Soil mechanics and the observational method: challenges at the Zelazny Most copper tailings disposal facility

This paper illustrates the geotechnical aspects of the development of one of the world's largest copper tailings disposal facilities, located at Zelazny Most in south-west Poland. Its operation commenced in 1977 and, by the end of 2013, 527 × 10⁶ m³ of tailings had already been stored within a confining embankment dam (‘ring dam') of about 14·3 km in total length. The foundations of the ring dam lie on Pleistocene deposits, underlain by a thick sequence of Pliocene sediments. The period of operation of the facility will continue until exhaustion of the ore body, which is estimated to occur in 2042, when the total volume of tailings stored will reach 1000 × 10⁶ m³. The development and the use of the Zelazny Most facility is affected by a number of significant geotechnical hazards, including the phenomenon of flow liquefaction, as is often the case for such structures. However, the most important geotechnical hazard at Zelazny Most is a consequence of its location in an area that, during the Pleistocene, experienced at least three major glaciations. The ice sheets, over 1000 m thick, which overrode the area, induced substantial glacio-tectonic phenomena, the most geotechnically important of which is the formation of extensive sub-planar shear planes in the Pliocene clays that extend to depths of around 100 m. The shear strength in these shear zones, which occur mostly in high-plasticity clays, is close to residual, thus controlling the stability of the confining dams. Moreover, the area of the disposal is also subject to a moderate degree of mining-induced seismicity. Discussion of this is omitted from this paper because of its relatively minor influence on the design of the depository. Given the large scale of the disposal facility, the complexity of the geological and geotechnical settings, and its anticipated life span, the owners KGHM (the Polish acronym for Copper Mine and Mill Company), on the advice of the World Bank, appointed an international board of experts (IBE). The IBE, supported by a Polish geotechnical expert as local liaison, have the responsibility to oversee the safe development of the tailings dams by applying Peck's observational method. This paper describes the monitored behaviour of the dams, with particular reference to the presence of the glacio-tectonic phenomena and the remedial measures adopted for their mitigation. The geotechnical characterisation of the foundation soils and of the tailings is described, and the stability analyses of the tailings dams are discussed in the context of the safe development of the whole facility.