Jiri Herza and John Phillips
The design of dams for mining projects requires processes and technology that are unfamiliar to many mine owners and managers. Dam designers rely on ANCOLD assessments of Consequence Category, commonly leading to a High rating for mining dams due to a combination of potential loss of life, impact on environment and damage to assets such as mine voids, process plants, workshops, offices, roads, railways etc.
From this High Consequence Category the relevant annual exceedance probabilities for design parameters and loading conditions such as earthquakes and floods are selected.
Mining companies have sophisticated methods available for assessing risk, yet for their assets they often adopt an order of magnitude lower security for earthquake and floods even though the consequences in terms of lives at risk and impact on project are similar.
The discrepancies in the design standards lead to situations where extreme dam loads are adopted to prevent damage and loss of life in assets that theoretically would have already collapsed under much lower loads.
One difference may be that some mining dams exist in an environment which is controlled by a single entity. Unlike other dams, failure of these mining dams would therefore impact only individuals and assets which fall under the responsibility of the same entity.
This paper discusses the discrepancies between the design of mining dams and the design of other mine infrastructure. The paper considers the impact of discrepancies on the overall risk to the mine and compares the degree of protection offered by a factor of safety and the influence of reliability of design input parameters, alternate load paths and design redundancy.
Keywords: Dams, tailings dams, mining, acceptable risk, factors of safety
T. Mortimer, J. McNicol, P. Keefer, W. Ludlow
CS Energy’s Kogan Creek Coal Mine located in the Surat Basin in Queensland, services the 750MW coal fired, Kogan Creek Power Station. Strip mining generates large volumes of mine waste which is typically used to construct waste dumps. Recent work at the mine has focused on using mine waste to construct an ash storage facility to store ash that is piped over 5 km from the power station as a dense phase slurry. The use of mine waste to construct the ash storage facility provides significant cost and time savings, however a range of design, construction and operation issues needed to be addressed to operate a facility of this type.
This paper describes some of the key design, construction and operation considerations for the ash storage facility. Design considerations include pipeline transport through environmentally sensitive areas, addressing the stability of the embankment and the use of a partial LLDPE geomembrane lining system to reduce the risk of seepage from the storage. Construction considerations include post construction (pre ash deposition) floor treatment to reduce potential settlement. Operational considerations include ash slurry deposition, water management of the decant pond and progressive rehabilitation of the final landform.
2011 – Design, Construction and Operation of a Partially Lined, Ash Storage Facility Constructed from Mine Waste
G. Hadzilacos, ML. Ng, K. Taske, A. Small and B. Loney
Alteration of flow patterns by constructing a dam may have an irreversible impact on ecosystems depending on the timing, duration and frequency of these flows. As part of an Environmental Impact Study, carried out for a proposed mining operation in Australia that included an earth dam on a pristine ephemeral creek, an appropriate waterway management scheme was proposed that required the establishment of measurable instream flow requirements. This paper describes an environmental flow analysis (EFA) carried out to identify flow regimes that achieve the desired ecological outcomes for the affected waterways. The EFA methodology was based on the range-of-variability approach using a calibrated rainfall-runoff model to form the hydrologic basis. The study established a relationship between flow components and ecological variables based upon which the flow requirements were estimated using a simple methodology.
2011 – A case study of an initial Environmental Flows Assessment for an earth dam on a pristine stream in Cape York
Robert Keogh RPEQ, CE Civil (Hon), Mal Halwala, Peter Boettcher, Renee Butterfield
SunWater is a Government Owned Corporation (GOC), operating in a competitive market on an equal commercial footing with the private sector. SunWater owns 23 referable dams. Over the last fifty years there has been significant development of the methodologies used to estimate extreme rainfall events. These have resulted in substantial increases in probable maximum flood (PMF) estimates for most of SunWater’s dams.
SunWater has undertaken a Comprehensive Risk Assessment program across its portfolio. SunWater now has a good understanding of the deficiencies and available risk reduction options for each dam under all load conditions. The total cost to rectify all deficiencies is several hundred million dollars and well beyond the financial capacity of the organisation in the short term.
ANCOLD and Regulators have different published opinions on decision making criteria for dam safety upgrades. Once the conditions for the tolerability of Societal and Individual Risk are satisfied the onus remains with the dam owner to meet the ALARP principle. The decision making process is complicated by uncertainties in inputs to risk assessments. The authors have considered these uncertainties as well as the legal implications, differing ANCOLD and Regulator requirements, and business and economic loss, in formulating the decision making process. The methodology is simplified but effective. If the process is followed the dam owner’s investments will meet ANCOLD, Regulatory, legal and business requirements.
This Paper details a logical decision making process designed to allow a non technical Board to balance social, legal and financial objectives. The process considers overall risk, tolerability, the ALARP principle, and project prioritisation. The process is being used by SunWater to determine the Acceptable Flood Capacity of each dam, which dams will be upgraded, priorities and scheduling of each upgrade.
How SunWater, as a commercial dam owner makes investment decisions for dam safety upgrades
David R Jeffery
In 2004 the Victorian Government announced the decision to proceed with Australia’s largest dam decommissioning project, the return of the 365,000ML capacity Lake Mokoan to a wetland.
The project has been completed and has resulted in significant river health benefits through liberating environmental flows in the Broken, Goulburn, Murray and Snowy Rivers. Decommissioning has allowed the recovery of water savings for return as environmental flow to the River Murray (30,000 ML/year) and Snowy River (21,000 ML/year).
With decommissioning complete, development of a significant wetland complex across the 8100 hectare site has commenced.
This project has been undertaken at a time when the Broken River basin was exposed to its worst drought conditions in over 100 years and within 11 years of the worst flooding experienced in the nearby Rural City of Benalla. These extremes of climatic conditions and their impacts on the local and irrigation communities have ensured considerable community and stakeholder interest in the decision to proceed with decommissioning and in the subsequent delivery of each of the project elements.
This paper provides an explanation of the drivers for the project, describes the process followed and some of the challenges experienced over the projects seven year life and presents some of the lessons learned along the way.
2011 – MOKOAN – RETURN TO WETLAND PROJECT