Cross Members Mean?
Cross members are the lateral supports provided under tunnel boring machines (TBM). As the TBM excavates, the cross members help support the machine and steer it forward. The shield is pushed forward by reaction of the supports into the tunnel wall. TBM's consist of a rotating cutting wheel, a thrust system and trailing mechanism. Depending on the type of geology, shield or open type TBM's are used.
Trenchlesspedia Explains Cross Members
An open shield TBM has the area behind the cutter head open for rock support. An open shield TBM provides only lateral support and can be single shield or double shield. A gripper system pushes against the tunnel walls to advance the machine forward. After a stroke is complete the supports are withdrawn and re-positioned to begin the next stroke. Closed shield TBMs use lateral and frontal support.
This method includes compressed air TBM, earth pressure balance machine and mixed confinement shield. These TBMs are used for a different type of geology such as soft ground or soft rock under water table and ground with high permeability.