The design and construction of foundations in permafrost imposes problems that are in addition to those encountered in temperate regions. The additional problems are brought about by a subsurface that has a temperature below freezing all year round. Therefore, consideration must be given to the temperature profile in the subsurface, its variation with time and any changes imposed on it, as well as to strength and deformation properties of the frozen soils.

Extent of permafrost

Before dealing with foundation design, a review of the location of the perennially frozen ground both areally and vertically is necessary. The permafrost map of Canada (Fig. 1) shows the general areal distribution. The permafrost is divided into two zones by a line trending northwesterly from the south edge of Hudson's Bay. North of this line is the Continuous Zone in which, as the name implies, permafrost is virtually everywhere. South of this line is the Discontinuous Zone in which localized areas of unfrozen ground appear and which gradually increase in areal extent until the temperate areas are reached.

Figure 2 shows schematically the distribution of permafrost in a vertical section as one proceeds in a northerly direction (Brown 1970). Note that the active layer (defined by Brown & Kupsch (1974) as the top layer of ground above the permafrost table that thaws each summer and refreezes each fall) decreases in thickness but the thickness of the permanently frozen mass increases northwards. The junction between the Discontinuous and Continuous Zones

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