A Novel Approach at Niagara
Robbins TBM Stands Up to Challenging Ground with Custom Lining and Ground Support By Desiree Willis

Canada’s new headrace tunnel beneath Niagara Falls will generate enough power to service 160,000 Ontario customers, but getting to that point has been anything but easy.

The world’s largest hard rock TBM, at 47.2 ft in diameter, is excavating the Niagara Tunnel Project in mixed ground including limestone, shale and sandstone, below a buried gorge and through fractured, stressed geology.
Removal of overbreak in the crown of the tunnel is a continual process. Geological conditions have ranged from competent ground – allowing world-record advance rates – to significant rock falls. Contractor Strabag AG and TBM manufacturer Robbins have worked together to control the difficult conditions through a modified ground support program including rock bolts, steel channels, wire mesh and shotcrete.

The finished 42 ft diameter tunnel will be fully lined with 24-in. thick continuously poured concrete and a polyolefin waterproof membrane to prevent leakage. The tunnel is being lined behind the Robbins TBM using separate invert and arch lining systems as well as a membrane laying machine. Invert lining operations are ongoing about 6,500 ft back from TBM excavation, while the 1,500 ft long arch and membrane operations trail farther behind.

Construction at the tunnel is 24 hours per day, which requires nearly 400 workers. Trucks at the busy jobsite are color-coded based on their contents. White trucks carry shotcrete to be applied directly behind the TBM, while yellow trucks transport concrete for the invert lining and arch carriers.

“By far the biggest challenge has been the concurrent operation of four different processes,” said Ernst Gschnitzer, Project Manager for Strabag AG. “We are simultaneously doing TBM tunneling, invert lining, overbreak restoration and arch lining.”

Project Background

The Niagara Tunnel Project was initiated in June 2004 by provincially owned company Ontario Power Generation Inc. (OPG). Two drill-and-blast tunnels were constructed beneath the city of Niagara Falls in the 1950s, but a third was deemed necessary to bolster supplies during peak summer months and provide for future electricity demand.

Roof drills on the Robbins TBM install 20 ft long rock bolts as part of an innovative ground support program at the Niagara Tunnel Project.

The 6.4-mile tunnel is being constructed by Austrian contractor Strabag AG, with Hatch Mott MacDonald as the designer and onsite OPG representative, and will add up to 17,700 cu ft/sec for hydroelectric generation by 2013. Tunneling began at the downstream end near the Sir Adam Beck Generating Station, and proceeded at a 7.82 percent decline below a buried gorge.

Maximum cover is up to 460 ft as it travels below the city to an intake in the Niagara River at the International Water Control Works. The structure is located about 1 mile upriver from the area’s famed Horseshoe Falls.

Ground conditions include a unique combination of limestone, dolostone, sandstone, shale and mudstone, with the vast majority (80 percent) of the tunnel initially situated in Queenston shale. The rock is non-abrasive and ranges from 2,100 to 26,000 psi UCS. The downgrades, as well as a planned steep upgrade at the intake, were taken in order to avoid a 328 ft deep, 1,300 ft wide buried canyon, known as St. David’s Gorge. Horizontal realignment of the tunnel was not possible due to existing water diversion tunnels already in place. The tunnel passes just 95 ft below the gorge at its lowest point.

Tough Ground

The Robbins TBM was launched in September 2006 following initial assembly at the jobsite. Robbins supplied a specialized supervisory and labor team, while Strabag supplied additional labor. The machine was assembled onsite, rather than in a manufacturing facility, less than 12 months from the date of contract signing using the process of Onsite First Time Assembly (OFTA).

Within the first 650 ft of tunneling, water inflows and the resulting drainage problems in the decline tunnel slowed excavation. Fines also clogged cutters and muck buckets until a foam spraying system was added, consisting of five openings in the cutterhead that allowed a mixture of foam, water and air to be plumbed in. The system brought progress up gradually to about 60 ft per day by April 2007.

After about 2,600 ft of excavation the TBM entered the Queenston shale formation. Large rock blocks fell from the crown before rock support could be placed, creating significant overbreak up to 10 ft above the cutterhead support. Cathedralling beyond the planned tunnel diameter was significant enough that a worker could stand on top of the roof shield.

Crews were required to perform systematic scaling to remove large quantities of loosened rock behind the lining. The volume and size of the scaling were beyond the design of the original ground support system, requiring a new strategy.

Modified Ground Support

Strabag ultimately designed a unique ground support system to cope with the geology. In extreme overbreak conditions, the system consisted of 30 ft long pipe spiles in an umbrella pattern at the crown of the tunnel. Using the new spiling method, overbreak was limited to about 3 ft above the normal tunnel diameter. Nearly 1,640 ft of very difficult ground was excavated using this method, at average rates of about 10 ft per day.

OPG and the contractor also opted to alter the vertical alignment of the tunnel. “We raised the alignment by 150 ft to bring the tunnel out of the Queenston shale and into more competent rock in order to reduce overbreak,” said Gschnitzer. After 6,500 ft, rock conditions were competent enough that spiling was no longer required. TBM progress has since significantly improved despite some continued overbreak from the tunnel crown.

In addition to spiling, the new plan required removal of several structures from the TBM. These included the ring beam erector, mesh erector, TBM roof fingers, work platforms, and muck lift buckets.

