Discussion of Determining Soil Plasticity Utilizing Manafi Method and Apparatus by Masoud S. G. Manafi, An Deng, Abbas Taheri, Mark B. Jaksa, and Nagaraj HB, published in Geotechnical Testing Journal 45, no. 4 (2022): 797-818

The authors presented an extrusion-based technique, which, they reported, can be used to quantify the workability of fine-grained soils. They also presented the calibration of a soil extrusion device to translate their workability parameter to the liquid limit (LL) and plastic limit (PL) states, i.e., employing the so-called Manafi method for this purpose. The discussers contend that, when considering a wide range of different fine-grained soils, the authors' pro-posed method is not appropriate for determination of the Atterberg PL. With toughness and workability being essentially synonymous, and with soil toughness at the Atterberg PL being variable between different soils (differing significantly when considering a diverse range of fine-grained soil classes), it follows that the work done in remolding these soils, or to cause extrusion of these soils, can also vary (and may differ significantly) at their Atterberg PL water contents. The discussers' reanalysis of the authors' data, presented for only seven test soils (all of them clay), also reveals a relatively narrow range of remolding toughness (of between 31.2 and 51.5 kJ/m3) at their Atterberg PL water contents. It is argued that the workability-based criterion of the Manafi method, with the authors assigning a fixed calibrated workability value of 86.3 J/s for Atterberg PL determinations employing their presented extrusion ap-paratus, is fundamentally not correct; rather, the Manafi method PL and the Atterberg PL are essentially different index parameters. It is contended that what the authors call workability (i.e., their workability parameter with units of J/s) is actually not really workability but seemingly rather an alternative assessment of undrained shear strength. Importantly, the Atterberg PL is identified based on the crumbling condition of the soil thread (i.e., it is not energy-based, nor is it related to the work done) during the standardized rolling-out procedure. For the LL, the authors' extrusion-type method, measuring more a sort of undrained shear strength, could produce good agreement between the Manafi method LL and the measured standard fall-cone LL because the latter is strength-based.

Automated summary

The authors presented an extrusion-based technique for quantifying the workability of fine-grained soils, but the discussers argued that this method is not appropriate for determining the Atterberg plastic limit of different fine-grained soils. They also questioned the authors' use of a fixed calibrated workability value and suggested that the workability parameter may actually be an assessment of undrained shear strength.