UFC 3-220-01N
15 AUGUST 2005
11-1.2
General Nature Of Design Problems. Generally, the design of
foundations in areas of only seasonal frost follows the same procedure as where frost is
insignificant 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.
11-1.2.1
Permafrost Areas. 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.
11-1.2.2
Seasonal Frost Areas. 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
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.
11-2
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