An interdisciplinary approach based on near-surface geophysics, stratigraphy and geotechnical measurements can provides a deep insight into the liquefaction susceptibility of the ground and related process. This approach has been tested in the the Quistello site, located in the Po valley, characterized by a thick sequence of Holocene fluvial sediments. This site is in the epicentral area of the 2012 Emilia earthquake and has been affected by several co-seismic effects, like local soil subsidence and sand boiling. The Quistello area has been studied in the framework of the Geomol European project and the Lombardy Region activities, aimed to analyze the Po valley at different spatial scales and with an interdisciplinary approach, also including seismogenic structures, geological site characterization for the seismic microzonation, and induced seismic effects. The main aim of the present work is the test of different geophysical methods for the definition and standardization of a protocol for the characterization of sites prone to seismic liquefaction. The Quistello test site was constituted by three sub-sites about 400 m far from each other. For all the three sub-sites, CPTU measurements up to 20 m depth (20 cm measurement steps) and surface geophysical acquisitions have been performed. In the central sub-site, a continuous core drilling to 25 m depth and a gamma ray log have been obtained, along with a seismic down-hole, (at 25 m depth), to estimate P and S seismic velocities. In all the three sub-sites seismic data acquisition including seismic refraction, MASW, microtremor recordings and electrical resistivity tomography (ERT) have also been performed. The seismic acquisitions allow the reconstruction of Vp and Vs models which contribute to the characterization of the site in terms of homogeneous units. The ERT acquisition integrates the site characterization with information on the electro-stratigraphic and hydro-geophysical features. The geophysical interpretation based on these data allows to define a detailed seismo-stratigraphy, verified with the core-drilling, the geological data and the geotechnical measurements, contributing to the conceptual geophysical model of sites prone to seismic liquefaction. In the Quistello area, this geophysical conceptual model identifies three physical parameters characterizing the site: 1. The Vs interval velocity, or the Vs seismic velocity of the homogeneous units, which is an indicator of belonging to a class of soil. 2. The Vp interval velocity, or Vp the seismic velocity of the homogeneous units , which is an indicator of the saturation degree of the soil. 3. The Vp/Vs interval ratio, or the Vp/Vs ratio of the homogeneous units, which is the indicator of the geophysical susceptibility of the site to exhibit liquefaction effects. The geophysical liquefaction susceptibility of the site was compared with the liquefaction susceptibility index calculated with a standard method based on CPTU measurements, used in the technical regulations, obtaining an acceptable agreement. On the basis of this conceptual model we analyze other Vp and Vs data available in several sites located in the epicentral area of the 2012 Emilia earthquake and the surrounding areas.
Geophysical characterization of prone sites to seismic liquefaction: The quistello Po PLain test site
R de Franco;G Caielli;A Corsi;G Boniolo;A Di Capua;G Norini;F Aghib;
2015
Abstract
An interdisciplinary approach based on near-surface geophysics, stratigraphy and geotechnical measurements can provides a deep insight into the liquefaction susceptibility of the ground and related process. This approach has been tested in the the Quistello site, located in the Po valley, characterized by a thick sequence of Holocene fluvial sediments. This site is in the epicentral area of the 2012 Emilia earthquake and has been affected by several co-seismic effects, like local soil subsidence and sand boiling. The Quistello area has been studied in the framework of the Geomol European project and the Lombardy Region activities, aimed to analyze the Po valley at different spatial scales and with an interdisciplinary approach, also including seismogenic structures, geological site characterization for the seismic microzonation, and induced seismic effects. The main aim of the present work is the test of different geophysical methods for the definition and standardization of a protocol for the characterization of sites prone to seismic liquefaction. The Quistello test site was constituted by three sub-sites about 400 m far from each other. For all the three sub-sites, CPTU measurements up to 20 m depth (20 cm measurement steps) and surface geophysical acquisitions have been performed. In the central sub-site, a continuous core drilling to 25 m depth and a gamma ray log have been obtained, along with a seismic down-hole, (at 25 m depth), to estimate P and S seismic velocities. In all the three sub-sites seismic data acquisition including seismic refraction, MASW, microtremor recordings and electrical resistivity tomography (ERT) have also been performed. The seismic acquisitions allow the reconstruction of Vp and Vs models which contribute to the characterization of the site in terms of homogeneous units. The ERT acquisition integrates the site characterization with information on the electro-stratigraphic and hydro-geophysical features. The geophysical interpretation based on these data allows to define a detailed seismo-stratigraphy, verified with the core-drilling, the geological data and the geotechnical measurements, contributing to the conceptual geophysical model of sites prone to seismic liquefaction. In the Quistello area, this geophysical conceptual model identifies three physical parameters characterizing the site: 1. The Vs interval velocity, or the Vs seismic velocity of the homogeneous units, which is an indicator of belonging to a class of soil. 2. The Vp interval velocity, or Vp the seismic velocity of the homogeneous units , which is an indicator of the saturation degree of the soil. 3. The Vp/Vs interval ratio, or the Vp/Vs ratio of the homogeneous units, which is the indicator of the geophysical susceptibility of the site to exhibit liquefaction effects. The geophysical liquefaction susceptibility of the site was compared with the liquefaction susceptibility index calculated with a standard method based on CPTU measurements, used in the technical regulations, obtaining an acceptable agreement. On the basis of this conceptual model we analyze other Vp and Vs data available in several sites located in the epicentral area of the 2012 Emilia earthquake and the surrounding areas.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
