LAUSR

The influence of non-plastic fines on undrained monotonic behaviour of sand was investigated over a wide range of fines content (FC = 0–40%), global void ratio and initial mean effective stress ( $$ p_{0}^{\prime } $$ p 0 ′  = 100 kPa to 500 kPa), by performing triaxial compression tests with isotropic and anisotropic consolidation. For the sand–silt mixtures, steady-state line in the e  − log( p ′) space was found to be dependent on fines content with the existence of a limiting fines content, which defines the transition from a “fines in sand” to a “sand in fines” soil fabric. It is demonstrated that by means of the concept of equivalent granular void ratio, e* , all steady-state/critical state data points can be well described by a unique relationship in the e*  − log( p ′) space called the equivalent steady-state line (EG-SSL), regardless of fines content. The equivalent granular state parameter ( ψ* ), defined in terms of e* , and the EG-SSL can be effectively adopted for predicting the undrained monotonic behaviour of sand–silt mixtures and the onset of static instability, irrespective of fines content and initial state of the sand–silt mixtures.