How it Works: Frost heave
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.
When freezing temperatures penetrate the ground, water trapped in voids in the soil forms ice crystals along the frost front. As it solidifies, this water expands by about 9%. In addition, the freezing process desiccates the surrounding soil, drawing unfrozen water from below the frost front through capillary action and vapor diffusion. This water freezes to the ice crystals that have formed above, thickening it to create an ice lens.
An upward force
As temperatures change, the depth of the frost front changes, leaving behind a series of ice lenses with layers of frozen soil between. As they grow, these ice lenses may attach themselves to vertical surfaces below ground, an action known as adhesion freezing, or adfreezing. The ice lenses continue to grow in the direction of the heat loss—that is, toward the surface—lifting soil and structures along the way.
When the air warms, thawing occurs from the ground’s surface downward. As the ice lenses melt, water saturates the soil, weakening it. Structures raised by the frost heave slide back down, often resting askew from the combination of weakened soil and shifting load forces above. The cumulative effect of repeated heaving may aggravate the situation, causing a structure to collapse.
Controlling frost heave
Footings and Piers
Code mandates that support structures either extend below the local frost line or be protected by insulation so that the bearing soil is not subject to freezing and, thus, heaving. Frost heave also can be controlled by backfilling around piers with gravel to promote drainage, using a sleeve to prevent ice from gripping the concrete, or pouring footing bases that resist upward movement.
Driveways, Walkways, and Patios
The occurrence of frost heave can be minimized by replacing fine-grain, frost-susceptible soil with coarse granular material that is not subject to heaving. Drainage measures can reduce the presence of moisture, which also prevents heaving. Providing a capillary break is another option; interrupting the capillary action that draws water toward the ice lenses can make frost heave less severe.
Frost heave can seriously damage a basement if the ground surrounding that basement freezes to the foundation walls. When this happens, heaving soil around the house can carry the walls with it. This situation does not occur with heated basements, however. That’s because a heated basement (insulated or not) loses heat to the soil surrounding it. This outward heat loss pulls moisture away from the foundation walls. Because moisture is required for adfreezing, less moisture means the frozen soil has a less tenacious grip on the foundation.
Drawings: Christopher Mills. Inset drawing: courtesy of Acta Materialia.