Processi di subsidenza nei depositi alluvionali olocenici nella Città di Roma. Caratteristiche stratigrafiche e geotecniche

The City of Rome is located in an area where long human activity and continuous transformation of the original terrain are tangible. The hydrographic networks of the Tiber's right and left banks have been modified more than once during historical times. Sometimes it has even been obliterated by urban growth (Capelli, G.,1999), so that today it is very hard, at least in the historical centre's area, to recognize the original terrain. Many of Rome’s streets run along the ancient courses of the Tiber's tributaries and many buildings lie upon alluvial deposits now buried by anthropogenic debris. Many structures overlying alluvium and debris have been damaged by subsidence and effects are visible in the buildings' uniform or differential settlement.In past, the alluvial deposits were considered as continuous bodies made of clayey-silty, sometimes sandy sediments. Instead, the formations are mainly heterogeneous and, as it will be made clear in this paper, consist of many facies (Kiersch, G.A,,1995). The geotechnical characterization of those units is mandatory for evaluating the geological environment's intrinsic hazard in urban areas, where the risk can reach very high values. This study has been carried out through the analysis of borehole and geotechnical data from three left-bank tributaries of the Tiber River the “Fosso del Velabro”, “Marrana della Caffarella”, “Fosso di Grotta Perfetta” and three right-bank tributaries (“Valle dell’Inferno”, a tributary stream of the “Fosso dei Tiradiavoli”, and the “Fosso della Maglianella”). Data about the Tiber’s alluvial deposit from various parts of the city were also included. Based on the geotechnical analysis, we created, for each deposit, a subdivision into lithotechnical units to make correlations and comparisons among the different deposits. The same level of detail has not been possible for all the stream valleys, since it is not always possible to obtain geotechnical data. Nonetheless, it is always been possible to define a stratigraphic series that would represent the examined deposit by subdividing it into units after stratigraphical and sedimentological observations