Assessment of new axial capacity design method against independent tests on open-ended steel piles driven in chalk

Chalk is widespread over large areas of NW Europe where it is frequently encountered at offshore windfarm and other development sites. Its sensitivity, brittleness and variability pose significant challenges for the installation and design of piles driven at such locations. The recently completed ALPACA and ALPACA Plus Joint Industry Projects (JIPs) addressed these difficulties and acquired new knowledge regarding monotonic and cyclic behaviour of axially and laterally loaded driven piles in chalk from comprehensive field, laboratory and theoretical investigations on open- and closed-ended tubular, square and sheet piles at the St Nicholas-at-Wade (SNW) test site. An axial capacity estimation method was developed that provided a far better overall fit than alternative methods to the ALPACA-SNW pile test outcomes. This paper describes how the new methodology was checked against an independent database of high-quality static and dynamic axial load tests conducted at five other sites in the UK, France and Germany. It reports how statistical evaluation of these new data led to a marginally more conservative long-term shaft radial effective stress formula being proposed for use with large open-ended piles. This updated approach, which is termed Chalk ICP-23 to save confusion, retains all other aspects of the ALPACA-SNW method. The paper also (i) re-emphasises the important influences on shaft capacity of pile age; slenderness ratio and compression versus tension loading in chalk and (ii) discusses further aspects of base capacity assessment.