One-Point Plastic Limit Test via a Modified Fall-Cone Penetration Method

Abstract

This paper developed a one-point testing method for determining the plastic limit of fine-grained soils using a fall cone penetration apparatus with a heavier, 240-gram cone, as proposed by C.P. Wroth and M.D. Wood (1978). The original method required multiple penetration trials at varying moisture contents to develop a regression line from points plotted on a semilogarithmic moisture content–penetration graph. The researcher simplified the testing procedure by deriving an equation that estimates the moisture content corresponding to a 20-millimeter penetration of the modified fall cone from a single instance of penetration. The value obtained, when combined with the liquid limit, can then be used to calculate the soil’s plastic limit. Statistical analysis of multipoint modified fall cone plastic limit tests on was used. The one-point method developed yielded plastic limit values that are in close agreement with those obtained by the multipoint penetration method . The values obtained via the one-point penetration method also had very little impact on the evaluation of the following geotechnical parameters: bearing capacity, consolidation, liquefaction, and AASHTO and USCS classifications. The method was also verified to be applicable to soils outside of the model equation’s development set, yielding plastic limit values statistically equal to their multipoint counterparts . Lastly, statistically significant time and sample savings of 65.27% and 47.60%, respectively, were observed when using the one-point penetration method instead of the multipoint penetration method.