A new wave-absorbing system is proposed that dissipates wave energy using damping produced by the movement of liquefied sand. Wave damping due to soft clay movement is generally known to be high. On the other hand, that due to sand movement is much less because of its high shear modulus, though when liquefaction occurs, the sand becomes soft and induces high damping similar to soft clay. The new system is primarily comprised of horizontal pipes buried in the sand bed. Water is pumped into the pipes, which increases the pore pressure in the sand bed and causes so-called boiling of the sand, i.e., a kind of sand liquefaction occurs since it behaves like a liquid. Consequently, the shear modulus of the liquefied sand is significantly decreased, and a large movement of sand occurs due to the wave action. Wave energy is consumed by the resultant friction between sand particles during their wave-induced movement.
A series of model experiments and some FEM calculations were carried out to investigate the fundamental characteristics of liquefied sand and the wave-damping effect. Wave damping due to a liquefied sand bed can be significant, even at a hydraulic gradient as low as about 0.5. When the water depth is shallow and the sand bed thickness is deep, then wave damping is large. Wave damping can be expressed by Biot's equation using the linear wave damping coefficient. The corresponding FEM-calculated results indicate good agreement with experimental ones.
The new wave-absorbing system can be employed as a wave barrier designed to produce a calm sea area, especially suitable for damping waves at a harbor entrance.
