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Best management practices of dispersive mine spoil

Monday, 01 June 2015

Sustainable closure of coal mines in Australia remains one of the industry's biggest environmental challenges.

This issue is amplified when spoil characteristics make rehabilitation difficult and costly. A significant proportion of mines in the Bowen Basin of Queensland have dispersive spoil. It is estimated that the current instantaneous liability for rehabilitating dispersive spoil dumps is $2 to $3 billion for the Bowen Basin alone and significantly more nationally.

Dispersive soils and mine spoil are characterised by weak aggregate stability, and spontaneous dispersion of clay particles in contact with water (Vacher et al., 2004; Minserve, 2004). This, in turn, results from their sodic properties, or an excess of sodium relative to calcium and magnesium (DNR, 1997).

Dispersive soils are a common feature of the Australian landscape, covering 25 per cent of Queensland (Shaw et al., 1994). Similarly, dispersive spoil material is a significant feature of sediments overlying coal deposits throughout the Bowen Basin and, to a lesser extent, the Clarence-Moreton Basin. Where they occur, dispersive spoils present problems due to their increased potential for both surface and tunnel erosion, as well as unfavourable conditions for plant establishment, compromising the ability to achieve the fundamental mine closure objectives of a safe, non-polluting, stable and sustainable post-mining land form.

Figure 1. Soil erosion resulting from sodic dispersive soil.

The preference for best practice management of dispersive spoils is for selective spoil placement to bury dispersive material at a depth sufficient to prevent contact with free water (Minserve, 2004; Henderson, 2008). However, many situations require management of dispersive spoil where it is not practical, economic, or possible to undertake selective spoil placement.

Study overview

A number of studies have previously investigated the rehabilitation of dispersive spoil material (eg the importance of vegetation cover, topsoil, slope length, gradient and interception structures); fundamental mechanisms of dispersion-related erosion; and the importance of spoil specification and landform design. Significantly, there remains no definitive guideline or set of best management practices to assist the industry in applying consistent, reliable, proven and cost-effective approaches to managing dispersive spoil.

 

  • a set of Best Management Practices (BMPs)
  • risk-based decision tools to equip practitioners with cost-effective guidelines to improve rehabilitation outcomes in relation to dispersive spoil
  • a decision framework and explanation of risks
  • quantification of the costs of alternatives
  • a process for understanding the trade-offs between risks and costs
  • a ‘business case’ for applying risk-based decision tools.  

The project has broad mining industry backing including in-kind support from Anglo American, Peabody Energy, Rio Tinto, Glencore and New Hope Group.

Research program and scope

The research program is focussed on development of a framework to support practical, cost-effective, risk-based best management practices for dispersive spoil where either:

  1. it is not practical, economic, or possible to undertake selective spoil placement, or
  2. dispersive spoil occurs as a legacy of prior rehabilitation efforts.

Figure 2. Draft decision framework for dispersive spoil management, outlining project scope (dotted box).

The objectives of the project will be met through a series of six research modules:

  1. Collation and review of prior research and industry experience in relation to the practice and cost of dispersive spoil management
  2. Field assessment across a range of site conditions and performance levels to provide data for development of a quantitative model of dispersive spoil behaviour
  3. Development of a quantitative explanatory model of dispersive spoil behaviour
  4. Establishment of a set of replicated trials and demonstration sites for long-term validation of management approaches and expected site responses
  5. Preparation of a set of Best Management Practices (BMPs), incorporating a costed, risk-based decision framework, for a range of final land-use outcomes, particularly for native vegetation communities
  6. Communication of project outcomes via a website that will make the risk-based decision tool and project outcomes accessible to the wider industry.

Expected project outcomes

Development of a quantitative model describing dispersive spoil behaviour will be a key output of the research program. By providing a means to predict performance for any configuration of site and management conditions, the model will facilitate application to new rehabilitation sites and situations. It will support ability to communicate an understanding of rehabilitation performance sensitivity to key design and management parameters. It will also provide a checklist of key factors that need to be understood and assessed in dispersive spoil rehabilitation design and management.

Equally important, by structuring around a decision tree framework and a costed, risk-based decision tool, the BMPs will assist practitioners to understand risks of various options, the costs of alternatives, trade-offs between risks and costs, and ultimately, to establish the business case for applying a given course of action.

Ultimately the project is aimed at making a positive contribution to improved environmental performance and community acceptance of post-mining landforms by developing tools that provide a consistent, reliable and informed approach to managing dispersive spoil.

Industry comment and more information

The research team is currently collating existing information in preparation for field assessment. If you have any experience to contribute, please don’t hesitate to contact Dr Glenn Dale, Managing Director and Chief technical Officer, Verterra Ecological Engineering; 07 3221 1102, This email address is being protected from spambots. You need JavaScript enabled to view it.

References

DNR (Department of Natural Resources) (1997). Salinity management handbook. Second edition. 214pp.

Henderson, S. (2008). Specification of Overburden for Use as Cover Spoil. ACARP Project C14042. Final Report, July 2008. 96pp.

Minserve (2004). Rehabilitation of Dispersive Tertiary Spoil in the Bowen Basin. ACARP project C12031. 62pp.

Shaw R., Brebber L., Ahern C., Weinand, M. (1994). A review of sodicity and sodic soil behaviour in Queensland. Aust J Soil Res 32(2): 143–172.

Vacher, C.A., Loch, R.J. and Raine, S.R. (2004a). Identification and management of dispersive mine spoils. Final Report. Australian Centre for Mining Environmental Research, Brisbane.