Combined use of cptu-sdmt and geophysical test to assess liquefaction: case studies in Emilia-Romagna (Italy)

Abstract

The presence of a non-liquefiable crust overlying a liquefiable layer plays a significant role in determining the occurrence of liquefaction damage, as originally formulated by Ishihara in 1985. Following the 2010-2011 Canterbury seismic sequence (New Zealand), almost no foundation deformation occurred in areas characterized by soils susceptible to liquefaction overlaid by at least 3 m-thick intact crust. In contrast, the 2012 Emilia-Romagna earthquake (Italy) provided evidence of liquefaction in silty-sandy layers below 3 to 9 m-thick crusts. Therefore, Ishihara’s approach and the variety of liquefaction severity indices need to be further tested to assess to what extent they can be considered reliable predictors of performance. This study aims at better understanding the role of non-liquefiable crusts in preventing damage to buildings and infrastructures. In this respect, in situ and laboratory tests were conducted at selected sites in EmiliaRomagna. The results of two case studies in Mirandola (Modena, Italy), which share similar soil profiles but exhibited different liquefaction evidences following the 2012 seismic sequence, are presented. Comprehensive geotechnical and geophysical surveys were performed at both the sites, by means of piezocone tests, seismic dilatometer tests, boreholes, laboratory tests, electrical resistivity tomography and multichannel analysis of surface waves. These surveys document the geotechnical and geophysical properties of the 5 m-thick non-liquefied (or potentially non-liquefiable) crust and of the liquefied (or potentially liquefiable) silty-sandy deposits. This effort is aimed at understanding how the surface layer properties contributed to the different behavior observed at the two sites during the earthquake events.

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