Editing Tunnelling shield (section)

{{Short description|Protective structure used during the excavation of tunnels}}
[[File:Thames_tunnel_shield.png|right|thumb|Side view of the tunnelling shield (far right) used to construct the [[Thames Tunnel]]; the permanent brick supporting structure is built immediately behind it.]]
[[File:Taipei MRT Xinyi Line Tunnel boring machine.jpg|right|thumb|The tunnelling shield used for the construction of the [[Tamsui–Xinyi line|Xinyi Line]] on the [[Taipei Metro]] system in [[Taiwan]].]]
A '''tunnelling shield''' is a protective structure used during the excavation of large, human-made [[tunnel]]s. When excavating through ground that is soft, liquid, or otherwise unstable, there is a potential [[Occupational safety and health|health and safety]] hazard to workers and the project itself from falling materials or a cave-in. A tunnelling shield can be used as a temporary support structure. It is usually in place for the short-term from when the tunnel section is excavated until it can be lined with a permanent support structure. The permanent structure may be made up of, depending on the period, bricks, concrete, cast iron, or steel. Although modern shields are commonly cylindrical, the first "shield", designed by [[Marc Isambard Brunel]], was actually a large, rectangular, scaffold-like iron structure with three levels and twelve sections per level, with a solid [[Structural load|load]]-bearing top surface. The structure protected the men from cave-ins as they laboured within it, digging the tunnel out in front of the shield.