Building on frozen ground: infrastructure in northern environments Stephen Robinson
The present distribution of permafrost in Canada.
Excess ice forms up to 80% of the volume of the upper 10m of ground in a meat locker excavated in permafrost at Tuktoyaktuk, Northwest Territories. The darker layers are nearly pure ice, while the lighter bands are sand.
p ermafrost, the presence of perennially frozen ground often in the near subsurface, covers more than 20% of the world’s land s surface, and about 50% of Canada. In southern permafrost regions (the discontinuous permafrost zone) near Thompson, Manitoba and Yellowknife, permafrost is sporadic, most often associated with a thick cover of organic material. Moving further north into the continuous permafrost zone, as in Iqaluit or Inuvik, permafrost begins to underly a greater proportion of the land, until virtually all of the terrain is frozen, often to depths of several hundred metres. Early settlers in Dawson City and Fairbanks built their structures directly upon the ground, and soon found their buildings shifting as the ground beneath melted. Subsequent measures that deal with perma- frost rely on the continued stability of frozen ground. The potential for a warming climate in the near future may have important repercussions — many structures have been planned to last many years on permafrost. Permafrost is most difficult to deal with when it contains significant quantities of ice that freezes and unfreezes. Changes in volume associ- ated with phase change in soils leads to the two basic rules to building upon permafrost. If you have permafrost, you want to preserve it. If a site doesn’t have permafrost, you don’t want to develop it. Many fine- grained silt and clay soils in permafrost areas actually contain more ice when frozen than they could water when unfrozen. Freezing here can lead to a heaving of the ground,with significant implications for struc - tural integrity. The melting of such thaw sensitive soils means a dramatic decrease in volume and structural integrity. In extreme cases, such as the subsurface conditions at Tuktoyaktuk, nearly pure ice lenses can occupy 80% of the upper 10m of ground. Spatial variability in the ice content of frozen ground leads to differential ground subsidence upon thawing. Bedrock, containing virtually no pore ice, has few permafrost- related difficulties.
In some communities where permafrost may be thin and warm, site clearing may allow permafrost to melt prior to construction, although the complete melting process may take several years. Alternatively, the excavation of thin permafrost and the replacement of the soil with sand or gravel can speed up the site preparation time. In the case of small projects, such as individual houses, patches of potentially unstable ground may simply be avoided, or construction may take place upon gravel pads that aim to keep summer thaw within the pad. Large proj- ects, such as airports, schools, and industrial and municipal buildings, and linear structures such as roads and pipelines, may not have the luxury of complete avoidance of such ground. The construction of buildings upon piles is the most commonly used construction technique to avoid thawing the permafrost. The gap between the soil and building is usually a minimum of 1 metre, which allows the dissipation of heat from the building without warming the ground. Additionally, the ground is shaded from the summer sun and a lack of snow insulation under the building keeps the ground frozen. Piles (iron or treated wooden poles) must be set at least 5m deep into permafrost. In the case of wooden piles, if the summer thaw progresses below the level of treated portion, the wood can start to rot if drainage is impeded. This is currently a concern in Inuvik. Adjustable screw jacks as the foundations of smaller buildings is becom- ing more common in northern communities. The police station in Tuktoyaktuk uses a triodetic foundation system that provides stability and adjustability, while at the same time keeping the gap between the building and soil.
On Site review 11
64
Spring 2004
Architecture of the Circumpolar Region
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