Tunnel Boring below Montreal: A Case Study of Urban Tunneling through Hard Limestone
Montreal, Quebec, Canada’s Rosemont Reservoir tunnel travels for 4.0 km below city streets, faulted rock, a disused quarry, and active subway. The story of the 3.0 m diameter Double Shield TBM’s successful breakthrough involves a careful analysis of geology, TBM operating parameters, and ground consolidation measures. Over the years, geologists conducted two diamond-drilling programs totaling 65 borehole tests to depths ranging from 21 to 65 m below residential and commercial neighborhoods along the tunnel alignment. The core sampling program indicated the presence of medium to thinly bedded limestone, with some shale and intrusive rocks, mainly dykes and sills. While the limestone averaged 50 to 300 MPa UCS, rock in the intrusives ranged from 100 to 430 MPa. More than 80 dykes and sills as small as a few centimeters wide and as large as 8 to 10 m wide were mapped along the 4.0 km tunnel. Contractor Foraction, Inc. took measures including cement injection of vertical boreholes in two suspected fault zones from the surface to a depth of 50 m. Even with these measures, fractured rock and water inflows, which had to be temporarily deviated, slowed progress and required alteration of the boring parameters in some sections. The crew were ultimately successful and made their final breakthrough with the TBM in November 2015. This paper will analyze TBM boring methods and performance based on the changing geological conditions below Montreal. Special attention will be paid to sections in fracture zones and below sensitive structures including the inactive quarry site and active Montreal subway. The authors will analyze how preliminary studies, combined with operational techniques and on-going geological monitoring, resulted in an ultimately very efficient tunnel boring project in a dense urban area.
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