Tunnel boring machines are designed to drill large-scale excavation holes into terrain that would otherwise be difficult or even impossible to penetrate. They are commonly used in the construction of underground tunnels and bypasses modern motorway and train tunnels are often built with them-and they are specially designed not just to cut through rock and earth, but crucially to support the tunnel in the process. Additionally, today's most advanced boring machines are engineered to remove debris as it is generated by the drilling process, with rocks and rubble transported to the rear of an assembly on conveyor belts for easy disposal.
Key to any tunnel-boring machine though is its colossal cutterhead, a cylindrical wall of disc cutters and drills that-in partnership with extreme pressure, which is generated by the bore's thrust cylinders-literally crush any material that sits in its path. The largest of these cutterheads currently in operation isa 15.2-meter (49.8-foot) across Herrenknecht-brand EPB Shield, a record-breaking piece of machinery that was used to carve out chunks of earth in the construction of Madrid's M-30 motorway north tunnel. The total assembly is huge and it weighs hundreds of tons.
This part can rotate around the machine's axisand drill holes into therockfor supporting metalbolts.
The rear of the bore and the back-up systems rest on the feet of the walking device. They are lifted as tunneling progresses and the back-up system follows.
These press the rotating cutterhead against the tunnel face.
The system's buckets transport the excavated rubble behind the cutterhead onto a conveyor belt system.
These are mounted in the cutterhead and roll in concentric circles over the tunnel face. The contact pressure crushes the rock.
All the excavation tools are mounted in the cutterhead, which also supports the tunnel face.