Using a Small Ring and a Fall-Cone to Determine the Plastic Limit

The plastic limit is an important property of fine-grained soils. The standard thread-rolling method for determining the plastic limit has long been criticized for requiring considerable judgements from the operator. This paper presents the design of a small specimen ring for fall-cone tests for determining the plastic limit. A small specimen ring of 20 mm in diameter and 20 mm in depth is designed and used for cone penetrations between 10 and 3 mm. For each soil tested, a linear relationship between the logarithmic cone penetration and the logarithmic water content is obtained by a regression analysis on four data points. This linear relationship is extended to determine the plastic limit at 2 mm of cone penetration. The fall-cone plastic limits of 21 soils are in good agreement with their thread-rolling plastic limits. In addition, a one-point fall-cone method is also recommended for determining the plastic limit. As the fall-cone has been recommended in several standards for determining the liquid limit, it is preferable that it is also used for determining the plastic limit.