An accurate, quick and simple method to determine the plastic limit and consistency changes in all types of clay and soil: The thread bending test

Abstract The standard thread rolling method for determining the plastic limit of soil, PL, has been widely criticized for requiring considerable judgment from the operator that carries out the test. In different studies other methods have been put forward; however, these methods cannot compete with the thread rolling test in simplicity, execution time and cost. In this article an accurate, quick and simple method is presented with a simple device for determining the plastic limit of soils, in which the subjective point of view of the operator is omitted. Soil threads which are 3 mm in diameter and 52 mm long are bent until they start to crack. The relationship between the bending produced, B, and water content, W, has been studied, in such a way that the plastic limit, PL, and another two new parameters (the stiff-soft limit, SSL, and the bend-breaking limit, BL) have been determined with minimal operator interference. These new parameters delimit other consistency states, which may be very useful in sectors such as the ceramics industry, agriculture or geotechnical engineering. The PL results obtained by the bending test in 24 soils concur to a great degree with those obtained in the thread rolling test by a highly experienced operator (R2 = 0.972). Moreover, these results have been endorsed by Shapiro–Wilk and Student's T statistics tests. Finally, the reliability of the method using only 3 and 1 experimental points has been studied too which have yielded very similar results to those obtained with 6 and 7 points.