Fully functional now, the tunnel is the realization of a long time dream of Canadians, who wanted to exploit optimum hydro-potential of the Niagara River, which forms an international border between Canada and the USA. Way back in the 1950s, the two countries had signed the Niagara Treaty to share the river's water. Barring Niagara Falls, which account for one third of the river, the remaining two-thirds of the water was equally divided between both the neighbouring countries.
Quite inclined towards tapping renewable energy, Canada strategically used the allotted water at the Toronto Power Generating Station, Ontario Power Generating Station, Rankine Generating Station, and the Sir Adam Beck Generating Stations. But, more than five decades later, only the Sir Adam Beck Generation Stations were functional and a substantial chunk of water from the Niagara River was in an unutilized state. This is when the Niagara Tunnel was conceptualized.
The Niagara Tunnel is a coveted possession not only because of its engineering credentials, but the fact that it facilitates production of clean and renewable energy. The tunnel can annually produce more power, on average, than cities the size of Niagara Falls or Kingston use every year with minimal maintenance costs.
Notably, OPG awarded the construction contract to Austria-based Strabag AG, which chose a 14.4 m (47.5 ft) diameter Robbins Main Beam Tunnel Boring Machine (TBM) named Big Becky 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, and behind the excavation, an in-situ placed concrete lining was put in place. The final lining includes a waterproofing membrane system to ensure that water does not seep from the tunnel into the rock and cause swelling.
Technicalities and Challenges
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 an average rate of about 3 m (10 ft) per day.
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 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.
A rotary-type ring beam erector was provided, with provisions to hydraulically lift the ring beam or channel section into place and hydraulically expand the steel sections against the bored rock.
Wire Mesh Erector/Material Handling Cart
A dual function handling cart, known as the donkey, is located on the top section of the TBM main beam. The donkey transports the steel sections and wire mesh forward into the L-1 working area.
Two Atlas Copcp 6.4-m- (21-ft-) long BMH 6000 series hydraulic drills with powerful COP 1532 hydraulic hammers were installed. The drills are installed on a rotary position locator, which allows independent operation of each drill. The position locator allows the various drill positions to be achieved to install the 6-m- (20-ft-) long rock bolts as per project requirements. Design of the system allows the bolts to be installed during the boring operation.
To assist the tunnel operating personnel in the installation of the rock support, there are various stationary and mobile work platforms located in the L-1 area. These platforms allow rock scaling, wire mesh and other ground support functions to be performed.
A shotcrete robot has been installed and integrated into the work platforms. The robot has been supplied by Rowa (Wangen, Switzerland)/Meyco-BASF (Switzerland, TX). It includes a boom to allow shotcrete coverage over a 180° section of the tunnel crown and at a rate of 15 m3 /h (530 cu ft/hour).
Other major changes to the TBM L1 area became necessary to reach the caved areas, including the addition of two hydraulic man baskets and special drill rigs, mesh and anchors. These changes were made incrementally as TBM progress allowed it. With the new method, the over-break could be limited to 0.5m - 1 m (1.6 - 3 ft), though the excavation process slowed to a maximum 5 m/d (16 ft/day).
A Dream Come True
Well, such is the status of the Niagara Tunnel that it has been showered upon a lot of accolades. At a ceremony to celebrate water flowing in the tunnel, OPG President and CEO Tom Mitchell congratulated the contractor STRABAG and remarked "The hundreds of men and women who worked with extremely difficult rock conditions to safely complete this engineering marvel. OPG Chairman, Jake Epp said, "The tunnel has claimed the attention of the entire world and it's a fitting representative of Canada's, Ontario's and OPG's great legacy of electricity development and operation." Meanwhile, Ontario's Minister of Energy, Bob Chiarelli commented, "This project is a source of pride as an engineering feat and as a practical solution for meeting Ontario's energy needs through clean sources." After all, it's a dream come true for Canada and the whole world should know it.