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 TM 5-852-4 and TM 5- 852-6/AFM 88-19, respectively.

(1)  Design depth of ordinary frost penetration.
 
     (a) For average permanent structures, the depth of frost penetration assumed for design, for situations not affected by heat from a structure, should be that which will occur in the coldest year in 30.  For a structure of a temporary nature or otherwise tolerant of some foundation movement, the depth of frost penetration in the coldest year in 10 or even that in the mean winter may be used, as may be most applicable. The design depth should preferably be based on actual measurements, or on computations if measurements are not available.  When measurements are available, they will almost always need to be adjusted by computations to the equivalent of the freezing index selected as the basis for design, as measurements will seldom be available for a winter having a severity equivalent to that value.

     (b) The frost penetration can be computed using the design freezing index and the detailed guidance given in TM 5-852-6/AFM 88-19.  For paved areas kept free of snow, approximate depths of frost penetration may be estimated from TM 5-818-2/AFM 88-6, or TM 852-3/AFM 88-19, entering the appropriate chart with the air freezing index directly.  A chart is also presented in TM 5-852-4/AFM 88-19, from which approximate depths of frost penetration may be obtained for a variety of surface conditions, using the air freezing index in combination with the appropriate surface index/air correction factor (n-factor).

     (c) In the more developed parts of the cold regions, the building codes of most cities specify minimum footing depths, based on many years of local experience; these depths are invariably less than the maximum observed frost penetrations.  The code values should not be assumed to represent actual frost penetration depths.  Such local code values have been selected to give generally suitable results for the types of
construction, soil moisture, density, and surface cover conditions, severity of freezing conditions, and building heating conditions  that are common in the area.

Unfortunately, the code values may be inadequate or inapplicable under conditions that differ from those assumed in formulating the code, especially for unheated facilities, insulated foundations, or especially cold winters.  Building codes in the Middle and North Atlantic States and Canada frequently specify minimum footing depths that range from 3 to 5 feet.  If frost penetrations of this order of magnitude occur with fine silt and claytype soils, 30 to 100 percent greater frost penetration may occur in well-drained gravels under the same conditions.  With good soil data and a knowledge of local conditions, computed values for ordinary frost penetration, unaffected by building heat, may be expected to be adequately reliable, even though the freezing index may have to be estimated from weather data from nearby stations.  In remote areas, measured frost depths may be entirely unavailable.

(2) 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 subparagraph a(2)(b) 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 TM 5-852-4/AFM 88-19. The thaw penetration can then be computed
using the detailed guidance given in TM 5-852-6/AFM 88-19.  Approximate values of thaw penetration may also be estimated from a chart of the air thawing index versus the depth of thaw in TM 5-852-4/AFM 88-19.  Degradation of permafrost will result if the average annual depth of thaw penetration exceeds the average depth of frost penetration.
 
(3) Thaw or freeze beneath structures.

     (a)  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 be not only the building heat but also the solar radiation, underground utilities, surface water, and groundwater flow.  TM 5- 852-4 and TM 5-852-6/AFM 88-19, respectively, provide guidance on procedures for estimating the depth of thaw under a heated building with time.

     (b) 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 water 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 TM 5-852-4 and TM 5-852-6/AFM 88-19, respectively.

     (c) Natural or forced circulation thermal piles or refrigeration points may also be used for overall foundation cooling and control of permafrost degradation.

 (4) 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, to conserve energy, to provide comfort, and to 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 thicknesses and placement may be determined by the guidance given in TM 5-852-4 and TM 5-852-6/AFM 88-19, respectively.

(5) Granular mats.  
In areas of significant seasonal frost and permafrost, a mat of non-frost- susceptible granular material may be used to moderate and control seasonal freeze-and-thaw effects in the foundation, to provide drainage under floor slabs, to provide stable foundation support, and to provide a dry,
stable working platform for construction equipment and personnel.  Seasonal freezing-and-thawing effects may be totally or partially contained within the mat.  When seasonal effects are only partially contained, the magnitude of seasonal frost heave is reduced through both the surcharge effect of the mat and the reduction of frost penetration into underlying frost-susceptible soils. TM 5-852-4/AFM 88-19, provides guidance in the design of mats.

(6)  Solar radiation thermal effects.  
The control of summer heat input from solar radiation is very important in foundation design in permafrost areas.

Corrective measures that may be employed include shading, reflective paint or other surface material, and sometimes live vegetative covering.  In seasonal frost areas, it may sometimes be advantageous to color critical surfaces black to gain maximum effect of solar heat in reducing winter frost problems.  TM 5-852-4/AFM 88-19, provides guidance on the control of solar radiation thermal effects.

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