The Proctor compaction test establishes the relationship between a soil’s moisture content and its dry density, defining the maximum dry density (MDD) and optimum moisture content (OMC) used to control earthworks compaction.
What the Test Measures
By compacting soil at several moisture contents with a standard energy, the test produces a moisture-density curve whose peak is the MDD at the OMC. Field compaction is then specified as a percentage of this MDD.
Apparatus Required
- Compaction mould (1000 cm³ or CBR mould) and collar
- Standard rammer (2.5 kg for D698) or Modified rammer (4.5 kg for D1557)
- Balance, drying oven and moisture tins
- Straightedge and extruder
NL Scientific manufactures the Proctor Test Apparatus and the full soil testing range, built to ASTM, EN, BS and AASHTO requirements with ISO/IEC 17025 accredited calibration.
Test Procedure
- Prepare soil at several target moisture contents.
- Compact each in the mould in the specified number of layers and blows (3 layers/Standard, 5 layers/Modified).
- Trim level, weigh the mould to obtain bulk density.
- Take samples to determine moisture content.
- Plot dry density against moisture content for all points.
Calculation & Reporting
Compute dry density for each point and plot the curve. The peak gives maximum dry density (MDD) and the corresponding optimum moisture content (OMC).
Acceptance Criteria
Earthworks specifications require field density to reach a percentage of MDD (commonly 90–98% depending on layer and standard). Modified Proctor gives higher MDD than Standard and is used for heavily loaded layers.
Frequently Asked Questions
Standard vs Modified Proctor — which to use?
Modified Proctor (D1557) applies higher compaction energy and suits pavements and heavily loaded fills; Standard (D698) suits general earthworks. The specification dictates which applies.
Why does dry density fall after the optimum?
Beyond the OMC, excess pore water carries part of the compaction effort and prevents further densification, so dry density decreases.
Need the right equipment for your laboratory? Contact NL Scientific for a quotation — Asia’s leading manufacturer of civil engineering and material testing equipment, shipping worldwide.
