TM 5-818-1 / AFM 88-3, Chap. 7
the area is still unobstructed by footings or other
(3) The compaction requirements for
construction. The adequacy of compacted fills for
clean, cohesionless, granular materials will be generally
supporting structures is dependent chiefly on the
higher than those for cohesive materials, because
uniformity of the compaction effort.
Compaction
cohesionless materials readily consolidate, or liquify,
equipment generally can be used economically and
when subjected to vibration. For structures with unusual
efficiently only on large areas.
Adverse weather
stability requirements and settlement limitations, the
conditions may have a pronounced effect on the cost of
minimum density requirements indicated in table 15-2
compacted fills that are sensitive to placement moisture
should be increased. For coarse-grained, well-graded,
content, i.e., on materials having more than 10 to 20
cohesionless soils with less than 4 percent passing the
percent finer than the No. 200 sieve, depending on
No. 200 sieve, or for poorly graded cohesionless soils
with less than 10 percent, the material should be
f.
Site problems. Small building areas or
compacted at the highest practical water content,
congested areas where many small buildings or utility
preferably saturated. Compaction by vibratory rollers
lines surround the site present difficulties in regard to
generally is the most effective procedure. Experience
maneuvering large compaction equipment. Backfilling
indicates that pervious materials can be compacted to an
adjacent to structures also presents difficulties, and
average relative density of 85 + 5 percent with no
practical difficulty. For cohesionless materials, stipulate
power hand-tamping equipment must be employed, with
that the fill be compacted to either a minimum density of
considerable care necessary to secure uniform
85 percent relative density or 95 percent of CE 55
compaction. Procedures for backfilling around structures
compaction effort, whichever gives the greater density.
are discussed in TM 5-818-4 / AFM 88-5, Chapter 5.
(4) If it is necessary to use fill material
having a tendency to swell, the material should be
15-3.
Compaction requirements.
compacted at water contents somewhat higher than
a. General.
Guidelines for selecting
optimum and to no greater density than required for
compaction equipment and for establishing compaction
stability under proposed loadings (table 15-2). The
requirements for various soil types are given in table 15-
bearing capacity and settlement characteristics of the fill
1. If fill materials have been thoroughly investigated and
under these conditions should be checked by laboratory
there is ample local experience in compacting them, it is
tests and analysis. Swelling clays can, in some
preferable to specify details of compaction procedures,
instances, be permanently transformed into soils of lower
such as placement water content, lift thickness, type of
plasticity and swelling potential by adding a small
equipment, and number of passes. When the source of
percentage of hydrated lime (chap 16).
the fill or the type of compaction equipment is not known
c. Compacted rock. Compacted crushed rock
beforehand, specifications should be based on the
provides an excellent foundation fill. Vibratory rollers are
desired compaction result, with a specified minimum
preferable for compacting rock. Settlement of fill under
number of coverages of suitable equipment to assure
the action of the roller provides the most useful
uniformity of compacted densities.
information for determining the proper loose lift
b. Compaction specifications.
For most
thickness, number of passes, roller type, and material
projects the placement water content of soils sensitive to
gradation. Compaction with a 10-ton vibratory roller is
compaction moisture should be within the range of -1 to
generally preferable. The rock should be kept watered at
+ 2 percent of optimum water content for the field
all times during compaction to obviate collapse
compaction effort applied. Each layer is compacted to
settlement on loading and first wetting. As general
not less than the percentage of maximum density
criteria for construction and control testing of
specified in table 15-2. It is generally important to
rock fill are not available, test fills should be employed
specify a high degree of compaction in fills under
where previous experience is inadequate and for large
structures to minimize settlement and to ensure stability
important rock fills.
of a structure. In addition to criteria set forth in TM 5-
818-4/AFM 88-5, Chapter 5, the following factors should
be considered in establishing specific requirements:
(1) The sensitivity of the structure to total
be placed in lifts no greater than shown in table 15-1,
and differential settlement as related to structural design
preferably 8 inches or less and dependingon the soil and
is particularly characteristic of structures to be founded
type of equipment available. No backfill should be
partly on fill and partly on natural ground.
placed that contains frozen lumps of soil, as later
(2) If the ability of normal compaction
thawing will produce local soft spots. Backfill should not
equipment to produce desired densities in existing or
be placed on muddy, frozen, or frost-cov-
locally available materials within a reasonable range of
placement water content is considered essential, special
equipment should be specified.
15-2
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