Test system and method for simulating cycle performance evolution of soft soil jacket pile foundation

Abstract

The invention discloses a test system and method for simulating cycle performance evolution of a soft soil jacket pile foundation, and belongs to the technical field of geotechnical engineering indoor model experiments. The lateral ultimate bearing capacity, the displacement distribution rule and the pile-soil interaction mechanism of a pile group system in soft soil are disclosed through a static load test, a theoretical basis is provided for evaluating the foundation stability, and the cyclic load test simulates alternating loads such as long-term waves and earthquakes in the marine environment. Evolution characteristics of soil body rigidity degradation, pile foundation accumulated damage and interface slippage are illuminated, the defects of traditional design on dynamic coupling effect and fatigue life prediction are overcome, design parameters of the jacket foundation in the soft soil foundation can be optimized through test results, safety and durability of offshore wind power, oil and gas platforms and other projects under extreme loads are improved, and the foundation performance is improved. And meanwhile, a test support is provided for a pile foundation performance degradation model under multi-disaster coupling.

Description

Technical Field

[0001] The invention belongs to the technical field of geotechnical engineering indoor model experiments, and in particular relates to a test system and method for simulating the cyclic performance evolution of a soft soil jacket pile foundation. Background Art

[0002] As my country's marine resource development strategy deepens, the dynamic stability of jacket pile foundations, a key supporting structure for major projects such as offshore wind power and cross-sea bridges, in complex marine environments has become a focus of engineering attention. According to statistics, widespread soft soil layers along my country's coast cover over 60% of the total coastline. These soils are characterized by significant structural weakness, low shear strength, and high dynamic sensitivity. Under typhoons, earthquakes, and cyclic wave loads, soft soil foundations are prone to cumulative deformation and weakening, directly threatening the service life of the superstructure. In recent years, numerous offshore structure capsizing accidents caused by pile-soil dynamic interaction have occurred worldwide. In 2022 alone, three jacket foundation failures were recorded in European waters, resulting in economic losses exceeding 500 million yuan. Although existing standards have established a mature system for static design, research on the dynamic coupling mechanism of soft soils and structures still lags behind. In particular, key issues such as the softening effect of the pile-soil interface and energy dissipation mechanisms under high-frequency cyclic loading lack systematic experimental verification.

[0003] While domestic and international researchers have made progress in model testing of the dynamic bearing characteristics of jacket pile foundations in soft soils, the detailed understanding of the underlying mechanisms under complex dynamic environments remains a research gap. In China, Zhang Jianmin, Li Guangxin, and others, through dynamic triaxial and centrifuge testing, have revealed the pore pressure accumulation and nonlinear stiffness degradation patterns of soft clays under cyclic loading, and constructed a constitutive model that accounts for the hysteretic properties of the soil. In response to the needs of marine engineering, teams from the Ocean University of China and Dalian University of Technology, among others, have employed shaking table testing and fiber Bragg grating (FBG) monitoring technology to investigate the lateral dynamic response of jacket piles under wave loading and the pile-soil interface slip mechanism. However, existing specifications (such as JTS167-4) still rely on empirical corrections for the dynamic design parameters of jacket piles. Foreign research focuses on extreme environment simulation and multi-scale coupling analysis. For example, Andersen et al. proposed a soil residual strain prediction model under cyclic loading based on North Sea soft clay data. Norway's NGI quantified the cumulative damage of pile foundations under storm loads using a centrifuge-coupled dynamic load device. Japan used the E-Defense large-scale vibration table to reveal the earthquake-soft soil-pile foundation kinematic interaction mechanism. However, international standards such as API and ISO have not yet formed a systematic design method for pile group effects, soil softening under long-term cyclic loading, and multi-hazard coupling.

[0004] In summary, existing research is relatively mature in single load path and short-term response analysis. However, model tests on the dynamic contact behavior of the soft soil-jacket pile interface, the cumulative deformation mechanism under asymmetric cyclic loading, and the creep-fatigue coupling effect are still insufficient. It is urgent to deepen the study of bearing characteristics under complex working conditions through multi-physics field coupling experiments. Summary of the Invention

[0005] In order to solve the problems raised in the above background technology, the present invention provides a test system and method for simulating the cyclic performance evolution of soft soil jacket pile foundations, so as to solve the problem that existing research on the dynamic contact behavior of the soft soil-jacket pile interface, the cumulative deformation mechanism under asymmetric cyclic loading and the creep-fatigue coupling effect model test is still insufficient.

[0006] To achieve the above object, the present invention provides the following technical solutions: A test system for simulating the cyclic performance evolution of a jacket pile foundation in soft soil, comprising: Reaction frame; the reaction frame is a frame structure with a base; Model box; the model box is arranged on the base of the reaction frame, and the model box is filled with model soil; Jacket pile foundation: The bottom of the jacket pile foundation is inserted into the model soil; Vertical loading device; the vertical loading device is detachably arranged on the top of the reaction frame, and the vertical loading device is provided with a linear actuating end, and the linear actuating end of the vertical loading device is arranged to point to the base of the reaction frame; Side loading device; the side loading device is detachably arranged on the side of the reaction frame, and the side loading device is provided with a linear actuating end, and the linear actuating end of the side loading device is arranged to point to the jacket pile foundation; Data acquisition system; the data acquisition system includes a data acquisition instrument, two displacement meters, tension and compression sensors and multiple strain gauges. The strain gauges are pasted on the bottom of the jacket pile foundation. The first end of the tension and compression sensor is connected to the top of the linear actuator end of the lateral loading device. During the test loading phase, the second end of the tension and compression sensor is connected to the jacket pile foundation through a connector. One end of a displacement meter is connected to the side of the reaction frame through a mounting frame, and the other end is connected to the bottom of the jacket pile foundation. One end of another displacement meter is connected to the top of the reaction frame through a mounting frame, and the other end is connected to the connector. The strain gauges, displacement meters and tension and compression sensors are all connected to the data acquisition instrument.

