Incorporating cyclic degradation into pile design for the Gull Wing Bridge, Lowestoft-Paper

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The paper describes a method for accounting for cyclic degradation in the piled foundations of the Gull Wing Bridge’s rolling bascule span in Lowestoft, UK. Opening and closing the bridge repeatedly changes the loads on individual piles, potentially reducing shaft resistance and causing long-term settlement. Over the 120-year design life, the bridge is expected to undergo about 876,000 load cycles.<br /><br />The foundations comprise large-diameter bored piles extending through the Happisburgh Formation into the dense silty sands of the Crag Group. Because cyclic pile testing and specialist constant-normal-stiffness soil testing were not practicable, the design drew on offshore pile research and cyclic load-test data from comparable dense sand at Dunkerque. The tests were used to develop cyclic stability diagrams that distinguish stable loading from conditions associated with progressive degradation or rapid failure.<br /><br />The designers created a cyclic capacity reduction factor by comparing the pile’s loading position with the stable cyclic boundary and static ultimate boundary on these diagrams. The factor reduces characteristic shaft resistance so that piles remain within the stable loading zone. It is calculated for each pile using its mean and peak cyclic loads, then incorporated into an iterative analysis of pile lengths, stiffnesses and group load distribution. The reduced shaft resistance is used in both ultimate and serviceability checks, while base resistance remains available in ultimate-limit-state calculations.<br /><br />The method distinguishes highly cyclic piles from piles with smaller load variations: the latter require little or no reduction, while highly cyclic piles may need substantial capacity reductions. Maintaining compression throughout the load cycle also limits degradation. The approach provides a targeted alternative to historically used high global safety factors, retaining code-based partial factors while directly addressing the resistance most affected by cyclic loading. The authors conclude that ignoring degradation could have led to unacceptable medium- to long-term settlement.

Keywords

cyclic degradation

piled foundations

rolling bascule span

shaft resistance

load cycles

cyclic capacity reduction

settlement analysis

bored piles

cyclic load testing

Dunkerque sand