SR99 Tunnel - Alaska Way Viaduct

Early owner and contractor interaction helped to secure Ballard’s hyperbaric team the Tunnel Boring Machine (TBM) support work for the single bore, 2,850-meter (1.8 miles) Alaska Way Viaduct tunnel. The 17.48-meter (57.3 feet) diameter Bertha TBM set a world record as the largest diameter TBM when the project commenced. The double-deck design, one deck for each direction with two lanes allowing circulation of both light cars and heavy trucks, was the solution for this mega project. The tunnel alignment is fifty meters (164-feet) under sea level and passes directly under downtown Seattle.

Scope of Work
Ballard’s hyperbaric team provided on-call compressed air workers for trimix and saturation interventions to perform maintenance and cutterhead replacement on the TBM. The team provided evaluation, setup, and integration of saturation equipment into the TBM system, all detailed in a custom project-specific hyperbaric saturation operations manual created for the project.

Innovative Solutions
The Seattle Tunnel Partners (STP) team consulted with Ballard’s hyperbaric support managers during the bidding process to develop a first-of-its-kind saturation habitat specifically for tunnel work. Early Contractor Involvement (ECI) ensured that Ballard’s hyperbaric systems fulfilled the needs of both the owners and the prime contractor while liaising on their behalf with state regulators regarding applicable codes, standards, and variances. The Ballard-owned saturation system provided the capability of housing up to twenty compressed air workers, and their life support crews.

Ballard schematic reviews, air consumption calculations, site assessments, and a hyperbaric operation manual all aided in creating a world-class hyperbaric tunnel support operation for STP.

TBM Type: Hitachi /EPB
Location: Seattle, WA
Design Pressure: 26 bar/87 PSI


•Developed project specific hyperbaric saturation operations manual
• Evaluation, setup, and integration of saturation equipment into TBM systems
• Provided on-call compressed air workers for trimix and saturation interventions
• Assisted in acquisition of necessary variances
• Contractor compressed air worker training
• Saturation
• Mixed Gas
• Welding/burning under pressure
• Confined space diving