Definition - What does Shield mean?
A shield is a protective cylinder used at the face of a tunnel during excavation. In trenchless construction, the term is sometimes used interchangeably with tunnel boring machines (TBMs), which may employ either open or closed shield technologies.
The primary purpose of a tunnel shield, as the name suggests, is to provide a safe barrier between workers and unstable ground.
Trenchlesspedia explains Shield
The ground encountered during tunnel construction can vary greatly in consistency. It may be soft and even semi-liquid, or it could be made of hard rock. A single tunnel might pass through many different types of soil. Since the earliest days of tunnel construction, workers have recognized the need to shield themselves from the dangers of potential cave-ins, rock falls, or encroaching soil or water.
One notable example is the Brunel Tunnel beneath the River Thames in London. The challenge of tunneling under a river had baffled engineers for centuries. How would they handle the inevitable seepage of water and the soft ground involved? The engineer Marc Isambard Brunel developed the first successful tunneling shield, a rectangular supporting structure, to enable the safe completion of the project in 1843.
Today, sophisticated tunnel boring machines use shield methods to excavate tunnels of great size. A TBM named Bertha, with a 57-foot diameter, was created in 2013 for a major tunneling project in Seattle. As these machines progress through dangerous ground, human workers safely conduct operations within their cylindrical body. Open-face shields allow for workers or excavating arms to have direct contact with the tunnel face.