Pile Design and Construction Practice, Fifth edition

The durability of piled foundations 479
industrial or household refuse, and floating rubbish discharged from ships or harbour
structures. The pattern of sea or river currents should be studied and water samples taken at
various stages of spring and neap tides or at dry-weather and at flood and dry discharge
stages in rivers. Particular attention should be paid to sampling water from currents
originating at the areas of contamination previously identified. Chemical and bacteriological
analyses should be made on the full range of samples to assess the daily or seasonal
variation in potentially aggressive substances. Other items for study include the presence
and activity of organisms such as weeds and barnacles, and molluscan or crustacean borers
(see Section 10.2.2).
10.2 Durability and protection of timber piles
10.2.1 Timber piles in land structures
Timber piles permanently below groundwater level have an indefinite life. There are numerous
examples of stumps of timber piles that are more than 2000 years old being found in
excavations below the water table. While timber does not decay from fungal attack if the
moisture content is kept below 20% it is impossible to maintain it in this dry condition
when buried in the ground above water level. Hence damp timber which does not have natural
durability is subject to decay by fungal attack, resulting in its complete disintegration.
Figure 10.1 shows an example of the decay of timber piles above the water table. Figure
10.1a shows the cavities left by the complete decay of the timber. The timber capping
beams have also decayed, allowing the stone lintels to sink down onto the ground surface.
Figure 10.1b is a view down a cavity which is partly filled by soil debris and fragments of
decayed timber. The piles were driven into clay fill in the early nineteenth century.
Preservative treatment can, however, give a useful life to timber piles in the zone above
groundwater level. If treatment is applied to properly air-seasoned wood at the correct
moisture content for the impregnation of the preservative, a life of several decades may be
achieved.
The Building Research Establishment (BRE) (10.2) has classified various grades of
durability in terms of their approximate life when in contact with the ground (Table 10.1).
Types of softwoods and tropical hardwoods suitable for timber piles as BS 5268 are listed
in Section 2.2.1 (Table 2.1). Their natural durability and ‘treatability’ is classified by BRE
Table 10.1 Natural durability classification of the heartwood of untreated timbers
Grade of durability Approximate life in ground contact European Standards Class
(years) (Resistance to fungal attack)
Very durable More than 25 1
Durable 15 to 25 2
Moderately durable 10 to 15 3
Non-durable 5 to 10 4
Perishable Up to 5 5