The failure of the Stava Valley tailings dams (Northern Italy): numerical analysis of the flow dynamics and rheological properties

Background: Tailings dams are made up of mining residue deposits, and they represent a high risk, in terms of mechanical instability. In the event of collapse, the tailings in such dams may be released and flow over long distances, with the potential risk of extensive damage to property and life. The traditional geotechnical assessment of tailings facilities has mainly concentrated on the stability of tailings dams, while relatively few studies have investigated the flow of tailings released after dam failure. In this context, it is possible to state that, if the complex rheological behaviour of the tailings material is captured correctly during the flow, numerical modelling can be used to contribute to a better comprehension of the flow characteristics and for the assessment of the possible extension of the impact area. Results: Considering the wide range of possible rheological behaviour that tailings flows can assume (from laminar to turbulent), this paper presents the new version of a computer model, which was designed to simulate the motion of rapid flow movements across 3D terrain. This new version integrates the existing rheological kernel (Frictional, Voellmy) with two new rheological laws (Bingham and Turbulent), and adds the possibility of changing the rheological properties of the flowing mass during the propagation process. The code has been applied to the disastrous flow that was caused by the failure of a pair of tailings impoundments in the Stava Creek Valley (Italy) in 1985. Since different interpretations on this flow behaviour already exist in literature, and since a large number of changes in the rheological values along the run-out path have been proposed to recreate its dynamics, new simulations, carried out with different rheological combinations, are presented and discussed here in order to obtain a better understanding of the flow dynamics and to identify the rheology that reproduces the phenomenon that occurred with the fewest possible changes in the rheological values along the runout path. The latter aspect is particularly important when numerical analyses are used for prediction purposes. Conclusions: The great rheological flexibility of the new code has allowed the Voellmy rheology and a combination of its parameters to be identified as the most suitable to describe the Stava flow, even where the run-out path presented critical characteristics.