Response spectra for transient pore-pressure in a sandy seabed under random waves: Frequency-filtering effect

Abstract

The real state of ocean waves is random in nature with various monochromatic components. Transient pore pressures in a sandy seabed under the action of the random-waves with JONSWAP spectrum and the regular-waves were physically modeled, respectively. The surface elevations and the corresponding transient pore pressures in the fine-sand were measured simultaneously in a wave flume. The frequency-filtering phenomenon was observed by Fourier analysis for random-wave induced pore pressure responses. In the Fourier energy spectra, both a primary-peak (high-frequency) and a secondary-peak (low-frequency) were identified within the relatively shallow soil layer. But the primary-peak kept declining rapidly, meanwhile the secondary-peak attenuated much more slowly with increasing soil depth, which can eventually lead to a shift between the high-frequency peak and the low-frequency one. As a consequence, the upgraded primary-peak frequencies of the dual-peaked spectra at the deeper soil layer would become smaller than the original significant frequencies, indicating that the pore pressure induced by long-wave components is of vital significance there. Furthermore, a filtering-depth parameter is introduced to characterize the frequency-filtering effect. Given the pore pressure at a certain soil depth, the quantitative relationship between the primary-peak and the secondary-peak can be predicted for an arbitrary soil depth.