Punch-through of spudcan foundations in two-layer clay

Hossain, M.S., Hu, Y., Randolph, M.F. and White, D.J. (2005) Punch-through of spudcan foundations in two-layer clay. Gourvenec, Susan (ed.) In First International Symposium on Frontiers in Offshore Geotechnics. Taylor & Francis. pp. 535-541 .

Abstract

Spudcan punch-through failure continues to be a major cause of foundation failure of offshore jack-up rigs. The resulting damage to the rig can range from minor structural damage of its leg and jacking mechanism to complete loss of the rig. Most punch-through failures happen during the jacking up and preloading and in stratified soil profiles with a relatively thin layer of sand or stiff clay overlying a weaker layer. Punch-through causes a reduction in bearing resistance with depth, causing instability during the load-controlled jacking-up process. To reveal the failure mechanism during punch-through, model spudcan foundation tests have been conducted on a two-layer clay sample (strong over weak) in a drum centrifuge. Half-spudcan model tests were carried against a transparent window to visualise the soil flow mechanisms around the spudcan during penetration. Particle Image Velocimetry (PIV) analysis was used to track the soil particle movements and hence obtain precise details of the failure mechanisms. Full-spudcan tests were also conducted to measure the vertical load-penetration responses. The study shows that punch-through failure and associated softening response are directly linked to the relative thickness of the top clay layer to the foundation diameter, and also the strength ratio between the two soil layers. A punching failure through the upper layer was observed as shear zones developed from the spudcan rim down to the layer interface. A soil plug was carried down beneath the spudcan. The thickness of the soil plug was ∼80% of the initial top layer thickness, and the cavity formed above the spudcan remained open until the spudcan fully penetrated into the soft layer. Softening penetration resistance profiles were observed.

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Contributors

Author: D.J. White

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