The document outlines the Proctor compaction test, a method developed in the 1930s to determine the maximum density of cohesive soils and its importance in construction. It details the procedures for sample preparation, compaction, and the calculation of the moisture-density relationship, including necessary equipment. The Proctor test is critical for ensuring soil stability and preventing issues like settlement or structural failure.

1. Proctor Compaction Test
A Basic Guide

2. Proctor Compaction Test History
 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.

3. 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

4. Proctor Test
 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.

5. 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.

6. Sample Preparation
 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.

7. 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.

8. 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.

9. Proctor Compaction Curve

10. Equipment
 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

11. Tips
 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.