The Unified Soil Classification System (USCS) is the internationally recognised framework for identifying, describing, and classifying soils in geotechnical engineering. Standardised under ASTM D2487, it provides a common language — a set of two-letter group symbols and standardised group names — that allows any geotechnical engineer anywhere in the world to read a borehole log and immediately understand what soil type is present in each layer, without ambiguity or regional terminology confusion.
Every soil description on a professional borehole log ends with a USCS group symbol in brackets: (CL), (SP), (GM), (CH). Getting that symbol right in the field — before laboratory testing is available — requires understanding the classification logic, knowing how to apply the field identification procedures, and being able to use the Casagrande plasticity chart correctly. This guide covers all of it.
USCS overview — the classification logic #
The USCS divides all soils into two major categories based on the dominant particle size, determined by the proportion of material passing the No. 200 sieve (0.075 mm — the boundary between sand and silt/clay):
| Category | Criterion | Primary letter | Classification method |
|---|---|---|---|
| Coarse-grained | More than 50% retained on No. 200 sieve (0.075 mm) | G (gravel) or S (sand) | Gradation curve: Cu and Cc, plus fines content |
| Fine-grained | More than 50% passing No. 200 sieve (0.075 mm) | M (silt) or C (clay) | Atterberg limits: LL and PI plotted on Casagrande chart |
Two additional special categories exist outside this split:
- Highly organic soils (Pt): Peat and highly organic materials classified on visual and manual evidence alone — dark colour, fibrous texture, organic odour, low unit weight, very high compressibility.
- Organic silts and clays (OL, OH): Fine-grained soils with significant organic content, identified by comparison of oven-dried and air-dried liquid limits.
The full classification is always performed from the particle size distribution curve (sieve + hydrometer analysis) and Atterberg limits. In the field, where laboratory results are not yet available, engineers use the field identification procedures described later in this article to assign a provisional USCS symbol that is later confirmed or revised when laboratory data becomes available.
Coarse-grained soils — gravels and sands #
Coarse-grained soils are further divided into gravels and sands based on the relative proportion of gravel-size and sand-size particles in the coarse fraction (the material retained on the No. 200 sieve):
| Subdivision | Criterion | First letter |
|---|---|---|
| Gravel | More than 50% of the coarse fraction retained on No. 4 sieve (4.75 mm) | G |
| Sand | More than 50% of the coarse fraction passing No. 4 sieve (4.75 mm) | S |
The second letter of the group symbol describes the gradation characteristics (W or P) or the nature of the fines (M or C), depending on the fines content:
| Second letter | Meaning | When applied |
|---|---|---|
| W | Well-graded (wide range of particle sizes) | Fines content < 5%; gradation criteria met (see below) |
| P | Poorly-graded (uniform or gap-graded) | Fines content < 5%; gradation criteria not met |
| M | Silty fines (non-plastic or low-plasticity fines) | Fines content 5–12%; or > 12% with fines plotting below A-line on Casagrande chart |
| C | Clayey fines (plastic fines) | Fines content 5–12%; or > 12% with fines plotting above A-line on Casagrande chart |
When fines content falls between 5% and 12%, a dual symbol is used — for example, SP-SM — indicating the soil has characteristics of both groups. This boundary zone is common in silty sands and sandy silts encountered in alluvial and estuarine deposits.
Gradation criteria — well-graded vs poorly-graded #
For coarse-grained soils with less than 5% fines, the W or P designation is determined from two coefficients calculated from the particle size distribution curve:
Coefficient of Uniformity: Cu = D60 / D10
Coefficient of Curvature: Cc = D30² / (D10 × D60)
where D10, D30, and D60 are the particle diameters at which 10%, 30%, and 60% of the sample (by mass) is finer.
| Soil type | Well-graded criteria (both must be satisfied) | Poorly-graded if |
|---|---|---|
| Gravel (G) | Cu ≥ 4 AND 1 ≤ Cc ≤ 3 | Either criterion not satisfied → GP |
| Sand (S) | Cu ≥ 6 AND 1 ≤ Cc ≤ 3 | Either criterion not satisfied → SP |
A high Cu indicates a wide range of grain sizes — the hallmark of a well-graded soil. A Cc between 1 and 3 confirms the gradation curve is smooth and concave upward with no gaps or steps. A soil with Cu = 2 is uniformly graded (all particles nearly the same size) — a classic SP or GP. A soil with a gap in the gradation curve may have a high Cu but a Cc outside the 1–3 range — also classified as poorly graded.
