ties are similar to those of unfrozen ground. Frozen ground 

 with a high moisture content, usually in the form of ice crystals 

 or lenses, requires special construction methods. 



PERMAFROST 



Permafrost is found in most of Alaska, northern Canada and 

 the Canadian archipelago, Greenland, and as far south as northern 

 Mongolia and Manchuria. It reaches farther south in the eastern 

 sections of the continents than in the western parts. The thick- 

 ness varies from a narrow wedge at the southern edge to depths 

 of several hundred feet in northern Alaska and reputedly over 

 1,000 feet in northeast Siberia. 



There are many factors that appear to be related to the forma- 

 tion of permafrost. A low annual mean temperature (minus 4° 

 to minus 6° C.) can produce and maintain permafrost. Its first 

 occurrence was probably during the excessive chilling caused by 

 the Pleistocene ice ages, about a million years ago. Inconclusive 

 evidence suggests that the permafrost layer is thickest in the areas 

 which were not glaciated during the ice age. Although the rela- 

 tionship with the ice age is not entirely clear, permafrost is related 

 to current climatic conditions. For example, at Port Nelson on 

 Hudson Bay after a lake was drained, the ground froze to a depth 

 of 8 feet the first year, to 20 feet the second year, and in the third 

 year the permafrost reached a depth of 30 feet, the average depth 

 for the area. 



The layer above the permafrost is usually subject to thawing 

 in the summer. This is called the active layer and its depth de- 

 pends upon the annual mean temperature, the surface exposure, 

 the heat distribution and composition of the ground, and the vege- 

 tation cover. Moss or peat act as insulators to limit the depth of 

 the active layer. If this cover is removed or the ground is dis- 

 turbed, the thawing will be increased and the active layer will 

 become thicker. Heavy snow cover and water bodies act as in- 

 sulators against the cold, and the underlying permafrost will be 

 at a deeper level or nonexistent. Thus the bottom surface of the 

 active layer and the top of the permafrost layer are quite irregular, 

 frequently having little relationship to the ground topography. 



Permafrost is impermeable to water. During the summer, a 

 considerable amount of the surface water percolates through the 

 thawed active layer. As the ground freezes in the winter from 

 the top downward, the water continues to flow in the unfrozen 

 layer which gradually becomes more restricted and the water 



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