General characterization of the mechanical behaviour of different volcanic rocks with respect to alteration

Physical–mechanical properties and the mechanical behaviour of volcanic rocks are extremely sensitive to their original structure and successive hydrothermal alteration. Various mechanical tests have been performed on different volcanic rocks to evaluate the relationships between chemical and mineralogical composition, microstructure and texture, and physical mechanical properties. A wide-ranging description of mechanical behaviour is obtained through a series of uniaxial, triaxial, isotropic and oedometric tests, and of pre- and post-failure non-destructive analyses. X-ray tomographies show deformation and compaction within the samples and the influence of porosity distribution. Results are interpreted in the key of degree of alteration (lava and tuff series) and of texture differences (pyroclastic and ignimbrite series); empirical relationships between strength and physical properties are presented and discussed, together with trends in change of an Et50 vs UCS ratio. The influence of facies and water saturation on strength and behaviour of ignimbrite rocks is discussed. A 45 to 85% loss both in strength and ultrasonic waves velocity is found for altered lava and pyroclastic rocks. Weak highly porous ignimbrite shows a 50% strength loss under water saturated conditions and the complete collapse of porous structure.