Study On Size Effect And Cyclic Bearing Capacity Of Large-diameter Lateral Loaded Piles In Sandy Soil

Single pile foundation is commonly used in offshore wind turbines(OWTs)and cross-sea bridge construction.With the upgrading of China’s energy industry structure and the vigorous development of transportation infrastructure,single pile foundation has the characteristics of large diameter,long buried depth and complex service environment.The lateral static and cyclic bearing capacity of single pile are important guarantee for the structural system to resist wind,ocean current and traffic vibration loads,and it is the key index of foundation design.The traditional design method is based on small-diameter flexible long pile,and its applicability to large-diameter single pile is widely questioned by the industry.Recently,domestic and foreign scholars have carried out a lot of research work on the static and cyclic lateral response of large-diameter piles in sandy soil.However,there are some shortcomings,which are summarized as follows:(1)For lateral bearing capacity of pile,there is no research systematically explaining the generation mechanism and influencing factors of‘size effect’,and no corresponding lateral design method has been proposed.(2)In the research of cyclic lateral response of single pile,the characteristics of external load are too focused,and the research on the internal factors of pile-soil is slightly insufficient.In addition,there is a lack of in-depth study on the cyclic pile-soil interaction,and no evaluation method for the cyclic bearing capacity of single pile and the natural frequency shift of the OWT system that can consider the cyclic mechanism has been proposed.In view of the above problems,this paper carried out the study of the‘size effect’and cyclic bearing capacity of large-diameter lateral loaded piles by means of field test,1-g model test,theoretical analysis and numerical simulation.The main work and research results are listed as follows:(1)Based on the Hangzhou-Shaoyong Expressway project,the in-situ lateral loading tests of large-diameter single pile with pile diameter D=1.2 m,1.8 m and 2.8 m are carried out in Shaoxing and Xiaoshan,and the corresponding load-displacement curves,pile bending moment,pile displacement curves and pile p-y curves are obtained.The applicability of the existing p-y models by API,S(?)rensen 2012 and Amar Bouzid is compared and analyzed,the shortcomings of conventional methods are revealed,the influence of pile diameter on the p-y initial stiffness is explored,and the modified p-y curve model suitable for local soil layer is established.(2)The three-dimensional finite element simulation of large-diameter lateral loaded pile in sandy soil foundation is carried out.The distribution characteristics of horizontal earth pressure,horizontal friction and vertical friction along the pile-soil interface are obtained.The Moment Contribution Ratio(MCR)of lateral soil resistance,shaft-resisting moment and pile tip shear is quantitatively evaluated by integrated analysis of interface resistance.The influences of the pile slenderness ratio,pile diameter and soil relative density on the MCR of each resistance are compared and analyzed.The mathematical expression of the MCR of each resistance in the initial elastic stage is proposed,and the transformation mechanism of pile-soil interaction from a single‘p-y’spring to a‘p-y+m-θ’double spring and finally to a‘p-y+m-θ+Q_b-y_b’three-spring model is revealed in the process of single pile developing from flexibility to rigidity.(3)Based on the finite element elastic model,the initial stiffness of p-y curve,m-θcurve and Q_b-y_b curve are studied,and the effects of pile diameter,slenderness ratio and soil elastic modulus on the initial stiffness of each curve are compared and analyzed.Based on the’equivalent pile head stiffness’method,the initial stiffness of the nonlinear distribution along the buried depth is transformed into a constant distribution,and the initial stiffness model of each curve in the homogeneous stiffness soil layer is obtained.The influence of the nonlinear distribution mode of soil stiffness is further considered,and the corresponding modified initial stiffness model is established.Considering the distribution forms of circumferential horizontal soil pressure,horizontal friction and vertical friction in the pile-soil interface,the distribution functions are fitted,and the calculation model of ultimate soil resistance of each curve is obtained based on integral solution.The’p-y+m-θ+Q_b-y_b’calculation model of lateral loaded pile is established by using nonlinear hyperbolic function as the backbone curve.The rationality of the calculation model is verified by engineering examples.(4)The 1-g model test of pile bearing capacity under lateral cyclic loading is carried out by using the self-designed cyclic loading device.The effects of cyclic amplitude ratio,cyclic loading ratio and pile-soil relative stiffness on the pile cumulative displacement,cyclic stiffness and the peak bending moment of pile body are analyzed.The differences of soil deformation modes around pile under one-way and two-way cyclic loads are preliminarily explored.Through the summary and analysis of the relevant test data,two empirical models considered respectively Reese and Poulos pile-soil relative stiffness are proposed to predict the accumulated displacement under one-way cyclic loads,and the rationality of the model is demonstrated by engineering application analysis.The rationality of geometric similarity ratio method,Reese and Poulos critical pile length similarity ratio method in 1-g model test design is discussed.(5)PIV model test of half-model single pile under lateral cyclic loading is carried out by using the improved cyclic loading equipment.The effects of cyclic loading ratio,cyclic amplitude ratio and pile-soil relative stiffness on the soil deformation characteristics around the pile are analyzed,and the deformation mechanism of subsidence-densification coupling is revealed.The geometric deformation mode and subsidence development model of soil around the pile under two-way cyclic loading are established.Based on the soil mass conservation principle between subsidence and densification region,the transformation of soil mechanical parameters before and after the cycle is deduced.Combined with the three-spring calculation model,the theoretical analysis method of pile bearing characteristics after cyclic loading and the analysis framework of natural frequency shift are proposed.Based on engineering cases,the parametric sensitivity analysis of soil relative density,pile embedded depth and cyclic amplitude ratio is carried out,and the cyclic response mechanism of single pile is revealed.