress-strain relationship of the soil model is:

[0023]

[0024] Where ψ is the expansion angle; ε is the shape parameter of the plastic potential surface; S is the shape of the plastic potential surface; R mw is the plastic potential surface control parameter; q is the spherical stress, which is used to reflect the size of the confining pressure; p is the shear stress, which is used to reflect the shear effect;

[0025] The steel bar model is set as a double oblique line model.

[0026] Preferably, in the prestressed hollow square pile model, the pile model is buried in the soil model, the top surface of the pile model is higher than the top surface of the soil model, the four side surfaces and one bottom surface of the pile model are in contact with the soil model, and the contact mode is set to face-to-face contact;

[0027] The contact attributes include normal contact attributes and tangential contact attributes. The normal contact attributes are set to hard contact, and the tangential contact attributes are set to penalty functions.

[0028] Preferably, in the step 3, the prestressed hollow square pile model is meshed in the horizontal direction, including meshing of a soil model, a pile model and a steel bar model, to obtain a prestressed hollow square pile model cell, specifically including a soil model cell, a pile model cell and a steel bar model cell;

[0029] First, mesh the soil model of the prestressed hollow square pile model;

[0030] Divide the soil model of the prestressed hollow square pile model in the horizontal direction, perform grid division along the horizontal orthogonal directions of the sides of the soil model and the pile model, and divide the soil model 0.1m away from the side of the pile model into cells of size 0.1m×0.1m in combination with the distance between the soil model and the side of the pile model, and divide the soil model 5 times the side length of the pile model into cells of size 0.2m×0.2m, and divide the rest of the soil model into cells of size 0.4m×0.4m, and then divide the soil model of the prestressed hollow square pile model in the vertical direction. According to the length of the pile model, divide the soil model within the length of the pile model into cells of size 0.5m in the vertical direction, and divide the rest of the soil model into cells of size 1.0m in the vertical direction, to obtain a plurality of soil model cells;

[0031] Then divide the pile body model of the prestressed hollow square pile model;

[0032] After the pile model is evenly divided into eight equal parts along the symmetry axis and two diagonals of the pile model in the horizontal direction, each side length of the pile model is evenly divided into ten equal parts, and then evenly divided along the axial direction of the pile model according to a length of 0.125 m to obtain multiple pile model cells.

[0033] Preferably, in step 4, based on the ODB analysis method, the soil model in the prestressed hollow square pile model is first treated as an independent model to be self-balanced under gravity load, and after obtaining the initial stress and initial displacement of the soil model, the soil model in the hollow structure of the prestressed hollow square pile model is removed, and a static analysis is performed on the soil model to obtain the stress distribution of the pile model. Then, the stress of the pile model is used as a predefined field to apply a gravity load to the pile model of the prestressed hollow square pile model, so that the prestressed hollow square pile model restores the ground stress balance, and the stress distribution of the soil model before and after the ground stress balance of the prestressed hollow square pile model is analyzed to determine the displacement of the soil model in the prestressed hollow square pile model.

[0034] Preferably, the minimum temperature of the steel bar model in the prestressed hollow square pile model is preset as shown in formula (3):

[0035]

[0036] In the formula, E is the elastic modulus of concrete, a is the expansion coefficient of the prestressed steel bar, and σ is the prestress that needs to be applied to the pile body;

[0037] The cooling method is used to apply stress to the steel bar model by cooling the steel bar model to a preset minimum temperature.

[0038] Preferably, the load is applied based on the coupled loading mode, firstly, a vertical load is applied to the top surface of the pile body model of the prestressed hollow square pile model according to a preset top pressure value, and then, a horizontal load is applied to the pile body model of the prestressed hollow square pile model under the action of the top pressure value of the pile body model;

[0039] The load loading and unloading method is low-cycle multi-cycle loading and unloading;

[0040] Estimate the ultimate horizontal bearing capacity of the prestressed hollow square pile model, and load each level step by step with 1 / 10 of the ultimate horizontal bearing capacity as the load amount to obtain the load value of each loading level;

[0041] For each level of loading, after adjusting to the preset load application value, the load application is maintained for a specified time, the displacement of the top surface of the pile model is recorded and then unloaded. After maintaining the no-load state for two minutes, the load is continued to be loaded to the load application value. After the cycle is repeated many times, the next level of loading program is entered to determine the horizontal displacement and residual displacement of the top surface of the pile model under each level of loading;

[0042] The loading stop conditions are that the horizontal displacement increase of the pile model under constant load exceeds a preset value, the horizontal displacement of the pile model exceeds 30 mm, and the pile model breaks. After stopping the load loading, the residual strain of the pile model is recorded to obtain the pile displacement-load-depth curve and load-bending moment-depth curve of the pile model.

[0043] Preferably, the step 7 includes the following sub-steps:

[0044] Step 7.1, using a prestressed hollow square pile model to conduct a soil mechanical response test, changing the internal friction angle of the soil model, simulating the top surface displacement of the pile model under different soil model internal friction angle conditions under various load conditions, and obtaining the relationship curve between the internal friction angle and the pile top displacement under various load conditions;

[0045] Step 7.2, using the prestressed hollow square pile model to conduct a soil mechanical response test, changing the Poisson's ratio of the soil model around the pile model, simulating the top surface displacement of the pile model under different soil model Poisson's ratios, and obtaining a curve of the top surface displacement of the pile model changing with the Poisson's ratio of the soil model.

