Thomas Vasconi, Glen Fergus
Abstract: This paper describes the design of an 80 m-high stepped chute spillway, in gabion material, that will be constructed on a tailings storage facility dam of a mine in South East Asia. This dam, constituted of two cells, will be raised progressively via a series of intermediate crest elevations as mining proceeds, and each lift will be equipped with an operational spillway. The design of such spillways was challenging since it had to integrate local topography configuration, dam design, water balance, wall raise sequence and structure interdependency parameters. The design included flood routing, spillway sizing, stepped spillway design, followed by hydraulic and civil/geotechnical computations. Challenging design aspects included optimizing the stepped spillway structure costs in light of the structure’s short service (estimated to be less than 5 years), and ensuring the stability component. The design incorporates an innovative solution which allows reduction in the rockfill quantity of up to 40% with associated cost benefits, and sustainability in terms of material usage. The lessons learnt in applying this innovative design are useful for other sites requiring adaptive construction and short service life spillways.
Keywords: tailings dam, stepped spillway, hydrology, hydraulics, mine water management, gabions.
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
Bob Wark, Alex Gower. Graeme Mann
Stirling Dam is a 53 m high extreme hazard zoned earthfill dam located in south west WA. Construction was completed in two phases between 1939 and 1947. Recent safety reviews confirmed that the societal risk exceeded the ANCOLD guideline tolerable limit due to inadequate spillway capacity and the lack of embankment filters. Remedial work would involve: widening the spillway; removing the downstream shoulder of the dam; adding downstream filters; and reconstructing the downstream shoulder fill. Rock from the spillway excavation would be used to provide the fill for the downstream shoulder. The works optimisation involved a 3 m raising of the embankment to provide the required spillway capacity.
The design criteria included: ensuring the risks of failure during construction were to be no higher than the risks prior to remedial works; maintaining reservoir operation during construction; and no river releases based on median monthly inflows. This required the spillway crest to be temporarily lowered during construction to provide adequate flood capacity while the embankment height was reduced. A key feature of the design had also been the scheduling of the storage drawdown and remedial works to manage the failure risk and probability of river releases during construction. Higher than average inflows after contract award resulted in water levels above the scheduled drawdown curve. This lead to river releases to prevent spillway flows and rescheduling the onstruction over two seasons.
Keywords: Stirling Dam, water conservation, embankment filters, spillway capacity, construction scheduling
Jonathon Reid, Chris Kelly, Bob Wark
One of the most important aspects in the construction of an embankment dam is to be confident that the filter materials placed meet the design intent. The design methodology for filters is now well documented.
However, all too often during construction the filter material, as placed, does not comply with the specified requirements and all parties are faced with costly decisions and delays to the works to determine correction measures and whether the work completed meets the design intent. This paper shares the knowledge gained over a number of projects the authors’ have been involved in and the methods used to improve the properties of the placed filters taking into account some of the practicalities of having these materials produced and placed in a commercial environment
Keywords: filters, specifications, manufacturing, construction, quality assurance.
Robert Kingsland, Glen Burton
The management and closure of tailings dams can present mines with a trailing liability potentially extending well beyond the life of the mine. The dilemma faced by mine operators is that a tailings storage facility (TSF) is usually required to be in service up until the last product is mined and processed, but the stored tailings may be too weak to support the capping of those facilities for some years after the last tailings deposition. This paper presents the authors’ experience in the geotechnical characterisation of tailings and failure mode analysis required for tailings dams cover design, with particular reference to coal mines in the Hunter Valley. Techniques for field and laboratory determination of strength and consolidation parameters are presented. Failure modes for capping cover and displacement cover alternatives are discussed. Alternative cover techniques including strategies for improving and/or accelerating tailings strength gain are also discussed. Finally, areas needing further study are noted.
Keywords: tailings storage facilities, tailings dams, closure, capping, cover design.
Alice Lecocq, Bob Wark, Paul Hurst, Michael Somerford
The justification for dam safety remedial works is often based on an assessment of life safety risk and financial losses defined at a discrete point in time. However these parameters are likely to change over time with demographic growth, land and industrial development. The Water Corporation has a number of dams upstream of major growth areas and an understanding of the future direct and indirect economic consequences of dam failure are required in order to define the change in risk profile over time.
This paper outlines the study framework adopted by the Water Corporation to review its capital expenditure on its remedial works programme. Dam failure consequence assessments for Wellington, Serpentine and Samson Brook Dams are presented and the paper describes the methodology adopted to forecast the likely development within the inundation areas. A framework to consistently estimate future changes to life safety and economic consequences is also presented.
Keywords: demographic growth, land and industry development, monetary assessment, future trends, consequence assessment.