Subgrade Construction Methodology & Quality Control
As per MoRTH Clause 305
Subgrade is the most critical structural layer of highway pavement. This guide provides a complete and practical methodology for subgrade construction as per MoRTH Clause 305 (5th Revision), including field quality control procedures, tolerances, inspection checklists, and acceptance criteria.
Quick Reference – Subgrade Requirements
- Compaction: ≥ 97% of Modified Proctor MDD
- Moisture Control: Within ±2% of OMC
- Layer Thickness: ≤ 200 mm (compacted)
- Finished Level Tolerance: ±20 mm
- Cross Slope: ±0.3%
1. Scope of Subgrade Construction
Subgrade refers to the prepared top layer of embankment or natural ground over which pavement layers such as GSB, WMM, DLC, or PQC are constructed. The objective is to provide a stable, uniform, and well-compacted foundation capable of distributing traffic loads safely.
2. Reference Standards
- MoRTH Specifications – 5th Revision (Clause 305 & 902)
- IRC:37 – Pavement Design Guidelines
- IS 2720 (Relevant Parts)
- Approved Project Drawings
3. Material Requirements
- Approved soil from embankment or borrow areas
- Free from organic matter, roots, trash, or deleterious materials
- Plasticity Index and gradation within specified limits
- CBR meeting project requirements
4. Construction Methodology
4.1 Preparation of Underlying Layer
- Underlying embankment compacted and approved
- Surface cleaned of loose material
- Camber and cross slope verified
4.2 Spreading of Subgrade Material
- Material spread uniformly using motor grader
- Loose thickness adjusted to achieve ≤200 mm compacted thickness
- Extra width provided for proper edge compaction
4.3 Moisture Conditioning
Water is added or material aerated to bring moisture content within ±2% of OMC as determined by Modified Proctor Test.
4.4 Compaction
- Compaction using 8–10 T vibratory roller
- Rolling from edges towards center
- Trial rolling conducted to finalize number of passes
5. Field Quality Control Procedures
5.1 Modified Proctor Test
Determines Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) for compaction control.
5.2 Field Density Test
Field density is checked using Sand Replacement or Core Cutter method as per IS 2720.
Acceptance Criteria: Minimum 97% of laboratory MDD.
5.3 CBR Verification
CBR tests are carried out to confirm load-bearing capacity of the subgrade.
6. Tests & Frequency
| Test | IS Code | Frequency |
|---|
| Gradation | IS 2720 (Part 4) | 1 / 3000 m³ |
| Atterberg Limits | IS 2720 (Part 5) | 1 / 3000 m³ |
| Modified Proctor | IS 2720 (Part 8) | 1 / Source |
| Field Density | IS 2720 (Part 28) | 1 / 500 m² / Layer |
| CBR | IS 2720 (Part 16) | 1 / 3000 m³ |
7. Tolerances & Acceptance Criteria
| Parameter | Permissible Tolerance |
|---|
| Compaction | ≥ 97% MDD |
| Moisture Content | ±2% of OMC |
| Finished Level | ±20 mm |
| Width | +100 mm / -50 mm |
| Cross Slope | ±0.3% |
8. Inspection Checklist
Before Construction
- Approved material source
- MDD & OMC available
- Underlying layer approved
During Construction
- Layer thickness controlled
- Moisture within limits
- Proper rolling pattern
After Construction
- Field density achieved
- Levels and slopes verified
- No loose or soft patches
9. Common Defects & Rectification
- Low Density: Scarify, adjust moisture, recompact
- Over-wet Layer: Aerate material before rolling
- Uneven Surface: Regrade and compact
