Changing Permafrost Landscapes

Evaluation of threshold freezing conditions for winter road construction over discontinuous permafrost peatlands, subarctic Canadian Shield

Tuesday, November 19, 2019 - 11:40am to 12:00pm Theatre Three


W.E. Sladen (Presenting)
Geological Survey of Canada
S.A. Wolfe
Geological Survey of Canada
P.D. Morse
Geological Survey of Canada

Winter roads are important transportation corridors in northern regions. The Tibbitt to Contwoyto Winter Road (TCWR) traverses the subarctic Canadian Shield and is the most heavily used winter road in Canada. In addition to lake-ice thickness, trafficability on the TCWR depends on adequate freezeback of overland portages, which primarily traverse peatlands underlain by discontinuous permafrost. We investigate threshold requirements for the initiation of winter road operation and assess the use of a recommended 305 °C-day air-freezing index (FDD305a) value to predict ground freezing at 30-cm depth, the standard depth for initiating winter road construction. Snow compaction and flooding enhanced freezeback of portages and early winter overland water flow had a similar effect. The majority of winter road portages were not frozen to a depth of 30 cm by FDD305a. The results indicate that an FDDa threshold of 1100 °C-days is more appropriate for drained and wet peatlands in this discontinuous permafrost environment. However, TCWR winter road operators presently plan the construction of the winter road by calendar date rather than by evaluation of the air-freezing index. This practice results in a conservative approach to the start of the construction season, close to 1100 °C-days, when a higher percentage of sites are frozen to 30-cm depth than would be if FDD305a was used. In addition, the use of low-pressure vehicles for snow compaction during the start of the construction season is an effective adaptation practice to speed up ground freezing.