International road maintenance handbook volume 2: Unpaved roads
"An unpaved road can be a track which has developed with use over a number of years or it can be a purpose built road that has a properly designed and engineered alignment, cross section and drainage. Earth roads are constructed using the natural soil found on the route. Gravel roads are surfaced with a layer of material which is stronger than the natural in situ soil. With proper maintenance, an unpaved road can provide a good level of service for traffic volumes up to several hundred vehicles per day. The two main purposes of maintenance are to repair or reduce the damage caused by the combination of the weather and traffic, and to provide a smooth passage for vehicles. The first objective of unpaved road maintenance is to keep the road in such a condition that it sheds water quickly. If the road does not shed water, the surface will become soft, and ruts and pot-holes will quickly appear. In this way earth roads can soon become impassable.