Liquefaction around coastal structure: role of soil gas content
Résumé
Wave action combined to sea level rise and coastline retreat may weaken coastal and port structures. The combination of energetic flows and loose seabed mechanical properties at the vicinity of a structure may trigger scour and liquefaction phenomena (Sumer et al. 2001; de Groot et al. 2006). The ocean waves can generate significant dynamic pressures on the sea floor, that further induce pore-water pressure variations and effective stresses within the seabed. With excess pore pressure and limited vertical effective stress, part of the seabed may become unstable or even liquefied. Once liquefaction occurs, the soil particles are likely to be carried away as a fluid by any prevailing bottom current (Jeng et al., 2010). Former field experiments in a macro-tidal environment have shown that air bubbles can be trapped within the sand bed, as the beach de-saturates at low tide and partially saturates during rising tide (Breul et al. 2008). In these field experiments (Mory et al. 2007), liquefaction has been observed at the toe of a vertical wall subjected to intense wave forcing. The phenomenon was identified as momentary liquefaction (Sakai et al. 1992) and promoted by air bubbles trapped inside the bed. The seabed gas content is of major influence on pore pressure transmission and hence on liquefaction occurrence (Michallet et al. 2009). Detailed measurements and visualization of the seabed are not easily obtained in the field. The present study aims at understanding the main physics of the seabed response to wave impact on a coastal structure with a physical model using light-weight sediment (Michallet et al., 2012). Both momentary liquefaction and liquefaction induced by pore-pressure build-up under cyclic loading are reproduced. It is shown that air bubbles trapped in the sediment bed is promoting the first type of liquefaction whereas it prevents the second type to occur.