Wave-induced pore pressure measurements near a coastal structure

摘要

Wave-induced pore pressures were measured at various depths below the sandy bed infront of a coastal structure. The results of a weeklong field experiment carried out inSeptember 2003 on the beach of Capbreton, in southwest France, are presented. Thecoastal structure was located in the intertidal zone of the beach. The transmission ofpressure variations inside the soil, as compared to the pressure variations produced by thewaves in the water layer, are analyzed in terms of both amplitude decay and phaseshifts and compared to theoretical models. The gas content inside the soil was alsomeasured. The results confirm that the gas content is a key parameter affecting thetransmission of pressure inside the soil. It is shown that a significant upward pressuregradient is generated during a wave period, which can liquefy a 30 cm deep superficiallayer of the soil. This is interpreted in terms of momentary liquefaction events. Themanner in which the phenomena change during the different tidal periods investigated isdescribed. The dependence of the results on wave height and bed level is discussed.Whereas the soil properties were not modified over a tidal period when the wave activitywas sufficiently low, a significant change in the transmission of pressure variationsinside the soil was observed when the structure was subjected to larger waves. This isinterpreted in terms of a change in the gas content in the superficial layer.