A tunnel boring machine also known as a mole, is a machine used to excavate tunnels with a circular cross section through a variety of soil & rock strata.
They can bore through hard rock, sand, & almost anything in between.
Tunnel diameters can range from a metre done with micro-TBMs to almost 16 metres to date.
Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs. Tunnel boring machines are used as an alternative to drilling & blasting methods in rock & conventional hand mining in soil.
TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall.
This significantly reduces the cost of lining the tunnel & makes them suitable to use in heavily urbanized areas.
The major disadvantage is the upfront cost. TBMs are expensive to construct, & can be difficult to transport.
However, as modern tunnels become longer, the cost of tunnel boring machines versus drill & blast is actually less this is because tunnelling with TBMs is much more efficient & results in a shorter project. The largest diameter TBM, at 15.43 m, was built by Herrenknecht AG for a recent project in Shanghai, China.
The machine was built to bore through soft ground including sand & clay.
The largest diameter hard rock TBM, at 14.4 m, was manufactured by The Robbins Company for Canada’s Niagara Tunnel Project.