[0007] Preferably, the reaction frame also includes four H-shaped steels and four square steel pipe cross braces. The H-shaped steels are vertically fixed on the base. The two ends of a square steel pipe cross brace are respectively fixedly connected to the top side surfaces of the two H-shaped steels. After the four directional steel pipe cross braces are installed, a rectangular structure is formed.

[0008] Preferably, the model box is a cylindrical structure, and the model box is rotatably mounted on the base, with the rotating shaft of the model box being its own central axis.

[0009] Preferably, the jacket pile foundation includes four model piles, a jacket platform and a tower. The four model piles are inserted into the model soil. The jacket platform is a tower-like structure including four tower feet, which is wide at the bottom and gradually narrows upwards. One tower foot of the jacket platform is fixedly connected to the top surface of a model pile by bolts, and the tower is fixedly installed on the top surface of the jacket platform.

[0010] Preferably, the vertical loading device includes a vertical actuator, a vertical motor control cabinet and a vertical loading beam. The two ends of the vertical loading beam are detachably arranged on two opposite square steel pipe supports. The vertical actuator is detachably installed on the vertical adjustable beam. The linear actuating end of the vertical actuator is arranged to point to the base. The vertical actuator is connected to the vertical motor control cabinet.

[0011] Preferably, the lateral loading device includes a lateral actuator, a lateral motor control cabinet and a lateral loading beam. The two ends of the lateral loading beam are detachably connected to the sides of the two H-shaped steels. The lateral actuator is detachably installed on the lateral adjustable beam. The linear actuating end of the lateral actuator points to the pile foundation of the conductor frame. The lateral actuator is connected to the lateral motor control cabinet.

[0012] A test method for simulating the cyclic performance evolution of a jacket pile foundation in soft soil comprises the following steps: S1: Four model piles are sunk into the model soil using a vertical actuator; S2: Perform a static load test, apply a static load to the jacket pile foundation through a lateral actuator, collect test data from the static load test through a data acquisition system, and obtain the allowable bearing capacity Pu of the jacket pile foundation; S3: Based on the allowable bearing capacity Pu obtained from the static load test, a cyclic load test plan is formulated: First, a force cycle test is carried out. The lateral actuator is adjusted to the pressure mode through the lateral motor control cabinet and controlled to load with four sets of cyclic loads of 0.25Pu, 0.5Pu, 0.75Pu and 1Pu respectively. The cyclic mode has two types: unidirectional cycle and bidirectional cycle, with a total of eight sets of tests. The test is terminated when the model pile reaches the predetermined number of cycles, is damaged, or has exceeded the deformation limit; S4: After completing eight sets of force cycle tests, according to the results of the force cycle tests, the lateral actuator is adjusted to the displacement mode through the lateral motor control cabinet and the lateral actuator is controlled to perform displacement unidirectional cycle tests with four sets of displacement data of 5mm, 10mm, 15mm and 20mm respectively. There are four sets of tests in total. When the model pile reaches the predetermined number of cycles, damage occurs or deformation exceeds the limit, the test is terminated to obtain the final experimental results.

[0013] Preferably, S1 is specifically as follows: four model piles are sunk diagonally in succession at preset positions in the model soil; during the pile sinking, a force transmission rod assembly is installed on the linear actuating end of the vertical actuator to squeeze and contact the top of the model pile; the vertical actuator is controlled by the vertical motor control cabinet to press each model pile to a specified depth at a speed of 30 mm / min; after the pile sinking is completed, the jacket platform and the tower are fixed to the model pile with bolts; and a level is used to check the verticality during the pile sinking process.

[0014] Preferably, S2 is specifically as follows: the estimated ultimate load value is Py, and graded loading is performed according to 10% of the ultimate load, i.e. 0.1Py. Each load level is observed by regularly observing the offset data until it stabilizes. If there is a sudden increase in offset, continuous instability, or the maximum test load is reached, the loading is terminated, and then the load is unloaded in stages and the rebound data is recorded. Finally, the ultimate bearing capacity and deformation parameters are determined by analyzing the load-offset curve, and the allowable bearing capacity Pu is calculated in combination with the safety factor.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This application constructs a scaled model test system that considers soil structure and fabric anisotropy. Static load tests reveal the lateral ultimate bearing capacity, displacement distribution, and pile-soil interaction mechanism of pile groups in soft soils, providing a theoretical basis for assessing foundation stability. Cyclic load tests simulate long-term alternating loads such as waves and earthquakes in marine environments, elucidating the evolutionary characteristics of soil stiffness degradation, pile foundation cumulative damage, and interface slip. This addresses the shortcomings of traditional designs in predicting dynamic coupling effects and fatigue life. The test results of this application can optimize the design parameters of jacket foundations in soft soils (such as pile diameter, spacing, and reinforcement measures), guide the improvement of dynamic response provisions for pile groups in international standards (such as API and ISO), and enhance the safety and durability of offshore wind power, oil and gas platform projects under extreme loads. Furthermore, the application provides experimental support for the performance degradation model of pile foundations under multi-hazard coupling (static-dynamic combined loads, scour-seepage effects). This has important engineering value and academic significance for promoting deep-sea resource development and achieving the "dual carbon" goals. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of the structure of the test system for this application; Figure 2 This is a schematic diagram of the calibration results of the strain gauge for pile 1; Figure 3 This is a schematic diagram of the calibration results of the strain gauge for pile 2; Figure 4 This is a schematic diagram of the calibration results of the strain gauge for pile 3; Figure 5 This is a schematic diagram of the calibration results of the strain gauge for pile 4; Figure 6This is a graph showing the variation of the penetration resistance of each pile in the structural clay model with the depth of the pile; Figure 7 is the time-varying load-settlement curve of the model pile; Figure 8 Schematic diagram of the pile axial force distribution of the model pile 1 and pile 2; Figure 9 Schematic diagram of the distribution of pile side friction resistance of the model of pile 1 and pile 2; Figure 10 Schematic diagram of the bending moment distribution of the model pile body of pile 3 and pile 4; Figure 11 The deformation curves of the model pile bodies of piles 3 and 4 are shown; The following are marked in the figure: 1-base; 2-H-shaped steel; 3-square steel pipe cross brace; 4-model box; 5-model soil; 6-model pile; 7-vertical actuator; 8-lateral actuator; 9-strain gauge; 10-jacket platform; 11-push plate; 12-tower; 13-mounting frame; 14-displacement meter; 15-tension and compression sensor; 16-vertical loading beam. DETAILED DESCRIPTION

