Soil Classification

Classification groups soils by behaviour. Learn grain-size analysis (sieve and hydrometer), uniformity and curvature coefficients, Atterberg limits (LL, PL, PI), the plasticity chart and A-line, and USCS / IS 1498 group symbols.

Before designing a foundation or road, engineers need a common language for soils: is this a well-graded gravel, a silty sand, or a highly plastic clay? Classification gives quick insight into strength, drainage, compressibility and workability.

Two simple laboratory tests — particle size distribution and Atterberg limits — classify most soils.

An analogy

Like sorting a bag of mixed beans by size and stickiness: size tells you about drainage and friction; stickiness (plasticity) tells you about clay behaviour.

Coarse vs fine soils

Gravel (> 4.75 mm), sand (0.075–4.75 mm), silt (0.002–0.075 mm), clay (< 0.002 mm). If more than half is coarser than 75 µm, the soil is coarse-grained (G or S); otherwise fine-grained (M, C, O). Peat (Pt) is organic.

Grading

Sieve analysis gives the particle size distribution curve. Well-graded soils (W) have a wide range of sizes that pack densely; poorly graded (P) or uniform soils have similar sizes. Fines content decides dual symbols like SM (silty sand) or SC (clayey sand).

Atterberg limits and plasticity chart

LL: water content where soil starts to flow; PL: where it crumbles when rolled to 3 mm. PI measures the plastic range. On the plasticity chart, soils above the A-line are clays (C), below are silts (M); LL < 35 low (L), 35–50 intermediate (I, in IS), > 50 high (H).

Using classification in design

Classification correlates with engineering behaviour: CH clays swell and shrink (expansive 'black cotton' soils), ML silts are frost-susceptible and erodible, GW/SW are excellent fill and drainage materials. Road subgrade design (CBR), embankment selection and foundation risk screening all start here.

Formulas

• Uniformity: C_u = D₆₀ / D₁₀
• Curvature: C_c = D₃₀² / (D₁₀ · D₆₀)
• Well graded: gravel: C_u > 4, sand: C_u > 6, and 1 ≤ C_c ≤ 3
• A-line: PI = 0.73 (LL − 20)
• Liquidity index: LI = (w − PL) / PI

Worked example: Grading of a sand

Given: D₁₀ = 0.1 mm, D₃₀ = 0.3 mm, D₆₀ = 0.8 mm; < 5 % fines

Find: C_u, C_c and symbol.

• C_u = 0.8/0.1 = 8 > 6
• C_c = 0.3²/(0.1 × 0.8) = 1.125 (1–3)
• Well-graded sand

Answer: SW

Worked example: Fine-grained soil

Given: LL = 45 %, PL = 25 %

Find: PI and classification.

• PI = 20
• A-line at LL 45: 0.73 × 25 = 18.25 → PI above A-line → clay
• LL 35–50 → intermediate plasticity

Answer: CI (IS 1498); CL in USCS (LL < 50)

Common mistakes

• Wrong: Classifying only by colour and feel. — Right: Use sieve and Atterberg tests; field descriptions must be confirmed.
• Wrong: Ignoring the fines fraction in sands. — Right: Over 12 % fines changes behaviour — SM/SC dual symbols.

Applications

• Site investigation: Borehole logs describe and classify every stratum.
• Earthworks: Material specifications for fills, sub-base and drainage layers.
• Problem soils: Identifying expansive, collapsible, organic and liquefiable soils early.

Key takeaways

• Coarse vs fine at 75 µm; G vs S at 4.75 mm.
• C_u and C_c decide well/poorly graded.
• Atterberg limits + A-line decide clay vs silt.
• Classification predicts engineering behaviour.

Going further

• AASHTO classification: Used for highway subgrades: A-1 (best) to A-7 (clays), with a group index.
• Activity: A = PI/(% clay) identifies swelling minerals like montmorillonite.
• Rock classification: RMR and Q-system rate rock masses for tunnels and slopes.