{"id":4733,"date":"2026-04-14T15:44:23","date_gmt":"2026-04-14T15:44:23","guid":{"rendered":"https:\/\/the-cornerstonegroup.com\/seattle-tunnel-sr-99-alaskan-way-viaduct-replacement\/"},"modified":"2026-04-14T15:57:15","modified_gmt":"2026-04-14T15:57:15","slug":"seattle-tunnel-sr-99-alaskan-way-viaduct-replacement","status":"publish","type":"page","link":"https:\/\/the-cornerstonegroup.com\/fr\/seattle-tunnel-sr-99-alaskan-way-viaduct-replacement\/","title":{"rendered":"Seattle Tunnel (SR-99 Alaskan Way Viaduct Replacement)"},"content":{"rendered":"
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Seattle Tunnel (SR-99 Alaskan Way Viaduct Replacement)<\/h1>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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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.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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Delivering One of the World\u2019s Most Complex Urban Tunnel Infrastructure Projects<\/h4>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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Client name<\/h6>\t\t\t\t<\/div>\n\t\t\t\t
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Washington State Department of Transportation (WSDOT)<\/h6>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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Project Value<\/h6>\t\t\t\t<\/div>\n\t\t\t\t
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2,8 milliards de dollars<\/h6>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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Location<\/h6>\t\t\t\t<\/div>\n\t\t\t\t
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Seattle, Washington, USA.<\/h6>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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Sector<\/h6>\t\t\t\t<\/div>\n\t\t\t\t
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Transportation Infrastructure \/ Heavy Civil Engineering<\/h6>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Project Overview & Strategic Intent<\/h2>\t\t\t\t<\/div>\n\t\t\t\t
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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.<\/p>

  • Bertha TBM:<\/strong> 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.<\/li>
  • Urban Preservation:<\/strong> 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.<\/li><\/ul>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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    Detailed Scope of Works<\/h2>\t\t\t\t<\/div>\n\t\t\t\t
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    The engineering scope required managing extreme geological and urban pressures:<\/p>

    • Bored Tunnel Construction:<\/strong> 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.<\/li>
    • Geotechnical & Settlement Monitoring:<\/strong> 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.<\/li>
    • Double-Deck Internal Structure:<\/strong> 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.<\/li>
    • Portal & Interchange Works:<\/strong> 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.<\/li><\/ul>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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      Role & Leadership: Alagie Sanyang<\/h2>\t\t\t\t<\/div>\n\t\t\t\t
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      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:<\/p>

      • Executive Safety Governance:<\/strong> Leading the corporate strategic vision for Environment, Health, and Safety (EH&S) across major North American projects.<\/li>
      • Risk Mitigation Oversight:<\/strong> Managing the high-stakes operational risks associated with massive-scale tunneling and the protection of adjacent high-value urban assets.<\/li>
      • Compliance & Certification:<\/strong> Ensuring the project met rigorous international standards, including ISO 45001 for Health and Safety, which he spearheaded during his executive tenure.<\/li><\/ul>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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        Environmental, Social & Urban Impact<\/h2>\t\t\t\t<\/div>\n\t\t\t\t
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        • Seismic Resilience:<\/strong> 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.<\/p><\/li>

        • Waterfront Revitalization:<\/strong> 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.<\/p><\/li>

        • Economic Vitality:<\/strong> Maintains a critical north-south freight and commuter corridor that supports the regional economy of the Pacific Northwest.<\/p><\/li><\/ul>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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          Key Stakeholders<\/h2>\t\t\t\t<\/div>\n\t\t\t\t
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          • WSDOT:<\/strong> Project owner and lead public agency.<\/p><\/li>

          • Seattle Tunnel Partners (STP):<\/strong> The joint venture (led by Dragados and Tutor Perini) responsible for design and construction.<\/p><\/li>

          • The Cornerstone Group Leadership:<\/strong> Executive-level project governance and safety systems oversight.<\/p><\/li><\/ul>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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            Critical Success Factors<\/h2>\t\t\t\t<\/div>\n\t\t\t\t
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            1. Technological Innovation:<\/strong> Successfully operating a record-sized TBM through variable soil conditions in a high-seismic zone.<\/p><\/li>

            2. Precision Geotechnics:<\/strong> Preventing catastrophic damage to historic Seattle landmarks through real-time feedback loops.<\/p><\/li>

            3. Governance Maturity:<\/strong> Utilizing executive leadership to manage the immense public, regulatory, and technical scrutiny inherent in a $2.8B mega-project.<\/p><\/li><\/ol>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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              Seattle Tunnel (SR-99 Alaskan Way Viaduct Replacement) The Seattle SR-99 Tunnel Project is…<\/p>","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-4733","page","type-page","status-publish","hentry"],"_hostinger_reach_plugin_has_subscription_block":false,"_hostinger_reach_plugin_is_elementor":false,"_links":{"self":[{"href":"https:\/\/the-cornerstonegroup.com\/fr\/wp-json\/wp\/v2\/pages\/4733","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/the-cornerstonegroup.com\/fr\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/the-cornerstonegroup.com\/fr\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/the-cornerstonegroup.com\/fr\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/the-cornerstonegroup.com\/fr\/wp-json\/wp\/v2\/comments?post=4733"}],"version-history":[{"count":4,"href":"https:\/\/the-cornerstonegroup.com\/fr\/wp-json\/wp\/v2\/pages\/4733\/revisions"}],"predecessor-version":[{"id":4737,"href":"https:\/\/the-cornerstonegroup.com\/fr\/wp-json\/wp\/v2\/pages\/4733\/revisions\/4737"}],"wp:attachment":[{"href":"https:\/\/the-cornerstonegroup.com\/fr\/wp-json\/wp\/v2\/media?parent=4733"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}