Cyclic and dynamic behaviour of a soft pyroclastic rock

The mechanical behaviour under cyclic and dynamic conditions of a weakly cemented pozzolana, taken from the subsoil of a historical town in a seismically active area, was investigated through laboratory tests and in situ measurements. The geotechnical characterisation at laboratory scale is of particular importance as the small- and large-scale behaviour of these deposits is comparable, owing to the large discontinuity spacing. These soft pyroclastic rocks, which are found in many volcanic regions worldwide, are particularly interesting as they are often alternated with stronger volcanic rocks in thick sequences, constituting a significant impedance contrast that could increase the groundshaking hazard. A cross-hole test and a spectral analysis of surface waves test, carried out on the ground surface and at the floor of an underground cavity, respectively, provided in situ vertical profiles of the shear wave velocity. Cyclic and dynamic properties were investigated in the laboratory through velocity measurements of ultrasonic pulses, cyclic simple shear, torsional shear and resonant column tests. Most testing procedures and devices, which were originally conceived for soils, posed some challenges. The collected data were used to analyse the influence of mean confining stress, strain amplitude and number of cycles on both shear stiffness and material damping.