The influence of meso-structure on the mechanical behaviour of marly clays from low to high strains.

On the 31st of October 2002, a ML=5.5 earthquake struck the Molise region in Southern Italy. The earthquake was followed by a comparable aftershock (ML=5.4) nearby on the day after, and by a series of seismic events of lower energy in the following month (Fig. 1). In the town of San Giuliano di Puglia, the distribution of the damage consequent to the mainshocks was strongly non-uniform: the old part, which was less damaged, lies on outcropping rock, while the most severe damages were surprisingly concentrated in the new part of the town, lying on fine-grained soils. This evidence suggested that differential site amplification significantly affected the seismic response in the town. This hypothesis has been confirmed by aftershock records logged by two temporary accelerometric stations, located both in the ancient and in the new portions of the town (Puglia, 2005). In order to develop a seismic microzonation of the area (Baranello et al., 2003), the Department of Civil Protection of the Italian Government committed a comprehensive investigation on the subsoil properties. The experimental programme included: boreholes, in situ piezometric measurements, CPT, cross-hole and down-hole tests and laboratory investigations on undisturbed samples, namely oedometer, triaxial and cyclic/dynamic torsional shear tests. The geotechnical laboratories of both the University of Naples and the Technical University of Bari were entrusted by the Department of Civil Protection with the laboratory investigations, aiming at the characterisation of the mechanical behaviour of the fine-grained soils. The present paper discusses the main experimental results of such laboratory investigations, as well as a comparison with seismic in-situ tests, giving evidence to the dependency of both the soil strength properties and stress-strain behaviour on the soil structural features.