Effect of Water Entrainment on Seabed Soils during Cyclic Pipe-Soil Interaction-Paper

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The study investigates how water entrainment affects seabed clay strength during cyclic movement of a pipe that repeatedly penetrates and withdraws from the seabed. This process matters for predicting pipeline embedment and the development of trenches around offshore risers and mooring chains.<br /><br />Researchers conducted 100 vertical cycles with a model pipe in two self-weight-consolidated kaolin samples. They then measured soil strength with a T-bar penetrometer and sampled water content at each test footprint. Results were compared with measurements from undisturbed soil and conventional cyclic T-bar tests, which represent remoulding without water entrainment.<br /><br />The pipe tests produced apparent soil sensitivities three to four times greater than those measured in conventional T-bar tests. Post-test measurements showed that soil in the pipe footprints had higher water content and lower strength than fully remoulded soil, confirming that water was drawn into the seabed when the pipe broke away from the mudline. The weakened zone extended below the trench, while strength and water content approached undisturbed values at greater depths.<br /><br />The findings indicate that cyclic interaction near the seabed is not purely undrained: volume increase from water entrainment compounds strength loss from remoulding. Remoulded strength was closely related to the soil’s liquidity index, suggesting a possible basis for a predictive soil-mechanics framework. However, the mechanisms controlling water entrainment and how to predict moisture changes remain unresolved. Further research is needed to apply the framework to offshore infrastructure and trench development.

Keywords

water entrainment

seabed clay strength

cyclic pipe movement

pipeline embedment

offshore trenches

mooring chains

T-bar penetrometer

kaolin samples

soil sensitivity

liquidity index