[0017] To facilitate those skilled in the art to understand the technical content of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and specific examples. It should be understood that the specific examples described herein are only used to explain the present invention and are not intended to limit the present invention.

[0018] like Figure 1 As shown, a test system for simulating the cyclic performance evolution of a jacket pile foundation in soft soil includes: Reaction frame; The reaction frame is a frame structure with a base 1. The reaction frame also includes four H-shaped steels 2 and four square steel pipe cross braces 3. The H-shaped steel 2 is vertically fixed on the base 1. The two ends of a square steel pipe cross brace 3 are respectively fixedly connected to the top side surfaces of the two H-shaped steels 2. After the four directions of the steel pipe cross braces are installed, a rectangular structure is formed. Model box 4; the model box 4 is filled with model soil 5, the model box 4 is a barrel-shaped structure, the model box 4 base 1 is provided with a turntable, the turntable and the base 1 are provided with a plurality of locking holes, the model box 4 is placed on the turntable, the central axis of the model box 4 coincides with the axis of rotation of the turntable, when there is no need to drive the model box 4 and the turntable to rotate, a latch is simultaneously inserted into a locking hole on the turntable and a locking hole on the base 1, when the turntable is required to drive the model box 4 to rotate, the latch can be pulled out, the entire test is carried out in a special model box 4, the model box 4 is made of acrylic plate as the main material, The internal dimensions are Ø800mm × 1000mm in diameter and height. The main structural stiffness of the model box 4 meets the test load requirements. The side walls and bottom plate of the model box 4 have been treated with anti-seepage and interface drag reduction treatments to effectively maintain the stability of soil moisture content and reduce the boundary constraint effect caused by side wall friction. In particular, four drainage pipeline systems are evenly distributed along the bottom plate of the model box 4, and controllable valves are configured to adjust the drainage status. During the self-weight consolidation stage, the valves are opened to achieve free drainage to simulate the actual consolidation process. During the test loading stage, the valves are closed to form a closed system, strictly meeting the requirements of pile foundation bearing characteristics research under undrained conditions. Soil Properties: The plain clay used in the model tests was obtained from the 61st layer of dark green silty clay in the plot south of Zhuying Road, Yingpu Subdistrict, Qingpu District, Shanghai. This soil is distributed throughout the site and is stable. The top elevation of the layer ranges from 1.45 to -0.46 m, and the soil layer thickness ranges from 3.00 to 4.40 m. The soil contains iron oxide streaks, humic plants, and ferromanganese nodules. The soil is relatively uniform, plastic, moderately compressible, and has no shaking response. It has a slight gloss, medium dry strength, and medium toughness. Laboratory measurements show its basic physical properties in the remolded state, as detailed in Table 1.

[0019] Table 1

[0020] Preparation plan for model soil 5: During the preparation process of model soil 5, in order to ensure that the artificial structural clay is uniform and saturated and meets the designed porosity ratio, the dry soil and water required to prepare the entire box of model soil 5 are divided into ten equal parts. Each time, the model soil 5 required to fill the model box 4 to a height of 10 cm is prepared. The detailed preparation steps are as follows: A1: The dark green clay retrieved from the site was sun-dried, air-dried and crushed. The crushed soil was passed through a 0.5 mm sieve to ensure uniform particle size.

[0021] A2: Determine the total height of the model soil 5 required for the test (1000mm), divide the total height of the fill into ten equal parts, and fill each part in the model box 4 to a height of 10cm.

[0022] A3: Lay geotextile at the bottom of the model box 4 to accelerate soil consolidation and prevent soil particles from clogging the drainage valve. At the same time, apply vaseline on the inner wall of the model box 4 to reduce the boundary effect.

[0023] A4: According to the mix ratio, measure the correct amount of crushed soil, place the soil in a centrifugal mixer, add appropriate amount of water according to the mix ratio provided in the table, seal the mixer, and then centrifuge for 2 hours to ensure uniform mixing and remove gas.

[0024] A5: Evenly fill the prepared model soil 5 in the first layer (10 cm) of the model box 4. Use a loading plate to slightly compact the model soil 5 to reach the designed height, ensuring that the model soil 5 meets the initial porosity ratio and is fully saturated.

