Home → Learn → Soil Classification
IS 1498 · IS 2720 · Geotechnical

Soil Classification — IS 1498 & IS 2720 Complete Guide with GATE MCQs

⏱ 18 min read📅 June 2026✅ IS 1498 & IS 2720🎓 GATE relevant
Before designing any foundation, you must know what type of soil you are building on. Is it sand, clay, silt, or a mixture? Is it plastic or non-plastic? Well-graded or uniform? Soil classification answers these questions systematically. India uses the IS classification system (IS 1498), which is closely modelled on the Unified Soil Classification System (USCS). The tests required for classification are standardised in IS 2720. This guide covers grain-size analysis, Atterberg limits, the plasticity chart, and how to assign the correct IS soil group — with three examples and 10 GATE MCQs.

📋 Table of Contents

  1. Introduction
  2. Concept and Theory
  3. IS Code Background
  4. Key Formulas and Definitions
  5. Classification Tables
  6. Classification Procedure
  7. Worked Examples (3)
  8. GATE MCQs (10)
  9. Common Mistakes
  10. Revision Summary
  11. Related Articles

1. Introduction

Soil is not a manufactured material with controlled properties — it varies enormously from site to site and even within a single borehole. Classification provides a common language: when a geotechnical report says the soil is "CI" (clay of intermediate plasticity), every engineer instantly knows its approximate behaviour — moderate compressibility, low permeability, moderate shear strength. Without classification, every soil description would be subjective and unreliable.

IS 1498:1970 classifies soils into groups based on two primary criteria: grain-size distribution (is it coarse-grained or fine-grained?) and plasticity characteristics (how does the fine fraction behave when wet?). IS 2720 standardises the laboratory tests needed to measure these properties.

2. Concept and Theory

The two-step classification logic

Step one separates soils into coarse-grained (more than 50% retained on the 75μm sieve — gravel and sand) and fine-grained (more than 50% passing the 75μm sieve — silt and clay). Step two further subdivides each group: coarse-grained soils are classified by their grading (well-graded vs poorly graded) and fines content; fine-grained soils are classified by their position on the Casagrande plasticity chart (plotting liquid limit vs plasticity index).

What is the plasticity chart?

The Casagrande plasticity chart has liquid limit (LL) on the x-axis and plasticity index (PI = LL − PL) on the y-axis. A sloping line called the A-line (PI = 0.73 × (LL − 20)) divides clays (above) from silts (below). A vertical line at LL = 50% divides low plasticity (L) from high plasticity (H). This gives four main fine-grained groups: CL (clay-low), CH (clay-high), ML (silt-low), MH (silt-high). There is also CI (clay-intermediate) and MI (silt-intermediate) when LL is between 35–50% in the IS system.

Why grading matters for coarse-grained soils

A well-graded soil has a good mix of particle sizes — large grains fill the skeleton, medium grains fill the gaps, and small grains fill the remaining voids. This gives higher density, better shear strength, and lower compressibility. A poorly graded (uniform) soil has particles of similar size — lots of voids, lower density, and weaker. The uniformity coefficient Cu and coefficient of curvature Cc quantify grading quality.

3. IS Code Background

Code/PartSubjectWhat it covers
IS 1498:1970Soil classificationThe master classification system: group symbols, naming, plasticity chart, grading criteria for coarse soils.
IS 2720 Part 4Grain-size analysisSieve analysis for coarse fraction, hydrometer for fine fraction.
IS 2720 Part 5Liquid limit & plastic limitCasagrande cup method for LL, thread-rolling method for PL.
IS 2720 Part 6Shrinkage limitMercury displacement method. SL used for volume change assessment.

4. Key Formulas and Definitions

Atterberg Limits
Liquid Limit (LL): water content at which soil transitions from plastic to liquid state (25 blows in Casagrande cup)
Plastic Limit (PL): water content at which soil crumbles when rolled into 3mm thread
Shrinkage Limit (SL): water content below which no further volume change occurs
Plasticity Index: PI = LL − PL
Liquidity Index: LI = (w − PL) / PI (current state relative to limits)
Grading Coefficients
Uniformity Coefficient: Cu = D60 / D10
Coefficient of Curvature: Cc = (D30)² / (D60 × D10)

