ABSTRACT

ABSTRACT: Excavations in jointed rock masses are commonly affected by the presence of keyblocks of rock formed by three or more joints on the periphery of an excavation. These blocks may be unstable if they exceed the strength of the joints unless an adequate support system is installed. Estimating the keyblock size that must be accommodated by the excavation-support system requires a model of the rock joints that considers all joint-set properties. The unit-cell method, which has previously been described in a three-dimensional form considering joint set spacing, is extended here to consider the effect of variations in joint-set orientation. Two models of tunneling cases are considered. The first case suggests that variations in joint-set orientation about a representative value makes only a small absolute difference in the probability of large keyblock formation. The relative error of the estimate is large, however, when large but unlikely keyblocks are considered. A second case is considered that produces a fairly large maximum keyblock with one joint set roughly parallel to the tunnel axis. In this case, the importance of considering variations in joint-set orientation is more clearly demonstrated.

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