“…The sample was then subjected to injection pressures of 69 and cm/s for Topopah Spring tuff (Anderson, A mixture of bentonite and ballast material (e.g. quartz sand and crushed rock) is being considered as backfill and sealant for nuclear waste repositories (Pusch et al, 1980;Nilsson, 1985;Holopainen, 1985;Yong et al, 1986;Williams and Daemen, 1987). Primary advantages for adding ballast material to bentonite are to increase thermal conductivity (Pusch et al, 1980;Dixon et al, 1985), to decrease shrinkage potential, and to increase the bearing capacity of the sealant, minimizing creep or settlement , a suitable composition for such sealant must fulfill the following requirements: (1) low permeability, ( 2 ) low compressibility, ( 3 ) small average pore size to prevent bentonite migration, and (4) some swelling potential.…”