Analytical and numerical modeling of progressive failure of onshore and offshore slopes with sensitive clay layer

Dey, Rajib (2015) Analytical and numerical modeling of progressive failure of onshore and offshore slopes with sensitive clay layer. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Landslides in both onshore and offshore environments are always a potential hazard and a great threat to many communities and infrastructure. There are some similarities and differences between failure mechanisms and potential causes of failure of slopes in these two environments. Large landslides in both environments are generally progressive in nature. This becomes more pronounced when the slide occurs in sensitive clays, generally assumed under undrained condition. Post-peak softening of sensitive clays is considered as one of the main reasons for pronounced progressive failure. Sensitive clays found in both environments show nonlinear post-peak strain softening behavior at large strain/displacement during undrained loading. Post-slide investigations show that failure patterns of many large-scale submarine landslides through marine clays could be very similar to onshore landslides through sensitive clays as encountered in Eastern Canada and Scandinavia (e.g. translational progressive slide, and spreads). As onshore slope failures in sensitive clays are better documented than submarine landslides in marine clays, information on onshore sensitive clays available in the literature can be utilized as the basis to model both submarine and subaerial landslides in sensitive clays. The main focus of the present study is to model slope failure through weak sensitive clay layers under undrained conditions. A nonlinear mathematical model for post-peak degradation of undrained shear strength of sensitive clay, applicable to small to large-strains, is proposed in this study based on available experimental results. The slope failure mechanisms are examined using the concept of shear band propagation. Various factors have been identified in the past that could trigger a large-scale slope failure in both environments. Among them, the effects of toe erosion, surcharge loading, and strength reduction in a section of a weak layer are considered in this study. In a marine environment, strength reduction in a section of a weak layer in an offshore slope might result in initiation and propagation of a shear band in both upslope and downslope directions at the same time. By incorporating a nonlinear post-peak softening model, an analytical solution is developed to examine a possible mechanism of failure of mild submarine slopes containing a weak zone of low shear strength. In comparison, due to toe erosion, a shear band formation could be initiated and propagated upward (inward) from the river bank which could lead to a spread type failure forming horsts and grabens. Upslope surcharge loading (e.g. the placement of fill) could also generate shear bands that might propagate down towards the river bank. Numerical modeling of these types of slope failure is considered as a large deformation problem. Finite element (FE) models in Lagrangian framework cannot model the complete process of these slides, as significant mesh distortion occurs. Coupled Eulerian Lagrangian (CEL), a finite element approach in Abaqus FE software is used in this study to model these progressive failures of slopes. A nonlinear strain softening model for undrained shear strength of sensitive clays is incorporated in the FE simulation. Upward progressive failure leading to spread due to toe erosion, downward progressive failure due to a construction load in the upslope area, and combined effects of upward and downward propagation of shear bands on stability of a river bank slope have been simulated in this study. Simulations are also carried out to model submarine landslides due to the existence of a weak layer. The FE simulated results and failure patterns of ground surface or seabed are compared with the slide morphology presented in the literature. The main advantages of the present FE modeling are: (i) extremely large strains in the shear bands can be successfully simulated without numerical issues, (ii) a prior definition of shearing zones with special mesh and/or element type is not required to capture extreme strains in the shear bands, and (iii) the FE program automatically identifies the location of critical shear band formation and direction of propagation.

Item Type: Thesis (Doctoral (PhD))
URI: http://research.library.mun.ca/id/eprint/11595
Item ID: 11595
Additional Information: Includes bibliographical references.
Keywords: Progressive failure, Sensitive clay, Slope stability, Strain softening behavior, Finite element modeling, Landslides, Submarine slope failure, Abaqus CEL
Department(s): Engineering and Applied Science, Faculty of
Date: May 2015
Date Type: Submission
Library of Congress Subject Heading: Landslide hazard analysis--Mathematical models; Slopes (Soil mechanics)--Stability--Mathematical models; Finite element method; Shear strength of soils--Testing

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