The art of digging a hole

Nov. 22, 2001
Digging tunnels has become easier and safer thanks to custom-designed tunnel-boring machines.

The 18.5-ft-diameter TBM nicknamed "Sandie" breaks through on one of the London Ring Main water tunnels, a project that consists of almost 50 miles of tunnels.

The 48-ft-diameter Groene Hart TBM is being built by NFM Technologies.

A Lovat hard-rock TBM uses an expanding thrust ring to anchor the machine and to provide a base from which thrust cylinders can push off of and drive the cutting head forward.

Richard and Rick P. Lovat, president and vice president of Lovat Inc., stand in front of the machine their company built to dig a subway line under Lisbon, Portugal. The 1,200-ton machine uses 1,800 kW to turn the cutting head.

A central curtain wall will separate the two rail tracks in the Groene Hart Tunnel. Openings in the upper part of this wall can be closed in case of fire. These windows can also be adjusted to compensate for changing air pressures as trains pass, making passengers and crew more comfortable.

A technician from Wirth Tunneling Ltd. in Scotland examines the grooves in hard rock made by a TBMs cutting discs.

Tunnel-boring machines (TBMs) could be the ultimate power tool. They're monstrous, weighing millions of pounds and exerting forces measured in thousands of tons, and they can dig through some of the hardest rock on earth. Plus, most come with an entire railroad and train as ancillary equipment. What more could power-loving handymen want?

In some cases, such as the English Channel Tunnel, the machines are so large and built to such unique specs that once they finish the job, they are stripped of usable equipment and abandoned underground. Companies also refurbish and resell some TBMs, usually the more standard sized machines with between 6.5 and 13-ft diameters. But for most projects, TBMs are custom built. As one TBM manufacturer says, "Every one of our machines is a prototype." One of the most important criteria in designing a machine is determining what it will be digging through.

In hard rock
Hard-rock TBMs are simpler than those designed for soft ground because the tunnel, once dug, supports itself. There's no need for liners to prevent the tunnel from caving in, and the TBM can push off the walls to supply forward pressure. Of course there are limits. Rock with compressive strengths greater than 30,000 psi, such as dense quartzites that have strengths to 60,000 psi, are still more economical to drill and blast through than to excavate with TBMs.

At the front of the TBM is the cutting head, a rotating drum with free-wheeling cutter discs mounted on the flat face. Cutter discs, an invention of The Robbins Co., Solon, Ohio, have a tungsten-carbide bit or rim that circles a hardened tool-steel wheel. Discs are spaced about 3 in. apart and in a pattern that covers the tunnel face. The wheels are turned by friction as the cutting head rotates and hydraulic cylinders push from behind. This thrust is a function of the number and size of the cutting discs.

The thrust a TBM can place on a disc goes up with the square of its diameter, according to John Turner, chief and the square of its diameter, according to John Turner, chief engineer at Robbins. "And while the largest discs were once only 12 in., we now use mostly 17 and 19-in. discs," The 17-in. discs are rated at 60,000 lb each, while 19-in. discs can handle 70,000 lb. "But individual cutters can see impulse loads 10 times their nominal ratings," notes Turner.

"How the discs actually work is not well understood," adds Turner. "We think the discs spall the rock as they turn, much like a glasscutter scores glass. The discs create circular crush zones. Cracks spread from one crush zone to the next, and chips then fall out. So the machines actually crush 20 to 30% of the rock, and the chips come for free."

Discs speed, measured in peripheral velocity, has increased over the years as bearings have improved. Speed was once limited to 300 ft/min, but now the discs travel up to 650 ft/min. For an 11.5-ft cutting head, 650 ft/min translates into 18 rpm, and with discs penetrating at 0.125 to 0.5 in./rev, TBMs can tunnel at between 11 and 45 ft/hr.

Discs are checked daily and changed about every 500,000 to 1,500,000 rolling feet, depending on the rock's hardness and abrasiveness. Worn cutters have their ring-shaped bits replaced. Today's tunnelers have space behind the cutting head that allows maintenance and cutter replacement while the TBM is still up against the tunnel face.

Adjustable screens on the face of the cutting head determine how much excavated rock, commonly called muck, passes behind the head. A screw conveyor or scraper blades inside the cutter head lifts muck onto a conveyor that passes through the main bearing. The belt takes the muck out of the tunnel or transfers it into a rail car.

The simplest hard-rock machines use a Kelly, a vertical beam that extends and presses against the tunnel ceiling and floor to anchor the TBM. The machine's main beam, which holds the cutting head and main bearing, goes through the Kelly and uses it as a thrust pedestal. The beam's thrust cylinders push off of the Kelly and press the cutter head against the rock face. There can be up to 10 thrust cylinders, with strokes of 4 to 25 ft. When the thrust cylinders reach full extension, the Kelly is relocated and the process repeated — stroke, reset, stroke, reset — moving as much as 0.6 miles per month.

