In geotechnical engineering, the soil strength is commonly related to the soil shear
strength. The soil shear strength can rather be determined as the maximum, residual
or the shear stress response at a certain deformation or strain. In low permeability soil
(e.g., clay or silt), the common strength parameter is the soil undrained shear strength,
since loading, unloading or in-situ testing are performed at relatively high rates than
the time requires for the pore water pressure to be dissipated. The measurement of the
undrained shear strength through laboratory or in-situ testing, may yield different shear strength values, due to the difference in shearing rates and the failure mechanism  in the soil associated with each test.
An analytical-experimental approach is presented to evaluate the reference, shear-rate
independent, undrained shear strength. Vane testing at different rotation rates have
been performed as part of this work, and T-bar tests in different penetration rates have
been inspected. By utilizing different strength rate-effect models, upon the testing results, the corresponding viscosity parameters have been evaluated. Using the viscosity  parameters and by considering the shear strain rates associated with the failure mechanism in vane testing, the reference undrained shear strength have been evaluated. The linkage between different tests; the T-bar penetration test and the vane shear test, is presented, as the viscosity parameters have been evaluated for the T-bar penetration at different rates, and implemented upon vane testing results.
Based on the developed analytical-experimental method, the reference (shear-rate independent) undrained shear strength is presented as a function of soil state parameters only (e.g., void ratio and liquidity index). The analysis shown enables a comparison between different (common) viscosity models.