"A large machine with a rotating cutter head located at the end of a hydraulically controlled boom. The cutter head spins rapidly with significant thrust and force, excavating and grinding the rock as it is pushed against the tunnel face. Some roadheaders have a loading apron which collects the excavated material, moving it onto a conveyer belt that dumps the material through the back of the machine into a truck."
Definition from the Caldecott Tunnel Fourth Bore Project website; photo by John Huseby, Caltrans.
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