Effect of initially inclined principle stress directions on undrained shear strength for anisotropic clay
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In this thesis is a continuation of Rabstad's and Isachsen's work on the user-defined soil model ADPXX is done. The model aims to include initially consolidated shear stressesin the yield criterion for anisotropic clays, or include an inclined anisotropy. A working safety analysis which reduces the undrained shear strength has been developed, based on the definition of strength introduced by Isachsen in her version of the ADPXX-model. To obtain this, a new iterative scheme is developed based on an implicit local integration that includes a correction procedure with updating of partial derivatives, rather than a explicit approach with a substepping routine like the old versions of the model. The new local integration forces stress increments violating the yield criterion back to the yield surface for each step by using only the information at the final and admissible stress state. The result is a more reliable and robust model, without any need of adjustment regarding numerical settings to obtain capacity and converging safety factors in Plaxis. The FORTRAN-code is uploaded to DAIM, and can be implemented in Plaxis as described in appendix B. To be able to study the effect of including inclined anisotropy, a new input parameter has been defined allowing the user to decide the ratio between the anisotropy parameters, η and ξ. Where ξ controls the anisotropy related to initial deviatoric stress, and η the anisotropy related to initial shear stress. The new parameter makes it possible to vary the order of inclined anisotropy, by gradually moving the yield surface centre towards a situation where η=ξ and obtaining maximum strength in initial stress direction. Verification of the constitutive model and safety analysis is completed by considering a simple bearing capacity problem for both a situation with constant shear strength (isotropic) and for a situation where the strength varies along the slip surface (anisotropic). The model corresponds very well compared to Tresca and NGI-ADP, both for bearing capacity and obtained safety factors. Effect of including inclined anisotropy is studied for a slope with geometry and strength parameters attempted to correspond to Strandajordet, Vestfossen. The obtained results show that the effect will vary for different assumption of consolidated initial stresses and for different evaluated slip surfaces (shallow or deep) with different initial mobilisation.A maximum relative change in safety of +9.5\% is obtained for the failure mechanisms evaluated.