Soil Mechanics: Rate effects, viscosity and creep behaviour in clay

It is widely accepted that there is a connection between the undrained shear strength and the strain rate. Thixotropy and creep behaviour are connected to the mechanical properties of clay. Thixotropy is the ability of clay to recover its shear strength over time when the shear stress is released. This phenomenon is reversible.

Creep is the continuous deformation at a constant shear stress. Creep behaviour consists of two stages; secondary and tertiary consolidation. Primary consolidation is the instant drainage and compression. Secondary and tertiary phases resemble the observations from the development of equilibrium in viscosity over time at a given stress level for a thixotropic fluid.

In rheology, this type of material is known as a non-Newtonian thixotropic fluid. A Newtonian fluid has no yield stress, resembling a strictly elastic material; whereas a non-Newtonian fluid cannot be expressed by a direct mathematical relationship. The aim of this work is to associate the observed behaviour during shear tests with the viscosity at a given shear stress and the creep at the same stress level.

In order to do so, the three tests are conducted at the same confined stress level. All three properties are expressed by stress and strain and their behaviour is convergent. Numerical models exist that simulate clay behaviour over time, the majority derived from Perzyna (1966). An empirical expression for the ‘index of viscosity’ was derived by Leinenkugel (1976).

This suggests the change of strain rate is proportional to the logarithm of their ratio. The consistency approach or the Perzyna and Leinenkugel work could be used to model and associate the three phenomena listed. A relationship between creep, the rate effect and the viscosity of a thixotropic fluid is apparent.

The poster is mainly based on numerous CAU-tests performed during M.Sc.