ABSTRACT

In this paper, a three-dimensional numerical model for wave-seabed interactions around a group of piled foundations is proposed. Unlike previous studies, both flow and seabed models are developed based on the open source library, OpenFOAM® (version 4.0). In the present model, the wave motion is governed by RANS equations, while the porous flow in the seabed is governed by dynamic poro-elastic u - p approximation. The present model is validated with the previous experimental data for a single pile. Then, the present model is further applied to a group of piles. Numerical results indicate that the wave characteristics as well as the configurations of a group of piled foundation can significantly affect the oscillatory pore-water pressures in the seabed.

INTRODUCTION

Piled foundations have been commonly used to support various offshore infrastructures such as platforms in shallow water, cross-sea bridge piers and offshore wind turbines, etc. For the design of a piled foundation, seabed stability (including soil liquefaction, scour and shear failure) in the vicinity of the structure needs to be taken into consideration. Furthermore, the wave-induced pore-water pressure is one of key factors for the estimation of the wave-induced seabed instability. With the increases of the pore-water pressure and the decreases of the effective stress due to the wave loading, part of the seabed may become unstable or even liquefied. Once the liquefaction occurs, the liquefied soil will behave like a heavy fluid and not provide any resistance to the pile foundation.

Two mechanisms of the wave-induced soil response have been observed in the previously laboratory experiments and field measurements (Zen and Yamazaki, 1990), depending on the manner that the pore pressure is generated. They are residual and oscillatory mechanisms. Among these, the residual mechanism is caused by the progressive nature of the excess pore pressure, which usually appears at the initial stage of the cyclic loading (Seed and Rahman, 1978; Sumer, 2014). The oscillatory mechanism is normally accompanied by the amplitude damping and phase lag to the pore-water pressures (Madsen, 1978). This type of soil response usually appears periodically during a storm sequence. In this study, we only focus on the oscillatory soil response.

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