Kelly-based TBMs let the cutting head move only fore and aft. The machines cannot be steered while operating. So if one runs into a particularly hard rock on the left side of the tunnel face, the head and rest of the TBM will want to bend, but can't. This generates huge bending loads on the structure, which tend to destroy the TBM.

Robbins refined the Kelly approach and puts grippers directly on the main beam. "We mount grippers on trunnions to give us six degrees of freedom between the machine and grippers," says Turner. "It is fully floating, so an operator can steer up, down, left, right, and roll the TBM while its boring." Grippers push outward with between 2.4 and 2.8 times the total forward thrust placed on the main beam. (Total applied thrust is calculated by multiplying the number of cutter discs times the thrust each can handle, then adding the weight of the machine, plus a friction factor to account for contact between the TBM and tunnel walls, and an additional safety factor.)

"Compared to a Kelly machine, the load paths with floating grippers are more efficient, but running them takes more operator skill," adds Turner.

There are hard-rock tunnels that require lining, such as those built for commuter trains or highways. No one wants to risk a stray rock falling on the tracks or through a windshield. For such jobs, some companies build single-shielded TBMs. On these, a cylindrical, metal covering or shield surrounds the hardware behind the cutting head. Behind the shield, a liner-erecting system carefully attaches precast concrete or steel liners around the tunnel's circumference. Thrust jacks on the back edge of the shield push off of the last liner ring to press the cutting head into the soil. The problem with this setup is that it uses liners for thrust, which means you can't bore while setting liners, and this wastes time.

"This led to the double-shielded TBM," says Turner. "These TBMs have a second shield right behind the cutting head. This second shield has grippers or a thrust ring that extend to the tunnel walls, so it doesn't rely on linings for thrust, so crews can set segments while boring."

These long, shielded TBMs are made easier to steer by putting articulations between shields, and, in some cases, on an additional tail piece. They let the shields move 2 to 3° and make the TBM less likely to get "iron bound," a condition in which the shield binds on the lining when the operator changes course. Iron-bound TBMs are trapped and cannot move fore or aft, presenting construction crews with few alternatives, none of them inexpensive or quick.

TBMs can make turns, but they're not exactly the hairpin variety. By carefully applying more pressure on one side of the cutting head, the machines can change course by about 0.125 in./ft. "A typical radius for a turn is between 300 and 400 ft," says Turner. "But we build special TBMs if jobs call for tighter turns. The tightest radius we ever dug was a 90° turn in 75 ft in a South African gold mine. One of our TBMs also dug the CERN particle accelerator in Europe, which is an ellipse. Its tunnel constantly turns at a constantly changing rate. Fortunately, it is a large ellipse with a seven-mile diameter."

In soft ground
"In hard rock, the difficulty is in excavating material at the face. The walls are self-supporting. In soft ground, however, the difficulty is not in excavating the face but in supporting the tunnel," says Marco Giorelli, product manager at Lovat in Etobicoke, Canada. Lovat specializes in soft-ground and mixed face TBMs, but has built machines for all types of ground conditions, including hard rock.

Like hard-rock machines, soft ground TBMs use a rotating cutting head, but it needs much less forward thrust, and doesn't use cutting discs. Instead, the head carries rippers and spade teeth. Rippers penetrate and excavate soil, while spade teeth collect and move it.

But soft-ground TBMs have to support the tunnel face, as well as the tunnel roof and walls, until the crew can erect liner segments that hold back the surrounding earth. Without this constant support, the TBM might create an " overexcavation," a situation in which too much ground or rock is excavated. "I've seen overexcavations open up 300-meter tall caverns over the TBM, and all that dirt fell right on the machine," says Turner. "And the cave-in can go all the way to the surface, which is a real disaster. Farmers get irritated when their cow sheds disappear."

"Overexcavations can be particularly harmful in cities," notes Giorelli. "They lead to settlement, and it doesn't take much settling to damage buildings."

Lovat uses muck as a pressurized fluid in a semipressurized mode to hold up tunnel faces in some soft ground tunnels. Muck is pushed into a chamber behind the cutting head where it collects and builds up the pressure to support the face. A mucking ring holds muck in place until a door, the pressure-relieving gate, periodically opens to drop muck onto the conveyor.

If the tunnel face needs more support, earth-pressurized balance (EPB) TBMs are used. They use pressurized muck, along with the force from the cutting head. A screw conveyor removes muck while ensuring pressure inside the head doesn't drop. If the operator puts too much force on the head, muck collects and pushes up, toward the surface, creating heave. Push too little, and the surface settles. The operator monitors forward progress and soil pressures to determine the right level of force.

