One of the most important design input parameter needed for geotechnical design is soil’s shear strength. ‘Shearing Strength’ of a soil is perhaps the most important of its engineering properties. This is because all stability analyses in the field of Geotechnical engineering, whether they relate to foundation, slopes of cuts or earthen dams, involves a basic knowledge of this engineering property of the soil. In practice, in selection of borrowed earth for construction of above mentioned structures, importance is paid to the quality of fines only as if quantity of fines is non-influential.It is necessary to understand the effect of quantity of fines on mechanical behavior of soil mixtures. With this objective, the plastic fines completely exclusive of coarse are added to the coarse fraction/sand completely free from fines in the proportion of 5% to 50% by weight of the sand. The compaction characteristics, shear strength values of mixtures in different proportion are determined. The results clearly indicated the fact that, there is a specific Critical Fines Content (CFC) for the mixture, such that the presence of plastic fines up to this critical fines content (CFC) is beneficial in terms of improving the gradation and increase in shear strength values. The CFC for the soil used in the study was found to be 30%. Hence, this study useful to the practicing Civil Engineers in predicting the behavior of soil mixtures based on the plastic fines content. Further, this study suggests the need for inclusion of quantity of plastic fines in addition to their quantity in relevant Codes of practice.

Keywords : plastic fines, soil-mixtures, compaction, Shear Strength, SPSS, regression equations.