Open Face Shield

Definition - What does Open Face Shield mean?

An open face shield is a type of tunnel boring machine (TBM) that has no frontal support. It is used in trenchless tunnel excavation with either manual or machine digging. Its use is limited. and is dependent on the ground being tunneled and the objectives of the project. A TBM with an open face shield has lateral support only and does not provide full protection for internal machinery.

Trenchlesspedia explains Open Face Shield

There are many different types of tunnel boring machines. One of the ways to classify TBMs is by the design of the face, or the front, of the TBM. There are open face and closed face shields. Among the closed face shields, classifications include compressed air TBM, slurry shield TBM and the earth pressure balance TBM. It is common to refer to a TBM by the type of shield. For instance, a slurry shield TBM could be referred to simply as a slurry shield.

The European TBM manufacturer Baumann-Tiefbau suggests several advantages of open face shield technology. With an open face, workers have direct visibility of the ground being tunneled. This allows for closer inspection of obstacles and easier documentation of the mining process. Open face machines can be used in either dry or soft soil, but can also be used for solid rock with the right drilling heads. As Baumann points out, walkability allows for easy service and repair.

Open face shields may use a gripper system which pushes against the tunnel walls in order to propel them forward. An open face shield TBM may use a variety of excavator arms to fulfill its purpose. Ripper teeth or a hydraulic hammer can be a part of the implementation. The Baumann website says that open face shield technology can be simplified to three steps: Loosening, loading and hoisting. Project designers have several options to accomplish these tasks.

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