The critical state soil mechanics provides robust framework to understand a wide range of stress-strain behaviour. Critical state line becomes the backbone of the framework and central part of the many advanced constitutive modeling in recent years. Understanding the critical state framework and the factors affecting the critical state line is imperative to understand the soil behaviour. In the literature, there are contradictory observations on the factors affecting critical state line. Among many factors, gradation has been identified as one of the factors affecting the critical state line significantly.  Two different observations, one is rotating about a pivot point and other is parallel downward shift have been reported in terms of effect of gradation. However, in the past studies, not only gradation, particle shape and mineralogy (different source) are also variable. Thus, the observations lack in expressing the influence of the gradation alone.

 This study aims at quantifying the effect of gradation on critical state line. Two soils, natural soil and Tailings, both from Ontario, Canada were chosen. A total of twelve (12) gradations, seven (7) from natural soil and five (5) from tailings were created using a wide range of particle sizes and include both uniformly graded and widely graded (mixtures). Particle sizes range between fine sand to fine gravel for natural soils and fine sand and less for tailings. For both materials, mineralogy and particle shape were rigorously studied to ensure the particle shape and mineralogy remain identical within natural soils gradations and tailings gradations. In both natural soil and tailings, gradation appears to have a significant influence on the CSL in  space. Uniformly graded soils have comparatively higher critical state line than well graded for both materials. Threshold gravel content and fines content determine the lowest critical state line for natural soil and tailings, respectively.