Compaction is the densification of soil materials by the use of mechanical energy. Soil is
compacted by removing air and water from its pore space. There exists a certain amount
of moisture that a soil can have where a maximum unit weight is obtained. When the soil
is compacted to this state it is referred to as the maximum moisture/ unit weight. The
maximum Moisture/ unit weight is used exclusively to determine the degree of
compaction of the earth backfill. There are several steps to determine the degree of
compaction of soil; the most widely used of those steps are described below:
Lab Analysis:
Generally before any soil compaction occurs various samples of the proposed
embankment or fill is analyzed in the Lab. One such lab test is Proctor compaction test.
The proctor compaction test is a test that compacts the soil material at various moisture
contents. There are two proctor tests that defined by the American Association of
Highway Officials and American Standard Testing Material. The Standard Proctor test
(ASTM D-698, AASHTO T-99), and the Modified Proctor test (ASTM D-1557 and
AASHTO T-180). The use of the Standard Proctor or Modified Proctor should be used at
the discretion of the Professional Engineer. The Standard Proctor Test is generally for
fills requiring the minimal sub-grade compaction, such as small parking lots, and building
structures and while the Modified Proctor is generally used to for fills that will support
large loads, such as Roadways, Airport Runways, and Concrete Parking Aprons.
The reader should study ASTM or AASHTO Standards for Modified and Standard
proctors to aid in understanding Example#1 as shown on the next page.
Field Compaction:
Field compaction of soils is mainly done with various types of rollers. The three most
common types of rollers are:
1. Sheepsfoot rollers, used mainly for clayey and Silty Soils.
2. Smooth-drum rollers, Used primarily for Granular Soils.
3. Vibratory Rollers, Used primarily for Granular Soils.
Several Factors affect the degree of field compaction. Those factors are moisture type of
soil and depth of fill. Soil is generally placed in lifts of 6” to 1 ft high. The size and type
of the compaction equipment and desired relative density is essential to determining the
depth of lift. It is at the discretion of the engineer to specify the fill lifts when compacting
backfill. Soil lifts should be specified as to give the required compaction with the least
amount of passes of the compaction equipment.