The behavior of a spread footing over reinforced ground with gravel interface during a strong earthquake

For the design of the Golden Ears approach bridge in Vancouver (Canada), a spread footing over a pile-reinforced ground with gravel interface was considered as an alternative to a conventional pile foundation. The mechanisms governing the behavior of the spread footing during strong earthquake events have been investigated in a numerical Finite-Element (FE) analysis using a (visco-) hypoplastic constitutive relationship. The FE model includes the soil, the piles, the footing and the gravel layer. The superstructure and the pier are represented by a point mass attached to the end of a vertical beam. Material parameters for the soil models were derived from available field and laboratory tests. The numerical model was validated using results from a large-scale in-situ test with a footing above a single pile. The goal of the numerical study was the investigation of the influence of pile spacing and gravel layer thickness on the dynamic response of the footing, the superstructure and internal pile forces during a strong earthquake. A significant kinematic decoupling between footing and improved soft soil through the gravel layer did not occur in the simulations. Analysis results show that the internal forces in the reinforcement piles are considerably smaller compared with those in a conventional pile foundation, particularly in the upper part of the pile. The pile spacing was found to have a minor influence on the internal pile forces. In the range investigated, a dependence between the bending moments and shear forces in the piles and the thickness of the gravel layer was not observed.