Seismic Analysis and Design of Pile Group Bridge Foundations in Soft and Liquefied Soil

A pile group foundation is a highly indeterminate system with non-linearities caused by soils and pile and cap materials. The cap footing is often sufficiently rigid to develop a linear variation in the pile axial forces induced by the imposed moments. However, a nonlinear foundation–soil interaction analysis is required to perform a reliable evaluation of capacity and displacement demands for seismic design. A detailed nonlinear analysis using advanced numerical techniques, such as the finite element method to model the soil response and soil–foundation interactions during a seismic event, can be complex and time-consuming. The complexity is significantly greater when the foundation supports high and multidirectional dynamic loads induced during seismic events, especially when the foundation soil is soft or predicted to liquefy. Because of time and resource constraints—and for the case of liquefied soils, considering the difficulties and uncertainties associated with developing constitutive models—such an analysis is often neither feasible nor desirable for small to medium-size projects. It is, therefore, necessary to develop simplified and practical analysis methods, with clearly defined applicability and limitations, and with appropriate built-in conservatism, for the seismic analysis and design of such foundations. This paper summarizes the results of a recent study on the subject. Applications of simplified two-dimensional and three-dimensional analysis methods are demonstrated. Effects of the footing rigidity and direction of the applied seismic shear and moment on the response are also presented.