Permafrost geotechnique for engineering design and land use planning

The vulnerability of infrastructure to permafrost degradation strongly depends on the physical, chemical, mechanical and thermal properties of the ground. The study of permafrost properties is essential to understanding how the ground will react to anthropogenic and climatic changes. This knowledge forms the baseline for permafrost geotechnique and its application provides answers to questions involving freezing and thawing processes such as slope movement, frost heave and thaw settlement. Permafrost geotechnique often uses field investigations to acquire specific knowledge on permafrost conditions. Among them, shallow and deep boreholes are the most common method. Physical, chemical and mechanical properties such as ice and water content, grain size, salinity, and creep parameters are determined from core samples. Boreholes can be used for monitoring ground temperature, hydraulic properties and slope movements by installing thermistor cables, piezometers and inclinometers, respectively. Geophysical methods, which use the electrical (e.g. resistivity or conductivity), electromagnetic (e.g. ground penetrating radar) and seismic properties of the ground, are useful to delineate frozen and unfrozen areas; in addition they provide an alternative tool to measure select physical and mechanical properties such as ice and unfrozen water content, and the Young's and Shear moduli. Finally, in-situ testing tools such as time domain reflectometry antenna, thermal conductivity probe and calorimetric testing provide information on physical and thermal properties while mechanical properties such as creep parameters are measured with pressuremeter and cone penetration test. Increased infrastructure in the north linked to development of natural resources and population growth emphasizes the need to incorporate permafrost sensitivity into engineering design and land use planning. Experience with the Norman Wells pipeline has highlighted the need to understand the pipeline distress that has occurred due to slow but ongoing creep deformation. Through field instrumentation and geophysical surveys, the GSC has been documenting the mechanical behaviour of frozen and thawing slopes. At a community level, land use planners have to deal with a broad range of terrain instability, which may be amplified by climate change, and that may threaten the integrity of current and future infrastructure. Using the geotechnique investigative methods, processes involved in permafrost degradation such as thaw settlement or thermal erosion are studied to assess permafrost sensitivity at the community scale. As an example of contribution, the integration of different layers of geotechnical and geophysical information has been use to build a 3D community-based thermal model for climate change impact assessment. A similar kind of methodology or tool for land management and decision making is currently being developed at the GSC through the Nunavut Landscape Hazard Mapping project. This tool incorporates into a GIS a variety of data layers including digital elevation models (DEM), surface geology, geotechnical properties, which are interpreted in terms of the permafrost sensitivity and the vulnerability of infrastructure to climate change. This research is currently being conducted in the communities of Pangnirtung and Clyde River, NU. The work is being conducted in partnership with GSC-Calgary (Dr. Rod Smith) and in collaboration with Université Laval (Centre d'études nordiques), Memorial University, the Canada-Nunavut Geoscience Office and the Government of Nunavut. Hazard risk assessment maps will be produced along with a methodology for conducting similar studies. Permafrost geotechnique is also being applied in activities conducted in collaboration with Parks Canada. Characterisation of the permafrost conditions at York Factory National Historic Site, northern Manitoba, was completed using similar methods discussed above. At a more regional scale, characterisation of the permafrost sensitivity with respect to different wetland types is being carried out in Wapusk National Park, MB. Parks Canada incorporates the results of these two projects in the development of their management plans. In general, permafrost geotechnique investigations are often restricted to point locations and a limited time frame. Some of the main challenges are to 1) interpolate between known data locations, 2) extrapolate this knowledge over a larger area, and 3) assessing the temporal variability in permafrost conditions. Because of the influence of surface characteristics (i.e. snow and vegetation cover) on permafrost conditions, there is the potential that high resolution satellite imagery could aid in overcoming some of these challenges.

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