[en] Any opening instabilities in a possible nuclear waste repository at Yucca Mountain, Nevada, are likely to be the result of movements along pre-existing joints in welded tuffs. Preliminary analyses suggest some potential for joint shear, but data are not available for evaluating the effect that large joint irregularities have on limiting displacements. Tests using sample sizes up to those that yield dilations and normal forces representative of in situ conditions are desirable for the proper characterization of shear behavior along irregular joints. A program of laboratory and field experiments will investigate the coupled effects of sample size and normal boundary conditions on shear behavior of joints in welded tuffs. This paper discusses this program. 15 refs., 4 figs