Soil Classification USCS AASHTO Calculator

Soil classification converts two cheap index tests, grain size analysis and Atterberg limits, into a group symbol that predicts engineering behaviour. The Unified Soil Classification System splits soils at the No. 200 sieve, an opening of 75 micrometres. If more than half is retained the soil is coarse-grained, called gravel when most of the coarse fraction is held on the No. 4 sieve at 4.75 mm, otherwise sand. Fines content decides the second letter: below 5 percent the soil is clean and graded W or P, above 12 percent it takes M for silty or C for clayey, and the 5 to 12 percent band gets a dual symbol such as SW-SM. Fine-grained soils go on the Casagrande plasticity chart, where the A-line, PI = 0.73 times LL minus 20, separates clays above from silts below, and a liquid limit of 50 divides low from high plasticity. AASHTO answers a different question: how good is this material as a road subgrade. Soils rank A-1 to A-7, granular A-1 and A-3 best and plastic A-7 worst, and the Group Index adds a penalty: the fines term (F - 35) times 0.2 plus 0.005 times (LL - 40), plus 0.01 times (F - 15) times (PI - 10). An index of 0 means excellent subgrade support, values near 20 very poor. The chief caveats are that both systems ignore in-situ density, structure, cementation and organic content, that classification never substitutes for strength testing, and that borderline soils near the A-line or the 12 percent fines boundary shift group with ordinary test scatter.

The index tests underlying both systems come from Albert Atterberg, a Swedish agricultural chemist who around 1911 defined the liquid, plastic and shrinkage limits to describe how the consistency of a fine soil changes with water content. Karl Terzaghi and later Arthur Casagrande recognised their value for engineering, and Casagrande standardised the liquid limit apparatus, the brass cup and grooving tool still specified today, in the early 1930s at Harvard, replacing an operator-dependent procedure with a repeatable one. The road-oriented scheme came first. Charles Hogentogler and Karl Terzaghi proposed a subgrade classification for the US Bureau of Public Roads in 1929, which the Highway Research Board revised in 1945 into the A-1 to A-7 groups and the Group Index formula that AASHTO now publishes as M 145. During the Second World War Casagrande was asked by the US Army Corps of Engineers to devise a rapid system for assessing airfield subgrades, and he produced the Airfield Classification System, described publicly in his 1948 ASCE Transactions paper Classification and Identification of Soils. In 1952 the Corps of Engineers and the Bureau of Reclamation revised it jointly and renamed it the Unified Soil Classification System, extending it beyond airfields to dams, foundations and embankments. ASTM later codified the procedure as D2487 for classification and D2488 for visual-manual identification, and the two systems have run in parallel ever since, USCS for general geotechnical work and AASHTO for pavement design.