A new method of measuring plastic limit of fine materials V. SIVAKUMAR, D. GLYNN, P. CAIRNS and J. A. BLACK (2009). Géotechnique 59, No. 10, pp. 813–823

B. C. O’Kelly, Trinity College Dublin The authors rightly state that the strength gain ratio over the plastic range (R 1⁄4 cu(PL)=cu(LL)) is often about 100 for many soils, although there is no theoretical basis for a universal value of 100, and that other researchers have postulated cu(PL) 170 kPa and R up to as high as 170. The LL, PL and R values largely depend on the grading and mineralogy, shape and surface texture of the constituent solids (Trauner et al. 2005); activity of the clay minerals (Wood, 1990); and test method (e.g. predefined cu(LL) value assigned to determine the fall-cone LL value). PL values determined using Casagrande’s thread-rolling method define the water content below which 3 mm diameter, saturated soil threads cannot be remoulded (stress and strain rate controlled; that is, soil-toughness dependent). The discusser agrees that there is a need for a more consistent and accurate procedure to determine PL, and in this regard, the authors have developed a pneumatic loading system to apply predetermined forces in a fast-static (‘almost instantaneous’) manner on a standard cone in the fall-cone apparatus. Knowing the LL and PL values of Speswhite kaolin, the authors determined experimentally that a faststatic force of 54 N applied to a 0.8 N, 30 cone produced 20 mm penetration at the PL for the kaolin test material. Next, the affects of loading rate on penetration depth were studied close to the PL state, with fast-static and quasi-static scenarios under 54 N loading producing similar penetration depths hs (Table 1). The fall-cone strength under the applied vertical cone force Q can be calculated as