These coefficients are calculated automatically by DartiGeo and Dartis Soil Lab once the particle size distribution data is entered from laboratory sieve and hydrometer testing. See the laboratory testing guide for the full grain size distribution test procedure.
Fine-grained soils — silts and clays #
Fine-grained soils cannot be classified by gradation — the particles are too small to measure by sieving, and the engineering behaviour of silts and clays is governed more by plasticity (the interaction of clay minerals with water) than by particle size alone. The USCS classifies fine-grained soils using Atterberg limits — specifically the Liquid Limit (LL) and Plasticity Index (PI = LL − PL).
The two key boundaries in fine-grained classification are:
| Boundary | Value | Splits |
|---|---|---|
| Liquid Limit = 50 | LL < 50 → low plasticity (L suffix); LL ≥ 50 → high plasticity (H suffix) | ML / CL from MH / CH |
| A-line on Casagrande chart | PI = 0.73 × (LL − 20); above A-line → clay (C); below A-line → silt (M) | CL / CH from ML / MH |
Combined, these two boundaries create four primary fine-grained USCS groups: ML, CL, MH, CH. Organic soils (OL, OH) are identified separately by comparison of oven-dry and air-dry LL, and plot as sub-categories below the A-line at the relevant LL.
The Casagrande plasticity chart #
The Casagrande plasticity chart is the definitive tool for fine-grained USCS classification. It is a scatter plot with Liquid Limit (LL) on the horizontal axis and Plasticity Index (PI) on the vertical axis. Each soil sample is plotted as a single point; the position of that point relative to the chart’s two key lines determines the USCS group.
The A-line #
A-line equation: PI = 0.73 × (LL − 20)
The A-line separates clays (above) from silts and organic soils (below). Clays plot above the A-line because the flat plate-like clay minerals generate high friction between particles, producing a high plasticity index relative to the liquid limit. Silts plot below the A-line because their rounder particles and different mineralogy produce lower cohesion and plasticity relative to their liquid limit.
The U-line #
U-line equation: PI = 0.9 × (LL − 8)
The U-line is the upper boundary of the chart — no naturally occurring soil should plot above it. Data points above the U-line indicate a laboratory testing error (usually in the liquid limit determination) and the test should be repeated. The U-line is a quality control check, not a classification boundary.
Classification zones on the Casagrande chart #
| Zone on chart | USCS symbol | Soil description |
|---|---|---|
| Below A-line, LL < 50 | ML | Silt of low plasticity; inorganic silt, very fine sand, rock flour |
| Above A-line, LL < 50, PI ≥ 7 | CL | Clay of low plasticity; lean clay; inorganic clay of low to medium plasticity |
| Above or below A-line, LL < 50, PI < 4 | CL-ML | Silty clay; boundary zone between CL and ML (4 ≤ PI ≤ 7 also sometimes dual-symbolled) |
| Below A-line, LL ≥ 50 | MH | Silt of high plasticity; elastic silt; micaceous or diatomaceous fine sandy or silty soil |
| Above A-line, LL ≥ 50 | CH | Clay of high plasticity; fat clay; inorganic clay of high plasticity |
| Below A-line, LL < 50, organic | OL | Organic silt or clay of low plasticity |
| Below A-line, LL ≥ 50, organic | OH | Organic clay or silt of high plasticity |
The CL-ML zone (PI between 4 and 7, plotting near the A-line) is a transition zone where the classification is uncertain from plasticity data alone. Additional consideration of grain size and field identification helps resolve the correct symbol in this zone.
