For open excavations, two main factors govern the angle of slope. First, consider the type of ground and, second, the possible damage caused by a slip. Flow of ground water or interference with the flow could initiate a slip. Drainage may be required to direct storm water away from the slope. Obviously, for example, if important property is close to the top of the excavation, there must be no risk of a slip and a high factor of safety must be adopted.

    Mass movements of the ground can occur due to mining subsidence, swallow holes, land slips on unstable slopes, rock falls, creep on clay slopes and frost or water damage to newly exposed faces. The sides of rock
excavations must not be assumed to stand vertical safely. Their stability depends on the angle of bedding planes and degree of shattering of unsound rock. Bedding planes sloping steeply towards an excavation provide
dangerously unstable conditions, especially when ground water lubricates the bedding planes. Only when the bedding planes are horizontal, or sloping away from the excavation, may stable conditions be assumed. It is to be noted that an apparently soundly bedded rock face can conceal a mass of shattered rock fragments behind, which are liable to fall out causing undermining and major collapse of overlying sound rock. See Fig. 7.2.