The upward or outward movement of the ground surface caused by the formation of Ice in the soil. Frost Action in fine-grained soils increases the volume of the soil not only by freezing of in situ pore water (9% expansion) but also by drawing water to the Freezing Front where Ice lenses form. Soils that have undergone substantial heaving may consist of alternate layers of Ice-saturated soil and relatively clear Ice lenses. The lenses are formed normal to the direction of heat flow and when freezing penetrates from the ground surface (which may be horizontal, sloping or vertical), they form parallel to that surface. When unrestrained, the amount of surface heave may be almost as much as the total thickness of the Ice lenses. Frost Heave can occur seasonally or continuously if freezing of the ground proceeds without interruption over a period of years. Differential, or non-uniform, frost heaving is one of the main aspects of the Frost Action process and reflects the heterogeneous nature of most soils, or variations in heat removal rate and groundwater supply over short distances. Depending on the degree of restraint, large Freezing Pressures (up to 1 MPa) can be developed as the ground freezes.