Water retention behaviour of tailings in unsaturated conditions

Tailing dams are complex geotechnical systems comprising of an embankment and a basin containing the waste products from the mining processes. These structures are characterized by a wide surface exposed to the atmosphere whose interaction governs the position of the phreatic surface within the basin. A detailed knowledge of the hydro-mechanical properties of the tailings is fundamental to reliably assess the stability of the tailing dams. While most of the previous studies have dealt with the response of tailings in saturated conditions, this research provides an extension of the hydraulic behaviour in unsaturated and nearly saturated state of tailings collected after the failure of the Stava basins. The hydraulic behaviour in unsaturated conditions was investigated by means of tests where the suction was imposed and the water content was monitored (axis translation technique and vapour equilibrium technique), and tests where the water content was imposed and the suction was measured with psychrometer (dew point method). To account for the in-situ heterogeneity of tailings, the dependency of the water retention relationship on the grain size distribution, the preparation technique and on the initial density / void ratio was studied. Denser tailings showed a higher water retention behaviour than that given in looser specimens. Similarly, the increase of the fine content was demonstrated to improve the water retention capability. As for standard soils, also statically compacted Stava tailings reveal lower retention capability than the slurry samples, thus confirming the importance of the preparation method in determining the hydro-mechanical response of such soils.