Prepared by Sandia National Laboratories for the U. S. Department of Energy under Contract No. DE-AC04-76DP00789 Authors: M. P. Hardy (AAI) and S. J. Bauer (Sandia National Laboratories) A design methodology is presented for assessing drift stability for a potential high-level radioactive waste repository. Excavation stability is required during construction, emplacement, retrieval (if required), and closure phases to ensure worker health and safety, and to prevent development of potential pathways for radionuclide migration in the post-closure period. Requirements for the design, site conditions, and stresses are considered in the methodology. Methods for evaluating empirical and analytical results in order to estimate ground support requirements are outlined.