The existing tunnels of the CBBT were constructed using an immersed tube method, in which U-shaped trenches were prepared in the Bay bottom for each of the tunnel sections that were previously fabricated and outfitted before being floated out and sunk into place.
There are many distinct differences between an immersed tunnel and a bored tunnel. Two primary advantages of a bored tunnel solution are: a) substantially less exposure to weather risks such as wind and wave action during construction as the deeper elevations of the tunnel are constructed under the surface of the Bay, and b) less channel and open water disturbance concern to commercial ships and the military as the tunnel is constructed under Thimble Shoal Channel and the auxiliary channel.
Construction will involve digging a circular passage under Thimble Shoal Channel using a mechanized tunnel boring machine (TBM). The TBM consists of a shield (a large metal cylinder) and trailing support mechanisms. At the front end of the shield is a rotating cutting wheel that can bore through mixed ground conditions that are typical in the Chesapeake Bay.
As the cutting wheel of the TBM breaks the ground material, the material is then transferred to a conveyor belt system. A hydraulic cylinder system presses the TBM forward continuously. The pressure avoids an uncontrolled penetration of soil and water. The TBM maintains or balances the pressure during construction.
Concrete segments are fabricated in an above-ground factory and they are then transported into the tunnel by mine cars. The TBM places these segments, one at a time, to form a complete tunnel ring. This cycle continues as the TBM drills forward.
To view a demonstrative video of a tunnel boring machine, click here.