INTRODUCTION
Rock mass discontinuities such as joints and faults often intersect to form discrete blocks that are individually stronger than the rock mass as a whole. When encountered during mining or construction, such blocks can cause stability problems (Yow, 1986). Displacement of the blocks around an excavation, even without failure of the excavation itself, can affect the rock mass in a hydrologic sense and in a mechanical sense. Hydrologic effects of block displacement can include the creation of preferred pathways for fluid flow or the creation of barriers to unsaturated flow of liquids (because of capillarity). Mechanical effects occur because unhealed joints cannot support tensile stress, tend to limit the shear stress that the rock mass can carry, and reduce the rock mass stiffness locally as they open.
Block theory (Goodman and Shi, 1985) and related analytical techniques have been developed to address problems of block identification and kinematic stability, but such methods are only beginning to be used by practicing engineers. Systematically applied, these techniques can identify and screen potentially troublesome blocks early in the design process. Although more detailed analyses can be applied in later stages of the design effort to develop block reaction curves (Yow and Goodman, 1987) or to evaluate the effects of loosening ground, the initial process of block identification and screening is the theme of this discussion.
DESIGN CONSIDERATIONS
Design considerations for excavations in jointed rock involve block locations, maximum possible block sizes, frequency of block occurrence, and block stability. Block locations in the excavation perimeter depend on the relative orientations of the joints and the constructed opening. Excavation size and orientation affect block size because these design parameters control the maximum size of blocks formed by joints intersecting the excavation. For a given site, the maximum attainable dimensions of any feasible block shape are determined by the joint orientations and extents and the size of the opening into which the block could move if displaced. The size and orientation of an excavation may be partially or totally constrained by other factors, such as the intended use of the opening, but maximum block size can be analyzed as a function of excavation orientation if desired. Figure 1 illustrates typical results from such an analysis. Block frequency is related to the length and frequency distributions of the joints and the probability of joint intersections, and will not be treated here.
Block stability is a function of block and excavation geometry, in situ stresses and degree of confinement, and joint stiffness and shear strength (Yow and Goodman, 1987). Block stability is examined during design or construction in order to anticipate and prevent excavation?