Material and slope failure in sensitive clays
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Recent and historic landslides in sensitive clays caused by an external disturbance point to the importance of understanding the acting failure processes. This includes the global slope behaviour and the local material response. Sensitive clay is a brittle material in the sense that it displays strain-softening behaviour under undrained shear deformation. In turn, strain-softening may lead to a progressive failure development in a slope. While traditional methods for slope stability assessments are based on limit equilibrium and rely on input on a strain independent soil strength, the understanding and modelling of progressive failure relies also on the soil stiffness, the rate of strain-softening and the residual resistance. These aspects have been exemplified and quantified in the present work. In particular, the initial stress state of a slope is identified as governing when considering the safety margin and potential failure extent. On the local material level, strain-softening holds the potential to cause strain localization and shear band formation. Two experimental setups have been utilized to examine the shear response of quick clay; a modified triaxial device where shear band formation is allowed and a field shear vane with over-coring of the sheared soil element. Strain localization and shear band formation was seen to initiate close to the undrained peak shear resistance. A clear effect of the displacement rate on the post-peak properties of sensitive clay is observed. Increasing the displacement rate increases the brittleness, both in terms of higher rate of strainsoftening and lower ultimate end-of-test resistance. The underlying mechanisms are argued to be related to the contractant behaviour of sensitive clay at failure and the corresponding generation of excess pore pressure. This situation can cause internal gradients of pore pressure and local drainage in the vicinity of the shear band. This hypothesis is supported by coherence between numerical simulations of local drainage and the experimental results as well as microscopy analyses which reveal a reduction in porosity in the shear band. The latter is a direct observation of contractant material behaviour and implies migration of water. The microscopy analyses have shown a mm-scale shear band as a non-smooth feature with complex inner systems of minor shears (μm size). The width of the shear band is a function of the applied displacement rate, presumably through processes of internal drainage of excess pore pressure. Higher rate yields a thinner band. The width of the minor shears is given by the microstructure. The strong brittleness of sensitive clay at high displacement rates under shear band formation motivates a recommendation to use a design approach close to a first yield criterion if the construction situation might include such high rates. The thesis provides insight for understanding slope and material failure in sensitive clays. It highlights and treats aspects needed for further work in the direction of an improved and unified methodology for slope stability assessments in these clays. In the extension of the work it is suggested to focus on the initial stress state of slopes as well as a study of the relation between material behaviour in laboratory experiments and full scale slopes.