Fine-grained soils, according to ASTM standard, are soils in which more than 50% of the particles pass-through sieve number 200 (< 0.075 mm). However, Atterberg limits tests used in designating fine-grained soils as silt or clay have been performed only by fractions passing sieve number 40 (425 mu m) and not with fractions passing sieve number 200. The question is whether it is appropriate to use soil fractions < 0.425 mm in a test used to classify a fine-grained portion of soil into silt or clay. To investigate this, consistency limits and Free Swell Index (FSI) tests were conducted on three genetically different lateritic soil fractions that pass-through sieve numbers 40 and 200. In addition, grain size analysis was conducted on soil fractions < 0.425 mm as well as the whole soil. The results showed that the lateritic soils were well graded with a significant amount of sand and mostly classified as inorganic clay of low to intermediate plasticity. The grain size distribution of fractions < 0.425 mm revealed that these fractions of soil contained a significant amount of medium-fine sand. In all the lateritic soils, the consistency limits of the soil fractions that pass-through sieve number 200 were found to be higher than those that pass-through sieve number 40. This increase in the consistency limits of the soil fractions < 0.075 mm with respect to soil fractions < 0.425 mm made the USCS classification as well as the level of plasticity of the lateritic soils change. The FSI tests also revealed that the soil fractions that pass-through sieve number 200 have higher FSI compared with the fractions that pass-through sieve number 40. This study has shown that lateritic soil fractions that pass-through sieve number 40 may contain a significant amount of medium-fine sand, and Atterberg limits obtained from such fractions may be underestimated and misleading.
