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 lying in a seismically active area, was investigated through laboratory tests and in-situ measurements. The interest for these soft pyroclastic rocks, widespread in many volcanic regions worldwide, is noticeable because they are often found alternated to stronger volcanic rocks in thick sequences, thus giving a significant impedance contrast that could increase the ground-shaking hazard. A cross-hole (CH) test and a spectral analysis of superficial waves (SASW) test, carried out from the ground surface and in the floor of a cavity respectively, provided in situ vertical profiles of the shear waves velocity. Cyclic and dynamic properties were investigated in the laboratory through velocity measurements of ultrasonic pulses (UPV), cyclic simple shear (DSDSS), torsional shear (TS) and resonant column (RC) tests. Most testing procedures and devices, originally conceived for soils, posed some challenges. The collected data allowed the analysis of the influence that mean confining stress, strain amplitude and number of cycles have on both shear stiffness and material damping. The exhaustive geotechnical characterization at the laboratory scale has an augmented importance because in these deposits the small- and large-scale behaviour are comparable due to the large discontinuity spacing.