This paper presents a mathematical modelling of the effects of initial fabric on the mechanical behaviour of sand. A stress-dilatancy model that incorporates microstructural aspects of sand is hereby obtained while writing energy conservation for an ensemble of particles over a representative elementary volume at micro- and macro-scales. The resulting stress-dilatancy model, when used within an elastoplastic framework, successfully reproduces certain aspects of sand behaviour that are reflective of its microstructure under both drained and undrained conditions. The role of microstructure in relation to the characterization of steady, quasi-steady, and phase-transformation states is discussed within the framework of the model. Numerical simulations obtained from the proposed model are generally very consistent with experimental observations and provide insightful information.Key words: sand, liquefaction, fabric, dilatancy, constitutive laws, granular materials, plasticity.