ABSTRACT
Wave-induced instability of seabed soil around a pile is an increasingly important subject for research in relation to the stability of offshore wind farm or other such facilities. There have been few studies which have focused on the liquefaction of seabed soil around a pile. This paper discusses the experimental findings pertaining to the instability of the offshore monopile foundations due to wave-induced liquefaction. The centrifuge wave testing with viscous scaling was applied to reproducing the build-up of residual pore pressures in sand beds leading up to liquefaction (Sassa & Sekiguchi, 1999).
Three series of wave tests using a drum centrifuge were performed under a centrifugal acceleration of 70 gravities. First series observed failure processes of a monopile in association with liquefaction in loose deposits of sand. Second series of the experiments investigated the effects of embedment of monopile in the dense layer on the stability of the pile. Third series was designed to investigate the variation of liquefaction resistance of sand bed due to the embedded pile.
The results from the first series of experiments demonstrated that the inclination and collapse of the monopile were in fact ascribed to the progress of liquefaction (Figure 1). It is seen that no significant movement of the pile was observed prior to the occurrence of liquefaction at shallow soil depth (see a point A in Fig. 1). However, as the liquefaction occurred and progressed to 1/3 of embedded depth of the pile D, the pile exhibited a significant vibratory motion and residual displacement started to develop (see a point B in Fig. 1). The residual displacement increased markedly in association with the progress of liquefaction zone in the course of continued wave loading, and finally the pile collapsed.
The second series showed that embedding the pile in the dense layer with an enough thickness brought about less development of residual displacement of the pile and prevented it from collapse. It was found that embedding the pile in the dense layer with a thickness D/2 or greater prevented significant inclination and collapse of a pile and can thus be a countermeasure against liquefaction.
In the third series, the experimental results showed that the liquefaction resistance of the sand bed with a monopile became larger than that of the level bed without a monopile. This indicates the importance of the rotation of the principal stress axes that occurred in the sand beds under the progressive fluid waves.
