Jiri Herza, Graeme Maher, Nihal Vitharana, Megan Evers, Michael Somerford
Abstract: Drakesbrook Dam is a 17m high earthfill embankment constructed in 1931 to provide water for the Waroona Irrigation District, 80km south of Perth. The dam has a storage capacity of 2.30 GL and is classified as a “High A” hazard dam according to ANCOLD Guidelines on Dam Safety Management.
A Dam Safety Review, undertaken in 2001 identified a number of deficiencies associated with main embankment, outlet works and the main spillway requiring remedial works. Detailed design of these remedial works is currently underway. Challenging features of this project are the design of a liner for the existing curved conduit and the design of a new spillway with an usual drop-type stilling basin.
This paper presents the salient aspects of the remedial works design along with the various design and construction criteria adopted to achieve an economical design of a new spillway and outlet conduit sleeve without compromising the safety of the dam.
Keywords: remedial works, earth embankment, conduit lining, spillway replacement
Paul Somerville, Gary Gibson
Abstract: This paper describes current methods for seismic hazard analysis and their application at Hinze Dam. Although Southeastern Queensland has experienced significant earthquakes in historical time, none of them are known to have caused surface rupture, and no active faults that could be used to represent earthquake sources have been identified in the region that surrounds the site. Under these conditions, we must estimate the seismic potential of the region using historical seismicity. Two alternative approaches to modelling future earthquake occurrence based on historical seismicity have been used. The first approach is based on the AUS5 source model of ES&S (2005), which uses geological criteria to identify zones of uniform seismic potential, and then uses historical seismicity to characterize the seismic potential of each zone. The second approach, developed by Hall et al. (2007) at Risk Frontiers, is based on the spatial smoothing of historical seismicity without identifying discrete source zones. Previous work by ES&S has shown that the attenuation of strong ground motion in Southeastern Australia is fairly well represented by ground motion models developed using strong motion data from western North America. The recently developed NGA ground motion models based mainly on data from Western North America represent the local site conditions using Vs30, the shear wave velocity averaged over the top 30 metres at the site. This provides a significant advantage over previous models, which were for broad site categories such as rock or soil, and did not provide for the use of more site-specific information. The left abutment, lower tower and valley section foundation at Hinze Dam are characterized by hard unweathered rocks with shear wave velocity of 2.0 km/sec estimated from P wave velocity measurements. The right abutment of the main embankment and the saddle embankment foundation consist of extremely weathered rock, with shear wave velocity of 0.45 km/sec estimated from P wave velocity measurements. This causes the ground motion response spectra estimated for the right abutment and neighbouring foundation components to be significantly larger than for the left abutment and neighbouring foundation components, by factors of 1.4, 2.0 and 2.3 for periods of 0 (PGA), 0.5 sec and 1 sec respectively.
Keywords: seismic hazard analysis.
Abstract: A number of SunWater’s dams are in the process of being upgraded to the acceptable flood capacity (AFC) to ensure the highest level of safety. The Fred Haigh Dam upgrade was completed in September 2006 and the Bjelke Petersen Dam upgrade was completed in October 2007. Borumba Dam is the latest upgrade being undertaken with construction commencing in April 2008 and is expected to be completed by December 2008. Each dam underwent a comprehensive risk assessment to identify and evaluate all risks with respect to the ANCOLD tolerability limits to ensure risks satisfied ALARP. The assessment identified the most cost effective upgrade solutions for detailed design.
The upgrade at Fred Haigh, Bjelke Petersen and Borumba Dams will enable them to pass an extreme flood equivalent to 50% of the Acceptable Flood Capacity (AFC). This is Stage 1 of a two stage upgrade to ultimately achieve 100% of the ANCOLD “Fallback” AFC which is the standard SunWater has adopted for its major dams. SunWater has prioritised spillway capacity upgrades to achieve a minimum dam portfolio standard of passing 50% Acceptable Flood Capacity inflow by 2015 and full Acceptable Flood Capacity inflow by 2025.
The most economic Stage 1 upgrade option for Fred Haigh, Bjelke Petersen and Borumba Dams was to maintain the existing spillway width and to raise the dam crest with a concrete parapet wall. For the Bjelke Petersen and Borumba Dams the spillway training wall heights were raised to allow for increased flow though the spillway. From the hydraulic model studies and flood routing a height of each different dam crest wall was obtained.
This paper will describe the different methods and considerations used for upgrading Fred Haigh, Bjelke Petersen and Borumba Dams to the 50% AFC.
Keywords: dam safety, spillway, Fred Haigh Dam, Bjelke Petersen Dam, Borumba Dam, SunWater, Queensland.
