Gravel road

A gravel road is a type of unpaved road surfaced with gravel that has been brought to the site from a quarry or stream bed. They are common in less-developed nations, and also in the rural areas of developed nations such as Canada and the United States. In New Zealand, and other Commonwealth countries, they may be known as metal roads. They may be referred to as "dirt roads" in common speech, but that term is used more for unimproved roads with no surface material added. If well constructed and maintained, a gravel road is an all-weather road.

Construction
Compared to sealed roads, which require large machinery to work and pour concrete or to lay and smooth a bitumen-based surface, gravel roads are easy and cheap to build. However, compared to dirt roads, all-weather gravel highways are quite expensive to build, as they require front loaders, dump trucks, graders, and roadrollers to provide a base course of compacted earth or other material, sometimes macadamised, covered with one or more different layers of gravel. Graders are used to "blade" the road's surface (pass frequently to mix and distribute the gravel) to produce a more extreme camber compared to a paved road to aid drainage, to produce an "A" shaped surface to the road called a "crown", as well as to construct drainage ditches and embankments in low-lying areas. Cellular confinement systems can be used to prevent the washboarding effect.

Construction of a gravel road begins with the base or subgrade layer. The expected road traffic volume and the average daily truck passage must be considered during the design process as they will influence the thickness of this layer, along with the balances of gravel and fines. Geotextile fabric may be laid to improve the stability of the subgrade layer. When geotextile fabric is used, a gravel layer with a minimum thickness of 6" (15 cm) is suggested to ensure the fabric remains unexposed. Road construction guidelines suggest that the crown in the road surface begins at the center point in the road, and does not exceed a 4% gradation from the center to the edge of the roadway.

The surface layer is constructed atop the subgrade layer. The amount of precipitation is taken into consideration for the selection of gravel size distribution. The surface layer will follow the crown established by the subgrade layer. Scarification of the subgrade layer prior to application of the surface gravel layer can be performed to increase the mixing and adherence between layers. Construction of the road surface is done gradually through multiple applications of layers of gravel, with compaction prior to the addition of the following layer. During reparation of a damaged road, ensuring that any washboarding, rutting, potholes, and erosion is adequately removed will minimize future need for reparation. Windrowing can be performed along the edges of roads in dry climates to allow easy access to gravel material for small repairs.

Materials
The gravel used consists of varying amount of crushed stone, sand, and fines. Fines are silt or clay particles smaller than .075 millimetres (0.0030 in), which can act as a binder. Crushed stone, also called road metal, is used because gravel with fractured faces will stay in place better than rounded river pebbles. A good gravel for a gravel road will have a higher percentage of fines than gravel used as a subbase for a paved road. This often causes problems if a gravel road is paved without adding sand and gravel sized stone to dilute the percentage of fines.

A gravel road is quite different from a 'gravel drive', popular as private driveways in the United Kingdom. This uses clean gravel consisting of uniform, rounded stones and small pebbles.
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