Corrosion and service life

The service life of a sheet piling structure is to a large extent dependent on the natural process of corrosion. Corrosion is the reaction of the steel to oxygen and the associated formation of iron oxide. Therefore, a continuous weakening of the sheet piling cross-section necessary for the stability of the wall takes place over several years. This weakening must be taken into account when analysing the serviceability and the ultimate load capacity. For corrosion in the atmosphere, i.e. without the effects of water or splashing water, a corrosion rate of approx. 0.01 mm/a is low. Also very low is the corrosion rate (on both sides) of sheet pile walls embedded in natural soils, which is also approx. 0.01 mm/a. The reason for this is the exclusion of oxygen. The same corrosion rate can be expected on sheet pile walls backfilled with sand. However, in this case it must be ensured that the troughs of the sections are filled completely with sand. A coating with a high protective effect forms in calcareous water and soils with a calcium carbonate content. Aggressive soils, e.g. humus, or aggressive groundwater should not be allowed to come into contact with the surface of a sheet pile wall. Furthermore, corrosion of the sheet piling can be promoted by bacteria in the soil. Considerably more severe corrosion can be expected in hydraulic structures, which is, however, not evenly distributed over the full height of the structure. The greatest weakening of the wall thickness and hence the resistance of the component takes place in the low water zone. When designing a sheet pile wall, care should be taken to ensure that the maximum bending moments do not occur at the same level as the main corrosion zones.

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EAU 2004 includes diagrams in section (R 35) with which the weakening of the wall thickness due to corrosion can be calculated. Using these diagrams, sheet pile walls can be designed for the mean and maximum losses in wall thickness if no wall thickness mea- surements are available from neighbouring structures. The areas shaded grey in the diagrams represent the scatter for structures investigated hitherto. To avoid uneconomic forms of con- struction, EAU 2004 recommends using the measurements above the regression curves only when local experience renders this necessary. For structures located in briny water, i.e. in areas in which freshwater mixes with seawater, the reduction in wall thickness can be interpolated from the diagrams for seawater and freshwater.

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According to current knowledge, adding a coating to the sheet piles can delay the onset of corrosion by more than 20 years. One way of virtually eliminating corrosion below the waterline is to employ an electrolytic method in the form of a sacri ficial anode. Another way of achieving protection against corrosion is to overdesign the sections, but in this case an economic analysis must be carried out first.

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