[0025] A6: The surface of the layered model soil 5 is scraped and brushed to allow the next layer of model soil 5 to be better connected with it and avoid stratification of the model soil 5.

[0026] A7: Repeat A4-A6 to prepare, fill and compact the remaining 9 layers of model soil 5 in the same manner as above to complete the preparation of a full box of model soil 5.

[0027] A8: Let the prepared model soil 5 stand for 28 days to ensure that the soil is fully consolidated.

[0028] It should be noted that the pressure applied in step 5 needs to be applied appropriately and evenly to each soil layer to avoid over-compaction of the model soil 5 and resulting in over-consolidation.

[0029] Jacket pile foundation; Jacket pile foundation includes, jacket platform 10 and tower 12, four model piles 6 are inserted into model soil 5 in the form of two rows and two columns of matrix distribution, the linear actuating end of lateral actuator 8 points in the direction perpendicular to the row of the matrix and parallel to the column, jacket platform 10 includes four tower feet and presents a tower-like structure with a wide bottom and gradually narrowed upwards, one tower foot of jacket platform 10 is fixedly connected to the top surface of a model pile 6 by bolts, tower 12 is fixedly mounted on the top surface of jacket platform 10, the main material of model pile 6 is made of aluminum alloy tube, aluminum alloy has excellent processing performance, good resistance Due to its corrosion resistance, high toughness, resistance to deformation during processing, and dense, defect-free material, it is an ideal test pile material. The model pile 6 has dimensions of 620 mm in length, 30 mm in outer diameter, 2 mm in wall thickness, and an aspect ratio of 20.7. To facilitate static pressure sinking, the pile end angle is set at 60°. To facilitate the attachment of strain gauges 9 to the model pile 6 and the routing and routing of all sensor signal lines within the pile, four strain gauges 9 are symmetrically attached to the outer wall of the model pile 6, starting from the pile base, every 100 mm. A 20 mm gap is left between the top strain gauge 9 and the pile top to facilitate line routing. A total of 28 strain gauges 9 are attached to seven sections of the pile body. The four strain gauges 9 at each section are connected using a Wheatstone full-bridge circuit. The Wheatstone full-bridge circuit eliminates strain errors caused by temperature changes and achieves temperature compensation. A small hole with a diameter of 2 mm is opened above each set of strain gauges 9. The signal lines of the strain gauges 9 are passed through the hole into the model pile 6 and exit at the top of the pile. All strain gauges 9 were calibrated and parameterized before being installed in the model piles 6. After installation, the strain gauges 9, signal lines, and small holes were covered and protected with epoxy resin. Using the aforementioned manufacturing process, four model piles 6 were fabricated. Two of these model piles 6 measured the axial force, and the remaining two measured the bending moment. Therefore, this model pile 6 system can monitor the distribution of the pile's lateral axial force and bending moment during the pile sinking process.

[0030] Furthermore, a pile layout plan for the model piles 6 was developed in a manner that symmetrically measures the distribution of axial force and bending moment: with the lateral actuator 8 as the reference visual point, the front right side is pile 1 for measuring the axial force of the pile body, the front left side is pile 3 for measuring the bending moment of the pile body, the back right side is pile 4 for measuring the bending moment of the pile body, and the back left side is pile 2 for measuring the axial force of the pile body. Before static pressure pile sinking, the four model piles 6 need to be calibrated: for piles 1 and 2 for measuring axial force, five levels of axial load are applied to the model piles 6 through the vertical actuator 7 to obtain the relationship between the axial force and voltage of each section. The calibration results are shown as follows: Figure 2 and Figure 3 As shown, for piles 3 and 4 measuring the bending moment, three sets of concentrated forces are applied in the middle of the simply supported single-span beam to obtain the relationship between the bending moment and voltage of each section. It can be seen that the calibration results are as follows: Figure 4 and Figure 5 As shown, the voltage at each cross-section of each pile exhibits a significant linear positive correlation with the axial force and bending moment, verifying the strain sensing system's excellent linearity and sensitivity, ensuring the accuracy and reliability of data measurements on model pile 6 during static and cyclic loading tests. The slope of the fitted trend line represents the calibration coefficient of strain gauge 9. Using linear equations, the voltage values monitored at each cross-section can be converted into the corresponding axial force and bending moment.

