Assessment of the Suitability of the Fall Cone Method to Replace the Casagrande Cup for Liquid Limit Determination of South African Soils

Widely used methods for estimating the swelling-shrinkage potential of soil in South Africa rely heavily on accurate determination of soil consistency limits. The liquid limit and plastic limit tests are usually determined using one of two techniques, the Casagrande percussion cup and the fall-cone (penetrometer). One or both of these have been adopted as the standard measurement approaches for the determination of liquid limit in most countries. The former method is implemented in South Africa as well as in the USA, whilst the fall-cone method is accepted in the UK and by Eurocode 7. The relatively large size of sample required for the fall cone test (approximately 300g) has made the fall-cone method rather unattractive in South Africa. Coupled with the care needed to fill the test cup while taking care not to include any air pockets this may explain the rather negative attitude of testing services to the fall cone. Lack of acceptance may have been aggravated by high spatial variability of properties of South African active clays leading to poor correlation between tests in some cases. This paper investigates the suitability of the fall-cone method to replace the Casagrande cup, tai-lored to the peculiarities of active clays, but extendable to soils of varied types.