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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. Refereed/Peer-reviewed

Fabric anisotropy and fines content (fc) in sands modify significantly their mechanical behaviour, particularly as related to static liquefaction under undrained conditions. The fabric anisotropy aspect, expressed by means of an evolving fabric tensor F, has been addressed in the recently developed Anisotropic Critical State Theory (ACST) that enhances the two critical state conditions on stress ratio (η) and void ratio (e) of the classical Critical State Theory by an additional condition on the critical state value of F in relation to loading direction; based on this concept it introduces the dependence of dilatancy on fabric anisotropy. Various models have been successfully developed within this framework for clean sands. The fc aspect has been addressed within the Equivalent Granular State Theory (EGST) that substitutes a properly defined equivalent granular void ratio (e*) for e in any model for clean sand in order to obtain the response of sand with fines without any other change of the model structure and constants. Along these lines, a constitutive model is constructed in this work in order to address the effect of both F and fc simultaneously, by a combination of these two powerful propositions. The idea is very simple: one takes a constitutive model developed within ACST for clean sands, hence it accounts for fabric anisotropy, and substitutes the e* for e, as well as the derivative quantities of such substitution, hence it accounts for fc. The result yields a model that can simulate data on the undrained response for a range of fc, with emphasis on static liquefaction. It is shown that the inclusion of fabric anisotropy improves previous similar simulations made within the EGST but without the framework of ACST. Refereed/Peer-reviewed