a. Types of areas.   For purposes of this manual, areas of  significant  frost  penetration  may  be factor in
foundation design.  Detailed requirements of engineering design in such areas are given in TM 5-818-2/AFM 88-6, and the Arctic  and  Subarctic  Construction series,  TM 5-852-1  through  9/AFM  88-19,  respectively.   Areas of significant  frost penetration may be subdivided as follows:

(1) Seasonal frost areas.  

     (a) Significant ground freezing occurs in these areas during the winter season, but  without
development of  permafrost.

In northern  Texas, significant seasonal  frost occurs about 1  year in  10.  A little  farther north it is experienced every year.  Depth of seasonal  freezing  increases northward with decreasing mean  annual and winter air temperatures until permafrost is encountered.  With still further decrease of air temperatures, the depth of annual  freezing  and thawing becomes progressively thinner.

     (b) The layer extending through both seasonal  frost and permafrost areas in which annual freeze-thaw cycles occur is called the annual frost zone.

In permafrost areas, it is also called the active layer.  It is usually not more than 10 feet thick, but it may exceed 20 feet.  Under conditions of  natural cover in  very  cold permafrost areas,  it may be as  little as 1  foot thick.  Its thickness  may  vary  over  a wide range even within a small area.  Seasonal changes in soil properties in  this layer are caused principally by the  freezing and thawing of  water contained in the soil.   The water  may be permanently  in  the  annual  frost  zone or may be drawn into  it  during  the  freezing process and released during thawing.    Seasonal  changes are also produced by
shrinkage and expansion caused by  temperature changes.

(2) Permafrost areas.

     (a) In these areas, perennially  frozen ground is  found below the annual  frost  zone.  In North America, permafrost is found principally north of latitudes 55  to  65 degrees, although patches of  permafrost are found much farther south on mountains where the temperature conditions are sufficiently low, including some mountains in the contiguous 48 States.  In areas of continuous permafrost,  perennially frozen ground is absent only at a few widely scattered locations, as at the bottoms of  rivers and lakes.   In  areas  of  discontinuous permafrost,  permafrost is found intermittently in various degrees.  There may be discontinuities in both horizontal and vertical extent.   Sporadic permafrost  is permafrost occurring in the form of scattered permafrost islands.  In the coldest parts of the Arctic, the ground may be frozen as deep as 2000 feet.

     (b) The geographical boundaries between zones of continuous permafrost, discontinuous permafrost,  and seasonal  frost without permafrost are poorly  de-  fined but are represented approximately in figure 18-1.

Frost and permafrost in North America.
Figure 18-1.  Frost and permafrost in North America.

b. General nature of design problems. 
Generally, the design of  foundations in areas of  only seasonal  frost  follows the same procedure  as  where frost is in- significant or absent, except  that precautions are  taken  to avoid winter damage  from  frost heave or thrust.  In the spring, thaw and settlement  of  frost- heaved  material  in  the annual  frost  zone  may occur differentially,  and a very wet, poorly drained ground condition  with  temporary but substantial loss of shear strength is typical.

(1) In permafrost areas, the  same  annual frost  zone phenomena occur, but the presence of  the underlying permafrost introduces additional potentially complex problems.   In permafrost areas, heat  flow  from buildings is a  fundamental  consideration,  complicating the design of all but the simplest buildings.  Any change from natural  conditions that results in a warming of  the ground beneath a structure can result in progressive lowering  of  the  permafrost table over a period of  years that is known as degradation.  If the permafrost contains ice in excess of  the natural void or  fissure space of  the material  when  unfrozen, progressive downward thaw may  result  in  extreme settlements or overlying soil and structures.  This condition can be very serious because such subsidence is almost invariably differential  and hence  very  damaging to a structure.  Degradation may occur  not  only  from  building heat but also  from  solar
heating,  as under pavements,  from  surface water and groundwater  flow, and  from  underground  utility  lines.

Proper  insulation will prevent degradation in some situations, but where a continuous source of heat is available, thaw will in most cases eventually occur.

(2) The more intense the winter cooling of the frozen layer in the annual frost zone and the more
rapid the rate of frost heave, the greater the intensity of uplift forces in piles and foundation walls.  The lower the temperature of permafrost, the higher the bearing capacity and adfreeze strength that can be developed, the lower the creep deformation rate under footings and in tunnels and shafts, and the faster the freeze-back of slurried piles.  Dynamic response characteristics of foundations are also a function of temperature.  Both natural and manufactured construction materials experience significant linear and volumetric changes and may fracture with changes in temperature.  Shrinkage cracking of flexible pavements is experienced in all cold
regions.  In arctic areas, patterned ground is widespread, with vertical ice wedges formed in the polygon boundaries.  When underground pipes, power cables, or foundation elements cross shrinkage cracks, rupture may occur during winter contraction.  During summer and fall, expansion of the warming ground may cause substantial horizontal forces if the cracks have become filled with soil or ice.

(3) Engineering problems may also arise from such factors as the difficulty of excavating and handling ground when it is frozen; soft and wet ground conditions during thaw periods; surface and subsurface drainage problems; special behavior and handling requirements for natural and manufactured materials at low temperatures and under freeze-thaw action; possible ice uplift and thrust action on foundations; condensation on cold floors; adverse conditions of weather, cost, and sometimes accessibility; in the more remote locations, limited local availability of materials, support facilities, and labor; and reduced labor efficiency at low temperatures.

(4) Progressive freezing and frost heave of foundations may also develop under refrigerated
ware- houses and other facilities where sustained interior below-freezing temperatures are maintained.  The design procedures and technical guidance outlined in this chapter may be adapted to the solution of these design problems.

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