A Visco-Hypoplastic Constitutive Relation for Soft Soils

Abstract

ABSTRACT Soils with soft angular particles are markedly viscoplastic. For constitutive modelling with hypoplasticity, the granulate hardness is replaced by a rate-dependent (argotropic) solid hardness hs. Use is made of the viscosity index IV. The state limits of isotropic compression, stationary shearing (critical state) and cyclic shearing are thus argotropic. They are asymptotic solutions of the proposed visco-hypoplastic relation, the core of which is a viscosity factor fv depending on an overconsolidation ratio pe/p’. It resembles a theory by Niemunis (1996), but has a wider variety of asymptotic solutions (attractors). For constant effective stress, the familiar volumetric creep is obtained, and a dependency of strain rate ratio on stress ratio as observed. The response to strain rate jumps is obtained realistically, and also the relaxation. The material parameters can be easily determined from standard tests and estimated from index tests. Further explanation is given with a back analysis of Henkel’s (1956) triaxial test results. Deviations are due to non-uniform sample deformations. Peak stress conditions are derived from instantaneous state values including strain rate. Extensions are briefly outlined: tensorial formulation, intergranular strain and polar terms. The microphysical background is explained with Persson’s (2000) theory of thermally activated shear melting. Realistic dislocation energies are thus obtained from observed Iv values.