Frost heave is a phenomenon whereby frozen ground expands and can cause severe structural cracking in houses, roads, and other structures. Areas with high moisture content soil (particularly clay soils) are most vulnerable to frost heave. If there is no permafrost layer, it is advisable to anchor the structure so that pilings run well below the extent of freezing.

Frozen ground, of course, expands because water takes on a lattice-like structure when it freezes. Hence, a given mass of water increases in size when chilled. (Go, try it and see for yourself!) Surprisingly (or not), this represents a powerful force; freezing water can explode rocks, collapse hillsides, topple trees, destroy houses, and create an effect on roads something like a natural speed bumb, as veins of water running underneath the roadway freeze and expand outwards, buckling the concrete/asphalt/anything else the road was made out of.

I remember learning about this in my mid-teens, as a girlfriend from a frosty northern place proprietarily explained what all the road signs bearing nothing but the cryptic inscruption "Frost Heaves" meant. In cold enough places, it can become a serious driving hazard...

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