October 29, 2026
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TL;DR
The Unified Soil Classification System (USCS, ASTM D2487) sorts soil into 15 basic group symbols, such as SW or CL, from the percent passing the No. 200 sieve, the gravel and sand split at the No. 4 sieve, and the liquid limit and plasticity index. AASHTO M 145 ranks soil from A-1 to A-7 as pavement subgrade.
Both run on the same lab data. Read the symbol, then read the spec: the geotechnical engineer decides what is acceptable as fill.
What soil classification tells a builder
Soil classification puts a soil into a group whose members behave alike, so a builder can predict how it will compact, drain and carry load before a truck is ordered.
FHWA's site characterization circular (GEC 5) states the premise: soils with similar classification can be expected to respond similarly to changes in load and environmental conditions.
Two systems dominate U.S. practice. Most boring logs for buildings and site work use the USCS, written as ASTM D2487. Highway agencies use AASHTO M 145 (also ASTM D3282), which grades soil as subgrade. Both classify only the material finer than 3 inches (75 mm), and both use the same two lab tests.
For an estimator, the symbol is a cost signal. A log full of SW and GW suggests a cut that can likely be reused as fill. A log full of CH or MH points to export, import or treatment, and to a hard look at the spec before bid day.
Types of soil: gravel, sand, silt & clay
Engineers sort mineral soil into four types: gravel, sand, silt and clay. Gravel and sand are split by sieve size. Silt and clay are told apart by how they behave with water.
Table: Particle sizes used in USCS logging (USBR)
| Type | Size range | Field clue |
| Gravel | Passes 3 in (75 mm), retained on No. 4 (4.75 mm) | Visible particles |
| Sand | Passes No. 4, retained on No. 200 (0.075 mm) | Gritty feel |
| Silt | Passes No. 200 | Little strength when dry |
| Clay | Passes No. 200 | Plastic when moist |
In the Bureau of Reclamation's field manual, the No. 10 (2.00 mm) and No. 40 (0.425 mm) sieves split sand into coarse, medium and fine sand, and the 3/4 in (19 mm) sieve splits gravel. Cobbles (3 to 12 in) and boulders are estimated by volume and left out of the symbol.
Because silt and clay both pass the No. 200 sieve, the USCS separates them by plasticity. A fine soil is clay when its plasticity index is above 4 and it plots on or above the A-line, and silt when it plots below. AASHTO puts the gravel and sand break at the No. 10 sieve instead, one reason the two systems do not map one to one.
Organic soils and peat sit outside the four types. Soil Connect's guide to the types of dirt in construction covers the trade names suppliers put on a ticket for these types of soil.
How sieve analysis sorts coarse soils
A sieve analysis shakes a dried sample through a stack of screens and weighs what each one holds. Percent passing at each size decides coarse or fine, gravel or sand, and how well the soil is graded.
Labs run it to ASTM D6913 or AASHTO T 88. Six sieves do most of the work:
Table: The sieves that matter for classification
| Sieve | Opening | What it decides |
| 3 in | 75 mm | Largest size classified |
| 3/4 in | 19 mm | Coarse vs fine gravel |
| No. 4 | 4.75 mm | Gravel vs sand (USCS) |
| No. 10 | 2.00 mm | Gravel vs sand (AASHTO) |
| No. 40 | 0.425 mm | Fraction tested for Atterberg limits |
| No. 200 | 0.075 mm | Percent fines |
Two numbers describe grading. Cu = D60/D10 and Cc = (D30)² / (D10 × D60), where D60 is the size that 60% of the sample is finer than. Gravel is well graded at Cu 4 or more with Cc from 1 to 3. Sand needs Cu 6 or more with the same Cc range. Miss either test and the soil is poorly graded.
GEC 5 notes that well-graded soils are generally easier to compact, stronger and less compressible, because small grains fill the voids between large ones. A uniform mason sand will not behave like a well-graded crusher run under a roller.
Screens stop at the No. 200 sieve. Finer particles are sized with a hydrometer when needed and classified by plasticity.
Atterberg limits & the plasticity chart
Atterberg limits are the water contents at which a fine soil turns from semisolid to plastic to liquid. Plotted on the plasticity chart, they sort silt from clay and low plasticity from high.
Labs run them to ASTM D4318 on the fraction passing the No. 40 sieve. Soil Connect's primer on the Atterberg limits explained covers what high values mean in the field.
Liquid limit, plastic limit & plasticity index
The liquid limit (LL) is the moisture content at which a groove cut in soil paste in a standard brass cup closes when the cup is dropped 25 times from 0.394 in. The plastic limit (PL) is the moisture content at which a thread rolled to 1/8 in starts to crumble. Subtract one from the other for the plasticity index: PI = LL - PL.
