What is the Unified Soil Classification System?

The Unified Soil Classification System (USCS), standardized as ASTM D2487, sorts a soil into one of fifteen groups using two laboratory results: the grain-size distribution and the Atterberg limits. Each group gets a two-letter symbol — the first letter names the dominant fraction (G gravel, S sand, M silt, C clay, O organic, Pt peat) and the second describes gradation or plasticity (W well graded, P poorly graded, M silty, C clayey, L low plasticity, H high plasticity).

How do you tell coarse-grained soil from fine-grained soil?

The No. 200 sieve (0.075 mm) is the dividing line. If more than 50 percent of the sample by weight is retained on the No. 200 sieve, the soil is coarse-grained and is classified as a gravel or a sand. If 50 percent or more passes the No. 200, the soil is fine-grained and is classified with the plasticity chart as a silt or a clay.

What are Cu and Cc in soil classification?

Cu is the coefficient of uniformity, D60/D10, and Cc is the coefficient of curvature, D30 squared divided by (D10 times D60), where D10, D30 and D60 are the grain sizes at 10, 30 and 60 percent passing. A gravel is well graded (GW) when Cu is at least 4 and Cc is between 1 and 3. A sand is well graded (SW) when Cu is at least 6 and Cc is between 1 and 3. Fail either test and the soil is poorly graded, GP or SP.

What is the A-line on the plasticity chart?

The A-line is PI = 0.73(LL − 20). Fine-grained soils plotting on or above the A-line are clays (CL or CH); those below it are silts (ML or MH). The vertical line at LL = 50 separates low plasticity (L) from high plasticity (H). The U-line, PI = 0.9(LL − 8), is an empirical upper bound — a point plotting above it usually means a testing error, not an exotic soil.

When do you use a dual symbol like SP-SM?

A coarse-grained soil with 5 to 12 percent fines gets a dual symbol: the gradation symbol first, the fines symbol second — GW-GM, GP-GC, SW-SC, SP-SM and so on. Below 5 percent fines only the gradation symbol applies; above 12 percent only the fines symbol applies. The other dual symbol, CL-ML, is a fine-grained soil with a liquid limit under 50 that plots above the A-line with a plasticity index between 4 and 7.

How does USCS compare to the AASHTO classification?

AASHTO M145 was written for subgrade rating and splits soils at 35 percent passing the No. 200 sieve rather than 50 percent, so a soil that USCS calls coarse-grained can land in an AASHTO silt-clay group. AASHTO ranks groups A-1 (best subgrade) through A-7 (worst) and adds a group index. USCS is the standard for geotechnical and dam engineering; AASHTO is standard for pavement subgrade. Cross-walks are approximate — run both if the design depends on it.

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Unified Soil Classification (USCS) — Reference

By Michael Flynn, PE · water resources & dam safety engineer

ASTM D2487 group symbols and names, the decision rules that produce them, and the AASHTO cross-walk. Everything below comes from two lab results: a sieve/hydrometer curve and the Atterberg limits. The symbol is not a soil property — it is a shorthand for a set of expected behaviors, listed at the bottom of the card.

The Two-Letter Code

First letter — dominant fraction	Second letter — modifier
G gravel · S sand · M silt · C clay · O organic · Pt peat	W well graded · P poorly graded · M silty fines · C clayey fines · L low plasticity (LL < 50) · H high plasticity (LL ≥ 50)

Decision Sequence

> 50% retained on No. 200 → coarse-grained → > 50% of coarse fraction retained on No. 4 → gravel, else sand
≥ 50% passing No. 200 → fine-grained → plasticity chart (LL vs. PI)

Sieve sizes that matter: No. 4 = 4.75 mm, No. 40 = 0.425 mm, No. 200 = 0.075 mm. Percent fines is percent passing the No. 200. Particles larger than 3 in. are excluded from the classification and reported separately as cobbles and boulders.

Coarse-Grained Soils (> 50% retained on No. 200)

Symbol	Group name	Criteria
GW	Well-graded gravel	< 5% fines; C_u ≥ 4 and 1 ≤ C_c ≤ 3
GP	Poorly graded gravel	< 5% fines; fails either C_u or C_c
GW-GM GW-GC	Well-graded gravel with silt / with clay	5–12% fines; meets GW gradation; fines plot below / above A-line
GP-GM GP-GC	Poorly graded gravel with silt / with clay	5–12% fines; fails GW gradation
GM	Silty gravel	> 12% fines; fines are ML or MH
GC	Clayey gravel	> 12% fines; fines are CL or CH
GC-GM	Silty, clayey gravel	> 12% fines; fines are CL-ML
SW	Well-graded sand	< 5% fines; C_u ≥ 6 and 1 ≤ C_c ≤ 3
SP	Poorly graded sand	< 5% fines; fails either C_u or C_c
SW-SM SW-SC	Well-graded sand with silt / with clay	5–12% fines; meets SW gradation
SP-SM SP-SC	Poorly graded sand with silt / with clay	5–12% fines; fails SW gradation
SM	Silty sand	> 12% fines; fines are ML or MH
SC	Clayey sand	> 12% fines; fines are CL or CH
SC-SM	Silty, clayey sand	> 12% fines; fines are CL-ML

C_u = D₆₀ / D₁₀ C_c = D₃₀² / (D₁₀ · D₆₀)

Fine-Grained Soils (≥ 50% passing No. 200)

