Jiri Herza and John Phillips
The design of dams for mining projects requires processes and technology that are unfamiliar to many mine owners and managers. Dam designers rely on ANCOLD assessments of Consequence Category, commonly leading to a High rating for mining dams due to a combination of potential loss of life, impact on environment and damage to assets such as mine voids, process plants, workshops, offices, roads, railways etc.
From this High Consequence Category the relevant annual exceedance probabilities for design parameters and loading conditions such as earthquakes and floods are selected.
Mining companies have sophisticated methods available for assessing risk, yet for their assets they often adopt an order of magnitude lower security for earthquake and floods even though the consequences in terms of lives at risk and impact on project are similar.
The discrepancies in the design standards lead to situations where extreme dam loads are adopted to prevent damage and loss of life in assets that theoretically would have already collapsed under much lower loads.
One difference may be that some mining dams exist in an environment which is controlled by a single entity. Unlike other dams, failure of these mining dams would therefore impact only individuals and assets which fall under the responsibility of the same entity.
This paper discusses the discrepancies between the design of mining dams and the design of other mine infrastructure. The paper considers the impact of discrepancies on the overall risk to the mine and compares the degree of protection offered by a factor of safety and the influence of reliability of design input parameters, alternate load paths and design redundancy.
Keywords: Dams, tailings dams, mining, acceptable risk, factors of safety
Chris Topham, Andrew Pattle, David Tanner, Oliver Giudici
Many owners around the world have dams that rely on grouted, post-tensioned rock anchors for stability. The anchors were installed during the original construction of the dams or retrofitted to improve stability during their operational life. The use of fully grouted post-tensioned anchors spanned the period of the 1960’s to 1980’s. The main issue with these un-sheathed grouted rock anchors is the question of integrity of the grout column protecting the anchor and concerns about possible corrosion of the high tensile wires from which the cables are constructed. While some of these anchors have corrosion monitoring systems installed, it is difficult to validate such data and there is considerable uncertainty over the condition of such anchors. To compound the problem, replacement of the anchors is technically complex, extremely costly and difficult to justify in the absence of known condition. For example, Hydro Tasmania has recent experience of work to cease reliance on such anchors at Catagunya Dam that cost $41m in 2009. With fifteen dams relying on some form of post-tensioned anchors, Hydro Tasmania has recently taken the unusual step of over-coring and extracting three post-tensioned rock anchors from operating dams in order to assess their condition. In what is believed to be a world first, a 42m long 70 strand high tensile anchor was overcored and removed from Meadowbank Dam in 2014. A further two anchors were successfully extracted from Repulse Dam in 2015, in conjunction with a group of international sponsors with similar anchors. This paper uses the 2015 work to illustrate the methodology used to extract the anchors, outlines the information gained from this unusual work, and presents the results of the condition of the extracted anchors. The paper concludes with some inferences for other owners with similar anchors and suggestions for further work.
Keywords: Grouted, post-tensioned rock anchor, ground anchor, corrosion, over-coring, extraction, dam safety.
Vicki-Ann Dimas, Wayne Peck, Gary Gibson and Russell Cuthbertson
Globally, reservoir triggered seismicity (RTS) is a phenomenon sometimes observed in newly constructed large dams worldwide, for over 50 years now. Over 95 sites have been identified to have caused RTS by the infilling of water reservoirs upon completion of their constructions worldwide. In Australia, there are seven confirmed sites with observed RTS phenomenon that are summarized by temporal and spatial means.
With almost 40 years of seismic monitoring, primarily within eastern Australia, several of Australia’s largest dams have monitored and recorded many RTS events. At present, twelve dams are 100 metres and above in height as possible candidates, with seven of these actually causing RTS and a disputed possible eighth dam.
Important factors of RTS are reservoir characteristics (depth of the water column and reservoir volume), geological and tectonic features (how active nearby faults are and how close to the next cycle of stress release they are temporally) and ground water pore pressure (decrease in pore volume under compaction of weight of reservoir and diffusion of reservoir water through porous rock beneath). RTS is an adjustment process often delayed for several years after infilling of reservoir before eventually subsiding within 10 to 30 years, when seismic activity then returns to its prior state of stress.
Generally there are two type of RTS events, either a major fault near the reservoir most likely leading to an earthquake exceeding magnitude 5.0 to 6.0, or more commonly, a series of small shallow earthquakes.
Seismic monitoring of all dams (except for Ord River) are presented with spatial and temporal series of maps and cross sections, showing the largest earthquake, build-up and decay of RTS events.
