Geophysical characterization of liquefied terrains using the electrical resistivity and induced polarization methods: The case of the Emilia earthquake 2012

Liquefaction phenomenon results in high water pressure being exerted on the soil particles due to earthquake shaking. Under certain condition this may result in permanent deformations leading to the formation of fractures, sand boils, lateral spreading on gently sloping landforms and landslides in highly sloping terrains. Some factors are known to affect liquefaction of saturated sands and silty sands such as materials type (i.e. compositional characteristics: particle size, shape, gradation, and relative density), in addition to, duration and amplitude of the dynamic excitation of one or several successive earthquakes. The geo-lithological factors are known also to affect other physical properties such as shear wave velocity, electrical resistivity and polarizability which constitute the backbone of seismic and electric geophysical methods. In this work, I highlighted the importance of subsurface electrical imaging in liquefaction related studies. Special emphasis has been given to usefulness of the IPT technique in localizing lithologic heterogeneities in alluvial plain lands where their presence is the rule and not the exception. The employment of the IPT completes and adds more independent information about the subsurface lithology which aid in the reconstruction of realistic geo-hydrogeological models. In addition, the gained information help in capturing post-seismic modifications. In particular, the IPT succeeded in mapping ruptures depth extension as well as the probable pathways followed during excess pore water pressure dissipation. It was interesting to note, in the second example (site B), how the liquefied sediments had moved horizontally due to the presence of a thick impermeable crust. This crust has facilitated the lateral dissipation of excess pore water pressure. Of course, this crust was removed to allow for the construction of the industrial building that had suffered a lot of damage leading to its demolition. The observed resistivity and IP anomalies are related to porosity and permeability modifications following liquefaction. These modifications can be easily explained by invoking the Archie (1942) empirical equation which explains that effective porosity, related to permeability, is inversely proportional to the formation factor (F defined as the ratio of formation resistivity and water resistivity) of clean sand (fine percent <10 %). The intrusion of liquefied sand mixed with water and fine materials has contributed in the increment of the IP response. The ERT/IPT may also be used to map and control consolidation effects normally used to mitigate the liquefaction risk. This last issue, however, requires further tests and analyses in order to evaluate possibilities and limitations. I think that the IPT technique may be used as a permanent monitoring tool to complement standard tools used for pressure monitoring in the future. The advantage of using these techniques resides in the fact that they can cover large volumes of subsurface materials. Hence reducing costs the society has to pay for post-seismic event reconstruction phase. I do believe that the best way to mitigate earthquake risk is to increase citizens perception to this type of risk that is related not only to the construction and its continuous maintenance but also to the spatial distribution ofsubsurface sediments. Such integration contributes in the process of smart underground monitoring of sites highly vulnerable to liquefaction. Last, the possibility to use the IPT to investigate the subsurface conditions of existing building is feasible and can be easily realized by installing the electrodes around the building itself. In this case, full 3D data acquisition protocols can be set up and acquired data can be processed using available 3D codes for data inversion.