Classification of Soil and Soil Compaction Civil Engineering GATE Notes

Classification of Soil and Soil Compaction is a core topic in civil engineering that helps students understand soil behaviour before construction. It is important for exams like GATE, SSC JE, and State AE, which focus on soil properties such as grain size, plasticity, and fine content.

The topic also explains soil compaction, grading, the plasticity chart, and Proctor test. It helps engineers select proper soil and compaction methods to improve the strength, stability, and safety of structures.

Classification of Soil and Soil Compaction Video Explanation

Watch this video to understand the Classification of Soil and Soil Compaction in a simple and clear way. The video explains soil types, plasticity chart, compaction methods, and Proctor test with easy examples for exam preparation.

Check: GATE Civil Engineering Notes

What Is Classification of Soil?

Classification of Soil and Soil Compaction starts with understanding soil classification. Soil classification means grouping soil based on its engineering properties. Soils with similar behavior are placed in the same group.

This system helps engineers decide whether soil is good for roads, buildings, embankments, or foundations. In India, the Indian Standard Soil Classification System (ISCS) is mainly used.

Soil classification is based on:

• Grain size

• Plasticity

• Liquid limit

• Percentage of fine particles

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Coarse-Grained and Fine-Grained Soil

In Classification of Soil and Soil Compaction, soil is first divided into coarse-grained and fine-grained soil.

If more than 50% soil particles are larger than 75 microns, it is called coarse-grained soil. If more than 50% particles are smaller than 75 microns, it is called fine-grained soil.

Coarse-grained soil mainly includes gravel and sand. Fine-grained soil includes silt and clay.

Before understanding symbols, it is important to know how soil types are identified in IS classification.

Soil Type Identification Table

Plasticity Chart and Atterberg Limits

Plasticity chart plays a big role in Classification of Soil and Soil Compaction. It uses liquid limit (WL) and plasticity index (IP) to classify fine soil.

Plasticity index is the difference between liquid limit and the plastic limit. The chart has two main lines:

• A-Line: Separates clay and silt

• U-Line: Upper limit where no soil exists

Soil plotted above A-line is clay. Soil below A-line is silt. Organic soil lies in a separate zone.

This chart helps students quickly identify soil type in exams.

Grading of Sand and Gravel

In Classification of Soil and Soil Compaction, sand and gravel are also classified based on grading. Grading tells how particle sizes are spread.

Two terms are used:

• Well graded soil

• Poorly graded soil

To decide this, two values are calculated:

• Coefficient of Uniformity (Cu)

• Coefficient of Curvature (Cc)

Before checking conditions, understand their formulas.

What Is Soil Compaction?

Classification of Soil and Soil Compaction is incomplete without compaction. Soil compaction means pressing soil particles closer using mechanical force.

During compaction:

• Air is removed

• Soil volume reduces

• Dry density increases

Compaction is a fast process and happens immediately after applying load.

It improves:

• Strength

• Stability

• Load carrying capacity

Difference Between Compaction and Consolidation

Many students confuse compaction with consolidation. In Classification of Soil and Soil Compaction, this difference is very important.

Compaction removes air using machines. Consolidation removes water slowly due to load.

Methods of Soil Compaction

Different soil types need different compaction methods. Classification of Soil and Soil Compaction helps choose the correct method. Cohesionless soil like sand and gravel is compacted using vibration. Cohesive soil like clay needs high pressure.

In partially wet sand, water forms thin films between particles. This creates a honeycomb structure, which reduces density at low moisture.

Important Classification of Soil and Soil Compaction Questions Asked in Exams 

Q1. The standard plasticity chart with different demarcated regions is shown in the given figure. Consider the following statements:

1. MH soil lies in V

2. CL soil lies in R

3. CI soil lies in Q

4. OI soil lies in T

Which of the above statements are correct?

A. 1, 2 and 3
B. 1, 3 and 4
C. 2, 3 and 4
D. 1 and 4

Q2. In a soil specimen, 70% of particles are passing through 4.75 mm IS sieve and 40% of particles are passing through 75 μ IS sieve. Its uniformity coefficient is 8 and coefficient of curvature is 2. As per IS classification, this soil is classified as:

A. SP
B. GP
C. SW
D. GW

Q3. Consider the following statements:

1. In clay soils, the maximum dry unit weight tends to decrease as plasticity increases.

2. In clay soils, the maximum dry unit weight tends to increase as plasticity increases.

3. Heavy clays with high plasticity have the minimum dry unit weight and high OMC.

Which of these statements are correct?

A. 1, 2 and 3
B. 1 and 2 only
C. 2 and 3 only
D. 1 and 3 only

Q4. The in-situ void ratio of a granular soil deposit is 0.5. If the maximum and minimum void ratios of the same soil were determined as 0.8 and 0.4 respectively, then the relative compaction (in %, round off to two decimal places) of the soil deposit is ________.

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