Case Study: Delay from Soil Instability on Seattle Viaduct Replacement (Washington, 2015)

Project Overview
• Name: Alaskan Way Viaduct Replacement
• Location: Seattle, Washington
• Year: 2015
• Project Size: $3.1 billion
• Scope: Replacement of elevated highway with tunnel and surface street improvements
• Lead Agencies/Contractors: Washington State Department of Transportation (WSDOT) /

Category of the Issue, Problem, or Challenge
• Geotechnical
• Project Delay

Summary of the Issue, Problem, or Challenge
Unanticipated soil instability and ground settlement caused delays during tunneling and foundation works.

Root Cause Analysis

• Inadequate Soil Investigations: Initial geotechnical surveys underestimated soil variability and weak zones.
• Design Underestimation: Foundation designs did not fully account for settlement risks.
• Construction Method Limitations: Tunnel boring and excavation techniques struggled with unstable soils.
• Monitoring Delays: Slow detection of soil movement delayed responsive action.

Impacts Due to the Issue, Problem, or Challenge

• Delay of 8 months in tunneling and foundation phases.
• Increased costs due to ground stabilization and design modifications.
• Risk of damage to adjacent structures requiring mitigation.

Corrective Actions Taken

1. Expanded soil investigations with deeper and more comprehensive testing.
2. Revised foundation and support designs with enhanced soil stabilization methods.
3. Implemented real-time ground movement monitoring systems.
4. Adapted tunneling methods with slower advance and ground conditioning.

Lessons Learned

• Thorough geotechnical investigation is critical for large underground projects.
• Construction techniques must be adaptable to ground conditions.
• Early detection and monitoring prevent escalation of soil-related issues.

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

• Are soil investigations sufficient and reflective of site conditions?
• Are monitoring systems in place for ground movement?
• Is there flexibility in construction methods to adapt to soil behavior?