Winter Sand, Road Salt and the Steep Driveway in Spring

What the city spreads, and why the rate changes

Duluth's approach to winter streets is published as a simple table on the DuluthStreams site. When the pavement is above 30 degrees, crews apply 200 to 400 pounds of salt per two-lane mile. As the pavement gets colder the rate climbs, reaching 1,200 to 1,500 pounds for dry snow below 15 degrees. The table also lists a blend of half salt and half sand, and plain sand in designated sand hazard areas when it is very cold. Salt melts; sand gives grip where melting is slow or impossible.

The city clears more than 530 miles of roadway, with published goals of 36 hours for main arterials after a snowfall ends, 48 hours to open residential streets to one lane, and 56 hours for alleys. On a hillside street, the sand and salt that go down in that process do not stay in the traffic lane. Plows push them toward the curb, tires track them, and meltwater carries the fines downhill, which is how some of that grit ends up on aprons and at the low ends of sloped driveways.

What the grit does to a sloped driveway

Sand sitting on top of sound pavement does little harm, and on a steep drive in January it is welcome. The trouble starts in the thaw. Meltwater running down the slope carries fine sand into every open crack and packs it into the seam where the asphalt meets the concrete apron. A crack full of sand stays damp longer, holds water to freeze on cold nights, and cannot be filled properly until the sand is blown out.

Grit also acts as an abrasive. Tires turning at the bottom of the drive grind sand into the surface, and a snowblower or shovel dragging sand across the pavement scuffs the top of the binder. Over a few winters that wear shows as a faded, rough band at the low end and along the tire paths. On a driveway sealed with sand in the mix, the coat's own sand is locked in place; loose road sand is a different thing and simply wears the surface down.

The timing makes it worse. NOAA's normals for the airport put 15.7 freeze-thaw days in March and 16.3 in April, the same weeks the snowbanks melt and send sand-laden water down every hillside driveway. By the time the last snow is gone, the cracks have had a month or more of water and grit working on them.

Where the salt goes

Salt dissolves and moves with the meltwater. Duluth's stormwater system collects it through 11,204 catch basins and 415 miles of pipe and delivers it to the city's 44 named streams, 16 of them designated trout streams. DuluthStreams gives the chronic chloride threshold for fish as 230 milligrams per liter over a tenth of an organism's life span, 100 to 200 days for brook trout, and a short-term maximum of 860. After one early-winter snowfall it recorded conductivity in Chester Creek jumping from a baseline near 300 to about 1,600 as salt runoff arrived.

For a homeowner, the practical point is restraint on private pavement. A steep driveway may need salt or grit on the worst days, but every extra scoop ends up in the same creeks. Sand, at least, can be swept up and bagged in spring; salt cannot be recovered once it melts. Using the least amount that makes the drive safe is kinder to the creeks and leaves less to clean up.

A spring cleanup order for a hillside driveway

Start at the top and work down, since everything will move downhill anyway. The order below keeps each step from undoing the last one, and it sets the driveway up for crack filling and, if it is due, a new coat of sealer later in the summer.

Once the surface is clean and dry, the marked cracks are ready for crack filling, and any soft spots need patching before sealer is considered. Sweeping road sand is on the city's own list of stormwater habits, alongside washing cars on the lawn rather than the driveway, so the cleanup helps the creeks as much as the pavement.

• Wait for a dry stretch after the last snowbanks have melted and the pavement has drained
• Sweep or blow loose sand from the top of the driveway toward the bottom, then bag it instead of leaving it in the gutter
• Clear the gutter and the catch basin in front of the property if sand has piled up there
• Blow sand and fines out of every crack and the joint along the concrete apron
• Walk the drive and mark new cracks, soft spots and places where water pooled
• Let the cracks dry fully before any filler goes in

Keeping traction without adding to the problem

A sealed driveway can carry some of its own traction. SealMaster's specification includes clean silica sand in the mix, for slip resistance, quicker drying and longer wear, and on a steep drive that sand is held in the coat rather than lying loose on top. It does not replace winter grit on icy days, but it makes a hillside driveway less slick in the wet shoulder seasons without anything extra spread on it.

For the icy days, use the least material that works: a narrow path of grit up the tire tracks and the walk rather than a blanket over the whole surface, and salt kept to what melts the ice. Anyone planning Duluth driveway sealing for next summer should add sand removal to the prep list and ask the contractor how much sand goes into each batch of sealer.

More from the blog

• Fitting a Seal Coat Into Duluth's Short Summer