Effect of the intermediate principal stress on the mechanical responses of binary granular mixtures with different fines contents

The effect of the intermediate principal stress (b quantifies its relative magnitude) on macroscale and microscale shear responses of binary granular mixtures with different fines contents (FCs) was investigated using the discrete element method. Mixtures were made up of coarse particles with real gravel shapes and fine particles with sphere shapes. True triaxial compression tests were conducted on their dense specimens. The shapes of coarse particles play an important role in the peak and critical friction angles and the peak dilatancy angles of binary granular mixtures. For mixtures with different FCs and an identical b, there is a linear relationship between the peak friction angle (or peak dilatancy angle) and the change in the void ratio. An acceptable equation was proposed to predict the peak dilatancy angle of a mixture. When b = 0.2 and 0.4, the coaxiality of strain increments and stresses is affected by the FC. A previous study classified mixtures as “underfilled”, “interactive-underfilled”, “interactive-overfilled” and “overfilled” based on the percentage contributions of coarse–coarse, coarse–fine and fine–fine contacts to the shear strength. This study shows that the b value has little effect on the classification of mixtures at the peak and critical states. Furthermore, it was observed that there is a linear relationship between the relative peak dilatancy angle and the difference between the partial coordination numbers based on the “subsphere–subsphere” approach at the peak and critical states.