Pyroclastic soil deposits, which cover a wide area of the Neapolitan metropolitan district (South Italy), have been often involved in catastrophic slope instability phenomena related to meteoric extreme events. For this reason, research attention on these soils was mainly devoted to the assessment of hydrogeological hazard of the Neapolitan area.
Nevertheless, the Neapolitan area is also characterised by a relevant seismic hazard, and extended coastal areas, where reworked pyroclastic soils are present at shallow depth in saturated conditions, are characterised by moderate to high susceptibility to liquefaction. Notwithstanding this, very few data are available on the cyclic behaviour of these soils.
In this study, the results of cyclic laboratory tests (torsional shear tests and cyclic triaxial) carried out on pyroclastic soil samples retrieved at different sites in the Neapolitan metropolitan district were collected and synthesised to analyse the main factors affecting their cyclic behaviour. The results indicate that due to the complex surface shape, the interlocking effect appears to be significant, resulting in higher linear threshold strain and initial damping ratio as well as higher liquefaction resistance when compared with hard-grained sands. Furthermore, the small and medium strain behaviour of pyroclastic soils seems to be ruled mainly by fine content. Samples exceeding 50% of fine content are plastic and 50% of fine content seems to be a threshold value beyond which soil behaviour is controlled by the fine matrix rather than by the granular skeleton. The peculiar cyclic behaviour of these soils significantly affects the seismic site response of the pyroclastic deposits, as shown by the results of site response analyses carried out on representative soil columns.
