UFC 3-220-01N
15 AUGUST 2005
8-3.4
Selection of Backfill Materials. Selection of backfill materials should be based
upon the engineering properties and compaction characteristics of the materials available.
The results of the field exploration and laboratory test programs should provide adequate
information for this purpose. The materials may come from required excavation, adjacent
borrow pits, or commercial sources. In selecting materials to be used, first consideration
should be given to the maximum use of materials from required excavation. If the excavated
materials are deficient in quality or quantity, other sources should be considered. Common
backfill having the desired properties may be found in borrow areas convenient to the site, but
it may be necessary to obtain select backfill materials having particular gradation
requirements, such as filter sands and gravels and pipe or conduit bedding materials from
commercial sources.
8-3.4.1
Primary Considerations. Primary considerations for borrow material sources
are suitability and quantity. Accessibility and proximity of the borrow area to the jobsite should
also be considered. The water contents of the borrow area material should be determined
seasonally, and a source of water should be located if the natural water contents are
considerably less than the required placement water content. If several sources of suitable
backfill are available, other factors to be considered in selecting the borrow materials are ease
of loading and spreading and the means for adding or reducing water. The need for
separating or mixing soil strata from excavation or borrow sources should be considered if
necessary to provide reasonably uniform engineering properties throughout the compacted
backfill.
8-3.4.2
Compaction Characteristics. If compaction characteristics of the major portion
of the backfill are relatively uniform, problems of controlling placement of backfill will be
significantly reduced since the inspector will be able to develop more rapidly the ability to
recognize the adequacy of the compaction procedures. In addition, the frequency of testing for
compaction control could be reduced. When available backfill materials are unusual, test
sections of compacted backfill are sometimes justified to develop placement procedures and to
determine the engineering characteristics to be expected in field-compacted materials.
8-3.4.3
Workability. An important factor in choosing backfill materials is the workability
or ease with which the soil can be placed and compacted. Material characteristics that effect
workability include: the ease of adjusting water contents in the field by wetting or aeration; the
sensitivity to the compaction water content with respect to optimum; and the amount of
compaction effort required to achieve specified densities.
8-3.4.4
Types of Backfill Material. A discussion of the many types of backfill and their
compaction characteristics is beyond the scope of this manual since soil types will vary on
each project. However, the compaction characteristics of several rather broad categories of
backfill (Table 8-3.1) are discussed briefly.
8-3.4.4.1
Coarse-Grained Soils. Coarse-grained soils include gravelly and sandy soils
and range from clayey sands (SC) through the well-graded gravels of gravelsand mixtures
(GW) with little or no fines (table 8-3.1). They will exhibit slight to no plasticity. All of the well-
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