TM 5-852-4/AFM 88-19, Chap. 4
ly on the ground, over permafrost. Figures 4-3 and 4-10
structure supported on piling in permafrost, columns
show data for large reinforced concrete structures, 3
supported on footings in permafrost, or posts and pads
stories and 5 stories, respectively, erected on clean,
over a gravel layer, as illustrated by figures 4-13 through
granular frozen soils which did not contain ice in such
4-23. For structures whose narrow width is not more
form as to cause significant settlement on thawing.
than 20 feet, the air space should technically not be less
Figure 4-11 shows the effects of various combinations of
than 18 inches and where the narrow building width is
insulation and granular mat on thawing beneath small
between 20 to 50 feet not less than 30 inches. However,
experimental buildings supported over frozen silt
since access to the air space may be required for
containing much ice. Insulation held back degradation
foundation adjustments such as jacking or shimming, for
initially but had little effect later. Figure 4-12 compares
inspection or repair of utilities, or for other reasons, the
the continuing degradation under a small building without
actual depth of the space should be enough so that it
foundation ventilation with the thermal stability achieved
may serve as a crawl space, nominally 30 to 36 inches
by supporting a structure on piles with an airspace.
minimum, regardless of size of structure. Beams, sills
(3) Ventilated foundations.
The most
and other supporting members may occupy part of this
space provided all parts of the foundation are accessible
widely employed, effective and economical means of
for maintenance, free paths of air circulation across the
maintaining a stable thermal regime in permafrost under
width of the foundation are maintained, and paths for
a heated structure is by use of a ventilated foundation.
direct conduction of heat from the building into the
In such a foundation, provision is made for either open or
foundation are kept minimal. In areas subject to snow
ducted circulation of cold winter air between the insulated
drifting, too little clearance, excessive numbers of piles
floor and the underlying ground. The air circulation
or excessive depth of framing members will reduce air
serves to carry away heat both from the foundation and
velocities and permit drifting and snow accumulation
from the overlying building, freezing back the upper
under buildings. For very wide structures or where
layers of soil which were thawed in the preceding
access to the air space is restricted, induced air
summer.
circulation by the use of plenums and/or stacks or
(a) Cold air passing through a simple
chimneys, or, less likely, by use of fans, may be required.
air space beneath a building or through a ducted
(c) When large buildings with heavy
foundation ventilation system is gradually warmed,
floor loads, such as hangars, garages and warehouses,
reaching the outlet side with a reduced air freezing index.
make provision of an open air space difficult, use of
Thus, freezeback in a ventilated foundation tends to
ventilation ducts below the insulated floor should be
progress from the intake toward the outlet side, as
considered. Examples of such designs are shown in
indicated by the asymmetrical curve of thaw penetration
figures 4-24 through 4-28.
Thermal calculation
depth in figure 4-28. In summer also, thaw tends to
procedures for ducted foundations are outlined in TM 5-
occur progressively across the foundation in the direction
14
852-/AFM 88-19, Chapter 6 . Ducted foundations are
of air flow. The freezing index at the outlet must be
normally much more expensive than open air-space
sufficient to counteract the thawing index at that point in
foundations, because of the relatively large volume of
order to insure annual freezeback of foundation soils. In
concrete and numbers of construction steps involved and
borderline discontinuous permafrost areas, this
because of the cost of pans, pipes, plywood or other
freezeback is more difficult to achieve than in colder
special duct-forming items left in place when these
climates and in these areas it may set a positive limit on
materials are used. The design shown in figure 4-24,
the feasible width of buildings for a given type of
which makes extensive use of simple prefabricated
ventilated foundation design.
Even under calm
members, demonstrates an effort to reduce the cost of
conditions, air circulation will be induced by heating of
ducted foundations. Because of the susceptibility to
the air below a building from both the ground and the
damage of ducted type construction from vertical
building. Stacks or chimneys may be used where
movements, special care must be taken that the
appropriate to induce increased circulation and they may
underlying gravel mat is of adequate thickness so that
be found to be a positive requirement. The stack or
freeze and thaw will remain within non-frost-susceptible
chimney height and the floor insulation are both very
materials to eliminate seasonal heave and settlement.
important variables in the foundation design. Increasing
(d) Ventilated
foundation
design
insulation thickness will permit lowering the stack or
should incorporate a safety factor which provides for
chimney height for the same insulation; increase in stack
complete freezeback of the underlying soil 30 days
height will increase the air flow. Potential permafrost
before the end of the freezing season, using the
degradation problems from flow of ground water in the
200
minimum site freezing index and allowing for any greater
annual zone must be carefully investigated .
freezeback requirement which may exist at the perimeter
(b) The
simplest
means
of
of the founda-
implementing foundation ventilation is by providing an
open air space under the entire building, with the
4-21
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