GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.3 433 information (stratigraphic, lithological, geomechanical, hydrogeological) on the shallow subsoil, say up to some tens of meters depth. Starting from the preexisting information stored in the database managed by the Regione Emilia-Romagna, it was subsequently conducted a systematic research in the archives of several professional geologists which worked in the broader area. The most important added value of this effort was the reconstruction and the upgrade of the geotechnical and geophysical archive of the municipality, that had been almost completely destroyed due to the collapse of the Sant’Agostino town hall. Thanks to the new data collection, more than three hundred new survey sites and associated metadata have been added, thus enabling also the Regione Emilia-Romagna to upgrade its database. At present are available about 700 sites (including penetration tests, water wells and geophysical surveys), that is more than twice existing before this investigation. The second phase of the work consisted in the digitalization of the available penetration tests, more than four hundred, in order to exploit their numerical content for microzonation purposes, i.e. for the analysis of the liquefaction hazard. Liquefaction occurs due to the shaking generated by moderate to strong earthquakes (M > 5.5), especially in floodplain and coastal areas that are often characterized by loose granular sediments and shallow aquifer. In the first 20 m depth, as a result of seismic stresses, the resistance of a saturated granular sediment (sandy silts, sands and sandy gravels) reduces to a critical level for which the sediments liquefy. During this process, the pore pressure increases and the water rises through preexisting and new fractures or by man-made structures such as water wells, pipelines and foundations. Liquefaction phenomena is the result of the combined effect of the triggering factors (characteristics of the earthquake in terms of magnitude and local acceleration) and the predisposing ones (subsoil conditions and geomorphological framework): in the case of the Sant’Agostino territory, liquefaction has been one of major causes of damage of buildings and facilities (ie. underground utilities) due to the concomitance of these factors. As a third step of the working flow, all available data have been analyzed in order to recognize and define the possible areas subject to liquefaction hazard. All the penetration tests have been preliminarly analyzed from a semi-quantitative point of view for detecting the presence of liquefiable horizons; in particular, the surveys have been classified on the basis of the thickness and depth of the liquefiable layer(s). Subsequently, we performed a quantitative assessment of the liquefaction hazard by calculating the Liquefaction Potential Index, LPI (Iwasaki et al ., 1978). This index is a synthetic parameter that quantifies the possible effects of liquefaction at a given site, taking into account a) the severity of the seismic event, in terms of magnitude and acceleration b) the depth, the thickness and the safety factor that characterizes the liquefiable layers. In the scientific literature several methods have been described for the estimation of the liquefaction hazard based on penetration tests, but based on a comparative analysis of different methods (Facciorusso et al ., 2013) that proposed by Idriss and Boulanger (2008) provides LPI values consistent with the observed effects. In order to calculate this index, we used the CLiq software (1.7.5.27). The input parameters for the calculation are a) the magnitude M Wmax , deduced from the seismogenic zonation ZS9 (Meletti and Valensise, 2004), equal to 6.14 for the Municipality of Sant’Agostino, b) the acceleration ag max_s (PGA*FA PGA ), computed by the Regione Emilia-Romagna following the regional guidelines (DAL 112/2007) c) the water’s depth, conventionally assumed equal to 1 m for interfluvial areas and 3 m in correspondence of the palaeo levees. The calculation has been performed for both electrical penetration tests and mechanical ones, despite the high degree of uncertainty in the measurement of the lateral friction characteristic of the latter type of investigation. This parameter is measured considering that there is a gap in the depth to which the tip resistance is measured compared to that of the lateral friction. Consequently, it is necessary to consider this altitude gap for both the evaluation of the friction that for the uncertainty degree due to the fact this value could be measured in different stratigraphic levels. Moreover, in the case of the Begemann tip, due to the junction at the bottom end of the sleeve, the friction is partly affected by a resistance provided by the material flowed