It is widely acknowledged that the stress-strain relationship of cohesive soils is governed by plastic behavior as well as by viscous characteristics. Viscous effects of soft clays are very important in the design and analysis of foundation, embankment, excavation, tunnel, and slope stability.
Laboratory tests for two remoulded soft marine clays were carried out to figure out the stress-strain-time relationship. Multiple-step constant load test and constant rate of deformation tests were performed. According to the test results, pore pressures generated at a given deviator stress are essentially independent of the strain rate, while the deviatoric strength is lowered as the strain rate is decreased.
The hyperbolic model is used to predict the deviatoric stress-strain relationship and it is extended to represent the influence of strain rate. Moreover, using the hyperbolic model, pore pressure-strain relationship can also be predicted from the deviatoric strength-pore pressure relationship at critical state. Model predictions show good agreements with experimental data. To unify phenomenological models for strain rate, creep, and stress relaxation, relationships between the different model parameters are studied and compared with experimental results.