Richard Herweynen, Robert Montalvo, John Ager
The choice of materials used in the construction of a dam is one of the most critical decisions in the design process. Our natural behaviour as engineers is to adopt materials which have proven performance, and which conform to Australian or international standards, which sometimes causes us to overlook the specific conditions and demands of the project at hand. In an environment where the majority of concrete produced is for structural purposes, the properties of these concretes is often vastly different to those desired for mass concrete structures such as dams and spillways.
The big question at Wyaralong Dam was could onsite aggregate be used in the Roller Compacted Concrete (RCC)? The Wyaralong Dam is located in the Gatton Sandstone (early Jurassic), predominantly feldspathic to lithic‐feldspathic sandstones with a clay matrix. Early analyses and tests suggested that the Gatton Sandstone was not suitable for RCC aggregate due to a 68% wet/dry strength reduction, high water absorption (5.2 – 7.5%) and petrographic interpretation that clay content was mainly swelling clay, leading to durability concerns.
Due to significant community, safety and cost issues with importing aggregate, Wyaralong Dam Alliance (WDA), during the development of the RCC mix design for Wyaralong Dam, chose to pursue the use of onsite quarried sandstone aggregate instead of importing aggregate. Additional petrographic and XRD analyses and extensive durability tests were undertaken on cores of sandstone and RCC samples, including wet‐dry cycles, soak tests in ethylene glycol, soaks in sodium hydroxide, and heating and cooling cycles. These tests indicated that, if swelling clays are present, they do not impact the durability behavior of the RCC aggregate.
The substantial effort put into testing the sandstone aggregate has paid off for WDA. Not only have the results indicated that the RCC mix performs remarkably well in terms of durability, but the very low modulus of elasticity of the mix has provided exceptional performance in terms of thermal loading; with all the related benefits in reduced restrictions to placement schedule and cooling requirements. Onsite sandstone was not only proven to be a feasible option, it has been demonstrated that it is the best option for the project. Details of the study are provided in this paper.
Keywords: Roller Compacted Concrete (RCC), Sandstone, Aggregate, Clay, Mix, Durability
Rick W. Schultz P.E.
The Corps of Engineers Risk Management Center is undergoing a nationwide assessment of its navigation and flood control projects. Development of the methodology and tools used to determine probability of failure of mechanical and electrical systems for dams is being presented in this document. Development of the Weibull formulas for specific use in dam will be addressed along with use of fault tree analysis to determine system reliability.
Keywords: Dormant-Weibull Formula, Fault Tree, Characteristic Life of Components, Beta Shape Parameters, Inspection intervals.
David Scriven, Errol Beitz, Aaron Elphinstone
The Bowen River Weir is located at AMTD 94.4 km on the Bowen River, some 25 km south of Collinsville in North Queensland. The weir is part of the Bowen/Broken Rivers Water Supply Scheme and it provides a pumping pool for pipelines serving two nearby coal mining developments and a power station, and also acts as a regulator for riparian water users downstream until it meets the Burdekin River.
The weir was constructed in 1982 and incorporated a fishway towards the southern (left) bank, the design of which was based on the old “pool and weir” fish ladder type layout, typical of that era, with 48 separate cells containing partial vertical slots and baffles. This design has since been found to be ineffective for Australian native fish. In addition it was often out of service due to cells becoming filled with river sediment and debris. For these reasons it was decommissioned and made safe in late 2008 on the condition that a new fishway be constructed.
In late 2008 agreement was reached with Fisheries Queensland to install a “fish lock” type fishway at the site. This type of fishway has in recent years proved to be reliable and effective (eg. successful fish locks at Neville Hewitt and Claude Wharton Weirs). The preliminary and then final design was undertaken by SunWater (Infrastructure Development) between September 2008 and March 2009. The construction was undertaken by SunWater direct management, commencing in July 2009 and completed in late 2010.
