Centrifuge Modelling And3-D Numerical Analysis Of Large Diameter Single Pile Under Lateral Loads

In recent years, with the development of offshore engineering, especially the rapid emergence of offshore wind power industry, pile foundation takes on the tendency of increase in both diameter and penetration. The most common foundation types used for offshore wind energy converters at home and abroad are the monopile, the tripod foundations, the jacket foundations and the pile group foundation. Among them, the diameter of monopiles can vary between3to8m, even if foundation piles for pile group can be up to1.5-2.5m.At present, the p-y curve method is recognized as the most effective method for the nonlinear lateral behaviour of the pile foundation, and has been adopted by American petroleum institute (API). The current Codes for pile foundation of harbour engineering (JTJ254-98) also recommend the p-y curve method, but is basically quoted API code. However the existing p-y curves are developed for pile tests with limited size. The current p-y curve approach given in the API code cannot be used for large diameter piles subjected to lateral loads.To deal with this problem, this paper investigates the behaviours of large diameter piles in sand under lateral static and cyclic loading based on several centrifuge model tests. The calculation approach for lateral displacement of pile shaft and soil reaction force using measured bending moment of the pile shaft is verified. Static p-y curves of large diameter piles in both dry and saturated sands are also obtained. By modifying the initial stiffness of p-y curves, a series of hyperbolic p-y curves are presented to compute internal force and deformation of large diameter piles. The change of deformation and internal force of large diameter pile foundations under one-way cyclic loading is disclosed. Test results show that the head displacement and the maximum moment of pile shaft and unloading stiffness are almost linearly increasing logarithmically with number of cycles. At last, the cyclic p-y curves of large diameter pile foundations according to bending moment of the pile shaft in each cycle are obtained, a cyclic degradation factor which is related to the cyclic stress ratio and a correspondingly approach for analysing cyclic deformation and internal force of the pile shaft are proposed.Further, drained triaxial compression test on dilatancy sand was conducted to obtain physical and mechanical characteristic parameters. Based on general-purpose finite element software ABAQUS, the behaviours of large diameter pile foundation under horizontal loading are analysed using a three-dimensional finite element model, stress paths for selected soil elements around a pile during loading are disclosed, the effects of soil properties, i.e., friction angle, dilatancy angle and the characteristic of pile-soil contact interface on the calculation result of the three-dimensional finite element model are also investigated. On this basis, a reasonable finite element computational model of large diameter pile foundation under horizontal loading including the parameter selection method of this model is established and verified by centrifuge model tests. With that, the suitability of existing methods for analysing horizontal bearing capacity and deformation of pile foundation is discussed. Finally, the method to determine the critical embedded length of monopile foundation is suggested.