Organic soils and peat #
Organic soils require special treatment in USCS classification because their LL is significantly reduced by oven drying — organic matter partially decomposes at 105°C, reducing the capacity of the soil to hold water. This provides a simple laboratory test to distinguish organic from inorganic fine-grained soils:
If LLoven-dried / LLair-dried < 0.75, the soil is classified as organic (OL or OH, depending on LL).
In the field, organic soils are typically identified by:
- Dark grey, brown, or black colour
- Organic or earthy odour — particularly when the sample is heated slightly
- Presence of visible plant matter, fibres, shells, or wood fragments
- Low unit weight and very high compressibility
- Slow rebound when squeezed
Peat (Pt) is classified separately from all other soils. It is identified visually and manually — no laboratory testing is required or meaningful. Peat is characterised by a predominantly organic texture (visible fibres, roots, recognisable plant material), very dark colour (dark brown to black), spongy texture, and an organic odour. Peat has extremely high compressibility and is almost always unsuitable as a bearing stratum for shallow foundations without ground improvement or displacement.
Complete USCS group symbol reference table #
The full set of USCS group symbols, covering all soil types encountered in geotechnical practice:
| Symbol | Group name | Typical characteristics |
|---|---|---|
| GW | Well-graded gravel | Cu ≥ 4 and 1 ≤ Cc ≤ 3; < 5% fines; good bearing, free draining |
| GP | Poorly-graded gravel | Cu or Cc criteria not met; < 5% fines; uniform or gap graded; free draining |
| GW-GM | Well-graded gravel with silt | Meets GW gradation criteria; 5–12% non-plastic fines |
| GW-GC | Well-graded gravel with clay | Meets GW gradation criteria; 5–12% plastic fines |
| GP-GM | Poorly-graded gravel with silt | Does not meet GW criteria; 5–12% non-plastic fines |
| GP-GC | Poorly-graded gravel with clay | Does not meet GW criteria; 5–12% plastic fines |
| GM | Silty gravel | > 12% non-plastic fines (below A-line); reduced bearing relative to GW/GP |
| GC | Clayey gravel | > 12% plastic fines (above A-line); cohesion added; lower permeability |
| GC-GM | Silty, clayey gravel | > 12% fines; PI between 4 and 7, plotting near A-line |
| SW | Well-graded sand | Cu ≥ 6 and 1 ≤ Cc ≤ 3; < 5% fines; good bearing, free draining |
| SP | Poorly-graded sand | Cu or Cc criteria not met; < 5% fines; uniform or gap graded |
| SW-SM | Well-graded sand with silt | Meets SW criteria; 5–12% non-plastic fines |
| SW-SC | Well-graded sand with clay | Meets SW criteria; 5–12% plastic fines |
| SP-SM | Poorly-graded sand with silt | Does not meet SW criteria; 5–12% non-plastic fines; common in alluvial deposits |
| SP-SC | Poorly-graded sand with clay | Does not meet SW criteria; 5–12% plastic fines |
| SM | Silty sand | > 12% non-plastic fines; lower bearing than SW; moderate permeability |
| SC | Clayey sand | > 12% plastic fines; cohesive, low permeability; susceptible to swelling |
| SC-SM | Silty, clayey sand | > 12% fines; PI between 4 and 7, plotting near A-line |
| ML | Silt, low plasticity | LL < 50; plots below A-line; low cohesion; susceptible to frost heave and liquefaction |
| CL | Clay, low plasticity | LL < 50; plots above A-line; lean clay; moderate compressibility |
| CL-ML | Silty clay | LL < 50; PI 4–7; near A-line; transition zone |
| MH | Silt, high plasticity | LL ≥ 50; plots below A-line; elastic; high compressibility; micaceous or volcanic |
| CH | Clay, high plasticity | LL ≥ 50; plots above A-line; fat clay; high compressibility; high shrink-swell |
| OL | Organic silt/clay, low plasticity | LL < 50; organic; plots below A-line after oven-dry LL comparison |
| OH | Organic clay/silt, high plasticity | LL ≥ 50; organic; typically dark, compressible |
| Pt | Peat and highly organic soils | Predominantly organic; fibrous; very dark; extremely compressible; classified visually |
Field identification without laboratory tests #
On the borehole, the geotechnical engineer must assign a provisional USCS symbol to each layer from visual and manual examination of the split-spoon sample — before laboratory grain size and Atterberg limits results are available. The following field procedures allow reliable provisional classification in most cases.
