Northgate Link | USA

Project summary

Service provided:  Monitoring the surrounding area for a major twin tunnel project
Location:  Seattle, Washington - USA
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Client:  Sound Transit
Period of the service:  2014-2016
Sixense solutions used: 

Images of the project

The N125 Northgate Link


Seattle’s Link Light Rail (the Northgate Link) is a rapid transit program implemented to improve transportation for the surrounding area. Before the update, the rail consisted of 16 miles of light rail with 13 stations.

In 2014, the public transportation network was developed to connect local urban centers in the area, and the existing Light Link Rail (metro) system was extended, connecting the Northern suburbs of King County to the city center and the SeaTac airport.

After being awarded the Alaskan Way Tunnel monitoring program in 2011 (the largest monitoring contract in US history), Sixense, Inc. was quickly awarded this major twin tunnel project.


Project Description

The new 3.6-mile section consisted of an alignment of 21-feet diameter twin tunnels, two new deep stations (Roosevelt and U-District), and a new portal (Northgate) at the North end, where the tunnels reach the surface.

Sound Transit, the local transport agency, gave the contract to the US joint venture JCM (Jaydee Coluccio Michels), who subcontracted Sixense.

As the subcontractor, the company was in charge of monitoring all the buildings along the 4.3 miles of the alignment, in addition to the two new stations and the launch portal. Sixense worked vigorously on the underground instrumentation (MPBX and inclinometers), as well as the installation of the monitoring instruments at the surface (RTS, structural instruments, etc.).


Access Limitation at Seattle’s Northgate Link

There were several critical locations, with highly sensitive buildings, located close to the tail walls and headwalls of the new deep stations, such as historical landmarks and the Washington University tower.

The design of the monitoring system not only considered the geotechnical risk but also the desire to reduce the intrusion of the construction-related crews in the nearby residential areas. For this reason, the instrumentation around the station was mostly automated, while the design of the monitoring system within the residential areas was a combination of automated and manual monitoring.



The whole instrumentation program was available to the main contractor, the owner, and all other partners involved in the project through the web-based GIS Gescope platform. Geoscope provided 24/7 real-time data from all the sensors installed on site.

Tunneling started in June 2014, and the instrumentation program was installed and base lined in due time by Sixense. The Geoscope platform provided useful information to the TBM crews as they went deep beneath the structure.


Project Challenges

This job offered the exciting challenges that come with most big projects like real-time data acquisition and settlement control.

Despite the complexity of this award-winning project, Sixense successfully finished work on the Northgate Link tunnel in 2016, and the structure opened for operation in 2021.


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Project Key figures

107 MPBX
24 Inclinometer in Slurry Walls
9 Inclinometers Drilled Around Stations
15 Tiltmeters
2 Automatic Crack Gauges
7 Convergence Meters

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