Page 286 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
Compaction 281

13.2 PROCTOR’S DISCOVERY

13.2.1 A Direct Approach

Proctor recognized that moisture content is a major variable influencing
compaction, and therefore devised a test that isolates the role of moisture content
by holding other variables constant. Compaction was accomplished in the
laboratory by ramming a soil into a standardized steel mold using a standardized
amount of energy, and it was discovered that there is an optimum moisture
content that gives the highest density for a given compactive effort. If a soil is too
dry, more energy is required to attain a particular density, and it is too wet, no
amount of energy will compact it to the same density. The optimum moisture
content is designated by OMC.

Proctor’s test was designed to simulate the action of a sheepsfoot roller. An earlier
field standard was if a roller ‘‘walked out’’ of a soil layer as it was compacted from
the bottom up, and some contractors still use this as a criterion for adequate
compaction. However, ‘‘walking out’’ measures bearing capacity, not density, and
a roller will walk out of a dry soil much more readily than a wet one. In a properly
compacted soil the air content is so low that the soil has very little capacity for
additional water.

The hand tamper devised by Proctor is shown in Fig. 13.1. The original tamper,
shown on the left, involved dropping a 5.5 lb (24.5 N) weight 12 in. (0.305 m) to
compact soil in three layers in a steel mold, using 25 blows per layer. The volume
3
3
of the mold is 1/30 ft (944 cm ). The lifting height is controlled by housing the
rammer inside a pipe; the bottom of the pipe is placed in contact with the soil and
the rammer lifted by hand until it contacts the top, when it is dropped.

13.2.2 AASHTO T99 Procedure

Four or five tests are performed with soil at different moisture contents, and
if the soil is not re-used (ASTM 698) about 5 lb (2.25 kg) will be required.
Some laboratories re-use soil after it has been thoroughly pulverized between
tests, but this is not an ASTM-approved procedure and if done should be so stated
in the test report. Soils containing significant amounts of mica should not
be re-used as the mica breaks down. Soils containing halloysite clay mineral
are irreversibly changed by drying, and should not be air-dried prior to testing.

Air-dried soils are moistened and allowed to age overnight to allow clay
expansion. The soil is compacted in three approximately equal layers using a
collar temporarily attached to the top of the mold to hold the soil. The collar is
removed, the top of the soil struck off with a knife or straight edge, and the net
weight and moisture content of the soil determined. The test then is repeated with

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