Plastic Limit Determination Using a Drop‐cone Penetrometer

Summary The drop‐cone method, in which a 30° cone of mass 80 g is allowed to sink into a soil sample at a selection of moisture contents, was used to determine the moisture content/cone penetration curve in the region of the plastic limit for eighteen soils. The moisture content corresponding to the minimum of the curve is shown to be always numerically less than, but to correlate closely with, the plastic limit. It is suggested that the plastic limit be redefined as the moisture content corresponding to the minimum of the moisture content/cone penetration curve, with the advantages that the test is more closely related to soil behaviour, less subjective, at least as reproducible as the Casagrande test and may be carried out simultaneously with the liquid limit test.