Case Study: Seattle Alaskan Way Viaduct Replacement Tunnel Design Adjustments (2014)

Project Overview
• Name: Alaskan Way Viaduct Replacement Tunnel
• Location: Seattle, Washington
• Year: 2014
• Project Size: $3.3 billion
• Scope: Construction of a deep-bore highway tunnel to replace the aging viaduct
• Lead Agencies/Contractors: Washington State DOT /

Category of the Issue, Problem, or Challenge
• Design
• Tunnel & Structural Engineering

Summary of the Issue, Problem, or Challenge
Unexpected soil settlement during tunneling required rapid design modifications to the tunnel lining and adjacent foundation supports to maintain structural integrity and avoid damage to nearby buildings. These design changes caused construction delays and cost increases.

Root Cause Analysis

• Initial geotechnical models underestimated soil variability and settlement risk.
• Design lacked sufficient flexibility to adapt to subsurface conditions.
• Limited real-time geotechnical monitoring integration into design process.
• Coordination delays between design engineers and construction teams.

Impacts Due to the Issue, Problem, or Challenge
• Construction delays of 5 months due to redesign and installation of additional supports.
• Cost overruns estimated at $12 million.
• Increased risk to adjacent structures and public safety concerns.

Corrective Actions Taken

1. Enhanced geotechnical investigation and monitoring during tunneling.
2. Developed adaptive design protocols for tunnel lining and supports.
3. Strengthened coordination and communication channels between design and construction teams.
4. Implemented advanced instrumentation to detect ground movement in real time.

Lessons Learned

• Geotechnical investigations must be comprehensive and continually updated.
• Design flexibility is critical for complex underground construction projects.
• Real-time monitoring data should inform ongoing design decisions.
• Strong collaboration between designers and contractors accelerates response to issues.

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

• Are geotechnical models validated and updated with field data during construction?
• Is the design adaptable to unforeseen subsurface conditions?
• Is real-time monitoring data integrated into design decision-making?
• Are design and construction teams collaborating effectively and promptly?