Bearing Behavior Of Pile Group Foundation Under Vertical-Torsional Combined Loads In Sand Subsoil

With the continuous development of marine resources in China,structures have constantly being built,such as offshore platforms and offshore wind turbines.Pile group foundations have become a common foundation form in large-scale offshore projects due to their high bearing capacity and mature construction technology.The pile group foundation which installed in the ocean,not only sustain the vertical load(V),but also the torque(T)which arising from eccentric lateral forces.However,the conventional design method follows the superposition principle of small deformation and always ignores the coupling effect of V and T,so the bearing capacity of pile group foundation can’t be evaluated correctly.Moreover,the geometric parameters of the pile group foundation have a great influence on its bearing characteristics.Therefore,in order to investigate bearing characteristics of V-T combined loaded pile group foundation,the relating model experiment and numerical simulations are carried out in this paper,and the main works of this paper are summarized as follows:Firstly,based on the self-designed pile group model and combined loading system,the static load test of pile group model with different pile spacing was carried out in the sand subsoil.Then,the vertical and torsional ultimate bearing capacity of the pile group foundation,the internal force,the displacement of the foundation,and the failure form of the pile-soil system,which was sustained single vertical load V,torque T or V-T combined loads were obtained respectively.Based on this,the influence of magnitude of pile-top V-T combined loads and loading sequence(V→T or T→V)on the bearing characteristics of pile group in sand subsoil were analyzed.The analyzed bearing characteristics includes the tip resistance ration,torque sharing ratio pile group foundation and the bearing failure mechanism of pile group foundation with V-T combined load.Based on the analyzed results,the bearing capacity envelopes and simplified calculation formula of pile group foundation which was sustained V-T combined load and installed in sand foundation were obtained,and it can be used for engineering design reference.Secondly,a series of three-dimensional numerical models of pile group foundation which was sustained V-T combined load and installed in sand subsoil,were established by using ABAQUS,The simulation results were compared with the laboratory model test results and existing research results to verify its accuracy.Then,the influence law of parameters,which include elastic modulus of foundation pile,length-diameter ratio of pile,number of piles,friction angle and pile spacing of foundation piles,on the bearing characteristics of pile group foundation with V-T combined loaded and the deformation of pile-soil system are investigated.And the bearing characteristics of V-T combined loads pile group foundation in different soil layers or different loading methods are compared and analyzed.Lastly,the engineering application analysis of a wind power pile group foundation in Hangzhou Bay is carried out.Through numerical simulation,the vertical and torsional bearing deformation characteristics of pile group foundation and the bearing characteristics of V-T combined loaded pile group foundation are analyzed and verified with the above conclusions.