In the previous chapter we have examined one geometrical requirement for plane failure in rock slopes: the basal discontinuity has to dip at a sufficiently steep angle to overcome friction. However, this is not the only condition for plane failure. The discontinuity has to be suitably oriented to allow detachment of the slipped rock mass and for its forward movement out of the slope. This condition is described as the requirement that the potential basal plane daylights on the slope.

Figure 45a illustrates the predicted direction of movement on a plane of weakness. The overlying rock will slide in the direction of dip of the plane of weakness. For plane failure to occur this down-dip direction has to be directed out of the surface slope. In Figure 45b, the discontinuity with down-dip direction labelled 2 will not give rise to movement because sliding in the direction opposite to the surface slope is prevented because of lack of space. Direction 1, on the other hand, corresponds to the down-dip of a plane that dips out of the surface slope, i.e. a plane that meets the daylighting condition for plane failure.

We use a stereogram to assess the daylight condition of any particular plane of weakness. On the stereogram we define the orientation field made up of planes that daylight. This daylight zone is bounded by a daylight envelope on the stereogram, a curve made up the poles of planes that are marginal in terms of the daylighting condition.