Every geotechnical report, boring log, and specification sheet in modern construction relies on the Unified Soil Classification System (USCS) to describe earth materials in a way that engineers everywhere understand. Arthur Casagrande proposed the original form in 1942 for airfield construction work carried out by the Army Corps of Engineers during World War II. The U.S. Bureau of Reclamation cooperated on a revision in 1952, and the system now sits under ASTM Test Designation D-2487. A lab report marked CH or SM reads the same in every state. Engineers who want the method laid out in sequence can follow the USCS classification walkthrough published on build-construct.com before moving into grain size and plasticity.
The system draws its first boundary at one sieve. More than 50 percent of the soil retained on the No. 200 sieve, which has a 0.075 mm opening, places the material in the coarse-grained family. More than 50 percent passing the same sieve places it in the fine-grained family. Peat and similar highly organic materials form a third group marked Pt. Every later decision, from the group symbol to the test selected, follows from that split.
Two Broad Categories: Coarse-Grained and Fine-Grained Soils
The USCS sorts soils into two broad categories first. Coarse-grained soils retain more than half of their particles on the No. 200 sieve, so the engineer can see and sort most of the material by eye. Fine-grained soils let more than half of their particles pass the same screen, so the behavior is controlled by particles too small to see and shows up only in plasticity testing.
Four fractions place a sample in this framework:
- Fine fraction: the percent passing the No. 200 sieve (0.075 mm opening)
- Coarse fraction: the percent retained on the No. 200 sieve
- Gravel fraction: the percent retained on the No. 4 sieve (4.75 mm opening)
- Sand fraction: percent retained on No. 200 minus percent retained on No. 4
Where the 50 Percent Rule Applies
The 50 percent boundary is a hard rule. A sample with 51 percent retained is coarse-grained even when it feels clayey, and a sample with 51 percent passing is fine-grained even when sand is visible. The sieve sizes that mark the boundaries matter as much as the percentages:
| Sieve | Opening size | What it separates |
|---|---|---|
| 76.2 mm sieve | 3 in. | Upper limit of gravel |
| No. 4 sieve | 4.75 mm | Gravel from sand |
| No. 200 sieve | 0.075 mm | Sand from silt and clay |
Field crews who need a quick check without lab equipment can compare their results against the OSHA field methods for soil type, which translate the same categories into hand tests that work at the bottom of a trench.
Information Required Before Classification Begins
Classification under ASTM D-2487 needs five pieces of information, each answering a different question about the material:
- Percent of gravel: the fraction passing the 76.2 mm sieve and retained on the No. 4 sieve
- Percent of sand: the fraction passing the No. 4 sieve and retained on the No. 200 sieve
- Percent of silt and clay: the fraction finer than the No. 200 sieve
- Uniformity coefficient (Cu) and coefficient of gradation (Cc) from the grain size curve
- Liquid limit and plasticity index of the portion passing the No. 40 sieve
Missing any of the five leaves the classifier guessing. A gravel percentage alone cannot distinguish a well-graded base course from a gap-graded fill. Reference sites such as Dream Civil publish detailed USCS breakdowns with worked examples for students and field engineers.
The Role of Atterberg Limits
The liquid limit and the plasticity index come from Atterberg limit tests run on the fraction passing the No. 40 sieve. The liquid limit is the water content at which the soil starts to flow, and the plastic limit is the water content at which it crumbles instead of rolling into threads. The plasticity index is the arithmetic gap between the two numbers, and that pair of values decides whether the fine fraction earns an L or an H suffix.
Why the No. 40 Sieve Matters
The No. 40 sieve has a 0.425 mm opening. Particles retained on it are too large to control the response to water, so testing only the passing fraction keeps the Atterberg results representative.
Group Symbols and Dual Designations
Every USCS group symbol is built from two letters. The primary letter names the material: G for gravel, S for sand, M for silt, C for clay, O for organic soil, and Pt for peat. The secondary letter describes its quality: W for well graded, P for poorly graded, M for non-plastic fines, C for plastic fines, L for low plasticity, and H for high plasticity.
| Primary symbol | Meaning | Secondary symbol | Meaning |
|---|---|---|---|
| G | Gravel | W | Well graded |
| S | Sand | P | Poorly graded |
| M | Silt | M | Non-plastic fines |
| C | Clay | C | Plastic fines |
| O | Organic soil | L | Low plasticity |
| Pt | Peat | H | High plasticity |
The two letters combine into groups such as GW, SP, CL, and CH. Alternative systems organize the same data differently: the MIT soil classification system sorts soils into particle size categories, which helps clarify what each USCS letter stands for.
