David Guest, George Samios, Richard Rodd
Tenterfield Creek Dam is a 15m high concrete gravity structure that was constructed in 1930 and raised by 1.83m and stabilised using 97 post-tensioned ground anchors in 1974.Recent stability assessments concluded that the dam does not satisfy the ANCOLD Guidelines for Stability of Gravity Dams and that the situation is likely to deteriorate given the questionable performance of the post-tensioning cables and on the grounds of continuing corrosion and demonstrated loss of load.Tenterfield Shire Council is committed to improving the stability of the dam to meet the requirements of the NSW Dam sSafety Committee and engaged Public Works Advisory to assist them achieve this outcome.
Public Works Advisory prepared a dam upgrade options study which selected two options for further consideration. The estimated costs of the two preferred options were found to be potentially close;therefore Tenterfield Shire Council requested that both options be taken to detail design and tender stage to allow the market to indicate which option was in-fact better value.Factors other than construction costs were also considered in the options evaluation process and these factors influenced the selection outcome. The two upgrade options of lowest cost were the conventional gravity dam strengthen solutions i.e. installation of new post-tensioned ground anchors and downstream mass concrete buttressing. The decision to proceed to tender with two options was supported by the other key funding stakeholder, DPI Water.
This paper provides some unique insight on the comparison of conventional upgrade options for concrete gravity dams and also examines some interesting design aspects encounter edduring the design development process
Andrew Northfield, Simon Lang, Peter Hill
Melbourne Water currently manages more than230retarding basins (RBs). A large portion of these are less than 4 metres high, and traditionally structures of this size have not been subject to intermediate or detailed ANCOLD Consequence Assessments. However, the need to understand the failure consequences for smaller structures has increased over time, as risk based approaches to managing safety have expanded from large dams to other water retaining assets.
Undertaking detailed consequence assessments for all Melbourne Water’s RBs would not be practical, given the costs and time involved. Therefore, this paper describes a method for assessing the level of ANCOLD Consequence Assessment that is justified, based on the structure’s attributes. It also presents an equation that was used to estimate peak outflows from RB failure. The peak outflow estimates can be used to model approximate failure inundation extents downstream of small dams and RBs.
The paper draws on work that HARC have recently undertaken for Melbourne Water to assess the failure consequences for 88 RBs. The outcomes are relevant to other organisations that own or manage significant numbers of small water dams or RBs.
Petros Armenis, Malcolm Barker, Peter Christensen, Graham Harrington
The Canterbury Earthquake Sequence in September 2010 and February 2011 caused large areas of land to change by differing amounts throughout Christchurch, New Zealand. Land levels fell by more than 300 mm in some areas. This increased flood risk in the tidal reaches of the Avon River. Urgent repairs were completed with the objective to restore the tidal river defences to a crest level equivalent to a 1% AEP tide level. This work needed to be completed prior to impeding spring tides.
The levees will be required for up to 20 years and then probably be rebuilt on a new alignment. To better understand the risks associated with the ongoing reliance of the levees for flood protection in the interim, a risk assessment was undertaken using conventional Australian National Committee on Large Dams (ANCOLD) practices and levee design procedures. Careful consideration was made to the performance of the existing levees under seismic, flood and tidal loading from which the societal and individual risk profiles were derived. The work included the following:
This paper will present the levee design and the process applied for the analysis of the levee and the upgrade options selection
Andrew Balme, Dan Forster, Tim Logan
The MW7.8 Kaikōura earthquake on 14 November 2016, ruptured over 20 faults during the initial shaking,which lasted nearly two minutes. A complex series of fault ruptures propagated northeast for nearly 180 km from the initial rupture location. Instrumentation from dams across New Zealand shows that whilst most dams did not suffer physical damage, piezometric responses were measured in dams and their foundations. Earthquake related changes in seepage regimes are not unusual and depend on the characteristics of the ground motions,and site specific characteristics that influence how a dam and its foundation respond to ground motions. The ability to measure a piezometric response in a dam or foundation is heavily influenced by the instrumentation network and method of monitoring. Data collected during events such as the Kaikōura earthquake provides valuable information for both characterising performance of a dam during the event, and assisting future analysis such as failure mode assessments. Careful consideration must be given to the scope of installed instrumentation and the frequency of monitoring in order to provide these benefits,and the robustness of the system to ensure it adequately survives the event.
Lisa J Neumann, Rod Westmore
In Australia construction of a new dam on a greenfield site is relatively uncommon and construction of a new dam on a brownfield site is even more unusual.This paper presents an innovative design solution to address the challenges associated with such a project.Ridge Park Dam is a new flood retarding dam located in a suburban recreation park, less than 10km south east of Adelaide, South Australia.The dam was constructed in 2014/15 and was designed to limit the peak flows in the creek downstream of the park under the 1 in 100 ARI event and to impound water as a component of the infrastructure required for the Managed Aquifer Recharge (MAR) scheme located in Ridge Park.The expectations of both the client and community and the technical issues encountered in the early stages of the project resulted in some unique design criteria. At the outset the client and community expectation was that the dam would improve the overall amenity of the park without impacting the existing vegetation or functionality of the park, including public access and safety.Identifying a dam type to suit the client and community expectations and address the technical issues was not straightforward.Typical dams types such as embankment dams, mass concrete gravity or concrete buttress structures, were found to be not suitable.A less typical, innovative solution was sought.The outcome was to construct a dam comprising a concrete core wall supported by rock filled gabion baskets.
Paul Somerville, Andreas Skarlatoudis and Don Macfarlane
The 2017 draft ANCOLD Guidelines for Design of Dams and Appurtenant Structures for Earthquake specify that active faults (with movement in the last 11,000 to 35,000 years) and neotectonic faults (with movement in the current crustal stress regime, in the past 5 to 10 million years) which could significantly contribute to the ground motion for the dam should be identified, and be accounted for in the seismic hazard assessment. The purpose of this paper is to provide guidance on the conditions under which these contributions could be significant in a probabilistic seismic hazard analysis (PSHA)and a deterministic seismic hazard analysis (DSHA).We consider five primary conditions under which identified faults can contribute significantly to the hazard: proximity, probability of activity, rate of activity, magnitude distribution, and return period under consideration