Liquefied sites of the 2012 Emilia earthquake: a comprehensive database of the geological and geotechnical features (Quaternary alluvial Po plain, Italy)

This paper presents a comprehensive geological and geotechnical study of the whole area affected by liquefaction following the 2012 Emilia earthquakes, including all the available information from the field reconnaissance surveys, in situ tests, and laboratory analyses. The compilation was performed at 120 liquefied sites to verify and validate the reliability of liquefaction charts in alluvial sediments, and to assess liquefaction induced by the 2012 seismic sequence in the Emilia plain. The results reveal a wide range of grain sizes (from clean sands to sandy silts) and compositional characteristics (quartz-rich to litharenitic) in the 2012 ejecta, and show a strong relationship between the liquefaction and stratigraphic architecture of the subsurface. The availability of in situ tests at the liquefied sites makes it possible to verify and validate the reliability of the liquefaction charts in alluvial sediments with respect to the real observations. For the analyzed Emilia case studies, the use of non-liquefiable crust provides better estimations of the liquefaction manifestations when coupled with the thickness of the liquefiable layer rather than with the liquefaction potential index. Altogether, this work makes available to the international scientific community a consistent liquefaction database for in-depth earthquake studies

Blast-induced liquefaction in silty sands for full-scale testing of ground improvement methods: Insights from a multidisciplinary study

In the engineering geology field increased attention has been posed in recent years to potential liquefaction mitigation interventions in susceptible sand formations. In silty sands this is a major challenge because, as the fines content increases, vibratory methods for densification become progressively less effective. An alternative mitigation technique can be the installation of Rammed Aggregate Pier\uae (RAP) columns that can increase the resistance of the soil, accounting for its lateral stress increase and for the stiffness increase from soil and RAP composite response. To investigate the influence of these factors on liquefaction resistance, full-scale blast tests were performed at a silty sand site in Bondeno (Ferrara, Italy) where liquefaction was observed after the 2012 Emilia-Romagna earthquake. A multidisciplinary team of forty researchers carried out devoted experimental activities aimed at better understanding the liquefaction process at the field scale and the effectiveness of the treatment using inter-related methods. Both natural and improved areas were investigated by in-situ tests and later subjected to controlled blasting. The blast tests were monitored with geotechnical and geophysical instrumentation, topographical surveying and geological analyses on the sand boils. Results showed the RAP effectiveness due to the improvement of soil properties within the liquefiable layer and a consequent reduction of the blast-induced liquefaction settlements, likely due to soil densification and increased lateral stress. The applied multidisciplinary approach adopted for the study allowed better understanding of the mechanism involved in the liquefaction mitigation intervention and provided a better overall evaluation of mitigation effectiveness

Sedimentary structures and textures in sand injectites. Insights from dikes and sand blows of the Holocene fluvial sediments (Emilia, Italy)

As discussed by Hurst et al. (2011), studies of sand injectites, including sand extrusions, are relatively scarce in the geological literature, although these phenomena were recognized in many geological settings (Quigley et al., 2013: Ross et al., 2014). A trench dug across dikes and sand blows formed as a consequence of the 2012 Emilia earthquake allowed a direct observation of liquefaction structures (Fontana et al., 2015, 2019). Sand dikes crosscut the fine-grained host sequence at high angles with a vertical extension of at least 5 m. The width of the fractures varied from a few cm to 30 cm. The injected sand showed complex sedimentary structures: the most common was a distinct banding, longitudinal to the dike length, or perpendicular to the dike margins. The bands oriented parallel to the dike were bounded by sharp contacts marked by thin clay veneers defined by differences in grain size and grain alignment. We observed both direct and inverse vertical grading from medium sand to mud. The fractures were rhythmically injected and filled of slurry sand and mud during the compression pulses end emptied by the rushing of the slurry back down deep into the fractures during the extension peak. The grain-size distribution along dikes and sand blows showed that some sorting occurred within injected dikes, probably due to pulse flows, and further segregation occurred as the material was extruded following the generated excess pore-water pressure. This may have caused the dispersion of the fine silt–clay content, producing highly sorted sand boils. The composition of sand dykes adds an important constraint in identifying the source layer. Regarding the possibility that selective mechanism due to flux variation may have influenced the sand composition, our data seem to indicate that no major variation was induced by injection phenomena

The evolution of the Miocene platform-basin system in the northern Apennines: what can we learn from seep-carbonates?

