Authors: Jantzer, I; Knutsson, S

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Jantzer, I & Knutsson, S 2010, 'Critical gradients for tailings dam design', in R Jewell & AB Fourie (eds), Proceedings of the First International Seminar on the Reduction of Risk in the Management of Tailings and Mine Waste, Australian Centre for Geomechanics, Perth, pp. 23-33.

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Knowledge on tailings dam design is often derived from conventional earth and rockfill dams that are designed for relatively short service periods. Tailings dams often differ in both structural design and service life, where a long-term stability of 1,000 years or more is demanded. One of the most important factors related to tailings dam stability and performance in long-term perspective is the prevention of failure from internal erosion, i.e. particle migration initiated by seepage pressure. Internal erosion is a process not yet completely understood; it is related to the seepage rate, which in turn is connected to the hydraulic gradient, the internal structure, particle size distribution, etc. The hydraulic gradient is therefore crucial for embankment stability and prevention of particle migration for a given material. A basic research question is: does a maximum hydraulic gradient in a given material exist, below which internal erosion cannot be initiated? If so, what maximum gradient can we allow for a tailings dam construction in order to prevent internal erosion with respect to the long-term stability of the construction? The maximum, or critical, gradient is often related to slope stability problems, heave, or blow out. Such critical values differ from a critical hydraulic gradient for internal erosion problems taking place inside the soil matrix. To be able to apply critical values on internal erosion problems, clear definitions and information on the origin of a given critical hydraulic gradient is needed. This paper presents a detailed literature survey on reported values of critical hydraulic gradients for the initiation of piping. These values are derived from laboratory tests and from experiences of conventional dam constructions, i.e. they have to be looked upon in a limited time perspective. This paper discusses the use of these values for tailings dams design. To take the long-term aspect into account, natural analogies to dam constructions have been analysed. These structures are formations from the last glaciations that have fulfilled the task of damming water. Such structures are especially interesting with regard to their obvious stability against internal erosion over long time periods. Natural analogies to dam constructions in Sweden are analysed to some extent and presented in this paper. Results show that they are stable under a hydraulic gradient between 0.02 and 0.05. Current guidelines on tailings dam design in Sweden allows a hydraulic gradient where neither the long-term stability of the construction, nor the possible degradation of the construction material is taken into account. To ensure a long-term stability of tailings dam constructions, a more thorough understanding of the critical hydraulic gradient is needed, and a modification of the present design guidelines is suggested.

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Critical gradients for tailings dam design I. Jantzer and S. Knutsson
34 Mine Waste 2010, Perth, Australia

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