Modified Proctor Test – OMC and MDD of Soil (IS 2720 Part 8)

Objective

Modified Proctor test Establishes the relationship between moisture content and dry density and identify:

• Optimum Moisture Content (OMC) — moisture at which MDD occurs
• Maximum Dry Density (MDD) — highest dry unit weight for given compactive effort

Principle (Short)

With increasing moisture the dry density first increases (water lubricates particles), reaches a peak (MDD at OMC), and then decreases as excess water fills voids.

Soil Sample Collection & Preparation

1. Collect a representative sample (minimum 30–40 kg) from the site; ensure material is passing 40 mm.
2. Break large lumps (hand rammer), oven-dry sample at 105–110°C for ~24 hours, then cool to room temperature and re-break lumps.
3. Sieves recommended: 37.5 mm, 19 mm (or 20 mm) and 4.75 mm.

Equipment Required

• IS Sieves: 37.5 mm, 19/20 mm and 4.75 mm
• Electronic balance (±1 g), oven (105–110°C)
• Proctor moulds (1000 cc & 2250 cc), rammer (2.6 kg or 4.89 kg)
• Measuring jar, straight edge, spatula, moisture containers (pre-weighed)

Sample Sieving & Fraction Analysis

After oven-drying, sieve the entire sample and record weights retained and passing on each sieve.

Reconstitution logic (code logic)

1. If ≤ 5% is retained on the 19 mm sieve, use the material passing 19 mm as-is.
2. If > 5% is retained on 19 mm, discard the oversized portion and replace with equal weight of the fraction passing 19 mm but retained on 4.75 mm.

Example

Since 1.41% < 5%, the sample is used as-is in this example.

Test Preparation

1. Take 5–6 kg of oven-dried and prepared sample for one specimen.
2. Measure Natural Moisture Content (NMC) and add water. Start first trial at about 3–4% above NMC (fine-grained soils often start at ~4%).
3. Mix thoroughly, cover with plastic/damp cloth, and rest for 15–30 minutes to achieve moisture equilibrium.
4. Divide conditioned soil into five equal parts for five-layer compaction.

Compaction Procedure (Modified / Heavy Compaction)

Use the 4.89 kg rammer, 450 mm drop height, compacting in 5 layers with 55 blows per layer (IS 2720 Part 8).

1. Weigh empty mould + base plate and note as W1.
2. Attach collar, fill mould in five equal layers, and compact each layer with 55 blows using the 4.89 kg rammer (450 mm drop).
3. After final layer, ensure the compacted surface is slightly above the mould top (bottom of collar).
4. Remove collar, trim flush with a straight edge, clean mould exterior and weigh (mould + compacted soil + base) as W2.

Moisture Content Determination

1. Immediately collect ~15–20 g representative sample from the compacted specimen in a pre-weighed container.
2. Oven-dry sample at 105–110°C for 24 hours to determine actual moisture content (w).

Repeat

Repeat the test for at least 5 specimens increasing moisture content by approximately 3% each time (e.g., 4%, 7%, 10%, 13%, 16%) until compacted weight (W2 − W1) falls after reaching maximum. Take at least two readings after the peak.

Observations & Calculations

Record for each trial: wet weight, moisture content, bulk density and dry density.

Formulas

Bulk density (γb) = (W₂ − W₁) / V
Dry density (γd) = γb / (1 + w/100)

Graphical Determination

Plot Dry Density (γd) on Y-axis against Moisture Content (ω) on X-axis. The peak gives:

• MDD = 2.325 g/cc
• OMC = 8.5%

Tip: Export your test data to CSV and draw the compaction curve using a spreadsheet for better accuracy and presentation.

Results

Discussion

The Modified Proctor Test employs higher compactive energy than the Standard Proctor Test, resulting in a higher MDD and lower OMC. It is therefore suited to heavy-duty construction like highways, runways and heavy pavement layers. Field compaction targets typically range 95–98% of laboratory MDD depending on MoRTH/NHAI specifications.

Standard vs Modified – Quick Comparison

Applications & Specification Limits

Use Modified Proctor results to set field compaction targets for:

• Highway embankments
• Airport runways and aprons
• Heavy pavement subgrades
• Earth dams and rail formation layers

Typical specification limits

Conclusion

The Modified Proctor Test is essential for quality control in heavy construction projects. Laboratory-determined MDD and OMC guide field compaction to achieve durable, stable pavement and embankment performance.

References & Standards

• IS: 2720 (Part 8) – 1983: Determination of Water Content–Dry Density Relation Using Heavy Compaction
• ASTM D1557: Laboratory Compaction Characteristics Using Modified Effort
• MoRTH / NHAI Specifications for Compaction

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