Hydro-Mechanical Behaviour of Tailings in Unsaturated Conditions

Many geotechnical problems are related with soils and structures in unsaturated conditions. Among them, a significant role is played by tailing dams that represent complex geotechnical systems. Because of their wide extension, tailing storage facilities are subjected to many external actions and interactions with the atmosphere, whose effects governing the position of phreatic surface. Hence, the low permeability deposited tailings are subjected to drying-wetting processes, leading variation in the extensions of the unsaturated zone above the phreatic surface with a significant role in stability of the embankment. The current research is aimed to study the hydro-mechanical behaviour and the water retention response of unsaturated silt and silt-sand tailings mixtures, respectively, in order to extend a previous characterization of the same soil performed in saturated conditions. This research is also aimed at investigating the occurrence of the critical state in unsaturated conditions, showing the range of state variables leading the static liquefaction. Finally, irradiation by means of gamma rays is proposed as an accelerated technique in order to simulate the natural ionizing radiations acting on stored tailings that, in many countries, are beginning to be re-used as backfill, landscaping material or feedstock for cement and concrete. Wet and dry silty samples were exposed to gamma rays and then characterized in order to study any possible physical and chemical modifications of the exposed matter. The starting point of this research is an available hydro-mechanical characterization of saturated tailings coming from Stava tailing dams collapsed in 1985 (Italy). The first series of experimental tests was carried out to study the hydraulic behaviour of the Stava tailings by means of water retention tests. To account for the heterogeneity of the tailing material within the basin, the dependency of the water retention relationship on grain size distribution and void ratio was investigated. In order to extend the limited range of matrix suction allowed by the axis translation technique, the water retention curves were also implemented with data obtained by indirect soil suction measurement and soil suction controlling methods, dew point technique and vapour equilibrium technique, respectively. Since the pre-shearing state of the soil has a relevant effect on the behaviour shown during shearing before reaching the critical state conditions, a second campaign of experimental tests, consisting in five mono-dimensional compression tests, was performed. Results were compared with those in saturated conditions obtained by Carrera (2008) in order to investigate the dependency of preconsolidation stress and stiffness of the soil on suction. Aimed to extend the hydraulic characterization and to approach the possible critical state in unsaturated conditions from drying and wetting sides, a third campaign of experimental tests was carried out by using a suction controlled triaxial cell and imposing different hydro-mechanical stress paths. Statically compacted silty samples were subjected at different suction levels, then isotropically consolidated at different net stresses, and finally sheared at constant water content or constant suction level by means of the axis translation technique or the vapour equilibrium technique. Finally, due to the importance of the static liquefaction phenomena in stability problems of tailing dams, a fourth series of experimental tests was performed by means of a triaxial cell. Silt specimens were tested in unsaturated or close to saturated conditions, in order to investigate some of the main factors (degree of saturation, initial density and preparation technique) that are supposed to influence the liquefaction response.