TUNNEL BORING MACHINE / REF / 4 / 2014

      

 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 can bore through anything from hard rock to sand. Tunnel diameters can range from a metre (done with micro-TBMs) to 19.25 m to date. Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs.

Tunnel boring machines 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.  However, as modern tunnels become longer, the cost of tunnel boring machines versus drill and blast is actually less—this is because tunneling with TBMs is much more efficient and results in a shorter project.

Modern TBMs typically consist of the rotating cutting wheel, called a cutter head, 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 amoun t of ground water present and other factors .

 The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. However, as modern tunnels become longer, the cost of tunnel boring machines versus drill and blast is actually less—this is because tunneling with TBMs is much more efficient and results in a shorter project. The largest diameter TBM, at 19.25 m, was built for a recent project in Orlovski Tunnel, St.Petersburg. The machine was built to bore through soft ground including sand and clay. Herrenknecht AG built also is the world's largest diameter hard rock TBM: "Martina" (excavation diameter of 15.62 mt, total length 130 mt; excavation area of 192 square mt, thrust value 39,485 t, total weight 4,500 tons, with total installed capacity of 18 MW;with yearly energy consumption of about 62,000,000 kWh).And the former largest diameter hard rock TBM, was 14.4 m.The machine was used to bore a hydroelectric tunnel beneath Niagara Falls. The machine has been named "Big Becky" in reference to the Sir Adam Beck hydroelectric dams to which it is tunnelling to provide an additional hydroelectric tunnel.

History

The first successful tunnelling 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' 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. Made of cast iron, it was known as Wilson's Patented Stone-Cutting Machine, after inventor Charles Wilson. It drilled 10 feet into the rock before breaking down. The tunnel was eventually completed more than 20 years later, and as with the Fréjus Rail Tunnel, by using less ambitious methods. The first successful use was on the Oahe Dam in 1952 by James S Robbins.

By the middle of the 19th century Western civilization was at the peak of its intoxication with railroads. Everywhere dreamers were bent over maps, drawing lines. As Walt Whitman wrote ...

Lo, soul! seest thou not God’s purpose from the first?
The earth to be spann’d, connected by net-work,
The people to become brothers and sisters,
The races, neighbors, to marry and be given in marriage,
The oceans to be cross’d, the distant brought near,
The lands to be welded together.

Alas, over and over these lines, and therefore these dreams, would bump into a mountain, or worse, a mountain chain, such as the Alps. Building over or around these obstacles was expensive and time- consuming. Logical minds yearned to poke straight through. Alas again, that meant tunneling, and the bill for tunneling was even higher. Tunneling by its nature offered only a tiny working area, not much larger than a bed sheet. Only a handful of people could labor within it at any one time, and much of every shift was wasted wrestling tools and work product out of each other's way. A project in Massachusetts at this time took twenty years to go five miles, and while that was worse than average, the famous formula T(ime) = M(oney) made tunneling costs appalling everywhere.

Bright engineers on two continents had the same thought: maybe their grandfathers would have had to tolerate such frustrations, but this was the 19th century, the acme of human progress, when science and engineering could be counted on to triumph over every sort of obstacle. The fix was obvious: just build a big machine. Amplify the power of labor. Bring the industrial revolution underground. How hard could it be?

The first to step up was a Belgian engineer named Henri- Joseph Maus, who in 1845 got the King of Sardinia to approve construction of the first railroad connecting France and Italy. Maus had an international reputation in mining engineering and the self- confidence to match. He shrugged off the idea of running a line up and over a pass, insisting that the right idea was to go straight through, specifically, through Mt. Frejus, near the famous pass at Mt. Cenis.

This must have raised eyebrows. A tunnel following the route Maus had in mind would have stretched for 40,000 feet, a highly implausible distance given the technology of the time. In this era the tunneling cycle ran as follows: drill holes in the face, pack them with gunpowder, light the fuse, run around a corner, wait for the explosion, run back carrying bracing timbers, hope you could hammer them into place before you got killed in a roof collapse, and shovel or toss the rock fragments into carts for removal.

The problem was that detonating gunpowder in a confined space saturated it with toxic fumes, so all this activity depended upon sucking the air polluted by the previous blast out of the tunnel in a reasonable length of time. Maus' tunnel was way too long for the ventilating technology of the

Description

Modern TBMs typically consist of the rotating cutting wheel, called a cutter head, 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.

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