....should read this whole Wiki page.
http://en.wikipedia.org/wiki/Frost_heaving
http://en.wikipedia.org/wiki/Frost_heaving
Frost heaving (or a frost heave) results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss (vertically toward the surface), starting at the freezing front or boundary in the soil. It requires a water supply to keep feeding the ice crystal growth; and the growing ice is restrained by overlying soil, which applies a load that limits its vertical growth and promotes the formation of a lens-shaped area of ice within the soil. Yet the force of one or more growing ice lenses is sufficient to lift a layer of soil, as much as 30 cm or more.