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.
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.
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 Size | 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 | 0-15 | 0-15 | - | - | 0-5 | 0-8 |
| 0.075 mm | < 5 | < 5 | < 5 | < 5 | - | 0-3 |
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).
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.
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.
| 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% | 2 tests / 3000 m³ |
| Free Swell Index (FSI) | IS:2720 (Part 40) | ≤ 50% | 1 test / source |
| Proctor Test (MDD & OMC) | IS:2720 (Part 8) | Required for density reference | 2 tests / 3000 m³ |
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.
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.
| 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 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.
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.
GSB provides a strong base for upper layers and distributes traffic loads evenly.
As per MORTH, the minimum CBR should be 30%.
Each compacted layer should not exceed 200 mm.
A vibratory roller of 80–100 kN is recommended.
By conducting the Proctor Compaction Test as per IS: 2720 (Part 8).
It should not be more than 6%.
MORTH Table 400-1 specifies the standard gradings.
Yes, with Engineer’s approval, crushed concrete or brick metal may be used.
At least 98% of Maximum Dry Density (MDD) should be achieved.
Surface finish is verified as per Clause 902 of MORTH specifications.
Standard construction methodologies for highway works as per MoRTH 5th Revision and IRC Specifications.
