The Niagara Tunnel Project is an ambitious project to tunnel 10.4 km (6.5 mi) from the Sir Adam Beck Generating Complex to above Niagara Falls. The new tunnel will increase the power supply for owner Ontario Power Generation (OPG) by 150 MW and will help to bolster the current power system, which is close to exceeding its capacity during peak months.
OPG awarded the construction contract to Austria-based Strabag AG, who chose a 14.4 m (47.5 ft) diameter Robbins Main Beam TBM to bore the tunnel. The setup included a 105 m (345 ft) long back-up system, which transported 1.7 million m3 (2.2 million cubic yards) of muck over three years via conveyor belt.
The tunnel is located predominantly in Queenston shale with some limestone, dolostone, sandstone and mudstone up to 200 MPa (29 ksi) UCS. The rock along the tunnel bore path is known to have high in-situ stress and there is potential for squeezing ground. An initial rock support lining of wire mesh, steel ribs, rock bolts, and shotcrete was installed as the TBM advanced. Behind the excavation, an in-situ placed concrete lining is being installed. The final lining will include a waterproofing membrane system to ensure that water does not seep from the tunnel into the rock and cause swelling.
The Main Beam TBM, the largest hard rock TBM in the world, was assembled at the jobsite using Onsite First Time Assembly (OFTA) in less than 12 months — ahead of a tight delivery schedule. The onsite assembly is considered unprecedented for such a large TBM. The TBM began boring in September 2006.
The Main Beam TBM was also the first ever to utilize back-loading 20-inch cutters, which increase cutter life and reduce cutter changes in hard rock. Both 19-inch and 20-inch cutters could be installed in the cutterhead. The machine had a cutterhead thrust of 18,462 kN (4,150,422 lb) and a maximum torque of 18,670,000 N-m (13,770,285 lb-ft).
After about 793 m (2,600 ft) of excavation the TBM entered the Queenston shale formation, where large rock blocks started to fall from the crown before rock support could be placed. In some cases, significant over-break up to 3 m (10 ft) above the cutterhead support was reported.
Strabag ultimately designed a unique ground support system to cope with the geology, which consisted of 9 m (30 ft) long pipe spiles in an umbrella pattern at the crown of the tunnel. Using the new spiling method, over-break was limited to about 0.9 m (3 ft) above the normal tunnel diameter. Nearly 500 m (1,640 ft) of very difficult ground was excavated using this method, at average rates of about 3 m (10 ft) per day.
The new ground support program, done for all excavated ground, consisted of 3 to 4 m (10 to 13 ft) long rock bolts, self-drilling (IBO) anchor bolts, steel straps, wire mesh and wire-reinforced shotcrete. Crews typically bored half a stroke, then began scaling down loose rock and installing rock bolts. After the full 6 ft stroke, the rest of the loose rock was scaled down before installing more rock bolts, wire mesh, steel straps and a layer of shotcrete.
OPG and the contractor also opted to alter the vertical alignment of the tunnel, raising it 46 m (150 ft) to move the tunnel out of the Queenston shale. After 1,981 m (6,500 ft), rock conditions were competent enough that spiling was no longer required.
After surpassing these challenging rock conditions, the machine achieved a world record-breaking month for any TBM 11 m (36 ft) in diameter or larger. During July 2009, the TBM excavated 468 m (1,500 ft) in one month and advanced 153 m (503 ft) in one week overcoming significant geological challenges.
May 13, 2011 marked the completion of the TBM’s drive. A well-attended ceremony celebrated the final breakthrough of the 14.4 m (47.2 ft) diameter Robbins Main Beam, following advance into a 300 m (1,000ft) long grout tunnel on March 1, 2011.
While the tunneling portion of the project has reached completion, two years of work still remain. Approximately 30% of the continuous concrete lining was completed during tunneling, with about two thirds of the work still to be done. The finished 12.8 m (42 ft) diameter tunnel will be fully lined with both 600 mm (24 in) thick cast in place concrete and a polyolefin waterproof membrane to prevent leakage. Other construction remaining includes the outlet structure, gates, and removal of the cofferdam in the Niagara River, as well as removal of the rock plug at the outlet end.