Proctor Compaction Test: A Basic Guide

Proctor Compaction Test: A Basic Guide

In the early 1930s, University of California, Berkeley student Ralph R. Proctor developed a method for determining the maximum density of soils. He established a laboratory procedure to define the moisture-density relationship of compacted, cohesive soils. Values from the Standard Proctor Test could be compared to unit weights and moistures of the same soils compacted in the field as structural earth fills to determine their degree of density and predict future performance. In 1958, the Modified Proctor Test was developed as an ASTM standard and it’s still used today concurrently with the standard test. This blog post will focus on the significance of soil compaction (particularly the Proctor Test), how the test is performed, necessary equipment and helpful tips.

Proctor Compaction Test Importance

Compaction is a method of mechanically increasing the density of soil, and it’s especially valuable in construction applications. If this process is not performed properly, soil settlement can occur, resulting in unnecessary maintenance costs or failure of the pavement or structure.

Compacting soil:

  • Increases load capacity and stability
  • Decreases permeability
  • Prevents settlement of the soils or damage from frost
  • Reduces water seepage, expansion and heaving

Proctor Tests

The Proctor Compaction Test and its variants are used to determine optimal moisture content for soils. This test is especially useful when determining the relationship between water content and the dry unit weight of soils to establish the maximum density of a soil needed for a fill area. The laboratory test serves a two-fold purpose by first determining the maximum density achievable for the materials in the field, as a reference. Secondly, it measures the effect moisture has on soil density. These values are often determined before earthwork begins to provide reference values for field testing.

Sample Preparation

A representative bulk sample is obtained for each type of material proposed for use in the earthwork operation. Back in the lab, processing of the samples begins with gradual air-drying to the desired moisture, usually around 10% or more below the anticipated optimum moisture. For cohesive soils, this can be expedited by breaking down clumps and spreading the sample out on open trays.

Once the soil is friable enough, the breakdown can continue more thoroughly. It is important to read and understand your particular test method carefully as there are a number of variables that can affect this stage of sample preparation. For most standard and modified Proctor variations, this means reducing the finer materials to pass through either a 4.75mm (#4) or 9.5mm (3/8in) sieve. Coarser materials are set aside for particle size determinations and in some cases for adding proportionally back into the final test specimens. At this stage, sample breakdown and coarse particle sizing are often performed concurrently.

Four or five specimens are prepared for the compaction points with increasing moisture contents and bracketing the estimated optimum water content. This requires some guesswork and experience is always helpful. Approximate optimum moisture of most cohesive soils can be estimated by manually squeezing a portion into a lump that will stick together, yet break cleanly into two sections when “bent”. Weight of the specimens should be about 5lb (2.3kg) each for 4in (102mm) molds and 13lb (5.9kg) for 6in (152mm) molds to ensure enough compacted volume to properly fill the molds. Water is added incrementally to increase the individual moisture contents by about 2% for each specimen and mixed thoroughly. The prepared specimens are set aside in closed containers for a prescribed amount of standing time ranging up to 16 hours for proper moisture conditioning. The containers can be sealed metal cans, but heavy-duty zip-closure bags work well for this step.

Sample Compaction

For each Proctor point, the operator compacts the specimen into the pre-weighed empty Proctor Mold in three to five layers (lifts) according to the method required. Special Manual Rammers of 5.5lb (2.5kg) dropped from a 12in (305mm) height, or 10lb Rammers with 18in (457mm) drop height are used for compaction. Automatic Compactors are also available to make this process easier. The collar is removed and any excess is carefully trimmed with a straightedge tool so the compacted soil is flush with the top of the mold. Small voids can be manually filled with excess sample. The mold with sample is then weighed and recorded and the soil is extruded from the mold. A sample of the specimen is obtained to determine exact moisture content by oven drying, and the process is repeated for subsequent samples.

For each of the initial points, the mass and the unit weight of the soil will increase as the increased moisture lubricates particles and allows them to be consolidated into a denser state from the same compactive effort. By about the fourth point (if you’re lucky and your estimates were correct) the mass of the sample will decrease as the volume of water reaches a point where it displaces soil particles in a given volume. This indicates that optimum moisture has been exceeded and having another point beyond that will make it a bit easier when constructing the final compaction curve.

Calculation and Plotting

The weight of each specimen is used to calculate wet unit weights and the oven-dried moistures are used to determine a dry unit weight for each point. The results are plotted on a graph as dry unit weight vs. moisture content and will show the curvilinear relationship that allows the maximum dry weight and optimum moisture for each type of soil to be established. These results are applied directly during field compaction tests to express the percent compaction of each test and determine if project design requirements are being met.


  • Soil Molds with either 4in or 6in diameters to hold compacted samples
  • Manual Soil Compactors enables compaction of an individual 4in Marshall asphalt sample into a stationary mold using a manually operated drop hammer
  • Mechanical Soil Compactors automatically count the number of hammer blows and shut off when a preset number is reached for improved accuracy, operation and reliability
  • Balance compliant with D4753 and 1-g readability to weigh the dry unit sample after compaction
  • Drying Oven that maintains uniformity to 230 ± 9°F (110 ± 5°C), to dry sample prior to testing
  • Straightedge to level and trim specimens held in the molds
  • E11 Sieves for particle size determinations
  • Pans for air drying, processing and mixing


  • Heavy duty plastic freezer bags are convenient for storage and moisture conditioning of individual compaction specimens.
  • When preparing individual compaction specimens, it takes little time to prepare a couple of extra, in case your estimates are a bit off. Set up one on the drier side and one on the wet side to cover the bases.
  • Some methods and materials require adding coarse material back into the final specimen. Set aside your plus size material as you break down the bulk sample. It’s also convenient to sieve the coarse material now to reduce handling.
  • Using a Sample Ejector expedites removal of compacted soil from the mold, increasing speed and efficiency and making it easier to obtain a representative moisture sample.
  • Running five or six points of a proctor test can be physically demanding, especially with a Modified Proctor using a 10lb hammer to compact five layers. If you didn’t qualify for Rio this summer, consider a Mechanical Soil Compactor. This device reduces effort and greatly enhances repeatability and accuracy.