A tunnel boring machine (TBM), also known as a “mole”, is a machine used to excavate tunnels. It tunnels through literally anything, even mountains, hard rock, wet or dry soil, or sand. TBMs have been customized to work with each material. TBM-bored tunnel cross-sections range from 1m (3.3 ft) to 17.6m (58 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 can be customized for u-shaped, horseshoe, square or rectangular tunnels
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 difficult to transport. These up-front costs become less significant for longer tunnels. Tunneling with TBMs is much more efficient and shortens completion times. TBMs are not used for tunnels in heavily fractured and sheared rock.
