Seattle Tunnel (SR-99 Alaskan Way Viaduct Replacement)
The Seattle SR-99 Tunnel Project is a global benchmark in civil engineering, involving the replacement of a seismically vulnerable elevated viaduct with a massive, double-deck underground corridor.
Delivering One of the World’s Most Complex Urban Tunnel Infrastructure Projects
Client name
Washington State Department of Transportation (WSDOT)
Project Value
2,8 milliards de dollars
Location
Seattle, Washington, USA.
Sector
Transportation Infrastructure / Heavy Civil Engineering
Project Overview & Strategic Intent
The primary objective was to decommission the Alaskan Way Viaduct, an aging structure prone to failure in a major earthquake. The project introduced a 2-mile, double-deck tunnelthat carries State Route 99 beneath the heart of downtown Seattle.
- Bertha TBM: At the time of launch, the project utilized “Bertha,” the world’s largest Earth Pressure Balance (EPB) Tunnel Boring Machine, featuring a record-breaking 57.5-foot diameter.
- Urban Preservation: The tunnel was bored beneath a dense corridor of more than 200 historic buildings and high-rise structures, necessitating unprecedented precision to avoid surface settlement or structural damage.
Detailed Scope of Works
The engineering scope required managing extreme geological and urban pressures:
- Bored Tunnel Construction: Excavation of the 2-mile alignment using the massive TBM, followed by the simultaneous installation of a precast concrete segmental lining to stabilize the earth and manage groundwater.
- Geotechnical & Settlement Monitoring: Implementation of a real-time instrumentation network that monitored hundreds of buildings and utilities 24/7. This system allowed the engineering team to adjust TBM face pressure in real-time to counteract ground movement.
- Double-Deck Internal Structure: Unlike traditional tunnels, the interior was designed as a two-level highway, with two lanes of traffic on each deck, requiring complex structural integration and fire-life safety systems on both levels.
- Portal & Interchange Works: Construction of the North and South portals, including sophisticated ventilation buildings and the highway tie-ins necessary to reintegrate the tunnel into the existing city grid.
Role & Leadership: Alagie Sanyang
Mr. Sanyang served as a Construction Executive on this multi-billion dollar project. Within the executive team (specifically at Dragados, a lead partner in the Seattle Tunnel Partners joint venture), his contributions included:
- Executive Safety Governance: Leading the corporate strategic vision for Environment, Health, and Safety (EH&S) across major North American projects.
- Risk Mitigation Oversight: Managing the high-stakes operational risks associated with massive-scale tunneling and the protection of adjacent high-value urban assets.
- Compliance & Certification: Ensuring the project met rigorous international standards, including ISO 45001 for Health and Safety, which he spearheaded during his executive tenure.
Environmental, Social & Urban Impact
Seismic Resilience: The tunnel is designed to withstand a 9.0-magnitude earthquake, providing a safe and reliable transit corridor that the elevated viaduct could not guarantee.
Waterfront Revitalization: By moving the highway underground, the city was able to reclaim the waterfront, leading to massive urban redevelopment, public parks, and improved aesthetic value for Seattle.
Economic Vitality: Maintains a critical north-south freight and commuter corridor that supports the regional economy of the Pacific Northwest.
Key Stakeholders
WSDOT: Project owner and lead public agency.
Seattle Tunnel Partners (STP): The joint venture (led by Dragados and Tutor Perini) responsible for design and construction.
The Cornerstone Group Leadership: Executive-level project governance and safety systems oversight.
Critical Success Factors
Technological Innovation: Successfully operating a record-sized TBM through variable soil conditions in a high-seismic zone.
Precision Geotechnics: Preventing catastrophic damage to historic Seattle landmarks through real-time feedback loops.
Governance Maturity: Utilizing executive leadership to manage the immense public, regulatory, and technical scrutiny inherent in a $2.8B mega-project.
