12.8 Tailings Impoundment Design
Mine tailings produced by the mill are usually in slurry form. Disposal of slurry tailings in impoundments made of local materials is the most common and economical method of disposal. There are four main types of slurry impoundment layouts; valley impoundments, ring dikes, in-pit impoundments, and specially-dug pits. Slurry tailings are sometimes disposed in underground mines as backfill to provide ground or wall support.
Open-pit backfilling is also practiced where tailings are deposited into abandoned pits or portions of active pits. In active pits, embankments may be necessary to keep the tailings away from the active area. However, since seepage from the tailings can adversely affect the stability of the pit walls or embankments, it is unusual to see disposal in active pits.
In general, tailings impoundments are designed using information on tailings characteristics, available construction materials, site specific factors (such as topography, geology, hydrology and seismicity) and costs, with dynamic interplay between these factors influencing the location and actual design of the impoundment.
Tailings composition, pulp density, grading, and other characteristics are used in the design of tailings impoundments in three basic ways: tailings analysis to assess the potential use of tailings sands in constructing the embankment, analysis of tailings to be placed in the impoundment to determine their potential impact on structural stability and seepage characteristics, and mineralogical analysis to determine the potential chemical aspects of seepage or other discharges from the impoundment. Tailings are considered to be soils, subject to traditional soil mechanics patterns of behaviour.