Fabrication is a stage in tunnel construction, and building these tunnel segments is referred to as "fabrication."
Tunnels for crossing rivers aren't fabricated beneath the surface in the bore the tunnel boring machine (TBM) has created. Instead, they are built ashore, alongside the river in a structure called a "graving dock," where their individual elements are readily available to engineers, construction workers and other specialists in an environment where those elements are accessible, visible and can be changed as needs merit.
Trenchlesspedia Explains Fabrication
The process of fabricating tunnel segments is a continuous production process. The materials for the tunnel segment, concrete and its supporting materials, such as rebar and steel mats, are shipped to the fabrication point where they are turned into the mass-produced tunnel segments,
As the segments are completed, they're staged in a special type of drydock, called a graving dock. The graving dock has direct access to the river that allows the dock to be flooded and the newly constructed tunnel segment floated into the river under tow by tugs. The tugs position the segment and sink it in the prepared trench in the riverbed, where it's buried in the trench.