A numerical simulation-based analysis method for the uplift bearing capacity of PHC spiral pipe piles

Abstract

The application discloses a kind of based on numerical simulation's PHC spiral pipe pile uplift bearing capacity analysis method;By adopting the intelligent rotary digging equipment equipped with special drill bit, the pile body is vertically rotated into the design elevation, and the penetration speed, rotation speed, torque and perpendicularity are monitored in real time;Record the penetration resistance-depth curve, torque-depth curve and pile body perpendicularity data;A three-dimensional refined numerical model containing real spiral blades is established, the key parameters of the soil are calibrated through parameter inversion algorithm, and the calibrated numerical model is obtained;Simulate the uplift load on the calibrated numerical model, analyze the ultimate uplift bearing capacity and failure mode;Based on the simulation results of multiple working conditions, the practical calculation formula of the ultimate uplift bearing capacity is obtained through regression analysis;Through construction data driven model inversion, combined with refined numerical simulation, the prediction accuracy of the uplift bearing capacity is improved, and reliable basis is provided for the uplift design of PHC spiral pipe pile.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical engineering pile foundation technology, and specifically discloses a method for analyzing the pull-out bearing capacity of PHC spiral pipe piles based on numerical simulation. Background Technology

[0002] PHC (prestressed high-strength concrete) spiral pipe piles have been widely used in various buildings, bridges, wind power foundations and other projects for compressive and tensile bearing capacity due to their advantages such as high pile strength, stable quality of factory production and high construction efficiency. The spiral blades on the outer wall of the pile can significantly increase the pile-soil contact area and effectively improve the tensile bearing capacity through mechanical interlocking with the surrounding soil. They are especially suitable for engineering scenarios that bear large uplift loads.

[0003] However, the analysis and design of the pull-out bearing capacity of PHC spiral pipe piles still face many challenges. Traditional analysis methods mainly rely on the following approaches:

[0004] Empirical formula method: This method estimates based on classical soil mechanics theory or regional empirical formulas. Although this method is simple, it often fails to adequately consider the unique reinforcement and anchoring effects of the helical blade. Furthermore, the values ​​of key parameters in the formula depend on the soil type and state, resulting in significant dispersion and regional limitations. It is difficult to accurately reflect the true interaction mechanism between the helical blade and the soil under complex geological conditions, leading to generally low prediction accuracy and conservative or unsafe designs.

[0005] The field static load test method directly determines the ultimate bearing capacity by conducting static pull-out tests on site. The results are the most reliable. However, this method is extremely expensive, has a long cycle, and has obvious point specificity. That is, the test results only represent the soil conditions at the test point. For large-scale projects or situations where the soil conditions vary greatly, it is difficult to represent the entire site with a limited number of tests. Therefore, it has poor economy and universality.

[0006] Conventional numerical simulation methods: With the development of computer technology, numerical methods such as the finite element method have been used to analyze the bearing capacity of pile foundations. However, existing simulations often involve significant simplifications, such as equating helical pipe piles to smooth straight-walled piles or using simplified equivalent models. This fails to accurately reflect the soil agitation, compaction effects, and soil arching effects between the helical blades during pull-out. Furthermore, the soil constitutive models used in conventional simulations are insufficient to accurately describe the nonlinear behaviors of soil under complex stress paths, such as dilatation and softening, leading to distortions in the simulation of pile-soil interaction and failure modes. More importantly, soil parameters in numerical models are usually directly derived from laboratory geotechnical tests, failing to consider the significant impact of actual construction techniques on the stress state and mechanical properties of the soil around the pile. This results in a disconnect between model parameters and the actual soil conditions on site, leading to significant discrepancies between the calculated results and actual conditions.

[0007] Therefore, it is necessary to invent a numerical simulation-based method for analyzing the pull-out bearing capacity of PHC spiral pipe piles to solve the above problems. Summary of the Invention

[0008] To overcome the aforementioned deficiencies in the prior art, this invention provides a numerical simulation-based method for analyzing the pull-out bearing capacity of PHC helical pipe piles. This method employs an intelligent rotary drilling rig equipped with a dedicated drill bit to vertically drive the pile to the design elevation, while real-time monitoring of the driving speed, rotation speed, torque, and verticality. The method records the driving resistance-depth curve, torque-depth curve, and pile verticality data. A refined three-dimensional numerical model incorporating real helical blades is established, and key soil parameters are calibrated using a parameter inversion algorithm to obtain the calibrated numerical model. Pull-out loads are simulated on the calibrated numerical model to analyze the ultimate pull-out bearing capacity and failure mode. Based on the simulation results under multiple working conditions, a practical calculation formula for the ultimate pull-out bearing capacity is derived through regression analysis, effectively solving the problems mentioned in the background technology.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a method for analyzing the pull-out bearing capacity of PHC spiral pipe piles based on numerical simulation, specifically including the following steps:

