Critical hydraulic gradients in tailings dams comparison to natural analogies

Abstract: The increasing demand of metals and minerals has made it economical for the mining industry to mine low-grade deposits, which results in immense quantities of by-products. In fact, such by-products, i.e. tailings, are the largest waste volumes produced on earth. Ore from sulfide rich geological formations result in sulfide rich tailings that can cause major environmental problems upon oxidation and acidic seepage. One way of economically and effectively preventing unwanted chemical reactions and leaching is the deposition in an impoundment surrounded by tailings dams, where tailings are allowed to settle and covered effectively by a water level against oxidation.In order to prevent environmental impacts and possible long term contamination from tailings dams, the Swedish Environmental Protection Agency (Naturvårdsverket), demands a long term stability without maintenance that refers to thousands of years or more, resulting in a design period of 1000 years for tailings dams. The stability of tailings dams in long term perspective depends, amongst others, on the prevention of internal erosion, a process that results from an exceeding seepage pressure causing particles in a dam to migrate, with possible consequences of damage and failure of the dam construction. Therefore, the main question in this thesis is: Which maximum hydraulic gradient can we allow for a tailings dam construction in order to prevent internal erosion in a long term perspective?With regard to the long term design of tailings dams, natural analogies to dam constructions are considered, i.e. formations from the last glaciation period 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, otherwise they did not exist today. Consequently, it is assumed that a critical hydraulic gradient exists, and that the material composition and compaction reaches an optimum which allows seepage without erosion. This study provides basic knowledge on tailings dam construction and a State-of-the-Art report on current knowledge on internal erosion. A summary of natural analogies to dam constructions that have been stable dams since the last glaciation in Sweden is presented. In the context of this work, a case study was conducted at the company area of Boliden in Gällivare, northern Sweden, with the aim to study the geotechnical properties of such a natural stable embankment. This case study includes field studies, ground water monitoring and sampling. Complementing laboratory analysis covers an analysis of the materials' properties with regard to composition, density, compaction and hydraulic conductivity. The formation consist of a well graded glacial till which is compacted to an optimum in situ above what could be obtained in laboratory conditions. The hydraulic conductivity of laboratory compacted samples shows a minimum of 2,60 ' 10-10, which implies that the material is practically impermeable, which may be an explanation for the absence of ground water during monitoring.Critical hydraulic gradients found in literature range between 4,8 and 14 %. Current tailings dam design guidelines in Sweden relate the maximum gradient to the internal angle of friction, thus resulting in gradients of about 12 to 27 %. Gradients in long term stable natural formations are between 2 to 5 %. The calculated hydraulic gradient in the case study is 6,7 %; however, the actual gradient could not be determined due to the absence of pore pressure measurements during ground water monitoring. With regard to long term stability, possible degradation and results from comparisons to long term stable natural analogies, a modification of the design criteria for Swedish tailings dams should be considered.