A wide PI is the clay signature, and it usually brings shrink and swell. A PI near zero means the soil goes from crumbly to soupy with little added water, typical of silt.
On the plasticity chart, LL runs across and PI runs up. Three lines carve it up:
- The A-line runs flat at PI 4 to LL 25.5, then follows PI = 0.73(LL-20). Clays plot on or above it, silts below.
- A vertical line at LL 50 splits low plasticity (L) from high (H).
- The U-line rises at LL 16 to PI 7, then follows PI = 0.9(LL-8). It marks the approximate upper limit for natural soils, and GEC 5 says results above it should be checked and probably repeated.
A hatched zone above the A-line with PI between 4 and 7 is CL-ML, silty clay. Soil with LL 45 and PI 25 sits above the A-line value of 18.3 and left of 50, so it is CL. Soil with LL 60 and PI 20 sits below the A-line value of 29.2 and right of 50, so it is MH.
If the liquid limit of an oven-dried sample is under 75% of its liquid limit before drying, the soil is organic and takes an O.
USCS soil classification step by step
USCS soil classification follows a short decision path: organic or not, coarse or fine, then grading or plasticity. Each step uses one number from the lab sheet.
- Screen for peat. Dark, fibrous, spongy soil with an organic odor is PT.
- Check the percent passing the No. 200 sieve. At 50% or more the soil is fine grained, below 50% coarse grained.
- For coarse soil, if more than half the coarse fraction stays on the No. 4 sieve, the first letter is G. Otherwise it is S.
- Check the fines. Under 5%, grade with Cu and Cc for W or P. Over 12%, plot the fines on the plasticity chart for M or C. From 5% to 12%, use a dual symbol such as SP-SM.
- For fine soil, LL under 50 gives L and 50 or more gives H. On or above the A-line gives C, below gives M, and a PI of 4 to 7 on or above it gives CL-ML.
- Add modifiers. A fine soil with 15% to 29% sand or gravel gets "with sand" or "with gravel", and 30% or more makes it "sandy" or "gravelly".
Table: The 15 basic USCS group symbols
| Symbol | Group name | Main test |
| GW | Well-graded gravel | Clean, passes grading test |
| GP | Poorly graded gravel | Clean, fails grading test |
| GM | Silty gravel | Over 12% silty fines |
| GC | Clayey gravel | Over 12% clayey fines |
| SW | Well-graded sand | Clean, passes grading test |
| SP | Poorly graded sand | Clean, fails grading test |
| SM | Silty sand | Over 12% silty fines |
| SC | Clayey sand | Over 12% clayey fines |
| ML | Silt | LL under 50, below A-line |
| CL | Lean clay | LL under 50, above A-line |
| OL | Organic silt or clay | LL under 50, organic |
| MH | Elastic silt | LL 50 or more, below A-line |
| CH | Fat clay | LL 50 or more, above A-line |
| OH | Organic clay or silt | LL 50 or more, organic |
| PT | Peat | Fibrous, organic odor |
Watch the mark between two symbols. A hyphen, as in SW-SM, is a dual symbol: the test data puts the soil on a defined boundary. A slash, as in CL/CH, is a borderline symbol, which the Bureau of Reclamation uses when a visual call could go either way.
The unified soil classification system is the one the Bureau of Reclamation logs to and most building boring logs use, while many state DOTs also log highway borings in AASHTO groups. The USCS reference on Wikipedia reproduces the criteria table, and Soil Connect's article on plasticity index & soil type shows how PI moves a soil between groups.
AASHTO soil classification for roads
AASHTO soil classification ranks soil as pavement subgrade, from A-1 (best) to A-7, plus A-8 for peat and muck. A group index in parentheses ranks soils within a group.
AASHTO's first split is 35% passing the No. 200 sieve. At 35% or less the soil is granular (A-1, A-3, A-2). Above 35% it is silt-clay (A-4 to A-7). So a sand with 40% fines is coarse grained in the USCS but silt-clay in AASHTO.
Table: AASHTO M 145 groups (Indiana DOT Geotechnical Manual, Table 5.4)
| Group | % passing No. 200 | LL | PI | Material |
| A-1-a | 15 max (No. 10: 50 max, No. 40: 30 max) | Not used | 6 max | Gravel, stone |
| A-1-b | 25 max (No. 40: 50 max) | Not used | 6 max | Coarse sand |
| A-3 | 10 max (No. 40: 51 min) | Not used | Nonplastic | Fine sand |
| A-2-4 | 35 max | 40 max | 10 max | Silty gravel, sand |
| A-2-5 | 35 max | 41 min | 10 max | Silty gravel, sand |
| A-2-6 | 35 max | 40 max | 11 min | Clayey gravel, sand |
| A-2-7 | 35 max | 41 min | 11 min | Clayey gravel, sand |
| A-4 | 36 min | 40 max | 10 max | Silty soil |
| A-5 | 36 min | 41 min | 10 max | Silty soil |
| A-6 | 36 min | 40 max | 11 min | Clayey soil |
| A-7-5 | 36 min | 41 min | 11 min, LL-30 max | Clayey soil |
| A-7-6 | 36 min | 41 min | Over LL-30 | Clayey soil |
The M 145 table rates the granular groups excellent to good as subgrade and the silt-clay groups fair to poor. A-7-5 has a PI of LL-30 or less, and A-7-6 a PI above LL-30.
