Shear Viscosity of Clays in the Fall Cone Test

Although viscometers are commonly used to determine the viscosity of clay soils, they are suitable only for determining the viscosity of soils for water contents greater than the liquid limit. Because soil flow can take place within the plastic range, the viscosity has to be determined through other methods. This paper investigates the use of a fall cone test to determine the viscosity of clay soils with a liquidity index of less than one. Eight soils, including kaolin and 6 natural clay soils, were thoroughly investigated using the fall cone test. The results of the study show that the fall cone test is suitable to determine the shear viscosity of clay soils. Excellent correlations were found between the liquidity index and the viscosity. A relationship exists between the dynamic equilibrium depth and the final penetration depth such that it is unnecessary to measure the rate of fall of the cone to determine the soil viscosity.