Abstract

Finite element models are developed using the poroelasticity formulation to analyze the instantaneous liquefaction potential of granular seabed around a benchmark rubble-mound breakwater under waves. Mathematical formulations in terms of governing equations of partially-dynamic and quasi-static cases are considered based upon the inertial terms associated with the solid phase. Liquefaction is modeled through the effective mean stress criterion. A number of parametric analyses carried out shows how much of seabed and wave parameters affect the depth and progression of instantaneous liquefaction. It is found that liquefaction may, for some instances, trigger instability of rubble-mound breakwater through the seabed under regular waves.

Introduction

Marine infrastructure plays a vital role in relation to energy, environment and sustainable development. Coastal and offshore structures built to protect coastal regions constitute a significant part of marine infrastructure. The instability of such coastal structures is induced primarily by the action of oscillatory and impact forces caused by waves and strong ocean currents. Geotechnical aspects play a significant role in the initiation of these instabilities. Thus, the evaluation of wave-induced response of seabed around structure foundation systems plays a key role in mitigation of the associated hazard. In this paper, we are presenting the preliminary outcomes of an ongoing research work focusing on the dynamic response and instability of a rubble-mound breakwater. Particularly, the instantaneous liquefaction of seabed around the rubble foundation of a typical breakwater cross-section is evaluated based on the mean effective stress criterion.

Analysis of breakwater-seabed system as a whole requires accurate modeling of soil dynamic response under cyclic wave action. Poroelasticity theory (Biot, 1941, 1955) has the most fundamental and mathematically sound background for numerical modeling of saturated porous granular soil under dynamic loads. Thus, it is employed in solving for the internal forces, deformations and pore pressure developments inside the porous structure of the system in such works. The variation of these variables with time and space determines whether the system will sustain its structural integrity under critical wave storms. Hence a stability condition called ‘instantaneous liquefaction’ where soil loses its mean effective interparticular stress at any time instant when the wave pressure is exerted on the surface of the seabed soil (particularly under the wave trough), is analyzed. To the best of our knowledge, instantaneous liquefaction around a rubblemound breakwater has not been analyzed through the poroelasticity theory before.

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