Group index: GI = (F-35)[0.2 + 0.005(LL-40)] + 0.01(F-15)(PI-10), where F is the percent passing the No. 200 sieve. Round to a whole number, and use zero if negative. For A-2-6 and A-2-7, use only the PI term.
Take soil with 60% passing the No. 200 sieve, LL 35 and PI 15. Over 35% fines, an LL of 40 or less and a PI of 11 or more make it A-6. Its group index is (60-35)[0.2 + 0.005(35-40)] + 0.01(60-15)(15-10) = 4.375 + 2.25 = 6.625, which rounds to 7, so the AASHTO classification reads A-6(7).
FHWA's pavement geotechnics manual calls a group index of zero good subgrade and 20 or more poor. A-1-a, A-1-b, A-2-4, A-2-5 and A-3 always carry zero, per NYSDOT. As a rough crosswalk, GEC 5 pairs A-4 with ML, A-6 with CL and A-7-6 with CH, though no conversion fits every soil.
Reading a soil classification chart on a geotech report
The soil classification chart in a geotech report is the key to the boring logs. It defines each symbol the logs use, and the lab summary table shows the numbers behind them.
Read the report in three passes: the key sheet, then each boring top to bottom with its depths and groundwater, then the lab summary, where percent fines, LL, PI and moisture tie each symbol to a test. Four details change how much weight a symbol deserves:
- Lab or field. A lab symbol follows ASTM D2487. A visual call follows ASTM D2488, and the log should say so.
- Hyphen or slash. SC-SM is a measured boundary. SC/CL means the logger was unsure.
- Plasticity of the fines. An SC with a PI of 25 behaves very differently from an SC with a PI of 8.
- Which system. A highway report may show A-6(12) where a building report shows CL.
Then hold the logs against the earthwork spec, which typically limits fill by percent fines, LL, PI or allowed groups and sets compaction against ASTM D698 or D1557. If the cut falls outside those limits, the estimate should carry import, export or treatment. Soil Connect's article on geotech reports decoded covers the cost side.
When the job needs import, the spec goes with the order. On Soil Connect's Find Dirt service, a contractor fills out a form and a Soil Connect Dirt Specialist makes contact within 1 business day. Its Dirt Pros check that the dirt meets the project specifications.
Why classify
Soil types
Sieve analysis
Atterberg limits
USCS steps
AASHTO
Geotech reports
Soil classification FAQ
Frequently asked questions
Is the unified soil classification system the same as USCS?
Yes. USCS is the short form of Unified Soil Classification System. ASTM D2487 is its laboratory standard, and ASTM D2488 is the visual-manual version used in the field. The Bureau of Reclamation logs soil to its own USBR 5000 and 5005 procedures, which it describes as consistent with both ASTM standards, so the symbols mean the same thing across reports.
How is OSHA soil classification different?
OSHA soil classification is for trench safety, not material quality. Appendix A to 29 CFR 1926 Subpart P rates deposits as Stable Rock, Type A, B or C, in decreasing stability, to choose sloping, benching or shoring.
Type A is cohesive soil with an unconfined compressive strength of 1.5 tsf or more. Type C includes cohesive soil at 0.5 tsf or less, gravel and sand. Type B sits between. A competent person makes the call from visual and manual tests, and disturbed soil can never be Type A.
Which soil types make good structural fill?
Your project geotechnical engineer's spec governs, so check it before ordering. Well-graded granular soils such as GW and SW generally compact more easily, and many specs also accept some SM, SC or GC within set fines and PI limits. High-plasticity clays (CH), elastic silts (MH), organic soils and peat are commonly excluded. That spec also sets the compaction test, ASTM D698 or D1557.
Do you need a lab to classify soil?
Not for every sample. Field logs use the visual-manual method of ASTM D2488, and the Bureau of Reclamation treats lab tests as a check on that work rather than a must for each sample. A lab classification under ASTM D2487, with a sieve analysis and Atterberg limits, is the norm when the symbol drives acceptance, such as a fill spec. Visual results should be labeled as visual.
Fill specs & soil testing guides
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Soil classification guide: USCS & AASHTO explained for builders
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