[0010] 1. A method for analyzing the pull-out bearing capacity of PHC spiral pipe piles based on numerical simulation, characterized by comprising the following steps:

[0011] S1. Construction Control Stage: Intelligent rotary drilling equipment with a two-way communication interface is used for the driving of PHC spiral pipe piles. The equipment is equipped with a special drill bit suitable for PHC spiral pipe piles and monitors and controls the pressing speed, rotation speed and torque in real time during construction. The ratio of pressing speed to rotation speed is controlled to match the advance of the pile body in one rotation with the pitch of the spiral blades, ensuring that the pile body is driven vertically and uniformly to the design elevation.

[0012] S2, Data Acquisition and Transmission Stage: Real-time acquisition and recording of the indentation resistance-depth curve, torque-depth curve, and pile verticality data formed in step S1;

[0013] S3. Model Construction and Inversion Stage: Establish a three-dimensional refined numerical model of the PHC helical pipe pile and surrounding soil containing the actual helical blade geometry. The soil adopts an advanced constitutive model that can reflect the shear dilatation and softening characteristics. Using the indentation resistance-depth curve and torque-depth curve obtained in S2 as targets, the key soil parameters in the three-dimensional refined numerical model are automatically calibrated through the parameter inversion algorithm to obtain a calibrated numerical model consistent with the actual working conditions on site.

[0014] S4. Bearing characteristic analysis stage: Simulate the tensile load on the calibrated numerical model, extract the load-displacement curve and soil stress-strain cloud map, and analyze its tensile ultimate bearing capacity and failure mode.

[0015] S5. Formula Derivation and Verification Optimization Stage: Based on the simulation results obtained in S4 under different working conditions, combined with the earth pressure balance theory, regression analysis is used to derive a practical calculation formula for the ultimate tensile bearing capacity of PHC spiral pipe piles; and its applicability and accuracy are verified.

[0016] The failure mode analysis method is as follows: the failure mode is identified by the soil stress-strain cloud map, and the failure mode includes soil shear, pile-soil interface slippage and soil loosening between the helical blades;

[0017] The simulation results under different working conditions include: pile body size parameters with different thread spacing, thread width, and thread inclination angle, as well as the ultimate tensile bearing capacity values ​​under different soil parameters such as cohesion, internal friction angle, and Poisson's ratio;

[0018] The practical calculation formula for the ultimate pull-out bearing capacity of the PHC spiral pipe pile is: an empirical formula obtained through multiple regression analysis with the soil internal friction angle, cohesion and shear dilatation angle as variables.

[0019] Preferably, the special drill bit is a sleeve-type drill bit that has a guiding function and can be reliably connected to the end of the PHC spiral pipe pile. The near-synchronous drilling refers to controlling the ratio of the pressing speed to the rotation speed so that the advance of the pile body in one revolution matches the pitch of the spiral blade.

[0020] Preferably, the pile verticality data includes: the real-time measured pile tilt angle, vertical deviation value, and offset direction.

[0021] Preferably, the advanced constitutive model is a hardened soil model, and the key soil parameters include the internal friction angle, cohesion, and dilatation angle.

[0022] Preferably, the analysis method for the ultimate tensile bearing capacity includes: extracting the ultimate load point from the load-displacement curve, and determining the ultimate tensile bearing capacity using the inflection point method or the displacement control method.

[0023] The technical effects and advantages of this invention are as follows:

[0024] 1. By collecting data on pressing resistance and torque curves in real time through intelligent construction equipment, and using this data to calibrate the numerical model, the model can truly reflect the actual soil conditions and construction impact of a specific site, greatly improving the accuracy of tensile bearing capacity prediction.

[0025] 2. By adopting an advanced constitutive model that reflects the shear dilatation and softening characteristics of soil and a three-dimensional refined model containing real helical blades, the pile-soil interaction can be simulated more accurately, and the pull-out failure mechanism can be clearly revealed.

[0026] 3. This method forms a complete closed loop from "intelligent construction → data acquisition → model inversion → simulation analysis → formula derivation". The derived practical calculation formulas are rooted in actual engineering data and have stronger universality.

[0027] 4. By emphasizing drilling and verticality control, and considering their impact from an analytical perspective, it helps to solve the problem of load-bearing capacity reduction caused by improper construction from both technical and quality control perspectives. Attached Figure Description

[0028] The present invention will be further described with reference to the accompanying drawings, but the embodiments in the drawings do not constitute any limitation on the present invention. For those skilled in the art, other drawings can be obtained based on the following drawings without creative effort.

