Pipeline floatation due to wave-induced liquefaction of trench backfill material
CEDA Dredging Days 2017 - Sustainable Dredging - Continued Benefits
Baelus L., Szengel V., Breughem A., Tavallali A., De Wit K.
Abstract: The backfill material of a pipeline trench can be liquefied during the service period. Liquefaction is characterised by material losing its strength and acting as a fluid, because the effective stresses between the individual grains have disappeared in response to an applied stress. In case of wave-induced liquefaction, the water pore pressure in the soil can build up due to the cyclic loading of waves to a level that exceeds the overburden pressure and eliminates the effective stresses. During the liquefied state of the trench backfill material, floatation of the pipeline can occur. To design the trench backfill material and avoid pipeline floatation after installation, IMDC has performed a detailed study of wave-induced liquefaction of the backfill material. The study is performed in two steps. First, the susceptibility of the backfill material to liquefaction is investigated based on the particle size distribution of the borrow area (for trench backfill). In a second step, a site specific assessment of the susceptibility to waveinduced liquefaction is performed based on the local wave conditions. Initially, the required soil characteristics are defined based on the particle size distribution of the borrow area (for trench backfill) and typical values given in literature, while in a later phase, results from laboratory test allowed a more precise definition. Additionally, a sensitivity analysis is executed on the physical and mechanical properties of the backfill material and the dimensions of the pipeline trench. Both the properties of the dredged material from the borrow area and the material disposing method are taken into account. At the end of the study, IMDC formulates recommendations to avoid liquefaction and pipeline floatation.
Key words: Wave-induced liquefaction, pipeline floatation, backfill material, trench