P060

Development of new Wave Absorbing System Using Liquefied Sandbed

A0134 Yoon-Koo Kang Port and Airport Research Institute

A0135 Shigeo Takahashi Port and Airport Research Institute

A0136 Kojiro Suzuki Port and Airport Research Institute

A0137 Kenichiro Shimosako Port and Airport Research Institute

 

To dissipate wave energy using damping produced by the movement of

liquefied sand, a liquefied sandbed wave barrier (LSWB) consisting of

horizontal pipes buried in the sandbed are being developed at Port and

Airport Research Institute (PARI). By pumping water through the pipes,

the pore pressure in the sand bed is increased and sand

liquefaction occurs;an effect that significantly decreases the shear

modulus of the sandbed by large movement of the sand due to wave

action causing wave energy dissipation by friction among the sand

particles during their wave-induced movement.

To investigate the wave damping effect of LSWB, we have carried out

the hydraulic model experiments with the small and large scale, and

the finite element method calculations. It was found that the wave-

damping effect of the LSWB is significant, i.e., the wave transmission

coefficient can be 0.2-0.4 through a LSWB with one wavelength and the

sandbed thickness of the water depth. When the water depth is shallow

and the sandbed thickness is deep, then wave damping is large. The

wave damping effect of LSWB is considerably stable for the period of

acting wave.

For the improvement of the wave damping effect, it is very important

to produce a homogeneously liquefied sandbed and to prevent compaction

of the liquefied sandbed by cyclic wave loading. We investigated the

optimal conditions for producing a uniform liquefied sandbed using a

basic sand tank generating only upward seepage flow. It results that

the small pipe interval produces a homogeneously liquefied sandbed.

It was also confirmed that the critical interval of water supply pipe

required under wave action is about 10% the thickness of the sandbed.

Based on experimental results, we propose a design method including

countermeasures against such problems and a prototype LSWB system is

constructed in a very large wave flume (184m long, 3.5m wide, and 12m

deep) at PARI. Experimental results also showed good agreement with

theoretical calculations.

The new wave-absorbing system can be employed as a wave barrier

designed to produce a calm sea area, and may be especially suitable

for damping waves at a harbor entrance.