Tailings Dam Stability

Tailings are waste materials from mining operations, which need to be disposed of and safely stored. They are commonly transported as slurry in pipes to the storage facility and surface impoundments through spigots. Different types of tailings dam construction and disposal method include tailings dams designed as water retention dams, and dams built using the upstream method, downstream method or centreline method. Several examples of recent tailings dam failures involved dams constructed by the upstream method, where the new embankments are founded on tailings, causing the dam to become progressively more dangerous as its height increases. From back-calculation of historical failures, two distinct failure mechanisms seem to be dominant. The first mechanism is related to the development of progressive failure in a weak soil layer in the dam foundation. The second dominant failure mechanism is related to static or dynamic liquefaction of loose tailings material at a critical state. Static or dynamic liquefaction of loose tailings may occur at a critical condition, where a rapid (undrained) small increase in the shear strain results in a large increase in pore pressure, reduced effective stresses and a dramatic reduction of shear strength. Typical for these types of failures is that they occur rapidly with no warning. There is often no sign of increased displacement rates or pore pressure increase in the days prior to dam failure. Failure is often initiated by a local instability at a critical location, where redistribution of stresses due to reduced shear strength upon further deformations rapidly develops into a global failure mechanism without any additional load actions.