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.