Soil Liquefaction

Under this condition, the soil (fine sand, silt) fails, thus precipitating failure of the structure. With the soil partially/completely liquefied, buried light pipelines may float to the surface of the seabed; heavier pipelines laid on the seabed may sink in the soil; offshore structures may settle; large individual blocks (such as those used for scour protection) may penetrate into the seabed; sea mines may enter into the seabed and eventually disappear.

Pore-water pressure and surface elevation in a progressive wave in the laboratory in a

water body

 

The key question here is: given the soil and given the waves, will there be any liquefaction potential risk for the soil supporting the marine structure, e.g. a pipeline, a gravity structure, a breakwater, a pier, a pile, or a scour protection structure? This question has stimulated research in the area of marine civil engineering over the past two decades.

Marine soil liquefaction is one of the areas covered by the Coastal and River Engineering Section in MEK. The following is a list of the topics studied under this theme:

• Sinking/flotation of pipelines and other objects in liquefied soil under waves.
• Liquefaction around a buried pipeline under waves.
• Influence of liquefaction (in partially/fully liquefied soil) on scour, another failure mode of structures.
• Liquefaction of soil under the rocking motion of a breakwater.
• Liquefaction of soil and resulting scour under the rocking motion of piles.
• Numerical modelling of wave-induced liquefaction.

Settlement caused by liquefaction at the Izmit Public Marina. Taken from Earthquake Spectra, 1999 Kocaeli, Turkey Earthquake Reconnaissance Report

The Section has also coordinated an EU research programme on this topic, namely Liquefaction Around Marine Structures (LIMAS), under the EU Framework Programme 5. The programme covers the period 2001-2004, and is undertaken by a consortium consisting of ten members, from academia and industry, from seven different European countries. The specific objectives of the programme are (1) to investigate the potential risks of failure of marine structures due to liquefaction; and (2) to prepare and disseminate practical guidelines, to be formulated from the present research programme, taking into consideration all state-of-the-art knowledge. Although the majority of the projects in LIMAS deal with wave-induced liquefaction, one project focuses on earthquake-induced liquefaction and its impact on coastal structures, with special reference to the 1999 Turkey earthquake. The following web address can be consulted for further information about the programme: http://vb.mek.dtu.dk/research/limas/limas.html