Nearing breakthrough at Niagara

Ontario Power Generation’s Niagara tunnel project includes the planning, design and construction of a 10.2km long, 12.7m internal diameter tunnel and associated facilities. This will divert an additional 500m³/sec of water from the Niagara River upstream from the Horseshoe Falls to the existing Sir Adam Beck generating stations near Queenston. The design / build contractor, strabag Inc, is using the world’s largest hard rock tunnel boring machine (14.44m diameter) to excavate the tunnel (supplied by Robbins). Niagara tunnel will deliver enough water to increase average annual energy output from OPG’s Sir Adam Beck stations by 1.6BkWhr – enough to supply 160,000 Ontario homes with clean, renewable hydroelectric power.

Big Becky, the name given to the TBM, is now 9.1km along the 10.2km tunnel route from Sir Adam Beck generating station complex toward the intake 2km upstream from the Horseshoe Falls at the International Niagara Control Works. With vertical realignment to minimise the remaining excavation in the overstressed Queenston shale starting in January 2009 at 3.3km from the outlet portal, the tunnel crown reached the much stronger Whirlpool sandstone in June 2009 at about 4.3km along the route. With minimal overbreak, the TBM advance often exceeded 15m per day when the Whirlpool sandstone was in the tunnel crown.

However, with a thickness of about 6m and the tunnel slope opposing the dipping rock, the tunnel crown passed through the Whirlpool sandstone in about 1km. Overbreak increased and progress slowed with the tunnel crown in the shaly lower portion of the Power Glen formation. There has been practically no crown overbreak since late April 2010 when the tunnel crown ascended into the sandstone portion of the Power Glen formation at about 6.7km along the route. Since that time, TBM mining has been progressing at an average rate exceeding 13m per day. To date, the best daily TBM advance was 26.46m on 1 May 2010 and the best weekly advance was 153.23m for the period ending 18 July 2010.

Initial tunnel support, comprised of steel ribs, wire mesh and rockbolts in the tunnel crown and a full circle layer of shotcrete, is applied as the TBM advances. The impermeable polyolefin membrane and 600mm cast in place unreinforced concrete liner are installed with separate invert and arch operations. Strabag workers have now advanced the invert concrete (the lower one-third of the permanent liner) by about 6.5km, while the arch concrete (upper two-thirds of the permanent liner) has advanced about 1.1km. The arch membrane is rolled out, fixed to the felt backing, heat welded to adjacent panels and fully tested to ensure no leaks before the arch concrete is placed.

Crown profile restoration, infilling overbreak areas of up to 5m in the Queenston shale, is carried out between the invert and arch concrete operations and has currently advanced about 3km. Profile restoration involves consolidation grouting of the overlying rock, installation of support dowels and sacrificial steel forms and placement of infill shotcrete or self-leveling concrete.

Contact grouting, between the membrane and concrete liner, is scheduled to begin in mid 2011. Pre-stress grouting, installed through the membrane to lock the liner into the surrounding rock and offset internal water pressure during tunnel operation, is scheduled to begin early in 2012.

At the intake structure, placement of primary concrete is nearing completion. Primary concrete for the outlet structure is scheduled for 2011. Fabrication of the intake sectional gates and the outlet gate and hoist are currently in progress and scheduled for delivery to site beginning in 2011.

The TBM is now expected to breakthrough at the intake portal ahead of the target schedule. In addition to the issues with crown overbreak, the project team has recently overcome a few other challenges along the route. The September 2009 fall of ground and the subsequent stabilisation of the area, delayed TBM mining for about seven weeks. About 100m3 of Queenston shale and initial shotcrete lining fell unexpectedly from the tunnel crown about 3.6km along the tunnel. All workers safely evacuated the tunnel without any injuries. Remedial work for the fall of ground required build-up of a gravel ramp in the lower half of the tunnel for access to the crown, followed by scaling of loose material, installation of additional rockbolts up to 6m in length, application of wire mesh reinforced shotcrete, and consolidation grouting of the host rock.

On 27 April 2010, a diesel compressor caught fire about half way along the tunnel and mining was delayed by a couple of hours. The fire was promptly extinguished and did not damage the shotcrete lining, however emergency procedures were implemented and all workers were safely evacuated from the tunnel until the smoke was cleared.

Currently, Strabag crews are repairing a crack that developed at a flange connecting two sections of the TBM’s main beam about 10m back from the cutterhead. Crews stopped mining on 3 December, and an outage scheduled for later in December has been moved forward to minimise schedule impacts associated with the TBM main beam repair. TBM mining was expected to resume by year end.

The Niagara tunnel is scheduled for completion in 2013.

Richard A Everdell, Project Director – Niagara Tunnel Project, Ontario Power Generation Inc. Email: rick.everdell@opg.com