Calculation of the daylight envelope for plane failure of rock slopes

Plane failure the downdip sliding on a single set of discontinuities-constitutes an important mode of failure of rock slopes. In assessing the potential for such failure, the orientation of the discontinuities relative to that of the face of the slope is a crucial consideration. Discontinuities that dip in towards the face cannot act as sliding surfaces because movement is prevented by the lack of space for the displaced mass (Fig. 1(a)). On the other hand, discontinuities that dip generally in the direction of the rock face are kinematically viable: such planes of weakness are said to daylight. It is common practice to use hemispherical projection techniques to establish the range of orientations of daylighting discontinuities from a consideration of the attitude of the rock face (Hoek & Bray, 1981; Bell, 1987; Lisle & Leyshon, 2004). On the projection, the normals of daylighting planes occupy an area bounded by a curve known as the daylight envelope. The envelope is defined by the poles of planes that are marginal in terms of the daylighting condition: these are planes whose downdip lines lie within the plane of the rock face (e.g. lines 3 and 4 in Fig. l (a)).