how to grade a gravel road

Gravel or dirt roads are common in South Africa and are an essential means of access for the farming, mining, conservation and forestry industries. And we’re not the only ones; roughly 80% of roads around the world are unpaved.

Gravel roads do take a fair amount of maintenance, especially in areas with heavy rainfall. Grading helps rejuvenate the road surface, eliminating problems such as corrugation and potholes.

The structure of a gravel road

The structure of a gravel road needs three basic elements in order to be suitable for ongoing maintenance. These are a crowned (canted with a sharp peak) driving surface, a shoulder that slopes away from the driving surface and a ditch on either side.

how to grade a gravel road

Source: Federal Highway Administration

This structure ensures proper drainage and helps prevent potholes caused by standing water. Gravel roads that don’t follow this basic construction, don’t perform well under the stress of constant traffic, especially heavy vehicles.

how to grade a gravel road

A badly graded road with potholes caused by poor drainage compared with a properly crowned road that drains well despite heavy rainfall.

Source: Federal Highway Administration

The basic principles of grading a gravel road

These are some of the things to be aware of when grading a gravel road. Following these general principles will help maintain the integrity of the road and avoid damage to the surface.

Moldboard angle

When grading a gravel road, the ideal horizontal angle for the moldboard is between 30 and 45 degrees. This gives you the best control over the loose aggregate and makes sure it doesn’t spill around the leading edge of the moldboard.

Moldboard pitch

The pitch of the moldboard is the angle that the blade edge touches the road surface at. If the pitch is too far back, the graded material won’t move along the blade to the discharge end. Too far forward, and the blade won’t scrape the road surface sufficiently.

Operating speed

The motor grader needs to be kept at a slow and steady pace no higher than 5-8 kph while grading. Excessive speed can lead to loping or bouncing which causes the blade to cut depressions and ridges in the road surface.

Crown

A crowned surface is necessary for drainage but if it’s too high, it can lead to unsafe driving conditions. An excessive slope will cause drivers to drift towards the shoulder. 1.27 cm of crown per 30 cm on the cross slope is recommended.

Shoulder

The shoulders need to be sloped away from the crowned surface of the road to help facilitate drainage into the ditches. The area is more sloped than the crown but not so much that a vehicle could drive on it in an emergency, such as regaining control if they drift. 

Ditches

The ditches on either side of the road need to be clear of debris to ensure proper drainage. The minimum recommended depth and width for these ditches is 30 cm. Areas with heavy water runoff will require larger ditches.

Potholes

In order to eliminate potholes, the road needs to be graded to the same depth as the pothole. Never fill a pothole in with loose aggregate, this won’t last under traffic wear and the pothole will quickly reform.

Reconditioned graders from KH Plant

A motor grader is a worthy investment for ongoing gravel road maintenance. Brand-new graders are expensive, however, and a lot of companies might not be able to shoulder the cost.

At KH Plant, we specialise in restoring Caterpillar 140G140H and 140K motor graders and components to as-new condition – so you can get the benefits of a new motor grader at a fraction of the cost of a new machine. 

Contact us online or call us on +27 83 274 4882 for more information.

Contact us for more information