Field identification of coarse-grained soils #
| Observation | What it tells you |
|---|---|
| Visible grain size | Particles visible to the naked eye: sand (0.075–4.75 mm); particles larger than about 5 mm: gravel. The No. 4 sieve boundary (4.75 mm) is roughly the size of a pencil eraser tip. |
| Gravel vs sand decision | If more than half the coarse particles (by eye) appear larger than 4.75 mm → gravel (G); if most particles are smaller → sand (S). |
| Gradation (visual) | If particles span a wide range of sizes with larger gaps filled by smaller ones → likely well-graded (W). If nearly all particles are the same size → likely poorly-graded (P). |
| Fines content (manual) | Squeeze a moist sample in the palm. If it leaves a dirty residue or has a sticky feel → appreciable fines present (M or C suffix). Wash over hand: if water runs clear immediately → <5% fines (W or P); if slightly cloudy → 5–12%; if turbid → >12%. |
| Fines type (M vs C) | Roll a small thread of the moist fines fraction between fingers. If it threads to 3 mm diameter without crumbling → plastic fines (C suffix). If it crumbles at 3 mm or cannot be threaded → non-plastic fines (M suffix). |
Field identification of fine-grained soils #
| Field test | Procedure | What it tells you |
|---|---|---|
| Dry strength test | Mould a small pat of soil, allow to air dry, then attempt to crush it between thumb and fingers. | High dry strength (cannot crush) → high plasticity clay (CH or MH). Low or no dry strength (crumbles easily) → silt or low-plasticity material (ML or CL). |
| Dilatancy (shake) test | Form a pat of soil at or near the liquid limit. Place on the open palm and shake horizontally, then squeeze between fingers. | Rapid appearance and disappearance of a shiny wet surface → silt (ML) or fine sand. Slow or no reaction → clay (CL or CH). |
| Thread rolling test | Roll soil between palm and glass plate to a thread of 3 mm diameter. | If thread reaches 3 mm without crumbling → plastic (CL or CH). If it crumbles above 3 mm → low plasticity (ML or CL boundary). If non-plastic and cannot be threaded → silt (ML). |
| Ribbon test | Squeeze soil between thumb and forefinger to form a ribbon. Observe the length achieved before the ribbon breaks. | Ribbon > 25 mm → high plasticity (CH). Ribbon 12–25 mm → medium plasticity (CL). Ribbon < 12 mm or crumbles → low plasticity (ML or CL-ML). |
| Shine test | Cut a dry pat with a knife blade and observe the surface. | Shiny cut surface → clay (CL or CH). Dull, rough surface → silt (ML or MH). |
| Organic check | Observe colour and smell; heat a small sample with a lighter. | Dark colour + musty or earthy smell + smokes when heated → organic (OL or OH). If fibrous texture visible → Pt. |
Experienced geotechnical engineers can assign USCS symbols quickly and reliably using these tests. For detailed guidance on writing the full soil layer description using these observations, see How to describe soil layers in a borehole log.
USCS group name vs group symbol #
The USCS provides two levels of identification for each soil: the group symbol (the two-letter code, e.g. CL) and the group name (a written description, e.g. “lean clay with sand”). The group name is more informative for the design engineer reading the log because it translates the symbol into plain language and includes information about secondary constituents.
