Observing the effects of sustained loading on spudcan footings in clay

Spudcan foundations of mobile jack-up rigs are penetrated into the seabed under seawater ballast preload, which is shed prior to rig operations commencing. During pauses in the installation process and during operation, soil beneath the spudcan foundations stiffens and strengthens due to consolidation. On the application of further loading or during spudcan extraction, this causes increased resistance, which in extremis can result in punch-though type failure. This note reports results from a series of experiments with particle image velocimetry measurements that were performed in a drum centrifuge to facilitate observation of the effects of a load-hold period on the soil movements around a model spudcan during subsequent further loading. The results show that the dimensionless load-hold period dominates the enhancement in the penetration resistance, due to significantly more soil being mobilised following a long load-hold period. These observations might be useful to (a) predict the enhancement in bearing capacity factor due to a load-hold period during installation or operation and (b) predict the footing extraction resistance during jack-up re-deployment.