[0029] Figure 1 This is a flowchart illustrating the overall steps of the present invention.

[0030] Figure 2 This is a flowchart of the parameter inversion steps of the present invention. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] This invention provides, for example Figure 1 The method shown is a numerical simulation-based analysis method for the pull-out bearing capacity of PHC helical pipe piles, such as... Figure 1 As shown, the specific steps include the following:

[0033] S1. Construction Control Stage: Intelligent rotary drilling equipment with a two-way communication interface is used for the driving of PHC spiral pipe piles. The equipment is equipped with a special drill bit suitable for PHC spiral pipe piles and monitors and controls the pressing speed, rotation speed and torque in real time during construction. The ratio of pressing speed to rotation speed is controlled to match the advance of the pile body in one rotation with the pitch of the spiral blades, ensuring that the pile body is driven vertically and uniformly to the design elevation.

[0034] Furthermore, in the above technical solution, the special drill bit is a sleeve-type drill bit that has a guiding function and can be reliably connected to the end of the PHC spiral pipe pile.

[0035] In a preferred embodiment of the present invention, the specific implementation of the construction control phase is as follows:

[0036] S101. Composition of Intelligent Rotary Drilling Equipment System:

[0037] The main unit uses a fully hydraulic rotary drilling rig with an electro-hydraulic proportional control system as the primary equipment. This main unit should possess strong torque and injection force, and its hydraulic system should be able to respond to precise electronic control commands. The fully hydraulic rotary drilling rig is equipped with an integrated intelligent control platform, which includes:

[0038] Programmable Logic Controller (PLC): As the control core, it receives sensor data and outputs control commands;

[0039] Human-Machine Interface (HMI): Used to set construction parameters, such as design depth, target rotation speed, target pressing speed, and display real-time data curves and equipment status;

[0040] Two-way communication interface: Adopting industrial Ethernet or CAN bus protocol, it realizes high-speed and reliable data exchange between PLC and sensors and actuators.

[0041] Sensor system:

[0042] Torque sensor: Installed at the drill rod power head, it monitors the torque value applied to the pile body in real time.

[0043] Pressure sensor: Installed in the hydraulic circuit of the main winch or pressurizing cylinder to monitor the pressing resistance in real time and convert it into pressing force.

[0044] Displacement sensor: Using a wire-type or GPS / BeiDou positioning system, it monitors the sinking displacement of the drill rod or pile head in real time, thereby calculating the pressing speed.

[0045] Rotary encoder: Installed on the power head motor, it monitors the rotation speed of the drill pipe in real time.

[0046] Inclination sensor: A high-precision inclination sensor is directly installed on the drill rod near the power source to monitor the inclination angle of the pile in two directions that are 90 degrees apart in real time.

[0047] Actuators: The hydraulic pump and proportional valve of the main unit receive instructions from the PLC and precisely adjust the rotation speed of the power head and the pressing speed of the pressurization system.

[0048] S102. Special drill bit connection and guidance:

[0049] Drill bit design: A sleeve-type drill bit is adopted, with a groove or threaded interface on its inner wall that matches the prestressed steel bar at the end of the PHC spiral pipe pile, ensuring reliable connection and synchronous rotation between the drill bit and the pile body.

[0050] Guiding function: The drill bit is designed with a guide tip at the front end, the diameter of which is slightly larger than the outer diameter of the pile body. It can cut into the soil in the early stage of drilling to form a guide hole, effectively restraining the initial deviation of the pile body.

[0051] S103. Implementation of drilling control logic:

[0052] Core parameter setting: Preset a key process parameter "infeed-speed ratio" on the HMI, which is the ratio of pressing speed to rotation speed. The theoretical benchmark value of this ratio is: for every 360° rotation of the pile body, its pressing depth should be equal to the pitch of the helical blade, that is, pressing speed ≈ rotation speed × pitch.

[0053] Closed-loop control process:

[0054] The PLC obtains the current pressing speed Va and rotation speed Na in real time through sensors.

[0055] The PLC calculates the actual ratio Ra=Va / Na and compares it with the preset "feed-speed ratio".

[0056] Feedback adjustment:

[0057] If Ra > Rt, it indicates that the advance is too fast, which may cause excessive compression or shearing of the soil between the threads. The PLC will fine-tune the proportional valve to slightly reduce the pressing speed or slightly increase the rotation speed.

