TM 5-818-1 / AFM 88-3, Chap. 7
FOUNDATIONS ON FILL AND BACKFILLING
b. Foundations partially on fill.
Types of fill.
Sloping ground surface or variable foundation depths
a. Fills include conventional compacted fills;
would result in supporting a foundation partially on
hydraulic fills; and uncontrolled fills of soils or industrial
natural soil, or rock, and partially on compacted fill,
and domestic wastes, such as ashes, slag, chemical
settlement analyses are required to estimate differential
wastes, building rubble, and refuse. Properly placed
settlements. In general, a vertical joint in the structure
compacted fill will be more rigid and uniform and have
should be provided, with suitable architectural treatment,
greater strength than most natural soils. Hydraulic fills
at the juncture between the different segments of
may be compacted or uncompacted and are an
foundations. The subgrade beneath the portions of
economical means of providing fill over large areas.
foundations to be supported on natural soils or rock
Except when cohesionless materials, i.e., clean sands
should be undercut about 3 feet and replaced by
and gravels, are placed under controlled conditions so
compacted fill that is placed at the same time as the fill
silty pockets are avoided and are compacted as they are
for the portions to be supported on thicker compacted fill.
placed, hydraulic fills will generally require some type of
stabilization to ensure adequate foundations.
investigations should be made to determine the suitable
b. Uncontrolled fills are likely to provide a
sources of borrow material.
Laboratory tests to
variable bearing capacity and result in a nonuniform
determine the suitability of available materials include
settlement. They may contain injurious chemicals and,
natural water contents, compaction characteristics, grain-
in some instances, may be chemically active and
size distribution, Atterberg limits, shear strength, and
generate gases that must be conducted away from the
structure. Foundations on fills of the second and third
for use in preliminary analyses are given in table 3-1.
groups (and the first group if not adequately compacted)
The susceptibility to frost action also should be
should be subjected to detailed investigations to
considered in analyzing the potential behavior of fill
determine their suitability for supporting a structure, or
material. The scope of laboratory testing on compacted
else they should be avoided. Unsuitable fills often can
samples depends on the size and cost of the structure,
be adequately stabilized.
thickness and extent of the fill, and also strength and
compressibility of underlying soils. Coarse-grained soils
Foundations on compacted fills.
are preferred for fill; however, most fine-grained soils can
a. Compacted fill beneath foundations.
be used advantageously if attention is given to drainage,
Compacted fills are used beneath foundations where it is
compaction requirements, compaction moisture, and
necessary to raise the grade of the structure above
existing ground or to replace unsatisfactory surface soils.
d. Design of foundations on fill. Foundations
Fills constructed above the natural ground surface
can be designed on the basis of bearing capacity and
increase the load on underlying soils, causing larger
settlement calculations described in chapter 10. The
settlements unless construction of the structure is
settlement and bearing capacity of underlying foundation
postponed until fill-induced settlements have taken
soils also should be evaluated. Practically all types of
place. Settlements beneath a proposed fill can be
construction can be founded on compacted fills,
computed using methods outlined in chapter 5. If
provided the structure is designed to tolerate anticipated
computed settlements are excessive, consider
settlements and the fill is properly placed and
surcharging and postponing construction until the
compacted. Good and continuous field inspection is
expected settlement under the permanent fill loading has
occurred. Extend the fill well beyond the loading area,
e. Site preparation.
The site should be
except where the fill is placed against a cut slope.
prepared by clearing and grubbing all grass, trees,
Where the fill is relatively thick and is underlain by soft
shrubs, etc. Save as many trees as possible for
materials, check its stability with respect to deep sliding.
environmental considerations. The topsoil should be
If the fill is underlain by weaker materials, found the
stripped and stockpiled for later landscaping of fill and
footings on the fill unless settlement is excessive. If the
borrow areas. Placing and compacting fills should
fill is underlain by a stronger material, the footings may
preferably be done when
be founded on the fill or on the stronger material.