Gravel Driveway Calculator

A gravel driveway is a layered structure, not a single dumping of stone, and the calculation reflects that. A base course of coarse angular material, commonly number 3 or number 2 stone at 1.5 to 2 inches, is placed 4 to 6 inches deep to spread wheel loads and break capillary rise from the subgrade. A surface course of dense-graded material such as crusher run or number 411, which contains stone dust that binds when compacted, is placed 2 to 3 inches deep to form the running surface. A total finished depth of 6 to 8 inches is typical for passenger vehicles. Volume is area times total depth in feet divided by 27, and conversion to tons uses bulk density: crusher run about 1.40 tons per cubic yard, number 57 and pea gravel about 1.35, river rock near 1.50. Because gravel is billed by the ton and delivered in 10 to 20 ton loads, the tonnage figure drives both cost and truck count. Order 10 to 15 percent extra, since dense-graded material loses roughly that fraction of loose depth under compaction. The main failure mode is aggregate migrating down into a soft subgrade, which a woven geotextile separation fabric prevents at a cost of about 0.15 to 0.50 dollars per square foot. A crowned cross section of 2 to 4 percent and a maintained ditch line matter more to service life than extra stone depth.

Layered aggregate road building predates modern equipment. Pierre-Marie-Jerome Tresaguet, appointed inspector general of French roads in 1775, specified a graded structure of hand-set foundation stone topped with progressively smaller broken stone. Thomas Telford refined the heavy stone foundation in Britain in the early 1800s. John Loudon McAdam, surveying for the Bristol Turnpike Trust from 1816, argued that the foundation was unnecessary if the subgrade was well drained and the stone was small, angular, and uniformly graded, producing the much cheaper macadam surface from which gravel road practice descends. Systematic guidance for unpaved roads in the United States is far more recent. The Federal Highway Administration published the Gravel Roads Maintenance and Design Manual in 2000, prepared with the South Dakota Local Transportation Assistance Program, and issued a revised Gravel Roads Construction and Maintenance Guide in 2015; it remains the working reference for crown, ditching, and surface gradation on the roughly 1.4 million miles of unpaved road in the country. Aggregate gradation for base layers was standardized in ASTM D2940 and AASHTO M 147. Synthetic geotextiles entered highway use in the late 1950s and 1960s, and their adoption as a separation layer beneath aggregate substantially reduced the recurring stone losses that had defined gravel road maintenance for the previous century.