What is Granular Sub-Base (GSB)?

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Granular Sub-Base (GSB) is one of the most essential layers in road construction. It is made of crushed stone, gravel, sand, and other well-graded granular materials. The GSB layer provides structural strength, enhances load distribution, improves drainage, and extends pavement life by preventing potholes, rutting, and deformation.

♦ What is Granular Sub-Base – GSB (Construction)?

The GSB layer is placed between the natural subgrade and upper pavement layers such as WMM and bitumen. It acts as a strong foundation that supports traffic loads and provides uniform stability.

Purpose: Provides a stable load-bearing foundation, distributes heavy vehicle loads, prevents road surface deformation, and improves drainage.

Composition: Typically made of 50% crushed stone, mixed with coarse sand, gravel, and well-graded fine aggregates.

GSB Grades: Classified into six grades by MoRT&H (India) based on particle size distribution and engineering requirements.

♦ What is the Composition of GSB Material?

GSB material composition depends on project specifications, traffic load, and local site conditions. It usually contains durable, hard, and well-graded aggregates.

A typical GSB mix includes:

≈ 50% crushed stones for load-bearing strength
≈ 30% CBR materials for structural rigidity
≈ 6% PI (Plasticity Index) materials for compaction
Water absorption: < 2%
Aggregate Impact Value (AIV): < 40%

This balanced mix ensures optimal compaction, stability, and long-term performance of the pavement.

♦ What are the Main Functions of GSB Material?

GSB performs several critical functions in road and highway construction:

Load Distribution: Transfers and spreads vehicle loads evenly to prevent subgrade failure.
Drainage: Acts as a permeable layer that prevents waterlogging and protects upper layers.
Subgrade Protection: Prevents pumping, rutting, and weakening of the underlying soil.

♦ What are the Purposes and Benefits of GSB?

GSB offers multiple engineering and long-term performance benefits:

Provides a stable and durable base for pavement layers
Improves road drainage efficiency
Cost-effective and suitable for all road types
Prevents potholes, cracks, rutting, and surface damage
Capillary Cut-off: Blocks upward moisture movement
Frost Protection: Prevents pavement damage in cold climates
Extends pavement life and reduces maintenance costs

♦ What are the Characteristics of GSB Material?

Composition: A mix of natural or manufactured aggregates that enhance strength and durability.

Grading: Well-graded aggregates ensure maximum density, stability, and reduced voids.

Compression: Requires proper compaction to achieve the required Maximum Dry Density (MDD).

Drainage: The material must allow water to pass through to prevent saturation and weakening.

Cost-effectiveness: Highly economical and widely used in modern road construction.

♦ What are the Types and Grading of GSB?

GSB is graded based on material size, density, compaction, and engineering application:

GSB 1: Well-graded, dense aggregate mix for heavy-traffic highways.

GSB 2: Medium-graded material suitable for moderate traffic loads.

GSB 3: Open-graded, highly permeable material ideal for drainage layers.

Detailed GSB Grading:

GSB 1: 0–40 mm aggregate, high structural strength
GSB 2: 0–63 mm aggregate, moderate strength
GSB 3: 0–80 mm aggregate, high drainage capacity

♦ What are the Physical Property Requirements for a GSB?

For optimal performance, the GSB layer must meet the following physical requirements:

CBR (California Bearing Ratio): Minimum 30% (soaked for 4 days)
Compaction: Minimum 98% Maximum Dry Density (MDD)
Plasticity Index (PI): ≤ 6%
Water Absorption: ≤ 2%
Aggregate Impact Value (AIV): ≤ 40%