Critical State Theory for Sand with Fines: A DEM Perspective

Abstract

Undrained triaxial simulations were carried out for three-dimensional assemblies of clean sand and sand with fines soils using the discrete element method (DEM). Similar to experimental observations, it was found that the critical state line (CSL) moved downward with increasing fines content (fc) in the e-log(p′) space; where e is the void ratio and p′ is the mean effective confining stress. The CSL for both clean sand and sand with fines was observed to provide an appropriate reference line to predict the dilative tendencies throughout undrained shearing. Soil elements with an initial state (i.e. e and p′ at the beginning of undrained shearing) located above the CSL were shown to exhibit contractant behaviour, whilst soil with initial states below the CSL yielded dilatant behaviour. However, this study observed that the classical e is not an appropriate density index for sand with fines. Instead, the equivalent granular void ratio, e* provided a more suitable density index of sand with fines soils, as it reflects the force skeleton structure for both clean sand and sand with fines.