Gregg A Scott
Abstract: The Bureau of Reclamation has been performing quantitative risk analysis as the primary dam safety decision making tool for well over a decade. This paper summarizes some of the key concepts and basic methodology currently used in the dam safety risk analysis process at Reclamation.
Keywords: dam safety, risk analysis, reliability analysis, event trees, subjective probability.
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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
Chris Topham, Paul Southcott, Tim Cubit
Abstract: Dee Dam is a 15 m high and 270 m long central core earth and rockfill dam on the upper reaches of Hydro Tasmania’s Tungatinah Power Scheme. The dam is assessed to have a High A hazard category. Hydro Tasmania’s portfolio risk assessment found that a risk based upgrade was warranted to protect against both piping and flood overtopping failure modes.
A $4.2M modernisation project was implemented in 2008 comprising the installation of a full height downstream filter with rockfill buttress, repairs to cracking in the diversion conduit and raising of the core for improved flood capacity. Lowering of the Dee Lagoon to facilitate full height excavation of the downstream shoulder of the dam was impractical to Hydro Tasmania for production, environment, cost and stakeholder reasons. Hence, careful analysis and risk management was required to ensure the safety of the dam during the construction of the above works against a full storage.
This paper presents the risk objective for the upgrade work, modelling and analysis undertaken to assess dam safety during the works. A wide range of construction risk mitigation measures were employed prior to and during the upgrade works. The response to latent conditions and potential dam safety incidents that occurred during construction are described. The continuous adaptation of the construction methodology to suit site conditions encountered during the project is also presented. The project was successfully completed in June 2008. The approach of detailed investigations and design coupled with a strong risk based approach during the construction proved to be effective in managing the dam safety risks of construction work with a full reservoir.
Keywords: earth and rockfill dam, filters, construction risk mitigation, dam safety, dam safety incidents.
Rick W Schultz PE
Abstract: Dams serve many purposes: flood control, fresh water intakes for municipalities/power plants, navigation, and recreation. Throughout history risk of catastrophic failure of dams has always been an issue which engineers have had to deal with. The potential for loss of life and property damage of residents living and working downstream of dams continues to grow as more and more development occurs downstream of dams.
The U.S. Army Corps of Engineers is embarking on a complete dam safety evaluation program which encompasses over 600 of its projects throughout the United States. This program is used to evaluate the risk and reliability of dams and will assist in making decisions for budgeting and repairs of dam projects throughout the Corps.
Corps of Engineers dams have a wide variety of ages, environments, structural and mechanical designs, and geological conditions.
This paper will discuss the condition assessment tools which are being developed to determine the reliability/probability of failure of the mechanical and electrical components and systems on dams. These tools, when combined with other tools such as geotechnical, structural, and hydraulic, are used to give an overall reliability of a dam.
Keywords: probability of failure, Weibull formula, characteristic life of mechanical/electrical components shape parameters, fault trees.
Brett J Heppermann
Abstract: As one of the main federal agencies with responsibility to build, operate, and maintain large dams in the United States, the US Army Corps of Engineers (USACE) is developing a risk based framework to better manage their portfolio of 600+ dams in terms of risk management and prioritization of funding. A key element to this effort is the development of risk-based analytical tools to evaluate primary features for applicable failure modes. These are used in conjunction with loading and consequence modules to assess the overall risk associated with the dam in terms of lives and economic damages. The focus of this paper is on the analytical tool being created for evaluating the probability of an uncontrolled reservoir release due to scour of concrete lined stilling basins and spillways.
This analysis module is broken down by potential failure modes that could initiate events that could lead to an uncontrolled release. The failure modes that are considered are Cavitation, Slab Uplift and Foundation Erosion, Ball Milling, Tunnel/Conduit Failure, Plunge Pool Erosion, Fuse Plug and Fuse Gate Failure, Hydraulic Loading and Profile, and Headcutting. Each failure mode analysis takes into account how the inlet structure, conveyance, and terminal structure is affected, if at all.
This paper gives and overview of the main characteristics of each failure mode and the methods used to quantify the risk associated with each.
Keywords: risk based analytical tools, concrete scour, spillway, stilling basin, risk assessment, U.S. Army Corps of Engineers.
Erik Bollaert, Nicola Bretz
Abstract: Gebidem Dam is a 122 m high double curvature arch dam situated in the canton of Wallis, Switzerland. The dam reservoir collects glacier water of the longest glacier in Europe, i.e. the Aletsch glacier (25 km length). The water is very rich in sediment. This results in huge amounts of fine sediments depositing into the reservoir. The 55 m3/s turbined by the Bitsch power station contain between 10 and 13 kg of sand, or an average of almost 40 tons per hour.
Hence, a yearly flushing is organized allowing to release about 400’000 m3 of solid material from the bottom outlets of the dam towards downstream. Moreover, the crest spillway of the dam is frequently used during summer months to release water from the reservoir, resulting in high-velocity jets impacting the downstream plunge pool. The latter consists of a series of flat concrete plates tied together by means of steel anchors and supported by a layer of underlying mass concrete.
Intensive jet impact on this concrete apron within the last 40 years has led to severe damage and, as such, replacement of the apron becomes necessary. The present paper describes the methodology applied for design of a new concrete apron. Especially the interaction between the hydrodynamic pressures exerted by overtopping jets and the steel anchorage necessary to prevent uplift of the new concrete apron is pointed out.
Keywords: scour, concrete apron design, anchorage