The Tunnel Monster

Early next year, the Canadian government will begin a drilling project to harness more of its share of the Niagara River . The new tunnel will run along two existing tunnels beneath the river and will span over 41 feet in diameter and run 6.5 miles long, diverting water to an existing hydroelectric plant—eventually providing electricity for an additional 160,000 Ontario homes every year. A project of this size (for comparison, the Chunnel's internal diameter was 25 feet) requires the largest hard rock tunnel boring machine (TBM) in the world, which is being constructed specifically for the size demands of the Niagara Project. Used for excavating ground for subway systems, traffic tunnels, and water or sewage canals, TBMs quickly transform rock face into conveyor-sized chips that can then be excavated. In an industry where the staggering risk of tunnel collapse makes innovations in TBMs incremental rather than drastic, this TBM, with the largest diameter cutting mechanisms ever used for hard rock, will require over 15,000 engineering hours to design and more than 50,000 hours to assemble.

Specs:

Cost: More than $31 million. Size: 59 feet long; 2,000 tons. Speed: 10 feet an hour, working 24-7 until the project's complete. Manpower: on site, at least 230 full-time construction jobs. Made by: The Robbins Company.

Parts:

Cutterhead and disk cutters: The teeth of the machine are 85 solid steel blades, each nearly two feet long. The blades are mounted to a 47-foot-wide steel face, which rotates up to seven times a minute. The stationary cutters chew through rock by boring concentric circles, much like a glass cutter does, causing faults that make the rock crumble in everything from fine dust to fist-size chunks. The TBM will produce over 1,600 tons of broken rock per hour, enough to fill a 1-ton pickup truck load every 2.25 seconds. Workers will typically spend 4-6 hours per day just replacing the worn-down the cutter blades, an average of 12-15 needing replacement daily.

A door gives access to the backside of cutterhead, and there's also another door through the front of the cutterhead, a 24 inch-diameter circle for a worker to crawl through.

Rock removal: Muck buckets are built into the cutterhead to collect the cut rock pieces. The pieces of cut rock fall to the ground, where they are scooped up by muck buckets, which are built into the cutterhead. The buckets contain the muck until it's overhead, where it's dumped out into the muck chute (funnel shaped), and travels via a troughed conveyor belt through the main beam to the rear of the machine. There the rock pieces are transferred to the Backup muck handling system, which then removes it from the tunnel. Unlike many other kinds of TBMs, the rock is never mixed with slurry, but it is sprayed with water simply to keep the dust down.

Power: 15 motors supply the TBM's power, each contributing 425 horsepower, for a total of 6,375 hp. Powered by electricity, the TBM pulls essentially the world's longest extension cord behind it through the tunnel, a cable carrying its 13,800 volts.

Movement: The TBM moves forward like an inchworm, with the back propelling the front. But to stay supported throughout, different parts take turns moving or support the machine. First, the half-circle shaped steel gripper shoes push out horizontally on each side toward the tunnel walls, the convex side of the gripper facing out to match the curve of the tunnel wall. While they're being pushed out to the sides by 300 horsepower hydraulic cylinders, they're also being pushed backward by another set of cylinders; this fully extends the machine's spine. Then two rear legs come down to support and stabilize the TBM so that the gripper shoes can retract from the tunnel wall while moving forward on the TBM, to the position where it started. The gripper shoes expand again to support the TBM while the rear legs lift clear of the tunnel floor, pushing the TBM forward form the rear. The process repeats, advancing about ten feet per hour.

Securing the tunnel walls: As the machine excavates it leaves a roof of dangerous, unsupported rock. Workers operate two rock drills that are usually located behind the cutterhead support—while the cutterhead is moving, the rock drills bore a pattern of holes in the tunnel's roof. Then workers install 10' long tensioned bolts to help hold large blocks of rock in place. If you can prevent any rock from falling, the overhead rock will squeeze together over time and form a supporting arch. But if any rock falls, the holes will cause the other rock to shift, possibly causing a tunnel collapse. So if the rock is not stable enough for the cutterhead to continue boring, the drills are moved to the front of the machine. Then, they drill holes around the shape of the TBM 40 feet in front of it, retract the drills, and pump those holes with chemical grout, which solidifies the rock into a solid, protective canopy before the cutterhead moves forward.

Control Cabin: Located just behind the grippers, hanging underneath the main beam. The Machine Operator is the “captain” of sorts, who controls the machine with state-of-the-art touch-screen controls. He has a complete view of the TBM, which is fitted with several cameras. From the control cabin, the Operator can watch video monitors and control the closed circuit television system, as well as monitor the critical operating parameters (cutterhead speed, advance rate, power consumption, various temperatures and pressures). This information from the control cabin is continuously transmitted to computers in the offices above-ground at the site.

Side supports: One on each side, half-moon shaped similarly to the gripper shoes, the round side is moved in and out hydraulically to snugly press against the side wall of the tunnel, which keep the machine stable during the aggressive rock-boring. They also help with horizontal steering—if one side support is moved in and the other moved out, it will further push the TBM in one direction.

Backup: (not made by Robbins; being constructed in Switzerland ) Nearly 800 tons of backup equipment is pulled through the tunnel behind the TBM. It will include trailers for electrical, hydraulic, muck removal, and ground support systems, offices, operator stations, water, compressed air, and firefighting systems, and even a lunchroom.