Natural stone Crush gravels

Colloquially, the term gravel is often used to describe a mixture of different size pieces of stone mixed with sand and possibly some clay.[1] The American construction industry distinguishes between gravel (a natural material) and crushed stone (produced artificially by mechanical crushing of rock.)[2][3][4]

The technical definition of gravel varies by region and by area of application. Many geologists define gravel simply as loose rounded rock particles over 2 mm (0.079 in) in diameter, without specifying an upper size limit.[5][6][7][8] Gravel is sometimes distinguished from rubble, which is loose rock particles in the same size range but angular in shape.[9] The Udden-Wentworth scale, widely used by geologists in the US, defines granular gravel as particles with a size from 2 to 4 mm (0.079 to 0.157 in) and pebble gravel as particles with a size from 4 to 64 mm (0.16 to 2.52 in). This corresponds to all particles with sizes between coarse sand and cobbles. the Soil Science Society of America define gravel as particles from 2 to 80 mm (0.079 to 3.150 in) in size, while the German scale (Atterburg) defines gravel as particles from 2 to 200 mm (0.079 to 7.874 in) in size. gravel as particles under 3 in (76 mm) in size that are retained by a number 4 mesh, which has a mesh spacing of 4.76 mm (0.187 in).[12][5] ISO 14688 for soil engineering grades gravels as fine, medium, and coarse with ranges 2 mm to 6.3 mm to 20 mm to 63 mm.[13]

The bulk density of gravel varies from 1,460 to 1,920 kg/m3 (2,460 to 3,240 lb/cu yd).[14] Natural gravel has a high hydraulic conductivity, sometimes reaching above 1 cm/s.[15]