In the last two decades extensive research has been undertaken to characterize the time and rate dependent response of soils and to describe it by means of a constitutive model. However, most of these constitutive equations have been dev.eloped for a single stress point and limited stress paths, and in engineering practice the time, dependent nature of soils is still commonly reduced to a single coefficient of secondary consolidation obtained from a 24h oedometer test. J This thesis describes the development and implementation, in the Imperial College Finite Element Program (I~FEP), of two elastic visco-plastic models based on the overstress theory to describe the stress-strain-time/rate behaviour of clayey soils. The models differ essentially in the adopted law to describe the variation of the creep ~ deformation with time. The fIrst model - Creep Model 1 - incorporates a linear logarithmic law to describe the variation ofthe creep defon:nation with time, while the second model- Equivalent Time (ET) Model - incorporates a non-linear logarithmic creep law, with a limit to the amount of volumetric creep strain that can occur. The implementation and performance of the models is validated through a series of simple fInite element analyses that mimic common l~boratory stress paths and show that the models are able to reproduce the' phenomena of primary and secondary compression, stress relaxation, primary and secondary creep and persistent rate effects on the stress-strain .response under Ko and triaxial stress conditions. The ET model is then used to investigate the increase in bearing capacity of preloaded footings on soft clay, taking into account both the effects of consolidation and soil hardening due to creep with time. Finally, a series of fInite element analyses are presented that mimic the loading tests performed on two instrumented rigid footings at the Bothkennar test site. These analyses highlight the importance of considering the soil viscous effects in engineering practice and the need to account for the soil creep non-linearity if good predictions of the long-term settlements of geotechnical structures are to be made.Imperial Users onl
