Compaction testing for road earthworks: Proctor tests and field density

Roads · 24 Oct 2026

A road is only as good as the layers under it. Poorly compacted subgrade or base settles under traffic, and the cracks show up in the surface later. Compaction control uses two kinds of test:

• a laboratory test that finds the best density the material can reach;
• a field test that measures the density actually achieved.

The laboratory test: moisture–density (Proctor)

Soil is compacted in a mould at several moisture contents, and the dry density is measured each time. Plotting the results gives a curve with a peak:

• the maximum dry density (MDD);
• the optimum moisture content (OMC) at which that density is reached.

There are two common levels of compaction effort.

The modified test uses more than four times the energy of the standard test. For the same soil it normally gives a higher MDD at a lower OMC.

The specification states which test applies to each layer. The percentage required is meaningless unless you know which test it refers to.

The field test: in-place density

After a layer is compacted, its density is measured in place. Common methods are:

• Nuclear density gauge (AASHTO T 310 / ASTM D6938). Fast, and widely used for production testing. It contains a radioactive source, so it needs trained operators and controlled handling under local radiation rules.
• Sand cone (ASTM D1556). Slower, but needs no radioactive source. It is often used to check nuclear gauge results.

For the nuclear gauge in direct transmission mode, one state transport agency's procedure (WSDOT SOP 615) also covers:

• keeping the gauge at least 6 in. (150 mm) from vertical surfaces such as trench walls;
• applying the manufacturer's corrections when testing closer than 24 in. (600 mm).

Percent compaction

The result is reported as percent compaction:

Percent compaction = field dry density ÷ laboratory maximum dry density × 100

Field dry density comes from the wet density and the moisture content:

Dry density = wet density × 100 ÷ (100 + moisture %)

Where the material contains coarse particles that were removed for the laboratory test, a correction for oversize material is applied. The WSDOT procedure sets out one method for doing this.

The specification sets the required percent compaction for each layer, often with a moisture range around the OMC. Values differ between projects and authorities, so always use the project figure.

Common reasons for failed tests

• Moisture far from optimum. Too dry and the material will not densify. Too wet and the roller pumps it.
• Layers too thick for the roller. The bottom of the layer stays loose.
• Wrong reference density. The field test is compared with the wrong Proctor curve, or the material has changed.
• Gauge problems. No daily standard count, or testing too close to a wall without correction.
• Testing too soon after rain, or on a disturbed surface.

A short checklist

• Proctor test done on the actual borrow or base material, with the method stated
• Moisture checked before rolling
• Layer thickness within the specification
• Field tests at the specified frequency and locations
• Gauge standard counts recorded daily
• Failed areas re-worked and re-tested, with the results logged

Summary

Compaction control compares what was achieved in the field with what the laboratory says is possible. Get the reference test right, control moisture and layer thickness, test at the agreed frequency, and keep clean records for each layer.

General guidance only. The authority having jurisdiction and the official text govern.