Excess pore pressures under cyclically loaded model jack-up foundations
Numerous centrifugal modelling research programmes have been carried out at Cambridge University to observe the drained response of circular foundations on sand. In this new series of centrifugal tests, the effects of excess pore pressure under spud-can foundations of a scaled model three-leg offshore jack up structure subjected to horizontal cyclic loadings were studied. The medium dense to dense sand layer under the foundation was saturated with viscous silicone oil to ensure that the transient flow could be measured and was close to the prototype response. The viscosity of the silicone oil used was about 3-4 times more than required for the prototype, to investigate the possibility of inducing liquefaction. The test series has shown that macroscopic partially drained behaviour was different from that expected in the drained or undrained conditions. Observations in the centrifuge tests indicate that there was a reduction of vertical and rotational stiffness of soil when the vertical loading during a cyclic event falls below its initial value (before the start of the cyclic event). Thus, structural design methods should if possible avoid the use of a single fixity value for design. However, the centrifuge experiments have shown that despite numerous cyclic loadings at different frequencies and amplitudes, the foundation of the model jack up structure did not fail. A comparison between the performance of Ilon-skirted and rigid vertical skirted flat spuds subjected to similar cyclic loadings was carried out to deduce the effects of suction under skirted foundations. The rigid vertical skirted foundation did not have increased fixity . The non-skirted foundations settled more than the skirted ones. During the pull-out event, much greater and more reliable selection forces were induced under the skirted foundations. The excess pore pressure behaviour under the foundation is extremely complex. There is no evidence of pore pressure building lip ill any of the events conducted in the nine tests. However, the excess pore pressure is a function of the cyclic loading amplitude, the cyclic frequency and the position under the foundation. Both double and single frequency pore pressure behaviours are present. The maximum and minimum pore pressure values do not coincide with the maximum and minimum loads. This pore pressure behaviour can be explained through the Characteristic State Concept (LlIong and Sidaller, 1981). The excess f pore pressure data can also be used to predict vertical permanent deformation and cyclic settlement profile.