Russell Paton, Peter MacTaggart, Lee Benson
The Nathan Dam project has been identified by the State Government of Queensland as a potential water supply option to facilitate future growth in central Queensland. The proposed storage is located approximately 69 km downstream of the township of Taroom and would have a storage capacity of 1,080,000 ML which would make it Queensland’s fourth largest storage.
The proposed dam arrangement includes a central concrete gated spillway section across the river in order to maximise the storage volume and limit the flood rise upstream such that flood levels at Taroom are not increased during major flood events. A high level fixed crest spillway, to assist in the passage of rare flood events, forms the right abutment portion of the dam wall. It is proposed that the bulk of the concrete sections of the dam be constructed using roller compacted concrete (RCC).
The investigations to progress Nathan Dam are complicated by the existence of the Boggomoss Snail (Adclarkia dawsonensis) within the proposed inundation area. The snail is listed as a critically endangered species under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act), and a proportion of the snail’s known population is located on a Boggomoss (the colloquial name for an artesian spring) that will be inundated should the project proceed.
SunWater has engaged Australia’s foremost expert on land snails to design a translocation process aimed at relocating the species to alternative habitat outside the inundation area. The process will seek to not only protect the snail from the dam development, but to increase both its numbers and distribution thereby reducing risks to the currently isolated population from threats such as fire and predation. It is the first time in Australia that such a trial has been attempted, and SunWater is working closely with the Federal Department of Environment, Water, Heritage and the Arts (DEWHA) to ensure that the process is consistent with their policies and guidelines.
The paper will discuss the engineering and environmental challenges of the dam and how the Environmental Impact Study process can influence the delivery of a project.
Keywords: Nathan Dam, Environment, Engineering
Tariq I.H. Rahiman, Amanda Barrett, Greg Dryden, Mike Marley, Cecile Coll
In this study we present the engineering geology of complex Late Carboniferous to Early Permian silicic volcanic rocks underlying the Connors River dam site located on the Connors River, at Adopted Middle Thread Distance (AMTD) 97.7 km. The initial investigation of the site by SMEC in 1976 characterised the bedrock as simple laterally continuous layers of rhyolite and pyroclastic rocks. Engaged by SunWater Limited since October 2007, Golder Associates have utilised a range of modern investigative techniques to reveal a more intricate bedrock geological model.
Geological mapping, targeted vertical and angle geotechnical drilling and trenching reveal that the dam site foundation consists of complexly laid felsic crystalline volcanic flow deposits, volcaniclastic (pyroclastic) deposits, and mafic intrusives. Petrographical tests depict a broad range of rock types that includes rhyolite, rhyodacite, dacite, basalt, volcanic breccia, lapilli tuff and tuff. Surface structural mapping and downhole acoustic televiewer profiling reveal that defects of varying orientations have developed in the rocks mainly as a result of tectonism. The rock defects are predominantly open joints and faults, and minor bedding, flow bands, decomposed seams and veins. The permeability of the bedrock, which appears to be primarily controlled by rock defects, was assessed using the results of Lugeon tests.
Rock stratigraphy, mineralogy and texture combined with high resolution seismic tomographic imaging were used to delineate three main engineering rock units. Unit 1, the oldest, occurs on the right abutment and consists mainly of slightly weathered to fresh, high to very high strength dacites and rhyodacites. Unit 2 occupies the central area of the dam foundation and overlies Unit 1. It comprises weakly bedded, slightly weathered to fresh, high to very high strength volcaniclastic rocks. Unit 3, consisting of variably weathered, high to very high strength flow banded and autobrecciated rhyolite, is the youngest unit and it overlies and partially intrudes Unit 2. All three rock units are intruded by slightly weathered to fresh and very high strength basalt, either as dykes or sills. The rock mass properties of the rock units were evaluated based on rock strength tests and the geological strength index (GSI).
Keywords: engineering geology, dam foundation, volcanic rocks, Connors River, dam site
Amanda Barrett, Mike Marley, Tariq Rahiman
The site of the Wyaralong Dam, west of Beaudesert, Queensland, has been investigated in progressive stages since 1991. The first stage of the investigation was a siting study and the second in 2006 was designed to gather sufficient geotechnical information to develop a preliminary design and provide input into the environmental approval process. The third stage of the investigation was designed to gather further information to allow the detailed design of the dam to commence. This iterative investigation approach has ensured confidence in the site geology and geotechnical model.
The site investigations have included diamond drilling, piezo-cone penetration testing, geophysics techniques, hydraulic conductivity testing, groundwater pumping tests, costean excavation, geological mapping and accompanying laboratory test programs. Investigations have been targeted to assess the foundation conditions for the proposed engineering structure and have been refined to the needs of the dam design as it has developed.
With sufficient data, a 3-dimensional geotechnical model has been developed using the computer modelling program Vulcan, to assess the position and influence of a number of key geological features observed in the site investigation. Assessment of engineering properties based on in situ and laboratory testing were then extrapolated across the site through application of the geotechnical model.
Keywords: Wyaralong Dam, geotechnical investigation, geology, 3-D Vulcan model, foundation.
In 2003, the Bureau of Meteorology revised the Probable Maximum Precipitation estimates and rainfall temporal patterns for Tinaroo Falls Dam using the Revised Generalised Tropical Storm Method. Based on the revised floods, the dam was assessed as having an ‘Extreme’ Flood Hazard Category rating. Subsequently a comprehensive risk assessment was undertaken in 2008 and this assessment recommended the dam be upgraded to pass the Fallback AFC which is the PMF event. The current spillway has a capacity for a flood with an AEP of 1 in 200. To achieve the AFC the concrete gravity Main Dam requires stabilising with post-tensioned anchors. The crest of the homogenous Saddle Dam needs to be raised by 300 mm and a filter and weighting zone needs to placed on the downstream face
Keywords: Tinaroo Falls Dam, mass concrete gravity dam, post-stressed anchors, Barron River, filter, weighting zone
Steve O’Brien, Christopher Dann, Gavan Hunter, Mike Schwermer
One of the principal geotechnical issues identified for the Hinze Dam Stage 3 project was the potential for internal erosion and piping within the extremely complex geology at the right abutment. A plastic concrete cut-off wall was selected as the best solution to reduce the risk of piping to acceptable levels and careful planning of this work was required to manage a range of key project risks that included complex technical challenges, potential risks to dam safety, the environment, the surrounding community as well as delivering the works on a tight construction schedule to an agreed budget value. Construction of the 220m long and up to 53m deep cut-off wall, the largest wall of this type constructed to date within Australia, was undertaken by Bauer Foundations Australia and completed in January 2009. A major key to the success of the project was the planning and risk reduction measures that were undertaken during both the design and construction phases, a summary of which is presented in this paper.
Keywords: Cutoff Wall, Plastic Concrete, Hinze Dam.