An experimental study on a partially saturated pyroclastic soil: the Pozzolana Nera from Roma

The paper presents the results of an experimental investigation on the mechanical behaviour of partially saturated Pozzolana Nera (Roma, Italy). Such natural deposits, which have been extensively studied in saturated conditions, are characterised by a marked heterogeneity in terms of grading, nature of grains, and inter-particle bonds. In situ, deposits of pozzolana are generally found in unsaturated conditions; this has an important practical relevance in the evaluation of the stability conditions of natural slopes and cuts. The experimental investigation consisted mainly of isotropic and triaxial compression tests on reconstituted samples, at increasing values of mean net pressures in the range 50 – 400 kPa, at different constant levels of suction (20 – 75 kPa). Volume pressure plate extractor tests were also conducted to obtain the water retention curve. In addition some wetting tests were carried out by means of conventional oedometer cells. The testing programme made it possible to examine the hydraulic and mechanical properties of unsaturated Pozzolana Nera at different values of the degree of saturation and to compare the mechanical behaviour of the same material in saturated conditions. Special attention was focused on defining the failure envelope at relatively low confining stress and assessing the influence of the degree of saturation on the failure conditions of the material