The site investigation for an underground cavern has to locate a sufficiently large mass of sound rock in which the cavern can be excavated. Because caverns usually are located at appreciable depth below ground surface, the rock mass often is beneath the influence of weathering and consequently the chief considerations are rock quality, geological structure and groundwater conditions. The orientation of an underground cavern usually is based on an analysis of the joint pattern, including the character of the different joint systems in the area and, where relevant, also on the basis of the stress distribution. It normally is considered necessary to avoid an orientation whereby the long axis of a caverrn is parallel to steeply inclined major joint sets (Hoek and Brown, 1980). Wherever possible, caverns should be orientated so that fault zones are avoided.

Displacement data provide a direct means of evaluating cavern stability. Displacements that have exceeded the predicted elastic displacements by a factor of 5 10 generally have resulted in decisions to modify support and excavation methods. In a creep-sensitive material, such as may occur in a major shear zone or zone of soft altered rock, the natural stresses concentrated around an opening cause time-dependent displacements that, if restrained by support, result in a build-up of stress on the support. Conversely, if a rock mass is not sensitive to creep, stresses around an opening normally are relieved as blocks displace towards the opening. However, initial movements may be...