Bearing Characteristics And Accumulative Deformation Of Monopile Foundations Under Cyclic Lateral Loading

With the rapid development of China’s economy,the problem of energy shortage is becoming increasingly serious.The development and utilization of new energy has become the focus of national energy development strategy.Wind power currently offers a very competitive source of renewable energy due to its abundant resources,stable wind speed and small environmental constraints.The offshore monopile in marine environment is subjected to horizontal cyclic load of millions to tens of millions of times in its life cycle,which is easy to cause accumulated deformation and inclination of foundation,and thus affect the service life.Therefore,the accumulating deformation characteristics of monopile under horizontal cyclic load are concerned in the industry.The method of reducing ultimate soil resistance in API code is used in engineering design,but the rationality of the design method is questioned because it fails to consider the influence of cycle times and cyclic load characteristics.This thesis presents experimental and theoretical research on the critical problems of monopile foundations subject to lateral cyclic loading.The results show that under the action of horizontal cyclic load,single pile produces irrecoverable accumulating displacement,which is affected by the characteristics of cyclic load,the number of cycles and the relative stiffness of pile-soil.A semi-empirical model of accumulating was proposed,which can consider the relative stiffness of pile-soil,the number of cycles and the characteristics of cyclic load.By comparing the unloading rigidity and the bending moment with the number of cycles,the influence of horizontal cyclic load on the bearing characteristics of pile foundation is revealed,as well as the competitive mechanism of the response of pile foundation to the accumulating deformation and the change of bearing characteristics of the pile foundation.Based on the test results,a cyclic backbone curve model is developed,which correlates the spring parameters with the number of cycles to capture the macroscopic response of the pile under cyclic load.the parameter sensitivity of pile foundations response are analyzed,with the relevant suggestions for improving the bearing capacity of pile foundation under long-term cyclic load.Propose a method for calculating long-term response of pile foundation under specific cyclic load based on static load backbone curve,which is verified by results of model tests.The results show that this method can accurately predict the accumulating displacement of monopile under cyclic load,and can take into account the effects of cyclic times and load characteristics.It is simple and accurate to calculate and can be applied in the design and calculation of monopile foundations for offshore wind farms.