Deve;opment of a Wave Absorbing System Using a Liquefied Sandbed

Abstract

A new wave-absorbing system, called the liquefied sandbed wave barrier (LSWB) system, is currently under development at the Port and Airport Research Institute (PARI) of Japan. The wave damping effect by the LSWB system is substantial,as confirmed by small-scale experiments and FEM numerical calculations, i.e., the wave transmission coefficient of the system is less than 0.2. Here, the results of large-scale experiments arediscussed in view of practical application. Although the LSWB system provides high wave damping, nearly equal to theoretical values, difficulty exists in obtaining a homogeneously liquefied sandbed, due to the occurrence of liquefied sandbed compaction by cyclic wave loading, which in turn, reduces excess pore pressure and the wave damping effect. These two phenomena primarily occur when the sandbed is composed of fine sand with small permeability. Based on experimental results, we propose a design method that includes countermeasures against such problems,and a prototype LSWB system is constructed in a very large wave flume at PARI. Wave damping by the prototype LSWB system is confirmed to be quite stable and high, as predicted by theoretical calculations.1. Introduction1.1 Concept of LSWB systemTo dissipate wave energy using damping produced by the movement of liquefied sand, we have been developing a liquefied sandbed wave barrier (LSWB) system consisting of horizontal pipes buried in the sandbed (Fig. 1), being an unique wave barrier in that it requires no structure. By pumping water through the pipes, the pore pressure in the sand bed is increased and "sand liquefaction (boiling)" occurs; an effect which significantly decreases the shear modulus of the sandbed such that large movement of the sand occurs due to wave action. This causes wave energy dissipation by friction among the sand particles during their wave-induced movement.