[0046] Preferably, the step 8 includes the following sub-steps:

[0047] Step 8.1, changing the magnitude of the top pressure force of the pile body model in the prestressed hollow square pile model, using the prestressed hollow square pile model to obtain the top surface displacement of the pile body model under each top downward pressure load, and determining the influence of the downward pressure on the displacement of the prestressed hollow square pile body;

[0048] Step 8.2, changing the pull-out force of the pile body model in the prestressed hollow square pile model, using the prestressed hollow square pile model to simulate the displacement of the top surface of the pile body model under each pull-out force, and determining the influence of the downward pressure on the displacement of the pile body of the prestressed hollow square pile;

[0049] Step 8.3, changing the horizontal load angle of the pile body model in the prestressed hollow square pile model, using the prestressed hollow square pile model to simulate the displacement of the top surface of the pile body model under various horizontal load angles, and determining the influence of the horizontal load angle on the displacement of the prestressed hollow square pile body.

[0050] The present invention has the following beneficial effects:

[0051] This method proposes a method for analyzing the horizontal bearing performance of prestressed hollow square piles. By constructing a prestressed hollow square pile model based on the finite element method for simulation, the effects of upper strata, lower strata, top load and load loading angle on the horizontal displacement of the prestressed hollow square pile are determined. By comprehensively analyzing the influence of soil conditions and external loads on the displacement of the prestressed hollow square pile body, the variation law of the horizontal bearing capacity of the pile body under different external conditions is determined, and the horizontal bearing performance of the prestressed hollow square pile is accurately obtained, which is beneficial to the study of the mechanical response characteristics of the prestressed hollow square pile and provides a basis for the performance analysis of prestressed hollow square piles in actual engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 The figure is a flow chart of a method for analyzing the horizontal bearing performance of prestressed hollow square piles.

[0053] Figure 2 Schematic diagram of the double-slash model.

[0054] Figure 3 Schematic diagram of the hard contact mechanics model.

[0055] Figure 4 Schematic diagram of the penalty function mechanics model.

[0056] Figure 5 Schematic diagram of mesh division of pile model. In the figure, (a) is the schematic diagram of mesh division of pile model cross section, and (b) is the schematic diagram of mesh division in the axial direction of pile model.

[0057] Figure 6 Schematic diagram of steel bar model mesh division.

[0058] Figure 7 Schematic diagram of the prestressed hollow square pile model before and after ground stress equilibrium. In the figure, (a) is the prestressed hollow square pile model before ground stress equilibrium, and (b) is the prestressed hollow square pile model after ground stress equilibrium.

[0059] Figure 8Schematic diagram of prestressing of prestressed hollow square pile model.

[0060] Fig. 9 The results of the relationship between the internal friction angle and the pile top displacement under various loads. In the figure, (a) is the relationship curve between the internal friction angle and the pile top displacement under various loads, and (b) is the displacement percentage-internal friction angle relationship curve.

[0061] Fig.10 This is the variation curve of the top surface displacement of the pile model with the Poisson's ratio of the soil model.

[0062] Fig.11 The top surface displacement of the prestressed hollow square pile model under different downward pressures; in the figure, (a) is the relationship curve between the pile top displacement and the vertical load, and (b) is the displacement percentage diagram.

[0063] Fig.12 It is the horizontal displacement of the pile body in the depth direction under different top pressures.

[0064] Fig.13 The top surface displacement results of the pile model under different uplift forces. In the figure, (a) is the relationship curve between the pile top displacement and the uplift load, and (b) is the relationship diagram between the uplift load and the displacement percentage.

[0065] Fig.14 The results of the influence of the horizontal load loading angle on the displacement of the prestressed hollow square pile body; in the figure, (a) is the relationship curve between the displacement in the direction of the horizontal force on the pile top and the angle of the pile body, (b) is the relationship between the angle and the displacement percentage, and (c) is a summary diagram of the displacement of the pile body along the depth direction under different loading angles. DETAILED DESCRIPTION

[0066] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and specific embodiments:

[0067] A method for analyzing the horizontal bearing performance of prestressed hollow square piles, specifically comprising the following steps:

[0068] Step 1: According to the soil experimental data at the construction site and the design scheme of the prestressed hollow square pile, a prestressed hollow square pile model including a soil model and a pile model is constructed based on the ABAQUS finite element software.

[0069] The soil model is constructed according to the soil test data of the construction site. The soil parameters are shown in Table 1. The soil test data include the soil layer number, layer bottom burial depth, soil type, density, Young's modulus, Poisson's ratio, friction angle and cohesion of all soil layers.

[0070] Table 1 Soil parameters

[0071]

[0072] A pile body model is constructed according to a design scheme of a prestressed hollow square pile, wherein the pile body model includes a concrete model and a steel bar model, and the design scheme includes the pile body size, hollow size, material, elastic modulus, Poisson's ratio of the prestressed hollow square pile, and material, elastic model, Poisson's ratio, and layout position of the concrete and the steel bar.

