Case Study: SR 99 Tunnel Project – Change Orders from Unanticipated Ground Conditions (2016)

Project Overview
• Name: SR 99 Alaskan Way Viaduct Replacement Tunnel
• Location: Seattle, Washington
• Year: 2016
• Project Size: $3.3 billion
• Scope: Bored tunnel replacement of aging elevated highway
• Lead Agencies/Contractors: Washington State DOT (WSDOT) /

Category of the Issue, Problem, or Challenge
• Contract Change Order
• Differing Site Conditions

Summary of the Issue, Problem, or Challenge
Bertha, the tunnel boring machine (TBM), struck an unexpected steel pipe casing left from a previous geotechnical borehole, causing severe mechanical damage. The repair delay triggered a series of change orders as both contractor and owner disputed responsibility and costs under differing site conditions clauses.

Root Cause Analysis

  • Incomplete subsurface utility information provided to contractor
  • Contract language on “unforeseen obstructions” was ambiguous
  • Delays in resolving scope of repair responsibilities
  • Lack of shared risk model for subsurface unknowns

Impacts Due to the Issue, Problem, or Challenge
• 2-year project delay
• $223 million in contractor claims
• Extensive legal disputes and cost-sharing arbitration

Corrective Actions Taken

  1. Amended contract language to clarify unforeseen condition thresholds
  2. Enhanced documentation of prior site investigations and installations
  3. Third-party risk allocator hired for future major tunneling efforts
  4. Added comprehensive geotechnical baseline reports (GBR) in similar projects

Lessons Learned

  • Differing site conditions must be explicitly addressed in contract language
  • Complete records of historical site activity must be shared
  • TBM projects need detailed baseline and risk sharing documents
  • Clear claim resolution pathways reduce change order escalation

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

  • Are differing site condition clauses clear and well-scoped?
  • Has all subsurface history been disclosed and verified?
  • Is a geotechnical baseline report included in bid documents?
  • Are dispute resolution procedures defined for subsurface issues?
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