ABSTRACT:
A 14.4 m diameter water conveyance tunnel is currently being excavated by TBM through a highly saline and highly stressed bedrock regime near Niagara Falls, Canada. Time-dependent deformations occur in certain rock units when exposed to fresh water and upon release of the high horizontal stresses. Historical data show that these deformations continue for decades. Consideration of the deformations has influenced all aspects of the planning for the excavation of the tunnel and design of the TBM and tunnel liner. The mechanics of the time-dependent phenomenon are reviewed, along with the various laboratory and field investigations undertaken for the project. Numerical modeling in the concept design phase led to various approaches being proposed to account for the time-dependent deformations. The approach adopted will incorporate a plastic membrane to prevent both fresh water from within the tunnel from contacting the rock, and ionic diffusion of salts from the rock into the tunnel, thus avoiding the potential for swelling.
1 INTRODUCTION
The Niagara Tunnel Facility Project, currently being implemented by Ontario Power Generation Inc. (OPG), comprises the design and construction of an approximately 10.4-km long water conveyance tunnel, and associated intake and outlet structures. The intake is located upstream from the Niagara Falls and the outlet is near the existing Sir Adam Beck (SAB) hydroelectric plant. The project is being constructed following the Design/Build delivery method, with Strabag AG as the main Contractor, and ILF Beratende Ing. as the designer. Hatch Mott MacDonald/Hatch Energy are acting as Owners Engineer. Tunnel construction is now underway, utilizing a Robbins Tunnel Boring Machine with a diameter of 14.44m, making it at this time the largest hardrock tunnel bore in the world. The project is described in Delmar et al. (2006). The intake and outlet structures are excavated in limestones and dolostones.
