Constitutive Model of Granular Soils Using Fractional-Order Plastic-Flow Rule

Traditional plasticity models of granular soils are usually established on different loading and plastic potential surfaces. This paper presents a nonassociated bounding-surface model by incorporating a fractional-order plastic-flow rule that is obtained by performing fractional-order derivatives on the bounding surface. It is found that the flow direction no longer necessarily coincides with the loading direction, even if the same potential and loading surfaces are used. The flexible ability of the model is then validated by simulating a series of experimental results of different granular soils, including ballast, rockfill, and sand, under both drained and undrained loading conditions. It is observed that the proposed model can well capture the key features (e.g., stress dilation, strain hardening/softening, and liquefaction) of various granular soils.