The powerhouse at the Portage Mountain dam is one of the largest single underground excavations in North America, being 890 feet long, 150 feet high and 67 feet wide. It has been excavated with its long axis parallel to the strike in gently dipping sandstones, siltstones, shales and coals of the Dunlevy Formation.

The powerhouse roof was designed to be a semi-elliptical arch in cross-section and to be supported by a pattern of grouted rock bolts spaced five feet apart. Rock mechanics studies used in the design of the arch and conducted prior to its excavation suggested the presence of residual stresses in the surrounding bedrock. During excavation of the powerhouse arch, significant downward movements of the overlying rock were recorded by a number of different methods. On the basis of these measurements supplementary rock bolting was installed including a change in the basic pattern to a four foot spacing.

Most of the movement is postulated to have occurred by opening of horizontal tension fractures due to blasting. The change in the basic spacing of the rock bolts was unnecessary. It is suggested that a rock bolt pattern which includes the immediate installation of wire mesh at the time of excavation would eliminate extensive supplementary rock bolting. It is further suggested that the excavation of a semi-elliptical arch in thinly-bedded sedimentary rocks is inadvisable.

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