Sand liquefaction induced by a blast test: new insights on source layer and grain-size segregation mechanisms (Late Quaternary, Emilia, Italy)

The composition and texture of liquefied sands represent an important tool for the recognition of buried source layers and for a better understanding of earthquake-induced liquefaction mechanisms. The earthquake simulating field experiment (blast test) carried out in 2016 in fluvial sediments of the Emilia plain induced subsurface liquefaction and the surface expulsion of sand as sand blows. The area was widely affected by liquefaction phenomena during the Mw 6.1 Emilia earthquake (2012). The grain size and composition of sand blows ejected during the blast test have been compared with various horizons of buried fluvial sediments as deep as 20 m, and with sands from two trenches in the blast site representative of co-seismic 2012 liquefied sands. The sands from the cores show a clear trend from shallow lithoarenitic to deeper quartz–feldspar-rich compositions. The sands at shallow depth (up to 7.7 m) are the most lithoarenitic, with fine-grained sedimentary rock fragments (shales, siltstones, and limestone) as the dominant lithic type. Lithic fragments derive mostly from erosion of sedimentary terrigenous and carbonate successions of Apenninic affinity. In contrast, deeper sands (at depth . 7.7 m) are enriched in quartz and feldspars and impoverished in lithic fragments, which are similar in character to Po River sands. The composition of ejected sands largely overlap that of the shallow litharenitic Apenninic sands, indicating that liquefaction processes affected mainly sand layers at relatively shallow depth (5.9–7.7 m). Textural parameters show that silty sands and silts characterized by relatively high content of fines can also liquefy. This is in contrasts to most of the literature, where fine-grained sediments are considered as incapable of generating the high pore pressures commonly associated with liquefaction. This result should be considered when estimating the liquefaction as a potential hazard. Moreover, we observe that there is a selective loss of fines in the clastic dikes and sand volcanoes relative to the source beds, indicating that the liquefaction process appears to preferentially select the diameters of the grains that reach the ground surface, probably following the generated excess pore-water pressure. This may have caused the segregation and dispersion of the fine silt–clay content, producing highly sorted sand boils. This effect is easily observable in both the blast-induced sand boils, and the co-seismic 2012 dikes and sand boils ejected in the same area.