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
- Installation of advanced dewatering and water control systems.
- Revision of hydrogeological models with real-time monitoring data.
- Modified tunnel lining design for improved water tightness.
- 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?