Theory and practice of the fall cone test

The fall cone is considered as a more reliable method for determining the liquid limit than the Casagrande method, and is standardised in many countries as the preferred liquid limit test method. In this paper the theory and practice of the fall cone test are described. First the penetration mechanism of a fall cone into clay is analysed, introducing the concept of dynamic strength to the static results. Next the applicability of dynamic analysis to the fall cone test is examined with regard to Hansbo's cone factor, K, for various cone angles. The theoretical K value for the 60° cone with a semi-rough surface is found to agree better with experimental results than is the case for the 30° cone. It is proposed that the liquid limit be redefined internationally as the water content at which a 60°, 60g fall cone penetrates 10 mm. Finally the applicability of the fall cone test as a device to relate the strength of a clay with the index properties is examined.