Experimental Evaluation of the Reference, Shear-Rate Independent, Undrained Shear Strength of Soft Clays

The soil's undrained shear strength is affected by the rate of shearing. This dependency can be inferred from mechanical laboratory testing at different operation rates or from using different in situ testing methods (associated with dissimilar shear-rate fields). However, all test results of a given soil can be correlated with a constant rate-independent shear strength by accounting the soil viscous behavior. This work investigates the viscosity model's role in evaluating the reference strength of soft clays. Three viscosity models are examined by their influence on the reference undrained shear strength values. The clay is tested by vane shearing at different rotation rates, in which the viscosity parameters are calculated based on strength ratios between tests at different rotation rates. The experimental results are compared with two types of analytical solutions, rigorous and simple, to examine the effect of the solution order on the obtained reference strength values. Among the examined viscosity models, this work finds the power-law model as more appropriate and applicable for rate-effect analysis in soft clays.