UFC 3-220-01N
15 AUGUST 2005
Before placement of any structure foundation is begun, the plans should be
rechecked to ensure that all required utilities and conduits under or adjacent to
the foundation have been placed, so that excavating under or undermining the
foundation to place utilities and conduits will not become necessary later.
Occasionally, it may be found upon completion of the excavation that if a
structure were placed as shown on the plans, it would be supported on two
materials with drastically different consolidation characteristics, such as rock and
soil, rock and backfill, or undisturbed soil and backfill. This situation could occur
because the pre-design subsurface information was inadequate, because the
structure was relocated or reoriented by a subsequent change in the plans,
because of an oversight of the design engineer, or because of the excavation
procedures followed by the contractor. Regardless of the reason, measures
such as over-excavation and placement of subsequent backfill should be taken,
where possible, and in coordination with the design office to provide a foundation
of uniform material. Otherwise, the design office should evaluate the differences
in foundation conditions for possible changes to the structural foundation
elements.
Preparing the area to receive the backfill consists of cleaning, leveling, and
compacting the bottom of the excavation if the foundation is in soil. All debris
and foreign material, such as trash, broken concrete and rock, boulders, and
forming lumber, should be removed from the excavation. All holes, depressions,
and trenches should be filled with the same material as that specified to be
placed immediately above such a depression, unless otherwise designated, and
compacted to the density specified for the particular material used. If the
depression is large enough to accommodate heavy compacting equipment, the
sides of the depression should have a positive slope and be flat enough for
proper operation of compaction equipment. After the area is brought to a
generally level condition by compacting in lifts in accordance with specifications,
the entire area to receive backfill should be sacrificed to the depth specified, the
water content adjusted if necessary, and the area compacted as specified. If the
foundation is in rock, the area should be leveled as much as possible and all
loose material removed.
All work in the excavation should be accomplished in the dry; therefore, the
dewatering system should be operated for the duration of this work. Under no
circumstances should the contractor be allowed to dry an area by dumping a
thick layer of dry material over it to blot the excess water. If soil exists at the
foundation level and becomes saturated, it cannot be compacted. The saturated
soil will have to be removed and replaced or drained sufficiently so that it can be
compacted. Any frozen material in the foundation should be removed before
placement of concrete footings or compacted backfill.
8-5
BACKFILL OPERATIONS
8-67
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