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

Winter roads provide an important transportation service in northern regions. The Tibbitt to Contwoyto Winter Road (TCWR), traversing subarctic Canada, is the busiest heavy-haul road in the world with as many as 10,900 truckloads per season. In addition to lake-ice thickness, trafficability on the TCWR depends upon adequate freezeback of overland portages, which are primarily peatlands underlain by discontinuous permafrost. We investigate threshold requirements for the initiation of winter road operations in this region and assess the use of a recommended 305 °C-day air-freezing index (FDD305a) value as an operational predictor of ground freezing at 30 cm depth, the desired depth to allow winter road construction to commence. Snow compaction and flooding were found to enhance freezeback of portages with early winter overland flow having a similar effect. The majority of winter road portages were not adequately frozen to a depth of 30 cm by FDD305a. Our results indicate that for drained and wet peatlands in this discontinuous permafrost environment, an FDDa threshold of 1100 °C-days is more appropriate than the 305 °C-day threshold. However, TCWR winter road operators presently plan the construction of the winter road by a 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 with a higher percentage of sites frozen to 30 cm depth than would be if the 305 °C-day air-freezing index was used as a guideline. In addition, the use of low-pressure vehicles for snow compaction during the start of the construction season is an effective adaptation practice to accelerate freezing penetration.