Case Study: Unexpected High Water Table Halts Tunnel Excavation in Washington State (2020)

Project Overview
• Name: Alaskan Way Viaduct Replacement Tunnel
• Location: Seattle, Washington
• Year: 2020
• Project Size: $3.3 billion
• Scope: Tunnel excavation beneath downtown Seattle for highway replacement
• Lead Agencies/Contractors: Washington State DOT /

Category of the Issue, Problem, or Challenge
• Geotechnical
• Differing Site Conditions

Summary of the Issue, Problem, or Challenge
During tunnel boring, a much higher-than-anticipated groundwater level was encountered, causing flooding risks and requiring additional dewatering measures.

Root Cause Analysis

  • Groundwater modeling underestimated seasonal and tidal effects.
  • Incomplete hydrogeological studies of subsurface water flows.
  • Lack of contingency planning for extreme water inflows.

Impacts Due to the Issue, Problem, or Challenge

  • Work stoppages and slowed progress during flood mitigation.
  • Additional costs for pumping systems and waterproofing.
  • Increased risk of tunnel face instability and worker safety hazards.

Corrective Actions Taken

  1. Installation of advanced dewatering and water control systems.
  2. Revision of hydrogeological models with real-time monitoring data.
  3. Modified tunnel lining design for improved water tightness.
  4. Enhanced safety protocols for working in wet conditions.

Lessons Learned

  • Groundwater studies must include extreme seasonal and tidal variations.
  • Contingency plans for water ingress critical in tunnel construction.
  • Real-time hydrogeological monitoring can mitigate unexpected inflows.

Audit & Prevention: Project Control Questions to Ask on Future Projects to Help Control the Situation

  • Are groundwater models comprehensive and validated?
  • Is there sufficient pumping capacity and waterproofing design?
  • Are safety measures in place for high water conditions during tunneling?
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