Seepage and critical hydraulic gradients in tailings dams and natural formations

Embankment dams, both water Retention Dams (WRD) and tailings dams, are constructed of granular material. Natural seepage flow through porous media, i.e. soil, rock, and tailings, occurs in all types of dams. During this process, particles in the porous media are exposed to hydraulic gradients. Under certain circumstances seepage can initiate internal erosion which can seriously damage the construction, eventually resulting in serious accidents or failure. The process starts when a critical hydraulic gradient is reached, which may vary with the way of construction, materials used and its properties and compaction. This is of special importance for dams constructed of mine waste, i.e. tailings dams, where the stability and function has to be guaranteed over very long time periods (>1000 years). Here, seepage, internal erosion and the corresponding critical gradient are fundamental parameters. It is not clear under which circumstances the process of internal erosion begins, i.e. when one particle in the system starts to move, thus creating a further process of particle transportation in the system. To common way to control seepage, filter layers are included in the construction. However, erosion, piping or sinkholes are still observed. Different models on the hydraulic conductivity and hydraulic gradient are presented and discussed. Properties of tailings in comparison with natural geological materials are identified. Observations from several natural geological formations in Sweden, which have fulfilled the function of a dam since the last glaciation, are presented and the critical hydraulic gradients with respect to vary long time periods are compared.Godkänd; 2007; 20071009 (isajan