Engineering Soil Classification
USCS group symbol and name and AASHTO group with group index, from gradation and Atterberg limits.
Classifies a soil sample by the two systems used in engineering reports: the Unified Soil Classification System (ASTM D2487) with its full group names and modifiers, and the AASHTO system (M 145) with the group index used for roads. It reads the gravel, sand and fines fractions, D10, D30, D60, Cu and Cc from the gradation, places the fines on the plasticity chart and states what the soil is good for.
Overview
Every borehole log, laboratory sheet and road design starts with the classification of each sample. Applying the flow charts of ASTM D2487 by hand is slow and error-prone, especially the dual symbols and the names with “with sand” or “gravelly”; the tool applies them exactly.
Organic soils are recognised from the oven-dried liquid limit, cobbles larger than 75 mm are excluded as the standard requires, and both figures are drawn ready for a report.
What you enter · What you get
What you enter
- Percent passing the standard sieves from 75 to 0.075 mm
- Liquid and plastic limits, or non-plastic
- Oven-dried liquid limit and peat (optional)
What you get
- USCS group symbol and group name
- AASHTO group and group index with subgrade rating
- Gravel, sand and fines; D10, D30, D60, Cu, Cc
- Gradation curve and plasticity chart
How it works in the field
Method and equations
USCS
Coarse-grained if less than 50 % passes 0.075 mm: gravel or sand by the larger coarse fraction; W or P by Cu (≥ 4 gravel, ≥ 6 sand) and 1 ≤ Cc ≤ 3; M or C by the plasticity of the fines; dual symbols for 5–12 % fines. Fine-grained otherwise, by the A-line, LL = 50 and the CL-ML band.
AASHTO
Groups A-1 to A-7 tested from left to right on the passing 2.0, 0.425 and 0.075 mm, LL and PI; A-7-5 or A-7-6 by PI against LL − 30.
Gradation
D10, D30 and D60 by logarithmic interpolation between sieves; fractions are referred to the material finer than 75 mm.
Accuracy and error limits
- The classification is exact for the numbers entered; its reliability is that of the tests. Liquid limit repeatability is about ±2–4 %, plastic limit ±2–3 %, and fines content ±1–2 %, so samples close to a boundary (A-line, LL = 50, 5 %, 12 % or 50 % fines) can change group.
- Interpolated D10 is uncertain when fewer than three sieves lie between 0.075 and 2 mm. Gypsiferous soils give misleading limits and gradation if dried at 105 °C; dry them at 60 °C.
- To reduce the error: wash the sample over the 0.075 mm sieve, use the full sieve stack, repeat the limits on borderline samples and report both symbols when a sample sits on a boundary.
Who uses it
Standards and references
- ASTM D2487-17(2025). Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System).
- AASHTO M 145-91 (2017). Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes.
- ASTM D4318-17. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils.
- Casagrande, A. (1948). Classification and identification of soils. Transactions of the ASCE, 113, 901–930.
- Holtz, R.D., Kovacs, W.D. & Sheahan, T.C. (2011). An Introduction to Geotechnical Engineering, 2nd ed. Pearson.
At a glance
- Category
- Engineering & geotechnics
- Runs on
- Phone, tablet and desktop browser
- Export
- Gradation curve and plasticity chart as PNG, JPG and PDF; data as CSV; full PDF report
- Access
- Free
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