[0031] Vertical loading device; the vertical loading device is detachably arranged on the top of the reaction frame, and the vertical loading device is provided with a linear actuating end, and the linear actuating end of the vertical loading device is arranged to point to the base 1 of the reaction frame, specifically: The vertical loading device includes a vertical actuator 7, a vertical motor control cabinet, and a vertical loading beam 16. The square steel tube cross brace 3 is provided with multiple threaded holes, and the vertical loading beam 16 is also provided with multiple threaded holes. The two ends of the vertical loading beam 16 are detachably mounted on two opposing square steel tube supports by bolts and nuts. The vertical actuator 7 is detachably mounted on the vertical adjustable cross beam, and the linear actuating end of the vertical actuator 7 is directed toward the base 1. The vertical actuator 7 is connected to the vertical motor control cabinet. Side loading device; the side loading device is detachably arranged on the side of the reaction frame, and the side loading device is provided with a linear actuating end, and the linear actuating end of the side loading device is arranged to point to the jacket pile foundation, specifically: The lateral loading device includes a lateral actuator 8, a lateral motor control cabinet, and a lateral loading beam. The H-shaped steel 2 is provided with a plurality of threaded holes, and the lateral loading beam is also provided with a plurality of threaded holes. The ends of the lateral loading beam are detachably connected to the sides of the two H-shaped steels 2 by bolts and nuts. The lateral actuator 8 is detachably mounted on the lateral adjustable crossbeam. The linear actuating end of the lateral actuator 8 is directed toward the jacket pile foundation. The lateral actuator 8 is connected to the lateral motor control cabinet. Both vertical actuator 7 and lateral actuator 8 utilize modular electric servo actuators, based on the principle of precision electromechanical conversion. A servo motor drives a ball screw, converting rotational motion into high-precision linear displacement. A closed-loop feedback control system enables precise three-dimensional control of thrust (±1% FS), speed (±0.5% of setpoint), and displacement (±0.01mm). The electric servo actuators include a servo motor with a high-precision encoder, a planetary reducer, a ball screw drive module, a 485 bus communication interface, a PID controller, an intelligent driver, and a dynamic force sensor. A PLC integrated control architecture enables multi-parameter coordinated operation. The two actuators employ differentiated control strategies. Vertical actuator 7 operates in a closed-loop displacement mode, achieving precise pile driving within a 0-330mm travel range with a speed of ≤10mm / s and a resolution of 0.01mm. Lateral actuator 8 operates in a force-controlled mode, with a maximum force of 5kN and a frequency response of 1Hz, simulating the complex load spectrum experienced during the service life of a pile foundation.

[0032] Data acquisition system; The data acquisition system includes a collection instrument, two displacement meters 14, a tension and compression sensor 15 and a plurality of strain gauges 9. The strain gauge 9 is pasted on the bottom of the jacket pile foundation. The first end of the tension and compression sensor 15 is connected to the top of the linear actuator end of the lateral loading device. During the test loading stage, the second end of the tension and compression sensor 15 is connected to the jacket pile foundation through a connector. One end of a displacement meter 14 is connected to the side of the reaction frame through a mounting bracket 13, and the other end is connected to the bottom of the jacket pile foundation. One end of another displacement meter 14 is connected to the top of the reaction frame through a mounting bracket 13, and the other end is connected to the connector. The strain gauge 9, displacement meter 14 and tension and compression sensor 15 are all connected to the collection instrument; Specific description of data acquisition system components: Strain gauge 9; This test uses a BF1K-3AA high-precision foil resistance strain gauge with a nominal resistance of 1002Ω±0.1Ω and a sensitivity coefficient of 2.0±1% to monitor the axial force and bending moment of the pile. The installation process for strain gauge 9 strictly adheres to ASTM E251: First, the predetermined measuring point area, covering an 8×5.5mm substrate, is polished with gradient sandpaper and ultrasonically cleaned with anhydrous ethanol to ensure a surface roughness of ≤0.8μm. Subsequently, cyanoacrylate quick-drying adhesive is applied using a micro-dispensing technique to achieve precise positioning of the strain gauge 9, and the strain gauge is triaxially waterproofed with waterproof adhesive. The signal transmission system is connected to shielded twisted-pair cables by soldering using gold-plated terminals, and the solder joints are protected with heat shrink tubing. To verify the reliability of the measurement system, axial compression calibration was performed on the model pile 6 during the pre-test phase to confirm that each strain gauge 9 met the required accuracy level.

[0033] Displacement meter 14; This test adopts YWJ type electromechanical composite displacement sensor, which is fixedly installed on the top or side of the reaction frame through the mounting frame 13. Based on the fusion design of mechanical dial indicator and full-bridge strain sensor module, dual-mode synchronous measurement of displacement is realized. The mechanical pointer resolution is 0.01mm, the electrical signal sensitivity is 0.1mV / mm&2V bridge voltage, the range is 0~50mm, the bridge resistance is 350Ω, the bridge supply voltage is less than or equal to 10V, the temperature range covers -35~70℃ and the basic error is ±5με. It has both mechanical intuitiveness and the advantages of electrical measurement digitization. It is magnetically installed through the mounting frame 13 to accurately monitor the settlement process. After calibration, it is verified that its linear error is ≤0.05%FS and the repeatability error is ±0.003mm, which meets the A-level accuracy requirements of civil engineering and is suitable for high-reliability displacement monitoring under static / dynamic loads.

[0034] Tension and compression sensor 15; This test uses the DYLY-103 S-type double-shear beam force sensor with a range of 0–10 kN and a sensitivity of 2.0 mV / V. Based on a full-bridge strain gauge design, it features bidirectional symmetrical tension and compression responses, a nonlinearity of ≤0.03% FS, a lateral interference of <1%, and a compact package of 51×26×76 mm. This complies with the GB / T7551-2008 standard. The tension and compression sensor 15 is mounted between the lateral actuator 8 and the jacket pile foundation via a universal joint. The displacement meter 14 is mounted by providing a push plate 11 at the second end of the tension and compression sensor 15. The two ends of the universal joint are connected to the push plate 11 and the jacket pile foundation, respectively, enabling accurate measurement of pile driving resistance and service load. Its wide temperature range and 150% overload protection ensure reliability under harsh working conditions. Calibrated with standard weights and a dynamic force hammer, the sensor achieves static accuracy of C3 and a dynamic error of ±0.5%, meeting the requirements for monitoring the full-cycle load spectrum of the pile foundation.

