Introduction All penetrations (e.g., boreholes, shafts, mine drifts and tunnels) in the vicinity of a high-level nuclear waste repository need to be sealed reliably to retard any radionuclide migration to the accessible environment (Hoffman 2004; USEPA 1995; US Nuclear Regulatory Commission 1983, 1985). Bentonite and bentonite/sand mixtures are considered as buffer and backfill materials for repositories of high-level nuclear waste in countries such as Sweden, Switzerland, Canada, Germany and France (Brenner 1988; Bucher et al. 1986; Coulon et al. 1987; Daemen and Ran 1996; Dixon et al. 1985; IAEA 1990; Pusch 1994; Pusch and Bergstro¨m 1980) and for sealing of shafts emplaced in the vicinity of the waste isolation pilot plant (WIPP) site in southeastern New Mexico (Daemen and Ran 1996; USDOE/WIPP 1995). Axial loads on seals emplaced in openings in the vicinity of a high-level nuclear waste repository that may be due to water, drilling mud, gas or backfill pressures may induce shear stresses along the contact between the seal and host rock which, under extreme conditions, could cause dislodging or slipping of seals leading to increased permeability through the seal (Akgu¨n and Daemen 1999). Hence, repository sealing requires that the sealing barrier possess adequate interface strength, a low per- meability, a long lifetime, a high resistance to erosion, mechanical and chemical stability, and compatibility with host rocks or materials (Akgu¨n and Daemen 1994, 2004; Daemen and Ran 1996). The required operative lifetime of barriers other than rock, such as clay, needs to be defined and assured. There are numerous examples of geologically well-defined clay materials that have undergone heating and loading without losing much of their sealing ability, which demonstrates that there are clay minerals that can be largely preserved for hundreds of millions of years under repository-like conditions (Pusch 1994). Bentonite and its soil mixtures have been used widely as a sealing material for waste containment structures such as landfills (Daniel 1993; Rowe et al. 1995) and as a principal sealing component for numer- ous nuclear waste repositories (Daemen and Ran 1996) H. Akgu¨n M. K. Koc¸kar O ¨ . Aktu¨rk Evaluation of a compacted bentonite/sand seal for underground waste repository isolation Received: 6 January 2006 Accepted: 7 February 2006 Published online: 7 March 2006 Ó Springer-Verlag 2006 Abstract This study investigates the performance of an optimum com- pacted bentonite/sand mixture seal for the isolation of underground waste repositories. Engineering geo- logical tests such as compaction, flow, swelling, mechanical and shear strength tests have been conducted to select an optimum mixture and to recommend a stable bentonite/sand seal length-to-radius ratio (L/a) as far as the factor of safety (F) is concerned. The results of the com- paction permeameter tests led to a recommendation to select an opti- mum compacted bentonite/sand mixture possessing a bentonite con- tent of about 20% to satisfy the minimum regulatory hydraulic con- ductivity requirement. Engineering geological analysis of the seal/rock mechanical interaction with regard to reduce the possibility of seal slip led to a recommendation to utilize a seal L/a of at least 25. Keywords Bentonite/sand mix- ture Æ Compaction Æ Hydraulic con- ductivity Æ Seal design Æ Shear strength Æ Shear stress Æ Factor of safety (F) Æ Underground waste repository sealing Environ Geol (2006) 50: 331–337 DOI 10.1007/s00254-006-0212-6 ORIGINAL ARTICLE H. Akgu¨ n(&) Æ M. K. Koc¸kar Æ O ¨ .Aktu¨ rk Faculty of Engineering, Department of Geological Engineering, Middle East Technical University (METU), 06531 Ankara, Turkey E-mail: hakgun@metu.edu.tr Tel.: +90-312-2105727 Fax: +90-312-2101263