What a Proctor test is
A Proctor test is a laboratory compaction test. A technician compacts samples of the same soil in a mold at several moisture contents, using a set amount of energy, and measures the dry density of each. Plotting dry density against moisture content gives a curve with a peak.
- Maximum dry density (MDD) is the dry density at the top of the curve.
- Optimum moisture content (OMC) is the moisture content at that peak.
Soil that is too dry resists compaction. Soil that is too wet holds water that cannot be squeezed out. The peak is the best balance for that energy. In the field, results are compared to these two numbers: percent compaction against MDD, and moisture against OMC.
Standard and modified Proctor
Two ASTM tests are common. They use the same idea with different compaction energy.
| Standard Proctor (ASTM D698) | Modified Proctor (ASTM D1557) | |
|---|---|---|
| Compaction energy | About 12,400 ft-lbf/ft³ (600 kN-m/m³) | About 56,000 ft-lbf/ft³ (2,700 kN-m/m³) |
| Resulting MDD | Lower | Higher |
| Resulting OMC | Higher | Lower |
| Typical use | Many general fills, embankments and some agency specs | Higher-demand work such as base courses, pavements and heavier loading |
The same soil has a different MDD under each test, so the specification must say which one applies. A requirement such as “95% of standard Proctor” is a different target than “95% of modified Proctor.” Always read the full spec wording.
Why the right curve matters
Each field test is compared to the Proctor for the material that was tested. A project can have several: different borrow sources, different layers, a cement-stabilized zone. If a field density is compared against the MDD of a different soil, the percent compaction is wrong even when the gauge reading is perfect.
Two checks help:
- The report names the material and the Proctor reference, and it matches the material placed at that location.
- The MDD and OMC on the field report match the laboratory Proctor report for that material.
When a result looks too good or too bad compared to its neighbours, check the Proctor assignment first.
Oversize particles
Proctor molds hold only part of the soil, because larger particles are removed. When a field soil has a meaningful amount of oversize material, an oversize correction may be applied to the reference values. ASTM D4718 covers this. If your soil is gravelly, ask your lab whether the reported MDD is corrected and make sure the same basis is used consistently.
How the reference values are used
percent compaction = field dry density / MDD x 100
moisture variance = field moisture content - OMC
For example, with an MDD of 123.0 pcf and an OMC of 10.5%, a field dry density of 119.3 pcf at 9.0% moisture gives about 97% compaction and a moisture variance of -1.5 points. Whether that passes depends on your spec, for example a 95% minimum and an OMC plus or minus 2 window.
Specification limits vary by agency, project and material. The numbers in this guide are examples, not recommendations. Your contract and engineer define what passes.
Next: how to read a nuclear density test report and tracking failed tests and retests.