Abstract This paper discusses the instability of an offshore monopile in association with wave-induced liquefaction of sand beds. Centrifuge wave tests in a drum channel were performed with viscous scaling introduced such that the time-scaling laws for fluid wave propagation and consolidation of the soil were matched. The relationships between liquefaction and the start and development of monopile instability in a sand bed under wave loading were investigated. The monopile started significant structural rocking when liquefaction occurred and progressed to one-third of the monopile embedment depth. The residual displacement of the pile increased markedly with the downward progress of the liquefied zone, eventually leading to the collapse of the pile. The characteristics of the onset of liquefaction around the monopile were also investigated. It was found that liquefaction first took place at the sides of the monopile rather than at the front or rear presented to the direction of the travelling waves, which highlights the importance of the rotation of the principal stress axes induced in the sand bed under the passage of progressive waves. The experimental results further demonstrated the effect of embedment of the pile in a dense layer. It was found that embedding the pile in the dense layer with thickness equal to or more than half of the pile embedment depth was effective in preventing the significant inclination and collapse of the pile. The present study also clarified the effects of scour protection around the monopile on wave-induced liquefaction and pile instability. It was found that the scour protection increased the liquefaction resistance depending on the diameter ratios of the scour protection and the pile, however, it could not prevent the collapse of the monopile once liquefaction occurred. These results show the importance of soil stratification on the wave-seabed-structure interaction in light of wave-induced liquefaction.

中文翻译：

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鼓式离心机中海上单桩的波浪诱导液化和不稳定性

摘要 本文讨论了与波浪引起的砂床液化相关的海上单桩的不稳定性。在滚筒通道中进行离心波测试，引入了粘性标度，以便匹配流体波传播和土壤固结的时间标度定律。研究了液化与波浪载荷作用下砂床中单桩失稳的开始和发展之间的关系。当液化发生并进展到单桩嵌入深度的三分之一时，单桩开始显着的结构摇摆。桩身残余位移随着液化区的向下推进而显着增加，最终导致桩身倒塌。还研究了单桩周围液化开始的特征。发现液化首先发生在单桩的侧面，而不是出现在行波方向的前部或后部，这突出了在通道下方沙床中引起的主应力轴旋转的重要性的进步浪潮。实验结果进一步证明了桩在密实层中的嵌入效果。研究发现，将桩埋入密度等于或大于埋桩深度一半的密实层，可有效防止桩的明显倾斜和倒塌。本研究还阐明了单桩周围的冲刷保护对波浪引起的液化和桩失稳的影响。发现冲刷保护根据冲刷保护和桩的直径比增加了液化阻力，但是，一旦液化发生，它就不能防止单桩倒塌。这些结果表明，鉴于波浪引起的液化，土壤分层对波浪-海床-结构相互作用的重要性。