[0058] If Ra < Rt, it indicates that the advance is too slow, which may cause excessive grinding and disturbance of the soil around the pile. The PLC will fine-tune the proportional valve to slightly increase the pressing speed or slightly decrease the rotation speed.

[0059] Verticality compensation: The tilt sensor feeds back the pile tilt data to the PLC in real time. Once the tilt angle exceeds the preset tilt angle threshold, such as 0.5%, the PLC will start the correction program. For example, if it tilts in the X direction, the PLC will apply a reverse correction force in the Y direction by controlling the fine adjustment cylinder of the drilling rig chassis or mast, and at the same time briefly adjust the rotation or pressing parameters on the corresponding side to make the pile return to a vertical state.

[0060] S2, Data Acquisition and Transmission Stage: Real-time acquisition and recording of the indentation resistance-depth curve, torque-depth curve, and pile verticality data formed in step S1;

[0061] Furthermore, in the above technical solution, the pile verticality data includes: the real-time measured pile tilt angle, vertical deviation value, and offset direction.

[0062] Furthermore, the core objective of this stage is to collect and record key data generated during construction in real time, including the indentation resistance-depth curve, torque-depth curve, and pile verticality data, to provide accurate and reliable input for subsequent numerical model inversion and calibration. The specific implementation method is as follows:

[0063] S201. Data Acquisition Content and Sensor Configuration:

[0064] Indentation resistance-depth curve:

[0065] Use pressure sensors installed in the hydraulic circuit of the main winch or pressurizing cylinder of the intelligent rotary drilling equipment to monitor the pressing resistance in real time and convert it into pressing force.

[0066] Combined with displacement sensors, such as wire encoders or GPS / BeiDou positioning systems, the sinking displacement of the drill rod or pile head is monitored, and the pressing speed is calculated.

[0067] By linking the indentation resistance and depth information, a continuous indentation resistance-depth curve is formed.

[0068] Torque-depth curve:

[0069] A torque sensor installed at the drill pipe power head is used to monitor the torque applied to the pile body in real time.

[0070] The rotational speed of the drill pipe is monitored by a rotary encoder and synchronized with the depth data;

[0071] Integrate torque and depth data to generate a torque-depth curve.

[0072] Pile verticality data: Using a dual-axis tilt sensor, directly mounted on the drill rod near the power head, the tilt angle of the pile in two directions that are 90 degrees apart is monitored in real time, and the vertical deviation value and offset direction are calculated to form a verticality dataset.

[0073] S202. Data Transmission and Processing:

[0074] Sensor data is transmitted in real time to the PLC of the intelligent control platform via a two-way communication interface. The PLC performs preliminary processing and quality checks on the data to ensure its continuity and accuracy. The data is also uploaded to the HMI, where the pressing resistance, torque, and verticality curves are displayed in a graphical manner in real time for operators to monitor.

[0075] S203. Data Recording and Storage:

[0076] The collected data, including timestamps, depth, indentation resistance, torque, and verticality datasets, are recorded in a local or cloud storage database. The data is saved in a standard format, such as JSON, for easy export for subsequent numerical model inversion.

[0077] S204. Data Quality Control:

[0078] Implement real-time data verification, such as by comparing the consistency of readings from multiple sensors to eliminate outliers;

[0079] A key data-triggered alarm mechanism is adopted to ensure controllable construction quality. For example, if the verticality data exceeds the preset tilt angle threshold, an early warning will be issued and a correction procedure will be triggered.

[0080] S3. Model Construction and Inversion Stage: Establish a three-dimensional refined numerical model of the PHC helical pipe pile and surrounding soil containing the actual helical blade geometry. The soil adopts an advanced constitutive model that can reflect the shear dilatation and softening characteristics. Using the indentation resistance-depth curve and torque-depth curve obtained in S2 as targets, the key soil parameters in the three-dimensional refined numerical model are automatically calibrated through the parameter inversion algorithm to obtain a calibrated numerical model consistent with the actual working conditions on site.

[0081] Furthermore, in the above technical solution, the advanced constitutive model is a hardened soil model, and the key soil parameters include the internal friction angle, cohesion, and dilatation angle.

[0082] According to the technical solution in the document, the core objective of step S3 is to establish a high-precision pile-soil interaction model using numerical simulation technology, and to perform parameter inversion calibration using construction data collected on-site to ensure that the model is consistent with the actual working conditions. The specific implementation steps are as follows:

[0083] S301. Construction of a refined three-dimensional numerical model:

[0084] Geometric modeling:

[0085] Use professional finite element software, such as ABAQUS, to create a three-dimensional model of the PHC helical pipe pile and the surrounding soil. The pile model needs to accurately reproduce the geometric features of the real helical blades, including parameters such as thread pitch, thread width, and thread inclination angle, to avoid simplification. The soil model should cover the influence range of the pile body. Usually, 5 to 10 times the pile diameter is taken as the lateral boundary dimension, and 3 to 5 times the pile length below the pile end is taken as the vertical boundary dimension to reduce boundary effects.

