Interaction between Liquefying Submerged Slope and Submerging Water Mass

Abstract

The thesis presents a numerical study on dredging induced undrained instability and subsequent static liquefaction of submarine landslides. For the study, a pre-existing hydro-dynamic uncoupled submarine slope numerical model, developed by Molenkamp (1999), has been modified to incorporate a fully hydro-dynamic coupled interaction between submerging water mass and submarine slope. The modified model is able to simulate transient quasi-static and dynamic phenomena up-till and including the immediate post-liquefaction behavior of submerged slopes of loose undrained homogenous fine sands in a 2 dimensional Updated Lagrangian (UL) finite element (FE) frame of work. To simulate soil behavior under dredge loading applications the model incorporates a Monot soil constitutive model and for submerging water behavior a Lagrangian expression of Navier stokes for nearly-incompressible visco-elastic, irrotational, fluid model.
The study primarily addresses the effect of dynamics of submerging water on the liquefying submerged slope. The research findings suggest that the dynamic motion of submerging water barely affects the occurrence of instability. However, it may decrease the rate of post-instability liquefied flow as compared to the commonly sorted uncoupled scenario, where dynamics of submerging water mass is ignored and only constant hydrostatic pressure heads due to water level is considered at the slope interface. Moreover, the findings suggest that about 50% of the loss in the potential energy of soil is consumed by the potential energy of the submerging water at the very initial stages of post-instability and that the contribution of kinetic energy of water amounts to mere 3.4%.
Next, as a secondary issue, the study also provides a valuable insight into the effect of the liquefying slope on the motion of the submerging water mass. The findings show a surface impulse wave formation post-instability, moving along the direction of landslide. Moreover, it shows a development of a distinct circular motion of fluid along the slope interface. Other than this, the thesis also attempts to provide some similarities and differences between the current findings and the published conventional research studies which make use of basic slide shapes such as viscous or rigid sliding wedge blocks.
Finally, the thesis also addresses some numerical shortcomings such as the hour-glass effect, the shake-down by the procedure to define the “initial state” effect etc., and thereby providing necessary recommendations useful for future computational modelling work.