The Influence of Friction Fatigue Effects on Large Diameter Driven Piles in the Bolders Bank Formation

Reliable driveability studies enable appropriate hammer selection, pile geometry optimization and successful, safe and cost-effective pile installation. Soil resistance around the pile shaft may temporarily change with increasing pile penetration, an effect generally termed Friction Fatigue. Factors affecting Friction Fatigue include soil type, geological characteristics, geotechnical properties and initial stress state. Understanding how soil resistance changes during driving is critical to a successful installation campaign.

This paper utilizes impact hammer driving records for large diameter monopiles which form the foundations for a Southern North Sea offshore wind farm. The site geology comprises Bolders Bank Formation glacial deposits overlying the Rowe Chalk Formation. This paper focusses on pile driveability within the Bolders Bank Formation which, in this area, comprises layered sequences of stiff, high strength, over-consolidated CLAY and dense to very dense SAND.

Driving records link dynamic soil resistance during driving with static soil resistance. Driveability assessments utilise static soil resistance in conjunction with soil quake, damping and friction fatigue. Quake and damping are widely generalized for many soil types, whereas friction fatigue, which arguably influences driveability more significantly, is not studied to the same extent. This paper evaluates the variability in soil resistance to driving and calibrates friction fatigue parameters for these soils and concludes that utilising friction fatigue improves pile driving predictions.