Studies On Modified P-y Curves Method Of Monopile Based On The Degradation Stiffness Model Of Sand

As an important means to further promote the revolution of energy production and consumption,as well as prevent and control the air pollution,offshore wind power generation has received national attention.Large diameter single pile is the most widely used foundation form of the offshore wind farm.These large diameter foundations will be subjected to long-term of horizontal cyclic low-frequency loads such as wind and wave loads.The cyclic stiffness of soil around piles tends to weaken,which leads to the decrease of bearing capacity of pile foundation and the accumulation of lateral displacement,it may even endanger the safety of offshore structures.At present,the p-y curve method is often used to analyze the horizontal load of single pile.However,most of the existing p-y curves are based on the loading tests of small diameter piles under limited cyclic times,which makes it difficult to accurately describe the displacement accumulation of large diameter piles under long-term horizontal cyclic loads and the cyclic weakening characteristics of soil.The main purpose of this paper is to study the p-y curve of large diameter pile foundation under medium and long-term cyclic loading of sand.The main work is listed as follows:(1)Based on the subroutine provided by ABAQUS,a stiffness degradation model of pile-soil system considering the long-term cyclic weakening effect of sand was established,and the p-y curves of large-diameter pile were obtained on the basis of numerical analysis..The effect of scale and circulation on p-y curves of pile foundation are analyzed,and the traditional p-y curves was proved not be able to adapted to analyze the mechanical characteristics of large diameter piles under long-term cyclic loading.(2)Based on the finite element analysis of three-dimensional large diameter single pile foundation,the scale effect caused by the change of pile diameter is studied.The ultimate soil reaction force and initial soil reaction modulus of the p-y curves increase with the increase of pile diameter.According to the numerical analysis results,the correction of the p-y curves expression considering the scale effect is proposed.(3)Based on the finite element model considering the cyclic weakening effect of sand,the influence of cyclic effect on the mechanical properties of large diameter piles is studied.The results show that the mechanical properties of large diameter piles are mainly determined by the initial modulus of foundation reaction.Under cyclic loading,cyclic weakening effect occurs in sand,and the initial foundation reaction modulus of large-diameter pile decreases continuously,the attenuation range is related to the load level H and the number of cycles N at the depth.(4)According to the results of finite element analysis,by introducing the concept of cyclic reaction ratio,the stiffness degradation factor of pile-soil system was obtained,finally,a modified p-y curves method based on the degradation stiffness model of sand was proposed.By comparing with the experimental results,the validity of this method is verified.This modified p-y curves,which is able to consider the effects of loading characteristics,number of cycles,cyclic weakening of sand stiffness,provides a concise and effective analysis method for the preliminary design of monopile foundation under long-term horizontal cyclic loading.