[0086] Grid generation:

[0087] A denser grid is used near the helical blades and in the pile-soil interface area. The structured grid with a size of 1 / 20 to 1 / 10 of the pile diameter is used. For example, a 600mm pile diameter corresponds to a 30 to 60mm grid to ensure accurate capture of stress concentration and deformation behavior.

[0088] The soil area uses a gradient grid, which gradually thins out as it moves away from the pile, in order to balance calculation efficiency and accuracy.

[0089] Constitutive model selection:

[0090] The soil is modeled using advanced constitutive models, such as the Hardening Soil Model, which can effectively simulate soil dilatation, softening, and stress path dependence.

[0091] The pile body is considered a linear elastic material, and its elastic modulus, Poisson's ratio, and other parameters are set according to the PHC pipe pile standard. For example, for C80 concrete, the elastic modulus E... p =3.8×10 4 MPa, v p =Poisson's ratio 0.2.

[0092] S302. Initial Parameter Settings

[0093] Initial values ​​of soil parameters:

[0094] Based on empirical values ​​and site survey reports, initial soil parameters are set, including internal friction angle φ, cohesion c, dilatation angle ψ, and tangential elastic modulus E. 50 Parameters such as Poisson's ratio (ν) will serve as the starting point for the inversion calculation. For example, for cohesive soil, the initial values ​​can be c=15kPa, φ=20°, ψ=0°, and v=0.3; for sandy soil, the initial values ​​can be c=0, φ=30°, ψ=10°, and v=0.25. The elastic modulus is set according to the soil type, such as E for silty clay. 50 =10~20MPa, medium sand E 50 =30~50MPa.

[0095] Boundary conditions and contact definitions:

[0096] The model has fixed constraints at the bottom and normal constraints on the sides to simulate semi-infinite soil.

[0097] The pile-soil interface adopts a surface-to-surface contact or frictional contact model. The normal behavior is set to "hard contact", and the tangential behavior adopts the Coulomb friction model. The friction coefficient is initially set according to the soil type, with a value range of [0.3, 0.5]. It is adjusted according to the soil properties, with a larger value for sandy soil and a smaller value for cohesive soil.

[0098] S303. Parameter Inversion Calibration:

[0099] like Figure 2 As shown, the parameter inversion process is as follows:

[0100] The indentation resistance-depth curve and torque-depth curve collected in stage S2 were imported into the inversion platform as calibration targets, and the soil parameters were automatically adjusted using a genetic algorithm.

[0101] The inversion process uses iterative calculations to minimize the error between the pressure resistance curve and torque curve obtained from the numerical simulation and the field curve;

[0102] Furthermore, the iterative calculation process is as follows:

[0103] The algorithm randomly generates a set of soil parameters, inputs them into a three-dimensional refined numerical model, calculates the simulated indentation resistance-depth curve and torque-depth curve, and calculates the root mean square error (RMSE) between the simulated curve and the field curve. , y in the formula si Let y be the simulated value of the i-th data point. fi Let be the field value of the i-th data point, and n be the number of data points. Using RMSE as the fitness function, the parameters are iteratively optimized through selection, crossover, and mutation operations of a genetic algorithm to reduce errors.

[0104] Furthermore, the genetic algorithm parameters are set as follows: population size: 100; crossover rate: 0.8; mutation rate: 0.05; termination condition: maximum number of iterations 100 or RMSE < 5%.

[0105] Calibration verification:

[0106] After each iteration, the fit between the simulated curve and the field curve is compared. When the error is lower than the preset fit threshold, such as when the error is lower than 5%, the iteration is stopped and the calibrated soil parameter set is output. If the number of iterations exceeds the preset upper limit, such as when it still does not converge after 100 iterations, the initial parameter values ​​or algorithm settings are adjusted and the inversion is performed again.

[0107] S304. Output of the calibrated numerical model

[0108] Save the calibrated model file for subsequent pull-out bearing capacity analysis.

[0109] S4. Bearing characteristic analysis stage: Simulate the tensile load on the calibrated numerical model, extract the load-displacement curve and soil stress-strain cloud map, and analyze its tensile ultimate bearing capacity and failure mode.

[0110] Furthermore, in the above technical solution, the analysis method of the ultimate tensile bearing capacity includes: extracting the ultimate load point from the load-displacement curve, and determining the ultimate tensile bearing capacity using the inflection point method or the displacement control method.

