Upstream construction methodology has been used to raise tailings dams in Western Australia (WA) for more than three decades, and the tailings storage facilities (TSFs) built in this manner have performed satisfactorily so far. The maximum design earthquake (MDE) for most of the existing, upstream-raised TSFs in WA was that corresponding to a 1-in-1,000 year annual exceedance probability (1:1,000 AEP). However, the recommended MDE loading for the High/Extreme Failure Consequence Category in the 2012 ANCOLD Guidelines on Tailings Dams is that of a 1:10,000 AEP. This more stringent seismic design criterion may restrict the use of upstream TSF construction in some areas of WA and Australia in general.
To evaluate the viability of upstream construction for a new or existing TSF, the effects of the earthquake design ground motion (EDGM) on the liquefaction and deformation response of the structure must be understood. The results of such analyses are an essential component in determining whether upstream raising will be feasible, or whether more robust but much more costly centreline or downstream construction methods are required.
A parametric study was completed to investigate the liquefaction and deformation behaviour of a typical, upstream-raised tailings dam under different earthquake design ground motions with different response spectra. The study utilized two-dimensional finite difference code FLAC2D effective stress dynamic analysis, in which the UBCSAND constitutive soil model was incorporated. Twenty-eight earthquake ground motions (matched and unmatched to the target response spectrum) were used in the study and the liquefaction response of the tailings dam model under those ground motions was analysed.
The results of the study demonstrate the importance of appropriate ground motion and response spectrum selection in assessing the seismic performance of an upstream-raised TSF. Liquefaction response was shown to vary with different response spectra, even though the corresponding EDGMs had similar peak ground acceleration (PGA) values. The importance of earthquake frequency content and duration, which in turn are affected by earthquake magnitude, distance and ground motion response, is emphasized. Scaling and matching the earthquake input motion to the uniform hazard response spectrum (UHRS) may result in overly-conservative design. Thus, selection of the most representative EDGM is essential to evaluating expected seismic performance for an upstream-raised TSF, and scaling or matching the earthquake input motions must be done cautiously.
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M.G. Webby and N.D. Sutherland
Repairs to the floor slab of the outlet transition section of the Pukaki Canal Inlet Structure in November 2009 were likely to have adversely affected the hydraulic jump behaviour in the transition section of the structure and therefore necessitated revision of the safe operating limits for the structure. Three separate series of flow trials were carried out at different lake levels over a period of about a year to carefully observe the behaviour of the hydraulic jump under a variety of gate operating configurations and discharges. New safe limits of operation for the structure were defined for the structure using the flow observations from the flow trials and the framework of analytical models for different types of hydraulic jump. The revised limits of safe operation were successfully implemented in 2013.
Bertrand Rochecouste Collet, Dawid van Wyk and Emmanuel Adanu
The preliminary design of the Kashimbila Multipurpose Dam on the Katsina-Ala River in the Taraba State, Nigeria was initially focussed solely on it functioning as a buffer dam in the case of failure of the natural embankment of Lake Nyos in Cameroon. The failure of Lake Nyos could generate an extreme flood endangering the population in south-eastern Nigeria. As the design process progressed with a more holistic and multipurpose approach, the capacity of the dam was increased to provide irrigation and potable water to the surrounding towns and villages, as well as the generation of hydropower. The dam is a composite structure consisting of a mass concrete gravity uncontrolled spillway, a clay-core rockfill embankment, a 40 MW hydropower station and an outlet works with twin 1.4 m diameter pipes feeding the irrigation pumpstation and water treatment works. This paper covers the design considerations of the Kashimbila Multipurpose Dam and Hydropower Station, with particular emphasis on hydrological challenges and related design solutions.
