Seismic Soil Structural Interaction for Foundation Piles Embedded in Rock with Top Layers of Dense and Loose Sands

The Eurocode defines several ground types for the seismic structural analysis, but what ground type has to be adopted for the bridge piers in water of the Volta Bridge in Ghana, founded on four/six raked piles embedded in rock and with overlying layers of dense and soft sand? The structure is well anchored in the rock (ground type A), but the presence of top layers of sand puts it in the nominal ground type of class D or other. The analysis has been carried out assuming ground type A, and considering the structural interaction with the sandy soil. Two analysis methods are used, as follows: 1) Pseudo-Static Analysis with the Mononobe-Okabe solution, typically applied for the seismic earth pressures on retaining walls; 2) Dynamic Modal Response Spectrum Analysis, modelling the soil with equivalent mass and stiffness.