Driveway

A driveway is a made vehicle route, shaped so that it carries the weight of a vehicle without rutting and sheds its own rain. The middle is built higher than the edges, a shape called a crown, at a slope of about 5 to 7 percent, so water runs off sideways instead of gathering in the wheel tracks. Grade, meaning how steeply the route climbs, is held to about 6 percent on flat or rolling ground and eases where the land is steeper. A travelled width of about 4.3 m (14 ft) suits one-way traffic and 6 m (20 ft) two-way. Holding that crown is machine work, not a hand-tool job.

A compacted running surface sheds most of the rain that lands on it and largely stops water soaking in underneath, so the road turns rain into runoff. Getting it off before it gathers volume is part of the road, not an addition to it. Shallow wide side drains carry it along the edge, and short drains angled away from the road throw it off at intervals set by rainfall, slope and soil, onto ground that can absorb what it brings. Where the route crosses a watercourse it takes a pipe or a hardened crossing rather than a dip in the surface.

The characteristic failure is the road becoming the stream. Ruts deepen, water collects in them rather than crossing the surface, and the flow runs along the road and cuts a gully. Side drains cut too steep scour into gullies themselves, and repeated grading that pushes material to the edge builds a low berm that traps water on the road. Surfacing stone is the expensive decision: a layer 50 to 70 mm (2 to 3 in) thick can more than double what the road costs, and traffic throws it off, so it is topped up rather than laid once. Maintenance is a cycle: hold the crown, keep the drains and pipe inlets clear, refill ruts with soil taken from outside the road bed, and take resurfacing material from the side drains so they widen rather than deepen.