Velocity diagrams for the mechanisms depicted in Figs 2 and 3: (a), (b) kinematics of a wedge-shaped shear zone that is in contact with a body undergoing rigid-body motion, such as a wall or soil rigid block; (c), (d) kinematics of wedge 2 in the two-wedges mechanism in Fig. 3(a); (e) kinematics of the logarithmic spiral sector in the mechanism depicted in Fig. 3(b)