An alternative to EPB, one borrowed from the drilling industry, is to pump slurry in front and behind the cutting head to supply support. The slurry usually contains bentonite, a thixotropic material, i.e., one that flows under pressure but solidifies when it stands still.

Some projects must go through "squeezing ground," in which the tunnel shrinks, pressing in with up to 300 tons/square meter, according to Turner. The trick, he says, is to keep moving and not let the ground trap the machine. To help, companies build cutting heads that expand mechanically to bore tunnels slightly larger than the barrel of the TBM. Cutting heads are also designed with cutting discs closer together at the outer edge and pointing outward to make a hole larger than the TBM.

"As you can see, it pays to know what kind of ground you are going through," Turner says. "If a hard-rock machine comes across soft ground, it's stuck because it can't push off the tunnel walls unless it is also erecting a suitable lining behind it. The crew will have to dig around the TBM and use grout and cement to build up the walls until they can take the pressure. Otherwise, that TBM isn't going anywhere." To further illustrate the value of knowing what you will be digging through, he talks of an infamous irrigation tunnel in Yacamb, Venezuela. "They started work on it 25 years ago and came across squeezing ground. Last I heard, they abandoned the project."

Choosing a power source
Some companies, like Lovat, use hydraulics to power TBMs. "They have high torque at low speeds, which is just what we want for tunneling in soft ground" says Giorelli. "In some instances, hydraulics also reduce the risk of electric problems in the wet environment found underground."

At Robbins, the engineers' experience is in electric drives. They've found variable-frequency control with pulse-width modulation lets electric drives behave much like hydraulic ones. "We routinely use three-phase ac squirrel-cage motors coupled through a clutch to a 30:1 gearbox. The clutch prevents repeated starting and stopping of motors and lets us start them one at time," says Turner. "We can put a dozen 315-kW motors on a TBMs cutting head. Bringing them all on line at once draws a huge load and really gets the attention of your electric company. And the PLC-controlled motors are built to standards that prevent leakage and provide ground-fault protection, so safety is not a large problem. Hydraulics, on the other hand, consume a lot of space, they're inefficient, and can contaminate the site."

One recent advance in soft-ground tunneling is soil conditioning. Construction crews inject soapy foams or long-chain polymers to plasticize sand and other loose soils, making them behave like clay, according to Lovat's Giorelli. They also inject water into clay to make it more manageable. And if crews suspect an underground lake or stream ahead, they check with forward probing drills. If they discover too much water, they pump grout and concrete into area to build up the ground and make it water tight, then tunnel through it.

Keeping it safe
When projects call for inclined tunnels, crews prefer to start at the bottom and drill uphill. It stops water from pooling at the drilling end of the tunnel and makes it easier for the muck-moving system to takes loads downhill. Of course the TBMs need more grip and thrust to counter gravity, as well as strong backstops to ensure the TBM doesn't come sliding out the tunnel.

"Four-percent grades are the limit for most standard TBMs due to the pulling limits of trains," says Giorelli. "But we've made machines for much steeper tunnels. We built a penstock for feeding water into a hydraulic power station at Sion in Switzerland. It has a 35° slant. The TBM cleared about 15 ft/day, a rate that would be about 100 ft/day if it were boring horizontally."

Taking TBMs to the vertical limit, one company worked with the government during the Cold War to build a machine that could bore straight up. The plan was to have machines resting atop the shaft, ready to drill through to the surface. Then, in case of war, TBMs would breakthrough to the surface and the shaft would serve as a missile silo, one that could not be destroyed before launching its missile.

Other safety features on TBMs include gas detectors that sniff out dangerous gases, such as methane. If gas is detected, more air is pumped into the tunnel to dilute it. There are also fire suppression systems for the electrical cabinets and hydraulic reservoirs. The hydraulic reservoir also has a nitrogen accumulator that maintains system pressure in case the hydraulics fail to lock doors and conveyors.

A soft-ground, earth-pressure-balanced TBM from Lovat was built to dig 18.8-ft-diameter subway tunnels in Kazan, Russia. It uses a 31-ft screw conveyor to move dirt from the cutting head to conveyor, and can deploy bentonite or foam to condition the soil ahead of the TBM. The 409-ton machine is 230 ft long and has a total thrust of 4,400 tons. The lining it erects is made up of seven pieces and a keystone, each about 3 ft long and 0.75-ft thick.

This 24-in. hard-rock boring unit from The Robbins Co. is pushed forward with 90,000 lb of force and turned with 7,800 ft-lb of torque when tunneling underground.

