Testing of a Full-Scale Bentonite-Based Tunnel Seal for Use in a Deep Geologic Repository: The Tunnel Sealing Experiment (1998-2004)

Abstract

The Tunnel Sealing Experiment (TSX) was a test of a full-scale simulation of two independent tunnel bulkheads installed at Atomic Energy of Canada's Underground Research Laboratory (URL). One bulkhead consisted of a single mass pour of low-heat, high-performance concrete into a tunnel segment that had been keyed. The second was an assembly of highly compacted bentonite-sand blocks filling a second keyed segment of the same tunnel. The clay bulkhead was 2.8-m-long, 3.5-m-high and 4.2-m-wide. The keyed portion was 1m in depth and 2 m in length. The resulting bulkhead was approximately 60 m3 in volume. An 11-m-long section of tunnel between these two plugs was filled with sand, flooded with water and pressurized. Following pressurization to 4 MPa, the water in the central region was heated and the influence of temperature on the tunnel plugs and the surrounding rock mass was monitored. Once clay bulkhead saturation was achieved, seepage past the clay bulkhead was Darcian and corresponded to an average hydraulic conductivity of approximately 10-11 m/s, regardless of temperature. End-of-test sampling identified considerable volume changes in the upstream portions of the clay bulkhead, associated with early consolidation of the block assembly under the applied hydraulic heads. The ability of the clay bulkhead to accommodate substantial movement and volume change in its upstream regions demonstrates the importance of using sealing materials having sufficiently high plasticity and swelling capacity to accommodate deformation. The TSX, with its extensive monitoring system, its well-defined initial state and carefully measured end-of-test conditions, has provided a valuable set of data on the performance and evolution of a bentonite-based tunnel seal.