The Effect of Clay Content on Liquid Limit from a Fall Cone and the British Cup Device

The many shortcomings of the Casagrande cup device, used almost univerally to determine the liquid limit of soils, led to the investigation of fall cones as alternatives. In two of the four types of fall cones in use, investigators have shown that the differences between fall cone liquid limit values and cup liquid limit values were not large. Evidence is produced to show that the fall cones of the same cone angle would give identical results if the appropriate depth of penetration is specified. Tests conducted using a British/French fall cone and a British cup device reveal that the difference in liquid limit values between these two devices is related to the amount of clay in the soil. A one point fall cone method is shown to be sufficiently accurate to deduce the liquid limit of soils. The fall cone method is a rational alternative to the cup device, and since engineers are interested in strength, the liquid limit could perhaps be defined as the water content at which a soil would have a specific undrained shear strength (say 1kN/m²).