frozen layer. When the surface heave was 55 mm the maximum swelling pressures of 54 to
91 kN/m^2 occurred at the base of the frozen soil layer.
Andersland and Ladanyi(9.10)offer a more comprehensive theoretical solution to the heave
rate and mobilized adfreeze stress taking account of climate data, soil thermal properties,
frost penetration and creep which compares well with the observations by Penner and Gold
and the other field researchers. Design for frost heave must ensure that uplift forces are not
sufficient to cause movement of the structure and that the adfreeze bond is not ruptured
causing increased rate of uplift in the permafrost zone.
Adfreezing forces on the shafts of piles and on the sides of pile caps and ground beams
can be eliminated or greatly reduced by removing frost-susceptible soils from around these
substructures to a depth equal to the maximum penetration of frost predicted. These soils are
replaced by suitable non-susceptible material such as clean sandy gravel or crushed and
graded rock. Open gravels should not be used since groundwater movements at periods of
thawing might wash fine soil particles into the voids in the gravel, leading to the formation
of a silty gravel susceptible to frost heave. Bond breakers such as grease or polyethylene
wrap on pile surfaces or an oil–wax mix in the annulus in the active zone can effectively
reduce uplift forces.
Uplift forces on the undersides of pile caps and ground beams can be reduced by
interposing a layer of compressible material between the substructure and the soil. Cellular
cardboard or low-density expanded polystyrene can be used for this purpose as shown in
Figure 7.15.
Instantaneous deformation and time-dependent deformation (creep) will occur in frozen
soil under load due to the breaking of ice bonds and the melting of a film of water around the
soil particles under compression. The degree of deformation is complex and will depend on
the stress applied, soil type and temperature. Piles in ice-rich frozen soil can be expected to
creep at a steady rate at stresses below the adfreeze strength.
9.4.3 Piling in permafrost regions
Piled foundations are generally employed where structures in permafrost regions are sited in
areas of frost-susceptible soils. Shallow foundations cannot normally be used because of the
massive volume changes which take place in the active layer under the influence of seasonal
freezing and thawing.
The general principle to be adopted when designing piled foundations is to anchor the
piles securely into a zone of stable permafrost (which can be difficult to locate) or into
non-susceptible material such as well-drained sandy gravel or relatively intact bedrock.
Where the piles are anchored into the permafrost layers their stability must be maintained
by conserving as far as possible the natural regime which existed before construction
was commenced in the area. Thus buildings must be supported well clear of the ground
(Figure 9.13) to allow winds at sub-zero temperatures to remove the heat from beneath the
buildings, and so prevent thawing of the active layer in the winter season.
The depth to which piles should be taken into the permafrost depends on the state of
stability of this zone. Consideration must be given to the recurrence of cyclic changes in the
upper layers, to the presence of layers of unfrozen water, and to the pre-treatment which can
be given to the permafrost by thawing, compaction of the soil, and re-freezing.
Compressive loads on the piles are carried almost entirely by adfreezing forces on the pile
shaft in the permanently frozen zone. Little end-bearing resistance is offered by the frozen
Miscellaneous piling problems 451