Overview of the evolution, performance and state of a bentonite-based tunnel seal after 5 years of operation

A major international experiment, demonstrating technologies for use in sealing the rooms and tunnels of a nuclear waste repository was conducted at Canada’s Underground Research Laboratory between 1998 and 2004. Two bulkheads separated by 12 m were installed in a 3.5-m-high by 4.25-m-wide tunnel located 420 m below the surface in an intact volume of rock within a granitic pluton. These bulkheads isolated a section of tunnel that was subsequently flooded, stepwise pressurized to 4 MPa and then following extended operation at ambient temperature the water in the flooded tunnel was heated. Heating occurred for a period of approximately one year prior to decommissioning of the TSX. One bulkhead consisted of high-performance concrete and the other of blocks of compacted sand-bentonite material. The performance of these two bulkheads was monitored throughout the experiment in order to evaluate the influence of elevated hydraulic head and tunnel temperature on these materials.This paper provides a brief overview of the evolution of the clay bulkhead portion of the TSX and generally discusses the water uptake, physical deformation and stresses developed and monitored in the course of TSX operation. At the end of 5 years of operation the TSX was dismantled and extensively sampled allowing for development of detailed density and water content profiles for the clay bulkhead. This also allowed the instrument responses to be compared to the physical state at the time of decommissioning.The majority of the very limited seepage past the clay bulkhead occurred at the outer perimeter regions of the clay bulkhead and that this was the region that will require particular attention when it comes time to construct bulkheads in an actual repository. Continual swelling at the upstream face of the bentonite-based clay bulkhead as downstream compression occurred meant that a positive contact was maintained between the clay bulkhead and the surrounding rock and other confining media. It also provided an excellent demonstration of the self-sealing capacity of bentonite-based materials.