The objective of this study is to assess the performance of a bentonite/sand mixture for the sealing of underground waste repositories through performing geotechnical laboratory tests such as compaction and flow tests. Swelling, mechanical and shear strength tests along with analyses of seal/rock mechanical interactions of an axially loaded seal in rock have been conducted to recommend an optimum compacted bentonite/sand mixture and a suitable bentonite/sand seal length-to-radius ratio (L/a) as a function of water load. The bentonite used in this study was a natural sodium based non-treated bentonite possessing a high swelling potential and containing at least 90% montmorillonite. The results of the compaction permeameter tests led to a recommendation to select a mixture with a bentonite content of about 30% since it possessed the lowest hydraulic conductivity. Analysis of seal/rock mechanical interaction to reduce the possibility of seal slip was performed as a function of the axial load generated from a water column on the seal for seal length-to-radius ratios (L/a) ranging from 2.0 to 20. (C) 2009 Elsevier B.V. All rights reserved.