Objective
- To determine the Optimum Moisture Content (OMC) of soil
- To determine the Maximum Dry Density (MDD) under heavy compaction
- To establish laboratory reference values for field compaction control
Standard Reference
- IS 2720 (Part 8): Determination of Water Content–Dry Density Relation Using Heavy Compaction
- MoRTH Specifications – Section 300
- IRC:SP:20 & IRC:36 (for embankment and subgrade)
Principle
When soil is compacted at increasing moisture content, its dry density increases due to lubrication of soil particles, allowing closer packing. A peak dry density is achieved at a particular moisture content known as Optimum Moisture Content (OMC). Beyond OMC, further addition of water reduces dry density as excess water occupies void spaces.
Equipment Required
- Modified Proctor mould – 2250 cc capacity
- Rammer – 4.89 kg weight with 450 mm free fall
- Oven maintained at 105–110°C
- IS sieves: 37.5 mm, 19 mm, 4.75 mm
- Mixing tools and trays
- Electronic balance (accuracy ±1 g)
Compaction Procedure
- Prepare soil sample passing 37.5 mm sieve and mix with known water content.
- Divide the soil into five equal layers.
- Compact each layer with 55 blows using a 4.89 kg rammer.
- Trim the excess soil and weigh the compacted mould.
- Determine moisture content for each trial.
- Repeat the test for at least five different moisture contents.
Observations and Calculations
| Trial No. | Moisture Content (%) | Bulk Density (g/cc) | Dry Density (g/cc) |
|---|---|---|---|
| 1 | 6.5 | 2.18 | 2.05 |
| 2 | 7.5 | 2.28 | 2.18 |
| 3 | 8.5 | 2.40 | 2.33 |
| 4 | 9.5 | 2.35 | 2.26 |
Formulas Used
Bulk Density (γb) = (W₂ − W₁) / V Dry Density (γd) = γb / (1 + w/100)
Where W₂ = weight of mould + soil, W₁ = weight of empty mould,
V = volume of mould, and w = moisture content (%).
Results
- Maximum Dry Density (MDD): 2.33 g/cc
- Optimum Moisture Content (OMC): 8.5%
Applications in Highway Engineering
- Design of embankment and subgrade layers
- Determination of field compaction requirements
- Quality control during earthwork construction
- Acceptance criteria for compaction as per MoRTH
Precautions
- Ensure uniform mixing of soil and water
- Maintain constant compaction energy
- Prevent loss of fines during preparation
- Determine moisture content accurately
Conclusion
The Modified Proctor Test provides reliable laboratory values of OMC and MDD corresponding to heavy compaction energy. These values are essential for achieving specified field density and long-term performance of highway embankments and pavement layers.
