The Fort Knox mine is an open pit mine in Alaska, USA. The tailings facility is a valley type impoundment and is contained behind a large, cross valley, zoned earthfill/rockfill embankment dam that has been raised in stages using the downstream method. The dam was completed to its original configuration in 2006 to a height of 324 feet. Since then, additional ore reserves were identified, which required a larger tailings capacity and further raises to the facility. This project presents many engineering and construction challenges including discontinuous permafrost and extreme seasonal variations in temperature that range from +95 to -60 degrees Fahrenheit (35 to -51 degrees Centigrade). The site is also in a region of high seismicity.
Knight Piésold has provided all engineering design, construction quality assurance/quality control (QA/QC), and permitting and operational assistance to the tailings storage facility since the onset of initial planning in 1992.
Tasks included detailed geotechnical investigations; characterization of mine waste, tailings and native soils; analyses of water and soil samples from creek and monitoring wells to characterize baseline water quality; assessment of impacts from historic mining activities; hydrological evaluations relative to tailings impoundment sizing; and design of a large zoned earthfill/rockfill embankment with internal filters and seepage recovery systems. Detailed finite element seepage analyses, detailed slope stability and seismic risk analyses, and permitting assistance (including 404, APP, and construction permits) were completed as part of the design and construction efforts.