Assessment of liquefaction susceptibility under complex stratigraphic conditions: the case of Terre del Reno (FE)

The phenomenon of liquefaction is nowadays sufficiently understood in terms of phenomenology and predisposing conditions. However, a better assessment of liquefaction risk is necessary to mitigate its effects and guide land-use planning choices, particularly in the context of post-earthquake reconstruction.

This evidence comes from some recent events (e.g., New Zealand, 2010-2011; Emilia-Romagna 2012; Palu, 2018), in which liquefaction induced effects were, in some instances, considerably more severe than expected. This is the case of Terre del Reno (Emilia-Romagna region, Italy) which experienced significant liquefaction phenomena during the 2012 Emilia-Romagna earthquake sequence, characterized by two main events: Mw 6.1 and 5.9. In this area sand eruptions, settlements, lateral spreading, and ground fractures were observed, resulting in extensive and irregularly distributed damage to structures and infrastructure.

This study deals with the evaluation of liquefaction susceptibility and development of liquefaction hazard map in complex stratigraphic condition through an integrated method and multilevel approach. The study area is characterized by complex geologic conditions and abrupt slope changes, typical of riverbank-channel systems.

The analysis of liquefaction potential was conducted using simplified semi-empirical methods. The safety factor against this phenomenon was estimated at different depths, relying on  soil properties obtained from penetrometric tests and seismic input. In addition to the calculation of liquefaction potential, the study also addressed the phenomenon of lateral spreading due to liquefaction. To date, the delimitation and representation of area prone to lateral spreading is not yet ruled by guidelines for the mitigation of liquefaction risks. Therefore, this study employed an empirical methodology based on original criteria and procedures to establish the perimeter of such areas.

The cross-analysis between the prediction of indicators of liquefaction potential and the evidence of damage found following the May 20, 2012 earthquake (Mw 6.1) showed a clear correlation between slope and damage frequency, suggesting the possibility of applying an empirical method to define the probability of lateral spreading occurrence.