Earthquake ground motions were developed for the Tekapo Canal Remediation Project, including both Canal and Bridge sites. This work involved the specifications of the parameters of active faults and seismic source zones, the development of an aftershock sequence, and the review and selection of suitable ground motion prediction equations. The seismic hazard at the project sites is dominated by earthquakes occurring on the Irishman Creek fault. The characteristics of an inferred active (unnamed) fault shown crossing the Tekapo Canal near Forks Stream and the hazard it poses to the canal were also assessed, and it was concluded that there was no need to further investigate it as part of the canal upgrade project. A probabilistic seismic hazard analysis was used to develop response spectra for mainshock events for the various return periods relevant to components of the canal system having different PIC categories. A deterministic seismic hazard analysis methodology was used to estimate the aftershock spectra. Depending on the PIC category, time histories were developed to represent the response spectrum for both mainshock and aftershock events at some canal sites.
Rob Campbell, Christopher Dann and Mark Foster
Queensland contains some of Australia’s most significant reserves of mineable metallurgical coal, which is an essential raw material used in the production of steel. The area also has large deposits of thermal coal, used for electricity generation.
For the many active open cut and underground coal mines in Queensland, the enduring operational focus is to maximise returns and productivity, while still meeting key safety and environmental responsibilities.
Maintaining open cut pits in a dewatered state is often a key factor in achieving optimal productivity of an open cut mine. In Queensland, for many mines it is not always practical to maintain all pits in a dewatered state, given the subtropical climate and significant rainfall that can occur during the wet season, between the months of November and March. In effectively managing mine water while maintaining production, it is not unusual for excess mine water to be temporarily stored in a designated open cut pit.
The typical scale and arrangement of open cut pits at mine sites in Queensland is such that relatively deep and high volume pits can be separated by relatively narrow “landbridges”, consisting of in-situ material or mine spoil. The situation can therefore arise where a significant volume and head of mine water is stored in one pit, with mining operations continuing in an adjacent pit, and the landbridge is required to perform as a water retaining structure. This is a scenario that might not have been considered when the landbridge was originally constructed. This paper presents a study of two such landbridges at either end of a mine pit in Queensland, over a 5 year period from 2008 to 2013, with mining activities in the pit ranging from dragline pre-stripping to open cut mining, to large scale construction works and underground mining. By employing a long term interactive approach with mine operations personnel and utilising quantitative risk management techniques, risks were effectively managed, helping the mine to maintain operations while meeting safety and environmental requirements.
Kinchant Dam is a zoned earth and rockfill embankment situated on the north branch of Sandy Creek, approximately 30 km southwest of Mackay in central Queensland. Kinchant Dam was constructed in stages. The ‘Initial Development Stage’ which consisted of an embankment length of approximately 3.3 km and full supply level (FSL) of EL 49.21 m AHD was completed in 1977. Further development completed in 1986 (Stage I) increased the FSL to EL 57.21 m AHD with an embankment length of 5.5 km and a maximum embankment height of 22.3 m. The dam has a storage capacity of 62,800 Ml and a 60 m wide emergency spillway with a fixed crest level of EL 58.21 m AHD, one metre higher than the FSL.
A series of investigations have been carried out since its construction as a consequence of both regulatory safety reviews and observed excessive pore pressures within the foundation that have led to wet patches developing at the toe of the dam. In one area at the toe, pore pressures were such that artesian conditions developed. This paper outlines the history of various stages of construction of the dam, the foundation investigations since construction and the safety review and comprehensive risk assessment process that lead to the upgrade design and construction of remedial works. The remedial works include the extension of the downstream filter material adjacent to the clay core and the provision of additional pressure relief wells at the downstream toe of the dam.
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.
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.
Jeong Yeul, Lim
For various historical reasons and some technical reasons, the safety of dams has been evaluated using an engineering standards-based approach, which was developed over many years. It was used initially for the design of new dams, but increasingly has been applied over the past few decades to assess the safety of existing dams. Some countries have carried out risk assessments of existing dams that included both the structural and hydraulic safety of the dam and social risk. These methods developed by other countries could be adapted to assist in decision-making for dam safety management. Unfortunately, methods for risk assessment of dams were not established in Korea. This study outlines a beginning risk analysis for structural safety management. The first stage consisted of research on the present domestic dam safety guidelines and reviewing operations for management systems of dam safety abroad. Also, dam risk analysis requires reliable data on dam failure, past construction history and management records of existing dams. A suitable risk analysis method of dams for structural safety management in Korea is use of event tree, fault tree and conditioning indexes methods. A pilot risk assessment was carried out for two dams. The dam risk assessment process was thus established, and we learned the importance of risk assessment. The future includes additional research and risk analysis to develop the system.