Subgrade Construction Methodology (As per MoRTH 5th Revision)
The subgrade forms the foundation for all pavement layers. Construction shall be done using approved materials, properly compacted to the specified density, and finished to the design level and slope.
1. Scope of Work
The work includes construction of subgrade using approved borrow materials, roadway excavation, or other suitable sources as per MoRTH Clause 305.
Before starting the subgrade preparation ensure that Embankment layer is constructed and approved. After approval of the Subgrade layer, proceed with the GSB as per MoRTH 401.
2. References
Contract Agreement
IRC: SP: 84-2014
MoRTH Specifications, 5th Revision (Clause 305)
Relevant Drawings
3. Setting Out
After site clearance and embankment, mark the subgrade limits by fixing pegs at regular intervals. Benchmarks shall be set outside the construction area.
4. Material Selection and Borrow Areas
Material: Soil, moorum, gravel, reclaimed pavement material, fly ash, pond ash, or any other approved material. Free from roots, logs, stumps, debris, and unsuitable materials.
Borrow Material: Samples collected and tested as per IS:2720. Approved areas shall be demarcated.
5. Material Testing & Frequency
Sl. No.
Item
Reference Code
Frequency
1.1
Sand Content / Gradation
IS:2720 (Part 4), MoRTH Clause 903.2
2 Tests / 3000 m³
1.2
Atterberg’s Limits
IS:2720 (Part 5), MoRTH Clause 903.2
2 Tests / 3000 m³
1.3
Modified Proctor Test (MDD & OMC)
IS:2720 (Part 8), MoRTH Clause 903.2
2 Tests / 3000 m³
1.4
Moisture Content
IS:2720 (Part 2)
1 Test / 250 m³
1.5
CBR Test for Subgrade
IS:2720 (Part 16), MoRTH Clause 903.2
1 Test / 3000 m³
1.6
Free Swell Index Test
IS:2720 (Part 40), MoRTH Clause 903.2
1 Test / 3000 m³
6. Equipment Required
Hydraulic Excavator
Dozer
Dumpers / Tipper Trucks
Motor Grader
Vibratory Roller (8–10 T)
Water Tanker
Tractor Trolleys
Survey Kit
7. Method of Operation
Mark subgrade limits after embankment completion with pegs. Construction toe line will extend beyond design toe line for edge compaction.
Level embankment top, scarify, mix with water, and compact to achieve minimum dry density (MoRTH Table 300-1). Remove unsuitable foundation material and replace with approved material.
Compact borrow material to minimum 97% MDD (MoRTH Table 300-2). If density is less, scarify, mix with water, and re-compact.
Spread subgrade material in layers: ≤250 mm (vibratory roller) or ≤200 mm (static roller). Blend water uniformly without flooding, or dry if too wet.
Compaction with vibratory or 8–10 T static roller. Conduct trial to determine required passes for density (MoRTH Table 300-2). Roll longitudinally with one-third overlap.
Subsequent layers placed only after acceptance of finished layer. Verify top levels against longitudinal and cross profiles (MoRTH Table 900-1).
Ensure safety equipment and signs are operational at all times along the highway.
8. Quality Control
Density testing by sand replacement method (IS:2720 Part 28)
Liquid Limit (LL) ≤ 40%, Plasticity Index (PI) ≤ 6%
CBR as per IS:2720 Part 16 & MoRTH Clause 903.2
Surface levels within ±20 mm longitudinally and ±15 mm transversely (MoRTH Table 900-1)
Quick Reference: Embankment & Subgrade Quality Control
Field Density Requirement: Embankment: 95% MDD, Subgrade & Shoulders: 97% MDD (IS:2720 Part 28)
