PLASTICITY INDEX EFFECTS ON MECHANICAL BIFURCATION: SOILS AND SOFT-SEDIMENT DEFORMATION

Abstract

Yielded specimens have various deformation patterns depending on loading stages and stress
ratios in spite of the same geomaterials (e.g. soils and soft-sediment). Failure patterns of materials
bifurcationally change to diamond, bulge and a pair of oblique shear patterns. Symmetry of deformation
patterns (e.g. shear band patterns) has been illuminated by bifurcation analysis of governing equation based
on Cam-clay model. Plasticity index tested by Casagrande liquid limit device and plastic limit instruments is
known to describe mechanical characteristics (e.g. compressibility) of soils. Relationship between plasticity
index and mechanical bifurcation controls the evolution of deformation patterns. From the view point of
concept of plastic potential, we show that plasticity index theoretically determines deformation patterns of
soils on the basis of the bifurcation analysis. Furthermore, because deformation facies of rocks are described
by mean ductility of rocks similar to the concept of plasticity index, we point out that mean ductility controls
mechanical bifurcation of the geologic materials such as soft-sediments.