Displacement response to axial cycling of piles driven in sand

Abstract

A review of the load applied to multi-pile offshore wind turbine foundations is presented, from which the need to consider the response to axial cyclic loading is emphasised. The paucity of available data on field tests on driven piles in sand is noted. A comprehensive data set of multiple axial cyclic and static tests conducted on seven industrial-scale steel pipe-piles at a marine sand site in Dunkerque, France, is re-examined in this paper. The effects of cycling on axial capacity are interpreted by reference to stable, metastable or unstable zones defined in a normalised cyclic stability interaction diagram. A detailed analysis is made of the load-displacement and stiffness response associated with each mode of cycling. It is shown that in all cases the piles' cyclic stiffnesses show only minor changes until cyclic failure is approached. The patterns of permanent cyclic strain accumulation are sensitive to the applied mean and cyclic loading levels. Whereas displacements accumulate rapidly over just a few cycles in the unstable zone, extended cycling in the stable zone leads to minimal accumulated displacements and constant transient cyclic displacements.

Advanced laboratory testing in research and practice: the 2nd Bishop Lecture

“…Also shown is the qc profile predicted by Zhang et al (2013) that is discussed later. Advanced laboratory testing in research and practice: the 2nd Bishop Lecture Jardine Rimoy (2013) describes more recent experiments with the same equipment, noting that axial capacities from multiple load tests agree encouragingly well with predictions made with the 'fieldcalibrated' capacity approach outlined by Jardine et al (2005b), which gave good results for the Dunkerque field tests. Jardine et al (2013aJardine et al ( , 2013b report and interpret the measurements made during installation, referring to these as the 'mini-ICP data set'.…”

“…High-level twoway (tension and compression) cycling could halve shaft capacity within a few tens of cycles. Rimoy et al (2013) discuss the piles' permanent displacement and cyclic stiffness trends, noting also that their non-linear cyclic stiffnesses depended primarily on Qcyc/QT and did not vary greatly with the number of cycles (N) until failure approached. The permanent displacement trends were more complex, depending also on Qmean/QT and N. Interactions were seen between the piles' ageing and cyclic behaviours: low-level cycling accelerated capacity growth, while high-level cycling slowed or reversed the beneficial capacity trend.…”

“…Jardine et al (2006), Jardine and Standing (2012) and Rimoy et al (2013) give details of the geotechnical profiles, pile driving records and testing methods. Static and cyclic axial loading tests were conducted on multiple piles, including six 19·3-m-long 457-mm-o.d.…”

“…Figure 41 presents an overall interactive diagram that compares directly with the field patterns in Figure 3. Tsuha et al (2012) and Rimoy et al (2013) report on the cyclic stiffness and permanent displacement trends. Broadly, they classify responses to cycling as ■ stable: capacity increasing slightly, displacements small and stabilising, over 1000 or more cycles ■ unstable: reaching failure with 100 cycles ■ metastable: falling between these limits.…”

“…Stiffness load-factor curves from first-time tests at Dunkerque conducted (except R6) around 80 days after driving (Rimoy et al, 2013) …”