TM 5-852-4/AFM 88-19, Chap. 4
tion. Since only about 5 percent of the freezing index is
circulation in ducts, but at the expense of increased
usually accumulated in the last 30 days of the freezing
mechanical complexity, increased operating costs, and
season, this is a very modest factor of safety. At one
necessity for alertness to make sure the system is turned
subarctic site, the complete freezeback of the soil on the
on and off and the air flow controls set correctly at proper
downwind side of a building with ducted foundation was
times (see further discussion of this point later in this
not completed until after the soil immediately under the
chapter).
foundation slab commenced its summer thaw. A slight
(h) Part or all of the air space of a
increase in the building's interior operating temperature
ventilated foundation has sometimes been used for
could have serious consequences under such a
unheated storage purposes, particularly when extra
situation, not only because of the risk of permafrost
height of air space has resulted from variations of the
degradation but also because of the possible lowering of
natural topography. However, air circulation at the
pile supporting capacity if the structure is pile-supported.
ground and foundation freezeback are easily impaired by
(e) Experience has shown that it is
such storage, and extra accumulation of snow may be
desirable that ventilated foundations be sufficiently
induced.
elevated, positioned and sloped relative to the
(i) In ducted foundations of the
surrounding terrain to avoid accumulation of surface
general types shown in figures 4-25 and 4-27, the
water, to drain away in summer thaw water from any
vertical concrete sections between individual ducts
accumulations of ice and snow from the preceding winter
should be kept relatively thin in order to minimize
and to prevent lateral migration of water through the
conduction of heat through these members directly to the
annual thaw zone. Figure 4-25 illustrates an elevated
foundation. In pile foundations, conduction of heat into
ducted foundation. Such ducts are also more immune to
the ground by the piles should be minimized by
techniques described in f below.
blocking with soil accumulations.
(f) Ducts depressed below the
(j) Experience has shown that where
ground surface as in figures 4-26, 4-27 and 4-28 are
blowing or drifting snow occurs in winter it is very
likely to collect water from the ground or ice and soil from
important to align and locate the structure so as to
snow and dust infiltration, which restrict or block air flow
minimize snow drifting which may in any way affect the
through the ducts. If ground water rises to the duct level,
structure. Unless ventilation openings of foundations are
soil may also be piped into the ducts. Blockage is often
placed and oriented so that they will not become blocked
unnoticed until after water in the ducts has frozen. Such
by snow, the snow drifting may restrict or prevent
obstructions are very difficult to remove. Steam thawing
necessary seasonal freezeback of the foundation. Size
may be required to open them; this is not only somewhat
and shape of structures and position with respect to
complicated but may also cause thermal damage unless
prevailing wind and to other structures, to fences and
carefully controlled. Condensation of ice crystals in the
vegetation affecting wind flow, and to adjacent snow
ducts from moist air may also block the ventilating ducts
removal operations are very important in determining
if they are kept in operation when air temperatures
snow drift patterns.
Everything else being equal,
become higher than the temperatures of the duct walls in
maximum drifting tends to occur on windward and lee
200
the spring. Tobiasson has pointed out that for below-
sides of obstructions. However, even if access of winter
grade duct systems, manifolds and perhaps the ducts
air to the foundation is completely shut off by snow on
themselves should be large enough to permit entry of
the windward and lee sides, ventilating action of an open
maintenance personnel for inspection and removal of
air space type foundation may still be satisfactory if the
blockage, and provisions should be incorporated to
other two sides remain open and so long as drifting of
minimize the amount of snow infiltration and to remove
snow into open space under the building itself remains
any material which does enter. Experience indicates that
insufficient to significantly insulate the foundation
when plenum and/or stacks or chimneys, as illustrated in
materials against freezeback. Open air space type
figures 4-24, 4-26, 4-27 and 4-28, are needed to
foundations subject to drifting therefore should be
increase air flow by the stack effect or to raise intakes
designed and oriented to depend on air flow through the
and outlets sufficiently to be above maximum snow
foundation at right angles to the wind direction; the
accumulation levels, a chimney space incorporated so as
shortest dimension of the foundation should then be at
to take advantage of the building heat, as in figure 4-24,
right angles to the prevailing wind. Provisions should
is preferable to independent exterior exhaust stacks in
also be made against other possible problems such as
which cooling of the rising air tends to diminish the
blocking of ground level ventilation intakes or outlets by
200
Insulation of the stacks can reduce this
draft.
accumulation of snow next to the foundation from roof
difference. Systems should be free of air leaks to insure
discharge or from snow plowing operations. If snow
maximum circulation effectiveness.
blockage problems cannot be practically avoided through
(g) Blower systems may be used
adjustment of orientation and location, use of
when conditions require increased volume of air
4-39
Previous Page
