Mechanics of the Casagrande liquid limit test

The liquid limit of soils can be measured using either the cup apparatus developed by Casagrande or by the fall-cone method, which is now the standard in much of Europe. This paper will demonstrate, based on a Newmarkian sliding block analysis of the Casagrande percussion test, that these two methods measure different mechanical properties of the soil; the cup test being a measure of specific strength, i.e., strength divided by density, whereas the fall cone is a direct measure of strength. Sources of error in the operation of the percussion test are quantified and a modification to the fall-cone test is proposed to standardize results between the two test methods.