Dynamic properties of low velocity alluvial deposits influencing seismically-induced shear strains: the Grottaperfetta valley test-site (Rome, Italy)

Abstract

The presence of peats and high compressibility inorganic clays within alluvial fills on the left-side tributaries of the Tiber River, close to Rome's historic center in Italy, is well documented in literature. Nevertheless, few literature data exist until now on the dynamic properties of these deposits by undisturbed bore-hole samples. The Galba test-site was set up to characterise dynamic properties of the alluvial deposits by using lab-tests as well as to derive velocity profiles by seismic noise measurements. These were performed in the Giustiniano Imperatore area located in the Grottaperfetta valley, about 2 km south of Rome's historic centre. The alluvial deposits filled a paleovalley excavated in the bedrock during the Wurm glacial (18-20 ky). The stratigraphic setting of the alluvial body was reconstructed along three geological cross-sections by means of the available logs; seven lithotecnical horizons can be distinguished within the alluvial body, some tens of meters thick, based on both log-stratigraphic data and in-site geotechnical tests. These horizons include peaty layers (T) and high compressibility inorganic clays (AGI), which characterise the alluvial deposits in the Grottaperfetta valley. They do not have direct correlation with the alluvial horizons which constitute the alluvial body of the main Tiber valley in Rome's historical centre. These alluvial horizons which are distinguished and characterised at the Galba test-site can be regarded as typical of other lateral valleys of the Tiber River in Southern Rome. They are characterised by the presence of similar high compressibility clayey deposits as well as peaty layers up to some meters thick. Undisturbed samples were also obtained at the Galba test-site for dynamic testing via resonant column and cyclic torsional shear tests. In order to attribute dynamic properties to the alluvial body at the Grottaperfetta valley, an extrapolation process was performed based on a detailed engineering-geology model of the alluvial body which was reconstructed along three transversal geological sections of the valley using bore-hole data. Three subsoil profiles, considered representative of the geological setting along the three reconstructed transversal cross sections, were analysed by seismic noise measurements performed specifically to derive S-waves velocity profiles. The results obtained show a very low velocity (< 180 m/s) for the layers T and AGI. 1D modelling of seismic shaking was performed by the code Shake91, in order to evaluate the influence of the low-velocity strata on maximum shear strains induced within the alluvial deposits under the maximum expected seismic action. The results of the numerical modelling indicate that the AGI and the T layers play a key role in: (i) concentrating the maximum shear strain along the subsoil profiles, even though the volumetric threshold is never exceeded; (ii) increasing the maximum shear strain along the subsoil profiles; (iii) causing the resonance frequency of the alluvial fill to assume an almost constant ...

The local seismic response of the Fosso di Vallerano valley (Rome, Italy) based on a high-resolution geological model

“…The here obtained Vs profile ( Fig. 2a) is largely in agreement with other literature data (Bozzano et al, 2008;Caserta et al, 2012) but points out a different value for the AL-VSC deposits (i.e. Vs value according to literature is 180 m/s, instead the value deduced from our analysis is 350 m/s) and a velocity gradient in the clay and silt deposits of the Santa Cecilia Formation (PT-CS), i.e.…”

“…Vs value according to literature is 180 m/s, instead the value deduced from our analysis is 350 m/s) and a velocity gradient in the clay and silt deposits of the Santa Cecilia Formation (PT-CS), i.e. corresponding to a Vs linear increment, according to Bozzano et al (2008) and Caserta et al (2012), from about 500 m/s up to 1100 m/s.…”

Geometry and seismic characterization of the subsoil below the Amphitheatrum Flavium, Rome

“…From a general point of view, for unconventional and historical structures like the Colosseum, the simplified approach suggested by the NTC08 for soil classification is not adequate and specific studies are needed to build a truly representative model of the subsoil and the buried part of the structure. Bozzano et al [2000], Bozzano et al [2008] and Caserta et al [2012] investigated the different lithotypes constituting the recent sedimentary fill of the Holocene Tiber River Valley under both static and dynamic conditions. Bianchi Fasani et al [2013] reported the typical ranges of shear-wave velocities observed in situ for the pyroclastic units within Rome city.…”

Predictive model for normalized shear modulus of cohesive soils

“…A wide-ranging database was compiled in the current study from the previously published laboratory tests (U.S. Army Corps of Engineers, 1978Engineers, , 1979Kokusho et al, 1982;Borden et al, 1994Borden et al, , 1996Lanzo et al, 1997;S&ME Inc., 1993, 1998Stokoe et al, 1995;Caserta et al, 2012). These studies include numerous tests conducted on cohesive soils to measure their dynamic properties at low and large strain levels using resonant column, simple shear, cyclic torsional shear, and cyclic triaxial tests.…”