Bowen River Weir Fishway – Design and Construction
An essential criterion for any new dam project in Australia is to provide for passage of fish past the structure in both the upstream and downstream direction. In recent projects with a relatively high barrier this has been provided by mechanised systems such as locks, lifts or a combination of both.
A nature-like fishway provides for passage of fish past a barrier by applying some of the features of natural streams. The concept has been increasingly applied to fishway designs in North America and Europe. A nature-like fishway will provide variable flow depths, velocities and turbulence across its width and along its length and is constructed using natural materials to simulate the natural stream characteristics. The variable flow conditions coupled with the use of natural materials inherently result in different channel substrates that support the passage of a large range and size of fish species as well as other aquatic species. Where fish habitat has been depleted, a nature-like fishway can also supplement and enhance aquatic habitat.
The performance of nature-like fishways can be difficult to quantify due the very nature of the system. However, qualitative assessments in North America are indicating that a wide range of species are using such fishways and that fish species that were previously extirpated from rivers are again migrating.
The nature-like fishway concept has been applied to in-stream structures up to four metres high in the eastern states of Australia. However, the substantial progress made with this design in North America and Europe has not as yet been applied in this country.
This paper analyses the advantages and disadvantages of nature-like fishways over mechanised systems, such as locks and lifts, and makes an assessment of the suitability of the concept to dams in Australia with relatively high walls.
Thomas Vasconi, Mike Gowan
This paper describes the methodology adopted for the design of a 180 m-high stepped chute spillway to be constructed on a mine tailings storage facility (TSF) in Africa. This TSF dam, constructed using the “downstream method”, will be raised progressively via a series of nine lifts as mining proceeds. The first eight will be equipped with an operational spillway sized for the 1in 10,000 AEP whilst the ninth will house the closure spillway sized for the Probable Maximum Flood. The problem, common to all staged tailings dams, is how to design the spillways for such raising sequence? The very steep ridge declivity favored locating them in a unique configuration rather than the more usual separate hillside spillway on each dam abutment. The design of such spillways was challenging since it had to integrate the TSF interdependency parameters (water balance, dam raise sequence) whilst including flood routing, spillway sizing, stepped spillway design components. Challenging aspects of the design also included optimizing the costs associated with the short service life of these spillways. Furthermore, the design was undertaken in a way that the operating stepped chute could be upgraded and reused at mine closure. The design incorporates an innovative solution which allows reduction in the rock armouring quantity of up to 40% with associated cost benefits, and sustainability in terms of material usage. The lessons learnt in applying this innovative and sustainable design are useful for other sites requiring adaptive construction and short service life spillways.
Keywords: Tailings storage facility, stepped chute spillway, hydrology, hydraulics, mine water management.
Peter A Ballantine, Christopher V Seddon
Massingir Dam, constructed in the late 1970’s on the Olifants River in Mozambique, is a 48 m high zoned earthfill dam. Due to various safety concerns, the dam was operated at a reduced full supply level of 110 masl, compared to the design full supply level of 125 masl. Between 2004 and 2006 remedial works were undertaken, including the construction of a berm on the downstream face of the dam, grouting and drainage of the foundations and installation of the spillway crest gates. From December 2005 the storage level of the dam was allowed to increase.
On 22 May 2008, with the reservoir storage level at 122.43 masl and the gates on the outlet conduits closed, the reinforced concrete conduits failed at the downstream end, releasing an estimated 1,000 m3 /s of water into the Olifants River.
A 2-D finite element analysis was undertaken in order to establish the safe load bearing capacity of the as-constructed conduits. On the basis of the analysis, it was concluded that the original design did not take proper account of the pressure that would develop within the thick concrete sections of the conduit. In view of assumptions regarding the load paths, the reinforcement was not placed in the most appropriate positions.
This paper describes the events leading up to the failure of the conduit, presents the findings of the investigation into the failure and makes recommendations on the basis of the findings.