Seattle Tunnel Partners has completed excavation of the 39.5m deep pit that will be used to access and repair Bertha, the 17.5m diameter Hitachi Zosen SR 99 TBM being used to bore the 2.7km long Alaskan Way Viaduct Relacement Tunnel. Crews removed the final scoop of soil from the pit on Friday, Jan. 30.
Approximately 20,000 cubic yards of material was removed from the ground over the course of excavation, which began in mid-October.
With excavation complete, crews can begin construction of the concrete cradle at the bottom of the pit. The cradle will support the machine after it moves through the pit’s southern wall, which is about 20 feet thick.
STP plans to tunnel through the concrete wall. The length of time it takes Bertha to reach the pit will depend largely on her ability to mine through and digest the concrete. If she’s unable to mine through the wall, STP will create an opening from within the pit to give her an unobstructed path forward. Once inside the pit, crews will use the massive red gantry crane (pictured) to hoist the front end of the machine to the surface for repairs. This narrated video (links to YouTube) explains the repair process in detail. You can also watch raw video of pit excavation on YouTube.
The dewatering system that controls groundwater in the pit will continue to operate at its current level as STP proceeds with its repair work. Settlement levels near the pit remain stable.
Crews building the highway inside the tunnel are also preparing for a big milestone: the first concrete pour on what will become the southbound lanes of the tunnel. Here’s what the southbound roadway currently looks like: