Construction season is underway, and road crews are out in force repairing frost heaves and potholes. Both of these driving hazards occur because ice is less dense than water (which is why ice cubes float), meaning water expands when it freezes. Frost is just another word for underground ice.
If you take a soil sample, you’ll find anywhere from 1% to 10% water content. But that’s not enough water to swell and damage a road or a building foundation when it freezes. The real problem occurs when there’s a source to replenish the water as it freezes, causing a buildup of ice. How might that happen?
Go deep enough underground and the soil isn’t affected by surface temperature swings, staying around 50 degrees Fahrenheit year round. Fine soils, like silts and clays, have pores that can draw water up from the water table. (This gravity-defying property is called capillary action, and it’s also how trees draw water from the ground up to their leaves) With a continuous supply of water freezing above the frost line, an ice-filled region forms underground in the shape of an oval or lens.
As an ice lens grows it displaces the soil around it, causing a frost heave. Then, in the spring, the ice melts and leaves a void. With nothing left to support it, any soil above collapses into the void… and there you have your pothole.
A frost heave (or pothole) can really mess up a work of construction, so engineers and builders design to avoid them. One way, of course, is to prevent the ground from freezing. If your house is heated, then you could rely on heat escaping through the floor to keep your foundation above freezing… but that means you’re wasting energy, and it leaves you forever paranoid about what happens if you lose your heat source. A better solution is to surround your soil with insulation – in the floor and around the perimeter – so it neither draws heat from your house, nor freezes.
Many buildings are built with a deep foundation wall extending below the depth of maximum frost penetration. As the map shows, that depth depends on your location. New York City and Long Island can expect less than 31 inches of frost, while the same state’s North Country designs for penetration up to 70 inches. (In Massachusetts most builders use 48 inches as a rule of thumb, though this depth is conservative for most of the state.) An alternative is to build a frost-protected shallow foundation extending out from the building on all sides.
Another important frost-protection technique is to prevent the capillary action that allows ice lenses to form. Coarse soils like sand and gravel have large voids between particles, too large to draw water. If your existing soil is finer, you can excavate and then fill below the structure with an appropriate soil.