Colleen Baker, Sean Ladiges, Peter Buchanan, James Willey, Malcolm Barker
Dam Owners and Designers are often posed with the question “what is an acceptable flood risk to adopt during the construction of dam upgrade works?” Both the current ANCOLD Guidelines on Acceptable Flood Capacity (2000) and the draft Guidelines on Acceptable Flood Capacity (2016) provide guidance on the acceptability of flood risk during the construction phase. The overarching principle in both the current and draft documents is that the dam safety risk should be no greater than prior to the works, unless it can be shown that this cannot reasonably be achieved.Typically with dam upgrade projects it is not feasible to take reservoirs off-line during upgrade works, with commercial and societal considerations taking precedent. It is therefore often necessary to operate the reservoir at normal levels or with only limited drawdown. The implementation of measures to maintain the risk at or below that of the pre-upgraded dam can have significant financial and program impacts on projects, such as through the construction of elaborate cofferdam arrangements and/or staging of works. This is particularly the case where upgrade works involve modifications to the dam’s spillway.The use of risk assessment has provided a reasonable basis for evaluating the existing and incremental risks associated with the works, such as the requirement for implementation of critical construction works during periods where floods are less likely, in order to justify the As Low As Reasonably Practicable (ALARP) position. This paper explores the ANCOLD guidelines addressing flood risk, and compares against international practice. The paper also presents a number of case studies of construction flood risk mitigation adopted for dam upgrades on some of Australia’s High and Extreme consequence dams, as well as international examples. The case studies demonstrate a range of construction approaches which enable compliance with the ANCOLD Acceptable Flood Capacity guidelines
Tian Sing Ng, David Gardiner
Spillway structures play an important part in regulating the designed reservoir water level and are paramount to protect the structural integrity of the dam structure. Impermeability and tight crack control are prime importance in the design and construction of the spillway lining in order to minimise the potential failure modes of cavitation damage and stagnation pressure related failure. A spillway chute is essentially continuously restrained by the roughness of the rock surface and the ground anchors. The provision of control joints, i.e. expansion, contraction and movement joints,are therefore of little benefit due to the restraint as open cracks will still occur. Steel fibre reinforced concrete has been used for resisting erosion of the surface due to abrasion and/or cavitation. Steel fibres combined with conventional reinforcement also provide an amazing synergy to effectively reinforce concrete due to their ability to provide an effective restraining tensile force across open cracks. For the spillway chute,this means any concrete panel size or shape can be considered, even when the chute is fully restrained. Most importantly, this cost effective solution can be constructed joint free while maintaining watertightness. This paper presents some basic principles governing the design of joint free dam spillways employing steel fibre combined with conventional reinforcement. The focus of this paper describes the design and construction of the 400 m long Happy Valley Dam Outfall Channel together with overseas project examples.
Chriselyn Kavanagh, Simon Lang, Andrew Northfield, Peter Hill
The U.S. Army Corps of Engineers have recently releasedHEC-LifeSim1.0, a dynamic simulation model for estimating life loss from severe flooding (Fields, 2016). In contrast to the empirical models that are often used to estimate life loss from dam failure, HEC-LifeSim explicitly models the warning and mobilisation of the population at risk, and predicts the spatial distribution of fatalities across the structures and transport networks expected to be inundated. This capability provides additional insights to dam owners that can be used to better understand and reduce the life safety risks posed by large dams. In this paper, we demonstrate the use of HEC-LifeSim to model the potential loss of life from failure of five large Australian dams. Particular attention is paid to how the predicted life loss varies with warning time, in a manner that depends on human response and the transport network’s capacity for mass evacuations, and the modelled severity of flooding. We also examine how the HEC-LifeSim estimates of life loss compare with those from the empirical Reclamation Consequence Estimating Methodology (RCEM).
