Soil Classification — IS System & USCS

1. Why Classify Soils?

Soil is one of the most variable engineering materials — two samples taken a metre apart on the same site can behave very differently. Classification provides a standardised shorthand that communicates the likely engineering behaviour of a soil from a small set of index tests. Knowing that a soil is CH immediately tells an experienced engineer to expect high compressibility, significant shrink-swell potential, low permeability, and potentially problematic foundation conditions — without needing a full suite of strength or consolidation tests.

The IS 1498:1970 system is used in India for all geotechnical reporting, site investigation logs, and design documents. Its group symbols appear in soil boring logs, laboratory test reports, and IS code tables for bearing capacity and settlement. For GATE CE, soil classification is tested both as a standalone topic (identify the group symbol) and as background knowledge needed to apply other geotechnical formulas correctly.

2. Grain Size Boundaries & Sieve Sizes

2.1 Particle Size Divisions (IS 1498)

2.2 Key Sieve Sizes for Classification

3. Gradation Coefficients

Two coefficients derived from the particle size distribution (PSD) curve determine whether a coarse-grained soil is well-graded or poorly-graded.

3.1 Well-Graded Criteria

3.2 Physical Interpretation

C_u = 1 means all particles are the same size (uniform). C_u = 10 means D₆₀ is ten times larger than D₁₀ — a wide range of sizes. C_c measures the shape of the gradation curve in the middle range; values between 1 and 3 indicate a smooth, continuous curve without gaps. A well-graded soil has a good interlocking of particle sizes, giving it higher density, lower void ratio, better shear strength, and lower permeability than a poorly-graded soil of the same material.

3.3 Effective Size and Hazen’s Formula

4. IS 1498 Classification System — Overview

4.1 Group Symbol Structure

Each two-letter group symbol consists of:

5. Coarse-Grained Soils (G and S Groups)

5.1 Gravel (G) vs Sand (S)

If more than half of the coarse fraction (particles > 75 μm) is retained on the 4.75 mm sieve → Gravel (G).
If more than half of the coarse fraction passes the 4.75 mm sieve → Sand (S).

5.2 Sub-Classification of Coarse Soils

5.3 Borderline (5 %–12 % Fines)

When fines content is between 5 % and 12 %, the soil plots in a borderline zone and may receive a dual symbol (e.g., GW-GM or SP-SC). The gradation coefficients are evaluated first; if well-graded criteria are met, the W symbol is used as the primary; the secondary symbol reflects the plasticity of the fines.

6. Fine-Grained Soils (M, C, O Groups)

Fine-grained soils are classified using the plasticity chart (LL vs PI plot). The A-line and the LL = 50 % boundary divide the chart into six zones.

6.1 Classification Table for Fine-Grained Soils

6.2 A-Line Equation (Casagrande)

7. Organic Soils and Peat

7.1 Identifying Organic Soils (OL and OH)

Organic matter in soil reduces the LL upon oven drying (organic matter dehydrates irreversibly at 105 °C). The organic criterion:

Organic soils typically have a dark colour (grey to black), a musty odour, and very high compressibility. They are generally unsuitable as foundation material without treatment.

7.2 Peat (Pt)

Peat consists primarily of decomposed organic matter (fibrous plant remains). It is identified visually by its dark colour, spongy texture, and organic odour. Peat has extremely high water content (often 300–1000 %), very high compressibility, and negligible shear strength. It is not classified further on the plasticity chart but is designated Pt and flagged as a problem soil requiring special treatment (excavation and replacement, preloading, deep foundations, etc.).

