Pile Design and Construction Practice, Fifth edition

452 Miscellaneous piling problems
Bored-andcast-in-situ
piles with
permanent steel
tube casing
Figure 9.13 Piling into permafrost.
Building
Piles frozen
into stable
permafrost
Wind at freezing
temperature
Active layer subject
to seasonal freezing
and thawing
Unstable permafrost
subject to cyclic
changes
ground due to the re-packing and re-crystallization of ice under pressure and the migration of
unfrozen water. Uplift forces on the piles which occur as a result of adfreezing in the active
layer in the winter season must be allowed for. Results of tests on laterally loaded piles in
permafrost and computer simulations of the displacements are described by Foriero et al. (9.17)
Generally, it is not recommended to drive piles into permafrost at temperatures less than
�5�C since this will cause splitting of the frozen ground, allowing thawing waters to
penetrate deeply into the cracks, and so upsetting the stable regime. However, reinforced
H-piles and tubular steel piles with wall thickness greater than 12.5 mm can be driven into
relatively warm permafrost (�1�C to �5�C) using vibratory hammers without pilot holes.
In a research project, Canadian Petroleum Engineering Inc has driven 500 mm diameter
steel piles 53 m into permafrost at �7�C using a high frequency pile driver. Generally,
adfreeze occurs earlier in driven piles, but driving resistance should not be used to calculate
long-term capacity of piles in permafrost. Driving can be easier in saline permafrost in
fine and coarse soils, because of the greater quantities of unfrozen moisture around the pile;
however, the bond is reduced due to the saline porewater pressure, reducing the pile capacity
by as much as 50% with salt concentrations of 10 g/litre. Enlarged base piles in ice-rich
permafrost formed using jet cutting are considered by Sego et al. (9.18) While improvements
in end-bearing capacity of 30% to 40% are claimed as a result of the belling, this is from a
low base value and when the costs of the high-alumina grout used to fill the bell are
considered, the benefits of the belling are marginal.
Drilled and cast-in-place piles are suitable but the concrete must not be placed in direct
contact with the frozen ground. North American practice is to use powered rotary augers to
drill into the permafrost to the required depth, but wear on bits will be high in silts and
sands. A permanent steel casing is then placed in the drill hole and filled with concrete. The
heat of hydration thaws the surrounding ground and as the concrete cools the freezing of
the melt water bonds the pile permanently to the permafrost. Water or a sand slurry can be
poured into the annulus between the casing and the permafrost to ensure full bonding, but
this may result in high creep. The annulus should be less than 100 mm to ensure adequate