Conditions for wedge failure

Wedge failure is the sliding of a wedge-shaped block, formed by two intersecting discontinuity planes, along their line of intersection. The conditions (Hoek and Bray) are:

1. Two planes (joints, bedding or foliation) intersect each other and the slope face, forming a wedge whose base (keel) is the line of intersection.
2. Daylighting: the line of intersection must daylight in the slope face, i.e. its plunge $\psi_i$ is less than the apparent dip of the slope face measured in the trend direction of the line, $\psi_{fi}$: $\psi_i < \psi_{fi}$.
3. Friction: the plunge of the line of intersection is greater than the friction angle, $\psi_i > \phi$. Hence the condition is

$$\psi_{fi} > \psi_i > \phi$$

4. Direction: the trend of the line must point out of the slope face (roughly toward the slope dip direction, within about $90^\circ$ of it).

        \  J1     J2  /
         \  \    /  /
          \  \  /  /
           \  \/  /  <- wedge slides along
            \ line of intersection

Stereonet test: the point where the two great circles cross (the line of intersection) must lie in the region between the slope great circle and the friction circle.