TM 5-818-4/AFM 88-5, Chap.5
pacted to provide a stable backfill, with the exception
(3) For coarse-grained soils containing only a
of certain bouldery soils and soils containing signifi-
small percentage (5 or less) of fine-grained particles,
cant amounts of soluble, soft, or organic materials.
maximum density is more readily obtained when the
soil is either dry or saturated. For water contents be-
B-2. Mechanics of compaction. The influ-
tween these limits, the water in the soil forms menisci
ence of the water content on compaction is markedly
between the particle contacts, which tend to hold the
different on coarse-grained, cohesionless soils and
soil particles together. This resistance to movement of
fine-grained, cohesive soils. As a result, the mechanics
particles into a more compact structure, termed appar-
or manipulation of soil grains in the two types of soil
ent cohesion or "bulking," results in lower densities
during the compaction process are different. The
than those for either a dry or saturated cohesionless
mechanics of compaction for the two soil types are dis-
soil under the same compaction effort.
cussed in subsequent paragraphs.
(4) It is to be noted that in the preceding para-
a. Compaction of coarse-grained soils. Compaction
graphs, the discussion has centered around the density
of coarse-grained soils that contain little or no fines
in weight per unit volume of coarse-grained soils with
and thus exhibit no plasticity (termed cohesionless
different gradation characteristics. A more realistic
soils) is achieved by causing the individual particles to
parameter that is often used is the relative density of
move into a closer, more compact arrangement, with
cohesionless coarse-grained soils. Relative density ex-
smaller particles filling in voids between larger parti-
presses the degree of compactness of a cohesionless
cles. The compaction energy overcomes friction at con-
soil with respect to the loosest and the densest condi-
tions of the soil that can be attained by specified
tact points between particles as they move past one an-
other into closer packing.
laboratory procedures. A soil in the loosest state would
(1) A loose volume of coarse-grained soil, such as
have a relative density of zero percent and in the dens-
gravel or sand, contains spaces or "voids" between in-
est state, a relative density of 100 percent. The dry
dividual particles that are filled with air and/or water.
unit weight of a cohesionless soil does not, by itself, re-
The density that can be obtained in such a soil under a
veal how loose or how dense the soil is due to the influ-
given amount of compaction effort depends on the
ence of particle shape and gradation on the density.
gradation and shapes of the particles and on the water
Only when viewed against the possible range of varia-
content. For a well-graded gravel or sand, the range of
tion, in terms of relative density, can the dry unit
particle sizes is sufficient to allow a fairly compact ar-
weight be related to the compaction effort used to
place the soil in a backfill or indicate the volume-
rangement of particles, with smaller particles filling in
change tendency of the soil when subjected to founda-
the voids between larger particles. For poorly graded
soil, either of uniform gradation or skip-graded (lack-
tion loads.
ing a specific range of particle sizes), the distribution
(5) Most coarse-grained soils can be compacted to
of particle sizes limits the density that can be ob-
a density such that detrimental additional consolida-
tained. Segregation of similar size particles in a skip-
tion will not take place under the prototype loading.
graded material tends to occur and prevents the voids
This factor is the first important consideration. An-
from being greatly reduced. In a uniform soil, point-to-
other important consideration may be that the com-
point contact occurs at very low compaction effort and
pacted soil be sufficiently pervious to provide good
low density results; further increase in density can
drainage. Proper consideration of these two basic
only be accomplished by crushing the grains. There-
factors will allow the use of most coarse-grained soils
fore, a well-graded, coarse-grained material can gener-
for backfill purposes.
ally be compacted to a greater density under a given
compaction effort than a poorly graded, coarse-
b. Compaction of fine-grained soils. The mechanics
grained soil. The increase in maximum density with in-
by which fine-grained soils are compacted is quite com-
crease in compaction effort will be greater for a well-
plex because capillary pressures, hysteresis, pore air
graded soil than that for a poorly graded soil.
pressure, pore water pressure, permeability, surface
(2) Rounded particle shapes facilitate movement
phenomena, osmotic pressures, and the concepts of ef-
and sliding of particles, while angular particle shapes
fective stress, shear strength, and compressibility are
restrict movement and sliding of grains in relation to
involved. Numerous theories have been developed in
one another. For either a well-graded, or a poorly grad-
an attempt to explain the compaction mechanics. The
ed, coarse-grained material, increase in angularily of
current state-of-the-art theories involving effective
grains requires a corresponding increase in compac-
stress give satisfactory explanations. The basic con-
tion effort to obtain a given density. However, a high-
cepts of these theories are discussed below.
er density can usually be attained with angular soils
(1) Fine-grained soils are compacted in a partially
because the particle shapes are more conducive to fill-
saturated state; therefore, voids or pores contain both
ing the voids.
pore air and pore water between the soil particles. Ini-
B-3
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