Time domain nonlinear lateral response of dynamically loaded composite caisson-piles foundations in layered cohesive soils

A nonlinear Winkler model for composite caisson-piles foundation (CCPF) is proposed by joining the two components, the caisson and the pile group, in which the nonlinear four-spring Winkler model is utilized for the caisson and the axial-lateral coupled vibration equations are derived for the pile group. Then the nonlinear lateral dynamic response of CCPFs embedded in layered cohesive soils and loaded at its top center is investigated by a simplified time domain method based on the model. All the impedance functions of the foundations are frequency independent, which make it easy to conduct a standard time domain analysis. The results are compared with 3D finite element simulations and the consistence convincingly verifies the reliability of the simplified method. The nonlinear Winkler model is also extended to consider the gapping and the cyclic degradation, and it is shown to be capable of reproducing various important nonlinear features such as oval-shaped or s-shaped hysteresis loops. Finally, the lateral dynamic responses of CCPFs with three different configurations of pile group are analyzed. The results indicate that the configuration of pile group beneath the caisson plays an important role in the lateral dynamic response of the CCPF. The results also convincingly illustrate the important role of considering the soil nonlinearity, cyclic degradation and gapping in the dynamic analysis of the CCPF.