Frost Heaving

The heaving of soil and other unconsolidated materials occurs as a result of two mechanisms: volumetric expansion associated with the water-ice phase change, and the migration of unfrozen pore water along a temperature gradient to the freezing front (usually upward toward lower temperatures) resulting in the formation of segregated ice lenses. It is the formation of segregated ice in cavities or lenses in the soil rather than the simple freezing of pore waters that is responsible for soil and regolith heaving. The growth of segregated ice continues as long as there is upward fluid migration and accompanying release of latent heat. Migration of the freezing front is generally episodic, rather than continuous. In the melt season, heat moves downward with accompanying moisture movement toward the frost and permafrost tables. During autumn freezeback, both the surface and the permafrost are colder than the central portions of the active layer. Consequently, the active layer is subject to freezing from two sides, leading to moisture migration in two directions with enhanced heaving at the top and bottom of the active layer. Frost heaving therefore is often greatest during the freeze-back period (Washburn, 1980).

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