Monday, February 10, 2014

Frost Heaves: Nature’s Speed Bumps

frost_heavesI’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 the Northeast, 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 forty percent of my time is spent dealing with suspension issues due to frost heaves and pot holes,” says one owner and operator of a local car repair shop. “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 ten 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  many stone walls. After the early settlers cleared almost all their 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. 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. 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, Vermont. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation:

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2 Responses

  1. Curt Austin says:

    Great article. I didn’t know that the Jack Frost/cryosuction phenomenon was involved.

    I’ve read that rocks move upwards through the action of frost by a rather simple fact: grains of sand and other small soil particles can find their way underneath a rock when soil is stirred by freeze-thaw cycles, but a big rock cannot similarly sneak under thousands of grains of sand in order to move downwards. There’s a way up for rocks, but not down.

    There’s another way to look at it: In cyclic phenomena, like freeze-thaw cycles, you look for something that is irreversible. Here, a grain of sand that falls down into a small pore in the soil left behind by melting ice cannot go back up during the next freeze. This amounts to a flow of sand downwards, so something else must go up: rocks, which are too big to be falling into pores.

    Gently shaking an old bag of sugar will bring the lumps to the top – same thing.

    But perhaps that’s not the whole explanation for rock gardens, maybe ice lenses are also involved. In any case, be careful about mentioning “cyclic phenomena” in casual conversation.

  2. Bev stellges says:

    Great article! However, why are they called frost “heaves”? Seems to me they should be called frost gullies or frost dips or frost Grand Canyons since most that I have driven over have sunk down not raised up (drive the Bloomingdale Bog Road, for example)!

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