What is the minimum field density required for embankment layers?
As per MoRTH Clause 305.3.4: compacted layers ≥ 95% MDD (Modified Proctor — IS 2720 Pt 8). Top 500 mm must be ≥ 97% MDD. Field check by sand replacement (IS 2720 Pt 28).
What should be the maximum layer thickness for embankment compaction?
Loose thickness ≤ 250 mm (MoRTH 305.3.2). Up to 300 mm may be allowed for granular soils with heavy vibratory rollers after trial and approval.
What is the permissible moisture content range during compaction?
Moisture should be within ±2% of OMC. Add water uniformly by tanker and mix with grader/rotavator; record pre/post compaction moisture.
What are acceptable limits for LL and PI of embankment material?
Typical MoRTH limits: LL ≤ 70%, PI ≤ 45%. Top 500 mm CBR ≥ 8%. Highly plastic soils should be blended or stabilised.
Which type of roller should be used for different soil types?
Cohesive soils: Sheep foot; Granular soils: Vibratory; Mixed: smooth drum + vibratory finishing. Maintain 15–20% overlap and sufficient passes determined by compaction trial.
How is the embankment top level checked for tolerance?
Tolerances (MoRTH 305.3.8): Level ±20 mm; Crossfall ±0.5%; Alignment ±100 mm. Use total station / auto level checks at regular intervals (e.g., 10 m).
What is the minimum CBR value required for embankment materials?
Top 500 mm: CBR ≥ 8%. Lower layers may be accepted ≥ 5% if approved by Engineer. Test by IS 2720 Pt 16.
What precautions should be taken during rainy conditions?
Stop filling when oversaturated; provide temporary drains; allow drying to near OMC; protect compacted surfaces; rework cracked or distressed layers.
How are borrow areas selected and managed?
Borrow pits must be approved (MoRTH 305.2.2). Avoid within 1.5× embankment height of toe; depth generally ≤1.5 m; restore or dress slopes after use to safe profiles.
What are the frequency and type of quality control tests as per MoRTH?
Representative frequencies:
| Test | IS Code | Typical Frequency |
|---|
| Field Density | IS 2720 Pt 28 | ~1 test / 500 m² / layer |
| Moisture Content | IS 2720 Pt 2 | ~1 test / 250 m³ |
| LL / PI | IS 2720 Pt 5 | 1 per source / change |
| CBR | IS 2720 Pt 16 | 1 per source / change |
| Gradation | IS 2720 Pt 4 | 1 per 1000–3000 m³ |
| MDD & OMC | IS 2720 Pt 8 | 1 per source / ~5000 m³ |
What is the minimum height of embankment above natural ground?
In flood-prone areas keep top ≥ 1.0 m above HFL (IRC:75) unless design specifies otherwise. In normal terrain follow the design profile.
What is an embankment?
A raised earthen structure carrying a roadway above natural ground; constructed using approved material compacted in lifts to required density and profile.
What materials are used for embankment construction?
Approved soils, moorum, gravel, reclaimed pavement materials, fly/pond ash, etc., free from organic/deleterious matter. PI and LL limits should comply with contract/MoRTH requirements.
What is the standard layer thickness for compaction?
Compacted (loose) layer ≤ 250 mm. Up to 300 mm for granular soils with heavy rollers after trial and approval.
What is the required degree of compaction?
Top 500 mm: 97% MDD; Remaining embankment: 95% MDD; Subgrade often targeted at 97–98% depending on spec.
How is the embankment foundation prepared?
Strip vegetation/topsoil (~150 mm), remove unsuitable material, scarify and compact natural ground to ≥95% MDD; stabilise weak soils (lime/cement/fly ash) as required.
Which field tests are used for density and how often?
Use Sand Replacement (IS 2720 Pt 28) or calibrated nuclear gauge. Typical frequency: 1 test per 500 m² per layer (adjust per contract and observed variability).
How is drainage managed?
Construct side drains and cross drains before reaching full height, design slopes appropriately (e.g., 1:4), protect slopes against erosion (turfing/stone pitching/geotextile).
How is the embankment surface finished and protected?
Final shape/trim to design, scarify top 150 mm and recompact for subgrade, protect slopes with turfing/jute/geotextile, and allow settlement before pavement works.
What are the best practice tips for site execution?
Maintain daily layer logs, run compaction trials, avoid compaction in adverse weather, bench into old embankments when widening, observe settlement and rework soft areas, and keep test records accessible for audits/claims.
