Multivariate Geostatistical Analysis of CPT Readings for Reliable 3D Subsoil Modeling of Heterogeneous Alluvial Deposits in Padania Plain

Urban planning and big infrastructure designing demand two novel and seemingly contrasting approaches: 1) a continuous description of subsoil nature and behavior under natural hazards, to increase the resilience of urban areas; and 2) a reliable characterization of subsoil hydro-mechanical properties and monitoring their working behavior. Both exigences can be addressed by reconstructing 3D mechanical models at a local scale by extracting from large databases several in-situ testings, already available for several urbanized territories worldwide. In this paper, 182 cone tip resistance qc, sleeve friction fs, and pore pressure u2 profiles, drawn from CPTs performed in the Bologna district (Padania Plain, Italy), have been used. Here, the alluvial deposits are mixtures of silt, clay, and sands, and they locally show gravel lenses where the ancient fans from the Apennines can be detected. Their heterogeneous hydro-mechanical characters cannot be described only through point investigations such as CPTs. Additionally, the variability of these mechanical profiles and the uncertainties due to the limited amount of data must be assessed and used in designing and hazard mapping. Thus, to draw a continuous 3D subsoil mechanical model based on these 182 CPTs, the Partially Heterotopic Co-Kriging technique (PHCK) has been applied. This approach is a multivariate technique that can be used when only some measurements are taken at the same locations. It allows for the estimation of the distribution of qc, fs, and u2 values in the studied domain by considering the spatial variability of the preceding random functions but also their spatial correlations. Differences in variance and spatial resolution between the measurements on the horizontal plane and along the vertical direction were accounted for by considering anisotropic spatial dependence models. As a result, this study has provided horizontal maps and vertical sections of qc, fs, and u2, as well as their 3D solid models.