Detailed step-by-step methodology for laying and compacting Granular Sub-base (GSB) in highway construction as per MORTH 5th Revision and IRC SP 84.

Scope of Work

The work of Granular Sub-base (GSB) involves laying and compacting well-graded material over a prepared subgrade. This layer forms a strong foundation for upper pavement layers and helps in distributing traffic loads evenly.
The work shall be carried out as per the Ministry of Road Transport & Highways (MORTH – 5th Revision) specifications, following the lines, levels, and cross-sections shown in the approved drawings or as directed by the Engineer.

Reference Documents

1. Contract Agreement
2. IRC: SP: 84–2014 – Manual of Specifications & Standards for Highway Works
3. MORTH Specifications (5th Revision)
4. Approved Construction Drawings

Materials

The materials used for GSB shall be natural sand, moorum, gravel, crushed stone, crushed slag, or a suitable combination of these depending on the required grading.
Other materials such as brick metal, kankar, or crushed concrete may be used only with the specific approval of the Engineer.
All materials should be free from organic matter or other harmful substances.

Key Quality Requirements (as per MORTH Table 400-1):

- Aggregate Impact Value (AIV): Max 40%
- Liquid Limit: Max 25%
- Plasticity Index (PI): Max 6%
- CBR Value: Minimum 30%

Water Absorption: If the water absorption of aggregates determined as per IS: 2386 (Part 3) is greater than 2%, the
aggregates shall be tested for Wet Aggregate Impact Value (Wet AIV) as per IS:5640.

Sieve Sizes		Grading 1	Grading 2	Grading 3		Grading 4	Grading 5	Grading 6
75.0 mm	100		–	100		–	100		–
53.0 mm	80-100		100	–		100	80-100		100
26.5 mm	55-90		70-100	55-75		50-80	55-90		75-100
9.50 mm	35-65		50-80	–		–	35-65		55-75
4.75 mm	25-55		40-65	10-30		15-35	25-50		30-55
2.36 mm	20-40		30-50	–		–	10-20		10-25
0.425 mm	Oct-15		Oct-15	–		–	0-5		0-8
0.075 mm	< 5		< 5	< 5		< 5	–		0-3
Aggregate Impact Value (AIV)					Max 40
Liquid Limit					Max 25
Plasticity Index					Max 6
CBR Value					Min 30%

Construction Methodology

1. Preparation of Subgrade

Before laying the GSB, the subgrade should be properly prepared as per MORTH Clause 301 or 305.
All vegetation, roots, and debris should be removed. The surface should be lightly sprinkled with water and compacted using a smooth wheeled roller (80–100 kN).

2. Mixing of Material

The GSB material shall be mixed in a mechanical mixer with controlled addition of water to ensure uniform moisture. Mixing must continue until a uniform blend is achieved.

3. Spreading and Leveling

The mixed material shall be spread evenly on the prepared subgrade using a motor grader. The blade should have hydraulic control for accurate slope and grade adjustments.
The compacted thickness of each layer should not be less than 150 mm.

4. Moisture Control Quality Control Tests Before Acceptance

Test Description	Code Reference	Acceptance Criteria	Frequency
Moisture Content	IS:2720 (Part 2)	Within OMC ± 2%	1 test / 250 m³
Field Density (by Sand Replacement Method)	IS:2720 (Part 28)	≥ 97% MDD for Subgrade ≥ 98% MDD for GSB	1 test / 500 m² / layer
CBR Value (Subgrade only)	IS:2720 (Part 16)	As per design requirement (usually ≥ 8%)	1 test / 3000 m³
Gradation / Particle Size Distribution (GSB)	IS:2386 (Part 1)	Within MoRTH Table 400-1 limits	1 test / 3000 m³
Atterberg Limits (Subgrade)	IS:2720 (Part 5)	LL ≤ 50%, PI ≤ 20% (for non-expansive soil)	2 tests / 3000 m³
Free Swell Index (FSI)	IS:2720 (Part 40)	≤ 50% (as per MoRTH Table 300-1)	1 test / source
Proctor Test (MDD & OMC)	IS:2720 (Part 8)	Required for density reference	2 tests / 3000 m³

5. Compaction

Compaction should be done immediately after spreading using:

- Smooth wheeled roller (80–100 kN) for layers below 100 mm

- Vibratory roller (80–100 kN static weight) for layers up to 200 mm

Rolling should start from the lower edge and proceed towards the upper edge. In double cross-fall areas, rolling should begin from the edges and move toward the center.