[0111] Furthermore, in the above technical solution, the failure mode analysis method is as follows: the failure mode is identified by the soil stress-strain cloud map, and the failure mode includes soil shear failure, pile-soil interface slippage and soil loosening between the helical blades.

[0112] Furthermore, this stage aims to simulate the response of PHC helical pipe piles under tensile loads using a calibrated numerical model, and to accurately analyze their ultimate tensile bearing capacity and failure mode. The specific implementation steps are as follows:

[0113] S401. Simulation of Pull-out Load:

[0114] On the calibrated numerical model, a monotonically increasing pull-out load is applied. The load is applied in a displacement-controlled manner to better capture the entire load-displacement curve. The load application point is located at the top of the pile to simulate the actual pull-out test conditions.

[0115] Furthermore, the displacement control method specifically involves applying vertical displacement in stages:

[0116] The elastic stage, where the estimated load has not reached 70% of the limit value: each displacement is 0.5mm, and the load is held for 0.1s after each loading to ensure that the soil stress is fully transferred;

[0117] In the plastic stage, when the load exceeds 70% of the limit value: each displacement is 0.2mm, and the step size is reduced to accurately capture the peak load.

[0118] S402. Data Extraction and Curve Generation:

[0119] During the simulation, the load value at the top of the pile and the corresponding displacement value at the top of the pile are extracted in real time to generate a load-displacement curve. At the same time, the stress field and strain field data of the soil area are extracted to generate a shear stress cloud map of the soil around the pile, a plastic strain cloud map of the soil, and a slip cloud map of the pile-soil interface, which are used to visualize and analyze the response and failure mode of the soil.

[0120] S403. Ultimate Pull-out Bearing Capacity Analysis:

[0121] Find the first derivative of the load-displacement curve and plot the slope-displacement curve. The point where the curve changes from approximately linear to a rapid decline is the inflection point. The tensile load corresponding to the inflection point is the ultimate tensile bearing capacity.

[0122] If the load-displacement curve has no obvious inflection point and the curve increases gradually, take the load corresponding to "pile top displacement = 4% pile diameter" as the ultimate tensile bearing capacity. For example, when D = 600mm, the load corresponding to a displacement of 24mm is:

[0123] Furthermore, the analysis needs to be combined with the convergence judgment of numerical simulation. If the calculation does not converge under a certain load, it usually indicates that the soil has failed. At this time, the load value of the previous load step can be regarded as the ultimate tensile bearing capacity.

[0124] S404. Failure Mode Analysis:

[0125] Failure modes are identified using soil stress-strain contour maps, with a focus on the following typical forms:

[0126] Soil shear failure:

[0127] Identifying features include the appearance of a "continuous high shear stress zone" in the shear stress cloud map, meaning the shear stress value is close to the shear strength of the soil, and the formation of a "plastic strain zone extending from the pile bottom to the ground surface" in the plastic strain cloud map, with a strain value >2%;

[0128] This phenomenon is most commonly seen in dense sandy soil and hard clay, where the soil around the pile is subjected to shearing during the pull-out process, forming a continuous shear surface.

[0129] Pile-soil interface slippage failure:

[0130] Identification features include the presence of "local slippage > 0.5 mm" on the pile surface or helical blade surface in the pile-soil interface slippage cloud map, with the slippage area expanding with increasing load, and a sudden drop in interface contact pressure;

[0131] This often occurs in soft clay soils and in cases of poor interface treatment, where the bond between the pile and the soil is insufficient, causing the interface to fail before the soil.

[0132] Soil loosening and failure between spiral blades:

[0133] Identifying features include a sharp decrease in principal stress in the soil on the upper and lower surfaces of the helical blades, and the appearance of local voids in the soil between the blades, which are characterized by dispersed plastic strain and no continuous shear bands.

[0134] This often occurs in piles with excessively large thread spacing, where the soil between the blades cannot form an effective interlock during pull-out, resulting in loosening and separation.

[0135] S405. Output Destruction Mode Determination Report:

[0136] Identify the dominant failure mode, whether it is a single mode or a combination of modes, mark the failure initiation location and development process, analyze the cause of failure in conjunction with soil parameters, and integrate the above information as the output of a failure mode determination report.

[0137] S5. Formula Derivation and Verification Optimization Stage: Based on the simulation results obtained in S4 under different working conditions, combined with the earth pressure balance theory, regression analysis is used to derive a practical calculation formula for the ultimate tensile bearing capacity of PHC spiral pipe piles; and its applicability and accuracy are verified.

