Enhanced Fractional Model for Soil–Structure Interface Considering 3D Stress State and Fabric Effect

Engineering structures in the field are exposed to three-dimensional (3D) stress-displacement conditions with fabric evolution occurring within the soil–structure interface. Upon 3D shearing, the fabric of the interface would change with shear displacement, which influences the dilatancy or plastic flow response of the interface. To consider the 3D stress state and fabric effect, an enhanced fractional model for the interface under monotonic and cyclic loads is developed by using a multiaxial loading vector, a fabric-dependent plastic flow vector, and a kinematic hardening rule. The developed model has three key features: (1) 3D stress-displacement response with shear coupling in different directions is considered by the 3D elastoplastic relation; (2) fabric-dependent dilatancy state line of the interface is captured via a fabric scalar; and (3) dependence of particle breakage on the critical state line of the interface is also considered by introducing a shift stress. To validate the proposed model, a series of test results obtained from the interface direct shear test and simple shear test are simulated. Discussions on model performances with and without considering the fabric effect are then carried out. It is found that the developed model can capture the key features, e.g., hardening/softening, dilatancy/contraction, and strength degradation, of different soil–structure interfaces. The evolution of the fabric scalar mobilized the dilatancy state of the interface under cyclic loads, through which the remarkable normal contraction response under cyclic loading is successfully captured when compared to those without considering fabric effect.