TM 5-818-4/AFM 88-5, Chap. 5
ditional preparation will be required prior to backfill
cur because the predesign subsurface information was
placement.
inadequate, because the structure was relocated or re-
oriented by a subsequent change in the plans, because
b. Good construction practices, and problems. As
of an oversight of the design engineer, or because of
mentioned previously, the problems associated with
the excavation procedures followed by the contractor.
foundation preparation are greatly reduced by follow-
Regardless of the reason, measures such as overexca-
ing such proper excavation procedures as maintaining
vation and placement of subsequent backfill should be
a dry excavation and planning ahead. The principles of
taken, where possible, and in coordination with the de-
good foundation preparation are simple, but enforcing
sign office to provide a foundation of uniform materi-
the provisions of the specifications concerning the
al. Otherwise, the design office should evaluate the
work is more difficult. Inspection personnel must
differences in foundation conditions for possible
recognize the importance of this phase of the work
changes to the structural foundation elements.
since, if not properly controlled, problems can result.
(5) Preparing the area to receive the backfill con-
(1) It is most important that a stable foundation
sists of cleaning, leveling, and compacting the bottom
be provided. Thus it may be necessary, particularly in
of the excavation if the foundation is in soil. All debris
the case of sensitive fine-g-rained materials, to require
and foreign material, such as trash, broken concrete
that the final excavation for footings be carefully done
and rock, boulders, and forming lumber, should be re-
with hand tools and that no equipment be allowed to
moved from the excavation. All holes, depressions,
operate on the final cut surface. To provide a working
and trenches should be filled with the same material as
platform on which to begin backfill placement on these
that specified to be placed immediately above such a
sensitive materials, it may be necessary to place an
depression, unless otherwise designated, and com-
initial layer of granular material.
pacted to the density specified for the particular ma-
(2) If the foundation is to be supported on rock,
terial used. If the depression is large enough to accom-
the soundness of the exposed rock should be checked
modate heavy compacting equipment, the sides of the
by a slaking test (soaking a piece of the rock in water
depression should have a positive slope and be flat
to determine the resulting degree of deterioration
enough for proper operation of compaction equipment.
(para 3-2b (6)) and visual observation to determine if
After the area is brought to a generally level condition
the rock is in a solid and unshattered condition. If re-
by compacting in lifts in accordance with specifica-
moval of rock below the foundation level is required,
tions, the entire area to receive backfill should be sacri-
the space should be filled with concrete. A qualified
ficed to the depth specified, the water content ad-
geological or soils engineer should inspect the area if it
justed if necessary, and the area compacted as speci-
is suspected that the material will deteriorate or swell
fied. If the foundation is in rock, the area should be
when exposed to the weather. If necessary, the mate-
leveled as much as possible and all loose material re-
rials must be protected from exposure using the
moved.
methods previously discussed in paragraph 4-1b (5).
(6) All work in the excavation should be accom-
(3) Before placement of any structure foundation
plished in the dry; therefore, the dewatering system
is begun, the plans should be rechecked to ensure that
should be operated for the duration of this work.
all required utilities and conduits under or adjacent to
Under no circumstances should the contractor be al-
the foundation have been placed, so that excavating
lowed to dry an area by dumping a thick layer of dry
under or undermining the foundation to place utilities
material over it to blot the excess water. If soil exists
and conduits will not become necessary later.
at the foundation level and becomes saturated, it can-
(4) Occasionally, it may be found upon completion
not be compacted. The saturated soil will have to be re-
of the excavation that if a structure were placed as
moved and replaced or drained sufficiently so that it
shown on the plans, it would be supported on two ma-
can be compacted. Any frozen material in the founda-
terials with drastically different consolidation charac-
tion should be removed before placement of concrete
tertistics, such as rock and soil, rock and backfill, or
footings or compacted backfill.
undisturbed soil and backfill. This situation could oc-
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