Frost heave is the upward movement of soil, and anything resting on it, caused by water in the ground freezing and expanding, and it's a leading cause of cracked footings and shifted walls in cold climates.
Water sitting in the pores of soil expands roughly nine percent when it freezes, and in soil that holds a lot of moisture, that expansion forms ice lenses that push everything above them upward. When the ground thaws, it doesn't always settle back evenly, so a footing, slab, or wall that got heaved in January can end up sitting slightly different than it did in November. Fine-grained soils like clay and silt hold more water and heave more than sandy, well-draining soil.
Because the foothills run through hard freeze-thaw cycles all winter rather than one freeze and one thaw, frost heave here is a repeated stress, not a one-time event, on any footing, walkway, or wall that isn't set deep enough or drained well enough. A footing below the frost line and proper drainage under hardscapes are what keep heave from becoming a cracked wall or a lifted patio.
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