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 be designed to bore through anything from hard rock to sand. Tunnel diameters can range from one metre (3.3 ft) (done with micro-TBMs) to 17.6 metres (58 ft) 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. TBMs can also be designed to excavate non-circular tunnels, including u-shaped or horseshoe and square or rectangular tunnels.
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. 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.
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