Coordination, Creativity, and a Lot of Concrete to Expand Seattle's Light Rail
N140 U District Station
Hoffman built a new 117,000 SF underground light rail station, one of three constructed as part of the 4.3-mile Northlink extension links UW Husky Stadium to Northgate in Seattle.
The building is mostly underground, so construction materials had to be fed in from the top. The project is designed to accommodate a transit-oriented development over the top of the station, with a maximum structure height of 280 feet.
Building a "Ship in a Bottle, Through a Storm Drain"
During construction, our team referred to the U District Station as the equivalent of building a ship in a bottle through a storm drain—the building is mostly underground, so everything had to be lowered into the excavation area through massive struts that spanned the station box.
On both stations, we utilized BIM and 4D sequencing to coordinate and sequence numerous large elements into the station box and ensure they would fit through temporary structure openings, including eight large escalators and steel escalator enclosure tubes that had to be rigged and loaded into the structure after the structure lid was completed. With the stations 90 feet below grade, we implemented extensive waterproofing systems to prevent water from entering and extensive ventilation systems that include tunnels with large 400 horsepower exhaust fans.
Working Seamlessly with a Complex Web of Contractors
With both stations on the critical path of the overall schedule for the Northgate Link Extension project, finishing the stations in time for this month’s grand opening required detailed coordination with many other Sound Transit contractors.
During three years of preconstruction, Hoffman worked through numerous logistical and phasing challenges to plan for construction to proceed in tandem with tunneling, rail, and systems happening in and around the station. Early initial planning and engagement with permitting officials helped all parties plan to allow permits to be signed off at completion of multiple systems with multiple interfaces.
Our U District team worked closely with the tunneling contractor to allow them to install the tunnel liner segments into the south third of the station box while Hoffman continued construction of the rest of the station box. The team also transferred control of the south end of the station to the rail contractor for six months to install the floating rail isolation slabs after Hoffman had only worked in that area for three months.
