Mount Polley — a Buried Glacial Clay Layer the Designers Never Found

Shortly after midnight on 4 August 2014, the Perimeter Embankment of the Mount Polley Tailings Storage Facility — a roughly 40-metre-high earth-and-rockfill dam impounding the wastes of a copper-gold mine near Likely, in the interior of British Columbia — broke open and discharged its contents into the headwaters of the Fraser River system. Some 7.3 million cubic metres of tailings, 10.6 million cubic metres of supernatant water and a further 6.5 million cubic metres of interstitial water — about 25 million cubic metres in total — surged out, scoured Hazeltine Creek from a metre-wide stream into a gouged channel tens of metres across, and emptied into Polley Lake and Quesnel Lake. No one was killed. What failed was not the dam but the ground beneath it: a continuous layer of weak glacial-lake clay, never identified by the foundation investigation, that lost its strength under load and slid the embankment off its base.

The facility had been raised almost annually since 1997 to keep pace with the mine. By 2014 the Perimeter Embankment stood about 40 metres high, with its downstream face built at a slope of 1.3 horizontal to 1.0 vertical — markedly steeper than the gentler profile a soft foundation demands. Beneath it, deposited in a lake between glaciations, lay a thin, continuous bed of glaciolacustrine silt and clay: the Glaciolacustrine Unit, or GLU. This clay was strong enough when its pore water could drain slowly, but when loaded faster than it could shed that water it behaved like a lubricated plane. The site characterisation had logged glacial sediments in places but never recognised the GLU as a single continuous, low-strength surface running under the breach location.

The trigger was load and geometry acting together. Each annual raise added weight; the steep downstream rockfill drove the load forward over the buried clay rather than spreading it back. As the embankment climbed past the GLU, the clay reached the point where it could no longer carry the stress without draining — and it could not drain. It failed in undrained shear, a sudden loss of strength, and a wedge of the embankment translated outward along the clay layer. The dam did not erode or overtop; it slid.

The Independent Expert Engineering Investigation and Review Panel, chaired by Norbert Morgenstern, reported on 30 January 2015 with a single unambiguous verdict: the dominant contribution to the failure resided in the design. The foundation investigation had failed to find the continuous GLU and to recognise its susceptibility to undrained failure, and the steepened downstream slope had removed the margin that might have saved it. Morgenstern’s metaphor became the case’s epitaph: building on the weak clay loaded the gun; building the steep slope pulled the trigger.

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