TM 5-852-4/AFM 88-19, Chap. 4
e. Handling of frozen soil from excavations is
the frozen layer may be required to accomplish the work.
Where excavation is in permafrost, this situation applies
often a problem because of the tendency of frozen lumps
year-round. Frozen soils may have strength properties
to freeze to each other or to the handling equipment
equivalent to those of lean concrete at only moderately
when temperatures are between about 15 and 32 F, or
low temperatures; at very low temperatures the
to thaw into mud in above-freezing temperatures.
f.
compressive strength may exceed 3,000 psi. Excavation
Excavation of borrow areas in frozen soil in
of frozen soils at low temperatures may be comparable
the spring and summer can also be accomplished by
to excavating concrete of low to moderate strength.
removal of thawed material periodically to promote
Frozen rock is stronger than when unfrozen.
By
thawing of underlying layers. On the other hand, if such
comparison ice may be comparatively easy to excavate.
incremental thaw depths of wet, soft materials are
A special problem is introduced by the tendency of
allowed to become deep enough to interfere with
frozen excavated materials of appreciable moisture
movement of construction equipment, construction may
content to adhere to equipment surfaces at below-
be seriously delayed or halted.
freezing temperatures or to exhibit fluid or semi-fluid
g. Frozen ground, including frozen soil and
properties when thawed. A number of research studies
highly fissured frozen bedrock, often can be broken more
have been performed on methods of penetrating,
economically and faster with a heavy bulldozer equipped
56,57,81,96,97
.
disengaging and handling frozen materials
with a sturdy ripper than with the use of explosives if a
Near-surface
frozen materials may be easier to
large area has to be excavated and the ground
excavate or penetrate in the summer months. However,
temperature is marginal (30 to 32 F). Operation over
excavation in winter is in some aspects easier than in
a large area allows maximum assistance to be gained
summer, because drainage and dewatering problems
from daily thaw increments in summer.
are reduced or eliminated. If shaped up prior to the first
h. Deep formations of frozen material can be
freeze, haul roads become as smooth and strong as
thawed to assist excavation. Cold water pumped into
pavement when frozen, requiring little maintenance
jets, placed in a grid fashion at 12 to 16-feet spacing, has
except snow plowing.
been used successfully in thawing operations and is the
c. Excavation at the face of the pit with a
most economical technique. Hot water and steam jets
dipper shovel is less affected by winter weather than
may also be used in a similar manner. Small electrically
excavation by scrapers, although when the seasonal
heated probes have local value. When excavation of
frost layer reaches a thickness of perhaps 1 1/2 to 2 feet
permafrost is required, the surface should be stripped
the soil may form slabs large enough to break a shovel
early in the summer to expose the frozen material.
boom unless the operator is careful. Various types of
Stripping of vegetative cover and removal of the annual
trench cutting equipment, as well as drop and pneumatic
thaw zone material in late summer or early fall will permit
hammers, may be used for cutting through or breaking
excavation to start at the permafrost table at the start of
up frozen soil in small areas. A snow cover over the
the following thaw season rather than at the top of the
area to be excavated limits the depth of frost penetration,
then frozen annual thaw zone, but removal of the
especially if the ground is covered early enough in the
overlying cover will have caused more intensive cooling
season and the snow is undisturbed. In the lower
of the permafrost during the winter, tending to slow the
latitudes of the cold regions, black polyethylene sheeting
daily rate of excavation. In areas where the annual thaw
placed over limited areas has been found effective in
zone is 12 to 18 inches under moss surface cover, 3 to 5
limiting freezing or promoting thawing by its absorption of
inches of thawed materials may be removed each day
solar radiation. Each day the surface cover should be
during the warm months and 25 feet may be excavated
114
removed only in the areas which are to be worked in that
in a 100-day summer operation . At bases where long-
day. In addition, for local areas the ground may be
range construction plans are known, it may be desirable,
covered with hot sand and tarpaulins or treated with salt
under certain conditions, to clear, strip, and provide
and covered with hay and tarpaulins 3 or 4 days before
drainage of future construction sites where permafrost
excavation. Salt in the soil moisture lowers its freezing
excavation will be required as far in advance of
point. Fires may also be built over the areas to be
construction as possible to minimize possible future
thawed. This method is slow and inefficient but has
subsidence and to make excavation easier. However,
been used to thaw up to 2 or 3 feet of frozen ground.
pre-thawing of finegrained soils to appreciable depth is
d. In the early winter, borrow areas, once
likely to be impractical; for example, it may produce too
opened, must be worked continuously from day to day or
soft or liquid a condition for the available construction
the frozen material will build up in thickness and be
equipment to operate effectively. Any introduction of
difficult to remove as well as present the problem of
heat into permafrost must be very carefully controlled
frozen inclusions in the fill. As the winter progresses and
and is general-
depth of freezing increases, excavating becomes more
difficult and may require special equipment or possibly
pre thawing of the frozen layer to accomplish the work.
6-2
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