[0073] In this embodiment, a prestressed hollow square pile is set according to "Prestressed Concrete Hollow Square Piles JG / T 197-2018"; the pile body size of the prestressed hollow square pile is 8000mm×450mm×450mm, the diameter of the hollow size is set to 250mm, and the length is 8000mm; the strength of the concrete in the prestressed hollow square pile is C80, the expansion angle is 30°, the eccentricity is 0.1, the viscosity parameter is 0.0001, the Young's modulus of the concrete is 41.04Gpa, and the Poisson's ratio is 0.2; the steel bars in the prestressed hollow square pile are according to "Prestressed Concrete Hollow Square Piles JG / T 197-2018" JG / T 197-2018 and "Steel Bars for Prestressed Concrete GB / T5223.3-2017" use 1420MPa low relaxation prestressed steel bars, Young's modulus 206GPa, Poisson's ratio 0.3, and the main reinforcement of prestressed concrete hollow square piles uses 1420MPa low relaxation prestressed steel bars. The prestressed steel bars are arranged at equal intervals. Considering the corrosion effect of groundwater, it is necessary to set the inner protective layer of the pile due to the presence of water in the pile. The thickness of the protective layer on both sides is 50mm. The stirrups use Q235 low carbon steel hot rolled round wire rods. The two sides of the pile end are the densification area. The stirrups in the densification area are arranged according to Ψ5@45. The other length intervals are non-densification areas and are arranged according to Ψ5@80. At the same time, the soil model in the prestressed hollow square pile takes 20 times the side length of the standard pile type in the horizontal direction and twice the pile length in the vertical direction. In addition, the top surface of the pile model is 20cm higher than the top surface of the soil model.

[0074] In this embodiment, the soil model is the Mohr-Coulomb elastoplastic model, and the relationship between the internal friction angle, soil cohesion and shear stress in the soil model is:

[0075]

[0076] Where τ is the shear stress at any cross section; is the friction angle of soil; c is the cohesion of soil;

[0077] The stress-strain relationship of the soil model is:

[0078]

[0079] Where ψ is the expansion angle; ε is the shape parameter of the plastic potential surface; S is the shape of the plastic potential surface; R mwis the plastic potential surface control parameter; q is the spherical stress, which is used to reflect the size of the confining pressure; p is the shear stress, which is used to reflect the shear effect.

[0080] The steel bar model is set to a double-slash model, such as Figure 2 As shown in the figure, the two oblique lines of the double oblique line model correspond to the stress-strain relationship of the material in the elastic stage and the inelastic stage. When the material reaches the elastic limit, microcracks inside the material begin to accumulate, but the material around the cracks continues to bear the load. Macroscopically, the stress increases slowly and the strain increases rapidly. The material enters a short strengthening stage. At this time, if the load is removed, the material strain will slowly recover. When the material stops recovering, the strain at this time is the residual strain.

[0081] Step 2: Set the contact mode and contact properties between the soil model and the pile model in the prestressed hollow square pile model.

[0082] In the prestressed hollow square pile model, the pile model is buried in the soil model, the top surface of the pile model is higher than the top surface of the soil model, the four side surfaces and one bottom surface of the pile model are in contact with the soil model, and the contact mode is set to face-to-face contact;

[0083] In this embodiment, the contact attributes include normal contact attributes and tangential contact attributes. In order to avoid the pile model being displaced by horizontal loads, resulting in a discrete gap between the pile model and the soil model, so that the soil model can no longer provide lateral friction resistance for the pile model and can no longer transmit normal stress, the normal behavior is set to hard contact, that is, the normal behavior is set to a hard contact mechanics model, such as Figure 3 As shown in , this contact behavior only allows normal stress to be transmitted when there is no gap between the two contact surfaces. As long as any of the two contact surfaces undergoes relative displacement, the cross section cannot transmit any normal stress. The tangential contact attribute is a penalty function, that is, the tangential contact attribute is set to a penalty function mechanical model such as Figure 4 As shown, it is used to define the penalty function friction formula; the penalty function allows relative penetration between contacts. Compared with the Lagrange multiplier, the penalty function allows relative limited slip before the shear stress reaches the limit. When slip occurs, the structure will "punish" it to generate friction on the contact surface. Further, the shear stress in the penalty function is set. In this embodiment, the shear force of the penalty function is set to unlimited, indicating that the penalty function mechanical model is in a bonding state under any shear stress size, and only limited slip can occur. When the shear stress is specified, if it is less than the shear stress value entered in the option, the contact surface is bonded and limited slip occurs. Once the shear stress on the contact surface is greater than the specified value, the contact surface undergoes relative slip, and the maximum shear stress provided is a preset value.

[0084] Step 3, performing three-dimensional spatial meshing on the prestressed hollow square pile model, including meshing of the soil model, meshing of the pile model and meshing of the steel bar model.

