Source layer identification of liquefied sands ejected during the blast-induced liquefaction test at Mirabello (Ferrara, Italy) through a quantitative compositional study.

It is well known that the occurrence of sand liquefaction phenomena may cause significant modifications of soil geotechnical properties and reduction of load-bearing capacity. Recent experiences (New Zealand, United States) have shown that liquefaction can be induced and monitored with field scale blast tests in order to study the related effects on soil chacacteristics. In this context the composition of sand blows represents an important tool to identify the liquefied layers. This study reports the composition of sands ejected during the blast-induced liquefaction test carried out in May 2016 at Mirabello (Ferrara, Italy). In 2012, the same area was largely affected by liquefaction phenomena during the Mw 6.1 Emilia earthquake. Sand samples analyzed in thin section (34 samples, 300 points for each sample, fraction 0125-0.250 mm) include blast-induced sand blows, sands from 2 cores at different depths in the subsurface (from 2 to 20 m) and sands from 2 trenches in the blast site representative of 2012 liquefied sands. The sands from the cores show a clear trend from lithoarenitic to quartz-feldspar-rich compositions. The sands at shallow depth (up to 7 m) are the most lithoarenitic, with sedimentary fine-grained rock fragments (shales and siltstones) as the dominant lithic type. Lithic fragments derive mostly from the erosion of sedimentary terrigenous and carbonate successions of Apenninic affinity. These shallow sands are well distinguishable from the deeper sands (at depth > 7 m) that show compositions slightly enriched in quartz and feldspars and impoverished in lithic fragments that suggest affinity with the Po river sands. The composition of ejected sands largely overlap that of the shallow Apenninic sands at depth from 6 to 7 m. Similarly the sands from the 2012 dikes show a composition of Apenninic affinity compatible with that of the shallow sands. Results from the blast test fit well with data obtained from the study of the sands ejected in the nearby area of San Carlo during the Mw 6.1 earthquake (Fontana et al., 2015). Also in this case, sand composition and fabric indicate that liquefaction processes affected mainly sand layers at relatively shallow depth ( 6.8-7.5 m). The study shows that composition of sands is crucial for a better understanding of earthquake-induced liquefaction mechanisms, in particular to identify the source layer of the sand blows and, more generally, for the recognition of critical levels prone to hazardous sand liquefaction phenomena.