| Due to the merits of high capacity,installation ease,and reliable quality,monopile foundation has been widely used in the construction of both onshore and offshore structure foundations.Different form closed-ended pile and grouted pile,plugging effect will occur during the installation process of open-ended pile,which in turn influence its penetration behavior and loading response.With the increase of pile foundation size and construction difficulty,the interplay between soil plug and pile will be more complicated.With increasing demand for marine resources,monopile foundation is gaining increasing popularity in the construction of marine structures.Different from onshore pile foundation,offshore monopile foundation not only supports the self-weight of the above structure,but also bears the long-term and irregular lateral load caused by wind,wave,currents,traffic,etc.This special loading mode will significantly influence the long-term interaction between monopile foundation and its surrounding soil,undermining the stability of the upper structure.With marine structures moving to deeper waters,simply increasing the size of monopiles is no longer feasible for both economical and practical purpose.It is necessary to propose more reliable and effective foundation type for marine structures.Considering the above background,model tests and numerical modelling were used to study the mechanism of soil plug formation and development during the installation and loading process of open-ended pile and analyze the load response of monopile and hybrid pile foundation and the migration pattern of surrounding soil under cyclic lateral load.The main works and conclusions are listed below:(1)A total of 42 sets of model tests were conducted to analyze the installation and loading response of pipe piles,with a focus on the influence of pile installation method,pile end condition,pile diameter,soil density.Tests results indicated that with increasing penetration depth and sand density,plugging effect would be more obvious,and the difference of installation resistance between open and closed-ended pile would be less noticeable.The jacked pile had the largest the vertical capacity,which is around twice that of the non-displacement pile,the capacity of driven pile was somewhere between.Under lateral load,model pile tended to rotate around a certain axis at the lower part of the pile,which is known as rotation point.The locations of the rotation point were in the range of 0.15?0.25 L deep.It is necessary to considering the influence of pile installation during pile capacity design.(2)Based on the model test setup,2D discrete element method was used to analyze the mechanism of soil plug formation and development.A linear fitting relation between PLR and IFR was obtained.Based on the comparison between the development of IFR and inside shaft resistance,an analytical model of the pile-soil interaction considering different plugging effect was proposed.According to the analysis of the deformation and loading response of soil plug,it assumed that the soil plug could be treated as a “soft pile”being pushed upwards during the pile loading process.Under static lateral load,soil plug seemed to rotate with the pile.(3)Based on self-designed loading device,lateral load tests on model pile foundation in saturated sand foundation were conducted.A total of 16 static load tests and 5 cyclic load tests was performed by considering the influence of pile stiffness and combination mode of hybrid pile foundation.The load-displacement response and failure pattern of the model foundation were systematically analyzed.At small displacement stage(with pile head displacement around 0.1 D),the increase of pile stiffness and application of hybrid foundation could effectively improve load resistance.At ultimate stage,the lateral capacity of stiff pile was close to that of the hybrid foundation.Power functions were used to estimate the development of accumulated lateral displacement against cycling number.Colored sand was placed around the foundation to track the particle migration trajectory.Test results indicated that particle migration pattern was highly related to the displacement magnitude and deformation pattern of the structure.(4)3D discrete element model of the monopile and hybrid pile foundation was constructed.The validity of the numerical model was further verified based on numerical triaxial tests and CPT.Based on the deformation contour of foundation and surrounding soil,the deformation pattern of soft pile under static loading gradually transferred from soft response to stiff-soft response,while the deformation pattern for stiff pile and hybrid pile foundation was in rotation mode.For hybrid foundation,it suggested that the pile length could be shortened to cut the construction cost and maintain similar lateral capacity of the foundation.The analysis of soil displacement pattern and soil resistance development revealed the 3D convective nature of the surrounding soil.It assumed that soil plug could be regarded as additional mass and moved together with the structure. |
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