Pile Design and Construction Practice, Fifth edition

Types of pile 15

Timber piles, when situated wholly below groundwater level, are resistant to fungal decay

and have an almost indefinite life. However, the portion above groundwater level in a struc-

ture on land is liable to decay and BSEN 12699 prohibits the use of timber piles above free

water level, unless creosote or other adequate protection is used. Therefore, it is the usual

practice to cut off timber piles just below the lowest predicted groundwater level and to

extend them above this level in concrete (Figure 2.1a). If the groundwater level is shallow

the pile cap can be taken down below the water level (Figure 2.1b).

Timber piles in marine structures are liable to be severely damaged by the mollusc-type

borers which infest the sea-water in many parts of the world, particularly in tropical seas.

The severity of this form of attack can be reduced to some extent by using softwood impreg-

nated with creosote or greatly minimized by the use of a hardwood of a species known to be

resistant to borer attack. The various forms of these organisms, the form of their attack and

the means of overcoming it are discussed in greater detail in Chapter 10.

Bark should be removed from round timbers where these are to be treated with preservative.

If this is not done the bark reduces the depth of impregnation. Also the bark should be

removed from piles carrying uplift loads by shaft friction in case it should become detached

from the trunk, thus causing the latter to slip. Bark need not be removed from piles carrying

compression loads or from fender piles of uncreosoted timber (hardwoods are not treated

because they will not absorb creosote or other liquid preservatives).

Commercially available timbers which are suitable for piling include Douglas fir, pitch

pine, larch and Western red cedar, in the softwood class, and greenheart, jarrah, opepe, teak

and European oak in the hardwood class. The timber should be straight-grained and free

from defects which could impair its strength and durability. BS8004 states that a deviation

in straightness from the centre-line of up to 25 mm on a 6 m chord is permitted for round

Timber pile

Timber

piles

Precast

concrete

(a) (b)

Pile cap

Lowest ground

water level

Head of timber

pile cast in

concrete before

driving

Figure 2.1Protecting timber piles from decay (a) By precast concrete upper section above water level
(b) By extending pile cap below water level.