Steep Asphalt Driveways on Duluth's Hillside

Duluth has been called the San Francisco of the Midwest, and the comparison is about topography. The city climbs roughly 820 feet from Sky Harbor Airport on Minnesota Point to Duluth International Airport on the ridge, and building on that slope gave the city many steep streets and, behind them, many steep driveways. Before World War II an incline railway even carried riders up the hill at 7th Avenue West.

A steep driveway is not a flat one tilted. Water moves faster and carries grit with it, the base often sits on rock or dense till, traction becomes a safety issue for months at a time, and sealer behaves differently on a grade. This guide takes those differences in order, using the city's own rules where they apply and NOAA's figures for the hill.

How steep is steep on the hillside

The city regulates only the street end. Duluth's driveway entrance detail, tied to Section 45-14 of the city code, caps the slope from the gutter up to the sidewalk at 10 percent for homes and 8 percent for commercial driveways, and it sets residential widths of 12 to 22 feet. Beyond the sidewalk the detail says nothing, so the grade of the private driveway is whatever the lot and its original builder made of it, whether that is on the Central Hillside, Observation Hill or the slopes above Lakeside.

Measuring is simple. Lay a straight board or a long level on the driveway, raise the downhill end until it reads level, and measure the gap: a 10-foot board that needs a 1-foot lift sits on a 10 percent grade. Take readings at the top, in the middle and just above the apron, because grade often changes along a hillside drive, and the transitions, where a steep run flattens at a garage or tips down toward the street, are where water, sand and cracking gather.

Grade changes the work itself. A crew has to plan how equipment will reach the top, where sealer or hot mix might run before it sets, and how to avoid walking back across fresh material. It also changes the maintenance priority list: on a steep drive, drainage and edge support come before appearance, because a handsome coat over a base that water is eroding will not last.

Where the water goes

Runoff is the main enemy of a sloped driveway. On June 19 and 20, 2012, about nine inches of rain fell on Duluth in 30 hours; streets turned into rapids, roads split apart and sinkholes opened across the city. Accounts of that flood tie the damage to the city's rocky sediments, hard soil and 44 streams and creeks. A driveway on the hillside is a small version of the same system: water arrives fast, has little chance to soak in, and looks for the weakest seam.

Good drainage on a grade has three parts. The surface should be shaped so water leaves the pavement along its length, through a slight cross-slope or crown, rather than running the full length and gathering speed. A trench drain or swale should intercept water before it reaches a garage slab or the apron. And the edges need support, since runoff that leaves the pavement will scour the shoulder and undercut the asphalt. The city's driveway detail adds a condition of its own: where drains are installed, they must be large enough that overflow does not damage property.

Runoff also carries a cost. Duluth's storm utility fee is based on impervious surface, a definition that names driveways, and the city's stormwater guidance asks residents to sweep road sand in spring and to wash cars on the lawn so soapy water soaks in instead of running down the driveway into the storm sewer. On a steep drive, everything left on the pavement ends up at the bottom, and then in a catch basin.

Rock, till and the base under a hillside drive

The hillside is built on the Duluth Complex, a gabbro intrusion from the Midcontinent Rift described as a sill about 16 kilometers thick, and the soils over rock like that are often thin. The USDA's Barto series, described for bedrock-controlled ground in northeastern Minnesota and typed in St. Louis County, reaches basalt, granite or gabbro at 20 to 51 centimeters, and the Mesaba series reaches fractured bedrock at 51 to 102 centimeters, both on slopes of 2 to 45 percent.

Thin soil over rock changes how a driveway base behaves. There is less room to dig out and replace poor material, and water that soaks into the base travels along the rock surface instead of draining down, so it can emerge at the bottom of the drive or stay trapped under the pavement to freeze. Where a driveway crosses from rock onto fill, the two support the pavement differently, and a crack tends to open along that line.

Upland lots can have the opposite problem: dense, reddish clay till. The Hibbing series of northeastern Minnesota has very slow permeability and perched saturation 1 to 2.5 feet down in spring and fall, the same seasons when Duluth's freeze-thaw days peak. A new base on ground like that needs to be thicker and drained to daylight, and the page on asphalt paving goes into how.

Traction, sand and sealer on a grade

Sealer on its own dries to a smooth film, and on a steep drive a smooth film is slippery when wet. The usual answer is sand. SealMaster calls for clean silica sand in the mix design and lists three benefits: slip resistance, faster drying and added durability. On a hillside, asking the contractor how much sand goes into each batch is as important as asking which sealer they use.

Thin coats matter more on a grade. A heavy coat can creep downhill before it sets, leaving the top of the drive starved and a thick, glossy band near the apron that stays soft longer and wears unevenly. Two thin passes, applied from the top of the drive down so no one walks back across wet sealer, avoid both problems. The coat, its timing and its cost are covered on the driveway sealcoating page.

Orientation affects drying. According to GemSeal, a coat that dries in a given time at 80 degrees can need twice that at 50, and humidity, cloud cover and a dew point within five degrees of the air temperature stretch it further. A north-facing drive shaded by the hill, or one close to the lake on a foggy morning, meets those conditions more often than a sunny, south-facing drive near the ridge, so it needs a longer dry spell before cars return.

Winter on a steep drive

The city's own winter approach sets the pattern. Duluth's published guidance ranges from 200 to 400 pounds of salt per two-lane mile when pavement is above 30 degrees to 1,200 to 1,500 pounds for dry snow below 15, with a half-salt, half-sand blend and plain sand in designated hazard areas. On a private hillside drive the same logic applies at a smaller scale: salt works in milder cold, sand gives grip when it is too cold for salt to do much, and both have to be cleaned up in spring.

Snow placement is regulated. It is illegal in Duluth to shovel snow into streets and alleys, and sidewalks have to be cleared, ice included, within 24 hours after a snowfall ends. On a steep lot, that means deciding before winter where the snow from a long driveway will be stacked on the property itself, preferably on the downhill side so its meltwater drains away from the pavement instead of across it.

Plows, blowers and shovels scrape edges and seams all winter. Marking the driveway edges before the first snow keeps blades off the unsupported shoulder, and setting snowblower skids slightly high protects a fresh coat. The spring routine that follows, sweeping sand off, checking the bottom of the drive for new cracks and filling them before the next fall, is laid out in the post on winter sand and salt.