In the context of "strength of rocks for rock slope design" this paper concentrates on the fact that rock masses are discontinua and, in many cases, have to be modelled as such. The paper, therefore, first describes various ways in which fracture systems can be described. Specifically, geometric, mechanical and geometric - mechanical models are discussed with emphasis on the latter. When considering slope stability, as well as rock mass behavior in general, it is very important to capture the mechanisms by which the discontinuities interact with each other. This interaction involves fracture coalescence which is the second major part of the paper. While all the preceding problems involve mainly non-persistent fractures, a few final comments are made on some special issues relating to persistent fractures.
Even if one considers single persistent discontinuities, as they are discussed in the companion paper, "Strength of Intact Rock and Rock Masses" by Mostyn and Johnson, slope stability is not a simple matter. Wittke (1965) in his path breaking Ph.D. thesis treated rock blocks and wedges with rigid body analysis considering translation and rotation (Figure 1). Many others followed with similarly complete (Londe et aI, 1969, 1970) or somewhat simplified approaches (John, 1968); see also Hoek and Bray (1981), for an excellent overview. Dynamic aspects were also considered by some authors (Whitman, 1971; Newmark, 1965). At this point it is sufficient to say that a variety of limit equilibrium methods exist and since they lend themselves very well to computer programs, many different commercial programs exist to solve rock wedge/block stability problems. In the context of this paper, it is important to point out a number of problems which affect the use of limit equilibrium approaches and how the problems have been or can be taken into consideration.