Deformation of a foliated medium

The deformation of a medium which contains a single family of parallel planes of weakness such as faults or foliation, is analyzed for plane strain conditions. It is shown that if slip along the planes of weakness and rotation of these planes are the main mechanisms of deformation, the mode of yielding of the medium depends mostly on the angle θ which the planes of weakness make with the maximum compressive stress. The medium yields in an unstable manner with a marked drop in stress for θ < 45° - ø/2, where ø is the friction angle. The medium yields stably with work hardening for θ ≥ 45°-ø/2. The predictions of this analysis are in agreement with the experimental results of deformation of Martinsburg slate by Donath (1964, 1968).