[0138] Furthermore, in the above technical solution, the simulation results under different working conditions include: pile body size parameters with different thread pitch, thread width, and thread inclination angle, as well as the ultimate tensile bearing capacity value under different soil parameters such as cohesion, internal friction angle, and Poisson's ratio.

[0139] Furthermore, in the above technical solution, the practical calculation formula for the ultimate tensile bearing capacity of the PHC spiral pipe pile is: an empirical formula obtained through multiple regression analysis with the soil internal friction angle, cohesion and shear dilatation angle as variables.

[0140] Furthermore, step S5, based on the simulation results of S4, uses the earth pressure balance theory to derive the practical formula for the ultimate tensile bearing capacity. The core implementation steps are as follows:

[0141] S501. Basic Data System Construction and Preprocessing:

[0142] Using the calibrated numerical model output by S3 as the calculation tool, and based on the correspondence between "pile parameters - soil parameters - tensile bearing capacity", the system generates a dataset covering commonly used ranges in engineering. The parameter value ranges are shown in Table 1.

[0143] Table 1. Data Acquisition Parameters for Multi-Condition Simulation of Ultimate Pull-Out Bearing Capacity of PHC Spiral Pipe Piles

[0144] Parameter type Specific parameters Range of values Interval step size Pile thread parameters Thread pitch s 100~500mm 50mm Thread width b 50~200mm 25mm Thread inclination angle α 10°～30° 5° Basic parameters of pile body Pile diameter D 300~800mm 100mm Pile length L 5～30m 3m Soil core parameters internal friction angle φ 5°～40° 5° Cohesion c 0~100kPa 10kPa Shear expansion angle ψ 0～20° 4° Pull-out bearing capacity parameters Ultimate tensile strength U Determined by S4 inflection point method / displacement control method

[0145] The 3σ criterion and engineering logic verification are used for dual screening. First, abnormal U values ​​that exceed the mean ± 3 times the standard deviation are eliminated. Then, data that do not conform to engineering rules, such as "thread pitch < 150mm but load capacity drops sharply" and "internal friction angle > 30° but U increases slowly", are excluded.

[0146] Dimensional parameters (s, b, D, L, c, U) are normalized according to "parameter value / maximum parameter value", while non-dimensional parameters (α, φ, ψ) retain their original angle values ​​and are converted to radians during calculation to eliminate the interference of dimensional differences on regression analysis.

[0147] S502. Decomposition of the force mechanism based on the theory of earth pressure balance:

[0148] Based on the theory of earth pressure balance, the three core sources of stress during the pull-out resistance of PHC helical pipe piles are identified, laying the theoretical framework for formula construction:

[0149] The end resistance U1 of the helical blade is the passive earth pressure generated by the uplift resistance of the soil below the blade, which is positively correlated with the soil φ, ψ, and the projected area of ​​the blade.

[0150] Helical blade side friction U2: The sliding resistance between the blade side and the soil, which depends on b, α, and the pile-soil interface friction coefficient;

[0151] The side friction resistance of the main body of the pile, U3, is the friction resistance between the cylindrical section of the pile and the soil, which is positively correlated with the pile diameter D, the pile length L, and the soil lateral pressure coefficient.

[0152] Based on the force decomposition, the theoretical expression for the ultimate tensile bearing capacity is determined as follows:

[0153] U = U1 + U2 + U3, where: ; ; ;

[0154] S503. Practical formula derivation of ultimate tensile strength:

[0155] SPSS analysis of variance was used to calculate the significance (P value) of the influence of each candidate variable on U. Insignificant variables with P value > 0.05 were eliminated. Soil elastic modulus E and Poisson's ratio ν had little influence on U and could be excluded. Finally, the core independent variables were retained: s, b, α, D, L, φ, c and ψ.

[0156] Because U has a nonlinear relationship with the independent variables, such as tanφ, sinα, and 1 / s, a multivariate nonlinear regression model is chosen. The model form is based on the theoretical framework of S502. In the formula: k1~k8 are the regression coefficients to be fitted;

[0157] Substitute the processed dataset from step S501 into the model and use the least squares method plus iterative optimization to solve for the regression coefficients.

[0158] By performing regression analysis on 256 sets of numerical simulation results covering typical engineering parameter ranges, a set of highly significant regression coefficients were obtained: k1=12.5, k2=0.85, k3=1.8, k4=0.025, k5=2.2, k6=0.15, k7=8.0, k8=-150;

[0159] S504. Formula Verification:

[0160] Select 3-5 PHC spiral pipe pile engineering cases that have completed static pull-out load tests, covering cohesive soil, silt, and sand. Substitute s, b, α, D, L, φ, c, and ψ into the formula to calculate the theoretical value of U. Compare it with the field test values. If the coefficient of determination R... 2 If the value is ≥0.92 and the root mean square error is <5%, output the practical calculation formula; otherwise, recalculate the regression coefficients.