[0085] In this embodiment, the soil model of the prestressed hollow square pile model is first meshed:

[0086] The soil model of the prestressed hollow square pile model is divided in the horizontal direction, and the grid is divided along the horizontal orthogonal directions of the sides of the soil model and the pile model. Combined with the distance between the soil model and the side of the pile model, the soil model 0.1m away from the side of the pile model is evenly divided into cells with a size of 0.1m×0.1m, and the soil model 5 times the side length of the pile model is evenly divided into cells with a side length of 0.2m×0.2m, and the rest of the soil model is evenly divided into cells with a side length of 0.4m×0.4m; then the soil model of the prestressed hollow square pile model is divided in the vertical direction. According to the length of the pile model, the soil model within the length of the pile model is evenly divided into 0.5m in the vertical direction, and the rest of the soil model is evenly divided into 1.0m in the vertical direction to obtain multiple soil model cells.

[0087] The pile body model of the prestressed hollow square pile model is divided as follows:

[0088] After the pile model is evenly divided into eight equal parts along the symmetry axis and two diagonals of the pile model in the horizontal direction, each side length of the pile model is evenly divided into 10 equal parts, such as Figure 5 As shown, the pile model is evenly divided along the axial direction according to a length of 0.125 m to obtain multiple pile model cells.

[0089] Finally, mesh the reinforcement model:

[0090] Since the steel bar can be regarded as a truss unit because it only transmits axial force, in this embodiment, only one unit is divided between every two nodes of the steel bar model, such as Figure 6 As shown, multiple steel bar model cells are obtained.

[0091] Step 4, performing geostress balance on the prestressed hollow square pile model, and analyzing the geostress distribution and displacement of the soil model in the prestressed hollow square pile model before and after the geostress balance.

[0092] Under the action of gravity, the internal force of rock and soil media is a relatively large value, especially when the burial depth is deep. At this time, it is necessary to balance the ground stress of the soil.

[0093] In this embodiment, based on the ODB analysis method, the soil model in the prestressed hollow square pile model is first used as an independent model to perform self-balancing under gravity load. After obtaining the initial stress and initial displacement of the soil model, the soil model in the hollow structure of the prestressed hollow square pile model is removed, and a static analysis is performed on the soil model to obtain the stress distribution of the pile model. Then, the stress of the pile model is used as a predefined field to apply a gravity load to the pile model of the prestressed hollow square pile model, so that the prestressed hollow square pile model restores the ground stress balance, and the stress distribution and displacement of the soil model before and after the ground stress balance of the prestressed hollow square pile model are analyzed. Figure 7 As shown, in this embodiment, the maximum displacement of the soil model in the prestressed hollow square pile model before and after the ground stress is balanced is 0.015 mm, and the ground stress balance effect of the soil model is better at this time.

[0094] Step 5: Apply prestress to the prestressed hollow square pile model based on the cooling method.

[0095] According to the expansion coefficient of the steel bar material, the steel bar shrinkage is achieved by lowering the steel bar temperature based on the cooling method. Since the prestressed bars are embedded in the soil, prestress is applied to the pile concrete.

[0096] The minimum temperature of the steel bar model in the prestressed hollow square pile model is preset as shown in formula (3):

[0097]

[0098] Where E is the elastic modulus of concrete, a is the expansion coefficient of the prestressed steel bar, and σ is the prestress that needs to be applied to the pile body.

[0099] In this embodiment, the pre-stressing method is used to apply a stress of 994 MPa. The steel bar model is cooled to a preset minimum temperature of 401°C and stress is applied to the steel bar model. The prestressed hollow square pile model after prestressing is as follows: Figure 8 As shown in the figure, the tensile stress generated by the steel bar model during the prestressing process is 919.2-989.2 MPa. During the shrinkage process, the effective prestress transmitted from the steel bar model to the pile model is 5-6 MPa, which is in line with the designed prestressing size. Due to the ground stress, the effective stress of the pile model at the bottom is significantly greater than that at the top.

[0100] Step 6: Set the load loading mode and stop loading conditions of the prestressed hollow square pile model.

[0101] In this embodiment, the loading point is set on the ground. In order to meet the actual working conditions, a vertical load is first applied to the top surface of the test pile during loading. The size of the vertical load is the design load of the pile body, 550kN. The pile is horizontally loaded under the top pressure of this load. In order to avoid stress concentration and stress singularity, this simulation adopts a coupled loading method. There are two loading points, one is RP-1 representing the upper load of the pile, and the other is RP-2 representing the lateral load. The two have a spatial relationship with the top of the pile.

[0102] At the same time, the load loading and unloading mode is set to low-cycle multi-cycle loading and unloading.

[0103] According to the estimated ultimate horizontal bearing capacity of the prestressed hollow square pile model, 1 / 10 of the ultimate horizontal bearing capacity is determined as the loading amount for each level, a total of nine loading levels, and the load value of each loading level is determined. In this embodiment, the ultimate horizontal bearing capacity of the prestressed hollow square pile model is determined to be 20kN.

[0104] For each loading level, after adjusting to the preset load application value, the load is applied for 4 minutes, and then the displacement of the top surface of the pile model is recorded and unloaded. After maintaining it in the no-load state for 2 minutes, it continues to be loaded to the load application value. After the cycle is repeated five times, the next loading program is entered to determine the horizontal displacement and residual displacement of the top surface of the pile model under each loading level.

