Pile Design and Construction Practice, Fifth edition

492 The durability of piled foundations
10.4 Durability and protection of steel piles
10.4.1 Steel piles for land structures
Corrosion of iron or steel in the electrolyte provided by water or moist soil is an
electro-chemical phenomenon in which some areas of the metal surface act as anodes
and other areas act as cathodes. Pitting occurs in anodic areas, with rust as the corrosion
product in cathodic areas. Air and water are normally essential to sustain corrosion but
bacterial corrosion can take place in the absence of oxygen, i.e. in anaerobic conditions.
Anaerobic corrosion is caused by the action of sulphate-reducing bacteria which thrive below
the sea or river bed in polluted waters, particularly in relatively impermeable silts and clays.
An exhaustive investigation of the corrosion rates of steel sheet piles and bearing piles in
soils was made by Romanoff (10.11) on behalf of the US National Bureau of Standards. Steel
piles which had been in the ground for periods of between 7 and 40 years were examined.
The soil types ranged from permeable sands to relatively impervious clays. Soil resistivities
ranged between 300 and 50200 ohm-cm and pH values between 2.3 and 8.6. Romanoff
concluded from observations of the condition of the piles that where they were driven into
undisturbed natural soil, the type and amount of corrosion was so small that it would not significantly
affect the strength or useful life of the piling to support structures. Some localized
pitting corrosion and loss of mill-scale were seen on steel surfaces but the loss of metal was
considered to have a negligible effect on the serviceability of the piles. Corrosion had
occurred in some instances where piles had been driven through fill above the water table,
or in the zone extending 0.6 m above and below the water table.
Romanoff pointed out that undisturbed natural soils are so deficient in oxygen that they
will not sustain the process of corrosion. Romanoff also found that determinations of soil
resistivity and pH-value had no relevance to the incidence of corrosion in the undisturbed
soil conditions covered by the Bureau of Standards research. He did not encounter any cases
of anaerobic corrosion by sulphate-reducing bacteria but the possibility of their occurrence
should not be overlooked at the site investigation stage. Undisturbed samples of the soil
should be sealed in their containers and submitted for bacteriological examination.
In a later study, Romanoff (10.12) examined steel sheet piles which had been driven through
fill material. Inspections were made at 13 locations where piles had been installed for
periods of between 11 and 30 years. With only one exception the piles showed only shallow
attack on the metal with some localized pitting corrosion. The single exception was at a site
where sheet piles had been driven through 6 m of clinker filling. Severe attack on the metal
and pitting up to 6 mm deep had occurred over large areas. However, it was pointed out that
these piles were continuing to give useful service 23 years after they had been driven.
Romanoff concluded that the relatively small amount of corrosion over the portion of the
pile in fill or in undisturbed soil above the water table is the result of the formation of a
galvanic corrosion cell between the upper part of the pile above the water table and the lower
permanently immersed part. The upper portion is small in volume compared with the
lower portion and it acts as a cathode, while the lower part in soil deficient in oxygen is
the anode. Because of the much greater mass of steel in the anodic portion only a small
proportion is sacrificed in protecting the cathodic part.
Similar corrosion rates for piling in land structures have been recorded by Morley (10.13) .
British Steel Corporation investigations of piles extracted from UK sites in the 1970s showed
corrosion losses below the soil line varying from nothing to 0.03 mm per year with a mean