In this chapter, fundamental results from Case 1 on the instability of the suction bucket foundation associated with wave-induced liquefaction are discussed first, followed by the results from Case 2.
Liquefaction Process with and without Suction Bucket Foundation
The measured waveforms in the free field are discussed first (Fig.
6). At shallow soil depth, the increase in wave severity resulted in a buildup of residual pore pressure
. In fact, the value of
reached the level of the initial vertical effective stress
there, indicating the occurrence of liquefaction at this soil depth. The criteria for the onset of wave-induced liquefaction such that
is described in detail in Miyamoto et al. (
2020). In the course of wave loading, liquefaction advanced downward [Figs.
6(c–e)]. Indeed, liquefaction occurred in sequence at shallow soil depth (
), at middle soil depth (
), at deep soil depth (
) and at the bottom (
). As a consequence of progressive liquefaction, the entire soil bed around the suction bucket foundation underwent liquefaction in the free field. This progressive nature of wave-induced liquefaction has been observed not only under gradually increasing sinusoidal waves as imposed in this study but also under severe irregular waves (
Miyamoto et al. 2020).
Next, the excess pore pressure responses inside and underneath the suction bucket are discussed (Fig.
7). The wave-induced fluid pressure fluctuation on the level of the soil surface was measured near the bucket [see inset in Fig.
7(a)]. The residual pore pressure built up in the bucket significantly but did not reach the level of initial vertical effective stress
, indicating that complete liquefaction did not occur. A closer observation of the pore-pressure response shows that the oscillatory component reached the
level instantaneously, followed by the start of dissipation of the residual pore pressure. This indicates that instantaneous liquefaction occurred inside the suction bucket but not complete liquefaction.
The liquefaction process in the level bed without a structure had been observed in past studies (
Miyamoto et al. 2021a). The previously mentioned liquefaction process in the free field of the sand bed with the suction bucket, in which the residual pore pressure built up and liquefaction occurred at shallow soil depth and then advanced downward, is the same pattern as the case without a structure.
Relationship between Wave-Induced Liquefaction and Instability of Suction Bucket Foundation
The wave-induced liquefaction in the free field was of a progressive nature, as shown in Fig.
6. The instability and collapse of the foundation bucket were in fact ascribed to the progress of liquefaction in the free field, which stemmed from the dynamic interaction between wave, liquefied soil and subliquefied soil (
Sassa et al. 2001). This aspect can be more clearly seen in Fig.
8 where the time histories of the advance of liquefied zone near the foundation bucket [Fig.
8(a)] as well as the displacements of the bucket tower [Figs.
8(b and c)] were shown. The plots in Fig.
8(a) represent the depths of the advancing liquefaction front based on the times of the occurrence of liquefaction shown in Figs.
6(b–e). The displacements of the tower in Figs.
8(b and c) were obtained from the observation of the movements of a marker on the tower located 60 mm above the bucket lid. No significant movement of the tower was observed until the occurrence of liquefaction at shallow soil depth [see point A in Figs.
8(b and c)]. As soon as the liquefaction advanced to the depth of the bucket tip, a significant vibratory motion started and was followed by the development of the residual horizontal displacement [see point B in Fig.
8(b)], as well as the vertical displacement [see point B in Fig.
8(c)]. The vertical displacement stems from the sinking and inclination of the foundation. The residual displacement increased in the course of continued wave loading, and the vibrating motion of the bucket decreased gradually, associated with sinking of the bucket. Finally, the foundation tilted about 90° and sank into the liquefied sand; that is, the foundation collapsed. This means that the progress of liquefaction has a crucial impact on the stability of the suction bucket.
The photograph shown in Fig.
8 indicates that the tilt direction of the tower was opposite to the direction of traveling waves. In the drum centrifuge experiments on monopile (
Miyamoto et al. 2021a) where the experimental conditions were almost the same with the conditions of the suction bucket in this study, the monopile tilted to the wave-traveling direction in association with wave-induced liquefaction. It might be worthwhile to investigate the relationships between waves and the tilting directions depending on the types of the structures.
By comparing Figs.
7(b and c) and
8(b), for the period where significant vibratory motion, namely rocking behavior, started to take place around 8.3 s, one can observe that the enhanced oscillatory pore pressures leading to the occurrence of instantaneous liquefaction inside the bucket were closely related to the onset of the significant vibratory or rocking motion of the bucket.
Next, the results from Case 2 will be discussed. In this case, whereas the tip of the suction bucket was supported on the dense layer, the entire bucket was set in the liquefiable layer. The experimental results were essentially the same as in Case 1, except for the pore-pressure responses in the dense sand layer. Following the advance of liquefaction in the free field, instantaneous liquefaction occurred in the bucket, followed by dissipation of the pore pressure. The dense sand layer below the bucket underwent a moderate buildup of residual pore pressure, indicating that liquefaction of the dense sand layer did not occur.
Displacement of the suction bucket associated with the progressive liquefaction was also observed, as shown in Fig.
9, which indicates the same pattern of behavior as in Case 1. Indeed, the foundation exhibited a vibratory motion, and residual displacement started to develop, just after the liquefaction zone advanced to the depth of the bucket tip [see point B in Figs.
8(a–c)]. This indicates that even if the tip of the suction bucket is supported on the dense layer where liquefaction could not occur, the suction bucket foundation could become unstable immediately when the liquefaction advances to the depth of the tip of the bucket in the free field around the bucket. The vertical displacement stems from tilting of the foundation on the dense layer.
Comparing Case 1 and Case 2 indicates that the horizontal and vertical displacements and the speed of development of the displacement were different between these cases. It is a serious concern that the final displacement in Case 2 was about 700 mm at the prototype scale, even though the foundation was supported on the dense layer.