Conditions for plane failure

Plane failure is the sliding of a rock block along a single plane of weakness (joint, bedding or foliation) that daylights in the slope face. According to Hoek and Bray, it occurs when all these conditions are satisfied:

1. Strike: the plane strikes parallel, or nearly parallel (within about $\pm 20^\circ$), to the slope face. That is, the dip directions of the plane and the slope differ by no more than about $20^\circ$.
2. Daylighting: the plane dips less steeply than the slope face, $\psi_p < \psi_f$, so that the plane is exposed in the face.
3. Friction: the dip of the plane is greater than its friction angle, $\psi_p > \phi$, so that the block can slide under gravity.
4. Release surfaces: the sliding block must be free at its sides, either through lateral release joints of negligible resistance or because the plane passes through a convex nose of the slope; a tension crack usually bounds it at the top.

   crest   tension crack
      \     |
       \    |__
   face \     \ \   sliding plane
         \     \ \ (dip psi_p)
          \_____\_\___

Stereonet test: on a great-circle plot, the dip vector of the plane lies between the slope great circle and the friction circle, within about $\pm 20^\circ$ of the slope dip direction; equivalently, on a pole plot, the pole lies inside the daylight envelope and outside the friction circle. If $\psi_p \leq \phi$, the slope is stable against plane sliding.