Geotechnical observations from the Niagara Tunnel Project: Numerical back analysis for application to shaft damage dimension prediction

During the excavation of the Niagara Tunnel Project challenges in Queenston Formation, a mudstone, were encountered which slowed the excavation advance. As the tunnel reached 140m below ground, large overbreak in the order of 3-4m began to occur due to stress induced spalling, which created a notch shaped geometry. The failure mechanism is the result of increased lateral strain around the excavation due to the anisotropic nature of the mudstone and the high stress concentration at the excavation boundary. Back analysis was conducted and determined at a Ko ratio of 4 a plastic yield zone of roughly 4m resulted using the Damage Initiation and Spalling Limit approach. Forward numerical prediction of damage in a shaft that will pass through the Queenston Formation at a different site was conducted to estimate the maximum depth of plastic yielding, which was found to be 1.9m.