Peter Buchanan, Damian Nott, Martin Weir and John Dymke
The Bulk Water Alliance (BWA) consisting of ACTEW and ACTEW-AGL, GHD, and John
Holland/Abigroup, was formed to deliver the Enlarged Cotter Dam project in Canberra,
ACT. This project consisted of the construction of an 87 m high RCC dam and two
saddle dams, 15 m and 20 m high, to provide additional capacity to the ACT‘s water
supply system. The project is scheduled to be completed in September 2013.
During construction, the dam site was subject to three significant flood events which
affected the construction program. The March 2012 flood, the largest of the three, also
indirectly caused the formation of longitudinal cracks at the top surface of the RCC, when
the dam had reached about 45 m in height.
This paper first looks at the consequence of the flooding on both the design and
construction of the dam; in particular the modification of the diversion strategy and the
impacts on the final dam arrangement. The risk mitigation strategies put in place,
including the construction of a significantly larger diversion conduit through the partially
completed dam, are also discussed. The paper then focusses on the formation of
longitudinal cracks in the dam; the cause of cracking, analysis of the likely extent of
cracking, and the treatment of the cracks to minimise the risk of any significant long-term
impacts on the safety of the dam.
Finally the paper will discuss lessons learned from constructing the Enlarged Cotter Dam
during a period of above average rainfall.
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Now showing 1-12 of 36 2977:
Lyndon Johnson, Alan White and Chris Topham
The integrity of foundation drainage systems is a key factor in minimising uplift pressures under concrete gravity dams. Contemporary industry practice for foundation drainage systems (and modern criteria presented in the imminent release of the concrete gravity dam guidelines) will lead owners with older concrete dams to consider enlarging foundation drainage systems via borehole drilling in the foundation. This paper presents the cautionary tale of a dam owner that undertook foundation drilling works in the gallery of a 67-m high concrete gravity arch dam and experienced borehole “blowout” in one of the drilled holes. Water under 90% of full reservoir head issued from the borehole and needed to be controlled. The context of the works is presented, followed by a description of the blowout, the risk mitigation measures that were planned prior to the work, and which ultimately had to be initiated. Management of the incident is discussed, including the use of blowout protection collars and valves, subsequent investigatory drilling, and pressure grouting programme. Dam safety concerns associated with the incident and their management are presented. The paper concludes with some recommendations to manage these risks for other owners considering a drilling programme in a concrete dam foundation.
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.
Peter Mulvihill and Ian Walsh
The Falls Dam was constructed in the 1930’s to provide storage for several irrigation schemes in the Manuherikia Valley situated in New Zealand’s South Island region of Central Otago.
The opportunity to retrofit a small hydropower plant to the concrete faced rock fill dam was taken in 2003, utilising existing tunnels complemented by an innovative syphonic penstock system. The key design and construction features of this integrated scheme are described, along with experience from the first 10 years of the generation performance.
Looking ahead, there may be further integration challenges as current investigation of irrigation storage requirements leads to major redevelopment at this dam site and substantial changes to generation parameters.
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.