Reading the Coefficients
Two coefficients describe the grain size curve. Cu equals D60 divided by D10 and measures the spread of particle sizes. Cc equals D30 squared divided by the product of D60 and D10 and checks whether the middle of the curve is well represented. A well-graded material satisfies both criteria; a poorly graded material fails at least one.
When Dual Symbols Are Required
Gravels and sands carrying 5 to 12 percent fines get dual symbols: GW-GM, GW-GC, GP-GM, and GP-GC for gravels, and SW-SM, SW-SC, SP-SM, and SP-SC for sands. The first symbol reflects the gradation of the coarse fraction, while the second reflects the behavior of the fines. Neither description alone captures the material, so the classifier writes both.
Coarse-Grained Soils: Gravels and Sands
Coarse-grained soils split first on the No. 4 sieve. If 50 percent or more of the coarse fraction is retained on that 4.75 mm screen, the material is a gravel; otherwise it is a sand. The gradation and fines content then decide the group symbol:
- GW: well-graded gravel, little or no fines, Cu over 4, Cc 1 to 3
- GP: poorly graded gravel, fails a gradation condition
- GM: silty gravel, non-plastic fines
- GC: clayey gravel, plastic fines
- SW: well-graded sand, Cu over 6, Cc 1 to 3
- SP: poorly graded sand
- SM: silty sand, non-plastic fines
- SC: clayey sand, plastic fines
Well Graded versus Poorly Graded
A well-graded soil contains a broad range of particle sizes, so smaller grains fill the voids between larger ones and the material compacts to a dense mass. A poorly graded soil is dominated by a narrow band of sizes, leaving voids unfilled and densities lower. The thresholds differ by material: gravels need Cu above 4 to qualify as well graded, while sands need Cu above 6.
Coarse particles are not the only earth materials that need a classification home. Rock masses are graded with the geomechanics classification system, which scores joints, weathering, and strength instead of grain size, and that system is the one used when a foundation sits on bedrock rather than soil.
Fine-Grained Soils and the Plasticity Chart
Fine-grained soils cannot be sorted by eye. The particles that control their behavior are too small to see, so the plasticity chart does the sorting. Liquid limit runs along the horizontal axis, plasticity index along the vertical axis, and the A-line, drawn at PI equal to 0.73 times (LL minus 20), separates clay behavior from silt behavior.
- CL: lean clay, liquid limit below 50, above the A-line
- ML: silt, liquid limit below 50, below the A-line
- CH: fat clay, liquid limit of 50 or above, above the A-line
- MH: elastic silt, liquid limit of 50 or above, below the A-line
- OL: organic clay or silt, low plasticity
- OH: organic clay or silt, high plasticity
- Pt: peat and other highly organic soils
The letters L and H mark the liquid limit boundary, not the stiffness of the material. Engineers can cross-check their chart readings against the plasticity chart notes on aboutcivil.org before committing a borderline symbol to a boring log.
The 50 Percent Liquid Limit Boundary
The L or H suffix depends on whether the liquid limit is below or above 50. A lean clay at a liquid limit of 38 is CL; the same clay at 54 becomes CH. The plasticity index matters too, because the A-line decides whether the point falls in clay territory or silt territory.
Borderline Cases in the Hatched Zone
When the plasticity index falls between 4 and 7 and the point lands in the hatched area of the chart, the classifier switches to a dual symbol. GC-GM and SC-SM cover coarse-grained soils whose fines hover between plastic and non-plastic behavior, and CL-ML covers the fine-grained equivalent.
Field Applications: Testing, Texture, and Excavation Safety
The group symbol on the boring log drives test selection. Project economics usually dictate which test actually gets run, and the best test for the friction angle is the one that most closely matches the problem at hand. For a square footing, the triaxial test is the best choice, because its stress conditions resemble the loading pattern under the footing.
Field crews do not always wait for lab results. Hand tests approximate the group symbol on the spot: the ribbon test and the toughness test separate clays from silts, the shake test detects silt by how fast water bleeds to the surface, and dry strength gives a fast read on plasticity. The same family of procedures appears in agriculture and landscaping as the soil texture classification used to sort fine fractions by feel.
Classification Language on the Job Site
Once the group symbol is fixed, it travels onto the excavation plan. Contractors use the classification to select slope angles, shoring, and protective systems, because a fat clay responds to a 3 m trench very differently from a silty sand. The regulatory framework that turns those differences into enforceable requirements is spelled out in the excavation safety regulatory standards, which tie soil type directly to protective system design.
The USCS works because it forces a decision: one sieve split, two gradation coefficients, two plasticity numbers, and one group symbol. That discipline is why letters proposed in 1942 still appear on modern boring logs and mean the same material in every state.