The hydro-mechanical behaviour of unsaturated pyroclastic soils: An experimental investigation

• Experimental research on the behaviour of pyroclastic soils affected by flowslides
• Laboratory investigation on the behaviour of a natural pyroclastic unsaturated soil
• Modelling of compressibility and critical shear strength of a pyroclastic soil
• Experimental demonstration of the convenience of the effective stress approach

An experimental investigation into the hydro-mechanical behaviour of a pyroclastic silty sand was carried out in the framework of a research project on flowslides in pyroclastic soils in the Campania region (Italy). Undisturbed soil samples were taken from the subsoil layers of a test site on a limestone relief mantled by a pyroclastic unsaturated soil cover. The lab investigation was focused on the hydro-mechanical behaviour of a pyroclastic silty sand originated from the Ottaviano eruption of the Somma–Vesuvius volcano some 8000 years ago. The main features of the hydraulic hysteresis domain were evaluated and the matric suction range showing the peak saturation rate with respect to suction was identified using water retention data obtained by means of evaporation and soaking tests. The mechanical behaviour of Ottaviano pozzolana emerged from stress-path controlled triaxial tests on saturated specimens and stress-path and suction controlled triaxial tests on unsaturated specimens. Average soil skeleton stress and matric suction were adopted as stress variables to present and interpret the experimental results. The effects of the unsaturated state on isotropic compressibility and on critical state shear strength are presented and discussed.