Road expansion is difficult and very expensive in Greater Seattle, due to its location on hilly ground between Puget Sound and Lake Washington.
The Central Puget Sound Regional Transit Authority (now known as Sound Transit) was founded by the Snohomish, King, and Pierce County Councils to provide a light rail alternative for moving people and goods.
The first planned line of Sound Transit’s light rail system was the Central Link, connecting downtown Seattle with the city’s airport.
Sound Transit retained Mott MacDonald to provide engineering services for the 5-mile (8-kilometer) Tukwila segment of the Central Link. Our successful performance led us to be hired for additional work on Section 755, between S. Boeing Access Road and S. 154th Street, and Section 770, known as the Airport Link.
The southernmost part of the line, Section 755 included 4.2 miles (6.8 kilometers) of elevated guideway, 0.8 miles (1.3 kilometers) of retained-earth guideway, and an elevated station. Long-span structures crossed Interstate 4, the Burlington Northern Santa Fe/Union Pacific main lines, and the Duwamish River.
Mott MacDonald was the prime civil and structural designer for the project, responsible for bid documents and design review during construction. We were also responsible for project management and controls, systems and project integration, utilities, drainage, permitting assistance, estimation of construction costs, and quality assurance.
Mott MacDonald proposed using precast segmental concrete for the entire elevated guideway, constructed using the overhead self-launching erection method. We provided a full-time Design Services During Construction (DSDC) coordinator on-site to support client design and line section managers.
With 0.9 miles (1.4 kilometers) of elevated guideway and 0.8 miles (1.3 kilometers) of at-grade light rail transit guideway, Section 770 extended the line from S. 154th Street to Seattle-Tacoma International Airport. The elevated guideway included three major long-span structures over International Boulevard, Route 518, and S. 170th Street, as well as a pedestrian bridge over International Boulevard and a station at the southern end of the alignment.
Mott MacDonald was responsible for the preliminary and final design of the Airport Link and for bid and design support during construction, which began in 2007. Our initial effort focused on architectural design and the development of alternative concepts to increase the maximum train speed.
Beacon Hill Station
Mott MacDonald was also retained to provide engineering services for the Beacon Hill Station and its one mile of twin tunnels. Our work on the project won several awards, including the 2010 Engineering Excellence Grand Award from the American Council of Engineering Companies.
As of 2020, the line runs for 20 miles from Angle Lake to the University of Washington.
Our value engineering on Section 755 saved the client approximately $23 million and shortened the duration of construction by eight months. Using an innovative approach, guideway runoff was directed to longitudinal diffusers along the low deck edge, allowing runoff to be dispersed to the natural environment along about 60% of the alignment. Flared piers and tulip-shaped columns added to the elevated guideway’s aesthetic appeal.
Mott MacDonald was able to build on its experience with the Tukwila and Section 755 segments of the line in used precast concrete segments on the Airport Link. Portions of the Airport Link formed part of the Port of Seattle North Expressway Realignment contract, and the design schedule was coordinated to meet the port’s construction program.