Finite Element Analysis Of Strain Localization In Natural Clay Using Elasto-viscoplastic Constitutive Model

In geotechnical engineering,strain localization and the formation of shear bands is regarded as initiator of failure as a result of non-uniform deformations within soil.In this research,the influence of confining pressure and strain rate on strain localization is studied numerically by analyzing their effects on the formation of shear bands,thickness as well as the orientation of shear bands in natural clay.Mesh dependency effect is also briefly studied in the finite element simulation.A new elasto-viscoplastic constitutive model not developed by the author that was implemented into a Finite Element Method commercial software ABAQUS,with the ability to accurately describe the time-dependent,over-consolidated and structural characteristics of natural soft clay is used to perform 2D plane strain compression test under undrained conditions on undisturbed Kyuhoji marine clay whose mechanical behaviour and micro-structure was studied experimentally in the past years showing anisotropic yielding properties and the influence of micro-structure properties in the soil’s compressive strength,stiffness and brittle behaviour.To test the ability of the constitutive model,first,one-dimensional consolidation test was simulated and the results were compared to experimental results available in literature.Results of pore water variation along depth were also compared to theoretical results to ascertain the capability of the constitutive model.Strain localization was observed under all conditions of confining pressure(100k Pa-500 k Pa)and strain rates(0.005%/min-0.5%/min)range chosen.Increase in confining pressure and in strain rate both increases the peak stress of the overall model as well as the individual elements inside and outside shear bands.Elements are classified with respect to their locations in the deformed mesh.Non uniform behaviour is observed in the same group of elements.Lateral and axial symmetric behaviour is shown in elements inside and outside of shear bands.Increase in confining pressure increases the softening rate of shear band elements but has no effect on the contraction and dilation rate of elements inside and outside shear bands respectively.Increase in strain rate increases the softening rate of shear band elements and has slight effect on the contraction and dilation rate of elements inside and outside shear bands respectively.Multiple crossing shear bands are observed whose pattern is unaffected by the variation in confining pressure and in strain rate.Increase in confining pressure decreases the inclination and thickness of shear bands whereas an increase in strain rate increases the inclination and thickness of shear bands.Mesh size shows no effect on the mechanical response of the model but affects the appearance of shear bands.Course mesh results to unclear shear bands with thicker bands at lower inclined angles.With finer mesh the shear bands,clearer and thinner shear bands are observed.