In this paper the liquefaction of sand bed under oscillating water pressure are treated as a basic study of the prevention works against the scouring around the hydraulic structures. The results of the former resurch show that the occurrence of the liquefaction depends on both properties of the oscillating water pressure and of the sand layer. Considering the latter properties, that is, the resistivity against the liquefaction increases with the increase of the permeability of the sand bed, we propose the displcement method as one of the prevention works, which is a method to displace the upper layer of the sand bed by the sand with large permeability. The effects of this method are investigated theoretically and experimentally. By the experimental study, it is shown that the proposed displacement method has the apparent effect to
prevent the liquefaction. The experimental results are explained fairly well by the theoretical analysis based on the theory of the flow through the elastic porous media.