Later this year construction of the Central Subway tunnels will begin on 4th Street beneath the I-80 overpass, within a major excavation known as a launch box. Excavation and construction of the launch box began last spring and is expected to continue through spring 2013. When it is complete, the launch box will reach a depth of about 50 feet and span almost the entire block between Harrison and Bryant streets. Tunnel boring machines will then excavate and construct the Central Subway tunnels, one for northbound trains and another for southbound trains.
The underground walls around the perimeter launch box have been completed. Now crews are excavating the launch box, and they recently completed construction of an access ramp.
Check out this video for a view beneath the roadway and to see the ongoing construction at the launch box site.