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 metre (done with micro-TBMs) to 19.25 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. Hard Rock Tunnel Boring Machine In hard rock, either shielded or open-type TBMs can be used. All types of hard rock TBMs excavate rock using disc cutters mounted in the cutter head. The disc cutters create compressive stress fractures in the rock, causing it to chip away from the rock in front of the machine, called the tunnel face. The excavated rock, known as muck, is transferred through openings in the cutter head to a belt conveyor, where it runs th
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