Richard Herweynen, Jamie Campbell, Mohsen Moeini
Hydropower storage plays an expanding role in integrated power systems internationally and can enable increased use of intermittent renewable energy sources such as wind and solar.With an increased amount of renewable energy within the Australian grid, pumped storage has gained increased focus in the past 2years. Entura have been working with Genex Power Ltd. to investigate, evaluate, optimise and design the Kidston Pumped Storage Project, located at the old Kidston gold mine in Northern Queensland. Through this design process, the final arrangement developed included an upper reservoir turkey’s nest dam to be built on the existing waste rock dump on the northern side of the Eldridge Pit, using the existing waste rock dump material and lining it with an HDPE liner. The original waste rock dump was formed during the mining operation by progressively dumping the waste rock predominantly from the Eldridge Pit excavation, with the haul truck traffic being the only compaction that occurred. Since the closure of the mine about 20 years ago, some consolidation of the waste rock dump has occurred.As a result, the key risks identified for the construction of the turkey’s nest dam on top of the waste rock dump were: (1) the stability of the slopes of the waste rock dump, which were generally at the angle of repose for the rockfill material; (2) the absolute settlement of the waste rock dump as the final dam crest level requires a settlement allowance in excess of the flood freeboard requirements; and (3) the differential settlement as excess differential settlement could cause fatigue stress cracking within the liner.This paper presents the investigation and modelling undertaken to confirm the feasibility of constructing this turkey’s nest dam on top of the existing rock waste dump, utilising the historical data on dumped rockfill dams. The paper also presents the feasibility design developed for the upper storage.
James Toose, Lelio Mejia, Jorge Fernandez
The recently completed Panama Canal Expansion project required construction of a new, 6.7-km-long channel at the Pacific entrance to the Panama Canal, to provide navigation access from the new Post-Panamax locks to the existing Gaillard Cut section of the Canal. The new channel required construction of four new dams adjacent to the existing canal, referred to as Borinquen Dams 1E, 2E, 1W, and 2W. The dams retain Gatun Lake and the Canal waterway approximately 11 m above the level of Miraflores Lake and 27m above the Pacific Ocean.The largest of the dams, Dam 1E, is 2.4km long and up to 30 m high. The dam abuts against Fabiana Hill at the southern end, and against the original Pedro Miguel Locks at the northern end. This paper provides an overview of the key challenges in construction of Dam 1E including the foundation, seepage cut-offs and embankment.
Gavan Hunter, David Jeffery and Stephen Chia
The Main Embankment at Tullaroop Dam in central Victoria is a 43 m high earthfill embankment with a very broad earthfill zone and rockfill zones at the outer toe regions. There has been an extensive history of cracking within the Main Embankment since formalisation of visual inspections in 1987.Widespread cracking has been observed on the crest and downstream shoulder. Cracking on the crest has mainly been longitudinal, but transverse cracks have also been observed. Cracking on the downstream shoulder has comprised longitudinal, diagonal and transverse cracking. In April 2004, a 60 mm wide diagonal crack opened on the downstream shoulder of the left abutment (from crest to toe) and Goulburn-Murray Water constructed a local filter buttress in 2005/06 on the left abutment. In 2011/12 a longitudinal crack opened up on the upper downstream berm toward the right abutment. The crack was initially 15m long and 10 to 215 mm wide, then propagated several months later to 70 m in length, 40 to 50 mm width and greater than 3 m in depth.In May 2011 three piezometers within the earth fill core recorded a very rapid rise in pore water pressure equivalent to 12 to 13 m pressure head above their previous readings. The piezometers were located on the same alignment (upstream to downstream) and were located below the crest and downstream shoulder, and the rise was to levels close to and above the embankment surface. The piezometers then showed a steady fall with time returning to the pre rise levels after 4 to 6 weeks.In 2015/16 Goulburn-Murray Water undertook dam safety upgrade works to reduce the risk of piping through the Main Embankment by extension of the filter buttress across the full width of the embankment. During these upgrade works, very deep (greater than 5 m) and extensive transverse cracks were observed in the embankment over relatively subtle slope changes on the right abutment.Thecracking and pore water pressure behaviour in the Main Embankment at Tullaroop Reservoir present an important case study. The paper provides details on the cracking and postulated crack mechanisms, and the rapid pore water pressure rise and postulated mechanisms. A summary of the upgrade works is also provided.