A tunnel boring machine (TBM), also known as a “mole”, is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They may also be used for micro tunneling. They can bore through anything from hard rock to sand. The head of a tunnel boring machine which has the rotating head and discs will press against the rock at the sides and exert a force on the rock, which allows the discs to chip at the solid rock. This falls on to buckets that take it to the conveyor belt that has been placed behind the machine and which conveys the rock to the tunnel face for further removal.
Tunnel diameters can range from one meter (3.3 ft) (done with micro-TBMs) to 17.6 meters (58 ft) to date. Tunnels of less than a meter or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs.
The History of Tunnel Boring Machines
Tunnel boring machines first started as tunneling shields first used in the Thames Tunnel in 1825. Digging was still done in the traditional way after the shield had pulverized the rock. An actual boring machine using a number of percussion drills was made in 1845 for a tunnel between France and Italy. Machines were made in the United States even in those days but rarely survived to be granted permanent status as a reliable method of tunnel excavation.
The first successful tunneling shield was developed by Sir Marc Isambard Brunel to excavate the Thames Tunnel in 1825. However, this was only the invention of the shield concept and did not involve the construction of a complete tunnel boring machine, the digging still having to be accomplished by the then standard excavation methods.
The first boring machine reported to have been built was Henri-Joseph Maus’s Mountain Slicer. Commissioned by the King of Sardinia in 1845 to dig the Fréjus Rail Tunnel between France and Italy through the Alps, Maus had it built in 1846 in an arms factory near Turin. It consisted of more than 100 percussion drills mounted in the front of a locomotive-sized machine, mechanically power-driven from the entrance of the tunnel. The Revolutions of 1848 affected the funding, and the tunnel was not completed until 10 years later, by using less innovative and less expensive methods such as pneumatic drills.
In the United States, the first boring machine to have been built was used in 1853 during the construction of the Hoosac Tunnel in northwest Massachusetts. Made of cast iron, it was known as Stone-Cutting Machine. It drilled 10 feet into the rock before breaking down. This machine anticipated modern TBMs in the sense that it employed cutting discs, like those of a disc harrow, which were attached to the rotating head of the machine. In contrast to traditional chiseling or drilling and blasting, this innovative method of removing rock relied on simple metal wheels to apply a transient high pressure that fractured the rock.
In the 1870s, John D. Brunton of England built a machine employing cutting discs that were mounted eccentrically on rotating plates.
The first TBM that tunneled a substantial distance was invented in 1863 and improved in 1875 by British Army officer Major Frederick Edward Blackett Beaumont (1833–1895).
During the late 19th and early 20th century, inventors continued to design, build, and test TBMs in response to the need for tunnels for railroads, subways, sewers, water supplies, etc.
The 1950s brought in a dam diversion project that required extensive tunneling work through shale. This is where James S. Robbins, who was the founder of the Robbins Company, entered the picture and made his first machine that was able to cut to a depth of 160 feet in a full day and night of work. His first machines used steel picks rotating in a circle, but these broke frequently and required replacement. This is when James Robbins came up with the idea of a rotating head mounted with disc cutters, an idea that persists to this day with refinements coming only in the propelling, thrusting, and rotating mechanisms, developments more to do with the stronger and harder materials now available. Cutting rates are now as much as 4000 feet in a month, and diameters for tunnels can be as high as 60 feet.
Today’s, TBMs are used as an alternative to drilling and blasting (D&B) methods in rock and conventional “hand mining” in soil. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas.
The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. The longer the tunnel, the less the relative cost of tunnel boring machines versus drill and blast methods. This is because tunneling with TBMs is much more efficient and results in shortened completion times, assuming they operate successfully. Drilling and Blasting however remains the preferred method when working through heavily fractured and sheared rock layers.
Different types of TBMs
Modern TBMs typically consist of the rotating cutting wheel, called a cutterhead, followed by a main bearing, a thrust system and trailing support mechanisms. The type of machine used depends on the particular geology of the project, the amount of ground water present and other factors.
In soft ground, there are three main types of TBMs: Earth Pressure Balance Machines (EPB), Slurry Shield (SS) and open-face type. Both types of closed machines operate like Single Shield TBMs, using thrust cylinders to advance forward by pushing off against concrete segments.
Earth Pressure Balance TBM (EPB) known as Bertha with a bore diameter of 17.45 meters (57 ft 3 in) was produced by Hitachi Zosen Corporation in 2013. These machines are used in soft ground with less than 7 bar of pressure. The cutterhead does not use disc cutters only, but instead a combination of tungsten carbide cutting bits, carbide disc cutters, drag picks and/or hard rock disc cutters. The EPB gets its name because it uses the excavated material to balance the pressure at the tunnel face.
Slurry Shield TBMs are needed in soft ground with very high water pressure or where ground conditions are granular (sands and gravels) so much so that a plug could not be formed in the Archimedes screw. The cutterhead is filled with pressurised slurry which applies hydrostatic pressure to the excavation face. The slurry also acts as a transport medium by mixing with the excavated material before being pumped out of the cutterhead back to a slurry separation plant, usually outside of the tunnel. Slurry separation plants are multi-stage filtration systems, which remove particles of spoil from the slurry so that it may be reused in the construction process. The limit to which slurry can be ‘cleaned’ depends on the particle size of the excavated material. For this reason, slurry TBMs are not suitable for silts and clays as the particle sizes of the spoil are less than that of the bentonite clay from which the slurry is made. In this case, the slurry is separated into water, which can be recycled and a clay cake, which may be polluted, is pressed from the water.
