Soil Compaction Calculator

Compaction control compares the dry density achieved in the field against the maximum dry density obtained from a laboratory Proctor test on the same material. Dry density is the mass of solids divided by total volume, so water is removed from the numerator but not from the volume. Moisture content is computed as the mass of water divided by the mass of solids, referenced to dry mass, which is why values above 100 percent are possible in organic or highly plastic soils. Relative compaction is field dry density divided by the Proctor maximum, times 100. The default mold volume of 943.3 cubic centimetres corresponds to the 4-inch Proctor mold, one thirtieth of a cubic foot. Most specifications require 95 percent of standard Proctor for building pads and general fill, and 95 to 100 percent of modified Proctor for pavement subgrade, base course and airfield work. Placement moisture is normally restricted to within about 2 percentage points of optimum, because the compaction curve is steep on the wet side and soil placed several points wet cannot reach target density under any amount of roller effort. Two limits matter in practice. Relative compaction above roughly 102 percent usually signals that the field material is coarser or differently graded than the sample the Proctor curve was run on, calling for a fresh curve. And density says nothing directly about shear strength, permeability or swell potential, which is why CBR, plate load or triaxial testing often accompanies it.

Systematic control of earthwork density is credited to Ralph R. Proctor, an engineer with the Bureau of Waterworks and Supply of the City of Los Angeles, who was building earth dams in southern California in the early 1930s. In a series of articles published in Engineering News-Record in 1933 he described compacting soil in a standard mold at several moisture contents and plotting dry density against moisture, revealing the characteristic bell-shaped curve with a single peak at what he named the optimum moisture content. Before this, fill was judged by appearance, by counting roller passes, or by proof rolling, with no reproducible acceptance criterion. The procedure was standardised as the standard Proctor test, now ASTM D698 and AASHTO T 99, using a 5.5 pound rammer dropped 12 inches over three layers for a compactive effort near 12,400 foot-pounds per cubic foot. Heavier aircraft and the wartime airfield construction programme of the 1940s made that effort unrepresentative of the rollers then coming into use, so the U.S. Army Corps of Engineers developed the modified Proctor test, ASTM D1557 and AASHTO T 180, using a 10 pound rammer falling 18 inches over five layers for roughly 56,000 foot-pounds per cubic foot. Field verification evolved in parallel, from the sand cone method of ASTM D1556 to the nuclear density gauges covered by ASTM D6938.