Symbol	Group name	Criteria (on the −No. 40 fraction)
CL	Lean clay	LL < 50; PI > 7 and on or above A-line
ML	Silt	LL < 50; PI < 4 or below A-line
CL-ML	Silty clay	LL < 50; 4 ≤ PI ≤ 7 and on or above A-line
OL	Organic clay / organic silt	LL (oven-dried) / LL (not dried) < 0.75
CH	Fat clay	LL ≥ 50; on or above A-line
MH	Elastic silt	LL ≥ 50; below A-line
OH	Organic clay / organic silt	LL ≥ 50; oven-dried LL ratio < 0.75
Pt	Peat	Primarily organic matter, dark, organic odor — visual/manual

A-line: PI = 0.73 (LL − 20) U-line: PI = 0.9 (LL − 8)

Plot the point (LL, PI). Above the A-line is clay, below is silt; LL = 50 splits L from H. A point above the U-line is an upper-bound violation — re-run the limits before you believe it. PI = LL − PL.

Group-Name Modifiers

Condition	Add to the group name
Coarse-grained soil, 15% or more of the other coarse fraction	“with gravel” / “with sand”
Fine-grained soil, 15–29% retained on No. 200	“with sand” or “with gravel” (whichever is larger)
Fine-grained soil, ≥ 30% retained on No. 200	“sandy” or “gravelly” prefix
Any soil with cobbles or boulders present	“with cobbles” / “with boulders”

AASHTO M145 Cross-Walk (approximate)

AASHTO	Description	Subgrade rating	Usual USCS equivalents
A-1-a	Stone fragments, gravel	Excellent	GW, GP, GM
A-1-b	Coarse sand	Excellent	SW, SP, GM, SM
A-3	Fine sand, nonplastic	Excellent to good	SP
A-2-4 A-2-5	Silty gravel & sand	Excellent to good	GM, SM
A-2-6 A-2-7	Clayey gravel & sand	Good to fair	GC, SC
A-4	Silt, LL ≤ 40, PI ≤ 10	Fair to poor	ML, OL
A-5	Elastic silt, LL > 40, PI ≤ 10	Fair to poor	MH, OH
A-6	Clay, LL ≤ 40, PI > 10	Poor	CL
A-7-5 A-7-6	Clay, LL > 40, PI > 10	Poor	CH, OH

GI = (F − 35)[0.2 + 0.005(LL − 40)] + 0.01(F − 15)(PI − 10)

Group index, with F = percent passing No. 200. Report as a whole number in parentheses after the group, e.g. A-6(9); a negative result is reported as 0. AASHTO splits granular from silt-clay at 35% passing the No. 200 — not 50% — which is why the cross-walk above can only ever be approximate.

Typical Engineering Characteristics by Group

USCS	γ_d,max (pcf)	φ′ (deg)	Drainage	Value as embankment / fill
GW	125–135	33–40	Excellent	Very stable; shell and drainage zones
GP	115–125	32–38	Excellent	Reasonably stable; pervious shell
GM	120–135	30–35	Fair to poor	Reasonably stable; impervious core if well compacted
GC	115–130	28–35	Poor	Fairly stable; impervious core and blanket
SW	110–130	33–38	Excellent	Very stable; shell and filter material
SP	100–120	30–36	Excellent	Reasonably stable when dense; liquefaction-prone if loose and saturated
SM	110–125	29–35	Fair to poor	Fairly stable; erosion- and piping-sensitive — filter it
SC	105–125	27–34	Poor	Fairly stable; usable core material
ML	95–120	26–32	Poor	Poor stability; frost-susceptible, highly erodible
CL	95–120	22–30	Practically impervious	Good stability; standard core material
OL	80–100	20–28	Poor	Not suitable — strip it
MH	70–95	23–30	Poor	Poor stability; high shrink/swell and compressibility
CH	75–105	17–25	Practically impervious	Fair stability; expansive — watch slope movement
OH	65–100	15–25	Practically impervious	Not suitable
Pt	—	—	—	Not suitable for any structural use

γ_d,max is standard Proctor (ASTM D698) maximum dry density; φ′ is the effective friction angle of compacted fill. Ranges are representative for preliminary work — site-specific testing governs final design.

The symbol is a starting point, not a design value. Two soils that both classify SM can differ by three orders of magnitude in permeability depending on how the fines are distributed. Classify to communicate and to bracket expectations; test to design. For embankment dams in particular, the piping and internal-erosion behavior of a SM/ML fill is governed by the gradation curve shape, not by the group symbol.

Sources: ASTM D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System); ASTM D2488 (visual-manual). AASHTO M145 / ASTM D3282. Typical-property ranges after NAVFAC DM 7.01, Soil Mechanics, and USBR Design of Small Dams.

Have a sieve curve? Get D₁₀/D₃₀/D₆₀, C_u and C_c → · Foundation soil? Bearing capacity.

Related cheat sheets and tools

The group symbol drives most other lookups on this site: permeability on the hydraulic conductivity card, allowable bearing on the IBC 1806.2 card, unit weight on the unit weights card, and hydrologic soil group on the curve number card. Run the numbers with grain-size distribution, slope stability, or filter criteria. For full watershed and stormwater design, see HydroComplete.

Unified Soil Classification (USCS) — Reference

The Two-Letter Code

Decision Sequence

Coarse-Grained Soils (> 50% retained on No. 200)

Fine-Grained Soils (≥ 50% passing No. 200)

Group-Name Modifiers

AASHTO M145 Cross-Walk (approximate)

Typical Engineering Characteristics by Group

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