Experimental results and constitutive model for time-dependent behaviour of sands under oedometric compression

Oedometric loading data of dry quartz sand with different initial relative densities was analyzed with respect to the time-dependent behaviour, specifically, creep, rate-dependency and stress relaxation. The test results revealed a significant stress and density dependency of the creep-rate, as well as non-permanent changes of the effective stress following sudden strain-rate changes during strain-rate controlled loading. The experiments with a loading of up to 7.5 MPa effective vertical stress were analyzed using the isotache-concept originally developed for soft fine-grained soils. In this concept, the time-dependent behaviour is governed by the viscosity index Iv, defined as the ratio of the coefficient of creep Cα to the compression index Cc. However, our experimental results show that the isotache-concept does not apply to granular materials. Particularly, the viscosity index Iv is not a material constant, but depends on density and stress. In order to describe the creep behaviour of granular soil under one-dimensional compression, a visco-hypoplastic model was modified. In this new model the viscous strain-rate is a function of the current density, the stress and the overconsolidation ratio according to a Norton-type creep law.