Factors Affecting Design of Foundations in Areas of Significant Frost Penetration.

a. Physiography and geology.   Physiographic and geologic details in the area of the proposed construction are a major factor determining the degree of difficulty that may be encountered in achieving a stable foundation.  For example, pervious layers in fine-grained alluvial deposits in combination with copious groundwater supplies from adjacent higher terrain may produce very high frost-heave potential, but clean, free-draining sand and gravel terrace formations of great depth, free of excess ice, can provide virtually trouble-free foundation conditions.

b. Temperature.   The most important factors contributing to the existence of adverse foundation conditions in seasonal frost and permafrost regions are cold air temperatures and the continual changes of temperature between summer and winter.  Mean annual air temperatures usually have to be  2º to 8ºF  below freezing for permafrost to be present, although exceptions may be encountered both above and below this range.  Ground temperatures, depths of freeze and thaw, and thickness of permafrost are the product of many variables including weather, radiation, surface conditions, exposure, snow and vegetative cover, and insulating or other special courses.  The properties of earth materials that determine the depths to which freezing-and-thawing temperatures will penetrate below the ground surface under given temperature differentials over a given time are the thermal conductivity, the volumetric specific heat capacity, and the volumetric latent heat of fusion.  These factors in turn vary with the type of material, density, and moisture content.  Figure18-2 shows how ground temperatures vary during the freezing season in an area of substantial seasonal freezing having a mean annual temperature of 37ºF (Limestone, Maine), and figure 18-3 shows similar data for a permafrost area having a mean annual temperature of 26ºF (Fairbanks, Alaska).

Ground temperature during freezing season in Limestone, Maine.
Figure 18-2.  Ground temperature during freezing season in Limestone, Maine.
Ground temperatures during freezing season In Fairbanks, Alaska.
Figure 18-3.  Ground temperatures during freezing season In Fairbanks, Alaska.

(1) For the computation of seasonal freeze or thaw penetration,  freezing-and-thawing indexes  are used based upon degree-days  relative to 32 F.  For the average permanent structure, the design indexes should be those for the coldest winter and the warmest summer in 30 years of record.  This criterion is more conservative than that used for pavements because buildings and other structures are less tolerant of movement than pavements.  It is important to note that indexes found from weather records are for air about 4.5 feet above the ground; the values at ground surface, which determine freeze-and-thaw effects, are usually different, being generally smaller for freezer conditions and larger for thawing where surfaces are exposed to the sun.  The surface index,  which is the index determined for temperature immediately below the surface, is n times the air index, where n is the  correction factor.   Turf, moss, other vegetative cover, and snow will reduce the n value for temperatures at the soil surface in relation to air temperatures and hence give less freeze or thaw penetration for the same air freezing or thawing index. Values of n for a variety of conditions are given in TM 5- 852-4/AFM 88-19.

(2) More detailed information on indexes and their computation is presented in TM 5-852-6/AFM
88-19.  Maps showing distribution of index values are presented in TM 5-852-1/AFM 88-19, and TM 5-818-2/AFM 88-6.

c. Foundation materials.   The foundation design decisions may be critically affected by the foundation soil, ice, and rock conditions.

(1) Soils.

     (a) The most important properties of soils affecting the performance of engineering structures under seasonal freeze-thaw action are their frost-heaving characteristics and their shear strengths on thawing. Criteria for frost susceptibility based on percentage by weight finer than 0.02 millimeter are presented in TM 5-818-2/AFM 88-6.  These criteria have also been developed for pavements. Heave potential at the lower limits of frost susceptibility determined by these criteria is not zero, although it is generally low to
negligible from the point of view of pavement applications.  Applicability of these criteria to foundation design will vary, depending upon the nature and requirements of the particular construction.  Relative
frost-heaving qualities of various soils are shown in TM 5-818-2/AFM 88-6.

     (b) Permafrost soils cover the entire range of types from very coarse, bouldery glacial drift to clays and organic soils.  Strength properties of frozen soils are dependent on such variables as gradation, density,
degree of saturation, ice content, unfrozen moisture content, temperature, dissolved soils, and rate of loading.  Frozen soils characteristically exhibit creep at stresses as low as 5 to 10 percent of the rupture strength in rapid loading. Typical strength and creep relationships are described in TM 5-852-4/AFM 88-19,.

