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be estimated from weather data from nearby stations. In remote areas, measured frost
depths may be entirely unavailable.
Design Depth of Ordinary Thaw Penetration. Estimates of seasonal
thaw penetration in permafrost areas should be established on the same statistical
measurement bases as outlined in paragraph 11-4.2.1 above for seasonal frost
penetration. The air thawing index can be converted to a surface thawing index by
multiplying it by the appropriate thawing conditions n-factor from UFC 3-130-01 and the
thaw penetration can then be computed using the detailed guidance given in that
criteria. Approximate values of thaw penetration may also be estimated from a chart of
the air thawing index versus the depth of thaw in UFC 3-130-01. Degradation of
permafrost will result if the average annual depth of thaw penetration exceeds the
average annual depth of frost penetration.
Thaw or Freeze Beneath Structures. Any change from natural
conditions, which results in a warming of the ground beneath a structure, can result in
progressive lowering of the permafrost table over a period of years. Heat flow from a
structure into underlying ground containing permafrost can only be ignored as a factor in
the long-term structural stability when the nature of the permafrost is such that no
settlement or other adverse effects will result. The source of heat may he not only the
building heat but also the solar radiation, underground utilities, surface water, and
groundwater flow. UFC 3-130-01 provides guidance on procedures for estimating the
depth of thaw under a heated building with time.
The most widely-employed, effective and economical, means of
maintaining a stable thermal regime under a heated structure, without degradation of
permafrost, is by use of a ventilated foundation. Under this scheme, provision is made
for the circulation of cold air between the insulated floor and the underlying ground. The
same scheme can be used for the converse situation of a refrigerated facility supported
on unfrozen ground. The simplest way of providing foundation ventilation is by
providing an open space under the entire building, with the structure supported on
footings or piling. For heavier floor loadings, ventilation ducts below the insulated floor
may be used. Experience has shown that ventilated foundations should be so elevated,
sloped, oriented, and configured as to minimize possibilities for accumulation of water,
snow, ice, or soil in the ducts. Guidance in the thermal analysis of ventilated
foundations, including the estimation of depths of summer thaw in supporting materials
and design to assure winter refreezing, is given UFC 3-130-01.
Natural or forced circulation thermal piles or refrigeration points may also
be used for overall foundation cooling and control of permafrost degradation.
Foundation Insulation. Thermal insulation may be used in foundation
construction in both seasonal frost and permafrost areas to control frost penetration,
frost heave, and condensation, and to conserve energy, provide comfort, and enhance
the effectiveness of foundation ventilation. Unanticipated loss of effectiveness by
moisture absorption must be avoided. Cellular glass should not be used where it will be
subject to cyclic freezing and thawing in the presence of moisture. Insulation thickness
and placement may be determined by the guidance given in UFC 3-130-01.