TM 5-852-4/AFM 88-19, Chap. 4
temperatures, inside building temperatures, floor and
to contain all thaw penetration and the required stack
duct or pipe system design, temperature and velocity of
height to insure freezeback of the pad on the outlet side
air in the system, and stack height. Cold air passing
of the ducts for the following conditions:
Duct length l, 220 feet.
through the ducts acquires heat from the duct walls and
Gravel pad: γd = 125 lb/ft , w = 2.5 percent.
experiences a temperature rise longitudinally along the
duct with a reduction in air-freezing index at the outlet.
Outlet mean annual temperature, 32 F. (This is
Field observations indicated the inlet air-freezing index to
a conservative assumption.)
closely approximate the site air-freezing index. The
Minimum site freezing index, 4,000 degree-days.
freezing index at the outlet should be sufficient to
Freezing season, 215 days.
counteract the thawing index and insure freeze back of
Thawing season, 150 days (period during which
ducts are closed).
Example: Determine the required thickness of a
gravel pad beneath the floor section shown in figure 4-29
U. S. Army Corps of Engineers
Figure 4-29. Schematic of ducted foundation14.