The group name is determined from the group symbol plus additional information about the proportions of secondary soil types. For example:
| Symbol | Example group names (depending on secondary components) |
|---|---|
| CL | “Lean clay”; “lean clay with sand”; “sandy lean clay”; “gravelly lean clay” |
| SW | “Well-graded sand”; “well-graded sand with gravel”; “well-graded sand with silt” |
| GM | “Silty gravel”; “silty gravel with sand”; “silty gravel with clay” |
| CH | “Fat clay”; “fat clay with sand”; “sandy fat clay” |
ASTM D2487 includes the complete decision tree for determining the correct group name from the symbol and secondary constituent fractions. DartiGeo and Dartis Soil Lab determine the group name automatically once grain size distribution and Atterberg limits data are entered — eliminating the need to work through the ASTM decision tree manually for each sample.
USCS vs AASHTO — which to use #
Two classification systems are in common use in geotechnical engineering. The USCS is the standard for most geotechnical applications worldwide; AASHTO is primarily used in highway and pavement engineering.
| Feature | USCS (ASTM D2487) | AASHTO (AASHTO M 145) |
|---|---|---|
| Primary use | Geotechnical engineering — foundations, embankments, slopes, earthworks | Highway subgrade assessment, pavement design |
| Output | Two-letter group symbol (GW, CL, etc.) + group name | Letter-number group (A-1 to A-7) + Group Index (GI) |
| Basis | Grain size + plasticity; reflects engineering behaviour broadly | Grain size + plasticity; reflects suitability as subgrade material specifically |
| Best soils | GW, GP (cleanest gravels and sands) | A-1-a (gravel or stone fragments, well-graded) |
| Worst soils | CH, Pt (fat clay, peat) — not ranked but understood as worst performers | A-7-6 (plastic clay) with high Group Index |
| Standards | International; referenced in Eurocode 7, ISO 14688 equivalent, and most national geotechnical standards | Standard in US and countries following AASHTO road design practice |
For borehole logging in geotechnical investigations, USCS is always the primary system and should appear on every borehole log. AASHTO classification may be added alongside USCS where the investigation includes highway or pavement subgrade work. Both USCS and AASHTO are determined automatically by DartiGeo’s laboratory module and by the free Dartis Classify tool (see below).
How USCS symbols appear on a borehole log #
On a formatted borehole log, the USCS classification appears in two places:
- At the end of the soil description for each layer: The group symbol is enclosed in brackets as the final element of the description. For example: “STIFF, grey, moist CLAY, fissured (CL)”. This is the universal standard position for the symbol in all borehole log formats.
- In the graphical soil column: The symbol pattern (hatching for clay, dots for sand, triangles for gravel) corresponds to the USCS group symbol assigned to that layer. DartiGeo assigns the correct ASTM D2487 graphical symbol pattern automatically when the soil type is selected for each layer.
Where laboratory data confirms or revises the field-assigned USCS symbol, the log description should be updated before the final report is issued. It is acceptable practice to note on the log whether the symbol is a field classification (F) or a laboratory-confirmed classification (L) — for example (CL-F) for field provisional and (CL-L) for laboratory confirmed.
For a detailed walkthrough of the complete soil layer description format including the USCS symbol, see How to describe soil layers in a borehole log. For the full borehole log format and column layout, see Borehole log format and required data fields (ASTM D5434).
How DartiGeo and Dartis Classify handle USCS classification #
Manual USCS classification — working through the ASTM D2487 decision tree for each sample, plotting each point on the Casagrande chart, and determining the correct group name — is time-consuming and prone to error when processing dozens of samples from multiple boreholes. DartisTech offers two tools that automate the classification process completely.
DartiGeo — integrated laboratory classification #
Within DartiGeo’s laboratory tests module, grain size distribution (sieve + hydrometer) and Atterberg limits data are entered for each sample. The software automatically calculates Cu and Cc from the gradation data, applies the ASTM D2487 classification logic and plots the sample on the Casagrande chart.
Dartis Classify — free USCS and AASHTO calculator #
For engineers who need a standalone classification tool without the full DartiGeo suite, Dartis Classify is a free desktop application that classifies soils by both USCS and AASHTO methods according to ASTM D2487 and exports results to Excel. Enter grain size distribution data and Atterberg limits for each sample, click Classify, and the group symbol, group name, and AASHTO group with Group Index are determined automatically. The tool supports batch entry of multiple samples and is designed for direct use in the office or field office alongside a borehole logging program.
