Marine structures such as breakwaters, seawalls, pile structures, sea barriers, pipelines, and rock armour systems of the type that are used to protect coastal communities against flooding and erosion could be at risk from the phenomenon which involves liquefaction of the seabed supporting these structures - with potentially catastrophic results. Pipelines laid on the seabed may sink into the seabed; large individual blocks (such as those used for scour protection) may penetrate into the seabed; and, as a result of wave motion, structures may undergo cyclical motions, resulting in local liquefaction around them, which may enhance scour, thus leading to the instability of structures.

Liquefaction is induced by effects such as waves, seismic action, and loads (such as sudden failure of a slope, or blasting effects (blast effect, wave slamming, and ship impact)).

Although a substantial amount of knowledge has been accumulated on flow and morphological processes around marine structures in the last decade or so, comparatively little is known about the impact of liquefaction on these structures. The topic has been little covered in recent EU research - which has substantially advanced the design of coastal structures, but not of their foundations.

The objectives of the project, which is due to be completed in 2004, are to investigate potential risks for failure of structures due to liquefaction; and prepare and disseminate practical guidelines for design and maintenance in order to avoid liquefaction.

In a paper given at the CoStructNet/Beach Processes Network Workshop conference Beach Structure Interaction , held earlier this summer, Scott Dunn and Jesper Damgaard of HR Wallingford in the UK explained that liquefaction can be defined as a loss of shear strength of the seabed due to increased pore pressures. They explained that there are two main types of liquefaction - residual, which occurs in loose sands, and momentary, which occurs when gases are present in the seabed material.

Liquefaction induced by the build up of pore pressure is called Residual Liquefaction;

liquefaction induced by a pressure gradient is called Momentary Liquefaction.

Dunn and Damgaard said liquefaction could adversely affect dredged channels, causing material to flow into a channel, potentially preventing access to a port or terminal. Problems could also occur with breakwaters and piles, because if liquefaction occurs the top 1-2m of the seabed cannot be relied upon for resistance, and rocks may sink or move and cause instability in a breakwater or pile. Given the widespread use of offshore windfarms, the project participants are also examining the potential consequences of liquefaction and scour around suction caissons in offshore windfarms.

Overall, said Dunn and Damgaard, the project participants would like to be able to determine whether liquefaction was likely in the vicinity of seabed structures, and whether it may be possible to mitigate its effects or prevent it.

More details about the project can be found at www.isva.dtu.dk/limas/limas.html