Tunnel boring machine

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A tunnel boring machine (TBM), also known as a "mole" or a "worm",^{[not verified in body]} is a machine used to excavate tunnels. TBMs are an alternative to drilling and blasting methods and "hand mining", allowing more rapid excavation through hard rock, wet or dry soil, or sand (although each requires specialized TBM technologies).^{[not verified in body]} TBM-bored tunnel cross-sections extend up to 17.6 meters (58 ft) (through June 2023).^[1] TBM tunnels are typically circular in cross-section, but may also be square or rectangular,^{[citation needed]} or U- or horseshoe-shaped.^{[2][3][full citation needed][4][better source needed]} Much narrower tunnels^{[clarification needed]} are typically bored using trenchless construction methods or horizontal directional drilling rather than by TBMs.^{[not verified in body]}

TBMs limit disturbance to the surrounding ground and produce a smooth tunnel wall, which reduces the cost of lining the tunnel;^{[not verified in body]} it also allows for tunneling in urban areas.^{[not verified in body]} Large TBMs are expensive and challenging to construct and transport,^{[not verified in body]} fixed costs which become less significant for longer tunnels.^{[not verified in body]} Tunneling speeds generally decline as tunnel size increases,^{[need quotation to verify]} but tunneling speeds using TBMs have nevertheless have increased over time;^{[citation needed]} the first TBM peaked at 4 meters per week,^[when?] which increased to 16 meters per week within four decades.^{[need quotation to verify]} By the end of the 19th century, speeds had reached over 30 meters per week.^{[need quotation to verify]} TBM speeds excavating through rock can, in the 21st century, reach over 700 meters per week, while soil tunneling machines can exceed 200 meters per week.^[5]