This lecture explores the current state of block theory. For blocky rocks, in which three fairly continuous and regular joint sets can intersect an excavation surface to generate potentially removable blocks, block theory provides methods that help make design decisions. Both translations and rotations of potential and real keyblocks have to be considered. Mode and stability analyses determine the most critical block types and when the excavation has been shaped, oriented, and sized, the support needs can be judged. Given sufficient freedoms, and stationary geologic structure, the wisest excavation orientation and size can be selected. Abutments and foundations of existing structures can be studied with respect to the degree of safety under defined load conditions, such as flood or earthquake. The main input data are the orientations and friction angles of the joint sets. The methods are fully three dimensional.
Block theory is a geometric approach for rock engineering, which is appropriate if one views the creation of an excavation as the introduction of a geometric space next to a rock mass; block theory evaluates the possibilities for the rock mass to invade this space. Block theory was developed to provide theoretical underpinning for layout and selection of supports of surface and underground excavations in blocky rock. It requires knowledge of three dimensional geologic structure and demands only minimal data the friction angles for the joints concerning the mechanical properties of the rockmass. The methods of block theory are generally inapplicable in non-blocky rock. Accordingly it will be useful to survey some of the different classes of rock in order to establish a context.
