David Hilyard, William Ziegler, Heather Middleton
New South Wales has a significant number of dams, including major water supply dams, located over or near mines. Mining near dams imposes dam safety risks including: mine subsidence, mine blast vibration, presence of mine personnel downstream, rapid changes in consequence during mining, and loss of stored waters. The NSW Dams Safety Committee(DSC) regulates mining near dams, using risk assessment to review applications to mine near dams. A structured approach allows rational, evidence-based decision making by stepping through a procedure involving: initial consultations, screening risk assessment, evaluation of technical arguments, risk assessment, and development of risk management strategies. The risk assessment for dam walls develops acceptance criteria, reviews 19 possible risks to dam walls, and site-specific hazards. For potential for loss of stored waters, four possible groups of flow paths from storage to underground mine are reviewed; flows are evaluated with Monte Carlo simulation in terms of tolerable loss. Risks are assessed from a dam engineering viewpoint, which may be more conservative than the perception of risk in the mining industry, considering both tolerable risks and operational time frames. Case studies include: a tailings dam 100 m upstream of an active open cut and underground portal was undermined by longwall mining, with about 1.5 m subsidence of parts of the embankment as each of four longwall panels was extracted; longwall mining beneath a major Sydney water reservoir, with no observed impact on the stored waters; and open cut mining immediately downstream of a mine water dam. Risk-based methodology has provided the DSC with increased confidence in reviewing applications to mine near dams.
Keywords: Mining, dams, risk assessment, New South Wales, Dam Safety Committee
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John Grimston, David Leong, Robin Dawson
The Angat Multipurpose Project, originally constructed in the 1960’s, is located 60 km north-east of Manila, and provides power, irrigation and domestic water supply and flood mitigation. The major water-retaining structures of the scheme are a 131 m high main rockfill dam and a 55 m high rockfill saddle dam.
Previous seismology studies have identified the presence of a possible branch of the West Valley Fault crossing under the saddle dam. If the fault dislocated, the branch under the saddle dam could produce horizontal and vertical shear displacements. Further, earthquake shaking poses a risk outside the fault zone. If the main dam/saddle dam were to fail in such an event, there would be major consequences in respect to both the water supply (serves a population of approximately 10 million) and the large population living below the dams. The dams are thus in the highest hazard category under any internationally accepted standard.
A study to investigate the dam safety aspects and identify remediation works which would bring the seismic performance of the main dam/saddle dam system up to an acceptable level was undertaken and included:
The main conclusions were:
Keywords: Dam, Remedial, Seismic, Fault, Spillway.
This paper highlights the importance of hydraulic diversion control structures during construction of large dams and the value of allocating sufficient resources during project planning and implementation.
The design of the diversion gate for construction of the Enlarged Cotter Dam presented various challenges, including operation for up to 38m head for discharge into a 3m diameter conduit and the need to serve as an upstream concrete form during eventual diversion closure.
The short duration of operation allowed acceptance of increased level of operational risk and a higher level of design uncertainty. The design used generally accepted gate design methods, but no hydraulic modelling. The hydrodynamic forces were estimated using published data. After installation, a 1 in 100 AEP flood event resulted in the gate being subjected to 90% of its design head while operating in conditions close to the maximum design down-pull force. Attempts to raise the gate succeeded only after increasing the hydraulic pressure above the design value.
Keywords: Guard gate design, outlet works, dam, construction.
Nicole Anderson, M. Tooley, N. Vitharana, D. Moore
There is a significant stock of aging concrete dams in Australia which do not meet the requirements of modern dam safety practices. Where no site-specific information exists, current practice requires unduly simplified, conservative assumptions to be made. In some cases, this results in theoretical dam failure for load conditions which the dam has already experienced and safely withstood.
This paper outlines a range of site-specific field and laboratory investigations undertaken to reduce uncertainties in the assessment of two concrete gravity dams. For one dam, a suite of lab tests was undertaken to determine the residual reactivity so that potential future Alkaline-Aggregate Reaction induced expansion can be incorporated into any upgrade design.
