Vinod et al. (2012) have presented valuable data on strength variation with water content. There are, however, several points that we would like to make regarding the assessment of strengths at and beyond the Atterberg limits.The authors attempted to measure soil strength both in the plastic range and in the brittle 'semi-solid' range using a vane shear apparatus. The vane shear test involves inserting a vane into a soil sample and measuring the resistance to rotation. The data are conventionally analysed by assuming that a constant shear stress is exerted over the entire surface area of the cylinder of rotating soil (ASTM, 2000). While this is appropriate for plastic materials in which strength will approach maximum strength at large strain, for brittle materials the variation in strain over the base of the cylinder will result in peak strengths not being achieved simultaneously. The maximum torque, when converted to a shear stress, will hence underestimate the peak strength of the soil. Further problems may also be created as the vane is inserted into the soil sample.ASTM D 4648-00 (ASTM, 2000) asserts that: 'The remoulded zone around a vane blade is generally assumed to be small and have little or no effect on the stress-strain properties of the sediment being tested.' This assertion is again valid only for plastic materials; the process of inserting the vane must result in yielding of the soil. For a brittle material this will result in cracking of the soil sample, and hence an underestimate of peak strength during subsequent shearing of the soil.As soils at water contents less than the plastic limit are by definition brittle, strengths measured using the vane shear apparatus at these water contents should be treated with caution. Strengths measured in triaxial compression, allowing constant strain to be accumulated within the sample and observation of the entire stress-strain relationship, would be a more appropriate approach.The strength observed at plastic limit for the nine soils tested is approximately 161 kPa, very similar to that predicted by Wroth and Wood (1978). As shown by Haigh et al. (2012) and Nagaraj et al. (2012), plastic limit does not correspond to a fixed soil strength, merely to the onset of brittleness. Based on a data set of 71 strengths at plastic limit reported in the literature, Haigh et al. (2012) showed that the strength at plastic limit had a range of 17-530 kPa, with an average value of 152 kPa and a standard deviation of 89 kPa. While the data for nine soils presented by Vinod et al. (2012) fall centrally within this range, it would be wrong to assume that the small scatter of strengths found was representative of a wide range of soils.The strengths observed at liquid limit were consistently found to be 5 . 8 kPa, much greater than the commonly reported range of 0 . 7-2 . 65 kPa (Wroth and Wood, 1978). It is unclear from the paper whether liquid limits were obtained using the fall-cone or Casagrande cup apparatus, which has a potential impact on the soil strength at the liquid ...
