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ASTM D2487: Classification of Soils for Engineering Purposes (Unified Soil Classification System)
The USCS group symbol and name — CL, SM, GP-GM, and so on — assigned from gradation and Atterberg results, which is how specifications, geotechnical reports, and fill acceptance criteria describe a soil.
| Designation | ASTM D2487 |
| Setting | Laboratory |
| Service line | Construction Materials Testing |
Testing is performed to the current edition of ASTM D2487 referenced by your project specification. This page is a plain-language overview, not a substitute for the published standard.
What ASTM D2487 does
ASTM D2487 is the Unified Soil Classification System. It is not a test; it is the set of rules that takes the results of a gradation and the Atterberg limits and assigns a two-letter group symbol and a group name: CL for lean clay, SM for silty sand, GW for well-graded gravel, CH for fat clay, and so on. That symbol is the shorthand every geotechnical report, boring log, and earthwork specification uses to describe a soil.
When a specification says fill shall be classified as GW, GP, SW, SP, GM, or SM, or requires a CL with a plasticity index under 20, this standard is the reference for what those letters mean.
How classification works, step by step
The fines content from ASTM D6913 or D1140 splits the soil into coarse-grained, less than 50 percent passing the No. 200, or fine-grained. Coarse-grained soils are further split by the gravel-to-sand ratio and by gradation coefficients into well-graded or poorly graded, with fines modifiers if the fines exceed 5 percent. Fine-grained soils are classified from the liquid limit and plasticity index by plotting them on the plasticity chart above or below the A-line, which separates clays from silts. Organic content and highly organic soils have their own categories.
What the classification means
The symbol carries engineering meaning. Well-graded coarse soils compact densely and drain; poorly graded ones do not lock up as well. Silty fines are moisture sensitive and pump under construction traffic. Fat clays, CH, are the expansive soils that heave slabs and foundations across the Gulf Coast and much of the South. A classification alone does not design a foundation, but it tells an engineer what to expect and it tells a contractor whether a borrow source is acceptable.
Common questions about ASTM D2487
What is the difference between D2487 and D2488?
D2488 is the visual-manual procedure used in the field and on boring logs to estimate the classification by hand. D2487 is the laboratory classification from measured test results. A report should say which was used.
Is USCS the same as AASHTO classification?
No. AASHTO M 145 is a separate system, A-1 through A-7, used by highway agencies. Many DOT specifications use AASHTO groups; building and geotechnical work generally uses USCS.
How we help with ASTM D2487 classification
Every gradation and Atterberg set our laboratory runs on a soil is reported with its USCS classification, so fill approval, subgrade evaluation, and geotechnical correlation can proceed from the same report.
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