Mine Tailings Storage: Safety Is No Accident

Storage dams – why are tailings dam failures more common than water dam failures?

Water dams have been known to fail with catastrophic consequences but in the last 40 years, failures have become very rare, whereas tailings dams have continued to fail. Is there something that can be learned from water dams to improve the safety of tailings dams? Unlike water- retaining dams, where the dam wall is usually constructed from concrete or some combination of engineered rock- fill and soil, most tailings storage facilities are built using designs that partially depend on the tailings themselves for support, a design feature not available to water supply reservoir dams (Chambers 2012). • Water-retaining dam s are built to full height, then filled and operated; tailings storage facilities are usually built incrementally and operated during this incremental building phase. • Construction of a tailings storage facility may take many decades until it reaches final design height, with a single tailings storage facility often being used for the entire life of the mine. During the operational period of a tailings Other key differences between water-retaining dams and tailings storage facilities are:

storage facility, there are likely to be many changes to the operating and management personnel, creating challenges that are often not addressed. • A water-retaining dam is regarded as an asset, typically for common use, while tailings storage facilities are seen as a cost – a means of storing waste rather than providing a service (Figure 8).

Component Tailings dam Differences between water-retention dams and tailing dams

Water-retention dam

Stored material

Tailings solids and processed water with various contaminant levels, run-off water Ministry of Mines, Ministry of Environment Limited operation life - 5 to 40 years Raised over the mines operating time Infinite closure period, aim for “walk away” design Medium to high level Varies: Owner and engineer may change frequently during the construction period Generally good for starter dam, variable levels during construction period. Can be at a low level for some companies Tailings debris flow resulting in physical damage and environmental contamination Can vary during the design life, e.g. transition to centreline or downstream

Water

Regulatory regime

Ministry of Public Works, Regional Authorities, National Dam Associations Typically designated as 100 years but “as long as required by society” Usually 1 to 3 years Often not addressed, but facility may be decommissioned High level Usually one engineering firm for design and construction

Operation life Construction period Closure Engineering Continuity of engineering Quality assurance and quality control Consequences of failure

High level

Water-inundation damage

Dam section

Usually a consistent section

Source: Adapded from McLeod and Murray 2003

Figure 8. Comparison of water dams and tailings dams

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