When the Snow Melts and the Water Stays

March is the hardest month on the pavement

At the Des Moines airport weather station, March averages 15.5 days when the temperature drops below freezing and then climbs back above it, tied with December for the most of any month in NOAA's 1991-2020 normals. It also averages 5.2 inches of snow and 2.30 inches of total precipitation, and its normal low of 29.7 degrees sits just under the freezing line while the normal high reaches 49.0. March delivers water and freezing together, day after day, and asphalt has no defense against that combination except a tight surface and a dry base.

April keeps the pressure on. It adds 3.86 inches of precipitation and 4.9 more freeze-thaw days, and the median last spring freeze does not come until April 17; in one year out of ten a freeze still arrives after May 1. That stretch, from the first March thaw to the last hard frost, is when winter damage shows itself, and when puddles that were hidden under the snow reveal where a drive is failing.

Why the water has nowhere to go

Ground thaws from the top down. For a period each spring the upper layer of soil under a driveway is soft and wet while the layer beneath it is still frozen, so meltwater cannot drain downward. It collects in the thawed zone, right where the stone base and the asphalt need firm support, and Pavement Interactive notes that a thawing subgrade loses much of its bearing capacity. Loads that do no harm in July can crush a weak spot in March.

The local geology makes it worse. The Iowa Geological Survey describes the Des Moines Lobe, the glacier that halted at the site of the city about 14,000 years ago, as a young landscape with sluggish, inefficient drainage, where nearly all of Iowa's natural lakes formed and where low spots still hold dark soils that mark old ponds. USDA soil descriptions bear that out: the poorly drained Webster soils of the flats and the ponded Okoboji soils of closed depressions both hold water close to the surface in spring, and Okoboji was established as a soil series here in Polk County.

Snow adds a local twist. City rules bar pushing driveway snow into the street, so a winter of plowed and shoveled snow sits in piles beside each drive. Normal snowfall at the airport station is 35.3 inches, and by March those piles are melting straight onto the ground the pavement depends on.

Reading the puddles

Standing water is a diagnostic tool. Walk the drive within an hour after rain or a warm afternoon of melting, then again a few hours later, and note where water sits and how long it lasts. A film that is gone within the hour is normal. Water that stands for half a day marks a depression, and Pavement Interactive traces depressions to subgrade settlement where fill was not compacted well. Water that seeps up through cracks, or leaves a line of fine silt on the surface as it dries, says the base itself is saturated.

Where the water sits says as much as how long it stays. The list below pairs the most common puddle locations on a Des Moines drive with their usual causes, starting at the house and working out toward the street.

• At the garage slab: a settled joint, or a slab that sheds its own water onto the drive
• Along one edge: lawn that has grown up above the pavement and dams the runoff
• In the wheel paths: rutting or fatigue in a base that turns weak when wet
• At the apron by the street: a low spot at the curb collecting salty slush
• In the middle of a flat drive: a depression over fill that settled
• Below a downspout or sump outlet: water the lot is adding to itself

Ice is the same warning in cold weather

The city's own guidance on downspouts and sump pumps spells out what happens when discharge runs across pavement: algae in warm weather and ice in freezing weather, with the property owner responsible for redirecting it. The warning is written about public sidewalks and streets, but a driveway that wears a sheet of ice every March morning is showing the same problem on private asphalt. That ice sits on the surface by day and works into every crack overnight.

An ice sheet that forms in the same place after every thaw is worth tracing back to its source. Often the source can be seen from the drive itself: a downspout extension that has come loose, a sump line that empties onto the pavement, or a snow pile on the uphill side.

Fixes that cost almost nothing

Start with the water the lot adds to itself. Des Moines recommends pointing downspout and sump outlets toward the front or rear of the house, into areas that drain away from structures, and ending them at least 15 feet from the building and from any property line. Extensions and splash blocks that meet that standard are inexpensive, and they cut the load on a driveway base right away.

Move the snow. Piles on the uphill side of a drive, or pushed against its edge, drain across the pavement as they melt; piles on lawn that slopes away do not. While the thaw is under way, push standing water off the drive with a broom before evening so it does not refreeze in the cracks overnight. Where the lawn has grown above the asphalt edge, cutting it back lets sheet water leave the pavement instead of pooling along the border.

Repairs to line up for May

Once the ground has firmed up and the last freeze has passed, the puddle map becomes a work list. Cracks that carried water all winter need crack filling before the next freeze season. Depressions that held water for hours, soft spots and patches of alligator cracking need saw-cut repair with fresh stone base. Only after that does a coat of sealer make sense, and because normal overnight lows at the airport station do not stay above 50 degrees until May, driveway sealing in Des Moines is a mid-May to mid-September job.

The one thing not to do is seal over a drive that is still holding water. Sealer is a thin coating that follows the surface beneath it. It will not lift a depression or dry out a base, and a fresh coat over a wet, settling spot only marks where the next pothole will open.

More from the blog

• The First Coat on a New Suburban Driveway