Open TBMs have no shield, leaving the area behind the cutterhead open for rock support. To advance, the machine uses a gripper system that pushes against the tunnel walls. Not all machines can be continuously steered while gripper shoes push on the walls, as with a Wirth machine, which only steers while ungripped. The machine will then push forward off the grippers gaining thrust. At the end of a stroke, the rear legs of the machine are lowered, the grippers and propel cylinders are retracted. The retraction of the propel cylinders repositions the gripper assembly for the next boring cycle. The grippers are extended, the rear legs lifted, and boring resumes. The open-type, or Main Beam, TBM does not install concrete segments behind with other machines. Instead, the rock is held up using ground support methods such as ring beams, rock bolts, shotcrete, steel straps, ring steel and wire mesh.
Open face TBMs in soft ground rely on the fact that the face of the ground being excavated will stand up with no support for a short period of time. This makes them suitable for use in rock types with strength of up to 10MPa or so, and with low water inflows. Face sizes in excess of 10 meters can be excavated in this manner. The face is excavated using a backactor arm or cutterhead to within 150mm of the edge of the shield. The shield is jacked forwards and cutters on the front of the shield cut the remaining ground to the same circular shape. Ground support is provided by use of precast concrete, or occasionally SGI (Spheroidal Graphite Iron), segments that are bolted or supported until a full ring of support has been erected. A final segment, called the key, is wedge-shaped, and expands the ring until it is tight against the circular cut of the ground left behind by cutters on the TBM shield. Many variations of this type of TBM exist.
While the use of TBMs relieves the need for large numbers of workers at high pressures, a caisson system is sometimes formed at the cutting head for slurry shield TBMs. Workers entering this space for inspection, maintenance and repair need to be medically cleared as “fit to dive” and trained in the operation of the locks.
Hard rock TBMs
In hard rock, either shielded or open-type TBMs can be used. Hard rock TBMs excavate rock with disc cutters mounted in the cutterhead. The disc cutters create compressive stress fractures in the rock, causing it to chip away from the tunnel face. The excavated rock (muck) is transferred through openings in the cutterhead to a belt conveyor, where it runs through the machine to a system of conveyors or muck cars for removal from the tunnel.
In fractured rock, shielded hard rock TBMs can be used, which erect concrete segments to support unstable tunnel walls behind the machine. Double Shield TBMs have two modes; in stable ground they grip the tunnel walls to advance. In unstable, fractured ground, the thrust is shifted to thrust cylinders that push against the tunnel segments behind the machine. This keeps the thrust forces from impacting fragile tunnel walls. Single Shield TBMs operate in the same way, but are used only in fractured ground, as they can only push against the concrete segments.
The world’s largest hard rock TBM, known as Martina, was built by Herrenknecht AG from the Italian Toto company.
Today a serious TBM often turns in a production rate of 4,000 ft/month. The newer models come with automatic lining installers, with a robot arm that picks up pre-cast lining segments and clicks them into place around the freshly created interior walls like so many Lego bricks. Such machines integrate all the functions of tunneling into a single device. They can cut through almost any kind of rock and often carry high-tech imaging devices that allow them look ahead. Some are more than forty feet in diameter.
There are now probably around 120 of these splendid machines working at any one time around the world. This number reflects not only the domination of tunneling by TBMs but the increasing reliance of the society on the underground’s virtues: primarily the huge amount of space down there. Almost every conflict between surface uses (for instance, between widening a highway and open space) can be dissolved by putting one of the proposed uses underground. You never run out.
The development of TBMs is nowhere near to plateauing. Right now TBMs are all hand-made to specific project specs and geologies. At some point in the near future the industry expects to achieve the holy grail of TBM design — a universal machine, powerful and versatile enough to handle any job. This will lower costs both by standardizing manufacturing design and improving the market for used machines. Improvements in materials science will soon allow cutter wheels and their bearings to be built of a supertough and microscopically flawless material that will allow the machines to run for hundreds of miles between pit stops. Finally, it should soon be possible to control the machines from the surface, eliminating all the expensive features and procedures now needed to keep humans safe. Small tunnels are already dug this way.
These improvements will open a new era of big dreams — a Korea- Japan tunnel, a Taiwan-Chinese mainland tunnel — but the biggest dream of all is the world subway. A maglev in an evacuated tunnel can crank to any speed you like: thousands of miles of hour or more. You can’t do this in an airplane; the costs of fighting all that air resistance would be intolerable, and even if they weren’t, surface communities would not accept a daily bombardment of dozens of sonic booms. (Underground trains are a lot harder to hit with a shoulder-fired missile, too.) In a hundred years we might have built a system in which every major city in the world would not be more than an hour’s trip from any other. Nothing would be more appropriate at its dedication than a stirring delivery of Whitman’s poem.
James Robbins made his first machine for boring tunnels for a dam diversion project in the 1950s. He developed the technique to make rotating heads and disc cutters a technology that exists even today. A tunnel boring machine is a safer, faster method of making tunnels than the earlier techniques.