Canada’s project of the year12 December 2013
As tunnels went to press the Tunnelling Association of Canada's annual awards were in full swing. The award for best Canadian tunnelling project for 2013 went to a project that had to endure some of the toughest ground conditions, consistently varying, and continually testing the skills and knowledge of the teams involved.
Utilising the largest hard rock TBM built to date, the 10.2km Niagara Tunnel project bored steep inclines through the Queenston shale. The conditions were not as expected and the high horizontal stresses of the shale located in the lower strata led to overbreak. "The biggest technical challenge was dealing with overbreak. In our case this means a break that exceeds 3m and we sometimes had the equivalent of a long tube above our heads like a 4m tunnel," Strabag project manager Ernst Gschnitzer told Tunnels in 2010. Progress slowed to just a metre a day when facing the worst ground conditions.
This meant contractor Strabag, designer Hatch Mott Macdonald and TBM supplier Robbins had to react and rethink much of the project. Handling a 14.4m diameter TBM presents enough challenges, couple that with the unexpected ground conditions, the need for grouting almost continuously and the ever present threat of water ingress, the project team are right to be congratulated.
The rock support system, which used rock bolts, wire mesh, steel ribs and shotcrete had to be adjusted for the encountered ground conditions, as did the methods for installation. "We had to do a series of adjustments and improvements on the rock support system and we had to change the original fixed working platforms. When there is a 3-4m overbreak above the TBM it is difficult to get there to support and prevent it deteriorating," explained Gschnitzer.
After losing months to the unexpected overbreaks the team decided that the only way to get the project back on schedule was to change the tunnel alignment. By the end of 2008 the new route was underway. "The main problem area was on the vertical alignment, 140m below surface in the Queenstown shale under the Barry St Davids Gorge, but we changed the alignment such that the tunnel was ramped up to 90m below the surface minimising the amount of remaining excavation in the Queenston shale," says Gschnitzer.
Project client Ontario Power Generation required a 100-year design life for the project. Achieving this meant fitting a permenant membrane that could cope with the 15 bar pressure found in parts of the tunnel.
The final membrane consisted of A geotextile fleece fixed to the shotcrete. A vacuum testable dual-layer polyolefin (FPO) membrane system used in the tunnel invert in rock formations with swelling potential and a prototype electrically testable FPO membrane used in the tunnel arch. The system can cope with 20 bar pressure.