8. Step-by-Step Classification Procedure

1. Determine % passing 75 μm sieve (from grain size analysis):
   • If ≥ 50 % passing → fine-grained soil → go to Step 5
   • If < 50 % passing → coarse-grained soil → go to Step 2
2. Determine G vs S: of the coarse fraction (> 75 μm), check what fraction is retained on 4.75 mm:
   • ≥ 50 % of coarse fraction retained on 4.75 mm → Gravel (G)
   • < 50 % of coarse fraction retained on 4.75 mm → Sand (S)
3. Check % fines (% passing 75 μm):
   • < 5 % fines → check gradation coefficients C_u and C_c → classify as W or P
   • 5–12 % fines → borderline → dual symbol (e.g., GW-GM)
   • > 12 % fines → classify fines by Atterberg limits → suffix M or C
4. For coarse with > 12 % fines: plot fines on plasticity chart:
   • Above A-line & PI > 7 → suffix C (e.g., GC, SC)
   • Below A-line or PI < 4 → suffix M (e.g., GM, SM)
   • PI between 4 and 7 → dual symbol (e.g., GC-GM)
5. For fine-grained soils: test for organic (compare oven-dried LL to undried LL):
   • LL_dried/LL < 0.75 → organic → OL or OH
   • Otherwise → inorganic → use plasticity chart
6. Plot (LL, PI) on plasticity chart:
   • Above A-line, LL < 50 %, PI ≥ 7 → CL
   • Above A-line, LL ≥ 50 % → CH
   • Below A-line, LL < 50 % → ML
   • Below A-line, LL ≥ 50 % → MH
   • Above A-line, LL < 50 %, PI = 4–7 → CL-ML
7. Check for peat: if visually organic, fibrous, very high water content → Pt.

9. Worked Examples

Example 1 — Classify a Coarse-Grained Soil

Problem: Grain size analysis of a soil gives: % retained on 4.75 mm = 35 %, % passing 75 μm = 4 %. D₁₀ = 0.18 mm, D₃₀ = 0.42 mm, D₆₀ = 1.20 mm. Classify the soil.

Step 1: Coarse or Fine?

Step 2: G or S?

Step 3: Check Fines and Gradation

C_u criterion met but C_c not met → Poorly-graded sand: SP

Example 2 — Classify a Fine-Grained Soil

Problem: A soil has 68 % passing 75 μm. LL = 55 %, PL = 28 %. The soil has no unusual colour or odour. Classify using IS 1498.

Step 1: Coarse or Fine?

Step 2: Organic Check

No unusual colour or odour, and oven-drying criterion not triggered → proceed as inorganic.

Step 3: Plasticity Chart

Example 3 — GATE-Style: Classify and State Engineering Implications

Problem (GATE CE type): A soil sample has the following properties: % passing 75 μm = 72 %, LL = 44 %, PL = 30 %, natural w = 38 %. Classify the soil and determine its liquidity index and consistency state.

Classification

Liquidity Index

LI = 0.57 → soil is in the plastic state, on the softer side (closer to LL than PL). In the field this would be a soft to medium stiff silt.

Example 4 — Borderline Coarse Soil with Fines

Problem: A sand has 8 % fines. The Atterberg limits of the fines give LL = 35 % and PL = 20 %. Gradation: C_u = 7, C_c = 1.8. Classify the soil.

Solution

10. Common Mistakes

Mistake 1: Comparing the Coarse Fraction to Total Sample Instead of to the Coarse Fraction

What happens: To decide G vs S, some students check what percentage of the total sample is retained on 4.75 mm, rather than what percentage of the coarse fraction (particles > 75 μm) is retained. If the soil has significant fines, this error can incorrectly flip G to S or vice versa.

Fix: G vs S criterion uses the coarse fraction only. Coarse fraction = 100 % − % passing 75 μm. Then check: if ≥ 50 % of the coarse fraction is retained on 4.75 mm → Gravel.

Mistake 2: Using C_u ≥ 4 for Sand (Should Be ≥ 6)

What happens: The well-graded criterion for gravel requires C_u ≥ 4, but for sand it requires C_u ≥ 6. Students apply the gravel criterion to sand and incorrectly classify SP as SW.

Fix: Gravel (GW): C_u ≥ 4 and 1 ≤ C_c ≤ 3. Sand (SW): C_u ≥ 6 and 1 ≤ C_c ≤ 3.

