Tunneling is generally performed by either boring or the open cut method. Bored tunnels are cut entirely underground. Chicago’s early tunnels used this method. Workers chopped away the clay by hand. Deeper tunnels in the limestone bedrock were drilled and blasted with explosives. Modern tunnels also use boring machines.
The open cut method involves digging a deep trench, building the tunnel structure, then filling in over. Segments of the CTA’s subways were built this way. It is the most common method for utility construction.
A variation called “raising the grade” is common in Chicago. The tunnel structure is built above ground, then earth heaped over it. The raised ground then becomes the new ground level. This method was used for Chicago’s first sewer system and Lower Wacker Driver. Sometimes you don’t know what to call the result. It may have a creepy tunnel like feel, but seems to be above ground.
A third method involves building a tube and dropping it in a trench in the bottom of the river. This method was used to build the three river crossings for the subway, and to rebuild the three currently unused street car tunnels.