D10, D30, D60 = particle diameters at 10%, 30%, 60% passing
Well-graded sand (SW): Cu > 6 AND Cc = 1–3
Well-graded gravel (GW): Cu > 4 AND Cc = 1–3
A-Line Equation (Plasticity Chart)
PI = 0.73 × (LL − 20)
If data point plots ABOVE A-line → Clay (C)
If data point plots BELOW A-line → Silt (M)

5. Classification Tables

IS Soil Group Symbols

SymbolDescriptionKey Property
GWWell-graded gravelCu > 4, Cc = 1–3, fines < 5%
GPPoorly graded gravelFails GW criteria, fines < 5%
GMSilty gravelFines 5–12%, plots below A-line
GCClayey gravelFines 5–12%, plots above A-line
SWWell-graded sandCu > 6, Cc = 1–3, fines < 5%
SPPoorly graded sandFails SW criteria, fines < 5%
SMSilty sandFines > 12%, plots below A-line
SCClayey sandFines > 12%, plots above A-line
CLClay — low plasticityLL < 35%, above A-line
CIClay — intermediate plasticityLL = 35–50%, above A-line
CHClay — high plasticityLL > 50%, above A-line
MLSilt — low plasticityLL < 35%, below A-line
MISilt — intermediate plasticityLL = 35–50%, below A-line
MHSilt — high plasticityLL > 50%, below A-line
OL, OI, OHOrganic soilsOrganic matter present, dark colour, organic odour
PtPeatHighly organic, fibrous, very compressible

Grain Size Boundaries (IS 1498)

FractionSize Range
Gravel4.75mm – 80mm
Sand (coarse)2.0mm – 4.75mm
Sand (medium)0.425mm – 2.0mm
Sand (fine)0.075mm – 0.425mm
Silt0.002mm – 0.075mm
Clay< 0.002mm

6. Step-by-Step Classification Procedure

  1. Perform sieve analysis (IS 2720 Part 4). Find % passing 75μm sieve.
  2. If > 50% retained on 75μm sieve → coarse-grained. Go to step 3.
  3. Coarse-grained: Is more retained on 4.75mm (gravel) or between 4.75mm and 75μm (sand)?
  4. Check fines: < 5% → check Cu and Cc for GW/GP or SW/SP. 5–12% → dual symbol. > 12% → check plasticity of fines for GM/GC or SM/SC.
  5. If > 50% passes 75μm sieve → fine-grained. Perform Atterberg limits.
  6. Plot on plasticity chart: LL on x-axis, PI on y-axis. Above A-line = Clay, below = Silt. LL < 35 = Low, 35–50 = Intermediate, > 50 = High.
  7. Assign group symbol: CL, CI, CH, ML, MI, MH, or organic variants.

7. Worked Examples

Example 1 — Coarse-Grained Soil Classification (Beginner)
Sieve analysis results: 70% retained on 75μm, 10% retained on 4.75mm, 3% fines. D10 = 0.15mm, D30 = 0.5mm, D60 = 2.0mm.
Step 1
70% retained on 75μm → Coarse-grained
Step 2
Most of coarse fraction between 75μm and 4.75mm → Sand
Step 3 — Fines
3% fines → < 5% → check grading
Step 4 — Grading
Cu = 2.0/0.15 = 13.3 > 6 ✅
Cc = 0.5²/(2.0×0.15) = 0.25/0.30 = 0.83 → Not between 1–3 ✗
Result
Fails Cc criterion → SP (Poorly Graded Sand)
Example 2 — Fine-Grained Soil (Intermediate)
Soil passes 75μm sieve: 62%. LL = 45%, PL = 22%.
Step 1
62% passes 75μm → Fine-grained
Step 2 — Plasticity Index
PI = 45 − 22 = 23%
Step 3 — A-Line Check
A-line PI at LL=45: 0.73 × (45−20) = 0.73 × 25 = 18.25
Actual PI = 23 > 18.25 → Above A-line → Clay
Step 4 — Plasticity Level
LL = 45% → between 35% and 50% → Intermediate plasticity
Result
CI (Clay of Intermediate Plasticity)
Example 3 — Dual Symbol Coarse Soil (Advanced)
Sieve: 55% retained on 75μm, 40% on 4.75mm. 8% fines. Fines have LL = 32%, PL = 24%.
Step 1
55% retained → coarse-grained. More on 4.75mm → Gravel
Step 2
Fines = 8% → between 5–12% → Dual symbol
Step 3 — Fines Character
PI = 32−24 = 8%. A-line at LL=32: 0.73×12 = 8.76
PI = 8 < 8.76 → Below A-line → Silty fines
Result
GW-GM or GP-GM (check Cu/Cc to decide GW or GP prefix)

