Determining the plastic limit of soil using the fall cone test and undrained shear strength

Properties of fine-grained soils, such as the liquid and plastic limits, are widely used as indicators of geotechnical behaviour. Unlike the liquid limit that can be reliably determined by mechanical testing, the determination of the plastic limit is significantly more complex, and the standard thread-rolling test is increasingly criticised for its subjective nature. Therefore, there is a need for a standardised, mechanical, more reliable and reproducible method to determine the plastic limit. This study compares several alternative methods with the standard rolling test, including tests using 240 g and 400 g cones, the cone penetration index (beta), and an empirical method based on undrained shear strength data. The accuracy of each method is evaluated using MAE, RMSE, and MAPE. The results show that the  method provides the highest reliability, while the empirical approach shows moderate accuracy.