Seattle, WA

Northgate Link Extension

Central Puget Sound Regional Transit Authority (Sound Transit)


The Northgate Link Extension light rail project connects Seattle’s Northgate business/​retail center with the University District. The project consists of 4.3 miles (6.9 km) of double-track light rail, 3.5 miles (5.6 km) in twin bored soft-ground tunnels, 0.8 mile (1.3 km) in retained cut portal structure, and an elevated trackway. The project also includes two underground transit stations (Roosevelt Station and University District Station), ancillary structures, an elevated multimodal station (Northgate Station), and an underground transit parking garage. 

Fast Facts
  • 4.3 miles (6.9 km) of double-track light rail
  • 3.5 miles (5.6 km) in twin bored soft-ground tunnels
  • 3 stations
  • 23 cross passages

Scope of Work

Delve Underground was the prime consultant for a team combining the talents of more than 30 firms, including local DBEs. We provided project management, station siting evaluation, tunnel and cross-passage design, station box excavation support design, station structural design, architectural management, civil design, geotechnical design/​baselines, and cost-estimating and scheduling services.

Challenges & Innovations

Project challenges included tunneling through dense, urban neighborhoods with complex subsurface and groundwater conditions, which required optimizing the location and alignment of the underground elements. Solutions included a state-of-the-art precast tunnel lining design and sequentially excavated cross passages with ground modification. In addition, the tunnels traverse beneath the University of Washington campus including numerous research laboratories with vibration sensitive equipment. 

The tunnel track slab was designed for an approximately 5 Hz resonance frequency to mitigate the operational train vibration to below agreed baseline levels during train operation. Underpinning and other innovative building protection measures were utilized adjacent to station excavations. In addition, our patented Tieback Anchor Alignment and Access Device was developed on this project.