Pile design in laterally spreading soil: Feedback from numerical predictions and model test results

Abstract

The main findings are summarized of a systematic research effort regarding the response of pile foundations in laterally spreading soils. The incentive of the research were findings from properly scaled (with regard to the pore fluid) centrifuge experiments suggesting that severe soil dilation may occur at the upper part of the pile, as a result of large soil-pile relative movement, causing soil pressures to significantly increase. In essence, this observation negates current design practice which univocally accepts that soil liquefaction drastically reduces seismic demands. To further explore this new evidence, a 3D nonlinear numerical methodology was developed and tested against the aforementioned experiments. On this basis valuable feedback was first gained with regard to current numerical techniques, with most important the need of a new type of tied-node boundaries for the simulation of submerged infinite slopes. Comparative analyses, with the old and the new boundaries, revealed that the former (which also reflect the kinematic response of the laminar box containers employed in model tests) can significantly underestimate soil pressures imposed to the foundation. In the sequel, the numerical methodology was applied parametrically (for various soil, pile and excitation characteristics) and a new set of multivariable relationships was statistically established for the practical estimation of ultimate soil pressures applied to the pile. Compared to existing empirical relationships, the new ones can capture the aforementioned dilation phenomena, which may develop at the upper part of the pile in the case of relatively low permeability soils (e.g. fine-grained, silty sands) and have an verall detrimental effect on pile response.