Excess pore pressures under cyclically loaded model jack-up foundations

Yu Sheng Hsu, Cambridge University
Geotechnical Engineering Group


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 was similar to the prototype response. 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 indicates that there was a reduction of vertical and rotational stiffness of soil when the vertical loading during a cyclic event is below the starting working load for the frequency range conducted during the tests. 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 non-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 suction 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 up in 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 (Luong and Sidaner, 1981). The excess pore pressure data can also be used to predict vertical permanent deformation and cyclic settlement profile.

A commercially available finite element programme, Abaqus, with coupled capabilities was employed to analyse the soil behaviour under a non-skirted flat plane strain foundation subjected to cyclic loadings similar to some typical experimental events. The constitutive model employed was unsuitable for high stress reversals problems but the objectives were to study the pore pressure generation pattern, to predict pore pressure values at different locations and to look for trends as observed in the experiments. It was concluded that the numerical model applied cannot correctly predict the observed behaviour, thus, indicating that the best option for studying soil-structural interaction properties was via centrifugal modelling or field tests.