Soil classification
Also known as: soil classification systems
Soil classification is the process of grouping soils into standard categories based on grain size distribution and plasticity so engineers can predict how they will behave. The Unified Soil Classification System and the AASHTO system are the two most widely used schemes.
Soil classification turns laboratory index tests into a short symbol that communicates expected engineering behavior. Two inputs drive every system: grain size distribution, determined by sieve analysis for coarse particles and hydrometer analysis for fines, and plasticity, described by the Atterberg limits — the liquid limit, plastic limit, and the plasticity index that is their difference. Coarse-grained soils are governed by particle size and packing; fine-grained soils are governed by water content and mineralogy.
The Unified Soil Classification System (USCS) is the standard for most geotechnical work. It splits soils at the No. 200 sieve: soils with more than half the material retained are coarse-grained, either gravels (G) or sands (S), with a second letter for gradation or fines — W for well graded, P for poorly graded, M for silty, C for clayey. So GW is a well-graded gravel and SC a clayey sand. Gradation is quantified by the coefficient of uniformity Cu = D60/D10 and the coefficient of curvature Cc. Fine-grained soils are classified with the plasticity chart against the A-line: M for silt, C for clay, O for organic, followed by L or H for low or high plasticity, giving symbols such as CL and MH. Peat is Pt.
The AASHTO system serves highway subgrade evaluation instead, running from A-1 (granular, excellent support) through A-7 (clayey, poor support) with a group index that penalizes fines and plasticity. The two systems sometimes disagree because they were built for different questions — foundation behavior versus pavement support. Classification also feeds directly into weight-volume relationships, since void ratio, porosity, water content, and unit weight are what actually control settlement, shear strength, and permeability.
The FE Civil exam tests soil classification in its geotechnical section. Expect to read a plasticity chart, compute Cu and Cc from a gradation curve, assign a USCS symbol, and connect the classification to weight-volume calculations.
Key takeaways
- Soil classification is based on grain size distribution and Atterberg limits (plasticity).
- USCS divides soils at the No. 200 sieve into coarse-grained gravels and sands and fine-grained silts and clays.
- USCS symbols pair a soil type letter (G, S, M, C, O) with a modifier (W, P, L, H), as in GW, SC, CL, and MH.
- The plasticity chart's A-line separates clays from silts among fine-grained soils.
- AASHTO classification (A-1 through A-7 with a group index) is used for highway subgrade evaluation.