2905, chemin des services
Higher air temperature and surface disturbance due to road construction alter the surface energy balance, increase underlying permafrost temperature and thawing, and even promote the development of taliks. Infrastructure embankments essentially interact with the underlying permafrost primarily through conductive heat transfers and phase change during freeze-thaw episodes. When the embankments are located in the path of groundwater flow, convective heat transfers may be an order of magnitude more important than conductive heat transfer in the embankment (Kane et al. 2001; Kurylyk et al. 2014, 2016). Heat advection by groundwater and its impact of the thermal regime of permafrost have been poorly documented. According to short-term field observations and numerical modeling results, water flow in the soils is a key factor in the dynamics of heat transfer and permafrost degradation (McKenzie et al. 2007; Woo et al. 2008; de Grandpré et al. 2010, 2012). Lateral water flow in the active layer triggers convective heat transfers that can significantly alter the temperature of the soils, increase active layer thickness, and create supra-permafrost talik beneath embankment. Connections of talik layers develop new groundwater flow paths, which enlarge with time (along with permafrost degradation). Clearly, groundwater flow will significantly reduce the long-term stability of embankments, increase maintenance costs, and very likely reduce the life cycle of the infrastructure.
In the Beaver Creek area, south-western Yukon, a highway research facility with a road test section has been constructed in 2008. This site is well-known for the ongoing thermal degradation of the permafrost below the embankment, demonstrates significant groundwater flow, and is heavily instrumented from previous work. The current road alignment intercepts a local drainage network and it was observed that heat advection from groundwater flow promotes permafrost degradation. As active layer below slopes and toes of embankment deepened over time, thawed permafrost (originally ice-rich) yielded ground subsidence and the road settled, unevenly, over time. As coarse embankment material (sand and gravel) with large interstitial voids subsides, it intercepts the water table, increases the permeability of the water storage media and yields more groundwater (positive feedback). In spring and summer, snowmelt water and rainfall saturate the thawed soil and water stored at the toe of embankment allows lateral flow in the active layer and in taliks. Furthermore, groundwater flow that gets trapped under the road before winter significantly delays freeze back of the active layer which contributes to general higher permafrost temperature. A network of groundwater monitoring wells, piezometers and thermistor cables was installed along the sides and under the centerline of the road to estimate groundwater flow and its thermal impact on the permafrost beneath the embankment.
The complex interaction between transportation infrastructure, groundwater flow, and underlying permafrost remains poorly understood and quantified and none of these studies have been based on long term field observations. Our study site provides an opportunity to investigate the convective heat transfer linked to groundwater, notably within taliks. The general objective of my project is to develop fully-coupled numerical thermal model based on field studies and to evaluate the potential response of permafrost and infrastructure to groundwater flow and climate change. The specific objectives are: 1) Improve the knowledge of groundwater flow paths and refreezing process in active layer and talik region beneath an embankment; 2) Develop a fully coupled heat-water-deformation model to investigate the impact of seepage on thermal regime and deformation of the subgrade; 3) Evaluate the long-term performance of mitigation techniques submitted to groundwater flow and warming climate. This study will also help to better understand the development of longitudinal settlement and to improve the design of mitigation techniques and drainage system in cold region.