Cold seep-carbonates documenting the expulsion of fluids enriched in methane have been identified in a variety of basin margins and tectonic-sedimentary settings. A number of geological factors influences methane seep activity, but primarily regional and local tectonics, mainly at the deformation front of accretionary prism and inner foredeep. Tectonics constrains the fluid circulation system, with faults and fractures serving as conduits and channelling water and methane up to the seafloor. Recent studies suggest that seep-carbonates occurrence is also controlled by climatic changes and their formation seems to correlate with cold periods and sea level low-stand. A drop of the hydraulic pressure on the plumbing system during sea level lowering in glacial phase could increase methane flows at seeps, inducing carbonate precipitation. The correlation between methane-derived carbonates and climate has been recently suggested for the Miocene of the northern Apennines by means of a sedimentological and biostratigraphic study of seep-carbonates and the enclosing hemipelagic marls (Vicchio Formation) (Fontana et al., 2013). The study suggests a correlation between the carbonate precipitation and the middle Miocene glacial cooling event (Mi3b). The triggering of the ascent and emission of methane-rich fluids may be related to the eustatic fall and in turn to the water pressure drop. A detailed stratigraphic and biostratigraphic study of seep-carbonates and enclosing marls in foredeep deposits of the Tuscan-Romagna Apennines has allowed a precise dating of seepage in slope and basinal successions. The analysis of the δ18O and δ13C records of carbonates, δ13Corg excursion of organic matter and TOC on the marls enclosing the authigenic carbonates has been performed in order to verify a correlative trend in correspondence of the climatic cooling event. A paleoecological study on benthic foraminifera assemblages has also allowed to detail these peculiar environments. The events defined from this study in basinal deposits have been correlated with depositional changes and discontinuity surfaces in the adjacent temperate-type carbonate platforms, and have contributed to the definition of modes and rates of the demise of carbonate deposition. The identification of cold phases and lowering of sea level in slope-basinal deposits and their detailed timing, may be a useful tool for correlation between deep depositional setting and coeval shallow-water successions. This approach may also provide important constrains in the reconstruction of the evolution of the Miocene platform-basin system in this complex compressive setting of the northern Apennines. Fontana D., Conti S., Grillenzoni C., Mecozzi S., Petrucci F. & Turco E. 2013. Evidence of climatic control on hydrocarbon seepage in the Miocene of the northern Apennines

Blast-induced liquefaction in silty sands for full-scale testing of ground improvement methods: Insights from a multidisciplinary study

In the engineering geology field increased attention has been posed in recent years to potential liquefaction mitigation interventions in susceptible sand formations. In silty sands this is a major challenge because, as the fines content increases, vibratory methods for densification become progressively less effective. An alternative mitigation technique can be the installation of Rammed Aggregate Pier\uae (RAP) columns that can increase the resistance of the soil, accounting for its lateral stress increase and for the stiffness increase from soil and RAP composite response. To investigate the influence of these factors on liquefaction resistance, full-scale blast tests were performed at a silty sand site in Bondeno (Ferrara, Italy) where liquefaction was observed after the 2012 Emilia-Romagna earthquake. A multidisciplinary team of forty researchers carried out devoted experimental activities aimed at better understanding the liquefaction process at the field scale and the effectiveness of the treatment using inter-related methods. Both natural and improved areas were investigated by in-situ tests and later subjected to controlled blasting. The blast tests were monitored with geotechnical and geophysical instrumentation, topographical surveying and geological analyses on the sand boils. Results showed the RAP effectiveness due to the improvement of soil properties within the liquefiable layer and a consequent reduction of the blast-induced liquefaction settlements, likely due to soil densification and increased lateral stress. The applied multidisciplinary approach adopted for the study allowed better understanding of the mechanism involved in the liquefaction mitigation intervention and provided a better overall evaluation of mitigation effectiveness