Design Life of Star Finned Screw Pile

The design life of the pile has been assessed based on the recommendations in Section 6 of AS2159-1995. The presence of the cable slot and open-ended shaft will allow corrosion to occur from the internal surface.

In assessing the design life, corrosion on both the internal and external surface was allowed for. The rate of corrosion will be dependant on the type of soil that the pile is installed in and presence of transient groundwater.

Classification of soils from non-aggressive to severe is given in AS2159-1995. For the purpose of this report, only non-aggressive, mild, and moderately aggressive soil types are considered.

Corrosion will increase and reach a maximum nat about 10 to 15 years for moderate to nonaggressive soils respectively.

Results of research have concluded that maximum corrosion rates of unprotected steel piles embedded in soils is about 0.02mm to 0.03mm a year. Based on this, the following design lives were estimated for particular soil types as a guide. The corrosion rates given represent the estimated rate at and immediately below the ground surface.

Galvanised Product

click to enlarge

Severe to very severe exposure classifications are limited to piles in seawater. We have therefore not considered this corrosion condition. If it was found to be necessary to install the piles in such ground, modification to the standard design may be adopted.

Modifications could also be included for less severe corrosion conditions if longer design life than those given above are required. These modifications may include:

  • Double corrosion protection with an inert coating; and Cathodic protection.

Maintenance of Star Finned Screw Piles

It is considered that the most critical portion of the pile susceptible to corrosion is the portion at and immediately below the ground surface. For the star finned screw pile system, this would include the hold down bolts to support the lighting column and the top portion of the fins.

It would be more effective and practical to have a long term maintenance program for the star finned screw piles. This would involve simple visual inspection of the critical areas of the piles at intervals dependant on the aggressiveness of the soil. If deterioration of the hold down bolts is observed they can be replaced without removing the lighting column.