TM 5-818-4/AFM 88-5, Chap. 5
mined from estimates of loads acting on the structure
available source of water, develop segregated ice in the
caused by heavy equipment and on the strength of the
form of lenses, layers, veins, or masses commonly, but
embedded structure members as discussed in d above.
not always oriented normal to the direction of heat
A 2-foot cover over small utility conduits and pipes is
loss. The expansion of the soil mass resulting from ice
adequate protection where proper bedding procedures
segregation is called frost heave. Frost heave of soil
are followed. The minimum cover requirements over
under and against structures can cause detrimental ef-
larger diameter (6 inches or greater), rapid and flexible
fects, which can be compounded during subsequent
pipes are presented in appendix II of TM 5-820-
thawing by differential movement, loss of density, and
4/AFM 88-5, Chapter 4.
loss of shear strength. Soils of this type should not be
placed during or immediately prior to freezing
f. Slopes and bracing. Where open excavation is
temperatures and must not be placed in critical areas.
planned, consideration should be given to the slopes to
Nonfrost susceptible soils should be used at the
which the materials to be encountered can be cut and
finished grade to the depth of frost penetration when
remain stable. The stability analysis should include
the finished grade serves as a load-bearing surface.
the strength of the materials, groundwater conditions,
(2) Additives, such as calcium chloride, can be
and any surcharge load that may be imposed as the re-
used to lower the freezing temperature of soil water,
sult of stockpiles being placed or equipment operating
but such additives will ordinarily also change the com-
near the crest of the excavation. Slope stability evalua-
paction and water content requirements. Therefore,
tion procedures are described in TM 5-818-1/AFM
additives must not be used without prior investigation
88-3, Chapter 7. Shoring and bracing should be used
to determine their effect on compaction and water con-
to support excavation slopes where it is not feasible to
tent requirements. Dry sand or sand-gravel mixtures
excavate to stable slopes (TM 5-818-l/AFM 88-3,
can be placed satisfactorily when temperatures are be-
Chapter 7). Requirements for shoring and bracing
low freezing without serious effects.
safety are presented in EM 385-l-l.
(3) Protection must be provided for in-place
g. Bedding for curved-bottom structures. Founda-
permanent backfill in critical areas, such as those
tions for pipes, conduits, access tunnels, fuel and wa-
around and under structures and embedded items al-
ter storage tanks, and other curved-bottom structures
ready placed. To preclude structural damage from pos-
constructed within the backfill are considered critical
sible frost heave, backfill materials around such struc-
zones that require special attention. Any bedding ma-
tures should be insulated with a protective covering of
terial used should be free of stones or other large parti-
mulch, hay, or straw. In some instances, loose lifts of
cles that would lead to nonuniform bearing. One of the
soil can be used for insulation. However, rock or sand
most important functions of any bedding procedure is
is too porous to provide sufficient insulation and too
to provide firm support along the full length of the
permeable to resist water penetration. If soil is to be
structure. For areas where it is difficult to perform
used as an insulating material, a material completely
field density control tests because of limited working
foreign to the permanent fill, such as straw or building
space, a procedure to ensure that proper compaction is
paper, should be laid down prior to placement of the
obtained must be employed. Several methods of ob-
insulation fill so that there will be a marked distinc-
taining adequate bedding are discussed in paragraph
tion between the permanent and the temporary insula-
5-1c (2).
tion fills. In this way, when the insulation fill is re-
h. Cold weather construction. Cold weather can
moved, the stripping limits can be readily discerned.
have a very adverse effect on backfilling operations
(4) Flooding of the excavation has also been used
and can cause considerable delay. If possible, the proj-
successfully to prevent frost penetration of the in-
ect should be planned to complete backfilling opera-
place permanent backfill. However, consideration
tions prior to any extended period of freezing tempera-
must be given to possible detrimental effects of
tures. The contractor and the resident engineer must
saturating in-place backfill and the delay of removing
keep up to date with weather data so that the con-
the water at the beginning of the next construction
tractor can plan the equipment and construction force
season if it freezes into a solid mass of ice.
required to meet the construction schedule and to pro-
(5) Concrete walls and floors of completed struc-
tect the work already accomplished.
tures provide poor insulation for the fill around and
(1) The designer must establish definite limita-
beneath these structures. Therefore, these structures
tions and requirements regarding placement of back-
should be enclosed as much as possible and kept closed
fill when the ambient temperature is below freezing.
during the winter when construction is halted because
Most inorganic soils, particularly silts and lean clays,
of adverse freezing weather. Reinforcing steel protrud-
containing 3 percent, by weight, or more of particles
ing from a partially completed structure will conduct
finer than 0.02 millimetre in diameter are frost sus-
cold through the concrete and increase the rate and
ceptible. Such soils, when frozen in the presence of an
depth of frost penetration beneath the structure.
2-8
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