OBSERVATION AND PREDICTION - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 140

given state of stress, it allows for several possible directions of plastic flow rather
than a unique direction, which coincides with one plastic potential in the Mohr-
Coulomb approach. Here, a special remark is made about the use of model
parameters. Since, as an example, the Mohr-Coulomb model is essentially different
from the Double Sliding model, one may not use parameters obtained from
laboratory tests by using Mohr-Coulomb for prediction with the Double Sliding
model. This is inconsistent and may lead to erroneous results. Model parameters
must be compatible with the constitutive model used for the calculation or
prediction.
Hyper-elastic or Green model is suitable when the current state of stress depends
on the current state of deformation and not on the history of strain. Hence, hyper-
elastic materials can be characterised by a strain-energy function, assuming
isotropic deformation (constant volume). Generally, it is suitable for materials that
respond elastically when subjected to very large strains and under proportional
loading conditions. This type of model fails to identify the inelastic and unloading
behaviour.
Hypo-elastic models exhibit nonlinear, but reversible, stress strain behaviour.
Similar to hyper-elasticity, the strain depends only on the stress, and not on the rate
or history of loading. In this line, incremental stress-strain or piecewise linear-
elastic laws are developed. Most of the plasticity models are in incremental forms
similar to those of hypo-elastic models. The application of hypo-elastic models
should be restricted to loading situations which do not basically differ from the
experimental tests from which the material constants were determined or curve-
fitted. Hypo-plastic models (Kolymbas, Gudehus) relieve the standard strain
decomposition into additive elastic and plastic parts and do not use yield or
potential surfaces and flow rules.
Time-dependent behaviour can be related to intrinsic viscous characteristics of
soil, known as creep, relaxation, rate sensitivity and secondary compression.
Viscous properties originate from the microscopic structure of soils like clay,
which are composed of small clay particles with a highly active ion-water system
in the micro-pores. Various visco-elastic models have been developed based on this
microscopic nature (Mitchell, Sekiguchi). Macroscopic modelling of viscous
behaviour follows the empirical approach, the visco-elastic approach or the visco-
plastic and elasto-visco-plastic approach. Visco-elasticity consists of an elastic
component and a viscous component where viscosity is a strain rate dependent on
time. It has characteristics like hysteresis, stress relaxation and creep. The explicit
introduction of time in an empirical approach when elaborating experimental tests
violates the principle of objectivity in continuum mechanics and it is therefore
limited to specific boundary and loading conditions. It is often used for one-
dimensional behaviour (see Chapter 6). In the visco-elastic approach, the Maxwell
model, the Voigt model and other similar arrangements are illustrative. In visco-
plastic models, the key assumption is that viscous effects become pronounced
when the material yields, and that the viscous effects are not essential in the elastic
domain. Sekiguchi proposed an elasto-viscoplastic model for normally
consolidated clay based on a non-stationary yield surface. Rate-dependent
behaviour such as creep rupture is then described by a visco-plastic potential.