This paper defines a methodology and criteria by which the stability of underground repository drifts in tuff is determined and from which the ground support system is designed. Preconstruction evaluations of stability are required for planning and to support the license application process. The emphasis is on analytical numerical methods because, at this time, empirical data are generally not available for excavations in welded tuff at elevated temperatures or in seismic environments. Observational methods are only applicable during construction. The methodology suggests analytical techniques for the range of structural conditions of the rock currently expected at the Yucca Mountain site: systematically jointed rock masses, randomly jointed rock masses, and widely spaced discrete joints. The analyses must also load on the rock in the vicinity of excavations that result from in situ stresses, thermal expansion, and seismic events. Large-scale field experiments at the Exploratory Shaft Facility (ESF), and laboratory experiments on ESF samples define the controlling deformation mechanisms and allows evaluation of in situ properties, the methodology presented is expected to evolve. 2 refs., 2 figs.