[0035] Data acquisition instrument; This experiment built a multi-physics field coupling data acquisition system that integrates static-dynamic strain testing and pile-soil interface parameter monitoring functions. The static test uses the YBY-4010 intelligent strain acquisition system, whose core consists of the following modules: (1) Hardware Architecture: Equipped with a 40-channel Wheatstone bridge interface, supporting full, half, and quarter bridges, a built-in 2VDC precision excitation source, a measurement range of ±19999με, a sampling rate of 10Hz in static mode, and adaptability to laboratory / field environments through a 420×300×100mm IP67 protection-grade chassis; (2) Software Ecosystem: Equipped with the DASPV10 analysis platform, it supports automatic calibration of sensor parameters and temperature compensation algorithms, and is compatible with both manual touch and computer-controlled dual modes; (3) System advantages: Based on ASIC chip-level signal conditioning technology, it achieves 0.5με resolution and ±0.1%FS nonlinearity, making it particularly suitable for long-term stable monitoring of quasi-static parameters such as pile axial force and pile end resistance. Dynamic testing uses the YBY-2001 high-speed acquisition system, which expands the 20-channel 100Hz synchronous sampling capability to meet the needs of synchronous analysis of pile top displacement and load phase under cyclic loading. The entire system is networked through a star topology, achieving 40+20 channel data fusion, completing multi-dimensional synchronous acquisition of pile-soil interface stress, pore water pressure, and structural response, with a data synthesis error of ≤0.2%.

[0036] Test system equipment connection: Before testing, the test instruments must be connected and debugged. The motor control cabinet's controller controls the driver's output current based on the difference between the set value and the force sensor's feedback. The driver, in turn, controls the motor's output torque. This torque is converted into linear tension and compression via a ball screw, ultimately acting on the pile side. This system can meet various functional requirements under static and cyclic loads, and can also be dynamically loaded, achieving independent control of the load amplitude, frequency, and load waveform, enabling accurate regulation of the cyclic load frequency and load waveform. The strain gauge 9, displacement meter 14, and tension and compression sensor 15 are connected to a signal amplifier. The amplified signals are collected by the corresponding signal acquisition instrument and ultimately processed by relevant analysis software in a computer. This experiment allows for synchronized data acquisition between the control system and the external data acquisition system.

[0037] A test method for simulating the cyclic performance evolution of a jacket pile foundation in soft soil comprises the following steps: S1: Four model piles 6 are sunk into the model soil 5 using the vertical actuator 7. Specifically, the four model piles 6 are sunk diagonally into the preset positions of the model soil 5. During the sinking process, a force transmission rod assembly is installed on the linear actuating end of the vertical actuator 7 to press the top of the model pile 6. The vertical motor control cabinet controls the vertical actuator 7 to press each model pile 6 to a specified depth at a speed of 30 mm / min. After the sinking is completed, the jacket platform 10 and the tower 12 are fixed to the model pile 6 using bolts. A level rod is used to check verticality during the sinking process. S2: Conduct a static load test. Apply a static load to the jacket pile foundation via the lateral actuator 8. Collect test data from the static load test via the data acquisition system to determine the allowable bearing capacity Pu of the jacket pile foundation. Specifically, estimate the ultimate load value Py, and perform graded loading at 10% of the ultimate load, i.e., 0.1Py. Regularly observe the deflection data for each load level until stability is achieved. If a sudden increase in deflection, persistent instability, or the maximum test load is reached, terminate loading. Subsequently, perform graded unloading and record rebound data. Finally, analyze the load-deflection curve to determine the ultimate bearing capacity and deformation parameters. Combined with the safety factor, calculate the allowable bearing capacity Pu. S3: Based on the allowable bearing capacity Pu obtained from the static load test, a cyclic load test plan is formulated: first, a force cycle test is carried out, in which the lateral actuator 8 is adjusted to the pressure mode through the lateral motor control cabinet and the lateral actuator 8 is controlled to load with four sets of cyclic loads of 0.25Pu, 0.5Pu, 0.75Pu and 1Pu respectively. The cyclic modes include unidirectional cycle and bidirectional cycle, for a total of eight sets of tests. The test is terminated when the model pile 6 reaches the predetermined number of cycles, is damaged, or is deformed beyond the limit; S4: After completing eight sets of force cycle tests, according to the results of the force cycle tests, the lateral actuator 8 is adjusted to the displacement mode through the lateral motor control cabinet and the lateral actuator 8 is controlled to perform displacement unidirectional cycle tests with four sets of displacement data of 5mm, 10mm, 15mm and 20mm respectively. There are four sets of tests in total. When the model pile 6 reaches the predetermined number of cycles, is damaged or deformed beyond the limit, the test is terminated to obtain the final experimental results.

[0038] Test results and analysis; Changes in pile penetration resistance: Figure 6 (a), (b), (c), and (d) are the curves showing the variation of the penetration resistance of piles 1, 2, 3, and 4 as a function of the depth of the piles during the sinking process. By analyzing the variation trends of the penetration resistance of the four piles during the sinking process, the following patterns can be observed: During the sinking process, the penetration resistance of the four piles gradually increases with increasing depth, and the variation trends are essentially the same. Furthermore, the order of pile sinking also affects the penetration resistance. The later-sunk piles are subject to the soil-squeezing effect of the earlier-sunk piles on the surrounding soil, which reduces the porosity and increases the density of the surrounding soil, changing the original stress and strain fields of the soil. Consequently, the penetration resistance of the later-sunk piles during the sinking process is greater than that of the earlier-sunk piles at the same depth.

