Mark Stephen Rynhoud, David Johns and Len Murray
The Hamata tailings storage facility at the Hidden Valley mine is being constructed in a remote, high rainfall, tropical environment in a mountainous region of Papua New Guinea. Implementation of the design hasrequired adapting the design in response to various challenges encountered on the site during the ongoing construction period, based on observations by the designers and site monitoring data which is continuously collected and compared against design assumptions. This paper describes some of the design and construction modifications which have been implemented since construction of the tailings facility started and provides a case history of some of the challenges facing designers and construction crews when mining in remote, tropical conditions.
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Paul Southcott,Suraj Neupane and David Krushka
TasWater owns and operates the water supply in Queenstown on the west coast of Tasmania. Anew water treatment plant was constructed downstream from one of the seven small dams that made up the original supply system, making the remaining six dams redundant.Two of these dams hada very high annual probability of failure and unacceptable societal life and financial risk due to their poor condition.Both dams required urgent attention (upgrade) to retain them as a lasting asset and legacy for the community or decommissioning to create a new ecological legacy.Roaring Meg Dam (6m high with a 9ML capacity) was constructed on Roaring Meg Creek around 1963. The Cutten Street Dam No 3 (10m high with a 2.4ML capacity) was constructed on Reservoir Creek around 1902to supply water to a growing mining community and had been in use since then. From a heritage perspective, the dam had some value as a timber crib and rockfill dam and its historical context as a key factor in the development of the town.There is limited guidance in the ANCOLD (2003) Dam Safety Management Guidelines on decommissioning and a process had to be developed in cooperation with the Regulator in this relatively new area of dam engineering. Detailed design of the decommissioning including diversion work during decommissioning, channel design to align with the original creek to help restore its ecological function and rehabilitation work on the exposed reservoir soils to stabilise them were undertaken. Aboriginal and historic heritage studies, flora & fauna studies and fluvio-geomorphological study at the dam sites were also undertaken to ensure that the decommissioning work did not interfere with the heritage, threatened species and riparian processes. The community were consulted to ensure acceptance of the changes to their town. Dam safety emergency management plans for the decommissioning of these dams was were also prepared. A significant issue in the decommissioning work was frequent and high rainfall due to the location of these dams on the west coast of Tasmania. The entire dam removal work had to be planned within the window of dry weather or very little rainfall. This paper presents the process, activities and lessons learned in successfully decommissioning these dams,to eliminate the unacceptably high risks posed by these dams and to restore the normal riparian processes.The general approach adopted for this project has applicability for other damsandis proposed as a starting point for an ANCOLD practice note in this area
Stefan Hoppe, Vicent J. Espert-Canet
Monitoring data has to be transformed into useful knowledge to provide owners and operators with valuable information about the safety status of their dams. This information should be up-to-date and easily accessible for all technicians and engineers involved inthe safety program,and directly linked to operation and emergency preparedness procedures.This article describes the main functions of a web-based software for the acquisition, processing,and evaluation of monitoring data. It runs on conventional internet browsers,and does not require the installation of any additional software. It provides appropriate tools for monitoring the safety status of dams and analysing dam behaviour.This article uses a case study to outline the experience gained from implementing and operating the software for 8 years to control more than 50 Spanish public dams owned by a river basin authority. The implementation involved completely revisingthe installed monitoring systems and recompiling all available information. This was used as a basis for an updated,goal-oriented definition of necessary variables, configuration of charts, SCADA views and threshold values. A key aspect of the software ́s successful implementation was the theoretical and practical training of all stakeholders.As a result of the software ́s implementation, the dam owner was able to use the data from their monitoring system more efficiently. The development of safety reviews and dam safety status evaluations were also considerably improved.
Although the total tailings dam failure frequency peaked in 1960s through 1980s, the failure rate of significant tailings dams has not dropped. The significant tailings dam failures the mining industry experienced in the recent history include: Merriespruit, South Africa, 1994; Los Frails, Spain, 1998; Kolontár, Hungry, 2010; Mount Polley, Canada, 2014; and Samarco, Brazil, 2015. The dam failures may be due to inadequate design, poor construction and inappropriate operations.This paper discusses the lessons learned and some recommendations and good practices to reduce the tailings dam failure risks. It addresses existing issues and provides some recommendations in risk based design, water management-integrity of facilities and water balance modelling, loading rates, tailings farming, adequate governance and roles and responsibilities of designers and nominated engineer.
David Guest, George Samios, Richard Rodd
Tenterfield Creek Dam is a 15m high concrete gravity structure that was constructed in 1930 and raised by 1.83m and stabilised using 97 post-tensioned ground anchors in 1974.Recent stability assessments concluded that the dam does not satisfy the ANCOLD Guidelines for Stability of Gravity Dams and that the situation is likely to deteriorate given the questionable performance of the post-tensioning cables and on the grounds of continuing corrosion and demonstrated loss of load.Tenterfield Shire Council is committed to improving the stability of the dam to meet the requirements of the NSW Dam sSafety Committee and engaged Public Works Advisory to assist them achieve this outcome.
Public Works Advisory prepared a dam upgrade options study which selected two options for further consideration. The estimated costs of the two preferred options were found to be potentially close;therefore Tenterfield Shire Council requested that both options be taken to detail design and tender stage to allow the market to indicate which option was in-fact better value.Factors other than construction costs were also considered in the options evaluation process and these factors influenced the selection outcome. The two upgrade options of lowest cost were the conventional gravity dam strengthen solutions i.e. installation of new post-tensioned ground anchors and downstream mass concrete buttressing. The decision to proceed to tender with two options was supported by the other key funding stakeholder, DPI Water.
This paper provides some unique insight on the comparison of conventional upgrade options for concrete gravity dams and also examines some interesting design aspects encounter edduring the design development process
Steven E Pells, Philip J N Pells
Junction reefs dam was designed in 1895 and constructed by 1897 as a multiple arch brick structure which was the first of its kind in Australia, and one of the earliest in the world. The dam was envisioned to provide mechanical and electrical power for gold mining. This paper provides an historical overview of the unique structure, and reassesses some of its engineering characteristics, such as the stress conditions in its unusual arches and reverse concrete gravity wing walls. The hydrology of the dam is re-assessed from the viewpoint of evaluating its potential as a mini hydro scheme. Commentary is also provided on the performance of its unlined spillway, which has been subject to regular spills for 120 years.