SR 99 tunnel walls under construction in Pierce County
The SR 99 tunnel’s exterior walls are made up of thousands of curved concrete segments connected in rings. Rubber gaskets will fill the long grooves on the outside edge of the segments to make the tunnel lining water-tight. Manufactured in Pierce County, these rings will be installed by Bertha, the SR 99 tunneling machine, as she digs underneath Seattle. Tunneling is set to start in summer 2013. Learn more about the Alaskan Way Viaduct Replacement Program at www.alaskanwayviaduct.org or follow Bertha on twitter @BerthaDigsSR99.