[0039] Static load settlement curve: Figure 7 Time-varying load-settlement curves (PS curves) for model pile 6 in model soil 5 at different consolidation time intervals (T) are presented. It can be seen that the load-settlement response of model pile 6 in clay exhibits a significant time effect: for the same settlement displacement, the applied lateral load increases with increasing consolidation time T. When the PS curve shows a stable trend, the starting point of the stabilization is generally taken as the ultimate bearing capacity of the pile. It can be seen that the ultimate bearing capacity of model pile 6 and its corresponding settlement value increase significantly with increasing consolidation time, and the growth trend slows down with time. This indicates that when the consolidation time reaches a certain limit, the overall properties of model soil 5 return to a steady state and will not change significantly. Furthermore, the dissipation of excess pore water pressure (EPWP) induced by pile driving increases the strength, stiffness, and shear strength of the soil surrounding the pile, thereby increasing the bearing capacity of the pile foundation over time.

[0040] Pile axial force and unit lateral friction: Pile 1 and Pile 2 are two piles for measuring axial force. Figure 8 (a) and (b) show the evolution of the pile axial force (Np) distribution when piles 1 and 2 are sunk 72 hours later, as the pile side load gradually increases. The unit pile side friction resistance distribution of model pile 6 is calculated based on the pile axial force distribution, as shown in the figure below: Figure 9 As shown, Figure 9 (a) and Figure 9 (b) are piles 1 and 2 respectively. In the static load test of model pile 6, the ultimate bearing capacity of model pile 6 was evenly divided into 6 levels of load, and then applied to the pile side step by step. During the graded loading process, the axial force of the pile body showed a decreasing trend along the depth, while the unit pile side resistance increased with the depth. As the pile side load increased, the axial force attenuation rate increased significantly, and the depth evolution effect of the corresponding pile side resistance also gradually increased. In the low load stage, the axial force distribution of the pile body was close to a linear mode, and the attenuation rate was slow. This is because the small relative displacement between the pile and the soil only mobilized the lateral friction resistance of the shallow soil, and its distribution was relatively uniform; when the load was increased to the medium and high stage, the mechanical response of the deep soil was gradually activated, and the nonlinear strengthening characteristics of the lateral resistance along the depth became more and more significant, and the axial force gradient change increased accordingly, reflecting the transition of the pile-soil interaction from the elastic stage to the elastoplastic stage, as shown in Figure 2. Figure 9 As shown in Figure 2 , as the pile side load increases step by step, the spatial variability of the shear strength of the pile-soil interface along the depth gradually dominates the axial force distribution pattern of the pile body: the progressive activation of the lateral friction resistance of the shallow soil causes the axial force gradient change rate to increase significantly with increasing depth. When the load level reaches the critical threshold, the upper part of the pile body exhibits lateral friction resistance saturation characteristics due to entering a fully plastic state, and the axial force attenuation rate in the corresponding section tends to stabilize, indicating that the pile-soil system has entered a stress redistribution stage where local failure and overall bearing capacity are coordinated.

[0041] Changes in pile bending moment and pile displacement: Pile 3 and Pile 4 are two piles for measuring bending moment. Figure 10 (a) and Figure 10 (b) shows that 72 hours after the sinking of piles 3 and 4, as the pile side load increases, the bending moment at different buried depths along the pile body shows an overall increasing trend, and the maximum bending moment point also gradually moves downward along the pile body as the load increases. The depth of the bending moment inflection point of the front and rear rows of piles is 40~50cm. According to the boundary conditions that the horizontal displacement of the overall model and the soil reaction force at the mud surface are both zero, the pile deformation curve can be calculated based on the turning bending moment measured by the pile body strain gauge 9. This application uses Matlab software to perform polynomial fitting on the measured bending moment of the pile body, and then solves the displacement formula based on the bending moment and performs a quadratic integration on it to obtain the horizontal deformation curve of the lower pile foundation of the jacket wind turbine model, as shown in the figure. Figure 11 As shown, Figure 11(a) is pile 3, Figure 11 (b) is pile 4.

[0042] Pile deformation curve Figure 11 As can be seen, the horizontal displacement of the pile gradually increases with increasing lateral load, with the primary deformation occurring within 20 cm of the pile tip. Comparing the fitted displacement with the actual displacement shows that the measured horizontal displacement is essentially consistent with the fitted displacement, demonstrating the accuracy of calculating the horizontal displacement of the pile using the measured bending moment. The horizontal displacements of the front and rear rows of piles are essentially consistent, indicating that the distance between the pile tops of the lower pile foundation of the jacket platform 10 remains essentially unchanged.

Claims

1. A test system for simulating the cyclic performance evolution of a soft soil jacket pile foundation, characterized in that: include: reaction frame; The reaction frame is a frame structure with a base (1); Model box (4); the model box (4) is arranged on the base (1) of the reaction frame, and the model box (4) is filled with model soil (5); Jacket pile foundation; the bottom of the jacket pile foundation is inserted into the model soil (5); A vertical loading device; the vertical loading device is detachably arranged on the top of the reaction frame, the vertical loading device is provided with a linear actuating end, and the linear actuating end of the vertical loading device is arranged to point to the base (1) of the reaction frame; Side loading device; the side loading device is detachably arranged on the side of the reaction frame, and the side loading device is provided with a linear actuating end, and the linear actuating end of the side loading device is arranged to point to the jacket pile foundation; Data acquisition system; the data acquisition system includes a collection instrument, two displacement meters (14), a tension and compression sensor (15) and a plurality of strain gauges (9), the strain gauge (9) is pasted on the bottom of the jacket pile foundation, the first end of the tension and compression sensor (15) is connected to the top of the linear actuator end of the lateral loading device, and the second end of the tension and compression sensor (15) is connected to the jacket pile foundation through a connector during the test loading stage, one end of a displacement meter (14) is connected to the side of the reaction frame through a mounting frame (13), and the other end is connected to the bottom of the jacket pile foundation, one end of another displacement meter (14) is connected to the top of the reaction frame through a mounting frame (13), and the other end is connected to the connector, and the strain gauge (9), the displacement meter (14) and the tension and compression sensor (15) are all connected to the collection instrument.

