Time-dependent deformations, such as creep and secondary compression, and time-dependent stress changes at constant deformation, such as stress relaxation, are important considerations in many geotechnical engineering studies. Practical situations in which these effects are important include long-term settlements of structures on compressible ground, deformations of earth structures, movements of natural and excavated slopes, and squeezing of soft ground around tunnels. Although methods for the analysis and prediction of one-dimensional and three-dimensional time-dependent consolidation of clay soils have been available for many years and have met with reasonable success in engineering practice, present ability to describe and predict time-dependent deformations and stress relaxations under deviatone (shear) stress states properly is very limited. Most analyses are made to determine limit equilibrium conditions; i.e., conditions at failure, or stress-deformation responses without consideration of time. Accordingly, the study of the rheological characteristics of clays at states of most interest in geotechnical engineering, i.e., in the consistency range from the plastic to the liquid limit, remain of great research interest.

Research on time-dependent deformation and stress phenomena in soils is important not only because of the immediate direct application of the results to analyses of practical problems, but also because the results of such studies can be used to obtain fundamental information about clay structure, interparticle bonding, and the mechanisms controlling strength and deformation behavior. In addition, the studies provide a basis for the formulation of constitutive relationships that can be used in analytical or numerical analyses of earth-system behavior.The purpose of th is chapter