15 AUGUST 2005
suitability for supporting a structure, or else they should be avoided. Unsuitable fills
often can be adequately stabilized.
Foundations on Compacted Fills
Compacted Fill Beneath Foundations. Compacted fills are used
beneath foundations where it is necessary to raise the grade of the structure above
existing ground or to replace unsatisfactory surface soils. Fills constructed above the
natural ground surface increase the load on underlying soils, causing larger settlements
unless construction of the structure is postponed until fill induced settlements have
taken place. Settlements beneath a proposed fill can be computed using methods
outlined in Chapter 4. If computed settlements are excessive, consider surcharging and
postponing construction until the expected settlement under the permanent fill loading
has occurred. Extend the fill well beyond the loading area, except where the fill is
placed against a cut slope. Where the fill is relatively thick and is underlain by soft
materials, check its stability with respect to deep sliding. If the fill is underlain by
weaker materials, found the footings on the fill unless settlement is excessive. If the fill
is underlain by a stronger material, the footings may be founded on the fill or on the
Foundations Partially on Fill. Where a sloping ground surface or
variable foundation depths would result in supporting a foundation partially an natural
soil, or rock, and partially on compacted fill, settlement analyses are required to
estimate differential settlements. In general, a vertical joint in the structure should be
provided, with suitable architectural treatment, at the juncture between the different
segments of foundations. The subgrade beneath the portions of foundations to be
supported on natural soils or rock should be undercut about 1 meters (3 feet) and
replaced by compacted fill that is placed at the same time as the fill for the portions to
be supported on thicker compacted fill.
Design of Foundations on Fill. Foundations can be designed on the
basis of bearing capacity and settlement calculations described in Chapter 10. The
settlement and bearing capacity of underlying foundation soils also should be evaluated.
Practically all types of construction can be founded on compacted fills, provided the
structure is designed to tolerate anticipated settlements and the fill is properly placed
and compacted. Good and continuous field inspection is essential.
Site Preparation. The site should be prepared by clearing and grubbing
all grass, trees, shrubs, etc. Save as many trees as possible for environmental
considerations. Strip and stockpile the topsoil for later landscaping of fill and borrow
areas. Placing and compacting fills should preferably be done when the area is still
unobstructed by footings or other construction. The adequacy of compacted fills for
supporting structures is dependent chiefly on the uniformity of the compaction effort.
Compaction equipment generally can be used economically and efficiently only on large
areas. Adverse weather conditions may have a pronounced effect on the cost of
compacted fills that are sensitive to placement moisture content, e.g., on materials