The timber
pile is probably the oldest pile foundation method and has been used for
many centuries. In some countries, and for particular applications, timber
piles are still a cost-effective solution. The main disadvantage of timber
piles is variable quality (strength and geometry) as well as the risk
of deterioration above the ground water. For modest loads and piling lengths
of about 12 m, timber piles are quite suitable, provided certain precautions
are taken. They are available in diameters of up to 0.40 m. Usually, pile
cross-sections are circular, although square sections can be used. Working
loads are unlikely to exceed 500 kN, partly because of difficulties in
driving piles to give sets appropriate to higher working loads. Also the
cross-sections of timber piles and lower compressive strengths limit the
load bearing capacity.
Timber piles
are frequently used in North America, China and Scandinavia in form of
trimmed tree trunks, driven butt uppermost. When employed in foundation
work, their light weight gives buoyancy to the foundation. Timber piles
are suitable for temporary works. The conical shape of timber piles increases
the bearing capacity.
Timber piles
are usually installed by drop hammers.
Damage as a result of "overdriving" manifests itself in "brooming" of
the pile head, cracking of the shaft or damage to the pile tip (which
may be difficult to detect). The driving process can crush and separate
the fibers at the head of the pile; hence the "broom" effect. A steel
band around the pile head may restrict this damage to acceptable levels,
but timber piles are not particularly suitable for driving through dense
strata, or strata with obstructions. A steel or cast-iron shoe is generally
attached to the tip of the pile.
For heavy
driving, the weight of the piling hammer should be equal to the weight
of the pile. When the driving is easy, it should be not less than half
the weight of the pile. Particular care should be taken to make sure that
the pile heads are square, that the hammer blows are axial to the pile,
that the pile is properly set up in the leaders, and not strained to bring
it vertical after a bad start to the drive.
In difficult
driving conditions, pre-driving or pre-boring
of a hole for the pile should be adopted rather than risking undetected
splitting or breakage of piles.
If timber
piles are kept permanently wet or permanently dry, for example installed
wholly below water or wholly above water level, they can have a long life.
They are however liable to decay in a zone of fluctuating water table.
In Scandinavia,
it has been common to use jointed timber piles, where the upper section
consists of a concrete element. Special care must be taken when splicing
the concrete pile to the timber pile.
In the case
of marine or river structures, the immersed portions of the piles are
liable to attack by marine organisms. Timber piles are also liable to
attack above the water level by fungi and ants or other wood-destroying
insects. Care in selection and treatment of timber piles can prevent or
minimize attack.
In some countries,
especially in the Far East, small diameter timber (Bakau piles) or bamboo
piles are used for support of excavations or road embankments. In spite
of their simplicity, these timber piles are cost-effective and optimal
as low-cost foundation solutions.
