effects of plasticity and fine percentage on the unstable behaviors of clayey sands

This paper covers the results of a comprehensive study on the unstable behaviors of clayey sands with special attention to the comparison of the fine content and its plasticity effects. tri-axial tests were conducted by applying two different densities and confining pressures on sands mixed with 0% to 15% of low and high-plasticity clayey materials. the results showed that for some combinations of sand and clay, with only a 5% increase in high plasticity clays, the peak deviatoric strength decreased by about 31% compared to the original value, while this amount was about 18%, using low plasticity clays. the initial increase in clay content made a considerable loss in the steady-state strength values, while the reduction was less significant for a greater increase in the clay content. adding only 5% of low plasticity clay has reduced the steady-state strength to about 60% compared to its initial value, while the continued increase in the percentage of clay up to 15% has sometimes caused the steady-state strength to increase again. the initial increase in the clay content significantly changed the clean sand structure to groups of clay and sandy particles with unstable connectors. greater clay content increase gradually made these unstable structures steadier due to the increase in plasticity and therefore cohesion between particles. at low percentages of clay content, it is the fines content that dominates the type of behavior, while as the clay content increases the role of the plasticity of fines gradually becomes more crucial.