TM 5-852-4/AFM 88-19, Chap. 4
dex of air passing through a ventilation space so long as
policy to avoid this in Army construction except for the
the air temperature is below the building temperature
outermost edge of the mat contiguous with the natural
even though the temperature differential is smaller and
terrain, where the organic material can reasonably be
the thawing index increase is of smaller magnitude than
expected to support a live vegetative cover. Another
the freezing index decrease in winter.
solution is to provide a more reflective surface by using a
(5) The surface of the mat should extend
light-colored reflective aggregate cover, if available, by
outside the perimeter of the structure or any foundation
spraying with whitewash or a very light spray coating of
members at least a distance of 5 feet before sloping
white paint, or by constructing white painted pavement in
down to the ground surface. In addition, greater width
the proper position near the structure. Such methods will
should be added for walkways or vehicular traffic or to
only be successful after construction activities have
handle construction equipment where required.
ended and accumulation of dirt and dust on the surface
(6) Allowable bearing values for footings
has ceased. An insulating course in the ground may
and sills supported on granular mats vary from 2,000 psf
sometimes be used to slow rate of thaw. Positive but
on shallow, poorly graded sandy mat materials to 6,000
potentially expensive solutions which shade both the
psf on deeper, well-graded gravels and clean crushed
granular pad close to the structure and the piles at the
rock. Thickness of granular material between footing
perimeter are special hinged plywood panels which can
and underlying natural soil must be sufficient to reduce
be extended from the side of the structure as shown in
concentrated stresses on the natural soils to tolerable
figure 4-18, or timber vanes as shown in figure 4-19;
levels, as discussed in paragraph 4-4.
while acting as sunshades, these do not interfere with
f.
Protection against solar radiation thermal
the flow of ventilating air under the structure. Actually,
effects.
simple white painted moisture-durable panels of metal or
(1) Solar radiation is a major factor in the
wood or other material can be used, even if only laid on
thawing of frozen ground, particularly at very high
the ground surface in the critical locations and anchored
latitudes. A very substantial proportion of the heat
down. Vegetation also can be an effective agent for
received in summer in those regions comes from this
control of radiation effects but may not be feasible within
source.
the time frame available or under the site climatic
(2) Because the portion of a granular
conditions.
Experience indicates that protection is
foundation mat extending beyond the building perimeter
usually needed only on the south and west sides of
tends to absorb substantial solar heat, thaw of
structures, although facilities located well north of the
permafrost under this extension, unless full protection
Arctic Circle may in fact receive substantial sunlight on
against combined heat input is provided, may cause
all sides during the height of summer. Areas of a well
settlement of this portion of the mat in time. At the
ventilated foundation which are fully shaded by the
extreme perimeter of the mat where it is tapered down to
structure against sunlight may experience a rise of the
the natural ground surface, the plane of seasonal thaw
permafrost table following construction. Cantilevering
penetration tends to be depressed because of the
the edge of the structure beyond piling, post or column
thinned granular cover.
On south-and west-facing
supports so as to provide positive shade for the
embankment edges the effect is intensified by increased
foundation will help to assure maximum foundation
solar heat absorption. Settlement or even sloughing of
stability on permafrost.
the embankment edge may result, and ponding of water
(3) Individual sun shades are sometimes
on thaw-depressed natural ground at the toe of the slope
used on the upper column part of an isolated steel pile.
may further intensify the condition by increasing the
These consist of sheet metal enclosures, usually of
absorption of solar radiation. Possible adverse effects
aluminum alloy or reflective-painted light gauge steel.
on the construction and possible increased maintenance
Care must be taken in fastening them to the column as
requirements from these perimeter conditions should be
wind damage has sometimes occurred. The air-space
anticipated in the design. Typical provisions include
between enclosure and column plus the reflective
extension of the mat sufficiently far beyond the structure
surface of the enclosure is very efficient.
foundations so that the latter cannot be affected by thaw
(4) Another expedient is to increase the
settlements, including those from surface and
pile length by about 2 feet to ensure adequate bond
subsurface drainage of thaw water; re-forming of the
length in permafrost when summer heat may increase
embankment cross-section with additional material from
the thaw depth by up to 12 to 18 inches around a steel
time to time during the life of the facility so as to
pile.
gradually build up extra thickness of granular material at
(5) Probably the simplest, yet adequate,
thaw-settlement locations; and use of heat-absorptive or
approach for controlling direct radiation absorption by
heat-reflective coverings. The area of mat exposed to
piling which may be exposed to the sun is to paint the
solar heat input is sometimes covered with moss or peat
exposed portions with a highly reflective white paint.
to minimize these effects, but because of the fire hazard
contributed by the dry organic material in summer, it is
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