Queenston Formation: Tunnel Back Analysis and Forward Shaft Prediction

The Niagara Tunnel Project (NTP) is a 10.1km long water diversion tunnel in Niagara Falls, Ontario, which was excavated by a 7.2m radiusTunnel Boring Machine. Approximately half the tunnel length was excavated through the Queenston Formation, which locally is a red shale to mudstone. A large notch formed, in the tunnel crown and invert, during excavation reaching 4m in height at the tunnel crown in areas. The failed zone was accurately modeled using the Damage Initiation and Spalling Limit approach utilizing the average laboratory testing results in the range of the tunnel depth, approximately 140m below ground surface. The stress ratios which correctly captured the observed notch were determined to be a K_o=4.0 and K_Hh=1.4. Using the same calibrated approach, average laboratory test results, a series of models were computed over the range of stress conditions for a shaft excavation in the Queenston Formation at the proposed Deep Geological Repository (DGR) for Low and Intermediate Level Nuclear Waste storage near Kincardine, Ontario. The maximum damage depth was determined to be 1.9m deep, with an average of 1.5m. Important differences are discussed between the tunnel orientation for NTP (parallel to bedding) and the DGR shaft (perpendicular to bedding). The models show that the observed normalized depth of failure at the NTP would over predict the depth of damage expected in the Queenston Formation at the DGR.