Granular Sub-Base (GSB) is the foundation of durable highways. This guide provides complete MORTH 401 specifications, material selection, gradation limits, CBR & compaction criteria, layer thickness, and QA procedures. Perfect for site engineers and QA/QC teams, it ensures stable pavements, accurate construction, and long-lasting performance.
| Property | Requirement |
|---|---|
| CBR (Soaked) | ≥ 30% |
| Aggregate Impact Value | ≤ 40% |
| Liquid Limit | ≤ 25% |
| Plasticity Index | ≤ 6% |
| Sieve | Gr-1 | Gr-2 | Gr-3 | Gr-4 | Gr-5 | Gr-6 |
|---|---|---|---|---|---|---|
| 75 mm | 100 | - | 100 | - | 100 | - |
| 53 mm | 80–100 | 100 | - | 100 | 80–100 | 100 |
| 26.5 mm | 55–90 | 70–100 | 55–75 | 50–80 | 55–90 | 75–100 |
| 4.75 mm | 25–55 | 40–65 | 10–30 | 15–35 | 25–50 | 30–55 |
| 0.075 mm | <5 | <5 | <5 | <5 | - | 0–3 |
Measured using speedy moisture meter or oven drying method.
Acceptance: OMC ±2%
| Parameter | Requirement |
|---|---|
| Degree of Compaction | ≥ 98% of MDD |
| Test Frequency | 1 test / 500 m² / layer |
Bulk Density = 1.58 g/cc
Degree of Compaction = 81% (Rejected)
| Item | Tolerance |
|---|---|
| Longitudinal Level | ±20 mm |
| Cross Slope | ±0.3% |
| Surface Evenness | ≤12 mm (3 m straight edge) |
| Layer Thickness | ≤200 mm |
Proper gradation, moisture control, compaction, and systematic quality control testing are essential for a durable Granular Sub-Base. Compliance with MoRTH 401 ensures long-term pavement performance and structural stability.
Granular Sub-Base (GSB) provides load distribution, drainage, and a stable foundation between the subgrade and base layers. It improves pavement performance and increases the overall lifespan of the highway.
Gradation limits are defined in MORTH Clause 401 with sieve-wise percentage passing requirements from 75 mm down to 0.075 mm. Proper gradation ensures strength, stability, and adequate drainage.
GSB must typically achieve a minimum of 98% of Modified Proctor Maximum Dry Density (MDD), and the soaked CBR value should generally be ≥ 30% as per project specifications.
Yes. This guide is structured for field engineers and QA/QC professionals, making it suitable for practical site inspections and compliance verification.
Yes. The information reflects current MORTH specifications and updated quality control practices relevant for 2026 highway projects.
