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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 can bore through anything from hard rock to sand. Tunnel diameters can range from a metre (done with micro-TBMs) to 19.25 m 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.

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Tunnel boring machine - Hydraulic jacks holding a TBM in place.by James Henry Greathead (6 August 1844 – 21 October 1896) was a civil engineer renowned for his work on the London Underground railway. https://en.wikipedia.org/wiki/James_Henry_Greathead