Nonlinear Seismic Response Analysis of Pile Foundations Interacting with Improved and Unimproved Soft Clay

Several ground improvement techniques, which have been proven to be effective and economical solutions for increasing the lateral stiffness and strength of weak soils around piles, can often result in unnecessarily conservative large volumes of soil improvement. Additionally, there are no rigorous techniques available to analyze the seismic behavior of piles in improved soils that can be utilized in day-to-day engineering practice. In this study, a stand-alone one-dimensional finite element computer code called DYPAC (Dynamic Piles Analysis Code), which uses the beams on nonlinear Winkler foundation (BNWF) approach, was developed to analyze the seismic response of a single pile in both improved and unimproved soils. The computer code models the pile as a beam element and the nonlinear soil behavior as springs and viscous dashpots using a nonlinear $p\text{-}y$ element, where $y$ = displacement; and $p$ = soil reaction per unit length of the pile. This study proposes and validates a method for modifying the $p\text{-}y$ curves to consider the limited lateral extent of ground improvement. The $p\text{-}y$ curves were inputted into DYPAC to analyze a series of dynamic centrifuge tests of single piles in soft clay improved using cement deep soil mixing (CDSM). Free-field site response analyses were performed using the DEEPSOIL computer program and the soil displacement-time histories were inputted into the free-field ends of the nonlinear $p\text{-}y$ elements. The predictions made by DYPAC were validated using the centrifuge data and the results show that the DYPAC predictions are reasonable. Furthermore, the proposed method for modifying $p\text{-}y$ curves to characterize CDSM-improved soil appears to be appropriate and practical. Moreover, the benefits of nonlinear DEEPSOIL free-field site response analyses during large earthquakes, as compared to equivalent linear analyses, were also demonstrated.