Shear Viscosity of Clays to Compute Viscous Resistance

Soils at critical state are like a visco-plastic fluids, which will flow for applied stresses greater than the critical state (CS) shear strength. Shear viscosity is a key parameter to understand the post-failure flow response of soils in geotechnical applications such as landslides, jacked piles and cone penetrometers. Viscosity of soil has been investigated for water contents greater than the liquid limit because at these water contents standard viscometers can be used. However, soils can reach CS for water contents in the plastic range and thereafter exhibit viscous behavior without further changes in water contents. There is no standard method to determine shear viscosity of clays at water contents lower that the liquid limit. A new analytical method is proposed to determine the viscous drag (force) on the penetrating cone in a fall cone test. Based on this analysis, it is possible to interpret the shear viscosity of clays with water contents less than the liquid limit from a penetration test. Time-penetration data from a fall cone test was used to determine the shear viscosity of clays with liquidity index less than 1.5. The test results show that the shear viscosity decreases exponentially with an increase in liquidity index.