ABSTRACT
This paper illustrates the results of the geognostic investigation made for the Centocelle Park station design (subway line C in Rome), in order to determine the deformability parameters of pyroclastic soils interested by the excavation. The in-situ investigation, consisting of seismic tests (RE.MI. Refraction Microtremor), permitted an estimate of the elastic modulus at small strains (from 0.01% to 0.1%) that are usually reached in the open pit station (in this case approximatively 27 meters). A comparison was made between the results obtained from traditional investigations (pressuremeter tests and, only for lithoid soils, unconfined compression tests) and those obtained from the seismic investigation. The analysis of the behavior of the deep excavations under working conditions is subordinate to the knowledge of the elastic modulus at small deformations (G0 or E0). The numerous researches made on the behavior of soils at small strains (Rampello & Silvestri, 1993; Stokoe et al., 1995), have shown that the G0 modulus can be taken as a “state parameter”, like the initial voids ratio e0, the earth pressure coefficient at rest K0 or the effective vertical lithostatic stress σ'v0 (Ghionna et al., 2006). For these reasons, in the design practice, there is a need to define the shear modulus G0 variation with depth for all layers interested by excavation. The nature of this “fundamental” parameter is better defined by the relation between G0 and propagation speed of shear waves provided by the elastic theory:
The whole thickness of Volcanites is represented by:
− The Superior Complex of Colli Albani volcanic products, composed by altered pyroclastics materials called Villa Senni Tuff (VS) deposited on top the Lionato Tuff (TL) and finally over the Black (PN) and Red Pozzolanas (PR).