[0105] In this embodiment, according to the on-site inspection regulations in the Technical Specifications for Building Foundation Pile Inspection (JGJ106-2016), when the pile model encounters any of the following conditions, the loading is stopped and the residual strain is recorded:

[0106] (1) Under constant load, the horizontal displacement increases sharply;

[0107] (2) Horizontal displacement exceeds 30 mm (40 mm for soft soil);

[0108] (3) The pile body is broken.

[0109] Under this premise, the displacement of the top surface of the pile model is controlled to monitor the top surface reaction force of the pile model. The reaction force of the loading point when the pile top displacement is 30 mm is set as the ultimate load of the pile model.

[0110] In this embodiment, the above-mentioned load loading and unloading method and loading stop condition are used to apply load to the prestressed hollow square pile model, obtain the axial displacement of the pile body model in the prestressed hollow square pile model, and obtain the pile body displacement-load-depth curve and load-bending moment-depth curve of the pile body model.

[0111] Step 7, using the prestressed hollow square pile model to conduct a soil mechanical response test to analyze the influence of soil conditions on pile displacement, specifically including the following sub-steps:

[0112] Step 7.1, use the prestressed hollow square pile model to conduct soil mechanical response test, change the internal friction angle of the soil model, simulate the top surface displacement of the pile model under different soil model internal friction angle conditions under various load conditions, and obtain the relationship curve between the internal friction angle and the pile top displacement under various load conditions, as shown in Fig. 9 shown.

[0113] In this embodiment, the internal friction angles of the soil model around the pile model are set to 14°, 16°, 18° and 20° respectively, and the prestressed hollow square pile model is used for simulation. The simulation results are as follows: Fig. 9 As shown, analysis Fig. 9 It can be obtained that when the pile model is subjected to a small horizontal load, the change in the internal friction angle of the soil model around the pile model will not affect the horizontal displacement of the pile model. When the load increases to 128 kN, the pile model begins to show obvious differences. Under this level of load, the maximum displacement and the minimum displacement of the pile model differ by 0.45 mm, accounting for 3.22% of the maximum displacement. As the load of the pile model increases, the gap further expands.

[0114] Further analysis was conducted to obtain the displacement percentage-internal friction angle relationship curve. It was found that under the action of 160kN load, the internal friction angle of the soil model around the pile model directly affected the horizontal displacement of the top surface of the pile model, and its displacement level increased with the increase of the internal friction angle of the soil model around the pile model. When the internal friction angle of the soil model around the pile model increased from 14° to 20°, the top surface displacement of the pile model decreased by 1.66mm, accounting for 6.26% of the former displacement.

[0115] Step 7.2, using the prestressed hollow square pile model to conduct a soil mechanical response test, changing the Poisson's ratio of the soil model around the pile model, simulating the top surface displacement of the pile model under different soil model Poisson's ratios, and obtaining a curve of the top surface displacement of the pile model changing with the Poisson's ratio of the soil model.

[0116] In this embodiment, the Poisson's ratio of the soil model around the pile model is set to 0.2, 0.25, 0.275 and 0.3 respectively, and the prestressed hollow square pile model is used for simulation. The simulation results are as follows: Fig.10 As shown by Fig.10 It can be obtained that when the Poisson's ratio of the soil model around the pile model is different, the displacement trend of the top surface of the pile model changes with the load. It can be seen from the figure that when the top surface of the pile model is subjected to horizontal load, the various change curves are basically parallel to the x-axis. The difference in the soil model around the top surface of the pile model does not change the displacement trend of the pile model. It can be considered that the Poisson's ratio of the soil model around the top surface of the pile model has little effect on the horizontal bearing capacity of the pile.

[0117] Step 8, applying external load to the prestressed hollow square pile model, analyzing the influence of the external load on the displacement of the prestressed hollow square pile body, and determining the influence of downward pressure, upward pull, horizontal load direction and pile side angle on the displacement of the prestressed hollow square pile body, specifically including the following sub-steps:

[0118] Step 8.1, simulating and determining the influence of downward pressure on the displacement of the prestressed hollow square pile;

[0119] By changing the magnitude of the top pressure of the pile model in the prestressed hollow square pile model, the top surface displacement of the pile model under different top pressure loads is obtained using the prestressed hollow square pile model, such as Fig.11 shown.

[0120] Depend on Fig.11 It can be obtained that the horizontal displacement of the top of the test pile after applying 350kN vertical pressure on the pile top is reduced by 0.49mm compared with the Model-8 without top pressure, accounting for 1.83% of its total displacement. This is because the concrete inside the pile body produces compressive stress under the vertical load, which limits the cracking degree of the pile body concrete when subjected to the bending moment caused by the horizontal load to a certain extent, thereby affecting the elastic strain of the steel bar and limiting the development of the displacement of the pile top. For the piles with the same top load, the displacement of the pile top when subjected to horizontal thrust decreases with the increase of the vertical load level, but its value reaches 0.01mm, accounting for 0.2‰ of the total displacement, which has little effect on the overall pile. This is because when the pile body is subjected to vertical pressure, the vertical bearing capacity of the pile is close to the limit, and the pile body will have a large vertical displacement instead of causing compressive stress in the pile body.

