Slope Stability Analysis in Soft Sensitive Clay at Heimdalsvegen

Abstract

In Scandinavia and Canada quick clay present a challenge, especially to the engineer that have to handle stability problems involving quick clay and other strain softening materials. It is also important in terms of public attention.Quick clay is marine clay formed after the last glaciation and transformed by seeping fresh water washing thesodium chloride out of the grain structure. This leaves a highly sensitive material.Quick clay is the extreme case of a brittle geo-material, having a remolded shear strength below 0.5 kPa. Norway has experienced many landslides in quick clay with consequences varying from insignificant to catastrophic. Quick clay has strain softening properties, i.e. the material loses strength if further deformation is applied after the material has been fully mobilized. This leads to behavior that seems to be unpredictable. Strain softening can lead to propagation of progressive failure mechanisms. This is in essence small instabilities that propagate, and can lead otherwise stable slopes to fail.Being able to model progressive failures would be of great benefit. But strain softening poses some problems tomodeling. Traditional limit equilibrium methods can take strain softening into account by applying high factors of safety, which is very conservative. The finite element method is not able to analyze strain softening materials, as the mathematics lead the outcome to be mesh dependent, and thus not objective. New methods are being developed with the aim of taking strain softening into account. This thesis looks at the extended finite element method specifically.The Norwegian Public Roads Administration is expanding the Heimdalsvegen road south of Trondheim. It runs partly through a ravine with quick clay. Cross sections from this project have been used to study the different calculation methods. The limit equilibrium methods may miss the critical failure surface if it is non-circular, while finite elements is able to catch it. Non of them are able to catch the progressive failure mechanisms that potentially reduce the factor of safety substantially.The new methods, based on finite elements, have potential to deal with strain softening behavior and produce mesh independent outcome. The current extended finite element method implementation is not working as required, but experiments with the interface analysis in this thesis have shown the potential of the method. The factor of safety was reduced by 23 %. This indicates that the true factor of safety for such cases may be closer to 1 than the factor of safety of 1.4 required by the Norwegian Water Resources and Energy Directorate.