Where the water table is high, that is near the surface, soils, such as silts, chalk, fine gritty sands and some lean clays, near the surface may expand when frozen. This expansion, or frost heave, is due to crystals of ice forming and expanding in the soil and so causing frost heave. In this country, ground water near the surface rarely freezes at depths of more than 0.5 m, but in exposed positions on open ground during frost it may freeze up to a depth of I m. Even in exposed positions during severe frost it is most unlikely that ground water under and adjacent to the foundations of heated buildings will freeze because of the heat stored in the ground under and around the building. There is, therefore, no need to consider the possibility of ground movement due to frost heave under and around heated buildings.
For unheated buildings and heated buildings with insulated ground floors, a foundation depth of 450 mm is generally sufficient against the possibility of damage by ground movement due to frost heave.