2008 – “When is a dam not a dam?”- Design of mine tailings storage facilities for closure

David Brett, Ben Hanslow. Rob Longey

Abstract: Mine tailings storages are among the largest man made structures in the world and often pose a considerable risk to the aquatic environment due to the nature of the stored materials. In particular, sulphide minerals are prone to oxidation when brought into contact with air and water. This leads to the formation of acidic conditions within the storages leading to dissolution of toxic metals, with seepage from these structures being known as Acid Rock Drainage (ARD). ARD is responsible for pollution of natural waterways in many areas of the world with some significant examples in Australia. Current practice in the mining industry is to attempt to exclude oxygen or water from tailings storages in order to prevent the oxidation process taking place. This involves capping of the storages with sophisticated soil covers or, where sufficient water is available, leaving a permanent water cover.

Mining operations have a relatively short life, usually around 5 to 10 years, although some can operate for over 100 years, as has the Mount Lyell Mine. Normal practice has been for companies to relinquish the mining lease on the cessation of mining, however governments are now realising the extent of liability involved with the “ownership” of large waste storage facilities. Bonds are placed by mining companies during operations, intended to cover the cost of “closure” of the mine. Often the major item covered by the bond is for the “closure” of the tailings storage facility. Following “closure”, the intention is that ownership of the lease, including waste storages, reverts to the State. State governments are now more aware of the potential liabilities in accepting the relinquishment of these leases and need to address the issues of their long-term management.

In Tasmania, Dam Safety legislation covers both water and also soil covered tailings storages, with the legislation requiring each type to meet various ANCOLD guidelines. In other jurisdictions this could well also be the case through common law requirements to meet common best practice. However, the current ANCOLD Guidelines are generally written around water storage dams and interpretation to include a waste storage facility is often not straightforward. As an example a tailings dam during operations with a water storage component is clearly a dam. Due to the environmental impact of failure it could well have a Significant or High-C hazard rating, which would require design for extreme floods and earthquakes. After closure, with say a soil cover and water diverted away, is it still a dam within an ANCOLD definition? Are ANCOLD guidelines relevant? The current ANCOLD (1998) Guidelines on Tailings Dam Design, Construction and Operation does not give specific guidance on these issues.

This paper explores these questions and suggests ways that ANCOLD could provide assistance with more guidance on the long term management aspect of tailings storages to assist designers, owners and regulators consider the closure phase.

Keywords: tailings, acid drainage, mine closure

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