[0121] Horizontal displacement of the pile body in depth under different top pressures, such as Fig.12 As shown by Fig.12 It can be seen that the magnitude of the vertical load on the pile top cannot change the deformation trend of the pile type. The deformation trends of the piles under different top pressures are consistent, and the number of displacement zero points is one. When the pile top load is 350kN, the part of the pile displacement curve above 3m is located on the far right of all curves, indicating that the positive displacement of the pile body under the horizontal thrust of 160kN is greater than that of other piles. The displacement curves of the piles below 3m are basically coincident. Therefore, changing the top pressure will only limit the cracks on the upper side of the pile body caused by tensile stress, and cannot affect the horizontal displacement of the lower side of the pile.

[0122] Step 8.2, simulating and determining the influence of downward pressure on the displacement of the prestressed hollow square pile;

[0123] The pull-out force of the pile model in the prestressed hollow square pile model is set to 200kN, 300kN and 400kN respectively. The top surface displacement of the pile model under different pull-out forces is obtained by using the prestressed hollow square pile model, as shown in Fig.13 shown.

[0124] Depend on Fig.13 It can be seen that in the early stage of pile loading, the displacements of the pile tops are almost the same, and the uplift load on the pile top has almost zero effect on the pile displacement. In the later stage of loading, the piles begin to have displacement differences, but the difference is smaller in the later stage. The main reason for this phenomenon is that the uplift effect on the pile limits the development of the maximum bending moment of the pile body, thereby reducing the displacement of the pile top. However, the pile top floats under the uplift effect, and the free end of the pile top is longer. The balance between the two causes the increase in the pile top displacement to be insignificant.

[0125] Step 8.3, simulate and determine the influence of the horizontal load angle on the displacement of the prestressed hollow square pile;

[0126] The horizontal load angles of the pile model in the prestressed hollow square pile model are set to 9°, 18°, 27°, 36° and 45° in turn. The top surface displacement of the pile model under different horizontal load angles is obtained using the prestressed hollow square pile model. Fig.14 shown.

[0127] Depend on Fig.14 It can be seen that as the angle between the horizontal loading direction and the length of the pile increases, the displacement along the loading direction shows a trend of first decreasing and then increasing. The maximum displacement is 26.28 mm, which occurs in the model with an orthogonal angle of 0° to the pile. When the angle increases to 9°, the displacement of the pile top begins to decrease significantly to 24.99 mm, accounting for 95.10% of the maximum displacement. When the angle increases to 18°, the displacement of the pile top further decreases to 23.83 mm, accounting for 90.70% of the maximum displacement. The reason for this phenomenon is that when the pile loading direction is orthogonal to the pile edge, the pile projection area along the load direction is the smallest. When the loading direction forms a certain angle with the pile, its projection area also increases. The increase in the pile-soil contact surface leads to a decrease in the average compressive stress of the soil, and the corresponding horizontal thrust is also increased accordingly, which can provide better support for the pile.

[0128] At the same time, as the angle increases, its effect on the displacement of the pile top is also decreasing. At 27°, this effect only reduces the maximum displacement by 2.25%, which is half of the first two levels. When the angle reaches 27°, its improvement effect on the displacement of the pile top is negligible. When it increases from 27° to 36°, the displacement only decreases by 0.03mm, which is 0.13% of the displacement at 27°; and when the angle increases from 36° to 45°, the displacement of the pile top begins to increase abnormally, and its displacement increases by 0.48mm compared to the angle of 36°. The essential reason why the displacement decrease effect decreases or even increases in the second half of the angle expansion is that as the angle gradually increases, the contact angle between the pile body and the soil will also decrease. The soil transmits less normal stress at the corners of the pile, but instead squeezes the soil to both sides. As the soil contact angle increases, the soil squeezing effect becomes gradually obvious. When the angle exceeds 27°, the soil squeezing effect and the effect of increased contact surface offset each other, resulting in basically unchanged soil displacement. When the angle increases from 36° to 45°, the effect of this effect is even greater than the weakening effect of the increase in its lateral area on the pile displacement, which manifests as the displacement starting to rise in this range.

[0129] Furthermore, analysis of the displacement of the pile body along the depth direction at different loading angles under the same load conditions shows that the deformation of the pile body along the depth direction at each loading angle is roughly the same, all concentrated in the upper half of the pile, and has little effect on the lower side of the pile. At each loading angle, the pile body will have a negative displacement, and the displacement will begin to decrease after quickly reaching the maximum, and finally there will be no second zero point at the bottom of the pile. That is, at different loading angles, the failure mode of the prestressed hollow square pile is consistent with that of the elastic pile with a free top.

[0130] Step 9, comprehensively considering the influence of soil conditions on the displacement of the pile model and the influence of external loads on the displacement of the prestressed hollow square pile body, and obtaining the horizontal bearing capacity of the prestressed hollow square pile.

