Proctor Compaction Test: OMC, MDD and Standard vs Modified Proctor
Compaction is the densification of soil by removing air from its voids using mechanical energy — rolling, ramming or vibrating. The Proctor test finds the water content at which a soil compacts best, so that embankments, road subgrades and foundation fills are made as dense and strong as possible.
Standard vs Modified Proctor test
| Item | Standard Proctor (IS 2720 Part 7) | Modified Proctor (IS 2720 Part 8) |
|---|---|---|
| Mould volume | 1000 cm³ | 1000 cm³ |
| Rammer mass | 2.6 kg | 4.9 kg |
| Height of fall | 310 mm | 450 mm |
| Number of layers | 3 | 5 |
| Blows per layer | 25 | 25 |
| Compaction energy | about 593 kJ/m³ | about 2,704 kJ/m³ (about 4.5 times) |
| Used for | Ordinary fills, light rollers | Highways, airfields, heavy rollers |
Energy per unit volume = (mass × g × height of fall × blows per layer × layers) / mould volume. For the standard test: 2.6 × 9.81 × 0.31 × 25 × 3 / 0.001 m³ = 593 kJ/m³.
Test procedure in brief
- Take air-dried soil passing the 4.75 mm sieve (for the 1000 cm³ mould) and mix it with a known amount of water.
- Compact it in the mould in the specified number of layers and blows.
- Weigh the mould with soil to get the bulk density, and take a sample for water content.
- Repeat with increasing water content (about 2–3% more each time) until the density starts falling.
- Plot dry density against water content: the peak gives the MDD and the water content at the peak is the OMC.
Key formulas
- Dry density: ρd = ρ / (1 + w)
- Zero air voids line: ρd = G ρw / (1 + w G) — the highest density possible at a water content, with no air left. The compaction curve always lies below it.
- Relative compaction: (field dry density / laboratory MDD) × 100
Worked example
In a standard Proctor test, the compacted soil in the 1000 cm³ mould weighs 1980 g at a water content of 14%. G = 2.68.
- Bulk density ρ = 1980 / 1000 = 1.98 g/cm³
- Dry density ρd = 1.98 / 1.14 = 1.74 g/cm³
- Zero air voids density at 14% = 2.68 / (1 + 0.14 × 2.68) = 1.95 g/cm³, so the sample still has some air voids, as expected
- If the field dry density is 1.62 g/cm³ and the MDD is 1.74 g/cm³, relative compaction = 1.62 / 1.74 × 100 = 93% — below a 95% specification, so more rolling is needed
Factors affecting compaction
- Water content: dry soil resists compaction; too much water fills the voids and lowers density. The OMC is the balance point.
- Compaction effort: more energy gives a higher MDD at a lower OMC.
- Soil type: well-graded sands and gravels reach high MDD at low OMC; clays have a lower MDD and higher OMC.
- Method: sheep-foot rollers suit clays, vibratory rollers suit sands and gravels, and smooth-wheel rollers suit granular bases and finishing.
Exam tip: Clay compacted dry of optimum has a flocculated structure (higher strength, more permeable); compacted wet of optimum it has a dispersed structure (lower permeability, more flexible). Core walls of earth dams are therefore compacted slightly wet of optimum.
Frequently asked questions
What is OMC?
Optimum moisture content is the water content at which a soil reaches its maximum dry density for a given compaction effort.
What is the difference between Standard and Modified Proctor tests?
Standard Proctor uses a 2.6 kg rammer falling 310 mm in 3 layers of 25 blows; Modified Proctor uses a 4.9 kg rammer falling 450 mm in 5 layers of 25 blows, giving about 4.5 times more energy.
What happens to OMC and MDD with more compaction effort?
Higher compaction effort increases the maximum dry density and reduces the optimum moisture content.
What is relative compaction?
The field dry density divided by the laboratory maximum dry density, expressed as a percentage. Specifications commonly ask for 95% or more.
Soil mechanics in full — phase relations, compaction, consolidation, shear strength and foundations — is in our Geotechnical Engineering notes.
See the Geotechnical notes ›