Force chains in 3D granular media*

Under the action of external forces, particles in granular materials form a complex network of contacts—these force chains transmit the external load through the medium. Understanding the emergence of these force chains and their spatial structure constitutes a fundamental goal of granular mechanics. For decades, our understanding of force chains has been derived from 2D experiments, using quasi-2D photoelastic particles with various shapes. Here, we introduce a new experimental technique, which integrates photoelasticimetry into tomography, to observe the evolution of 3D force chains under isotropic compression, triaxial shear and rotary shear. Our experimental study unveils the astonishing 3D nested, parallel and helical force chain patterns in the granular medium under isotropic, triaxial shear and rotary shear conditions, respectively. We also show the length and orientation statistics of the force chains are closely correlated to the 3D loading conditions.