8. GATE MCQs

Q1. The A-line on the plasticity chart is defined by:
  1. (a) PI = 0.73(LL − 20)
  2. (b) PI = 0.50(LL − 25)
  3. (c) PI = LL − PL
  4. (d) PI = 0.73 × LL
Answer: (a)
The A-line equation PI = 0.73(LL − 20) separates clays (above) from silts (below).
Q2. A soil with LL = 60% and PI = 35% is classified as:
  1. (a) CL
  2. (b) CH
  3. (c) MH
  4. (d) CI
Answer: (b)
LL > 50% → High plasticity. A-line at LL=60: 0.73×40 = 29.2. PI=35 > 29.2 → above A-line → Clay. Result: CH.
Q3. For a well-graded sand (SW), the criteria are:
  1. (a) Cu > 4, Cc = 1–3
  2. (b) Cu > 6, Cc = 1–3
  3. (c) Cu > 6, Cc > 3
  4. (d) Cu > 10, Cc = 1–3
Answer: (b)
SW: Cu > 6 AND Cc between 1 and 3. For gravel (GW): Cu > 4. Don't mix sand and gravel criteria.
Q4. The 75μm sieve separates:
  1. (a) Sand from gravel
  2. (b) Coarse-grained from fine-grained
  3. (c) Clay from silt
  4. (d) Organic from inorganic
Answer: (b)
The 75μm (0.075mm) IS sieve is the boundary between coarse-grained (sand + gravel) and fine-grained (silt + clay).
Q5. Plasticity index (PI) is defined as:
  1. (a) LL + PL
  2. (b) LL − PL
  3. (c) LL − SL
  4. (d) PL − SL
Answer: (b)
PI = LL − PL. It represents the range of water content over which the soil is in a plastic state.
Q6. The 4.75mm sieve separates:
  1. (a) Sand from silt
  2. (b) Sand from gravel
  3. (c) Coarse from fine
  4. (d) Clay from silt
Answer: (b)
4.75mm is the boundary between sand (below) and gravel (above) in the IS system.
Q7. If Cu = 3 for a sand with less than 5% fines, the classification is:
  1. (a) SW
  2. (b) SP
  3. (c) SM
  4. (d) SC
Answer: (b)
Cu = 3 < 6 → fails the well-graded criterion. With <5% fines → SP (poorly graded sand).
Q8. Liquid limit test uses the Casagrande apparatus with the standard number of blows:
  1. (a) 10
  2. (b) 20
  3. (c) 25
  4. (d) 50
Answer: (c)
LL = water content at which the groove closes over 13mm length in exactly 25 blows of the Casagrande cup.
Q9. A soil plotting below the A-line with LL = 55% is classified as:
  1. (a) CH
  2. (b) MH
  3. (c) ML
  4. (d) OH
Answer: (b)
Below A-line = Silt (M). LL > 50% = High plasticity (H). Result: MH. (Check for organic content to rule out OH.)
Q10. The IS boundary between low and intermediate plasticity fine-grained soil is at LL =
  1. (a) 25%
  2. (b) 35%
  3. (c) 50%
  4. (d) 40%
Answer: (b)
IS system: L (low) = LL < 35%, I (intermediate) = 35–50%, H (high) = > 50%. The USCS uses only 50% as the boundary (no intermediate category).

9. Common Mistakes

Mistake 1: Confusing IS classification (3 plasticity levels) with USCS (2 levels). IS 1498 has Low/Intermediate/High (35% and 50% boundaries). USCS has only Low/High (50% boundary). GATE may use either system — read the question carefully.
Mistake 2: Forgetting to check BOTH Cu AND Cc for well-graded. A soil can have Cu > 6 but Cc outside 1–3 — it is still SP, not SW. Both conditions must be satisfied simultaneously.
Mistake 3: Mixing up Cu criteria for sand and gravel. Sand: Cu > 6. Gravel: Cu > 4. Students often use 6 for both.
Mistake 4: Plotting PI vs PL instead of PI vs LL on the plasticity chart. The x-axis is LL, not PL. This changes the position relative to the A-line completely.

10. Quick Revision Summary

Memorise:

Explore more geotechnical topics

StructLearn Pro covers soil classification, bearing capacity, foundation design and more — all with IS code compliance.

Start Free →