TM 5-818-4/AFM 88-5, Chap. 5
APPENDIX B
FUNDAMENTALS OF COMPACTION, FIELD COMPACTION TEST METHODS,
AND FIELD MOISTURE-DENSITY TEST METHODS
Section B-1. FUNDAMENTALS OF COMPACTION
B-1. Factors influencing compaction. Soil
tory compaction method may be more appropriate in
compaction is the act of increasing the density (unit
determining the maximum density. The compaction
effort used for design purposes should be the basis for
weight) of the soil by manipulation by pressing, ram-
ming, or vibrating the soil particles into a closer state
construction control.
(2) A compaction curve is developed in the impact-
of contact. The most important factors in soil compac-
compaction test by plotting densities (dry unit
tion are type of soil, water content, compaction effort,
weights) as ordinates and the corresponding water
and lift thickness. It is the purpose of field inspection
contents (as percent of dry soil weight) as abscissas.
to ensure that the proper water content, lift thickness,
For most soils the curve produced is generally para-
and compaction effort are used for each soil type so
bolic in form. Figure B-1 shows a compaction curve.
that the desired degree of compaction is obtained.
The water content corresponding to the peak of the
When the water content, lift thickness, or compaction
curve is the optimum water content. The dry unit
effort being used does not produce the desired degree
weight of the soil at the optimum water content is the
of compaction, changes may be necessary. The deter-
maximum dry density. The zero air voids curve repre-
mination of the necessary changes of these factors to
sents the relation between water content and dry den-
produce the desired degree of compaction requires
sity for 100 percent saturation of the particular mate-
knowledge of the principles governing the compaction
of soils. Therefore, it is important that inspection per-
rial tested. Thus, it shows the dry density for a given
sonnel have a general understanding of the fundamen-
water content based on the condition that all the air is
forced out of the voids by the compaction process.
tals of compaction.
a. General. It has been established through research
b. Influence of soil type. Compaction characteristics
and construction experience that there is a maximum
vary considerably with the type of soil. Figure B-2
density to which a given soil can be compacted using a
shows four compaction curves representing the water
content-density relation for four general soil types for
particular compaction effect, For each soil and a given
standard compaction. The maximum dry density for a
compaction effort, there is a unique water content,
which is called the optimum water content, that pro-
uniform sand occurs at about zero water content, al-
though density approaching maximum can be obtained
duces the maximum density. The purpose of the lab
when the sand is saturated. A very sharp peaked curve
oratory compaction test is to determine the variation
in density of a given soil at different water contents
of dry density versus water content is usually obtained
for a silt, and water content is critical to achieving
when compacted at a particular effort or efforts. Nor-
mally, the soil to be used is compacted in the labora-
maximum density. A small change in water content (as
tory over a range of water contents using the impact-
small as 0.5 percentage point) above or below optimum
compaction procedures given in MIL-STD-621A and
causes a significant decrease in the density (as much as
ASTM D 1557. The compaction effort used is selected
2 to 4 pounds per cubic foot) for a given compaction ef-
on the basis of the requirements of the structure. In
fort. The compaction curve for a lean clay is not as
foundation or backfill design for most major struc-
sharp as that for the silt, and water content control is
tures, the CE 55 (also termed modified) compaction ef-
not as critical. Optimum water contents for silts and
fort that produces approximately 56,000 foot-pounds
lean clays generally range between 15 and 20 percent.
per cubic foot of compacted soil should be used.
The compaction curve for fat clays is rather flat and
(1) For some cohesionless soils, a greater maxi-
water content is not particularly critical to obtaining
mum density can be obtained using vibratory-type
maximum density; a 2 to 3 percentage point change in
compaction procedures given in EM 1110-2-1906 and
water content from optimum for fat clays causes only
ASTM D 2049 than can be obtained using
a small decrease (1 pound per cubic foot or less) in den-
MIL-STD-621A or ASTM D 1557 impact-compaction
sity. The maximum dry density, as obtained in labora-
procedure. Thus, there may be cases where the vibra-
tory compaction tests using MIL-STD-621A and
B-l
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