Effects of over-consolidation history on flow instability of clean and silty sands

This article presents an experimental program under the constant volume condition to investigate the influence of over-consolidation on flow instability of clean and silty sand specimens prepared by a systematic increase in fines content (FC), where fines are particles smaller than 0.075 mm of non-plastic minerals. An extensive series of constant volume Direct Simple Shear (DSS) tests on normally and over-consolidated clean and silty Firoozkuh No. 161 sand specimens were carried out. The testing program covers a range of initial void ratios, effective vertical stresses, and FCs. Analysis of the laboratory data indicates that over-consolidation history leads to an increase in the slope of the instability line and a rise in normalized constant volume shear strength at the onset of flow instability. Furthermore, at the same normalized constant volume shear strength, brittleness index and maximum excess pore-water pressure ratio increase with over-consolidation. Even though an increase in FC leads to a family of non-unique critical state lines in the void ratio versus effective vertical stress plane, it is observed that the slope of the instability line and normalized constant volume shear strength vary almost uniquely with state parameter at the onset of flow instability irrespective of FC.