Pile Design and Construction Practice, Fifth edition

486 The durability of piled foundations
The methods of preparing, air-seasoning and preserving timber against borer attack are
the same as those described for fungal decay in Section 10.2.1 above. However, great care
is necessary to avoid making incisions through which borers can enter the untreated wood
in the interior of the pile. The timber should be handled by slings rather than hooks or dogs
after creosoting, and purpose-made devices should be used to give pressure impregnation of
the bolt holes after drilling.
Chellis (10.3) describes the following other methods of protecting timber piles against
attack by borers:
(1) Tipping stone around the piles (this protects only the length covered by the stones)
(2) Sleeving the timber with galvanized iron, copper, or aluminium sheeting
(3) Encasing the piles
(4) Jacketing the piles with precast concrete tubes and filling the space between the timber
and the tubes with cement grout and
(5) Coating the piles with cement–sand mortar, applied with a spray gun (e.g. the ‘Gunite’
process).
Reliable methods of repairing decayed marine timber piles to provide substantial recovery
of original strength are not available, not least because of the difficulty in gaining access to the
critical zones. Experimental techniques which first remove the decayed material, treat the
remaining wood with preservative, and infill the void with epoxy resin mortar followed by
wrapping with glass fibre have shown some small-scale success. Voids left by rotting timber
piles below the Royal Scottish Academy in Edinburgh were successfully treated by Keller
Ground Engineering using their ‘Soilfrac’ process. This entailed installing horizontal tubes-àmanchette
2 m below the pile cap stonework from a trench around the building so that each
41 m long tube intersected an average of 15 piles. This allowed about 40% of the piles to be
directly injected using a low viscosity grout, with the remaining piles filled by overflow and
pressure grouting. The stability of the building was extensively monitored during the process.
In tropical and sub-tropical countries timber piles can be destroyed by termites above the
waterline unless a resistance species is used, or they are given the usual preservative treatment.
Also the end grain at the heads of piles is particularly susceptible to attack by fungi
or beetles when in a damp condition. The pile heads can be protected by heavy coats of
hot-applied creosote followed by capping with metal sheeting, bituminous felt or glass fibre
set in coal-tar pitch.
Some species of wood corrode iron fastenings by the secretion of organic acids. Either
non-ferrous fastenings should be used or steel components should be heavily coated with tar
or sheathed in plastics. Stainless steel fastenings can be used if the type of steel is resistant
to corrosion by seawater.
The abrasion of timber piles by shingle on the sea bed has been mentioned. While
protection by metal sleeving can be adopted, non-ferrous metal is expensive and it may be
preferable to use sacrificial timber strapped around the main bearing piles, or to accept the
cost of periodical renewal.
10.3 Durability and protection of concrete piles
10.3.1 Concrete piles in land structures
Properly mixed concrete compacted to a dense impermeable mass is one of the most permanent
of all constructional materials, and gives little cause for concern about its long-term