The Unified Soil Classification System (USCS), standardized as ASTM D2487, is the most widely used geotechnical soil classification framework in the United States and internationally. It assigns every soil a two-letter group symbol (e.g., SW, CL, GM) that encodes both the soil type and its engineering behavior. Geotechnical engineers use USCS classifications to quickly communicate soil properties, select appropriate foundation types, evaluate fill suitability, and compare laboratory test results against published engineering property databases.
The Unified Soil Classification System was developed by Arthur Casagrande in the 1940s for the U.S. Army Corps of Engineers and was later standardized as ASTM D2487. It divides soils into two primary groups: coarse-grained soils (gravel and sand, with less than 50% passing the No. 200 sieve) and fine-grained soils (silt and clay, with 50% or more passing the No. 200 sieve).
Coarse-grained soils are further classified by grading (well-graded vs. poorly-graded, using the uniformity coefficient Cu and coefficient of gradation Cc) and by fines content and plasticity. Fine-grained soils are classified using the Casagrande plasticity chart, which plots plasticity index (PI) versus liquid limit (LL) and uses the A-line boundary (PI = 0.73(LL - 20)) to separate clays (above the A-line) from silts (below the A-line).
Atterberg limits quantify the water content boundaries at which fine-grained soil transitions between states. The liquid limit (LL) is the water content at which soil transitions from plastic to liquid behavior; the plastic limit (PL) is the water content at which soil transitions from semi-solid to plastic. The plasticity index (PI = LL - PL) is the range of water content over which the soil is workable and plastic.
High PI values (above 20) indicate highly plastic clays with significant shrink-swell potential and poor engineering behavior. Low PI values (below 7) suggest silty or low-plasticity materials. The USCS A-line on the plasticity chart separates clayey materials (PI plots above the line) from silty materials (PI plots below the line), while the LL = 50 boundary separates low-plasticity (L) from high-plasticity (H) designations.
The 50% passing the No. 200 sieve (0.075 mm) is the fundamental dividing line in USCS. Soils with more than 50% fines are classified as fine-grained (M or C group) and require Atterberg limit testing. Soils with less than 50% fines are coarse-grained (G or S group) and are primarily classified by gradation.
Within coarse-grained soils, the 5% and 12% fines thresholds create three zones: less than 5% fines yields a clean gravel (GW/GP) or sand (SW/SP); 5 to 12% fines requires a dual symbol; more than 12% fines produces a gravel with fines (GM/GC) or sand with fines (SM/SC), where the fines character is determined by PI.
USCS classification predicts a soil's probable behavior across the key engineering properties: shear strength, compressibility, permeability, and frost susceptibility. GW and SW soils are excellent for foundations and fill — they have high strength, negligible compressibility, and high permeability. CH soils (fat clays) represent the worst-case engineering soils: very low permeability, very high compressibility, and moderate to high shrink-swell potential.
For foundation design, USCS classification feeds into presumptive bearing capacity tables, liquefaction susceptibility screening (fine sands SP/SM are most vulnerable), and earthwork compaction specifications. Many specifications (e.g., AASHTO subgrade classification, highway embankment specs) accept USCS group symbols as the basis for material acceptance criteria.
GW stands for "Well-graded Gravel." The G indicates the soil is gravel-dominated (less than 50% passing the No. 4 sieve and more than half of the coarse fraction retained on the No. 4). The W indicates the gravel is well-graded, meaning it has a uniformity coefficient (Cu) of 4 or greater and a coefficient of gradation (Cc) between 1 and 3. GW gravels have excellent engineering properties: high bearing capacity, negligible settlement, and high permeability. They are ideal for base courses, drainage layers, and foundation support.
USCS (ASTM D2487) and AASHTO M 145 serve similar purposes but use different frameworks. USCS divides soil into 15 groups based on grain size and plasticity, emphasizing geotechnical engineering behavior. AASHTO classifies soils into groups A-1 through A-7, primarily for highway subgrade evaluation. AASHTO places more emphasis on the effect on pavement performance, while USCS is preferred for foundation engineering and general geotechnical work. A CL soil in USCS most closely corresponds to A-6 or A-7-6 in AASHTO, but direct conversion is not always straightforward since the classification boundaries differ.
Both ML and MH are inorganic silts plotting below the A-line on the Casagrande plasticity chart. ML (Silt of Low Plasticity, or "lean silt") has a liquid limit below 50 and low plasticity index — typically less than 7. MH (Silt of High Plasticity, or "elastic silt") has a liquid limit of 50 or above. MH soils are particularly problematic: they are highly compressible, highly susceptible to frost heave, and can have significant swelling potential. They are often confused with clays because of their high LL, but their position below the A-line confirms the dominant mineralogy is silt-like (low plasticity relative to LL).
Per ASTM D2487, a fine-grained soil is classified as organic when the oven-dried liquid limit is less than 75% of the air-dried liquid limit: LL(oven) < 0.75 × LL(air-dry). This test (ASTM D4318, oven-dry method) detects organic matter because organic particles lose plasticity when dried. Organic soils with LL below 50 are classified OL; those with LL of 50 or above are classified OH. Visually, organic soils typically appear dark (gray to black), have an organic odor, and may contain visible plant fibers. They are unsuitable as structural fill or foundation support due to very high compressibility and secondary compression (creep).
USCS classification directly influences foundation type selection and sizing. Clean gravels and sands (GW, GP, SW, SP) support high bearing pressures (150-400+ kPa) and are suitable for spread footings at shallow depth. Silty sands (SM) and clayey sands (SC) require more conservative bearing values and attention to settlement. Lean clays (CL) typically support 75-150 kPa but require evaluation of consolidation settlement, especially for saturated conditions. Fat clays (CH) and elastic silts (MH) may require deep foundations, ground improvement, or mat foundations to manage both bearing capacity and long-term settlement. Organic soils (OL, OH, Pt) essentially preclude direct foundation support.