Opportunity
Sound Transit’s East Link project is a $2.8 billion, 14-mile (22-kilometer) light rail transit extension that will connect downtown Seattle with the rapidly developing Eastside cities of Bellevue and Redmond. The South Bellevue to Overlake Transit Center segment is 7 miles (11 kilometers) long.
The most politically and technical challenging aspect of the segment is the portion that runs through downtown Bellevue. The proposed Downtown Bellevue Tunnel will run along 110th Avenue NE, a major thoroughfare that includes the 25-story Skyline Tower, the 26-story City Center Plaza Tower, and the Bellevue City Hall, known for its award-winning aesthetics.
Preliminary engineering for the project assumed the construction of a cut-and-cover tunnel, with a temporary traffic decking system to maintain surface traffic during construction. The tunnel will also house the new Bellevue Transit Center station.
Construction impacts to pedestrian and vehicular surface traffic, utilities, and businesses were expected to last more than three years.
Solution
Mott MacDonald’s long association with Sound Transit gave our team the insight needed to provide an innovative solution. Beginning in 2000, Mott MacDonald has worked with Sound Transit on its ambitious Central Link project, including the Central Link, the 1.7-mile (2.7-kilometer) Airport Link, and the Beacon Hill Station with its mile of twin tunnels. Our work won several awards, including the 2010 Engineering Excellence Grand Award from the American Council of Engineering Companies.
In 2012, Sound Transit retained us as part of a joint venture to provide engineering services for the East Link project, including the following:
- Seven miles (11 kilometers) of double-track light rail transit line
- 8,900 feet (2.7 kilometes) of at-grade line, including storage track
- 16,200 feet (4.9 kilometers) of aerial guideway
- 400 feet (123 meters) of trestle structure
- Long-span bridges over Interstates 90 and 405
- Four at-grade stations
- Two elevated stations
- One retained cut station
- Two parking structures and one surface lot
- Approximately 2,100 feet (640 meters) of retained fill
- Creek relocation and park
In addition, we were responsible for project management of Area 2, including the Downtown Bellevue Tunnel. Noting that the excavations to either side of the proposed Bellevue station were relatively deep, and that local geology was favorable, we recommended a formal study to weigh the proposed cut-and-cover tunnel against a tunnel constructed by the Sequential Excavation Method (SEM) that would eliminate the majority of the surface disruption to 110th Ave NE.
Mott MacDonald completed a comprehensive feasibility study, including development of an appropriate tunnel cross-section and excavation sequences and initial support needs based on anticipated ground conditions, assessment of ground movements, impact on buildings and utilities, and final tunnel lining requirements. All were assessed in sufficient detail to provide accurate cost estimates.
Primary risks for the cut-and-cover tunnel included utility relocation and impacts, and the extent of traffic decking and disruptions required on 110th Avenue NE. Primary risks for the SEM tunnel included geotechnical conditions, productivity, allowable working hours, and market conditions. Mott MacDonald and Sound Transit identified, ranked, and provided mitigations for significant risks associated with each option.
The feasibility estimated construction cost of an SEM tunnel at $104 million, with completion in 36 months. The cut-and-cover tunnel was estimated at $118 million and 40 months. This conclusion, coupled with risk analysis, led Sound Transit and the city to approve the SEM tunnel concept in April 2013.
Benefits
According to Sound Transit’s project manager for the East Link project, the SEM tunnel has made Sound Transit “look like heroes” to city officials, who were fearful of the magnitude of surface disruption and public reaction that would be caused by the cut-and-cover option.
Mott MacDonald is continuing to refine its design for the tunnel, using laser profiling to establish excavation lines and analyze the use of spray-applied waterproofing membrane and a final lining of steel-fiber-reinforced shotcrete.