Soil is subjected to shear stress and/or displacements in numerous geotechnical applications. This leads to a large number of numerical models being developed to represent its behaviour under various stress paths. This paper aims to present a sound mechanical formulation for finite strains and to introduce a rational constitutive model to match realistically the behaviour of soil where, for a high range of deformations, strains localise and the phenomenon of grain breakage is usually observed in granular soil. The reference model is the ECP’s constitutive model (also known as “Hujeux’s model”), which is an elastoplastic multimechanism model. An overview is made of laboratory results and constitutive relations that consider finite deformation, and the grain breakage phenomenon that is usually associated with it in granular materials. The performance of the proposed constitutive model is demonstrated simulating constant volume ring shear tests carried out on Ottawa (OT) and Illinois River (IR) sands up to very high shear strains and comparison of the obtained results with laboratory testing data.