Effect of cyclic history on the ageing of shaft friction of driven piles in sand

It is now well established that the shaft friction developed by driven piles in sand increases with time after installation. First time load tests on field-scale pipe piles at sites such as Dunkirk (France) and Blessington (Ireland) have shown that shaft capacities after 1 year can be almost three times higher than those shortly after installation. While such increases in capacity can lead to significant economies, they have not yet been incorporated into design practice because of the uncertainty regarding the influence of cyclic loading during the life of a pile on the ageing characteristic. This paper presents the results of a systematic experimental study on the effects of cyclic history on the set-up of shaft friction. Twenty six 4m long pipe piles with diameters ranging from 70mm to 450mm were driven into siliceous Perth sand in late 2018. All piles were driven using an air hammer, applying approximately 50 blows per metre. Tension tests were then performed at a variety of times after installation. At least half of the piles were subjected to various levels of cycling and then tested at specific intervals after cycling. This paper presents the shaft capacity set-up trends up for a maximum ageing period of 9 months. These trends are compared with existing trends reported in the literature to provide designers with information that can assist the assessment of shaft friction for in-service piles.