Numerical modeling of progressive and retrogressive failure of submarine slopes with a sensitive clay layer

Roy, Shubhagata (2018) Numerical modeling of progressive and retrogressive failure of submarine slopes with a sensitive clay layer. Masters thesis, Memorial University of Newfoundland.

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Abstract

Submarine landslides are major concerns in offshore oil and gas development activities, as they can displace an enormous amount of sediment compared to onshore landslides. Among the different triggering factors identified in the past, the presence of a weak strain-softening clay layer and earthquake loading have been considered the primary causes of many large-scale landslides. Depending upon geometry, loading conditions and sediment properties, different types of failure patterns (e.g., slab, spread, ploughing, run-out) have been observed in the field. In the present study, a Eulerian-based finite-element (FE) approach in Abaqus software is used to study large-scale submarine landslides for varying slope angles. The main objective of the present study is to numerically simulate the failure of submarine slopes with strain-softening clay layers triggered by two factors: i) shear strength reduction in a thin zone of soil which could be occurred due to gas hydrate dissociation, and ii) earthquake loading. Two approaches are used to model earthquake loading: (i) complete dynamic modeling base on acceleration–time history and (ii) modified pseudostatic loading with a simplified pulse of horizontal acceleration. A model for strain-softening behaviour of clay is implemented in the FE simulation, and analyses are performed for varying slope angles over a large seabed section. The FE simulations show that an initial shear band of sufficient length parallel to the seabed in the steeper part of the slope could cause a large-scale landslide by progressive formation of failure planes, both in upslope and downslope areas. In upslope areas, the failure pattern is similar to spreads commonly observed in sensitive clay slopes in onshore environments. In downslope areas, the failures tend to be similar to ploughing. The obtained failure patterns, ploughing distances, and retrogression distances for both types of earthquake loading are compared. Simple pulse-type pseudostatic loading can be used to reasonably model earthquake-induced landslides.

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