Download Dartis Classify free →
For integrated borehole logging, laboratory classification, SPT processing, CPT interpretation, and foundation design in a single platform:
Download a free 14-day trial of DartiGeo →
Frequently asked questions #
What does CL mean in soil classification? #
CL is the USCS group symbol for a clay of low plasticity — also called “lean clay”. The C means the soil is classified as a clay (plastic fines dominating behaviour, plotting above the A-line on the Casagrande plasticity chart), and the L means its liquid limit is below 50% (low plasticity). CL is one of the most commonly encountered fine-grained USCS symbols in temperate-climate site investigations. It indicates moderate compressibility and moderate sensitivity to moisture changes — suitable as a bearing stratum for most light to medium structures when in a stiff to very stiff state, but requiring careful evaluation for settlement when soft or firm.
What is the difference between ML and CL in USCS? #
Both ML (silt of low plasticity) and CL (clay of low plasticity) have liquid limits below 50%. The difference is in their position on the Casagrande plasticity chart relative to the A-line: CL plots above the A-line (PI > 0.73 × (LL − 20)), indicating clay minerals dominate and the soil exhibits plastic behaviour over a relatively wide moisture range. ML plots below the A-line, indicating silt-like behaviour — lower plasticity relative to the liquid limit, less cohesion, quicker dilatancy response, and higher susceptibility to frost heave and liquefaction. In the field, the dry strength test and shake test (dilatancy) are the quickest way to distinguish ML from CL when lab data is not yet available.
Can a soil have more than one USCS symbol? #
Yes — dual symbols are used when a soil falls in a transition zone between two groups. They are used in two specific situations under ASTM D2487: for coarse-grained soils with fines content between 5% and 12% (e.g. SP-SM, GW-GC); and for fine-grained soils with PI between 4 and 7 plotting near the A-line (CL-ML). Dual symbols are not used arbitrarily to express uncertainty — they apply only at the specific transition boundaries defined by ASTM D2487. If a soil has ambiguous characteristics outside these zones, the geotechnical engineer uses professional judgement to assign the single most representative symbol and notes the uncertainty in the remarks.
What is the difference between USCS and the British Soil Classification System (BSCS)? #
The British Soil Classification System (BSCS), defined in BS 5930:2015, uses a similar logic to the USCS but with different plasticity chart boundaries (the A-line and plasticity class limits differ slightly), different sieve sizes, and different group symbol conventions. For example, BSCS uses a finer subdivision of plasticity — low (L, LL < 35), intermediate (I, 35–50), high (H, 50–70), very high (V, 70–90), extremely high (E, >90) — compared to the USCS two-way low/high split. Engineers working on projects following Eurocode 7 may encounter BSCS terminology; those following ASTM standards use USCS. DartiGeo is aligned with ASTM D2487 (USCS) and AASHTO M 145.
Does the USCS group symbol change if the soil is disturbed or remoulded? #
No — the USCS group symbol is a classification of soil type based on grain size distribution and Atterberg limits, which are intrinsic properties of the soil minerals. These properties do not change when the soil is disturbed, remoulded, or recompacted. What does change with disturbance is the soil state — its void ratio, consistency, moisture content, and strength. A CL that is very stiff in its undisturbed state becomes soft CL when remoulded; it remains CL in both states. The USCS symbol and the consistency or density descriptor on the borehole log are independent pieces of information that together describe both the soil type and its current condition.
Related articles #
- Borehole logging — complete guide
- What is a borehole log? Elements, format and standards
- How to describe soil layers in a borehole log
- Borehole log format and required data fields (ASTM D5434)
- Borehole log software — features to look for
- Geotechnical laboratory testing — complete guide
- Standard Penetration Test (SPT) — complete guide
- Foundation design — bearing capacity and settlement guide
