Plastic Limit and Other Consistency Parameters by a Bending Method and Interpretation of Plasticity Classification in Soils

Although plastic limit (PL) is one of the parameters more widely used in the field of geotechnical engineering, the existing method for its determination (the thread-rolling test) is characterized by its low accuracy, repeatability, and reproducibility. These shortcomings have led to a great deal of criticisms, and in fact, several authors have tried to develop alternative methods to obtain the PL in soils. Despite that, the standardized thread-rolling method remains strong because of its simplicity, low cost, and quick performance. In previous studies, an alternative method, which was based on bending soil threads of 3mm in diameter and 52mm long until they start to crack, was presented by the authors. The relationship between the water content and the measured bending deformations allows the PL to be calculated with an equation. Apart from this, two other additional parameters associated with the soil consistency changes were defined: the bend-breaking limit (BL) and the stiff-soft limit (SSL). In the present study new equations are presented to calculate these parameters with only one experimental point. By means of their respective equations, PL, BL, and SSL have been calculated in 55 soils. The results of PL and plasticity index (PI) achieved with this test are highly correlated with those obtained through the standardized rolling test by a highly experienced laboratory operator. The Casagrande chart has been interpreted according to the PL, BL, and SSL in order to uncover more information about this system for soil classification, and after detecting some incongruities, a new proposal is presented.