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Cover System Design

The overall purpose of a mine waste cover system is to restore the surface of a waste deposit to a stable, natural condition while minimizing degradation of the surrounding environment following closure of the facility. Cover systems over waste material can have numerous objectives, including but not limited to:

  • Isolation of waste;
  • Limiting influx of atmospheric oxygen;
  • Limiting influx of atmospheric water;
  • Controlling erosion of waste material;
  • Control upward movement of process-water constituents / oxidation products; and
  • Providing a medium for establishing sustainable vegetation.
Cover system design alternatives.

Illustration by Derrill Shuttleworth.

Our design philosophy for a cover system is one that integrates the waste material within its environmental context. A mine waste cover system must be designed as an unsaturated system exposed to the atmosphere, the performance of which will be significantly influenced by seasonal, annual, and long-term site climate conditions. A key factor to consider during the design process is the anticipated climax vegetation species that will develop on the cover system, and how this may positively or negatively influence the performance and integrity of the cover system.

OKC has the unique fortune to be involved and to lead the design of cover systems within a large range of climatic conditions around the world.  More importantly, OKC is responsible for monitoring over 70 of these locations, and is responsible for continuously managing and interpreting detailed monitoring data.  Our constant monitoring and large diversity of locations allows for continual evolution of design methodologies by learning through interpretation of detailed monitoring data.  Hence, OKC’s design methodology uses an adaptive approach where a methodology implemented for a past project is constantly improved and immediately implemented in the future.  This ensures that all cover designs are tailored to site-specific processes while still using prior experience for designing the most optimal design.

At many mine sites, the design and construction of a cover system for closure of a waste storage facility represents the single biggest issue, not only with respect to environmental impact and cost, but also public and regulatory scrutiny. In our experience, there has been a general tendency by regulators to develop performance criteria for cover systems that are tied directly to various cover design objectives. In many cases, this practice has led to the development of single, often very conservative, numerical values of cover performance criteria such as ‘net percolation’, ‘rate of oxygen ingress’ and/or ‘plant density/mixture’. In our opinion, there is a need to develop cover performance criteria on a case-by-case basis and with due consideration of the short-term and long-term impacts on the receiving environment at a particular site. Further details on this approach can be found in the following paper.

 

O’Kane, M. and Wels, C., 2003. Mine Waste Cover System Design – Linking Predicted Performance to Groundwater and Surface Water Impacts. Proceedings of 6 th International Conference for Acid Rock Drainage, Cairns, Qld., Australia, July 2003.