[0161] Furthermore, R 2 The calculation formula is In the formula, S res S is the sum of squared residuals. tot The total sum of squares;

[0162] Furthermore, the formula for calculating the residual sum of squares, Sres, is as follows: The total sum of squares S tot The calculation formula is U in the formula ia U represents the ultimate tensile bearing capacity obtained through numerical simulation under the i-th working condition, i.e., the actual value; ip The predicted value of the ultimate tensile bearing capacity is obtained by substituting the pile and soil parameters of the i-th working condition into the regression formula. R is the average of the actual values ​​of the ultimate tensile bearing capacity obtained from numerical simulations under all n working conditions. 2 The value range of is [0, 1]. The closer its value is to 1, the stronger the explanatory power of the regression formula on the data, and the higher the degree of agreement between the predicted value and the actual value.

[0163] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A numerical simulation-based method for analyzing the pull-out bearing capacity of PHC spiral pipe piles, characterized in that, Specifically, the following steps are included: S1. Construction Control Stage: Intelligent rotary drilling equipment with a two-way communication interface is used for the driving of PHC spiral pipe piles. The equipment is equipped with a special drill bit suitable for PHC spiral pipe piles and monitors and controls the pressing speed, rotation speed and torque in real time during construction. The ratio of pressing speed to rotation speed is controlled to match the advance of the pile body in one rotation with the pitch of the spiral blades, ensuring that the pile body is driven vertically and uniformly to the design elevation. S2, Data Acquisition and Transmission Stage: Real-time acquisition and recording of the indentation resistance-depth curve, torque-depth curve, and pile verticality data formed in step S1; S3. Model Construction and Inversion Stage: Establish a three-dimensional refined numerical model of the PHC helical pipe pile and surrounding soil containing the actual helical blade geometry. The soil adopts an advanced constitutive model that can reflect the shear dilatation and softening characteristics. Using the indentation resistance-depth curve and torque-depth curve obtained in S2 as targets, the key soil parameters in the three-dimensional refined numerical model are automatically calibrated through the parameter inversion algorithm to obtain a calibrated numerical model consistent with the actual working conditions on site. S4. Bearing characteristic analysis stage: Simulate the tensile load on the calibrated numerical model, extract the load-displacement curve and soil stress-strain cloud map, and analyze its tensile ultimate bearing capacity and failure mode. S5. Formula Derivation and Verification Optimization Stage: Based on the simulation results obtained in S4 under different working conditions, combined with the earth pressure balance theory, regression analysis is used to derive a practical calculation formula for the ultimate tensile bearing capacity of PHC spiral pipe piles; and its applicability and accuracy are verified. The failure mode analysis method is as follows: the failure mode is identified by the soil stress-strain cloud map, and the failure mode includes soil shear, pile-soil interface slippage and soil loosening between the helical blades; The simulation results under different working conditions include: pile body size parameters with different thread spacing, thread width, and thread inclination angle, as well as the ultimate tensile bearing capacity values ​​under different soil parameters such as cohesion, internal friction angle, and Poisson's ratio; The practical calculation formula for the ultimate pull-out bearing capacity of the PHC spiral pipe pile is: an empirical formula obtained through multiple regression analysis with the soil internal friction angle, cohesion and shear dilatation angle as variables.

2. The method for analyzing the pull-out bearing capacity of PHC spiral pipe piles based on numerical simulation as described in claim 1, characterized in that: The special drill bit is a sleeve-type drill bit that has a guiding function and can be reliably connected to the end of the PHC spiral pipe pile.

3. The method for analyzing the pull-out bearing capacity of PHC spiral pipe piles based on numerical simulation as described in claim 1, characterized in that: The pile verticality data includes: the real-time measured pile tilt angle, vertical deviation value, and offset direction.

4. The method for analyzing the pull-out bearing capacity of PHC spiral pipe piles based on numerical simulation as described in claim 1, characterized in that: The advanced constitutive model is a hardened soil model, and the key soil parameters include the internal friction angle, cohesion, and dilatation angle.

5. The method for analyzing the pull-out bearing capacity of PHC spiral pipe piles based on numerical simulation as described in claim 1, characterized in that: The analysis method for the ultimate tensile bearing capacity includes: extracting the ultimate load point from the load-displacement curve, and determining the ultimate tensile bearing capacity using the inflection point method or the displacement control method.

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