COVER STORY work on the tunnel happened 16 to 24 metres below the surface with little sign of anything happening at street level. Each roadheader had a cutting profile of 9.1-metre width and 6.6-metre height, and each cutterhead contained 78 picks made from tungsten carbide. In action, SEM involves advancing in increments (headings and rounds), which are supported with shotcrete (projected concrete), followed by the installation of rein- forced shotcrete and steel support elements. “The concrete lining has a defined stiffness and allows rock and soil settling to increase its self-supporting capacity until a safe and stable opening is created,” Ferrer says. Tunnel excavation was finished in mid- February 2017 and, in total, about 285,000 cubic metres of excavated materials (mostly bedrock and limestone) were removed. The finished tunnel spans 2.5  kilometres and averages 15 metres below the surface. It is 10 metres in diameter and the stations (Lyon, Parliament and Rideau) are 16 metres by 13 metres in diameter and 120 metres in length. Since completion, “crews have been constructing the concrete slab, pulling rails, and installing dowels to advance west of Lyon  Station and in the station cavern,” Ferrer notes. “East of Parliament Station, crews have water- proofed the arch and have completed the starter walls.” Final concrete lining work throughout the tunnel was completed in fall 2017. CHALLENGES Although project leaders employed every possible strategy to minimize disrup- tions of all kinds, Ferrer notes that all the underground stations are located in urban areas, and therefore impacts on other parties (utilities, buildings, bridges, roads and the public) were a concern. In addi- tion to settlement and possible impacts on existing structures and utilities, there was a need to control noise, dust and vibra- tion. “Consideration was given to expected 10 AVENUES WINTER 2018 Many companies were involved in the massive construction project, including four Ottawa-area businesses contracted for aggregate