Assessment of Soil Shaking Features in Urban Areas

Abstract

The city of Cosenza is located in the southern part of the Crati Basin (Calabria, Italy), an asymmetric graben filled by sedimentary marine covers mainly consisting in sands, gravels and clays from Miocene to Pleistocene, and alluvial deposits of Crati river and its tributaries. Many geological studies were performed in the Crati Basin (Tortorici et al., 1995, 2002, Van Dijk et al., 2000; Spina et al., 2012), with the aim to reconstruct the actual stratigraphy of the area and to define its structural geological setting. The historical centre of Cosenza has been affected by numerous earthquakes during last year causing many damages to its architectural heritage; nevertheless, until now, no microzonation studies were performed in it, and no detailed geotechnical studies on the near-surface geology are available in literature. In order to fill this gap and better understand and quantify the expected ground shaking within the city of Cosenza, a multidisciplinary research project called pon-MASSIMO project (http://openmap.rm.ingv.it/ponmassimo/) was carried on. One of the purposes of the MASSIMO project is to study and monitor the response of different types of architectural buildings to seismic stresses considering the near surface geology, the topographic characteristics and the dynamic behaviour of soils in relation to the monument architectural and static preservation. The Test Site chosen by the MASSIMO project is the San Agostino Church and the annex Monumental Complex of Bretti&Enotrii. The choice of Test Site was made by Cultural Heritage considering its historical and architectural importance. The purposes of this PhD Thesis, taking part of the MASSIMO project, are realize a geological reconstruction of the top of the metamorphic bedrock and assess possible site effects in the area of the Test Site chosen by the MASSIMO project. To reach the aforementioned aims a detailed analysis of the physical and mechanical properties of the different lithotypes constituting the recent sedimentary fill of Crati river Valley, as well as the corresponding bedrock, under both static and dynamic conditions was made. The study was performed by the drilling of 5 boreholes (S1-S5) in the urban area of the city of Cosenza, down-hole and laboratory tests on sampled soils. The results of the boreholes revealed a critical positions for the Test Site, located between two different lithologies, marine sands and alluvial deposits, potentially able to generate differential oscillation modes in the building (Gaudiosi et al., 2015), and a deepening of the contact between sedimentary covers and metamorphic bedrock from the area of the Test Site (South) to Site S5 (North). Use of ambient vibrations (Nakamura technique, 1989, seismic noise arrays) allowed to define the resonance frequencies where the soil shaking is amplified and potential able to damage the civil infrastructures, and also obtain a complete shear-wave velocity profile for one site (S5), able to reach the depth of the metamorphic bedrock forming the basement of the Crati Basin. Undisturbed samples collected during boreholes allowed to obtain decay curves G(ϒ)/G0 and damping curves D(ϒ) through resonant column (RC) and cyclic torsional-shear test (TTC) at 1 Hz for layer SCS (silty-clayed sands) in site S5. Simulations were performed with the software EERA in order to reconstruct vertical profiles of the maximum shear deformations that could be obtained in the sedimentary covers in response of the maximum expected earthquakes for the city of Cosenza (PGA of up to 0.005 g). The 1D modelling obtained for the site S5, located in central part of the High Crati Valley, shows that non-linear effects can be induced on the aforementioned layer of silty-clayed sands (level SCS). Finally, the amount of data provided during boreholes, supplied a useful tool for a preliminary liquefaction assessment of the study area. Testing liquefaction risk following the simplified procedure proposed by Andrus and Stokoe,(2000) revealed that none of the sites is susceptible to liquefy.