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Coal Mine Open Pit Final Void Closure and Relinquishment - Addressing Uncertainty in Coal Mine Environmental Planning

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Published: June 17Project Number: C25030

Get ReportAuthor: David Salmon | Amanzi Consulting

This report addresses uncertainties faced by coal mine operators when assessing and planning options and implementing rehabilitation designs and methods during the closure of opencast coal mine voids to ensure safe, stable, non-polluting, and sustainable post mining void uses.

Guidance material is given in the form of a knowledge base of existing coal mine void closure management practice, general principles in planning and implementing void closure activities and a basically sequential and phased decision making process. This information will enable mine operators to assess pit void closure options when planning to relinquish a mining lease.

Closure planning has become a focus of attention for Australian coal mines as they mature and as some mines face early closure. Relinquishment of the mining lease to the State is a common closure strategy that mines will adopt. Existing guidance on mine closure is often generalised and may even be inappropriate for coal mines. Guidelines, specific to coal mines, are needed to support decisions made by coal mine staff planning and implementing closure activities.

The production of closure and relinquishment guidance for coal mine voids, developed by this ACARP project, involved a collaboration with six mining companies. These companies provided information on existing practices on mine sites and shared the challenges when planning for closure and relinquishment of coal mine final voids.  Information was also sourced from international and Australian literature, from mining company corporate environmental staff and from thirteen open cast coal mine sites, located in Queensland and New South Wales, that were visited as part of this project.

The review of information confirmed the uncertainty that exists on what is required, what is acceptable, what process should be followed to enable closure and relinquishment and what makes a void safe, stable, sustainable and non-polluting. Uncertainty is amplified because the goals may vary according to site specific conditions and they are temporal, changing over time and making the application of closure criteria and estimation of residual risk difficult to assess. Understanding these goals is essential to achieve closure and relinquishment. Relinquishment is often the stated aim of closure, but it is the least understood of the mine closure strategies available. Therefore, it is hardly surprising that relinquishment of coal mine voids has rarely, if ever, been achieved.

An assessment of the information collected produced a number of generic findings. Some of these findings included the identification of variability in the characteristics of mine sites; the complexity of coal mine final void systems and that these systems are not always well understood; the variability in existing closure practice; the need for stakeholder engagement to support successful closure plan implementation; the uncertainty present when applying existing guidelines to site specific characteristics of coal mine voids; the identification of safety and health issues that must be addressed in the closure of mine voids; the variation in regulatory requirements and apparent inconsistency in the application of them; and the lack of relinquishment of coal mines may be due to these findings.

Two main gaps in knowledge were determined that compound uncertainty in risk assessment and implementation of coal mine void closure management options. One gap is the lack of empirical data and understanding of coal void backfill hydrogeology. The second gap is the lack of geotechnical characterisation and classification of void wall material and definition of its stability.

The aspects to be considered when planning activities to enable relinquishment were identified and are listed. Some of these aspects include, characterisation of coal mine pit void type; uses of void land and void water; void catchment and final landform design; pit void backfilling; highwall and low wall types and stability; environmental impacts and risk assessment; and closure and rehabilitation activities and their costing. Other actions required in the process, such as options analysis and risk assessment, and the general principles behind the guideline process are described in this report.

It is recommended that investigations are undertaken to determine basic hydrogeological parameters of coal mine spoil placed in voids. The results of these studies should be used to improve hydrogeological modelling assessment of residual risks. It is also recommended that geotechnical investigations are done to characterise and classify final void wall stability. These investigations would address the two main knowledge gaps found during this project.  

The guideline provides a source of information for mining companies and also any interested and affected party, government agency, non-governmental organization, community and the general public. The information is provided to improve the understanding of what is needed and what is practical for closure of coal mine voids and to assist all parties in reaching agreements in closure and relinquishment planning. It should reduce the risk of extended questioning of proposed mine closure actions and also reduce demands for impractical, alternative management options.

The information on existing practices that mines apply, combined with the process methodology provided in this report can assist in planning and implementing activities to achieve closure goals and final void relinquishment. This information can be used to support the selection of rehabilitation practice appropriate to a particular mine site or void within a mine site and reduce uncertainty in what should be done to move toward closure and relinquishment. Selecting the appropriate practice will help to avoid excessive or unnecessary expenditure on investigations and application of rehabilitation strategies that are inappropriate for a particular mine site.


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