Keeping a gravel road in shape: grading, washboarding and where dust control fits

Why gravel roads fall apart when they lose their fines

A gravel surface is stone, sand and fines. The fines are the smallest part, small enough to pass a No. 200 sieve and too small to see one by one, the USDA Forest Service guide says. They're also the glue. The same guide says fine particles, dust included, are what help hold an unpaved road's surface together.³

So that dust rolling up behind a truck is your fines leaving, and after more than 20 years treating Southern Oregon roads, that's how I read it. The FHWA gravel roads manual spells out what comes next.¹

"When the fines are lost from a gravel surface, the stone and sand-sized particles that remain will tend to remain loose on the surface, leading to some distresses like washboarding and reduced skid resistance. It will become very hard to maintain."

And it adds up. The same manual points to studies that put the loss as high as one ton of gravel per mile each year for every vehicle that uses the road daily. Rainfall, gravel quality and other things move that number up or down.¹

What causes washboarding

Washboarding (engineers also call it corrugation) is that row of ripples across the road that rattles your truck. The FHWA's 2013 handbook says the gravel that washboards is usually poorly graded or gap-graded. It's short on fines and plasticity, so the particles don't bind together. The handbook also says empty trucks and trailers running fast can break the crust and speed up washboarding and raveling.²

It gives a simple yardstick, the share of the surface gravel that passes a No. 200 sieve. Under about 10 percent, the road washboards and needs regular grading. Over about 20 percent, it's dusty when dry and can turn slippery when wet. For good performance, the handbook says "a fines content of between 12 and 15 percent is typically required."²

Crown and drainage keep water off the road

Water is what turns a soft spot into a pothole. The gravel roads manual is blunt about it.¹

"Make sure the road has a good crown in the driving surface. Also, make sure there is good shoulder drainage. Standing water anywhere in the roadway will cause the surface to soften and fail."

A crown just means the road sits a little higher in the middle than at the edges, so rain runs off to the sides and doesn't sit in the wheel tracks. The Forest Service guide talks about it as draining the road surface with crown and crossfall.³

It's also why no spray fixes a pothole. The FHWA handbook warns you not to trust anyone who says their product will prevent potholes, because potholes are a road shape and drainage problem that no chemical treatment can solve.² I'd tell you the same.

Grading reshapes the surface

Grading, or blading, means running a grader over the road to reshape the surface. The FHWA handbook says a road built of gravel that washboards or ravels can still serve slow, light traffic, as long as someone is ready to blade or drag it regularly. It also says to weigh what that upkeep costs against bringing in better gravel.²

For a private road to a few homes, it adds a low-tech option. "The homeowners may be willing to tow a simple tire drag themselves to smooth washboarding and raveled areas."²

Before a dust treatment, the Forest Service guide lists the road work to do first. Repair unstable surfacing, crown the road for drainage, take off poorly graded, boney surface material, and grade deep enough to get rid of ruts, potholes and erosion gullies.³ My prep guide walks through that in order.

Regravel or treat?

If the gravel itself is wrong, no binder makes up for it. The FHWA handbook says that on gravel with too little fines and clay, chemical treatments generally don't work, because it would take an uneconomic amount of product to fill the gaps between the stones. On coarse, gap-graded gravel they'll keep the dust down at first, but they won't stop the raveling.²

On cost, the handbook says "it is usually cheaper to use fines to fill voids … than to use a chemical."² So get the gravel right first, then treat it. Lignin is especially good on roads with more dirt and sand in them.

Where dust control fits

On a road with decent gravel, a dust treatment works alongside the maintenance. It doesn't replace it. The FHWA handbook puts it this way.²

"Chemical treatments can be effectively used on roads with these materials to minimize dust and limit fines loss, reduce the rate of gravel loss, and increase the intervals between grader maintenance."

The gravel roads manual says the same from the maintenance side. A surface that stays tightly bound and stable needs a lot less blading. Getting a road ready takes some extra blading and shaping, it says, but after that the overall need for blade work should drop a good deal.¹

There are two limits I want you to know about with lignin sulfonate, the one product we apply.

• It controls dust. It isn't a stabilizer. The FHWA handbook says lignin-type products work well against dust but don't add enough strength to count as soil stabilizers. It also says roads treated with them can still be maintained the usual way for gravel roads.²
• It's a once-a-year job. The manual notes that nearly every method of dust control needs treating again each year.¹ Ours is built to last the season.

Who grades your road

That depends on who maintains it. On a county-maintained gravel road in Jackson County, the county does. It may grade the road before a dust treatment under its program, and it keeps the right to grade or gravel later whenever it decides the road needs it. The county says that "will significantly diminish the effectiveness of the dust abatement treatment."⁴ See the Jackson County program.

On a private or shared road, the folks who use it arrange the work themselves. See county road or private road? and road associations and shared roads.

Not sure whether your road needs grading or gravel before it's treated? Call me and we'll talk your road through. More on how we work on shared roads is on our private road and road association page.