Tunnel boring machine

A tunnel boring machine (TBM), also known as a "mole", 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). TBM-bored tunnel cross-sections extend up to 17.6 meters (58 ft) (through June 2023). TBM tunnels are typically circular in cross-section. Specialized tunnel boring machines have been developed to excavate non-circular tunnels, including rectangular and horseshoe-shaped profiles. Much narrower tunnels are typically bored using trenchless construction methods or horizontal directional drilling rather than by TBMs.

TBMs limit disturbance to the surrounding ground and produce a smooth tunnel wall, which reduces the cost of lining the tunnel and allows for tunneling in urban areas. Large TBMs are expensive and challenging to construct and transport, and have fixed costs which become less significant for longer tunnels. Tunneling speeds generally decline as tunnel size increases, but tunneling speeds using TBMs have nevertheless increased over time. 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.