2. The test system for simulating the cyclic performance evolution of soft soil jacket pile foundation according to claim 1, characterized in that: The reaction frame also includes four H-shaped steels (2) and four square steel pipe cross braces (3). The H-shaped steels (2) are vertically fixed on the base (1). The two ends of a square steel pipe cross brace (3) are respectively fixedly connected to the top side surfaces of the two H-shaped steels (2). After the four directional steel pipe cross braces are installed, a rectangular structure is formed.

3. The test system for simulating the cyclic performance evolution of soft soil jacket pile foundation according to claim 1, characterized in that: The model box (4) is a cylindrical structure. The model box (4) is rotatably mounted on the base (1), and the rotation axis of the model box (4) is its own central axis.

4. The test system for simulating the cyclic performance evolution of soft soil jacket pile foundation according to claim 1, characterized in that: The jacket pile foundation includes four model piles (6), a jacket platform (10) and a tower (12). The four model piles (6) are inserted into the model soil (5). The jacket platform (10) is a tower-shaped structure including four tower feet, which is wide at the bottom and gradually narrows upward. One tower foot of the jacket platform (10) is fixedly connected to the top surface of one model pile (6) by bolts. The tower (12) is fixedly installed on the top surface of the jacket platform (10).

5. The test system for simulating the cyclic performance evolution of soft soil jacket pile foundation according to claim 2, characterized in that: The vertical loading device includes a vertical actuator (7), a vertical motor control cabinet and a vertical loading beam (16). The two ends of the vertical loading beam (16) are detachably arranged on two opposite square steel pipe supports. The vertical actuator (7) is detachably installed on the vertical adjustable crossbeam. The linear actuating end of the vertical actuator (7) is arranged to point to the base (1). The vertical actuator (7) is connected to the vertical motor control cabinet.

6. The test system for simulating the cyclic performance evolution of soft soil jacket pile foundation according to claim 2, characterized in that: The lateral loading device includes a lateral actuator (8), a lateral motor control cabinet and a lateral loading beam. The two ends of the lateral loading beam are detachably connected to the sides of the two H-shaped steels (2). The lateral actuator (8) is detachably installed on the lateral adjustable beam. The linear actuating end of the lateral actuator (8) points to the pile foundation of the conductor frame. The lateral actuator (8) is connected to the lateral motor control cabinet.

7. A test method for simulating the cyclic performance evolution of a soft soil jacket pile foundation, applied to the test system for simulating the cyclic performance evolution of a soft soil jacket pile foundation according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1: four model piles (6) are sunk into the model soil (5) by means of a vertical actuator (7); S2: Perform a static load test, apply a static load to the jacket pile foundation through the lateral actuator (8), collect the test data of the static load test through the data acquisition system, and obtain the allowable bearing capacity Pu of the jacket pile foundation; S3: Based on the allowable bearing capacity Pu obtained from the static load test, a cyclic load test plan is formulated: first, a force cycle test is carried out, and the lateral actuator (8) is adjusted to the pressure mode through the lateral motor control cabinet and the lateral actuator (8) is controlled to load with four groups of cyclic loads of 0.25Pu, 0.5Pu, 0.75Pu and 1Pu respectively. The cyclic mode has two types of unidirectional cycle and bidirectional cycle, with a total of eight groups of tests. The test is terminated when the model pile (6) reaches the predetermined number of cycles, is damaged or deformed beyond the limit; S4: After completing eight sets of force cycle tests, according to the results of the force cycle tests, the lateral actuator (8) is adjusted to the displacement mode through the lateral motor control cabinet and the lateral actuator (8) is controlled to perform displacement unidirectional cycle tests with four sets of displacement data of 5mm, 10mm, 15mm and 20mm respectively. There are four sets of tests in total. When the model pile (6) reaches the predetermined number of cycles, is damaged or deformed beyond the limit, the test is terminated to obtain the final experimental results.

8. The test method for simulating the cyclic performance evolution of a soft soil jacket pile foundation according to claim 7, characterized in that: S1 specifically comprises: sinking four model piles (6) in a diagonally sequential manner at preset positions in the model soil (5); during the sinking process, a force transmission rod assembly is installed on the linear actuating end of the vertical actuator (7) to squeeze and contact the top of the model pile (6); the vertical actuator (7) is controlled by the vertical motor control cabinet to press each model pile (6) to a specified depth at a speed of 30 mm / min; after the pile sinking is completed, the jacket platform (10) and the tower (12) are fixed to the model pile (6) with bolts; and a level ruler is used to check verticality during the pile sinking process.

9. The test method for simulating the cyclic performance evolution of a soft soil jacket pile foundation according to claim 7, characterized in that: S2 is specifically as follows: the estimated ultimate load value is Py, and graded loading is performed according to 10% of the ultimate load, that is, 0.1Py. The offset data of each load level is observed regularly until it stabilizes. If there is a sudden increase in offset, continuous instability, or the maximum test load is reached, the loading is terminated. Then, the load is unloaded in stages and the rebound data is recorded. Finally, the ultimate bearing capacity and deformation parameters are determined by analyzing the load-offset curve, and the allowable bearing capacity Pu is calculated in combination with the safety factor.

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