Pyroclastic flow deposits from the Colli Albani. The example of the Pozzolana Nera

This paper presents a selection of the results of an extensive programme of laboratory testing on intact and reconstituted samples of a pyroclastic weak rock from the volcanic complex of the Colli Albani (Central Italy). In situ, these deposits are often found in unsaturated conditions; this has an important practical relevance on the stability of slopes and cuts. The deposit selected for the investigation is known as Pozzolana Nera and may be assimilated to a bonded coarse grained material. The nature of bonds and the micro-structural features were examined by means of diffractometry, optical and electron microscopy. As bonds are made of the same constituents of grains and aggregates of grains, bond deterioration and particles breakage upon loading are indistinguishable features of the mechanical behaviour. The testing programme consisted mainly of one-dimensional and drained and undrained triaxial compression tests in a wide range of confining pressures. Sustained loading oedometer tests revealed significant creep. As the confining stress increases, the mechanical behaviour of the material in triaxial compression changes from brittle and dilatant to ductile and contractant; for both brittle and ductile behaviour failure is associated with the formation of shear surfaces separating the sample in several parts at the end of test. The experimental stress-dilatancy relationships were first compared with the classical stress-dilatancy theories for a purely frictional material and for a material with friction and cohesion between particles. The analysis of the data indicates that peak strength in the stress-strain curve does not correspond to the maximum rate of dilation. The testing programme made it possible to examine the hydraulic and mechanical properties of unsaturated Pozzolana Nera.