Easier method for determining Plastic Limit had been introduced by Feng (2000). By utilizing this method, there is no necessity to conduct rolling test, as for this test, the result might be dependent on the operator.

This new method can be simplified as follows:

• Plot results from fall cone method in log scale for both x- and y-axes. X-axis represents penetration (mm) and y-axis represents water content (%).
• Construct the best straight line from the points.
• Extend the straight line to y-axis. The point where the line cross y-axis (when penetration = 1mm) is known as C.
• Measure the slope of the line, m.
• Plastic Limit, PL = C(2)^m.

Example are as shown in figure below (Budhu, 2010):

References:

1. Budhu, M. (2010). Soil Mechanics and Foundations: John Wiley and Sons.
2. Feng, T. W. (2000). Fall-cone penetration and water content relationship of clays. Geotechnique, 50(2), 181–187.