Keywords: Seismic monitoring, reservoir triggered seismicity (RTS), earthquake cycle
Phillip Kennedy, Robert Murphy, Pat Russell, Chi Fai Wan
Central Highlands Water (CHW) owns thirty four dams varying significantly in size, age, and condition. Thirty of the dams are used for water supply purposes with the remainder providing storage for wastewater reuse schemes. Out of the thirty-four dams, eighteen are more than one hundred years old. They are zoned earthfill embankments, some with a puddle clay core. Fourteen of the dams have been assessed as having potentially high to extreme consequences if the dam fails. The key safety issues among these high consequence dams are inadequate flood capacity, slope instability, and high potential for piping.
CHW’s management policy includes a commitment to identify, assess, prioritise improvements to, and periodically review the safety of its dams, and implement a dam safety upgrade works program. CHW’s Water Plan 3 (2013 – 2018 economic regulatory period) includes nine dam safety upgrade projects, which were identified from risk assessments and investigations carried out over several years.
In 2013, CHW and MWH formed a Delivery and Operational Efficiency Review (DOER) Group to refine and confirm priorities for the proposed dam safety upgrades. The main objectives of the DOER Group were to identify solutions to meet current ANCOLD guidelines and any opportunities to achieve 10% – 20% reduction in capital expenditure costs during planning or delivery of the works for Water Plan 3, while achieving the intended risk reduction. The key elements of the DOER were to (1) form a working group to cover operational, planning and executive management considerations together with dam safety consultants and Victorian dam management experience; (2) closely scrutinise previous assessments; (3) challenge the justification for the project; (4) understand the priorities whilst aiming to deliver a major works program; and (5) identify additional investigations.
Initial investigations of the DOER Group developed a revised program of works allowing confirmed capital works to proceed while investigations into other projects were carried out. The follow-up investigations have identified optimal outcomes through a program of cost-effective solutions for CHW.
This paper aims to share the experience from planning the DOER, and the further investigations that resulted in the development of an optimised delivery strategy for the upgrade projects.
Keywords: Delivery and Operational Efficiency Review, Risk.
Bronson L McPherson, Eric J Lesleighter, David C Scriven, Erik F R Bollaert
A number of medium to major floods in Queensland caused substantial scour around spillway structures. This included the Paradise Dam primary spillway which experienced significant scour of the rock body below the spillway during flooding in January 2013. The occurrence has led to a series of evaluations of the geology, and the prevailing hydraulics behaviour as part of a process to determine the scour mechanism, and to determine the response of the spillway and areas downstream to future floods of larger magnitude. Part of the process has been to utilise a large-scale physical model to obtain transient data which together with the detailed geologic assessment would be incorporated into the comprehensive scour modelling procedures developed by Dr Erik Bollaert, AquaVision Engineering, Switzerland.
The paper will describe the design and construction of the physical model with special features to obtain pressure transients from more than 60 transducers, and velocity transients in more than 40 locations using Acoustic Doppler Velocimeter (ADV) instrumentation. The features of the rock scour will be discussed and the geology of the area below the spillway apron will be described. The range of discharges, and the model’s results including the pressure and velocity characteristics will be described in detail to illustrate the violent nature of the turbulence in the energy dissipation zone. The paper will go on to describe the computational scour modelling procedures of calibration and application, demonstrating a “system” approach to spillway scour analysis for plunge pools and similar situations with energy dissipation on natural materials.
Keywords: Spillways, flood hydraulics, hydraulic modelling, rock scour, transients, numerical analysis, energy dissipation.
Kim Robinson, Andrew Pattle and Thomas Shurvell
Rowallan Dam is a 43m high clay core rock fill dam located in Northern Tasmania. The dam impounds 121GL used for hydro power generation and has a High A consequence category.
Over the summer of 2014/15 major reconstruction works were carried out on the dam to repair a piping incident from 1968. The work entailed reconstructing two sections of the dam down to foundation level and the upper 7m of the 568m dam crest. During the work, the dam was temporarily exposed to a significantly increased flood overtopping risk.
A range of measures were taken to manage the overtopping risk; such as increasing the dewatering capacity of the dam, lake draw down, installation of a sheetpile wall, development of emergency backfill procedures and a flood forecasting system.
The focus of this paper is on the flood forecasting system and how this was integrated into the overall management of overtopping risk during construction. The forecast models were run automatically on a 2 hour schedule using the latest BoM forecast, telemetered lake levels and rainfall from 7 gauges surrounding the catchment. The system provided a continuous 7 day lake level forecast which guided the site team on when to release water to manage the storage.
In the event that the lake level forecast reached a predetermined trigger level, the dam safety team would have been automatically notified and various emergency procedures would have been triggered in response to the flood warning.
This paper discusses the measures that were taken to manage the flood risk, how it worked in practice and conclusions which are applicable more generally to managing overtopping risk during dam works.
Keywords: dam construction flood risk, flood forecasting