A numerical study on the filtering action of piles in the soft clay of Maliakos Gulf, Central Greece

The potential of piles to reduce the seismic motion transmitted to the superstructure with respect to the free field motion, due to the kinematic interplay between soil and pile, is explored by considering a single pile embedded in the soft marine clay of Maliakos Gulf subsoil in central Greece. The soil profile consists of a very soft normally consolidated clay layer with nearly-zero stiffness at ground surface, increasing proportionally with depth, underlain by a stiffer layer of sandy clay. The problem is tackled numerically by means of Equivalent linear (EL) and strength-controlled non-linear (NL) constitutive laws to model the behavior of the soil under seismic excitation. The latter is chosen on a magnitude-epicentral distance (M-R) basis from the European Strong-motion Database (ESD), taking into account the seismotectonic regime of the broader area under study. Seven (7) motions recorded at soil type A, according to EC8, were chosen to define an average code-compatible spectrum. The average mobilized stiffness of the free field soil obtained from EL and NL analyses is then introduced into a finite-element model of the soil-pile system to investigate the kinematic-induced filtering action of the pile as affected by pile cap embedment depth and pile diameter. The above is quantified in terms of ratios of spectral accelerations at the pile head and free field at ground surface. Results highlight the importance of considering NL response for such type of soils and conclude on the beneficial role of piles as reflected in a large reduction of the free field spectral acceleration, even for piles with relatively small diameter.