Classification of Soils in Soil Mechanics – A Complete Guide for JKSSB Civil Aspirants -

Introduction

Soil classification is the systematic arrangement of soils into different groups or categories based on their physical and engineering properties such as particle size, texture, plasticity, moisture content, and behavior under applied loads. It forms the foundation of geotechnical engineering and is essential for effective communication among engineers, accurate soil analysis, and selecting appropriate construction techniques.

This classification helps in:

Predicting how soil will behave in different construction environments
Ensuring structural stability and preventing failures
Optimizing material use and construction cost

Civil engineering projects such as road construction, dam building, tunneling, and foundation design require a deep understanding of soil types. For JKSSB Civil JE aspirants, questions based on classification systems, Atterberg limits, and field identification tests are common, making this topic a scoring section in the exam. Understanding this topic not only boosts theoretical knowledge but also sharpens the decision-making ability of future engineers in real-world site conditions.

Importance of Soil Classification in Civil Engineering

Determines load-bearing capacity, helping engineers assess whether the soil can safely support structures like buildings, bridges, and dams.
Assists in choosing appropriate foundation types such as shallow foundations for stable soils and deep foundations for weaker ones.
Guides drainage and compaction strategies to improve soil strength and reduce settlement or water retention issues.
Essential for road and earthwork projects by identifying suitable borrow materials and ensuring long-term pavement performance.
Aids in slope stability analysis, especially in hilly or landslide-prone regions, by understanding soil cohesion and internal friction.
Helps identify problematic soils such as expansive, collapsible, or marine soils, allowing for proper soil treatment or replacement.
Facilitates communication and standardization in design by using internationally recognized classification systems (IS, USCS, AASHTO).
Provides vital data for laboratory and field tests like permeability, compaction, and shear strength.

In short, soil classification forms the basis of informed decision-making and risk mitigation in geotechnical and civil engineering design.

Classification Based on Particle Size (Textural Classification)

According to IS 1498:1970:

Soil Type	Size Range
Boulders	> 300 mm
Cobbles	80 mm – 300 mm
Gravel	4.75 mm – 80 mm
Sand	0.075 mm – 4.75 mm
Silt	0.002 mm – 0.075 mm
Clay	< 0.002 mm

IS Soil Classification System (IS 1498:1970)

A. Coarse-Grained Soils (more than 50% retained on 75-micron sieve)

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

B. Fine-Grained Soils (more than 50% passing 75-micron sieve)

Classified using Atterberg limits:

Liquid Limit (LL)	Classification
LL < 35%	Low (L)
35% ≤ LL ≤ 50%	Intermediate (I)
LL > 50%	High (H)

Soil Type	Code
Low plasticity clay	CL
High plasticity clay	CH
Low plasticity silt	ML
High plasticity silt	MH

C. Organic Soils

Soil Type	Code
Peat, muck	Pt

Unified Soil Classification System (USCS)

Used internationally, USCS classifies soils based on grain size distribution and Atterberg limits.

Coarse-Grained: GW, GP, SW, SP, GM, GC, SM, SC
Fine-Grained: CL, CH, ML, MH

Example Code	Description
GW	Well-graded gravel
CL	Low plasticity clay
SM	Silty sand

AASHTO Soil Classification (Used in Highway Engineering)

Group	Soil Type	Typical GI Range
A-1	Gravel and sand	0–4
A-2	Sand with fines	0–10
A-3	Fine sand	0–8
A-4-A-7	Silts and clays	0–20+
A-8	Peat and organic soil	Unsuitable

Group Index (GI) Formula:
GI=(F−35)×(0.2+0.005×(LL−40))GI = (F-35) \times (0.2 + 0.005 \times (LL – 40))

Atterberg Limits and Plasticity

Liquid Limit (LL): Moisture at which soil flows
Plastic Limit (PL): Moisture at which soil crumbles
Plasticity Index (PI): LL – PL

PI Value	Plasticity Category
< 7	Low
7–17	Medium
> 17	High

Field Identification Tests

Test Name	Indication
Feel Test	Sticky = Clay, Gritty = Sand
Ribbon Test	Longer ribbon = More clay
Dry Strength	High = Clay, Low = Silt
Dilatancy Test	Quick reaction = Silt
Shaking Test	Glossy surface = Silt

Special Soil Types (Engineering Classification)

Type	Description
Collapsible Soils	Lose strength on wetting (Loess)
Expansive Soils	Swell upon wetting (Black Cotton Soil)
Marine Soils	High salt content, weak
Alluvial Soils	River-deposited, fertile, loose

Comparison Table: IS vs USCS vs AASHTO

System	Application	Soil Symbols	Main Use
IS	Indian Standard	CL, GW, SC	General construction
USCS	International	CL, CH, GW	Soil behavior analysis
AASHTO	Road Engineering	A-1 to A-8	Subgrade classification

Previous Year JKSSB MCQs (Examples)

What is the size range for silt?
- A: 0.002 mm – 0.075 mm ✅
CL in soil classification stands for?
- A: Clay with Low plasticity ✅
Which soil is expansive in nature?
- A: Black cotton soil ✅

Conclusion

Understanding soil classification helps in proper planning, design, and execution of civil engineering works. It enables engineers to assess soil behavior under various environmental conditions and loads, which is critical for ensuring the safety, durability, and cost-effectiveness of structures. Additionally, it assists in selecting construction methods, determining suitable materials, and designing earthwork operations.

For JKSSB Civil exams, aspirants must focus not only on IS, USCS, and AASHTO systems but also on interpreting Atterberg limits, understanding the logic behind dual-symbol classifications (e.g., SC-SM), and recognizing soil characteristics through quick field tests. Knowledge of specific soils like black cotton soil, loess, and marine deposits is also important. A deep grasp of classification systems not only helps solve MCQs but also enhances conceptual clarity essential for real-life project execution.