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 microtunneling. They can bore through anything from hard rock to sand. Tunnel diameters can range from a meter (done with micro-TBMs)to 19.25 meters to date. Tunnels of less then a meter or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather then TBMs.
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 of limiting the disturbance to surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost lining the tunnel, and makes suitable in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. However, as modern tunnels become longer the cost of tunnel boring versus drill and blast is actually less. This is because tunnelling with TBMS is more efficient and results in shortened completion times (when they operate successfully).
Tunnel Borers dig underground to make tunnels, usually for trains and trams to go through. They can go through some of the hardest rock in the world! Fact: Some people even call it a "mole"!