CONSTRUCTION OF THE TAILINGS STORAGE FACILITY FRESHET MANAGEMENT EMBANKMENT CUT OFF WALL AT MOUNT POLLEY MINE-Paper

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Following the August 2014 breach of Mount Polley Mine’s Tailings Storage Facility (TSF), a temporary rockfill berm was built to limit tailings loss. Because it was not designed to retain water, a seepage barrier was needed before the 2015 spring freshet. Golder Associates designed a deep-mixed cutoff wall downstream of the berm, constructed by Mount Polley Mining Corporation and Golder Construction.<br /><br />The wall comprised 132 cutter soil mixing (CSM) panels, up to 27 metres deep, embedded into foundation till and tied into the existing dam abutments. The panels mixed cement and bentonite slurries with placed gravel aggregate. Design targets included unconfined compressive strength (UCS) of 1–2 MPa, hydraulic conductivity no greater than 1 × 10⁻⁶ cm/s, and sufficient ductility to withstand at least 5% strain without loss of strength or increased permeability.<br /><br />Coarse aggregate limited penetration, so an auger rig pre-drilled holes where the CSM equipment could not advance beyond about 12 metres. Cold-weather protection and a scheduled preventative-maintenance program helped sustain production and limit equipment downtime.<br /><br />Quality assurance included monitoring slurry properties and installation data, and collecting 2,553 wet-grab samples. Of these, 1,395 were tested for UCS, averaging 1.89 MPa at 28 days. Hydraulic conductivity averaged 1.92 × 10⁻⁸ cm/s across 39 tested specimens, and all 21 specimens tested for ductility met project requirements. Core recovery from four panels was limited, but optical and acoustic televiewer logs helped assess material consistency.<br /><br />The project demonstrated that CSM can create a cutoff wall through coarse, porous aggregate under tight schedule constraints. The authors also highlight the value of pre-drilling, tailored sampling methods, and preventative maintenance when mixing coarse-grained materials.

Keywords

Mount Polley Mine

tailings storage facility

cutoff wall

cutter soil mixing

seepage barrier

deep-mixed wall

hydraulic conductivity

unconfined compressive strength

pre-drilling

quality assurance