Gravel Driveway Maintenance in Montana

Why do gravel driveways fall apart?

Water is the main reason. The Gravel Roads Maintenance and Design Manual, prepared by the South Dakota Local Transportation Assistance Program for the Federal Highway Administration, calls standing water anywhere in the road's cross section one of the major reasons for distress and failure. It also says gravel surfaces rut more easily in wet weather, and that traffic displaces gravel from the surface toward the shoulder and ditch in dry weather.

So a gravel surface is never finished. A paved surface keeps its shape for a long time once built. Gravel needs its shape put back on a regular basis. That manual was written for public roads, and a driveway carries far less traffic, but the three things it asks for still apply: a crowned surface, a shoulder that slopes away from it, and somewhere for the water to go.

What is crown, and how much does a driveway need?

Crown is the raised center that sheds water to both sides, like a shallow roof. The manual says problems develop quickly without it: water collects on the surface and softens the crust, which leads to rutting, and traffic pounds small depressions into potholes. Its recommendation is at least 1/2 inch of crown per foot of cross slope, or approximately 4 percent, in a straight line from shoulder to centerline.

On a driveway 12 feet wide, that works out to about 3 inches of rise from each edge to the center. A rounded shape is a known fault: the manual says a parabolic crown leaves the middle too flat to drain, and potholes and ruts form there. It also warns against too much crown, which is uncomfortable to drive and adds risk on snow and ice. A drive on a side hill can instead slope one way across its width, as long as the water leaves toward the low side and not down the wheel tracks.

What kind of gravel belongs on the surface?

Not the same gravel that goes under a patio. The manual explains that good surface gravel is a blend of stone, sand and fines. Stone carries loads, sand fills the gaps, and plastic fines, usually natural clay, bind the mix so it forms a crust and sheds water. It says the stone should be fractured.

Base gravel and fill are made for the opposite job. The manual says base material has larger stone and few fines so that it drains, and it will not form a crust. Sandy fill drains well and stays loose and unstable on a road. Its sample specification allows gravel surfacing 4 to 15 percent passing a #200 sieve, with a plasticity index between 4 and 12, while base course can drop to a plasticity index of 0. When you order, say the material is for a driving surface and ask the pit what they sell for that. Clean, round drain rock on a drive will roll underfoot and never tighten up.

What causes washboard?

The manual lists three primary causes: driving habits, lack of moisture and poor quality gravel. Washboard shows up where drivers accelerate or brake hard, such as hills, curves and driveway entrances. In dry weather the crust loosens and the stone and sand float, then line up in the familiar ripple under traffic. It adds that graders are often blamed but seldom cause it.

On a driveway, you control more of this than a county does. Slowing down on the steep section helps. Better gravel helps most. Dragging a dry washboard only smooths it for a while; the manual says to rework the area when moisture is present, cutting at least an inch below the depressions, mixing, reshaping and compacting so the crust can form again.

How should potholes be fixed?

Filling a pothole with a shovel is fine for this week, but a pothole is usually a symptom. If the surface around it is flat or dished, water will gather there again, and the patch will pound out. A lasting repair loosens the surface to below the bottom of the holes, reshapes the crown and compacts it, adding gravel where material has been lost.

Work when the gravel is damp. The manual's advice on washboard applies here too: dry material can only be smoothed temporarily. In western Montana the workable window often comes after spring melt, once the surface has firmed but still holds moisture, and again after a soaking rain. Wait until a soft, frost-affected surface has firmed before bringing in heavy machines.

What are high shoulders, and why do they matter?

Look along the edge of your drive for a ridge of gravel or grass, higher than the driving surface. The manual calls this a secondary ditch. It blocks water from leaving the surface. On level ground the water sits and seeps in, softening the road. On a slope it runs down the edge, carrying gravel with it and eroding into the subgrade. The manual says it pays to work hard to keep these to a minimum.

High shoulders build from blading losses, loose gravel thrown to the edge by traffic, and, in northern regions, the buildup of winter sand. A plow pushing gravel to the edge can add to it. The recovered material is often good gravel that belongs back on the surface. Keep grass from building a curb, and pull the edge back onto the drive during reshaping. Our guide to snow plow damage prevention covers what plows can do to edges.

What about ditches and culverts?

Water needs a place to go once it leaves the crown. Where a drive has ditches, keep them shaped and clear, and check any culvert under the drive before spring melt, because the manual lists clogged culverts among the problems to watch for. A plugged culvert in spring can send water over or along the drive and wash out a section quickly.

If water reaches your drive from uphill, fix that first. Our guides to yard drainage at spring melt and erosion control on slopes cover moving water around a property, and a rock-lined swale can carry it past the drive, as explained in dry creek beds. Work that alters a stream or its banks is another matter: Montana DNRC says that needs a 310 permit from the local conservation district.

How do you control dust?

Dust is your fines blowing away. The manual points out that fine material lost as dust is the same material that binds the surface, so dust is a maintenance problem as well as a nuisance. Slower driving helps, and so does better gravel. Its dust chapter covers commercial chloride treatments, including calcium, magnesium and sodium chloride. If you consider a treatment, follow the product label and keep it away from wells, streams and plants you care about.

How wide should a Montana driveway be?

Wide enough for what must reach the house. The Firewise USA fact sheet from the National Fire Protection Association says driveways should be at least 12 feet wide with a vertical clearance of 15 feet for emergency vehicle access. Local fire districts can set their own requirements, and our guide to driveway access for fire trucks goes further. Overhanging limbs are often what fails the clearance test, and our tree services can lift them.

What should you do before winter?

Fall is the time to deal with soft spots and low areas, because anything left will hold water and freeze. A drive frozen with ruts in it is hard to plow without gouging. Mark edges and culverts with stakes before the first snow, as described in snow stakes and markers. Plowing technique has its own guide, plowing a gravel driveway without wrecking it, and long shared drives raise the questions in who is responsible for private road plowing.

What can HJ do for a gravel drive?

We plow gravel drives and private roads in winter through our residential snow removal and private road plowing services, and we handle grading and surface drainage around homes as part of our hardscaping and landscaping and design work. We are not a road builder, so new road construction, major reshaping with a motor grader, or engineered culverts belong with an excavation or road contractor. This guide is part of the Montana Landscaping, Lawn & Irrigation Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the site with you.

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