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.

1. Scope of Subgrade Construction

The work shall consist of construction of Subgrade with approved and specified materials obtained from approved borrow areas or suitable material obtained from roadway excavation and drain excavation and in accordance with clause 305 of MORTH specifications.

2. Reference Standards

• MoRTH Specifications – 5th Revision (Clause 305 & 902)
• IRC:37 – Pavement Design Guidelines
• IS 2720 (Relevant Parts)
• Approved Project Drawings

Setting Out

After completion of site clearance/embankment the limits of sub-grade shall be marked by fixing pegs on both sides at regular intervals. The working bench mark shall be set outside the limits of construction areas.

3. Material Requirements

1. Material: The material used in Sub-grade shall be soil, moorum, gravel, reclaimed material from pavement, fly ash, pond ash, a mixture of these or any other material approved by the AE. It shall be free of logs, stumps, roots, rubbish & any other material likely to deteriorate or affect the stability of subgrade. The material for subgrade shall be obtained from the approved source with preference to the material becoming available from nearby roadway excavation or any other excavation under the contract. The material requirements shall be in accordance with clause 305.2 of MORTH specifications.
2. Borrow Material: Samples shall be collected from the identified borrow area & tested as per IS 2720. On confirmation of the suitability material the area shall be properly demarcated.

4. Construction Methodology

1. After completion of embankment, the limits of subgrade shall be marked by fixing pegs at regular intervals before commencing the earthwork. Construction toe line will be more than the design toe line to ensure proper compaction on edges.
2. The embankment top shall be levelled to facilitate placement of first layer of subgrade, scarified, mixed with water and then compacted by rolling so as to achieve minimum dry density as specified in Table 300-1 of MORTH specifications. Subgrade treatment specified for subgrade shall be carried out as per approved design and drawing. Unsuitable material occurring in subgrade foundation shall be removed as per direction and approval of AE and replaced by approved materials laid in layers and suitably compacted to the required and specified degree of compaction. Excavation at approved borrow areas shall be carried out with excavator and transportation of excavated material shall be done by dumpers.
3. M.D.D. of borrowarea soil as determined in lab, and when the same be laid for subgrade preparation and tested for dry density should not have relative compaction less than 97% of M.D.D. (as specified in MORTH specifications Table 300-2), if found less, the layer shall be scarified mixed with water and compacted to achieve minimum specified dry density.
4. The subgrade material shall then be spread in layers not exceeding 250 mm (when compacted using vibratory roller / compactor) or up to 200mm (when compacted using 80-100 kN static roller) thicknesses over the entire area with the dozer and finished by a motor grader. When the existing subgrade to be widened continuous horizontal benches shall be cut out into the old slope to ensure adequate bond. Where water is required to be added the same shall be sprinkled from a water tanker fitted with sprinkler, uniformly on the surface but without flooding and shall be mixed thoroughly in soil by blending or harrowing until uniform moisture content is obtained throughout the depth of the layer. If the material delivered to the roadbed is too wet, it shall be dried by aeration and exposure to the sun till the moisture content is accepted for compaction.
5. The compaction shall be done with the help of vibratory roller or 8 - 10 tonne static roller. Compaction trial shall be carried out on a suitable stretch to determine the no. of passes required for particular type of soil, dumped for subgrade, to achieve densities specified in the Table 300-2 of MORTH specifications. Based on the trial actual compaction will be carried out with required no. of passes of vibratory roller or static roller until the specified density is achieved. Rolling shall progress parallel to the centreline of the road uniformly overlapping each preceding track by one-third width. Rolling shall be continued till the specified density is achieved as per Table 300-2 of MORTH specifications. In case specific compaction is not achieved the material in the soft area shall be removed and replaced with approved material. Densities shall be tested by sand replacement method.
6. Subsequent layers shall be placed only after the finished layer has been tested as per the clause 903.2.2 and accepted. The top levels of the subgrade shall be checked with reference to the longitudinal and cross profile of the road as per drawing to keep the variation within tolerance limit as per table 900-1.
7. The Safety Officer shall make frequent patrols along the highway to ensure that the safety equipment and signs are operational at all time.

5. Tests & Frequency

Test to be conducted for Subgrade

6. Tolerances & Acceptance Criteria

7. Inspection Checklist

8. Common Defects & Rectification

• Low Density: Scarify, adjust moisture, recompact
• Over-wet Layer: Aerate material before rolling
• Uneven Surface: Regrade and compact

9. Frequently Asked Questions (FAQ)