University Link Light Rail Extension Project Tunneling

Michels Trenchless, Inc. was part of an integrated joint venture responsible for executive management field supervision and engineering on the Central Puget Sound Regional Transit Authority’s University Link Light Rail Extension Project. The project extends light rail service from Capitol Hill to downtown Seattle and the airport.

Michels scope of work included the excavation and installation of twin precast concrete segment tunnels, northbound and southbound for the LRT extension. The tunnels were excavated using a 21-foot diameter Earth Pressure Balance Tunnel Boring Machine (EPTBM) with a finished interior diameter of 18 feet, 10 inches. Each tunnel spans 3,800 linear feet, for a total of 7,600 linear feet, downhill at a 4.9 percent grade. The depth of the tunnels generally ranges 50-130 feet below ground except for the crossing beneath Interstate 5, where the depths are 13-25 feet. I-5 carries approximately 250,000 vehicles per day in that area.

Nearly the entire tunnel alignment consists of horizontal and vertical curves with the tightest curve radius being only 550 feet. Five cross-passages connecting the twin tunnels were constructed using the sequential excavation method (SEM). The project was accomplished without settlements affecting structures, with minimal disruption to the community.