Aleatory variability and site effects: the problem of soil classification

The seismic excitation to consider for buildings assessment requires the preliminary soil classification of the site, based on the uppermost 30 m shear-wave velocity, and the conse-quent assumption of the Code spectra. In some cases, however, the soil classification is not easy to pursue, due to the variability of the soil within the area of interest. This paper deals with the aleatory variability of the soil with reference to a real site in Tuscany (Italy), whose soil properties have been determined through a detailed investigation, made of four different methods. Indeed, a seismic refraction test, a down-hall test, a multi-channel analysis of surface waves combined to an extended spatial auto-correlation analysis, besides some single station recordings of the horizontal to vertical spectral ratio for ambient vibrations have been made on the area. Despite the large amount of collected information, an unequivocal soil characterization has not been possible, due to the large variation of the soil stratigraphy. Therefore, the evaluation of seismic demand has been made by performing a Site Response Analysis on alternative cross-sections, representing the different soil profiles obtained from the investigation. The seismic input at the bedrock has been represented by seven ground motions, provided by two alternative databases. Furthermore, two alternative convolution analyses, based on different soil modelling, have been applied to find the surface seismic input. The effects of the aleatory uncertainty related to the soil variability has been compared to the ones of the epistemic uncertainty, related to the choices made to perform the seismic site response analysis.