Workers then installed new structures to contain overbreak, including man baskets and a crown-mounted hoist for delivery of straps and mesh. A horizontal drill jumbo was added for spiling and rock scaling, while the controls for these machines were relocated farther back on the machine where they would be under fully supported sections of rock. Two invert cleaning conveyors were also added to deliver broken rock and debris from the invert to the TBM belt conveyor. A mini-excavator loaded the conveyors during invert cleaning. Changes were made incrementally as the TBM progressed.

A waterproof, continuous concrete lining is being installed in the upper two-thirds of the tunnel using a PVC-laying membrane carrier (red structure) and arch carrier (yellow structure).

The new ground support program, done for all excavated ground, consists of 10 to 13 ft long rock bolts, self-drilling (IBO) anchor bolts, steel straps, wire mesh and wire-reinforced shotcrete. Crews typically bore half a stroke, and then begin scaling down loose rock and installing rock bolts. After the full 6 ft stroke, the rest of the loose rock is scaled down before installing more rock bolts, wire mesh, steel straps and a layer of shotcrete. “We are very satisfied with the modified support system. It allows for tunneling in difficult ground while maintaining decent production rates,” said Gschnitzer.

Recent Obstacles and Successes

Obstacles and successes have marked the project throughout the drive. In July 2009, the Robbins machine set a record in its size class of 46 to 49 ft after excavating 1,535 ft in one month. The achievement came after revisions to the initial ground support system had been completed. “There were excellent ground conditions in the month of July with no overbreak – these were the main factors in the high advance,” said Gschnitzer. The lowered tunnel alignment left the crown of the tunnel in more stable Whirlpool sandstone.

By September 2009, crews had stopped the machine following a 130-cu yd, 300-ton rock fall about 1.2 miles behind the TBM. No workers were injured, and investigations found the collapse was caused by an old structure that was intersected. The 20-year-old borehole had collapsed after deterioration, requiring significant overbreak repair. Repairs included grouting the borehole and applying standard overbreak restoration procedures, including 13 ft rock bolts, 20 ft self-drilling (IBO) anchor bolts, and wire-reinforced shotcrete from a rock ramp built to gain access to the crown by filling the bottom half of the tunnel. A planned maintenance outage was combined with the repairs, and the machine began boring again in December 2009.

As of April 2010, the machine was operating in a mixed face, with the upper half of the tunnel in 7,300 psi UCS Power Glen Shale, and the lower half down to the tunnel invert in 29,000 psi Whirlpool Sandstone. “Vibrations in the mixed face are causing some wear in the machine parts and cutters. We are happy with the current rock conditions, however, as we haven’t been short of challenges in the past. Invert concreting has reached 3 miles, and we are currently starting up our arch lining operations,” said Gschnitzer.

Current advance rates are topping out at more than 80 ft per day, with monthly advance ranging between 980 and 1,300 ft depending on the amount of overbreak. By the end of April the machine had completed approximately 4.2 miles, or about 67 percent, of the tunnel length.
Simultaneous lining is taking place behind TBM tunneling using specialized arch and invert carriers. The invert structure is cast and set under a 285 ft long bridge equipped with two working windows – a setup that allows rubber-tired supply vehicles to travel over the installation area. Behind the invert casting operations is the membrane carrier, which began the arch lining process in April 2010. The setup is used to install polyolefin water-proofing membrane to the upper two-thirds of the tunnel. “A machine applies the rolls of membrane, while the rest of the carrier is used for logistics, equipment, and material storage,” said Pedro Rojas of Strabag AG.

Behind the membrane carrier is the 1,150 ft long arch shutter carrier, due to start up in May. The arch carrier allows workers to install cast-in-place concrete in the upper reaches of the tunnel, and is capable of moving up to 82 ft per day.

Moving Forward

The 47.2 ft diameter Robbins Main Beam TBM is the world’s largest hard rock tunnel boring machine.

“This project is important for the tunneling community to know about. It is unique, not only in terms of diameter for an open gripper TBM, but also for its difficult ground conditions and logistical challenges operating four distinct processes from one tunnel portal,” said Gschnitzer.
The geological conditions have also spurred improvements in large diameter TBM design and ground support. “We have learned a lot in the last few years on how to install ground support more effectively, from both Niagara and other worldwide projects. This is an area of our TBM design that we have given particular focus,” said Lok Home, Robbins President.

New Robbins Main Beam TBMs are now being designed without roof shield fingers for high cover tunnels. Ground support programs in large diameter (greater than 26 ft) tunnels are also moving away from complete ring beams in favor of steel straps with rock bolts. Two new 33.5 ft TBMs, for example, were launched at China’s West Qinling tunnels in spring 2010. The rail tunnels are located below over 3,300 ft of soft sandstone and phyllite rock. The machines do not have roof fingers, and instead come equipped with mesh pockets for installation of wire mesh panels beneath the safety of the roof shield. “We need to have the lessons learned at Niagara and other large diameter projects transmitted to future tunnel designers. The specification of ground support by ring beams in large diameter tunnels certainly is called into question based on recent experience,” said Home.

Desiree Willis is a technical writer for The Robbins Company based in Kent, Wash.