ABSTRACT:

A major international experiment, demonstrating full-scale tunnel sealing technologies and methodologies is being conducted on the 420 Level of Canada's Underground Research Laboratory (URL) with experiment partners from Canada, Japan, France and the USA. The Tunnel Sealing Experiment consists of two instrumented bulkheads, one of low heat high performance concrete, the other of highly compacted bentonitc-sand blocks which seal a central section of the tunnel filled with permeable sand. The central portion will be pressurized incrementally via piping water through boreholes to 4 MPa, to characterize the performance of the bulkheads in preventing axial flow along the tunnel. The bulkheads were designed with cut-off keys to stop flow through the excavation damage zone. The design of these keys required a thorough understanding of the in situ stress tensor. The role of rock mechanics in the design of the Tunnel Sealing Experiment is reviewed along with the construction of this experiment.

INTRODUCTION

The Underground Research Laboratory (URL) is a geotechnical research and development facility constructed by Atomic Energy of Canada Ltd. (AECL) as part of the Canadian Nuclear Fuel Waste Management Program. The URL, located in south- eastern Manitoba, is constructed in the Lac du Bonnet granite batholith and provides a representative geological environment in which to conduct in situ multidisciplinary experiments, to assess the feasibility and safety of deep geological disposal of nuclear fuel waste.

Many countries, including Canada, Japan, France and the United States, are developing concepts for the deep geological disposal of radioactive waste materials. The safety of the respective disposal systems relies on the combined performance of the natural barriers (host rock) and engineered barriers (the waste form, the waste container, the buffer barrier, the room, tunnel and shaft backfill material). Engineered barrier concepts include using bulkheads or plugs to seal shafts and emplacement rooms or both. Bulkheads would be designed as a barrier to water flow and thus potential transport of radionuc!ides. Although the bulkhead would act as a barrier to flow through the backfilled tunnel or shaft, it would also have to be designed to prevent flow through the near-field Excavation Damaged Zone (EDZ) in the rock. Thus it is important to minimize the connected EDZ at the bulkhead location.

The Tunnel Sealing Experiment (TSX) has been designed to characterize the sealing potential of well-constructed bulkheads from the perspectives of both engineering performance and safety assessment. Elements of the experiment design include not only the construction of the bulkheads but also the excavation of the tunnel and bulkhead keys. One bulkhead is composed of hand-placed, highly compacted bentonite-sand blocks, while the second has been constructed using Low-Heat High- Performance Concrete (LHI-IPC). A permeable sand fill was placed between the bulkheads and is being pressurized via piping water through boreholes to 4MPa, a value representative of the natural porewater pressure at the 420 Level. Pressure in the tunnel is being increased in a stepwise fashion over a five month period, and the experiment will be operated at full pressure for about one year. At the time of writing, construction is complete and pressurization has begun.

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