Offshore windfarms are being developed at sites where jacket substructures will be founded on large diameter steel piles drilled and grouted into rock. Although very little experience exists regarding these conditions, the piles must sustain high compressive, tensile and irregular cyclic loading. Their design will be governed by their shaft capacities and axial performance. This paper describes a campaign of static and cyclic tests performed on 14 steel piles grouted into rock sockets with diameters ranging from 330mm to 1300mm, drilled in a meta-sandstone at a well-characterised nearshore site in Northern Brittany, France to inform the design of an offshore windfarm. The field tests identified the loading conditions under which stable or metastable responses were developed under one-way axial cyclic loading. They also demonstrated that the shaft shear stresses developed at failure vary inversely with pile diameter, as captured by models that treat shaft resistance as developing through constrained dilation at the grout-to-rock interface, which is governed by the drilled shaft’s roughness and the rock mass stiffness. More conventional "/spl alpha/ – UCS" shaft capacity design methods lead to unsafe shaft capacity estimates for larger diameter piles when calibrated to small onshore tests unless the observed inverse relationship with diameter is addressed.
Jamie Klapper, Sebastien Manceau, Alexander Hall, Richard Jardine and Pedro Barbosa Moreira
Keywords: rock socket, pile design, drilled and grouted, pile test, cyclic loading
Publisher: DFI