Control of Movement and Distortion - Areas of Significant Frost Penetration..

The amount of movement and distortion that may be tolerated in the support structure must be established and the foundation must be designed to meet these criteria. Movement and distortion of the foundation may arise from seasonal upward, downward, and lateral displacements, from progressive settlement arising from degradation of permafrost or creep deflections under load, from horizontal seasonal shrinkage and expansion caused by temperature changes, and from creep, flow, or slide of material on slopes.  Heave may also occur on a nonseasonal basis if there is progressive freezing in the foundation, as under a refrigerated building or
storage tank.  If the subsurface conditions, moisture availability, frost penetration, imposed loading, or other
factors vary in the foundation area, the movements will be nonuniform.  Effects on the foundation and structure may include various kinds of structural damage, jamming of doors and windows, shearing of utilities, and problems with installed equipment.

(1) Frost-heave and thaw-settlement deformations.

     (a) Frost heave acts in the same direction as the heat flow, or perpendicular to the freezing plane. Thus, a slab on a horizontal surface will be lifted directly upward, but a vertical retaining wall may experience horizontal thrust.  Foundation members, such as footings, walls, piles, and anchors, may also be gripped on their lateral surfaces and heaved by frost forces acting in tangential shear.  Figure 18-5 shows an
example of frost-heave forces developed in tangential shear on timber and steel pipe piles restrained against upward movement.

AVERAGE ADFREEZE STRESS vs TIME
AVERAGE ADFREEZE STRESS vs TIME
Figure 18-5.  Heave force tests, average tangential adfreeze bond stress versus time, and timber and steel pipe piles
placed with silt-water slurry in dry excavated holes.  Piles were installed within annual frost zone only, over permafrost, to depths from ground surface of 3.6 to 6.5 feet.

     (b) In rivers, lakes, or coastal water bodies, foundation members to which floating ice may adhere may also be subject to important vertical forces as water levels fluctuate.

     (c) Among methods that can be used to control detrimental frost action effects are placing nonfrostsusceptible soils in the depth subject to freezing to avoid frost heave or thrust; providing sufficient embedment or other anchorage to resist movement under the lifting forces; providing sufficient loading on the foundation to counterbalance upward forces; isolating foundation members from heave forces; battering or
tapering members within the annual frost zone to reduce effectiveness of heave grip; modifying soil frost susceptibility; in seasonal frost areas only, taking advantage of natural heat losses from the facility to minimize adfreeze and frost heave; or cantilevering building attachments, e.g., porches and stairs, to its main foundation.

     (d) In permafrost areas, movement and distortion caused by thaw of permafrost can be extreme and should be avoided by designing for full and positive thermal stability whenever the foundation would be adversely affected by thaw.  If damaging thaw settlement should start, a mechanical refrigeration system may have to be installed in the foundation or a program of continual jacking may have to be adopted for leveling of the structure.  Discontinuance or reduction of building heat can also be effective.  Detailed guidance is given in TM 5-852-4/AFM 88-19.

(2) Creep deformation. 
Only very small loads can be carried on the unconfined surface of ice- saturated frozen soil without progressive deformation.

The allowable long-term loading increases greatly with depth but may be limited by unacceptable creep deformation well short of the allowable stress level determined from conventional short-term test.  Present practice is to use large footings with low unit loadings; support footings on mats of well-drained non-frost- susceptible granular materials, which reduce stresses on underlying frozen materials to conservatively low values; or place foundations at sufficient depth in the ground so that creep is effectively minimized.  Pile foundations are designed to not exceed sustainable adfreeze bond strengths.  In all cases, analysis is based on permafrost temperature at the warmest time of the year.  For cases which require estimation of foundation creep behavior, see TM 5-852-4/AFM 88-19.

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