GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 2.2 417 0.1-1s at S2 site whereas it is limited within 0.5-0.8s at S1 site (Fig. 2). Comparing Fig. 1b and Fig. 2 it can be drawn that the local amplification due to the soil deposits involves a large range of periods and can affect a broad range of structures. At S1 site, where a liquefaction prone area is evidenced at level 1, a spectral acceleration peak of 0.9g has been calculated. This value can be used to calculate the liquefaction instability of this zone through a simplified approach based on the Factor of Safety FSL. Liquefaction instability. The high seismic hazard associated to the activity of the Sulmona fault system suggests the possible occurrence of high magnitude earthquakes exceeding acceleration threshold for liquefaction phenomena. Moreover, during the 2009 L’Aquila earthquake liquefaction phenomena occurred within the Holocene alluvial deposits in the Sulmona plain near the Vittorito village even if they affected limited areas. The first level of microzoning studies defined liquefaction prone areas according to the following predisposing conditions (NTC08): (1) the presence of sandy deposits (some quantitative limits on granulometric composition are provided), (2) the presence of the water table at depth higher than 15 m, (3) the expected magnitude Mw > 5, (5) the expected peak ground acceleration (a max ) at surface > 0.1g. These conditions are all matched at Sulmona city center. The identified prone areas to liquefaction are located nearby the railway station and along the main arterial road to the city. These sites are characterized by Late Pleistocene-Holocene alluvial deposits that host a water table at depth higher than 15 m. These deposits are mainly composed of fine loose sandy-silt deposits, falling in the grain size range of susceptible soils to liquefaction (S1 in Fig. 2a). Conversely, the older alluvial deposits that show 20-30m elevation over the valley floor are composed of gravelly soil deposits (S2 in Fig. 2a). However, due to the scarce data available and the high variability in soil properties and grain sizes characterizing these deposits, the liquefaction susceptibility must be evaluated in several points, at least with simplified methods, to be correctly assessed. Analytical methods to estimate the factor of safety against liquefaction. Only at the S1 site, where some physical and mechanical soil properties were available, the Factor of Safety against Liquefaction (FSL) has been calculated (Fig. 2c). For this calculation the following equations have been used (Youd and Idriss, 2001): (1) (2) (3) where a max is the peak ground acceleration calculated through 1D seismic local response at S1 and S2 sites by site response analyses. The CRR 7.5 is referred to a 7.5 M earthquake. In order to scale to the actual value of the earthquakes at Sulmona site, the following equation is used: (4) where MSF is the magnitude scaling factor and K σ is factor of the effective overburden stress. Results from these calculations, that are simplified studies of level 2 microzoning activity, S1 site shows not to be susceptible to liquefaction. Conclusions. Microzoning studies are divided into three levels of details. Passing from the 1st to the 3rd level those areas that are highlighted and bordered within MOPS as unstable to liquefaction must be studied through analytical and numerical methods. The present study shows a liquefaction analysis at level 2 carried out at Sulmona urban area. Here, due to the variability of physical and mechanical properties of recent alluvial deposits several boreholes have been performed to characterize the deposits. Starting from two available stratigraphic