Discussion of “Mechanics of the Casagrande liquid limit test” 1 Appears in the Canadian Geotechnical Journal, 49(9): 1015–1023 [doi:10.1139/t2012-066

Abstract

The discusser wishes to express his appreciation to the author for shining a more useful light on the theoretical issues related to the conventional Casagrande percussion method, which is used for determining the liquid limit of fine-grained soils worldwide with the exception of a few countries where the fall cone method is preferred. Particularly, the author deserves a compliment for demonstrating theoretically that the results obtained from the Casagrande percussion and fall cone methods represent two different mechanical properties of the soil. This finding is in concurrence with the outcome of research by the discusser and his colleague presented in many of their publications (e.g., Sridharan and Prakash 1998, 1999, 2000; Prakash and Sridharan 2006). However, the basis and approach adopted by the two groups (i.e., the author on one side and the discusser with his research colleague on the other) to achieve the same goal are different. The author has shown theoretically through the Newmarkian analysis of the Casagrande percussion test that the Casagrande percussion liquid limit is a measure of soil specific strength, while the fall cone liquid limit is a direct measure of the undrained strength of the soil. The clay minerals composing the fine-grained soils are no longer inert, but are physicochemically active by virtue of their surface charge deficiency. The extent to which the fine-grained soils are active depends upon their clay mineralogical composition, which controls their behavior. It is unfortunate that the clay mineralogy of soils is ignored in most of the present-day research dealing with fine-grained soils even though most of the field problems associated with these soils can be attributed to their clay mineralogical composition. The liquid limit behavior has been proven to be a dominant function of the soil clay mineralogy. While the liquid limit of montmorillonitic soils is due primarily to the diffuse doublelayer-held water (Sridharan et al. 1986), that of kaolinitic soils is mainly due to the soil fabric as governed by the interparticle attractive forces (Sridharan et al. 1988). The undrained shear strength of fine-grained soils has been shown to have two components: viscous shear resistance and frictional shear resistance (Sridharan and Prakash 1999, 2000). An exhaustive experimental study of the mechanisms that come into play during the Casagrande percussion and fall cone methods of determining the liquid limit of soils has shown that the value of the liquid limit obtained by the Casagrande percussion method is controlled primarily by the viscous shear resistance while that obtained by the fall cone method is controlled essentially by undrained frictional shear resistance (Sridharan and Prakash 2000; Prakash and Sridharan 2006). The above illustrations reveal very clearly that the liquid limit values obtained by the Casagrande percussion and fall cone methods cannot be the same and hence, the liquid limits obtained by the two methods represent two different soil properties controlled by two different mechanisms. This conclusion is very much in line with that of the author, proven by a different approach altogether. The discusser also agrees with the author in that there is a growing tendency towards using the fall cone method to determine the liquid limit without understanding its limitations. In that direction, the author’s effort in proposing a methodology using the fall cone method itself to arrive at the liquid limit equivalent to that by the Casagrande percussion method is welcome. However, while doing so, the author is trying to equate the undrained shear strengths of the soils at the Casagrande percussion and fall cone methods’ liquid limits, which is not fully justifiable. This is because the undrained shear strengths mobilized during testing by the two methods are not of the same nature, as indicated by the discusser in the previous paragraphs. The discusser is aware that the author’s approach is theoretical, which does not take into account the physicochemical nature of liquid limit of fine-grained soils and the controlling mechanisms that are operative during the two testing procedures. Bringing these complex parameters into the theoretical formulation is not that simple. Apart from this issue, the discusser wishes to bring the following major discrepancies in the paper under discussion to the attention of the author for his comments: • Equation [6] appears to be incorrect. For a saturated soil mass, the correct relation should have been the following equation (where w is the mass density of water):