Neil Jacka, Christopher Dann, Jeremy Eldridge
The Tekapo Canal Remediation Works were undertaken to extend the life of the canal and enhance its seismic and environmental resilience. The deterioration of the canal lining in specific reaches has been the consequence of internal erosion of the lining under operating conditions.
The remedial works comprised installation of a supplementary geomembrane liner over selected sections of the canal, reconstruction of a culvert where the embankment had suffered piping, installation of filters in the Maryburn Fill, strengthening of the bridges across the canal and replacement of irrigation off-takes.
This paper presents a summary of key issues resolved during the design of the remediation works, in particularly the design of the geomembrane ballast system, the cofferdams and the management of side slope stability during drawdown for the works. A number of construction trials were carried out to confirm design assumptions and test construction techniques. The trials were a significant factor in the successful completion of the first season of work ahead of programme.
Keywords: Canal, Lining, Geomembrane, Cofferdam, Design, Seismic resilience
Peter Hill and Rory Nathan
The ANCOLD Acceptable Flood Capacity (AFC) guidelines were published in 2000 and provide guidance on the selection of design flood capacities for dams and specifically a deterministic fallback provision for spillway capacities. Since the guideline was published, there has been a continual evolution in dam safety management practices and related guidelines, including the 2003 ANCOLD guidelines on risk assessment and the current revision of Australian Rainfall and Runoff by Engineers Australia. This paper describes the scope of the current AFC guidelines and perceived opportunities for refinement. A survey of users was used to test and identify issues and gauge the need for the guideline to be updated. A number of topics were identified that would benefit from clarification or further guidance. These topics include consistency with other ANCOLD guidelines, clarity on the selection of the AFC, definition of the dam crest flood, freeboard and application to gated structures.
John Duder, David Bouma and Paul McCallum
The authors have been involved in the safety inspection and remediation of many older (pre-dating the 2004 Building Act) farm dams over the past decade coupled with considerable corporate knowledge from dams inspected by Tonkin & Taylor Ltd in its 50+ year history. This paper presents a summary of the varied benefits and risks of these older dams and the difficulties encountered in bringing them into alignment with current practice.
The many farm dams around New Zealand provide considerable benefit to the owners and often to the environment and wider community including the obvious stock water and irrigation, but also micro hydro, recreation, flood detention, release of environmental flows and flows for downstream users, and wetland habitat.
However, when applying current dam safety practice, and looking forward to the implementation of the Dam Safety Regulations, some of the older farm dams have significant dam safety issues that are often challenging to address. Although there is a high degree of variability, typical issues include:
Little or no documentation of geotechnical investigations, design or construction,
Design standards, particularly for spillway capacity have generally increased,
Little or no formal surveillance or maintenance carried out or recorded since commissioning,
Many farm dam owners have a poor understanding of their obligations under the Building Act and the Conditions of their Resource consents,
Consent conditions may not require dam safety related monitoring and maintenance, and/or the conditions may not have been historically enforced.
Many of these farm dams have been constructed by small contractors at the request of the farmers, often with only “standardised” engineering design and little specific geotechnical investigation. Typically there are no as-built records and the dam owners have been left with a general lack of understanding of owner’s responsibilities to monitor and maintain the dam.
Given that there are often very limited funds available for upgrade work, it has proved important to apply sound engineering judgement and a high degree of pragmatism to realise the greatest possible reduction in dam safety related risk for the available funds. Good cooperation between the Regional Authority, the Building Consent Authority for dams (often they are different organisations), the dam owner, and the dam engineer, together with a pragmatic approach is vital in moving toward current best practice for management of these dams.
Case studies are presented for the Northland Region, where the farm dams are typically homogenous earth fill dams in the order of 8 to 12 m high, fulfilling functions as irrigation, stock water supply, recreation and flood detention structures. The findings are considered relevant to earth fill farm dams across the country.
Joseph Camuso, Bruce Howse, Vaughan Martin and Don Tate
The proposed Kotuku Flood Detention Dam has been designed to reduce flooding within Whangarei City. This paper describes the potential benefits and the impact of the project on the community and the environment. It also covers the engineering challenges encountered during the design phase of the project. In particular, the dam site is located within a complex geological area, including a basalt lava flow on the left abutment, and site constraints required a twin emergency spillway design. If the risks associated with the dam are managed effectively, the proposed dam will provide a valuable asset to the community.
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.
Chris Topham, Eoin Nicholson and David Tanner
A number of Australian dams have spillways with reinforced concrete training walls designed in the 1950/60s to the standards of the day, but which could be considered under-designed according to modern criteria. Such walls commonly retain significant depths of earth and rockfill embankment materials, where structural failure of the wall could seriously compromise the safety of the dam. This paper presents the journey to mitigate the risk of such training walls, drawing primarily on experience in managing structurally deficient spillway training walls for a High Consequence Category dam in northern Tasmania. Reflections from each step of the risk management process are presented, including how the portfolio risk assessment contributed to a focus on the dam as a whole, and how that led to more detailed analysis and evaluation of the training wall risk. The use of instrumentation and enhanced surveillance for risk monitoring is discussed, including how real-time deformation data ultimately led to installation of temporary wall bracing works and enhanced contingency planning. The long-term risk treatment for the walls is presented, comprising a $6m structural upgrade to the training walls completed in 2013. The paper concludes with the learnings from the risk management journey and highlights the range of interventions available to owners with similar spillway training walls.