A knowledge of shear viscosity is important in understanding the viscous resistance in the dynamic penetration of soils for geotechnical applications such as jacked piles and cone penetrometers. Viscosities of soils with water contents greater than the liquid limit have been investigated using viscometers. There is, however, no standard method to determine shear viscosities of clays at water contents lower than the liquid limit. In this paper, the post failure (critical state) response of finegrained soils is modelled as if they were visco-plastic fluids. In addition, the theory of the fall cone test, currently used to interpret the index and strength properties of fine-grained soils, is re-evaluated to discern the viscous drag as the cone penetrates the soil. This reevaluation shows that the shear viscosities of clays at liquidity indices less than 1 . 5 can be estimated from a continuous record of time-cone penetration depth data. Fall cone test results on kaolin show that the shear viscosity decreases exponentially with increase in liquidity indices.

The penetration of rigid objects such as piles and penetrometers into soils creates a zone of soil disturbance around them. The extent of this disturbed zone influences the resistance of the moving rigid body. This paper presents a theoretical framework to analyze the resistance in the disturbed zone created by a shaft penetrating a clay soil. The soil is modeled as a viscous material after it reaches failure [critical state (CS)]. The results of this analysis show that the viscous drag stress component on the shaft surface is influenced by the size of disturbed zone that has reached CS around the shaft, the shear viscosity of the soil and the velocity profile (or strain rate) in the CS zone around the shaft. The size of CS zone, the velocity profile and the viscosity of soil are interdependent. Large variation in viscous drag occurs when the size of the CS soil zone is less than four times the shaft's radius. Limiting drag occurs when the size of the CS soil zone exceeds six times the shaft's radius. The theoretical velocity distribution of the movement of soil in the CS zone shows that the soil is dragged along with shaft in the near field (close to the shaft surface) and moves upwards in the far field.