TM 5-852-4/AFM 88-19, Chap. 4
(Courtesy of Building Research Advisory Board, NAS-NRC)
Figure 4-6b. Thaw vs time, Canadian locations (after Sebastyan188). (Period of thaw vs freezing index.
Data based on depth of thaw determined by soundings at 38 locations in northern Canada.)
seasonal thaw penetration in high density, extremely
ground ice under a uniformly heated structure proceeds
well-drained granular materials may be substantial and in
most rapidly near the center of the structure and more
marginal permafrost areas may reach as much as 20
slowly at the perimeters, tending to produce a bulb-
feet. Thaw depths under non-paved areas reach typical
shaped thaw front and dish-shaped settlement surface.
values as illustrated in figure 4-7 and thaw may vary
Interior footings in such a structure tend to settle
seasonally from place to place on an airfield site as
progressively in the same dish-shape, at about the same
shown in figure 4-8. The air thawing index to be used in
rate as the melting of the ice. However, a rigid
the estimate of seasonal thaw penetration should be
foundation slab tends to develop a space under it, at
established on the same statistical basis as outlined in
least for a time, after which abrupt collapse may occur.
(1) above for seasonal frost penetration. The air thawing
The larger the structure the larger the potential ultimate
index can be converted to surface thawing index by
depth of thaw; however, in initial stages of thaw, the rate
multiplying it by the appropriate thawing-conditions n-
of thaw advance under the center of the structure is not a
factor from table 4-1. The thaw penetration can then be
function of structure size. For small temporary buildings
computed using the detailed guidance given in TM 5-
it is seldom necessary to completely preclude differential
14
852-6/AFM 88-19, Chapter 6 . Approximate values of
seasonal movements even though it'may be relatively
thaw penetration may also be estimated from figure 4-4a
easy to do this; most construction camp buildings, for
for soils of the density and moisture content ranges there
example, can be maintained easily and the movements
represented. If average annual depth of thaw exceeds
brought about by frost action and thaw can be equalized
average annual freeze depth, degradation of the
by the use of jacks and shims.
permafrost will result.
(c) For large structures intended for
c. Estimation and control of thaw or freeze
long term use, maintenance requirements must be kept
beneath structures on permafrost.
at a much lower level, consequences of progressive
(1) General.
thawing may be more severe, and achievement of
adequate ground cooling and thaw depth control with a
(a) Heat flow from the structure is a
foundation ventilation system is more difficult.
major consideration in the design of a foundation in a
(2) Building floor placed on ground. When
northern area. Only when no settlement or other
the floor of a heated building is placed directly on the
adverse effects will result can heat flow from the
ground over permafrost, the depth of thaw is determined
structure to the underlying ground be ignored as a factor
by the same method as that used to solve a multilayer
in the long term structural stability.
problem when the surface is exposed to atmospheric
(b) Figure 4-9 presents an idealized
effects, as explained in TM 5-852-6/AFM 88-19, Chapter
diagram of the effect of size on both total depth of thaw
14
6 , except that the thawing index is replaced by the
and rate of thaw under a heated structure placed directly
product of the time and the differential between the
on frozen material. Thawing of uniformly distributed
4-13
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