Compaction shall continue until at least 98% of Maximum Dry Density (MDD) is achieved as per IS 2720 (Part 8).
The compacted surface should be uniform, well-closed, and free from cracks or loose patches. Any defective areas should be corrected and re-compacted.

Surface Finish and Quality Control

The finished surface must meet the tolerance requirements of MORTH Clause 902.
Quality control tests should be conducted regularly on both materials and the constructed layers to ensure compliance with Section 900 of MORTH.

Geometric and Surface Level Checks

Parameter	Requirement (as per MoRTH Table 900-1)	Frequency
Longitudinal Level	± 20 mm	10 m intervals
Cross Slope / Camber	± 0.3%	10 m intervals
Surface Evenness	Max 12 mm when tested with 3 m straight edge	Random checks
Layer Thickness (after compaction)	GSB ≤ 200 mm Subgrade ≤ 250 mm	Random sections

Any deviations should be corrected immediately under the direction of the Engineer.

Conclusion

Following the correct methodology for GSB construction ensures a durable and stable pavement foundation. Proper material selection, moisture control, and compaction are key to achieving long-term performance as per IRC: SP:84–2014 and MORTH (5th Revision) standards.

Top 10 FAQs on Granular Sub-base (GSB)

1. What is the purpose of GSB in road construction?
  GSB provides a strong base for upper layers and distributes traffic loads evenly.

1. What is the minimum CBR value required for GSB?
  As per MORTH, the minimum CBR should be 30%.

1. What is the maximum layer thickness for GSB?
  Each compacted layer should not exceed 200 mm.

1. Which roller is used for compaction?
  A vibratory roller of 80–100 kN is recommended.

1. How is Optimum Moisture Content (OMC) determined?
  By conducting the Proctor Compaction Test as per IS: 2720 (Part 8).

1. What is the acceptable Plasticity Index (PI) for GSB material?
  It should not be more than 6%.

1. Which grading table is referred for GSB?
  MORTH Table 400-1 specifies the standard gradings.

1. Can recycled materials be used for GSB?
  Yes, with Engineer’s approval, crushed concrete or brick metal may be used.

1. What field density should be achieved?
  At least 98% of Maximum Dry Density (MDD) should be achieved.

1. How is the surface tolerance checked?
  Surface finish is verified as per Clause 902 of MORTH specifications.

Quick Reference: Granular Sub-Base (GSB) Quality Control

Field Density Requirement: ≥98% of Maximum Dry Density (MDD) as per IS:2720 Part 8 & 28
Layer Thickness: Each compacted layer ≤200 mm (80–100 kN static roller) or ≤250 mm (vibratory roller)
Moisture Control: Maintain moisture content within ±2% of OMC (IS:2720 Part 2 & 8)
Material Testing: Gradation (IS:2386 Part 1), AIV / LAA (IS:2386 Part 4), Liquid Limit & PI (IS:2720 Part 5), CBR (IS:2720 Part 16), Water Absorption & FSI
Surface Tolerances: Top level ±20 mm longitudinally, cross slope ±0.3%, surface evenness ≤12 mm with 3 m straight edge
Compaction Method: Vibratory or Static Roller with trial passes to determine required compaction effort
Defective Areas: Remove soft spots or segregated areas, replace with approved material, re-compact, and re-test
Testing Methods: Sand Replacement Method for field density, straightedge & leveling instruments for surface finish