Foundations on Compacted Fills.

a. 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 previously.  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 stronger material.

b. Foundations partially on fill.   Where a Sloping ground surface or variable foundation depths
would result in supporting a foundation partially on 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 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.

c. Location of borrow.   Exploratory investigations should be made to determine the suitable
sources of borrow material.  Laboratory tests to determine the suitability of available materials include natural water contents, compaction characteristics, grain- size distribution, Atterberg limits, shear strength, and consolidation.  Typical properties of compacted materials for use in preliminary analyses are given in table 3-1.

Typical Engineering Properties of Compacted Materials
Table 3-1.  Typical Engineering Properties of Compacted Materials

The susceptibility to frost action also should be considered in analyzing the potential behavior of fill
material.  The scope of laboratory testing on compacted samples depends on the size and cost of the structure, thickness and extent of the fill, and also strength and compressibility of underlying soils.  Coarse-grained soils are preferred for fill; however, most fine-grained soils can be used advantageously if attention is given to drainage, compaction requirements,  compaction moisture,  and density control.

d. Design of foundations on fill.   Foundations can be designed on the basis of bearing capacity and
settlement calculations described previously.  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.

e. 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.  The topsoil should be stripped and stockpiled 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, i.e., on materials having more than 10 to 20 percent finer than the No.  200 sieve,  depending on gradation.

f. Site problems.   Small building areas or congested areas where many small buildings or utility lines surround the site present difficulties in regard to maneuvering large compaction equipment.  Backfilling adjacent to structures also presents difficulties, and power hand-tamping equipment must be employed, with considerable care necessary to secure uniform compaction.  Procedures for backfilling around structures are discussed in TM 5-818-4 / AFM 88-5.

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