In August of 2014, a mixed earth and rockfill dam impounding a tailings pond at the Mount Polley Mine in Canada breached1. Over the following four days, c. 4.5 million m3 of tailings slurry was released into Polley Lake. An expert inquiry reviewed potential causes of the breach: cracking, overtopping, foundation failure, and human intervention. The inquiry noted that “the presence of a glacially pre-sheared surface in the dam foundation posed significant uncertainty throughout the design process”, and, after eliminating overtopping and human intervention, assigned maximum likelihood to the scenario of foundation failure stemming from preferentially oriented glaciolacustrine deposits underlying the dam.
While glaciofluvial deposits and glacial tills were also present beneath the dam, the fine silt and clay of the glaciolacustrine deposits made them the most likely culprit for instability. The presence of glaciolacustrine deposits was well documented in borehole records. In c. 2005 the mine operator (Mount Polley Mining Corporation) recorded that “the glaciolacustrine deposit encountered in [borehole] GW96-1A is a discontinuous unit and will not adversely affect the dam stability”. The breach occurred c. nine years later 300 m due west of borehole GW96-1A.
Although the Mount Polley Mine is located more than 50 km away from present-day glaciers, the site was covered by the Cordilleran Ice Sheet during the last glaciation, which reached a maximum c. 22 kaBP. During the subsequent deglaciation, which lasted until c. 11 kaBP, proglacial rivers and lakes evidently left substantial lacustrine deposits as the ice margin retreated through the site. Despite the last deglaciation ending millennia ago, the strong residual imprint of glacier processes on local stratigraphy compels them to be considered in the design of sensitive infrastructure in formerly glacierized areas.
The Kumtor Mine, Kyrgyzstan, shares some analogous geotechnical challenges with the Mount Polley Mine. At the Kumtor Mine, an earth dam impounds a c. 3.4 million m2 tailings pond, which is located c. 7.5 km downstream of the Petrov Glacier. The Petrov Glacier terminates in the proglacial Petrov Lake, which is itself impounded by glacial moraines and tills. Given the equilibrium line lowering and growth of glaciers during the past glaciation2, it is very likely that glaciolacustrine and glaciofluvial deposits are present in the vicinity of the Kumtor tailings pond. The growth of Petrov Lake upstream of the tailings pond, from 1.8 to 4.3 million m2 between 1977 and 2014 (due to climate change enhancing glacier retreat and melt), presents an additional geotechnical hazard: glacial lake outburst floods upstream of the tailings pond3.
When existing infrastructure is confronted with such unique geotechnical challenges associated with operating in a proglacial setting, adaptive engineering solutions are often be employed. For example, deformation and creep of glaciolacustrine sediment rich embankments can be monitored with cm-scale accuracy using spaceborne radar, and mm-scale accuracy with ground-based radar. While this may potentially allow embankments to be reinforced as needed, given that the Mount Polley tailings pond instability progressed to a complete breach in just a few days, monitoring alone may be insufficient to avoid a breach. Perhaps the lesson from the Mount Polley Mine, for sites like the Kumtor Mine, is to ensure that unstable glacial sediment is comprehensively identified and factored into robust hazard management and infrastructure design plans!
1Mount Polley Review Panel. 2015. Independent Expert Engineering Investigation and Review Panel: Report on Mount Polley Tailings Storage Facility Breach. Province of British Columbia.
2Koppes et al., 2008. Late quaternary glaciation in the Kyrgyz Tien Shan. Quarternary Science Reviews. 27: 846-866.
3Jansky et al., 2009. The evolution of Petrov Lake and moraine dam rupture risk (Tien-Shan, Kyrgyzstan). Natural Hazards. 50: 83-96.