Determination of Shear Strength, Shear Viscosity and Liquidity Index Using Fall Cone Penetrometer

Abstract

In the usual case, the fall cone test has been used to estimate the undrained shear strength of insensitive remoulded clays. Its concept was based on the critical state soil mechanics and is well established. It is only recently that the use of this simple laboratory equipment has been extended to estimate the shear viscosity of soils which are well below their liquid limit. At these water contents, the viscous strength helps to understand the resistance of soils to flow in penetration tests, pile driving and landslides. This paper reports the results of fall cone tests under different loads. In this study, the effect of water content on shear viscosity was investigated by varying the weight of the cone. All the tests were conducted using speswhite kaolin clay. The samples were thoroughly mixed with water and then kept overnight in air-tight containers to ensure proper mixing of the clay with water. The fall cone apparatus was modified to accommodate an LVDT connected to a high-speed data logger which enabled the cone penetration to be logged at every 0.01 s. The mass of the standard cone was increased by 390 to over 1000 g with the use of stainless steel discs. In some tests, the cone was not permitted to fall freely but was driven at a constant rate of 1.25 mm/min. The load versus penetration resistance measured at different water contents showed that the penetration increased with the increase in LI and at a given load, the results were consistent with the triaxial test results. Shear viscosity was calculated using the rate at which the cone penetrated into the soil and was shown to decrease with the increase in LI. Design curves were obtained for the different weight of the cone which can be implemented in the selection of soil characteristics, for example, in estimating the capacity of different driven piles.