Why was the tunnel alignment changed?
Initial progress of the tunnel boring machine was much slower than expected due to difficulties safely excavating and installing initial rock support in the Queenston shale formation. This resulted in significant overbreak (excess rock removal) in the tunnel crown. In December 2008, the contractor initiated a realignment of the tunnel to minimize remaining excavation in the Queenston shale formation. This realignment reduces the tunnel length by about 200 metres, to 10.2 kilometres.
