TM 5-818-4/AFM 88-5, Chap. 5
may come from required excavation, adjacent borrow
STD-619 should be studied for more detailed informa-
pits, or commercial sources. In selecting materials to
tion.
be used, first consideration should be given to the
(1) Coarse-grained soils. Coarse-grained soils
maximum use of materials from required excavation.
include gravelly and sandy soils and range from clayey
If the excavated materials are deficient in quality or
sands (SC) through the well-graded gravels of gravel-
quantity, other sources should be considered. Common
sand mixtures (GW) with little or no fines (table 3-l).
backfill having the desired properties may be found in
They will exhibit slight to no plasticity. All of the well-
borrow areas convenient to the site, but it may be nec-
graded soils falling in this category have fairly good
essary to obtain select backfill materials having par-
compaction characteristics and when adequately com-
ticular gradation requirements, such as filter sands
pacted provide good backfill and foundation support.
and gravels and pipe or conduit bedding materials
(a) One difficulty that might arise with soils in
from commercial sources.
this category would be in obtaining good compaction
of the poorly graded sands and gravels. These poorly
a. Primary considerations. Primary considerations
graded materials may require saturation with down-
for borrow material sources are suitability and quan-
ward drainage and compaction with greater compac-
tity. Accessibility and proximity of the borrow area to
tion effort to achieve sufficiently high densities. Also,
the jobsite should also be considered. The water con-
close control of water content is required where silt is
tents of the borrow area material should be deter-
present in substantial amounts. Coarse-grained mate-
mined seasonally, and a source of water should be lo-
rials compacted to a low relative density are suscepti-
cated if the natural water contents are considerably
ble upon saturation to liquefaction under dynamic
less than the required placement water content. If sev-
loads.
eral sources of suitable backfill are available, other fac-
(b) For sands and gravelly sands with little or no
tors to be considered in selecting the borrow materials
fines, good compaction can be achieved in either the
are ease of loading and spreading and the means for
air-dried or saturated condition. Downward drainage
adding or reducing water. The need for separating or
is required to maintain seepage forces in a downward
mixing soil strata from excavation or borrow sources
direction if saturation is used to aid in compaction.
should be considered if necessary to provide reason-
Consideration may be given to the economy of adding
ably uniform engineering properties throughout the
cement to stabilize moist clean sands that are particu-
compacted backfill.
larly difficult to compact in narrow confined areas.
b. Compaction characteristics. If compaction char-
However, the addition of cement may produce zones
acteristics of the major portion of the backfill are rela-
with greater rigidity than untreated adjacent backfill
tively uniform, problems of controlling placement of
and form "hard spots" resulting in nonuniform
backfill will be significantly reduced since the in-
stresses and deformations in the structure.
spector will be able to develop more rapidly the ability
(c) Cohesionless materials are well suited for
to recognize the adequacy of the compaction proce-
placement in confined areas adjacent to and around
dures. In addition, the frequency of testing for com-
structures where heavy equipment is not permitted
paction control could be reduced. When available back-
and beneath and around irregulary shaped structures,
fill materials are unusual, test sections of compacted
such as tunnels, culverts, utilities, and tanks. Clean,
backfill are sometimes justified to develop placement
granular, well-graded materials having a maximum
procedures and to determine the engineering char-
size of 1 inch with 95 percent passing the No. 4 sieve
acteristics to be expected in field-compacted materials.
and 5 percent or less passing the No. 200 sieve are ex-
c. Workability. An important factor in choosing
cellent for use in these zones. However, a danger exists
backfill materials is the workability or ease with which
of creating zones where seepage water may accumulate
the soil can be placed and compacted. Material charac-
and saturate adjacent cohesive soils resulting in unde-
teristics that effect workability include: the ease of
sirable consolidation or swelling. In such cases, provi-
adjusting water contents in the field by wetting or
sions for draining the granular backfill, sealing the
aeration; the sensitivity to the compaction water con-
surface, and draining surface water away from the
tent with respect to optimum; and the amount of com-
structure are necessary.
paction effort required to achieve specified densities.
d. Types of backfill material. A discussion of the
Inorganic clays (CL) of low to medium plasticity (grav-
many types of backfill and their compaction character-
elly, sandy, or silty clays and lean clays) and inorganic
istics is beyond the scope of this manual since soil
silts and very fine sands (ML) of low plasticity (silty or
types will vary on each project. However, the compac-
clayey fine sands and clayey silts) are included in this
tion characteristics of several rather broad categories
category. The inorganic clays are relatively impervious
of backfill (table 3-1) are discussed briefly. MIL-
and can be compacted fairly easily with heavy compac-
3-5
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