Time-dependent processes influencing offshore foundations in clay: an experimental study on plate anchors

Motivation for this paper stems from experimental investigations that consider how the vertical capacity of a horizontal circular plate anchor in clay changes due to consolidation. These experiments produced interesting time-dependent measurements that prompted a follow-on study, designed to explore the underlying mechanisms. The new experimental data indicate that changes in anchor load during consolidation under a fixed anchor displacement are linked to three distinct mechanisms. The first is a ‘stress-relaxation’ reduction in anchor load that occurs quasi-instantaneously after the initial anchor movement stops, with a magnitude that is linked to the average strain rate associated with the initial anchor movement. The second is a local consolidation effect that causes a reduction in anchor load over durations that scale with the anchor diameter. The final mechanism occurs simultaneously with the second, but at a slower rate, such that the resulting increase in anchor load becomes apparent at larger values of time. This increase in anchor load is due to dissipation of excess pore pressures developed in the wake of the anchor during the initial anchor movement. These time-dependent changes are considered relevant for the post-installation capacity of offshore anchors and for the capacity of anchors following a large movement event.