natural ‘freeze-back’around the pile. Korhonen and Orchino(9.19)point out that freezing of
the concrete need not be the sole reason for rejecting the pile: concrete in cast-in-place piles
that has frozen after attaining a certain minimum strength and remains frozen will continue
to gain strength while frozen. Biggar and Sego(9.20)comment on the use of high-alumina
cement-based grout for infilling the annulus at temperatures of 10 C.
Timber piles installed in pre-drilled holes or driven in conjunction with steam jetting have
been used for many years in northern Canada. Timber piles will generally remain well
preserved in permafrost, but must be protected against deterioration in the active zone.
In Russia a form of reverse-circulation drilling with steam or gas burners is used to thaw
the soil and flush the cuttings to the surface before freezing-in the permanent steel casing.
Holes should be provided in the casing to drain off the water before the concrete is placed.
If this is not done freezing of the water inside the casing may cause bursting of the steel.
Precast concrete piles driven into pre-formed holes are extensively used in Siberia, but low
concrete tensile strength may result in cracking during frost heave. Prestressed concrete
piles perform better.
‘Thermal piles’are piles on which natural convection or forced circulation cooling
systems have been installed to retain the adfreeze during warmer weather and lower the pile
surface temperature during the autumn. Johnson(9.21)describes two types of thermal piles,
including piles used to support the above-ground section of the Trans-Alaska oil pipeline.
9.5 Piled foundations for bridges on land
9.5.1 Selection of pile type
Bridge construction for developed countries is subject to many constraints concerned with
access to sites and environmental conditions. These have an important influence on the
selection of a suitable pile type and equipment for installation. In undeveloped territories the
constraints are fewer and selection of suitable pile types is influenced mainly by the ground
conditions.
When constructing new main highways it is desirable to complete under- and over-bridges
at an early stage in the overall construction programme in order to facilitate the operation of
earthmoving and paving equipment along the length of the highway without the need for
detours or the use of existing public highways by construction equipment. Hence, access
to bridges will be difficult at this early stage and it may be impossible to route the piling
equipment and material deliveries along the cleared highway alignment without interfering
with the early earthmoving operations.
In the case of small bridges, such as those carrying minor roads over or beneath the main
highway, it is desirable to use light and easily transportable equipment to install a number
of small- or medium-diameter piles rather than a few large-diameter piles requiring heavy
equipment. Suitable types are precast concrete or steel sections which have the advantage
over bored piles of the facility to drive them on the rake thus providing efficient resistance
to lateral forces, which are an important consideration in most bridge structures. Only small
angles of rake are feasible with bored piles (see Section 3.4.11) and it is usually preferable
to provide only vertical bored piles suitably reinforced to resist horizontal loads and
bending moments.
Some of the most difficult access problems are involved with bridges in deep cuttings
where the bridge is constructed in an isolated excavation in advance of the main
Miscellaneous piling problems 453