Brunner Wolfgang G, Bi Arthur, Chang William, Zong Dung Feng
ABSTRACT: In the South of the Sichuan province the 240 MW Yeleh hydroelectric power project is under construction by Sichuan Nanya River Basin Hydraulic Power Development Company Limited, a state-owned enterprise, and China Gezhou Ba Water & Power Group Company Ltd.. This project sets out to develop the mountain cascade of the Nanya river, a tributary of the Dadu River, provide superior electricity and adjust flood peak and frequency. The specialist contractor Foundation Engineering Company of China Water Resource and Hydropower (FEC) was awarded the contract of foundation treatment at the right bank, which included the construction of a 75 m deep concrete cut-off wall inside a 6.0 m x 6.5 m tunnel. The requirement for 19 317 m3 cut-off wall to be constructed in permeable and very dense gravel/cobble formations and a demanding project program led FEC to adopt the BAUER Low Headroom Cutter CBC25/MBC30 in conjunction with the overlap cutter joint.
Keywords: hydroelectric project, dam, dam tunnel, cut-off wall, trench cutter
David Brett, Ben Hanslow. Rob Longey
Abstract: Mine tailings storages are among the largest man made structures in the world and often pose a considerable risk to the aquatic environment due to the nature of the stored materials. In particular, sulphide minerals are prone to oxidation when brought into contact with air and water. This leads to the formation of acidic conditions within the storages leading to dissolution of toxic metals, with seepage from these structures being known as Acid Rock Drainage (ARD). ARD is responsible for pollution of natural waterways in many areas of the world with some significant examples in Australia. Current practice in the mining industry is to attempt to exclude oxygen or water from tailings storages in order to prevent the oxidation process taking place. This involves capping of the storages with sophisticated soil covers or, where sufficient water is available, leaving a permanent water cover.
Mining operations have a relatively short life, usually around 5 to 10 years, although some can operate for over 100 years, as has the Mount Lyell Mine. Normal practice has been for companies to relinquish the mining lease on the cessation of mining, however governments are now realising the extent of liability involved with the “ownership” of large waste storage facilities. Bonds are placed by mining companies during operations, intended to cover the cost of “closure” of the mine. Often the major item covered by the bond is for the “closure” of the tailings storage facility. Following “closure”, the intention is that ownership of the lease, including waste storages, reverts to the State. State governments are now more aware of the potential liabilities in accepting the relinquishment of these leases and need to address the issues of their long-term management.
In Tasmania, Dam Safety legislation covers both water and also soil covered tailings storages, with the legislation requiring each type to meet various ANCOLD guidelines. In other jurisdictions this could well also be the case through common law requirements to meet common best practice. However, the current ANCOLD Guidelines are generally written around water storage dams and interpretation to include a waste storage facility is often not straightforward. As an example a tailings dam during operations with a water storage component is clearly a dam. Due to the environmental impact of failure it could well have a Significant or High-C hazard rating, which would require design for extreme floods and earthquakes. After closure, with say a soil cover and water diverted away, is it still a dam within an ANCOLD definition? Are ANCOLD guidelines relevant? The current ANCOLD (1998) Guidelines on Tailings Dam Design, Construction and Operation does not give specific guidance on these issues.
This paper explores these questions and suggests ways that ANCOLD could provide assistance with more guidance on the long term management aspect of tailings storages to assist designers, owners and regulators consider the closure phase.
Keywords: tailings, acid drainage, mine closure
Marius Jonker, Mike Taylor and Glen Hobbs
Abstract: One of the activities authorized by organizations such as ANCOLD is the development of guidelines to enhance the ability of organizations to assure that adequate dam safety programs and practices are in place.
However, due to the absence of a single recognized guideline covering the various design aspects of dam outlet works, there is currently great inconsistency in the underlying principles for design and review processes for these facilities. A single, nationally recognized “standard” would lead to greater consistency between similar project designs, facilitate more effective and consistent review of proposed designs, and result in increased potential for safer more reliable facilities.
The need for a design standard is further underscored by the fact that poorly designed and/or constructed outlet works have been identified as a significant contributor to the occurrence of embankment dam failures. The failure of critical components in the outlet works of dams, not leading to an uncontrolled release of water, is generally considered less important due to apparent lesser consequences compared with a dam failure. However, the drought experienced over the past decade in Australia has highlighted the value of water and securing it for the future. The potential impact caused by the inability of a primary storage to supply a town or city, due to a failure in the outlet works, potentially affecting thousands of people and industries, could be devastating.
This paper summarises the current state of practice with regard to outlet works for dams concerning design, construction, inspection and evaluation, as well as maintenance and renovation. It also underlines some commonly occurring deficiencies encountered at existing dams. It provides a basis for further discussions of the state of practice for these topics in order to work towards consistent and unified outlet works design guidelines for dams.
Keywords: outlet works, state of practice, design guidelines