Capacity of plate anchors in clay under sustained uplift

The use of plate anchors on permanent offshore facilities with taut or semi-taut moorings requires a thorough understanding of their long-term capacity under sustained loading. In this paper, a programme of centrifuge tests and numerical simulations is reported to investigate the capacity of a plate anchor deeply embedded in normally consolidated clay during and after vertical sustained loading. Experimental data show that a sustained load higher than 85% of the ultimate monotonic anchor capacity leads to anchor failure, characterised by acceleration of anchor movement with a large accumulation of vertical displacements towards the soil surface. At sustained loads lower than 85% of the monotonic anchor capacity, the undrained anchor capacity increases due to consolidation and hence strengthening of the soil mobilised by the anchor. The maximum increase in capacity relative to the monotonic capacity is 23% when the sustained load is approximately half the monotonic anchor capacity. A gap-filling technique was proposed and incorporated into large deformation finite element analyses, which are shown to reproduce the experimental observations well.