ABSTRACT
UNSTABLE SLOPES Each rock material has its own equilibrium slope angle. Clays are generally unstable at 10, roughly φ/2. Most rocks of moderate or greater strength can be stable in vertical walls 100 m high if they are massive with only vertical and horizontal fractures. Granite forms a vertical wall 700 m high on Half Dome California, and the vertical cliffs 150 m high at Beachy Head, Sussex, are formed in much weaker chalk. Minor stone fall is always a hazard on these high faces. Planar weaknesses – bedding planes, joints, etc. – inclined towards the slope create potential slip surfaces in any rock; slopes degrade back to any major fractures with dip φ (may be 20 for clay infilling; cohesion and water pressure are also significant). Densely fractured or thin bedded rocks weather back to slopes of 20-40. Potential failure can be assessed on any of the above criteria in context of local data. Rock slides are mostly related to existing planar weaknesses (bedding, joints, faults, cleavage or schistosity) that daylight within a surface slope (i.e. they have unfavourable orientations and are exposed at outcrop at their lower ends).
