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Top1. Introduction
The latter half of the last century witnessed a great spurt in the mining activities all over the world. Different milling processes were introduced in the last few decades to extract different metals. In general, the mined ores are finely powdered and subjected to metallurgical processes to extract the mineral values. After the extraction, the remaining part of the mined material will be disposed of usually in the slurry form termed as tailings. In the beginning, when the productions were small, and when environmental concerns were nonexistent, the tailings were discharged into the natural streams and valleys. Such is not the case now, as the tailings discharges are huge, and environmental concerns are very serious since the tailings contain chemicals and contaminants. As a result, large surface impoundments are created for the tailings by constructing an all-round dyke system called as tailings dam.
Three different methods are in use for the construction of the tailing dams namely, upstream method, downstream method and centerline method. The suitable method is selected depending on the availability of land, characteristics of the settled tailings, and the cost of construction. The upstream method is the most cost-effective; but cannot always be adopted due to the stability issues. In this method, the new section has to be constructed on the settled tailings with low shear strength and highly variable properties. The downstream methods are known for higher stability; but are costly. The centerline method is in between the upstream and downstream methods in terms of cost and stability. Figure 1 shows the schematic construction sequence of the different methods.
Figure 1. Different raising schemes: (a) Upstream method; (b) Downstream method; (c) Centerline method
Over the past decades, failures of the large tailing ponds have triggered serious disasters and drawn public attention to the safety of tailings dams (Rico et al., 2008a). The major cause for the tailing dam failure are slope instability, seepage, in sufficient free board, rise in the phreatic surface, foundation failure, excessive rainfall and seismic liquefaction etc. Rico et al. (2008b) carried out the failure analysis of 147 tailing dams and reported that the 85% of the failure occurred in the active tailing dams. Meteorological factors such as uneven rainfall and snow were found to be the major cause. It was also observed that the majority of the failed dams were constructed using the upstream method of embankment raising. The routine stability analyses may not suffice while adopting the upstream method of embankment raising. More robust and comprehensive analysis considering all the critical aspects has to be carried out in those cases. The present study proposes the reliability based seismic stability analysis for the dams. The reliability-based methods consider the spatial variability of the mine tailings and the soil (e.g. Hong and Roh, 2008; Griffiths et al., 2009; Wang et al., 2010; Wang, 2012; Liang Li et al., 2014).