How do frost heaves form?

I’m driving to work too fast, late as usual, trying to make up for those last five minutes I spent puttering around my house when I should have gotten out the door. I lean on the accelerator a little and grab my trusty travel mug, lifting it to my lips just as my wheels hit a bumpy, rippled section of the pavement. I hit the brakes. The tires make painful washboard sounds, and coffee splashes out of my cup and all over the steering wheel.
Living in New England, you get used to the spilled coffee and car repair bills. It’s a fact of life here — come winter, the roads are going to get rough, and your struts and brakes (and wallet) are going to pay.
“I’d guess 40 percent of my time is spent dealing with suspension issues due to frost heaves and pot holes,” says Jeff Whynot, owner and operator of the Autobarn car repair shop in Hartland. “Bent wheels, ball joints, tire rods … the roads around here are not the greatest.” Snow, ice and freezing rain all contribute to poor road conditions, but frost heaves make winter driving like a video game. Dodge and weave a heave? Twenty points! Hit a heave? Lose 10 points and call a mechanic.
So what is exactly happening under the surface to make our roads look like a crumpled blanket? It’s a matter of physics and fluid dynamics. When the temperatures fluctuate during cold months, previously fallen snow or ice will melt and trickle down into the soil below. When that soil has good drainage properties (for example, when it contains a lot of sand) melting poses no problems. The water continues to trickle down, away from the road. However, when the soil has a denser structure (such as clay loam) it tends to capture water and hold it under the pavement.
When the temperature drops again and the water in the soil re-freezes, this sometimes promotes ice crystal formations appropriately called “Jack Frost” formations by soil scientists. These formations are just like the frost typically seen on your windowpane. For hundreds of years, Jack Frost has been the Western cultural personification of winter, snow and ice — he’s that mischievous sprite responsible for your car battery dying when the temperature hits zero. In the case of Jack Frost formations in soil, the actual structure of the ice crystals take up much more space than a solid block of ice. As the frozen water expands, it creates a separate ice layer between the soil below and the pavement, called an ice lens. The ice lens grows by freezing water fed by a process called cryosuction. Water creates a negative pressure in the soil as it freezes, sucking up moisture from deeper, warmer soil by capillary action. Eventually, pressure from the ice lens pushes up the road material. The repeated formation of ice lenses turns roads into “wavement.”
One reason why our roads are so susceptible to frost heaves is the way they are built. While there are proven ways of constructing roads to minimize heaving, these techniques and materials are often too costly. Many roads are thin sections built on top of whatever soils are present, which usually consist of silt and clay. The low-drainage properties of these soils, combined with our seasonal weather, make roads very vulnerable to frost heaving.
Roads aren’t the only victims. Frost heaves in garden soils can push up plants and expose their roots to wind and winter temperatures, damaging or even killing shrubs and perennials. Concrete foundations can also be affected, leading to cracks and structural damage.
Frost heaves are also responsible for the creation of one of our most iconic Yankee structures — the stone wall. After the early New England settlers cleared almost all of the original tree cover for farming, erosion denuded the once-rich topsoil. Repeated frost heaving pushed up a bumper crop of ancient rocks deposited by glaciers thousands of years ago. These “New England potatoes” were a nuisance in the fields, and with wood in short supply, farmers piled up the stones to make walls.
These seasonal phenomena are a fact of life living in this neck of the woods, and in true Yankee style, frost heaves have become part of our area’s unique character and identity. Jack Frost is everywhere this time of year, living up to his reputation as a real pain in the neck, and the wallet. But winter doesn’t last forever. Soon enough the frost heaves will sink back down and spring will be on its way. In the meantime, we’re at Jack’s mercy. His, and the mechanic’s, of course.
Leah Burdick is a freelance writer and gardener who lives in Hartland. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlandsmagazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: [email protected].

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