LIQUEFACTION POTENTIAL STUDY BASED ON INTEGRATED GEOELECTRICAL IMAGING AND GEOTECHNICAL DATA – A CASE STUDY (Extended Abstract)

Abstract

Liquefaction of saturated sand and silty-sand sediments represents one of the most important co-seismic site effects, that occur below the water table, generally at depths less than 20m. Undesired consequences are ground failures and damage to the built environment. To reduce such risks, mapping and characterizing the liquefiable layers is a priority for assessment and mitigation, to increase safety and preserve economy. Non-invasive geophysical methods help in expanding over large subsurface volumes punctual information gathered by well-established geotechnical methods, whose application remains necessary to determine liquefaction potential. Besides seismic methods, geoelectromagnetic methods are sensitive to texture and saturation and can easily capture contrasts in the subsurface properties related to lithological heterogeneities. In a selected test area, heavily damaged by liquefaction phenomena as a consequence of the Ml 5.9 earthquake that struck the Emilia-Romagna plain (Northern Italy) on 20 May 2012, 2D and 3D electrical resistivity (ERT) and induced polarization tomography (IPT) from surface and cross-hole were performed, to analyze their capability to map two liquefiable layers detected by previous geotechnical tests in a range of 15m depth. To our knowledge, the use of IP method in liquefaction studies is poorly documented in literature, or completely absent. Achieved preliminary results can be summarized as follows: 1)The subsurface is composed of a sequence of sub-horizontal layers, alternatively more or less conductive, in the range 10-50 Ohm.m; the liquefiable, silty-sand layers show resistivities ranging from 25 to 50 Ohm.m; the most conductive values identify clay layers, 2)2D and 3D ERT images obtained from surface failed to map even the shallowest liquefiable sandy layer, but 3)IPT was able to image the shallowest liquefiable layer, as easily understood if we consider that confining layers were non-polarizable, while the liquefiable one presents a remarkable membrane polarization effect, 4)Cross-hole ERT and IPT mapped exhaustively both liquefiable layers