The subsoil of a large area surrounding the city of Rome (Italy) consists mainly of pyroclastic flow deposits, locally known as pozzolane. The physical and mechanical properties of these deposits are highly variable depending on depositional environment and diagenetic alteration. Quarries of pozzolana with stable subvertical open cuts as high as 20–30 metres are frequently encountered in the area south east of Rome, due to high characteristic values of effective peak cohesion and friction angle. However, cohesion may be highly variable in the deposits and it deteriorates on loading, leading to precarious conditions of stability. Sub-vertical cuts in pozzolana may also collapse due to the presence of discontinuities hidden in the rock mass, such as tension cracks at the top of the cut, or thin layers of different geological origin, formed during quiescent stages. Because the mechanical behaviour of the material at ambient stresses in the field is brittle, collapse may occur suddenly with little warning signs. The paper is a parametric study of the conditions of stability of typical cuts in pyroclastic deposits; the sub-vertical cut face at Fioranello, a quarry 20 km south-east of Rome, is used as an example. Although the degree of fissuring of the soft rock at this site is generally low and there are no major discontinuities within the rock mass, the analysis takes into account the presence of tension cracks as the triggering factor for an instability phenomenon.
Pyroclastic deposits originated from the explosive activity of the Colli Albani volcanic complex (Upper Middle Pleistocene or about 500,000 years ago) form the subsoil of a large area south-east of the city of Rome. Due to their geological origin the deposits are highly heterogeneous in particle size distribution, mineralogical composition, and micro-structural features depending on different depositional environments and diagenetic alteration.