15 AUGUST 2005
strength occurs. In seasonal frost areas, a heaved foundation may or may not return to
its before-heave elevation. Friction on lateral surface or intrusion of softened soil into
the void space below the heaved foundation members may prevent full return.
Successive winter seasons may produce progressive upward movement. Therefore,
when the soils within the maximum depth of seasonal frost penetration are
frost-susceptible, foundations in seasonal frost areas should be supported below the
annual frost zone, using conventional foundation elements protected against uplift
caused by adfreeze grip and against frost overturning or sliding forces, or the structure
should be placed on compacted non-frost-susceptible fill designed to control frost
effects (Figure 11-4).
Foundations in Permafrost Areas. Design on permafrost areas must
cope with both the annual frost zone phenomena described in paragraph 11-4.1 and
those peculiar to permafrost.
11-22.214.171.124 Permafrost Foundations not Adversely Affected by Thaw. Whenever
possible, structures in permafrost areas should be located on clean,
non-frost-susceptible sand or gravel deposits or rock that is free of ground ice or of
excess interstitial ice, which would make the foundation susceptible to settlement on
thaw. Such sites are ideal and should be sought whenever possible. Foundation
design under these conditions can be basically identical with temperate zone practices,
even though the materials are frozen below the foundation support level, as has been
demonstrated in Corps of Engineers construction in interior Alaska. When conventional
foundation designs are used for such materials, heat from the structure will gradually
thaw the foundation to progressively greater depths over an indefinite period of years. In
5 years, for example, thaw may reach a depth of 40 ft. However, if the foundation
materials are not susceptible to settlement on thaw, there will be no effects on the
structure from such thaw. The possible effect of earthquakes or other dynamic forces
after thawing should be considered.
11-126.96.36.199 Permafrost Foundations Adversely Affected by Thaw. When
permafrost foundation materials containing excess ice are thawed, the consequences
may include differential settlement, slope instability, development of water-filled surface
depressions that serve to intensify thaw, loss of strength of frost loosened foundation
materials under excess moisture conditions, development of underground uncontrolled
drainage channels in permafrost materials susceptible to bridging or piping, and other
detrimental effects. Often, the results may be catastrophic. For permafrost soils and
rock containing excess ice, design should consider three alternatives, as indicated in
Figure 11-4: (1) maintenance of stable thermal regime, (2) acceptance of thermal
regime changes and (3) modification of foundation conditions prior to construction.
These approaches are discussed in UFC 3-130-01. Choice of the specific foundation
type from among those indicated in Figure 11-4 can be made on the basis of cost and
performance requirements after the development of details to the degree needed for
Foundation Freeze and Thaw and Techniques for Control. Detailed
guidance for foundation thermal computations and for methods of controlling freeze-
and-thaw penetration is presented in UFC 3-130-01.