If you’re a fan of our construction cams, this summer has been all about the pits.
Up at the SR 99 tunnel’s north portal, our disassembly pit camera gave viewers a front-row seat for the SR 99 tunneling machine’s remarkable disappearing act. Down at the south portal, cam fans have seen something equally remarkable – the transformation of Bertha’s launch pit into a section of the SR 99 tunnel.
The timing of these twin transformations is no coincidence. During tunnel mining, the launch pit served as the access point for delivering tunnel ring segments and supplies to the machine. The pit also housed a major piece of the conveyor belt that delivered muck from the machine to barges docked at Terminal 46 on Seattle’s waterfront.
After tunnel mining concluded in April 2017, contractor Seattle Tunnel Partners began the transformation of Bertha’s launch pit into a section of the SR 99 tunnel. In June, after removing the conveyor belt, crews started constructing this section of the tunnel. Three months later, the transformation from pit to tunnel is nearly complete. This slideshow shows how they did it.
Step 1: Build lower interior walls (red scaffolding) on top of the lower roadway.
Step 2: Build upper roadway on top of the lower interior walls.
Step 3: Build upper interior walls on top of the upper roadway.
Step 4: Build the tunnel lid on top of the upper interior walls.
Putting a lid on it
The last phase of the launch pit transformation is underway now. Crews are building the top of the tunnel, a 4 1/2-foot-thick slab of reinforced concrete known as the lid. Building the lid makes it possible to build streets on top of the tunnel. In 2018, crews will rebuild part of Alaskan Way South directly atop what once was Bertha’s launch pit. When the tunnel opens, three levels of roadway will move people north and south across what was once the launch pit.
This graphic shows a cutaway view of the future tunnel just west of the south portal operations building.