TM 5-818-4/AFM 88-5, Chap. 5
CHAPTER 5
BACKFILL OPERATIONS
5-1. Placement of backfill.
achieve the required densities will usually depend
upon the type of backfill material being compacted
a. General. Backfill construction is the refilling of
and the type of zone in which the material is placed.
previously excavated space with properly compacted
(a) In open zones, coarse-grained soils that ex-
material. The areas may be quite large, in which case
hibit slight plasticity (clayey sands, silty sands, clayey
the backfilling operation will be similar to embank-
gravels, and silty gravels) should be compacted with
ment construction. On the other hand, the areas may
either sheepsfoot or rubber-tired rollers; close control
be quite limited, such as confined areas around or be-
of water content is required where silt is present in
tween and beneath concrete or steel structures and
substantial amounts. For sands and gravelly sands
areas in trenches excavated for utility lines. Prior to
with little or no fines, good compaction results are ob-
construction of the backfill, the inspection personnel
tained with tractor compaction. Good compaction can
should become thoroughly familiar with the various
also be achieved in gravels and gravel-sand mixtures
classes of backfill to be used. They should be able to
with either a crawler tractor or rubber-tired and steel-
readily identify the materials on sight, know where
wheeled rollers. The addition of vibration to any of the
the various types of material should be placed, and be
means of compaction mentioned above will usually im-
familiar with the compaction characteristics of the soil
prove the compaction of soils in this category. In con-
types. Compaction characteristics of various soil types
fined zones, adequate compaction of cohesionless soils
are discussed in appendix B.
in either the air-dried or saturated condition can be
b. Good construction practices, and problems. Prob
achieved by vibratory-plate compactors with a static
lems with placement of backfill will vary from one con-
weight of at least 100 pounds. If the material is com-
struction project to another. The magnitude of the
pacted in the saturated condition, good compaction
problems will depend on the type of materials avail-
can be achieved by internal vibration (for example, by
able such as backfill, density requirements, and the
using concrete vibrators). Downward drainage is re-
configuration of the areas in which compaction is to be
quired to maintain seepage forces in a downward di-
accomplished. Problems should be expected during the
rection if the placed material is saturated to aid in
initial stages of backfill compaction unless the con-
compaction.
tractor is familiar with compaction characteristics of
(b) Inorganic clays, inorganic silts, and very
backfill materials. The inspector can be of great as-
fine sands of low to medium plasticity are fairly easily
sistance to the contractor during this period by per-
compacted in open zones with sheepsfoot or rubber-
forming frequent water content and density checks.
tired rollers in the 15,000-pound and above wheel-load
The information from these checks will show the con-
class. Some inorganic clays can be adequately com-
tractor the effects of the compaction procedures being
pacted in confined zones using rammer or impact com-
used and point out any changes that should be made.
pactors with a static weight of at least 100 pounds pro-
(1) Backfilling procedures. Problems associated
vided close control of lift thickness and water content
with the compaction of backfill can be minimized by
is maintained.
following good backfilling procedures. Good back-
(c) Fine-grained, highly plastic materials,
filling procedures include: processing the material
though not good backfill materials, can best be com-
(para. 3-4) before it is placed in the excavation; plac-
pacted in open zones with sheepsfoot rollers. Sheeps-
ing the material in a uniformly spread loose lift of the
foot rollers leave the surface of the backfill in a rough
proper thickness suited to the compaction equipment
condition, which provides an excellent bond between
and the type of material to be used; applying the neces-
lifts. In confined areas the best results, which are not
sary compaction effort to obtain the required densi-
considered good, are obtained with rammer or impact
ties; and ensuring that these operations are not per-
compactors.
formed during adverse weather. Proper bond should be
(3) Lift thickness. The loose-lift thickness will de-
provided between each lift and also between the back-
pend on the type of backfill material and the compac-
fill and the sides of the excavation.
tion equipment to be used.
(2) Compaction equipment, backfill material, and
(a) As a general rule, a loose-lift thickness that
zones. The type of compaction equipment used. to
will result in a 6-inch lift when compacted can be al-
5-l
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