Pile Driving - Durability- Steel Structures.

It is important that the long-term performance of the structure is considered both in the choice of structural form and in the design of construction details. Failure to do so may result in maintenance problems requiring costly repair.

1 Corrosion allowances
The means for countering the effect of corrosion of steel piles are well developed.
Guidance is given in the British Steel Piling Handbook.

BS 80028 considers that the end of the effective life of a steel sheet pile occurs when the loss of section, due to corrosion, causes the stress to reach the specified minimum yield strength. A pile section chosen for the in-service condition has to be adequate at its end-of-design-life, at which time the effective pile section will have been reduced by corrosion.

As the corrosion loss allowance varies along the pile according to the corrosion environment, the designer needs to be aware that the maximum corrosion may occur at a different level to that of the maximum forces and moments, and should allow for this accordingly.

Also, since redistribution of earth pressures may occur as a result of increased flexure of a corroded section, the end-of-design-life condition may be a critical design load case in the selection of the sheet pile section.

2 Corrosion and protection of steel piles
The design life requirements for proposed buildings and individual components or assemblies are defined in BS 7543 Guide to durability of buildings and building elements, where a building design life can range from 10 years for a building with a ‘short’ life to 120 years for civic and other high quality buildings. A retaining wall which is part of a building structure, i.e. a basement, must therefore comply with these requirements and be designed with sacrificial thicknesses  applied to each surface, depending on the exposure conditions. The exposure conditions are based on the advice given in BS 8002 clause and are shown in Table 29.4.
 Table 29.4 Sacrificial thicknesses for piling according to BS 8002
Sacrificial thicknesses for piling according to BS 8002
Another way of allowing for sacrificial thickness is to use a higher strength steel than would be required if no corrosion were assumed (i.e. use steel grade S355GP, to BS EN 10248, in a wall designed for steel grade S270GP).This permits a greater loss of metal before stresses become critical

It should be noted that the corrosion allowances apply to unprotected steel piles.
Although it is generally cost effective to provide the sacrificial steel thickness, consideration can alternatively be given to the following corrosion protection options:

• Protective coatings, particularly in the exposed section of the pile.
• Cathodic protection in soil below the water table or in a marine environment.

Details of these options are given in the British Steel Piling Handbook.

3 Corrosion in fill or industrial soils
Buildings can be constructed in areas of recent fill or industrial soils. Corrosion protection of the steel in contact with the fill material may be required, and this can be assessed by testing the material for pH and resistivity.

The nature of in situ fill soils can be variable, and a full soil analysis is required to assess the likely corrosion performance of steel in the environment. Soil tests to determine the pH of the soil should be in accordance with BS 1377-353 and as directed by the contract to determine resistivity. Other tests may be relevant, and
most of these are reviewed in CIRIA’ s series of reports on contaminated land (contact CIRIA for further details).

In a controlled fill, no special measures are required, and the same corrosion rates as in natural undisturbed soils can be assumed.

Corus (formerly British Steel) has undertaken significant research and development into corrosion of steel and corrosion protection. Further advice on corrosion assessment and protection can be obtained from Corus Construction Centre or from
The Steel Construction Institute.

4 Corrosion and structural forces
It is not immediately obvious whether the start of in-service life case or the end-of-design-life case will be the most critical for the structural design of retaining walls.

At the end-of-design-life, the reduced stiffness of the corroded steel pile will permit increased deflection, which will in turn reduce the soil pressures acting upon it (and therefore the induced moments and shears).

As the corrosion loss allowance varies along the pile according to the corrosion environment, the designer should consider that the maximum corrosion may not occur at the same level as the maximum section stresses.

The reduced (corroded) section properties can be obtained either by calculation or from the British Steel Piling Handbook or other Corus brochure.

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