Theory: Proctor Compaction
What the Proctor compaction test measures, how the compaction curve is drawn, and how the maximum dry density and optimum water content are used to control fills.
Compaction
Compaction densifies a soil by expelling air with mechanical energy, which raises its strength and lowers its compressibility and permeability. Ralph Proctor showed in 1933 that, for a given effort, the dry density reached depends on the water content: it rises to a maximum and then falls.
On the dry side, water lubricates the grains and helps them pack; on the wet side, water fills voids that compaction can't drive out. The peak of the curve gives the maximum dry density (ρd,max) and the optimum water content (wopt).
The test
The soil is compacted in layers in a mold by a rammer dropped from a fixed height. This is repeated at increasing water contents, usually five points, two below and two above the optimum.
| ASTM D698 (method A) | NBR 7182 (normal) | |
|---|---|---|
| Mold | 4 in, 944 cm³ | Small, 1000 cm³ |
| Rammer | 24.4 N (5.5 lbf), 305 mm drop | 2.5 kg, 305 mm drop |
| Layers × blows | 3 × 25 | 3 × 26 |
| Effort | ≈ 600 kN·m/m³ | ≈ 580 kN·m/m³ |
The modified effort (ASTM D1557) uses a heavier rammer, a higher drop and more layers, and gives a higher density at a lower optimum water content.
Calculations
The curve is drawn through the points (w, ρd). This page fits it by least squares — a cubic, which follows the curve's usual skew, with five points or more, and a parabola with three or four — and takes its highest point between the driest and the wettest.
With S = 100%, it's the zero-air-voids curve: no point can plot above it. The wet side usually runs parallel to it, at S between 80 and 95%.
Line of optimums
The greater the compaction effort, the higher the maximum dry density and the lower the optimum water content. The peaks of curves at different efforts fall roughly on a straight line, the line of optimums, which runs nearly parallel to the saturation curves, at S of about 80 to 90%.
It divides the dry and wet sides for any effort: a soil compacted left of it is dry of optimum, where more effort still raises the density; right of it, more effort does little, since the remaining air can't get out.
Field control
In the field, the dry density of the compacted layer is compared with the test's maximum by the relative compaction:
Specifications usually ask for 95 to 100% relative compaction, with the water content within ±2% of the optimum. Compacting dry of optimum gives a stiffer soil, more prone to collapse; wet of optimum, a softer and less permeable one.
Limitations
- The fit only approximates the curve; with few or scattered points, the position of the peak is uncertain.
- Without points on both sides of the optimum, the peak isn't defined and more specimens must be compacted.
- Soils with much gravel call for the 6-in mold (methods B and C) or a correction for oversize particles (ASTM D4718).
References
- ASTM. D698: Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort. West Conshohocken, 2021.
- ASTM. D4718: Standard Practice for Correction of Unit Weight and Water Content for Soils Containing Oversize Particles. West Conshohocken, 2015.
- Proctor, R. R. Fundamental principles of soil compaction. Engineering News-Record, v. 111, 1933.
- Das, B. M.; Sobhan, K. Principles of Geotechnical Engineering. 9th ed. Boston: Cengage, 2018.