Selection of Foundation Type - Areas of Significant Frost Penetration..

Only sufficient discussion of the relationships between foundation conditions and design decisions is given below to indicate the general nature of the problems and solutions. Greater detail is given in TM 5-852-4/AFM 88-19.

(1) Foundations in seasonal frost areas.

     (a) When foundation materials within the maximum depth of seasonal frost penetration consist of clean sands and gravels or other non-frost-susceptible materials that do not develop frost heave or thrust, or
thaw weakening, design in seasonal frost areas may be the same as for nonfrost regions, using conventional
foundations, as indicated in figure 18-4.  Effect of the frost penetration on related engineering aspects, such as surface and subsurface drainage systems or underground utilities, may need special consideration.

Permanent Construction, Temporary Construction
Figure 18-4.  Design alternatives.

Thorough investigation should be made to confirm the nonfrost susceptibility of subgrade soils prior to design for this condition.

      (b) When foundation materials within the annual frost zone are frost-susceptible, seasonal
frost heave and settlement of these materials may occur.

In order for ice segregation and frost heave to develop, freezing temperatures must penetrate into the ground,
soil must be frost-susceptible, and adequate moisture must be available.  The magnitude of seasonal heaving is dependent on such factors as rate and duration of frost penetration, soil type and effective pore size, surcharge, and degree of moisture availability.  Frost heave in a freezing season may reach a foot or more in silts and some clays if there is an unlimited supply of moisture available.  The frost heave may lift or tilt foundations and structures, commonly differentially, with a variety of possible consequences.

     (c) When thaw occurs, the ice within the frostheaved soil is changed to water and escapes to
the ground surface or into surrounding soil, allowing overlying materials and structures to settle.  If the water
is released by thaw more rapidly than it can be drained away or redistributed, substantial loss in soil 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.

     (d) 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 (fig 18-4).

(2)  Foundations in permafrost areas.
Design on permafrost areas must cope with both the annual frost zone phenomena described in paragraph 18-4a(l) and those peculiar to permafrost. 

     (a) 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 are 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 feet.  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.

     (b)  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 intensity thaw, loss of strength of frostloosened 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 18-4: maintenance of stable thermal regime, acceptance of thermal regime changes, and modification of foundation conditions prior to construction.  These approaches are discussed in TM 5-852-4/AFM 88-19.  Choice of the specific foundation type from among those indicated in figure 18-4 can be made on the basis of cost and performance requirements after the development of details to the degree needed for resolution.

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