Last reviewed 16 Sept 2026 · 6 min read
Compaction
Compaction is the densification of soil by expelling air from the voids through mechanical energy (rolling, ramming, vibration), with little change in water content. It is used for embankments, earth dams, road subgrades, backfills and foundations on fill.
Objectives: increase shear strength and bearing capacity; reduce compressibility and future settlement; reduce permeability; reduce swelling and shrinkage potential; improve stability of slopes.
| Compaction | Consolidation |
|---|---|
| Expulsion of air | Expulsion of water |
| Quick (dynamic loading) | Slow, time-dependent (static loading) |
| Mainly in unsaturated soils during construction | In saturated clays under sustained load |
| Artificial process | Natural process under structures |
Proctor compaction test
Soil at various water contents is compacted in a standard mould with a standard energy; bulk density and water content give the dry density:
| Item | Light compaction (Standard Proctor, Part 7) | Heavy compaction (Modified Proctor, Part 8) |
|---|---|---|
| Mould volume | 1000 cm³ | 1000 cm³ |
| Rammer mass | 2.6 kg | 4.9 kg |
| Height of drop | 310 mm | 450 mm |
| Layers | 3 | 5 |
| Blows per layer | 25 | 25 |
| Energy (approx.) | ≈ 593 kJ/m³ | ≈ 2700 kJ/m³ (about 4.5 times) |
(A 2250 cm³ mould with more blows is used for soils containing coarse particles.)
The plot of dry density against water content is the compaction curve:
- Optimum moisture content (OMC) — the water content at which the maximum dry density is obtained.
- Maximum dry density (MDD) — the peak of the curve.
Why a peak? At low water content, the soil is stiff and particles resist rearrangement; adding water lubricates grains and lets them pack closer. Beyond OMC, extra water occupies space that solids could fill (water cannot be expelled by rapid compaction), so dry density falls.
Zero-air-voids and air-voids lines
- With (fully saturated): zero-air-voids (saturation) line. No compaction curve can cross it; the wet side of the curve runs roughly parallel to it.
- Lines of 5% and 10% air voids are also drawn; compacted soils at OMC typically retain about 1–5% air voids (more for clays).
Factors affecting compaction
- Water content — governs the curve shape.
- Compactive effort — higher effort → higher MDD and lower OMC; the peaks of curves for different efforts lie on a line of optimums roughly parallel to the zero-air-voids line.
- Type of soil — well-graded coarse soils reach high MDD at low OMC; clays have low MDD and high OMC; uniform sands have flat curves.
- Method of compaction — static, kneading (sheepsfoot), impact, vibration: vibration works best for granular soils, kneading for clays.
- Admixtures — lime, cement, fly ash change compaction characteristics.
Effect of compaction on clay properties
| Property | Compacted dry of OMC | Compacted wet of OMC |
|---|---|---|
| Structure | Flocculated (random) | Dispersed (oriented) |
| Permeability | Higher | Lower (preferred for dam cores and liners) |
| Strength (as compacted) | Higher, brittle | Lower, ductile |
| Strength after soaking | May drop considerably | Changes less |
| Swelling on wetting | More | Less |
| Shrinkage on drying | Less | More |
| Compressibility at low stress | Lower | Higher |
| Construction pore pressure | Lower | Higher |
| Flexibility / cracking | More likely to crack | More flexible, adjusts to settlement |
Choice: clay cores of dams and landfill liners are compacted slightly wet of optimum (low permeability, flexibility); pavement subgrades and fills near structures where strength matters are compacted at or slightly dry of optimum, with attention to swelling.
Field compaction equipment
| Equipment | Best suited to |
|---|---|
| Smooth-wheeled (static) rollers | Granular bases, finishing surfaces, bituminous layers |
| Sheepsfoot rollers (tamping feet, kneading) | Clays and silty clays (not for sands) |
| Pneumatic-tyred rollers | Wide range: sands, silts, clayey sands; kneading action |
| Vibratory rollers | Granular soils (sand, gravel), rockfill |
| Vibrating plates, rammers, tampers | Confined areas, trenches, behind walls |
| Grid rollers | Weathered rock, well-graded coarse soils |
Compaction is done in layers (lifts), typically 150–300 mm loose thickness depending on equipment, with water added or soil aerated to reach the specified moisture range.
Field control
- Specification usually requires a relative compaction (field / laboratory MDD), e.g. 95% of Standard or Modified Proctor MDD, within a moisture range around OMC.
- Field density by core cutter or sand replacement; water content by rapid methods (calcium carbide).
- Proctor needle (penetration resistance) for quick checks in cohesive soils.
- Number of roller passes and lift thickness established by test strips.
Worked examples
Standard Proctor test results (mould 1000 cm³):
| (%) | 8 | 10 | 12 | 14 | 16 |
|---|---|---|---|---|---|
| Wet mass (kg) | 1.80 | 1.92 | 2.00 | 2.02 | 1.98 |
Find the OMC and MDD. = 2.68.
Solution. :
| (%) | 8 | 10 | 12 | 14 | 16 |
|---|---|---|---|---|---|
| (g/cm³) | 1.667 | 1.745 | 1.786 | 1.772 | 1.707 |
MDD ≈ 1.79 g/cm³ at OMC ≈ 12.5% (reading the fitted curve peak slightly past 12%).
At = 12%: ; ; air voids .
Find the dry unit weight on the zero-air-voids line at = 15% for = 2.70.
Solution.
A field test gives bulk density 2.05 g/cm³ at = 13%. Laboratory MDD = 1.90 g/cm³. Find the relative compaction.
Solution. g/cm³ → RC
Frequently tested points
- Compaction removes air; consolidation removes water.
- Standard Proctor: 2.6 kg, 310 mm, 3 layers × 25 blows; Modified: 4.9 kg, 450 mm, 5 layers × 25 blows (≈ 4.5× energy).
- More effort → higher MDD, lower OMC.
- Zero-air-voids line .
- Dry side: flocculated, more permeable, stronger, swells more. Wet side: dispersed, less permeable, shrinks more.
- Sheepsfoot → clays; vibratory → sands/gravels; smooth wheel → finishing and granular bases.
- Plotting bulk density instead of dry density.
- Using sheepsfoot rollers on clean sands.
- Assuming higher water content always gives better compaction.
- Compaction densifies soil by removing air, improving strength and reducing permeability and settlement.
- Proctor tests give OMC and MDD; effort and soil type shift the curve.
- Compaction water content controls structure and engineering properties of clays.
- Choose equipment by soil type and control fill by relative compaction and moisture.