Physical and Mechanical characterization of altered volcanic rocks for the stability of volcanic edifices

Geomechanical characterisation of altered volcanic rocks and their role in flank volcanoes stability are evaluated in this study. Physical and mechanical properties and their variation with the degree of alteration are described in detailed. A series of multidisciplinary tests were performed to identify and quantify the progressive degradation of the properties. They are as follow: 1) petrographycal and chemical studies (thin-sections, x-ray diffractions and x-ray fluorescence); 2) effective and total porosity (standard test procedure, mercury intrusion porosimetry, pycnometer tests, two-dimensional and x-ray CT image analysis); 3) Ultrasonic pulse velocity measurements; 4) uniaxial compressive tests (with p-wave measurements, cyclic loading); tensile tests (with strain gauge measurements); and 5) triaxial tests (single-stage and multi-stage). Preliminary numerical modelling was mainly focus on the effect of altered rocks content and gravity effects, even if different perturbations such as pore water pressure (e.g. rainfall, vapour and gas), and regional or local tectonics (e.g. faults, earthquakes and dynamic loading) are presented in volcanoes nature. Collected samples are representative of four different classes of volcanic deposits: i) trachytic lava with abundant crystals; ii) pyroclastic deposits, with lava clasts and pumice elements with different sizes; iii) Green tuff, constructed prevalently by pumice clasts; and iv) ignimbrite deposits characterized by low density. Petrographical and chemical characteristics, in particular weathering indexes reveal large differences not only between lithotypes, but also between samples. These differences are well quantified by physical properties, in particular porosity and shear wave velocity values. Decay of the properties, well represented by regression analysis with significant correlation parameter (R2>85), is observed when average values of the compressive strength, tensile strength and Young’s modulus are compared with the average porosity value, fractal dimension and grade of alteration. Failure of rocks were well documented by the evolution of elastic properties, differences between each lithotype are discussed. Post-failure reconstruction of samples reveals that the nature of deformation is controlled by textural properties (e.g. grains, pores, and cement) and the behaviour strongly influences the response of the specimen. Anisotropy of rocks is clear represented by triaxial tests post-failure reconstruction, abrupt differences between fresh and altered samples are observed. Finally, a simplified 2-D numerical stress‐strain modelling was carried out in order to visualize the effects of rock properties degradation in volcanic flank failure. Modelling was aimed at clarifying the role of the altered volcanic rocks in the evolution of volcano stability. The results, in terms of maximum computed values of shear strain and displacements, show that degradation of rock properties is capable of defining and controlling large zones of instability.