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The tension capacity of suction caissons in sand under rapid loading Guy Houlsby, Richard Kelly and Byron Byrne.

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The tension capacity of suction caissons in sand under rapid loadingGuy Houlsby, Richard Kelly and Byron Byrne

MOTIVATION: if suction caissons are used for tripod or tetrapod foundations, then the upwind caissons may be subjected to tension under extreme conditions. We present a means of determining the tensile capacity in sand

This research has been supported by the DTI, EPSRC, SLP, Fugro, Garrad Hassan, GE Energy, Shell and Vestas

CALCULATION CASES: the tensile capacity depends on the suction developed within the caisson, which in turn depends on the rate of pull-out –dh/dt

COMPARISONS WITH EXPERIMENTS: 300mm diameter model caissons were subjected to rapid pull-out in a testing chamber which simulated the ambient pressure at the seabed.

Four different cases are identified for analysis, depending on whether the pull-out rate is sufficient to cause liquefaction, and whether or not cavitation within the caisson occurs first

Oxford University Civil Engineering – ISFOG, Perth, September 2005

CASE 1: very slow pull-out In this case the suction is calculated from a seepage calculation, and the frictional capacity by a “Ktand” approach. This case will only occur rather rarely

If the ambient pressure is high the pull-out load is limited by liquefaction inside the caisson

CASE 2: Liquefaction without cavitation

Onset of liquefaction occurs when the suction reaches a critical value, and after that friction on the inside is ignored

At lower ambient pressures cavitation occurs and the suction is limited to a fixed value, which determines the pull-out load

CASE 3: Cavitation without liquefaction

Cavitation occurs at a different critical suction, which depends on the water depth

CASE 4: Cavitation and liquefaction

Both liquefaction and cavitation occur in many cases once the pull-out rate is large enough

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