Loksewa - Geotechnical Engineering - Chapter 3

Soil Classification

Clean sand and soft clay are both "soil" - but they behave nothing alike underground. Classification is how engineers turn a vague word into a precise, predictable engineering material.

Clean Sand high permeability Soft Clay low permeability, plastic

Same word, "soil" - opposite engineering behavior. That's why classification exists.

Learning objectives
  • Understand why we classify soil
  • Grain-size boundaries (boulder to clay)
  • Coarse vs fine-grained soil
  • USCS two-letter symbols
  • Well-graded vs poorly graded soil
  • Plasticity chart and A-line
  • Solve Loksewa MCQs on classification
01

Why Do We Classify Soil?

Classification lets an engineer look at a two-letter code and instantly predict bearing capacity, permeability, compressibility, shear strength, drainage, and foundation suitability - without re-testing from scratch every time.

Definition: Soil classification is the process of grouping soils having similar engineering properties based on grain size and plasticity characteristics.

02

Objectives of Soil Classification

Goal

Identify soil type

Goal

Predict engineering behavior

Goal

Select foundation type

Goal

Estimate permeability

Goal

Estimate settlement

Goal

Common engineering language

03

Major Soil Classification Systems

The Loksewa syllabus lists five systems - but USCS and IS Classification (IS 1498) are the most important for the exam.

SystemNotes
Descriptive ClassificationPlain-language description
Textural ClassificationBased on % sand/silt/clay (triangle chart)
IS Classification (IS 1498)Indian Standard, widely used in South Asia
MIT ClassificationOlder grain-size based system
USCSUnified Soil Classification System - most important
04

Classification Based on Grain Size

Particle size runs from massive boulders down to microscopic clay - and one single sieve size decides where "coarse" ends and "fine" begins.

Boulder > 300 mm Cobble 80-300 mm Gravel 4.75-80 mm Sand 0.075-4.75 mm Silt 0.002-0.075 mm Clay < 0.002 mm 0.075mm sieve (No. 200)

This single line separates coarse-grained soils (left) from fine-grained soils (right).

05

Coarse-Grained vs. Fine-Grained Soil

> 50% retained on 0.075mm sieve

Coarse-Grained Soil

  • Types: gravel, sand
  • High permeability
  • Low compressibility
  • Good bearing capacity
  • Rapid drainage
> 50% passes 0.075mm sieve

Fine-Grained Soil

  • Types: silt, clay
  • Low permeability
  • High compressibility
  • High settlement
  • Slow drainage
06

Gravel vs. Sand

GravelSand
Larger particlesSmaller particles
Higher bearing capacityGood bearing capacity
Excellent drainageGood drainage
Used in filtersUsed in concrete and fills
07

Silt vs. Clay

This is a favorite interview question.

SiltClay
Non-plastic or slightly plasticHighly plastic
Feels smoothFeels sticky when wet
Lower cohesionHigher cohesion
Low shrink-swellHigh shrink-swell
08

Unified Soil Classification System (USCS)

USCS labels every soil with a two-letter symbol: the first letter says what the soil basically is, the second says how it's graded or what fines it contains.

Coarse-Grained Soils

GW
Well-graded gravel
GP
Poorly graded gravel
GM
Silty gravel
GC
Clayey gravel
SW
Well-graded sand
SP
Poorly graded sand
SM
Silty sand
SC
Clayey sand

Fine-Grained Soils

ML
Low-plasticity silt
MH
High-plasticity silt
CL
Low-plasticity clay
CH
High-plasticity clay

Organic Soils

OL
Organic silt/clay (low plasticity)
OH
Organic silt/clay (high plasticity)
Pt
Peat
-

Memory Trick

G
-> Gravel
W
Well graded
P
Poorly graded
M
Silt
C
Clay
S
-> Sand
W
Well graded
P
Poorly graded
M
Silt
C
Clay
09

Well-Graded vs. Poorly Graded Soil

Wide range of particle sizes

Well-Graded Soil

  • Dense packing
  • Lower void ratio
  • Higher bearing capacity
  • Better compaction
Particles nearly the same size

Poorly Graded Soil

  • Higher void ratio
  • Lower density
  • Lower stability
10

Plasticity Chart

Fine-grained soils are further classified using Liquid Limit (LL) and Plasticity Index (PI). The A-line on this chart separates silts from clays.

PI LL A-line Clay Silt

For Loksewa, remember it as a simple rule:

  • Above the A-line -> Clay
  • Below the A-line -> Silt
-

Engineering Importance

Foundation design
Pavement design
Earth dam construction
Embankment stability
Filter design
Drainage systems

Memory Box

  • Soil classification predicts engineering behavior
  • 0.075mm sieve divides coarse and fine soils
  • Gravel + Sand = Coarse-grained soils
  • Silt + Clay = Fine-grained soils
  • GW = Well-graded gravel
  • SW = Well-graded sand
  • CL = Low-plasticity clay
  • CH = High-plasticity clay
  • Above A-line = Clay
  • Below A-line = Silt
-

Interview Questions

Q. Why is well-graded soil preferred over poorly graded soil for foundations?
Q. Why is the 0.075mm sieve important in soil classification?
Q. Explain the difference between silt and clay based on engineering behavior.
Q. What information does the USCS symbol "SC" convey?
Q. Why is soil classification the first step before any geotechnical design?
Q

Practice MCQs

Tap a question to reveal the correct option.

Which sieve separates coarse and fine soils?
  • A. 4.75 mm
  • B. 2.36 mm
  • C. 0.425 mm
  • D. 0.075 mm - Correct
Symbol "GW" stands for:
  • A. Clayey gravel
  • B. Well-graded gravel - Correct
  • C. Silty gravel
  • D. Poorly graded gravel
CH represents:
  • A. High-plasticity clay - Correct
  • B. High-plasticity silt
  • C. Clayey sand
  • D. Clayey gravel
SW represents:
  • A. Silty sand
  • B. Well-graded sand - Correct
  • C. Poorly graded sand
  • D. Sandy clay
Which soil generally has the highest permeability?
  • A. Clay
  • B. Silt
  • C. Sand - Correct
  • D. Organic soil