Investigations of granular specimen size effect in interface shear box test using a micro-polar continuum description

Parametric studies were carried out to investigate the effects of specimen size and interface friction angle on the mechanical behaviour of a cohesionless granular material in the interface shear box test under constant normal load and plane strain conditions. For a description of the pressure and density dependent properties of granular materials such as sand, a micro-polar hypoplastic continuum model is used. Numerical simulations were performed to obtain the evolution of the deformations and the shear resistance for three different specimen lengths and interface friction angles using the finite element method in the updated Lagrange frame. Due to the presence of lateral rigid boundaries of the shear box, the deformations and the stresses became significantly inhomogeneous within the granular specimen. Correspondingly, the mobilised interface shear resistance and the deformation field within the specimen were not uniform along the interface. In the case of higher interface friction angles, the evolution of the mechanical quantities along the interface can also be influenced by the specimen size. It is shown that the calculated average friction angle mobilised along a rough interface is influenced by the shear box length and it can be lower than the prescribed one. In addition, the influence of sidewall friction inside the shear box frame was explored in terms of the response of the specimen. The results of the micro-polar hypoplastic model were also compared with those obtained from the corresponding non-polar version of the model to clarify the role of polar effects within the granular body. The numerical results provide new insights into the microstructural effects of the granular material on the mobilisation of the interface friction angle and indicate the complexities and difficulties to precisely determine and interpret the interface friction angle, obtained from the classical interface shear box experiment.