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
B. Ghahreman Nejad, H. Taiebat, M. Dillon and K. Seddon
One of the causes of tailings dam failure has been seismically induced liquefaction during earthquakes. Liquefaction, if mobilised, significantly reduces the stiffness and strength of affected soils in the embankment dam or its foundation and may lead to large deformations and dam failure. This paper reports the results of seismic liquefaction assessment and deformation analyses of Bobadil tailings dam located in Tasmania. The tailings dam consists of a perimeter rockfill starter dam which has been raised in stages using the “upstream” construction method. The embankment raises (formed by clay or coarse tailings) are constructed over a foundation of previously deposited tailings in the impoundment which is potentially susceptible to liquefaction. Extensive field and laboratory tests were carried out to assess the tailings liquefaction potential and also to determine the material properties required for seismic stability and deformation analyses. Numerical modelling of seismic liquefaction and deformation analyses were carried out to predict the magnitude and pattern of deformations that may lead to uncontrolled release of tailings. The results of these analyses are presented and compared with literature report of those observed during past earthquakes.
2011 – Numerical Modelling of Seismic Liquefaction for Bobadil Tailings Dam
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
Craig Messer, Francisco Lopez, and Manoj Laxman
The Enlarged Cotter Dam is a new 80m high Roller Compacted Concrete Dam being constructed to augment the water supply for the Canberra region. Due to the size of the main dam and the extreme climatic variations in the ACT, where temperatures range from sub zero in winter to in excess of forty degrees in summer, it is expected that significant stresses will be generated during the cooling of the structure. For this reason it is essential that an understanding of the magnitude of these stresses is developed through the initial strength development period and at critical periods such as the first and second winter when the temperature differential between ambient conditions and the core of the structure may be greatest. The development of thermal stress within the structure has critical impacts on both the RCC mix design and the dam construction equipment and methodology.
For the Enlarged Cotter Dam, thermal stresses were investigated using both two and three dimensional finite element transient heat transfer analyses, making use of the thermal properties derived from laboratory testing including instrumented thermal blocks, as well as established literature. Modelling of the thermal stresses in the dam required the development of time dependent concrete properties, such as strength, stiffness and heat generation, with the latter based on test results and calibrated to actual measured values. Additionally, site dependent conditions for ambient temperature, external conduction, convection and radiation factors, dam foundation temperatures and restraint, dam construction sequence, formwork, joint spacing, insulation and timing of reservoir filling were also modelled.
Initial thermal modelling of the dam demonstrated that significant tensile stresses and potential cracking could develop within the structure, at both early and mature concrete ages. Subsequent analyses were developed to investigate methods of reducing these stresses to within acceptable limits. This paper presents the results of the thermal analyses, including the methods to be employed during and after construction to minimise cracking without impacting construction costs and even optimising the speed of construction.
Finite Element Transient Thermal Analysis of the Enlarged Cotter Dam
Richard R. Davidson, Joergen Pilzand Bruce Brown
Recent earthquakes in Chile, New Zealand and Japan have created a new focus on the safe design of tailings dams in seismic regions of the world. Building sand and rockfill embankments to sustain large ground motions and provide crucial drainage of excess pore pressures remain daunting challenges at each site. Are conventional hydraulic deposition practices still viable? What new technologies can be considered? Addressing seismic stability of existing upstream method tailings dams whether currently in operation or closed is stretching our seismic geotechnical engineering profession to its limits of understanding of behaviour. Creating a safe, secure environmental storage must also be integrated with the geotechnical and hydrologic concerns. Is there a viable risk context to consider these competing issues? This paper will raise these issues within the international context and suggest a prudent path forward.
2011 – The Challenges of Building Tailings Dams in Seismic Regions