Risk Management Strategies for the Surrender of Open Cut Dragline Spoil Pile Areas

Forestry and native habitat are more sustainable post-mining land uses for central Queensland mines than grazing or cropping.  The emerging government vision is driven by the principles of ecological sustainable development and is enshrined in the Environmental Protection Act, which protects and enhances environmental values, including air, downstream water quality and land management. Native habitat or bushland is now the preferred post-mining land use for about half the Bowen Basin open cut coal mines. It is recognised that unmined landforms are irregular, while mined landforms tend to be regular (engineered) and that post-mining landform design should aim to blend the mined landform with the surrounding unmined landform. The primary aim of this project was to develop a multi-disciplinary risk management tool to select rehabilitation strategies for the surrender of Bowen Basin open cut coal mine spoil areas, using the most cost-effective and environmentally acceptable approaches. The methodology comprised:

• Literature review and commentary, based on available published information, carried out during the first half of the project.
• Collection of data and observations on spoil rehabilitation from Bowen Basin open cut coal mines.
• Collection of the views of stakeholders on spoil rehabilitation and future land use.
• Development of a risk assessment and cost-effectiveness tool for spoil rehabilitation and future land use, taking into account the available information and views of stakeholders.
• Application and demonstration of the risk assessment and cost-effectiveness tool through focused workshops.  
• Through ACARP, widely disseminate the results of the research throughout the Bowen Basin.

Researchers collected available data on the key parameters, including topography, climate, spoil type and distribution, water quality, the performance of unrehabilitated spoil piles, and the design, costs and performance of a range of approaches to spoil pile rehabilitation and revegetation. For unrehabilitated spoil piles, observations on natural reshaping and revegetation, the stability of steep slopes in durable spoil, the instability of erodable spoil, and armouring were collected. For rehabilitated spoil, researchers collected observations on erosion, revegetation success, and the potential for land use.

Findings

The key causes of land use failure and, by inference a failure to achieve lease surrender, are landform instability, surface water impact, groundwater impact, vegetation failure, animal failure, poor perception and negative socio-economic impact.

Some of the key points derived from the literature review and commentary are:

• Historically, the overriding aim of spoil rehabilitation was a return to productive land use, with a focus on grazing, through future grazing on spoil is now generally seen as impractical in the Bowen Basin.
• Aesthetics must be considered in designing spoil rehabilitation, along with safety, stability and land use.
• Most Bowen Basin spoil materials are geotechnically stable at a slope angle of 33 percent or steeper and elevated spoil materials on stable foundations will be stable at their angle of repose.
• The Queensland Department of Minerals and Energy target erosion rate for rehabilitated spoil of 12 to 40t/ha/a is typically 30-fold higher than natural erosion rates and river sediment yields, typically four-fold higher than erosion rates from agricultural land, but typically 10-fold lower than erosion rates from construction sites.
• Erosion loss increases with slope angle and slope length, and decreases with vegetative, litter and coarse particle cover, and with profile concavity.
• The average cost of conventional rehabilitation in the Bowen Basin is about $19,000/ha (ranging from $3,000 to $35,000).
• The remaining potential spoil rehabilitation liability is around $825 million of a $1 billion liability.
• The major shortcomings are entrenched perspectives, inadequate definition of rehabilitation success criteria, inadequate understanding and knowledge of spoil hydrology, the lack of field data on erosion from steep spoil slopes, paucity of acid mine drainage and salinity data, lack of proven spoil rehabilitation strategies, and paucity of data on the hazards and impacts of alternative land use surrounding mine sites.
  

Where To From Here

While the project is directed specifically at spoil areas in the Bowen Basin coal fields, the results may have application to other disturbed areas such as final voids, tailings dams and coarse reject dumps, and to other open cut coal mining regions.