Open Shield Mean?
Open shield refers to a type of tunnel boring machines (TBM) in which the cutting face of the shield is open, allowing direct access to the soil face. Open shields are used to provide lateral support and can also accommodate manual excavation as opposed to other types of TBM that provide frontal support.
Open shields may also be called an open face shield.
Trenchlesspedia Explains Open Shield
Open shield machines provide no frontal support. These machines use a gripper system instead, that pushes against the walls of the bore for lateral support. The grippers provide some steering ability and react to the force of the gripper as the machine thrusts forward. As the legs of the grippers reach full extension, the cylinders are retracted and the shoes are re-positioned in preparation for the next stroke. Open shield machines do not install concrete liners behind them as most other machines do.