[0131] In this embodiment, the influence of soil conditions on the displacement of the pile model and the influence of external loads on the displacement of the prestressed hollow square pile are comprehensively considered to obtain the horizontal bearing capacity of the prestressed hollow square pile through qualitative analysis. The analysis results show that:

[0132] (1) The soil above the pile has a significant impact on the horizontal displacement of the pile. Increasing the strength of the upper soil layer can effectively reduce the displacement of the pile. If possible, the strength of the upper soil layer around the pile under the ultimate horizontal bearing capacity should be verified to determine the pile parameters. If the soil does not yield under this load, the pile bearing capacity can be effectively improved.

[0133] (2) The soil below the pile will also affect the displacement of the pile top, but it mainly affects the displacement of the upper part and has little effect on the lower part. The increase in the internal friction angle will increase the horizontal bearing capacity of the pile body, but its initial impact on the pile body is small. The increase in the horizontal bearing capacity of the pile by Poisson's ratio can be ignored.

[0134] (3) When the pile is subjected to vertical load, the horizontal bearing capacity of the pile can be improved to varying degrees because the development of the bending moment of the pile body can be effectively limited. The vertical bearing capacity of the pile body can be improved more effectively under downward pressure. When the pile body is in the upward direction of the vertical load, the free end length of the pile top increases, resulting in a smaller increase in bearing capacity.

[0135] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for analyzing the horizontal bearing performance of prestressed hollow square piles, characterized in that: The specific steps include: Step 1, based on the soil test data at the construction site and the design scheme of the prestressed hollow square pile, a prestressed hollow square pile model including a soil model and a pile model is constructed based on finite element software; Step 2, setting the contact mode and contact properties between the soil model and the pile model in the prestressed hollow square pile model; Step 3, performing three-dimensional space meshing on the prestressed hollow square pile model to obtain a meshed prestressed hollow square pile model; Step 4, performing ground stress balance on the prestressed hollow square pile model, and analyzing the ground stress distribution and displacement of the soil model in the prestressed hollow square pile model before and after the ground stress balance; Step 5, applying prestress to the prestressed hollow square pile model based on a cooling method; Step 6, setting the load loading and unloading mode and the loading stop condition of the prestressed hollow square pile model; Step 7, conducting a soil mechanical response test on the prestressed hollow square pile model, analyzing the influence of soil conditions on the displacement of the pile model, obtaining a curve of the relationship between the internal friction angle and the displacement of the top surface of the pile model and a curve of the displacement of the top surface of the pile model changing with the Poisson's ratio, and determining the influence of the internal friction angle and the Poisson's ratio of the soil around the top surface of the pile on the horizontal displacement of the prestressed hollow square pile; Step 8, applying external load to the prestressed hollow square pile model, analyzing the influence of the external load on the displacement of the prestressed hollow square pile body, and determining the influence of downward pressure, upward pull, horizontal load direction and pile side angle on the displacement of the prestressed hollow square pile body; Step 9, comprehensively considering the influence of soil conditions on the displacement of the pile model and the influence of external loads on the displacement of the prestressed hollow square pile body, and obtaining the horizontal bearing capacity of the prestressed hollow square pile.

2. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 1, characterized in that: The soil model is constructed based on soil test data at the construction site, wherein the soil test data includes soil layer numbers, layer bottom burial depths, soil types, densities, Young's modulus, Poisson's ratio, friction angles, and cohesion of all soil layers; A pile body model is constructed according to a design scheme of a prestressed hollow square pile, wherein the pile body model includes a concrete model and a steel bar model, and the design scheme includes the pile body size, hollow size, material, elastic modulus, Poisson's ratio of the prestressed hollow square pile, and material, elastic model, Poisson's ratio, and layout position of the concrete and the steel bar.

3. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 2, characterized in that: The soil model is the Mohr-Coulomb elastoplastic model. The relationship between the internal friction angle, soil cohesion and shear stress in the soil model is: In the formula, is the shear stress at any cross section; is the friction angle of soil; c is the cohesion of soil; The stress-strain relationship of the soil model is: Where ψ is the expansion angle; ε is the shape parameter of the plastic potential surface; S is the shape of the plastic potential surface; R mw is the plastic potential surface control parameter; q is the spherical stress, which is used to reflect the size of the confining pressure; p is the shear stress, which is used to reflect the shear effect; The steel bar model is set as a double oblique line model.

4. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 1, characterized in that: In the prestressed hollow square pile model, the pile model is buried in the soil model, the top surface of the pile model is higher than the top surface of the soil model, the four side surfaces and one bottom surface of the pile model are in contact with the soil model, and the contact mode is set to face-to-face contact; The contact attributes include normal contact attributes and tangential contact attributes. The normal contact attributes are set to hard contact, and the tangential contact attributes are set to penalty functions.

5. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 1, characterized in that: In the step 3, the prestressed hollow square pile model is meshed in the horizontal direction, including meshing of a soil model, a pile model and a steel bar model, to obtain a prestressed hollow square pile model cell, specifically including a soil model cell, a pile model cell and a steel bar model cell; First, mesh the soil model of the prestressed hollow square pile model; Divide the soil model of the prestressed hollow square pile model in the horizontal direction, perform grid division along the horizontal orthogonal directions of the sides of the soil model and the pile model, and divide the soil model 0.1m away from the side of the pile model into cells of size 0.1m×0.1m in combination with the distance between the soil model and the side of the pile model, and divide the soil model 5 times the side length of the pile model into cells of size 0.2m×0.2m, and divide the rest of the soil model into cells of size 0.4m×0.4m, and then divide the soil model of the prestressed hollow square pile model in the vertical direction. According to the length of the pile model, divide the soil model within the length of the pile model into cells of size 0.5m in the vertical direction, and divide the rest of the soil model into cells of size 1.0m in the vertical direction, to obtain a plurality of soil model cells; Then divide the pile body model of the prestressed hollow square pile model; After the pile model is evenly divided into eight equal parts along the symmetry axis and two diagonals of the pile model in the horizontal direction, each side length of the pile model is evenly divided into ten equal parts, and then evenly divided along the axial direction of the pile model according to a length of 0.125 m to obtain multiple pile model cells.

6. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 1, characterized in that: In step 4, based on the ODB analysis method, the soil model in the prestressed hollow square pile model is first used as an independent model to perform self-balancing under gravity load. After obtaining the initial stress and initial displacement of the soil model, the soil model in the hollow structure of the prestressed hollow square pile model is removed, and a static analysis is performed on the soil model to obtain the stress distribution of the pile model. Then, the stress of the pile model is used as a predefined field to apply a gravity load to the pile model of the prestressed hollow square pile model, so that the prestressed hollow square pile model restores the ground stress balance, analyzes the stress distribution of the soil model before and after the ground stress balance of the prestressed hollow square pile model, and determines the displacement of the soil model in the prestressed hollow square pile model.

7. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 1, characterized in that: The minimum temperature of the steel bar model in the prestressed hollow square pile model is preset as shown in formula (3): In the formula, E is the elastic modulus of concrete, a is the expansion coefficient of the prestressed steel bar, and σ is the prestress that needs to be applied to the pile body; The cooling method is used to apply stress to the steel bar model by cooling the steel bar model to a preset minimum temperature.

8. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 1, characterized in that: Loading is performed based on the coupled loading method. First, a vertical load is applied to the top surface of the pile model of the prestressed hollow square pile model according to the preset top pressure value. Then, a horizontal load is applied to the pile model of the prestressed hollow square pile model under the action of the top pressure value of the pile model. The load loading and unloading method is low-cycle multi-cycle loading and unloading; Estimate the ultimate horizontal bearing capacity of the prestressed hollow square pile model, and use 1 / 10 of the ultimate horizontal bearing capacity as the loading amount for each level to load each level, and obtain the load value for each loading level; For each level of loading, after adjusting to the preset load application value, the load application is maintained for a specified time, the displacement of the top surface of the pile model is recorded and then unloaded. After maintaining the no-load state for two minutes, the load is continued to be loaded to the load application value. After the cycle is repeated many times, the next level of loading program is entered to determine the horizontal displacement and residual displacement of the top surface of the pile model under each level of loading; The loading stop conditions are that the horizontal displacement increase of the pile model under constant load exceeds a preset value, the horizontal displacement of the pile model exceeds 30 mm, and the pile model breaks. After stopping the load loading, the residual strain of the pile model is recorded to obtain the pile displacement-load-depth curve and load-bending moment-depth curve of the pile model.

9. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 1, characterized in that: The step 7 includes the following sub-steps: Step 7.1, using a prestressed hollow square pile model to conduct a soil mechanical response test, changing the internal friction angle of the soil model, simulating the top surface displacement of the pile model under different soil model internal friction angle conditions under various load conditions, and obtaining the relationship curve between the internal friction angle and the pile top displacement under various load conditions; Step 7.2, using the prestressed hollow square pile model to conduct a soil mechanical response test, changing the Poisson's ratio of the soil model around the pile model, simulating the top surface displacement of the pile model under different soil model Poisson's ratios, and obtaining a curve of the top surface displacement of the pile model changing with the Poisson's ratio of the soil model.

10. The method for analyzing the horizontal bearing performance of prestressed hollow square piles according to claim 1, characterized in that: The step 8 includes the following sub-steps: Step 8.1, changing the magnitude of the top pressure force of the pile body model in the prestressed hollow square pile model, using the prestressed hollow square pile model to obtain the top surface displacement of the pile body model under each top downward pressure load, and determining the influence of the downward pressure on the displacement of the prestressed hollow square pile body; Step 8.2, changing the pull-out force of the pile body model in the prestressed hollow square pile model, using the prestressed hollow square pile model to simulate the displacement of the top surface of the pile body model under each pull-out force, and determining the influence of the downward pressure on the displacement of the pile body of the prestressed hollow square pile; Step 8.3, changing the horizontal load angle of the pile body model in the prestressed hollow square pile model, using the prestressed hollow square pile model to simulate the displacement of the top surface of the pile body model under various horizontal load angles, and determining the influence of the horizontal load angle on the displacement of the prestressed hollow square pile body.

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