ABSTRACT

In this study, a series of sea-wave loading tests were conducted on model monopiles settled on a rubble mound with a scour countermeasure around the monopile foundations. The results revealed that local scouring around the structure could be suppressed by installing rubble mound countermeasures, which reduce the bottom shear stress and prevent sediment transportation. However, excess pore water pressure in the seabed was generated by waves, regardless of the countermeasures. Therefore, simply placing a highly permeable crushed stone above the seabed was insufficient for suppressing the generation of excess pore water pressure in the seabed.

INTRODUCTION

Sediment transportation induced by ocean waves is important for disaster prevention and environmental management in coastal areas. In particular, scour and erosion around coastal structures due to sea waves are critical for the stability of such structures. For example, monopile foundations have been used for offshore wind turbines at depths of >30 m. For monopile foundations that are subjected to wind loads, scouring and erosion, which can cause structural instability, around the pile foundation must be prevented. Because fluctuating water pressures due to sea waves affect monopiles and the seafloor, in many cases, coastal structures become unstable owing to the reduced integrity and strength of the seabed. A previous study on scouring around upright cylindrical structures owing to waves was focused on the maximum scour depth and final scour profile (Summer et al., 1992). Recently, studies on the stability of the monopile foundations of wind turbines have been conducted (Mayall et al., 2018; Miyamoto et al., 2018). Miyamato et al. (2018) showed the process of wave-induced liquefaction to have a progressive nature using a centrifuge model test.

In previous studies on scour and erosion, many researchers have considered tractive forces for predicting topographical changes due to water flow (Madhav, 1955; Iwagaki, 1965). Conversely, geotechnical researchers have focused on stress changes in the seabed induced by waves (Madesen, 1978; Yamamoto, 1978; Zen and Yamazaki, 1990; Miura et al., 2004). The movement of soil sediment induced by sea waves is caused by complex interactions between tractive and seepage forces. Matsuda et al. (2018) conducted a series of wave flume experiments with a movable bed, focusing on the influence of tractive and seepage forces on soil movement. The movable bed was selected based on Dean number equivalency, and the behaviour of pore water transmission inside the soil sediment was accurately represented. Moreover, the change in the phase angle according to excess pore water pressure with depth was determined. The observed maximum excess pore water pressure ratio was similar to the exact solution. The soil layer at the surface was transported upward with an increasing excess pore water pressure ratio (EPWPR). Matsuda et al. (2021) investigated the scour process around a cylindrical structure by considering the shear stress on the seabed surface and the effective stress response of the seabed. Their results showed that excess pore water pressure in the seabed was generated by waves and that the change in the effective stress response of the seabed differed at each location around the structure. In particular, the seabed was scoured on the sides of the cylindrical structure. Therefore, measures for preventing scouring and seabed instability caused by waves around cylindrical structures must be considered.

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