Miners power through difficult rock to dig massive tunnel under Niagara Falls
By definition, ‘mining’ means (in part): “Excavation in earth….” and nowhere is that more accurate than when talking about the recently completed “Niagara Falls Tunnel Project” in Niagara Falls, ON., where contractors tunneled for seven years under the city to build a 12.7-m diameter pipe designed to carry 500 m3/ second of water.
By any standards, mining or not, that’s a huge tunnel and to build it using a single 14.4 m diameter tunnel boring machine (TBM), along a twisting 10.4 km route, is even more remarkable. In fact, over the seven years it took to dig and bore the tunnel, the machine chewed its way through more than 1.6 million m(3) of rock: enough to fill the Rogers Centre, home of the Toronto Blue Jays.
As excavations go, the Niagara Falls Tunnel is huge and called for just about every skill required to build any conventional underground mining project.
From initial soil sampling and rock drilling along the route, to more detailed and in-depth geologic analysis of mineral formations in the area, to dealing with the existing underground infrastructure in the popular tourist city, contractors faced a number of monumental challenges that caught the attention of ‘mining’ contractors around the world.
“The concept of building a tunnel of this size under one of the most popular tourist attractions in the world without disrupting the day-to-day operation of a major city up to 140 m above was a logistical challenge in itself. And, using the world’s largest hard rock boring machine for the first time, just compounded that challenge,” said John Tait, Project Manager for Hatch Mott MacDonald.
“The size of the TBM (tunnel boring machine) was a bit intimidating because many of the crew had never seen or worked with such a monster of a machine. Anything with a cutting head that is more than 14 metres in diameter and is capable of eating the rock the way it did is a ‘marvel’ in itself.”
Tait said the crews quickly learned to respect the power of the boring machine and as the tunneling progressed below the city, they encountered and successfully overcame a number of obstacles, including extreme challenges posed by the host rock.
Groundwater, especially in the very permeable upper levels of shale where crews tapped aquifers, was of particular concern but so too, were the natural gases (methane) detected in various formations
“But in the end,” says Tait, “there wasn’t a single moment when the crew or machine was in any serious danger and it’s a credit to everyone who ever set foot on the site over the years that safety and awareness of the potential dangers was a first priority.”
As mentioned earlier, the scope of the $1.6 Billion Niagara Tunnel Project was massive but moreover, courageous because of the capacity and stress of the water the 12.7-m diameter pipe was intended to carry.
Feeding the Sir Adam Beck generating stations with a 27 per cent increase in water, resulting in a 14 per cent increase in an average energy output by 1.6 billion kWh, involved connecting the Niagara River with the Sir Adam Beck stations.
In addition to the many feats of engineering that will be mentioned later, the job depended on “Big Becky,” the name affectionately given to the TBM by the crews who worked the machine to connect the River’s water with the Stations’ turbines.
Rick Everdell, Project Director for the owners, Ontario Power Generation, explained that “Big Becky” started arriving on site in June, 2006, and after three months of assembly, it started its seven-year journey in September by punching its way towards the Sir Adam Beck stations at a rate of 15 m a day.
As the machine progressed, Everdell explained that the tunnel (depending on the host rock conditions) was supported with wire mesh, rockbolts, steel ribs and shotcrete. An impermerable polyolefin membrane to prevent swelling of the host shales was also installed, followed by an unreinforced, 600-mm-thick, cast-in-place pre-stressed, permanent concrete liner.
For safety purposes, all tunnel work was staged at various intervals behind the TBM. Installing the invert membrane and concrete took place 3000m behind the boring machine, while tunnel profiling work was an additional 1500m back, followed by arch membrane and concreting 1500m farther to the rear, and liner grouting yet another 1000m back.
A total of 400,000 m(3) of concrete was delivered to the site using 15 m(3) agitator trucks to line the tunnel. Laser scanning was used to monitor deflections to +/-0.5 mm in the concrete lining.
With the tunnel secured, a convoy of utilities advanced with the TBM drive including the fresh air duct, conveyor, power and communication cables, lighting, and clean and dirty piping.
As the tunnel progressed, turning and parking platforms were also built for use by non-construction (visitor) vehicles and safety/rescue equipment.
While much of the Niagara Tunnel story focuses on the underground workings of the project, surface work was also ongoing over the seven-year construction period; most notably the in-water work in the Niagara River.
In 2006, demolition crews removed an old accelerating wall in the river and in the next year built a new approach wall as well as drill and blast for the excavation of a 100-m by 20-m intake channel .
Following the excavation, an 8-m by 7-m, 400-m-long drill and blast grout tunnel was built while on the surface, five, 130-m deep dewatering shafts and an associated outfall pipe and structure were built.
A total of 12 concrete pours for the approximately 35-m high intake structure were required as well as more than 570 tonnes of reinforced steel was placed within the structure.
When the TBM finally reached its goal after its seven year journey and broke through at the Beck end of the project, much of the final work involving the interior of the tunnel was right behind it and was either complete, or on schedule and well underway.
Among the more notable and final achievements saw the completion of the invert lining of the tunnel and the placement of the last of 95,000 m(3) of concrete, 94 per cent of which was produced at an on-site batch plant, and the completion of the arch concrete, which included the installation of the arch portion of the tunnel’s waterproofing membrane.
Culminating the work and one of the more momentous events was the installation of the intake service gates which allowed the flooding of the intake channel and filling the tunnel.
At the end of the day, almost 2000 of them to be exact, The Niagara Tunnel project was complete and it was time to move to another project for the estimated 450 contractors on the job knowing that their work will be instrumental in helping produce an estimated 1.6 million kilowatt hours of ‘new’ electricity for Ontario annually. That’s enough to serve 160,000 homes.
With the project completed, Ontario Power Corporation faced one final challenge and that was to demobilize and get rid of the tools and other equipment used during the construction of the project.
Some was stockpiled for future use, some was sold for scrap, while more than 1,600 pieces went up for sale at an auction conducted by Ritchie Bros. Auctioneers of Bolton, ON.
Some of the equipment sold included more than 40 mobile structures that were used for of
fices, portable camps including bedrooms, and washrooms and kitchens.
About 80 trucks, mixers and a multitude of pumps and hoses were also on the block during the two-day auction and not surprisingly, many of the items went to bidders watching the auction from around the world on Ritchie Bros. EquipmentOne online telecast.
And, as mentioned at the outset, The Niagara Tunnel Project caught the world’s attention and held its interest to the very end.
Comments