GNGTS 2015 - Atti del 34° Convegno Nazionale

2D numerical modelling of seismically induced strain effects in a complex geological system hosting a recently urbanized neighbour of Rome C. Varone 1 , L. Lenti 2 , S. Martino 1 , G. Scarascia Mugnozza 1 , J.F. Semblat 2 1 Department of Earth Sciences and Research Center for the Geological Risks (CERI), University Sapienza, Rome, Italy 2 Université Paris-Est LCPC/Institut Français des Sciences et Technologies des Transports, de l’Aménagement et des Réseaux (IFSTTAR)/ Departement GERS, Champs sur Marne, France Introduction. The Fosso di Vallerano valley (Rome, Italy) was selected as case study to evaluate the Site-City Interaction (SCI - Kham et al. , 2006; Semblat et al. , 2008), i.e. the influence of buildings on the local seismic response and on the seismically-induced effects of alluvial fills. The valley was chosen as it is characterized by a highly heterogeneous geological setting and it is one of the most recent urbanized areas in Rome. More in particular, the Fosso di Vallerano valley hosts the “Europarco Business Park” i.e. the highest buildings (120 m) in Rome. The first phase of the study was focused on the reconstruction of the engineering- geological model of the valley as well as on 1D numerical modeling of the seismo-stratigraphic setting of the alluvial body (Bozzano et al. , 2015; Varone et al. , 2014) that was used to calibrate the dynamic properties of the local seismostratigraphy. In a second phase, 2D numerical models were performed to analyze the local seismic response and the inducted strain effects assuming visco-elastic and visco-plastic modelling conditions for the free-field. Preliminary considerations were obtained on possible interactions between site and buildings even if more specific models will be implemented to better highlight the Site-City Interaction of the area. Engineering-geological model. The subsoil geology of the Fosso di Vallerano valley was reconstructed thanks to 250 borehole logs as well as in-site geomechanical investigations, available from technical reports and official documents (Varone et al. , 2014). The geological model was integrated by a geophysical data set available from field surveys in order to provide a high-resolution engineering-geological model of the valley. Based on such data four main lithotechnical units were distinguished: i) Plio-Pleistocene marine deposits (Marne Vaticane Formation) composed by high consistency clays with silty- sandy levels; ii) Pleistocene alluvial deposits of the Paleo Tiber 4 River (650-600 kyr) composed by soils including graves, sands and clays; iii) volcanic deposits of the Alban Hills and of the Monti Sabatini Volcanic Districts (561-360 kyr) consisting of highly heterogeneous tuffs; iv) recent alluvial deposits that filled the valley incisions since the end of the Würmian regression (18 kyr-Present), characterized by a basal gravel level and including by different soft soils from sands to inorganic or peaty clays. In particular, the Plio-Pleistocene marine deposits represent the local geological bedrock. Mechanical and dynamic properties (Tab. 1) were attributed to each lithotechnical unit according to literature data (Bozzano et al. , 2015 and references therein). Numerical modeling. The numerical modeling actually represents the main tool to estimate local seismic response and seismically inducted effects, particularly in the urban area where the geophysical measurements are often not suitable for highlighting the local seismic response. A proper 2D numerical modeling was performed through CESAR-LCPC code implemented by the Institute of Paris IFSTTAR based on the Finite Element Method (FEM) considering the geological cross section shown in Fig. 1 (bottom). Calibration of the absorbing boundary conditions for 2D numerical modeling. The numerical analysis of elastic wave propagation in unbounded media can be difficult due to spurious waves reflected at the mode artificial boundaries; this point is particularly critical for the analysis of wave propagation in heterogeneous or layered systems as in the present study. In this regard, Semblat et al. (2011) proposed an absorbing layer solution, based on Rayleigh/Caughey damping formulation that considers both homogeneous and heterogeneous damping in the absorbing layers. The efficiency of the method was tested through 1D and 2D FEM simulations, GNGTS 2015 S essione 2.2 177