Frost heave occurs when freezing temperatures penetrate the ground, causing subsurface water to form ice structures that displace the soil along with anything that rests on or in that soil. While it was once thought that frost heave happens because water expands as it freezes, the process is actually more complicated, involving not only expansion due to freezing, but also the accumulation of additional layers of ice as liquid water is drawn up from below the frost line.
Frost-susceptible soil—finegrained, moist soil in certain climates—is the first prerequisite for frost heave. Engineers define this type of soil as either that in which more than 3% of the grains (by weight) are 0.02 mm in dia. or smaller, or that in which 10% of the grains are 0.075 mm or smaller.
Water is another requirement, as are subfreezing temperatures that penetrate beneath the surface. The depth to which freezing temperatures penetrate the ground is referred to as the freezing plane or frost front. The depth to which they can potentially extend in any given region is the frost line. Frost lines range from a few inches in Florida to more than 6 ft. in the northern United States.
If not controlled, frost heave can seriously damage buildings and other structures in cold climates. Mitigation typically involves removal of one of the three elements (frost-susceptible soil, freezing temperatures, or water) required for frost heave to occur. Here’s how it works.