Tunnel boring machine

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A tunnel boring machine (TBM), also known as a "mole", is a machine used to excavate tunnels. Tunnels are excavated through hard rock, wet or dry soil, or sand, each of which requires specialized technology.

Tunnel boring machines are an alternative to drilling and blasting (D&B) methods and "hand mining".

TBMs limit the disturbance to the surrounding ground and produce a smooth tunnel wall. This reduces the cost of lining the tunnel, and is suitable for use in urban areas. TBMs are expensive to construct, and larger ones are challenging to transport. These fixed costs become less significant for longer tunnels.

TBM-bored tunnel cross-sections range from 1 to 17.6 meters (3.3 to 57.7 ft) to date. Narrower tunnels are typically bored using trenchless construction methods or horizontal directional drilling rather than TBMs. TBM tunnels are typically circular in cross-section although they may be u-shaped, horseshoes, square or rectangular.^{[1][2][3][4][5][6]}

Tunneling speeds increase over time. The first TBM peaked at 4 meters per week. This increased to 16 meters per week four decades later. By the end of the 19th century, speeds had reached over 30 meters per week. 21st century rock TBMs can excavate over 700 meters per week, while soil tunneling machines can exceed 200 meters per week. Speed generally declines as tunnel size increases.^[7]