Many waste isolation programs propose using bentonite as primary sealing materials for shaft, emplacement rooms and holes, access drifts and ramps. This investigation is to study the possibility of using only bentonite to seal fractures in rock. The sealing behavior of the bentonite when used for sealing fractures includes permeability, stability, self healing, washing out and resaturation. Bentonite suspension with 19 to 23% solids content was injected into a single fracture in clear acrylic plastic models and in rock. The hydraulic conductivity of the bentonite grout is affected by the applied pressure head, drying, particle wash out, self-healing and time for over 200 days. After some grout (particles in channels and shrinkage cracks) have been washed out twice and subjected to varied pressure gradients for over 200 days, the bentonite grout can maintain low hydraulic conductivity.
Bentonite has widely been proposed and considered as primary potential sealing material for containment structures. Bentonite as sealing component provides a barrier in the repository access excavations, to block fluid (water or brine and gas) flow into or out of the repository. In addition, bentonite can generate swelling pressure when water or brine penetrates the clay. Swelling of the seals increases the internal supporting pressure in the shaft and fractures, and should accelerate healing of any disturbed rock zone. Swelling of bentonite also assist in sealing fractures caused by structural damage or by rock block displacement by self penetration into the fractures, and should help in obtaining tightness between seals and host materials. Fracture sealing requires that the sealing barriers have a low permeability, a long lifetime, a high resistance to erosion, and chemical stability. Bentonite has excellent sealing performance and has been selected as a major sealing component for numerous waste con- tainment projects. Bentonite has an extremely low hydraulic conductivity, self-healing ability, good chemical stability that would provide effective long-term sealing (Gnirk, 1988). Bentonite can penetrate rock fractures either by viscous flow or by expansion (Pusch, 1978). The injection of bentonite suspensions into minute artificial fractures (Ran and Daemen, 1991, 1992a) and rock fractures (Ran and Daemen, 1992b) shows that bentonite can reduce fracture permeability greatly and can produce gels at low solids concentration. Some disadvantages have been observed in model tests (Ran and Daemen, 1991), such as channeling and shrinkage cracks in the grout and that some grout can be washed away when the pressure gradient is high enough. This paper presents an experimental assessment of the sealing properties of bentonite suspension in fractures. After having been washed out twice and subjected to varied pressure gradients for over 200 days, the bentonite grout can maintain low hydraulic conductivity.
A laboratory test program was performed to study the sealing properties of bentonite suspensions for fractures, including hydraulic conductivity, washing out, channeling, resaturation after having been dried, and self-healing. Bentonite suspension with 19 to 23% solids content was injected into a single fracture in clear acrylic plastic models and in rock samples.