The main purpose of the investigations was to reduce inherent uncertainties surrounding the design assumptions for strength and uplift pressures. This in turn reduced uncertainties relating to the risk profile of the dams.
The findings of this investigation will be of interest to dam designers and owners faced with upgrading concrete dams where a single traditional assumption can result in the difference between no upgrade or an upgrade worth several million dollars.
Keywords: Concrete gravity dams, testing, upgrade, Alkali Aggregate Reaction, dam design guidelines.
Dr. Mark Locke, Jiri Herza
Gördes Dam is a nickel and cobalt mine tailings dam situated in a seismically active zone in Manisa Province, Western Turkey. The dam is a conventional cross valley earthfill structure with a fully lined storage basin. The starter embankment with a maximum height of 50 m will be raised in downstream lifts to an ultimate height of 90 m. The total storage capacity is 19 million m3. Construction of the starter embankment is planned to commence in late 2012 and the dam will be commissioned in June 2013.
The tailings will be discharged from the dam crest and return water will be collected by a floating decant pump at the opposite site of the storage. Decant water has high calcium sulphate levels and will require treatment before re-use in the plant or release. The tailings contain about 33 % of solids and are classified as high plasticity silts and clays with more than 90 % of particles passing the 0.075 mm sieve.
The dam is founded on a complex formation of altered sedimentary and metamorphic rocks including mudstones, siltstones, limestones and serpentines. The mudstone blocks, the predominant foundation materials, are juxtaposed with siltstones and serpentines via a complex arrangement of faults. Where exposed, the mudstones are highly to completely weathered with a well-developed structure of smooth bedding surfaces leading to anisotropic strength characteristics. Several landslides, likely associated with the anisotropic character of the mudstones, were identified within the area including a significant landslide under the upstream shoulder of the dam.
Mining development in Turkey has a complex legislative environment. There is also standard practice which is not legislated but expected, this can be considerably different to normal design practice in Australia. The Turkish legislation is based on waste management guidelines and may be more appropriate to landfills than large tailings storages. The legislation is very prescriptive in some aspects and silent in others, with little consideration of risk or consequence based design.
This paper discusses the design difficulties associated with the challenging foundation conditions, which have been magnified by the requirements and limitations embedded in the approval documentation and the legislative environment in Turkey. It will also address some of the key differences between the design philosophy in Australia and in Turkey with a focus on the major risk elements of the design.
Keywords: Tailings, Turkey, Liner, HDPE, Nickel laterite
A concrete-rockfill composite dam consists of two zones: a slender concrete gravity section and a rockfill embankment section. Each zone behaves according to its stiffness and geometry during earthquake shaking. At the abrupt interface a structure behavioural discrepancy results. To mitigate such this discrepancy, a transition interface is introduced by gradually tapering the concrete section down and burying into the central part of rockfill embankment. However the behaviour of the interface is complex due to the two intermeshing of the different materials. Previously, the interface was not designed with any serious theoretical approach, but with the intuitive belief that the transition structure can play the role of mitigating behavioural difference between concrete and rockfill sections. This study seeks to characterize the dynamic behaviour of each section and to understand the performance of the interface using centrifuge model test and numerical analyses. The centrifuge model, which was reproduced by scaling down D dam in Korea, were loaded with adjusted seismic forces based upon seismic coefficient of 0.098g and 0.154g required in the dam design criteria. The legitimacy of the model test was verified by the comparison of the test results with those of numerical analyses, and the most appropriate input values for the interface elements were proposed through a systematic parametric analysis. The key findings of the paper are as follows: Numerical parameters study of the interface-element was carried out, the friction angle depends on rockfill zone material and normal and shear stiffness coefficient of the two materials (concrete and rockfill), the average values were found to be the most appropriate. The findings of this study can be used to design new composite dams, rehabilitate current dams, or design additional spillways to current rockfill dams.
Keywords: Composite dam, Centrifuge, Interface-element