Not all TBMs are built to drill railroad tunnels through mountains or to put multilane highways under rivers. Some companies, such as Akkerman Inc. in Brownsdale, Minn., specialize in microtunnelers, TBMs that measure as little as 4 in. in diameter. These TBMs are commonly used to lay pipe, also called pipejacking, under city streets, runways, and buildings. With TBMs, there's no need to dig a trench that would disrupt surface traffic, hence this method has been dubbed "trenchless technology."

In pipejacking, remotely controlled boring machines attach themselves with expanding grippers into the end of a pipe. The boring head excavates, while dirt enters the head through adjustable doors and is whisked away on conveyor belts or screws, or mixed with water and pumped out. The entire pipe, which acts as a liner, is hydraulically "jacked" forward, pushing the cutting head into new dirt. In pipes up to a foot in diameter, the jacks exert up to 20 tons of thrust.

Pipe sections are welded onto the original pipe to make a continuous liner. The finished pipe can carry sewer and gas lines, as well as utility and telecommunications cables and fiber optics. The pipe must withstand the jacking forces, so it is commonly built of concrete with lots of steel reinforcement, steel, or a sand and polyester composite.

A carefully set-up laser beam passes through light-sensitive arrays and projects a steering beam onto the face of the excavation. Signals from the arrays are used to calculate TBM position, pitch, yaw, and roll. The pilot, usually on the surface, monitors these readings and makes corrections. In simple mini-TBMs, the pilot controls which section of the face gets excavated and, therefore, which direction the small TBM will go. In more sophisticated versions, the TBM is articulated and the front end can move 5° or less in any direction. The pilot directs the TBM along the proper path by cutting head pressure, putting more on one side than the other.

In larger tunnels, up to 168 in. in diameter, microtunnelers act like their larger cousins, erecting liner segments as they dig through soft ground, and pushing against liners to hydraulically advance the cutting head.

Trenchless technology is slightly more expensive than digging a trench, unless requirements call for placing pipes or utility lines deeper than usual or through extremely wet ground. "But it is still a viable alternative to tearing up the streets and gridlocking traffic," says Carl Neagoy, an Akkerman executive. "City engineers don't include the social costs of wasted time and fuel, and road rage, that digging trenches cause, so TBMs always seem too expensive. But if those costs were included in engineering calculations, and the public was aware they didn't have to put up with all those inconveniences, TBMs would be used much more often to install utilities."

Tunnel diameter
Shield length
Shield weight
Length of support train
Cutting head power
Thrust force
Total power
Total weight
Rotation of cutting head
Max. rate of advance

48.8 ft
40.6 ft
1,800 ton
393 ft
3,500 kW
18,430 tons
9,540 kW
3,520 tons
1.4 rev/min
1.6 in./min

When the Thalys high-speed rail line north of Rotterdam is complete, Amsterdam, the northern end of the line, will be only 2 hr by train from Paris. On its way north of Rotteredam, the rail line passes into the Groene Hart (Green Heart), thousands of acres containing lowland canals, channels, and a countryside dotted with windmills. The area also houses an important bird sanctuary. All of this land is polder, ground reclaimed from the sea, and considered culturally and environmentally important to almost everyone in the Netherlands.

A design team, led by Bouygues Construction, quickly decided on tunneling to keep the environmental and visual impact to a minimum. They also chose to build a single, 43.6-ft-diameter tunnel rather than two 30.5-ft-diameter tunnels. The single-tunnel approach eliminates the need for costly safety passages every 985 ft connecting the two subterranean rail lines. Using just one tunnel also shrinks the infrastructure needed to direct traffic in and out of the tunnel, reduces the amount of material excavated, and means the project will finish sooner and cost less.

The tunnel will measure 4.4 miles long, with half-mile approaches at each end, and travel 100 feet below the surface. To dig the tunnel, NFM Technologies, a subsidiary of Framatome, is building the world's largest earth-pressure tunnel-boring machine. It will have a cutting head 48.8 ft in diameter. The TBM will erect precast concrete liners positioned to within millimeters as the tunnel is dug. During digging, pressure in front of and around the cutting head will be maintained at 4.5 bar to prevent the face and unlined walls from collapsing. When completed, 36,000 liners will form a seal that keeps the relatively shallow water table from mixing with the deeper one connected to the ocean. Environmental constraints on tunnel construction include noise limits of 55 dB, surface settlement of less than 0.78 in., excavation material containing less than 2% bentonite, a mining material used to maintain uniform pressures around the TBM, and absolutely no contamination of the upper water table. If all goes to schedule, the TBM will "breakthrough" in May 2004, and the railway will be completed May 2005.

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