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...