TM 5-852-4/AFM 88-19, Chap. 4
building at Ft Wainwright (Ladd AFB), Fairbanks,
allowance for these in design. Modification of
100
No adverse effects could be detected. The
Alaska.
foundation conditions prior to construction. This
settlement indicated by the earlier set of reference points
includes the alternatives of removing and
in figure 4-3d may be attributed to compression of 2 to 6
replacing unacceptable foundation conditions,
feet of gravel backfill which had been placed beneath the
and thawing to eliminate permafrost.
footings and of the 3 to 4 feet of underlying gravelly soil
The principles of these three alternative methods are
which was at that time thawed to a depth of 10 feet. In
explained in the following paragraphs.
(1) Existing thermal regime to be
special cases, such as of very important or critical
maintained.
structures which can tolerate only minute settlement or
which transmit significant vibratory stresses to the
(a) This
design
approach
is
foundation, or where effects of thaw after construction
applicable for both continuous and discontinuous
would be otherwise unacceptable, it may be necessary to
permafrost zones.
employ pre-thawing (b below) followed by foundation soil
(b) In surface construction, it is
consolidation and/or stabilization in accordance with the
possible to utilize the low temperatures of the freezing
same principles and techniques as applicable under
season to maintain permanent frozen soil conditions in
similar situations in non-frost areas.
the finegrained soil at and below the depth of the
(2) In some cases, in areas of low
foundation support by providing for circulation of cold
precipitation, fine-grained soils may be encountered
winter air through a foundation ventilation system or by
which are free of ground ice and sufficiently dry and
some other method of foundation cooling. In some
compact so that they may in theory be treated in the
circumstances artificial refrigeration systems may be
same way as granular non-frost-susceptible soils.
necessary.
However, the possibility that moisture may be introduced
(c) In order for natural cooling
into such soils later, during or following construction such
methods to be practical, it is necessary to cool the upper
as from roof drains, dry wells or condensate discharge,
foundation soils sufficiently during the winter so that the
must be considered.
foundation materials thawed in the preceding summer
b. Construction when foundation supporting
will be completely refrozen, progressive annual lowering
conditions will be adversely affected by thaw.
of the permafrost table will be prevented, and there will
Permafrost in which the soil is fine-textured or contains
be sufficient "storage of cold" so that maximum
significant fractions of silt or clay frequently contains
temperatures of permafrost do not exceed limits for safe
significant amounts of ground ice in various forms such
foundation support. The latent heat of fusion of the ice
as lenses, veins, or wedges.
Bedrock also often
produced by the winter refreezing of the moisture
contains substantial ground ice. Any change from
contained in the upper soil layers will be a major factor in
natural conditions which results in a warming of the
restricting summer thaw to a shallow depth. When
ground can result in progressive lowering of the
seasonal frost heave and settlement of the soil under the
permafrost table over a period of years, known as
structure must be controlled and summer thaw must be
degradation. Thawing of high ice content materials may
prevented from reaching into underlying unsatisfactory
produce large volume reduction and settlement of
foundation materials, sufficient thickness of non-
overlying soil and structures. Consolidating soils may
frostsusceptible granular material should be placed to
have greatly reduced shear strength.
Degradation
achieve the desired effect. The flow of heat from a
subsidence in soils containing ground ice is almost
building to the permafrost is retarded, and the refreezing
invariably differential and hence potentially very
of foundation materials aided, by placing insulation
damaging to a structure. The local thaw-depression
between the floor of the building and the underlying
produced in the permafrost will tend to form a collection
foundation ventilation or cooling system.
sump for ground water, and underground components of
(d) To minimize disturbance of the
the construction may encounter a difficult water control
subsurface thermal regime, the existing vegetative cover
problem. Under some conditions lateral soil movements
and seasonal frost zone material. should be protected
may develop. Degradation may occur not only from
and preserved in non-work are. In the areas of actual
building heat but also from solar heating, in positions
construction, however, a mat of granular non-
which sunlight can reach, from ground water flow, and
frostsusceptible material should be placed over soft
from heat from underground utility lines. During the
vegetative cover to serve as a working surface, unless
winter, seasonal freezing of frost-susceptible materials
the work can be accomplished in winter without essential
may produce substantial frost heaving. For locations in
damage to the surface materials. Since it is generally
areas of fine textured soils, design should consider the
not feasible to remove such a mat later and restore the
following alternatives, as shown in figure 4-2.
vegetative cover to its original condition, the mat should
Maintenance of existing thermal regime.
Acceptance of the changes in the thermal
regime and foundation conditions which will be
caused by the construction and facility and
4-4
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