Assessing compressibility characteristics of silty soils from CPTU: lessons learnt from the Treporti Test Site, Venetian Lagoon (Italy)

This paper explores the capability of cone penetration testing as an effective tool to estimate the compressibility characteristics of silts, silt mixtures and sand mixtures. The study uses part of the large amount of piezocone (CPTU) data and subsoil deformation measurements assembled over approximately 6 years at the Treporti Test Site (Venice, Italy), within an extensive research project aimed at thoroughly analyzing the stress-strain-time response of the complex assortment of sandy and silty sediments forming the Venetian lagoon subsoil. The combined analysis of piezocone data and subsoil deformations measured beneath a full-scale test embankment primarily showed that the existing interpretation approaches developed for 'standard' sands or clays - and thus based on the idealized assumption of fully drained or fully undrained testing conditions - generally result in invalid estimates of soil compressibility parameters. Therefore, site-specific correlations have been developed in order to correctly predict the compressibility of Venetian sediments, both primary and secondary, and consequently obtain the best fit of the measured settlements. By taking this very well documented case study as a base, the proposed paper aims at discussing some key issues on the use of piezocone tests for the geotechnical characterization of silts and other sedimentary soils with very scattered grain size distributions, thus falling in the so-called intermediate permeability range. The idea is basically to provide a critical appraisal of the CPTU-based approaches currently available for the mechanical characterization of natural soils, and explore potential advances in the interpretation methods, which might take account of possible partial drainage phenomena around the advancing cone during the test.