Mistake 3: Classifying Soil Above A-Line with PI Between 4 and 7 as CL

What happens: When PI falls between 4 and 7 and LL < 50 %, the correct classification is the borderline symbol CL-ML, not simply CL. Ignoring the borderline zone is a common GATE trap.

Fix: CL requires PI ≥ 7. If PI = 4–7 and the point plots above or near the A-line with LL < 50 %, the symbol is CL-ML.

Mistake 4: Forgetting to Check the Organic Criterion for Fine-Grained Soils

What happens: A soil with LL = 40 % and PI = 8 % that plots below the A-line is classified as ML without checking whether it is organic. If the oven-dried LL is, say, 28 % (ratio = 28/40 = 0.70 < 0.75), it should be classified as OL.

Fix: For any fine-grained soil, always check the organic criterion (oven-dried LL / undried LL < 0.75) before finalising the symbol. In GATE problems this will be explicitly stated if the organic check is required.

Mistake 5: Treating IS 1498 and USCS as Different Systems

What happens: Students study the two separately and get confused when switching between them, applying different criteria for the same classification step.

Fix: IS 1498 and USCS use identical group symbols, the same A-line equation, and the same gradation criteria. The only differences are in the sieve sizes (IS vs ASTM) and some terminology. For GATE CE purposes, treat them as one and the same system.

11. Frequently Asked Questions

Q1. What does it mean for a soil to be “well-graded” — is it always better than poorly-graded?

A well-graded soil contains a wide range of particle sizes, which means smaller particles fill the voids between larger ones, resulting in a denser packing, lower void ratio, higher shear strength, and lower permeability compared to a poorly-graded (uniform) soil of the same material. For most engineering applications — subgrade, embankment fill, granular base course, and aggregate for concrete — well-graded soils are preferred because they compact more readily and achieve higher density. However, for drainage applications (filter layers, French drains, drainage blankets) a uniform (poorly-graded) gravel or sand is preferred precisely because its large uniform voids allow free water flow without clogging.

Q2. Why does the USCS use 4.75 mm as the gravel-sand boundary rather than 5 mm?

The 4.75 mm boundary corresponds to the No. 4 ASTM sieve (4.76 mm opening), which was the standard sieve available in American laboratories when Casagrande developed the system in 1942. IS 1498 adopted the same boundary (IS sieve 4.75 mm) to maintain compatibility. In practice, the difference between 4.75 mm and 5 mm is negligible for engineering purposes — no soil property changes discontinuously at this grain size. The boundary is a classification convenience, not a physical threshold.

Q3. Can a soil be classified as both GW and GM? What does a dual symbol mean?

A dual symbol (like GW-GM or SP-SC) is used when a coarse-grained soil has 5–12 % fines — the borderline zone where both gradation and the nature of fines influence behaviour. The first symbol reflects the gradation (W or P), and the second reflects the plasticity of the fines (M = silty, C = clayey). For example, GW-GM is a well-graded gravel with 8 % non-plastic silt fines. It is not that the soil is “both” — the dual symbol is a standard notation for soils in this intermediate range, and their properties lie between the two end members.

Q4. How does soil classification relate to foundation design decisions?

Soil classification gives a first-order prediction of engineering behaviour that guides the scope of further investigation and the preliminary design approach. GW and GP soils (clean gravels) — high bearing capacity, low compressibility, free-draining — are excellent foundation materials; shallow footings typically suffice. CH soils (high-plasticity clays) — high compressibility, potential for significant long-term settlement, sensitivity to moisture — require detailed consolidation and strength testing; deep foundations or ground improvement may be needed. ML soils below the water table may be susceptible to liquefaction under seismic loading; special seismic design provisions apply. Pt (peat) almost always necessitates either complete excavation and replacement or deep piles bypassing the organic layer. Classification is the starting point, not the end, of geotechnical investigation.