Stability of tailings dams focus on water cover closure

Abstract: Mining activities have been ongoing for thousands of years within Sweden. As the results of previous activities are evident today, i.e. for example acid mine drainage (AMD), the focus and interest on closure and remediation of mine sites and tailings storage facilities (TSFs) has increased. At present all mines in operation have closure plans according to legal requirements. The purpose of a closure plan is to make sure that the site is safe when the mine comes to an end and the mining company abandons the site. The need for remediation of TSFs, where the fine (crushed and milled) waste material, i.e. tailings, from the process plant is stored, is important with regard to the consequences for the future environment. The composition of the tailings vary, i.e. content of chemicals, minerals etc., from mine site to mine site. Unwanted processes taking place in the tailings within the TSF may result in acidic leachate and leaching of metals and heavy metals from the TSF. These processes will be harmful for the environment and must therefore be prevented or reduced to levels that the environment can handle. They need to be controlled through a proper remediation and closure design of the facility. One method to control these processes is by covering the impoundment with water. One of the most important requirements when using this method is the stability of the tailings dams surrounding the impoundment. Without stable tailing dams the water cover will not stay. Long term stability of tailings dams has been the focus of this research project. Long term has in this case been set to 1000 years or more, which has become an international praxis in the last couple of years. To understand what we need to know in order to achieve long term stable tailings dams several areas have been studied. One idea used, was that we need to know, and understand, how tailings dams in operation perform today in order to understand how they may perform in a long term perspective. This resulted in studies of tailings dam safety in Sweden (see Benckert , 2003 and 2004) and of incidents and failures at Swedish tailings dams (see Bjelkevik, 2005b and 2005c). Another field of importance is the properties of tailings and for dam stability purposes the mechanical properties of tailings as a construction material. Sampling and laboratory tests where performed in 2002/2003 in order to compare the properties of tailings with natural materials (see Bjelkevik and Knutsson, 2005a). The conclusion drawn is that tailings have different properties compared to natural materials and the way of testing tailings material need to be calibrated for these differences. It was also concluded that this is a field requiring much more attention in the future. The focus of this thesis has been the long term dam stability and the factors and processes affecting this. In the State of the Art report (Bjelkevik, 2005d) this is covered and discussed. The most important factors for long term dam stability are: the hydraulic gradient and its relation to internal erosionextreme events like floods, drought, high winds, earthquakes etc. slow deterioration processes like erosion, weathering, frost and ice forces etc. These aspects have been analysed and discussed within the thesis. One valuable source for improving our knowledge on long term stability is natural analogies that have been stable since the last glacial period. Another valuable source is ancient structures, like man made earthen mounds and dams. Existing knowledge of some of these types of structures are presented in the thesis. The author, however, believes that there are a lot more information and knowledge to gain from analysis of these types of structures. Finally, the conclusions from this research project are: no Swedish tailings dams can be regarded as long term stable today it is possible to design long term stable tailings dams more knowledge can be gained from natural analogies and ancient structures the most challenging aspects in designing long term stable tailings dams are probably not the technical aspects, but the non-technical In order to be able to define criteria for long term stable tailings dams several areas need further attention and research. Examples of these areas are: internal erosion long term changes in material properties the effect of the hydraulic gradient on slope stability interaction between tailings material and sealing elements/foundation within the tailings dam external erosion seepage points