Shear behaviour of a grain assembly in a shear box under the condition of no externally applied load was investigated numerically using the Discrete Element Method. The assembly was a monodisperse system comprising 3000 circular elements, 1 mm in diameter with an initially random configuration. Both macroscopic and microscopic behaviour was investigated including particle movement in the box, the mechanics of critical failure of the assembly and the relationship between inter-particle forces and the total normal force acting on the wall of the fixed upper box (i.e., the quantity defined as the shear force in a standard shear box test). Particle movement including a number of micro-scale failures occurred in the assembly of the upper box. An increase in the sum of contact forces and a reduction in interlocking were observed prior to the critical failures. There was a good correlation between the shear force measured at the side wall of the box and the sum of contact forces in the upper box.