Simon Lang, David Stephens, Peter Hill, Mark Arnold and Tommie Conway
Considerable thought has been given in recent years to managing the risks associated with floods during the construction of new dams and dam upgrades. Both ANCOLD and the NSW DSC provide some limited advice on how this risk should be managed, with many dam owners aiming for societal risk during construction to be no higher than pre-construction. One approach to do this is to draw down the reservoir such that sufficient airspace is created to reduce the probability of overtopping the construction works to be equal to that of overtopping the dam crest pre-construction. However, this frequently leads to very large releases of valuable water resource being required. This approach also fails to consider that the conditional probabilities of failure may be quite different during construction than during normal operation. A risk-based approach was applied for the recent upgrade of Tarago Reservoir. Existing event trees from a failure modes analysis were adjusted to reflect the construction conditions. In some cases, the event probabilities increased (for example as a result of excavation of the dam embankment), however some also decreased (for example as a result of more rapid means of detecting and intervening in breach formation during construction). The conditional probabilities of failure during construction were then used to estimate the overall seasonal probability of failure, and it was found that a limited draw down of the reservoir would be sufficient to ensure that risks were no higher during construction than pre-construction. To reinforce this, the cost-to-save-a-statistical life was estimated for further drawdown of the reservoir and used to demonstrate that the risks were as low as reasonably practicable.
Alan Collins and Michelle Archer
The Waikato River is the longest river in New Zealand. Mighty River Power operates nine dams on the river with a combined net head of 335 m. The reservoirs have limited storage capacity so that the Waikato Hydro System is effectively a continuous run of the river scheme, providing constant generation for the New Zealand electricity grid. The river is also the habitat of the New Zealand Longfin and Shortfin Eel. Before the dams were constructed, eels naturally migrated as small elvers and lived as far upstream as the Arapuni gorge, where a waterfall prevented them from travelling further upstream. The commissioning of the Karapiro dam in 1947 reduced the natural habitat of the eels. In recent years, the eel population has been declining through a variety of anthropogenic factors and protective status is being called for. An elver catch and release program commenced at Karapiro Dam in 1992. This transferred elvers as far upstream as Lake Ohakuri and significantly increased the available habitat for the elvers to grow into adult eels. Spawning adults migrate downstream and back out to sea and as a result most of these eels are killed by turbines at the hydro stations. While consent conditions don’t stipulate it, Mighty River Power is committed to being an environmentally responsible custodian of the Waikato River and is dedicated in efforts to preserve the eel fishery. Mighty River Power recognises the importance of eel to local iwi; particularly highlighted by the emphasis on eel in the Waikato River Independent Scoping Study. The Karapiro eel bypass project, started in 2010, sought to investigate and research means to assist downstream eel migration. Research was gathered into eel searching patterns, timing of eel migration, durability in high velocities and other survival factors. This information was used to design, construct, and test a prototype downstream eel bypass at the Karapiro dam, something that had not been built on a dam this size before. In the 2013 migration season, three eels safely used the bypass. Plans are in place to improve the performance of the bypass in the coming seasons. Mighty River Power wishes to share the lessons learnt from this project with other dam operators for the conservation of this important species.
Lelio Mejia and Ethan Dawson
The 26 km long Tekapo Canal is a major component of the Upper Waitaki Power Scheme in the Mackenzie Basin in New Zealand’s South Island. The canal, commissioned in 1977, conveys water from a power plant at Lake Tekapo to a power plant at Lake Pukaki. To support a re-lining and repair works project along sections of the canal, seismic deformation analyses were performed. Earthquake-induced settlements and deformations for three critical embankment sections were estimated. Two dimensional, nonlinear, dynamic numerical analyses were performed with the computer code FLAC. Analyses were performed for the Maximum Design Earthquake (MDE), the Serviceability Level Earthquake (SLE) and for aftershocks of various magnitudes. A critical feature in the repairs and upgrades was a geosynthetic liner to be placed along portions of the canal. Seismic performance of this liner would be affected by cracking in the underlying embankment. Crack size estimates (width and depth) were developed based on evaluation of the computed deformations and the empirical correlations of Fong and Bennett (1995) and Pells and Fell (2002). Calculated deformations were generally small, and indicative of adequate seismic stability. Recommended design crack widths for the MDE ranged from 20 to 50 mm, while recommended design crack depths for the MDE ranged from 1.0 to 2.5 m.