Excess pore pressures induced by installation of suction caissons in NC clays

Suction caissons have been used as mooring systems for offshore structures since the mid-1980s due to their advantages over driven piles and drag embedment anchors. Over the last five years, suction caissons have become the foundation of choice for anchoring deepwater floating structures (eg, SPARs, DDCVs, semis) or supporting seabed founded systems (eg, well heads, SDUs, UTAs). The installation of suction caissons induces excess pore pressures in soft soils, such as normally consolidated (NC) clays. The distribution of the excess pore pressures and their dissipation with time has a significant effect on the caisson's pullout capacity, and very few studies have been conducted on these subjects. In order to investigate the behaviour of suction caissons installed in NC clay, a series of centrifuge tests was carried out at C-CORE's centrifuge facility. The model caissons were installed in carefully-prepared clay,test-beds through a combination of self-weight and active suction. This paper first presents briefly C-CORE's centrifuge facility and describes the centrifuge tests, including the clay test-bed and model caissons, the multi-function test set-up, the testing procedure and the monitoring of excess pore pressures. The results of the centrifuge tests regarding the distribution of excess pore pressures induced during installation of the caisson and their dissipation with time are then discussed., The experimental results are analysed using the cavity expansion theory, which was first used for driven piles. Finally, the experimental dissipation results are compared with a modified version of the Bogard and Matlock's empirical hyperbolic equation for consolidation time, corrected for the diameter and wall thickness of the caisson.