(2) Ice.  Ice that is present in the ground in excess of the normal void space is most obvious as more or less clear lenses, veins or masses easily visible in cores, and test pits or excavations, but it may also be so uniformly distributed that it is not readily apparent to the unaided eye.  In the annual frost zone, excess ice is formed by the common ice segregation process, although small amounts of ice may also originate from filling of shrinkage cracks; ice formations in this zone disappear each summer.  Below the annual frost zone, excess ice in permafrost may form by the same type of ice segregation process as above, may occur as vertical ice wedges formed by a horizontal contraction-expansion process, or may be "fossil ice" buried by landslides or other events.  Although most common in fine- grained soils, substantial bodies of excess ice are not uncommon in permanently frozen clean, granular deposits.  The possible adverse effects of excess ice are discussed in paragraph 18-4a(2)(b).

(3) Rock.   Bedrock subject to freezing temperatures should never be assumed problem-free in
absence of positive subsurface information.  In seasonal frost areas, mud seams in bedrock or concentrations of fines at or near the rock surface, in combination with the ability of fissures in the rock to supply large quantities of water for ice segregation, frequently cause severe frost heave.  In permafrost areas, very substantial quantities of ice are often found in bedrock, occurring in fissures and cracks and along bedding planes.

d. Water conditions.

(1) If free water drawn to developing ice segregation can be easily replenished from an aquifer
layer or from a water table within a few feet of the plane of freezing, heave can be large.  However, if a freezing soil has no access to free water beyond that contained in voids of the soil  immediately  at or below the plane of freezing, frost heave will necessarily be limited.

(2) In permafrost areas, the supply of water available to feed growing ice lenses tends to be
limited be- cause of the presence of the underlying impermeable permafrost layer, usually at relatively
shallow depths, and maximum  heave may thus be less than under otherwise similar conditions in seasonal frost areas. However, uplift forces on structures may be higher because of lower soil temperatures and consequent higher effective tangential adfreeze strength values.

(3) The water content of soil exerts a substantial effect upon the depth of freeze or thaw penetration that will occur with a given surface freezing or thawing index.  Higher moisture contents tend to ntities of
water for ice segregation, frequently cause severe frost heave.  In permafrost areas, very substantial quantities of ice are often found in bedrock, occurring in fissures and cracks and along bedding planes.

d. Water conditions.

(1) If free water drawn to developing ice segregation can be easily replenished from an aquifer layer or from a water table within a few feet of the plane of freezing, heave can be large.  However, if a freezing soil has no access to free water beyond that contained in voids of the soil  immediately  at or below the plane of freezing, frost heave will necessarily be limited.

(2) In permafrost areas, the supply of water available to feed growing ice lenses tends to be limited be- cause of the presence of the underlying impermeable permafrost layer, usually at relatively shallow depths, and maximum  heave may thus be less than under otherwise similar conditions in seasonal frost areas. However, uplift forces on structures may be higher because of lower soil temperatures and consequent higher effective tangential adfreeze strength values.

(3) The water content of soil exerts a substantial effect upon the depth of freeze or thaw penetration that will occur with a given surface freezing or thawing index.  Higher moisture contents tend to reduce penetration by increasing the volumetric latent heat of fusion as well as the volumetric specific heat capacity. While an increase in moisture also increases thermal conductivity, the effect of latent heat of fusion tends to be predominant.  TM 5-852-6/AFM 88-19, contains charts showing thermal conductivity relationships.

e. Frost-heave forces and effect of surcharge.
Frost- heave forces on structures may be quite large.
For some engineering construction, complete prevention of frost heave is unnecessary and uneconomical, but for most permanent structures, complete prevention is essential.  Under favorable soil and foundation loading conditions, it may be possible to take advantage of the effect of surcharge to control heave. It has been demonstrated in laboratory and field experiments that the rate of frost heaving is decreased by an increase of loading on the freezing plane and that frost heaving can be completely restrained if sufficient pressure is applied.

However, heave forces normal to the freezing plane may reach more than 10 tons per square foot.  Detailed information on frost-heaving pressures and on the effect of surcharge is presented in TM 5-852-4/AFM 88-19f.

f. Type of structure.   The type and uses of a structure affect the foundation design in frost areas as in other places.  Applicable considerations are discussed in TM 5-852-4/AFM 88-19.

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