The kinematics of wedge failure, like plane failure, can be analysed from data consisting of the angle of sliding friction, φ, and the orientation of the rock slope.

With plane failure, dangerous orientations of planes of weakness map onto the stereogram in terms of their plane normals (poles). In contrast, when considering wedge failure we consider the orientation of the direction of wedge sliding, parallel to the intersection line of two sets of discontinuities (Fig. 46a).

Friction cone

To overcome frictional resistance under dry conditions, the plunge of the intersection line of the two discontinuities must exceed the sliding friction angle. All intersection lines in dangerous (steep) attitudes lie inside a cone consisting of all lines with a plunge equal to φ (Fig. 46b). This is the friction cone which gives a small circle on the stereogram (Fig. 46c).

Daylighting

Intersection line 1 in Figure 46d allows the possibility of wedge failure because it plunges (or at least has a component of plunge) in the direction of the natural slope and has an angle of plunge less than the apparent dip of the slope in the plunge direction. Intersection line 2 (Fig. 46d) would not permit wedge failure because it has a component of plunge into the slope. Lines 3 and 4 are intermediate cases where the intersection line lies with the plane of the slope. Therefore, on the stereogram, the great circle representing the plane of the slope corresponds to the daylight envelope.