The E330 Downtown Bellevue Tunnel (DBT) was selected as a 2020 CMAA National Project Achievement Award winner in the category “Transportation: Construction Value Greater than $50 Million.” It was also chosen by Engineering News Record Northwest as the best regional project in the “Airport/Transit” category.
The $121 million DBT is part of Sound Transit’s $3.7 billion East Link Extension Program, a 14-mile (22.5 km) extension of the Sound Transit light rail transit system from downtown Seattle, across a floating bridge over Lake Washington, to the cities of Mercer Island, Bellevue, and Redmond. It was the first East Link project Sound Transit issued and posed a significant construction risk. McMillen Jacobs Associates, as subcontractor to HDR, Inc., was the Construction Manager on this project.
The DBT project consists of a 250-foot-long (76 m) cut-and-cover portal structure at the southern end, a 1,983-foot-long (604 m) soft ground tunnel constructed using the Sequential Excavation Method (SEM), and a mid-tunnel access shaft with a short adit connecting it to the tunnel. The DBT is an ovoid-shaped tunnel with a typical excavated cross section that is 37.7 feet wide by 30.5 feet high (11.5 x 9.3 m). This cross section is enlarged to 42.3 feet wide by 37.7 feet high (12.9 x 11.5 m) near the tunnel’s midpoint, to provide space for an emergency ventilation fan room above the tracks.
During construction, several value engineering improvements were made, including elimination of pipe canopy support on the North Portal, allowing the follow-on E335 Contract to continue construction of the Bellevue Downtown Station without interrupting its work for E330 to install the canopy. Several requirements, including prescriptive installation of tunnel presupport, were eliminated, and presupport was installed where necessitated by the ground. The team also investigated new technologies and materials, leading to use of macro-synthetic fiber in the shotcrete and spray-applied waterproofing in the tunnel. Also, shotcrete, not commonly used as a permanent lining, was placed in the enlarged tunnel section where the emergency fans are located.
These measures resulted in a cost savings to Sound Transit of over $2.7 million and establishment of a 30-day float in the project schedule. With this proactive and open construction management style, the CM team facilitated an additional $2 million in savings for unused SEM “toolbox items” during construction. The project was completed under budget and on time.
Read More: Bellevue Tunnel Holes Through