Design of Large Underground Caverns for Hydroelectric Projects With Particular Reference to Structurally Controlled Failure Mechanisms

Two distinct types of failures occur in the roof and walls of excavations in rock. In weak or very heavily jointed rock or in massive hard rock subjected to very high stress, failure of the rock mass surrounding the excavation is the dominant failure mode. In hard rock excavations at shallow depth, gravity controlled falling or sliding of blocks or wedges defined by intersecting structural discontinuities is the most common type of failure. In the former case, support by means of pattern bolting with shotcrete and mesh is the most common means of stabilizing the excavation with steel sets or concrete lining being used in extreme cases. In the case of structurally controlled failures, rock bolts or cables designed to support the weight of individual blocks or wedges are generally the most effective and economical form of support. The differences between these two types of rock mass failure is examined and some of the practical aspects of underground excavation design involving these types of failure are discussed.